摘要:

CONTENTS PAGE PRESIDElVTIAL ADDRESS. TRANSITION-METAL COMPLESES OF SOME MOLECULAR COMPLEXES OF WATER I N ORGANIC SOLVENTS AKD I N THE VAPOUR PHASE. By Sherril D. Christian Ahrned A. Taha and Bruce W. Gash . . 20 PHOTOCHEMICAL REACTIONS I N NATURAL PRODUCT SYNTHESIS. By P. G. Sarnrnes 37 THE CO-ORDINATION OF AMBIDEXJATE LIGANDS. By A. H. Norbury and A. I. P. Sinha . . 69 WAVE FUNCTIONS FOR SMALL MOLECULES BASED ON LINEAR COMBINATIONS OF ATOMIC ORBITALS. By R. G. Clark and E. Theal Stewart . . 95 DEVELOPMENTS IN THE CHEMISTRY OF DIAZO-ALKANES. By G. W. Cowell and A. Ledwith. . 119 STRUCTURE AND PROPERTIES OF AQUEOUS SALT SOLUTIONS. By M. J. Blandamer 169 THE THERMAL DECOMPOSITION OF HYDROUS LAYER SILICATES AND T H ~ R RELATED HYDROXIDES. By N. H. Brett K. J. D. MacKenzie and J. H. Sharp .. 185 THE STEREOCHEMISTRY OF POLYSULPHIDES. By R. Rahman S. Safe and A. Taylor 208 THE STUDY OF SIMPLE LIQUIDS BY COMPUTER SIMULATION. By I. R. McDonald and K. Singer . . . 238 VOLATILE COMPOUNDS OF THE HYDRIDES OF SILICON AND GERMANIUM WITH ELEMENTS OF GROUPS V AND VI. . 263 TRIMETHYLENEMETHANE AND RELATED CY,CY’-DISUBSTITUTED ISOBUTENES. By Francis Weiss . . I . 278 PERFLUORO-LIGANDS. By Sir Ronald Nyholrn F.R.S. . . . I By John E. Drake and Chris Riddle . ORGANOTHALLIUM CHEMISTRY. By A. G. Lee . . . . 310 AND CALCIUM. By R. J. P. Williams . . . . 331 TILDEN LECTURE. THE BIOCHEMISTRY OF SODIUM POTASSIUM MAGNESIUM CENTENARY LECTURE. ROADS TO CORRINS. By A. Eschenmoser . . 366 THE VAN DER WAALS FLUID A RENAISSANCE. By M. Rigby . . 416 THE SEPARATION OF POLAR STERIC AND RESONANCE EFFECTS I N ORGANIC RE- ACTIONS BY THE USE OF LINEAR FREE ENERGY RELATIONSHIPS.By J. Shorter . 433 APPLICATION OF COMPUTERS I N CHEMICAL ANALYSIS AMINO-ACID ANALYSIS AND SEQUENCE DETERMINATION. By B. Sheldrick . . . . . . 4 5 4 Bartlett . . 473 By E. W. Abel and F. G. A. Stone . . 498 ACIDIC AND BASIC AMIDE HYDROLYSIS. By Charmian O’Connor . . 553 THE PHOTOLYSIS OF SIMPLE INORGANIC ANIONS IN SOLUTION. By Malcolm F. Fox 565 BASE-CATALYSED ISOMERISATION OF ACETYLENES. By R. J. Bushby . . 585 DIFFUSION IN TONIC SOLIDS. By J. M. Pollock . . 601 THE REACTIONS OF HYDRAZINE WITH TRANSITION-METAL COMPLEXES. By F. Bottornley . . . . 617 CENTENARY LECTURE. MECHANISMS OF CYCLOADDITION. By Paul D. THE CHEMISTRY OF TRANSITION-METAL CARBONYLS SYNTHESIS AND REACTIVITY. CONTENTS PAGE PRESIDElVTIAL ADDRESS.TRANSITION-METAL COMPLESES OF SOME MOLECULAR COMPLEXES OF WATER I N ORGANIC SOLVENTS AKD I N THE VAPOUR PHASE. By Sherril D. Christian Ahrned A. Taha and Bruce W. Gash . . 20 PHOTOCHEMICAL REACTIONS I N NATURAL PRODUCT SYNTHESIS. By P. G. Sarnrnes 37 THE CO-ORDINATION OF AMBIDEXJATE LIGANDS. By A. H. Norbury and A. I. P. Sinha . . 69 WAVE FUNCTIONS FOR SMALL MOLECULES BASED ON LINEAR COMBINATIONS OF ATOMIC ORBITALS. By R. G. Clark and E. Theal Stewart . . 95 DEVELOPMENTS IN THE CHEMISTRY OF DIAZO-ALKANES. By G. W. Cowell and A. Ledwith. . 119 STRUCTURE AND PROPERTIES OF AQUEOUS SALT SOLUTIONS. By M. J. Blandamer 169 THE THERMAL DECOMPOSITION OF HYDROUS LAYER SILICATES AND T H ~ R RELATED HYDROXIDES. By N. H. Brett K. J. D. MacKenzie and J. H. Sharp . . 185 THE STEREOCHEMISTRY OF POLYSULPHIDES.By R. Rahman S. Safe and A. Taylor 208 THE STUDY OF SIMPLE LIQUIDS BY COMPUTER SIMULATION. By I. R. McDonald and K. Singer . . . 238 VOLATILE COMPOUNDS OF THE HYDRIDES OF SILICON AND GERMANIUM WITH ELEMENTS OF GROUPS V AND VI. . 263 TRIMETHYLENEMETHANE AND RELATED CY,CY’-DISUBSTITUTED ISOBUTENES. By Francis Weiss . . I . 278 PERFLUORO-LIGANDS. By Sir Ronald Nyholrn F.R.S. . . . I By John E. Drake and Chris Riddle . ORGANOTHALLIUM CHEMISTRY. By A. G. Lee . . . . 310 AND CALCIUM. By R. J. P. Williams . . . . 331 TILDEN LECTURE. THE BIOCHEMISTRY OF SODIUM POTASSIUM MAGNESIUM CENTENARY LECTURE. ROADS TO CORRINS. By A. Eschenmoser . . 366 THE VAN DER WAALS FLUID A RENAISSANCE. By M. Rigby . . 416 THE SEPARATION OF POLAR STERIC AND RESONANCE EFFECTS I N ORGANIC RE- ACTIONS BY THE USE OF LINEAR FREE ENERGY RELATIONSHIPS. By J. Shorter . 433 APPLICATION OF COMPUTERS I N CHEMICAL ANALYSIS AMINO-ACID ANALYSIS AND SEQUENCE DETERMINATION. By B. Sheldrick . . . . . . 4 5 4 Bartlett . . 473 By E. W. Abel and F. G. A. Stone . . 498 ACIDIC AND BASIC AMIDE HYDROLYSIS. By Charmian O’Connor . . 553 THE PHOTOLYSIS OF SIMPLE INORGANIC ANIONS IN SOLUTION. By Malcolm F. Fox 565 BASE-CATALYSED ISOMERISATION OF ACETYLENES. By R. J. Bushby . . 585 DIFFUSION IN TONIC SOLIDS. By J. M. Pollock . . 601 THE REACTIONS OF HYDRAZINE WITH TRANSITION-METAL COMPLEXES. By F. Bottornley . . . . 617 CENTENARY LECTURE. MECHANISMS OF CYCLOADDITION. By Paul D. THE CHEMISTRY OF TRANSITION-METAL CARBONYLS SYNTHESIS AND REACTIVITY.

ISSN:0009-2681    

DOI:10.1039/QR97024FX005

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Quarterly Reviews No 2 Vol24 1970 Page Structure and Properties of Aqueous Salt Solutions By M. J. Blandamer 169 The Thermal Decomposition of Hydrous Layer Sili- cates and their Related Hydroxides By N. H. Brett K. J. D. MacKetizie and J. H. Sharp 185 The Stereochemistry of Polysulphides By R. Rahman S. Safe and A. Taylor 208 The Study of Simple Liquids by Computer Simulation By I. R. McDonald and K. Singer 238 Volatile Compounds of the Hydrides of Silison and Germanium with Elements of Groups V and VI By John E. Drake and Chris Riddle 263 Trimethylenemethane and Related a,a'-Disub- stituted Isobu tenes By Francis Weiss Organo thallium Chemistry By A. G. Lee 278 310 The Chemical Society London Quarterly Reviews contains articles by recognised authorities on selected topics from general physical inorganic and organic chemistry.The Journal and Annual Reports interest primarily the research worker Quarterly Reviews is designed for a wider range of readers. It is intended that each review article shall be of interest to chemists generally and not only to workers in the particular field being reviewed. The submission of reviews for publication is welcomed but intending authors are advised to write in the first place to the Editor The Chemical Society Burlington House Piccadilly London W 1V OBN. Such pre- liminary communications should be accompanied by an outline of the ground to be covered (about two quarto pages) rather than by the completed manuscript. Price to non-fellows E4 10s. Od. per annurn. The Chemical Society-Endowed Lectures. The Council of The Chemical Society has decided that the Endowed Lectures of the Society shall in future be published in full in Quarterly Reviews to ensure that there will be a permanent record. These will be in addition to the normal Review articles. 0 Copyright reserved by The Chemical Society 1970 Published by The Chemical Society Burlington House London. Printed in England by Eyre & Spottiswoode Ltd Thanet Press Margate

ISSN:0009-2681    

DOI:10.1039/QR97024FP006

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Ami no-acids Peptides and Proteins Volume I The Chemical Society announces the publication of Volume 1 in this the third title in their series of Specialist Periodical Reports. This volume reviews and evaluates progress reported during 1968 to a depth and comprehensiveness not available elsewhere. The scope is indicated by the principal chapter headings Amino-acids ; Structural investigation of peptides and proteins ; Peptide synthesis ; Peptides of abnormal structure; The relationship between structure and biological activity of peptides and proteins ; Metal derivatives of amino-acids peptides and proteins. The coverage includes naturally-occurring synthetic and chemically modified materials and chemical physical stereochemical analytical structural and synthetical studies. Publication will be annual authorship being undertaken by a team of eight scientists led by Dr.G. T. Young of Oxford as Senior Reporter. Specialist Periodical Reports are designed to assist the research worker or specialist in his own field t o give the non-specialist a concentrated but complete view of the topic being reported and to provide libraries with a useful source book. Size 83" x 53" Pages xii + 308 Cloth Bound SBN 85186 004 4 Price per volume Fellows of The Chemical Society f 3. 0.0 (US $7.20) Non- Fellows f4.10.0 (US $1 0.80) This publication may be ordered from the Publications Sales Officer The Chemical Society Blackhorse Road Letchworth Herts England. Ami no-acids Peptides and Proteins Volume I The Chemical Society announces the publication of Volume 1 in this the third title in their series of Specialist Periodical Reports.This volume reviews and evaluates progress reported during 1968 to a depth and comprehensiveness not available elsewhere. The scope is indicated by the principal chapter headings Amino-acids ; Structural investigation of peptides and proteins ; Peptide synthesis ; Peptides of abnormal structure; The relationship between structure and biological activity of peptides and proteins ; Metal derivatives of amino-acids peptides and proteins. The coverage includes naturally-occurring synthetic and chemically modified materials and chemical physical stereochemical analytical structural and synthetical studies. Publication will be annual authorship being undertaken by a team of eight scientists led by Dr. G. T. Young of Oxford as Senior Reporter. Specialist Periodical Reports are designed to assist the research worker or specialist in his own field t o give the non-specialist a concentrated but complete view of the topic being reported and to provide libraries with a useful source book. Size 83" x 53" Pages xii + 308 Cloth Bound SBN 85186 004 4 Price per volume Fellows of The Chemical Society f 3. 0.0 (US $7.20) Non- Fellows f4.10.0 (US $1 0.80) This publication may be ordered from the Publications Sales Officer The Chemical Society Blackhorse Road Letchworth Herts England.

ISSN:0009-2681    

DOI:10.1039/QR97024BX007

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Structure and Properties of Aqueous Salt Solutions By M. J. Blandamer DEPARTMENT OF CHEMISTRY THE UNIVERSITY LEICESTER LE 17RH Many properties of aqueous salt solutions can be qualitatively linked with molecular models for these solutions. This review examines some properties of salt solutions with reference to the properties and possible structures of the solvent water.112 Initially the properties of salts in standard states in solution and methods for evaluating single ion properties are considered. Modification of these properties by ion-ion interactions in real solutions are then examined. This review will only examine aqueous solutions of closed shell ions e.g. alkali metal halide and alkylammonium ions. This greatly simplifies the task of interpretation since in the first limit these ions can be characterised by their size shape and charge.Once the properties of these ions in water are understood then the properties of other ions e.g. those formed by transition metals can be examined although here quantum mechanical forces assume a greater import- ance. 1 lnterionic Distances A simple model for a salt so1ution3a assumes that the ions of a 1 1 electrolyte are randomly distributed at the centres of cubes volume d3. If the concentration (mol m-3) of salt is c the average distance between ion centres equals 9.399(~)-”~ nm. This distance d can be compared with ionic radii ri estimated from molecular models 3b and crystallographic data,4 e.g. r[(C,H,),N+] + r(Br-) = 0.689 nm. Although radii of ions in solution are not precisely known,s this simple calculation shows that where c > lo3 d < 0.94 nm there can be few water molecules (approx.diam. 0.28 nm) between the ions. There is indeed a close link between the properties of these systems6 and corresponding solid salts. However in very dilute solutions e.g. c = d w lo3 nm the ions are separated by many water molecules. Thus the properties of these solutions will probably be extensively influenced by the structure of water and the struc- ture of water by the ions. J. D. Bernal and R. H. Fowler J . Chem. Phys. 1933,1 515. For recent reviews of models for liquid water see for example (a) H. J. C. Berendsen ‘Theoretical and Experimental Biophysics’ Arnold London vol. I 1967; (b) D. J. G. Ives and T. H. Lemon Roy. Inst. Chem. Rev. 1968 1 62; (c) D. Eisenberg and W. Kauzmann ‘The Structure and Properties of Water’ Oxford University Press Oxford 1969.* R. A. Robinson and R. H. Stokes ‘Electrolyte Solutions’ Butterworth’s Sci. Publ. London 2nd edn. (revised) 1965 (a) p. 15 (b) p. 125 (c) p. 73 ( d ) p. 133 (e) p. 245. * L. Pauling ‘The Nature of the Chemical Bond‘ Oxford Press 1960 3rd edn. (a) p. 514 (6) p. 469. K. H. Stern and E. S. Amis Chem. Rev. 1959,59 1. (a) C. Deverell and R. E. Richards Mol. Phys. 1966 10 551; (6) F. Fister and H. G. Hertz Ber. Bunsengesellschaft Phys. Chem. 1967 71 1032. 169 Structure and Properties of Aqueous Salt Solutions 2 Analysis of Experimental Results A valid approximation for very dilute solutions (e.g. c < 1 for 1 1 electrolytes in water at 298 K where d > 9.4 nm)’va is to assume that ion-ion interactions occur through long-range electrostatic forces.The effects of these interactions on thermodynamic properties of salt solutions can be examined using the Debye-Huckel the~ry.~C Properties of a salt in its standard state in solution i.e. in the absence of ion-ion interactions can then be calculated from the properties of real salt solutions Thus the Debye-Huckel theory can predict the slope of a plot of apparent molar volume #v (determined from the densities of a series of salt solutions of differing concentrations) vs. (c)lI2 which can then be extrapolateds to give the standard partial molar volume V2* e.g. V2*(KCl) = 26.90 x Unfortunately in some cases there is no simple theoretical model on which to base an analysis of the results. The simplest procedure is to plot the variation of an observed quantity against salt concentration.In favourable instances the plot is a straight line and the slope is measured. If the plot is a smooth curve the limiting slope as c - 0 is measured. These slopes depend on and for given set of observations characterise each salt. This procedure has been used to analyse dielectric datall and n.m.r. spectrz .68913913 m3 mol-l at 298.15 K.l0 3 The Solvent-Water The term ‘hydration’ can apply either to the interaction between an ion and the nearest neighbour water molecules the primary hydration shell or to the inter- action between an ion and the solvent (solution) over the range 0 < Y < 00. Indeed the arrangement of water molecules ‘the structure of water’ is affected at some distance from an ion i.e. beyond the nearest neighbour water mole- c u l e ~ .~ * - ~ ~ The failure of the simple Born equation for Gibbs functions for ionic hydration17 and of Walden’s rule for ionic mobilitiesaJa shows that the solvent in aqueous solutions cannot be treated as a bulk continuum. Water is not a typical liquid.2 For example the molar volume of liquid water is smaller than the solid ice-Ih (common ice) at 273-15 K and 101 325 N m-2. H. S. Frank and P. T. Thompson ‘The Structure of Electrolyte Solutions’ ed. W. J. Hamer Wiley New York 1959 p. 113. * H. S. Frank ‘Chemical Physics of Ionic Solutions’ ed. B. E. Conway and R. G. Barradas Wiley New York 1966 p. 60. 10 F. Franks and H. T. Smith Trans. Faraday SOC. 1967,63,2586. l1 (a) R. Pottel ref. 8 p. 584; (b) G. H. Haggis J. B. Hasted and T. J. Buchanan J . Chem. Phys. 1952,20,1454; (c) J.B. Hasted D. M. Ritson and C. H. Collie ibid. 1948,16 1. la (a) J. Burgess and M. C. R. Symons Quart. Rev. 1968,22,276; (0) J. F. Hinton and E. S. Amis Chem. Rev. 1967 67 351. l4 H. S. Frank Z. phys. Chem. (Leipzig) 1965 228 364. l6 H. S. Frank and M. W. Evans J . Chem. Phys. 1945,13,507. l6 H. S. Frank and W-Y. Wen Discuss. Faraday SOC. 1957,24,133. l7 D. Feakins and P. Watson J . Chem. SOC. 1963,4734. l8 (a) R. L. Kay ‘Trace Inorganics in Water’ (Advances in Chemistry no. 73) American Chemical Society 1968 p. 1 ; (b) R. L. Kay and D. F. Evans J . Phys. Chem. 1966 70 2325. 0. Redlich and D. M. Meyer Chem. Rev. 1964,64,221. 0. Engel and H. G. Hertz Ber Bunsen~esellschaftphys. Chem. 1968,72,808. 170 Blandamer Further the liquid contracts on warming until T = 277.13 K (the temperature of maximum density) and then expands.The molar heat capacity Cpis significantly larger for the liquid than for the solid and has a minimum near 310 K. At 298 K the viscosity decreases as the pressure is increased and only increases the normal behaviour when the temperature exceeds 300 K. The larger relative permittivity and high melting and boiling points of water have long been taken as evidence for the importance of intermolecular hydrogen bonding. From this basic premise many detailed molecular theories have been evolved. Nevertheless most theories fall into one of two general classes,a mixture1 and uniform16 models. Uniform models propose that each water molecule has the same molecular environment as every other water molecule in the liquid. For example Pople’s model (see reference 2c) a classic of this type requires that every water molecule is hydrogen bonded to four other water molecules and that each hydrogen bond can in- dependently bend without breaking.However by far the greater number of models are mixture models and these have been extensively used in rationalizing the properties of water and more importantly in the present context of aqueous solutions. It is envisaged that no (or little) hydrogen bond bending can occur and that there are many broken hydrogen bonds in liquid water. Consequently water molecules exist in different molecular environments in water. For example there may be water molecules involved in 4 3 2 1 or 0 hydrogen bonds.lga An important proposaP is that hydrogen bonding between water molecules is a co-operative process.Association of two water molecules by hydrogen bonding stimulates association of these molecules with other water molecules the lifetime of a cluster of water molecules being ca. lo-%. In water at low temperatures there are low-density (large molar volume) networks of hydrogen-bonded water molecules. Consequently at a particular temperature and pressure liquid water has some degree of structure. With increase in temperature the extent of inter- molecular hydrogen bonding decreases i.e. the intensity of water-water interactions decreases and the structure breaks down. Conversely with decrease in temperature the structure increases. These changes in temperature result in ‘structure breaking’ and ‘structure making’ respectively. The structure of water can also be affected by adding a solute and one aim in this field is to bring together the effects of added polar and non-polar solutes in a series of related general models for these solutions.The low solubility of non-polar solutes in water is linked with a large entropy decrease which accompanies d i ~ ~ 0 1 ~ t i o n . ~ ~ ~ ~ ~ J ~ ~ ~ ~ ~ This makes dC (and dGe) positive. A characteristic of some aqueous systems is the importance of entropy changes. A major part of the entropy decrease is attributed to enhancement of water-water interactions around solute particles. This enhancement sometimes called hydrophobic hydration means that these solutes are ‘structure formers’. The structure of this water does not necessarily resemble the structure of ice-Ih l9 G. Nkmethy and H. A. Scheraga (a) J . Chem. Phys.1962,36,3382; (b) ibid. 1962,36,3401; ( c ) ibid. 1964 41 680; ( d ) J. Phys. Chem. 1968 66 1773. 2o H. S. Frank and F. Franks J. Cheni. PJrys. 1968 48,4746. 171 Structure and Properties of Aqueous Salt Solutions or the many other ice polymorphs.21 Other possible structures include those based on pentagonal dodecahedra of water molecules cJ the structure of clathrate hydrates containing non-polar guest^.^^^^^ In real solutions regions of enhanced water structure interact and can through co-operative interactions mutually enhance the water structure.lO 4 Ionic Properties An important problem in this subject is the derivation of standard partial molar quantities for ions from the corresponding quantities for salts e.g. V*(K+) and Ve(CI-) from Ve(KCI). The determination is not straightforward and several techniques have been applied.23 A.Relative Quantities.-The most direct approach assumes that the value for one ion usually the hydrogen ion is zero e.g. V*(H+) = 0 and the standard partial molar entropy S*(Ii+) = 0. Similarly in the interpretation of the ionic molal shifts cf. 170 n.m.r. shifts of salts in Ha170 8(NH,+) has been equated to zero24 in a number of investigations. B. Theoretical Analysis.-Experimental data can be fitted to equations derived from theoretical models for ion-water interactions. In most cases the ionic radius is a variable parameter. Gibbs functions for ionic solvation can be derived assuming that the Born equation26 is valid. The ionic parameters are obtained by subdividing the values for the salts into the corresponding ionic solvation parameters such that when plotted against the reciprocal of ‘corrected’ radii the points fall on the predicted straight 1ine.26v27 Clearly the validity of derived ionic values depends on the validity of the model adopted at the outset.C. Intuitively Derived Quantities.-An important example of this approach concerns analysis of B-viscosity coefficients. Viscosities of dilute salt solutions 7 relative to that of pure water can be fitted to an equation of the form:28 B-Viscosity coefficients are characteristic of each salt and each B-coefficient can be divided in principle into individual B-ionic coefficients. These ionic co- efficients reflect the effect of each ion on solvent structure. Since K+ and C1- are approximately the same size (and isoelectronic) it was suggested that in water at 298 KZ9 and over the range 288-318 K,30 B(K+) = B(C1-).21 B. Kamb ‘Structural Chemistry and Molecular Biology’ ed. A. Rich and N. Davidson Freeman San Francisco 1968. z2 G. A. Jeffrey and R. K. McMullan Progr. Znorg. Chem. 1967 8,43. z8 B. E. Conway R. E. Verrall and J. E. Desnoyers Z. phys. Chem. (Leiprig) 1965 230 157. z4 Z. Luz and G. Yagil J . Phys. Chem. 1966,70 554. 26 M. Born Z. Physik 1920 1 45. 26 W. M. Latimer K. S. Pitzer and C. M. Slansky J . Chem. Phys. 1939,7 108. 27 For other comments see K. J. Laidler and C. Pegis Proc. Roy. SOC. 1957 A 241 81. 28 R. H. Stokes and R. Mills ‘Viscosities of Electrolytes and Related Properties’ Pergamon Press London 1965. 29 R. W. Gurney ‘Ionic Processes in Solution’ McGraw-Hill New York 1953. ao M. Kaminsky Discuss.Faraday SOC. 1957 24 171. 71qo = 1 + A ( c ) ~ ’ ~ + BC (1) 172 Blandamer Such ionic coefficients can be used to calculate other ionic quantities. If the standard partial ionic entropies Sie and B-viscosity coefficients are both in- dicative of the effects of an ion on water structure then Si” and Bi should be linked If Se(H) equals -23.03 J mol-1 K-l rather than zero then in a plot of Si” vs. Bi data points for both cations and anions fall on one line.29 These new ionic entropies depend on the initial assumption that B(K+) = B(C1-). An extension of this approach involves examining a series of related salts e.g. with common anion and developing a graphical analysis such that the required ionic property is obtained as an inte~cept.~lJ~ For example,31b the standard partial molar volumes for tetra-alkylammonium iodides P(R,N+I-) in water (and water-ethanol fall on a straight line when plotted against the molecular mass of the cation M+.The intercept at M+ = 0 gives the standard partial molar volume of iodide Ve(I-) = 42.3 -t 0.2 x m3 mo1-1 82a D. Absolute Ionic Quantities.-Some ionic quantities can be unambiguously obtained. The molar conductance of a salt (1 together with the transport number of one ion ti in a solution gives the ionic conductance xi = tiA and Aie = tjeLle.3933a933b Absolute ionic partial molar volumes can be obtained by measuring ultrasonic vibration potential~,~~C e.g. Ve(H+) at 298.1 5 K = - 5.5 -I 0.5 x ms mol-l. Some experimental techniques measure the properties of single ions in extremely dilute solution where there is little evidence of changes due to ion-ion interactions.The charge-transfer-to-solvent (c.t.t.s.) spectra of iodide ions in water are extremely intense and the frequency of absorption maximum for the low-energy absorption band3 is very sensitive to temperature solvent and pressure. An additional advantage is that the solvent plays an important part in defining the energy of the excited state and c.t.t.s. spectra can therefore be used to probe the arrangement of solvent molecules around an ion. 5 Ion-Solvent Interactions The interaction between an ion an3 water is extremely intense.35 The enthalpy (a) H. P. Bennett0 and D. Feakins ‘Hydrogen Bonded Solvent Systems’ ed. A. K. Coving- ton and P. Jones Taylor and Francis London 1968 p. 235; (6) B. E. Conway R. E. Verrall and J. E. Desnoyers Trans.Faraday SOC. 1966,62,2738. 32 I. Lee and J. B. Hyne Canad. J. Chem. 1968,46,2333. 32a See also estimation of absolute enthalpies of hydration H. F. Halliwell and S. C . Nyburg Trans. Faraday SOC. 1963 59 1126. 35 (a) H. S. Harned and B. B. Owen ‘The Physical Chemistry of Electrolyte Solutions’ 3rd edn. Reinhold New York 1958; (b) J. Barthel Angew. Chem. Infernat. Edn. 1968 7 260; (c) R. Zana and E. Yeager J. Phys. Chem. 1966,70,954 (see also P. Mukerjee J. Phys. Chem. 1966,70,2708). 34 M . J. Blandamer and M. F. Fox Chem. Rev. in press. 38 (a) B. E. Conway and J. O’M. Bockris ‘Modern Aspects of Electrochemistry’ vol. I Butterworths London 1954 p. 47; (b) W. M. Latimer Chem. Rev. 1936 18 349; (c) B. E. Conway Ann. Rev. Phys. Chem. 1966,17,481; ( d ) B. E. Conway J. E. Desnoyers and A.C. Smith Proc. Roy. SOC. 1964 A 256 389. 173 Structure and Properties of Ayireous Salt Solutions changes AH^ for the process salt + water -+ solution are small and sometimes owing to the similarity between the interaction energies for an ion with a solvent and a salt lattice. Many treatments of ion-water interactions accept that the intense electric field ca. loa V m-l at a distance of 0.6 nin from the centre of an ion aligns nearest neighbour solvent molecules such that the dipolar axis passes close to or through the ion centre. These solvent molecules are called electrostricted water molecules. Arising from the charge on an ion the enthalpies of evaporation for ions (solution - water + ion in gas phase) are very different from those for non- polar solutes having similar size.15 However the corresponding entropies AS,* (at 298.15 K and 101-325 N m-2) for both classes of solutes are very similar.Thus for potassium chloride dSvO is 217 J mol-1 K-l while for two moles of argon the corresponding entropy change is 253 J mo1-1 K-l. Release of electro- stricted water molecules from ions should on evaporation contribute a sig- nificant increase in entropy. Nevertheless the entropy change for the salt is less than that for the argon atoms. Therefore in a solution of potassium chloride there is too much entropy and this excess is traced to some degree of disorder in the arrangement of water molecules beyond the layer of electrostricted water molecules. These observations are geiieralised for all simple ions in aqueous solutions by use of a model which identifies three zones of water structure around each ion.l6 Two concentric spheres are drawn around the centre of an ion.The zone between the surface of the ion and the first sphere zone A contains all those water molecules for which ion-water interactions dominate i.e. the electro- stricted water molecules. Zone C extends from the outer sphere to infinity and includes those water molecules which have essentially the same arrangement as in pure water. These water molecules experience a weak electrostatic field from the ion and the associated entropy contribution to dSvO can be calculated by treating this liquid as a bulk continuum. The water between the two concentric spheres zone B is subjected to the competing demands of water structures associated with zone C bulk water and with zone A the hydration shell.Generally these two influences will differ and the water structure in this fault zone36 is broken down. This structure-broken layer is the source of the excess entropy in the example discussed above potassium chloride in water. This rather formal structural model represents essentially an instantaneous picture of the arrangements of water molecules around an ion in the same way that mixture models tackle the structure of liquid water. Zone B increases with increase in ion size and consequently the extent of water structure breaking increases. Conversely with decrease in ion size zone B contracts until for small ions e.g. lithium and fluoride zone B disappears. In these systems the hydrated ions are accommodated within the structure of water. Such ions are (electro- strictive) structure formers.36 E. Wicke Angew. Chem. Internat. Edrt. 1966 5 106. 174 Blandamer The above model rationalises in a straightforward way possible competing influences of ion-water and water-water interactions. The importance of this balance is apparent for example in the relative enthalpies of solution for salts in water-alcohol mixtures i.e. essentialiy AHe (salt + solvent -+ solution) - AH^ (salt + water -+ aqueous solution). Addition of small amounts of an alcohol to water particularly t-butyl alcohol and to a lesser extent ethanol enhances water-water interaction^.^^^^^ When small amounts of ethanol are added to water the relative enthalpies of solution for potassium chloride potas- sium bromide and potassium iodide become increasingly end~therrnic.~~ Here ion-water interactions become weaker as the water-water interactions become more intense.6 Application of Structural Models These qualitative models have been extensively used in the interpretation of experimental data and have also been fruitfully extended to include the properties of solutes which are biochemically important.40 A few examples are given to show how the model is used. The standard partial molar heat capacities of non-polar solutes are positive and increase with increase in size of the solute m01ecule.l~~ In contrast these heat capacities for alkali-metal halides in water at 298.15 K are negative and become increasingly negative with increase in ion size. This is associated with structure breaking in zone B. (Above 310 K heat capacities show a reverse trend with increase in ion size.3s) Tracer amounts of H,lsO diffuse more rapidly through aqueous solutions of potassium iodide than of potassium chloride:41 this is expected if iodide breaks the structure more effectively than chloride.Furthermore added potassium iodide increases the diffusion coefficient more markedly the lower the tempera- ture showing that at the lower temperature the ions have more structure to break. Interpretation of translational properties of ions in water is however not straightforward. There is for example evidence that alkali-metal cations and halide ions diffuse by different me~hanisms.~9~~ An example of a clear link between thermodynamic data and structural models is found in the transfer functions AG9 AH" and TAP for salts between deuterium oxide and The properties of these two solvents are very 37 F.Franks and D. J. G. Ives Quart. Rev. 1966,20 1. 38 M. C. R. Symons and M. J. Blandamer ref. 310 p. 21 1. 39 (a) E. M. Arnett and D. R. McKelvey Rec. Chem. Progr. 1965 26 185; (b)E. M.Arnett 'Physico-Chemical Processes in Mixed Aqueous Solvents' ed. F. Franks Heinemann London 1967 p. 105. 40 See for example (a) M. Kennerley H. J. V. Tyrell and M. Zaman J. Chem. SOC. 1966 1041 ; (b) M. Kennerley and H. J. V. Tyrell ibid. 1968 607. 4J (a) J. H. Wang J. Phys. Chem. 1954 58 686; (b) J. TamBs and K. OjszBszy Actu Chirn. Acad. Sci. Hung. 1966,49 377. 42 J. N. Agar and J. C. R. Turner Proc. R0.v. SOC. 1960,255 A 307. 43 J. Greyson Desalination 1967,3,60; J. Phys. Chenr. 1962,66,2218; 1967,71,259. 175 Structure and Properties of Aqueous Salt Solutions similar except that at a given temperature and pressure more structural order exists in deuterium oxide than in water.l9& For transfer of salts from standard states in deuterium oxide to standard states in water dGze is negative i.e.the transfer is thermodynamically favourable. In water water-water interactions are less intense than in deuterium oxide and so as expected ion-water inter- actions are relatively more intense. Nevertheless A G ~ is relatively insensitive to the salt whereas more marked differences occur in AH# and TASe quantities. The trends agree with the model discussed above in that the structure breaking sequences are K+ > Naf > Li+ and I- > Br- > Cl- > F-. A structure breaking salt is more effective in deuterium oxide where there is more structure to break.The relative insensitivity of the Gibbs function in the above example coupled with a greater sensitivity in the associated entropy and enthalpy quantities is not uncommon and these phenomena are more widely known as compensation In fact the effects of structural changes in water on ionic properties are generally not readily apparent in Gibbs functions and associated parameters such as equilibrium and rate constants. They are much more noticeable in associated AH TAS45 and quite markedly in G&quantitie~.~~ Although the properties of solutions containing alkali-metal cations and halide ions have been used in the above examples the properties of other ions have been discussed in a similar fashion. For example NO3- and C104- are water structure breakers.The planar structure of NO3- cannot be easily accom- modated into the water structure. The concept of a structure-breaking ion has also been related to the idea of ‘negative hydrati~n’.~~ This approach to ionic solvation considers the motion of solvent molecules in the neighbourhood of an ion. If in comparison to those in pure water the motion is more rapid the ion isnegativelyhydratedwhereas if less rapid the ion is positively hydrated. This model is particularly useful in the interpretation of proton TI relaxation times in aqueous 7 Arrangement of Water Molecules in Zone A The assumption that every simple ion in water is surrounded by a layer of electrostricted water molecules is widely but not universally held.6bJ4~z9~4s~47 Some authors suggest that for iodide in water zone A is absent so that zone B extends from the surface of the ion and the water adjacent to the ion is more fluid than in pure The existence of an electrostricted layer of water molecules is supported however by other observations.For example the decrease in relative permittivity of water when salts are added is attributed to the re- striction of dipolar reorientation for a number of water molecules by electro- 44 D. J. G. Ives and P. D. Marsden J. Chem. SOC. 1965,649. 45 S . Lindenbaum J. Phys. Chem. 1966,70 814. 46 0. Ya. Samoilov (a) Discuss. Faruduy SOC. 1957 24 141; (b) ‘Structure of Aqueous Electrolyte Solutions and the Hydration of Ions’ Consultants Bureau New York 1965. 47 D. Rosseinsky Chem. Rev. 1965 45 467. 48 H. G. Hertz and M. D. Zeidler Bey. Bunsengesellschnft Phys. Chem. 1963,67,774.176 Blandamer strictive f o r ~ e ~ . ~ ~ ~ ~ ~ ~ ~ ~ Similarly the effect of added salts on the Raman spectra has been linked to the electrostriction of water particularly by anions.so These electrostricted water molecules form a potential energy well which provides a well-defined energy level for the excited state of c.t.t.s. absorption spectra of anions.34 The n.m.r. spectra of ions in some non-aqueous solvents have detected solvent molecules attached to cations and it has been possible to determine the number of attached solvent molecules and their rate of exchange with the bulk solvent . l 2 8 61 The arrangement of water molecules in zone A is however still unknown.47 In some models the dipolar axis of each solvent molecule passes through the ion centre e.g.0 +-+ and @ ++ where ++ represents the molecular dipole moment of one of the nearest neighbour water molecules. This structure is generally used in the context of cation hydration However some treatments use a model where the interaction for anions occurs through one hydrogen of water @--H-O(H) such that one hydrogen can still hydrogen bond with other water molecules outside the primary hydration layer.62g53 Interaction between a cation and a nearest neighbour water molecule might also involve a non-bonding doubly filled orbital on oxygen. Some insight into the arrangement of water molecules around an ion is given by the structures of various hydrates. In tetrabutylammonium fluoride clathrate hydrate Bun,N+F-,32.8H20 each fluoride ion replaces a water molecule in the host lattice and has tetrahedral ~o-ordination.~~ The structure of the monoclinic hydrate KF,4H20 contains K+(H20) and F-(H,O), octahedra; each water molecule has a tetrahedral co-ordination of either two fluoride ions one potassium ion and one water molecule or two potassium ions one fluoride ion and one water A related problem to that concerning arrangement of water molecules in the electrostricted layer concerns possible differences in the hydration energies of a cation and an anion having the same size.Most authors accept that the difference is quite large.*' However if the hydration energies are compared with ionic radii derived from electron density measurements on crystals the differences between these hydration energies are This conclusion seems to be supported by analysis of the mass spectra of hydrated ions a novel but important way of studying ion-water interact ions.67 49 F. E. Harris and C. T. O'Konskii J. Phys. Chem. 1957,61,3 10. so (a) G. E. Walrafen J. Chem. Phys. 1962 36 1035; 1966 41 1546; (b) R. E. Weston Spectrochim. Acta 1962 18 1257. 61 J. H. Swinehart and H. Taube J. Chem. Phys. 1962,37 1579. 6z E. J. W. Verwey Rec. Trav. chim. 1942 61 127. 64 R. K. McMullan M. Bonamico and G. A. Jeffrey J . Chem. Phys. 1963,39,3295. 66 G. Beurskens and G. A. Jeffrey J. Chem. Phys. 1964,41,917. 66 M. J. Blandamer and M. C. R. Symons J . Phys. Chem. 1963,67 1304. s7 (a) P. Kebarle Adv. Chem. Ser. 1968 72 26; (b) P. Kebarle M. Arshadi and J. Scar- borough J . Chem. Plrys. 1968 49 817. F. Vaslow J . Phys. Chem. 1963 67 2773. 177 Structure and Properties of Aqueous Salt Solutions 8 Alkylammonium Ions A great deal of interest has been aroused by the often strange properties of alkylammonium salts in aqueous s o l u t i o n ~ .~ ~ ~ ~ ~ Ce5* For example the apparent partial molar heat capacity of tetra-n-butylammonium bromide in water at 293.15 K is large and positive being more characteristic of non-polar solutes in water than ionic solutes.16 By comparison with ion size and heat capacities of alkali-metal halides a large negative value would be expected. This break in a pattern of behaviour is found in other properties. The B-viscosity coefficients increase and the electrical mobilities decrease with increase of size of the cation.18 In addition the Walden products X0q for alkylammonium ions are markedly dependent on temperature. There is a close link between the properties of alkylammonium ions and those of non-polar solutes.For example the partial molar volumes V*(R,N+Br-) increase through the series (R =) methyl < ethyl < n-propyl < n-butyl < n - a m ~ l . ~ ~ The increase in volume per CH2 group is similar to the increase observed for the partial molar volumes of n-alkanes in These observations are explained by a structural model in which the alkyl groups enhance water-water interactions the extent of this enhancement increasing with increase in size of the alkyl group. (This model is widely but not universally accepted.60) These alkylammonium ions are therefore structure formers. These ions enhance water structure for probably very similar reasons to non- polar solutes. The lack of a readily available site on a large alkylammonium ion for specific ion-water interactions means that the interactions between the water lattice and ion are weak van der Waals forces.These forces stabilise those water molecules involved in a hydrogen bonded lattice type structure such water structures being able to form sufficiently large cavities to accommodate the alkyl chains. This structure forming is consequently quite different from the electrostrictive structure forming of lithium and fluoride ions. The extent of structure making for alkylammonium ions according to the above model follows the order Me,Nf < Et,N+ < Prn4Nf < Bun,N+ and this order agrees with most experimental observations. Actually tetramethylammonium is thought to be an electrostrictive structure breakerl0~ls (cf action of Csf 61) whereas in the tetraethylammonium ion the structure enhancement by the ethyl groups and the electrostrictive structure breaking effects cancel.When a site for specific ion-solvent interactions is introduced into the alkyl group these special properties of alkylammonium ions are The hydroxy- groups in tetra-(2-hydroxyethyl) ammonium (cf. tetrapropylammonium - an ion of similar size) destroys the hydrophobic character of the alkyl chains. The B-viscosity coefficient is now relatively insensitive to temperature and there is Ks F. Franks ref. 31a p. 31. 69 W-Y. Wen and S. Saito J . Phys. Chem. 1964,68,2639; 1965,69,3569. 6o H. E. Wirth,J. Phys. Chem. 1967,71,2922. Y-C. Wu and H. L. Friedman J . Phys. Chem. 1966 70 2020. 178 Blandamer some evidence that the tetraethanolammonium ion should be considered a structure breaker.ld The above model derived for alkylammonium ions has been successfully extended to account for properties of aqueous solutions of trialkylsulphonium ions6a (e.g.Prn,S+ is a structure former whereas Me,S+ is a structure breaker) and of sodium tetraphenylboron Na+B-Ph4.63 9 Very Dilute Aqueous Salt Solutions (0 < c < 0.1) So far we have considered ion-water interactions where the ions are effectively separated by an infinite amount of solvent. When the properties of real solutions are considered the effects of ion-ion interactions on the properties of each ion must be included in a discussion of structural models for real solutions. The derivation of ionic properties can also involve new problems. As noted above single ionic conductances can be calculated from a knowledge of molar con- ductance and a transport number of one ion in the solution.The spectrum of an ion in solution also provides information about ionic properties. However the case for evaluating single-ion activity coefficientQ4 remains seriously in The Debye-Huckel treatment of ion-ion interactions is satisfactory for these dilute solutions and equations for the activity coefficient of the salt and osmotic coefficients of the solvent in a solution can be conveniently calculated. In general the limiting law is used and the predictions are compared with the properties of real solutions. The limiting law is quite successful in accounting for the variation in properties of solutes in real solutions at low concentrations Even for example the apparent molar volume of alkylammonium salts in water at very low concentrationslO follows closely the trend predicted.However the experimental points rapidly deviate from the predicted dependence as the concentration of salt is increased. The deviations for other salt solutions are not so large but are well outside experimental error. The limiting law can also be used to predict the dependence of thermodynamic excess functions on composition.66 These excess functions e.g. Gex Hex and T S e x refer to the difference between the property of the real solution and the corresponding ideal solution. If the properties of the real solution follow the limiting law values of Gex Hex and TSeX can be calculated. These are compared in the Table for an aqueous solution and a salt solution in a hypothetical solvent for which the temperature and pressure coefficients of the relative permittivity6' are zero.62 (a) D. F. Evans and T. L. Broaawater J. Phys. Chem. 1968,72 1037; (6) S. Lindenbaum J . Phys. Chem. 1968,72,212. O3 S . Subramanian and J. C. Ahluwalia J. Phys. Chem. 1968,72,2525. 64 M. Alfenaar and C. L. DeLigny Rec. Trav. chim. 1967,86 829. 65 E. A. Guggenheim J. Phys. Chem. 1929 33 842. 66 R. H. Fowler and E. A. Guggenheim 'Statistical Thermodynamics' Cambridge University Press 1949 Chap. 9. 67 H. L. Friedman J. Chem. Phys. 1960,32 1351. 179 Structure and Properties of Aqueous Salt Solutions Table Thermodynamic excess functionsa for salt solutions at 298.1 5 K according to the Debye-Huckel limiting law (reJ 67). Water Hypothetical Solvent -3874Z3I2 J -3874Z3I2 J f1975I3l2 J - 58 1 2Z3I2 J +19-7113'2 J K-' -6~4891~'~ J K-l UExcess functions derived with respect to a kilograrii of solvent.The total Gibbs function decreases with increase in ionic strength I and this decrease in water is a result of an increase in enthalpy and an increase in entropy. This indicates again the importance of entropy changes in aqueous solutions. The increase in entropy predicted by the limiting law can be linked with the release of solvent molecules from the electrostatic fields since the fields from anions and cations will partially canceP7 as the concentration increases. 10 Higher Concentrations Agreement between theory and experiment can be extended to higher salt concentrations if the full Debye-Huckel equations are used and also by including additional terms in the expressions relating log yrt and salt concentration.Another a p p r ~ a c h ~ ~ * $ ~ ~ treats the solution as an expanded salt lattice. The predicted linear interdependence of log yk and (c)lI3 is shown by potassium chloride in water at 298.15 K where 1 < c< 100. Another method3e is to add to the Debye-Huckel equation for log yrt another term which measures the number of moles of water h bound to the two ions. Unfortunately the dependence of h on salt is not easily accounted for (see also ref. 3%). Compensation effects between enthalpy and entropy quantities also occur in real solutions such that the corresponding Gibbs function is relatively insensitive to the salt. This is shown by the excess functions (per mole of salt) for salts in water and deuterium oxide (Figure l).69$70 The differences between Gex for a given salt in the two solvents are much smaller than in the associated Hex and TSeX quantities.Despite these compensation effects the patterns shown in plots of log yk vs. (m2)lI2 where ma is the molality show several interesting trends. For example when m = 2.25 the order of log yrt for chlorides is Li > Na > K > Rb > Ca i.e. increasing with decreasing size of the cation. However the order for lithium salts is I > Br > C1 i.e. increasing with increase in anion size but for caesium salts C1> Br > I.71 68 (a) J. C. Ghosh J . Chem. SOC. 1918,113,449,707; (b) E. Gluekauf ref. 7 p. 97; (c) M. H. Lietzke R. W. Stoughton and R. M. FUOSS Proc. Nut. Acad. Sci. 1968,59 39. 6B Y-C. Wu and H. L. Friedman J. Phys.Chem. 1966,70,166. 70 See also trends in enthalpies of dilution (a) 'The Structure of Electrolyte Solutions' ed. W. Hamer Wiley New York 1959 p. 135; (6) E. Lange and A. L. Robinson Chem. Rev. 1931,9 89. 71 R. M. Diamond J . Amer. Chem. SOC. 1958,80,4808. 180 Blandamer 600 3 00 -300 Li CP Na I t 0 I 2 3 0 1 2 3 0 1 2 3 Aquamolality Figure 1 Total excess thermodynamic functions XeXper mole of solute for three salts in water (-) and deuterium oxide ( - - - ) at 298- 15 K; aqua molality is the number of moles of salt in 55.5 1 moles of solvent (taken front ref. 69). (Reproduced by permission from J. Phys. Chem. 1966 70 166.) One structural mode114$29 which aims to rationalise these trends examinzs the response of water structure when ions approach each other i.e. as the concentration increases.Each ion is placed at the centre of a co-sphere of water molecules ; the sphere encompasses all those water molecules whose arrangement differs from that of bulk water. With increase in salt concentration these co- spheres start to overlap. The problem is to decide how the various structural influences associated with each ion react in the region of overlap.* The experi- mental data are fairly consistent with the following generalisations. Anions and cations having similar structural influences (e.g. both electrostrictive structure makers or both electrostrictive structure breakers) salt each other in i.e. decrease the activity coefficient. For example log y (CsCl) < log y (LiCl). Two ions having different structural influences salt each other out i.e. increase the activity coefficient.For example log y (LiI) > log y (CsIj. In the above context an electrostrictive structure former e.g. fluoride and a hydrophobic structure 181 Structure and Properties of Aqueous Salt Solutions former e.g. Bun4N+ have different structural influences and salt each other out Changes in (co-operative) water-water interactions coupled with ion-water interactions are particularly important in the properties of alkylammonium salt solutions. Following an increase in the relative apparent molar volume +2 - Ve2 for Bun4N+Br- with increase in salt concentration as required by the limiting law the volume decreases and+ - Ve2 has a minimumclose to M = 1.0 corresponding to a stoicheiometry Bun4N+Br- ,(60 5 10)H20. This decrease reflects a mutual stabilisation of water structure around the alkyl groups as the alkylammonium ions come together.1° The structures of the salt solutions particularly at low temperatures probably resemble those of the corresponding alkylammonium clathrate hydrates,22 e.g.Bun,N+Br-,34H20. In these crystals the alkyl groups are held in cages formed by hydrogen bonded water molecules. When the temperature is increased the minimum in t$2 - Ye2 becomes less well defined due to the competing thermal breakdown of water When more salt is added there is insufficient water to form these structures. New properties are now observed. The solubility of benzene in these s01utions~~ and the ultrasonic absorption of these increase rapidly as more salt is added.75 11 .Mixed Salt Solutions The foregoing models have also been used in discussions of the properties of mixed salt solutions particularly solutions of two salts having one common ion.The trends in heats of mixing of two solutions containing alkali-metal halides lead to a set of general is at ion^.^^ Mixing of two solutions containing either electros tric t ive structure makers or electros t ric t ive structure breakers is endo- thermic (assuming Na+ is a structure maker) whereas a structure breaker and a structure maker give exothermic mixing. However if one solution contains a large alkylammonium ion e.g. tetra-n-prcpylammonium the effect of this ion dominates and the mixing is exothermic. The importance of these ammonium ions on thermodynamic properties is confirmed in an analysis of volumes of mixing7? involving alkylammonium and potassium bromide solutions.When 72 For further discussions on effect of alkylammonium salts on water solutions see (a) H. G. Hertz and M. D. Zeidler Ber. Bunsengesellschuft Phys. Chem. 1964,68 821 ; (6) F. J. Miller0 and W. Drost-Hansen J. Phys. Chem. 1968,72 1758; (c) R. Gopaland and M. A. Siddiqi J. Phys. Chern. 1968,72 1814; ( d ) W-Y. Wen S. Saito and C. Lee J. Phys. Chern. 1966,70 1244; (e) B. E. Conway and R. E. Verrall J . Phys. Chem. 1966,70,1473; (f) R. M. Diamond J. Phys. Chem. 1963 67 2513; ( g ) S. Lindenbaum and G. E. Boyd J. Phys. Chem. 1964 68 911; (h) K. W. Bunzl J . Phys. Chem. 1967 71 1358; (i) J. D. Worley and I. M. Klotz J . Chem. Phys. 1966,45,2868;(j) G. E. Boyd A. Schwarz and S. Lindenbaum J . Phys. Chern. 1966,70,821; 1967,71,573. 7Q H. E. Wirth and A. LoSurdo J. Phys.Chem. 1968,72 751. 7p M. J. Blandamer M. J. Foster N. J. Hidden and M. C. R. Symons Trans. Faraduy SOC. 1968 64 3247. 75 For other evidence of structural effects in real solutions see F. Vaslow J. Phys. Chem. 1966 70 2286; 1967 71 4585. 76 R. H. Wood and H. L. Anderson J. Phys. Chem. 1966,70,992,1877; 1967,71,1869,1871. 77 W-Y. Wen and K. Nara J. Phys. Chem. (a) 1967 71 3907; (and for solutions in D20) (b) 1968,72 1137; (c) W-Y. Wen K. Nara and R. H. Wood J . Phys. Chem. 1968,72,3048. 182 Btandamer these two solutions are mixed in different proportions but at constant ionic strength there is an overall increase in volume (Figure 2). The volume increase is a maximum when the component salts have equal concentrations. Here the 0.8 A v e x (ml mol-l) 0.4 0.2 0 -0.1 I = 0.50 (HOC2H4)NBr 0 0.2 0.4 0.6 0-8 1.0 Y Figure 2 Excess volumes of mixing for quaternary ammonium salt solutions and potassium bromide solutions at 298.15 K and a total ionic strength 0.5; y is fraction of ionic strength I due to the salt R,N+Br- in the mixed salt solution (taken from ref.77a). dmVex(y,Z) = Vex(y,l) - (1 - y ) VeX(0,I) - yVex(1 I ) where Vex(y,Z) is the excess vohme whosecomposirionisspecijed by y and Z (Reproduced by permission from J. Phys. Chem.. 1967,71 3907.). 183 Structure and Properties of Aqueous Salt Solutions structure-breaking potassium bromide does maximum damage to water structure enhanced by the alkylammonium ions. The importance of these structural influences as opposed to ion size effects is confirmed by the essentially zero excess volumes of mixing for (HOCzH4)4N+ and potassium salt solutions (Figure 2). In surveying a large subject within the limits of the space allowed I have had unfortunately to omit many important contributions by workers in this field. I thank colleagues in this Department and Dr. F. Franks for their valuable comments. 184

ISSN:0009-2681    

DOI:10.1039/QR9702400169

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

The Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides By N. H. Brett K. J. D. MacKenzie and J. H. Sharp DEPARTMENT OF CERAMICS WITH REFRACTORIES TECHNOLOGY UNIVERSITY OF SHEFFIELD 1 Introduction Several years ago two excellent reviews1 s 2 concerned primarily with crystallo- graphic aspects of the thermal decomposition of hydroxides and hydrous silicates were published. There have been many subsequent investigations of these reactions using X-ray and other techniques and these are the subject of this Review. The structures of the mineral silicates are usually classified according to Bragg’s system which is based on the observation that silicon is almost always found in tetrahedral co-ordination with respect to oxygen (except in rare instances e.g. stishovite).This classification over-emphasises the importance of the SiO tetrahedron as a structural unit whereas the cation polyhedra are neglected. Yet chemically related elements such as Mg and Ca form silicates of formulae MSi03 and M2Si04 which have different structures. Furthermore similarities between the sheet structures of the layer silicates and the closely related hydroxides of Mg Cay Al etc. which are based on the cadmium iodide structure (Figure 1 A) and between the layer and chain silicates (Figure lB) can easily be overlooked in terms of Bragg’s classification. This Review is concerned with the decomposition of these structurally related compounds. The principal cation in the silicates is usually Mg Al or Fe; those silicates in which Ca is the major cation have different structures and their decompositions have been reviewed by Brindley2 and Tay10r.~ 2 Hydroxides and Oxyhydroxides The layer hydroxides are probably the simplest class of compounds in which the thermal decomposition process has been thoroughly investigated using a wide variety of techniques.When orientation relationships develop during thermal decomposition between the crystal structure of the starting material and that of its product the reaction is described as topotactic and the pheno- menon observed as topotaxy. A three-dimensional correspondence between the crystal structure of the starting material and product is implied in a topotactic reaction in contrast to epitaxy where the correspondence is two-dimensional. L. S. Dent F. P. Glasser and H. F. W. Taylor Quart. Rev. 1962,16 343. G.W. Brindley ‘Progress in Ceramic Science’ Pergamon 1963 vol. 3 p. 1 . H. F. W. Taylor ‘Progress in Ceramic Science’ Pergamon 1961 vol. 1 p. 89. 185 Brucite (Cd 1 Structure) 4 0 + 2 (OH) 6 0 4 Si Vacant 6 Mg 4 Si 6 (OH) 4 0 + 2 (OH) 6 0 Serpentine (1 :1 Layer Silicate) 6 0 6 0 6 (OH) Vacant 2 K Vacant - GO 6 0 6 0 4 Si 3 Si + 1 A1 4 Si 4 0 + 2 (OH) 4 0 + 2 ( O H ) ~ 4 0 + 2 (OH) 6 Mg 6 Mg 6 Mg - 4 0 + 2 (OH) 4 0 + 2 ( O H ) - 4 0 + 2 (OH) 4 Si 3 Si + 1 A1 4 Si 6 0 6 0 6 0 Vacant Talc Phlogopite 6 (OH) Mg6Si80 20(OH)4 K2Mg6(Si CA12)02 0(OH)4 - 6 (OH) 6 Mg Ideal Chlorite (2 1 Layer Silicate) (Mica) Mg6Si4010(0H)8 Figure 1A Schematic representation of the structural relationship bet ween hydroxides and hydrous layer silicates 12 0 12 0 12 0 12 0 12 0 8 Mg 8 Si 8 Mg 8 Si Enstatite (Pyroxene) Mg8Si8024 11 0 + 1 (OH) 1 1 0 + 1 (OH) 7 Mg 8 Si 7 Mg 8 Si Anthophyllite Mg&O 2 ,(OH) 2 (Amphi bole) 11 0 + 1 (OH) 11 0 + 1 (OH) 11 0 + 1 (OH) - 1 2 0 12 0 8 0 + 4 (OH) Vacant 8 Si 12 Mg 8 Si Talc (2:l Layer Silicate) 8 0 + 4 (OH) 12 0 Mg6Si80 2doH)4 Figure 1B Schematic representation of the structural relationships between pyroxenes amphiboles and 2 1 layer silicates Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides Shannon and Rossi4 suggest that one may describe a continuum of degrees to topotaxy since the degree of related orientation found in practice varies tremendously.A. Brucite Mg(OH),.-In the conversion of brucite to periclase MgO the re- action takes place in an ordered manner as the hexagonal close-packed structure of oxygen ions rearranges to a cubic close-packed structure.A high degree of topotaxy is preserved and the orientation relationships have been confirmed by several worker^.^,^,^ These relations are not limited to the crystallographic structure of the starting material and its product but extend to the macro- scopic structure of the materials. Thus magnesium oxide prepared by the thermal decomposition of nesquehonite (MgCO,,- 3H@) retains the needle-like struc- ture of the carbonate whilst that prepared from brucite has the form of hexagonal platelets which the hydroxide itself exhibits. * From a knowledge of the structure of the parent and product of a decomposi- tion reaction and their orientation relationships it is an obvious step to postulate the reaction mechanism. Investigations of the dehydroxylation of brucite have resulted in two differing models for the decomposition process.In the firstY6 it is assumed that water is lost in a similar manner from all regions of the crystal. Hydroxy-groups from adjacent layers combine to form water which can then diffuse away; the main shrinkage occurs in the c-direction with a smaller shrinkage in the basal plane. Kinetic and optical microscope studiesg suggest that reaction commences at an internal surface and advances towards the centre of the crystal. This is known as the homogeneous mechanism. The second model the inhomogeneous mechanism was proposed independently by Ball and Taylor,’ and by Brindley,2 who assumed that the reaction takes place simul- taneously throughout the bulk of the crystal which develops ‘donor’ and ‘acceptor’ regions’ during the decomposition process.Mg2+ ions migrate from the former to the latter with a counter-migration of protons. The ‘acceptor’ regions thus become MgO crystals whilst protons combine with hydroxyl ions to form water molecules which escape from the ‘donor’ regions leaving inter- crystalline pores. The strengths of the inhomogeneous mechanism are twofold. Firstly a good explanation of topotaxy is provided since less disturbance of the structure would be expected if cations and protons rather than water molecules were the migrating species. Secondly the formation of a porous decomposition product is explained by the creation of donor and acceptor regions in the brucite lattice. Its weakness lies in the ad hoc nature of the postulate of donor and acceptor regions and the corollary of counter-migration of two positively charged ions.R. D. Shannon and R. C. Rossi Nature 1964,202 1000. J. Garrido Ion Rev. Espan. Quim. Apli. 1951,11 206,220,453. M. C. Ball and H. F. W. Taylor Mineralog. Mag. 1961 32,754. R. M. Dell and S. W. Weller Trans. Faraday SOC. 1959 55 2203. * J. F. Goodman Proc. Roy. SOC. 1958 A 247 346. @ P. J. Anderson and R. F. Horlock Trans. Faraday SOC. 1962 58 1993. 188 Brett MacKenzie and Sharp The original evidence for the inhomogeneous mechanism for the decomposi- tion of brucite was based not only on the observed topotaxy and porous product but also on a reported intermediate phase.' X-Ray powder and single crystal photographs of brucite crystals which had been decomposed at about 800" exhibited 'spinel-like' reflections indicating the presence of tetrahedral mag- nesium ions which must therefore be migrating.More recently this observation of spinel reflections during the decomposition of natural brucite has been shown to be an artefact arising from the presence of impurities.1° Pure samples of Mg(OH)2 powder gave no extra reflections on decomposition and it was con- cluded that the experimental basis for the inhomogeneous mechanism is invalid. Other workersll have reported the existence of an intermediate hexagonal phase but this too has been rejected in an electron and optical microscope study by Gordon and Kingery.12 From kinetic data these authors13 interpreted the decomposition process as a nucleation and growth process in which MgO nuclei form coherently with the brucite matrix.This interpretation is similar to that of Pampuch,14 who considered that the formation of new phases in topotaxial decomposition reactions proceeds through an intermediate stage of hybrid crystals. These crystals contain domains of the product phase in a homogeneous matrix of the parent crystal instead of independent nuclei separated from the matrix by phase boundaries. The latter mechanism is reserved for reactions where the structure of the starting material and the product are considerably different. Recently Freund15 has proposed a hydrogen-bond model for the decomposi- tion of ionic hydroxides. In the case of brucite the broadening of the 0-H band in the i.r. spectrum at temperatures just below 300°C was interpreted as the commencement of OH-OH interaction at the surface of crystals.Proton transfer by a tunnelling mechanism is then supposed to occur between adjacent hydroxyl groups followed by removal of water molecules by diffusion. Tunnel- ling can take place at low temperatures when the energy levels of two adjacent OH groups overlap but at higher temperatures where widely spaced OH groups exist proton jumping leads to the removal of OH groups with the loss of hydrogen.15 Freund's interpretation of the dehydroxylation process is consistent with the advancing interface concept rather than the inhomogeneous mech- anism since the reaction of OH groups in adjacent layers6s9 is satisfactorily explained by proton tunnelling. Moreover no counter migration of cations is required to preserve electrical neutrality. B. Portlandite Ca(OH),.-This has a similar structure to that of brucite (CdI layer structure) with no evidence of OH bonding although Freund considers that lo N.H. Brett and P. J. Anderson Trans. Faraday SOC. 1967 63 2044. l1 R. R. Balmbra J. S. Clunie and J. F. Goodman Nature 1966 209 1088. l2 R. S. Gordon and W. D. Kingery J. Amer. Ceram. SOC. 1966 49 654. l3 R. S. Gordon and W. D. Kingery J . Amer. Ceram. SOC. 1967 50 8. l4 R. Pampuch Proc. IXth Conf. Silicate Ind. Budapest 1968 p. 143. l5 F. Freund Angew Chem. Znternat. Edn. 1965 4 445; F. Freund and H. Gentsch Ber. dmt. keram. Gesellschaft 1967 44 51. 189 Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides proton tunnelling occurs near the decomposition temperature.15 In contrast to brucite the decomposition product CaO is poorly oriented when the reaction takes place in air.l6 The degree of topotaxy is improved if the dehydroxylation takes place in vacuo rather than in air;17s18,1s the absence of an intermediate phase and the observation of a reaction i n t e r f a ~ e ~ * ~ ~ ~ ~ ~ ~ which moves in from the edges of the crystal are consistent with a homogeneous mechanism.Various explanations for the differing topotaxial behaviour of brucite and portlandite have been suggested viz. the disruptive effect of the larger Ca2+ ion,l12 the lowering of the decomposition temperature of portlandite in vacuo and the consequent lower thermal vibration,lB and the high nucleation rate of CaO crystallites in air which increases the chance of mismatch with the parent crystal.18 C. Hydroxides and Oxyhydroxides of Aluminium and 1ron.-The structure of the hydroxides and oxyhydroxides of aluminium and iron like that of brucite is based on the approximate close packing of anions in double layers with cations occupying intermediate octahedral sites.Important differences are however discernible. The hydroxyl and oxygen ions can assume either cubic or hexagonal arrangements forming a framework which persists during dehydroxylation and results in oxide products with preferred orientation. Hydrogen bonding occurs between hydroxy-groups in adjacent layers with a consequent shortening of the OH-OH distance. In gibbsite cubic AI(OH), where two-thirds of the available octahedral sites are filled this results in a different stacking arrangement of the double layers and a less tightly packed structure than in brucite.21 The thermal transformations which occur in these compounds are complex particularly in the case of aluminium where several metastable oxide phases exist; the transformations are summarised in Table 1.The confusion in the earlier literature was due in part to the difficulty in recognising mixtures of the oxide products in X-ray powder patterns (see for example Lippens and De Boerz2 for recent diffraction data on the y 9- 6- and 8-forms of Alg,) and to the lack of well characterised hydroxide starting materials. The presence of water vapour in the ambient atmosphere or hydrothermal treatment markedly . affects the phases formed and their recrystallisation rate (the tetragonal oxide 8-A1203 is not observed in vacuum dehydration products nor is the x-K sequence in the dehydration of gibbsiteZ3).The effect of micropore systems in partially decomposed powders on the transformation of the disordered spinel- type 9- or y-Al,O into more crystalline phases has also been n ~ t e d ; ~ ~ ~ ~ this l6 C. D. West Amer. Mineralogist 1934 19 281. l7 S. Chatterji and J. W. Jeffery Mineralog. Mag. 1966 35 867. l8 N. H. Brett Mineralog. Mag. 1969 37 244. lo N. Datta S. Chatterji J. W. JelTery and A. L. MacKay Mineralog. Mag. 1969 37 250. zo R. Sh. Mikhail S. Brunauer and L. E. Copeland J . Colloid Interface Sci. 1966 21 394. 21 A. F. Wells ‘Structural Inorganic Chemistry’ 3rd edn. Oxford University Press London 1962 p. 552. 22 B. C. Lippens and J. H. De Boer Acta Cryst. 1964 17 1312. 23 J. Beretka and M. J. Ridge J. Chem. Soc. ( A ) 1967 2106. 24 D. Aldcroft G. C.Bye J. G. Robinson and K. S. W. Sing J . Appl. Chem. 1968,18 301. 190 <02p T L T - 6' larger C 300°C in a i L boehmite 45O0C,y-Al2O3 750°C 6-A1,03 I.OoOf~e+a 1200°C crystals or hydrothermaily Y-A~O.OH in air in air in vacuo y-AlO.OH -cubic phase in vacuo in vacuo in vacuo in vacuo. diaspore 500°C in air A1203 25O"g boehmite 425°C disorganised 120 ~ ~ ~ ~ o " ~ f ? - A 1 2 0 3 1O5O0C LU-AIO.OH in air) 230"~q-A120 8 5 0 ' 6 O-AI,O 1200°C 4 gibbsite N O W 3 bayerite a a corundum a-AlzOx b b C ferrous hydroxide <200"c in an wiistite magnetite WOW2 inert atrnospheEFe0 in air Fe30 =I < 200 "C 4 in air in air y-Fe203 -XF-+ ~ maghemite 400°C lepidocrocite 300°C y-FeO.OH 6-FeO.OH hydrothermal goethite 300°C intermediate > 300°C treatment- a-FeO.OH in air) product? in air* heating under low water vapour pressure < 300°C in air maghemite i * y-Fe203 P-FeO.OH C f -b !? hematite g -b % ar-~e,03 h a I a 0 Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides was considered to be due to inhibition of the diffusion of aluminium ions into better ordered positions by the surface of the pores.z2 In nordstrandite [a form of AI(OH)3 first prepared syntheticallyz5 but which has since been found in nature26] the formation of fine cylindrical pores in a direction normal to the original hydroxide sheet during dehydration was taken to indicate an inhomo- geneous mechanism.27 This evidence alone is insufficient to establish the mech- anism since the formation of porous products is a general phenomenon in decomposition reactions.Recent work has centred on the mechanism of dehydroxylation in aluminium hydroxides and oxyhydroxides.Following their studies on micas and kaolinite (see section 3B) Fripiat and co-workers have carried out similar investigations with boehmite y-AlO.OH using i.r. and dielectric measurements.28 A continuous decrease in the intensity of the fundamental 0-H vibration bands was observed the effect being reversible on cooling when the temperature had not exceeded 400°C. To explain this a proton delocalisation process was postulated facilitated by the oxygen-hydroxyl zigzag chain structure of boehmiteYzg y-AlO.OH where proton tunnelling takes place through a potential barrier. Later work using n.m.r. techniques30 indicated that two proton spin populations could be differen- tiated in accordance with the suggestion that AI-OH2 units are formed.At the dehydroxylation temperature a substantial proportion of protons (- 10 %) is associated with these defects resulting in a strong repulsion which en- hances the diffusion of water molecules from the lattice. Proton tunnelling during the initial stages of the dehydroxylation of gibbsite has also been sug- gested followed by diffusion and subsequent desorption of water After 90% reaction has taken place the isolated OH groups which are more difficult to remove because the probability of proton tunnelling is decreased are eliminated as molecular hydrogen. The persistence of adsorbed hydroxyl ions during the dehydration of y-A120 was also observed although the migra- tion of OH groups possibly through proton transfer occurred readily at 400-600”c.31 Several excellent reviews have been written on the structure phase trans- formations and topochemical relationships of iron hydroxide and oxy- hydroxide~,~~~~ and only a short account of more recent work will be given here.In contrast to aluminium iron has a variable valency exhibits fewer inter- + 26 R. A. Van Norstrand W. P. Hettinger and C. D. Keith Nature 1956 177 713. 26 J. R. D. Wall E. B. Wolfenden E. H. Beard and T. Deans Nature 1962,196 264; J. C. Hathaway and S. 0. Schlanger ibid. 196 265. 27 J. D. C. McConnell ‘International Symposium on Reaction Mechanisms of Inorganic Solids (Abstracts)’ Aberdeen 1966. 88 J. J. Fripiat H. Bosmans and P. G. Rouxhet J. Phys. Chem. 1967 71 1097; A. Mata Arjona and J. J. Fripiat Trans. Faraday SOC. 1967 63 2936. 2s K.A. Wickersheim and G. K. Korpi J . Chem. Phys. 1965,42 579. 8o J. J. Fripiat and R. Touillaux Trans. Faraday SOC. 1969 65 1236. 31 J. B. Peri J. Phys. Chem. 1965 69 21 1. sa A. L. Mackay Proc. 4th Internat. Symp. Reactivity of Solids Amsterdam 1960 p. 571 ; H. P. Rooksby ‘The X-Ray Identification and Crystal Structures of Clay Minerals’ ed. G. Brown 2nd edn. Mineralogical Society 1961 p. 354. 192 Brett MacKenzie and Sharp mediate oxide structures and forms the stable oxide hematite a-Fe,O, at sub- stantially lower temperatures (Table 1). No trihydroxide of iron analagous to gibbsite or bayerite has been reported but ferrous hydroxide Fe(OH), crystal- lises with the layer structure of CdzI,. The dehydration behaviour of ferrous hydroxide is similar to that of bnrcite provided that air is excluded during the and activation energies of the same magnitude have been Four forms of the oxyhydroxide FeO.OH are known; goethite a-FeO.OH and lepidocrocite y-FeO.OH are the most common and are isomorphous with diaspore and boehmite respectively.Recent evidence suggests that intermediate structural states are formed during the dehydration of goethite (and diaspore) prior to the formation of hematite (and although no disruption of the oxygen ion packing is involved in this transformation. P-FeO.OH (akananeite) has been found in limonite deposits36 and shows interesting features in its fine structure in aqueous di~persions;~~,~~ low temperature calcination leads to the formation of a spinel phase (probably y-Fe,03) prior to the recrystal- lisation of a-Fe,0,.32 Hydrothermal treatment of 8-FeO.OH gives rise to g ~ e t h i t e ~ ~ with which it has structural ~imilarities.~~ The end product of the heat treatment of iron oxyhydroxides is hematite and the intermediate phase changes are topotactic with the tendency to retain the existing oxygen framework.3 Hydrous Layer Silicates The layer silicates are usually divided into two groups the 1 :1 layer silicates in which there is one tetrahedral sheet for every octahedral sheet and the 2:l layer silicates containing two tetrahedral sheets per octahedral sheet (Figure la). An altexnative classification is based on the number of available octahedral sites occupied; those minerals containing divalent cations being called tri- octahedral since all the available cation sites are filled. Minerals containing trivalent cations are called dioctahedral having only two-thirds of the octa- hedral sites filled.Although i somorphous replacement invariably occurs in these minerals their thermal decomposition can conveniently be divided into two series those containing only magnesium in the octahedral sites and those containing aluminium. The magnesium minerals decompose at higher temperatures than their aluminium analogues but recrystallise almost immediately whereas the alu- minium minerals tend to form stable dehydroxylates which recrystallise only after further heating. The decomposition reactions of the magnesium silicates show good topotactic reIations and are usually interpreted in terms of an inhomogeneous mechanism but the decomposition of aluminium silicates is generally more complex.33 J. D. Bernal D. R. Dasgupta and A. L. Mackay Clay Minerals Bull. 1959 4 15. 34 I. F. Hazel1 and R. J. Irving J . Chern. SOC. (A) 1966 669. 35 J. Lima-de-Faria and P. Gay Mineralog. Mag. 1962 33 37. 36 A. L. Mackay Mineralog. Mag. 1962 33 270. 37 J. H. L. Watson R. R. Cornell and W. Heller J. Phys. Chem. 1962,66 1757. 3* S. Okamoto J. Amer. Ceram. SOC. 1968 51 594. 193 Table 2 Tliermal deconiposi:ion of the layer silicates serpentine 600"g forsterite* Mg3Si 2O,j(O H)4 Mg,SiO 1100°C forsterite + enstatitc Mg,SiO MgSiO - 500°C anhydrous modification 90OCC,enstatite* Nao.,Mg,(Si7.,A~o.7)O~otO~)~ MgSiO ___) 400-800"~nhydrous 90o0Ctenstatite* Mgo.7(Mg,Fe11r,Al)6(Si,P,1),0,,(OH)~,~~~~ modifications MgSiO phlogopite 1 ~oO"C,olivine +leucite + kalsilite I< z( Mg FeI1),( S i ,A1 ?)o 20( 0 H)4 (Mg.Fe),SiO KAISi,O KAlSiO.kaolinite 550°C metakaolinite 980'GAl-Si spinel* 1 10Oc~mullite* Al,Si,0,(OH)4 Al,Si,O AI,Si,O, AI,jSi,Ol allophane 700-900"~muIlite 1200"c+m~IIite + coiundinn A1 ,SiO,,nH,O A1,Si Al,Si,O 13 cu-Al,O pyrophyllite 750"gpyrophyllite dehydrosylaie 1 209"~mullite* niontmorillonite anhydrous montmorillonite 1000"&spinel* I\iao.,(A13.,Mgo.7)Si,0,0(OM)4,,2H,0 modification dehydioxylate muscovite 700°~muscovite 105O"Caa spinel + sanadine 1300"&corundurn* AI,Si,O,,(OH) Al,Si,O AI,Si20,3 K,AI,(Si6AI Z)O,,(OH) dehydroxylate phase KAISi,O a-AIZO:3 -t mullite A1,Si2Ol3 *With separation of silica or a silica-rich phase. Ref. 1. Ref. 2. Ref. 48. Ref. 49. Ref. 51. * Ref. 75. g Ref. 81. 1' Ref. 52. iRcf. 84. j Ref. 87. Ref. 88. Ref. 89. Ref. a b a,b C d e a b f a b g h i j k7 1 Brett MacKenzie and Sharp A.Layer Silicates of iMagnesium.-Sevpentine Mg,Si,O,(OH), has the structure shown in Figure la and exists in three forms lizardite antigorite and a fibrous modification chrysotile. The first decomposition product of all forms is forsterite (Table 2) which usually appears almost immediately after dehydroxylation although chrysotiles of high surface area form an X-ray-amorphous phase which remains stable for at least 150°C;39 enstatite appears only at higher temperatures. Under hydrothermaI conditions serpentine transforms into talc above 500°C.40 In early work on the reaction41 a homogeneous mechanism was proposed in which dehydration and recrystallisation of farsterite were viewed as a combined operation. This however explained neither the escape of water molecules with- out disordering the structure nor the expulsion of silica.An inhomogeneous reaction sequence was later proposed for chrys~tile,~~ in which a separate dehydration stage occurs by countermigration of protons and magnesium and silicon ions in a more or less unchanged oxygen framework. The disordered product serpentine dehydroxylate Mg3Siz0, then separates into magnesium- rich regions (later forsterite) and silicon-rich regions (later enstatite). This mechanism does not explain the delayed appearance of enstatite and from a quantitative X-ray analysis of the amount of forsterite formed from massive ~erpentine,~~ a modified inhomogeneous mechanism was suggested in which the products of the initial countermigration are disordered forsterite and silica.The forsterite recrystallises at 800°C and above 1000°C reacts with the silica to form some enstatite. The kinetics of dehydroxylation of serpentine4 under constant water vapour pressures of < to 47 mm. Hg fitted a diffusion-controlled rather than first- order model. The apparent activation energies of 68-120 kcal/mole reflected retardation by surface coverage of chemisorbed water. Kinetic studies of for- sterite development from the dehydr~xylate~~ have yielded from a first-order model an ‘activation energy spectrum’ from 80-1 00 kcal/mole. This kinetic analysis is recognised as unrealistic although the dehydroxylate characteristically behaved as a chemically damaged solid. The decomposition of talc Mg6Si8020(OH)4 (Table 2) exhibits three-dimen- sional orientational relation^.^^ One unit cell of talc transforms into one unit cell of enstatite both of which contain 48 02- ions.In terms of an inhomogeneous mechanism the acceptor region loses 8 protons and is compensated by the gain of 4 magnesium ions to become enstatite.2,46 Simultaneously water is lost from the donor region which becomes silica-rich. 38 A. W. Naumann and W. H. Dresher Amer. Mineralogist 1966 51 1200. 40 N. L. Bowen and 0. F. Tuttle Bull. Geol. So,. Amer. 1949 60 439. *l G. W. Brindley and J. Zussman Amer. Mineralogist 1957 42 461. 42 M. C. Ball and H. F. W. Taylor Mineralog. Mag. 1963 33,467. 4s G. W. Brindley and R. Hayami Mineralog. Mag. 1965 35 189. 44 G. W. Brindley B. N. Narahari Achar and J. H. Sharp Amer. Mineralogist 1967 52 1697. 45 0. W. Brindley and R. Hayami Clays and Clay Minerals 1964 12 35.46 M. Nakahira and T Kato Clays and Clay Minerals 1964 12 21. 195 Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides The other 2:l magnesium layer silicates differ from talc in the extent to which isomorphous replacement occurs in either octahedral sites or tetrahedral sites or both. The replacement of some of the tetrahedral silicon of talc by aluminium leads to a deficiency of positive charge which is made up in saponite by the inclusion of sodium in high co-ordination sites be tween the tetrahedral layers. Further substitution of aluminium for silicon leads progressively to vermiculite and phlogopite mica (Figure la) with more inter-layer cations providing a charge balance. Sodium is the most common inter-layer cation in saponite and related minerals (srnectites) magnesium commonly occurs in vermiculite and potassium in micas.Although much attention has been paid to the dehydration of inter-layer water in smectites:’ less is known about their high temperature products. In air ~ a p o n i t e ~ ~ and vermic~lite~~ form enstati te (Table 2) whereas under hydro- thermal conditions saponite forms anthophyllite and forsterite.60 The inter-layer cations in smectites are exchangeable and determine the composition of the resulting aluminosilicate; when sodium is present nepheline NaAISiB, and albite NaAlSi,O, are frequent products. Phlogopite K2Mg6A12020(OH)4 forms olivine and potassium alumin~silicates~~ (Table 2) although spinel has also been On the basis of optical and electron microscopy Nakahira53 has suggested that this decomposition follows an inhomogeneous mechanism in contrast to muscovite mica.B. Layer Silicates of Aluminium-Kaolinite A12Si,05(OH)4 is the aluminium analogue of serpentine and has three polymorphs kaolinite dickite and nacrite differing only in their stacking sequence. A hydrated form halloysite with tubular morphology contains additional water accommodated between the layers. The decomposition sequence of all these minerals follows that given for kaolinite in Table 2 but the transformation temperatures vary somewhat. Under hydrothermal conditions hydralsite and pyrophyllite are formed instead of metaka~linite.~~ The key to the understanding of the mechanism of the reaction sequence lies in the nature of metakaolinite which is the first product of dehydroxylation.The diffuse X-ray reflections of this phase have so far made a direct structure analysis impossible but three crystal structures have been p r o p o ~ e d . ~ ~ ~ ~ 47 G. Brown (ed.) ‘The X-Ray Identification and Crystal Structures of Clay Minerals’ Mineralogical Society Monograph 1961. 48 J. D. Russell and V. C. Farmer Clay Minerals Bull. 1964 5 443. 49B. S. Bobrov Yu. E. Gorbatyi and M. B. Epelbaum Fiz-Khim. Issled Gidroslyud Mater. Soveshch ‘Prob. Primen. Vermikulita stroit’ Chelyabuisk 1965 p. 53. 6o J. T. Iiama and R. Roy Clay Minerals Bull. 1963 5 161. 61 H. S. Yoder and H. P. Eugster Geochim. Cosmochim. Acta 1954 6 157. 62 R. Roy J. Amer. Ceram. SOC. 1949 32 202. M. Nakahira Amer. Mineralogist 1965 50 1432; M. Nakahira and M. Uda ibid. 1966 51 454. 54 R.Roy and E. F. Osborn Amer. Mineralogist 1954 39 853. 65 L. Tscheischwili W. Bussem and W. Weyl Ber. deut. keram. Gesellschaft 1939 20 249. 56 G. W. Brindley and M. Nakahira J. Amer. Ceram. SOC. 1959 42 31 1. R. Pampuch Prace Mineralogiczne 1965 No. 6 53. 196 Brett MacKenzie and Sharp These were deduced from the structure of kaolinite based on the observed topotactic relations density changes on heating and Pauling’s rules. Common features of these structures are that the Si-0 network of kaolinite remains largely intact but the aluminium changes from octahedral to tetrahedral co- ordination. Tscheischwili et aZ.,65 proposed a structure based on chains of edge- shared A104 tetrahedra. A second model with different oxygen packing and alternately corner and edge-shared A104 tetrahedra was suggested by Brindley and Nakahi1-a.5~ The third model due to Pampuch5’ contains 12% residual hydroxyls in alternate corner and edge-shared Al-0-OH tetrahedra.The sug- gestion that metakaolinite is not anhydrous was based on i.r. evidence although weight-loss measurements indicate that the figure of 12% is probably high. TayloP8 questioned the equating of ‘X-ray density’ of so poorly crystallised a substance as metakaolinite with the observed density of material prepared at 800°C. A further weakness of all these models is that such crystalline structures should give much better X-ray patterns than those observed for metakaolinite. This led Taylor5a to suggest that crystalline order such as it is exists chiefly in the oxygen packing whilst the cations are distributed randomly among the lattice sites.The random cation configuration is compatible with the X-ray pattern and changes in oxygen packing on transforming to the next phase are quite small preserving structural continuity. This model could accommodate areas of residual hydroxyls ; in such regions the original aluminium configura- tion might be preserved albeit highly dist~rted.~~ The only direct experimental evidence for A P in metakaolinite is from X-ray fluorescence spectroscopy,60~68 but recent works1 has indicated that the alu- minium phosphate used as a calibration standard for AlIV in earlier work60,68 is unsatisfactory. When a zeolite is used as a standard,s1 both tetrahedral and octahedral aluminium are indicated in agreement with i.r. which suggest the presence of both A l I V and Alvl in metakaolinite but in a very dis- ordered state.A metakaolinite structure containing a high concentration of defects ‘frozen in’ when the water is removed was proposed by Freund62 on the basis of density measurements which revised the c-parameter from 6.3 8 to 7.0 A. X-Ray fluorescence results were again taken to indicate AIIV throughout the very open structure. At about 980°C a cubic phase appears considered by most earlier workers to be y-alumina. The lattice parameter (7-886 A) was measured by Brindley and Nakahi~-a,~~ who deduced that it was an aluminium-silicon spinel rather than y-alumina (measured parameter 7.906 A). The suggested spinel was Si8All,,. 05.3032 with tetrahedral silicon octahedral aluminium and some octahedral sites vacant. This stoicheiometry may not be invariable but the 58 H.F. W. Taylor Clay Minerals Bull. 1962 5 45. 8o G. W. Brindley and H. A. McKinstry J. Amer. Ceram. SOC. 1961 44 506. 82 F. Freund Ber. deut. keram Gesellschaft 1967 44 5 . K. J. D. MacKenzie J . Amer. Ceram. SOC. 1969 52 635. J. J. Fripiat and A. Leonard personal communication. 197 Therma I Decompositiori of Hydrous Layer Silicates aiid their Related Hydroxides model gives structural continuity with the next phase mullite. However the variation in lattice parameter does not conclusively prove silicon inclusion in the spinel phase because of reported variations of 7.73 8 to 8.06 A in the y- alumina parameter due to variations in the oxygen and aluminium content.63 Yamada and Kimura have suggested a variable proportion of silicon in the cubic An alternative explanation of the 980°C reaction was given by Freunde2 in terms of the ‘healing’ of defects left in the metakaolinite structure resulting in a ‘metastable high pressure modification’ not a spinel or even 7-alumina although of similar parameter.Freund argues that no close-packed alumina-silica structure is known but gives no explanation for the cubic structure of the ‘metastable modification’. Parnp~ch~~ attributes the 980°C reaction to structural instability caused by the removal of residual hydroxy- groups. Although a sharp exotherm is observed by differential thermal analysis at 980”C it is not accompanied by any sudden change in X-ray or i.r. structural properties so its cause cannot at present be uniquely established. It is becoming increasingly apparent that protons play an important role in the reaction mechanisms.Conductance studies of dehydroxylatione5 have yielded a temperature coefficient of 4 kcal at 100-360°C. Since this value is comparable with proton energies in ice or water the conductance process was identified with proton rearrangement (delocalisation) in the octahedral orbitals. Below 400°C a ‘predehydroxylation’ state exists in kaolinite in which the i.r. hydroxyl bands reappear on Above 450°C the hydroxyl bands have almost disappeared and do not reappear on cooling although a small number of hydroxy-groups must be retained even up to 1100°C because mass spectro- scopic studies have shown hydrogen evolution at these temperatures.16 Pampuchl* considers that proton retention influences the crystallinity of the products; those phases in which protons are retained to high temperatures being poorly crystalline.Similarly when proton mobility in kaolinite is increased by the application of an electric field the crystallinity of the final product mullite is enhan~ed.~’ Mechanism of thermal decomposition of kaolinite. In the Brindley-Nakahira mode15* (Table 3) loss of water occurs more or less uniformly from all unit cells (the homogeneous mechanism) with a decrease in the number of oxygen atoms per unit cell and a change in the co-ordination of aluminium from six in kaolinite to four in metakaolinite. The n.m.r. and acid dissolution studies of Gastuche et al.,Ss have been taken as further evidence in support of the homogeneous mechanism. In the Al-Si spinel proposed by Brindley and Nakahira the alu- minium co-ordination reverts to six while in mullite it is both four and six.K6 This sequence of changes in the aluminium co-ordination seems implausible.63 H. Konig Naturwiss. 1948 35 92. 64 H. Yamada and S. Kimura Yogyo Kyokui Shi 1962,70 65. 65 J. 3. Fripiat and F. Toussaint J. Phys. Chem. 1963 67 30. 66 J. J. Fripiat and F. Toussaint Nature 1960 186 627. 67 K. J. D. MacKenzie J . Appl. Chem. 1970 20 80. 68 M. C. Gastuche F. Toussaint J. J. Fripiat R. Touilleaux and R. Van Meersche Clay Minerals Bull. 1963 5 227. 198 Brett MacKenzie and Sharp Table 3 Schematic view of the various proposed kaolinite-mullite reaction mechanisms MODEL 650 "C 980°C 1 100°C Water lost 7 0 population I decreased I- J formed. LA+ [ Al-Si spinel 7 + silica I Spinel-+mullite 3 Silica-+ 1 I A1IVfVI I BRINDLEY- NAKAHIRA Ref.a 7 APV. f I 0 framework r (Si Al) and Hf counter- migration. Water lost from pores 0 frame- work unchanged. Cations random. TAYLOR Ref. b PAMPUCH Refs. c and d FREUND Refs. e and f MACKENZIE Ref. g I lattice unchanged. I I area are Cations I 1 random. random. J diffusion. Structure ordered. J 1 Si Alcounter-1 c migration. I I Water lost; I J someproton 1-j 1 ret;;;::? I distorted and I disordered. I Further A13+ and H+ counter- diffusion. Alumina separates out Si-A1 spinel formed. Silica separates out. Defects 'heal'. Stable phase forms (not spinel). Exoelectrons + protons-thydrogen. APV ? Residual protons lost as hydrogen (in vacuo). Si- doped cubic phase formed. AlVIfIV. Silica separates. 1 cristobalite. J Spinel-+ 3 [ mullite. I Si-rich ). Ccristobalite. J -+I areas-+ mullite.i ).--+{ Silica+ I alumina-+ 1 I 1 mullite? J J i Mullite 1 1-,1 formed. 1 J 1 J Increase in 1 [ 1 Silica-+ J 1 cristobalite. I APV. a Ref. 56. Ref. 58.&C Ref. 14. Rgf. 57. Ref. 15. Ref. 62. g Ref. 59. An alternative inhomogeneous mechanism proposed by Taylor58 involves migration of aluminium and silicon to the acceptor regions and protons to the donor regions which later become pores from which water is lost. No oxygen is lost from the acceptor regions which segregate into aluminium-rich and silicon-rich areas becoming spinel and cristobalite respectively. The mechanism does not require drastic changes in oxygen packing hence topotactic and X-ray observations are better accounted for. Taylor does not discuss changes in aluminium co-ordination during the reaction sequence.The inhomogeneous mechanism has been modified by Pampuch14 to take account of residual hydroxy- groups. 199 Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides Freund16 has extended the concept of proton tunnelling to explain how de- hydroxylation of kaolinite occurs at temperatures much lower than those neces- sary to rupture the 0-H bond. Fripiat's of proton delocalisation provides an alternative explanation which may however differ more in termino- logy than in substance. The overall reaction sequence proposed by Freund (Table 3) involves tetrahedrally co-ordinated aluminium in metakaolinite partially changing to octahedral co-ordination in mullite. This necessitates tetrahedral aluminium in the metastable cubic phase formed at about 980°C. The almost identical i.r.spectra of metakaolinite and the 980°C phase have led MacKenzie to suggest that these structures are very similar particularly with respect to aluminium co-~rdination.~~ He considers (Table 3) that the cubic phase is silicon-doped y-alumina containing octahedral and tetrahedral alu- minium. Therefore metakaolinite is also thought to contain both octahedral and tetrahedral aluminium albeit extremely distorted. The model proposed involves a gradual increase in the amount of tetrahedral aluminium throughout the reaction sequence. Kinetics of kaolinite decomposition. This has been studied extensively by iso- thermal weight-loss measurements dynamic thermogravimetry and differential thermal analysis methods. The dynamic techniques are less reliable since there must always be a temperature gradient within a sample when the temperature is increasing continuously.Most data obtained prior to 1962 were interpreted in terms of first order kinetics usually attributed to a random nucleation process.69 These experiments were almost always carried out in air with no attempt to control the partial pressure of water vapour around the sample. Since 1962 it has been established that in vacu0~~9'~ and under controlled water vapour pressures71 the kinetics are best described by a two-dimensional diffusion equation. With increase in water vapour pressure the reaction rate decreases markedly whereas the apparent activation energy increases. Attempts have been made to explain these observations in terms of a chemisorbed surface water layer.'l~~~ Many of the earlier studies previously explained by first order kinetics can probably be re-interpreted in terms of a diffusion process although the results of Toussaint et aZ.,72 are an important exception.Recent studies of the kinetics of formation of mullite from kaolinite suggest that the rate determining step is nucleation c o n t r ~ l l e d . ~ ~ ~ ~ Other aluminium silicates. Allophane which is composed of silica and alumina gels is strictly not a layer silicate although Wada7* considers that it might possess some structural order as does the fibrous variety im~golite.~~ The P. Murray and J. White Trans. Brit. Cerarn. SOC. 1955 54 137; G. W. Brindley and M. Nakahira J. Amer. Ceram. SOC. 1957 40 346. 70 J. B. Holt I. B. Cutler and M. E. Wadsworth J. Arner. Ceram. SOC. 1962,45 133. 71 G.W. Brindley J. H. Sharp J. €1. Patterson and B. N. Narahari Achar Arner. Mineralogist 1967 52 201. 72 F. Toussaint J. J. Fripiat and M. C. Gastuche J. Phys. Chem. 1963 67 26. 73 J. F. Duncan K. J. D. MacKenzie and P. K. Foster J . Arner. Ceram. SOC. 1969 52 74. 74 K. Wada Amer. Mineralogist 1967 52 690; J. D. Russell W. J. McHardy and A. R. Fraser Clay Minerals 1969 8 87. 200 Brett MacKenzie and Sharp principal decomposition product of both minerals is mullite (Table 2) with minor feldspar phases formed from imp~rities.~~ Alumina-rich samples give corundum at higher temperatures. Pyrophyllite A12Si4010(OH)2 decomposes in air at about 800°C to form a stable dehydr~xylate~~ (Table 2) which is sometimes called metapyrophyllite by analogy with metakaolinite but the name pyrophyllite dehydroxylate is perhaps preferable.Orientational relationships e ~ i s t ~ ~ ~ ~ ~ although they are difficult to observe since good single crystals of pyrophyllite are rare and pyrophyllite dehydroxylate gives a disordered X-ray pattern. 1.r. spectroscopy suggests that aluminium in pyrophyllite dehydroxylate is in five- or six-fold co-~rdination,~~ ruling out one proposed based on a homogeneous mechanism and involving tetrahedral aluminium. At 1100°C or above pyro- phyllite dehydroxylate is transformed into mullite with a definite orientation relationship and cristobalite which has been variously reported to be formed in random ~rientation~~ and Opinions differ as to whether the de- hydroxylation proceeds by a homogene~us~~ 9 7 s or inhom~geneou~~~ mechanism. Under hydrothermal conditions at pressures between 13.5 and 27 kilobars pyrophyllite forms kyanite and coesite above 1000"C.80 Montmorillonite Nao.7(A13.3Mgo.7)Si8020(OH)4,nH20 is a common clay mineral which decomposes according to the simplified scheme in Table 2 although cordierite enstatite and anorthite have also been r e p ~ r t e d .~ ~ ~ ~ ~ Grim and Kulbickisl have suggested that there are two modifications of montmoril- lonite (Wyoming and Cheto) which differ in their high temperature reactions. The Wyoming-type decomposes essentially as in Table 2 whereas the Cheto- type forms quartz at 850°C cristobalite at 1000°C and cordierite at 125O0C but no mullite. The hypothesis of two types of montmorillonite has been questioneds2 and recent works3 indicates that completely hydrogen-exchanged montmoril- lonites all form mullite rather than cordierite without the appearance of quartz.The i.r. spectrum of montmorillonite dehydr~xylate~~ is similar to that of pyrophyllite dehydroxylate suggesting a resemblance between their layer structures although X-ray evidence indicates that the layer stacking must differ. The X-ray pattern of montmorillonite dehydroxylate is even more diffuse than that of pyrophyllite dehydroxylate. The dioctahedral mica muscovite K2A~4(Si6A~2)020(OH)4 decomposes essentially as shown in Table 2. Slight differences reported in the high tempera- 75 K. J. D. MacKenzie Clay Minerals 1970 8 359. 76 L. Heller Amer. Mineralogist 1962 47 156. 77 L. Heller V. C. Farmer R. C. MacKenzie B. D. Mitchell and H. F. W. Taylor Clay Minerals Bull. 1962,5 56; V. Stubican and R.Roy J. Phys. Chem. 1962,65 1348. 'I3 W. F. Bradley and R. E. Grim Amer. Mineralogist 1951 36 182. 79 W. D. Johns Bull. Amer. Ceram. Soc. 1965 44 682. 46 976. 82 A. Varty and D. White Clay Minerals Bull. 1964 5 465. 83 J. D. Hancock personal communication. A. A. Giardini J. A. Kohn D. W. Eckart and J. E. Tydings Amer. Mineralogist 1961 R. E. Grim and G. Kulbicki Amer. Mineralogist 1961 46 1329. 201 Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides ture products are probably due to variations in the composition of the mineral especially in the Si :A1 ratio.2 EberhartE4 proposed a homogeneous dehydroxylation mechanism based on a one-dimensional Fourier analysis of the X-ray pattern of muscovite dehydro- xylate. The product formed at 700°C is thought to be little changed from the parent mica except for an increase in the cell parameters.In his model neigh- bowing pairs of hydroxy-groups interact to form a molecule of water and the left-over oxygen ion occupies the vacant site in the octahedral layer leaving the cations in five-fold co-ordination. NicolE5 investigated the reaction under mild hydrothermal conditions when kalsilite (in epitaxial relation to muscovite) and unoriented corundum are formed. He suggests that an inhomogeneous mechanism fits his and Eberhart's observed topotactic relations better than the homogeneous mechanism. More recently the homogeneous mechanism has again been favoured by Nakahi~-a~~ (see phlogopite) and Vedder and Wilkins,86 whose i.r. spectroscopy showed that when muscovite is successively dehydroxylated and re-hydroxylated the OH ions return to their original sites consistent with a homogeneous mechanism.However a more refined structure determination of muscovite dehydroxylate will be needed to establish the mechanism conclus- ively. Under atmospheric conditions the mica structure breaks down above 1O5O0C giving products variously reported as a spine152,84~87~89 which is formed topota~tically,~~ c ~ r u n d u m ~ ~ ~ ~ ~ 9 87 9 88,89 mullite 84,89 l e u ~ i t e ~ ~ sana- dine,87,88 and trid~rnite.~~ The spinel with a = 7.9-8.0 A has been reported as ~ - a l ~ r n i n a ~ ~ ~ ~ ~ but may be similar to the silicon-containing spinel formed from kaolinite. Fripiat et al.,90 have reported a predehydroxylation state for the micas mus- covite phlogopite and biotite in which the protons become delocalised.The activation energy for the transition of protons to the excited state is 4.3 kcal/ proton g. for all the micas studied. Such a low value suggests that a tunnelling mechanism is operative similar to that of Freund.15 The dehydroxylation kinetics of muscovite have been investigated in airs1 and in V ~ C U O . ~ ~ Kodama and Brydong2 report that in vacuo the rate is controlled by two-dimensional diffusion in the reacted product with an activation energy of 54 kcal/mole. Both the model and the activation energy are closely similar to those reported for kaolinite," ~erpentine,~~ and cro~idolite.~~ C. Other Related Layer Silicates-Few if any of the silicates discussed above 84 J. P. Eberhart Bull. SOC. Franc. Mineral. Crist. 1963 86 213. 85 A. W. Nicol Clays and Clay Minerals 1964,12 1 2 .a6 W. Vedder and R. W. T. Wilkins Amer. Mineralogist 1969 54 482. 87 N. A. Toropov E. S. Sheo and A. I. Boikova Izvest. Akad. Nauk S.S.S.R. Neorg. Materialy 1966 2 1487. H. S. Yoder and H. P. Eugster Geochim. Cosmochim. Acta 1955,8,225. 89 N. Sundius and A. M. Bystrom Trans. Brit. Ceram. Soc. 1953 52 632. J. J. Fripiat P. Rouxhet and H. Jacobs Amer. Mineralogist 1965 50 1937. 91 J. B. Holt I. B. Cutler and M. E. Wadsworth J. Amer. Ceram. SOC. 1958,41 242. 9aH. Kodama and J. E. Brydon Trans. Faraday SOC. 1968,64 3112. 93 M. W. Clark and A. G. Freeman Trans. Faraday SOC. 1967,63 2051. 202 Brett MacKenzie and Sharp are found in nature with the ideal compositions listed. Extensive isomorphous replacement occurs especially in the octahedral sites with iron@) iron(m) nickel lithium and other cations substituting for part or all of the aluminium and magnesium.In addition fluoride ions often replace hydroxide especially in the micas. Silicates containing divalent ions such as Fe2+ and Zn2+ of similar size to Mg2 + decompose analogously to magnesium silicates5*~ 94 whereas those con- taining Fe3+ behave like aluminium ~ i l i c a t e s . ~ ~ ~ ~ ~ Since the decompositions are usually investigated in air oxidation of iron(@ occurs before or simultaneous with dehydro~ylation.~~~ gspg7 The reaction (equation 1) involves the conversion of OH- to 02- while the liberated protons form water with oxygen from the atmo~phere.~~ The most important iron-rich layer silicates are the micas e.g, biotite K2(Mg FeII FeIII A~)~(S~~A~,)O~O(OH F)4 and zinnwaldite K,(Fe Li Al)6- (Si6A1,)0,,(OH F)4.On heating in air at about 600"C a typical biotite of composition KMgFe112(Si3AI)010(OH) would form the oxybiotite KMgFeII1,- (Si3A1)OI2 whereas in vacuum or inert atmosphere it would not dehydroxylate until about 1000°C when a mixture of phases of overall composition KMgFe11,- (Si3AI)Ol would be formed. In other respects biotite and zinnwaldite behave similarly to phlogopite although hematitee* and iron(m)-containing spinels are formed above 1000°C. The vapour species evolved from biotite heated to 1100°C in vacuo have been identified in a mass ~pectrometer.~~~ 99 In addition to water considerable amounts of hydrogen and traces of other gases were detected. The hydrogen may result from an internal oxidation processQ7 (equation 2) which has also been reported for amphiboleslOO and chlorite~.~~ Proton delocalisation before dehydroxylation has been discussed under muscovite.Four other iron-containing 1 1 layer silicates chamosite (FeI1,AI)- (SiAI)O,(OH), greenalite Fe113Si205(OH), amesite (Mg Fe11)2(SiAI)05(OH)4 and cronstedtite (FeI1,Fe1II) (SiFe111)0,(OH)4 should be mentioned. The thermal decomposition of chamosite and greenalite have not been studied further since the previous reviews.1$2 4Fe2+ + 40H- + O2 = 4Fe3+ + 402- + 2H20 (1) 2Fe2+ + 20H- = 2Fe3+ + 202- + H2 (2) O4 H. F. W. Taylor Amer. Mineralogist 1962 47 932. 86 S. Orcel and P. Renaud Compt. rend. 1941 212 918; G. W. Brindley and R. F. Youell Mineralog. Mag. 1953 30 57. 98 C. C. Addison W. E. Addison G. H. Neal and J. H. Sharp J. Chem. SOC.1962 1468. O7 A. D. White and J. H. Sharp unpublished data. O* A. L. Litvin Zap. Ukr. Otdel. Vsesoyuz. Mineralog. Obshchestva Akad. Nauk Ukr. S.S.R. 1962 1 38. 88 C. G. Barker Nature 1965 205 1001. loo A. A. Hodgson A. G. Freeman and H. F. W. Taylor Miireralog. Mag. 1965 35 445. 203 Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides Brindley et al.,lol heated amesite to 600°C and obtained a strong X-ray pattern which was tentatively identified as an iron-rich olivine but has subse- quently been shown to be a diffuse reflection caused by stacking disorder.102 Above 9OO"C a spinel phase appears.lol A recent study of cronstedtite by Steadman and Topo3 has confirmed the published reaction sequence,l but has shown that the intermediate phases depend on the layer stacking of the original polymorphs.Thus at about 700"C either a spinel a hexagonal ferrite or a phase having an ill-defined structure may be formed. The spinel phase was formed in small 'zones' rather than nuclei,103 an observation in accordance with an inhomogeneous mechanism. Bradley and Grim7s have reported that the lithium-containing smectite hec t orit e Na .7 (Mg 5. ,Li .,)Si,O 20(0H)4 while not forming a stable anhydride does not dehydroxylate until above 700"C considerably higher than for mont- morillonite. Under hydrothermal conditions it is converted into talc or antho- phyllite.104 The iron smectite nontronite Na0.,FeI1I4(Si7. 3A10.7)020(OH)4 dehydroxylates at lower temperatures than montmorillonite and forms mullite cristobalite and an iron-containing spinel at 1300°C.7s The chlorites which are mixed-layer minerals in which talc-like units are regularly interspersed with brucite-like units (Figure la) have the same ideal formulae as serpentine and related minerals although extensive isomorphous replacement occurs.On heating a well-crystallised chlorite to about 600°C an intermediate phase is produced by loss of water from the brucite units.ls2 Struc- tural hydroxy-groups are lost from the talc-like units above 800°C with forma- tion of olivine (or forsterite if the mineral is almost purely magnesian) as the principal product; spinel phases and enstatite have also been reported. Hydrogen evolution probably involving internal oxidation has been observed by Orcel and Renaud. B6 The predominantly magnesium minerals sepiolite and palygorskite (some- times called attapulgite) which have some of the characteristics of talc and of the amphibole anthophyllite decompose to form enstatite and cristobalite4' while the aluminium content of palygorskite leads to the formation of some ~illimanite.~' Amphiboles.Figure 1 b shows that amphiboles and pyroxenes have structures closely related to the layer silicates although the tetrahedral sheets are not continuous. This structural resemblance provides a convenient explanation for the frequent occurrence of a pyroxene (usually enstatite) among the reaction products of the magnesium layer silicates. The amphiboles may be considered as intermediate between micas and pyroxenes; of the four cation sites in amphi- boles two (M and M3) are equivalent to the cation sites in micas whereas the other two (M2 and M4) are not co-ordinated to OH groups and are similar to lol G.W. Brindley B. M. Oughton and R. F. Youell Acta Cryst. 1951,4 552. lo2 H. Steinfink and G. D. Brunton Acta Cryst. 1956,9 487. lo3 R. Steadman and M. Toy 2. Krist. 1965 122 321. lo4 L. B. Sand and L. L. Ames Clays and Clay Minerals 1959 2 392. 204 Brett Mackenzie and Sharp the cation sites in pyroxenes. Furthermore the equivalent of the inter-layer site filled by potassium in the micas is also present as the A-site in the amphiboles which can be vacant but is frequently occupied as in the hornblendes. There are however some unexplained differences between the micas and the amphib01es.l~~ The total number of cations per 24 0 atoms is always close to 7 (or 8 if the A-site is occupied) in amphiboles but in the micas it can be 4 or 6.Thus there are no cation vacancies in the amphiboles (cf. trioctahedral micas) which led Wilkins and Vedderlo5 to predict that amphiboles should decompose without the formation of stable dehydroxylates. This prediction is upheld (Table 4) except for crocidolite which forms a d e h y d r ~ x y l a t e ~ ~ ~ ~ ~ ~ ~ stable over a range of 200°C. Table 4 Decomposition products of amphiboles Mineral Anthophyllite Mg,SisO22(OH)2 Amosite (fibrous grunerite) Fe115.5Mgl.5Sis022(OH)2 Tremolite-actinolite Ca2(Mg,Fe11)5Sis022(OH)2 Pargasite (hornblende) NaCa2Mg,A13Si602,(OH) Glaucophane Na2Mg3A12Si8022(0H)2 Crocidolite (fibrous riebeckite) Na,(Mg,Fe11)3Fe1112Si802z(OH)2 } Products P + s P + sp + H + s P + P2 + s PI + Sp1 + 0 + Fi + F2 P + 0 + F3 600°C inert A atmosphere +P3 + Sp + H + S L g R efi a b e Key P = pyroxene.P = (Mg,FeI1)SiO3 P2 = CaMgSi20, P = NaFe1I1Si,O6. Sp = a spinel phase. Sp = MgAl,O, Sp = Fe304. F = feldspar. F = NaAISiO, F2 = CaAl,Si,O, F3 = NaA1Si308. 0 = olivine (Mg,FeII),SiO, H = hematite Fe203 S = silica SO2. A = crocidolite dehydoxylate Naz(Mg,Fe11)3Fe1112Si8023. a Ref. 111. b Ref. 100. C Ref. 110. e W. G. Ernst Amer. J. Sci. 1961,259 735,. Ref. 107. g Ref. 108. F. R. Boyd ‘Researches in Geochemistry’ Wiley 1965. An alternative hypothesislo7 that minerals containing trivalent ions in octa- hedral sites form stable dehydroxylates is not necessarily supported by the existence of crocidolite dehydroxylate since the iron(n1) ions are located chiefly in the M2 site which is not co-ordinated to the OH group.The temperature at which dehydroxylation occurs depends largely on the cations occupying the MI and M3 sites; this temperature rises as magnesium replaces iron(r~).~O~ Most studies of amphibole decompositions have been under hydrothermal lo5 R. W. T. Wilkins and W. Vedder ‘Reactivity of Solids’ Wiley 1969 p. 227. lo6 W. E. Addison and J. H. Sharp J. Chem. SOC. 1962 3693. lo’ A. A. Hodgson A. G. Freeman and H. F. W. Taylor Mineralog. Mag. 1965 35 5. lo8 J. H. Patterson Mineralog. Mag. 1965 35 31. log A. G. Freeman Mineralog. Mag. 1966 35 953. 205 Thermal Decomposition of Hydrous Layer Silicates and their Related Hydroxides conditions but those shown in Table 4 are at atmospheric pressure. A pyroxene is always formed its composition varying with that of the reactant. In the absence of a framework aluminosilicate silica is formed as a separate phase.Good topotactic relations are frequently observed between the amphiboles and their decomposition ~ ~ o ~ u c ~ s . ~ ~ ~ ~ ~ ~ ~ J ~ ~ J Inhomogeneous mechanisms have been postulated but not proved for some of these reactions. When amosite,loO actinolite,lll or anthophyllitelll is heated in an inert atmo- sphere hydrogen evolution is so pronounced that it can be detected from differences between measurements of weight loss and water liberated. The mechanism is probably that proposed for biotite (equation 2). Reduction of iron(rrr) occurs on heating crocidolite in hydrogen and it has been suggested that iron(0) is formed within the structure at 450°C.106 This unstable arrangement leads to the destruction of the amphibole structure at 530°C when metallic iron is one of the The dehydroxylation kinetics of crocidolite in vacuo show a change in mech- anism at 55OoC;ll2 above this temperature a diffusion equation is ~ b e y e d ~ ~ ~ ~ similar to that discussed for muscovite.Attempts have been made to establish the presence of iron(0) in reduced cro~idolite~~~ and the co-ordination of iron in crocidolite dehydroxylatell* by Mossbauer spectroscopy. The results are inconclusive although the iron atoms in crocidolite dehydroxylate are probably in distorted octahedral sites. Moss- bauer spectroscopy may prove more valuable in future investigations of iron- containing silicates. 4 Summary This Review emphasises the conflicting interpretations of the decomposition mechanisms of related hydrous compounds.The experimental basis (viz. the spinel intermediate) for the inhomogeneous mechanism of decomposition of brucite has been invalidated. Kinetic and microscopic evidence favours a homogeneous mechanism although the difficul- ties7 pointed out previously are still not explained. Further attempts to elucidate the reaction mechanism may rest on a less rigid approach which marries the important concepts of the previous theories. At the intermediate stage of de- composition the structure which has already lost the bulk of its water is amorphous to X-rays yet retains a memory of its previous structure. Local cation migration may then take place as magnesium ions pass through positions of tetrahedral co-ordination into their final octahedral sites.2 The principal evidence for an inhomogeneous mechanism in the decomposition of hydrous magnesium silicates is the observed topotaxy between the reactants 110 A.G. Freeman and H. F. W. Taylor Silikat Tech. 1960,11 390. ll1 A. A. Hodgson ‘Fibrous Silicates’ R.I.C. Lecture Series 1965 No. 4. l12D. J. O’Connor and J. H. Patterson Conf. on Phys. and Chem. of Asbestos Minerals Oxford 1967 Abst. 1-7. l13T. C. Gibb and N. N. Greenwood Trans. Faraday SOC. 1966,61 1317. 114 H. J. Whitfield and A. G. Freeman J . Inorg. NucZear Chem. 1967 29 903. 206 Brett Mackenzie and Sharp and products; this evidence was interpreted in terms of an inhomogeneous mechanism by analogy with brucite when the spinel intermediate was accepted. Since the latter has now been discredited a re-examination of the mechanism postulated for magnesium silicates seems appropriate particularly in view of possible complications presented by the presence of a second product phase. Similarly the dehydroxylation of aluminium- and iron-containing minerals may not rigorously follow either a homogeneous or inhomogeneous mechanism. 207

ISSN:0009-2681    

DOI:10.1039/QR9702400185

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

The Stereochemistry of Polysulphides By R. Rahman S. Safe and A. Taylor NATIONAL RESEARCH COUNCIL OF CANADA ATLANTIC REGIONAL LABORATORY HALIFAX CANADA The existence of linear chains of sulphur atoms and also rings of such atoms e.g. S8 has been known for a long time. The discovery of restricted rotation about the -S-S- bond had obvious stereochemical implications but examples of this isomerism were rare until recent years. Similarly the asymmetry of quadrivalent sulphur was appreciated at the time of the discovery of methionine sulphoxide but further examples were described only this year and the implication of such stereochemistry to the controversy about the existence of branched i.e. -S-S- sulphur compounds has not been explored. Recently 3. S a few natural products having asymmetric polysulphide moieties have been isolated and improved methods of synthesis have enabled other asymmetric polysulphides to be secured.The availability of these compounds has enabled n.m.r. studies to be made and these investigations have shown that the chemical shift of atomic nuclei adjacent to a sulphur group depends on its asymmetry. In addition these compounds exhibit different circular dichroism. These advances have led to an increase in our knowledge of the stereochemistry of sulphur compounds and it therefore seems appropriate to present a review of the subject at this time. We have divided the review into four sections dealing with sulphur acyclic polysulphides organic cyclic polysulphides and inorganic cyclic polysulphides merely to aid access to particular information. 1 Elemental Sulphur A sulphur melt contains innumerable forms of molecular sulphur which exist as a complex equilibrium mixture.Thirty different allotropic forms of sulphur have been described but the precise structure of only a few of these is known. From a stereochemical and molecular point of view two distinct classes of sulphur allotropes have been defined those aliotropic forms which differ in their stereo- chemical assemblage of atoms i.e. intramolecular allotropy; and those which differ in the assembly of their molecules i.e. intermolecular allotropy. In this review we are concerned with the intramolecular a1lotropes.l The three most common intramolecular allotropes of naturally-occurring sulphur are cyclo-octasulphur (SA) cyclohexasulphur (Sp) and a complex mixture of non-cyclic polymeric forms.The structure and properties of poly- B. Meyer Chem. Rev. 1964 64 429; ‘Elemental Sulfur’ John Wiley New York 1965. 208 Rahman Safe and Taylor sulphur (or fibrous sulphur) vary with the conditions under which it is formed. It invariably occurs and is produced in liquid sulphur melts together with many other allotropes. Many other molecular forms of sulphur have been detected; at least ten different molecular species have been detected in sulphur vapour2 and some have recently been identified mass spectr~metrically.~ Several authors have reported S2 Ss S4 and S but their extreme instability has prevented detailed study.* Cyclododeca- cyclodeca- and cyclohexa-sulphur have recently been synthesised by Schmidt and his co-~orkers.~ A. Cyclo-Octasulphur (S, l).-Cyclo-octasulphur (S,) is the most common intramolecular allotrope of sulphur and the orthorhombic (S,) crystalline modification is the most stable form.The X-ray crystallographic structure determination of S has been reported by several groups of workers* and the following molecular constants have been obtained 16 S molecules in the unit cell; average S-S bond length 2.048 A; S-S-S bond angle 107" 54'; and S-S-S-S dihedral angle 98" 42'. As suggested by Pauling,' the structure depends on the two latter constants. Monoclinic sulphur (SB) is a second modification of cyclo-octasulphur and it is the stable form at temperatures greater than 95.4". X-Ray crystallographic studies* have shown 48 atoms in the unit cell having an average bond distance of 2.06 A. Some structural information has been reporteda on a third modification of cyclo-octasulphur (S,) and de Haan has proposed a structure based on the packing of only four s molecules in the unit cell.Several other allotropes of H. Staudinger and W. Kreis Helv. Chim. Acra 1925 8 71. J. Berkowitz and J. R. Marquart J. Chem. Phys. 1963 39 275; J. Berkowitz and W. Chupka ibid. 1964,40,287; J. Berkowitz and C. Lifshitz ibid. 1968,48,4346; D. Cubicciotti J. Phys. Chem. 1963 67 1385. K. Ikenoue J. Phys. SOC. Japan 1953 8 646; H. Lux and E. B~hrn Chern. Ber. 1965,98 3210; J. A. Poulis and W. Derbyshire Trans. Faraday SOC. 1963,59 559. M. Schmidt and E. Wilhelm Angew. Chern. Internat. Edn. 1966 5 964. S. C. Abrahams Acta Cryst. 1955,8,661; A. S. Cooper W. L. Bonds and S. C. Abrahams L. Pauling Proc. Nat. Acad. Sci. U.S.A.1949 35 495. J. Donohue A. Caron and E. Goldish J . Amer. Chern. SOC. 1961,83,3748; Y. M. de Haan ibid. 1961 14 1008; A. Caron and J. Donohue ibid. 1965 18 562. * J. T. Burwell Z . Krisr. 1937 97 123; D. E Sands J. Arner. Chem. SOC. 1965 87 1395. Physica 1950 24 855. 209 The Stereochemistry of Polysulphides cyclo-octasulphur have been described but few structural details reported. The major variations are intermolecular allot ropes prepared either by crys tallisation from a variety of organic solvents e.g. a-pinene fenchenone and o-xylene,l0 by in.adiation,ll or by controlled generation of sulphur in a chemical reaction.6* l2 B. Cyclohexasulphur (Sp 2).-Cyclohexasulphur occurs naturally and has been synthesised but little data has been given about the synthetic material.The naturally-occurring S exists as a staggered six-membered ring with the following molecular constants S-S bond length 2.059 A; S-S-S bond angle 102" 12'; and S-S-S-S dihedral angle 74"; three S molecules were found in the unit ce11.13 lo M. G. Wolf J. Chem. Educ. 1951 28 427. l1 (a) P. D. Bartlett and G. Meguerian J. Amer. Chem. Soc. 1956 78 3710; P. D. Bartlett E. F. Cox and R. E. Davis ibid. 1961 83 103; (6) F Fehkr and D. Kurz 2. Naturforsch. 1969,24b 1089; (c) F. Feh6r and W. Becker ibid. 1965,20b 1125; F. Fehtr B. Degen and B. Sohngen Angew. Chem. 1968,80 320. l2 W. A. Pryor in 'Mechanisms of Sulphur Reactions' McGraw-Hill Co. New York 1962. l3 J. Donohue A. Caron and E. Goldish Nature 1958,182,518; A. Caron and J. Donohue J. Phys. Chem. 1960 64 1767; S. H. Goodman and J.Donohue ibid. 1964 68 2363; C. Frondel and R. E. Whitfield Acta Cryst. 1950 3 242; J. D. H. Donnay. ibid. 1955 8 245. 21 0 Rahman Safe and Taylor C. Cyclododecasulphur (3).-Schmidt and his co-workers5 have developed a new and promising synthetic method for the preparation of specific sulphur molecules viz Sue12 + HZSz = 2HC1 + S(z+y) . Thus condensation of dihydro-octasulphide ( x = 8) and tetrasulphur dichloride ( y = 4) in carbon disulphide gave a stable cyclododecasulphur S12. X-Ray crystallographic analysis14 gave the following molecular constants 2 molecules/ unit cell; S-S bond distance 2.055 A; S-S-S bond angle 106" 30'. The ring exists as a zig-zag conformation with the sulphur atoms lying in three planes. This approach to sulphur chemistry seems to be of great versatility since cyclohexa- and cyclodeca-sulphur have been prepared by the method.The reaction of triarylphosphines with cyclic sulphur compounds has received some attention.llU~ Thus it has been shown that the rate of the reaction of diphenyl-o-tolylphosphine with S6 S 7 SI2 and S decreases in the order given; the rate difference between the reaction of S and S with the nucleophilic reagent is at least 10,000 and this has been attributed to the strain in the six-membered ring shown by the value of the dihedral angle (74") as compared to 98" 42' in S,. D. Polysulphur (4).-Polysulphur is a complex mixture of non-cyclised sulphur chains and several allotropic forms have been described. Very little structural data has been reported but the proposition has been made that they consist of helices of sulphur atoms as found in metallic selenium and tel1~rium.l~ It should be noted that chain elongation occurs by trans addition of sulphur atoms.l4 A. Kutoglu and E. Hellner Angew. Chem. Infernat. Edn. 1966 5 965. J. A. Prins J. Schenk and L. H. J. Wachters Physica 1957 23 746; J. A. Prins ibid. 1954 20 124; J. A. Prins and F. Tuinstra ibid. 1963 29 329 884; C. W. Thompson and N. S. Gingrich J . Chem. Phvs. 1959 31 1598. 21 1 The Stereochemistry of Polysulphides E. Charge Transfer Complexes of Sulphur.-Charge transfer complexes of sulphur are well known and X-ray crystallographic determination of the structures of the iodine16- and iodoforrnl7- sulphur complexes have been reported. The average bond distances and bond angles (2.043 A and 107" 42' respectively) for the iodoform-tricyclo-octasulphur complex are comparable to the data obtained for S,; the slightly smaller S-S bond distances of the complex are undoubtedly due to charge transfer interactions.2 Inorganic and Organic Linear Polysulphides The structure and stereochemistry of acyclic polysulphides has been the subject of a large number of investigations. There has been particular controversy on the question of whether or not the chains of sulphur atoms are branched (i.e. 5 or 6). The chemical evidence can be interpreted to support either type of R-S-R R-S-S-R J. S (5) (6) structure. Thus those sulphur atoms bonded only to other sulphur atoms are extremely reactive. This may be illustrated by the reaction of a nucleophile18 e.g. CN- viz R-SSSS-R + CN- -+ R-SSS-CN + S-R- R-SSS-CN + CN- + R-SS-CN + SCN- R-SSS-CN + RS- 4 R-SS-R + SCN- .It has been suggested that a branched sulphur chain would react more readily with the nucleophile and hence account for the fast reaction of trisulphides and higher sulphides. The reaction of triphenylphosphine with tris~lphidesl~ to give disulphides is formally analogous to its reaction with sulphoxides when the sulphide is the principal product.20 Another analogy with sulphoxide chemistry is the reaction of diphenyl disulphide with dihydrogen disulphide to give polysulphides,21 in the same way that the sulphides give sulphoxides when treated with hydrogen peroxide. Recent studies by Barnard and his co-workers22 describe the facile thermal racemisation of di-ally1 polysulphides (7 9). Because the racemisation of (7) does not take place with disproportionation or allylic rearrangement and rate studies indicate the absence of homolytic side reactions the authors believe the l6 J.Jander and G. Turk Chem. Ber. 1964,97 25. l7 T. Bjorvatten Acta Chem. Scand. 1962 16 749. J. L. Kice Accounts Chem. Res. 1968 1 58. 2o H. H. Szmant and 0. Cox J . Org. Chem. 1966 31 1595. 21 S. Safe and A. Taylor J. Chem. SOC. (C) 1970,432. 22 D. Barnard T. H. Houseman M. Porter and B. K. Tidd Chem. Comm. 1969 371. 0. FOSS in 'Organic Sulfur Compounds' vol. 1 Pergamon Press Oxford 1961 p. 83; C. G. Moore and B. R. Trego Tetrahedron 1962,18 205; 1963,19 1251. 212 Rahman Safe and Taylor ty S- %L s = s I S P branched compound (8) to be an intermediate. The similarity of this reaction with the sulphenate (10)-sulphoxide( 1 1) rearrangementz3 isclear and is illustrated in the following scheme.S=0 / CCI 3 ,s =o Cl C When disulphides prepared from a 35S-labelled thiol and a different unlabelled thiol are reduced both of the thiols obtained are equally radioactive. Wieland and Schwahnz4 propose an equilibrium between the linear disulphide and its branched isomer i.e. 23 S. Braverman and Y. Stabinsky Chem. Comm. 1967 270; Israel J . Chem. 1967 5 125. 24 T. Wieland and H. Schwahn Chem. Ber. 1956,89 422. 21 3 The Stereochemistry of Polysulphides R'SS*R2 =1 R1S*R2 -+ R1SR2 + RfS*SR2 \c S" J. S to account for this phenomenon. Thus all of this chemical evidence supports the view that disulphides (and presumably polysulphides) exist in solution in equilibrium with their branched isomers. Raman26 and i.r. spectroscopic results2') all favour unbranched sulphur chains and these conclu- sions are supported by X-ray crystallographic data.In the following sections some of this physical evidence will be presented. Physical measurements (that is dipole moment A. Disulphides.-(i) Inorganic disulphides. Linear disulphides are non-planar with an X-S-S-X dihedral angle of about go" although there are under- standably considerable deviations from this average value. The 90" dihedral angle affords the molecule a configuration in which the repulsion between the sulphur lone pairs of electrons is at a minimum. There are thus two possible enantiomorphic forms of a disulphide. Winnewasser and Haase2* have recently reported electron diffraction measurements of dihydrogen disulphide (12). The data are typical of many disulphides.The compound disulphur difl~oride~~ (1 3) has a comparable dihedral 90' 37' F angle but a much shorter S-S bond length (1.888 8,) which is similar to that found for S (1.889 8,); disulphur dichloride and disulphur dibromide also have short S-S bond lengths (1.97 8 and 1.98 8 re~pectively).~~ Kuczkowski2g has suggested that this phenomenon may be due to efficient pd orbital overlap (possible with a dihedral angle of 90") to given bonding. This bonding might occur in the dihalogen disulphides since the electronegative substituents may distort the sulphur 3d orbitals. A second isomer (14) which represents the stable branched form can be isolated from the preparation of disulphur difluoride. 25 C. C. Woodrow M. Carmack and J. G. Miller J . Chem. Phys. 1951 19 951.26 F. Feher G. Krause and K. Vogelbruch Chem. Bet-. 1957,90 1570. 27 L. Schotte Avkiv Kemi 1956 9 361. 28 M. Winnewasser and J. Haase Z . Naturforsch. 1968 23a 56. 29 R. L. Kuczkowski J . Amer. Chem. SOC. 1964 86 36 17. 30 E. Hirota Bull. Chem. SOC. Japan 1958 31 130. 214 Rahman Safe and Taylor The structures of the barium31 and sodium of tetrathionic acid (1 5) have been determined by X-ray crystallography. The barium salt crystallises in enantiomorphous space groups with only one enantiomorph/unit cell. The length of the middle S-S bond is 2.02 A; the bond angle of the two divalent sulphur atoms is 103" and the dihedral angle is 90". A related derivative dimethanesulphonyl di~ulphide,~~ has similar structural properties. S II %..h%sg O2 (ii) Organic disrrlphides. The structures of a large number of organic disulphides including diphenyl di~ulphide,~~ 5,5'-dithiobis-(2-nitrobenzoic 2,2'- biphenyl di~ulphide,~~ L-cystine hydr~chloride~~ and hydrobr~mide,~~ NN'- diglycyl-L-cystine dih~drate,~~ ~-cystine,~O formamidinium disulphide di-iodide monohydrate41 and tetramethylthiuram di~ulphide,~~ have been determined by X-ray crystallography.The S-S bond distances in all cases lie between 2.03 and 2.05 A and the -C-S-S-C- dihedral angles vary from 74" to 105". Distortion of the structures from the 90" dihedral angle is usually due to steric or electronic effects e.g. the dihedral angle in di-t-butyl disulphide is larger than normal due to some extent to the steric interactions of the bulky t-butyl groups. 43 The n.m.r. spectra of the diphenyl disulphides (16 17) change with temperat~re;~~ o-alkyl groups become magnetically non-equivalent at - 27 a in the case of (16) and at - 55" for (17).These experimental results show that at low temperatures the rotation about the S-S bond is restricted and hence the n.m.r. spectrum of each of the two enantiomers is observed. This explanation recalls the suggestion of Strem et al.,45 that the unusually large specific rotation 31 0. Foss S. Furberg and H. Sachariasen Acta Chem. Scund. 1953 7 230; 1954 8 459 32 0. Foss and A. Hordvik Acta Chem. Scand. 1958,12 1700; 1964 18 662. 33 0. H. Sorum Acta Chem. Scand. 1953,7 1. 34 I. M. Dawson A. McL. Mathieson and J. M. Robertson J. Chem. SOC. 1948 322. 35 E. Shefter and T. I. Kalman Chem. Comm. 1969 1027. 36 I. Bernal and J. Ricci Actu Cryst. 1966 21 Suppl.A105. 37 L. K. Steinrauf J. Peterson and L. H. Jensen J. Amer. Chem. Soc. 1958 80 3835. 38 J. Petersen L. K. Steinrauf and L. H. Jensen Acta Cryst. 1960 13 104. 39 H. L. Yakel jun. and E. W. Hughes Acta Cryst. 1954 7 291. 40B. M. Oughton and P. M. Harrison Actu Cryst. 1959 12 396. 41 0. FOSS J. Johnsen and 0. Tvedten Acta Chem. Scand. 1958,12,1782. 42 K. Marcay Actu Chem. Scand. 1965 19 1509. 43 H. P. Koch J. Chem. Soc. 1949 394. 44H. Kessler A. Rieker and W. Rundel Chem. Comm. 1968 475. 45 J. Strem Y. S. R. Krishna-Prasad and P. J. A. Schellman Tetrahedron 1961 13 176. 21 5 The Stereochemistry of Polysulphides of L-cystine was due to the presence of relatively stable geometrical isomers. Similarly the optical rotations of the anomeric deoxynucleosides (18 19) vary (16) R' = R2 = Pri (17) R' = Et R2 = H 0 How - OH inversely with temperature and X-ray crystallographic analysis indicates that rotation about the S-S bond is hindered by both the sugar and base moieties.The presence of thionucleotides has been reported in certain soluble ribonucleic acids and whilst the biological function of such nucleotides is unknown the results discussed above suggest that they may introduce another element of asymmetry into these complex molecules.4s B. Trisu1phides.-(i) Inorganic trisulphides. Trisulphides of the general formula X-SSS-X can exist as two stable rotational isomers. The X groups are rotated approximately 90" out of the plane of the sulphur atoms; rotation of the X groups on the same side of the plane gives the cis form whereas rotation of these groups on the opposite side of the plane gives the trans isomer.The latter configuration exists in two enantiomorphous forms. The pentathionate ion and the isomorphous monoseleno- and monotelluro- derivatives have been extensively investigated and show both cis (20 X = S (21) b (20) 46 C. Szantay M. P. Kotick E. Shefter and T. J. Bardos J. Amer. Chem. SOC. 1967 89 713; M. N. Lipsett J . Biol. Chem. 1967 242 4067 and references cited therein. 216 Rahman Safe and Taylor Se or Te) and trans (21 X = S Se or Te) isomers in the solid state. The nature of the cation and its bonding with the pentathionate ion largely determine the preferred configuration. Barium pentathi~nate~' (20 X = S) its seleno (20 X = Se),49 and telluro (20 X = Te)48 analogues exist in the cis configuration whereas the corresponding potassium salt exists in the trans form.5o Cyanogen tris~lphide~l and the isomorphous cyanogen tri~elenide~~ are neutral and both occur in the cis configuration in the crystalline form.A detailed study of the i.r. and Raman spectra of dihydrogen trisulphide (22) by Weiser and his co-w~rkers,~~ resulted in the conclusion that the most stable conformer in solution is cis. The assigned conformation of (22) accounts for the hydrogen bonding which occurs in solution. (ii) Organic trisulphides. The structures of di-24odoethyl trisulphide 54 di- benzenesulphonyl and -toluene-p-sulphonyl tri~ulphides,~~ and perchlorodimethy 1 tris~lphide~~ have been determined by X-ray crystallography. In all cases the sulphur chains are unbranched and in the trans configuration. Dipole moment (p) and theoretical studies by Woodrow and his co-worker~~~ concluded that the p value for hexadecyl trisulphide (p = 1-63) was indicative of a mixture of linear rotational isomers.C. Tetra- and Poly-sulphides.-Tetrasulphides of the general structure X-S-S-S-S-X can exist in six non-superimposable forms or three different pairs of enantiomers (i.e. 23 24 and 25). Woodrow Carmack and Miller25 suggested their presence to account for the dipole moment of di-n-hexadecyl tetrasulphide. They concluded that the observed dipole moment (p = 2-16) could be accounted for by assuming that equal proportions of the enantiomers were present in solution but it is conceivable that unequal enantiomer 47 0. Foss and 0. Tjomsland Acta Chem. Scand. 1956,10,424; 1958,12 44. 48 0. Foss and 0.Tjomsland Acta Chem. Scand. 1958 12 52. 4g 0. Foss and 0. Tjomsland Acta Chem. Scand. 1954 8 1701. 50 K. Maray Acta Chem. Scand. 1969 23 338. 61 F. FehCr and K. H. Linke 2. anurg. Chem. 1964 327 151. 52 0. Aksnes and 0. Foss Acta Chem. Scand. 1954 8 1787. 5s H. Wieser P. J. Kreuger E. Muller and J. B. Hyne Canad. J. Chem. 1969 47 1633. 5 4 I. M. Dawson and J. M. Robertson J. Chem. Soc. 1948 1256. 55H. J. Berthold Z . anorg. Chem. 1963 325 237. 21 7 The Stereochemistry of Polysulphides populations would also be possible. Examination of models shows that (23) is in the cis@ configuration (24) in the trans,cis (or cis,trans) and (25) in the trans,trans configuration. The former configuration is present in cyclo-octa- sulphur whilst the latter is the configuration of the helices of fibrous sulphur.X-Ray crystallography of potassium barium he~athionate~~ shows that the molecule exists in the cis,cis form (26) presumably due to the bonding influence of the barium cation. By contrast trans-dichlorobis-(ethylenediamine)cobalt(m) hexathionate dihydrate (27)57 crystallises in the trans,trans configuration similar to fibrous sulphur and caesium hexasulphide.68 3 Organic Cyclic Polysulphides A. Disulphides-Cyclic disulphides of the general formula -C-SS-C- are composed of eauimolar concentrations of the two enantiomeric forms (28) 66 0. Foss and K. Johnsen Acta Chem. Scand. 1965,19,2207. 67 0. Foss and K. Marray Acta Chem. Scand. 1965 19,2219. 68 S. C. Abrahams and E. Grison Acta Cryst. 1953 6 206. 218 Rahman Safe and Taylor and (29). The two conformers are optical isomers which are in equilibrium.The rate of interconversion of the two conformers depends on the height of the barrier hindering rotation about the S-S bond; in most cases thedGf for the process is 10-15 k~al./mole.~~ This value is too small to allow separation of the enantiomers. The height of the barrier to free rotation varies with ring size degree of unsaturation substitution pattern and the size of the angle 8 (when 8 = 90" the energy barrier has a minimum value).6o Lipoic acid (30) a substituted 1 ,Zdithiolan derivative is the coenzyme required for oxidative decarboxylation of pyruvate.61 Its discovery prompted considerable interest in the dithiolan ring system. Calvin and his co-workers62 showed that the disulphide was unbranched thus negating previous work where a branched structure was proposed.The strain energy in dithiolan derivatives is considerable and has been estimated to be as high as 27 kcal/mole. Bergson and Schottes3 calculated a minimum value of 16 kcal/mole based on the X-ray diffraction analysis of the structure of 1,2-dithiolan-4-carboxylic acid (31).64 The dihedral angle of this compound (31) was shown to be 26" 36' which is considerably less than the value (90") for unstrained disulphides. Calvin has attributed this high strain to be a major factor in the mode of action of lipoic acid (30) which undergoes ready photolysis and reduction; the formation of the dithiol is essential for coenzyme activity. The structures of several unsaturated five-membered cyclic disulphides e.g. 59 G. Claeson G. Androes and M. Calvin J . Amer.Chem. SOC. 1961 83 4357. 6o M. Calvin Fed. Proc. 1954 13 697. L. J. Reed in 'Advances in Enzymology' Interscience Publishers New York 1957,18 p. 319. 62 J. A. Barltrop P. M. Hayes and M. Calvin,J. Amer. Chem. SOC. 1954,76,4348; M. Calvin and J. A. Barltrop ibid. 1952 74 6153. 63 C. Bergson and L. Schotte Acta Chem. Scand. 1958 12 367. 0. Foss and 0. Tjomsland A m Chem. Scand. 1957 1 1 1426. 219 The Stereochemistry of PolysuEphides (32)65 and (33),s6 have been determined by X-ray crystallography. These compounds are planar with a high degree of stability typical of aromatic systems; the aromaticity of these unsaturated rings is also dependent on the ring substituents. A recent review67 discusses this aspect of valence-shell expansion in sulphur heterocycles. S - s (32) (33) An interesting example of planar disulphides is found in the thiothiophthene system (34).68 This compound is prepared by the reaction of a diacylacetone e.g.diacetylacetone with phosphorus pentasulphide. The structure of (34) was established by X-ray cry~tallography~~ and showed the two S-S distances are nearly equal with the S-S bond lengths being about 2.36 A. However in the phenyl-substituted thiathiophthens (35 and 36) the S-S bond lengths are no longer equidistant e.g. in the compound (35)70 values of 2-297 and 2.355 8 were found and in (36)'l the distances were 2.233 and 2.433 A. It is possible s-s-s Ph (3 5) s-s- S Ph Ph O 5 A. Hordvik and E. Sletten Acta Chem. Scand. 1966 20 2043. A. Hordvik and J. Sletten Acta Chem. Scand. 1966 20 1907. W. G. Salmond Quart. Rev. 1968 22 253.F. Arndt P. Nacktwey and J. Push Ber. 1925 58 1638. S. Bezzi M. Mammi and C. Carbuglio Nature 1958,182 247. 70 A. Hordvik Acta Chem. Scand. 1968 22 2397. 71 P. L. Johnson and I. C. Paul Chem. Comm. 1969 1Q14. 220 Rahman Safe and Taylor that the latter result is due to the non-planarity of the phenyl substituents with the thiathiophthen nucleus. The kind of resonance hybrid found in the substituted thiathiophthens also occurs in the system containing four thioketone groups (e.g. 37a 37b).72 From the foregoing discussion concerning five-membered ring disulphides it will be gathered that only small deviations from planarity occur and that the resulting strain accounts for some of the interesting chemistry of these compounds. Turning now to six-membered ring disulphides it is apparent from an examination of models that the S-S dihedral angle is about 60" and hence the ring system can exist in two enantiomorphic forms.The existence of such enantiomers was first shown by Calvin; interconversion between the optical antipodes of [4,4,5,5-2H,]-1,2-dithian (38) and 3,3,6,6-tetramethyl-l,Zdithian (39) was studied by the n.m.r. technique. The n m r . spectra of both (38) and (39) D D Me S - S Me were temperature dependent (Figure 1). At room temperature the methylene protons of (38) appear as a singlet; on cooling the signal broadens and at temperatures below - 50" the signal appears as an AB quartet. The AB pattern is typical of the coupling of magnetically non-equivalent methylene protons. This data was plausibly interpreted to support the notion that these molecules exist as enantiomeric conformers with their interconversion at - 65" being slow.The interconversion rate constant ( k ) can be calculated from the n.m.r. data73 since k = .rrd VAB Jz and substitution of this value into Eyring's equation gives a free energy of inversion of 11.6 kcal/mole. For the dimethyl compound (39) the methyl resonances appear as a singlet at room temperature but at lower temperatures the chair-chair interconversion is slow and two singlets for 72 E. Klinsberg J. Heterocyclic Chem. 1966 3 243. 73 H. S. Gutowsky and C. H. Holm J. Cliem. Pliys. 1956 25 1228. 221 The Stereochemistry of Polysulphides 46 (PPm) H- Figure 1 the axial and equatorial methyl groups are observed. The dGf. for conformer interconversion for (39) is 13.8 kcal/mole. This n.m.r.procedure has been extensively used (notably by Liittringhaus and his c011eagues~~) in studies of a large number of cyclic disulphides (and polysulphides) in order to determine the effect of structure on the conformational inversion process. The dGt values obtained for the bicyclic compounds (40) and (41) were 14.4 and 12-5 kcal/mole respectively whereas the monocyclic D D (42) (43) (44) 74 (a) A. Luttringhaus S . Kabuss W. Maier and H. Friebolin Z. Naturforsch. 1961 16b 761 ; (b) S. Kabuss Dissertation University of Freiburg 1962; (c) H. Freibolin Dissertation University of Freiburg 1963; ( d ) U. Hess Dissertation University of Freiburg 1965. 222 Rahman Safe and TayIor disulphides (42) (43) and (44) had dGt values of 12.3 12-6 and 12.5 kcal/mole respectively. The effect of substituents on the free energy of conformer inversion is illustrated by the n.m.r.studies of cis- and trans-3,6-dimethy1-1,2-dithian~.~~d At 25" the cis isomer (43 with an axial and equatorial methyl group gives a H H sharp doublet at r 7-5 for the methylene protons indicating rapid inversion On the other hand the trans isomer (46) with two equatorial methyl substituents. H Me Me exhibits a complex pattern for the methylene signals and thus appears to be fixed into a single conformation. This would be expected since inversion would give the energetically unfavourable diaxial conformer. B~shweller~~ has examined tetramethyl-s-tetrathian (47) whose n.m.r. spectrum showed three methyl signals at 0"; two smaller peaks of equal area occurred at r 8-47 and 7-97 and a larger peak at r 8.32; only one signal was observed at higher temperatures at T 8.37.The data was rationalised by assigning the two smaller peaks to the axial and equatorial methyl groups of the rigid chair conformers (47a and 47c) and the larger resonance to the more flexible twist-boat conformer (47b). The substitution of four sulphur atoms for carbon in the molecule thus allows the relatively strain-free twist-boat conformer to predominate over the usually more stable chair form. X-Ray crystallographic studies of the structure of cyclic disulphides have provided data in agreement with that obtained by spectroscopic investigations of the same compounds in solution. X-Ray crystallographic analysis'6 of C. H. Bushweller J . Amer. Chem. SOC. 1967 89 5978; ibid. 91 6019. 76 A. Fredga Acta Chem. Srand.1958 12 891. 223 The Stereochemistry of Polysulphides Me Me Me Me S Me etramethyl-s-tetrathian (47) shows that the compound exists in the boat form which is energetically similar to the twist-boat conformer which predominates in solution. The structure of 1,2-dithian-3,6-dicarboxylic acid was determined H COOMe H by Foss and co-workers;77 the S-S bond distance was 2-07 A and the dihedral angle was 60". The molecules were arranged in infinite chains held together by hydrogen bonding between the carboxyl groups. The chains consist of alternating right- and left-handed conformers and thus the data reflect in a static sense the dynamic equilibrium in solution. To summarise; in most of the six-membered cyclic disulphides discussed so far the low energy barrier to rotation (- 10 kcal/mole) does not allow the separation of optical antipodes although their presence may be inferred from n.m.r.data. If however there is an additional asymmetric centre in the molecule it is possible that unequal populations of enantiomers exist. Thus Clae~on'~ showed that at -65" the axial and equatorial carboxymethyl conformers of methyl (R)-( - )-1,2-dithian-4-carboxylate (48) were present in 77 0. FOSS K. Johnsen and T. Reistad Acta Chem. Scand. 1964 18 2345. 78 G. Claeson personal communication ; Laboratory of Chemical Biodynamics Quarterly Report Lawrence Radiation Laboratory (LCBQ-16 1967 5). Rahman Safe and Taylor the ratio of 1 3. The more stable equatorial form predominates at low temperatures; at higher temperatures the two conformers approach equimolar concentration.This is also reflected in the optical rotatory dispersion properties of (49) which vary markedly with temperature. Its c.d. dispersion curve has a Cotton effect at 286 nm coincident with the U.V. absorption maximum associated with the disulphide When the c.d. of a series of disulphides is examined [open chain (Amax 250 nm; 0 = dihedral angle = go") 1,Zdithians (Amax 255 nm; 8 = 60°) 1,Zdithiolans (Amax 330 nm; 8 = 27") gliotoxin (52; Amax 340 nm; 0 = 14°)82a] a relationship between 8 and the lowest energy transition in the U.V. is observed. C.d. data on three cyclic disulphides [(95',10S)-trans-2,3- dithiadecalin (49) (4R,5R)-4,5-isopropylidenedioxy-1,2-dithian (50) and the corresponding diol (51)] have been reported.so Using the Cahn Ingold PrelogS1 H rules compound (49) has left-handed helicity whilst the other two compounds are right-handed.The c.d. and U.V. absorption of the lowest energy disulphide absorption band occurs at 285-290 nm for all but the dithiadecalin (49) has a negative c.d. band whereas the two dithians (50 51) have positive c.d. bands. It was concluded that 'in simple 1,Zdithian ring systems a positive c.d. band corresponding to the lowest frequency U.V. absorption band of the disulphide 79 G. Bergson G. Claeson and L. Schotte Acta Clzem. Scand. 1962,16 1159. M. Carmack and L. A. Neubert J. Amer. Chem. SOC. 1967 $9 7134. R. S. Cahn C. Ingold and V. Prelog Angew. Cliem. Internat. Edn. 1966 5 385. 82 (a) A. F. Beecham J. Fridrichsons and A. McL. Mathieson Tetrahedron Letters 1966 3131 ; (b) J. Fridrichsons and A. McL. Mathieson Acta Cryst.1967 23 429. 225 The Stereochemistry of Polysulphides group (in the case of 1,Zdithians in the range 280-290 nm) is associated with a right-handed (P) screw sense of the helix containing the atoms C-S-S-C and a negaiive c.d. band is associated with a left-handed (M) screw sense of the helix’. The structures of the naturally-occurring epidithiadioxopiperazines have been determined by X-ray crystallography. The absolute stereochemistry of gliotoxin (52),s2b sporidesmin (53 x = 2),s3 and acetylaranotin (54)84 is the same at the QTACO OH I ‘s I O C y N M e CH* OH Me0 co OMe Me asymmetric centres of the dioxopiperazine rings. The conformation of the piperazine ring in gliotoxin (52) and sporidesmin (53 x = 2) is the boat form due to the constraint imposed by the sulphur bridge; in acetylaranotin the skewed-boat conformation is preferred as a result of the additional strain imposed by the fused pyrrolidine rings.The circular dichroic dispersion of several of these compoundss6 shows a negative band at about 340 nm thus indicating a 83 J. Fridrichsons and A. McL. Mathieson Acta Cryst. 1965 18 1043. s4 D. B. Cosulich N. R. Nelson and J. H. van den Hende J. Amer. Chem. SOC. 1968 90 5519. s5 H. Herrmann R. Hodges and A. Taylor J. Chem. SOC. 1964 4315; A. F. Beecham and A. McL. Mathieson Tetrahedron Letters 1966 3139; R. Nagarajan N. Neuss and M. M. Marsh J . Ainer. Chem. Soc.. 1948,90 6518. 226 Rahman Safe und Taylor left-handed helicity of the disulphide-bridged system. Thus the c.d. results and the X-ray data are in complete accord. N.m.r.studies of the conformations of seven-membered rings containing a vicinal disulphide group have been reported.86 The n.m.r. spectrum of the disulphide (55) at 25" shows a singlet for the methylene protons at positions 3 and 7. At - 110" the signal appears as an AB quartet indicating the magnetic non-equivalence of the methylene hydrogens. In the deuteriated compound (56) Me S S I Me Me Me Me D D at - 110" the 5-gem-dimethyl group and the 4- and 6-methylene protons appear as 2 singlets and a quartet respectively. However an additional signal is also present at low temperatures and this has been attributed to a twist-boat form of the seven-membered ring. ThedGt. values for (55) and (56) are 11 and about 9 kcal/mole respectively which are significantly higher than those for the corresponding cycloheptane analogues.In cycloheptene vicinal disulphides three main conformations are present the chair form (57) the boat form (58) and the twist-boat form (59). The chair (57) S (59) conformer is rigid whereas the boat and twist-boat forms are interconvertible. The n.m.r. spectrum of (60 R = H) below 0" shows a quartet for the 86 S. Kabuss A. Luttringhaus H. Friebolin and R. Mecke 2. Naturforsch. 1966. 21b 320; H. Friebolin and S. Kabuss in 'Nuclear Magnetic Resonance in Chemistry' ed. Biagio Pesce Academic Press New York 1965 p. 125; K. von Bredow Dissertation University of Freiburg 1968. 227 The Stereochemistry of Polysiilphides 3-methylene group as well as a singlet in the ratio 7 3. The former signal corresponds to the axial-equatorial protons of the chair conformer whereas the singlet is attributed to the more process may proceed as shown 74 vi chair + boat The dGt for inversion for (60) flexible conformers.The overall equilibrium twist-boat boat + chair vi twist-boat was 13.5 kcal/mole whereas the dGt for pseudorotation (boat $ twist-boat) was 10.4 kcal/mole. When the 3-methylene protons are replaced with a gem-dirnethyl group (60 R = Me) a dG7 for inversion of 12.1 kcal/mole is observed. This value is about 1.4 kcal/mole less than (60 R = H) but about 1 kcal/mole greater than the comparable effect of methyl-group substitution in the corresponding carbocyclic analogues. The preparation and properties of 4,5,6,7-dibenzo-1,2-dithiacyclo-octadiene (61) have been de~cribed.~' Its n.m.r. spectrum showed an AB quartet at 7 6.13 (JAB = 14 Hz ~ V A B = 60 Hz) at 25" and even at 130".The estimated dGt of inversion (28.8 kcal/mole) was of sufficient magnitude to allow the first \ Hz c s-s a7A. Luttringhaus U. Hess and H. J. Rosenbaum 2. Naturforsch. 1967 22b 1296; A. Luttringhaus and H. J. Rosenbaum Monatsh. 1967,98 1323; H. J. Rosenbaum Dissertation University of Freiburg 1968. 228 Rahman Safe und Taylor separation of the optical isomers of a vicinal disulphide. The right-handed (P) form had a positive c.d. band at 244 nm whilst the left-handed ( M ) form had a negative band in accord with the conclusions of Carmack and Neubert (see above) . Several nine-membered rings containing vicinal disulphides have been synthesised by EckhardLS8 At low temperatures the ether (62 R = H X = 0) R R showed signals attributable to both the chair and boat forms.At -50" an AB quartet at r 6-07 (JAB = 14 Hz ~ V A B = 14 Hz) and a weaker AB quartet at T 5.91 (JAB = 14 Hz ~ V A B = 50 Hz) were observed for the chair and boat conformers respectively. Similar results were obtained for other members of the series (62 R = H X = CH2 S SO2). The substituted compound (62 R = Me X = SO,) gave a considerably higher d G f value (19.7 kcal/mole) probably because the methyl groups cause considerable interaction in the chair form. Thus the AB quartet observed at 35" is probably due to the boat conformer. (i) Macrocyclic disulphides. A large number of macrocyclic disulphide natural products are known. Those whose structures have been determined include oxytocin (269 vasopressin (20) insulin (20 85) ribonuclease (26 187) hen's egg-white lysozyme (53 368) papain and chymotrypsinogen A.89 (Figures in parenthesis indicate the smallest and largest numbers of atoms in the disulphide rings of the molecules.) The physiological properties of these molecules depend on the stereochemistry of the disulphide bonds since the topography of the peptide is determined by their presence. There are four disulphide bridges in hen's egg white lysozyme and their chirality has been determined. It is of in- terest that whilst all the asymmetric carbon centres have the same (L) configu- ration the asymmetric disulphides are present in both stereoisomeric forms ; the disulphides linking amino-acid residues 6 k 8 0 and 76-+94 are the mirror images of the bridges linking 6-+127 and 30+115. The disulphide bridge be- tween residues 6 and 127 is not required for biological activity; however when 88 P.Eckardt Dissertation University of Freiburg 1968. D. G. Smytli Ann. Reports 1964 61 507; 1965 62 488. 229 The Stereochemistry of Polysulphides that linking residues 30 and 115 is also broken the molecule is no longer biologically a ~ t i v e . ~ ~ ~ ~ ~ The first four polypeptides mentioned above have been synthesised. The formation of the disulphide groups from the corresponding dithiols is usually the last step of the synthesis but yields at this step have been very low. The ease of cyclisation may be dependent on the conformation of the ring e.g. the cyclisation of the linear peptide (63) to give a 22-membered ring proceeds in better yield than the cyclic oxidation of reduced oxytocin where a 20-membered ring is formed.92 OH NHZ CH;! Me I I I / \ HS co I HS CONHz NH \ I / CHz - CHCONHCHCONHCHCHEt CH2 CHNHCOCHZ CHZ CHNHCOCH(CH2)2 CONHZ I CO-Pro-Leu-Gly NHz B.Trisu1phides.-Conformational equilibria in six- and seven-membered rings containing the 1,2,3-trithia-group have been studied by n.m.r. spectro- sc0py.*~9 939 O4 TheLfGt values for the six-membered trithians (64 R = H Me) C. C. F. Blake G. A. Mair A. C. T. North D. C. Philips and V. R. Sarma Proc. Roy. SOC. 1967 B 167 312. 91 P. Jollies Proc. Roy. SOC. 1967 B 167 350. 92 C. Ressler and V. du Vigneaud J. Amer. Chem. Soc. 1957 79 451 1. O3 S. Kabuss A. Luttringhaus H. Friebolin H. G. Schmid and R. Mecke Tetrahedron Letters 1966 719. 94 B. Milligan and J. M. Swan J. Chem. SOC. 1965 2901. 230 Rahman Safe and Taylor were 13.2 and 14.7 kcal/mole respectively which is 1.6 and 1.1 kcal/mole greater than the value for the corresponding dithians.These values reflect the greater rigidity imposed on the ring when a sulphur atom is substituted for a methylene group. The seven-membered ring trisulphides (65 66 67 R = H) give dGt values R (6.7 8.9 and 17.4 kcal/mole respectively) for inversion that are also higher than the values obtained for the corresponding carbocyclic compounds. The n.m.r. spectrum of the trithia-compound (67 R = H) is complex (Figure 2). At 153" a singlet is observed for the protons of the methylene groups but at 25" the signal assigned to the methylene hydrogens appears as an AB quartet at 7 5.70 +153"C 1 I I I I I I I I Figure 2 23 1 The Stereochemistry of Polysulphides (chair conformer) and a singlet at T 6.0 in the ratio 17 3.At -60" the peak at 7 6.0 is broadened and is assigned to the flexible twist-boat or boat conformer probably representing an average value. von Bredowsa has prepared several substituted 1,2,3-trithia-5,6-benzocycloheytenes (67 R = OMe Me Ph) whose n.m.r. spectra indicate both the chair and boat forms. ThedGf values are high (dGt 19.8 20.0 and 21.2 kcal/mole for 67 R = OMe Me and Ph respectively). For the dimethyl derivative (67 R = Me) the resonances due to the methyl groups and aromatic protons appear as two singlets whose ratio is proportional to the conformer population. Sporidesmin E (53 x = 3)95 is a toxic metabolic product of Pithomyces chartarum. It can be converted into sporidesmin (53 x = 2) and it is probably a 3,6-epitrithia-2,5-dioxopiperazine.Its n.m.r. spectrum is complex (Figure 3a) but it is not a mixture of isomers because the relative intensity of the pealcs in 2 4 6 8 10 T Figure 3 I ' ~ ' ~ I 6.5 6 -7 6-9 2- Figure 4 96 R. Rahman S . Safe and A. Taylor J . Chern. Soc. ( C ) 1969 1665. 232 Rahman Safe and Taylor the spectrum depends on the solvent used (Figure 4) and also on temperature (Figure 3b). A model of an 3,6-epitrithiadioxopiperazine reveals that the unbranched sulphur bridge can exist in two conformations depending on the position of the central sulphur atom. In one case this is towards the centre of the open-book-like molecule and in the other at the edge of the right-hand page. From the intensity of the N-methyl peaks in the n.m.r. spectrum the ratio of the two principal conformers was calculated to be 2 3 in chloroform but 1 3 in methanol.This change in conformer population is also reflected in the c.d. in which the asymmetric transition at 306 nm in chloroform is shifted to 312 nm in methanol. Thiadehydrogliotoxin (69 x = 3) has been prepared21 and has similar properties. C. Tetrasu1phides.-Few cyclic tetrasulphides are known. The perfluoro- compound (68)96 has a complex lSF n.m.r. spectrum which showed an AB quartet each line of which was further split into five peaks. Thus several conformers probably exist in solution. Sporidesmin G (53 x = 4) and dithiadehydrogliotoxin (69 x = 4) have been prepared from sporidesmin and F F FQ F CH2 OH dehydrogliotoxin respectively.21 Their n.m.r. spectra suggest that only one conformation is present.D. Penta- and Other Poly-sulphides.-The compounds S,CH and S,CH are known but little information on their chemistry is available. Recently compounds having two tetrasulphide bridges linking the 1,l'- and 4,4'-positions of two benzene rings have been des~ribed.~ Such compounds e.g. (70) are thought to be stabilised by charge-transfer interactions between alkoxy substituents ortho to the polysulphide chains. 4 Inorganic Cyclic Polysulphides A number of cyclic compounds of sulphur are known in which none of the atoms in the ring are carbon. Some of these compounds have been known for many O6 C. G. Krespan and W. R. Brasen J. Org. Chem. 1962,27 3995. O7 Z. S. Ariyan and R. L. Martin Chem. Comm. 1969 847. 233 The Stereochemistry of Polysulphides S S /s-s \ S /s s-s \ years (e.g.71)98 but their structural recognition has been the result of improved techniques of X-ray crystallography. A few five-membered rings have been proposed. These compounds are the heterocyclic equivalents of the still unknown cyclopentasulphur. The product obtained@@ from the reaction of dichloro-bis-cyclopentadienylmolybdenum with ammonium pentasulphide was assigned the structure (72) on the basis of analysis and the equivalence of the cyclopentadienyl protons in the n.m.r. spectrum. The dianil of diacetyl and cystinamine forms a tetraco-ordinated complex with Ni2+ and Egen and Krause1O0 pointed out that the geometry of the sulphur ligands was such that they might be expected to react with disulphur dichloride to give a five-membered 2,3,4,5tetrathianicke1(11) ring. One of the products isolated from such a reaction had the composition CgH14C12N2Ni2+$q @* K.A. Hofmann and F. Hochtlen Ber. 1903,36 3090. H. Kopf Angew. Chem. Internat. Edn. 1969 8 962. loo N. B. Egen and R. A. Krause J . Inorg. Nucleur Chern. 1969 31 127 234 Rahmun Safe and Taylor expected for such a five-membered ring system (73). A number of 2,3,5-trithia- diboralanes (74 R = Br I Ph) have been preparedlof whose llB n.m.r. spectra suggest that the boron atoms are identical and that the ring is planar. A number of six-membered ring systems have now been synthesised. Two groups of workers1O2* lo3 reported the preparation of a dicyclopentadienyl- titanium(iv) pentasulphide (75 X = S) having a six-membered ring of five R (75) X = S Se (74) sulphur atoms and a titanium atom. The n.m.r. spectrum of this compound at 30" shows two sharp singlets at T 3.58 and T 3.90 for the cyclopentadienyl ligands of the chair conformer.At higher temperatures the rate of conformational inversion increases and at 120" a single peak at T 3.74 was observed. These results are very similar to those reported above for the organic cyclic poly- sulphides. An X-ray crystallographic analysis of the platinum complex lol M. Schmidt and W. Siebert Angew. Chem. Internat. Edn. 1964,3 637; 2. anorg. Chem. 1966,345 87; Chem. Ber. 1969,102,2752. l02 H. Kopf B. Block and M. Schmidt Chem. Ber. 1968,101,272. Io3 R. Ralea C. Ungurenasu and S. Cihodaru Rev. Roumaine Chim. 1967 12 861. 235 ?-he Stereochemistry of Polysulphidcs (NH4)2PtS15,2H20 (71)lo4 has been reported. The anion is an octahedrally co-ordinated complex having S-S-S bond angles of average value 104" 48' and S-Pt-S bond angles of 92" 48'.Thus three chains of sulphur atoms are co-ordinated to the platinum making three six-membered rings each in a chair conformation. Several polythiaimides have been described prepared by the action of ammonia on a polysulphur dich10ride.l~~ The structures of heptathiainiide (76) and hexathiadi-imide (77) were established by X-ray crystallography.loG Their structures are analogous to cyclo-octasulphur sulphur atoms being replaced by NH groups. The di-imide studied by Weiss was the symmetrical isomer but the other isomeric di-imides,lo7 some of the tri-imideslo8 and the alternating tetraimide are all known.lo9 In most cases the S-S-S bond angles are about 107" and the S-NH-S angles about 120".The bicyclic polythianitride (78) has been synthesised1l0 by reaction of hexathia-l,3-di-imide with pentasulphur dichloride. (78) lo* P. E. Jones and L. Kaiz Chem. Comm. 1967 842. lo5 J. Weiss Angew. Chem. 1959 71 246. lo6 J. Weiss Z. anorg. Chem. 1960 305 190. lo' H. G. Heal Nature 1963 199 371 ; P. Taw H.-J. Schulze-Steinen and J. E. Colchester J. Chem. SOC. 1963 2555. H. Garcia-Fernandez and H. Heal Compt. rend. 1968,266 B 1449; H. Heal and J. Kane Nature 1964 203 97 I . lo9 M. Goehring Quart. Rev. 1956 10 437. 110 €I. G. Heal M. S. Sahid. and H . Garcia-Fernandez Chcm. Comm. 1969 1063. 236 Rahman Safe and Taylor 5 Conclusions It is clear that only human ingenuity prevents the development of a chemistry of sulphur quite as subtle and complex as the better known chemistry of carbon. We might therefore extrapolate the current dogmas of astronomy and bio- chemistry and suggest the thaumaturgical presence perhaps on a remote heavenly body of a self-replicating system based on sulphur! 237

ISSN:0009-2681    

DOI:10.1039/QR9702400208

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

The Study of Simple Liquids By Computer Simulation I. R. McDonald PHYSICAL LABORATORY UNIVERSITY OF DUBLIN TRINITY COLLEGE DUBLIN K. Singer (UNIVERSITY OF LONDON) ENGLEFIELD GREEN SURREY DEPARTMENT OF CHEMISTRY ROYAL HOLLOWAY COLLEGE 1 Introduction The derivation of the physical properties of macroscopic systems from the intermolecular potentials is a major but largely unattained objective of molecular theory. For dilute gases and certain crystalline solids the problem is greatly simplified by the fact that these systems may be satisfactorily represented by models consisting of entities particles or oscillators which are independent of each other. No such simplification is possible in the case of compressed gases liquids and solutions and the mathematical difficulties of the many-body problem are obstacles to progress.It is possible to by-pass these difficulties to some extent by making use of powerful computer simulation techniques which have been developed in recent years. Such methods have proved particularly valuable for the study of simple liquids i.e. liquids composed of molecules obeying the laws of classical mechanics and interacting through potentials which are spherically symmetric and pair-wise additive. The substances which most closely resemble this ideal are the heavier rare gases (A Kr Xe) and liquids composed either of certain diatomic molecules (e.g. N2 02 CO) or of poly- atomic molecules which are approximately spherical (e.g. CH,). These systems display the characteristic features of the liquid state without the problems arising from more complicated modes of interaction.Argon is the example most frequently quoted because a wealth of accurate experimental data is available. This Review is concerned with the application to the study of simple liquids of the techniques of computer simulation known as the Monte Carlo method and the method of molecular dynamics. In each case the properties studied are those of a model system representing a fluid at fixed temperature and density and containing typically tens or hundreds of particles. In the Monte Carlo method devised originally by Metropolis et all and later developed by Wood and his ~011aborator~,~-* a set of molecular configurations is generated in the Metropolis Rosenbluth Rosenbluth Teller and Teller J. Chem. Phys. 1953 21 1087. W. W. Wood and F. R. Parker J . Chem. Phys.1957,27,720. W. W. Wood F. R. Parker and J. D. Jacobson Nuovo Cimento suppl. X 1958,9 133. W. W. Wood ‘Physics of Simple Liquids’ ed. H. N. V. Temperley J. S. Rowlinson and G. S. Rushbrooke North-Holland Publishing Co.. Amsterdam 1968. 238 McDonald and Singer computer by random displacements of the particles of the model; the configura- tions are accepted or rejected according to a criterion which ensures that a given configuration occurs with a probability proportional to the Boltzmann factor exp (- p@) for that configuration. (@ is the total potential energy of the configuration and fi = l / k ~ T . ) The equilibrium value of a physical property X may then be found by taking an average value {X) over the whole set of configurations. In the method of molecular dynamics developed by Alder and Wainwright6* the equations of motion of the particles of the model are solved by step-wise numerical integration.The equilibrium properties of the system are calculated from averages taken over a sufficiently long time interval. Time- dependent phenomena may also be studied. The Monte Carlo and molecular dynamics methods therefore provide respectively solutions of the many-body problem in classical statistical mechanics and classical kinetic theory. Their success is due to the fact that at least for homogeneous phases the average properties per particle of a relatively small model are generally very close to those of the macroscopic system which the model is chosen to represent. A complete Monte Carlo or molecular dynamics calculation may be regarded as a computer ‘experiment’ which yields accurate information on the consequences of a given intermolecular force law.The results of such an ‘experiment’ can be used to test the adequacy of pair potentials proposed for some real system or conversely to determine the ‘best’ pair potential for a real system by comparing measured and computed thermodynamic and transport properties. Statistical mechanical theories may be tested unambiguously by comparing results derived by means of a theory from a given pair potential with those obtained for the same potential in a Monte Carlo or molecular dynamics calculation. It is also possible to obtain data which are experimentally inaccessible or nearly inaccessible. For example the motion of molecules in a simple liquid can be studied in great detail in a molecular dynamics ‘experiment’; equilibrium and time-dependen t pair distribution functions can be calculated without the considerable errors involved in determining these functions by means of X-ray and neutron diffraction experiments; and for systems of two components A and B say the total potential energy and the total radial distribution function can be resolved into contributions from A-A B-B and A-B pairs.At the time of the last review of this subject in these pages’ the Monte Carlo and molecular dynamics methods had been applied to the study of systems of hard elastic spheres and a promising beginning had been made with Monte Carlo calculations for a Lennard-Jones 12-6 potential. This early work has also been reviewed elsewhere.** * Assemblies of hard spheres and other highly idealised systems are of theoretical interest and have continued to receive attention.This review however will be mainly concerned with ‘realistic’ pair B. J. Alder and T. W. Wainwright J. Chem. Phys. 1959,31,459. M . A. D. Fluendy and E. B. Smith Quart Rev. 1962,16 241. I. 2. Fisher Soviet Phys. Uspekhi 1960,2 783. 13 B. J. Alder and T. W. Wainwright ‘The Many-Body Problem’ ed. J. K. Percus Interscience New York 1963. 239 The Study of Simple Liquids By Computer Simulation potentials such as the 12-6 and exp-6 functions. (These potentials are described in section 3.) Thermodynamic properties including phase equilibria and the properties of mixtures structural properties and time-dependent phenomena are discussed in separate sections. 2 Computational Details The same basic computer model is used both in Monte Carlo calculations and in molecular dynamics.A system of N particles is confined within a cell usually a cube of volume V and the co-ordinates which define the position of each particle within the cell are stored in the computer. The number of particles in the cell is generally less than a thousand and in order to simulate as closely as possible the behaviour of a macroscopic system a periodic boundary condition is used the fundamental cell is surrounded by replicas of itself; each replica contains N particles which occupy the same relative positions as those in the fundamental cell. It is also advantageous to choose Nand the shape of the cell in such a way that the periodic boundary condition generates a perfect lattice appropriate to the system under study when the particles in the fundamental cell are arranged in a suitably ordered manner.Argon for example crystallises in a face-centred cubic lattice and for this system it is therefore convenient to use a cubic cell and choose N = 4n3 where n = 1 2 3 4 5 6 . . . i.e. N = 4 32 108 256 500 864. . . . The particles are assumed to interact through a pair potential 4 ( r ) ; the corresponding pair virial function # ( Y ) for potentials without discontinuities is given by The total potential energy @ and total virial of the model are calculated as the sums of pair terms. Interactions between the particles in the fundamental cell and particles in adjacent cells are included in the sums. The contributions from particles separated by a distance greater than some chosen cut-off value are not calculated explicitly but usually by integration over a uniform particle density.In the Monte Carlo method a particle of the system is chosen either serially or at random and given a random displacement. Let the increase in total potential energy of the system be A@. If A@ is negative the move is accepted and the new configuration replaces the old one. If d@ is positive the move is accepted only with the probability exp (- FA@). Repetition of this procedure generates a chain of configurations which are distributed in phase space with a probability density proportional to the Boltzmann factor exp (- /I@). (If a move is rejected the previous configuration is counted again.) The overall chain average of any function of the particle co-ordinates (e.g. @ ‘I?) therefore converges to the canonical ensemble average of the same quantity as the chain length increases.The molar configurational internal energy U(V T) where V is the molar volume is proportional to the average potential energy <@) of the computer model and the pressure P (V T ) is calculated from the average virial {Y} by means of the virial theorem. Thus 240 Mc Donald and Singer P V = NokBT - (No/3N)(Y) (3) where No is the Avogadro number. The radial distribution function and other equilibrium properties may also be determined. In the method of molecular dynamics the particles are given initial velocities and the subsequent motion of the system is studied by numerical integration of Newton's equations. Equilibrium properties are calculated as time-averages and information is also obtained on time-dependent phenomena and transport coefficients.The total energy of the system remains constant apart from small fluctuations caused by the use of a finite time interval in the numerical integrations. The temperature associated with a particular 'experiment' is calculated from the mean kinetic energy. The particle velocities may be changed in the early stages of a calculation if it is found that the temperature has drifted far from the region of interest. As it is desirable to minimise the fluctuations in temperature the model of a macroscopic system used in a molecular dynamics calculation generally contains a larger number of particles than that used in a Monte Carlo study of the same system. Apart from the choice of initial co-ordinates and velocities the molecular dynamics calculation contains no probabilistic elements.The great advantage of the method of molecular dynamics is that it allows the study of transport processes. This is not possible with the Monte Carlo method but the latter possesses features which are of value in certain applications. For example the method can be extended to the calculation of average values in other types of statistical mechanical ensemble. The procedure outlined above is one appropriate to the usual Gibbs canonical or NVT-ensemble but calculations in the isothermal-isobaric or NPT-ensemble have also been r e p ~ r t e d . ~ ~ a-11 The fact that the temperature is a fixed parameter in a Monte Carlo calculation makes this method particularly suitable for the study of iso- thermal processes. The efficiencies of the two methods measured by the total computing time required to give averages of equal statistical reliability 'are much the same.A typical Monte Carlo calculation for a system such as liquid argon requires the generation of ca. 300,000 configurations of a model containing 108 particles. An equivalent calculation by molecular dynamics would require the integration of the equations of motion of 864 particles for ca. 1500 time intervals of ca. 10-14 sec. In either case the cut-off distance in the energy summations would be ca. 9 A. The machine requirements are severe. The data given in Table 1 provide some indication of the attainable speeds on a number of computers. These figures can be regarded as only a rough guide because they take no account of different programming practices.W. W. Wood J. Chem. Phys. 1968,48,415. lo I. R. McDonald Chem. Phys. Letters 1969 3 241. l1 I. R. McDonald 'Proceedings of Culham Conference on Computational Physics' UKAEA Culham Laboratory and IPPS 1969 Vol. 2 July paper 38. 24 1 The Study of Simple Liquids By Computer Simulation Table 1 Machine time used on various computers in simulation studies of the 12-6 potential at liquid densities Column (i) number of time steps per hour in molecular dynamics calculation. Column (ii) number of Monte Carlo configurations generated per hour. Computer N (0 (ii) IBM 7M2 32 19,Ooo IBM 7042 108 6500 IBM 7M60 250 90 CDC 36oO6O 864 75 CDC 660013 864 1500 UNIVAC 1 10713 864 150 UNIVAC 1108a 864 400,000 ICT ATLAS’ 108 260,000 ICT ATLASb 256 180,Ooo a personal communication from Dr.Levesque; b unpublished work. 3 Thermodynamic Properties and Intermolecular Forces A. One-component Systems.-Computer ‘experiments’ on systems of hard spheres have confirmed the intuitively plausible proposition that for such systems there exists only a solid and a gas-like phase.4 In the absence of cohesive forces there is no liquid phase and a pair potential which allows for both cohesion and repulsion must be used in studies of the liquid state. In an important paper published in 1957 Wood and Parker2 described the results of a series of calculations by the Monte Carlo method of thermodynamic properties of argon along a supercritical isotherm. They used the 12-6 potential 4 (r) = 4 E [((T/r)l2 - (o/r)6] (4) where r is the intermolecular separation E is the depth of the potential well at the minimum in# (r) and (T (the ‘collision diameter’) is the value of r for which 4 (r) is zero.The values chosen for the interaction parameters E and a were those deduced by Michels Wijker and Wijkerl2 from measurements of the second virial coefficients at high temperatures viz. E/kB = 119.8 K and a = 3-405 A. This work represented the first successful application of computer simulation to the study of a real fluid. Extensive investigations of the thermodynamic properties of the 12-6 fluid have recently been carried out both by molecular dynamics13 and by the Monte Carlo r n e t h ~ d . ~ * - ~ ~ There is good agreement between the two sets of l3 A. Michels H. Wijker and H. Wijker Physica 1949 15 627. l8 L. Verlet Phys. Rev. 1967 159 98. lti I. R. McDonald and K. Singer Discuss.Faraday SOC. 1967 43 40. l6 I. R. McDonald and K. Singer J. Chem. Phys. 1967 47 4766. l7 I. R. McDonald and K. Singer J. Chem. Phys. 1969,50 2308. L. Verlet and D. Levesque Physica 1967 36 254. 242 McDonald and Singer results. Attention has been mostly centred on the calculation of pressure and internal energy as a function of density and temperature. Data on other thermodynamic properties including specific heat compressibility and thermal pressure coefficient have also been reported. The values of these latter quantities are determined by the magnitude of fluctuations in the potential energy and virial and it is sometimes difficult to attain a high accuracy. No serious problems are encountered in the calculation of pressure and internal energy in the liquid range except in the neighbourhood of a phase change.If the computer model contains several hundred or inore particles there is a tendency for the system to separate into two phases in the region of the liquid-vapour transition. This leads to large fluctuations in the calculated properties. The isotherms display van der Waals loops and negative pressures are therefore obtained at sufficiently low temperatures and densities. In the melting region the isotherms generally have two distinct branches. One of these branches corresponds to the fluid state and the other corresponds to the solid. Near the critical point there is the additional complication that the small size of the model and the imposition of a periodic boundary condition suppress the large fluctuations in density which characterise the critical region in macroscopic systems.One effect of this is to increase the critical temperature of the 12-6 fluid in the computer model by ca. 7%18 and another is that the specific heat is underestimated.la The results obtained for the 12-6 potential with the parameters of Michels et a1.12 are found to agree closely with experimental properties of argonl9 throughout the range between the triple point (83.8K) and the critical temperature (150.7 K) and also at higher temperatures. Figure 1 for example shows the excellent agreement between the experimental equation of state and the molecular dynamics calculations of Verlet ,13 There are small systematic discrepancies between the experimental and calculated internal energies but these are removed if the depth of the potential well is reduced by only 2X.l' The same computer results could be used to assess the adequacy of the 12-6 model for other simple liquids (e.g.Kr Xe N2 02 CO CHo) by application of the law of corresponding states and it is rather surprising that calculations of this type have not so far been reported. Levesque and Vieillard-Baron20 have carried out a series of calculations for other potentials and conclude that the 12-6 function provides the best correlation between computed and measured properties of argon. The success of this potential requires some explanation because it is known that the 12-6 function fails to account satisfactorily for the experimental low-temperature second virial coefficients of argon.21 Furthermore the coefficient of the r-6 la D. Levesque and L. Verlet Phys.Rev. 1969 182 307. (a) Clark Din Robb Michels Wassenaar and Zwietering Physica 1951 17 876; (b) F. Din 'Thermodynamic Functions of Gases' vol. 2 Butterworths London 1956; (c) J. M. H. Levelt Physica 1960 26 361; (d) A. Van Itterbeek 0. Verbeke and K. Staes Physica 1963 29 742; (e) W. van Witzenburg and J. C. Stryland Canad. J . Phys. 1968 46 811; (f) R. K. Crawford and W. B. Daniels Phys. Rev. Letters 1968 21 367; (g) W. B. Streett and L. A. K. Staveley J. Chem. Phys. 1969 50,2302. eo D. Levesque and J. Vieillard-Baron Physica 1969 44 345. 21 Jones Rowlinson Saville and Weir Trans. Faraduy SOC. 1967 63 1320. 243 The Study of Simple Liquids By Computer Simulation 4 3 2 F- a 3 1 0 -1 1.428 1.478 - 0. I I I 0 0.5 1 .o 1.5 2.0 1 OO/T (K-') Figure 1 Compressibility factor of argon as a function of inverse temperature for several isochores.The curves are the results of molecular dynamics calculations (ref. 13) based on the 12-6 potential with the parameters of Michels et al. (ref. 12). The curves are labelled with the density in g cm-3. The dots are experimental data (refs. 19c d e) term (representing the London dispersion energy) in the empirical potential of Michels et aZ.lZ is larger than that predicted by quantum mechanical calculations by a factor of about two.2zs z3 Careful analysis of the experimental data shows clearlyzz that the true pair potential between argon atoms has a well which is narrower and deeper than that of the 12-6 potential and has a maximum depth of ca. 150 k~ K. The simplest proposed representation of the true pair interaction in argon is the Kihara core potential.This resembles the 22 J. A. Barker and A. Pompe Austral. J. Chem. 1968 21 1683. 23 J . S. Rowlinson Quart. Rev. 1954 8 168. 244 McDonald and Singer 12-6 potential except that the intermolecular separation is taken to be the distance between the surfaces of atomic hard cores. A third parameter is therefore introduced to describe the size of the atomic core. A number of more complicated multi-parameter potentials have also been put forward.22 Monte Carlo calculations based on the Kihara potential are found to lead to pressures and internal energies which are much lower than the experimental values for liquid arg0n.l'~ 24 It seems plausible to ascribe these differences to the neglect of non-additive i.e. many-body interactions. On the other hand the 12-6 potential may be used to calculate the properties of argon accurately over a range of temperature and density which includes the solid liquid and gaseous states.This suggests that the form of the many body interactions is such as to lead to an effective pair potential which is almost state-independent and approximately of the 12-6 type. It is possible however that the calculation of equilibrium thermodynamic properties does not provide a very sensitive test of a proposed potential function and that a number of different functions will give satisfactory results if suitable values are chosen for the parameters. For simple liquids at normal densities it is thought that the induced dipole-dipole-dipole interaction studied by Axilrod and Tellerz5 is the dominant and probably the only important many-body effect.This interaction has a similar physical origin to the London dispersion force between a pair of molecules. Table 2 shows the results of Monte Carlo calculation^^^ for liquid Table 2 Contribution of triple-dipole forces to thermodynamic properties of argon24 T(K) ~ ( g c m - ~ ) a b c d e a b c d e UIRT P VIRT 87.9 1.390 -7.96 -8.53 +0.44 -8.09 -7.96 0.00 -1.99 +1.33 -0.66 -0.04 105.5 1.271 -5.94 -6.32 $0.28 -6.04 -5.92 0.02 -1.31 $0.83 -0.48 0.00 123.1 1.134 -4.45 -4.69 +0*18 -4.51 -4.48 0.05 -0.85 +0.54 -0.31 0.02 140.6 0.934 -3.16 -3.31 $0.10 -3.21 -3.14 0.11 -0.49 +0*30 -0.19 0.08 a experimental valueslD; b based on Kihara potential of ref. 26; c contribution of triple- dipole forces; d sum of columns b and c; e based on 12-6 potential of ref.17. argon based on the Kihara potential with parameters derived from measured viscosities and second virial coefficients of the dilute gas.26 Also shown are the contributions of triple-dipole forces estimatedz4 from an analysis of small numbers (ca. 50 for each state) of Monte Carlo configurations. The sums of pair and three-body contributions are in very much better agreement with the experimental results than the values obtained from the Kihara potential alone but poorer than that obtained when the 12-6 function is used. The remaining discrepancies may result either from the neglect of other many-body forces or from inaccuracies in the assumed form of the true pair potential. At very 24 I. R. McDonald and L. V. Woodcock J . Phys. (C) in the press. 26 B. M. Axilrod and E. Teller J .Chern. Phys. 1943 11 299. 28 J. A. Barker W. Fock and F. Smith Phjw. Fluids 1964 7 897. 245 The Study of Simple Liquids By Computer Simulation high densities non-additivity of the repulsive part of the pair potential is likely to be important. Theoretical work suggestsz3 that the pair repulsion is more accurately described by an exponential function than by the r-12 term which appears in the 12-6 potential. An exp-6 potential which has an exponential repulsion term and an inverse sixth power attraction term has been used by Ross and Alder2’ to obtain results for argon by the Monte Carlo method for comparison with shock compression data. They conclude that for liquid argon compressed two-fold non-additive effects act in such a way as to increase the repulsive part of the effective potential between pairs of molecules by ca.30%. The results obtained from various studies of many-body interactions are by no means conclusive and much work remains to be carried out. Computer simulation may be expected to play an important r61e in such calculations. B. Liquid Mixtures.-There exists only a limited number of liquid mixtures in which the molecules may be expected to interact according to the 12-6 potential. These are mixtures of the rare gases and of substances obeying the law of corresponding states. Even in such cases there is considerable uncertainty about the values to be used for the parameters which characterise the interaction between unlike molecules. These factors together with a lack of reliable experimental data on some of the simple mixtures which do exist have hampered the development of quantitatively satisfactory theories of mixtures.In the case of a mixture of two 12-6 liquids A and B say with interaction parameters EAA OAA and EBB (TBB it is usual to assume that the potential between A-B pairs is described by a 12-6 function with parameters defined by the Lorentz-Berthelot rules EAB = (EAAEBB)~ ; (TAB = 9 (UAA + (TBB) (5) The Monte Carlo method may be used to calculate thermodynamic properties of mixtures of 12-6 liquids over an arbitrary range of potential energy parameters. Apart from providing basic information on the properties of such mixtures the results may be used as quasi-experimental data in testing theories of mixtures. Alternatively the comparison of the computer results with measured values of the properties of mixtures should prove to be a reliable method for the determination of EAB and (TAB in real systems.The changes of pressure and internal energy on mixing at constant volume may be determined from Monte Carlo calculations for the pure components and for the mixture. The method of molecular dynamics is less useful here because the temperature at which a molecular dynamics ‘experiment’ is performed can not be accurately predicted in advance. As the mixtures under consideration are very nearly ideal it is in any case difficult to obtain accurate values of the excess properties. It therefore seems advisable to avoid the additional error which is introduced by uncertainties in the value of the temperature at which mixing takes place. If sufficient Monte Carlo data are obtained the M.Ross and B. Alder J. Chem. Phys. 1967 46 4203. 246 McDonald and Singer changes in volume and enthalpy at constant pressure may be determined by extrapolation. In order to calculate the change in free energy the initial and final states of the system are linked by a reversible path along which the potential parameters vary continuously together with an ideal mixing process at an appropriate point in the path. The excess Gibbs free energy GE enthalpy HE and volume VE have been calculated2* in this way for a series of liquid mixtures in which EAA/EBB and OAA/OBB are varied systematically. A selection of the results which have been obtained are shown in Table 3. The cross-interaction parameters are calculated from the Lorentz-Berthelot rules and have the same values in all cases viz.E A B / ~ B = 133.5 K and OAB = 3.596 A. These are values appropriate to the cross-interaction between molecules of argon and methane. It is possible to make a number of generalisations on the basis of these calculations. Perhaps the most interesting result is that the excess properties of mixtures of molecules which differ only in size (i.e. EAA/EBB = 1) are very small. This is in agreement with the conclusion reached on different grounds by Leland Rowlinson and Sather.2v The excess energy is positive when the ratios EAA/EBB and OAA/(JBB vary in opposite senses. When they vary in the same sense the excess free energy goes through a negative minimum when plotted as a function of EAA/EBB. The depth of this minimum becomes greater as the ratio CTAA/(TBB increases. The excess volume is negative in all cases.Table 3 also shows results of calculations based on the ‘one-fluid’ (If) and ‘two-fluid‘ (2f) versions of the Average Potential Model (APM) developed by Prigogine and his ~011aborator~~~~ 31 and the recently proposed van der Waals approximation ( V ~ W ) ~ ~ 32 In the ‘one-fluid’ versions of both theories the properties of the mixture are assumed to be those of an imaginary pure liquid characterised by the average potential parameters Z a”. In the ‘two fluid’ versions the properties of the mixture are those of an ideal mixture of two imaginary pure components the molecules of which experience average potentials described by the parameters Gl and z2 G2. The two theories differ in the recipe for the determination of the average potential parameters.In the APM theory the averaging is based on the assumption of random mixing whereas in the van der Waals approximation it is carried out in the spirit of the original proposal for the calculation of the van der Waals constants for fluid mixtures. For mixtures of molecules of equal size the two theories are identical and agree well with the Monte Carlo calculations. In other cases the computer results strongly favour the van der Waals theory and perhaps surprisingly the ‘one fluid’ version is somewhat superior to the ‘two fluid’ model. The extrapolation to constant pressure of Monte Carlo data obtained at 28 K. Singer Chem. Phys. Letters 1969 3 164. The data in Table 3 differ from the original as it contains errors which will be corrected. 29 T. W. Leland J. S. Rowlinson and G. A. Sather Trans.Faraday SOC. 1968 64 1447. 30 1. Prigogine ‘The Molecular Theory of Solutions’ North-Holland Publishing Co. Amsterdam 1957. 31 A. Bellemans V. Mathot and M. Simon Adv. Chem. Phys. 1967 11 117. 32 Leland Rowlinson Sather and Watson Trans. Faraday SOC. 1969 65 2034. 247 Table 3 Excess thermodynamic properties of equimolar mixtures of 12-6 liquids calculated by a Monte Carlo method.28 T = 97 K P = 0; EAB/KB = 133.5 K (T = 3.596 A. % 4 b B B 1 so00 1 -062 1 -062 1 -062 €AA/€BB 1.235 0.810 1 -000 1.235 GE MC 34.7 59.7 - 2.4 3.2 (J mol-l) APMlf 37.9 139 96 129 APM2f 34.7 85.5 48 80.6 vdWlf 37.9 67.7 - 1.8 4.0 vdW2f 34.7 49-2 - 0.8 17-7 HE MC (J mol-l) APMlf APM2f vdWlf vdW2f V E MC (~rn~rno1-l) APMlf APM2f vdWlf vdW2f 35 31 48 31 48 - 0.13 - 0.16 - 0.07 - 0.16 - 0.07 72 177 121 74 68 - 0.25 0.03 - 0.04 - 0.28 - 0.20 6 146 73 0 0 - 0.01 0.048 0.21 - 0.04 - 0.04 - 11 175 119 - 12 27 - 0.09 0.60 0.32 -0.13 - 0.04 1.128 0.810 83.1 410 227 92.7 61.3 125 607 337 117 90 - 0.36 1 -24 0.44 - 0.49 - 0.41 1.128 1 -000 - 7.0 357 185 0 - 4.8 24 566 286 0 0 - 0.02 1.97 0.86 - 0.18 -0.18 1.128 1,235 - 29.0 381 21 1 - 34.3 - 2.4 - 39 582 327 - 55 4.8 - 0.05 2-33 1.09 - 0.19 - 0.11 Estimated (mean) accuracy of the MC data GE & 6 HE 20 VE rfi 0.08 McDonald and Singer constant volume may be a source of error if the changes in volume are large.One way of avoiding this difficulty though it requires more machine time lies in the use of Monte Carlo calculations in the NPT-ensemble. The pressure of mixing is then a fixed parameter. Excess functions for 12-6 mixtures corresponding to some real systems (A + Kr A + N2 A 3.CH4 CO + CH,) have been computed by this method.l0* l1 The results again strongly favour the van der Waals approximation. Agreement with experimental values on the whole is poor. This is almost certainly due to departures from the Lorentz- Berthelot rules in real systems. Recent has shown that even in the simplest mixtures the value of EAB is ca. 1 % less than that given by the geometric mean of EAA and EBB. Such a change would be sufficient to bring the calculated excess properties into agreement with measured values. C. Phase Equilibria.-Phase equilibria in one-component systems are determined by the equality of the Gibbs free energy per mole of the two phases at a given temperature and pressure. In order to compare the free energies of two fluid phases by means of the usual basic data obtained in a computer experiment i.e.U (V T) and P (V T) it is necessary to link the two states by a reversible path along which the change of A (Helmholtz free energy) and hence of G (Gibbs free energy) can be evaluated by numerical integration. In this way one could for example determine the change of G along the stable isochores and isotherms V Z T - t F’z T’+ V g T’-t r/ T where T’ is a supercritical temperature and the suffixes I and g denote respectively liquid and gas. For the computer model there is the additional possibility of integrating along the unstable isotherm VZ T - V T on which the P (V T ) data exhibit van der Waals loops. This is the method adopted in the Monte Carlo calculations of Hansen and Verlet.33 (Molecular dynamics is less convenient to use here because changes along an isotherm are required.) Very large pressure fluctuations and slow convergence of the average values were encountered in the unstable part of the isotherm.This difficulty which arises because of the tendency of the system to separate into two phases was overcome by the deliberate suppression of large inhomogeneities within the model. Subsequent calculations revealed that this somewhat arbitrary removal of accessible configurations while greatly improving the statistical convergence had no effect on the calculated pressures. When the change of G along the isotherm is known the liquid-vapour coexistence line can be determined because it is then possible to relate the free energy of either phase to that of the dilute gas.The method described above cannot be used to investigate the solid-vapour and solid-liquid transitions because the lattice arrangement disappears irreversibly on expansion. Hansen and VerIets3 therefore used an approach 33 J. P. Hansen and L. Verlet Phys. Rev. in the press. 249 The Study of Simple Liquids By Computer Simulation due to Hoover and in which the computer model is expanded in such a way that the particles are constrained to remain within the expanded lattice cells. The pressure is calculated at different volumes and when a density corresponding to the dilute gas is reached the walls of the lattice cells are removed. The only effect of this last step is the appearance of the communal entropy equal to NkB. The changes of A up to and beyond this point are evaluated by the integration of P (Y) d V.By combining these data with the free energies of the fluid phases which were calculated in treating the liquid-vapour 3 00 250 2 200 W w d 5 p! 150 8 i+ 1 oc 5c MELTING SOLID --/ FREEZING LIQUID- LIQUID-VAPOUR EQUILIBRIUM CRITICAL POINT TEMPERATURE I I I I 0.5 1 .o 1.5 2.0 DENSITY (g ~ r n - ~ ) Figure 2 Solid-liquid-vapour phase diagram for argon. The full curves are the results of Monte Carlo calculations (ref. 33) based on the 12-6 potential of Michels et al. (ref. 12). The dashed curve is the experimental liquid-vapour coexistence line (ref 19a). The open circles and dots are experimental melting data (refs. 19e f) 34 W. G. Hoover and F. H. Ree J . Chem. Phys. 1967,47,4873. 250 McDonald and Singer transition it is possible to determine the melting curve.Hoover and have themselves used the cell expansion method to establish the existence of a first-order solid-fluid transition for hard sphere molecules. The solid-liquid- vapour phase diagram for the 12-6 potential of Michels et aZ.12 is shown in Figure 2. Agreement with experimental data for argon is good except in the critical region where as has already been remarked the effect of the suppression of large-scale density fluctuations in the computer model is to raise the critical temperature. This work is important not only because it supplies additional proof of the surprising excellence of the 12-6 potential for argon but because it also shows that the quantitative study of phase equilibria by computer simulation is possible. 4 Radial Distribution Function and Liquid Structure The structure of a simple liquid is usually described in terms of the radial distribution function.Let n ( r ) be the number of particles situated at a distance between r and (r + dr) from a reference particle. The radial distribution function g ( r ) is defined as If the intermolecular potential is spherically symmetric and pair-wise additive then the internal energy pressure and other thermodynamic properties which can be derived from these may be calculated if g ( r ) is known as a function of density and temperature. The calculation of the radial distribution function is therefore a fundamental problem in the statistical thermodynamics of simple liquids. Several theories have been proposed in which g ( r ) is obtained from the intermolecular potential function by solving an integral equation.3s The most successful of these at least at high temperatures is that of Percus and Yevi~k.~' The structure factor S (k) for the liquid is defined in terms of g ( r ) through a Fourier transform S(k) = 1 + i ( k ) = 1 + (N/v)le-" k ( r ) - l l d r (7) V The importance of S (k) arises from the fact that (for k # 0) it is proportional to the differential cross-section for scattering of electromagnetic radiation.ss The quantity k is the difference in wave vector between the incident and scattered radiation though in liquids the structure factor is a function only of the magnitude of k.Neutron and X-ray scattering experiments are therefore valuable sources of information on the radial distribution function in liquids. Alterna- tively the usual form of the Percus-Yevick or some other integral equation may 35 W.G. Hoover and F. H. Ree J. Chem. Phys. 1968,49 3609. 36 G. S. Rushbrooke 'Physics of Simple Liquids' ed. H. N. V. Temperley J. S. Rowlinson and G. S. Rushbrooke North-Holland Publishing Co. Amsterdam 1968. 37 J. K. Percus and G. J. Yevick Phys. Rev. 1958 110 1. P. A. Egelstaff 'An Introduction to the Liquid State' Academic Press London 1967. 251 The Study of Simple Liquids By Computer Simulation be inverted to allow the derivation of the pair potential in a liquid from measured scattering intensities. Work of this type has been reported both for argon3# and for liquid In the latter case the calculation leads to an effective ion-ion interaction potential. Some of the difficulties associated with this potential inversion problem are discussed below.The radial distribution function for liquid argon shown in Figure 3 is taken from the early work of Wood Parker and Jacobson3 in which the 12-6 2 .o z 1.5 !i 2 2 z 1.0 t; Ei 2 z 0 b U GI i3 4 p? 0.5 0 1 I I I 2 4 6 8 10 r (& Figure 3 Radial distribution function for liquid argon at T = 126.7 K p = 1.099 g cm-s. The curve shows the results of a Monte Curlo calculation (ref. 3) bused on the 12-6 potential of Michels et al. (ref. 12). The dots are results from X-ray scattering experiments (ref. 41) 39 P. G. Mikolaj and C. J. Pings J. Chem. Phys. 1967 46 1412. 40 (a) M. D. Johnson P. Hutchinson and N. H. March Proc. Roy. Soc. 1964 A 282,283; (b) P. Ascarelli Phys. Rev. 1966 143 36. 252 McDonald and Singer potential of Michels et ~ 1 1 .~ ~ was used. The main peak in g ( r ) represents a shell of nearest neighbours and a peak corresponding to a second shell may also be seen. Agreement with the experimental curve derived from X-ray scattering data by Eisenstein and Gingrich41 is qualitatively good but quantitatively is rather poor. The radial distribution function and structure factor of the 12-6 fluid have since been calculated by Verlet by means of the method of molecular 2.c 1.5 - 25 v) 1 .o 0.5 0 0 1 3 4 5 6 k (i-') Figure 4 Structure factor of liquid argon at T = 99.1 K p = 1.260 g ~ r n - ~ obtained from a molecular dynamics calculation (ref. 42) based on the 12-6 potential with the parameters of Michels et al. (ref. 12). The dots are the results of neutron scattering experiments (ref. 43) on liquid krypton in a nearly-corresponding state 41 A.Eisenstein and N. S. Gingrich Phys. Rev. 1942 62 261. 253 The Study of Simple Liquids By Computer Simulation dynamics.42 These calculations cover a wide range of density and temperature. At high densities the structure factor has the characteristic shape shown in Figure 4. The curve is dominated by a large peak at ko = 27r/ro where ro is approximately equal to the value of 0 in the 12-6 potential. From the properties of the Fourier transform it follows that the peak in S (k) is responsible for the oscillations in g(r) and that the position of this peak determines the period ro = 27r/k0 of the oscillations. The value of To which has the character of a hard core decreases slowly with increasing density but is almost independent of temperature.Ve1-1et~~ shows that the main features of the structure factor of the 12-6 fluid are accurately reproduced by a hard sphere model in which the only adjustable parameter is the diameter of the hard spheres. These results emphasise the importance of the repulsive part of the potential in determining the form of the radial distribution function and show that the structure of a simple liquid is determined primarily by geometric effects arising from the presence of a hard core. The adequacy of the Percus-Yevick approximation may be tested by comparing the theoretical radial distribution function with those obtained in computer ‘experiments’. A more illuminating test of the theory is provided by solving the potential inversion problem in order to recover the 12-6 potential from the structure factors computed by Verlet.It is found that at densities near the critical point the potential calculated from the Percus-Yevick equation has a bowl which is ca. 1% shallower than that of the 12-6 function used in the molecular dynamics ‘experiments’. The error increases rapidly as the density rises and it is concluded that the Percus-Yevick theory cannot be used to obtain quantitatively reliable information on the two-body interaction in dense systems. Figure 4 shows that there is good agreement between the structure factor calculated by computer simulation and the results of neutron scattering experiments on liquid krypton.43 It is found in particular that the neutron data lead to structure factors which display the regular oscillations at high k values which are a prominent feature of the computer results.The structure factors obtained from X-ray scattering experiments4* show a more erratic behaviour and agreement with the molecular dynamics calculations is not so good. Levesque and Verlet45 suggest that the available X-ray data contain some systematic errors. The radial distribution function is concerned only with pair correlations and therefore falls some way short of providing a complete description of the structure of the liquid. Multiple correlations can in principle be studied by com- puter simulation but this requires an extremely large amount of computing time. An alternative method of analysis in terms of Voronoi polyhedra has been developed by Bernal for the study of his random close-packed hard sphere 42 L. Verlet Phys.Rev. 1968 165 201. 43 G. T. Clayton and L. Heaton Phys. Rev. 1961 121 649. 44P. G. Mikolaj and C. J. Pings J. Chem. Phys. 1967 46 1401. 45 D. Levesque and L. Verlet Phys. Rev. Letters 1968 20 905. 254 McDonald and Singer model of the liquid 47 It may equally well be applied to the analysis of computer generated configurations. The Voronoi polyhedron for a partide i is the smallest closed convex polyhedron surrounding i which is formed by the set of planes which bisect the vectors linking i to all other particles. A typical two-dimensional ‘Voronoi polygon’ is shown in Figure 5. The construction is & Figure 5 A typical two-dimensional ‘ Voronoi polygon’. The dots represent particles of the fluid unique and the set of Voronoi polyhedra fill the whole volume occupied by the particles.The structural features of a system may be described in terms of various distributions including the number of faces of the polyhedra the number of sides of the polygons which form these faces and the volume of the polyhedra. In liquid-type systems it is found4** 4g that polyhedra with 14 or 15 faces and faces with five sides are dominant. This type of analysis is useful for the pictorial insight into the structure of the liquid which it provides though it offers no 40 J. D. Bernal Proc. Roy. Soc. 1964 A 280 299. 47 J. D. Bernal and J. Finney Discuss. Faraday SOC. 1967 43 62. 48 J. Finney Thesis University of London 1968. ‘CI A. Rahman J. Chem. Phys. 1966,45 2585. 255 The Study of Simple Liquids By Computer Simulation obvious route to the calculation of thermodynamic properties.It also proves useful as later discussion will show in describing the process of diffusion 5 Time-dependent Phenomena The method of molecular dynamics permits the study of microscopic time- dependent phenomena in liquids to a degree of detail which experimental techniques cannot yet approach. In the early work of Alder and Wainwright,6 for example much attention was devoted to the problem of the rate at which a system of interacting particles approaches equilibrium starting from some arbitrary non-equilibrium state. The initial condition used most often was one in which the particle velocities are of equal magnitude but have different directions. The results revealed a marked difference between the behaviour of hard spheres and particles interacting through potentials more representative of real systems.For hard spheres the equilibrium Maxwell velocity distribution develops extremely rapidly and is essentially complete after each particle has collided two to four times with its neighbours. This appears to be true at all densities. By contrast for molecules interacting through a square well potential a Maxwell distribution again appears after a short time but the mean velocity of this distribution then moves slowly to reach its final equilibrium value after ca. 60 collisions per particle. The reason for this difference in behaviour is that the attainment of equilibrium in systems with cohesive potentials requires the interchange of kinetic and potential energy. This is a relatively slow process. At equilibrium the total energy being constant in a molecular dynamics calculation the mean square fluctuations in kinetic and potential energy are equal.The motion of particles in a system at equilibrium may be described in a variety of ways. Consider a particle i which at time t = 0 is located at a position ri(0) and has a velocity vi (0). The mean square displacement at a time t is defined as and the velocity autocorrelation function Z ( t ) is defined as The form of the velocity autocorrelation function is determined by the rate at which the velocities of the particles change as a result of interactions with other particles. The angular brackets in equations (8) and (9) denote averages over an equilibrium ensemble of initial conditions. The two functions may also be defined in terms of a time average and in practice in the molecular dynamics ‘experiment’ the averages are calculated by considering a number of different time origins.Both the mean square displacenient and the velocity autocorrelation function 256 McDonald and Singer are related to the process of diffusion in the liquid. Thus the coefficient of self-diffusion D may be expressed in terms of Z ( t ) by the equation The function Z ( t ) is equal to one at t = 0 and decays to zero as t h e advances and the memory of the initial conditions is lost. Figure 6 shows the velocity 1 .o 0.8 z 0 b 3 0.6 w d d U 8 f3 F s 9 2 0.4 G 0.2 0 0.1 0 0.5 1 .o 1.5 TIME in lo-’ sec Figure 6 Velocity autocorrelation function for liquid argon at T = 94.4 K p = 1.374 g ~ r n - ~ obtained from a molecular dynamics calculation (re6 50) based on a 12-6 potential (see text).The dots show the Langevin type of velocity autocorrelation (m is the atomic mass) 257 The Study of Simple Liquids By Computer Simulation autocorrelation for liquid argon calculated by Rahmanso from a 12-6 potential ( E / ~ B = 120K (3 = 3.4 A) by the method of molecular dynamics. The autocorrelation decays to zero in ca. 2 x 10-12 sec but the most striking feature of the curve is that Z ( t ) becomes negative after 0.33 x 10-la sec and remains essentially negative as the decay goes to zero. The physical significance of a negative autocorrelation is that it represents ‘back scattering’ of particles.6 At sufficiently high densities a particle is trapped for a time within a cage formed by its neighbours. The motion of the particle is therefore characterised by frequent reversals of velocity within a narrow range of angles.This results in a negative correlation or anti-correlation of velocity. In the extreme case of an harmonic oscillator Z ( t ) is a cosine curve. It is clear that simple oscillation does not contribute to self-diffusion and inspection of equation (10) confirms that the effect of negative regions in Z (t) is to reduce the value of D. The behaviour of (r2) as a function of time for the system studied by Rahmanso is shown in Figure 7. The slope of the linear portion of the graph is proportional to the coefficient of self-diffusion. Thus D may be calculated by means of the equation Lt (r2> = 6Dt + C (1 1) t-+ 00 where C is a constant. Figure 7 shows that this asymptotic behaviour is already reached at 10-l2 sec. The calculated value of D is 2.43 x cm2 sec-l which is ca.15 % lower than the experimental value for argon at the same temperature and density. Agreement with experiment is improved when an exp-6 potential is It may also be seen from Figure 7 that the root mean square displace- ment after 2.5 x 10-l2 sec [by which time Z ( t ) is effectively zero] is only 1.9 A. This is approximately one-half the nearest neighbour distance in the liquid. The persistence of short range order which is suggested by this result may be described more precisely by means of a time-dependent pair correlation function Gd (r t ) . Let n (r t ) be the number of particles situated at time t at a distance between r and (r +dr) from the position occupied by a reference particle at t = 0. Then Gd (r t ) is defined as The subscript d (symbolising ‘distinct’) indicates that the reference particle is not included in the number n (r t ) .The function Ga (r 0) is equivalent to the radial distribution function g(r). Rahmanso finds that the height of the first peak in Gd (r t ) for liquid argon at 94-4 K at t = 0 1 and 2.5 x 10-l2 sec is respectively 2-8 1.5 and 1-1. Remnants of the first shell of neighbours therefore persist for at least 2.5 x 10-l2 sec. Fluctuations in short range order are closely related to the phenomenon of self-diffusion. Rahman49 shows that for liquid argon near the triple point the decay time T of the fluctuations in the shape of the Voronoi polyhedra is ca. 0.5 x 10-12 sec. In this time interval the particle tends to ‘slip’ along the 60 A. Rahman Phys. Rev. 1964,136 A405. 258 McDonald and Singer 1 .o 2.0 3.0 TIME in sec Figure 7 Mean-square displacement of atoms in liquid argon (ref.50). For details of the calculation see caption to Figure 6 direction corresponding to the elongation in the polyhedron to which the fluctuations give rise. As these fluctuations result from the correlated motion of many particles it is clear that this description is one truly appropriate to the liquid state and does not rely on concepts borrowed from the study of gases or solids. The direction along which a particle moves in the decay time r may be used to resolve the total velocity autocorrelation into two parts corresponding to ‘slipping’ and ‘rattling’ motion of the particles. Thus where S ( t ) (the ‘slipping’ part) is the autocorrelation of the component of velocity parallel to the direction of displacement in the time T and R ( t ) (the ‘rattling’ part) is the autocorrelation of the velocity component perpendicular 259 The Study of Simple Liquids By Computer Simulation to this direction.The function S ( t ) may be further resolved into two parts S,+ ( t ) and S,- ( t ) which represent the contributions of particles which at t = 0 are moving respectively with a positive velocity component along the direction of displacement in the time interval r and with a positive velocity component in the opposite direction. Both S - ( t ) and R,(t) represent an oscillatory type of motion. Figure 8 shows this resolution for a system 0.6 0.4 z t- d 14 p d 0 U 3 < 2 4 0.2 8 b 0 5 s u iri > -0.2 -0.4 I I I 0 0.5 1 .o 1.5 TIME in lo-'* sec Figure 8 Components of the velocity autocorrelation function for liquid argon corresponding to 'slipping' and 'rattling' motion of the atoms.The curves are obtained from a molecular dynamics calculation (ref. 49) at T = 85.5 K p = 1.407 g ~ r n - ~ based on an exp-6 potential representing argon at 85.5 K. The sum [R ( t ) + S,- ( t ) ] decays to zero in ca. sec after passing through a deep negative minimum. The motion described by these components of the total velocity autocorrelation contributes little to the process of self-diffusion and is analagous to the vibrations of atoms 260 McDonald and Singer in an anharmonic solid. The function S,+ ( t ) decays more slowly and becomes only weakly negative. The major contribution to D comes from this component of z (t). Other transport properties including viscosity and thermal conductivity may be expressed in terms of appropriate time-correlation functions.51 These could be calculated in a molecular dynamics ‘experiment’. The tumbling motion of diatomic molecules may also be characterised by various correlation functions. Harp and Berne have recently calculated the angular momentum auto- correlation function for liquid carbon monoxide by molecular dynamics. 52 The autocorrelation has an oscillatory behaviour but the detailed structure of the function is dependent on the strength of the non-central part of the pair po- tential used in the calculations. The angular momentum autocorrelation is im- portant in determining the shape of n.m.r. signals in liquids. Similarly the i.r. and Raman spectra for diatomic molecules are related to the form of the autocorrelation functions which describe the rotation of the molecular axis.53 The importance of time-correlation functions in statistical mechanics makes it certain that more work of this type will be carried out.6 Final Remarks Computer simulation has also been used in studies of a number of other systems. Monte Carlo calculations of thermodynamic properties of liquid water 54 based on a rather simple pair potential and of liquid potassium based on potentials which are the sums of an exp-6 function and a Coulomb term have been briefly reported. Systems of hard ellipsoidal particle^,^^ which can serve to simulate the properties of liquid crystals and the pair correlation function in liquid have also been studied. The equilibrium number of lattice defects in solid argon has been determined58 by a method which could be generalised to yield absolute chemical potentials in liquids and liquid mixtures.Work on increasingly complex systems including for example ionic solutions will no doubt be carried out in the near future; and it is probable that what has been achieved for the system of 12-6 molecules i.e. a knowledge of the thermodynamic properties including the phase diagram will also be achieved for systems in which less simple pair potentials are operative (e.g. polar molecules). It is reasonable to hope that systematic work of this kind will lead to a thorough understanding of the relationship between the bulk properties of a system and the form of the intermolecular pair potentials. By contrast the study of systems in which many-body forces cannot be treated as small 51 R.Zwanzig Ann. Rev. Phys. Chem. 1965 16 67. 62G. D. Harp and B. J. Berne J . Chem. Phys. 1968 49 1249. 53 R. G. Gordon Adv. Magnetic Resonance 1968 3 1. 54 J. A. Barker and R. 0. Watts Chem. Phys. Letters 1969 3 144. 65 L. V. Woodcock and K. Singer ‘Proceedings of the Culham Conference on Computational Physics’ UKAEA Culham Laboratory and IPPS 1969 Vol. 2 July paper 25. 66 D. Levesque D. Schiff and J. Vieilland-Baron J. Chem. Phys. 1969 51 3625. 57 A. Paskin and A. Rahman Phys. Rev. Letters 1966 16 300. 6eD. R. Squire and W. G. Hoover J . Chem. Phys. 1969 50 701. 26 1 The Study of Simple Liquids By Computer Simulation would make computational demands which for some time to come will be prohibitive. The same is true of time-dependent phenomena characterised by a relaxation time greater than say lo-* sec. No general method has as yet been devised for the computer simulation of non-classical systems. This is the most fundamental and from the chemist’s point of view the most frustrating limitation. If this problem were to be solved it would be possible to examine in great detail all kinds of elementary chemical processes. 262

ISSN:0009-2681    

DOI:10.1039/QR9702400238

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Volatile Compounds of the Hydrides of Silicon and Germanium with Elements of Groups V and VI By John E. Drake and Chris Riddle UNIVERSITY OF WINDSOR ONTARIO CANADA 1 Introduction Discussion is centred around volatile ternary hydrides represented by the general formula MxM’yHz (M = Si or Ge; M’ = an element of Group V or VI). Hydrides with organic groups attached to M or M will also be referred to when relevant to any discussion. The development by Stock’ of vacuum line techniques for handling air- sensitive compounds provided the initial impetus to study of the parent binary silanes and germanes. Subsequent progress has been covered by several texts including those of MacDiarmid,2 Stone,3 EbsworthY4 Mackay,6 Aylett,g and Gl~ckling,~ which have influenced our treatment of the ternary hydrides.Of outstanding interest in these hydrides is the possible participation of the vacant d-orbitals on silicon or germanium in additional bonding. In several molecules filled orbitals on a Group V or VI atom are apparently of the correct symmetry and energy to allow 7.r-interactions. Qualitatively changes in bond-lengths bond-angles and donor properties have been related to the degree of p-character in bonds and in turn to the extent ofn-bonding. A comprehensive summary of the evidence onn-bonding has been presented by Ebsworth,* and his co-workers have recently reported some pertinent structural work.Q Dynamically too as in the stabilisation of reaction intermediates there appear to be good reasons for invoking d-orbital participation. The formation of additional a-bonds to silicon increases its co-ordination number beyond four and possibly reduces the activation energy of an overall reaction process.Such mechanistic routes would not be available for carbon. A further feature that reflects differences in the chemistry of species containing Si-H and Ge-H rather than C-H is the change not only in the degree of polarity of the M-H A. Stock ‘Hydrides of Boron and Silicon’ Cornell University Press 1933. A. G. MacDiarmid Adv. Inorg. Chem. Radiochem. 1961 3 207. F. G. A. Stone ‘Hydrogen Compounds of the Group IV Elements’ Prentice Hall 1962. K. M. Mackay ‘Hydrogen Compounds of the Metallic Elements’ Spon 1966. 8. J. Aylett Adv. Inorg. Chem. Radiochem. 1969 12 249. ’ E. A. V. Ebsworth ‘Volatile Silicon Compounds’ Pergamon Press 1963. ’ F. Glockling ‘The Chemistry of Germanium’ Academic Press 1969.* E. A. V. Ebsworth in ‘Organometallic Compounds of the Group IV Elements’ Dekker 1968. (a) B. Beagley A. G. Robiette and G. M. Sheldrick Chem. Comm. 1967,601 ; (b) D. W. H. Rankin A. G. Robiette G. M. Sheldrick B. Beagley and T. G. Hewitt J. Inorg. Nuclear Chem. 1969 31 2351 ; (c) C. Glidewell D. W. H. Rankin A. G. Robiette G. M. Sheldrick S. Cradock E. A. V. Ebsworth and B. Beagley Inorg. Nuclear Chem. Letters 1969’5 417. 263 Volatile Compounds of the Hydrides of Silicon and Germanium bond but also in its direction. Thus nucleophilic attack at silicon and germanium is expected to take place much more readily. 2 Hydrides Containing a Group V Element Bound to Silicon or Germanium A. Silicon-Nitrogen Compounds.-Several such compounds are now known.Comparison of these with the analagous carbon compounds provides good evidence for additional bonding between Si and N. Trisilylamine (SiH3)3N a volatile liquid that spontaneously ignites in air was first prepared by Stock and Somieski from reaction of SiH3Cl and NH3.1 They proposed the reaction was step-wise (1-3) although they were unable to isolate mono- and di-silylamine. Varying conditions give quantities of a polymeric solid and SilanelO proposedly by reaction (4). 6SiH3Cl + 12NH3 = 6SiH3NH2 + 6NH4Cl 6SiH3NH2 = 3(SiH3)2NH + 3NH3 3(SiH3)2NH = 2(SiH3),N + NH3 n(SiH,),NH = nSiH + [SiH2(NH)Jn Subsequently it was suggested that the formation of (SiH3),N is indicative of the donor properties of disilylamine ( 5 ) and also that the instability of the intermediates results from ammonia-catalysed decomposition similar to the base- SiH SiH3 SiH I H3Si - Nf - SiH I I H3Si - N - H3Si - N I H I H = I H - (SiH&N etc.- +SiH3NH2 + SiH3NH- (5) catalysed SiH redistribution observed with (SiH3)20.11 Substantial verification of the mechanism has followed the recent isolation of (SiH&NH (Table 1).l2 This although stable as a gas up to 150" disproportionates (3) at 0" to tri- silylamine and also yields the latter on reaction with SiH,I. Its donor property is confirmed by the formation of a complex with Me3B and in the presence of NH3 at 130" it indeed polymerises with evolution of SiH4 (4). The structure of (SiH3)3N is unique amongst the trisilyls and trigermyls (Table 2) in that the Si3N skeleton is ~1anar.l~ The simplest bonding 'picture' suggests qP-hybridisation of the nitrogen orbitals for o-bonding to silicon.The remaining p-orbital interacts with the appropriate vacant d-orbitals on the silicon atoms to give (p - d)r-bonding for which the planar structure allows optimum overlap (Figure). The 'short' Si-N bond and the 'high' force constant required to describe the skeletal vibrations provide physical The lo A. B. Burg and E. S. Kulijan J. Amer. Chem. Soc. 1950 72 3103. l1 See Ref 4; p. 110 l2 B. J. Aylett and M. J. Hakim J. Chem. SOC. (A) 1969 639. l4 See Ref. 4 p. 161; Ref. 8 p. 86. K. Hedburg J. Amer. Chem. SOC. 1955,77 6491. 264 Drake and Riddle Table 1 Preparative and Reaction Routes for Silicon-Nitrogen Hydrides / s (SiH3)3N.BX3 SiH3Cl (a) BIH,- (SWINBX (SiH3)2NBH - (SiH3)lNBlH adduct ( e ) / T Me2 BN(SiH2 Br)l (9) SMlI (b) /-+ SiHlN3 +SiHN (h) r-3 (a) SiH,CI tNH,--ISM3NH,I-c(SiH,),NH-(SiH3),N ':Iadduct below-800(i) (SiH3 1 N.Me Al (i) SiN bond cleaved (a) no reaction with:- Si,-Cl,-l (a) (NHi ,LM (SiH,NSiH2)3 amhe') BlHs ( 0 HCI Me,B 0 ) (b) PhZNSiHs + NH3 -/ \b) "jag \- HCI /-7 .t (SilNH),? (k) (a) SiH,CI + NH - (SiH2 NH) SiH,I( donor) (d) SiH,I+NPH -t(SiH3),N "Oreaction (d) Si2H,X+NH3-%(Si2H,),N -%I:ladduct (1) ~ ~ ~ p l o d e s in air reduces Cu & Hg salts (d) (Q) A.Stock and C. Somieski Chem. Ber. 1921,54 740. ( 6 ) Ref. 12. (C) B. J. Aylett and H. J. Hakim Inorg. Chem. 1966,5 167. ( d ) Ref. 19. (e) Refs. 10 17. v) Ref. 16. (Q) Refs. 10,15a,c. (A) Ref. 21; J. F. Ogilvie and S. Cradock Chem Comm. 1966 364. (f) Ref. 15d. (4) Ref. 15b. (k) Ref. 3 p. 29. (2) M. Abedini and A. G. MacDiarmid Inorg.Chem. 1963,2,608. Table 2 Structural Data for Trisilyl- and Trigermyl-Hydrides Compound rM-H(R) m-M'(A) MM^Mo Predicted Ref. (SiH,),N 1.54 20.05 1.73820.02 119.6 2 1 1.77 a (SiH3)3P 1-50 L-0.02 2*247+0*005 95 + 2 2.25 b (SiH,),As 1.48 kO.02 2.352+0.005 91.5 + 2 2.36 b (SiH,),Sb 1-3942 0.027 2.557 2 0.004 88.6 2 0-2 2.57 C (GeH,),P 1.510f 0.008 2-308 f 0.003 95-39 k 0.05 2.31 C (GeH,),N (GeH,),As and (GeH,),Sb all believed to have non-planar M3M' skeletons from vibrational spectra. (a) Ref. 13. (b) Ref. 9a. (c) Ref. 9b. r M-M' (A) Figure The (p-+d)n-bond in (SiH,)N. Shading indicates filled nitrogen p-orbital. 265 Volatile Compounds of the Hydrides of Silicon and Germanium effective donation of electrons from N to Si required by thenbonding would be expected to decrease the 'availability' of the lone-pair on N and hence reduce its donor properties.Its reactions towards boron Lewis acids are less extensive than those of Me3N so that no adduct is formed by (SiH,),N with B2HB or Me3B and only weak adducts with BF and BC13.10+ A re-investigatiod6 of the SiH3CI + NH reaction identified among the expected products NN'N" trisilylcyclotrisilazane (SiH,NSiH,), which may also be formed in the liquid-phase reaction between (SiH,),N and NH,. Deuterium labelling established that there is no protonic exchange between ammonia and trisilylamine and that the reaction is probably intermolecular. The kinetics indicate the formation of a weak complex with an ammonia attached to each silyl group N[(SiH,) . NH,], so that (SiH,),N can also act as a Lewis acid. This mechanism requires 5-co-ordinate silicon possibly through d-orbital participation and hence is not found in the methylamines.The reaction of ammonia with disilanyl halides gives condensation to tris-(disilanyl)amine (Si,H6),N,l7 while with SiH2C12 SiHCI, and SiCl poIymeric species are f0rmed.l A recent review of silicon-nitrogen polymers1* includes work on systems such as (SiN2H2)n. Hydrazine reacts with SiH,I to give (SiH3)*N2 l9 which apparently does not show typical acid or base properties. An interpretation of its vibrational spectrum20 supports possible N-Si (p - d)n- bonding. Whereas the non-linear skeleton claimed for silyl azide SiH3N3,21 suggests that any .rr-bonding is not strongly stereochemically significant. By contrast SiH,NCO 22 and SiH,NCS 23 are linear but it seems likely that complete methyl-substitution at silicon (Lee Me,SiNCO and Me,SiNCS) leads to a bending of the Si-N-C angle2 as is found with Me3SiN3.25 These results suggest that the differences in energy between linear and bent heavy-atom skeletons are small so that comparatively minor electronic changes may have significant effects on bond-angles.B. Germanium-Nitrogen Compounds.-These are less extensive than those of silicon so that the very unstable trigermylamine (GeH3),N has only recently been prepared by a carefully controlled reaction between GeH,Cl and NH,.2s In contrast to (SiH,),N the preliminary i.r. spectrum suggests that ( p -+ d)n- l5 (a) J. M. Gamboa Anales de Quim. 1950 72 3103; (b) S. Sujishi and S. Witz J. Amer. Chem. SOC. 1954 76 4631; (c) S. Sujishi and S. Witz J.Amer. Chem. SOC. 1957 79 2447; (d) H. M. Manasevit US. Dept. Corn. Office Tech. Serv. P.B. Report 143 572 1959 1. l6 R. Schaeffer and R. L. Wells J. Amer. Chem. SOC. 1966 88 37. 17 L. G. L. Ward and A. G. MacDiarmid J. Znorg. Nuclear Chem. 1961 21 287. Is B. J. Aylett J . Inorg. Nuclear Chem. 1956 2 325. 2o B. J. Aylett J. R. Hall D. C. McKean R. Taylor and L. A. Woodward Spectrochim. Acta 1960 16 747. 21 E. A. V. Ebsworth and M. J. Mays J . Chem. SOC. 1964 3450. 22 M. C. L. Jerry J. C. Thompson and T. M. Sugden Nature 1966 211 846. 23 D. R. Jenkins R. Kewley and T. M. Sugden Trans. Faruday SOC. 1962 58 1284. 24 K. Kimura S. Katada and S. H. Bauer J . Amer. Chem. SOC. 1966 88 416. 26 J. S. Thayer and R. West Inorg. Chem. 1964,3 889. 26 D. W. H. Rankin Chem. Comm. 1969 194; J. Chem.SOC. (A) 1969 1926. B. J. Aylett Organometallic Chem. Rev. 1968 3 151. 266 Drake and Riddle bonding if present does not stabilise a planar structure although clarification should come with electron diffraction data. The vibrational spectrum of the azide prepared from GeH,F + Me,SiN, indicates a non-linear GeH,N as does that of GeH,NC0.26 The only other volatile Ge-N hydrides reported are GeH,NH and GeH,(NH,) which were mass spectroscopically detected among the products from the hydrolysis of CaGeN alloys,29 and Ge(NH,) which has been proposed as an unstable intermediate that polymerises to Ge(NH)z.30 C. Phosphorus Arsenic and Antimony Compounds.-The first primary species SiH,PH2 was synthesised in 1953 by the pyrolysis of SiH4 and PH3.31 Further interest was stimulated by the introduction in 1961 of electrical discharge techniques as a synthetic route in this area., Of the preparative routes now available (Tables 3-7) for the phosphines and arsines ‘exchange reactions’ (6) have been used for the preparation of specifically deuteriated species.33 Table 3 Preparative and Reaction Routes for Monosilyl-phosphine and -arsine (b) SiII + vH3 <r s11131+ As SIH Br + KM‘H2 (d) (d) + PH & SiH,X + LiAl(Pl12)4 (e) (f) (Sit Siti MH -13 )20 t MI13 - SiH,PHZ.BH3 0 ) C’PH + Sill4 + (SilIzNH)x -t H2 (k) -c S1H3PH2’BF (I) + [SiH,OHI + oxyhydrides (k) PH3 + 11 + [Si(OH) ] (k) (f) (SiH3)JM t- MH (i) (SiH3)3M + MH * 110 reaction with:- ASH (I) (O SiH3Br + MH3 LiH,LiCI (I) (a) Ref.31. ( b ) Ref. 32. (C) B. J. Aylett Ph.D. Thesis Cambridge University 1954. C. GlidewellandG.M. Sheldrick,J. Chem. SOC. (A) 1969,350. (e) A. D. Norman Chem. Comm. 1968 812. (f) Ref. 51. (g) Ref. 52a,b. ( h ) J. E. Drake N. Goddard and J. Simpson Znorg. Nuclear Chem. Letters 1968 4 361. (0 Ref. 50. (3) Refs. 52a 54; G. E. Bagley Dissertation Abstr. 1959 20 66. (k) G. Fritz Angew. Chem. 1966 78 80. (2) Ref. 52c. (na) S. D. Gokhale and W. L. Jolly Inorg. Chem. 1965,4,596. (n) G. Fritz 2. Anorg. Chem. 1955,280,332. 27 S. Cradock and E. A. V. EbswoIth J. Chem. SOC. (A) 1968 1420. 28 J. E. Griffiths and A. L. Beach Chem. Comm. 1965,437. 20 P. Royen and C. Rocktaschel 2. anorg. Chem. 1966,346,290. ao D. Rustad and W. L. Jolly Znorg. Chem. 1967 6 1986. a1 G. Fritz 2. Nufurforsch. 1953 8B 776. 32 J. E. Drake and W. L. Jolly Chem. and Znd. 1961,1470. a3 (a) J. E. Drake and C.Riddle J . Chem. SOC. (A) 1968,1675; (b) J. E. Drake and C. Riddle J. Chem. SOC. (A) 1968 2452. 267 Volatile Compounds of the Hydrides of Silicon and Germanium SiH,PH2 3. GeD,Cl = GeD,PH + SiH3Cl (6) Three antimony compounds are known SiH,SbH, formed in the reaction of SiH,Br with KSbH234 which unlike the corresponding reactions with KPH2 and KAsH will not give the trisilyl derivative; (SiH,),Sb formed when silyl halides react with Sb 35 or Li3Sb;34 and (GeH,),Sb,formed by exchange between GeH,Br and (SiH ,) ,S b.,* Detailed vibrational spectra studies have been reported for the primary hydrides MH3M'Ha (M = Si or Ge; M' = P or which conform to Table 4 Preparative and Reaction Routes for Disilyl-phosphine and -arsine (a) SM4 +PH (b) SiH + SMSPHZ SiH3 C1+ PH3 (b) (4 LiAl(AsH2 + SiHa Br (c) KPH2 + SiH3Br (a) S.D. Gokhale and W. L. Jolly Inorg. Chem. 1964,3 1141. ( b ) S. D. Gokhale and W. L. Jolly Inorg. Chem. 1965 4 596. (c) C. Glidewell and G. M. Sheldrick J. Chem. SOC. (A) 1969,350. Cssymmetry like MeNH and MePH,. In early work on the spectra of (SiH,),P and (SiH,),As it was ~uggested,,~ from the activity and relative intensities of the skeletal modes that the heavy-atom skeleton was planar. Subsequently electron diffraction work showed that it was in fact pyramida1.O It has been pointed out that structural conclusions based on the relative intensities of bands also proved incorrect for (SiH3)20.40 The reactions of (SiH3),P (Table 5 ) suggest it is a weaker nucleophile than Me,P which may be indicative of some delocalisation of the phosphorus lone-pair into silicon d-orbitals.The low Si-P-Si angle of 96.5" has led to speculation that this additional bonding is not through (p -f d)?r-bonds but through (s-+ d)a-bonds in which the essentially s-character lone-pair of P interacts with dz2-orbitals of Si (where the z axis is along the P-Si bond).41 Reference to additional bonding in discussing the bond-angles bond-strengths 34 E. Amberger H. D. Boeters and M. R. Kula Angew. Chem. 1964,76 573. 36 B. J. Aylett H. J. Emel6us and A. G. Maddock Research 1953 6 30 S. E. A. V. Ebsworth D. W. H. Rankin and G. M. Sheldrick J. Chem. Soc. (A) 1968,2828. 3' J. E. Drake and C. Riddle Spectrochim. Actu in press. K. M. Mackay K. J. Sutton S. R. Stobart J. E. Drake and C. Riddle Spectrochim. Actu 1969,25A 925; 1969 25A 941. 90 G. Davidson L. A.Woodward E. A. V. Ebsworth and G. M. Sheldrick Spectrochim. Actu 1966,22 67; 1967 23A 2609. 4O D. C. McKean Spectrochim. Acta 1968 MA 1253. 41E. A. V. Ebsworth C. Glidewell and G. M. Sheldrick f. Chem. SOC. (A) 1969 352. 268 Drake and Riddle Table 5 Preparative and Reaction Routes for Trisilyl-phosphine and -arsine GeH3 Br no reaction with:- CS2 CCl ,H2S CH3& SMiIy BaCI2 CH30H PF (g) (a) Ref. 35. ( b ) E. Amberger and H. D. Boeters Angew. Chem. 1962,74 32 and 293. (C) Ref. 2 p. 247. ( d ) Refs. 50,52. (e) S. Cradock G. Davidson E. A. V. Ebsworth and L. A. Woodward Chem. Comm. 1965,515. (f) W. L. Jolly and A. D. Norman Preparative Inorganic Reactions 1968,4,35. (8) Ref. 41. and reactions of ihese mixed hydrides may be misleading. The minimum H-H distances for hydrogen atoms not attached to the same atom in (SiH3)3P and (SiH,),As are both essentially the same being ca.twice the van der Waal's radius of hydrogen. This may be an important controlling factor on the Si-M-Si angles so that it is unnecessary to search for extraordinary features in the bonding unless deviations from this distance are unusually large. This is the case for (SiH3),N in which the minimum H-H distance is much larger suggesting that here additional bonding is important particularly since the corresponding distances are smaller in Me,N and Me,P. Also as is shown (Table 2) it is only for (SiH3)3N that the M-M' bond is significantly shorter than the predicted length a further indication that additional bonding is not important in the related species. Table 6 Preparative and Reaction Routes for Disilanyl-phosphine and -arsine (a) S.D. Gokhale and W. L. Jolly Inorg. Chem. 1965,4 596. ( b ) Ref. 32; S. D. Gokhale and W. L. Jolly Znorg. Chem. 1964 3 1141. (C) J. E. Drake N. Goddard and J. Simpson Inorg. Nuclear Chem. Letters 1968 4 361. (d)A. D. Norman Chem. Comm. 1968 812. (C) Ref. 46. (f) W. L. Jolly and A. D. Norman Preparative Inorganic Reactions 1968 4 32. (8) Ref. 45. 269 Volatile Compounds of the Hydrides of Silicon and Germanium The lH n.m.r. spectra of the ternary hydrides unlike those of most binary hydrides are often first order. In addition the variations in chemical shift are fairly predictable so that species may be readily characterised. Reactions can be followed over a wide temperature range under non-destructive conditions that require only small samples.The values of the coupling constants for directly bonded nuclei (e.g. J29s,K and J31p,) 4 2 9 4 3 have been related as with 44 to the degree of s-character in the bonding orbitals and in turn to the shape of the molecule. The spectra of several silyl- and germyl-phosphines show that the value of the direct coupling constant Jp, varies only slightly from that in PH,. Thus no marked changes in 's' character and hence in the shapes of the basic skeletons are suggested for SiH3-,45 GeH3-,33 GeH,Cl-,4s or Si2H5- 46 substitution in PH3 a further indication that additional bonding need not be involved for these molecules. In a given series the chemical shift Table 7 Preparative and Reaction Routes for Germyl-phosphines -arsines and -stibines GeH Br + LiAI(MHz) \ GeH Br + KAsH / 3 E p (GeH,),E + MH (E=S Se) (0 GcH,PHz.BH3 -+ decomposes (g) b (GeH,),MH + (GeH,),M + MIf (h) MH +GeH,EH +(GeH,),E+H2E GeH,MH,.BCI +decomposes (f) GeH,I + ASH + As €I (GeH,),AsH + (GeH,),As+ AsH3 (i) (i) B H + adduct(?) (f) GeH Br i PH4Br (e) GeH I t ASH (e) ( e ) GeH Br + (SiH,),M' GeIb + polyniers e.g.[(GeH3)zP]2 GeH (e) (e) GeH,Br + Li3Sb at 20" M = P AS M' = P AS Sb.(a) D. C. Wingleth and A. D. Norman Chem. Comm. 1967 1218; Ref. 49. ( 6 ) P. Royen C. Rocktaschel and W. Mosch Angew. Chem. 1964 76 860. ( c ) J. E. Drake N. Goddard and J. Simpson Znorg. Nuclear Chem. Letters 1968 4 361. ( d ) Ref. 32. (e) Refs. 36; S. Cradock E. A. V. Ebsworth G. Davidson and L. A. Woodward J . Chem. SOC. (A) 1967 1229. (f) Ref. 74. (g) Ref. 33a. (h) Refs. 49,73. Ref. 33b. 4a J. W. Emsley J.Feeney and L. H. Sutcliffe 'High Resolution Nuclear Magnetic Resonance Spectroscopy' Pergamon Press 1966 1052. d3 G. Mavel Progr. N.M.R. Spectroscopy 1966 1 251. P4 See Ref. 42; p. 191. 46 J. E. Drake and N. Goddard J. Chem. SOC. (A) 1969 662. 46 J. E. Drake N. Goddard and C. Riddle J. Chem. Soc. (A) 1969 2074. 270 Drake and Riddle of a MH,-M’ group (M = Si or Ge; M’ =Group V element) is progressively to higher field as the electronegativity of the M’ element is decreased. Thus for the series (SiH3)3-N,47 -P -As -Sb 48 the chemical shifts are 5.56 6.08 6.20 and 6.37 T respectively and for (GeH,),-N,26 -P -As,49 -Sb 36 are 5.09 5.96 6-02 and 6-39 T. The same effect is found where the chemical shifts are known for (MH3),M’H and MH3M’H2 species.4g The tendency of the primary species to condense to the tertiary decreases below the first row element.Whereas SiH,NH is unknown SiH3PH2 SiH,AsH, and SiH,SbH disproportionate very s10wly.~~~ 5o MacDiarmid has suggested that this is associated with a decrease in the positive charge induced at silicon so that formation of the internuclear bond (7) is more difficult.2 H H H I SiH - M - SiH + MH3 etc. (7) I I I I SiH - M -+ SiH - M = .. H H Equally well the ‘availability’ of the lone-pair of P or As could be less than that of N. The trend exists for the germyl analogues but is less marked. GeH3PH condenses readily and the rates of disproportionation of both it and GeH,AsH are markedly increased by the introduction of water.49 The silyl analogues react with water immediately giving (SiH,),O and MH3.51 Condensation of the silyl species as well as the germyl is facilitated by the introduction of diborane or boron trifluoride in the liquid phase.,,* 62 Deuteriation studies of the SiH3PH2/BF3 reaction indicate that entire SiH3 groups are transferred.A four-centre mechanism (8) is possible in which H H H H P H,Si i BX = H-P + PH,BX etc. H,Si __.______.__ j \ I I H,Si - P \ I + BX = H-P-H H SiH H-p ___________ (X = H F) (8) I I SiH SiH the activation energy of disproportionation is lowered by adduct formation. 53 47 G. Rocktaschel E. A. V. Ebsworth D. W. H. Rankin and J. C. Thompson 2. Naturforsch. 1968,23B 598. 48 E. A. V. Ebsworth and G. M. Sheldrick Trans. Faraday SOC. 1966 62 3282. 48 J. E. Drake and C. Riddle J . Chem. SOC. (A) 1968 2709. C. Riddle and J. Simpson unpublished observations.61 J. Simpson Ph.D. Thesis Southampton Univ. 1967. (a) J. E. Drake and J. Simpson Znorg. Chern. 1967,6 1984; (b) J. E. Drake and J. Simpson J. Chem. Soc. (A) 1968 1039; (c) C. R. Russ Ph.D. Thesis University of Pennsylvania 1965; ( d ) C. R. Russ and A. G. MacDiarmid Angew. Chem. 1966,78,391. 68 A. D. Norman and W. L. Jolly private communication 1969. 27 1 Volatile Compounds of the Hydrides of Silicon and Germanium With the arsines there is no direct evidence for adduct formation but the condensations are indeed step-wise and disilyl- and digermyl-arsines can be isolated so (8) is probably appropriate here.62b*33b B2H6 reacts with the stoicheiometric amounts of phosphines to give the triple mixed hydrides MH3PH2BH3 (M = Si or Ge) as is confirmed by their IH n.m.r. ~ p e c t r a .~ ~ ~ With excess SiH3PH2 disproportionation occurs to (SiH,),P as expected (8) but neat SiH3PH2BH3 when sealed under pressure decomposes after about 12 hr. to give silane and an intractable polymer. The adducts formed by SiH3PH2 and SiH,AsH with boron halides also decompose to give a volatile silane and polymer possibly through the formation of an intramolecular transition state ( 9 ) . 5 2 9 5 3 There is an interesting gradation in that under similar M’H 2 / \ 2 / H3Si BX2 = SiH,X + ~/~I(M’H,BX,)~ (9) (M = P or As; X = H F C1 or Br) X conditions BF also encourages the condensation process (8); BCl gives SiH,Cl quantitatively; and BBr gives more SiH2Br2 than SiH,Br possibly through the intermediate formation of SiH2BrPH2BH,. Recently it has been shown that species MH2XPH2 can be formed during exchange processes so that SiH3PH2 reacts with GeH,X2 to form GeH,XPH (X = C1 Br).46 Thus there is experimental evidence in support of the intermediates.That over half the compounds in this section were first reported as recently as 1968 means that much is still to be learnt of their physical and chemical properties. With new preparative routes being developed the opportunities to examine them in greater depth should emerge. 3 Hydrides Containing a Group VI Element Bound to Silicon or Germanium A. Silicon Compounds.-The extensive chemistry of silicon bound to oxygen is dominated by the tendency to form linkages involving alternate Si and 0 atoms. The simple volatile ‘parents’ of these polymeric species were discovered by Stock who coined the name siloxanes.’ He isolated (SiH,),O and (Si,H,),O while subsequently the condensation reactions of the former have led to the preparation of (SiH30)$iH2 and (SiH30)3SiH.1s Despite extensive work on organo derivatives no silanol with hydrogen bound to the silicon atom has yet been characterised.SiH,OH SiH,(OH), and SiH(OH) have never been isolated and though hydrolysis of SiHzX2 (X = C1 or Br) may proceed via SiH,(OH) this then forms polymeric (H,SiO)n.l Similarly Si2H,X2 hydrolyses to high molecular weight polymers although Si,HSX gives (Si2Hs)20 possibly via Si,H50H.l The hydrolysis of most SiH,-containing species usually results in the formation of (SiH3),0 54 J. E. Drake and J. Simpson Chem. Comm. 1967 249. 272 Drake and Riddle possibly via SiH,OH. With SiH3X the yield is only ca. 40% suggesting that hydrogen halides may catalyse an alternative decomposition route of SiH,OH (1 1) reducing the amount of disiloxane formed The mode of condensation proposed for the silylamines (7) is equally applicable to this disproportiona- tion (10).H2O SiH3X -+ [SiH30H] --+ (SiH3)20 + H20 (10) (1 1) L SiH4 + (SiH20)n Disiloxane is quite thermally stable and in contrast to most SiH,- containing species is stable in air unless ignited.l (Si,H,),O more typically is spontaneously inflammable.ss In general Lewis acids cleave the S i - 0 bond (e.g. A12Cls,67 Table 8 ) while Lewis bases (e.g. NH3 68) encourage condensation. Table 8 Preparative and Reaction Routes for Silicon-Oxygen Hydrides (a) Ref. 71. ( 6 ) S. D. Gokhale and W. L. Jolly Inorg. Chem. 1965,4 596. (C) B. Sternbach and A. G.MacDiarmid J . Amer. Chem. Soc. 1961 83 3384. ( d ) A. Stock and C. Somieski Chem. Ber. 1917 50 1739. (e) Ref. 51. (f) C. H. Van Dyke and A. G. MacDiarmid Inorg. Chem. 1964,3,747. (8) A. Stock and C. Somieski Chem. Be?. 1920,53,759. (h) A. Stock and F. Zeidler Chem. Ber. 1923,56B 986. (0 Ref. 59. ( I ) Ref. 60a. (k) H. J. Emelkus and M. Onys- zchuk J. Chem. Soc. 1958 604. ( l ) Ref. 58. (m) A. Stock and C. Somieski Chem. Be?. 1923 56B 132. ( n ) M. Onyszchuk Canad. J . Chem. 1961 39 808. (0) Ref. 57. (P) Ref. 15c. (4) A. Stock C. Somieski and R. Wintgen Chem. Ber. 1917,50,1754. titi See Ref. 4 p. 127. 66 L. G. L. Ward and A. 0. MacDiarmid J. Amer. Chem. Soc. 1960,82,2151. W. A. Kriner A. G. MacDiarmid and E. C. Evers J. Amer. Chem. Soc. 1958,80 1546. T. Yoshioka and A. G. MacDiarmid Inorg.Nuclear Chem. Letters 1969 5 69. 273 Volatile Compounds of the Hydrides of Silicon and Germanium Van Dyke has shown5@ that the amphoteric phosphorus(II1) halides cleave SiH,OMe at low temperature but not (SiH,),O indicating a reduction in Lewis basicity with additional silyl substitution as is also found for the series Me,N through to (SiH3)3N.15b With B,H 6o and Me,Ga 15d as reference acids it is clear that (SiH,),O is a weaker nucleophile than Me,Q but stronger than (SiH,),N. This is expected since the result of several investigations indicate that the skeletal Si-0-Si angle is ca. 144°,61 (Table 9). A marked degree of ( p 4 d)n-bonding is implied by such a wide angle although it would need to be 180” to make the lone-pairs as ‘unavailable’ as in trisilylamine. Table 9 Molecular Parameters for (MM3)2M’ (M=Si Ge; M’=O S Se) Molecule rM-M‘(A) rM-H (A) MG MO Predicted Ref.(SiH,),O 1.634 1.486 144.1 1 -75 a (SiH3)2S 2-1 36 1 -494 97.4 2.14 b (SiH,),Se 2.273 1.516 96.6 2.28 C (GeH,),O 1.774 1.53 125.6 1.83 d (GeH,),S 2-205 1.53 99.1 2.21 d (a) Ref. 61. (b) Ref. 62. (C) Ref. 63. (d) Ref. 9c. r M-M‘(A) By contrast the skeletal angles in (SiH,),S 62 and (SiH,),Se 63 are close to go” and as with (SiH,),P and (S~H,),AS,~~ the H-H distance for hydrogen atoms on adjacent silicons is ca. 2 A. Despite surface tension measurements which were interpreted as indicating a large amount ofn-bond character in the Si-S linkages,64 the Si-S bond-length is as expected for a single bond.62 Long-range H-H coupling is observed in the lH n.m.r. spectrum of (SiH3)2S.65 Since such coupling is not observed with (SiH,),O it has been suggested that it may be associated with d-orbitals on sulphur rather than with ( p - 4 ) ~ - bonding.Support comes from a recent survey on the absolute values of J, coupling which increases along the series of C-S-M (M C 4 Si+ Ge) linkages (Table The trend in chemical shifts as with the group V ternary hydrides is for the MH resonance to shift progressively to higher field when the electronegativity of the M’ atom is decreased. Thus for the series 59 C. H. Van Dyke J. Iiirorg. Nuclear Chem. 1968 30 8 I . 6o (a) S . Sujishi E. L. Gasner and A. D. Payton 133rd Meeting Amer. Chem. SOC. 1958 Communication 524.; ( 6 ) S. Sujishi E. L. Gasner and A. D. Payton Ofice of Ordnance Research Project No. TB2-0001 (817). 61 A. Almenningen 0.Bastiansen V. Ewing K. Hedberg and M. Traetteberg Acta Chent. Scand. 1963 17 2455. 62 A. Almenningen K. Hedberg and R. Seip Acta. Chem. Scand. 1963 17 2264. 63 A. Almenningen L. Fernholt and H. M. Seip Acta Chem. Scand. 1963 22 51. 64A. G. MacDiarmid J. Inorg. Nuclear Chem. 1956 2 323. g6 J. T. Wang and C. H. Van Dyke Chem. Comm. 1967 612. E. A. V. Ebsworth and J. J. Turner J. Chem. Phys. 1962,36,2628. 274 Drake and Riddle (SiH,)2-0,67 -S -Se and -Te68 the chemical shifts are 5-39 5.65 5.91 and 6.33 7- respectively. The same trend is found for (GeH,),M' SiH,M'H and GeH M 'H . * Table 10 Long-Range Coupling in some Sulphide Systemsss JER (Hz) 0.30 0.45 0.70 0.60 not observed Compound Me,§ SiH,Me (SiH,),S GeH,SMe (GeH,),S Studies on the Si-S -Se and -Te systems have centred around the nature of the heavy-atom bonding.The syntheses (Table 11) are mainly from earlier preliminary work although like the Group V mixed hydrides there is a current revival of interest. Recently (SiH,),Te was prepared from SiH,I and Li2Te 69 while (SiH3)&3 and (SiH,),Se were prepared from the cleavage of the Si-P bond in (SiH,),P by S and H,§e respectively (Table 5).41 There is apparently no direct action of silyl halides with H2S although Stock reported a volatile product possibly SiH,SH when SiH,C1 reacted with H,S at 150" for several Table 11 Preparative and Reaction Routes for Silicon-Sulphur and Silicon- Selenium Hydrides 'I involatile solids (el Refs. 35 71. ( b ) Ref. 17. ( c ) M. Onyszchuk Canad. J. Chem. 1961 39 808. ( d ) Ref. 60a. (e) A. G. MacDiarmid J.Inorg. Nuclear Chem. 1963,25,1534. (f) Ref. 4. E. A. V. Ebsworth and J. J. Turner J. Phys. Chem. 1963 67 805. 6sC. Glidewell D. W. H. Rankin and G. M. Sheldrick Trans. Faraday SOC. 1969 65 1 409. 69 H. Burger and U. Goetze Innorg. Nuclear Chem. Letters 1967 3 549. 275 Volatile Compounds of the Hydrides of Silicon and Germanium days in the presence of A12C16.70 A conversion series2 which indicates which silyl compound can be converted specifically into another by means of a silver or heavy-metal salt is partially shown (12). The hydrolysis of (SiH,),S gives (SiH3)20 in over 90% yield71 which is twice as good as the yield from the hydrolysis of the halides discussed previously. This suggests either that the former reaction is a simple exchange process not involving the intermediate formation of silanol (lo) or that H2S is not acting as a catalyst for the polymer formation (ll).66 Several reactions of (SiH3),S indicate that it is a weak electron donor weaker even than (SiH3)20 with BF3 as reference acid.? It is concluded however that the Si-S bond is an essentially single bond of mainly p-charac t er.SiH,I + (SiH,),Se -+ (SiH3)2S + SiH,Br - SiH3Cl -+ (SiH3)20 (12) B. Germanium Compounds.-The introduction of two preparative routes making use of (i) reactions with H2S S H,Se and Se 49,73,74 and (ii) exchange reactions76 has enlivened interest in the germanium-Group VI mixed hydrides. The former reagents react with either germyl-phosphine or -arsine to give (GeH,),S and (GeH,),Se via GeH,SH and GeH,SeH as is proved by lH n.m.r. spectroscopy. The exchange reactions of (SiH,),Se and (SiH,),Te with germyl- bromide give (GeH,),Se and (GeH,),Te (Table 12).The hydrolysis of organo-germanes R,GeX gives (R,Ge),O except in a few cases e.g. for R = Ph PhCH2 and Pri when R,GeOH is formed. However GeH,OH has not been detected and although GeH,M'H (M' = S,49 Se,? Te 6a) disproportionate to (GeH,),M' and M'H2 very readily there is less tendency towards disproportionation below the first row elements as was noted with the Group V species. The attempt by Dennis and Work in 1933 to prepare (GeH,),O by hydrolysis of GeH,CI 76 failed to yield a volatile product presumably because even if (GeH,),O forms it is unstable and its condensation to polymeric species is probably water ~atalysed.~? Condensation is evident to a small extent in (GeH,),Se leading to (GeH,Se),GeH and probably (GeH ,Se) ,GeH and (GeH ,Se) 4Ge,73 The observed PR separations in the i.r.spectra of (GeH,),O and (GeH,),S have been used to calculate the skeletal angles as 11 1 ~ ~ X " and 1 16_+,2:" re- spectively which was interpreted as indicating a lack of n-bond character in both species.?' Recently a more detailed i.r. study using a Valence Force Field approach estimated the apex angles as 139" and 100" re~pectively,~~ whilst 70 A. Stock and C. Somieski Chem. Ber. 1923 56B 247. 71 H. J. Emelbus A. G. MacDiarmid and A. 0. Maddock J. Inorg. Nuclear Chem. 1955 1 194. 78 J. E. Drake and C. Riddle J. Chem. SOC. (A) 1969 1573. ?* C. Riddle Ph.D. Thesis Southampton Univ. 1969. 76 L. M. Dennis and R. W. Work J . Amer. Chem. SOC. 1933,55,4486. 17 T. D. Goldfarb and S. Sujishi J.Amer. Chem. SOC. 1964 86 1679. ?*S. Cradock J . Chem. SOC. (A) 1968 1426. M. Onyszchuk Canad. J. Chem. 1961,39,808. S. Cradock E. A. V. Ebswotth and D. W. H. Rankin J. Chem. SOC. ( A ) 1969 1628. 276 Drake and Riddle Table 12 Preparative and Reaction Routes for Germanium-Group VI Hydrides E = S,Se 2 (a) GeH31 H 2 0 10 hrs 25' Ic) GeHzCN S. Sujishi and W. Ando U.S. Dept. Corn. Office Tech. Serv. PB Rept. 143 572 1959 93; S. Sujishi Abstracts of XVIIth Intern. Congr. of Pure and Appl. Chem. 1959 53. ( b ) Refs. 52 73. (C) T. D. Goldfarb and S. Sujishi 140th Meeting Amer. Chem. SOC. 1961 Com- munication N92. @ ) S . Cradock and E. A. V. Ebsworth J. Chem. SOC. (A) 1968 1422. (e) T. D. Goldfarb and S. Sujishi J. Amer. Chem. Soc. 1964,86,1679. electron diffraction measurements now place the angles at 126" and 99" respecti~ely.~c That the bond length in the oxide is shorter than the predicted covalent bond-length as in disiloxane coupled with the higher angle is indicative that some multiple bond character is present.The H-H minimum distance in the sulphide is about twice that of the van der Waal's radius of hydrogen (ca. 2.2 A) whereas in the oxide it is ca. 2.7 A. Thus as was seen in Group V the simple stereochemical consideration is adequate for elements below the first short period but for nitrogen and oxygen it is reasonable to consider some degree of wbonding. 4 Conclusion The literature reflects a rapid growth of interest in ternary hydrides. Although extensive chemical properties are largely unknown it is some compensation that fairly detailed lH n.m.r.and vibrational spectroscopic studies for which these hydrides are particularly suited have been made. However caution is essential in interpreting this data to give physical parameters. Bonding theories have provoked discussion but at the moment it is prudent to merely indicate trends and allow speculation to stimulate further studies. Hopefully as more researchers accept the vacuum techniques that are standard for the study of these volatile hydrides more synthetic routes will be discovered and their chemical and even fairly simple physical properties examined in greater depth. 277

ISSN:0009-2681    

DOI:10.1039/QR9702400263

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Trimethylenemethane and Related 01 a’ = Disubstituted Is0 bu tenes * By Francis Weiss 69 P I E R R E - B ~ N I T E F R A N C E UGINE KUHLMANN C E N T R E D E R E C H E R C H E S D E LYON 1 Introduction Formally trimethylenemethane is a diradical (I) with one methylene group attached to the central carbon atom by a double bond the other two being single bonded and bearing an unpaired electron. It thus constitutes the backbone of compounds of general structure (2) and (3) which are a a’-disubstituted derivatives of isobutene. Trimethylenemethane has received much theoretical attention. It has been found by calculation’ and experiment2 to be in a triplet ground state with a strongly delocalised 4.rr-electron system. The interesting consequence of the three- fold symmetry of the structure is that all the resonance structures are equivalent diradical species which distinguishes this n system either from the related straight chain 4.rr-electron system of butadiene (4) an isomer of (l) forming two con- jugated double bonds as a stable resonance structure or from the cyclic 4n-electron system of cyclobutadiene (5).The latter presents a triplet diradical character but can also appear in the form of two double bonds (Scheme 1). *This article is based on lectures given at Manchester Symposium on ‘New Olefin Reac- tions’ on June 26 1968 at Miilheim Ruhr on November 21 1968 and at Strasbourg on March 14 1969. ’ (u) W. E. Moffitt footnote in C. A. Coulson J. Chim. phys. 1948,45 243; (b) W. Moffitt Trans. Furuduy SOC. 1949,45,373; (c) H. C. Longuet-Higgins J. Chern. Phys. 1950,18,265; ( d ) H.H. Greenwood Trans. Furaduy SOC. 1952,48 677; (e) J. D. Roberts A. Streitwieser jun. and C. M. Regan J. Amer. Chem. SOC. 1952 74 4579; (f) J. D. Roberts ‘Notes on Molecular Orbital Calculations,’ W. A. Benjamin New York 1961 p. 56; (g) A. Streitwieser jun. ‘Molecular Orbital Theory for Organic Chemists’ J. Wiley New York 1961 p. 43 57; (h) H. M. McConnell J. Chem Phys. 1961,35 1520; ( i ) A. D. McLachlan Mol. Phys. 1962 5 51; 0‘) D. P. Chong and J. W. Linnett ibid. 1964,8 541; 1. Chem. SOC. 1965,1798; ( k ) J. G. Burr and M. J. S. Dewar J . Chem. SOC. 1954 1201. * (a) P. Dowd J. Amer. Chem. SOC. 1966,88 2587; (6) P. Dowd ana K. Sachdev ibid. 1967 89 715; (c) P. Dowd A. Gold and K. Sachdev ibid. 1968,90 2715. 278 Francis Weiss \ coupling products Scheme 1 Equivalency of diradical resonance structures will disappear by unsymmetrical substitution of one or more hydrogens in (1).The only way for (1) to achieve stabilisation is by forming o-bonds with the unpaired electrons thus losing the 4~electron structure. Depending upon the electronic structure the o-bond may be formed intramolecularly leading to methylenecyclopropane (6) or intermolecularly giving various coupling pro- ducts. Species of this kind have been postulated as intermediates both in the formation and also in several rearrangements of methylenecy~lopropanes.~ But like other highly reactive compounds e.g. cycl~butadienes,~ trimethylene- (a) E. F. Ullman J . Amer. Chem. SOC. 1959,81,5389; 1960,82,505; (b) E. F. Ullman and W. S . Fanshawe ibid. 1961 83 2379; (c) H. M. Frey Trans.Faraday SOC. 1961 57 951; (d) J. P. Chesick,J. Amer. Chem. SOC. 1963 85,2720; (e) T. C. Shields B. A. Shoulders J. F. Krause C. L. Osborn and P. D. Gardner ibid. 1965 87 3026; (f) A. C. Day and M. C. Whiting J. Chem. Sac. (C) 1966,464; Proc. Chem. SOC. 1964 368; ( g ) S. D. Andrews and A. C. Day Chem. Comm. 1966,667; (h) J. K. Crandall and D. R. Paulson J . Amer. Chem. SOC. 1966 88 4302; ( i ) R. J. Crawford and D. M. Cameron ibid. 1966 88 2589; ( j ) T. Sanjiki H. Kato and M. Ohta Chem. Comm. 1968,496; (k) W. R. Dolbier jun. Tetrahedon Letters 1968 393; ( I ) J. J. Gajewski J. Amer. Chem. SOC. 1968 90 7178; (m) T. Sanjiki M. Ohta and H. Kato Chem. Comm 1969 638. (a) G. F. Emerson L. Watts and R. Pettit J. Amer. Chem. SOC. 1965 87 131 ; (b) M. P. Cava and M. J. Mitchell ‘Cyclobutadiene and Related Compounds’ Academic Press New York London 1967.279 Trimethylenemethane and Related a a'- Disubstituted Isobutenes methane species can be successfully trapped in complexes with transition-metal compo~nds,~ a field of investigation which raises considerable interest. Outstanding characteristics of compounds (2) are the allylic nature of both substituents and their 1,3 relative position which allows a number of addition substitution and cyclisation reactions. But the combination of these structural features sometimes entails unusual behaviour. Thus a number of cyclisation reactions of 1,3-~ubstituted compounds to small ring compounds while being easy with saturated derivatives fails with compounds of type (2).6 The threefold symmetry of (l) from which they are precursors gives to compounds (2) and (3) very interesting possibilities in the numerous reactions involving allylic re- arrangement.This comes from the fact that principally a double rearrangement can occur since if we consider a stepwise sequence the first rearrangement dis- places the double bond to a position which is equivalent to the original one so that a second rearrangement can take place with the other substituent (Scheme 2). Such rearrangements seem to have both synthetic and mechanistic value X A Y Y Y' Scheme 2 ti (a) G. F. Emerson K. Ehrlich W. P. Giering and P. C. Lauterbur J. Amer. Chem. Soc. 1966,88 3172; (b) A. Almennigen A. Haaland and K. Wahl Chem. Comm. 1968 1027; (c) M. R. Churchill and K. Gold ibid. 1968,693; ( d ) A. C. Day and J. T. Powell ibid. 1968 1241; (e) R.Noyori T. Nishimura and H. Takya ibid. 1969 89; (f K. Ehrlich and G. F. Emerson ibid. 1969 59; G. F. Emerson K. Ehrlich W. P. Giering and D. Ehntholt Trans. New York Acud. Sci. 1968,30 1007. (a) D. E. Applequist and J. D. Roberts J. Amer. Chem. Soc. 1956 78 4012; (b) I. D'Yakonov J. Gen. Chem. (U.S.S.R.) 1940 10 402; (c) J. T. Gragson K. W. Greenlee J. M. Derfer and C. E. Boord J. Amer. Chem. SOC. 1953,75,3344; ( d ) R. G. Doerr and P. S . Skell ibid. 1967 89 3062; P. S. Skell and R. G. Doerr ibid. 1967 89 4688. 280 Francis Weiss especially in the case of cyclic rearrangements like Cope and Claisen’ rearrange- ments or 1,5-transfers of hydrogen atoms.8 Scheme 2 indicates the scope of this chemistry and emphasises its specific interest. Despite this obvious interest the chemistry of trimethylenemethane and related compounds has been little studied until recent years so that the available knowledge is still fragmentary a situation which should stimulate further work.2 Nomenclature No name can adequately express the true structure of three methylene groups of equal resonance and the absence of a free valence at the central carbon atom. The systematic name 2-me t h y lenet r imet h y lene discriminates between the three peripheral CH groups whilst trismethylenemethyl suggests some reactivity at the central carbon atom. The completely trivial name trimethylenemethane is therefore used in this article as being without structural significance. Compounds (2) are 2-methylene-l,3-~ubstituted propanes or 2,3-substituted propenes while compounds (3) are met h ylene-subs t i t u t ed heterocyclic com- pounds.Yet these systematic names do not suggest the close relationship nor do they point out the real structural characteristics of these compounds. They are to be considered as a,a’-disubstituted isobutenes and this term may be a generic one for this class of compounds. 3 Preparation of a&-Disubstituted Isobutenes Ally1 halides and allyl alcohol are the basic intermediates from which most other allyl compounds can be obtained by a proper substitution reaction. In the same way the dichloroisobutene (8) and the isobutenediol (15) are convenient inter- mediates for the preparation of other a,a’-disubstituted isobutenes. The methods of preparation can therefore be divided into two groups those involving building up the structure and those which are simple substitution reactions starting from other a,a’-disubstituted isobutenes.The latter group will be more conveniently discussed in the section on chemical properties. The simplest method of preparing these compounds is from isobutene itself by substituting each methyl group. This can be done by chlorination either directl~,~ or better in two steps via methallyl cloride (7).1° However even the two-step method gives poor results because the chlorination of methallyl chloride is unselective giving only 20-30 % of (8),11 together with equal amounts (a) F. Weiss and A. Isard Bull. SOC. Chim. France 1967 2033; (b) A. hard and F. Weiss ibid. 1967 2038; (c) J. P. Schirmann A. Isard and F. Weiss Tetrahedron 1968,24 6475. (a) F. Weiss A. Isard and R. Bensa Bull. SOC. chim.France 1965,1358; (b) J. M. Morgen ThBse Universitk de Strasbourg 1966. (a) A. 0. Rogers and R. E. Nelson J. Amer. Chern. SOC. 1936,58 1029; (6) I. D’Yakonov and D. Tishchenko J. Gen. Chem. (U.S.S.R.) 1939,9 1258. l o (a) J. Burgin G. Hearne and F. Rust Ind. Eng. Chem. 1941 33 385; (b) L. F. Hatch J. J. RUSS and L. B. Gordon J . Amer. Chem. SOC. 1947 69 2614; (c) W. S. Ropp C. W. Gould H. M. Engelmann and G. E. Hulse ibid. 1951,73 3024; ( d ) See also ref. 6a 6c 6 4 and 60. For Toxicity of (8) see V. V. Stankevitch Gig. Tr. Prof. Zabol. 1968 12 (l) 33-7 (Chenr. Abs. 1968 68 8254). 28 1 Trimethylenernethane and Related a a I- Disubstituted Isobutenes of the close-boiling isomeric 1,3-dichlor0-2-methylpropene (9) and of the saturated trichloro-compound (10). Several routes based on degradation reactions of pentaerythritol halohydrins12 and of tri(hydro~ymethy1)nitrornethanel~ may be used to prepare (8) or (1 5) but they are often cumbersome and give low yields.A three-step method giving good overall yields starts from acrolein (ll) which is first treated with cyclopentadiene (12)14 to give the Diels-Alder adduct (13). The unreactive a-hydrogen atom of acrolein becomes reactive in (13) while the double bond is protected and this permits acrossedCannizzaro reaction with formaldehyde giving 2,2-di(hydroxymethyl)-5-norbornene (14).16 Iso- butenediol(l5) (2-hydroxymethyl-2-propen-1-01) is finally obtained by pyrolysis of (14) with recovery of the cyclopentadienel6 (Scheme 3). Other dienes can be used instead of cyclopentadiene for example anthracene,16 which will give the adduct (17) or even acrolein itself.” The di(hydroxymethy1) adduct (17) can also be prepared by reducing the anthracene adduct of diethyl methylenemalon- ate (16) with lithium aluminium hydride.6a The adducts (14) and (17) can be converted into a number of derivatives such as carboxylic esters ethers acetals etc.as a route to the corresponding isobutenediol derivatives for the pyrolysis step.ls Applequist and Roberts6a have thus prepared 3-methyleneoxetan (19) by pyrolysing the oxetan (18) which was obtained from (17) instead of small ring compounds in attempts to cyclise a,a’-disubstituted isobutenes (Scheme 4). Little indication exists as to the suitability of these methods for making homo- logues which seems to be limited by the decreasing reactivities in the different steps.Thus only moderate yields of 2-hydroxymethyl-2-buten-1-01 (20) are obtained when crotonaldehyde is used instead of acrolein.lS la (a) A. Mooradian and J. B. Cloke J . Amer. Chem. SOC. 1945 67 942; (6) R. Lukes and J. Plesek Chem. Zisfy 1955,49 1826 (Chem. Abs. 1956,50,9288); (c) C. Issidorides and A. I. Matar J . Amer. Chem. SOC. 1955 77 6382; ( d ) A. S. Matlack and D. S . Breslow J. Org. Chem.. 1957,22 1723; (e) F. Nerdel Ber. 1958,91,938; (f) K. Watanabe T. Sugihara and M. Tanaka Yuki Gosei Kagaku Kyokai Shi 1966,24 651 (Chem. Abstr. 1965 65 20082); K. Watanabe and T. Sugihara Makromol. Chem. 1966,99,141. la H. Kleinfeller Ber. 1929 62 By 1582 1590. l4 0. Diels and K. Alder Annalen 1928,460,98. l6 H. A. Bruson W. D. Niederhauser and H. Iserson U.S. P. 2,417,100/1947 (Chem.Abs. 1947 41 3819). l6 F. Weiss A. Isard and R. Bensa Bull. SOC. chim. France 1965 1355; Ugine Kuhlmann F. P. 1,350,723/1962 and 1,387,099/1963. l 7 C. W. Smith ‘Acrolein’ J. Wiley New York 1962 p. 198. 282 Francis Weiss (13) 95-100% f CH,O/OH- 1 CH OH CH OH ,CH2 OH CH,-C ‘CH20H +o ‘n (15) 90-9576 (14) 90% Scheme 3 -+____+ CH,-CC -)o ‘CH ‘CH OH V‘ Scheme 4 Another precursor of (i) 4-methylene-1-pyrazoline (21) can be prepared by the reaction of diazomethane with allene at room ternperat~re.~a~~~~~* This product is highly sensitive to water heat light and oxygen and is best handled in vacuo.2a Similarly 3-substituted 4-methylene-1-pyrazolines (22) can be obtained from substituted diazomethanes and allene.3m lo I. D’Yakonov J. Gem Chem. (U.S.S.R.) 1945,15,473. 283 Trimethylenemethane and Related a a'- Disubstituted Isobutenes 4 Substitution and Addition Reactions Practically no kinetic or mechanistic studies of the reactions of cc,a'-disubstituted isobutenes have been published so far.Broadly speaking substitutionand addition reactions will be normal for allylic compounds of this kind and will give in the case of substitution the new disubstituted product. It appears also possible to limit some reactions to monosubstitution and thus to obtain mixed disub- stituted isobutenes. For example 2-chloromethyl-2-propen-l-ol(23) can be prepared in fairly good yield from gaseous hydrogen chloride with (15) in dioxan,lga and (2-chloro- methy1)allyl phenyl ether (24) is obtained from stoicheiometric amounts of (8) phenol and sodium hydroxide.19b ,CH* c1 'CH OH CH,=C Various intramolecular eliminations which should give small 3- or 4-membered ring compounds are difficult or even impossible.The problem of the synthesis of 3-methyleneoxetan (19) has been mentioned and that of methylenecyclo- propane will be considered later. There is a strong difference with saturated 1,3-disubstituted compounds which readily undergo such cyclisation reactions. How the double bond of a,a'-disubstituted isobutenes obstructs the cyclisation step is a problem which remains to be studied the large bond angle of 120" on the central carbon atom,sa as well as resonance structures of the intermediates certainly are important factors. Substitution reactions which have been described include halogen ex- change,6de20 hydro1ysis,l2a and etherificatiorP reactions of dichloroisobutene @) and esterification,l2cJs or polyesterification,21 of isobutenediol (1 5).Hydrogen cyanide reacts with (8)22 or (15)19a to give 3-methyleneglutaronitrile (25). Dichloroisobutene's reactions with metals and organometallic compounds will be considered in detail in section 5. Two alkylation reactions should be men- tioned the reaction of (8) with cyclopentadienylsodium gives the biscyclo- (a) A. Isard and F. Weiss unpublished results; (b) F. Pt. 1,503,900/1966. 8 o N. V. de Bataasche Petr. Maat. B. P. 672,757/1952. *l (a) T. W. Evans U.S. P. 2,435,429/1948; (b) W. Gumlich and G . Schaefer (Huls) 0. P. 1,012,457/1957; (c) K. W. Doak and H. N. Campbell J. Polymer Sci. 1955 18 215. a x (a) R. C. Schreyer U.S. P. 2,609,385/1952; (b) P. Kurtz and 0. Bayer G.P. 925,774/1955. 284 Francis Weiss pentadienylisobutene (26) which is in thermal equilibrium with one or more intramolecular Diels-Alder a d d ~ c t s . ~ ~ In a similar manner (8) reacts with .rr-allylnickel complexes [e.g. (2711 to give products like (28).24 CH2CN CH2-C' 'CH,CN Hydrogenation of (8)13 and (1 5)26 proceed normally but catalytic hydrogena- ation of S-methylene-l,3-dioxans (29) over palladium has been reported to be stereoselective giving the less stable cis-isomer rather than the trans-isomer of the saturated dioxan (30) (Scheme 5).2s A co-ordination of the oxygen atoms to the catalyst probably causes this selectivity. H (29) H H cis (93-95%) trans (5'7%) (30) R = Me But Ph Scheme 5 Chlorine and bromine add on to the double bond of (8)13 or (15)l9a but with a marked steric hindrance effect.Thus the following relative rates of halogena- A. Lakodey and F. Weiss unpublished results. J. E. Anderson F. G. Riddell J. P. Fleury and J. M. Morgen Chem. Comm. 1966 128. p4 E J. Corey and M. F. Semmelhack Tetrahedron Letters 1966 6231. * 6 F. Weiss and A. Lantz F. P. 1,398,960/1964. 285 Trimethylenemethane and Related 01 x'- Disubstituted Isobrrtenes tion have been determined :27 ethylene I allyl chloride 0.45 (8) 0.055. In reverse epoxidation is faster than for unsubstituted allyl compounds. Dichloroiso- butene (8) is easily epoxidised by carboxylic peracids (e.g. p-nitroperbenzoic acid19a and performic acidz8) to 2-chloromethylepichlorhydrin (31), while 2-(hydroxymethy1)glycidol (34) is obtained in 90 % yield by epoxidation in solution of (15) with aqueous hydrogen peroxide and sodium t ~ n g s t a t e .~ ~ The value of the rate constant for epoxidation of (15) is close to the published values for crotyl and methallyl alcohols,30 showing a similar activating effect of hydroxy- methyl and methyl groups (Table 1). Table 1 R1 RZ R' R2 \ I H202-Na2WO4 ,/ v -+ C - C-CHzOH \ I / C=C-CH,OH H O H pH5 20 "C R' R2 k1(min-l) x lo4 Ref. H H 3.2 30 Me H 71 30 H Me 45 30 H CH2OH 58 29 Chloromethylepichlorhydrin (31) which can be also obtained by the reaction of diazomethane with 1,3-dichlor0-2-propanone,~~ reacts with potassium cyanide to give 3-cyanomethyl-3-hydroxy-glutaronitrile (33),31 and can be polymerised to a high molecular weight crystalline polyether (32) with triethylaluminium catalysts.32 Hydrolysis of (34) and direct hydroxylation of (15) with hydrogen peroxide and tungstic(vr) easily yield isoerythritol(35) which is formed also but impure in the hydrolysis of by-products from the chlorination of methallyl The Prins reaction with formaldehyde in acid works well with (8) and gives 4,4-di(chloromethyl)-1,3-dioxan (36),35 whereas with (1 5 ) which is of poor B.E. Swedlung and P. W. Robertson J . Chem. SOC. 1947,630. A. Lakodey A. Isard and F. Weiss unpublished results. (a) Z. Raciszewski J. Amer. Chern. SOC. 1960,82 1267; (b) H. C. Stevens and A. J. Kaman F. Johnson J. P. Panella and A. A. Carlson J . Org. Chem. 1962 27 2241. S. Kambarra and A. Takahashi Makromol. Chem. 1962,58 226. F. Weiss and A. Isard Bull. SOC. Chim. France 1965 1364. G. Hearne and H. W. de Jong Ind. Eng. Chem. 1941,33,940.'* E. Steininger U.S. P. 3,176,026/1962; Cliem. Abs. 1965 63 1775. 'I A. Isard and F. Weiss F. P. 1,464,139/1965. ibid. 1965,87 734. 286 Francis Weiss CH CN I CH C1 HO-C-CH CN - f C H - L & CH I C1 CH I CN CH OH ,CH2 OH 1 HO-C-CH OH I CH -C \' / 'CH OH 0 CH OH stability in strong acid complex mixtures are obtained in which the spiro- bisdioxan (37) has been identified.lga 3-Methyleneoxetan (19) has a planar structure according to the infrared and Raman Berezin3' studied its chemical properties particularly the thermal cycloaddition of tetrafluoroethylene giving the fluorinated oxetan (38) the epoxidation with carboxylic peracids to the epoxyoxetan (39) and the hydroxylation with hydrogen peroxide to the dioI (40) which was oxidised with periodic acid to the very reactive 3-oxetanone (41).Almost nothing is known about free-radical reactions in this field. Poly- merisation certainly is difficult as it is for most allylic compounds. Benzoyl- peroxide-induced reaction of the dichloroisobutene (8) mainly gives a dimer in 33 % yield and trimers in 43 % yield; this is probably due to steric hindrance and s(l J. R. Durig and A. C. Morrissey J. Chem. Phys. 1966,45 1269. 37 G. H. Berezin S 161 152nd ACS Meeting Sept. 1966 Abstracts of Papers; U.S. P. 3,297,719/1967. 287 Trimethylenemethane and Related a a'- Disubstituted lsobutenes to chain-transfer.lOC Extremely low yields are found on the other hand in the free-radical additions of carbon tetrachloride to isobutenediol (1 5),38 and of butyraldehyde to the diacetate of (15).3B Nevertheless polyesters of (15) are said to be crosslinked by copolymerisation with styrene.21b 5 Allylic Rearrangements A.Claisen-type Double Rearrangements.-While these reactions can occur step-wise by two successive normal rearrangements there is an intriguing R2 I I R2 CH2 I U I R1-C-COO-CH2-C-CH2--OOC-C-R' H H R1 R2 (43) (44) R1=R2 = Me 38%yield = Ph 68% " R' +R2 = CH;! -CH=CH-CH CH2 50% Scheme 6 A. Lantz and F. Weiss unpublished resuIts. a' B. P. 635,934/1950; Chem. Abs. 1950,44,7867. 288 Francis Weiss possibility that compounds of structure (2) might undergo a direct double rearrangement through the single bicyclic transition state suggested in Scheme 2 which is related to the ‘trimethylenemethane’ structure of the skeleton. Mech- anistic studies should be done to elucidate this interesting point.The ease with which the reactions already investigated proceed and some qualitative arguments suggest that such a one-step process may occur at least in part. The known rearrangement of ally1 carboxylic esters to y-ethylenic acids by reaction with sodium meta1,4Oor sodium hydride,*l has been successfully extended to the rearrangement of isobutenediol carboxylates (42) to y-methylenepimelic acids (45).7b Reaction of the esters (42) with sodium hydride presumably gives the bis-sodium enolates (43) which rearrange to the sodium pimelates (44). Acidification of the reaction mixture then liberates the free acids (Scheme 6). I (49) (52) Scheme 7 (53) 40 R. T. Arnolds and S. Searles J. Amer. Chem. Soc. 1949,71 1151. S. Julia M. Julia and C. Linstrumelle Bull. SOC. Chim. France 1966 3499.(a) K. C. Brannock H. S. Pridgen and B. Thompson J . Org. Chern. 1960,25 1815; (6) 289 Trimethylenemethane and Related 01 01‘- Disubstituted Isobutenes Thermal rearrangement of a,a’-bisaryloxyisobutenes7a is an application of the well known Claisen rearrangement of allyl phenyl By heating a,”- diphenoxyisobutene (46) at 200 “C in N,N-dimethylaniline cc,or’-bis-o-hydroxy- phenylisobutene (47) is obtained in 66% yield. In the absence of a basic solvent an 80% yield of coumaran (48) is obtained by a cyclisation of (47) during the treatment. This clean and rapid cyclisation which compares with those often observed during allyl phenyl ether rearrangement^,^^@^^^ could also occur with a monorearrangement product (49) to give a coumaran (50) and thus prevent the rearrangement of the second phenyl ether group (Scheme 7).As no appreciable amounts of monorearrangement products are formed one can infer that both steps are closely related and may very weH merge into one another with a quasi-chair-chair bicyclic transition state (51) which appears to favour easy migration according to recent stereochemical studies of the Claisen re- arrangement .428944 Double para-rearrangement occurs as expected when the orrho-positions are substituted. Thus the bis-(2,6-dimethylphenoxy)isobutene (52) gives the para- para’bisphenol (53) in 73 % ~ i e l d . ~ a * ~ ~ In 3-methylene-l,5-benzodioxepan (54) the allylic double bond is blocked by the seven-membered ring at a distance from the aromatic ring which makes unlikely a concerted bond breaking and formation process.Nevertheless the rearrangement proceeds smoothly at 200 “C to give the cyclohexadienone derivative (56) which suggests a step-wise Claisen rearrangement via a diradical ( 5 5 ) . ’ c ~ ~ ~ The intermediate (56) has not been isolated; it reacts during the treatment with starting compound (54) to give an adduct (57) or with other dienophiles if the reaction is effected in their presence [e.g. with acrylonitrile giving (58) or maleic anhydride giving (59) (Scheme S)]. When the product (54) is heated at 450 “C in the vapour phase 2-(4’-methyl-2’- furyl)-2-cyclopenten-l-one (63) and 3a,4,7,7a-tetrahydro-5-methyl-1,7-inden- edione (64) are formed in 39 and 10 % yields respectively. This result has been interpreted in terms of a secondary free-radical rearrangement of the cyclo- hexadienone (56) (Scheme 8).The diradical (60) resulting from ring opening cyclises to give the diradical (61) which can proceed to structure (63) by mere hydrogen shifts and to structure (64) by cyclisation and hydrogen shift. Thus the mechanism of this reaction would be closely related with that of thermal and “ ( a ) D. S. Tarbell ‘The Claisen Rearrangement in Organic Reactions’ ed. R. Adams J. Wiley New York 1944 vol. 11 p. 1 ; (b) S. J. Rhoads ‘Rearrangements Proceeding through “No Mechanism Pathways”,’ in ‘Molecular Rearrangements’ ed. P. de Mayo Interscience New York 1963 vol. I p. 655. 43 C. D. Hurd and W. A. Hoffmann J. Org. Chem. 1940,5,212. ‘* B. Capon M. J. Perkins and C. W. Rees ‘Organic Reactions Mechanisms (1965),’ Inter- science London 1966 p. 173. 46 For a similar double para rearrangement of bis-l,4-(2,6-dimethylphenyl)-2-butene the linear isomer of (52) see B.S. Thyagarajan K. K. Balasubramanian and R. Bhima Rao Chem. and Znd. 1967,401. 4’ For an interesting case of a retro-Claisen rearrangement in a similar system see (a) M. F. Ansell and V. J. Leslie Chem. Comm. 1967 949; (b) A. Jefferson and F. Scheinmann Quart. Rev. 1968 22 409. 290 Francis Weiss - 2000 - 450' *-& 0 a Scheme 8 photochemical isomerisations of o-cyclohexadienones to bicyclo[3,1,01 hex en one^.^' A bicyclo[3,1 ,O]hexenone (62) has however not been found; its formation at high temperatures is unlikely but it may be an intermediate in the reaction to (63). Ring opening of (56) to a keten which would be a reso- nance structure of (60) could also be considered although such a keten has not been detected during pyrolysis in the presence of methanol.7c '' (a) A.J. Waring in 'Advances in Alicyclic Chemistry' ed. H. Hart and G. J. Karabatsos Academic Press New York 1966. vol. I p. 241; (b) J. Griffith and H. Hart J. Amer. Chem. SOC. 1968,90 3297. 29 1 Trimethylenemethane and Related a a'- Disubstituted Isobutenes B. Rearrangements involving Hydrogen Transfer.-Double 1 $hydrogen trans- fers of the type indicated in Scheme 2 have not so far been reported; probably they should occur for instance in the thermolysis of dialkyl ethers of (15) (compare the thermolysis of simple alkyl ally1 Several types of hydrogen transfer occur during the thermolysis at 400- 600 "C of 5-methylene-l,3-dioxans (29) and (67) depending on the substituents present.8 High yields of isomeric methallyl esters (65) are obtained from the acetals [29; R = hydrogen or an aliphatic or an aromatic group (Table 2)].Since the conversion of various acetals into esters is known to be a free-radical reaction induced by peroxide~,4~ u.v.-light,SO or heat,bl a free-radical mechanism such as (A) can be considered as operating here too (Scheme 9). However since saturated 1,3-dioxans are very heat-stable in these conditions,8b or undergo other types of rearrangernent,S2 the effect of the double bond has to be taken in account. Steric hindrance should prevent a normal hydrogen transfer through a six-membered transition state unless it can be assumed that the intramolecular process (B) begins with the homolytic cleavage of an allylic bond in the ring giving a diradical (66a).A kinetic study now in progress should elucidate this point .63 Table 2 Thermolysis of 5-methyZene-l,3-dioxans at 450-550 "C R R' % Dioxan Ester (65) Dialdehyde (72) reacted % yield* % yleld* (29a) H - 26 62 - (29b) Me - 11 85 - (29c) Et - 40 80 - (29d) t-CdH9 - 92 70 - (29e) Ph - 59 79 - (29f) CH2=CH- H 74 26 46 (29g) CH=C(CH&- Me 48 38 22 (67a) Me - 22 (67b) t - - c - - 0 * yo of the theoretical yield based on consumed dioxan t R + R' = [CH& (ketal of cyclohexanone) The diradical(66) can split to the parent carbonyl compound and a diradical (68) which rearranges to methacrolein (69). In fact methacrolein and the parent carbonyl compound are normally by-products of these thennolyses and are the '* (a) R. A. Malzahn Diss. Abs. 1963,23,2698; (b) R. C. Cookson and S.R. Wallis J. Chem. SOC. (B) 1966,1245. 'v (a) L. P. Kuhn and C. Wellman J. Org. Chem. 1957,22,774; (b) E. S. Huyser and D. T. Wang ibid. 1964,29,2720; (c) E. S. Huyser and Z. Garcia ibid. 1962,27,2716. 6 0 D. Elad and R. D. Youssefyeh Tetrahedron Letters 1963 2189. &,(a) J. H. Davies and P. Kirby Chem. and I d 1964 194; (6) H. Chafetz U.S. P. 3,079,42911961 ; Chem. Abs. 1963,59,2654. ** C. S. Rondestvedt and G. J. Mantell J. Amer. Chem. SOC. 1962,84 3307 3320. 6J F. Mutterer Thesis Strasbourg 1969; F. Mutterer P. Baumgrrten and J. P. Fleury Bull. Soc. Chim. France in the press. 292 Francis Weiss Rx;I)= H Scheme 9 sole if any products from thermolysis of the ketals (67) which cannot give esters and are rather resistant to thermolysis. This behaviour compares with that of 5-methylene-l,3-dioxan-2-one (70) which at 200 "C over potassium carbonate gives methacrolein (69) instead of the expected 3-metliyleneoxetan (19).54 When R in the dioxan (29) is a vinylic group a third rearrangement appears concurrent with the former two which leads to 4-methyl-2-methyleneglutaral- dehydes (72).This reaction is easily interpreted as the result of a concerted 54 P. E. Throckmorton and S . Searles S 102 152nd ACS Meeting Sept. 1966 Abstracts of Papers. 293 Trimethylenemethane and Related a a'- Disubstituted Isobutenes - 2 0 -0 a - P - 2P transfer of a hydrogen on C-4 to the vinylic double bond and a subsequent Claisen rearrangement of the intermediate ally1 vinyl ether (71) (Scheme 9 ) . s 9 s s - - - 6 Trimethylenemethane A. Structure.-Trimethylenemethane is important in theoretical chemistry because its central carbon atom has the maximum bond order N=1.732 and the minimum free valence index F=O possible for an sp2 carbon atom; l b ~ l d ~ l e these values are the basis of the calculations of free-valence indexes of carbon compounds.This molecule has been used as a model in calculations of negative spin densities in triplet stafes,lhs1t in comparison of different methods for calculatingthe energy of 7~ systems,li and for estimation of the energy involved in the ring opening of cyc10propanone.1k~6s Molecular-orbital calculationslf~l~ predict that trimethylenemethane will have a triplet ground state (Figure 1) with a strong delocalisation energy of 1-46 /3 relative to the classical structure of one double bond and two localised electrons (1).The complete configuration interaction treatment of (1) also predicts a triplet ground state.li t -+ ff Figure 1 HMO ?r energy levels of trimethylenemethane 6s F. Mutterer J. P. Fleury and F. Weiss Tetrahedron Letters 1968 4225. For a recent review on cyclopropanones see N. J. Turro Accounts Chem. Res. 1969,2,25. 294 Francis Weiss Recently Dowd and his co-workers prepared trimethylenemethane by matrix photolysis of 4-methylene-1-pyrazoline (21) at - 185 "Cpa and of 3-methylene- cyclobutanone (73) at - 196 "COb (Scheme 10). The e.s.r. spectrum then confirmed that trimethylenemethane has a triplet ground state. The proton hyperbe splitting of the e.s.r. spectrum observed with an irradiated crystal of 3-methylene- cyclobutanone (73)2c is maximum when the magnetic field is perpendicular to the molecular plane.In this orientation all of the hydrogens are equivalent with respect to the magnetic field and each of the two major lines is split into seven and only seven hyperfine lines the intensities of which are close to the expected binomial coefficient ratios for an electron interacting whith six equivalent protons. The splitting between the peaks is 8.9 G which agrees with the value calculated on the basis of valence-bond theory.lh (73) hv A t t I H H Scheme 10 The e.s.r. spectrum is stable for a month after termination of the irradiation,2a provided that the temperature of the sample is maintained at the boiling point of nitrogen ; this indicates a remarkable stability for a simple aliphatic diradical. The peaks of the triplet disappear immediately and irreversibly if the tempera- ture is raised to - 150 "C.B. Trimethylenemethane as a Radical Intermediate.-The problem is to under- stand the behaviour of this diradical and to deduce its structure from the course of the reactions in which it is thought to be an intermediate. Skell and Doerr,6d studying the production of trimethylenemethane by the reaction of potassium vapour with 3-iodo-2-iodomethylpropene (76) obtained 1,4-dimethylenecyclohexane (77) and p-xylene (78) as a secondary product from (77) virtually no methylenecyclopropane (6) being formed. In contrast a 76.7 % yield of methylcyclopropane (75). but no coupling products was obtained from reaction of the corresponding saturated di-iodide (74) under the same conditions (Scheme 1 1). 295 Trimethylenernethane and Related a a I- Disubstituted Isobutenes This suggests the intermediacy of triplet trimethylenemethane and its unusual behaviour compared with that of other 1,3-radicals has been explained by the large delocalisation energy which makes it the favoured state but renders its cyclisation to triplet methylenecyclopropane endothermic by ca.25-50 kcal/mole whilst its cyclisation to singlet methylenecyclopropane (ca. 20 kcal/ mole exothermic) is strongly spin-forbidden.6d Thus since no favourable unimolecular path is available trimethylenemethane (1) accumulates until its concentration becomes high enough for bimolecular coupling to give 1,4- dimethylenecyclohexane (77). Coupling with other triplet species provides further evidence for the triplet state for (1) the reaction of potassium vapour with a mixture of 3-iodo-2-iodomethylpropene (76) and di-iodomethane gives some (74) (75) 76% (6) + 1 % (78) 16% Scheme 11 methylenecyclobutane (80) by addition of trimethylenemethane (1) to triplet methylene (79) and with a mixture of (76) and di-iodotetramethylcyclobutene an adduct (82) of trimethylenemethane (1) and the triplet cyclobutadiene (81) is obtained (Scheme 1 l).6d 296 Francis Weiss 1,4-Dimethylenecyclohexane (77) but not methylenecyclopropane (6) was also obtained in the high-temperature pyrolysis of the oxalate (83),67 suggesting the same triplet (1) intermediate.Lastly trimethylenemethane formed by photolysis of 3-methylenecyclobutanone (73) has also been shown to give the dimethylenecyclohexane (77) and further undergoes a photochemical 1,2-cyclo- addition to but adiene giving the 1 -met hylene-3 -vin ylc ycl open t ane (84).* However cycloaddition to ethylene was not observed under thermolysis con- ditions,sd which is consistent with the triplet structure of (1) (see for example the study by Day and Powell Scheme 15). In contrast to these facts trimethylenemethane species have been considered as intermediates both in formation and in rearrangements of cycl~propanes.~ Obviously it will not then have the same structure. The outstanding contrast is given by the results of decomposition of 4-methylene-1 -pyrazoline (21) which while giving the triplet trimethylenemethane by photolysis,2a quantitatively gives methylenecyclopropane (6) by thermoly~is.~f Deuterium labelling showed in this case that the original methylene group in the pyrazoline was at least partially equilibrated with the methylene groups in the cyclopropane ring.A similar equilibration was observed in the formation of alkylidenecyclopropanes from the photolysis and pyrolysis of 4-alkylidene-l- pyrazoline~,~f*~~~~3~~m and in the rearrangement of substituted alkylidene- cyclopropane~.~a,~ bs3dp3h Borden6g rationalised these results by discussing the correlation diagrams of the photolytic and thermal decompositions of 4-methylene-l-pyrazoline (21). In the photolysis the allowed n + n*(N=N) transition gives rise to a state which is correlated with the lowest antisymmetric state of trimethylenemethane in a conrotatory mode and with a symmetric state in a disrotatory reaction. Inter- system crossing then leads to the ground state triplet of trimethylenemethane.These trimethylenemethane species are themselves correlated with excited states of methylenecyclopropane of higher energies so that ring closure cannot occur. The thermally produced singlet trimethylenemethane by decomposition of J. P. Schirmann and F. Weiss Tetrahedron Letters 1967 5164. P. Dowd G. Sengupta and K. Sachdev ORGN 147 156th ACS Meeting Sept. 1968 Abstracts of Papers. 6@ W. T. Borden Tetrahedron Letters 1967 259. 297 Trimethylenemethune and Related a a'- Disubstituted Isobutenes S symmetric to C A antisymm II I) IA 3A 'S Con tro ta to ry p h o to ly sis S symmetric through plane A antisymm 11 N ' S 3s 'S ' A 3A a,2,* - 3A Disro tatory pho tolysis --- --- lA1 1s Jahn-Teller distortion lS lE' 1 1 s 'S Con- and Dis- rotatory thermolysis Figure 2 Partial Correlation Diagrams 298 Francis Weiss ground-state pyrazoline might undergo intersystem crossing to the ground-state triplet too but this competes with the ring closure to the ground-state methylene- cyclopropane which is possible in either a conrotatory or disrotatory fashion.Moreover the Jahn-Teller distortion of the lE’ state from D,ll to CZv symmetry can move two of the carbon atoms together and orient the molecule favourably to a quick disrotatory closure to methylenecyclopropane (Figure 2). C. Non-radical Trimethylenemethane Intermediates.-The synthesis of methylene- cyclopropane (6) has long been an elusive problem because of the difficulties of cyclisation to small rings already mentioned. Thus the classical halogen elimination from 1,3-dihalogeno-cornpounds with zinc which gives a 91 % yield of methylcyclopropane (75) with 1,3-dihalogen0-2-methylpropane failed to give any methylenecyclopropane with dichloroisobutene ( 8 ) ‘ j b s 6 (see also results of Skell).On the other hand the method involving the preparation and retrodienic cracking of Diels-Alder intermediates also failed with the anthracene adduct (85) of methylenecyclopropane which appeared to be thermally stable.’ja The first successful synthesis of methylenecyclopropane (6) was realised by Gragson et uL6C who obtained a 17 % yield by the reaction of dichloroisobutene (8) with magnesium. This low yield was later improved to ca. 30% by Anderson,Boa by using tetrahydrofuran as solvent (see also ref. 60b). Recent re-examinafion6l of the reaction has confirmed that 1,4-dimethylene- cyclohexane (77) is in fact the main product.The yield may rise as high as 60% but has poor reproductibility apparently because of the great versatility of the reaction. Interestingly a full range of macrocyclic ‘oligomers of trimethylene- methane’ were found also e.g. 1,4,7-trirnethylenecyclononane (86) 1,4,7,10- tetramethylenecyclododecane (87) 1,4,7,10,13-pentamethylenecyclopentadecane (88) and 1,4,7,10,13,16-hexarnethylenecyclooctadecane (89). Each of these cyclic compounds is accompanied by a small amount of the open-chained analogue e.g. (90)-(94). Yields from a typical experiment are shown in Scheme 12. This reaction thus appears to be both an intramolecular and an intermolecular cyclisation by a Wiirtz procedure and macrocyclisation may be favoured by the ‘rigid‘ structure of the dihalide (8).This compares with the similar behaviour of a,a’-dihalogenoxylenes in coypling reactions with sodium.s2 The mechanism possibly is a step-wise formation of successive Grignard intermediates having a pseudocyclic structure by intramolecular co-ordination of the magnesium atom with the second chlorine atom. This process would consist in an intermolecular elimination of magnesium chloride which results in an insertion of a trimethylenemethane unit to give the @ O (a) B. C. Anderson J . Org. Chem. 1962 27 2720; (b) Methylenecyclopropane can be prepared also in 60% yield from diazomethane and allene; see A. T. Bloomquist and D. J. Connolly Chem. and Ind. 1962 310. O * K. Ziegler in Houben Weyl ‘Methoden der Organischen Chemie’ G. Thieme Stuttgart 1955 vol. IV part 2 p.733. A. Lakodey G. Bonnard and F. Weiss to be published. 299 5 Trimerhylenemerhane and Related a a’- Disubstituted Isoburenes ( 6 ) 15% (77) 20% (86) 7% (87) 1% (88) 1.5% (90) n = 2 0.8% (91) n = 3 0.2% (92) n = 4 0.2% (93) n = 5 0.25% (94) n = 6 0.2% (89) 0.8% Scheme 12 next higher intermediate Grignard structure (95) + (96) + (97) + (98) e t ~ . ~ ~ (Scheme 13). Two competing reaction pathways can then take place with each intermediate an intramolecular elimination giving the corresponding cyclic hydrocarbon or reaction with magnesium to give the cyclic magnesium compound e.g. (99) which is postulated to form the open-chain hydrocarbons in the subsequent hydrolysis of the mixture. An analogous cyclisation by a mechanism not discussed has been discovered by Corey and Semmelha~k~~ in the reaction of dichloroisobutene (8) with nickel carbonyl which affords a 54% yield of the trimethylenecyclononane (86) together with 11 % of (77) and 5% of 3,6-dimethylenecycloheptanone (100).No higher ‘oligomers’ are mentioned. A good preparative method for methylenecyclopropane (6) may finally have ‘8 These allylic Grignard intermediates possibly are in a dynamic equilibrium (see J. E. Nordlander and J. D. Roberts J. Amer. Chem. Soc. 1959 81 1769) but this should not affect the qualitative course of the reaction for symmetry reasons. 300 Francis Weiss I c1 + Mg -MgCI -MgCI I 4 -Mg CI + ( 9 5 ) 1 Ji Scheme 13 been found in the reaction recently described by Koster et a1.,84 of methallyI chloride (7) with potassium amide in tetrahydrofuran which affords a 36 % yield of (6).The reaction with sodium amide formerly reported to give l-methyl- cyclopropene (101),66 has been found to give a mixture of (6) and (101). As R. Koster S. Arora and P. Binger Angew. Chem. 1969,81 186; Angew. Chem. internat. Edn. 1996 8 205. O 6 F. Fisher and D. E. Applequist J. Org. Chem. 1965,30 2089. 301 Trimethjhemethane and Related a a ‘-Disubstituted Isobutenes potassium amide quantitatively isomerises (101) to (6) the former may well be an intermediate in this case (Scheme 14). Scheme 14 ID. Iron Tricarbonyl Complexes.-A number of molecules which are unstable in the free state can be stabilised as ligands in transition metaln-complexes for example cyclob~tadienes,~ cyclopentadienone,66a$ 2,4-~yclohexadienone,~~~ carbene,66d o-quinodimethane,66 and benzyne.66f Trimethylenemethane like cyclobutadiene having an unfilled nonbonding orbital has also proved to be an excellent acceptor in transition-metal complexes.A tricarbonyl-n-allyl(acy1)iron complex (102) is formed when iron ennea- carbonyl reacts with isobutenediol (15)67 whereas the reaction of iron enneacarbonyl with 3-chloro-2-chloroinethylpropene (8) gives the tricarbonyl- (trimethy1enemethane)iron (103) in 30 % yield.5a More recently EhrIich and Emerson5f observed that (103) could also be formed in a disproportionation of tricarbonyl-2-methallyliron chloride (1 03 and found a convenient one-step synthesis of (103) in the reaction of an excess of 2-methallyl chloride (7) with iron enneacarbonyl followed by fractional distillation without isolation of the 66 (a) M. L.H. Green L. Pratt and G. Wilkinson J. Chem. SOC. 1960 989; (b) E. Weiss R. Merhyi and W. Hiibel Chem. andInd. 1960 407; (c) A. J. Birch P. E. Cross J. Lewis and D. A. White ibid. 1964 831 ; (d) E. 0. Fischer and A. Maasbol Chew. Ber. 1967 100 2445; (e) W. R. Roth and J. D. Meier Tetrahedron Letters 1967 2053; (f) E. W. Gowling S . F. A. Kettle ana G. M. Sharples Chem. Comm. 1968 21. ‘7 H. D. Murdoch Helv. Chim. Acta 1964,47,936. 302 Francis Weiss intermediate (105) which gives a yield of 14-20% of theory based on Fe (CO) 9' Several substituted trimethylenemethaneiron complexes have been prepared by these authors using the same procedure with substituted ally1 halides the tricarbonyl(phenyltrimethy1enemethane)iron (1044 was obtained in 32 % yield from 3-chloro-2-methyl-l-phenylpropene while 36% of a 1 3 mixture of tricarbonyl(methyltrimethy1enemethane)iron (104d) and tricarbonyl(is0prene) iron (1066) were obtained by reaction of an excess of Fe,(CO) with a mixture of 1- and 3-chloro-2,3-dimethylpropene.Substituted tricarbonyl(triniethy1- enemethane)iron complexes [(104a) (1046) and (104c)l can also be prepared by reaction of Fe,(CO) with 2-substituted methylenecyclopropanes (107a) (107b) and (107c) in 40-60% ~ield,~e Methylenecyclopropang (6) itself however gives tricarbonyl(butadiene)iron (106a) ca. 2 % rather than tricarbonyl(trimethy1enemethane)iron (103) in the same condition^.^ e In contrast methylenecyclopropane (6) and the 2,2-diphenyl derivative (1074 react with palladium chloride to give the .rr-allylpalladium complexes (108) and (log) respectively.68 Gas-phase electron-diffraction investigation of tricarbonyl(trimethy1ene- methane)iron (103)5b and single-crystal X-ray diffraction study of (104a)6C show a close similarity in molecular conformation of the two complexes.The iron 68 R. Noyori and H. Takaya Cliem. Conzrn. 1969,525. 303 Trimethylenemethane and Related a a '- Disubstituted Isobutenes Me atom is located directly beneath the central atom C1 of the trimethylenemethane skeleton (the Fe-C distances being respectively 1.938 and 1.929 A) and is rr-bonded to all four carbon atoms of this ligand. The carbon skeleton of the ligand forms a tetrahedron with C1 displaced away from the iron atom relative to the C2-C3-C4 plane by ca. 0.34 A for (103) and by 0.31 A for (104a). The trimethylenemethane ligand and the Fe(C0) moiety adopt a mutually staggered conformation forming a trigonal antiprism (Figure 3).It also appears that the plane of the phenyl ring makes an angle of ca. 59" with the plane defined by C2-C3-C4 which seems to indicate that there is little conjugation between phenyl and trimethylenemethane systems. This seems to be the case even in solution according to the i.r. spectra.6e oc 0 0 ( 1 04a) O H Figure 3 Preliminary studies of some reactions of tricarbonyl(trimethy1enemethane)iron (103) have been made. Protonation of the ligand occurs in strong acids as with tricarbonyl(butadiene)iron and derivatives,sg and treatment of (103) with sulphuric acid followed by concentrated hydrochloric acid gives rise to complex (105) in high yieId.6a Addition of bromine to (103) gives the complex (IlO) 6 9 0.F. Emerson and R. Pettit J . Amer. Chem. Soc. 1962 84 4591. 304 Francis Weiss Fe -Br (CQ which disproportionates to re-form (103) instead of yielding complex (104e) as would be expected from a reaction similar to the disproportionation of (105).5f Day and Powelljd studied the photolysis of (1 03) in several solvents as a possible method of generating (1) and obtained results which strongly suggest the inter- mediacy of free trimethylenemethane in some of the reactions (Scheme 15). In pentane dimers of (l) e.g. (77; 6%) and isomers like (111; 9%) as well as p-xylene (78; 2073 and even 3 % of [2,2]paracyclophane (1 12) were formed a result which is similar to those obtained by Skell and DoerrGd (cf. Scheme 11). In cyclopentene and in cyclopentadiene the formation of these dimerisation products and derivatives was completely suppressed and cycloaddition of (1) to the olefins occurs (113) was obtained in ca.5 % yield in cyclopentene [cf. D o w d ' ~ ~ ~ result with butadiene -+ (84)] and (114) was obtained in 23% yield in cyclopentadiene. The main products of these complex photolyses in cyclo- pentene were the three cyclobutane derivatives (115; 29%) (116; 11 %) and (117; 3 %) which were probably formed by carbonylation of (1) to a primary product believed to be 3-methylenecyclobutanone (73). These carbonylation products [CJ (8) -+ (100) in the presence of nickel carbonyl] are probably formed within the ligand sphere of the iron atom and not at the expense of free trimethylenemethane (1). 305 Trimethylenemethane and Related a 01 '- Disubstituted Isobutenes E.Heteroanalogues of Trimethy1enemethane.-Although it will not be discussed here the possible existence of heteroanalogues of trimethylenemethane (1) is mentioned without implication as to their electronic structures. They may occur for example in the isomerism depicted in Scheme 16 in which if A = B = C = D = carbon we have the isomerism of methylenecyclopropanes. The much discussed valence tautomerism of allene oxides (118) and cyclo- propanones (1 19)56 recently received experimental ~upport,~~a and substituted diaziridine imines (120) have been shown to be in thermal equilibrium possibly through structure (121).'Ob D II A-B / \ R " R' K /N ON\ R Scheme 16 0 R H l2' x Nl- N R' 'R / t N II C >N I 4. 7 Conclusion This article aims to provide an introductory review of a new field of organic chemistry which already promises much interest.The close relation of trimethyl- enemethane and cyclobutadiene clearly demands a comparison of the two systems their reactivities analogies and differences. Study of complexes with 7 D (a) R. L. Camp and F. D. Greene J . Amer. Chem. SOC. 1968,90,7349; J. K. Crandall and W. H. Machleder ibid. 1968,90,7347; (6) H. Quast and E. Schmitt Angew. Chem. 1969,81 429; Angew Chem. internat. Edn. 1969 8 449; Cliem. Ber. 1970 103 1234. 306 Francis Weiss transition-metal compounds the syntheses and chemistry of complexes with metals other than iron with substituted trimethylenemethanes study of ligand transfers etc. should be particularly fruitful. The possibility of producing ionic trimethylenemethane species like the dication (122) the dianion (1 23) and especially the 1,3-dipolar species (124) also deserves attention.The chemistry of ar,a'-disubstituted isobutenes in general whether or not trimethylenemethane itself is implicated as an intermediate should continue to find both synthetic and mechanistic interest especially in the broad field of reactions involving some allylic rearrangement. In all cases the influence of various substituents on the properties of the systems for example on the stability of trimethylenemethane species should be investigated for a better understanding of this chemistry. II Note added in Proof. - Several recent papers on these subjects deserve mention. Photochlorination of methylenecyclopropane (6) gives dichloroisobutene (8) in 42% yield among a variety of chlorinated products but not the expected 2-chloromethylenecyc10propane.71 Condensation of (8) and large-ring cyclo- alkanones (125) in the presence of sodium hydride provides substantial yields of 1,s-polymethylene-bridged 3,7-dimethylenebicyclo [3,3,1 Inonan-9-ones (126) - h v b n = 9 (1 26) Scheme 17 'lA.J. Davidson and A. T. Bottini J . Org. Chem. 1969 34 3642. (127)n = 10 307 Trimethylenemethane and Related a a '-Disubstituted Zsobutenes and u.v.-irradiation of (126a) causes a smooth intramolecular cycloaddition to cage compounds (127) (Scheme 17).72 Conformational studie~~~aJ~b and additional data on the thermal rearrange- ment73bJ3c of 5-methylene-l,3-dioxans (29) and (67) have been published. A recent calculation indicates a positive value for the zero field splitting parameter D of trimethylenemethane (l).74 The closely related thermal rearrangements of 2,2-diphenyl-l -methylene- M cycloyropane (107c) to 2-methyl-3-phenylindene (128) 7Sa and of l-methylene- 2-vinylcyclopropane (129) to 3-methylenecyclopentene (1 30) 75b~76c may well proceed through trimethylenemethane intermediate^.^^^ The thermal behaviour of the more complex cyclopropylidenecyclopropane (131) and the related methylenespiropentane (132) is also interpreted by a mech- product Scheme 18 72T.Mori K. Kimoto M. Kawanisi and H. Nozaki Tetrahedron Letters 1969 3653. 73(u) Ph. Desaulles Thbse Strasbourg 1969; (6) F. Mutterer J. M. Morgen J. M. Biedermann J. P. Fleury and F. Weiss Bull. SOC. chim. France 1969 4478; Tetrahedron 1970 26 477; (c) J. M. Biedermann J. P. Fleury and F.Weiss Bull. SOC. chim. France 1970 1187. 74A. Gold J. Amer. Chem. SOC. 1969 91 4961. 75(a) J. Maitland jun. M. E. Hendrick J. C. Gilbert and J. R. Butler Tetrahedron Letters 1970,845; (b) T. C. Shields W. E. Billups and A. R. Lepley J . Amer. Chem. SOC. 1968,90 4749; T. C. Shields and W. E. Billups Chem. and Ind. 1969 619; (c) H. D. Roth PETR 2 Abstracts of Papers. 159th ACS National Meeting Feb. 1970. 308 Francis Weiss anism based on trimethyleneniethane-type intermediates (e.g. 133) as summarized in Scheme 18.76 The bis-ally1 diradical (1 34a) (‘tetramethylene ethane’) has been observed recently by Dowd“’ in the low-temperature photolysis of 3,4-dimethylenecyclo- pentanone and of its perdeuteriated derivative. The e.s.r. spectrum confirmed the ground-state triplet structure of (1 34a) which was predicted,lc and which determines a close relation between this species and trimethylenemethane (1) (see references cited in ref.77 for example). Irradiation of tricarbonyliron complexes (1 03) and the bromo-analogue of (105) in t-butylbromide gives mainly the hydrocarbon (135) which is a dialky- 1 at i on product of t rime t h y lenemet hane . 78 H H I thank Professor R. N. Haszeldine and MM. R. Lichtenberger and P. Terestchenko for encouragement. I also thank my co-workers for valuable assistance especially Miss R. Mounier Mrs. S. Passot MM. P. Debard E. Ghenassia G. Prudhomme J. Radix R. Rolland and G. Viviant. 7eJ. K. Crandall D. R. Paulson and C. A. Bunnell Tetrahedron Letters 1969 4217. 77P. Dowd J. Amer. Chem. SOC. 1970,92 1066. 7eE. Koerner yon Gustorf F. W. Grevels and J. C. Hogan Angew. Chem. 1969 81 918; Angew. Chem. Internat. Edn. 1969 8 899. 309

ISSN:0009-2681    

DOI:10.1039/QR9702400278

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Or g ano t hallium Chemistry By A. G. Lee UNIVERSITY CHEMICAL LABORATORY LENSFIELD ROAD CAMBRIDGE THE element thallium was discovered in 1861 by William Crookes and very soon after in 1870 the first organothallium compound was prepared.l This compound diethylthallium chloride was found to be remarkably stable being unaffected by air and water. A large number of such dialkylthallium compounds were prepared around the turn of the century and found to be similarly stable. The substitution of the third group on thallium by an alkyl group however had to wait until 1930 when it was found that the reaction of diethylthallium chloride and ethyl-lithium gave triethylthallium.2 Since that time there have been sporadic forays into organothallium chemistry but few concerted attacks. Most of the work has predictably concentrated on the air stable diorgano- thallium derivatives although with the advent of vacuum and dry-box techniques methods of handling the very air-sensitive triorganothallium compounds have been developed.1 Triorganothallium Compounds A. Preparation.-Triorganothallium compounds are usually prepared by the reaction between a dialkyl- or diaryl-thallium halide and an organolithium c o m p o ~ n d . ~ ~ ~ R2TlX + LiR-+R,Tl + LiX The reaction between thallium(Irr) chloride and a Grignard reagent in diethyl ether stops after the introduction of two organic groups the third group can be substituted by carrying out the Grignard reaction in tetrahydrofuran as ~olvent.~ The most convenient preparation of trimethylthallium however is by the reaction between thallium(1) iodide and methyl-lithium in the presence of methyl i~dide.~a~b The overall reaction is 2 MeLi + Me1 + TI1 + Me,Tl + 2 LiI.The first step of the reaction is considered to be the formation of monomethyl- thallium(I) followed either by disproportionation or by reaction with methyl iodide C. Hansen Chem. Ber. 1870,3 9. H. P. A. Groll J. Amer. Chem. SOC. 1930,52 2998. E. G. Rochow and L. M. Dennis J . Amer. Chem. SOC. 1935,57,486. 0. Ya. Okhlobystin K. A. Bilevitch and L. I. Zakharkin J. Organometallic Chem. 1964 2 281. “(a) H. Gilman and R. G. Jones J. Amer. Chem. SOC. 1946 68 517; (b) H. Gilman and R. G. Jones J. Amer. Chem. SOC. 1950 72 1760. 310 Lee MeLi + TI1 4 LiT -t. MeTl 3 MeTl + Me3T1 + 2 T1 MeTl + Me1 -+ Me,TII 4 Me,Tl + LiI MeLi The thallium metal formed (in a finely divided state) can then react with methyl iodide Me1 + 2 TI 4 MeTI + TI1 Although methyl iodide does not react with massive thallium or with a thallium- sodium alloy,3 ethyl chloride and a thallium-sodium alloy react to give low yields of triethylthallium2 and thallium reacts with phenyl iodide to give thallium(1) iodide and possibly phenylthaliium derivatives.'j B.Properties.-The first point of interest concerning triorganothallium com- pounds is their state of association. Freezing point measurements show that trimethyl- triethyl- and triphenyl-thallium are monomeric in benzene s o l ~ t i o n . ~ a ~ ~ 1.r. and Raman studies of trimethyithallium also suggest that it is monomeric in benzene solution and in the vapour phase but that it is associated in the crystaL8 This has now been confirmed by a single crystal X-ray study.s In the crystal the three nearest neighbours of each thallium are three methyl groups in a plane (I) so that little rearrangement takes place when the crystal vaporises as monomer.Association to give a polymeric three-dimensional net- work occurs through unsymmetrical Me-T1-Me bridges the co-ordination about thallium being a distorted trigonal bipyramid with three short equatorial and two long axial bonds. MeK Me /d Me 3.16 90' \ 2.22 T1 Me 1:: Me J. F. Spencer and M. L. Wallace J. Chem. SOC. 1908,1827 ' W. Strohmeier K. Humpfner K. Miltenberger and F. Seifert Z . Elektrocheni. 1959 63 527. A. J. Downs and A. G. Lee to be published. G. M. Sheldrick and W. S. Sheldrick J . Chem. SOC. ( A ) 1970,28. 31 1 Organothallium Chemistry The methyl bridges are long ( N 3.2 A) and must clearly be weak in comparison to those in the trimethylaluminium dimer.Although no stable methyl-bridged dimer is formed by trimethylthallium such a dimer has been postulated to be the transition state for intermolecular exchange of methyl groups in solution this rapid intermolecular exchange is evidenced by the collapse of the 2osT1-H and 203Tl-H doublets in the n.m.r. spectrum. The exchange is concentration and solvent dependent and being second order in the concentration of trimethyl- thallium has been postulated to proceed via a methyl-bridged dimer (2 X = Me).l0 The energy of activation for the exchange is 6 & 1 kcal mole-l in solution in dichloromethane. At low temperatures ( N - 70") the exchange could be stopped but all the methyl groups were equivalent in the n.m.r.spec- trum so that (2) can only be a transition state higher in energy than the monomeric trimethylthallium molecules. Dimerisation of trimethylaluminium is favoured by low inner shell repulsion between the two aluminium atoms when separated by the internuclear distances demanded by the geometry of the dimer and also by strong electronic configurational interaction between the aluminium atoms. For thallium the first factor will probably be greater and the second smaller (TI-TI bonds are expected to be weak) so that dimeris- ation will be less favourable for thallium than for aluminium. A number of organothallium compounds seem to have trigonal-bipyramidal structures (see later) so that a basic D3h symmetry for thallium in trimethylthallium is not so surprising; distortion from the idealised geometry is demanded by the re- quirements of crystal packing coupled with the requirement that the basic TIC3 planar skeleton should be maintained.The structure adopted by trimethyl- thallium is closely analogous to that found for trimethylindium.ll As well as undergoing intermolecular exchange of methyl groups trimethyl- thallium undergoes rapid methyl exchange with dimethyl zinc.12 Between triphenylthallium and n-butyl-lithium however there is a rapid redistribution of groups.13 J. P. Maher and D. R. Evans J. Chem. Soc. 1963 5543. E. L. Amma and R. E. Rundle J . Amer. Chem. Soc. 1958 80 4141. I* A. G. Lee to be published. l3 H. Gilman and R. 0. Jones J. Amer. Chem. SOC. 1940,62,2357. 312 Lee Ph,TI + 3 BunLi -+ 3 PhLi + Bun,TI Similarly in the reaction between bis(trimethylsily1)mercury and triniethyl- thallium there is a rapid redistribution to form tris(trimethy1~ilyl)thallium.~~ 3(Me3Si),Hg + 2 Me,TI -+ Z(Me,Si),TI + 3Me2Hg Little work has yet been done on reactions of triorganothallium compounds with metal halides but it is known that both trimethyl- and triphenyl-thallium will react with mercury :6a*1z 2 R3TI + 3 Hg + 3 RzHg + 2T1 This is of interest in view of the fact that both gallium and indium react with dimethylmercury to give the corresponding metal trialkyls.Although trimethylthallium is a much weaker acceptor than trimethylindium or trimethylgallium it will form 1 1 complexes with Lewis bases. Thus with trimethylamine trimethylthallium forms a crystalline complex Me,TI. NMe which is however extensively dissociated in the vapour phase:15 the adduct formed between trimethylamine and triphenylthallium is much less readily dissociated.1° The decidedly weak acceptor properties of triorganothallium compounds have important consequences for their chemistries.Many reactions of organo- aluminium compounds for example are postulated to proceed via preliminary adduct formation. Thus trimethylaluminium first forms a 1 1 adduct with acetonitrile and then reacts to form (MezC:NAlME,)2,16 whereas trimethyl- thallium fails to form an adduct with acetonitrile and does not react." The two most important factors in the chemistry of these compounds are probably however the weakness of the TI-C bond and the great stability of diorganothallium derivatives. Although no value for the mean TI-C bond dissociation energy has yet been measured the dissociation energy of the first TI-C bond D(Me,TI-CH,) has been calculated as 27.4 kcal mol-1 from kinetic data for the pyrolysis of trimethylthallium.ls This compares with bond dissoci- ation energies (D 1) of 59.5 and 47.2 kcal mo1-1 for trimethylgallium and trimethyl- indium re~pectively.~~a~ b The reactivity of triorganothallium compounds has been attributed to their ability to ionise to R2TI+R- .20 Thus hydrolysis to give RzTIOH and RH has been considered a unimolecular electrophilic substitution (SE~).However the only experimental evidence reported to support this mechanismzo has been shown to be incorrect.21 In the absence of any evidence to the contrary it seems l' E. A. V. Ebsworth A. G. Lee and G. M. Sheldrick J.Chem. SOC. (A) 1969,1052. l5 G. E. Coates and R. A. Whitcombe J. Chem. SOC. 1956 3351. J. R. Jennings J. E. Lloyd and K. Wade J. Chem. SOC. 1965 5083. l 7 A. G. Lee and 0. M. Sheldrick J . Organometallic Chem. 1969 17 481. l 8 M. 0. Jacko and S. J. W. Price Canad. J. Chem. 1965,43 1961. l o (a) M. G. Jacko and S. J. W. Price Canad. J. Chem. 1963,41 1560; (b) M. G. Jacko and S. J. W. Price Canad. J . Chem. 1964 42 1 198. C. R. Hart and C. K. Ingold J . Chem. SOC. 1964,4372. F. R. Jensen and D. Heyman J. Amer. Chem. SOC. 1966 88. 3438. 313 Organothallium Chemistry most likely that the mechanism adopted is a concerted displacement of the leaving group by the entering group (often loosely referred to as an s E 2 mech- anism) as is found for reactions of many other organoinetallic compounds.But whatever the mechanism trimethylthallium has been shown to react with a wide variety of compounds containing an acidic hydrogen to give methane and the corresponding dimethylthallium derivative.l Me3Tl 3- HX -+ Me,TIX -t- CH4 The reaction proceeds not only with such obvious acids as the hydrogen halides alcohols and thiols but also with less obvious acids such as cyclo- pentadiene22 and’ HMn(CQ) 5.12 With halogenomethanes the correspond- ing dimethylthallium halide is formed:12,17 thus with chloroform Me,TlCl is formed. The order of reactivity of the halogenocarbons I > Br > C1 > F and CHX3 > CH2X2 > CH3X is similar to that found for the basic hydrolysis of the halogenocarbons involving intermediate halogenocarbon anions. If any CX derivatives of thallium are first formed in these reactions then they must be very unstable.Thus dimethylthallium ethoxide reacts with chloroform to give Me,TlCI whereas mercury alkoxides and chloroform react to give compounds of the type RHgCC13.23 The compound Ph,TI[C(CN),] can however be made by the reaction between Ph,TlF and K[C(CN),].24 There is no reaction between trimethylthallium and dimethylamine or diethy1amine,l5>l7 or between triethyl- thallium and ammonia. In a variety of other reactions of triorganothalliurn compounds a diorgano- thallium derivative is formed Et3Tl 4- GIN -+ Et,TIN + EtCl Me,Tl + SO -+ Me,TlQS(Q)Me (reference 25) (reference 26) It requires reagents such as the halogens or interhalogen compounds to cleave the final two Even in oxidation reactions diorganothallium species can be formed.Thus phenol biphenyl and diphenylthallium oxide have all been found in the products of the slow reaction of a benzene solution of triphenyl- thallium with dry air.28ayb The reason for the great stability of the diorgano- thallium group is unclear. 2 Diorganothalli~rn(1rr) Derivatives A. Preparation.-The most versatile routes to diorganothallium derivatives are by reactions of triorganothallium compounds of the type detailed above. Starting from thallium(Ir1) halides however dialkylthallium halides can be prepared by t a A. G. Lee and G. M. Sheldrick Chem. Comin. 1969 441. 24 W. Beck H. S. Smedal and H. Kohler Z . anorg. Chem. 1967,354 69. G. Holan Tetrahedron Letters 1966 1985; and references therein. J. Muller and K. Dehnicke J . Organometallic Chem.1968 12 37. A. G. Lee Chem. Comm. 1968,1614. 27 A. E. Goddard J. Chem. SOC. 1923 1161. ** (a) S. F. Birch J . Chem. SOC. 1934,1132; (b) H. Gilman and R. G. Jones J . Amer. Chem. SOC. 1939 61 1513. 314 Lee the Grignard r e a ~ t i o n ~ ~ ~ ~ although yields are often poor owing to oxidation of the Grignard reagents by the thallium(rII) halides TlX + 2 RMgX R2TlX + 2 MgX Diarylthallium halides can be very conveniently prepared by the reaction between thallium(m) halides and aryl boronic acids. b$ 30 Starting from the diorganothallium halides a wide variety of other diorgano- thallium derivatives can be prepared by anion exchange with silver salts alkali-metal salts and thallium(1) salts. Thus halide can be exchanged for anions such as nitrate chromate thiocyanate trichloroacetate and even more exotic groups :31 Liq NHs K,[Fe(CN),(CiCPh),NO] I- Ph,TlBr --+ KBr + K[Ph,Tl] [Fe(CN),(CiCPh),NO] NH Another much used procedure is to convert the diorganothallium halide to the hydroxide (using aqueous silver oxide) and then to neutralise with the appropriate acid.In the same way that triorganothallium compounds with substituents in the alkyl chain are unstable so are the analogous diorganothallium compounds. Electrolysis of ICH,CH,CN using a sacrificial cathode of thallium produces the unstable compound (CNCN2CH2),TlI.32 Reaction of thallium(rn) chloride with diazomethane produces (C1CH2),T1C1 as an explosive solid with diazoethane the product was too unstable to B. Properties-Dialkyl- and diaryl-thallium(II1) derivatives are amongst the most stable and least reactive organometallic compounds known.They are unaffected by water and oxygen and in many ways resemble both the iso- electronic diorganomercury compounds and the corresponding thallium(1) derivatives. They can thus be used as a ‘pseudo-TlI’ cation. Vibrational and lH n.m.r. spectra of the organic group can be used as a probe to study the interactions of the thallium with the other ligands. This is particularly useful since the thallium(1) cation itself has relatively few easily measured properties which are sensitive to the nature of the interaction of the thallium with ligands. Most of the thallium compounds of this type give highly conducting solutions in water. Conductance measurements indicate a high degree of dissociation for the hydroxide Me2T10H34 with the formation of Me2T1(H,0)sf and OH- ions.In 0.2 molar aqueous solution about 10% of the thallium is present as the dimer 2 e (a) R . J. Meyer and A. Bertheim Chem. Ber. 1904 37 2051; (b) F. Challenger and B. Parker J . Chem. Soc. 1931 1462. 3 o F. Challenger and 0. V. Richards J . Chem. Soc. 1934,405. s1 R. Nast K. W. Kruger and G. Beck Z . anorg. Chem. 1967,350,177. a 2 A. P. Tomilov Yu. D. Smirnov and S. L. Varsharskii Zhur. obschei Khirn. 1965,35 391. sa A. Yu. Yakubovich and V. A. Ginsburg Doklady Akad. Nauk S.S.S.R. 1950,73 957. F. Hein and H. Meininger 2. anorg. Chem. 1925,145,95. 315 Organothallium Chemistry (2 X = OH) 36% as Me,TlOH and 54% as Me2TI(H20)z+.35 The n.m.r. spectra of aqueous solutions of a wide variety of dimethylthallium derivatives suggest that the same thallium-containing species is present in all ~ ~ I u t i o n s ~ ~ presumably Me2TI(H20)z+.Water having a high dielectric constant and being a good electron donor is almost an ideal solvent for the production of solutions of organometallic cations :37 dimethylformamide is a less ideal solvent and conductivity measurements suggest that Me,TlI is incompletely dissociated in this The Raman spectra of aqueous solutions of dimethylthallium perchlorate and nitrate show that the C-TI-C skeleton in the dimethylthallium hydrate [Me,Tl(H,O)z+] ion is linear.3a No information regarding the number of solvent molecules co-ordinated to the metal atom is available but it seems probable that there will be four producing an overall octahedral configuration for the thallium. The dimethylthallium ion is only a very weak aquo-acid indicating that the bonds to water molecules in the first co-ordination sphere are very weak.34~36~40 Supporting evidence for weak polar bonds to the water molecules comes from the Raman spectra of aqueous solutions of the dimethylthallium ion which show no lines attributable to stretching of T1-0 bonds.Even the spectrum of 2.3 M Me,TlOH shows no lines attributable to TI-0 bond stret~hing.~~ The dimethylthallium halides were investigated by X-ray crystallography in 1935;41 the structure consists of layers in which a linear Me,TI group is sur- rounded by four halogen atoms and each halogen is surrounded by four Me,Tl groups (3). It is a moot point whether the structure should be described in terms of an ionic lattice or in terms of bridging halogen atoms and thallium- halogen bonds with a reasonable degree of covalency.1.r. studies of the dimethyl- thallium derivatives of CN- C104- and NO3- in the solid state are all consistent with structures containing free linear Me,T1 cations and free anion^.^^^,^ The i.r. spectrum of Me,TINO, for example shows only a small degree of splitting of the doubly degenerate Y ionic nitrate mode indicating that the nitrate ion is only weakly perturbed. Furthermore the spectrum shows only one band in the TI-C stretching region at 557 cm-l assignable to the antisymmetric stretching mode in a linear Me,TI g r o u ~ . ~ ~ B For these dimethylthallium derivatives therefore in aqueous solution or in the solid state there can be distinguished two distinct types of bond from the central thallium to the ligands the kinetically inert and relatively non-polar s6 J.K. Lawrence and J. E. Prue Internat. Conference on Co-ordination Chem. London 1959 Chem. SOC. Special Publ. No. 13. p. 186. J. V. Hatton J. Chem. Phys. 1964,40 933. s 7 R. S. Tobias Organometallic Chem. Rev. 1966,1 93. J. E. Prue and P. J. Sherrington Trans. Faraday SOC. 1961,57 1795. s B P. L. Goggin and L. A. Woodward Trans. Faraaby SOC. 1960,56,1591. 'O A. J. Berry and T. M. Lowry J . Chem. SOC. 1928,1748. 'l H. M. Powell and D. M. Crowfoot 2. Krist. 1934,87 370. J. H. S. Green and R. S. Nyholm J. Chem. SOC. 1965,341 1. (a) G. D. Shier and R. S. Drago J . Organometallic Chem. 1966,5 330; (b) G. B. Deacon 316 Lee bonds to carbon in the alkyl groups and the kinetically labile and highly polar bonds to the electronegative group.It is unlikely in the case of the halides at least that these latter bonds can be regarded as purely ‘hard-sphere’ Coulomb Me Me Me Me //I Me Me I / T1- - - - T1 Me I Me interactions and there will be a certain amount of covalent character to them. The example of aluminium trichloride comes to mind although the solid has a largely ionic distorted chromyl chloride layer-structure it is present in the vapour phase as dimeric Although the structures of the dialkyl- thallium halides in the vapour phase are unknown the mass spectra of both Me,TICl and Me,TICN show the presence of dimeric molecules in the vapour phase for Me,TlBr and Me,TlI parent ions are observed but no dimeric species.44 Little is known about the structures of the dialkylthalliuni fluorides *a A. F. Wells ‘Structural Inorganic Chemistry’ Oxford University Press 1962.a* A. G. Lee Internat. J. Mass Spectrometry Ion Phys. 1969 3 239. 317 Organothallium Chemistry which by comparison with other organometallic fluorides might well be expected to adopt fluorine-bridged structures. Di-isobutyl- di-isoamyl- and di-n-hexyl- thallium fluorides have been shown to be highly associated in benzene and the i.r. spectrum of Me,TlF shows both a symmetric and an antisymmetric Tl-C stretch so that unlike the other dimethylthallium halides the C-Tl-C skeleton must be n~n-linear.~~ A number of dimethylthallium compounds are known which are dimeric in solution. Molecular weight measurements show that Me,TlOEt is largely dimeric in benzene although n.m.r. spectroscopy suggests that some monomer is also present in toluene Similarly such groups as OPh SMe NMe, and S0,Me are good bridging g r o ~ p s .* ~ ~ ~ ~ As already mentioned the methyl group would seem to have little capacity for bridge formation. Thus n.m.r. spectra of mixtures of Me,TI and Me,TlOEt show no evidence for the formation of mixed dimers with methyl bridges.48 In solutions of (Me,Si),Tl intermolecular exchange of trimethylsilyl groups occurs and in this case a dimeric transition state with trimethylsilyl bridges was invoked.50 Rapid exchange of alkyl groups occurs in mixtures of Me,Tl and Et3Tl at room temperature at - 85" additional lines are seen in the n.m.r. spectrum due to the mixed alkyls TlMe,Et and TlMeEt, but again there was no evidence for the formation of dimeric species.l0 The exchange of vinyl groups on thallium is faster than that of methyl groups and it has been suggested that this is due to stabilisation of the dimeric transition state by bridging vinyl groups; the non-bonding molecular orbital for the three-centre bridging system has the appropriate symmetry to interact with the n-orbitals of the vinyl group.With Me,TlCiCPh and Me,TlC,H5 however the situation is rather different. Here no redistribution of groups occurs on the thallium in solution; furthermore there is no exchange of methyl groups between them and trimethylthalli~m,~~.~~ They resemble dimeric dimethylthallium compounds such as Me,TlOEt rather than 'dimethyl- alkylthallium' compounds such as Me,TlEt. N.m.r. results suggest that both Me,TlCiCPh and Me2TlC5H5 exist in solution as a mixture of monomer and dimer,51 although molecular weight measurements suggest that Me,TlCi CPh is largely monomeric in aniline The properties of dimethylthallium compounds of the type Me,TlX are obviously very sensitive to the nature of the group X.Unfortunately no crystal structure determinations have been reported for those diorganothallium compounds which have been shown to be dimeric in solution so that little can be said about solid-state structures. 1.r. spectroscopy suggests that the C-Tl-C skeleton in a number of these derivatives is bent but 4 5 E. Krause and P. Dittmar Chem. Ber. 1930,63B 1953. 46 W. Beck and E. Schuierer J. Organometallic Chem. 1965,3 55. O 7 R. C. Menzies and A. R. P. Walker J. Chem. SOC. 1934 1131. 4 8 A. G. Lee and G. M. Sheldrick to be published. 4 0 H. Kurosawa K. Yasuda and R. Okawara Bull.Chem. SOC. Japan 1967,40 861 3 0 A. G. Lee and G. M. Sheldrick J . Chm. SOC. (A) 1963 1055. 51 A. G. Lee J. Chem. Suc. (A) in the press. 51 R. Nast and K. Kab J. Organometallic Chem. 1966 6 456. 318 Lee cannot distinguish the degree of association. Thus i.r. measurements indicate that the acetate group in Me,TlO,CMe is probably bidentate but cannot distinguish between a dimeric structure and a polymeric In such a case where the substituent attached to thallium has the possibility of chelating three structural possibilities must be considered; (i) the compound is largely ionic (ii) the compound is chelated and (iii) it is bridged to give a dimer or polymer. The very weakly polarising effect of the weak acid Me,TI+ is indicated by its slight tendency to hydrolyse.Stability constants for complex formation with /3-diketone ligands in aqueous solution are small only one B-diketone being co-~rdinated.~~ 1.r. spectra of solid dimethylthallium acetylacetonate suggest that the C-TI-C skeleton is linear and thus presumably only weakly ~ h e l a t e d . ~ ~ This is also consistent with the observation that very rapid exchange of acetyl- acetone occurs between dimethylthallium acetylacetonate and free acetyl- In benzene solution although some dialkylthallium /3-diketonates (including dimethylthallium hexafluoroacetylacetonate) are monomeric others consist of a mixture of monomer and dime~.~’~,b Again i.r. spectra suggest that Me,TISSPMe is ionic in the solid [Me,Tl] + [SSPMe,] -,58 whereas Me,TlSSP(OR) is monomeric in and may well have a chelated structure and Me,Tl(SSPEt,) is a mixture of monomer and dimer in chloroform Finally diethylthallium salicylaldehydate is monomeric in chloroform solution but an X-ray crystallographic study61 has shown that in the solid state it has polymeric structure.Monomer units such as (4) are associated by phenolic (at 1) and aldehydic (at 2) bridges to form infinite chains of nearly coplanar TI-0-TI-0 rings (5). Each thallium is thus six-co-ordinate with two ethyl groups and four oxygen atoms. Unlike many of the corresponding dialklyaluniinium compounds there is no tendency for dialkylthallium compounds to disproportionate to give mono- alkylthallium(m) derivatives. The dimethylthallium group would seem to be kinetically very inert it is possible to ring many changes with the anionic group while leaving the diorganothallium group unchanged.Thus it is possible to insert into the thallium-substituent bond leaving the dialkylthallium group unchanged,62 Me,TlOPh + SO -+ Me,TlOS(O)OPh Me,T1C5H5 + SO -+ Me,T10S(0)C5H No intermolecular exchange of alkyl groups occurs in solutions of dialkyl- G. B. Deacon and J. H. S. Green Spectrochim. Acta 1968,24 A 885. 6’ J. R. Cook and D. F. Martin J . Inorg Nuclear Chem. 1964,26 1249. 6 6 H. Kurosawa K. Yasuda and R. Okawara Znorg. Nuclear Chem. Letters 1965 1 131. G. E. Glass and R. S. Tobias J . Organometallic Chem. 1968 15 481. 6 7 (a) E. R. Wiltshire and R. C. Menzies J . Chem. SOC. 1932 2734; (h) C. Z. Moore and W. H. Nelson Inorg. Chem. 1969 8 143. 6 B G. E. Coates and R. N. Mukherjee J. Chem. SOC. 1964 1295. 6 n F. Bonati and G. Minghetti Znorg.Chirn. Acta 1969 3 161. 6 o F. Bonati S. Cenini and R. Ugo J. Organometallic Cheni. !%7 9 395 61 G. H. N. Milbam and M. R. Truter J. Cheni. SOC. ( A ) 1967 648. @ * A. G. Lee J . Chem. SOC. ( A ) 1970,467. 319 Organothallium Chemistry thallium derivatives and mixed dialkylthallium cations R1R2Tl+ have been prepared and found to be stable towards disproportionation to R21TI+ and Rz2T1+ 63 Et The thallium-carbon bonds in dialkylthallium groups can be cleaved by halogens and diorganothallium derivatives will also act as alkylating and arylating agents. Dialkylthallium chloride reacts with tin(@ halides to form a tin(rv) deri~ative:~ and diphenylthallium bromide reacts with mercury to give diphenylmercury :28 0 SnCl + R,TlCl+ R2SnCl + TIC1 Ph,TIBr + Hg 3 Ph2Hg + TlBr The Lewis acidity of diorganothallium derivatives has received only slight attention.For those dialkylthallium compounds which are associated complex formation with Lewis bases must be preceded by dissociation of the dimer or polymer or the latter process must at least occur simultaneously. As thallium compounds are in general weak Lewis acids anyway it is not surprising that dimeric dimethylthallium derivatives [Me,TlX X = OMe SMe SeMe,15 or C5H5 51] do not form stable complexes with trimethylamine. Dimethylthallium perchlorate and pyridine however react to give MezTICIO,.py which on the basis of i.r. evidence has been formulated as [Me,TIpy] + Clop- in which the M. Tanaka H. Kurosawa and R. Okawara Inorg. Nuclear Chern. Letters 1967 3 565. I' A. N. Nesmeyanov A. E. Borsiov and N.V. Novikova Zzvest. Akad. Nauk S.S.S.R. Otdel. Khim Nauk 1959 644. 320 Lee cation has a slightly distorted T-shape the Me,Tl portion being not quite linear.ss An X-ray crystallographic analysis of dimethylthallium( 1 10-phenanthroline) perchlorate has been performeds6 and the structure has been shown to consist of a distorted pentagonal bipyramid with one equatorial position vacant (6). The two covalent C-TI bonds and four other bonds definitely do not form the usual octahedral configuration expected for six-co-ordination. There is a 12" departure from linearity for the Me,TI group. . oc10 3 Mono-organothallium(1n) Derivatives In contrast to the readily prepared diorganothallium(rn) derivatives much controversy has surrounded mono-organothallium(n1) derivatives until very recently.Early claims to have prepared monoalkylthallium dihalides by reaction between dialkylthallium halides and thallium(Ir1) halides must now be discounted. Hot aqueous solutions of Me,TICI and TICI give impure TI3TlCl6 and various organic Monophenylthallium dichloride can however be readily prepared by reaction of phenylboronic acid and excess thaIlium(r~~) chlor- ides29b 30 PhB(OH) + TICI + H20 + PhTlC12 + B(OH)3 + HCl Phenylthallium dibromide can similarly be prepared from thallium(m) bromide but PhTlI is unknown addition of KI to either the dichloride or dibromide results in the rapid formation of iodobenzene. Pseudohalogen derivatives such as PhTl(CN) and PhTI(N,) can be made from PhTICI and the corresponding a1 kali-metal salt It would seem that mono-organothallium(I1I) derivatives have a survival problem in solution.Successive ionisation of a mono-organothallium salt RTlX2 would produce the cation RT12+ which could then fragment to produce R+ and thallous salts RTlX2 -+ R+ + TIX + X- 6 6 I. R. Beattie and P. A. Cocking J. Chem. SOC. 1965 3860. T. L. Blundell and H. M. Powell Chem. Comm. 1967 54. 6 7 D. Sarrach Z . anorg. Chem. 1962 319 16. 321 Organothallium Chemistry To stabilise a mono-organothallium(II1) derivative therefore the charge density on thallium must be reduced. Thus monophenylthallium(1n) derivatives will be more stable than monoalkylthallium(Irr) derivatives and an appreciably covalent thallium-substituent bond will also serve to stabilise the species. Thus the first monoalkylthallium(nI) derivatives to be isolated were diacetates.Methyl- thallium diacetate was prepared by the exchange reaction ? Me,TlOAc + Hg(OAc) + MeTl(OAc) + MeHgOAc Methylthallium diacetate reacts with Me,SnCN to form MeTI(CN) (0,CMe) this compound having a covalently bound CN group contains a C-TI-C group and thus shows the stability associated with such a group.69 This compound can also be prepared by the reaction of Tl(O,CMe) and ammonium methylpenta- fluorosilicate in aqueous solution in the presence of cyanide ion. In the absence of the cyanide ion the monomethylthallium(IJr) species formed reacts further to give a dimethylthallium species. 70 Methylthallium diacetate reacts with halide ions to produce a thallium(1) halide and methyl halides.68 Clearly if a methyl- thallium dihalide is first formed in these reactions then it must be too unstable under these conditions to be isolated.Mono-organothallium(1n) derivatives can also be formed by the so called ‘thallation’ and ‘oxythallation’ reactions. In the thallation reaction a thallium- group replaces a hydrogen attached to carbon to form a thallium-carbon bond. Thus heating thallium(m) isobutyrate with benzene produces phenylthallium di-isobutyrate in high yield (90 %) thallium(n1) trifluoroacetate reacts in an exactly analogous manner but under milder conditions :71a9 b T1(0,C*CX3) + C6H6 + PhTI(02C*CX3) + H20CCX3 Similarly thallium(n1) isobutyrate reacts with dibenzofuran to give dibenzofuryl thallium di-isobutyrate. 72 When thallium(n1) chloride reacts with dibenzofuran however bis(4-dibenzofuryl) thallium chloride is formed.73 L _I2 6 8 H. Kurosawa and R.Okawara J. Organometallic Chem. 1967 10 211. H. Kurosawa M. Tanaka and R. Okawara J. Organometallic Chem. 1968 12 241. 7 0 H. Hurosawa T. Fukumoto and R . Okawara Inorg. Nuclear Chem. Letters 1969,5,473. 71 (a) V. P. Glushkova and K. A. Jocheshkov Izvest. Akad. Nauk S.S.S.R. Otdel. Khim. Nauk 1957 1186; (6) A. McKillip J. S. Fowler M. J. Zclesko J. D. Hunt E. C. Taylor and G. McGillivray Tetrahedron Letters 1969,2423. 7 2 V. P. Glushkova and K . A. Kocheshkov Izvest. Akad. Nauk. S.S.S.R. Otdel. Khim. Nauk 1957 1391. 7 3 H. Gilman and R. K. Abbott J . Amer. Chem. SOC. 1943 65 122. 322 Lee Organothallium derivatives of the type RTl(Q,CMe) have also been prepared by oxythallation reactions of styrene o-allylphenol norbornadiene and nor- bornene. 74a9 b For example with styrene TI(OAc).q PhCH:CH + PhCH:CH - Tl(OAC) I MeOH OMe These organothalliuin compounds dethallate rapidly in acetic acid to yield both normal and rearranged products consistent with the intervention of carbonium ion intermediate^,^^ e.g.- TlOAc P~CH-CH~-T~(OAC)~ + Ph-CH-CH,+ I - OAC OMe I OMe + Ph-CH-CH,-OAc I OMe OAc / \ + PhCH,CH OMe Monoalkylthallium(n1) species also appear to be involved in the oxidation of olefins by thallium(rI1) salts. The kinetics of the oxidation of ethylene under aqueous conditions has been studied in detail and the following mechanism suggested 76 CH, CH2 + T13+ -+ H2C 3+ +Ha0 7 2+Tl-CH2CH20H + Hf " HOCH2CH20H + MeCHO - HSO t / ,*- H2C A number of other reactions of this type are known and agree with the above scheme they have been the subject of a recent review.77 A derivative similar to the postulated 7-intermediate may possibly be formed in the reaction between 74 (a) H.J. Kabbe Annalen 1962 656 204; (b) K. C. Pande and S. Winstein Tetrahedron Letters 1964 3393. '* P. H. Henry J. Amer. Chem. SOC. 1965,87 4423. 7 7 W. Kitching Organometallic Chem. Rev. 1968 3 61. R. Criegee Angew. Chem. 1958,70 173. 323 Organothallium Chemistry MaTICsH and hydrogen halides. The highly coloured compounds formed have been postulated to be .rr-complexes (7) on the basis of n m r . evidence. H *4 Many of these monoalkylthallium(m) derivatives appear to be associated in solution. Methylthallium di-isobutyrate is dimeric in chloroform solution and it has been suggested that both chelating and bridging butyrate groups are present .68 Methylthallium dioxinate is monomeric in solution and the thallium is probably five-co-ordinate.ss The monoarylthallium dihalides are stronger Lewis acids than the correspond- ing diarylthallium halides.Pyridine and triphenylphosphine both form 1 1 adducts with phenylthallium dihalides although no adduct is formed with the diphenyl thallium halides .3 O p 78 4 Cationic organothallium compounds Halide anions being Lewis bases can act as donors to give complex organo- thallium salts M+[R2TlXz]- M+2[RzT1X,]2- M+[RTlX,]- and M+2[RTlX4]2-.70a~b These derivatives can be prepared by direct union of the components thus 2 Bun4NI + Me2TlI 3 (Bu~~N)~(M~,TII,). In acetone solvent [Me2T113]2- ions are present but n.m.r. spectra of solutions in pyridine suggest that disproportionation to the starting materials has occurred.A preliminary report has appeared of the preparation of the [TIMe,]- ionso and cationic species containing the Tl(CiCPh),- ion have also been prepared:s2 T1Cl3.4NH3 + NaCiCR Na+[Tl(CiCR)J- 3T1I + 4 NaciCR -+ 2 T1 + Na [TI(CiCR),] + 3 NaI 5 Vinyl and Acetylenic Derivatives Trivinylthallium itself has not yet been reported; there is no reaction between 7 8 J. M. Davidson and G. Dyer J. Chem. SOC. (A) 1968 1616. '* (a) G. Faraglia L. R. Fiorani B. L. Pepe and R. Barbieri J. Organometallic Chem. 1967 10 363; (b) 0. Faraglia L. R. Fiorani B. L. Pepe and R. Barbieri Inorg. Nuclear Chem. Letters 1966,2 277. 80 C. A. Wilkie and J. P. Oliver quoted in 'Organometallic Compounds' vol. 1. G. E. Coates and K. Wade Methuen 1967 p. 373. 324 Lee divinylthallium chloride and vinyl-lithium.Divinylthallium halides can be prepared by treatment of thallium(n1) halides either with a vinyl Grignard reagent or with divinylmercury. 81 The reaction of di-l-alkenylmercurials and thallium(m) halides in ether has also been used to prepare di-cis and di-trans- propenylthallium bromide in high yield. 82a* b Cleavage of tetravinyltin by thallium(m) chloride gave a mixture of divinylthallium chloride and vinylthallium dichloride. 83 Redistribution of vinyl groups between divinylthallium halides and thal- lium(m) halides in aqueous solution gave vinylthallium dibromide and dichloride.81 (CH,:CH),TlCI + TIX3 3 2(CH,:CH)TICl The reductive cleavage of divinyl- and di-l-alkenyl-thallium halides by metallic mercury (at 40") and by tin(@ bromide (at 200") have been reported (CH,:CH),TIBr + Hg -+ (CH,:CH),Hg + TlBr (CH,:CH),TlBr + SnBr 3 (CH,:CH),SnBr + TIBr.The divinyl derivatives are probably stronger Lewis acids than the corresponding dialkylthallium derivatives. Thus (CICH:CH),TICI forms a stable 1 1 adduct with pyridine and ~iperidine.~~ The only acetylenic thallium derivatives reported are Me,TlCi CR and Me,TICiCTlMe, and the salts of the [TI(CiCR),]- ion which have already been discussed. 6 Pentafluorophenyl Derivatives Pentafluorophenyl derivatives of thallium can be prepared from the Grignard reagent in an analogous manner to that employed for the alkyl derivatives toluene T1Cl3 + 3C,F6MgBr + (C,F,),TI TlCI + 2C,F6MgBr -+ (C,F,),TIBr ether (reference 85) (reference 42b) Reaction between (C,F,),TlBr and silver or sodium salts allows the preparation of a variety of bis(pentafluoropheny1)thallium derivative~.~~bs 86 Because of the greater 'electronegativity' of the pentafluorophenyl group as compared to those of the alkyl groups pentafluorophenyl derivatives of thallium *I A.N. Nesmeyanov A. E. Borisov 1. S. Savel'eva and E. I. Golubeva Zzvest. Akad. Nauk. S.S.S.R. Otdel. Khim. Nauk 1958 1490. (a) A. N. Nesmeyanov A. E. Borisov and N. V. Novikova Zzvest. Akad. Nauk. S.S.S.R. Otdel. Khim. Nauk 1959,1216; (b) A. N. Nesmeyanov A. E. Borisov N. V. Novikova and E. I. Fedin J . Organometallic Chem. 1968,15,279. A. E. Borisov and N. V. Novikova Zzvest. Akad. Nauk. S.S.S.R. Otdel. Khim. Nauk 1959 1670. R. Kh. Freidlina A. K. Kochetkov and A. N. Nesmeyanov Zzvest. Akad. Nauk. S.S.S.R. Otdel. Khim. Nauk 1948,445. 86 J.L. W. Pohlmann and F. E. Brinckmann Z . Naturforsch 1965 20b 5 . I6 G. B. Deacon Austral J . Chem. 1967 20 459. 3 25 Organothallium Chemistry would be expected to be stronger Lewis acids than the corresponding alkyl derivatives. This is reflected in dimerisation and adduct formation. Tris(penta- fluoropheny1)thallium is monomeric in benzene solution. It does however form a 1 1 adduct with ether which is stable at room temperature although it is completely dissociated at Both (C6F5)2T1Cl and (C6F5),T~Br are di- meric in benzene presumably with bridging halogen atom~.~~b# 87 Bis(penta- fluoropheny1)thallium sulphate is thought to have the structure c6F5 0 CsFS \ II / \ C6F6 II / T1-O-S-0-TI 0 CSFS although dimethylthallium sulphate seems (on the basis of i.r. evidence) to be largely 2[Me,T1]+ SO4,-.A series of 1 1 complexes of (C,F,),TICl and (C6F5),TIBr with Ph,PO Ph,AsQ Ph3P and Ph,As have been prepared.s8aBb They are monomeric in benzene solution and presumably contain four-co-ordinate thallium. Bis(penta- fluoropheny1)thallium bromide dissolves in pyridine and the complex (C,F5),T1 py ,Br can be isolated. The five-co-ordinate complexes (C,F,),L,TlNO (L = Ph,PO or Ph,AsO) dissociate extensively in solution to (C6Fs)zTlLNOI and L.88a Cationic complexes containing four-co-ordinate thallium [Ph,P+ or Et 4N+] [(C,F5),T1X2]- (X = CI or Br) can readily be prepared,42b although the cor- responding iodide [(c6Fs)zT~Iz]- could not be formed.89 Furthermore (c,F,),TII is less stable towards decomposition to thallium(1) halide than are the other bis(pen tafluorophenyl) compounds.Presumably as a result of the ‘electronegativity’ of the pentafluorophenyl group exchange will occur with other metal phenyl derivatives. Thus (C,F,),TIBr reacts with both Ph,Hg and NaBPh to produce Ph,T1Br.42b There is no exchange of organic groups between dimethylthallium halides and NaBPh, and indeed the compound Me,TlBPh can be isolated. As with other organothallium derivatives (C6F5)2T1Br will react with a variety of metal halides to give thallium(1) bromide and the pentafluorophenyl derivative of an oxidation state which is two higher than that of the metal in the original compound. Thus with tin(@ chloride SnCl + (C6F5),T1Br -+ (C6F5),SnCI + TlBr. 87 G. B. Deacon J. H. S. Green and W. Kynaston J . Chem. SOC. (A) 1967 158. 8 6 (a) G. B. Deacon and R. S.Nyholm J. Chem. SOC. 1965 6107; (b) G. B. Deacon and J. H. S. Green Spectrochim. Acta 1969,25A 355. 8 9 G. B. Deacon and J. C. Parrott J. Orgunomefallic Chem. 1968,15 11. R. S. Nyholm and P. Royo Chem. Conim. 1969,421. 326 Lee 7 Or gano t hallium(1) Derivatives A. CyclopentadienylthalIium(I).- Cyclopentadienylthallium(1) is precipitated when aqueous thallium(1) hydroxide is shaken with cycl~pentadiene:~~ TlOH + C5Hs + TlCgH5 The methyl and higher-alkyl cyclopentadienyls hydropentalenide and iso- dicyclopentadienide derivatives can similarly be prepared from thallium(1) hydroxide but unlike TlC,H these products are air sensitive. sza9b The i.r. spectrum of TlC5H5 in the vapour phase suggests a highly symmetric moIec~le,~~ and the microwave spectrum has been interpreted in terms of a symmetric-top model with a planar C5H5 ring the distance from the thallium to the centre of the ring being 2.4 This relatively large distance suggests that the compound has considerable ionic character.In the solid state TlC5H5 consists of an infinite lattice containing zig-zag chains of -Tl-C5H5-T1-,94 and is thus quite similar to the low-temperature form of thallium(1) iodide.95 The standard free energy of formation of TlC,H has been measured as AGf"(298) = 42.3 k 0.5 kcal The unexpected stability of cyclo- pentadienylthallium toward water is merely a reflection of the low affinity of thallium for oxygen and the solubility of TlOH in water. Cyclopentadienylthallium(1) has recently been finding favour as a very mild reagent for the synthesis of .rr-cyclopentadienyl-transition-metal derivatives.97 Thus [C,H,Mo(NO)I,] + 2TlC5H5 + 2(C5H,),Mo(NO)I + 2 TI1 B.Other Organothallium(1) Derivatives.-Although monoalkyl- and monoaryl- thallium(1) derivatives have been postulated as reaction intermediates they have never been isolated. The reaction of thallium@ iodide and methyl-lithium in the presence of methyl iodide to form trimethylthallium has been postulated to proceed via methylthallium(1). The reaction of thallium(1) chloride with phenyl-lithium gives triphenylthallium and thallium metal even at - 7Oo.l3 With ethylmagnesium bromide thallium(1) chloride produces ethane + ethylene + thallium or alternatively diethylthallium bromide depending on the con- ditions employed. 9 8 Q 9 k c Reaction of aryl- and s-alkyl-magnesium bromides with thallium(1) bromide produces biphenyls and bialkyls respectively.With 91 F. A. Cotton and L. T. Reynolds J . Amer. Chem. SOC. 1958 80 269. and J. J. Mrowca J. Amer. Chem. SOC. 1967 89 1105. 93 J. K. Taylor A. P. Cox and J. Sheridan Nature 1959 183 1182. (a) L. T. Reynolds and G. Wilkinson J. Znorg. Nuclear Chern. 1959 9 86; (6) T. J. Katz E. Frasson E. Menegus and C. Panattoni Nature 1963 199 1087. G. Wykhoff 'Crystal Structures' vol. 1 2nd edn. 1965. 9 6 H. Hull and A. G. Turnbull Inorg. Chem. 1967 6 2020. R. B. King Inorg. Chem. 1968,7 90. 9 8 (a) R. C. Menzies and I. S. Cope J . Chem. SOC. 1932,2862; (b) H. Gilman R. G. Jones and L. A. Woods J . Amer. Chem. SOC. 1954 76 3615; (c) A. McKillop L. F. Elsom and E. C Taylor J . Organornetallic Chem. 1968 15 500. 327 Organothallium Chemistry ortho-substituted aryl Grignard reagents however the dialkylthallium bromide is formed rather than the coupled When triphenyl-thallium is heated above its melting point or when refluxed in xylene decomposition occurs to give metallic thallium and biphenyl.loO When carbon dioxide is passed into a boiling solution of Ph3Tl in xylene benzoic acid and biphenyl are formed in equivalent amounts.It has been suggested that phenylthallium(1) is first formed and then immediately carbonated COI Ph,Tl-+ Ph2 + PhTl+ PhCO2Tl No reaction occurred with carbon dioxide at room temperature and no biphenyl was formed when diphenylthallium benzoate (a possible alternative intermediate) was refluxed with carbon dioxide.13 Phenylthallium(1) has also been postulated as an intermediate in the reaction between Ph,Tl and benzophenone or benzonitrile.Electro-reduction of both dialkyl- and diaryl-thallium cations at the dropping mercury electrode gives bivalent and monovalent organothallium species but these undergo metallation reactions to give diorganomercury compounds and thallium amalgam.lOla~ The failure to isolate monoalkyl- and monoaryl-thallium(1) species is not so surprising. The stability of TIC,H is clearly associated with the stability of the cyclopentadienyl anion and TlC,H is largely ionic. An alkyl derivative of thallium(I) on the other hand would be expected to have a largely covalent TI-C bond because of the lower stability of the alkyl anion. Covalent thallium(1) compounds however are always stabilised by use of the empty p orbitals on thallium for intermolecular bonding (or conceivably for intramolecular bonding).Thus the thallium(1) alkoxides are tetrameric with bridging alkoxide groups. It has already been seen that the methyl group has little ability for bridging thallium atoms in thallium(m) derivatives and it is reasonable to postulate that the same will be true for thallium(1) derivatives. If then the thallium(1) akyls are going to be unstable three modes of decomposition must be considered (i) polymerisation to a thallium-thallium metal-bonded polymer (RTl)s (ii) decomposition to thallium metal and trialkylthallium and (iii) decomposition to thallium metal and decomposition products of the alkyl group. The first possibility is ruled out by the expected weakness of the Tl-Tl bond (although one compound containing such a bond K[T12Me,] has been the subject of a pre- liminary leaving the second and third possibilities both of which have been observed.8 Concluding Remarks In this Review I have tried to put together some of the scattered observations *@ A. McKillop L. F. Elsom and E. C. Taylor J. Amer. Chem. SOC. 1968 90 2423. loo H. Gilman and I. Haiduc J. Amer. Chem. SOC. 1968,90,5912. lol (a) G. Costa Ann. Chim. (Italy) 1950 40 559; (b) J. S. DiGregorio and M. D. Morris Analyt. Chem. 1968,40 1286. 328 Lee that have been made on organothallium chemistry to see whether any coherent picture can be made to emerge. It is obvious that even allowing for the recent increase in publications a great deal of structural work must yet be done before a completely satisfying description of organothallium chemistry can be achieved.Further at the present time the only organothallium derivatives to have been studied in any detail are the methyl and pentafluorophenyl. Studies of a wider range of organothallium compounds would be very worthwhile. Although a firm foundation for a study of organothallium chemistry has by now been laid much is still unaccountable. As Williams James said ‘round about the accredited and orderly facts of every science there ever floats a sort of dust-cloud of exceptional observations’ and in organothallium chemistry the light is only just beginning to break through the clouds. 329

ISSN:0009-2681    

DOI:10.1039/QR9702400310

出版商:RSC    

年代:1970

数据来源: RSC