摘要:
1132 J.C.S. DaltonPolarities and Directional Polarisabilities of Trimethylamine Adducts ofBoron Trihydride and Boron Trihalides. Stereospecif ic Solute-SolventInteractionsBy Robert S. Armstrong," Graeme J. Peacock, and Keith R. Skamp, School of Chemistry, University ofRaymond J . W. Le FBvre, School of Chemistry, Macquarie University, North Ryde, New South Wales,Sydney, Sydney, New South Wales, AustraliaAustraliaMolar Kerr constants are reported for the boron trihydride and the boron trihalide adducts OF trimethylamine assolutes in dioxan and benzene. The anisotropic electron polarisabilities of the adduct molecules are derived fromthe measurements in dioxan. The changes in polarisability on co-ordination are discussed. Estimates of thepolarisability parameters for a tetrahedrally disposed BX3 (X = H, F, CI, Br, and I ) are derived. The molar Kerrconstant data in benzene are interpreted in terms of stereospecific solute-solvent interactions.THE refractivity (and therefore mean polarisability) of asystem is markedly reduced by the development ofstituent pairs with polarity increments of three or moredebye units and refractivity contractions beyond prob-strong polarity within it.l The anisotropy of suchBX, (X = H, F, C1, Br, or I), formed from their con-1 R.J. W. Le Fbvre, in ' A4dvances in Physical OrganicChemistry,' ed. V. Gold, Academic Press, London and New York, diminutions is usually unknown. The molecules1965, VOI. 3, pp. 30, 641973 1133able errors of observations,l.2 are convenient for studysince their structures are axially symmetric and theirpolarisability ellipsoids should be ones of revolution.Accordingly we have made the measurements, herereported, of molar Kerr constants, dipole moments, etc.,of these complexes in dioxan or benzene, and from them(a) computed principal polarisabilities for each solute,and (b) estimated overall and anisotropic changes in thepolarisability resulting from co-ordination.A furtheranalysis provides group polarisabilities for the tetra-hedrally disposed BX, species. Interpretation of thedata for the adducts is in favour of stereospecific adduct-benzene interactions .EXPERIMENTALMaterials and Appuratus.-The solutes were prepared bypublished procedures : 2-4 trimethylamine-borane, sub-limed, m.p.92-94 OC, trimethylamine-boron trifluoride,vacuum sublimed, m.p. 146-147 @C, trimethylaniine-boron trichloride, white needles (from dry ethanol), m.p.TABLE 1E l 4 (.l)D 10713, 1012~rr',Dioxan 2.2090 1.0280 1.4202 0.068 0.0116Benzene 2.2725 0-87378 1-4973 0.410 04756238-2 3 9 "C, trimethylamine-boron tribromide, wlii teneedles (dry ethanol), m.p. 232-234 O C , and trimethyl-amine-boron tri-iodide, white powder (dry chloroform-lightwas checked by microanalysis (C, H, and halogen) and i.r.and n.m.r. spectra.Apparatus, techniques, symbols used, and methods ofcalculation have been described,5-* with the exception thatthe refractive indices for the dioxan solutions were deter-mined using a Wild precision spectrometer.The split-beam method for calculating n was employed.9 The dataof Table 1 apply a t 25 "C for the solvents used. Themeasurements are sumniarised in Table 2.Previous Measurements.-The following dipole momentestimates are recorded in the literature (which contains novalue for Me,N,BI,): v(Me,N,BH,) = 4-62 (C6H6),3 4.72y(Me,N,BCl,) = 6.29 (C,H,) ; p(Me,N,BBr,) = 6.60(C6H6)., The molar Kerr constants have not previouslybeen measured.(COH,), 4.66 (C4H,0z); lo p(Me,N,BF,) = 5.81 (C6H6) ;DISCUSSIONDioxan was chosen as a non-interacting solventenvironment. The chemical shifts of the sensor protonsin these adducts in dioxan were of similar magnitude tothose recorded in ' inert ' solvents such as chloroform l1and dichloromethane.12 No resonance was observedfor free trimethylamine at 139-5 Hz downfield fromtetramethylsilane.The possibility of dissociation ofthe adduct, with dioxan acting as a competing base forthe borane and boron trihalides, is thus excluded. Car-bon tetrachloride was precluded as a solvent by solu-bility limit at ions.TABLE 2(from observations on solutions a t 25 "C) *Polarisations, refractions, dipole moments, and molar Kerr constants of adducts Me,N,BX, (X = H, F, C1,Range ofX Solvent 1 06w2 a=1 P Y 6 coP,/cm3 R=/crn3 p / ~ a13 Dioxan 199-1679 36.1 -0.465 -0.013 42.5 462 25.6 4-62F Dioxan 72-258 32.9 0.162 - 37.5 717 23-8 5.81C1 Dioxan 11 0-509 28.3 0.212 0.034 -48.7 861 38.6 6-33Br Dioxan 306-1099 17.7 0.511 0.045 -123 945 47.5 6-61I Dioxan 197-539 13.2, 0.624 0.054 -227 1031 59.5 6-88H Benzene 451-1417 31.4,C -0.211 -0.073 C -25.9 462 - 4-62 eF Benzene 122-46 7 28.5' 0.132 J - - 46.9 713 - 6.81C1 Benzene 409-1265 24.7' 0.359 J - - 31.5 846 - 6.29Br Benzene 611-1905 15.3C 0.543C 0.037e -29.5 938 - 6.60 eI Benzene 207-525 10.3 0.692 0.061 -42.2 922 1 6-50 *Br, and I)1 o1 ( mK,22.733-4 f- 126- 472- 1219-217-571 f- 665 f- 849- 1592* The coefficients were derived from the observed quantities AE, Ad, An, and AB (the incremental changes in dielectric constant,density, refractive index, and Kerr constant, respectively, for solutions having solute weight fractions w,) by use of the followingrelations: = XAd/d,Cw,; y = ZAn/n,Cw,, and 6 = ZAB/BlXw2.a Calculated on the basis of DP = 1.05X~.Calculated value, determined as follows: RD(Me3N,BF3) = RD(Me,N:)T+RD(BF3) + RD(N-B), where RD(Me3N:) = 20.2 cm3 (R. J. W. Le FBvre and P. Russell, Trans. Favuduy Soc., 1947, 43, 3741,RD(BF,) = 6-0 cm3 [R. Gillis, Rev. Pure Afipl. Chern. (Austvalia), 1960, 10, 211 and Ru(N-B) = -2.3, (the mean value determinedfrom data concerning the chloride and bromide; see text). d Coefficients calculatedfrom data in ref. 2. The moment of the iodideadduct in dioxan is considered to be more accurate as experimental difficulties were encountered with the benzene solutions.f 101200 (mK2) derived neglecting contributions from y terms.= ZAE/ZW,;C Coefficients calculated from data in ref. 3.C Determined using the refractions a t the Na-D line, listed for dioxan as solvent.petroleum), m.p.cu. 200 "C (decomp.). All solutes wereprepared in a dry nitrogen atmosphere and their purity2 G. M. Phillips, J. S. Hunter, and L. E. Sutton, J . Chem. SOC.,1945, 146.3 C. M. Bax, A. R. Katritzky, and L. E. Sutton, J . Chem. Soc.,1958, 1258.4 E. L. Muetterties, J . Inorg. Nztclear Chem., 1960, 15, 182.6 C. G. Le Fhvre and R. J. MT. Le Fbvre, Rev. Pure Appl. Chem.6 R. J. W. Le FBvre, ' Dipole Moments,' Methuen, London,7 R. J. W. Le Fbvre and G. L. D. Ritchie, J . Chem. SOC., 1963,(Australia), 1955, 5, 261.3rd ectn., 1953.4933.Molecular Polarisabilities of the Adduck.-Each adductis assumed to have a tetrahedral disposition of valencies* C. G. Le Fbvre and R. J. W. Le FBvre, in ' Physical Methodsof Organic Chemistry,' ed.Weissberger, Interscience, New York,3rd edn., vol. 1, ch. !6, p. 2459.L. C. Martin, Optical Measuring Instruments,' Hackie,London, 1924.lo J. R. Weaver and R. W. Parry, Inorg. Chew., 1966, 5, 713.l1 J. M. Miller and M. Onyschuk, Canad. J . Chem., 1963, 41,l2 A. G. Massey and A. J. Park, J . Organometallic Chem., 1966,2898.5, 2181134 J.C.S. Daltonabout the nitrogen and boron atoms when it exists as asolute in an inert solvent environment. Structureanalyses support this upp position.^^ l4 The polarisabilityellipsoid associated with each of the molecules Me,N,BX3is one of revolution with b, # b2 = b,, where b, is theTABLE 3Proton magnetic resonance measurements (chemical shifts) *of the methyl protons in Me,N,BX,, in different solventmediaX Dioxan Chloroform l1 Dichloromethane l 2H 153.5 159-5 155F I52 157.3 156c1 175.5 180.2 182Br 186.5 190.8 189I 197.5 - 201* The spectra were determined on a Varian A60 spectro-meter.The solution concentrations were of the order of1-274. Tetramethylsilane was taken as internal reference,and the data recorded is the upfield chemical shift in Hz.electro-optical polarisability in the direction of themolecular three-fold symmetry axis and b, and b, arethe electro-optical polarisabilities perpendicular to thisaxis. In each case the permanent electric moment islocated along b,, so that p, = p(observed), p 2 = p, = 0.+FIGURE 1The molecular quantities bi and pa may be related to theelectronic polarisation E P and the molar Kerr constantby wav of the Lorentz-Lorenz and Langevin-Bornequations which, for the molecules Me3N,BX3 simplify to(1) and ( 2 ) .The terms EP, =P, N , k , and T refer, inEP = 4xN (b1 + 2b,)/9,K = 4TcN (DP(b, - b2)2/kT3P + (1)pob?(b1 - b2)/@T2)/405 (2)turn, to the electron polarisation, distortion polarisation,the Avogadro constant, the Boltzmann constant, andthe absolute temperature. In the absence of refractivitydispersion data cannot be directly evaluated and weassume in the calculations that EP = 0-95RD and theratio D P : Z P = 1.1. Substitution in equations (1) and( 2 ) of the and values of Table 2 (which were* The polarisability semi-axes of bonds or groups, bL, bT, orb, or of molecules b,, b,, or b, are quoted throughout in k3 units.t These values were calculated by use of the Cauchy relation-ship, from refractive index data taFulated in Landolt-Bornstein' Physikalisch-Chemische Tabellen, Springer, Berlin, 4th edn.,1912, p. 1021.$ Extrapolated from Rc = 19.791 and RG = 20.406 to giveEP = 19-34 cm3 (cf. A. I. Vogel, J . Chem. SOC., 1952, 514).obtained from measurements in dioxan) leads to themolecular polarisability semi-axes * in Table 4. Themeasured values in benzene were not used in the calcu-lations since solute-benzene associations are known tooccur.TABLE 4Polarisability semi-axes of the molecules Me,N,BX,X b, b, (= 63) bllb, x:bsc1 14.32, 14.61 0-98 43-54,H 9-71, 9.62 1.01 28.95F 9.02 8-93 1.01 26-88Br 17.23 18.2 1 0-95 53.65I 20.85 23.18 0.90 67.21Cha.Yzges ifz Polarisability 03t Co-ordination.-The re-fractions (at the Na-D line) of trimethylamine (20-2 cm3)and boron trichloride (20.9 cm3) t or boron tribromide(29.5 cm3) j- together exceed those measured for thecompounds Me,N,BCl, (38.6 cm3) or Me3N,BBr, (47.5 cm3)respectively.The refractivity contraction is ca. 2.5 cm3for the chloride and ca. 2-2 cm3 for the bromide. Experi-mental data are not available for computation of refrac-tion changes for the hydride, fluoride, and iodide. Theoverall reduction in refractivity, and hence polaris-ability, is better appreciated from the directional polaris-ability variations on formation of the N-B bond. Thefollowing approach was necessitated to account for thecontribution of the lone pair of electrons to the tri-methylamine group polarisabilities. Tolkmith l5 esti-mated the nitrogen lone-pair refraction to be 2.6 em3.It being assumed that BP = 0.95R~ for the nitrogen lonepair, the sum of the semi-axes, bi for trimethylamine,excluding the lone pair contribution, is extracted fromthe difference .P(Me&)*, - $(N:), by use of equation(1). From Kerr constant measurements of Me,N: inblMeaN + 2b2Me3N = 20.06 (3)cyclohexane l6 as solvent, the nitrogen lone pair beingassumed to be isotropically polarisable, equation (4) isobtained.Solution of equations (3) and (4) gives theb l x e a - b MW = -0.27' true ' semi-axes for the Me3N group (without the lonepair), b, = 6-51, b, = b, = 6.78. It is now possibleto derive values for AbihleaNsBX* = bi(obs)Me~N~BX~ -(biMesN + bfBX3>, which will indicate the directionalchanges in polarisability consequent upon co-ordinate-bond formation.Results are in Table 5. On formationof the N-B bond in these adducts there is an increase inpolarisability along the bond and a decrease in polaris-ability perpendicular to the bond. This is consistentwith a decrease in the angle subtended by bIAB-x with b,and an increase in the angle subtended by t5TB-X andbVB-X with b, (bL*-X > bTB-X = bVB-=; refer to Table 7 ) .Attempts to estimate the bond anisotropy of the N-B13 P. S. Bryan and R. L. Kuczkowski, Inorg. Chew., 1971, 10,14 P. H. Clippard, R. C. Taylor, and J. C. Hanson, J . Cryst.15 H. Tolkmith, Ann.N . Y . Acad. Sci., 1959, 79, 187.l6 R. S. Armstrong and K. R. Skamp, unpublished results.(4) 2200.Mol. Structure, 1971, 1, 3631973 1135link were unsuccessful, because of the apparent near-zero refraction of this b0nd.l' However assuming theB-X polarisability parameters are invariant in progress-ing from the 'isolated' BX, molecules to the co-ordinated BX, species, then we can obtain the longitu-dinal bond polarisability, bLN-B, by resolution : bluesNpBX8= blMeaN + 3 b ~ ~ - = cos2 704" + 3b~B-x sin2 70i" +bLx-B. Thus bLN-B emerges as 0.53 (fluoride adduct),1-22,(C1), 1.53(Br), and 1*23(H, assuming b ~ ~ - ~ =bTB-H = bVB-H = 0.61; see below).* The gradual in-crease in bLN-B is associated with the polarity incrementsin these adducts and a decrease in the N-B bonding these bond polarisabilities to the tetrahedral BH,situation, the semi-axes bi for the ' Me3NB ' moiety arebi(' Me,NB ') = bi(Me,N,BH,) - bi(BH,) so that b, =7*88,, b2 = b, = 7.79.Table 6 contains the values thusderived for the bi's of the BX, group and also comparesthem with the appropriate calculated and experimentalvalues for tetrahedral BX,, isolated BX, molecules, andthe analogous tetrahedral CX, group. The etheratespecies of BF, and BC1, represent a structurally andelectronically similar situation to those of the tri-methylamine adducts. Agreement is best in this case.In all instances the value for ZbiBX8 extracted from theTABLE 5Computation of directional polarisability changes arising from the formation of the adducts Me,N,BX,x b1(BX3) b2(= bJ(BX3) b, (calc.) b,(= b,) (calc.) b Abl 4(= b3)5- 1.71 0 2-57 0 8.22 9.35 +0.80 - 0.42c1 5.92 8.99 12.43 15.77 + 1.89, - 1.16Br 7-98 13-08 14-49 19.86 f2.74 - 1-65a R.S. Armstrong, M. J. Aroney, A. Hector, and R. J. W. Le FBvre, J. Chem. SOG. (B), 1968, 1203. b Calculated assuming theconstituent molecules are isolated, i e . , bicac = bi(Me,N) + bl(BX,), where b, lies along the 3-fold symmetry axis of Me,N andBX,. Determined using approximate link polarisabilities for bLB-F z 1.14, brB-F % 0.67 (ref. a).TABLE 6Tetrahedral BX, group polarisabilities compared with values derived from structurally similar situationsBX,, tetrahedral BX,, tetrahedral a BX,, tetrahedral(in Me,N,BX,) (in C,H,02,BX3) (calculated) CX,, tetrahedral cx bl bz = b3 Zbr b1 b, = b3 Cbg bl b2 = b, Zbi b, bz = b3 ZbiF 1-13, 1-14 3.41, 1.3 1.9 5.1 1-92 2.44 6.80 2.00 2-18 6.36 d c1 6.44 6.82 20.08 7.0 7.6 22.2 6-60 8.65 23.90 6.28 8-25 22.78 cBr 9.34, 10.42 30.18, 9.11 12.51 34.13 8.46 11-93 32.32fI 12.96, 15.39 43-74, 15.25 17-20 49.65 f6-43 8-33 23-09a See ref.a Table 5 . b Calculated assuming blB-5 remains invariant when the hybrid state of boron changes from s$2 t o sp3.c Extracted from data in refs. d, e, and f. Comparisons involving values determined from solution measurements in benzene arecomplicated by known solute-solvent interactions, viz., ref. e. d R. J. W. Le FBvre and C. G. Le FBvre, J . Chem. SOG., 1954, 1577;gaseous state data, CCI,F and CHF,.0 R. J. W. Le FBvre, D. V. Radford, G. L. D. Ritchie, and P. F. Stiles, J . Chem. SOC. (B),1968, 148; carbon tetrachloride solution data, HCC1, and H,CCCl,. f R. J. W. Le FBvre and G. L. D. Ritchie, J. Chent. SOC.,1963, 4933; benzene solution data, HCBr, and HCI,.distance 1 3 9 1 4 in progressing from the fluoride to theiodide. The nitrogen lone pair thus appears to retainits isotropy to a greater degree in the fluoride, relativeto the bromide. This apparent deformation of the lonepair is in line with the trends in the electron-pairacceptor properties of the BX, molecules.18Group Anisotropy of the Tetrahedyal Species BX,.-Forbond anisotropies inaccessible via the Kerr effect, LeF h r e 19,20 predicted a quantity Q = (l/~~~)~(bL*~/&f)s,where YAB is the internuclear distance of the bond AB,b L a the longitudinal polarisability of AB, is thereduced mass, and Q is empirically related to the i.r.stretching frequency vAB, by v = 92733 - 254.For theB-H link, utilising data for vlOB-H, ~llB-H,~l the YBHdistance,22 and assuming the B-H link to be isotropic,we find bLB-H = b,B-H = bTB-a = 0.61. Approximat-* Estimates have been made for bLN-B in the adducts Me,N,-BX,, by use of the ' infrared rule ' (see Discussion section), fromvalues of v(N-B) (cf. R. L. Amster and R. C. Taylor, SFectro-claim. A&, 1964, 20, 1487) and the N-B distances.13s14 Theylie between 1.1 and 1.4 (X = H, F, C1, and Er).l7 M. J. Aroney, R. J. TV. Le F h r e , and J. D. Saxby, Austral.J .Chem., 1965, 18, 253, and references therein.adducts are smaller than those from the isolated mole-cules and this may be caused by loss of p,.,-p, bondingin the B-X link and a change in the hybrid state ofboron consequent upon co-ordination. Further analysisof the BX, group anisotropy, by resolving B-X bondpolarisabilities, exemplifies these two changes. Thelongitudinal polarisability of the B-X link is markedlyreduced in the adduct, relatively to the free boron tri-halides, and this is characteristic when such a variationin charge distribution occurs (cf. Table 7). Thesecalculations are based on the assumption that the N-Banisotropy is invariant with X. It has been shownabove that this may not necessarily be the case, particu-larly when X = F.18 T.D. Coyle and F. G. A. Stone, in ' Progress in BoronChemistry,' ed. H. Steinberg and A. L. McCloskey, PergamonPress, London, New York, 1964, vol. 1, ch. 3 (and referencestherein).19 R. J. W. Le FBvre, PYOG. Chem. SOL, 1959, 363.20 R. J. W. Le FBvre, Austral. J . Chern., 1961, 14, 312.2 l W. Gordy, H. Ring, and A. B. Burg, Phys. Rev., 1950, 78,22 G. W. Bethke and M. K. Wilson, J. Chem. Phys., 1957, 26,512.11181136 J.C.S. DaltonSolute-Solvent Interactions.-Variation in the magni-tude of the Kerr constant of a solute measured indifferent solvents is a sensitive probe of solute-solventinteractions (see ref. e, Table 6). The molar Ken-constants in Table 2 point to benzene being an interact-ing solvent with these adducts. In each case them(mK2) in benzene is algebraically more negative thanin the inert solvent dioxan.That this association iselectrostatic is inferred when the dipole moment of the,Kc for the hypothetical 1 : 1 complex with the experi-mentally observed value implies that there is a probableequilibrium mixture of the type &1e3N ,BX, + C6H,Me,N,BX,,C,H,. Finally trimethylamine-borane showsstereospecific interactions with the solvent benzene, atboth ends of the molecule, as the deshielding of theborane sensor protons illustrates: 25 6 (in Hz) (solvent),-99(FzC1CCC1,F) ; -98-4(CDC13) ; -97-2(1,4-dioxan) ;-97*2(MeCN) ; -127*8(C$6). In the present work itTABLE 7The B-X bond polarisability semi-axesb LX bLB-" brB-X( = bvB-X) b; XbiH 0.61 0.61 1.00 1.83F 0.38 0.38 1.00 1.14c1 2.49 2.10 1.19 6.69Br 4.07 2.99, 1.36 10.06I 6-48 4.05 1.60 14-58Derived from adducts Me,N,BX,adduct is related to the difference in the upfield chemicalshift of the methyl protons in the n.m.r.spectra ofbenzene solutions relatively to dioxan solutions(A@Ege = P o x a n - BBenzene). This relationship isFIGURE 2approximately linear, as has been shown for other systemswhere a dipole-induced dipole mechanism is operating.23As suggested by Armstrong et aLZ4 the most favoured1 : 1 solute-solvent collision complex would have theTABLE 8Chemical shift and dipole moment data for Me,N,BX,molecules *X A8;:;& ILlDH 30.5 4-62F 42 5-81 c1 53 6-33Br 55.5 6-61I 57 6-88* Refer to footnote, Table 3.configuration which is favoured by (a) attraction of thepositive end of the Me,N,BX, dipole and the nucleo-philic n-system of the benzene ring and (b) repulsiveinteraction between the benzene x-electrons and the lonepairs of the X atoms.A comparison of the calculated23 T. L. Brown and K. Stark, J . Amer. Chem. SOG., 1965, 69,2679.b L -bLB"X bTB-X( = bvB-X) bT ZbiDerived from isolated BX, molecules, see ref. a, Table 5- - I I1-14 0.57 ca. 2 2-274-02 1.97 2.04 7.9611.38 6.06 2.66 2.28- - - -was found that 6 = -138 Hz for a ca. 3% benzene solu-tion. As the molar Kerr constant is still negative suchNMe' !\\Me i Mea bFIGURE 3an association can be sterically less important than thatpreviously mentioned although the sign does suggestTABLE 9Comparison of calculated molar Kerr constants for hypo-thetical 1 : 1 collision complexes Me,K,BX,,C,H,,1012,KC,,~,, with experimentally derived molar Kerrconstants, 1012, (mKz)X 1012,,KCa'C 1012co (rnKZ)PI: - 852 -217F - 1346 -571 c1 - 1745 - 565Br - 2254 - 849I - 3158 - 1592that the orientation of benzene molecules around theborane protons may be other than perpendicular to the24 R. S . Armstrong, M. J. Aroney, R. J. W. Le F&vre, R. K.Pierens, J. D. Saxby, and C. J. Wilkins, J . Chem. SOC. ( A ) ,1969. 2735. - I ~ - -2, C. W. Heitsch, lnorg. Ckem.. 1966, 4, 10191973 1137dipole moment direction (see Figure 3b). A cluster ofbenzene molecules could be so orientated about the BH,’sas to allow deshielding of these hydrogens whilst onlypartly offsetting the anisotropic contribution of the C6H6molecules at the positive end of the dipole.We thank Mr. R. K. Duffin for measuring the n.m.r.spectra. The award of a Commonwealth Research Student-ship (to K. Re s-) and of a grant by the Australian ResearchGrants Committee is acknowledged.[2/2039 Received, 30th August, 1972
ISSN:1477-9226
DOI:10.1039/DT9730001132
出版商:RSC
年代:1973
数据来源: RSC