摘要:

Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L. Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A.Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L.Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A. Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2

ISSN:0300-9599    

DOI:10.1039/F198985FX045

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Orga nising Com mitte e Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Or J. W. Goodwin Professor H. Hoff mann Professor A.L. Smith Professor P. Stenius Dr Th. F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and order-disorder phenomena will form topics for discussion.The preliminary programme is now availablemay be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-18 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e ) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Orga nising Com mitte e Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Or J. W. Goodwin Professor H. Hoff mann Professor A.L. Smith Professor P. Stenius Dr Th.F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and order-disorder phenomena will form topics for discussion. The preliminary programme is now availablemay be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-18 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e ) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.

ISSN:0300-9599    

DOI:10.1039/F198985BX047

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 85( 1 2) 3901 -4366 ( I 989) 390 1 3913 3927 3939 3953 3963 3973 3987 3995 401 1 401 9 403 1 4039 4047 4053 130 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS The first 20 papers of this issue were given at the 22nd International ESR Conference at Sheffield on 10th-14th April 1989, including the fourth Bruker Lecture by Professor J. S. Hyde. The Bruker Lecture. Alternatives to Field Modulation in Electron Spin Resonance Spectroscopy J. S. Hyde, P. B. Sczaniecki and W. Froncisz Dramatic Tensor-axis Non-coincidence Effects in the Electron Spin Resonance Spectra of some Low-spin Manganese(r1) Complexes R. D. Pike, A. L. Rieger and P. H. Rieger Structural Information from Powder ENDOR Spectroscopy. Possibilities and Limitations D.Attanasio The Use of Electron Paramagnetic Resonance Techniques in the Molecular Approach to Heterogeneous Catalytic Processes on Oxides M. Che, C. Louis and 2. Sojka Reactions of Scandium Atoms in Hydrocarbon Matrices J. A. Howard, B. Mile, C. A. Hampson and H. Morris Electron Spin Resonance Study of the Reaction of Group 11 Atoms with Ketene F. Genin, J. A. Howard, B. Mile and C. A. Hampson Reactions of Ag' Ions in Alcohols after Radiolysis at 77 K R. Janes, A. D. Stevens and M. C. R. Symons Experimental Evidence for the Hyperfine Interaction between a Surface Superoxide Species on MgO and a neighbouring Hydroxylic Proton E. Giamello, E. Garrone, P. Ugliengo, M. Che and the late A. J. Tench Mixed-ligand Complexes of Cul'- 1,lO-o-phenanthroline and its Analogues Characterized by Computer-aided Electron Spin Resonance Spectroscopy Y.Yang, R. Pogni and R. Basosi Electron Spin Resonance, ENDOR and TRIPLE Resonance of some 9,lO- Ant hraquinone Radicals in Soh tion. Part 2 .-Ant hraquinonesulp hona tes R. Makela and M. Vuolle Electron Paramagnetic Resonance Spectra of the Fe,(CO); Radical trapped in Single Crystals of PPN+FeCo(CO); J. R. Morton, K. F. Preston, Y. Le Page and P. J. Krusic Electron Paramagnetic Resonance Study of Transition-metal-ion Impregnated Brookite Titanium Dioxide Powders A. Amorelli, J. C. Evans and C. C. Rowlands Electron Paramagnetic Resonance Study of Monoclinic Zirconium Dioxide Polycrystalline Powders doped with Paramagnetic Transition-metal Ions J. C: Evans, C.R. Owen and C. C. Rowlands Binuclear Radical Complexes of Heavy-metal Fragments containing Ruthenium, Osmium, Rhodium and Gold S. Koblmann, V. Kasack, E. Roth and W. Kaim Quantitative Analysis of the Electron Paramagnetic Resonance Spectrum of a Uranium(m) Compound E. J. Soulie and P. C. Lesieur FAR IContents Electron Addition to Xanthine Oxidase. An Electron Spin Resonance Study of the Effects of Ionizing Radiation M. C. R. Symons, F. A. Taiwo and R. L. Petersen Stabilization of Biochemically Interesting Intermediates by Metal Coordi- nation. Part 5.--Complexes of Zn", Cur, Re' and Ru" with Singly Reduced 2,5-Diacetylpyrazine C. Bessenbacher, S. Ernst, S. Kohlmann, W. Kaim, V. Kasack, E. Roth and J. Jordanov Novel Electron Paramagnetic Resonance Signals from an Fe/S Protein containing Six Iron Atoms W.R. Hagen, A. J. Pierik and C. Veeger High-temperature Superconductivity and Electron Paramagnetic Resonance Spectroscopy J. R. Morton Electron Spin Resonance Investigation of the Electronic Structure of Hopping Centres and the Polaronic Conduction in Iron-containing Phosphate Glasses D. Narducci, M. Lucca, F. Morazzoni and R. Scotti 4063 407 5 4083 409 1 4099 - 4111 41 19 4129 4139 4147 41 57 4167 4179 4193 420 1 421 1 4227 4237 4247 Electron Paramagnetic Resonance Study of the Effect of Temperature upon Copper-impregnated Titanium Dioxide Powders A. Amorelli, J. C. Evans and C. C. Rowlands Effect of Magnetic Field on Radical Yields During the Photoreduction of Xanthene Dyes in Viscous Media E. S. Klimtchuk, G. Irinyi, I.V. Khudyakov, L. A. Margulis and V. A. Kuzmin Photoelectron and X-Ray Emission (PAX) Spectroscopy and Electronic Structure. Thiourea Ionic Solvation. Part 6.-Standard Potential of Ag/Ag+Cryptand (222,22 1 and 21 I) Electrode in Aprotic Media A. Lewandowski Influence of Ionic Association on the B coefficient of the Jones-Dole Equation for NaI in water-t-Butyl Alcohol Mixtures at 26 "C A. Kacperska, S. Taniewska-Osinska, A. Bald and A. Szejgis Temperature-programmed Desorption Study of Activated Chemisorption Involving a Precursor State: Desorption of Water from TiO, P. Malet and G. Munuera Intercalation Mechanism of Nitrogenated Bases into V,O, Xerogel B. Casal, E. Ruiz-Hitzky, M. Crespin, D. Tinet and J. C. Galvan Spectroscopic Studies of Molybdate Species deposited on a Nb,O, Support Y. S.Jin, A. Auroux and J. C. Vedrine Salt Effects in the Kinetics of the Formation of the Iron(1II) Thiocyanate Complex M. J. Capitan, E. Muiioz, M. M. Graciani, R. JimCnez, I. Tejera and F. Sanchez Molecular Self-diffusion of Methane in Zeolite ZSM-5 by Quasi-elastic Neutron Scattering and Nuclear Magnetic Resonance Pulsed Field Gradient Technique H. Jobic, M. BCe, J. Caro, M. Biilow and J. Karger Solubilization of Pentanol in Sodium Dodecylsulphate Micelles. Interpretation of Calorimetric Results Using a Theoretical Model I. Johnson, G. Olofsson, M. Landgren and B. Jonsson Viscosities of Alkali-metal Chlorides and Bromides in 2-Methoxyethanol at 25 and 35 "C D. Nandi and D. K. Hazra Kinetics of the Reaction between Substituted Thioureas and the Palladium(r1) Aquocomplex of ltl,7,7-TetraethyIdiethylenetriamine in Water and in Aqueous Micellar Solutions F.P. Cavasino, C. Sbriziolo, M. Cusumano and A. Giannetto In situ Spectroelectrochemical Study of the Passivation of Iron in Alkaline Solutions J. C. Rubim S. Luck, R. Foerch, K. D. Sales and D. S. UrchContents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L. Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D.Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A. Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution.Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L. Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A. Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2

ISSN:0300-9599    

DOI:10.1039/F198985FP155

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chern. Soc., Furuduy Trans. I, 1989, 85(12), 3901-3912 The Bruker Lecture Alternatives to Field Modulation in Electron Spin Resonance Spectroscopy James S. Hyde,*t Pawel B. Sczanieckit and Wojciech Fronciszs National Biomedical ESR Center, Department of Radiology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, U.S.A . In conventional ESR spectrometers, high-frequency magnetic field modu- lation is transferred to microwave amplitude and frequency modulation by the spin system, thereby overcoming low-frequency noise arising from the oscillator, the detection system and the environment. A serious dis- advantage of field modulation arises from the compromise forced on the spectroscopist by lineshape and signal-amplitude considerations. Over the years we and others have investigated alternatives to field modulation, but success has been minimal.The invention of the loop-gap resonator (LGR) with its very high energy density per watt of incident power and its low Q gives rise to technological opportunities that have led us to reinvestigate several of these alternatives, and these studies are reported here. It is true that the properties of the LGR give rise to improved performance in some of these alternative strategies, compared with cavity resonators. More significantly a new approach for avoidance of field modulation in ESR spectroscopy has been discovered which is called Electron Paramagnetic Rotary Resonance. The method is based on resonazce that occurs when the difference of two incident microwave frequencies matches the precession frequency in the rotating frame, following an analysis of Bloch. Pure absorption or dispersion lines (i.e.not derivatives) are observed with good baseline stability and freedom from microwave source noise. It is a transverse magnetization detection scheme, and not longitudinal. The hypothesis is made, based on first experimental results, that the method is a general one that can provide a viable alternative to field modulation. Magnetic field modulation was introduced into continuous-wave (CW) magnetic resonance spectroscopy by Bloch et a1.l The use of 100 kHz field modulation in EPR spectroscopy was introduced commercially by Varian Associates in 1959. It served to overcome three sources of noise. (1) The klystron has a high level of phase noise that decreases approximately as 1 /f, where f is the distance from the carrier.(2) Point-contact detector diodes have 1 /$dependent noise below 100 kHz and frequency-independent noise above. (3) 100 kHz is well above environmental acoustic noise and there is relatively little environmental electrical noise at this frequency. Field modulation satisfies the ‘transfer of modulation’ principle: 100 kHz appears as sidebands on the microwave carrier because of resonance in the spin system. There are numerous problems associated with field modulation. Most serious is the lineshape-line amplitude compromise. Only if the modulation amplitude is comparable to the linewidth is an optimum signal obtained, but in this circumstance resonance t The fourth Bruker Lecturer.$ On leave from the Institute of Molecular Physics, Polish Academy of Sciences, Poznan, Poland. 4 On leave from the Department of Biophysics, Institute of Molecular Biology, Jagiellonian University, Krakow, Poland. 390 13902 The Bruker Lecture information is found in higher harmonics of the field modulation. Although in principle they could be captured, in general they are lost. The resulting lineshape distortion cannot be recovered. Most spectroscopists give up a factor of 5 to 10 in signal amplitude (i.e. signal-to-noise) in order to avoid lineshape distortion. Another serious difficulty is loss of SIN for broad lines, particularly from organometallic complexes in unordered solids. There are also various minor complications including eddy-current effects in the resonator walls.For these reasons there has been a long and intensive search by numerous investigators for alternatives to field modulation. The early schemes are surveyed in the first and second editions of Poole's treatise.2* We decided to revisit this old and well explored area of ESR technology, and report our results here. Our working hypothesis was that recent developments in low-noise microwave amplifiers, sample containing structures (particularly loop-gap resonators') and oscillators with lower phase noise5 should solve some of the problems that gave rise to the early failures. The study has led us to what appears to be a new spin-physics experiment that may well open up a new branch of ESR spectroscopy. That experiment involves irradiation of the sample with the two coherent microwave frequencies that lie within the spin packet.The response of an NMR spin system under these conditions has been analysed by Bloch,6 Anderson,' Baldeschwieler* and Bucci et aZ.' Bloch pointed out in ref. (6), footnote 14, the very close relationship between this experiment and Redfield's rotary saturation experiment. lo Abragam" in his discussion of the Redfield experiment refers to it as 'rotary resonance', and indeed the phenomenon of saturation does not seem critical. Therefore, we propose that experiments of this general nature be classified as electron paramagnetic rotary resonance. Chiarini et all2 introduced the technique of 'longitudinal detection of electron spin resonance', LODESR, in which the effect of two incident microwave fields is monitored by a pickup coil along the polarizing magnetic field B, to detect time-dependent changes in longitudinal magnetization.Pinzino13 in a recent paper describes an ENDOR extension of LODESR, and gives a number of relevant citations to the method. In our view, LODESR would be a special sub- technique of electron paramagnetic rotary resonance, a field introduced by Redfieldlo and applied by him not only to NMR but also to ESR spectro~copy.~~ As will be discussed in more detail, if two coherent microwave species are applied to the spin system (call one the pump and the other the observe) the observe, according to ref. (6)--(9), has two resonances. For strong pump, both resonances approach zero amplitude. Moreover for the usual ESR conditions, the two lines are nearly perfectly overlapped and of equal and opposite intensities.Thus the presence of the pump destroys the observed ESR resonance. If the pump is square-wave amplitude modulated, the observed line is modulated. A transfer of modulation has occurred from the pump to the observe, and thus by detection of modulation riding on the observe carrier, the condition is satisfied for an ESR detection scheme with good baseline stability. Anderson' observed suppression of resonance in the presence of a strong second radiofrequency signal in NMR, see his fig. 3. This idea, which has occurred recently and about which we are enthusiastic, arose in the course of a long and somewhat disappointing series of experiments in which other modulation schemes were investigated.We choose, in the structure of this paper, to report these other experiments not because they led to our ideas of electron paramagnetic rotary resonance historically, but rather because they provide considerable insight into the technique as it finally evolved. Experimental Experiments were performed initially with a Bruker ER200D-SRC spectrometer equipped with ER 047 MRP microwave bridge. This bridge contains a pin-diode switch,J. S. Hyde, P. B. Sczaniecki and W. Froncisz 3903 level-set phase detector crystal I 1 L G R t- loop-gap resonator ii Fig. 1. (a) Reference arm bridge with locus shown for microwave modulation schemes that are physically external to the bridge. (b) Internal modulation locus. suitable for microwave amplitude modulation, and also an external reference cavity for the AFC.Later experiments were performed with a Varian E4 equipped with an El01 bridge to which an external reference cavity had been added for AFC. Both bridges were modified in various ways depending on the experiment, as described in more detail later. Key microwave components were : Anaren single sideband modulator, model 90338- DC, Triangle Microwave model XN8 1 HAD2 single pole double throw pin diode switch, and Triangle MP-72 biphase modulator. A Hewlett Packard model 8566 spectrum analyser was used to characterize the various modulated microwave signals. A PAR model 124A lock-in amplifier was used to demodulate audio signals, All experiments were performed at X-band using a loop- gap resonator as described by Hubbell et aI.l5 Results and Discussion Straightforward Amplitude Modulation Fig.1 shows two possible positions at which microwave modulators of various types can be inserted into a conventional reference-arm bridge. The reader is referred to ref. (16) for a contemporary discussion of this type of bridge. The various improvements described there can readily be incorporated into the simplified diagrams of fig. 1. In the figures below, blocks A or B will be given that fit into either fig. 1 (a) or 1 (b). Fig. 2 shows a simple scheme based on a microwave single-pole-double-throw switch3904 The Bruker Lecture Fig. 2. SPDT pin-diode modulation scheme. If the output is matched, modulation of reflected power is reduced compared to an SPST switch. The device can be used at either position A or position B.for modulating the power incident on the sample. It can be at either position A or position B. Fig. 3(a) shows a scheme for modulating the power reflected from a resonator without affecting the incident power. Fig. 3(b) shows a biphase 180" modulator for modulating the power incident on the resonator. The biphase modulator and the SPDT switch in fig. 3 could be interchanged. The use of a biphase modulator gives a factor of two higher signal intensity than the use of a SPDT switch. All of these circuits have been built and evaluated using 100 kHz modulation. The advantage of the structure of fig. l(b) is that it is external to the bridge so that an expensive piece of equipment, namely the microwave bridge itself, is not at risk because of the switching around of microwave components.The power reflection coefficient of a matched resonator close to resonance can be written where the so-called frequency tuning parameter 6 is given by The loop-gap resonator has a very favourable efficiency parameter A, defined by A = B,/P%. ( 3 ) It is between 5 and 10 times higher than for the usual cavity resonators. Combining these equations, one obtains Consider a modulation sideband 100 kHz from the carrier incident on the resonator, and compare a loop-gap resonator with a cavity resonator readjusting Po until B, at the sample is in each case the same (say a level that satisfies a well defined condition of partial microwave power saturation). P,(cavity) - Qi(cavity) A~(LGR) P,(LGR) Qi(LGR) A2(cavity) 'J. S.Hyde, P. B. Sczaniecki and W. Froncisz 3905 biphase + A IA A Fig. 3. (a) Biphase modulator. The double circulator scheme of this figure is equivalent to putting the biphase modulator at position B of fig. 1(b). (b) The SPDT switch modulates the signal reflected from the resonator without affecting the incident power. The biphase modulator could, alternatively, modulate the reflected signal. Or, as still another alternative, the SPDT switch could be used to modulate the incident power. Values of Qo typically differ by a factor of 10, so this ratio is ca. 40 dB. Modulation sidebands are enormously suppressed using LGRs, and it is this point that led us to build and evaluate the structures of fig. 1-3. One can ask if it makes any difference whether one modulates incident or reflected microwave levels.The above calculation is only for the first sideband, when square-wave microwave amplitude modulation is employed. Since the higher harmonics decrease linearly and the resonator reflection coefficient increases quadratically as a function of resonator tuning parameter 6 = (cu - w ~ ) / Q ~ , higher harmonics can cause a lot of trouble.3906 The Bruker Lecture T l r n m Fig. 4. LGR two-loopone-gap cross-section. The structure is made from machineable glass- ceramic and silver plated. Microwaves are coupled by a coaxial line to the larger loop and the sample goes in the smaller loop. See ref. (15). (a) Re-entrant pick-up loop, (b) multi-turn pick-up coil. The lowest possible modulation frequency should be used and the rise and fall times smoothed out to reduce higher harmonics.Enhancement of higher harmonics with incident modulation is absent when modulation of the reflected wave is employed. In general, straightforward AM fails to satisfy the transfer of modulation principle. Microwaves are modulated in the same way as the signal. There is one possible aspect of the technique that is favourable, however. The resonator can turn AM to FM (and vice versa), depending on its Q and the sample can also turn AM into FM (i.e. dispersion) depending on T,. If Q/a, + T,, detecting dispersion could result in improved baseline stability relative to detection of absorption. This argument is illustrative of the concept of transfer of modulation, i.e. from AM to FM. Of course ‘transfer of modulation’ by the spin system can only be effective if microwaves incident on the resonator are modulated.J.S. Hyde, P . B. Sczaniecki and W, Froncisz A B 777Hz SWITCH - 3907 Fig. 5. Microwave circuits used for electron paramagnetic rotary resonance. They are equivalent. The circuit of 5(b) was used here. We report that none of these schemes worked well enough in our hands to warrant encouraging another investigator to try them. There was drift and excess noise. The noise seemed to arise from up-conversion of noise within 1 Hz of the carrier to the modulation frequency and the drift seemed to arise from thermal variation of the bridge balance or cavity match. Of course we were able to obtain good signals from DPPH because of its great intensity.However, because the transfer of modulation principle is violated, straightforward amplitude modulation cannot be considered a viable general purpose ESR spectroscopic method. Detection of Mz Poo1e2T3 gives several citations where bridges analogous to fig. 2 and 3 were employed and the resulting time variation of the z component detected with a pick-up coil oriented parallel to B,. The same pick-up coil technique was employed by Chiarini et a1.12 and more recently by Schweiger and Ernst. l7 This technique eliminates microwave source noise and bridge instabilities completely, and is the ultimate ' transfer of modulation ' method : from microwave to audio frequency via the spin system. These various authors3908 The Bruker Lecture Fig. 6. Superheterodyne- type detection of electron spin resonance (Fremy's salt).No modulation of any kind was used. 90" Difference in 100 kHz reference phase. generally report good success and illustrate their papers with nearly noise-free spectra. We therefore built the structures shown in fig. 4 using a two-loop-one-gap X-band resonator as described in ref. (1 5). Fig. 4(a) has very good filling factor for the pick-up loop and fig. 4(b) permits more turns. Coil Qs in these small structures were not very good (of the order of 10). Although signals were detected, the sensitivity was poor. Very careful shielding of the pick-up coil was necessary. We did not build a buck-out coil assembly, as suggested by Feng and Bloembergen," but probably that would be helpful. In any event, signal levels were low.Abragamll gives the following expression for the signal-to-noise ratio for a crossed coil in a CW magnetic resonance induction experiment [ref. (1 I), p. 83, eqn (76)]: Herefis noise from the detection apparatus, q the filling factor, Av the bandwidth of the receiver (typically in ESR 1 Hz), 6 is the sample volume, and Ho the polarizing magnetic field. This equation has been discussed in considerable critical detail by Hoult and Richards.lg It occurs to us that this equation can be used to estimate the relative sensitivity for a pick-up coil that detects a modulation of the z-component of magnetization compared to another pick-up coil that detects magnetic induction. We ask the reader to accept the assumption that detection of ESR by induction or by a reflection bridge must be essentially similar.It is further assumed that M,, My and M, are similar in magnitude (the condition of partial saturation), and that f and Av are the same. We have Y(induction) q(LGR) v(RF) Q(LGR) 5 Y(M, pick-up) = [ q(coi1) v(AM freq.) Q(coi1) I ' The highest AM frequency is of the order of l / q , say lo6 typically. The LGR Q is ca. 500. The geometry of fig. 4 suggests a microwave filling factor ca. 5 times higher than the pick-up coil filling factor. At v = lolo, (7) Y (induction) Y(M, pick-up) = i.6x 103.J . S. Hyde, P. B. Sczaniecki and W. Froncisz 3909 Fig. 7. Superheterodyne-type detection of electron spin resonance (Fremy’s salt) as a function of pump power attenuation (dB) at a frequency 100 kHz from the observe frequency.The geometry of fig. 4 is exceptionally unfavourable for detection of time-varying changes in M,. Quite generally this is true for all geometries. One does not develop schemes to detect M, if the principal motivation is spectroscopy. It is an insensitive technique. However, as numerous authors have pointed out, it can be a useful technique for obtaining information on longitudinal relaxation. 2o If one wanted to detect M,, one should not use an LGR because of the appearance of in eqn (6). A large microwave structure should be used, the larger the better. Not only does increase, but the Q of the pick-up coil will improve as it gets larger. Detection of time-modulated M, is more natural if the available sample is large, and detection of radiofrequency magnetization using a loop-gap resonator is more natural if the sample is small. As was stated at the beginning, this paper is directed towards improved general purpose spectroscopy.It is apparent that the LODESR technique of Chiarini et al. does not have the requisite sensitivity for general usage. However, their good success in spite of low sensitivity encouraged us to look at schemes involving two incident microwave signals but with r.f. detection. The next section reports the results. Electron Paramagnetic Rotary Resonance We consider the microwave circuits of fig. 5, which are equivalent. The microwave signal produced by the single sideband modulator is the observe and the original carrier signal is the pump. Consider first the situation with no pump power incident on the sample.An ordinary ESR signal is produced at the single sideband microwave frequency. Let us assume that the modulator is driven at 100 kHz. After microwave detection, there will be an ordinary ESR signal at 100 kHz, and after amplification and phase-sensitive detection at 100 kHz, a signal exists that can be displayed on the recorder. Fig. 6 shows such a signal. As the 100 kHz phase is changed by 90°, the lineshape goes from absorption to dispersion. This is the familiar action of an ordinary superheterodyne receiver where the local oscillator is fixed an intermediate frequency away from the signal oscillator, and, after amplification at the intermediate frequency, i.f., phase-sensitive detection occurs. One can go between dispersion and absorption not only by changing the microwave reference39 10 I .. . . . . . . . I . . . . . . . . . 1 . . . . . . . . . 1 . . . . . . . . . I . . . . . . . . . I . . . . . . . . . a . . . . . . . . . I . , . . . . . . . I . . . . . . . . ~I . . . , . . . , Fig. 8. Electron paramagnetic rotary resonance signal. The same sample was used as for fig. 6. The sample is below the microwave power saturation limit for both pump and observe frequencies. The final integrating time constant was 0.1 s. 777 Hz pump AM. phase, but also by changing the 100 kHz phase, which is a familiar superheterodyne characteristic. There is nothing special about fig. 6 except that it is in fact a d.c. spectrometer display, and it is not too bad. Note that the noise changes on going from absorption to dispersion. Apparently oscillator phase noise is suppressed by the correct setting of the microwave and 100 kHz reference phases.Consider next the application of pump microwave power to the display of fig. 6 (see fig. 7). As the pump level increases, the signal disappears with a slightly asymmetric residual. This result can be understood referring to the papers of Bloch' and Anderson.' These collaborators independently solved the problem of two radiofrequencies simultaneously incident on a single transition and published their calculations back-to- back in Physical Review, cross-referencing each other. Considerable insight can be gained in going back and forth between the calculations. In the presence of a strong coherent pump, two resonances are obtained at frequencies given by ( c o ~ - c o , ) ~ = (W,--W~)~+Y~H;.(9) Here w, is the resonant frequency in the absence of the pump, HI is the pump level at frequency w, and w2 is the observe frequency. The intensities of the signals are SIGNALS k cos e,(i k cos e,)2 (10) where Y s *l tan 8, = -. a 2 -a1 In the limit that the pump level HI goes to zero, only one resonance at w, is obtained, as expected. This is ordinary ESR as seen in fig. 6. As the power increases, the term cose drives both signals to zero. Moreover, since they are shifted by an amount much smaller than the linewidth (w2 -wl corresponds to 36 mG), the residual terms cancel to a good approximation. The predictions of eqn (9)-(11) seem well fulfilled in fig. 7. Consider finally periodic amplitude modulation of the pump.A frequency of 777 Hz was used to avoid accidental beats of harmonics with 100 kHz. An auxiliary lock-in amplifier at 777 Hz was used to amplify and phase sensitive detect the output of theJ . S. Hyde, P . B. Sczaniecki and W. Froncisz 391 1 70 I 60 50 en 0) 1- v) 8 40 C 2 30 2 El 20 !? l o t 0 ‘ I I 0.1 1 pump power/mW Fig. 9. Electron paramagnetic rotary resonance signal from Fremy’s salt. A, The, ratio of Ppump/Pobs was held constant and they were varied together. The abscissa is in (mW)r for Pobs. B, The observe level was held constant at one of the indicated levels and the pump power varied. Po,,/mW: (a) 0.14, (b) 0.07, (c) 0.035. 100 kHz receiver. Thus the signal level goes back and forth between the two extremes of fig. 7. The results are shown in fig.8. The same sample was used for both fig. 6 and 8. Fig. 9 shows preliminary results of physical studies of the resonance. We observe that as long as the ppmp power is greater than the observe power, the signal is linear in (observe power)z. This, of course, is gratifying since it is the familiar experience of ordinary ESR spectroscopy. In fig. 9 B, the observe power was held constant at level (a), (b) or (c) and the pump power varied. It does in fact seem desirable to keep the pump power ca. 10 times higher than the observe power. In another experiment we checked to see if any source noise was coming through the amplifier train. Increase of noise could be seen at incident levels of the order of 5-10 mW. Because of the high microwave efficiency parameter A of the LGR, this level corresponds favourably to a cavity resonator.However, analysis of noise sources and further refinement of the equipment is appropriate.3912 The Bruker Lecture Conclusions It might be said that we should have called the effect studied here ‘anti-electron paramagnetic rotary resonance ’. We are not observing a rotary resonance signal; we are depending on the fact that the process of rotary resonance destroys the ordinary ESR signal. Nevertheless, we defend the nomenclature because the existing theoretical provides an excellent guide to understanding the phenomenon. It is emphasized again that modulation of the second coherent pump microwave signal turns the resonance on and off because of specific spin-sensitive processes.Thus it is argued that we have satisfied the transfer of modulation principle in an optimum manner. The work presented in this Bruker lecture is very new. The range of conditions needs to be explored in much greater depth. We will be guided in this effort by the more extensive experience in the LODESR technique. On samples where it has worked, the present method should work. The low Q of the loop gap resonator will facilitate introduction of two microwave levels of rather different frequencies, which is an obvious parameter to vary. The high-energy density per watt of the LGR will facilitate the requirement that the pump level be much greater than the observed microwave level. This work was supported by grants GM27665 and RRO1008 from the National Institutes of Health. Electrical engineer T. Camenisch’s skill in organizing the equipment is gratefully acknowledged. References 1 F. Bloch, W. W. Hansen and M. Packard, Phys. Rev., 1946, 70, 474. 2 C. P. Poole Jr, Electron Spin Resonance (Interscience, New York, 1967). 3 C. P. Poole Jr, Electron Spin Resonance (Wiley, New York, 2nd edn, 1983). 4 W. Froncisz and J. S. Hyde, J . Magn. Reson., 1982, 47, 515. 5 R. A. Strangeway, T. K. Ishii and J. S. Hyde, IEEE Trans. on Microwave Theory and Techniques, 6 F. Bloch, Phys. Rev., 1956, 102, 104. 7 W. A. Anderson, Phys. Rev., 1956, 102, 151. 8 J. D. Baldeschwieler, J. Chem. Phys., 1964, 40, 459. 9 P. Bucci, M. Martinelli and S. Santucci, J . Chem. Phys., 1970, 53, 4524. 1988, 36, 792. 10 A. G. Redfield, Phys. Rev., 1955, 98, 1787. 1 1 A. Abragam, The Principles of Nuclear Magnetism (Clarendon Press, Oxford, 1961). 12 F. Chiarini, M. Martinelli, L. Pardi and S. Santucci, Phys. Rev. B, 1975, 12, 847. 13 C. Pinzino, J. Magn. Reson., 1989, 81, 318. 14 G. Whitefield and A. G. Redfield, Phys. Rev., 1957, 106, 918. 15 W. L. Hubbell, W. Froncisz and J. S. Hyde, Rev. Sci. Instrum., 1987, 58, 1879. 16 J. S. Hyde and J. Gajdzinski, Rev. Sci. Instrum., 1988, 59, 1352. 17 A. Schweiger and R. R. Ernst, J. Magn. Reson., 1988, 77, 512. 18 S.-Y. Feng and N. Bloembergen, Phys. Rev., 1963, 130, 531. 19 D. I. Hoult and R. E. Richards, J . Magn. Reson., 1976, 24, 71. 20 K. J. Standley and R. A. Vaughan, Electron Spin Relaxation Phenomena in Solids (Plenum Press, New York, 1969), pp. 132-142. Paper 9/01607D; Received 17th April, 1989

ISSN:0300-9599    

DOI:10.1039/F19898503901

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Farada-y Trans. I , 1989, 85(12), 3913-3925 Dramatic Tensor-axis Non-coincidence Effects in the Electron Spin Resonance Spectra of some Low-spin Manganese( 11) Complexes Robert D. Pike, Anne L. Rieger and Philip H. Rieger* Department of Chemistry, Brown University, Providence, Rhode Island 02912, U.S.A. ESR spectra are reported for frozen CH,Cl,/CICH,CH,CI solutions of 14 low-spin manganese(I1) cations : [(q5-C,H5)Mn(CO)L,]+, L = 1 /2 Ph,PCH,CH,PPh, (dppe), 1 /2 Me,PCH,CH,PMe, (dmpe), 1 /2 Ph,PCH,PPh, (dppm), PMe,, PPh, ; [(q5-MeC5H,)Mn(CO)dppe]+; [(q5-6-exo-PhC,H,)Mn(CO)L,]+. L = 1 /2 dppe, 1 / 2 dmpe, PMe, ; [(q5-6-em-PhC,HMe,)Mn(CO)dppe]+; [(q5-6-exo-PhC,H,)Mn(CO)dppe]+; [(q5-C,H,-Mn(CO),PPh,l'; and [(q5-6-exo-PhC,H,)Mn(CO),L]+, L = PMe,, P"Bu,. The spectra show anisotropic 55Mn hyperfine coupling and nearly isotropic 31P hyperfine coupling.There are dramatic departures from first-order line spacings which result from non-coincidence of the X and 2 principal axes of the g and the j5Mn hyperfine tensors. In most cases, the ESR parameters can be interpreted in terms of a semi-occupied molecular orbital (SOMO) primarily d,z-yz in character, but rotated about the y axis to avoid an antibonding interaction with the dienyl ring. The spectrum of [(q5-6-exo-PhC,H,)Mn(CO)(PMe3),]+ shows non-equivalent 31P coup- lings, suggesting an unsymmetrical dienyl ring conformation. This spectrum, and those of the monophosphine cations, are best explained in terms of a SOMO primarily d,, in character. Elucidation of the mechanism of a reaction initiated by one-electron oxidation of an organotransition metal complex depends, at least in part, on understanding the nature of the highest occupied molecular orbital (HOMO).There have been several attempts to characterize this orbital in the manganese(1) complexes, [(q5-dienyl)Mn(CO),-rL L,]. Thus, for example, Connelly and Kitchen' obtained some insights through a correlation of CO stretching frequencies and electrochemical oxidation potentials for a series of such complexes with n = 1,2 and dienyl = C,H,-, Me,, x = 0, 1,5. Such data are available for Mn' complexes with other dienyl ligands.' An X-ray diffraction study2 of structural changes accompanying one-electron oxidation has led to further insights into the nature of the HOMO in [(y5-6-exo-PhC,H,)Mn(CO)dppe]. There have been several studies of this and related systems using extended Huckel molecular-orbital theory calculations.', Although ESR spectra have been recorded for solutions thought to contain the 17- electron Mn" they were poorly resolved, and no serious attempt has been made to obtain anisotropic ESR spectra, the parameters of which would allow a detailed characterization of the semioccupied molecular orbital (SOMO).Low-spin Mn" complexes are relatively rare, but there is ample precedent for high-quality ESR spectra of such species. Thus, for example, isotropic ESR spectra have been reported for series of complexes such as [(q5-C5H,)Mn(NO)L,], where L, is an anionic dithi~lene,~ and [(q5-C5H5)Mn(CO),L], where L is an anionic nitrogen base.5 The isotropic 55Mn hyper- fine couplings observed in these studies ranged from 45 to 60 G, calling into question the spectra with (aMn) = 93 G reported for [(y5-MeC,H4)Mn(CO)L,lf (L = 1/2 dppe, PPh,, PMePh,).' Since Mn(ClO,), in acetonitrile gives a spectrum with (AMn) = 94 G,6 the observed spectra were probably due to high-spin Mn" decomposition products.39133914 Tensor- axis Non - coincidence Efe c ts There is also precedent for high-quality anisotropic ESR spectra. Several ESR studies of [Mn(CN),NO]'- in dilute crystals and in frozen solutions were reported in the 1960s.' More recently, Wilkinson and co-workers have reported ESR spectra of frozen toluene solutions of [Mn(o-(CH,),C,H,} (dm~e),],~ and [MnMe,(dm~e),],~ and Connelly and co- workerslO have reported a well resolved ESR spectrum of [trans-Mn(CO)(CNBu)(dppe),]+ in a CH,Cl,/tetrahydrofuran glass.The spectrum of the o-xylylene complex exhibits tensor-axis non-coincidence effects" similar to those we will discuss here. There have been several ESR studies of related Cr' species.12.13 The recent dilute single-crystal ESR study13 of CpCr(CO), and CpCr(CO),PPh, is most definitive, but the failure to resolve hyperfine structure in the PPh, derivative leaves the detailed electronic structure of this species in some doubt. In this paper, we report the frozen-solution ESR spectra of 14 low-spin Mn" species of the type [(y5-dienyl)Mn(CO)L,]+ and [(y'-dienyl)Mn(CO),L]+, where L is a phosphine ligand. The spectra exhibit particularly dramatic effects resulting from non-coincidence of the principal axes of the g and nuclear hyperfine tensors.The interpretation of the ESR parameters led us to an unusually detailed picture of the SOMOs in these molecules. Experimental Instrumentation ESR spectra were recorded using a Bruker ER220D spectrometer, equipped with a Bruker variable-temperature unit, a Systron-Donner 6246A microwave frequency counter, and an ASPECT-2000 computer. The magnetic-field calibration was checked by measuring the DPPH resonance. NMR and IR spectra were recorded using Bruker WM250 and Mattson Alpha Centauri FT-IR spectrometers. Chemicals Solvents were obtained from Fisher ; acetonitrile, dichloromethane, and 1,2-dichloro- ethane were purified by distillation under nitrogen from CaH,. Tetrahydrofuran (THF) was distilled under nitrogen from benzophenone ketyl.(y5-Cyclopentadi- eny1)manganesetricarbonyl and (~5-methylcyclopentadienyl)manganesetricarbonyl were used as obtained from Strem. (y6-Benzene)manganesetricarbonyl hexafluorophos- phate, l4 (y5-6-exo-phenylcyclohexadienyl)manganesetricarbonyl, l5 (y5-6-exo-phenyl- pentamethylcyclohexadieny1)manganesetricarbonyl l6 and (y5-6-exo-phenyl-cyclo hep ta- dieny1)manganesetricarbonyl '' were prepared by literature methods. Trimethyl- phosphine (1 mol dm-, in THF), bis(dipheny1phosphino)methane (dppm), triphenyl- phosphine, tri-n-butylphosphine and silver tetrafluoroborate were supplied by Aldrich, bis(dipheny1phosphino)ethane (dppe) was obtained from Strem, and bis- (dimethy1phosphino)ethane (dmpe) was obtained from Alfa.Preparation of Manganese(1) Complexes The preparation, purification and reactions of the following complexes were carried out in a tube sealed under argon or in an argon-filled glove box. The complexes (y5-6-exo- PhC,H,(CO)L,, L = 1/2 dppe (la), 1/2 dmpe (lb), and PMe, (Id), (y5-6-exo- PhC,HMe,)Mn(CO)dppe (2a), (y5-6-exo-PhC,H8)Mn(CO)dppe (3a), CpMn(CO)L,, L = 1/2 dppe (4a), 1/2 dmpe (4b), 1/2 dppm (&), and PMe, (4d), and (q5-MeC,H,)Mn(CO)dppe (5a) were prepared by UV photolysis (at 350 nm in a Rayonet photochemical reactor) of the THF solutions of the corresponding tricarbonyl and the appropriate ligand (1 : 2 mole ratios).". l9 CpMn(C0) (PPh,), (4e) was prepared similarly except that a large excess of the ligand was required; CpMn(CO),PPh, (4e') wasR.D. Pike, A . L. Rieger and P . H. Rieger 391 5 Table 1. Infrared dataa compound neutralb catione ref. (PhC,H,)Mn(CO)dppe (la) (PhC,H,)Mn(CO)dmpe (lb) (PhC,HMe,)Mn(CO)dppe (2a) (PhC,H,)Mn(CO)dppe (3a) CpMn(CO)dppe (4a) CpMn(C0)dmpe (4b) CPMn(CO)dPPm (44 MeCpMn(C0)dppe (5a) (PhC,H,)Mn(CO),PMe, (la') (PhC,H,)Mn(CO),P"Bu, (If') (PhC,H6)Mn(C0) ('1 'pMn(') 'pMn(") (PPh3), CpMn(CO),PPh, (4e') 1830 (1 840) 1812 1814 1825 1853 1833 (1 8209 1810 1837 (1850d) 1810 (1831d) 1835 (1824) 1831 (1826) 1858, 1927 1856, 1924 (1875, 1935") 1864, 1932 (1863, 1931) 1941 (1941b) 1913 1920 1 906b 1940b 1932 1913 1939' 1918 1932 (1918b) 1926 (1926b) 1954, 1998b 1952, 1993 1965,2046 (1965, 2045b) - 2 - - 28 28 28 1 1 - - - 29 1 - ' Literature values in parentheses. In CH,Cl,.In CH,Cl,/C,H,Cl, unless otherwise noted. In THF. " In pentane. obtained when a lower ratio was employed.' The complexes were purified by chromatography on a neutral alumina column from which they were eluted with hexane+ther. 2a was also recrystallized from hexane at - 78 "C; la, 3a, 4a and 4c were further purified by Soxhlet extraction with hexane. The yield of l b was low as the major product eluted from the alumina column proved to be (q5- 1 -PhC,H,)Mn(CO)dmpe, as shown by an X-ray crystal structure determination.20 Precedents for such a thermal isomerization have been reported.21 The complexes (~5-6-exo-PhC,H,)Mn(CO)2L, L = PMe, (Id') and PBu, (If') were prepared by reacting [(C,H,)Mn(CO),]PF, with the appropriate ligand in CH,CN under incandescent lamp illumination to give the monophosphine derivative.22 The product was precipitated with ether and treated with phenyl Grignard reagent in CH,C12 to give the desired products16 which were purified by chromatography on neutral alumina and elution with hexane+ther.IR data for all 14 complexes, as well as the corresponding Mn" cations, are given in table 1. Analyses (by Schwartzkopf Laboratories, Woodside, NY, or Galbraith Laboratories, Knoxville, TN), uncorrected melting points and NMR data are given in table 2 for the previously unreported compounds. Preparation of Radical Cations The purified Mn' complexes were dissolved in a 1 : 1 mixture of dichloromethane and 1,2- dichloroethane in an argon-filled glove box. An approximately equimolar amount of AgBF, was added and the mixture shaken until all the silver appeared to have precipitated.The silver was removed by filtration through a Celite plug in a Pasteur pipette and the solution placed in an IR cell or ESR sample tube. ESR tubes were capped with a ground-glass stopper and sealed with Parafilm, removed from the glove box as quickly as possible, and frozen by immersion in liquid nitrogen. The frozen samples were then transferred to the ESR cavity, precooled to 120 K, and spectra recorded. Isotropic spectra were obtained later after melting the solution.3916 Tensor-axis Non-coincidence Efects Table 2. Analytical and NMR data compound m.p./"C analysis (YO)' lH NMR [d(ppm), J/Hzlb lb 66 C, 58.3 (58.8) H, 7.01 (6.96) Id 79 C, 58.6 (58.5) H, 7.71 (7.44) 2a 45 C, 77.0 (74.8) H, 7.19 (6.37) 3a 155 (dec) C, 70.7 (73.8) H, 5.48 (5.69) 4b 82 (dec) C, 48.2 (48.3) H, 7.01 (7.05) Mn, 18.2 (18.5) I d 94 C, 59.9 (59.6) H, 5.96 (5.85) 7.0-7.4 (m, Ph), 5.40 (br t, H3) 4.36 (br t, H2v4), 3.32 (t, 5.1, H6), 2.44 (br t, H1*5), 1.80 (d, 22, CH,), 1.43 (dd, 22, 7, CH3)C 6.9-7.4 (m, Ph), 4.90 (br t, H3), 4.38 (br t, H2*4), 3.40 (br t, H6) 2.52 (br t, H1v5), 1.43 (br t, Me)d 7.0-7.6 (m, Ph), 2.95 (br s, CH,), 2.62 (s, Me3), 2.28 (s, Me2g4), 1.91 (s, M e l ~ ~ ) ~ 6.9-7.7 (m, Ph), 5.35 (br t, H2), 4.88 (br t, H4), 4.21 (t, 7.8, H1) 4.14 (t, 11, H5), 3.17 (d, 8.3, H') 2.2 (m, CH,), 1.7 (m, H 6 y e n d o ) 0.5 (td, 12, 3.1, H6,ezo)c 4.06 (t, 1.9, Cp), 1.33 (dd, 3.1, 5.8, Me), 1.2 (m, CH,) 0.88 (dd, 3.0, 5.0, Me)" 7.0-7.2 (m, Ph), 5.46 (t, 6.4, H3), 4.58 (9, 5.2, H2*4), 3.73 (t, 6.0, Hs) 3.09 (t, 6.0, H1+5), 1.29 (d, 8.5, Me)d a Calculated values given in parentheses.multiplet, br = broad; for ring-numbering systems, see ref. (15x17). s = singlet, d = doublet, t = triplet, q = quartet, m = In CD,Cl,. In CDCl,. In C6D6. Results Only three of the isotropic ESR spectra were sufficiently resolved to permit extraction of g values and hyperfine coupling constants; the parameters are given in table 3. Isotropic spectra of the other cations cover approximately the same field range and apparently have very similar parameters. In contrast, all 14 Mn" cations gave reasonably well resolved and qualitatively similar, frozen-solution ESR spectra. The spectra of la+, 3a+, and 4d+, which are typical, are shown in fig. 1-3. In each case, apparent 'parallel' features (1 : 2: 1 triplets or 1 : 1 doublets) at the low- and high-field ends can be distinguished from the ' perpendicular ' features in the centre. The triplet or doublet spacing is essentially the same for all features and is equal to the 31P coupling in the corresponding isotropic spectra when these were resolved; thus the 31P coupling is nearly isotropic.Careful examination of a spectrum revealed that the 'parallel ' features are unevenly spaced (even after accounting for second-order shifts). Furthermore, although the high- and low-field features have similar spacings, typically ca. 120 G, they do not belong to a single six-feature sequence. If the 55Mn hyperfine tensor components are to average to the isotropic coupling, 50-60 G, there can be only one component as large as 120 G.This behaviour can be understood, and analysed quantitatively, by assuming that two of the 55Mn hyperfine tensor axes (labelled x and z ) are rotated, relative to the g-tensor axes, through an angle p. The qualitative effect of this rotation is to produce two sequences of 'parallel' features with centres determined mainly by g, and g,, but with spacings determined mainly by A, (assuming A, < A,). Each feature corresponds to a different molecular orientation which minimizes or maximizes the resonant field. The positions of these features thus depend strongly on four parameters (gz, g,, A, and 8) andR. D. Pike, A . L. Rieger and P. H. Rieger Table 3. Isotropic ESR parameters" la+ 2.052 62 30 1 d' 2.048 56.7 24.4, 32.0 4d' 2.057 50.6 30.9 a Hyperfine coupling constants in units of lo-* cm-'.3917 3000 3200 3400 3600 3800 B /G Fig. 1. (a) ESR spectrum of [(y5-6-exo-PhC,H,)Mn(CO)dppe]+ (la+) in CH,Cl,/CICH,CH,Cl at 120 K ; (b) computer simulation using the parameters of table 4. weakly on A,. Given a value of A,, the other four parameters can be determined quite accurately from the 'parallel' features alone. Fortunately, most of the spectra have two or three well resolved 'perpendicular ' features, the positions of which depend more strongly on A,, permitting an accurate and unique fit. The remaining parameters (g, and A,) were usually less obvious. Only the spectrum of 4d+ is sufficiently well resolved to permit the unique determination of g , and A,. Many of the other spectra have a large, rather broad, 'perpendicular' triplet which appears to be the y feature with A , z 0; the position of this triplet gives g,, after correction for the second-order hyperfine contribution.The ESR parameters, least-squares-fitted to the field positions of the spectral feature~,'~ are given in table 4. Computer simulation of the spectra using these parameters gives, in most cases, a very satisfactory account of the positions and relative intensities of the spectral features. In those cases where A , z 0, the simulations would3918 Tensor-axis Non-coincidence Efects I 1 I 1 1 1 1 , , # I 1 1 , , 3000 3200 3400 3600 3800 B /G Fig. 2. (a) ESR spectrum of [($-6-exo-PhC,H8)Mn(CO)dppe]+ (3a+) in CH,Cl,/ClCH,CH,Cl at 120 K; (b) computer simulation using the parameters of table 4.2800 3000 3200 3400 3600 B /G Fig. 3. (a) ESR spectrum of [(q5-C,H,)Mn(CO) (PMe,),]' (a+) in CH,Cl,/ClCH,CH,Cl at 120 K; (b) computer simulation using the parameters of table 4.3919 R. D. Pike, A . L. Rieger and P. H . Rieger Table 4. Anisotropic ESR parametersa 1 a+ 1 b+ Id+ 2a' 3a+ 4a+ 4b+ 4c+ 4d' 4e' 5a+ Id'+ If" 4e'+ 2.080 2.072 2.082 2.080 2.050 2.188 2.187 2.164 2.141 2.190 2.179 2.085 2.090 2.188 2.057 2.050 2.058 2.058 2.033 2.02 1 2.02 1 2.015 2.032 2.023 2.023 - 2.034 1.994 1.996 2.006 1.991 1.996 2.001 2.000 1.992 1.997 2.001 2.000 2.0 1 2.009 2.002 39.9 0 141.8 27.4 27.3 42.7 0 142.1 29 28.7 ca. 13 ca. 25 129.2 24, 32 CQ. 64 44.2 0 142.0 25.9 21.9 44.7 0 146.9 24.0 21.0 37.6 0 114.2 28.5 43.0 35.6 0 115.3 31.0 44.5 43.2 0 124.0 30.0 29.3 16.0 16.5 119.0 31.7 55.0 42.0 0 114.4 28.4 43.5 41.7 0 116.1 28.2 45.0 104 31 ca.75 ca. 10 12. I - 105.1 28.9 76.5 ca. 10 32.9 98.4 29.8 73.9 - a Hyperfine coupling constants in units of cm-'. not be significantly altered for A , = (0-10) x cm-l, but since most of the intensity of the observed triplet comes from the m, = - 1/2 component, gy is also somewhat uncertain. The parameters given in table 4 have g , > g, and A , < A,, which gives p < 45" in most cases. Interchanging either g , and g , or A , and A , would change the non-coincidence angle to (90" -p). The spectrum of Id+ is well resolved with many assignable features which gave a good least-squares fit. Nonetheless, there is a significant discrepancy between the experimental and computer-simulated spectra.It appears that all of the g and hyperfine tensor principal axes are non-coincident in this case. Thus there is significant mixing of the nominal x and y features in the perpendicular region of the spectrum which was not accounted for in the analysis or simulation. This case is unique in several other respects : (i) as noted above, the isotropic spectrum of Id+ shows non-equivalent 31P couplings; in the anisotropic spectrum, the central features of the 1 : 2 : 1 triplets are very broad, again suggesting non-equivalent 31P nuclei. The apparent asymmetric cyclohexadienyl ring conformation applies to the neutral parent as well since the 'H NMR spectrum of Id shows two distinct PMe, resonances at 198 K.24 (ii) As in the case of the monophosphine cations, Id'+, If'+ and 4e'+, the non-coincidence angle p is very large, suggesting that the x and z labels should be interchanged ; furthermore, Id+ is the only monocarbonyl cation with g, > g,.There is a different kind of discrepancy between the experimental and computer- simulated spectra of all the cyclopentadienyl cations, including 4d'+ and 5a+. In these spectra, the positions of the features are well fitted, but the low-field 'parallel' features are much less intense than expected, with the amplitudes decreasing with decreasing Im,l ; the high-field ' parallel ' features have normal amplitudes, as do the ' perpendicular ' features. The effect is not simply due to linewidth variations. In all the spectra, the ' parallel ' features are broader on the low-field end and there is a small increase in width with decreasing lmll ; the high-field ' parallel ' features are narrower, but the widths increase with increasing lmJ.In the case of the cyclopentadienyl cations, the amplitudes on the low-field end decrease much more rapidly than expected from the increasing linewidths. The molecular orientations corresponding to these features may be a clue to these effects. For la+, for example, the low-field 'parallel ' features (m, = - 5/2, - 3/2, - 1/2) correspond to B = 50, 62 and 80°, while the high-field features (m, = 1/2, 3/2, 5/2) correspond to 8 = 5, 11 and 15", where B is the polar angle measured from the3920 Tensor-axis Non-coincidence Eflects g-tensor z axis. Thus the widths appear to increase, and the amplitudes decrease, as the molecular orientation approaches the g-tensor x axis.We have no explanation for this effect. The average of the 55Mn hyperfine tensor components is close to the measured isotropic coupling in those cases where the isotropic spectra are resolved; we assume that this is a general result and that all tensor components have the same sign. We also assume that the isotropic 55Mn hyperfine coupling arises primarily from spin polarization and thus is negative. Discussion Symmetry Considerations The X-ray structure of la+ shows that the saturated carbon of the cyclohexadienyl ligand lies above the carbonyl ligand ;’ thus, neglecting the small distortions resulting from the ethene bridge of the dppe ligand, the cation has approximate C, symmetry.The cyclopentadienyl-substituted cations should also have C, symmetry (4c+, 4df, 4e+ and 4e’+ nearly exactly, 4a+ and 4b+ at least approximately). Id+, which has non-equivalent 31P couplings, clearly does not have even approximate C, symmetry and so must have a different cyclohexadienyl ring conformation from la+. The symmetries of the other cations are less clear, but we will assume C, symmetry. The normal to the reflection plane, the y axis in our coordinate system, is necessarily a principal axis of both the g and 55Mn hyperfine 25 In a low-spin d5 pseudo-octahedral metal complex, the semi-occupied molecular orbital (SOMO) is expected to be one of the ‘ tZg’ set (d,,, d,, or d,2-,2). In C, symmetry, the metal d,2, d,2-,2 and d,, orbitals belong to the a’ representation and the d,, and d,, orbitals to the a’’ representation so that the metal contributions to the SOMO are given either by la’) = a1x2 - y 2 ) + blxz) + cJz2) (1 a) or by la”) = djyz) + elxy).(1 b) Extended Huckel MO calculations for [(q5-C,H,)Mn(CO) (PH3)z]+2g 26 or [(q5-C5H5)Mn(CO) (PH3),]+26 suggest an a’ SOMO with a > b, c. Similar calculations for [(q5-C5H5)Mn(CO)2PH3]+3~ 26 suggest an a” SOMO with d > e. Interpretation Assuming an a’ SOMO For a SOMO given by eqn (1 a), the g-tensor components are given by where the parameters 6, are given, for example, by where CMn is the spin-orbit coupling constant and ck,,, is the LCAO coefficient of d,, in the kth MO with energy Ek. Since the ‘eg’ orbitals are expected to be significantlyR.D. Pike, A . L. Rieger and P. H. Rieger 392 1 higher in energy than the ‘t2g’ set, the parameters, a,, and d,~, are probably negligible compared with d,,, d,, and d,~-,2. Neglecting terms in d,, and d,~, the principal values of the g tensor are given by g , = g, - 2[b2 + (a + 2/3 c)‘] a,, g y = g, - 2[(a - ~ ’ 3 c)28x2 + b2dZ2-,2] gz = ge. (4 c) The neglected term in d,, would be expected to make g, somewhat less than ge, in agreement with most of the experimental results. The g-tensor X and 2 axes are displaced from the corresponding molecular axes by the angle Pg, which is given by 2(a + .t/3 c) b (a + 2/ 3 c) - b2 * tan28, = The hyperfine tensor components, corrected for spin-orbit coupling, are given by the (6 a) (6 b) (6 c) (6 d) equations A,, = A , + P[f(a2 + b2 - c2 - 2 1 / 3 ac) + Agxx -&(Ag,, + Agzz)] A,, = A , + P[5(a2 - 2b2 - c2 + 2 2/3 ac) + Aggy -&(Agxx - Agz,)] A,, = A , + P[$( - 2a2 + b2 + 2c2) + AgZz +&(Agxx + Ag,,)] A,, = P[+b(3a + .\/3 c) +gx,] where for 55Mn, P = 207.6 x lo-* ~ m - ’ .~ ~ The tensor is diagonalized by rotation about the y axis by the angle PA, given by 2b(3a + 4 3 c) + Zg,, (3a + 2/3 c) (a - 2/3 c) + yAgXx -&,, -yAg,, * tan2PA = - (7) Like the angle P, required to diagonalize the g tensor, PA increases with d,, admixture. However, since the angles are of opposite sign, the experimental angle p is the sum of the magnitudes of D, and PA. The principal values derived from eqn (6) give A - ‘ A ’ = (a2 - 2b2 - c2 + 2 2j3 ac) + iAgyy - gAgxx - AAg,, ;P r*)2 = [(3a2 - 3c2 - 2 2/3 ac) + $Agyy - yAgxx + yAgz,]2 + [2b(3a + .\/3 c) + 2gxZl2 which were used with experimental values of A,, A , and A , and an assumed ratio, r = a/c, to obtain a set of LCAO coefficients (a, b and c).(It was found that a and c must have the same sign, but the relative sign of b is indeterminate.) The angles B, and PA were then computed using eqn (5) and (7). The calculation was repeated for a range of r values until /? = l&+ IpAI. Initially, it was assumed that Ag,,, Agyy and AgZ2 were equal to Agx, Agy and Agz; subsequently, Agxx and Ag,, were corrected using D, and the fitting procedure iterated until & and PA were constant. The results of this analysis are given in table 5. The hybridization of the metal contribution to the SOMO varies considerably among the cations listed in table 5, and this variation masks an important feature.If an orbital of the same shape were constructed using only dX2-,2 and d,*, then, in all cases the hybrids would be more than 90% dX2-,2 in character. Admixture of small amounts of d2z character into dX2-,2 with LCAO coefficients of the same sign has the effect of increasing (8 b)3922 Tensor-axis Non-coincidence Efects Table 5. Composition of semi-occupied orbitals la+ 1 b+ 2a+ 3a+ 4a+ 4b' 442' 4d' 4e+ 5a+ 0.707 0.707 0.696 0.728 0.680 0.689 0.678 0.664 0.679 0.679 90.0 5.2 4.8 17.6 9.7 88.6 5.7 5.7 18.7 10.0 92.1 3.3 4.6 14.2 7.7 92.9 3.0 4.1 13.5 7.5 69.0 7.4 23.6 33.7 9.3 68.3 8.3 23.4 34.7 9.8 80.7 4.3 15.0 21.7 7.5 62.8 21.8 15.4 37.4 17.6 68.2 6.4 25.4 35.0 8.5 68.6 7.2 24.2 36.0 9.0 the electron density in the y 2 lobes and decreasing that in the x2 lobes; some electron density appears along the z axis, but this is really an extension of the x2 lobes, which can be thought of as an unsymmetrical doughnut in the xz plane (as the d,z character increases, the doughnut becomes more symmetrical until, at 25 70 d,n, it is round and the hybrid has the dZZ shape, but with the major lobe along the y axis).Increased electron density in the y 2 lobes suggests an antibonding interaction of the x2 lobes. With this in mind, consider the orientation of the x2 lobes. If we assume that the molecular z axis is defined by the strongly interacting carbonyl ligand, PA = 0 corresponds to one lobe of d,l-yt bisecting the P-Mn-P bond angle, consistent with the Mn-P d n 4 n bonding interaction implied by the X-ray structures of l a and The other x2 lobe is directed towards the ring carbon atom above the carbonyl group.The rotation of the SOMO about the y axis, PA = 18O, thus would reduce the apparent antibonding interaction with the dienyl ring, as shown in fig. 4. A similar conformation for lb+, 2a+ and 3a+ would be consistent with the comparable rotations of the SOMOs in these cases, 19, 14 and 14", respectively. In la+, the ring carbon above the carbonyl is tilted up out of the dienyl plane. In the cyclopentadienyl cations, 4a+, 4b+, 4c+, 4d+, 4e+ and 5a+, the corresponding carbon is in the plane of the ring and the antibonding interaction is probably much more important; a larger SOMO rotation might be expected.For these cations, PA ranges from 34 to 37" for the monodentate and five- membered ring bidentate phosphines; 4c+ has PA = 21", but with a four-membered dppm ring, this case is most likely far from the idealized geometry. This explanation receives some support from another feature of the parameters of table 5 . In the cyclohexadienyl and cycloheptadienyl cations, the average d-electron spin density is 0.71 0.01 whereas in the cyclopentadienyl cations, the average spin density is 0.67f0.01. Although the difference is small, it appears to be real and may reflect increased delocalization of spin density into one of the carbonyl n* orbitals in the case of the cyclopentadienyl cations. Although our arguments are based entirely on the ESR parameters, the conclusions are consistent with those reached by Connelly et a1.,2 from extended Huckel MO calculations.Perhaps our most surprising result is that the SOMO shape and metal character are relatively insensitive to the nature of the dienyl and phosphine ligands. Indeed, the biggest difference among the various cations is the change in SOMO orientation with dienyl ligand. Cations with a'' SOMO While the above analysis gives a coherent explanation of the ESR spectral parameters for most of the cations studied, the bis-PMe, cation, Id+, and the monophosphineR. D. Pike, A. L. Rieger and P. H. Rieger 3923 Fig. 4. Schematic drawings showing relief of the antibonding interaction of the metal dz2-y2 orbital with the dienyl ring through rotation about the y axis by PA = 15" for [(q5-C,)Mn(CO)L,]+ and 8, = 35" for [(q5-C,)Mn(CO)L,]+; atom coordinates in drawings are based on X-ray structure of la+ cations, Id'+, If'+ and 4e'+, do not fit the pattern.Although the ESR parameters can be forced through the data analysis, the predicted SOMOs are largely d,2 with values of PA of the order of 60-70". It seems much more reasonable to interchange the hyperfine tensor X and 2 labels so that X is the major axis, consistent with a SOMO mainly d,, in character. The non-coincidence angle then becomes 90" -/? (26" for Id+, ca. 15" for the three monophosphines). The slightly larger values of g , also suggest that the a' SOMO may not be correct for these species. If the SOMO has the form of eqn (1 b), the g-tensor components are given by g,, = g , - 2[d2(d,2-,2 + 36,~) + e26,,] gyy = ge - 2[d2dzy + e2dyeI g,, = g , - 2[d 2dzz + 4e2d,~-u~] g,, = 2de[6,, + 26,2-,2].(9 4 (9 b) (9 4 (9 d) Although d,, and 6 , ~ can probably again be neglected, the resulting simplification is not as great as for the a' case; the rotation angle 8, depends on the ratio, R = Bzz/6z~-y2, as . - 2e 2 f R d R - 1 +(4-R)(e/d)2' shown in the equation tan28, = - Neglecting spin-orbit coupling perturbations, the hyperfine-tensor components are given by the equations A,, = A,+$P(-2d2+e2) A,, = A, +$P(d2 + e2) A,, = A,+$P(d2-2e2) A,, = $Pde.3924 Tensor-axis Non-coincidence Efects The hyperfine tensor is diagonalized by rotation about the y axis by the angle PA, given by the equation 2e/ d 1 - ( e / d ) 2 * tan2P, = - For R z 1 the angles P, and PA are opposite in sign so that even small values of e / d should give observable values of p = [&I + I/3,I.Unfortunately, while 8, is extremely sensitive to R, PA is nearly independent of R. Thus, without another measure of R, we cannot separate the rotations of the g and hyperfine tensor axes. Although the hyperfine tensor is axial when spin-orbit coupling effects are neglected, the g-tensor anisotropy is sufficiently large to explain easily the observed departure from axial symmetry for Id+ and the monophosphine cations. With inclusion of an approximate spin-orbit coupling correction, eqn (1 1 b) leads to the equation which can be used to estimate the d-electron spin densities. Although none of the monophosphine cation spectra were sufficiently well resolved to give complete parameter sets, that of 4e'+ has only one uncertain parameter; for this cation, eqn (13) gives a d-electron spin density, d2 + e2 = 0.38.Similarly the approximate parameters for Id+ give d2+e2 = 0.48. Although the ESR parameters do not provide a complete picture for the monophosphine-substituted cations, the above result is consistent with extended Huckel MO theory prediction^^'^^ and with the conclusions, based on ESR parameters, for the isoelectronic species, C P C ~ ( C O ) ~ P P ~ ~ . ' ~ Our results indicate that in the series [(q5-dienyl)Mn(CO),_,(PR3),]+, the metal character of the SOMO increases as carbonyl ligands are replaced by phosphines, presumably through decreased delocalization into carbonyl n* orbitals.This conclusion appears to conflict with that of Morton et a1.,13 who computed a metal d-orbital spin density of ca. 0.7 for [(r5-C5H5)Cr(CO)3]. In fact, their analysis of the 53Cr hyperfine tensor is in error by a factor of 2/3, so that the correct value should be ca. 0.4, consistent with our results for the Mn" complexes. We are grateful to Prof. D. A. Sweigart for his supportive interest and for helpful discussions of this work. References 1 2 3 4 5 6 7 8 9 10 11 12 N. G. Connelly and M. D. Kitchen, J. Chem. SOC., Dalton Trans., 1977, 931. N. G. Connelly, M. J. Freeman, A. G. Orpen, A. R. Sheehan, J. B. Sheridan and D. A. Sweigart, J. Chem. SOC., Dalton Trans., 1985, 1019. T. A. Albright, P. Hofmann and R. Hoffmann, J. Am. Chem. SOC., 1977,99, 7546; B.E. R. Schilling, R. Hoffmann and D. L. Lichtenberger, J. Am. Chem. SOC., 1979, 101, 585; see also T. A. Albright, J. K. Burdett and M-H. Whangbo, Orbital Interactions in Chemistry (Wiley-Interscience, New York, 1985), chap. 20. J. A. McCleverty, T. A. James and E. J. Wharton, Inorg. Chem., 1969, 8, 1340. R. Gross and W. Kaim, J. Chem. SOC., Faraday Trans. 1, 1987, 83, 3549. R. M. Nielson and S . Wherland, Inorg. Chem., 1985, 24, 3458. D. A. C. McNeil, J. B. Raynor and M. C. R. Symons, Proc. Chem. SOC., 1964, 364; J . Chem. SOC., 1965,410; J. J. Fortman and R. G. Hayes, J. Chem. Phys., 1965,43, 15; P. T. Manoharan and H. B. Gray, Inorg. Chem., 1966, 5, 823. C. G. Howard, G. S. Girolami, G. Wilkinson, M. Thornton-Pett and M. B. Hursthouse, J. Chem. SOC., Dalton Trans., 1983, 2631.G. S. Girolami, G. Wilkinson, A. M. R. Galas, M. Thornton-Pett and M. B. Hursthouse, J. Chem. SOC., Dalton Trans., 1985, 1339. G. A. Carriedo, V. Riera, N. G. Connelly and S. J. Raven, J. Chem. SOC., Dalton Trans., 1987, 1769. P. H. Rieger, in Organometallic Radical Processes, ed. W. A. Trogler (Elsevier, Amsterdam, in press). T. Madach and H. Vahrenkamp, Z. .Vaturforsch., Teil B, 1978, 33, 1301.R . D . Pike, A . L. Rieger and P . H. Rieger 3925 13 J. R. Morton, K. F. Preston, N. A. Cooley, M. C. Baird, P. J. Krusic and S. J. McLain, J. Chem. Soc., 14 G. Winkhaus, L. Pratt and G. Wilkinson, J. Chem. SOC., 1961, 3807. 15 Y. K. Chung, P. G. Williard and D. A. Sweigart, Organometallics, 1982, 1, 1053. 16 R. D. Pike, T. J. Alavosus, C. A. Camaioni-Neto, J. C. Williams Jr and D. A. Sweigart, Organo- 17 P. L. Pauson, F. Haque, J. Miller and J. B. P. Tripathi, J . Chem. Soc. C, 1971, 743. 18 W. Strohmeier and C. Barbeau, 2. Natwforsch., Teil B, 1962, 17,848; W. Strohmeier and F. J. Miiller, 19 R. S. Nyholm, S. S. Sandhu and M. H. B. Stiddard, J. Chem. SOC., 1963, 5916. 20 R. D. Pike, W. J. Ryan and D. A. Sweigart, unpublished work. 21 W. Lamanna and M. Brookhart, J. Am. Chem. SOC., 1980, 102, 3490; G. A. M. Munro and P. L. 22 L. A. P. Kane-Maguire and D. A. Sweigart, Inorg. Chem., 1977, 18, 700. 23 P. H. Rieger, J. Magn. Reson., 1982,50,485; J . A. DeGray and P. H. Rieger, Bull. Magn. Reson., 1987, 8, 95. 24 R. D. Pike, T. J. Alavosus, N. S . Lennhoff, J. Van Epp, C . H. Bushweller and D. A. Sweigart, to be submitted. 25 F. K. Kneubuhl, Phys. Kondens. Muter., 1963, 1, 410; 1965, 4, 50; J. R. Pilbrow and M. R. Lowrey, Rep. Prog. Phys., 1980, 43, 433. 26 P. H. Rieger, unpublished work. 27 J. R. Morton and K. F. Preston, J . Magn. Reson., 1978, 30, 577. 28 D. Rehder and A. Kececi, Inorg. Chim. Acta, 1985, 103, 173. 29 Y. K. Chung, D. A. Sweigart, N. G. Connelly and J. B. Sheridan, J. Am. Chem. Soc., 1985, 107,2388. Paper 9j01593K; Received 17th April, 1989 Faraday Trans. I , 1987, 83, 3535. metallics, in press. Chem. Ber., 1967, 100, 2812. Pauson, J. Chem. Soc., Chem. Commun., 1976, 134.

ISSN:0300-9599    

DOI:10.1039/F19898503913

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1989, 85(12). 3927-3937 Structural Information from Powder ENDOR Spectroscopy Possibilities and Limit at ions Donato Attanasio Istituto di Teoria e Struttura Elettronica e Comportamento Spettrochimico dei Composti di Coordinazione del C.N.R., P.O. Box 10, 00016 Monterotondo Staz., Roma, Italy The methodology for extracting structural information from powder ENDOR spectra is briefly reviewed. The procedure, based upon tensor transformation of hyperfine data obtained from spectra recorded at the EPR ‘turning points ’, is described in some detail, whereas other techniques, such as fitting of spectra obtained as a function of the observing field, are only mentioned. Examples from the recent literature are presented and discussed. It is shown that in many cases single-crystal measurements do not offer appreciable advantages, as far as the geometry of magnetic nuclei surrounding the paramagnetic probe is concerned.Continuous-wave electron nuclear double resonance spectroscopy (ENDOR),14 when performed on magnetically diluted single-crystals, typically yields highly accurate data concerning weak magnetic interactions between a probing paramagnetic centre and surrounding magnetic nuclei. Often these hfs tensors measured by ENDOR are dominated by a simple dipolar contribution, and the coordinates of the interacting nucleus, with respect to the magnetic axes of the central unpaired spin, can be extracted from the experimental data. In the past beautiful examples of ‘ENDOR crystallography’ have been provided by the work of Hutchison Jr et al.,5-7 who applied this technique in the determination of proton coordinates both in model compounds and biomolecules.However, in many cases it is convenient or essential to obtain this kind of information from the spectra of non-oriented samples, even though considerable loss in resolution and accuracy may result. ENDOR spectroscopy is often one of the few techniques available to give an insight into the electronic and geometrical structure of metal-containing, complex materials of biochemical or catalytic interest. A conventional powder ENDOR experiment is performed by choosing a suitable value of the external magnetic field B, (i.e. a suitable position within the powder EPR absorption envelope) and then sweeping a radiofrequency field. At certain values of the applied r.f., nuclear transitions, detected as intensity changes of the EPR absorption, may be observed, assuming favourable values of the electron and nuclear relaxation times.** Clearly the ENDOR response of a powder sample arises only from the subset of molecules which contributes to the EPR intensity at the chosen value of the observing field B,.If EPR turning points are selected, namely magnetic field values which correspond to defined molecular orientations, so-called single-crystal-like ENDOR spectra are obtained.1° The possibility of using this selection technique depends upon the anisotropy and relative orientation of the various interacting tensors. In some favourable cases three components of the magnetic tensor can be obtained for each set of equivalent, interacting nuclei.However, these experimental parameters do not correspond to 39273928 Powder ENDOR Spectroscopy Fig. 1. General orientation of the principal axis system for g and proton hyperfine tensors. principal values of the magnetic tensors, but rather represent the extrema of the interaction along the magnetic axes of the central unpaired spin. Therefore a further step in data analysis or additional experimental data, such as collection of spectra as a function of B,, is required before the principal values and hence structural data may be obtained. Recently several groups have shown renewed interest in powder ENDOR spectro- scopy, with the aim of taking full advantage of the wealth of information contained in the spectra.11-20 Proper interpretation of the experimental data may yield the principal values of the magnetic interactions in general cases, thus providing access to the geometrical arrangement of ligand nuclei in the vicinity of the paramagnetic centre. Several, slightly different approaches have been proposed and are briefly discussed below.Data Analysis Fig. 1 shows the principal axis systems of a generic proton hfs tensor and of the g tensor of the central paramagnetic probing ion. Generally speaking, the two sets of magnetic axes are completely independent and the effective hyperfine coupling constant can be writ ten2' where and g2 = c g i " 1: a (3) Our coordinate reference frame is the principal axis system of the g tensor, with i = 1-3 = x-z. The arbitrary orientation of the hyperfine tensor principal axis system, (x', y', z'), with respect to the reference frame, is represented by the polar angles (01, O,, e3).M (el, O,, 0,) is the Euler transformation matrix 22 and Zi are the direction cosines defining the orientation of the external magnetic field. If we restrict ourselves to the simple case of an axially symmetric g tensor the above Euler transformation reduces to a simple rotation of an angle 8 around the x' axis.D. Attanasio 3929 II(III A195.97 MO ( I = 5/2) I I I I I 310 330 350 BO /mT Fig. 2. Frozen-solution X-band EPR spectrum of [MoOCl,(H,O)] in 1 : 1 glycerol : HCl (2 mol dm-3). The spectrum consists of a central intense g feature and satellite hyperfine lines, respectively due to the presence of non magnetic (ca.75 %) and magnetic (ca. 25 %, I = p) Mo nuclei (see text). z and xy indicate the turning points saturated to obtain parallel and perpendicular ENDOR spectra. Approximate positions of the perpendicular hyperfine lines are given. Fig. 2 shows a typical MoV, dl, EPR powder spectrum for such an axially symmetric situation and in the absence of any metal hyperfine interaction. Note that the central g pattern is due to molecules containing the non-magneticg6* Mo nuclei (natural abundance 75%). ENDOR spectra recorded with the external field set at the two EPR turning points, as indicated in the figure, selects subsets of molcules with their z axes respectively parallel and perpendicular to the external magnetic field. These spectra may yield, for each proton hfs, three different couplings A,,, A,,, and A,, which correspond to the extrema of the hyperfine interaction along the z-axis and within the xy plane.Fig. 1 clearly shows that A,, coincides with a principal value of the the hfs tensor, whereas rotation of the unknown angle 0 must be performed on the two other components to obtain A , and A,. Rotation formulae can easily be obtained on the basis of the above equations and below we quote the results reported by Hofmann et al. :21a AEy - (Atu +A&) sin2 9 1 -2 sin2 9 We now assume the electronic spin to be 1a;gely localized at the central metal ion and the metal-proton distance to be at least 2 A: Under these conditions the point dipole approximation is valid, i.e. the interacting tensors are axially symmetric, and the additional condition : A , = A , = A,, (7) 131 FAR I3930 Powder ENDOR Spectroscopy is valid.Thus an estimate of the rotation angle 8 can be obtained: Alternatively, direct measurement of the hyperfine principal value may be possible by recording ENDOR spectra as a function of the observing field. Moving the field from B, to B, the ENDOR lines corresponding to A,, move outwards until they reach a maximum splitting when B, coincides with the direction of the principal value A,(see fig. 1). The measured splitting gives the A, value, whereas the rotation angle can be easily obtained from the measured effective g value. In the literature such a direct procedure has been found to be viable for the spectra of very simple paramagnetic systems.2o Otherwise fitting of field-variation spectra is necessary. l6 In addition sensitivity problems often complicate or completely hamper recording of ENDOR spectra outside the EPR turning points. The above techniques allow derivation of the hfs principal values. If the relative signs of the tensor components can be assessed their average gives the isotropic contribution, whereas the anisotropic principal value may be used to derive the metal-proton distance according to the simple dipole formula,22 which assumes unit spin density at the metal : (9) In this way the proton spectroscopic coordinates are obtained as r, the distance of the proton from the paramagnetic ion, and 8, the angle that the metal-proton vector makes with the g, reference axis. In the case of axial systems the second polar angle @ is not accessible, but estimates of its value have been obtained for strongly rhombic compounds.16 In the case that the tensor principal values are obtained through tensor transformation two further points must be stressed.The first is the possibility of error propagation from the calculated value of 8 to the calculated value of r. Determination of 8 is based upon the difference between A,, and A,,, which are similar in value, and is therefore affected by rather large errors. However, it is easy to show, on the basis of eqn (4)-(8), that the value of the rotation angle affects only the isotropic part of the tensor, whereas it leaves completely unaffected the dipolar component A,,. This means that the calculated Y values do not depend upon the tensor transformation we perform.23 The second point to be noted is the ambiguity in the sign of 8.The rotation angle enters eqn (4)-(6) as a squared sine function. Therefore its absolute value can easily be derived, but we cannot discriminate between clockwise and anticlockwise rotations. Although reasonable guesses can often be made this appears to be the major drawback of this kind of analysis. A , = A,-A,, = pbpngeg,(3COS2~- 1 p 3 . Results VO(sa1en) Several characteristics make the dl V02+ ion one of the most suitable transition metals to be used as an ENDOR paramagnetic probe." Its o non-bonding ground-state implies high localization of the unpaired electron, thus extending the validity of the point-dipole approximation. In addition highly orientational selective and well resolved ENDOR spectra can often be obtained from powder samples of vanadyl compounds at liquid- nitrogen temperatures or above.This is due, respectively, to the large anisotropy of the A" tensor and to the fact that vanadyl EPR transitions can easily be saturated. lass together with the 11 and I ENDOR spectra recorded at the indicated B, settings,323gSix, out of seven sets of non-equivalent protons gave measurable couplings with the central Fig. 3 shows the frozen-solution EPR spectrum of VO(sa1en) in a dmf-d7/CDC1D. Attanasio 393 1 I 1 ' 1 I I I I I ] 280 320 360 T/K I -2 -1 0 1 2 (v - VP )/MHz 1 I I I 1 I -2 -1 0 1 2 (v - v p )/MHz Fig. 3. Frozen-solution parallel and perpendicular proton ENDOR spectra of VO(sa1en) at 100 K. Different proton lines are labelled as follows: a and e, axial and equatorial ethylenediamine protons , 7, 6, 3, and 5 are H(7), H(6), H(3) and H(5) protons. X has not been identified.Table 1. Experimental and transformed proton hyperfine splitting parameters (MHz) of VO(sa1en)" assign- men t A , A , A , A , , A l Aiso Adi, rb 9 r" 8' H3 1.03 1.01 2.02 2.03 -1.01 - 2.03 427 97 438 98 H, 1.66 1.55 3.14 3.21 -1.51 0.06 3.15 370 104 385 100 0.47 0.47 1.01 1.01 -0.47 0.02 1.03 535 90 528 89 0.26 nmd nm 0.52 -0.26 - 0.52 662 - 651 97 H6 H5 CHzes 2.26 2.3 3.91 4.03 -2.11 -0.06 4.09 338 103 324 108 CH,,, 2.68 2.59 4.40 4.74 -2.51 -0.09 4.81 310 78 305 79 a From ref. (23). In units of pm. Crystallographic values. nm = not measured. unpaired spin, and the experimental splittings were assigned to specific protons essentially on the basis of chemical substitution at selected positions.Table 1 summarizes the experimental data and includes the results of a tensor transformation performed according to the procedure outlined above. Keeping in mind the rather crude approximations used, comparison between spectroscopic and crystallographic results appears to be quite satisfactory. In addition it turned out that experiments with randomly oriented vanadyl complexes may provide structural information with accuracy comparable to detailed single-crystal 25 In the case of frozen-solution spectra, the use of deuterated solvents allows easy detection of lines due to quite distant protons, e.g. H, with r = 650 pm, which in the solid are generally obscured by strong matrix lines due to surrounding, non-interacting pro tons.Another outcome of the spectra concerns the conformation of the ethylenediamine bridge. X-Ray data26 for these complexes indicate a bridged conformation intermediate between gauche and eclipsed, with the two equatorial protons at largely different 131-23932 Powder ENDOR Spectroscopy A?- ' VI -2 - 1 0 1 2 (v - v p Y M H Z Fig. 4. Frozen-solution parallel and perpendicular proton ENDOR spectra of [MoOCl,(CH,OH)]- at 15 K. Magnification shows the OH proton lines due to second solvent shell molecules. distances from the central metal ion. However, ENDOR spectra give a single set of lines, clearly suggesting equivalence of the two equatorial protons, therefore implying that in solution the bridge conformation relaxes to the more usual eclipsed configuration.[MoOCl,(S)]- Powder ENDOR spectroscopy of the dl MoV ion could give a substantial contribution to the structural characterization of the active sites in a number of Mo-based chemical and enzymatic catalysts. In this context single-crystal ENDOR2' and multifrequency EPR data2* have recently been reported. However, a number of additional problems may be expected from the powder ENDOR spectra of Mo-containing compounds. Among them the low g-tensor anisotropy, the superposition of different EPR lines, due to the presence of different magnetic isotopes, and the very low symmetry of most of these complexes may be recalled. Discussion of the ENDOR spectra of the particularly simple compounds derived from dissociation and solvent interaction of the [MoOC1J2- ion conveniently illustrates the actual problems.The frozen-solution and proton ENDOR spectra of different [MoOCL,(S)]- species, including S = H20, CH,OH, C2H,0H and CH,CHO, have been mea~ured.~' All the ENDOR spectra were recorded at temperatures between 10 and 20 K. Above this value their quality rapidly deteriorated, although the hydroxyl proton lines could still occasionally be measured up to ca. 110 K. Fig. 4 and 5 report the I( and I ENDOR spectra of [MoOCl],(CH,OH)]- as well as their variation as a function of B,. It turns out that the parallel spectra always show the presence of rather intense perpendicular lines. On the other hand fig. 5 clearly indicates that these perpendicular features can not be simply explained in terms of low g- tensor anisotropy, i.e.low orientational selectivity. Inspection of fig. 2 indicates thatD. Attanasio 3933 I 1 I I -3 -2 - 1 0 1 (v - v p )/MHz Fig. 5. Magnetic field dependence of the frozen-solution ENDOR spectrum of [MoOCl, (CH,OH)]-. The B, values used to saturate the EPR line are shown in the insert. Purely parallel ENDOR spectra are obtained only in a narrow field interval around gll. Outside this range intense perpendicular features appear. For B, .< BI, their intensity is ascribed to superposition of g,l and A , hyperfine lines due to magnetic Mo isotopes. superposition of the 11 g line, due to molecules containing non-magnetic Mo isotopes, with the M, = -$ I hyperfine line, due to molecules containing magnetic Mo isotopes ( 9 5 M ~ , I = 4, 15.72 % ; 9 7 M ~ , I = 4, 9.46 %), is a common feature of these EPR spectra. This means that nuclear irradiation at B, = hv/g,P simultaneously involves molecules containing both even and odd Mo isotopes, respectively oriented parallel and perpendicular to B,.Since superposition of the EPR lines is not perfect, small shifts in the observing field greatly alter the relative line intensities, allowing almost pure 11 or I spectra to be obtained. In spite of this, more complex spectra would inevitably require the use of isotopically pure molybdenum. Table 2 summarizes the experimentally obtained proton couplings. Axial coordination of the solvent molecule is easily deduced from the absolute values of the OH proton splittings and from the fact that the largest coupling is measured along the molcular axis.The I ENDOR should give two different pairs of peaks, measuring the extreme of the hfs interaction in the equatorial plane. Of course these two values are expected to be quite similar, and they are in fact unresolved, giving rise to a sharp doublet split by 2.38 MHz. This apparent axial symmetry of the interaction prevents determination of the rotation angle 8. Derivation of an approximate Mo-H distance implicitly assumes a small 8 value, so that measured and principal magnetic values approximately coincide. The couplings of the three inequivalent CH, protons could also be measured, showing the presence of one strongly interacting and two, quasi-equivalent, weakly interacting protons.In addition, variable-temperature measurements showed that at ca. 100 K the CH, line pattern reduces to a single broad pair of lines split by ca. 0.9 MHz, suggesting3934 Powder ENDOR Spectroscopy Table 2. Experimental and transformed proton hyperfine splitting parameters (MHz) of (NH,)[MOOC~,(CH,OH)]~ assignment Arb AYb Aiso A , rc OH 5.17 2.38 0.14 5.03 315 n.m.d 5.75 - n.m. 4.92 - n.m. 3.85 - - - - - - - 2.23 1.22 -0.07 2.30 410 0.79 0.51 -0.07 0.86 570 CH,’ 0.45 n.m. - - CH,” - CH3 From ref. (29). In units of pm. 11 and I refer to the g-tensor principal axes. n.m. = not measured. (v - vp U M H Z Fig. 6. Parallel (a) and perpendicular (6) proton ENDOR spectra of VIV/VOPO,(H,O), at 4 K. Labels 1, 2 and 3 simply show ordering from the largest to the smallest hfs.the presence of a freely rotating CH, group. Finally, additional broad lines ascribed again to hydroxyl proton interactions could be identified, although only in the parallel spectrum. Three different proton couplings were measured and they suggest the presence of a well ordered second solvation shell sphere. They correspond to at least three different CH,OH molecules, approximately oriented along the molecular plane and probably connected to the chlorine atoms via hydrogen bonds. VOPO,(H,O), The approach described above, tested in the case of simple coordination compounds of known crystal structure, has been applied to a more complex situation with the aim ofD. Attanasio 3935 Table 3. Experimental and transformed proton hyperfine splitting parameters (MHz) of VOPO,(H,O), at 4 K" assignment A , A , A , A,, A , Aiso Adi, rb 9 H(1) 2.58 0.70 0.70 2.58 -0.70 0.63 2.66 390 90+5 H(3Id 0.19 - 0.18 - 0.75 0.70 3.28 3.29 -0.70 0.39 2.19 416 c - - 0.19 940 - Ht2) a From ref. (32).determination of 9. different orientations (see text). In units of pm. ' Quasi-axial orientation of this protein prevented direct This coupling is assigned to different protons with similar distances but determining the geometrical arrangement of intercalated water molecules in the layered materials VOPO, (H,O), and VOPO, H20),. Vanadyl (V) phosphate dihydrate3', 31 consists of infinite layers of corner-sharing octahedra and tetrahedra. The vanadium atom lies on a fourfold axis and is surrounded by six oxygen atoms to give a distorted octahedron.The four equatorial oxygens are provided by four different tetrahedra, whereas the axial ones are a terminal 0x0 ligand and a water molecule. Each phosphate tetrahedron connects four different octahedra and vice versa, to make up the layers, which are then weakly connected, at a distance of 715 pm, by the two water molecules coordinated or hydrogen bonded to the vanadyl ion and to the layer oxygens. Interest in this and related materials has recently increased, after realizing that they readily swell and intercalate different guest molecules, providing novel expanded, pillared porous materials of catalytic interest. Of course detailed knowledge of the structural relationships between the host matrix and the intercalated or pillared molecules is of primary importance.On the other hand, VOPO, is particularly amenable to paramagnetic resonance techniques in that it always contains small amounts of VIV. Therefore determination of the weak interactions between the Vrv ion and the magnetic nuclei of the intercalated molecules is possible and may provide information upon their arrangement inside the layers. On this basis the EPR and proton ENDOR spectra of the simplest possible vanadyl phosphate intercalates, i.e. VOPO,(H,O), and VOPO,(H,O),, have been Fig. 6 shows the 11 and I ENDOR spectra of VOPO,(H,O), measured at 4 K, whereas no spectrum was detected from the dihydrate analogue in the temperature range 4- 100 K. This result was unexpected since axially coordinated and outer-sphere, hydrogen- bonded water molecules have been frequently and easily measured in the spectra of different vanadyl compounds.ll~ 2o Together with the rather poor quality of the spectra in fig.6 this is an example of the problems which can be met on going from simple model compounds to chemically relevant complex systems. From the fact that VOPO,(H,O), gives no observable spectrum, apparently because of unfavourable relaxation effects, we conclude that all the signals measured in the spectra of the pentahydrate are due to the three extra intercalated water molecules. Couplings due to three different proton sets, which we ascribe to at least two distinct interlayer water molecules are listed and analysed in table 3, according to the usual procedure. H(1), found at 416 pm in a quasi axial position, belongs to an outer-sphere water molecule, probably hydrogen bonded to the undetected, vanadyl-coordinated H20.Owing to the poor spectral resolution the H(l) lines may well account for both the quasi-equivalent water protons. H(2), at 390 pm and relatively close to the [vO(O),] equatorial plane (6' = 5"), identifies a second outer-sphere water molecule. Lines due to the second proton of H,0(2) could not be identified with certainty. Fig. 7, drawn taking into account the coordinates of VOP0,(H20)230 and the location of the two intercalated3936 Powder ENDOR Spectroscopy Fig. 7. Sketch of the expanded interlayer suggested to be present in VOPO,(H,O),. Re-drawn on the basis of the X-ray data reported for VOPO,(H,O),, [ref. (30)] and of the position of the two interlayer water molcules as derived from ENDOR.water molecules as derived above, reconstructs the expanded interlayer suggested to be present in VOPO,(H,O),. H,O( 1) and H,0(2), together with the V02+ coordinated water molecule, complete pillaring of the layers allowing for the 317 pm expansion observed on going from the dihydrate to the pentahydrate compound. The spectra of fig. 6 show the presence of a third proton coupling with an almost constant splitting of ca. 0.19 MHz both in the 1) and I spectra. We suggest that these lines are due to different protons having similar distances (ca. 940 pm) and different orientations. Possible assignments are the protons of H,O(3) as well as proton H(2) from an adjacent molecular unit in the layers. References 1 L.Kevan and L.D. Kispert, Electron Spin Double Resonance Spectroscopy (John Wiley, New York, 2 A. Schweiger, Struct. Bonding (Berlin), 1982, 51. 3 Multiple Electron Resonance Spectroscopy, ed. M. M. Dorio and J. H. Freed (Plenum Press, New 4 H. Kurreck, B. Kirste and W. Lubitz, Angew. Chem. Int. Ed. Engl., 1984, 23, 173. 5 C. A. Hutchison Jr and D. B. McKay, J. Chem. Phys., 1977, 66, 331 1. 6 C. A. Hutchison Jr and T. E. Orlowski, J. Chem. Phys., 1980, 73, 1. 7 C. A. Hutchison Jr and D. J. Singel, Proc. Natl Acad. Sci., 1981, 78, 6883. 8 G. Rist and J. Hyde, J. Chem. Phys., 1968,49, 2449. 9 G. Rist and J. Hyde, J. Chem. Phys., 1969, 50, 4532. 10 G. Rist and J. Hyde, J. Chem. Phys., 1970, 52,4633. 11 H. van Willigen, J. Magn. Reson., 1980, 39, 37. 12 B. Kirste and H.van Willigen, J. Phys. Chem., 1982, 86, 2743. 13 H. van Willigen and T. K. Chandrashekar, J. Am. Chem. SOC., 1983, 105, 4232. 14 R. A. Venters, M. J. Nelson,P. A. McLean, A. E. True, M. A. Levy, B. M. Hoffman and W. H. Orme- 15 A. E. True, M. J. Nelson, R. A. Venters, W. H. Orme-Johnson and B. M. HofEman, J. ,4m. Chem. 16 (a) G. C. Hurst, T. A. Henderson and R. W. Kreilick, J. Am. Chem. SOC., 1985, 107, 7294; (b) T. A. 17 P. J. O'Malley and G. T. Babcock, J. Am. Chem. Soc., 1986, 108, 3995. 1976). York, 1979). Johnson, J. Am. Chem. SOC., 1986, 108, 3487. SOC., 1988, 110, 1935. Henderson, G. C. Hurst, and R. W. Kreilick, J. Am. Chem. SOC., 1985, 107, 7299.D. Attanasio 3937 18 M. Baumgarten, W. Lubitz and C. J. Winscom, Chem. Phys. Lett., 1987, 133, 102. 19 D. Gourier and E. Samuel, J. Am. Chem. Soc., 1987, 109, 4571. 20 D. Mustafi and M. W. Makinen, Inorg. Chem., 1988, 27, 3360. 21 (a) B. M. Hoffman, J. Martinsen and R. A. Venters, J. Mugn. Reson., 1984, 59, 110; (b) B. M. Hoffman, R. A. Venters and J. Martinsen, J. Mugn. Reson., 1985, 62, 537. 22 A. Schweiger, G. Rist and Hs. H. Gunthard, Chem. Phys. Lett., 1975, 31, 48. 23 D. Attanasio, J. Phys. Chem., 1986, 90, 4952. 24 A. Schweiger and Hs. H. Gunthard, Chem. Phys. Lett., 1978, 32, 35. 25 S. Kita, M. Hashimoto and M. Iwaizumi, Znorg. Chem., 1979, 18, 3432. 26 D. Bruins and D. L. Weaver, Inorg. Chem., 1970, 9, 130. 27 N. H. Atherton and R. D. S. Blackford, Mol. Phys., 1987, 61, 443. 28 G. R. Hauson, G. L. Wilson, T. D. Bailey, J. R. Pilbrow and A. G. Wedd, J. Am. Chem. Soc., 1987, 29 D. Attanasio, M. Funicello and L. Suber, Chem. Phys. Lett. 1988, 147, 273. 30 H. R. Tietze, Aust. J. Chem., 1981, 34, 2035. 31 M. Tachez, F. Theobald, J. Bernard and W. Hewat, Rev. Chim. Miner., 1982, 19,291. 32 L. Alagna, D. Attanasio, T. Prosperi and A. A. G. Tomlinson, J. Chem. SOC., Faraday Trans. I , in press. 109, 2609. Paper 9/01634A; Received 18th April, 1989

ISSN:0300-9599    

DOI:10.1039/F19898503927

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1989, 85(12), 3939-3952 The Use of Electron Paramagnetic Resonance Techniques in the Molecular Approach to Heterogeneous Catalytic Processes on Oxides Michel Che* and Catherine Louis Laboratoire de Re'activite' de Surface et Structure, UA 1106 CNRS, Universite' P. et M. Curie, 4 place Jussieu, 75252 - Paris Cedex 05, France Zbigniew Soj ka Jagellonian University, ul. Karasia 3, Krakow, Poland This review shows that EPR spectroscopy can be a powerful tool for catalysis research. This is illustrated in the particular case of highly dispersed Mo/SiO, catalysts prepared by grafting. The different steps of the preparation may be identified : e.g. the EPR changes observed in the Mo5+ coordination sphere during the grafting process of MoCl, onto silica, showed that the grafted species is probably SiOMoCl? (8, = 1.952, g,, = I .968).The catalyst was also characterized after activation treatment such as thermal reduction. Three different Mo5+ species have been detected. Their coordination sphere was determined using probe molecules such as water, methanol and natural or 13C-enriched carbon monoxide. These Mo5+ species are molybdenyl compounds surrounded by oxygen ligands in distorted octahedral (Maid) (gl = 1.944, g,, = 1.892), square-pyramidal (MoXd) (8, = 1.957, g,, = 1.866), and distorted tetrahedral symmetry (MO,~;) (8, = 1.926, g,, = 1.755). The subscripts 6c, 5c and 4c stand for hexa-, penta- and tetra-coordinated, respectively. The EPR results obtained on UV-irradiated oxidized Mo/SiO, on which methanol was preadsorbed, are also consistent with the variation of selectivity in methyl formate and formaldehyde with Mo dispersion observed during methanol oxidation. They extend the reaction mechanism previously proposed on the basis of kinetic data.EPR revealed the formation of a hydroxymethyl radical 'CH,OH. The deactivation of this radical by an 'external' redox process is favoured on highly dispersed Mo catalysts such as grafted Mo/SiO,. The process involves radical migration on silica where it can react with SiOCH, groups, forming methyl formate. The main steps in the development of a catalytic process involve the control of all the preparation steps of the catalytic system, the characterization of its bulk and surface physico-chemical properties and finally the measurement of its catalytic properties (essentially activity, selectivity and stability). These steps are usually repeated until the best catalytic system is obtained by trial and error.The present paper will illustrate how the EPR techniques have contributed to improve our understanding, at a molecular level, of these main steps for catalytic reactions occurring on oxide systems. The use of powder catalysts generates a number of difficulties particularly the frequent presence of several EPR-active species.' In order to determine their magnetic parameters, it is often convenient to employ adapted techniques such as spectral simulation,l isotopic labelling,l Q-band2 and third-derivative spectra. This paper is a review of the EPR results obtained on silica-supported molybdenum catalysts prepared in our laboratory, either by the classical impregnation method, or the 39393940 EPR of Catalytic Processes on Oxides grafting method.The latter method involves the reaction of molybdenum pentachloride with the silanol groups of a high-surface-area silica support. The experimental details and the full description of the results, if already published, can be found in the original references. In the present review, the surface and coordination properties of the grafted molybdenum ions using various probe molecules will be emphasized before presenting the main steps of the oxidation of methanol used as the catalytic test reaction. Reduced Mo catalysts exhibit EPR signals owing to the presence of paramagnetic Mo5+ ions (4dl).If the latter are in interaction, the dipolar coupling broadens their signal and prevents their detection. As illustrated below, the Mo5+ signal often exhibits some hyperfine lines on the low-field side. This hyperfine structure is due to the interaction between the unpaired electron and the nuclear spin ( I = 5/2) of the ’,Mo and 9 7 M ~ isotopes which have about the same magnetic moment (natural abundance 15.8 and 9.5 %, respectively). Results and Discussion Catalyst Preparation The first EPR studies of supported Mo catalysts were performed with samples prepared by impregnation of the support with (NH,),Mo,02,.4H20, calcined at 773-873 K and then reduced.*-* The alumina support, which is of interest from a catalytic point of view, gives ~pectra’’~ whose poor resolution may be attributed for the most part to the superhyperfine splitting due to the interaction of the unpaired electron of Mo5+ with 27Al nuclei ( I = 5/2).Thus, most of the EPR characterization work has been performed with silica, which does not lead to this problem. Impregnat ion Method The reduction by H, (100 Torr) at 773 K for 20 min of impregnated Mo/Si02 catalysts prepared by the incipient wetness method, led to an EPR Mo5+ signal at g , = 1.940 and g,/ = 1.882, characteristic of an axial crystal fieldg? lo (table 1). After reduction by CO (100 Torr’f) at 873 K for 3 h, another Mo5+ species was detected in addition to the species described above’’ (table 1). By comparison with the EPR signals of the isopolyanion Mo,O:; (gl = 1.930, g , = 1.9 19), 95Mo-enriched Mo/SiO, catalysts and various molybdenyl compounds, Che et al.9711 showed that both species possess a molybdenyl character.They also showed that the Mo5+ coordination spheres were composed of oxygen ligands, in distorted octahedral (Moi:) and square-pyramidal symmetry (Moi;). In the case of MoZT, the vacancy in the Mo5+ coordination-sphere was located in the trans position with respect to the ‘yl’ oxygen. The following models were proposed for these two species: 0 0 0 Grafting Methods This method involves a chemical reaction, under air- and water-free conditions, between surface hydroxyl groups of silica and MoCl,. In contrast to impregnation, where the Mo becomes bonded to the support during calcination, the grafting reaction produces this t 1 Torr = 101 325/760 Pa.M .Che, C. Louis and Z . Sojka 3941 Table 1. g-values of Mo5+ EPR signals and characteristics of reduced Mo/SiO, catalysts samples Mo (wt %) reduction g, gll ref. impregnated 0.17, 2 H2/773 K 1.940 impregnated 0.17, 2 C0/873 K 1.961 1.942 impregnated 1.03 H,/873 K 1.956 1.945 pseudografted 0.15, 1.56 H2/773 K 1.958 1.941 grafted 0.18, 0.33, 1.05 H,/873 K 1.944 1.957 1.926 1.882 9, 10 1.861 10 1.891 1.859 12, 14, 17 1.889 1.856 10 1.885 1.892 12, 14, 17 1.866 1.755 Experimental error: Agl = k0.002; Agl, = f0.004. bonding directly and is expected to prevent Mo migration and aggregation during calcination and lead to a better dispersed Mo. In the first study," MoCl, was treated with a suspension of silica in chloroform. The catalyst was then washed with chloroform, further hydrolysed with water vapour and finally dried at 393 K.The samples obtained were blue. After thermal reduction under H, or CO, the same Mo5+ species were detected as in the case of impregnated samples reduced under CO (table I). The blue colour of the samples was due to the presence of molybdenum blues (mixed-valence Mo5+-Mo6+ hydroxides) in weak interaction with silica, since these hydroxides could be eliminated by washing with water. As a matter of fact, the molybdenum was shown not to be really grafted,l2>l3 hence the term pseudo- grafted in table 1. The grafting method was improved by the use of MoCl, as vapour12* 14, l5 or dissolved in cy~lohexane,l~*'~ a solvent less polar than chloroform. The choice of MoC1, is particularly attractive since it is monomeric and paramagnetic in the vapour phase and in cyclohexane.16 If not altered during deposition, the Mo5+ oxidation state can be used as a very effective probe to monitor, via its EPR spectrum, the changes in the coordination sphere that occur during the grafting process.14 The results presented below concern the preparation in vapour phase.The reactor used for this kind of preparation was equipped with an EPR tube which permitted the monitoring of the grafting process by EPR.l2l 14* l5 When silica (Spherosil XOA 400, RhGne Poulenc, 400 m2 g-') was heated in the presence of only MoCl, at 473 K, a Mo5+ EPR signal with axial symmetry (gl = 1.952, g,, = 1.968, A , = 37 G, A , , = 70 G) appeared and increased in intensity (fig.I). The change of the Mo5+ EPR signal before (the monomer MoC1, gives an isotropic signal at g = 1.952), and after grafting indicates that the Mo5+ coordination sphere is affected by the grafting process. The parameters of the EPR signal and the UV-visible spectrum of Mo after grafting were found to be very similar to those of the [MoOClJ ion, suggesting the following grafting reaction :12, 1 4 3 l5 MoCl, + SiOH -+ SiOMoC1, + HCl. (1) The samples turned from red-orange after grafting, to a blue colour when exposed to ambient air. This change arose from the hydrolysis and the partial oxidation of loosely bonded MoCl, into molybdenum b 1 ~ e s . l ~ ~ ~ ~ The latter were eliminated by washing with water or 1 mol dmP3 ammonia solution and the samples turned brown, the colour characteristic of grafted molybdenum.This washing step turned out to be all the more effective because the pH was more basic, increasing the surface negative charge density, thereby removing the negatively charged molybdenum blue species by electronic rep~lsion.'~3942 EPR of Catalytic Processes on Oxides u i/'l Fig. 1. EPR spectra recorded at 77 K of Mo/SiO, catalysts obtained after grafting with MoC1, vapour at 473 K: (a) first-derivative and (b) third-derivative. After standard thermal reduction, i.e. in H, (200 Torr) at 873 K for 2 h and further evacuation at 873 K for 30 min, the grafted samples exhibited a complex asymmetric line with a mean g value of 1.944 [fig. 2(a)]. The third-derivative spectrum [fig. 2(b)] revealed the presence of three signals1' for which g values are listed in table 1.These signals correspond to three Mo5+ species in axial symmetry with g , > g,,. While two signals were found similar to those of Mog,f and Mo:,f of impregnated samples (table l), the third one Mot: was new. Molybdenum Coordination Chemistry In order to characterize the coordination sphere of the Mo:: species and to confirm the symmetry previously attributed to Mot: and Moi,+, the approach was to use probe molecules such as water, carbon monoxide,127 1 4 7 l7 and methanolls to complete the coordination sphere of coordinatively unsaturated Mo5+ ions. As shown below, water and methanol could only provide information on the presence of a coordinatively unsaturated state, whereas l2C0 and 13C0 could give information on the number of coordination vacancies through the superhyperfine structure due to the nuclear spin When adsorbed at room temperature, water, carbon monoxide and methanol act only as ligands and not as redox agents.This was inferred from the constant number of spins before and after adsorption. Methanol is non-dissociatively adsorbed on Mo below 350 K." I = 1/2 of 13C.M. Che, C . Louis and Z . Sojka 3943 Fig. 2. EPR spectra recorded at 77 K of a reduced grafted Mo/SiO, catalyst (0.33 wt YO) : (a) first- derivative ; (b) third-derivative. Fig. 3. EPR spectra recorded at 77 K of a reduced grafted MolSiO, catalyst (0.33 wt YO): (a) after reduction; (b) after water adsorption at 300 K (1 Torr); (c) after water adsorption at 300 K (18 Torr). Water and Methanol Adsorption Water adsorption at room temperature led to several transformations in the EPR spectrum recorded at 77 K:l2v1' first, the MO:: signal disappeared whereas that of MoZZ increased [fig.3(a)], then the MoZZ decreased to disappear while that of Moi: increased [fig. 3(b)]. Water desorption at room temperature led to the reappearance of the Moi: signal intensity. At 773 K, the water desorption was complete since the three initial Mo5+ signals were observed again. Similar results were observed with methanol.l* Variable temperature experiments were performed to monitor the intensity changes of the three signals (fig. 4).3944 1.0 EPR of Catalytic Processes on Oxides 4 77 120 170 210 250 T/K Fig. 4. Temperature dependence of the Mo5' signal intensities after methanol adsorption on reduced grafted Mo/SiO, catalysts (0.33 wt %).The arrows indicate the transformation of less coordinated species to more coordinated ones. Examination of fig. 3 and 4 shows that H20 and methanol adsorption involve a two- step p r o c e s ~ . ~ ~ ~ ~ ~ * ' ~ The introduction of water or methanol into the MoZ: coordination sphere transforms its EPR signal into one analogous to that for Mo::. The introduction of additional molecules completes the coordination sphere of MO~,;, and of the partially coordinated Moi:. This induces the disappearance of their EPR signals and the increase in intensity of a signal similar to Moi:. These results suggest that Moi: can adsorb more than one molecule of water or methanol and indicate that Moi: contains more than one vacancy in the coordination sphere.The Mot: ion is then the most unsaturated species of the three Mo5+ species. As its signal transforms into signals corresponding to molybdenyl ions, the Moi: species is expected also to possess a molybdenyl character. Carbon Monoxide Adsorption The counting of coordination vacancies in transition-metal ions has been shown to be possible with 13C-enriched C0,3* 12, 17, l9 particularly for ground-state orbitals like d,z or d,g-yl, whose lobes point along the metal-ligand bonds.lg The adsorption of carbon monoxide12* l7 induced the disappearance of the Moi: signal and the appearance of a signal composed of two lines at g = 1.965 and 1.969 whose intensities increased with pressure (up to 100 Torr), suggesting that Mot,+ can also coordinate CO (fig. 5).It was difficult to estimate whether CO could enter in the coordination sphere of Mo:: since some of its signal remained visible after CO adsorption. Note that the perpendicular components of the Mo:: and Moi: signals are less intense than the associated parallel components (fig. 5). This is in contrast to the situation before CO adsorption (fig. 2) or when water was adsorbed (fig. 3). This may be explained by the superimposition of the g,, (or g3) components of the new signal(s) induced by CO adsorption either on the g , components of the Mo;: and Moi: signals,3945 M. Che, C. Louis and Z . Sojka 5 0 G c-, Fig. 5. EPR spectra recorded at 77 K of a reduced grafted Mo/SiO, catalyst (0.33 wt %) after l2C0 adsorption (200 Torr/300 K) : (a) first-derivative ; (b) third-derivative.leading to a decrease in their intensity, or on their g,; components, inducing in this case an increase of intensity. When 13C0 was adsorbed, the signal located at g = 1.965 was broader than with l2C0 and the MoZ,f signal less well resolved [fig. 6(a)]. The third-derivative spectrum shows that, instead of two lines, the spectrum is now composed of a quartet, i.e. four lines of different intensities [fig. 6(b), 7(b)], separated from each other by ca. 7.5 G and located at magnetic fields corresponding to apparent g values of 1.974, 1.969, 1.965 and 1.960. The two central lines of the quartet are located at the same field values as the two lines obtained after l2C0 adsorption. These four lines were not resolved in Q-band,12.17 suggesting that they arise from a superhyperfine coupling between the unpaired electron of Moi: and the nuclear spin ( I = 1/2) of 13C.Indeed, the hyperfine coupling constant is independent of the microwave frequency,2 therefore the splitting observed in X-band (7.5 G) is too weak with respect to the linewidth in Q-band (60 G) to be detected. Assuming similar linewidths, these four lines were decomposed into two superimposed triplets centred at g = 1.965 and 1.969, and with a hyperfine constant of 7.5 G (fig. 7). Using the relative intensities of the two lines obtained after l2C0 adsorption, the superposition of the two triplets with relative inner intensities 1 : 2 : 1 compares well with the intensities of the experimental spectrum (fig.7). Each triplet arises from the interaction between the Mo5+ unpaired electron and the 13C nuclear spin of two apparently equivalent 13C0 molecules. The Moicf coordination sphere is then completed on admission of two CO molecules. It was therefore deduced that Moicf is a tetracoordinated species : 0 +2co - OC-Moi: - co 03946 l 2 C O t EPR of Catalytic Processes on Oxides I- 1.9 6 5 DPPH Fig. 6. EPR spectra recorded at 77 K of a reduced grafted Mo/SiO, catalyst (0.33 wt 70) after I3CO adsorption (200 Torr/300 K) ; (a) first-derivative ; (b) third-derivative. Fig. 7. Third-derivative EPR spectra recorded at 77 K of the Moi,+ ion coordinated by (a) l2C0 and (b) I3CO; ( c ) is the analysis of these signals. The Coordination Sphere of Mo5+ Ions after Adsorption of Gas-phase ligands On the basis of the knowledge of the MoiZ coordination sphere, the mechanism of water adsorption can be discussed.It occurs in two steps: (i) the admission of a first water molecule, probably located in the equatorial plane since the EPR signal of the partially hydrated Mo5+ ion is similar to that of MoZ,+, which possesses a square-pyramidalM . Che, C. Louis and 2. Sojka 3947 structure; (ii) the admission of a second water molecule in axial position, since the EPR signal of the fully hydrated Mo:,f is similar to that of Mo:,f. The following adsorption mechanism was proposed : 1 2 ~ l7 On the other hand, one water molecule can be adsorbed on MoXl: H H \ 0 J \ 0 J T T MO;; MoZ On the same basis, the following mechanism of methanol coordination by reduced Mo/SiO, samples was proposed :I8 H H H CH,OH I CH3OH I I * CH3O-Moz -XH3 T> 190K Moz - CH,O-MO~ 120- 190K H CH,OH I MO: CH3O-Moz T > 190K An excess of H 2 0 or CH,OH may be adsorbed in the second sphere of Mo5+ ions by means of hydrogen bonding or on the silica support by different modes of coordination.The g values of the three initial Mo5+ species and of the hydrated forms can be interpreted on the basis of a g, versus g , , diagram (fig. 8) plotted from the g-tensor components known for a number of molybdenyl compounds of C4" symmetry :11 [MOO L,LJ"-, (x = 4, 5, y = 0, 1, n = 1, 2, with L,L' = monodentate ligands). In line with the theoretical expressions of the g-tensor components, fig. 8 illustrates the dramatic variation of g,i, mainly due to the spin-orbit coupling constant R, of L, in contrast to that observed for gl.ll The following conclusions may be drawn: (i) molybdenyl compounds with the same type of ligand L are found in the same region of the diagram, demonstrating the preponderant influence of AL ; (ii) the molybdenyl halide series expands over both parts of the diagram, with A, varying from 270 cm-l for3948 EPR of Catalytic Processes on Oxides A 2 0 g l 19 18 - ./ / gl ’ gll / gll ’ gl / / I / / oxyqen c o n t a i n i n g / halide liqonds / / / tonic c h a r a c t e r / I / / / / / / / f / / / c o v a l e n t c h a r a c t e r 17 1.8 1.9 2.0 2 1 2 2 gll Fig. 8. Representation of various molybdenyl compounds is the g,, gll plane : (1) Mo,O,,H, ; ( 2 ) MoO(HSO,),; (3) MoOJTeO,; (4) MoO(H,PO,),; (5) MoO(H,AsO,),; (6) M0,O;;; ( 7 ) PMoW,,; (8) MoO(NCS),; (9) MoOFi-; (10) MoOClE-; (11) MoOCl,(H,O)-; (12) MoOC1;; (13) MoOBrE-; (14) MoOIg-; (15) Moil; (16) Moi:; (17) Moil; (18) SiOMoCl,.Compounds 1-14 from ref. (1 1) and references therein. fluorine to 5060 cm-l for iodine; (iii) a comparison between MoOCl,, MoOCC-, and MoOCl,(H,O)- shows that the axial ligand has little influence on the g-tensor components; (iv) the oxygen-containing ligands are characterized by g , > g ; A,, the spin-orbit coupling constant of oxygen is the same whether the oxygen is an oxide 02- ion or belongs to an hydroxyl group, a water or a methanol molecule. This is the reason why the partially and fully coordinated MoZ; species possess EPR signals similar to those of Moi: and Mo::, respectively [eqn (3)-(5)].By contrast, when a ligand is connected to the central ion by a non-oxygen atom, the spin-orbit coupling constant changes and affects the g,, value. This is the case of CO connected to Moi: by the carbon atom (A, = 28 cm-’ instead of A, = 152 cm-l). Moreover, the CO adsorption lowers the Mo symmetry which becomes C,. The two lines at g = 1.965 and 1.969 observed after CO adsorption are therefore attributed to the g, and g, components, respectively, of an orthorhombic signal; the g , component of the Mo5+ carbonyl species, MoZ: (CO),, as stated above overlaps with the MoZ; and Moil signals and is thus ill-defined. Therefore, the magnetic parameters of the Mo:; (CO), are as follows: g, = 1.965, g, = 1.969, g , = ill-defined, A , = 7.5 G, A , = 7.5 G, A , = ill-defined.The hypothesis of two different Mo5+ carbonyl species in axial symmetry may be discarded, since after admission of two CO ligands in its coordination sphere, Moi: no longer preserves its axial symmetry. In addition, the two CO appear equivalent (same superhyperfine constant of 7.5 G) and are thus probably located in the equatorial plane. Two models for 2 CO coordinated to Mo:: can be proposed: 0 0 0 0M. Che, C. Louis and Z. Sojka 3949 Fig. 9. EPR spectra recorded at 77 K of the Mo/SiO, catalysts prepared by grafting with MoC1, vapour at 473 K, followed by evacuation: (a) at 473 K for 30 min; (b) at 573 K for 30 min; (c) at 773 K for 30 min; ( d ) third-derivative spectrum of (c). The cis model is more probable since it is formed from Mo:: bonded to the surface and that attack by two CO is likely to yield the cis isomer for steric reasons.Eflect of Evacuation Temperature directly after Grafting As described above, after grafting in the vapour phase, the sample exhibits an EPR signal with g,l > g , (fig. 1) owing to the formation of the SiOMoC1, species [eqn (l)]. If after grafting, the sample was directly evacuated at increasing temperatures without any intermediate exposure to air, the following changes were observed : (i) at ca. 473 K, the relative values of the g components are reversed with g , > g,l [fig. 9(a)]. The spectrum is similar to that of pure Moi; species obtained after water adsorption on reduced sample [fig. 3(c)]; (ii) at 573 K, the spectrum exhibits the signals of both MoZ: and Moi,f species [fig.9 (b)] ; (iii) at 773 K, the spectrum becomes similar to that of the reduced samples (fig. 2), i.e. with three Mo5+ species [fig. 9(c)].' The spin concentration measurements indicated that the number of Mo5+ ions remains constant during evacuation, suggesting that the spectral changes arise only from modifications within the Mo5+ coordination sphere. Fig. 8 shows that the inversion of the g values on sample evacuation at 473 K is due to the replacement of the chlorine ligands of SiOMoC1, by 0,- ions of hydroxyl groups or water molecules arising from the silica support, leading to the formation of MoZ,+. On subsequent increase of temperature, some oxygen ligands are lost, leading to the formation of vacancies in the Mo5+ coordination sphere, i.e.to Mog; and Mo~:.~~. 1 4 7 l5 Conversely, the Mo:: signal disappeared when the sample was left in static vacuum at room temperature for ca. 24 h12,15 or when water was adsorbed : its signal was first transformed into one similar to Mo;: and then into one similar to Moi:. Upon water adsorption, it was shown previo~slyl~ that these changes are due to the admission of a first then a second water molecule within its coordination sphere. The binding with H,O is via the available doublet of oxygen of the water molecule, the molybdenum remaining paramagnetic. The similarity between the signals of hydrated Moi: species and those of Mo;; and Moil3950 EPR of Catalytic Processes on Oxides means that MoER and Moi: ions are also bound to oxygen ligands via dative bonds. These oxygen ligands can belong to OH hydroxyl groups of silica and act as H20.Methanol Oxidation Experiments have shown that Mo is much better dispersed on grafted samples than on impregnated samples and in strong interaction with silica.12-15 These characteristics are very attractive to investigate the effect of isolated Mo ions which are believed to be the active sites of certain catalytic reactions. The influence of the Mo relative distance for lower Mo dispersions can also be studied. Impregnated and grafted Mo/Si02 samples have been compared in methanol oxidation2' which is known to be structure-sensitive. 21, 22 While on polycrystalline MOO,, the main product is formaldehyde, on orientated crystallites the selectivity to formaldehyde, dimethoxymethane or dimethyl ether depends on the face exposed.In an earlier study20 the selectivity in formaldehyde and methyl formate, the main products on supported Mo, was shown to depend upon the molybdenum content of grafted catalysts. As the Mo content decreases, i.e. when the Mo dispersion increases, the selectivity in methyl formate increases while that in formaldehyde decreases. In contrast, the dependence is not so clear for impregnated catalysts because of the lack of reproducibility in their preparation. However, their main product is formaldehyde, and this appears to be due to a lower Mo dispersion. The study of the formation of methyl formate using different reaction mixtures and kinetic calculations led us to propose a reaction mechanism.On Mo sites, methanol leads to formaldehyde, which spills over to silica, where it further reacts with methoxy groups to form methyl formate via a hemiacetal intermediate. 2o An EPR study was performed with the aim to establish the elementary steps of this mechanism'' and to investigate the cycle of model reactions, which may reproduce the most important features of the real catalytic process. As shown above, methanol adsorbed on reduced grafted Mo/Si02 enters into the coordination sphere of MoZL [eqn (5)]. No EPR signal appeared when methanol was adsorbed on the oxidized sample. However, when the sample was heated above 373 K, an Mo5+ signal corresponding to Moi: species was observed, indicating that Mo6+ ions were reduced. The molybdenyl bond Mo6+=02- of oxidized Mo is known to be activated as [Mo5+-0-]* not only by electron transfer during UV-irradiation at low temperature (77 or 300 K)23324 but also by vibrational excitation during thermal 26 The advantage of the low-temperature UV-irradiation is to produce stable paramagnetic species, which disappear at higher temperature and are therefore undetectable after thermal treatment.When the oxidized grafted Mo/Si02 samples on which methanol was first adsorbed, were UV-irradiated at 77 K, two species were detected by EPR : MoiZ (gl = 1.946, g,, = 1.90) as in the case of thermal activation, and another one as a sharp triplet (gaV = 2.003, A , = 23, A , = 29 and A , = 13 G), attributed to the hydroxymethyl radical 'CH20H (fig. 10). Above 140 K, the triplet signal decreased in intensity while that of Mo:: increased.The same experiments performed on the pure silica support led to the formation of an EPR signal as a doublet g,, = 2.007 and A = 140 G, corresponding to 'CH20 radicals generated from methoxy groups : SiOCH, 2 SiOCH; + H' (7) The EPR results with the Mo/Si02 system are consistent with the photoinduced formation of an excited state ria ligand-to-metal charge transfer. This is in agreement with earlier which showed that methanol reacts with the [Mo5+-O-]* tripletM. Che, C. Louis and Z . Sojka 395 1 A H - @) 4 Fig. 10. EPR spectra recorded at 77 K, obtained after 10 min of UV-irradiation at 77 K with preadsorbed methanol outgassed to Torr: (a) reduced grafted Mo/SiO, catalyst (0.33 wt YO); (b) SiO,. excited state.In our case, methanol was adsorbed first at room temperature and then UV-irradiated at 77 K. The following reaction scheme is proposed : CH,OH CH,OH 'CH,OH The last step is a ligand-to-ligand hydrogen-transfer process. It is in accordance with the increase of the Mo5+ and 'CH,OH signals upon irradiation and their mutual dependence. At temperatures above 140 K, the 'CH,OH radical becomes unstable. Its decay can be explained in two different ways: (i) deactivation by an 'internal non-redox' bi- and/or uni-molecular decay of 'CH,OH radicals resulting in EPR-invisible species. This process, however, does not involve any change in the Mo5+ ion concentration, in contrast to the experimental results observed; (ii) by an 'external' redox decay via interaction with Mo6+ ions resulting in their reduction as evidenced by the increase of the Mo5+ signal intensity and formation of adsorbed formaldehyde by a ligand-to-ligand hydrogen-transfer process : T>140 K Mo6+=02- + 'CH,OH CH,O-Mo5+-0H- (9) This study has shown that the oxidation of methanol is a multi-step process. The order in which these steps occurs, depends on the temperature and the sequence of reactants adsorption, the dispersion and the oxidation state of molybdenum. Other experiments which, for the sake of brevity are not presented here, have confirmed the above processes.18 They involve the adsorption of methanol onto (Mo6+-O-) formed by N,O decomposition onto reduced Mo/SiO,.In the case of thermal activation, the reaction obviously is more complex than for UV-irradiation since methanol can dissociate, leading to the reduction of Mo6+ into Mo5+. This EPR study is consistent with the variation of selectivity in methyl formate and formaldehyde with the Mo dispersion observed during methanol oxidation and extends the reaction mechanism proposed on the basis of kinetic data.20 Indeed, on highly dispersed Mo catalysts such as grafted Mo/SiO,, the decay of the *CH,OH intermediates by an 'external' redox process is favoured.As a consequence, these radicals formed at3952 EPR of Catalytic Processes on Oxides the first stage of the reaction may have enough time to spill over to silica before the final deactivation takes place. This makes it possible for the side reactions with support surface groups, such as SiOCH,, to occur during migration, giving rise to the formation of methyl formate as discussed above. On the other hand, for bulk MOO, or low dispersed Mo/SiO, catalysts prepared by impregnation, where the Mo adsorption sites are in close interaction and involve at least pairs of reducible Mag+ ions, the two deactivation processes (‘internal ’ and ‘external redox ’) are coupled.No migration of ‘CH,OH is required for its deactivation to occur and CH,O becomes the main product of methanol oxidation. Conclusion The present review has shown that EPR can be a powerful tool: during the catalyst preparation, to identify at a molecular level, the different steps of the preparation process. In the case of grafting of MoCl, to silica, the changes in the Mo5+ coordination sphere have been observed and analysed ; to characterize the catalyst after activation treatment such as thermal reduction.Three different Mo5+ species have been detected and their coordination spheres determined using probe molecules such as water, methanol and natural or 13C-enriched carbon monoxide ; to understand the mechanism of the elementary steps that occur during the catalytic reaction, as illustrated here for methanol oxidation. The EPR results have extended the reaction mechanism proposed on the basis of kinetic data. It is, however, wise to remember that other techniques can complement the conclusions based on the EPR technique. Owing to its high sensitivity towards paramagnetic species, EPR may at times give a very incomplete, not to say wrong, picture of the main process(es) occurring on catalytic surfaces.References 1 M. Che and E. Giamello, Spectroscopic Analysis of Heterogeneous Catalysts, ed. J. L. G. Fierro, 2 M. Che and Y. Ben Taarit, A h . Colloid Interface Sci., 1985, 23, 179. 3 M. Che, B. Canosa and A. R. Gonzalez-Elipe, J. Phys. Chem., 1986, 90, 618. 4 G. K. Boreskov, V. A. Dzis’ko, V. M. Emel’yanova, Yu, I. Pecherskaya and V. B. Kazanskii Dokl. 5 J. Masson and J. Nechtschein, Bull. SOC. Chim. Fr. 1968, 3934. 6 K. S. Seshadri and L. Petrakis, J. Phys. Chem., 1970, 74, 4102. 7 K. S. Seshadri and L. Petrakis, J. Catal., 1973, 30, 195. 8 M. Dufaux, M. Che and C. Naccache, J. Chim. Phys., 1970, 67, 527. 9 M. Che, J. C. McAteer and A. J. Tench, J. Chem. Soc., Faraday Trans. I , 1978, 4, 2378. Elsevier, Amsterdam, 1989, in press. Akad. Nauk. SSR, 1963, 150, 829. 10 M. Che. F. Figueras, M. Forissier, J. C. McAteer, M. Perrin, J. L. Portefaix and H. Praliaud. Proc. 6th 11 M. Che, M. Fournier and J. P. Launay, J . Chem. Phys., 1979, 71, 1954. 12 C. Louis, Doctoral Thesis (Paris, 1985). 13 C. Louis, M. Che and F. Bozon-Verduraz, J . Chim. Phys., 1982, 79, 803. 14 M. Che, C. Louis and J. M. Tatibouet, Polyhedron, 1986, 5, 123. 15 C. Louis and M. Che, J. Catal., to be submitted. 16 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (Wiley, Chichester, 4th edn, 1980), p. 17 C. Louis and M. Che, J. Phys. Chem., 1987, 91, 2876. 18 Z. Sojka and M. Che, J. Phys. Chem., in press. 19 L. Bonneviot, D. Olivier and M. Che, J. Mol. Catal., 1983, 21, 415. 20 C. Louis, J. M. Tatibouet and M. Che, J. Catal., 1988, 109, 354. 21 J. M. Tatibouet and J. E. Germain, J . Catal., 1981, 72, 375. 22 J. M. Tatibouet, J. E. Germain and J. C. Volta, J. Catal., 1983, 82, 240. 23 V. B. Kazansky, Proc. 6th Znt. Cong. Catal., 1976, London, 1977, 1, p. 50. 24 M. Anpo, I. Tanahashi and Y. Kubokawa, J . chem. SOC., Faraday Trans. 1, 1982, 78, 2121. 25 M. Che and A. J. Tench, Adv. Catal., 1982, 31, 77. 26 V. B. Kazansky, Kinet. Katal., 1983, 24, 1338. Int. Congr. Cataf., 1976 (The Chemical Society, London, 1977), vol. I , p. 261. 862. Paper 9/0 16 16C ; Received 17th April, 1989

ISSN:0300-9599    

DOI:10.1039/F19898503939

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1989, 85(12), 3953-3962 Reactions of Scandium Atoms in Hydrocarbon Matrices? James A. Howard* and Brynmor Mile* Division of Chemistry, National Research Council of Canada, Ottawa, Canada KIA OR9 Carl A. Hampson and Harry Morris” School of Natural Sciences, Liverpool Polytechnic, Liverpool L3 3AF Scandium atoms have been deposited in inert (adamantane, cyclohexane and deuteriocyclohexane) and reactive (benzene, deuteriobenzene and cyclohexene) matrices on a rotating cryostat at 77 K. The main para- magnetic product in the former matrices is a weakly bound Sc...H complex, whereas in the latter two types of complex are formed. When carbon monoxide is co-deposited with Sc the biscarbonyl is the main product. The ESR parameters of these species are reported, and possible structures are discussed.There have been only a few matrix isolation studies involving reactions of scandium in pure matrices, the most notable being those of Knight et al.,l’, who obtained the paramagnetic clusters Sc,, Sc, and Sc,, by deposition of zero-valent Sc atoms in neon and argon matrices. ESR resonances attributed to the complex Sc(H,O) were also observed, but no lines corresponding to isolated atoms were detected even though Singer and Grinter have recorded electronic and magnetic circular dicroism spectra3 and Weltner et al. the UV-visible spectra of the atoms in rare-gas matrices.* These studies were extended to Sc04 and ScS5 by Weltner et al. and to complexes of the type ScX, (where X is H, D, CN, F, Br or C1) by Knight et a1.6-8 In this paper we report the results of a series of experiments in which zero-valent scandium atoms were co-deposited in inert (adamantane and cyclohexane) and more reactive (cyclohexene and benzene) hydrocarbon matrices on a rotating cryostat.We also report the effect of co-depositing carbon monoxide into the Sc/adamantane and Sc/cyclohexane systems. Experiment a1 Scandium atoms were produced by vaporizing scandium foil (99.9 YO Alpha Products Ltd) from a resistively heated tantalum envelope (0.025 mm sheet, Alpha Products Ltd) and were condensed on to previously deposited adamantane (Aldrich Gold Label, 99Y0), at 77 K on the surface of a stainless-steel drum rotating at 2000 r.p.111.’ ca. 5 mg of metal were deposited with 500 mg of matrix over a period of ca.30 min. The material was transferred from the drum still under vacuum at 77 K into an ESR tube. ESR spectra were recorded at 77 K on a Bruker ER200-SRC spectrometer, the field positions were measured with an NMR gaussmeter (Bruker, ER035-M) and microwave frequencies with a frequency counter (Systron Donner, 6245A). The microwave power was typically 2 mW. Variable-temperature studies were made with the aid of a temperature controller (Varian, 4540). Essentially the same experimental arrangement was used with cyclohexane (Aldrich Gold Label), deuteriocyclohexane, C,D,, (Aldrich Gold Label), benzene (BDH Chemicals Ltd, 99.99 YO) and deuteriobenzene, C,D, t Issued as NRCC no. 30524. 39533954 Scandium Ions in Hydrocarbon Matrices I 400 G I f 3300 C SCX Fig.1. The ESR spectrum (9405.3 MHz) of 45Sc atoms in adamantane at 77 K. (Aldrich, 99.9 YO) as matrix. For cyclohexene (Aldrich, 99.9 YO) the temperatures of the cyclohexene and adamantane reservoirs were such that the vapour pressures of both reactants were equal and for the experiments involving l2C0 (BDH) and 13C0 (Amersham International Plc, 99 atom YO) the CO: matrix ratio was ca. 1 : 500. In a preliminary set of experiments scandium was evaporated from alumina-coated tungsten wire baskets. In these cases spectra showed, in addition to Sc based resonances, lines of Al,, presumably from the reduction of alumina. Similar problems were experienced in the evaporation of Rh. Results Sc and Adamantane When scandium atoms ( I = i) were co-condensed with adamantane the ESR spectrum of the resulting dark-green deposit, shown in fig.1, was obtained. This spectrum consists of two main features, a broad central band with g = 2.0053, labelled C, and an eight-line orthorhombic spectrum labelled ScX. The latter results from the interaction of an unpaired electron with a single scandium nucleus and shows a marked second-order effect. An exact analysislO of the lines gave A,(45S~) = 146 G, A,(45S~) = 159.3 G and A,(45S~) = 161.5 G, g , = 1.9590, gy = 1.9838 and g, = 2.0007. Some of the lines are shown at an expanded field range in fig. 2. A computer simulation by using the Belford set of programs'' confirmed these parameters. On warming, the orthorhombic set of lines decreased in intensity rapidly above 170 K and disappeared completely at 230 K.On recooling to 77 K the ScX spectrum did not return which suggests that this species had completely decomposed. In contrast the central band was reduced in intensity by ca. 40% on annealing. When the microwave power was varied all of the lines from the orthorhombic spectrum showed the same intensity variation ; the central band was, however, more sensitive to incident microwave power and increased in intensity considerably, relative to ScX, at higher powers. In similar experiments at lower Sc concentrations, the intensity of the central band (C) was reduced relative to the orthorhombic spectrum and had a more asymmetric appearance. At both high and low Sc concentrations band C possessed some hyperfme structure, although this was insufficiently well resolved to be assigned to any particular species.However, the power and concentration dependences indicate a species with several Sc nuclei.J. A . Howard, B. Mile, C. A . Hampson and H . Morris 3955 1 2900G 2 3500G hJ X 2 -X r t 50G Fig. 2. The M , = 4 (a), -: (b), and -3 (c) ESR transitions (9213.0 MHz) of 45Sc in adamantane at 77 K. Scandium and Cyclohexane When the experiments were repeated with cyclohexane and perdeuteriocyclohexane as the matrix identical spectra were obtained. In both the adamantane and cyclohexane experiments weak isotropic lines were observed in the outer reaches of the spectrum that were assigned to ScO. Further experiments were performed to determine the effects of impurities on the Sc-hydrocarbons systems. Such experiments included the addition of air, water vapour and CO,.These resulted in spectra with an intense axial central feature (g,/ = 2.0328 and g , = 2.0081). There was no increase in intensity of the ScX lines, indicating that they did not result from traces of impurities in the adamantane, and no extra transition were observed. Scandium and Benzene When scandium atoms were co-deposited with benzene the resulting brown deposit gave an ESR spectrum [fig. 3(a)] consisting of a broad central feature flanked by eight broad weaker features that at first sight appeared to be triplets and were due to an unpaired electron interacting with a single Sc nucleus. When the experiment was repeated with C6D6 the ESR spectrum shown in fig. 3 ( b ) was obtained. In this spectrum the central feature was better resolved and the eight outer lines were resolved into anisotropic multiplets.This difference in resolution was attributed to hyperfine interactions with protons or deuterons of benzene. On warming, the central feature proved to be less stable than the eight outer multiplets. The various components of the multiplets also proved to have different stabilities and the spectrum behaved as though it was due to a single species trapped in four different sites [fig. 3 ( c ) ] . The most intense of the four sites, 2, proved to be the least stable, followed by 3 and 4, which had comparable stability. The most stable site was 1, and at 235 K only species in this site remained (fig. 4). On recooling to 77 K none of the lines due to sites 2, 3 or 4 returned, while a sharp signal3956 2(1) ' MI = 112 -- Scandium Ions in Hydrocarbon Matrices M1=17121 ,5:2, :{: fl -1/2 -312 -512 -712 Ill I l l I l l I I 1 (4 I 2 ( 1 ) M/ = 512 f---A-\ l(1) ' 3 ( 1 ) 4(1) t (.> centred at g = 2.0048 was seen to grow in the centre of the spectrum.This was not the same central feature as was observed in the original low-temperature spectrum as it contained no hyperfine structure. The ESR parameters for the various sites are presented in table 1. When scandium atoms were deposited with equal mixtures of adamantane and benzene or adamantane and C6D6 the ESR spectra shown in fig. 5(a) and 5(6), respectively, were obtained. The latter spectrum was better resolved and was analysed as an axial spectrum of eight parallel and perpendicular lines superimposed on a conduction ESR signal with g = 2.0075. There was a direct correspondence between lines in the two spectra, although those in C,H6 were somewhat broader, indicating a superhyperfine interaction with the protons of the benzene molecule.The g and A values of this species are given in table 1 ; a computer simulation of the Sc/adamantane/C,D, spectrum with a superimposed conduction band (Sc: conduction 1 : 1) is shown in fig. 5 (c). When the above spectra were compared with those obtained for Sc in pure C6H6 and C,D6 matrices there were similarities between the central regions of the two sets of spectra. The latter were, however, complicated by overlap from the species with the larger hyperfine interaction. The central region of both sets of spectra were thus tentatively assigned to the same species.J. A .Howard, B. Mile, C. A. Harnpson and H. Morris 3957 I I I I I I I MI= ? I 2 512 -112 -312 -512 -712 Fig. 4. The ESR spectrum (9287.9 MHz) of 45Sc in C6D6 at 235 K. Table 1. ESR parameters of Sc species in hydrocarbon and rare-gas matrices' gl g1 ref. species All A , Aiso 'dip ScH, : Sc H ScF,: Sc F s c o SCS ScXb Sc(C6H6)C,ld 2 3 4 Sc(C6H6)" SC(CO), : sc 13C - 75.7 4.3 - 735.7 691.9 161.5 88.4 - 24.2 23.2 5.3 -45.7 4.3 78.8 10.0 708.6 650.4 152.9 93.1 94.2 87.2 80.6 12.1 16.1 5.3 - 55.7 - 10.0 4.3 1.980 - 717.5 664.3 155.8 91.5 - 8.9 13.0 2.85 - 1.6 - 2.000 1.9975 2.0007 1.9922 ca. 0 - 12.1 - 2.0092 1.997 1 1.987 1.995 1.9965 1.991 5 1.9714 2.0023 2.0038 2.0020 1.9994 2.0074 2.0023 - - - 6 7 4 5 this work this work - - - - - this work this work - a Units of hyperfine interaction are MHz.' g,, = g,, g, = (g,+g,)/2, A,, = A,, A, = (A,+A,)/2. Outer complex. Site. Inner complex. Scandium and Cyclohexene In a further experiment Sc atoms were codeposited with an equal mixture of adamantane and cyclohexene. The ESR spectrum of the resulting brown deposit is shown in fig. 5(4. The spectrum contained only a broad central band that had features which corresponded to the perpendicular features of the Sc/adamantane/C,H, spectra. The spectrum clearly showed the outer four parallel features of an eight-line spectrum characteristic of a paramagnetic species containing a single Sc nucleus. The hyperfine interaction of the parallel lines was greater than that obtained in benzene with A,,(Sc) = 43 G and g,, = 1.9986.The central line of the spectrum had g = 2.0088 and was probably a conduction ESR band.3958 Scandium Ions in Hydrocarbon Matrices A Fig. 5. The ESR spectrum of Sc/C,H,/adamantane (9419.5 MHz) (a), Sc/C,D,/adamantane (b), simulated spectrum of Sc/C,D,/adamantane (9419.5 MHz) (c), and Sc/C,H,,/adamantane (9413.0 MHz) (d). Scandium and Carbon Monoxide When Sc atoms were co-condensed with natural CO and adamantane the ESR of the resulting brown deposit, shown in fig. 6 (a), was obtained and consisted of two octets of parallel and perpendicular features which were due to an axial species containing one Sc nucleus. The g and A parameters were measured as A,,(Sc) = 23.2 G, A,(Sc) = 16.1 G, g,l = 1.9971 and g , = 2.0023.These resonances appeared only when both Sc and CO were present and were consequently attributed to an Sc(CO), complex. The spectrum also contained a weak isotropic octet of lines with g = 2.0049 and A,, = 22.5 G labelled ScX’. The power saturation characteristics of the two sets of lines showed that they originated from two different species. On annealing, the Sc(CO), and ScX’ spectra disappeared rapidly leaving a fairly intense axial spectrum with no resolved hyperfine structure. The spectrum at 213 K was characteristic of a peroxyl and was attributed to either ScOO or Sc(C0)OO formed by reaction of Sc atoms or Sc carbonyl with small amounts of oxygen in the system. The value of gl, = 2.0280 and g , = 2.0045 compared favourably with those of metal peroxyls and alkylperoxyls in previous matrix-isolation Some lines due to Sc(CO), could still be observed, but an isotropic spectrum of the quality previously obtained for Al(CO), and Cu(CO), was not obtained.l5* l6 The Sc(CO), spectrum could be simulated using the g and A parameters given above with the relative concentrations of Sc(CO),: ScOO = 1.When the experiment was repeated with 13C0 the spectrum shown in fig. 6(b) was obtained. In this case the resonances from the peroxyl were more intense than those of SC(~~CO), and to a certain extent spoiled the spectrum. Fortunately, the outer reaches of the spectrum showed hyperfine interaction with 13C and the MI = --; perpendicular line was split into a broad overlapping triplet. Computer simulations of this region gave reasonable results with x = 2 and A,,(13C) = A,(13C) = 5.3 G.Other simulations with x = 1 and 3-6 gave poor simulations and as a result the spectrum was assigned to Sc(CO),. The ScX’ species was also present in the 13C0 spectrum but showed no 13C hyperfine coupling.J. A . Howard, B. Mile, C. A . Hampson and H. Morris 3959 -ll-hl-in A , , I 1 SCX’ Fig. 6. The ESR spectrum of Sc/12CO/adamantane (9423.0 MHz) (a), Sc/13CO/adamantane (9418.5 MHz) (b) and the M , = -f of (b). Discussion Scandium in Adamantane and Cyclohexane The results obtained in this work indicate that when Sc atoms are deposited in inert hydrocarbon matrices a species ScX containing a single scandium nucleus is formed in addition to small microcrystallites of scandium. There are several possible assignments that can be made for ScX.These are (1) a product from reaction of Sc atoms with an unknown impurity, (2) a crystal-field-type interaction in a weak van der Waals complex, ScX,, within the matrix resulting in a lifting of the degeneracy of the Sc 3d energy levels of the type observed by Ammeter and Schlosnagle” for A1 and Ga in rare-gas matrices and (3) a weak hydrogen-bonded complex between Sc atoms and one or more of the protons or deuterons of the matrix. The first possibility can be discounted as the same species is formed in three different matrices, each of which was of high purity. Similarly the second possibility seems unlikely, although it could be envisaged that the Sc atom could be trapped in a site in which it interacts with one or more of the surrounding matrix molecules to lift the degeneracy of the five 3d orbitals such that one of them, possibly the dZ2, is unique and contains the unpaired electron.It is interesting to note that no ESR spectra of Sc atoms have been observed in rare-gas matrices even though their existence has been demonstrated by other r n e t h ~ d s ~ , ~ and the magnetic circular dichroism spectra suggest that there is significant interaction between the metal atoms and the lattice, especially in the Sc/Xe ~ystem.~ In general the perturbation of the metal3960 Scandium Ions in Hydrocarbon Matrices atoms tends to be lower in hydrocarbon than in rare-gas matrices, as illustrated by the fact that neither A1 nor Ga show ESR transitions in adamantane or cyclohexane.The fact that the same species is formed in adamantane and cyclohexane argues against a van der Waals complex since the packing and interactions are different in these two matrices. We believe (3) to be the most likely situation in which a hydrogen bond is formed between the electron-rich Sc atom and the electron-deficient proton. If the interaction is such that only one d orbital is involved then the degeneracy of the energy level containing the unpaired electron will be lifted. Detailed analysis of the position and intensity of the satellite (spin flip) lines in Na, K and Ag in adamantanel’ indicate that the metal atom is associated with only one proton at a distance of ca. 0.25 nm. It therefore seems reasonable to assume that some weak bonding takes place.This would explain the similarity of the ScX spectra in three different matrices. Any small hydrogen or deuterium hyperfine interaction would be lost in the rather large linewidths (ca. 20 G) observed in the spectra. An approximate value for A,,, (Sc) can be calculated from the relationship (A,, - 4 ) 3 Adip = where for the orthorhombic spectrum of ScX, A,, = A , and A , = (A, + A,)/2. This leads to a value of Adi, (Sc) = 2.85 G assuming that A,, and A, have the same positive sign. This is quite low compared to the calculated value of 12.25 G” for unit spin population in an Sc 3d orbital and gives an unpaired 3d spin population, p3,, of 0.24. A,, and A, cannot be of opposite sign since an impossibly high value of Adi, would result. Comparison of Adi, (Sc) with the experimental value of 16.1 G from gas-phase optical data for the 2D state of Sc atom^,^ gives P3d = 0.18.The low value of pZd requires some 4s orbital contribution to the singly occupied atomic orbital and some 4p orbital contribution which gives a dipolar interaction of opposite sign to that of the 3d orbital. The relationship gives Aiso = 155.8 G, and comparing this with the calculated atomic value of A:so = 1008.2 G,l’ gives p4s = 0.154. This value seems too high to arise from spin polarization alone and suggests that the unpaired electron in ScX has some s character. The contribution from 4p orbitals cannot be determined but could account for the departure of the total unpaired spin population, p3d+p4s = 0.39 from unity. A dsp hybridization is implied. When Adi, is calculated for other quenched atoms, only boron and aluminium give reasonable p values.Thus for B, Al, Ga, Br and I atoms the p values are 0.85, 0.98, 1.7, 0.62 and 0.29, r e s p e ~ t i v e l y . ~ ~ ~ ~ ~ The ESR parameters for ScX are compared to other Sc compounds in table 1. Aiso = (A,, +2A,)/3 Scandium and Benzeme Two distinct paramagnetic products are formed in a benzene matrix, one possessing a large Sc hyperfine interaction, that only occurs in pure benzene and a second with a much smaller Sc hyperfine interaction that occurs in both pure and dilute matrices. The former is attributed to a benzene complex of the type Sc(C6H,), where n = 1 or 2, in which the Sc atom lies on the c6 axis of the complex. Analogous species have been observed when Cu, Ag and Au atoms are deposited in benzene.’ Metal hyperfine interactions in these cases are 1040% less than those in inert matrices such as adamantane, cyclohexane or rare-gas solids.’.21 Buck et al.’ have suggested that bonding takes place between the II orbitals of benzene and s, p, and d orbitals of the group 1 1 metal atom. This structure is consistent with the lack of any resolved hydrogen hyperfine interaction in the spectra. If A,, and A , are both positive Adip(SC) = - 1.6 G and P3d = 0.065. For opposite signsJ. A . Howard, B. Mile, C. A . Harnpson and H. Morris 3961 impossibly large values of P3d are obtained. The corresponding value of Aiso is 91.5 G, giving p4s = 0.09. The remaining unpaired spin population must reside in 4p orbitals of the Sc or on the benzene rings.The second Sc/C,H, species has a much smaller Sc hyperfine interaction. Kasai and McLeod have studied reaction of matrix-isolated A1 atoms with ethylene22 and benzene.23 The similarity between the two spectra led these workers to conclude that the A1 atom interacts with only one double bond in the benzene molecule. A similar complex may be formed between Sc and C,H, and this appears to be substantiated by the fact that a similar spectrum is obtained with cyclohexene. The larger A,,(Sc) hyperfine interaction in the cyclohexene case suggests less interaction between the ligand and the metal atom when only one double bond is present. A similar trend was observed by Kasai and McLeod22r 23 for the Al/C2H4 and Al/C,H, systems in which the A1 unpaired spin population of Al(C,H,) is double that in Al(C,H,).An analysis of the hyperfine interactions gives Adip (Sc) = 4 G for like signs of A,, and A , and 12 G for opposite signs. Comparison with A&p gives p3d = 0.163 or 0.49. Aiso is calculated to be either 16.1 or 0 G, giving an unpaired s spin population of < 0.02 which probably arises through core spin polarization. Although this Sc(C,H,) spectrum is not sufficiently well resolved to determine the hydrogen hyperfine interaction, the increase in line width of the perpendicular features in Sc(C,H,) compared with those of Sc(C,D,) suggests a value of ca. 5 G, assuming that the Sc atom couples to two protons. The ESR parameters of the species found in benzene are presented in table 1. Scandium and Carbon Monoxide Reaction of Sc atoms with CO in hydrocarbon matrices gives principally the biscarbonyl rather than the tetracarbonyl which one might expect on the basis of a coordinatively saturated argument.This stoichiometry is similar to that obtained for the Al/CO and Ga/C0l5p 2 4 3 25 systems and is reasonable in view of the similar chemistry of group 3 and 13 elements. Scandium has the outer electron configuration 3d14s2 and for Sc(CO), the unpaired electron must be in a non-degenerate orbital for it to give rise to an ESR signal. The two possible structures for this species are linear and bent. For a coordination number of two sp and dp hybridization give a linear arrangement while p2, ds, and d2 give a bent arrangement.26 Bonding by sp or ds hybridization is unlikely since there is an excess of electrons to be accommodated, two from the Sc 4s orbital and four from the 5a orbitals of two CO ligands.A linear Sc(CO), molecule with dp hybridization would be ESR-silent since there are two degenerate dZ2-,2 and d,,orbitals fo,r occupation by the unpaired electron. A bent molecule with the unpaired electron largely in a d,2 orbital at right angles to the molecular plane with either d2 or p2 hybridization for CO bonding is consistent with the observed ESR spectrum and parameters. Although the symmetry of the d,2 orbital is not suitable for back-donation of electron density into the n* orbitals of the CO ligands, such back-donation could be facilitated by the filled 3p orbitals on the Sc atom. The possibility of ds hybridization between the 3d,~-,~ and 4s orbitals, which would also give rise to a bent complex, can be discounted since this, depending on the energies of the d,z, d,,, and d,, orbital, would lead to a molecule with three unpaired electrons or an electron in a degenerate level, contrary to the ESR evidence.A trigonal-planar arrangement of hybrids with one of the hybrids containing the lone pair, as is the case for Al(CO), and Ga(CO),, also seems unlikely since the only form of hybridization involving 3d and 4s orbitals which gives this arrangement is ds2. In the case of Sc this would involve the 5s orbital which is much higher in energy. Sc(CO), is thus thought to have a bent, C2", configuration with the Sc atom d2 hybridized and the unpaired electron in a d,z orbital.I32 F A R I3962 Scandium Ions in Hydrocarbon Matrices Since an isotropic spectrum is not observed the signs of A , (Sc) and A , (Sc) cannot be determined but if they are of the same sign Adi, (Sc) = 2.4 G or if the signs are different, 13.1 G. These give unpaired spin populations, P3d = 0.1 and 0.53, respectively. The latter value seems more reasonable because it is similar to those in Mn(CO), (0.51) and Fe(C0); (0.55)277 28 and suggests that A,, and A , have opposite signs, unlike Al(CO), and Ga(CO),. The values of AIl(l3C) and A1(13C) of ca. 5.3 G gives Adi, (13C) = 3.6 G if the signs are different and ca. 0 G if they are the same. The latter value would give pap of ca. 0 which is consistent with the proposed structure of Sc(CO), and suggests that there is no significant back-donation of unpaired spin population into the ;TI* orbitals of the ligands and that the unpaired spin on carbon arises by bond polarization.Note Added in Proof: Van Zee and Weltner2’ have reported the ESR spectrum of ScCO(*C) prepared from Sc atoms in an argon matrix at 4 K. We found no evidence for this species in hydrocarbon matrices at 77 K. C.A. H. thanks S.E.R.C. for providing a research grant. C.A. H. and H. M. thank N.R.C.C. for Visiting Scientists’ Research Awards. We also thank N.A.T.O. for a collaborative research grant. References I L. B. Knight Jr, R. J. Van Zee and W. Weltner Jr, Chem. Phys. Lett., 1983, 94, 296. 2 L. B. Knight Jr, R. W. Woodward, R. J. Van Zee and W. Weltner Jr, J. Chem. Phys., 1983, 79, 5820. 3 R. J. Singer and R.Grinter, Chem. Phys., 1987, 113, 99. 4 W. Weltner Jr, D. McLeod Jr and P. H. Kasai, J. Chem. Phys., 1967, 46, 3172. 5 N. S. McIntyre, K. C. Lin and W. Weltner Jr, J. Chem. Phys., 1972, 56, 5576. 6 L. B. Knight Jr, M. B. Wise, T. A. Fisher and J. Steadman, J. Chem. Phys., 1981, 74, 6636. 7 L. B. Knight Jr and M. B. Wise, J. Chem. Phys., 1979, 71, 1578. 8 L. B. Knight Jr, M. B. Wise, and T. A. Fisher, Inorg. Chem., 1981, 20, 2623. 9 A. J. Buck, B. Mile and J. A. Howard, J. Am. Chem. SOC., 1983, 105, 3381. 10 G. Breit and I. I. Rabi, Phys. Rev., 1931, 38, 2082. 11 R. L. Belford and M. J. Niges, EPR Symposium, 21st Rocky Mountain Conference, Denver, CO, 12 J. A. Howard, R. Sutcliffe and B. Mile, J. Phys. Chem., 1984, 88, 4351. 13 J. E. Bennett, B. Mile and A. Thomas, Symp. Combust., 1966 (The Combustion Institute, 1967), 14 P. H. Kasai and P. M. Jones, J. Phys. Chem., 1986, 90, 4239. 15 J. H. B. Chenier, C. A. Hampson, J. A. Howard, B. Mile and R. Sutcliffe, J. Phys Chem., 1986, 90, 16 J. A. Howard, B. Mile, J. R. Morton, K. F. Preston and R. Sutcliffe, J. Phys. Chem., 1986, 90, 1033. 17 J. H. Ammeter and D. C. Schlosnagle, J . Chem. Phys., 1973, 59, 4784. 18 C. A, Hampson, Ph.D Thesis (Liverpool Polytechnic, 1988). 19 K. F. Preston and J. R. Morton, J. Mag. Reson., 1978, 30, 577. 20 W. Weltner Jr, Magnetic Atoms and Molecules (Van Nostrand Reinhold, New York, 1983). 21 P. H. Kasai and D. McLeod Jr, J. Chem. Phys., 1971, 55, 1566. 22 P. H. Kasai, J . Am. Chem. SOC., 1982, 104, 1165. 23 P. H. Kasai and D,. McLeod Jr, J. Am. Chem. SOC., 1979, 101, 5860. 24 C. A. Hampson, J. A. Howard, B. Mile and R Sutcliffe, J. Phys. Chem., 1986, 90, 4268. 25 J. H. B. Chenier, C. A. Hampson, J. A. Howard and B. Mile, J. Chem. Soc., Chem. Cornrnun., 1986, 26 G. E. Kimball, J. Chem. Phys., 1940, 8, 188. 27 J. A. Howard, J. R. Morton and K. F. Preston, Chem. Phys. Lett., 1981, 83, 226. 28 S. A. Fairhurst, J. R. Morton and K. F. Preston, J. Chem. Phys., 1973, 77, 5872. 29 R. J. Van Zee and W. Weltner Jr, J. Am. Chem. Soc., 1989, 111, 4519. Aug. 1979 (unpublished). vol. 11, p. 853. 1524. 730. Paper 9/01611B; Received 18th ‘4pri1, 1989

ISSN:0300-9599    

DOI:10.1039/F19898503953

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1989, 85(12), 3963-3972 Electron Spin Resonance Study of the Reaction of Group 1 1 Atoms with Ketenet Franqois Genin,$ James A. Howard" and Brynmor Mile* Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR9 Carl A. Hampson Department of Chemistry and Biochemistry, Liverpool Polytechnic, Liverpool L3 3AF lo7Ag atoms react with ketene in a rotating cryostat at 77 K to give two major paramagnetic products, a monoligand 7t-complex, Ag[CH,CO] and a 2-silver- 1 -0xally1, CH,C(Ag)O and one minor product formed by addition of Ag to the carbonyl function that has tentatively been identified as a-silver oxyvinyl. These paramagnetic species have been examined by ESR spectroscopy and their magnetic parameters are reported.Au atoms also add to CH,CO to give 2-gold-1-oxallyl [CH,C(Au)O] and a n-complex, Au[CH,CO]. The spectrum from Cu and CH,CO is poorly resolved at low temperatures but the spectrum of 2-copper- 1 -oxally1 appears upon warming. This species seems to have two non-equivalent hydrogens at low temperatures which become equivalent above 273 K. We have recently demonstrated1i2 that Group 11 (Cu, Ag, and Au) and Group 13 (A1 and Ga) metal atoms (M) in solid hydrocarbon matrices add almost exclusively to the central carbon of allene at 77 K to give 2-metal substituted allyls [eqn (1 a)] and do not give detectable addition to the terminal carbons to produce metal substituted vinyls . . . . . . . . . . [eqn (1 b)l- This regioselective addition has been attributed to the presence of the low-lying empty p orbital of the metal atoms2 and the terminal addition that occurs for atoms, alkyls and thiyls arises because of the absence of empty p orbitals in these reagents.The isoelectronic cumulene ketene, CH2C0, is an important ligand in organometallic chemistry, and together with diazomethane (CH,N,) has been used to generate methylene on metal surfaces so as to study its contribution to Fischer-Tropsch ~ynthesis.~-~ Reactions of ketene with metal atoms are of interest because this represents the simplest metal-ketene system and because of the different metal fixation, fragmentation and coupling reactions that could occur. Addition to the terminal carbon would give a metal-substituted acyl [eqn (2 a)] which might undergo p-scission to give an organometallic carbene complex, MCH,, while addition to the oxygen would give an a- substituted vinyl [eqn (2b)l.Alternatively metal atoms could add to the central carbon t NRCC no. 30433. $ NRCC summer student 1985. 3963 132-23964 Reaction of Group 11 Atoms with Ketene to form a 2-metal substituted- 1-oxallyl [eqn (2 c)]. Hydrogen atoms react exclusively by reaction (2 a) under the experimental conditions of the rotating cryostat.' M + c H 2 c o J : r cH2=toM (2) . . . . . . . . . . . CH2-C -0 I M Margrave and coworkersS have recently reported a FTIR matrix-isolation spec- troscopic study of the reaction of Cu atoms with CH,N,, which is isoelectronic with ketene, in Ar and N, at 12 K. They found that these atoms insert spontaneously into the CN bond to form N,CuCH,.CuCH, and Cu[CH,N,] are also formed, the former probably by decomposition of the insertion product while the latter may be the initial reaction product. Because of our interest in the regioselective and stereospecific factors which govern the addition of coordinatively unsaturated neutral metal atoms to cumulenes we initiated an ESR study of the reaction of Group 11 atoms with ketene. The results of this study are reported here. Experimental The rotating cryostat and furnace used to vaporize Cu, Ag and Au have been described previo~sly.~~ lo The sequence of deposition on the spinning cold drum was matrix, metal and CH,CO. ESR spectra were obtained on Varian E-4 and E-12 instruments equipped with a Systron-Donner 6054 microwave frequency counter and a Bruker ER 035 proton magnetometer.Temperature control was achieved with an Oxford Instruments ESR 9 liquid-helium cryostat and cold nitrogen gas controller. The computational methods used to analyse ESR spectra have been described previously.'' Copper oxide enriched to 99.89% in 63Cu and lo7Ag were obtained from Oak Ridge National Laboratory, TN. Y u O was reduced to 63Cu by hydrogen at 500 "C. Au was a gift from Dr C. M. Hurd (NRCC). Ketene and perdeuterioketene were prepared by pyrolysis of acetic anhydride and perdeuterioacetic anhydride by the method of Ho and Noyes.ll Results Silver lo7Ag atoms ( I = i) and CH,CO (0.05 Torrt) in adamantane gave a brown deposit that turned purple on standing at 77 K. The spectrum given by this deposit is shown in fig.1 (a) and consists of a major doublet A with the magnetic parameters alo7 = 1566.6 MHz and g = 2.0008 and a triplet B centred at g = 2.0046 with a spacing of 45.6 MHz. Ag atoms and CD,CO also gave a doublet A with lines that were slightly narrower than those from CH,CO and were in fact narrow enough that satellite lines on either side of the main transitions from the simultaneous flipping of the electron and matrix protons were resolved [fig. 1 (b)]. CD,CO also gave a single line centred at g = 2.0046. indicating that the triplet spectrum (B) from Ag/CH,CO resulted from hyperfine coupling with t 1 Torr = 101 325/760 Pa.F. Genin, J. A . Howard, B. Mile and C. A . Hampson 3965 9125 MHz 100G, 4 3250 G Fig. 1. The ESR spectrum given by lo7Ag atoms and CH,CO (a) and CD,CO (b) in adamantane at 77 K.two equivalent hydrogen nuclei. The minor doublet A' had a hyperfine interaction of 1802 MHz and was readily assigned to the lo9Ag analogue of A from lo9Ag impurity in the lo7Ag. There were other weak rather broad features on either side of triplet B, but the resolution did not improve upon cooling to 4 K or warming to 250 K, and they have so far eluded analysis. As the sample from Ag/CH,CO in adamantane was warmed in the cavity of the spectrometer the central feature B became more isotropic until at 204K the almost isotropic triplet of doublets shown in fig. 2 was obtained. Analysis of this spectrum gave aH(2) = 32.3 MHz, alo7(l) = 6.2 MHz and g = 2.0046. At 204 K, but at higher spectrometer gain, an almost isotropic doublet of doublet of doublets (C) was apparent in the spectrum from Ag/CH,CO (fig.3). The carrier of this spectrum had three Z = nuclei with hyperfine interactions of 399, 33.8 and 56.9 MHz. The largest hyperfine interaction is almost certainly associated with a silver nucleus, and the other two are associated with hydrogen nuclei. The spacing of doublet A increased noticeably with temperature. Thus at 153 K it had increased to 1598.7 MHz, and at 192 K it had increased to 1613.3 MHz. Photolysis of the ketene during deposition with light of wavelength > 300 nm gave in addition to the doublet A, a doublet with a107 = 1676 MHz and g = 2.002 and a doublet with a107 = 1696.5 MHz and g = 2.002. The doublet with the smaller Ag h.f.s. is readily assigned to isolated Ag atornslO while the larger doublet has parameters similar to those of AgC0.123966 Reaction of Group 11 Atoms with Ketene 3252.5 G Fig.2. The central part of the ESR spectrum (B) given by lo7Ag atoms and CH,CO in adamantane I I t 3199.2 G at 204 K. , 4 0 G , Fig. 3. The central part of the ESR spectrum (B) given by lo7Ag atoms and CH,CO in adamantane at 204 K and at high gain. Gold Au atoms ( I = i) and CH,CO (0.05 Torr) in adamantane at 77 K gave a spectrum (fig. 4) that consisted of three quartets, D, with the magnetic parameters aAu = 2852.6 MHz and g = 2.0016, aAu = 2880.4 MHz and g = 2.0033, and aAu = 2929.5 MHz and g = 2.0018 that are readily assigned to Au atoms isolated in adamantane," and a triplet, E, shown at a lower scan range in fig. 4(b), centred at g = 2.0047 with a major spacing of 53 MHz and the indication of a small h.f.s.of 9 MHz. In addition there were two peak quartets, F, with aAu = 161 1 MHz and g = 1.993 and aAu = 1554 MHz and g = 1.990. In the case of Au and CD,CO the central feature centred at g = 2.0047 consisted of a single line. Copper 63Cu atoms ( I = :) and CH,CO (0.05 Torr) in adamantane at 77 K gave a complex powder ESR spectrum centred about g = 2 that was dominated by a poorly resolvedF. Genin, J. A . Howard, B. Mile and C. A . Hampsoli J 1 F 7 400 G t---4 I 3967 Fig. 4. The ESR spectrum given by Au atoms and CH,CO (a) full spectrum and (b) central region in expanded field range. quartet. Upon annealing the sample to 173 K an almost isotropic quartet of doublets developed [fig.5(a)] that could be analysed in terms of one I = $ nucleus with a coupling of 39 MHz and one I = nucleus with a = 55 MHz centred at g = 2.0025. Upon further annealing to 243 K a further hyperfine interaction of 8.4 MHz is resolved from an I = f nucleus [fig. 5(b)] suggesting a transient with one copper and two magnetically different hydrogens. At 283 K the major spectrum was a quartet of triplets with a,, = 32 MHz and a,(2) = 35 MHz [fig. 5(c)]. Cu atoms and CD,CO in adamantane gave a similar powder spectrum to that of Cu/CH,CO, but upon annealing to 273 K only one line developed. The presence of one copper in the transient from Cu and CH,CO was therefore not confirmed in this experiment. Discussion It is apparent from this study that the group 11 atoms, Cu, Ag and Au, react readily with ketene in solid hydrocarbon matrices at 77 K to give a variety of paramagnetic products.Of the three metal atoms Ag is the most straightforward and is discussed first. The EPR parameters of the two major species (A and B) and the minor species (C) given by Ag atoms and CH,CO are presented in table 1, along with those from reaction of CH,CO with Au and Cu atoms. Species A has an Ag h.f.s. and g factor less than those of the isolated atom in adamantanelO (alo, = 1676.6 MHz and g = 2.0021) and by analogy with other Ag atom organic/inorganic substrate complexes,1o. 13-16 it is most probably the monoligand n-complex Ag[CH,CO]. The difference in linewidths of Ag[CH,CO] and Ag[CD,CO]3968 Reaction of Group 11 Atoms with Ketene Fig. 5.The central part of the ESR spectrum given by 63Cu and CH,CO in adamantane at 173 K (a) 243 K (b) and 283 K (c). Table 1. ESR parameters of the paramagnetic species given by group 1 1 atoms and ketene at 77 K" species aM a H €! Ag[CH,COl A 1566.6 - 2.0008 CH,C(Ag)O B 6.2 32.3(2) 2.0046 CH,COAg C 339 33.8, 56.9 1.9940 1.9900 1.9930 CH,C(Au)O E ca. 9 53 2.0047 32 35 2.0025 Au[CH,CO] F 1554 - CH,C(Cu)O - - F 161 1 - " Hyperfine interactions in MHz. (= AHpp x 2G) suggests that there is some unpaired spin population in ligand orbitals, but there is not enough to produce a resolved H h.f.s. Dividing alo7 = 1566.6 MHz by the atomic parameter for unit spin population in the lo7Ag 5s orbital," A = 1831 MHz, gives an unpaired 5s spin population, p5s, of 0.85. This is similar to p5s for the side-on- bonded monoligand n complexes Ag[HCN]15 and Ag[C,H,],1° but is smaller than the values of 0.9 1-0.96 for Ag[C,H,] and Ag[C,H,].13 Similar monoligand complexes haveF.Genin, J. A . Howard, B. Mile and C. A . Hampson 3969 been reported from a FTIR matrix-isolation study of the reaction of Cu atoms and diazomethane.* The bonding in metal-atom n-complexes has been discussed in terms of the Dewar-Chatt-Duncanson mode1l8? l9 in which there is electron donation from the ligand into an empty metal orbital of the correct symmetry and back-donation from the metal d orbitals into the empty n* orbitals of the ligand. In the case of ketene the HOMO is the 2B, n, orbital with most of the electron population located on the terminal oxygen and carbon nuclei, and the LUMO is the 3B, nzo orbital with most of the spin population located on the oxygen and central carbon nuclei.20 The lower 5s spin population in Ag[CH,CO] over that of Ag[C,H,]13 could result from free electron transfer to ketene and/or by hybridization at the metal to include p-orbital contribution to the SOMO.In ketene there are three filled ligand n-orbitals, not two as in allene. The extra orbital originates from the lone-pair orbital on the oxygen which can combine in a bonding or antibonding fashion with the C=C n orbitals to produce the lowest n, (1 B,) orbital and the HOMO n, (2B,) orbital, respectively. The nco (2B,) and nzo (3B,) orbitals are lower than those in formaldehyde by ca. 2 eV, although the separation between the energy levels is unchanged. Two structures, 1 and 2, are possible for the silver-ketene complex.‘. H. 00 .... :.::.. ‘.’.’.’.:.:.’ :::::::::A:. ....... ... ....... .,... ..>>: H, ‘ c = c = o ?Y H’ 1 2 Structure 1 has the silver atom bonded to the CO moiety and located perpendicular to the ketene plane containing the CH, group. There is overlap between the n, orbital and an s or sp hybrid orbital on the metal with donation of electrons from the filled n, orbital to the metal, but in this structure the n,*, orbital which lies in the molecular plane cannot overlap with either silver 5s or 4d orbitals to allow back-donation of electrons from the metal into the nEo orbital. Structure 2 has the silver atom also bonded to the CO end of the molecule but now lying in the molecular plane with overlap and electron transfer from the lower-energy nco orbital to the silver atom. The disposition of the n,*, orbital and the 4p/4d orbitals enable symmetry allowed overlap and hence provides a channel for back-donation.This disposes us towards structure 2; it is worth noting that the energy of the nco orbital is similar to that of the 5 0 orbital in CO which binds effectively with Cu and Ag atoms. However, the difference in energy between the two structures may be small and indeed the increase in Ag h.f.s. with increase in temperature may arise from a rotation of the Ag atom around the C=O from structure 2 to structure 1. Similar complexes have not been observed for reaction of Ag atoms with allene; the filled 2E orbital is similar in energy to the 2B, orbital of ketene and the lB,, orbital of ethene, but the n* orbital of CH,CO is 1 and 2 eV lower in energy than the n* orbitals in CH,CCH, and C,H,, respectively.The relative energies of the frontier molecular orbitals for allene, ketene and ethene are shown in fig. 6. The second major species, B, has magnetic parameters similar to those for 2-silver ally1 CH,C(Ag)CH$ [a,(2) = 39.2 MHz, aH(2) = 42 MHz, alo7 = 19.6 MHz and g = 2.00451 and different from those expected for AgCH,CO and AgCH,. Thus the H h.f.s. of AgCH,CO and AgCH, should be similar to those for CH3C021 and CH3,,, i.e. 15 and 65 MHz, respectively, while Ag h.f.s. of AgCH,CO and AgCH, are predicted to be 17 and 80 MHz, respectively, from the values of A for Ag and H.17 In support of these3970 Reaction of Group 11 Atoms with Ketene allene ketene ethene -5 i H%..*@ @*..a H c-c - 1B1 Fig.6. Frontier orbital energies of allene, ketene and ethene. predictions Knight23 has found that CuCH, has Cu and H h.f.s. close to those expected for a planar radical. We, therefore, conclude that B is the 2-silver- 1-oxallyl CH,C(Ag)O. To our knowledge this is the first example of an organometallic oxallyl. The carrier of the spectrum C, with a large Ag h.f.s. and two different H h.f.s. is more difficult to identify. Dividing the isotropic h.f.s. by the one-electron pararneter~l~ A = 1420 and 1831 MHz for H and "'Ag, respectively, gives plS = 0.024 and 0.040 and p5s = 0.22, where pns is the unpaired spin population in the ns orbital. The two most likely structures for C are the silver-substituted acyl, 3, and the a- silveroxyvinyl, 4 : AgcH2b CH2=C 0 3 4 Structure 3 can readily be discounted because it would be expected to have two equivalent proton h.f.s. even if it takes the bridged form 5: H 5F.Genin, J. A. Howard, B. Mile and C. A. Hampson 397 1 protons of 4 would not be equivalent, but the H h.f.s. do seem low when compared with the values of 192 and 96 MHz for CH,CH.24 It is, however, possible that 4 takes the partially bridged form 6, thus reducing the unpaired spin population on the methene function and increasing it on the Ag nucleus. As with vinyl itself the two 6 This superficially resembles the n-complex Ag[CH,CO] but must involve a-bonding from ketene to Ag rather than n-bonding. In summary, three paramagnetic products have been detected in the reaction of Ag atoms with ketene, a monoligand n-complex, Ag[CH,CO], 2-silver- 1-oxallyl and a- silveroxylvinyl.All of these products are consistent with attack of Ag atoms at the carbonyl and not the vinyl function of ketene. This is the opposite to the reaction of H atoms which add to the CH, end of ketene. It further continues the opposite behaviour of metal atoms and H atoms now observed in a number of reactions, e.g. (i) addition at the central and terminal carbons of allene and (ii) much more rapid addition of H atoms to alkenes than alkynes and metal atoms to alkynes than alkenes.' In the case of Au, only two paramagnetic products can be identified. The central feature in the EPR spectrum with aH(2) = 53 MHz and no resolved Au h.f.s.can almost certainly be assigned to the 2-gold-l-oxyallyl, CH,C(Au)O. The carriers of the two quartets with the large Au h.f,s. of 1554 and 1611 MHz and g factors below the free- electron value of 2.0023 are almost certainly Au[CH,CO] in two trapping sites." Dividing the experimental values of a by A = 2876 MHzl' gives unpaired s spin populations, pGS, similar to those for other Au monoligand n-complexes.'o~ 1 4 3 l6 In the Cu/CH,CO system the spectrum of a species with a Cu coupling constant similar to that of CH,C(Cu)CH, resolves upon warming and is probably due to 2-copper- 1 -0xally1. Unlike CH,C(Ag)O and CH,C(Au)O this species has non-equivalent hydrogens in the range 173-253 K that become equivalent only above 273 K. They then have a coupling constant similar to other organometallic allyls. This suggests that the Cu atom on the central carbon interacts with one of the hydrogens on the terminal carbon at low temperatures, an interaction that does not appear to be available to Ag and Au.The only problem with the assignment of this spectrum to CH,C(Cu)O is the g value that is low for an organometallic allyl. In conclusion it is possible that a monoligand q2-(C,0) ketene complex is the primary product from reaction of coinage-metal atoms and ketene, and the metal atom migrates to opposite sides of the C=O group to form the 2-metal oxallyls and metal oxyvinyls. The higher M-0 bond strengths (ca. 15&200 kJ mol-l) compared with the weaker M-C bonds of the coinage-metal atoms favour attack at the terminal oxygen, but the delocalization stability in the oxallyls favours attack at the central carbon atom as in allene.In ketene the lBl and 2B, orbitals are already arranged so that there is no energy needed to twist the 71 bonds, thus favouring attack at the central carbon atom over that at the terminal CH, group. J.A.H., C.A.H. and B.M. thank NATO for a collaborative Research Grant (no. 42/32).3972 Reaction of Group 11 Atoms with Ketene References 1 J. H. B. Chenier, J. A. Howard and B. Mile, J. Am. Chem. SOC., 1985, 107, 4190. 2 B. Mile, J. A. Howard and J. S. Tse, Organometallics, 1988, 7, 1278. 3 M. A. Henderson, P. L. Radloff, J. M. White and C. A. Mims, J. Phys. Chem., 1988, 92,4111. 4 G. E. Mitchell, P. L. Radloff, C. M. Greenlief, M.A. Henderson and J. M. White, Surf. Sci., 1987, 5 P. H. McBreen, W. Erley and H. Ibach, Surf. Sci., 1984, 148, 292. 6 R. C. Brady I11 and R. Pettit, J. Am. Chem. Soc., 1980, 102, 6181; 1981, 103, 1287. 7 J. E. Bennett and B. Mile, J. Chem. SOC., Faraday Trans. I , 1973, 69, 1398. 8 S-C. Chang, 2. H. Kafafi, R. H. Hauge, W. E. Billups and J. L. Margrave, J . Am. Chem. Suc., 1987, 9 J. E. Bennett, B. Mile, A. Thomas and B. Ward, A h . Phys. Org. Chem., 1970, 8, 1. 183, 403. 109, 4508. 10 A. J. Buck, B. Mile and J. A. Howard, J. Am. Chem. SUC., 1983, 105, 3381. 11 S-Y. Ho and W. A. Noyes Jr, J. Am. Chem. SOC., 1967, 89, 5091. 12 J. H. B. Chenier, C. A. Hampson, J. A. Howard and B. Mile, J. Phys. Chem., 1988, 92, 2745. 13 P. H. Kasai, D. McLeod Jr and T. Watanabe, J. Am. Chem. SOC., 1980, 102, 179. 14 J. A. Howard, R. Sutcliffe, J. S. Tse and B. Mile, Organometallics, 1984, 3, 859. 15 J. A. Howard, R. Sutcliffe and B. Mile, J. Phys. Chem., 1984,88, 5155. 16 J. A. Howard, R. Sutcliffe, H. Dahmane and B. Mile, Organometallics, 1985, 4, 697. 17 J. R. Morton and K. F. Preston, J. Magn. Reson., 1978, 30, 577. 18 J. Chatt and L. A. Duncanson, J. Chem. Soc., 1953, 2939. 19 M. J. S. Dewar, Bull. SOC. Chim. Fr., 1951, 18, C71. 20 W. L. Jorgensen and L. Salem, The Organic Chemists Book of Orbitals (Academic Press, New York, 21 J. E. Bennett and B. Mile, Trans. Faraday SOC., 1971, 67, 1587. 22 F. A. Neugebauer, in Landolt-Bornstein, New Series, ed. H. Fischer (Springer-Verlag, Berlin, 1984), 23 L. B. Knight Jr and J. Ott, Faraday Discuss. Chem. Suc., 1988, 86, 71. 24 R. W. Fessenden, J. Phys. Chem., 1967, 71, 74. 1973). vol. 17, part b, pp. 8-12. Paper 9/01601E; Received 17th April, 1989

ISSN:0300-9599    

DOI:10.1039/F19898503963

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1989, 85(12), 3973-3985 Reactions of AgI Ions in Alcohols after Radiolysis at 77 K Robert Janes,y Andrew D. Stevens and Martyn C. R. Symons* Chemistry Department, University of Leicester, Leicester LEI 7RH Solutions of silver perchlorate in methanol, ethanol, ethane- 1,2-diol, isopropyl alcohol, allyl and propargyl alcohols have been exposed to 6oCo y-rays at 77 K and the resulting radicals studied using ESR spectroscopy. In addition to CH,OH solvent radicals, methanolic solutions gave Agrr centres and a number of discrete Ago species. The latter exhibited a range of hyperfine couplings, reflecting various degrees of solvation. Their relative yields were shown to vary according to initial Ag' concentration and with added sodium perchlorate. After annealing to ca.120 K, Ago gave way to the molecular cluster cations Agl, Agi+, Agi+ and Ag;', previously identified by ESR and optical spectroscopy in a range of systems and shown to be formed by the sequential addition of Ag' ions. Agt+ formation was favoured either by high initial Ag' or high counterion (nitrate or perchlorate) cqncentration. During the silver aggregation process the solvent radicals (CH,OH) decayed and the adduct [Ag . CH,OH]+ was formed [A,,,(Ag) = 128 GI. On annealing to the softening point of the matrix these species were lost. No higher-nuclearity molecular centres were detected. However, a weak singlet in the free-spin region grew in. This is tentatively assigned to conduction electron spin resonance (CESR) in pseudo-metallic, AgO, particles.On further melting the AgO, signal disappeared and the sample became black, indicating the eventual formation of colloidal silver. Similar reactions were observed in ethanol, ethane- 1,2-diol and isopropyl alcohol. The allyl alcohol system did not give any clearly defined Ago centres, only solvent radicals being observed at 77 K. Two possible silver-solvent adducts were detected after annealing, one with a larger A,,,(Ag) value than expected (ca. 360 G). Results for propargyl alcohol were similar, except that only one silver-solvent species was formed. Possible structures for these centres are discussed. When both systems were annealed to the matrix softening point, the adduct signals decayed and an isotropic singlet in the free-spin region was revealed.This signal was particularly strong in the propargyl system. The g value and linewidth are similar to values recently reported for CESR signals from AgO, latent-image particles formed in silver halide matrices by photolysis. The study of silver atoms and silver clusters is of great interest in the fields of surface physics and chemistry, heterogeneous catalysis and the photographic process. These have been prepared in a variety of matrices including inert gases, zeolites, organic and aqueous media.' Silver atoms and the paramagnetic molecular cluster cations Aga, Ag:+, Agi+ and Ag%+, formed after the radiolysis of aqueous and alcoholic Ag' solutions at 77 K have been characterised using conjoint ESR and optical technique^.^-^ Conduction electron spin resonance (CESR) signals, which have some characteristics in agreement with theoretical electronic ' have been reported for pseudo-metallic silver agglomerates formed in CO, and organic matrices at 77 K' using an atomic beam technique'.' and photolytically in silver halide matrices between 90 K and room temperature.lo Various organo-silver radicals have also been prepared from the reaction t Present address : Research Centre in Superconductivity, University of Cambridge, West Cambridge Site, Madingley Road, Cambridge CB3 OHE. 39733974 Reactions of Ag' in Alcohols of silver atoms and ions with a number of saturated and unsaturated organic molecules at cryogenic temperatures. 11-15 Here we have used ESR to study solutions of Ag' ions in methanol, ethanol, isopropyl alcohol, ethane- 1,2-diol, allyl and propargyl alcohol, exposed to ionizing radiation at 77 K.A more complete picture of the reactions of Ag' ions in these systems has emerged. In some cases, a complete pathway which includes the initial formation of silver atoms and, on subsequent annealing, the formation of molecular silver clusters and pseudo- metallic silver particles has been established. Several novel silver-solvent radical adducts formed during the course of these annealing studies have also been detected. Experimental Salts and solutions were of the highest grade available and were used without further purification. lo7AgC10, was prepared by refluxing lo7Ag powdered metal (98.2 70 enrichment, Oak Ridge Laboratories, Tennessee) with perchloric acid (Hopkin and Williams).13C-enriched methanol was obtained from Amersham International. Samples were either placed into quartz tubes prior to freezing to 77 K or frozen as small beads by dropping into liquid nitrogen. They were irradiated at 77 K using a Vickrad 6oCo y-source with doses of ca. 0.8 Mrad. Typical exposures were ca. 30 min. ESR spectra were recorded at ca. 77 K using a Varian El09 X-band spectrometer. Samples were annealed above 77 K either using a commercial variable temperature attachment or by decanting liquid nitrogen from the quartz insert Dewar and allowing the sample to warm, with continuous monitoring of the ESR spectrum, before re-cooling to 77 K. Results and Discussion It is convenient to consider each type of centre in turn, going from atom-like centres to cationic clusters, to silver aggregates and finally to solvent-radical silver adducts.Silver-atom Centres One of the most remarkable results of studies of Ago centres formed by electron capture (1) has been the multiplicity of subtly different centres formed :2* 16, l7 In our view, these differences reflect the fact that Ag+ ions are strongly solvated in water and alcohols, and the solvent shell is not initially lost on electron addition. Loss of solvent molecules should occur in stages, giving rise to different centres, some being asymmetric.2 Ultimately, the unsolvated atoms are formed. In general, as solvent is shed, the atomic character increases. This was clearly established by measuring the shift in the optical spectra. However, in some systems, Ai,O('OSAg) actually decreases on annealing. We have interpreted this in terms of s-p mixing, induced by the asymmetry of the solvates.In the present work, we have studied the effect of increasing the concentration of AgC10, in methanol and CD,OD, and have extended the range of alcohols to include isopropyl alcohol, ethane- 1,2-diol and the unsaturated alcohols allyl alcohol and propargyl alcohol. Methanol The effect of increasing [AgClO,] is shown in fig. 1 . There is a remarkable difference between dilute and concentrated solutions. For the former, two species dominate. One is the 'unsolvated' atom and the other must be quite strongly solvated. The former hasR. Janes, A . D. Stevens and M. C . R. Symons 1 30006 3975 ‘ O 9 A g ( 0 ) + u)’AgloI hf, = - Y2 Fig.1. First derivative, X-band, ESR spectra for a range of Ago centres in alcohols formed after the exposure of unenriched AgC10, in CD,OD at various concentrations (a)-(d) and with added NaClO, (e) to “Co y-rays at 77 K. Each centre gives rise to low- and high-field doublets corresponding to the isotopes Io9Ag and lo7Ag, ( I = 1/2) which have approximately equal natural abundance. For the sake of clarity, only the low-field (- 1/2) features are shown. sharp features, as expected for trapped atoms, but so has the latter, which is most unexpected and contrasts with results for more concentrated solutions. The fact that CD,OD was used must contribute to the lack of broadening, but the implication must be that only one, highly specific solvate, is present.This is presumably the primary Ag+(CD,OD), solvate. This means that n is a precise number, probably 4, by comparison with other data.ls* l9 As [AgClO,] increases, so ion-ion interaction becomes important, and a range of slightly different solvates are clearly present. Also, features for Agi can be detected. At these concentrations reaction (2) becomes important : Ago + Ag+ + Ag - Ag+ (2)3976 Reactions of Ag' in Alcohols Table 1. ESR data for Ago centres observed after the exposure of AgCIO, solutions to 6oCo y-rays at 77 K solvent species -A,,,('09Ag)/Ga~b -Aiso('07Ag)/Ga'b giso _ _ _ _ ~ ~ MeOD I I1 I11 IV EtOD vii"( = 11) ix"( = 111) PrOH ( = I ) CH,OHCH,OH - ( = 11) 615 625 655 722 62 1 66 1 616 560 638 533 54 1 568 624 538 573 533 485 553 2.002 2.000 2.000 2.002 1.999 1.999 1.996 1.998 - a 1 G = lo-, T.Silver has two magnetic isotopes, 1°'Ag (p, = -0.1 130) and Io9Ag (p, = -0.1299). They have almost equal abundance. Note that their moments are negative so that the numbers quoted herein are actually - A values. This convention is commonly used to stress the fact that positive spin densities are involved. Nomenclature taken from ref. (2). The fact that unsolvated atoms are no longer detected suggests that reaction (2) primarily involves attack by unsolvated Ago rather than by the solvated entities. Another remarkable fact is that, for dilute solutions, desolvation seems to be 'all or nothing', in that only the two centres are significant. The fact that they coexist even on slight annealing is interpreted in terms of desolvation occurring for the 'hot' Ago centre before it has time to relax to its equilibrium state.Once this relaxation has occurred, the shedding of solvent molecules requires considerable thermal energy. The changes induced by increasing salt concentration are clearly due to cation-anion interactions, rather than cation<ation interactions, since NaC10, had a very similar effect [fig. 1 (41. Species I11 is favoured, with properties quite close to gas-phase values, although the lines remain very broad. We assume this is a specific ion pair, which is probably less strongly solvated than the normal cations. Data for these centres are given in table 1. Isopropyl Alcohol Only one major Ago centre was obtained for this solvent [see outer features in fig. 5(a) below]. This has parameters close to those for methanol centre 11.Ethane- I,2-diol Two species with almost equal concentrations were initially detected. One of these has parameters quite close to those for the methanol centre 11. The other has a value for A,,,(Ag) significantly lower than methanol centre I. No unsolvated atomic centres were obtained. Ally1 and Propargyl Alcohols It is interesting that neither of these solutions gave significant yields of either ,Ago centres or cluster centres such as Agl or Agi+. This can be understood in terms of reactions (3) and (4), which we have established for the radiolysis of the pure alcohols. . - - - - - - - - - - - -. CH,=CHCH,OH + e- + CH,-CH-CH, +OH- (3) .- ----- --. HC=CCH,OH + e- -+ HC--C-CH, + OH-. (4)R. Junes, A . D.Stevens and M. C. R. Symons 3977 For simple alcohols, ROH, reaction with electrons is slow, and electron trapping (solvation) is favoured. However, for these alcohols, no e; centres were detected, but good yields of the allyl and propargyl radicals, well characterised by their ESR spectra were obtained. Clearly, Ag+ ions are unable to compete significantly with reactions (3) or (4), so Ago centres are not important. Silver Cluster Cations Silver atoms, formed from the cations in a range of solvents, show a remarkable tendency to form cationic clusters : Agi + Ag+ -+ AgtS Agi+ + Ag+ + Ag:+ (6) [see also reaction (l)]. All the evidence suggests that these are formed in a stepwise manner, the Ag:+ cluster being the most stable. Typical spectra are shown in fig.2 and data are given in table 2. Neutral clusters of up to seven atoms are well established,2w22 and are structurally quite well understood. However, it is remarkable that one electron is able, apparently, to hold four cations together, despite the larger charge repulsion. Some degree of solvation must help, but we have never seen any clear explanation for this effect. We have therefore attempted to show that these well defined, stable centres really are Ag:+ units and not Agi with three bonding electrons. The stoichiometry can be gauged by following the loss of Ago to give Agl, and the loss of Agl to give Ag, species. The results establish that each Ago gives rise to one Agi unit, and each Agl gives rise to one Ag, unit. This points strongly to the Ag:+ representation.In another series of experiments, we annealed irradiated samples to give solutions characterised by intense quintets for Ag, units and exposed these samples at 77 K to high radiation doses. Marked loss of the quintets was observed, but unfortunately we have not achieved two-electron addition to give the Ag,+ species (unless this cluster is not detectable by ESR spectroscopy). Even more surprising has been the identification of a species thought to be Ag:+.5 This centre has a far more complex ESR spectrum than those of the other cluster cations [fig. 2(d)]. Computer analysis, after subtraction of features due to Agi ions, gave a reasonable fit for three strongly coupled silver nuclei (140 G) and two weakly coupled nuclei (55 G) (table 2). As usual for these cationic clusters, there are significant shifts to low g values, indicating important contributions from p orbitals, although the hyperfine anisotropy is always small.Although we were able to detect Agi+ and Ag$+ centres under carefully controlled conditions, there is no doubt that the most stable cluster is Agi+, having four equivalent silver nuclei and, probably, a tetrahedral structure. This centre was the final product for all the saturated alcohols. For allyl alcohol and propargyl alcohol, extremely weak features, probably due to various clusters, were observed, but these are not important intermediates. This accords with the absence of Ago centres, which are, presumably, precursors. This means that the species responsible for the intense singlet obtained from the propargyl alcohol system on annealing, which we assign to large silver agglomerates (see below) is not formed from stable cation clusters of the type discussed here.Silver Agglomerates When the irradiated solid solutions were annealed close to their softening points, all signals were lost irreversibly, but in some cases single lines close to free-spin were detected. The best example was for the propargyl alcohol system, which is surprising in view of the absence of Ago centres (fig. 3 and table 3). These signals were not obtained3918 Reactions of Ag' in Alcohols Fig. 2. First-derivative X-band ESR spectra for the silver-cluster cations (a) Agi, (b) Ag;', (c) Agt+, (d) Agt+, after the exposure of unenriched AgC10, (0.05-0.50 mol dm-3) in CD30D to "Co y-rays at 77 K and subsequent annealing in the region ca.100- 130 K. In (a) and (b), features resulting from the various possible lo9Ag and "'Ag isotopic combinations can be clearly seen [in (c) and (d) the linewidths were too broad to resolve these]. (Central solvent features are not shown.) in the absence of silver ions. Also, on annealing to the melting points and re-freezing, the signals were lost, but the samples became black from the formation of colloidal metal particles. The singlet obtained with the propargyl alcohol system has a width (1 8 G) and g value (2.0036) similar to CESR signals obtained from silver particles deposited in CO,, C,H, and C,,H,, matrices using an atomic beam technique.',' We have obtained very similar signals from latent-image silver particles formed during the photolysis of silver halide microcrystals.10 Data for all these centres are given in table 3.R.Janes, A . D. Stevens and M. C. R. Symons 3979 Table 2. ESR data for molecular silver clusters observed after the exposure of AgC10, solutions to 'OCo prays at 77 K and subsequent annealing - A(logAg)/G species solvent A , A , , A,, g, gll g,, Ag2 MeOD EtOD PrOH Agt+ MeOD Ag:' MeOD Ag4,+" MeOD EtOD EtOD 307 312 310 307 312 309 300 308 303 195 150 150 155 152 - - 140( 3) - - - - 55(2) - 140(3) 5 5 a - 1.970 1.997 1.979 1.977 1.999 1.984 1.968 1.992 1.976 - 1.975 - 1.962 1.951 1.967 1.956 1.979 1.946 1.968 - - 1.973 1.943 1.963 a Data derived from a computer simulation. 1 Fig. 3. First-derivative X-band ESR spectra for pseudo-metallic silver agglomerates formed after the exposure of AgClO, (0.50 mol dm-3) in EtOD and CHCCH,OH to 6oCo prays at 77 K and annealing to the matrix softening point.3980 Reactions of Ag' in Alcohols Table 3.X-Band ESR data for AgO, centres observed after the exposure of AgClO, solutions to ,OCo prays at 77 K and extended annealing (CESR data for AgO, centres formed in other systems is also given) matrix giso AH,,IG EtOD 2.002 9 CHCCH,OH 2.0036 18 c 0 , g 2.008-2.016 15-26 CO,, C,H,, C18, HZN9 2.008-2.016 30 AgCl emulsion1o AgBr 2.0035 19-21 2.0042-2.005 1 12- 1 8.5 Solvent Radical Adducts Saturated Alcohols Some time ago a well defined doublet centre was detected after annealing irradiated solutions of silver salts in methanol.ll This was assigned to Ag+-OCH, centres.We have also detected centres of this type, but suggest that they are not Ag+-OR units but are best interpreted in terms of the structure (1) formed by the reaction R,COH + Ag+ -+ Ag+ CR,(OH). (7) Some examples are given in fig. 4 and data in table 4. The major reason for this new assignment is that Ag+-OMe is better written as Ag2+-OMe-. Given that other solvent ligands are also present, such centres should have ESR signals typical of Ag" complexes. Indeed, AgII centres are often detected as minor products in the systems described herein. Also, RO' radicals, although formed from alcohol molecules on irradiation following electron ejection [reaction (8)] : (8) are unstable and convert to R,COH radicals very rapidly at 77 K [reaction (9)]: R,CHOH + R,CHOH+ -+ R,CHO + (H+) R,CHO -+ R,COH.(9) Since the silver-radical adducts are not formed at 77 K, but only begin to grow in around 130 K, they cannot be formed from RO' radicals, nor is there any obvious route starting with Ago centres. However, the adduct doublets grow in as the R,COH signals are lost, in a 1 : 1 manner. As shown below, all the ESR evidence supports the Ag+ .CR,(OH) formulation, and this structure is used from hereon. Notable results are : (i) the Ag+ CH,(OH) adducts give no detectable proton coupling, but the lines narrow on going to Ag+ - CD,(OD). (ii) A clear doublet splitting appears on each feature for Ag+-l3CH2(OH). (iii) For Ag+.CH(CH,)(OH) an extra ca. 40 G doublet is found, and for Ag+ - C(CH,),(OH) a 38 G triplet is observed (fig. 5).These splittings must be due to one specific proton in the CH, groups. This means that -CH, rotation is restricted specifically, such that there is good overlap for one C-H bond andR. Janes, A . D. Stevens and M. C. R. Symons 1 320OG A I +Y* 398 1 I + Y2 Fig. 4. First-derivative X-band ESR spectra for various silver-solvent adducts formed after the exposure of AgC10, (0.05-0.50moldm-3) in (a) MeOH, (b) EtOH, (c) CH,OHCH,OH, (d) CH,CHCH,OH, (e) CHCCH,OH to s°Co y-rays at 77 K and annealing to ca. 120 K (residual solvent features are not shown). Table 4. ESR data for silver-solvent adducts observed after the exposure of AgClO, solutions to "Co y-rays at 77 K and subsequent annealing A(13C)/G adduct -A('OgAg)/G a('H)/G A,, A, g ~ ~~ Ag+ -CH,OH 128 - 80 30 2.000 Ag+ * CH(Me)OH 132 40(1H) - - 2.000 - Ag+ - CH(CH,OH)OH 107 - - 2.002 Ag+ * C(Me,)OH 115 38(2H) - - 2.002 - - 1.993 - 1.994 - Ag+ CH,CHCH, 180 Ag+ CH,CCHa 90 - - Ag * H+,' 1 04 302 - - - a Ref.(24).3982 Reactions of Ag' in Alcohols 1 32176 109 1 LL LLI /I I-I-J LL - __r "9; -I$ +r, +1 4 U+& Ag; ( ' H I +I '09A9 + '07A9 1 I I Fig. 5. First-derivative X-band ESR spectra for silver centres formed after the exposure of unenriched AgClO, (0.05 mol dm-3) in (Me,)CHOH to 6oCo y-rays at 77.K. (a) Immediately after irradiation, showing lo9Ag0 and lo7Ago doublets and central (Me,)COH features, (b) after annealing to ca. 120 K showing features from Ag+.C(Me,)OH radicals and Agi cluster cations. The (- 1) and (+ 1) features from the latter are clearly resolved into 1 : 2 : 1 triplets corresponding to the three possible isotopic combinations (logAg-lo9Ag)+, (lo9Ag-lo7Ag)+ and (107Ag-107Ag)+.The ( - 1, - 1 /2) and ( + 1, + 1 /2) features for the Ag+ * C(Me),OH radicals are concealed under the central features for Agi. poor overlap for the other two. Such restricted rotation is typical of pyramidal Unfortunately, these structures were lost before ?ny onset of rotation able to make all the methyl protons couple equally. That the -CR,OH unit is pyramidal is expected from the postulated structure, and is required by the small aH coupling, which would have been much larger had the radicals come close to planarity, as they are prior to bonding to Ag+. The centre Ag'+CMe,(OH) is of particular interest in the light of a recent report by HengleinZ5 that small silver clusters of remarkable stability are formed by radiolysis of solutions of AgClO, in water and isopropanol solvent.He suggests that reaction (10) occurs, followed by aggregation of silver atoms. Ag+ + (CH,),COH -+ Ago + (CH,),CO + [H+]. (10)R. Janes, A . D. Stevens and M. C. R. Symons 3983 Our results show that this is not a one-step reaction, that the adduct, Ag'+CMe,(OH), is relatively stable and shows no unimolecular tendency to give Ago. Probably aggregation occurs via reactions between these adducts. Possibly the (Ag), species formed in propargyl alcohol is also formed from the silver-radical adducts. It is interesting to compare these novel d radicals with those formed from silver atoms and ethenes (1 1).These have been extensively studied by Kasai and co-worker~.~~* 24 They are clearly symmetrical units with equal bonding to both carbon atoms. The link between the two types of radicals is protonation (12). Ago + CH2=CH2 - 1- H?C-CH2 As yet, such protonation of ethene complexes, or deprotonation of the present o1 complexes has not been observed. Ally1 and Propargyl Alcohols Both form similar doublets, but the splitting has increased to ca. 180 G for ailyl alcohol and decreased to ca. 90 G for propargyl alcohol. If the complexes are d species comparable with those discussed above, i.e. A g Y Ccentys, it is not easy to understand these large differences. For the allyl alcohol doublet, it is possible that there are other features hidden by the central lines. However, these radicals have a larger negative g shift which accords with a larger spin density on silver, so we reject this concept.The problem is more complex than for the saturated alcohols since two different radicals could be involved. Analysis of the spectra of these two ,alcohd! after irradiation26 shows that for allyl alcohol, CH2cHCH2 radicals and CHiCHCHOH radicals are formed in comparable yields. On annealing, the former radicals are lost as the silver adduct signals grow in, leaving the latter radicals still trapped. Thus, the 180 G centre [centre A, fig. 4(d)] is probably formed from allyl radicals. If these are the units (11) they could well have a higher spin-density on silver. Alternatively, they may have a cyclic structure such as (111). //y 'CH2 YCH2 (111) However, other results suggest that such radicals might have only small silver hyperfine couplings.263984 Reactions of Ag’ in Alcohols A second doublet centre was also detected for the propargyl alcohol systems [centre B, fig. 4(d)] with a splitting of ca. 360 G, and a large negative g shift. We have no clear assignment for this unusual centre, but stress that these might be the outer features of a multiplet, rather than being a doublet. Possibly some form of silver aggregation is involved. Only one centre, with A(Ag) = 90 G was seen for the propargyl alcohol . - - - - solutions. - . A@n2-_Lhere are two initial solvent radicals, thought to be H,CC=CH and HC=HCCHOH. The former were lost as the doublet grew in, so the adduct could be (IV). If so, it is difficult to see why, in this case, the silver splitting is reduced./* Ag’ - C -H \C=CH (IV) We stress that the o1 structure for all these adducts is unusual, although o;at (or a*) radicals such as Clip are well established. Ag.Ag+ was probably the first such radical studied by ESR spectroscopy. The prototype for the present species is Ag H+, formed from hydrogen atoms in sulphuric acid gla~ses.~’ The silver splitting of ca. 104 G is clearly comparable with the present centres (except that for ally1 alcohol) which accords well with expectation based on orbital energies. The features were narrow and showed a clear anisotropy for the silver splitting (- 12.8, - 11.6, +24.4 G). This means that there is considerable 5p, contribution to the SOMO. The proton coupling of 302 G shows that the spin density on hydrogen is ca. 60 YO.The 5s character is ca. 15 %, leaving ca. 25 YO Sp-orbital character. Conclusions These systems are excellent examples of the power of ionizing radiation coupled with ESR and optical spectroscopy to give detailed information on charge-transfer and radical chemistry. Thus, electron addition to Ag+ gives Ago centres in a range of solvation states. These give Agi and other cluster centres on warming, and very small metallic particles on further annealing. Solvent radicals, formed by radiolysis, attack Ag+ to give novel silver-alkyl radicals not previously detected. An overall example of this is shown in fig. 5 for AgClO, in isopropyl alcohol. In fig. 5(a) one dominant Ago centre is present together with (CH,),COH radicals, first studied many years ago.28 On annealing [fig.5(b)] the Ago centres clearly form Aga centres, whilst the Me,COH radicals form Ag+ * CMe,(OH) radicals. We thank Drs R. S. Eachus and M. T. Olm (Eastman Kodak Co., Rochester, N.Y.) for many helpful discussions. References I See, e.g. M. D. Morse, Chem. Rev., 1986,86, 1049; J. A. Howard and B. Mile, Ace. Chem. Res., 1987, 2 A. D. Stevens and M. C. R. Symons, J. Chem. Soc., Furaday Trans. I, in press. 3 A. D. Stevens, Ph.D. Thesis (Leicester University, 1985). 4 A. D. Stevens and M. C. R. Symons, Chem. Phys. Lett., 1984, 109, 514. 5 R. Janes, A. D. Stevens and M. C. R. Symons, J . Chem. SOC., Chem. Commun., 1988, 1454. 6 R. Kubo, J. Phys. SOC. Jpn, 1962, 17, 975. 7 A. Kawabata, J . Phys. SOC. Jpn, 1970, 29, 902. 8 R. Monot, C. Narbel and J. P. Borel, Nuevo Cimento, 1974, 19B, 253. 9 A. Chatelain, J. L. Millet and R. Monot, J. Appl. Phys., 1976, 47, 3760. 20, 173. 10 A. D. Stevens, M. C. R. Symons and R. S. Eachus, Phys. Stat. Sol. (a), in press. 1 1 L. Shields, Trans. Faraduy SOC., 1966, 62, 1042.R. Janes, A . D. Stevens and M. C. R. Syrnons 12 R. S. Eachus and M. C. R. Symons, J. Chem. SOC. A, 1970, 1329; 1336. 13 A. J. Buck, B. Mile and J. A. Howard, J. Am. Chem. SOC., 1983, 105, 3387. 14 P. H. Kasai, D. McLeod and T. Watanabe, J. Am. Chem. SOC., 1980, 102, 179. 15 M. C. R. Symons, R. Janes and A. D. Stevens, Chem. Phys. Lett., submitted. 16 D. R. Brown and M. C. R. Symons, J. Chem. Soc., Furuday Trans. I , 1977, 73, 1490. 17 B. L. Bales and L. Kevan, J. Chem. Phys., 1971, 55, 1327. 18 C. K. Aylesbury and M. C. R. Symons, J. Chem. Soc., Faraduy Trans. I , 1980,76, 244. 19 M. C. R. Symons and C. K. Aylesbury, J. Chem. SOC., Furuduy Trans. I , 1982, 78, 3629. 20 J. A. Howard, K. F. Preston and B. Mile, J. Am. Chem. SOC., 1981, 103, 6226. 21 J. A. Howard, R. Sutcliffe and B. Mile, J. Phys. Chem., 1983, 87, 2268. 22 G. A. Ozin and H. Huber, Inorg. Chem., 1978, 17, 155. 23 C. Gase, B. C. Gilbert and M. C. R. Symons, J . Chem. Soc., Perkin Trans. 2, 1978, 235. 24 P. H. Kasai, J. Phys. Chem., 1982, 86, 3684. 25 A. Henglein, Chem. Phys. Lett., 1989, 154, 473. 26 R. Janes, A. D. Stevens and M. C. R. Symons, unpublished results. 27 R. S. Eachus and M. C. R. Symons, J. Chem. SOC. A , 1970, 1336. 28 J. F. Gibson, M. C. R. Symons and M. G. Townsend, J. Chem. SOC., 1958, 269. 3985 Paper 9/0 1605H ; Received 17th April, 1989

ISSN:0300-9599    

DOI:10.1039/F19898503973

出版商:RSC    

年代:1989

数据来源: RSC