Some spectroscopic studies of derivatives of the Group III elements

In this thesis a systematic study of the structures adopted by the trimethyl compounds of the Group III elements under a variety of conditions is described. Oligomerization is a feature of common and apparently irrational occurrence amongst these compounds and the investigation was undertaken with the aim not only of determining the structures themselves but also of elucidating the factors influencing structure within the series. The investigation is presented in four main sections: Introduction (Chapters 1 and 2), Experimental (Chapters 3, 4 and 5), Results (Chapters 6, 7 and 8) and Discussion (Chapter 9).
The background to the problem is discussed in Chapter 1. Previous investigations of the Group III trimethyl compounds have been haphazard and have concentrated, in general, on the elucidation of the structure of one compound in one phase. The fluid phases have received most attention and whilst trimethylboron, trimethylgallium and trimethylindium are all monomeric, the aluminium compound adopts a symmetrical dimeric structure with bridging methyl groups. In the former three compounds the MC3 skeleton is planar and, since the methyl groups are able to rotate freely, the overall point group of the molecules is D3h; the trimethylaluminium dimer belongs to the D2h point group. The structure of trimethylthallium in the fluid phases has not been reported.
In the solid state trimethylaluminium has been shown by X-ray crystallography to retain the bridged, dimeric structure, whereas trimethylindium adopts an unusual structure with a polymeric network of tetramer units in which each indium atom is penta-coordinated by methyl groups. The tetrameric unit is held together by unsymmetrical methyl bridges and the tetramers also interact, but wore weakly. Within each tetramer nearly trigonal Me3In 'monomer' units can still be distinguished and the binding within these units is much stronger than the binding between them. Thus the strength of the methyl group to indium bonds in the trimethylindium crystal falls in the order:
Intra-'monomer' and#x226b; intra-tetramer andgt; inter-tetramer.
Recently trimethylthallium has been reported to adopt a similar solid state structure, but with more nearly equivalent inter and intra tetrameric bonds. For both trimethylindium and trimethylthallium the point group of the tetrameric unit is S4 and the overall symmetry of the crystal is C44h. The structures of trimethylboron and trimethylgallium in the solid state have not been investigated.
The concept of electron deficiency in the context of the present investigation is briefly discussed and the factors conceivably influencing the equilibrium:
n (monomer) and#x21cc; oligomer
are considered in some detail. For the specific case of dimer formation five factors arc isolated and, as far as possible, their relative importance assessed for each of the Group III elements:
the M-M distance in the dimer;
the M-C bond enthalpy;
the sandrarr;p promotion energy;
the van der Waals repulsion between non-bonded atoms in the dimer;
the stability of the monomer.
In the light of these factors it is concluded that the relative tendency amongst the trimethyl compounds of the Group III elements towards dimer formation in particular, and oligomer formation in general, is given:
Al andgt; In andgt; Ga and#x224f; Tl and#x226b; U
The requirements of the technique for structural elucidation are considered:
applicability to all three phases;
applicability over a very wide temperature range;
feasibility of examining highly reactive compounds;
adaptability to special circumstances (e.g. matrix isolation);
and it is concluded that only vibrational spectroscopy is able to meet all these stringent requirements. In consequence the techniques of Raman and infrared spectroscopy were used almost exclusively in the structural investigations described in the thesis. The interpretation of the spectroscopic results is briefly discussed and the value of vibrational spectroscopy as a structural tool critically reviewed in Chapter 2. The solid state is given particular attention and it is concluded that the level of sophistication appropriate to the interpretation of the spectra of solid compounds is dependent upon both the nature and degree of intermolecular interaction involved. The structure of solid trimethylindium is used to exemplify three distinct approaches to the interpretation of solid-phase spectra:
the site symmetry approach;
the aggregate unit approach;
the factor group approach.
By way of introduction to the experimental section of the thesis, the general problems involved in the examination of compounds which react avidly with oxygen and moisture are discussed. The salient features of the instruments used in the investigation are then described. A Cary 81 spectrophotometer equipped with either a helium-neon or an argon-ion laser was employed to obtain the bulk of the Raman data. A Perkin-Elmer 225 spectrophotometer and a Beckman I.R.11 spectrophotometer were used to obtain the infrared spectra in the 4000andndash;200 cmandminus;1 and 600andndash;50 cmandminus;1 regions, respectively. The chapter is concluded by a description of the experimental assemblies developed during the course of the investigation for the examination of the Raman spectra of liquid samples over the temperature range andminus;150 to +150anddeg;C and for the examination of the infrared and Raman spectra of solid samples at andminus;196anddeg;C. The preparation and manipulation of the five compounds are described in Chapter 4, together with details of the simpler sampling devices developed during the course of the investigation.
In Chapter 5 the nature, scope and validity of the matrix-isolation method in the context of the present investigation are discussed in SORO detail. It is concluded not only that the method provides a powerful and valid tool for the investigation of the structures of both stable and unstable species but also that, by allowing the matrix to become mobile, it is possible to use the method to follow oligomerization processes in a novel and potentially highly-informative way. The experimental assembly and technique for the examination of the infrared spectra of species isolated in noble gas or nitrogen matrices at 20anddeg;K are described. The assembly comprises in essence a Perkin-Elmer 225 spectrometer coupled to a miniature Joule-Thomson liquid hydrogen cryostat bearing a caesium iodide window and encased within a suitable vacuum system. The potential advantages of obtaining the Raman spectra of matrix-isolated species are discussed and the development of an assembly for use in conjunction with the Cary 81 instrument is described.
The results of the fluid-phase investigations are presented in Chapter 6. The infrared and Raman spectra of pure trimethylboron wore fully examined at room temperature and, in the case of the Raman spectrum, at temperatures down to andminus;150anddeg;C, no change in the spectrum being detected. Trimethylaluminium was examined both pure and in solution at room temperature and, in the case of the Raman investigation, at elevated temperatures both in solution and in the vapour phase. In particular, it proved possible, by examining the vapour at 170anddeg;C, to obtain the major features of the Raman spectrum of monomeric trimethylaluminium for the first time. The infrared and Raman spectra of pure trimethylgallium were fully investigated and the Raman and proton magnetic resonance spectra of trimethylgallium in dichloromethane solution were examined at temperatures down to andminus;100anddeg;C, no change being detectable in either case. Trimethylindium was investigated in the vapour phase by infrared spectroscopy and in solution, both at room temperature and at andminus;23anddeg;C, by Raman spectroscopy, again no new features being observed. The vibrational spectra of neat trimethylthallium were fully investigated at 40anddeg;C, the compound being monomeric at this temperature.
The spectra of the compounds in the solid phase are described in Chapter 7. The infrared spectrum of solid trimethylboron was examined at 20anddeg;K and did not differ appreciably from that of the compound in the vapour phase. The Raman and infrared spectra of the compounds of the four heavier elements were all examined at andminus;196anddeg;C, and whilst trimethylaluminium yielded results closely similar to those obtained in the fluid phases, the spectra of trimethylgallium, -indium and -thallium were all very much more complicated than their fluid-phase counterparts. The overall spectral patterns for these latter three compounds were closely similar to one another and the examination of microcrystalline samples of trimethylgallium and trimethylindium by X-ray crystallography confirmed that trimethylgallium is isostructural with trimethylindium in the solid state.
The spectra of matrix-isolated trimethylboron, -aluminium and -gallium are reported in Chapter 8. The infrared results for monomeric trimethylboron and trimethylgallium and for dimeric trimethylaluminium agreed well with their vapour-phase counterparts and it also proved possible to obtain the infrared spectra of virtually pure monomeric trimethylaluminium for the first tine. When the matrices were allowed to diffuse the spectrum of trimethylboron showed no alteration, but a new set of features appeared in that of trimethylgallium and two new sets in that of monomeric trimethylaluminium. One of the two new sets of features for trimethylaluminium belonged to trimethylaluminium dimer, whilst the other set, as far as could be ascertained, corresponded to the set given by trimethylgallium. The major features of the Raman spectra of matrix-isolated trimethylgallium monomer and of matrix-isolated trimethylaluminium monomer and dimer were also recorded.
In the final chapter of the thesis the results arc correlated and the chemical information they contain discussed under four headings: monomeric species, dimeric species, diffusion species and larger aggregates. In the first section the results for all five monomers are compared and it is concluded that although the bonding in trimethylaluminium is appreciably weaker than would be expected on the basis of a direct interpolation of the strengths of the bonds in trimethylboron and trimethylgallium, it is in reasonable accord with the high polarity of the aluminium-carbon bond. Comparison is made to the methyl compounds of the other Main Group elements and product rule, force constant and bond length calculations are presented.
The spectra of dimeric trimethylaluminium are compared both with the results of published force constant calculations and with the published results for analogous systems and the assignment for the skeletal nodes of the molecule is considerably extended. The validity of certain ideas recently postulated for the structure of solid trimethylaluminium are also investigated.
The nature of the new species produced by the interaction of trimethylaluminium monomer units and trimethylgallium monomer units in nitrogen matrices at 34anddeg;K are considered and it is concluded that the spectroscopic results arc best interpreted in terms of species containing pyramidal MC3 units. It is suggested that direct metal-metal interactions nay be involved in the formation of the new diffusion species:
[For the diagram omitted here, please consult the PDF]
In the discussion of the spectra of the solid compounds of trimethylgallium, -indium and -thallium, it is concluded that whilst a site symmetry approach is definitely inappropriate it is not possible to decide which of the two extended approaches (the aggregate unit or the factor group) is the more suitable. It is further concluded that although the overall degree of intermolecular interaction in solid trimethylgallium is less than that in solid trimethylindium and probably more nearly comparable to that in solid trimethylthallium it is unfortunately not possible to rake any definite deductions regarding the relative strengths of inter- and intra-tetrameric bonds in solid trimethylgallium.
In the concluding section of the thesis the tendency to oligomer formation for each of the Group III trimethyl compounds is discussed in the light both of the theoretical considerations of Chapter 1 and of the experimental results. It is concluded that the tentative sequence of oligomerization tendency suggested on theoretical grounds for the trimethyl compounds of Group III elements is substantiated by experiment:
Al andgt; In andgt; Ga and#x224f; Tl and#x226b; B