1002
J . Phys. Chem. 1985,89, 1002-1006
Matrix Isolation Infrared Spectroscopic Study of the 1:l Molecular Complexes between the Sulfur Fluorldes and Oxyfluorldes and Selected Bases Craig S. Sass and Bruce S. Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: August 27, 1984)
The 1:l molecular complexes between SF,, SOF2, and S02F2with a wide variety of amines and oxygen-containing bases have been isolated and characterized in nitrogen matrices. All complexes are characterized by a red shift in the S-F stretching modes from that of the parent sulfur fluoride or oxyfluoride,as typified by the bands at 675,830, and 841 cm-' for the SF4.NH3 complex. The magnitude of these shifts to lower energy is dependent upon the strength of the base. In addition, perturbed vibrational modes of the base subunit in the complex were noted for certain complexes, such as the symmetric deformation mode of NH3 in SF4-NH3complex, at 1050 cm-'. These shifts suggest that these complexes are covalently bonded from the sulfur to the nitrogen or oxygen on the base with the sulfur fluoride or oxyfluoride acting as a d?r* acceptor of electron density from the lone pair on the base. In addition, the spectra indicate an order of Lewis acidity of SF4 > SOF2 > S02F2, where only very weak complexes were observed.
Introduction excellent technique for the study of relatively weakly bound molecular complexes and therefore a study was undertaken to Lewis acid-base complexes have been known for years'*2and isolate and spectroscopically characterize the complexes of SF4, many examples are known, particularly for strong acids and bases SOF2, and SOzF2with amines, pyridine, and acetone to obtain such as BF3334 Interest in such molecular complexes has increased a better understanding of the nature of these complexes. in recent years, in part due to their possible intermediacy in chemical reactions, and in part due to theoretical interest in their Experimental Section structure and stability. Within the past few years, the matrix All of the experiments in this study were conducted on a isolation technique has been applied successfully to study molecular complexes, including those which are only weakly b o ~ n d . ~ . ~ conventional matrix isolation system which has been described in detail previously.16 N H 3 (Matheson), (CH3)3N(Matheson), Recently, 1:l complexes of SO2 with amines' were isolated and pyridine (MCB), and acetone (M. J. Daly) were all used after characterized in nitrogen matrices, which provided some insight as to the nature of the sulfur-nitrogen coordinate bond in these purification by one or more freeze-thaw cycles at 77 K. SF4 (Matheson) was found to contain considerable amounts of SOF2 complexes. However, little is still known of this coordinate linkage impurity and was purified by distillation from a dry ice/chloroform and its role in molecular complexes. bath. SOFz (PCR) was found to contain large amounts of SO2 Sulfur tetrafluoride, SF,, is a highly reactive species which can and was purified by distillation from a dry ice/acetone bath. Even serve either as Lewis acid or base. In this latter capacity it is with these attempts at purification, matrix samples of N2/SF4 known to react with Lewis acids8 to form ion pairs of the form contained substantial SOF2 which contributed to the clutter in S F 3 + A F ,much less is known about its ability to serve as a Lewis the S-F stretching region of the spectrum. CH3NHz(Matheson) acid. A few workers have reported sketchy data which suggests and (CH&NH (Matheson) were found to contain NH, impurity the existence of molecular complexes of SF4with pyridine? but and were distilled from a dry ice/chloroform bath before use. few details are known, presumably due to its high reactivity. At S02F2was prepared by allowing SOz (Matheson) and F2 the same time, SF4is known to react with NH3 in the synthesis (Matheson) to react in a vacuum line at room temperature for of S4N4,and a molecular complex may be formed in the initial several minutes and was then purified by distillation from a dry step of the The identification of such a complex ice/chloroform bath. NH3, CH3NHz,and (CH3)2NHwere found would provide information on the reaction pathways of these to adsorb strongly on the stainless steel vacuum line, necessitating processes in addition to providing more information on the nature conditioning prior to each experiment. ND3 (99% D, Merck) was of the sulfur-nitrogen coordinate bond. In addition, SF4is known used after purification by freeze-thaw cycles, and the line deuto selectively fluorinate ketones to -CF2 groups, suggesting that terated by repeated exposure to D20. Even then the ND3 exa SF4.0C(CH3), complex might well be stabilized under low changed with hydrogen-containing impurities in the vacuum temperature conditions. system and a final enrichment of 65 atom % D was observed. The sulfur oxyfluorides SOFzand SOzF2are known to exhibit Nitrogen was used as the matrix gas in all experiments and was Lewis acid character through involvement of the sulfur d orbitals12 used without further purification. and are known to react with the strong Lewis base to form In a typical experiment the two reactants were mixed with the SOF3-and S02F
