INVESTIGATIONS INTO THE STERIC INFLUENCES ON THE REACTION-MECHANISM OF CO2 INSERTION INTO METAL OXYGEN BONDS - COS ACTIVATION AS A MODEL FOR CO2

Steric influences on the insertion of CO2 into metal-oxygen bonds have been investigated. The compounds [Et4N][W(CO)5O-2,6-Ph2-C6H3] (1) and [Et4N][W(CO)4(PPh2Me)OPh] (2) have been synthesized and their reactivities toward CO2 studied. Complex 1 showed no reactivity toward CO2 even at elevated pressures (900 psi). The anion of 2 demonstrated a retarded rate of reaction with CO2 as compared to the parent complex, [Et4N][W(CO)5OPh], a result ascribed to the steric bulk of the cis phosphine ligand. In general, it was found that the reactivity of metal-oxygen bonds toward CO2 was adversely affected by the increased steric demands of substituents on either the metal center or the aryl group of the ligated aryloxide. Complexes 1 and 2 were characterized in the solid state by X-ray crystallography. Complex 1 crystallized in the space group P21/c with cell dimensions a = 12.658 (3) angstrom, b = 15.136 (3) angstrom, c = 33.471 (6) angstrom, beta = 95.388 (15)-degrees, V = 0.6385 (2) angstrom 3, Z = 8, and R(F) = 6.4%. Complex 2 crystallized in the space group P2(1/n) with cell dimensions a = 11.309 (7) angstrom, b = 16.9389 (8) angstrom, c = 16.888 (13) angstrom, beta = 102.39 (5)-degrees, V = 3160 (3) angstrom 3, Z = 4, and R(F) = 4.3%. These compounds were also reacted with COS as a model for CO2. [Et4N] [W(CO)5O-2,6-Ph2-C6H3] reacted cleanly under 1 atm of COS and at ambient temperature to afford the product of COS insertion into a metal-oxygen bond, [Et4N][W(CO)5SC(O)O-2,6-Ph2-C6H3] (3). enhanced reactivity of 1 toward COS as compared to CO2 is proposed to be in part due to W...S bond formation in the transition state. Complex 3 was subjected to X-ray crystallographic investigation. Complex 3 crystallized in the space group Pcmn (No. 62) with cell dimensions a = 11.803 (6) angstrom, b = 14.844 (8) angstrom, c = 18.752 (11) angstrom, V = 3285(3) angstrom 3, Z = 8, and R(F) = 6.3%.