Acid-catalyzed hydrogenolysis of osmium-alkyl bonds

Article abstract of DOI:10.1021/om00114a021

Hydrogenolysis of the osmium-carbon bond in fac-L₃Os(H) (η²-CH₂PMe₂) (1, L = PMe₃) and cis-L₄Os(H)Me (8) to form cis-L₄OsH₂ (2) has been found to proceed at 80°C in benzene or THF under less than 1 atm of H₂ upon catalysis by [L₄OsH₃](OTf) (2-H⁺). The catalytic cycle, e.g. for 1, involves transfer of a proton from 2-H⁺ to 1; 1-H⁺ becomes [L₄OsH]⁺ (4); 4 takes up H₂ to form 2-H⁺; and 2-H⁺ again donates one of its protons to 1 liberating 2. The hydrogenolysis is retarded by excess L, which reversibly traps 4 to form [L₅OsH]⁺ (5). Steps in this cycle have been corroborated by carrying out model reactions. The kinetic basicity of 1 and 8 (L₄Os(H)R) is demonstrated by the following reactions. Treatment of L₄Os(H)R with [L₄OsH₃]Cl yields 2 and L₄Os(H)Cl; L₄Os(H)R and [HPMe₃]OTf form [L₅OsH]OTf (5); and treatment of L₄Os(H)R with HOTf generates cis-L₄Os(H)OTf (7). Complex 7 rapidly reacts with H₂ to form 2-H⁺ and with L to form 5. An equilibrium between 2-H⁺ + L and 5 + H₂ has been demonstrated. A linear dependence of the rate constant (zero order in 1 or 8) on the concentration of 2-H⁺ has been shown.