Hydride-Carbyne to Carbene Transformation in an Osmium-Acetate-Bis(triisopropylphosphine) System: Influence of the Coordination Mode of the Carboxylate and the Reaction Solvent

The bis-solvato hydride-allenylidene complex [OsH(CCCPh2)(CH3CN)2(PiPr3)2]BF4 (1) reacts
with acetic acid to give the hydride-carbyne [OsH(κ2-O2CCH3)(⋮CCHCPh2)(PiPr3)2]BF4 (2), which in
1,2-dichloroethane under reflux is stable and does not evolve into its five-coordinate carbene isomer. In
acetonitrile at room temperature, complex 2 is in equilibrium with [OsH{κ1-OC(O)CH3}(⋮CCHCPh2)(CH3CN)(PiPr3)2]BF4 (4; ΔH° = −6.4 ± 0.3 kcal·mol-1, ΔS° = −22.9 ± 1.1 eu). At 353 K,
complex 4 is transformed into the carbene [Os{κ1-OC(O)CH3}(CHCHCPh2)(CH3CN)2(PiPr3)2]BF4
(5; 40%), which is obtained in high yield (88%) by reaction of [Os(CHCHCPh2)(CH3CN)3(PiPr3)2][BF4]2 with sodium acetate in 2-propanol. The hydride-carbyne to carbene transformation is analyzed by
DFT calculations.