Assembly of an Allenylidene Ligand, a Terminal Alkyne, and an Acetonitrile Molecule:  Formation of Osmacyclopentapyrrole Derivatives

Treatment in acetonitrile at −30 °C of the hydride−alkenylcarbyne complex [OsH(⋮CCHCPh2)(CH3CN)2(P i Pr3)2][BF4]2 (1) with t BuOK produces the selective deprotonation of the alkenyl substituent of the carbyne and the formation of the bis-solvento hydride−allenylidene derivative [OsH(CCCPh2)(CH3CN)2(P i Pr3)2]BF4 (2), which under carbon monoxide atmosphere is converted into [Os(CHCCPh2)(CO)(CH3CN)2(P i Pr3)2]BF4 (3). When the treatment of 1 with t BuOK is carried out in dichloromethane at room temperature, the fluoro−alkenylcarbyne [OsHF(⋮CCHCPh2)(CH3CN)(P i Pr3)2]BF4 (4) is isolated. Complex 2 reacts with terminal alkynes. The reactions with phenylacetylene and cyclohexylacetylene afford [Os{(E)-CHCHR}(CCCPh2)(CH3CN)2(P i Pr3)2]BF4 (R = Ph (5), Cy (6)), containing an alkenyl ligand beside the allenylidene, while the reaction with acetylene in dichloromethane at −20 °C gives the hydride−allenylidene−π-alkyne [OsH(CCCPh2)(η 2-HC⋮CH)(P i Pr3)2]BF4 (7), with the alkyne acting as a four-electron donor ligand. In acetonitrile under reflux, complexes 5 and 6 are transformed into the osmacyclopentapyrrole compounds (R = Ph (8), Cy (9)), as a result of the assembly of the allenylidene ligand, the alkenyl group, and an acetonitrile molecule. The X-ray structures of 2, 5, and 8 are also reported.