Repercussion of a 1,3-Hydrogen Shift in a Hydride-Osmium-Allenylidene Complex.

An unusual 1,3-hydrogen shift from the metal center to the C β atom of the C 3 -chain of the allenylidene ligand in a hydride-osmium(II)-allenylidene complex is the beginning of several interesting transformations in the cumulene. The hydride-osmium(II)-allenylidene complex was prepared in two steps, starting from the tetrahydride dimer [(Os(H···H){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]}) 2 (μ-Cl) 2 ][BF 4 ] 2 ( 1 ). Complex 1 reacts with 1,1-diphenyl-2-propyn-1-ol to give the hydride-osmium(II)-alkenylcarbyne [OsHCl(≡CCH=CPh 2 ){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]}]BF 4 ( 2 ), which yields OsHCl(=C=C=CPh 2 ){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]} ( 3 ) by selective abstraction of the C β -H hydrogen atom of the alkenylcarbyne ligand with K t BuO. Complex 3 is metastable. According to results of DFT calculations, the migration of the hydride ligand to the C β atom of the cumulene has an activation energy too high to occur in a concerted manner. However, the migration can be catalyzed by water, alcohols, and aldehydes. The resulting alkenylcarbyne-osmium(0) intermediate is unstable and evolves into a 7:3 mixture of the hydride-osmium(II)-indenylidene OsHCl(=C IndPh ){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]} ( 4 ) and the osmanaphthalene OsCl(C 9 H 6 Ph){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]} ( 5 ). Protonation of 4 with HBF 4 leads to the elongated dihydrogen complex [OsCl(η 2 -H 2 )(=C IndPh ){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]}]BF 4 ( 6 ), while the protonation of 5 regenerates 2 . In contrast to 4 , complex 6 evolves to a half-sandwich indenyl derivative, [Os(η 5 -IndPh)H{κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]}][BF 4 ]Cl ( 7 ). Phenylacetylene also provokes the 1,3-hydrogen shift in 3 . However, it does not participate in the migration. In contrast to water, alcohols, and aldehydes, it stabilizes the resulting alkenylcarbyne to afford [Os(≡CCH=CPh 2 )(η 2 -HC≡CPh){κ 3 - P , O , P -[xant(P i Pr 2 ) 2 ]}]Cl ( 8 ).