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

[Image: see text] 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).