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 Cβ atom of the C3-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)­{κ3-P,O,P-[xant­(PiPr2)2]})2(μ-Cl)2]­[BF4]2 (1). Complex 1 reacts with 1,1-diphenyl-2-propyn-1-ol to give the hydride-osmium­(II)-alkenylcarbyne
[OsHCl­(CCHCPh2)­{κ3-P,O,P-[xant­(PiPr2)2]}]­BF4 (2), which yields OsHCl­(CCCPh2)­{κ3-P,O,P-[xant­(PiPr2)2]} (3) by selective abstraction of the Cβ–H hydrogen atom of the alkenylcarbyne ligand
with KtBuO. Complex 3 is
metastable. According to results of DFT calculations, the migration
of the hydride ligand to the Cβ 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(CIndPh)­{κ3-P,O,P-[xant­(PiPr2)2]} (4) and the osmanaphthalene
OsCl­(C9H6Ph)­{κ3-P,O,P-[xant­(PiPr2)2]} (5). Protonation
of 4 with HBF4 leads to the elongated dihydrogen
complex [OsCl­(η2-H2)­(CIndPh)­{κ3-P,O,P-[xant­(PiPr2)2]}]­BF4 (6), while the protonation
of 5 regenerates 2. In contrast to 4, complex 6 evolves to a half-sandwich indenyl
derivative, [Os­(η5-IndPh)­H­{κ3-P,O,P-[xant­(PiPr2)2]}]­[BF4]­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CPh2)­(η2-HCCPh)­{κ3-P,O,P-[xant­(PiPr2)2]}]Cl (8).