Hydroosmiation of Allenes and Reductive Elimination of Olefin in Unsaturated Osmium(IV) Polyhydrides: Hydride versus Chloride

The replacement of a chloride in
OsH2Cl2(PiPr3)2 (1) by a hydride, to afford OsH3Cl­(PiPr3)2 (2), produces dramatic changes in the behavior of the
system. Complex 1 hydrogenates the sterically most hindered
C–C double bond of gem-disubstituted allenes.
In contrast, the trihydride 2 reacts with 1,1-dimethylallene
and 1-methyl-1-(trimethylsilyl)­allene to give the alkenyl derivates
OsH2Cl­{C­(CH3)C­(CH3)­R}­(PiPr3)2 (R = CH3 (3), SiMe3 (4)), which do not
undergo reductive elimination of olefin in spite of the coplanar cis disposition of the metalated carbon atom and one of
the hydride ligands. The X-ray structure of 3 proves
that the α-methyl substituent of the alkenyl group cancels the
unsaturated character of the metal center. Complex 2 also
reacts with ethyl allenecarboxylate. The reaction leads to the metallafuran
OsH2Cl­{κ2C,O-[C­(CH3)CHC­(O)­OEt]}­(PiPr3)2 (5), which has been also characterized
by X-ray diffraction analysis. DFT calculations suggest that the formation
of 3 and 4 is a consequence of the coplanar cisoid disposition of the hydride ligand and the CH2 group of the allene in a hydride–dihydrogen−π-allene
intermediate. The DFT calculations also show that 3 and 4 are thermodynamic sinks and that the reductive elimination
of the olefin is kinetically disfavored.