Cationic Dihydride Boryl and Dihydride Silyl Osmium(IV) NHC Complexes: A Marked Diagonal Relationship

The complex [OsCl­(η⁶-p-cymene)­(IPr)]­OTf (1; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolylidene, OTf = trifluoromethanesulfonate) reacts with pinacolborane (HBpin) and catecholborane (HBcat) to give the dihydride boryl osmium­(IV) derivatives [OsH₂(Bpin)­(η⁶-p-cymene)­(IPr)]­OTf (2) and [OsH₂(Bcat)­(η⁶-p-cymene)­(IPr)]­OTf (3), which undergo hydrolysis to afford the trihydride [OsH₃(η⁶-p-cymene)­(IPr)]­OTf (4). In agreement with the boron–silicon diagonal relationship, complex 1 also reacts with silanes, HSiR₃. The reactions lead to the dihydride silyl species [OsH₂(SiR₃)­(η⁶-p-cymene)­(IPr)]­OTf (SiR₃ = SiPh₃ (5), SiEt₃ (6), SiHPh₂ (7)), which also undergo hydrolysis to yield 4. Complexes 2 and 5 have been characterized by X-ray diffraction analysis. Their four-legged piano-stool geometries with transoid hydride ligands are similar. In solution, the arene rotates over the remaining four ligands. The activation barrier of the process depends upon the size of the boryl or silyl groups. The nature of the Os–B and Os–Si interactions has been investigated by means of computational methods. Both of them are σ bonds with small or negligible π back-donation from the metal to the ligands.