C(sp²)−H Activation of RCHE−py (E = CH, N) and RCHCHC(O)R‘ Substrates Promoted by a Highly Unsaturated Osmium−Monohydride Complex

The 14-valence-electron monohydride OsH(SnPh2Cl)(PiPr3)2 (a), generated from (1), activates a C(sp2)−H bond of 2-vinylpyridine, (E)-N-(phenylmethylene)-2-pyridinamine, and α,β-unsaturated ketones. The activation of 2-vinylpyridine affords the elongated dihydrogen (dH-H = 1.41 Å) derivative (2) in equilibrium (ΔH° = −2.5 ± 0.2 kcal mol-1 and ΔS° = −14.8 ± 1.0 cal mol-1 K-1) with the tautomer (3), where the stannane is bonded to the transition metal by an Os−H−Sn three-center bond (JH-Sn = 183 Hz). The activation of (E)-N-(phenylmethylene)-2-pyridinamine gives rise to (4; dH-H = 1.32 Å), whereas the treatment of 1 with methyl vinyl ketone and benzylidenacetone leads to (R = H (5), Ph (6)), which show blocked rotation of the dihydrogen ligand (dH-H = 1.45 (5), dH-H = 1.42 (6) Å) on the NMR time scale (ΔH⧧ = 11.7 ± 0.4 kcal mol-1 and ΔS⧧ = −1 ± 1 cal mol-1 K-1 (5), ΔH⧧ = 10.8 ± 0.5 kcal mol-1 and ΔS⧧ = 0.4 ± 1 cal mol-1 K-1 (6)). Complex 1 also reacts with benzylidenacetophenone. The reaction initially gives (7), which isomerizes into (8), resulting from the o-CH activation of the PhCO aryl group. Complex 8 is other example of blocked rotation of the dihydrogen ligand (dH-H = 1.45 Å) on the NMR time scale (ΔH⧧ = 10.6 ± 0.6 kcal mol-1 and ΔS⧧ = −6.3 ± 1.5 cal mol-1 K-1). The structures of 2 and 5 have been determined by X-ray diffraction analysis.