Cβ(sp2)-H bond activation of α,β-unsaturated ketones promoted by a hydride-elongated dihydrogen complex:: Formation of osmafuran derivatives with carbene, carbyne, and NH-tautomerized α-substituted pyridine ligands

Complex [OsH(eta(2)-H-2)(eta(2)-CH2 = CH-o-C5H4N)((PPr3)-Pr-i)(2)]BF4 (1) reacts with methyl vinyl ketone in the absence of solvent to give [OsH{CHCHC(O)CH3}(2)((PPr3)-Pr-i)(2)]BF4 (2), which can be described as two osmafurans joined by a common [OsH((PPr3)-Pr-i)(2)](+) fragment. The hydride ligand of 2 is fairly acidic. Thus, its treatment with sodium methoxide produces the deprotonation of the metal center to give OS{CHCHC(O)CH3}(2)((PPr3)-Pr-i)(2) (3). The reaction is not reversible; the protonation of 3 with HBF4 center dot OEt2 leads to [OS{CHCHC(O)CH3}[= CHCH2C(= O)CH3)((PPr3)-Pr-i)(2)]BF4 (4), which in dichloromethane is unstable and evolves into [(Os{CHCHC(O)CH3}Cl{= CCH=C(CH3)OH}((PPr3)-Pr-i)(2)]BF4 (5), containing an enolcarbyne ligand. In the solid state the anion of 5 is associated with the OH-hydrogen of the enol, by means of an H center dot center dot center dot F hydrogen bond. In dichloromethane the H center dot center dot center dot F hydrogen bond is broken and DFT calculations suggest that the OH-hydrogen atom forms an H center dot center dot center dot Cl hydrogen bond with the chlorine ligand. In the absence of solvent, complex 1 reacts with benzylideneacetophen one and benzylideneacetone to give [Os{C(Ph)CHC(O)R} (eta(2)-H-2) {kappa-C-[HNC5H3Et]}((PPr3)-Pr-i)(2)]BF4 (R = Ph (6), CH3 (10)), containing a NH-tautomerized 2-ethylpyridine ligand. Complexes [Os{C(Ph)CHC(O)R}(eta(2)-H-2){(CH3CN)}((PPr3)-Pr-i)(2)]BF4 (R = Ph (8), CH3 (11)) and [Os{C6H4C(O)CH = CHPh}(eta(2)-H-2){(CH3CN)}((PPr3)-Pr-i)(2)]BF4 (9) have been also isolated and characterized. The X-ray structures of 2, 5, and 6 are reported.