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

Abstract

Complex [OsH(η2-H2)(η2-CH2═CH-o-C5H4N)(PiPr3)2]BF4 (1) reacts with methyl vinyl ketone in the absence of solvent to give [OsH{CHCHC(O)CH3}2(PiPr3)2]BF4 (2), which can be described as two osmafurans joined by a common [OsH(PiPr3)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(PiPr3)2 (3). The reaction is not reversible; the protonation of 3 with HBF4·OEt2 leads to [Os{CHCHC(O)CH3}{═CHCH2C(═O)CH3}(PiPr3)2]BF4 (4), which in dichloromethane is unstable and evolves into [Os{CHCHC(O)CH3}Cl{≡CCH═C(CH3)OH}(PiPr3)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···F hydrogen bond. In dichloromethane the H···F hydrogen bond is broken and DFT calculations suggest that the OH-hydrogen atom forms an H···Cl hydrogen bond with the chlorine ligand. In the absence of solvent, complex 1 reacts with benzylideneacetophenone and benzylideneacetone to give [Os{C(Ph)CHC(O)R}(η2-H2){κ-C-[HNC5H3Et]}(PiPr3)2]BF4 (R = Ph (6), CH3 (10)), containing a NH-tautomerized 2-ethylpyridine ligand. Complexes [Os{C(Ph)CHC(O)R}(η2-H2){(CH3CN)}(PiPr3)2]BF4 (R = Ph (8), CH3 (11)) and [Os{C6H4C(O)CH═CHPh}(η2-H2){(CH3CN)}(PiPr3)2]BF4 (9) have been also isolated and characterized. The X-ray structures of 2, 5, and 6 are reported.