Activation of C(sp2)-H and reduction of C=E (E = CH, N) bonds with an osmium-hexahydride complex:: Influence of E on the behavior of RCH=E-py substrates

The hexahydride complex OsH6((PPr3)-Pr-i)(2) (1) reacts with 2-vinylpyridine to give the trihydride derivative OsH3(NC5H4-o-CH=CH)((PPr3)-Pr-i)(2) (2). The beta- and gamma-positions of the pyridine ring are quantitatively and selectively deuterated by addition of 1 (5%) to toluene-d(8) and benzene-d(6) solutions of 2. Under hydrogen atmosphere, 2 reacts with (HBF4OEt2)-O-. to afford [HNC5H4-o-Et]BF4 and regenerate 1. Under argon atmosphere, the addition of (HBF4OEt2)-O-. to dichloromethane solutions of 2 leads to the hydride-dihydrogen [OsH(eta(2)-H-2)(eta(2)-CH2=CH-o-C5H4N)((PPr3)-Pr-i)(2)]BF4 (3), which catalyzes the hydrogenation of 2-vinylpyridine to 2-ethylpyridine. In dichloromethane 3 evolves into the neutral chloro-dihydrogen compound Os(NC5H4-o-CH=6H)Cl(eta(2)-H-2)((PPr3)-Pr-i)(2) (4). Complex 1 also reacts with N-methylene-2-pyridinamine and (E)-N-(phenylmethylene)-2-pyridinamine. The reaction with the first pyridinamine leads to a 3:1 mixture of the trihydride complexes OsH3(NC5H4-o-NCH3)((PPr3)-Pr-i)(2) (5) and OsH3(NC5H4- o-N=CH)((PPr3)-Pr-i)(2) (6), while the reaction with the second one selectively affords OsH3(NC5H4-o o-NCH2Ph)((PPr3)-Pr-i)(2) (7). Similarly to 2, the addition of 1 (5%) to toluene-d(8) and benzene-d(6) solutions of 6 produces the quantitative and selective deuteration of the beta- and gamma-positions F_ of the pyridine ring. The addition of (HBF4OEt2)-O-. to diethyl ether solutions of 5 leads to [OsH3-(NC5H4-o-NHCH3)((PPr3)-Pr-i)(2)BF4 (8). Isotope labeling experiments suggest that the reduction of the C C double bond of 2-vinylpyridine takes place by concerted addition of a dihydrogen ligand, while the reduction of the C=N double bond of N-methylene-2-pyridinamine occurs by sequential addition of H- and H+. In solution the hydride ligands of these compounds undergo two different site exchange processes. Their activation parameters have been calculated by H-1 NMR spectroscopy. Complexes 2, 3, and 5 have been characterized by X-ray diffraction analyses.