A MHC6 complex of a platinum group metal with a capped octahedral arrangement of donor atoms around the metal center has been characterized. This osmium compound OsH{κ(2) -C,C-(PhBIm-C6 H4 )}3 , which reacts with HBF4 to afford the 14 e(-) species [Os{κ(2) -C,C-(PhBIm-C6 H4 )}(Ph2 BIm)2 ]BF4 stabilized by two agostic interactions, has been obtained by reaction of OsH6 (PiPr3 )2 with N,N'-diphenylbenzimidazolium chloride ([Ph2 BImH]Cl) in the presence of NEt3 . Its formation takes place through the C,C,C-pincer compound OsH2 {κ(3) -C,C,C-(C6 H4 -BIm-C6 H4 )}(PiPr3 )2 , the dihydrogen derivative OsCl{κ(2) -C,C-(PhBIm-C6 H4 )}(η(2) -H2 )(PiPr3 )2 , and the five-coordinate osmium(II) species OsCl{κ(2) -C,C-(PhBIm-C6 H4 )}(PiPr3 )2 .
Small modificationso nt he co-ligands of complexes containing two coordinatedh ydrogen atoms can determine the elongated dihydrogen versus compressed dihydride nature of these species and therefore their chemical behavior.2 ,6-diphenylpyridinef avors the formation of the osmium(IV) cation [OsH 2 (C 6 H 4 pyPh)(PiPr 3 ) 2 ] + , whereas 2-phenoxy-6-phenylpyridine, which containsa n oxygena tom between the heterocycle and one of the phenylg roups, stabilizes the osmium(II) elongated dihydrogen species[ Os(C 6 H 4 pyOPh)(h 2 -H 2 )(PiPr 3 ) 2 ] + .I nc ontrast to the latter,t he former showsamarked tendency to undergo reductivee limination of the heterocycle.
The preparation of new osmium-hydride complexes, starting from OsH 6 (P i Pr 3) 2 (1) and OsH 2 Cl 2 (P i Pr 3) 2 (2), and their catalytic activity in acceptorless dehydrogenative coupling of alcohols and amines and in dehydrogenation of cyclic amines are reported. Complex 1 reacts with acetylacetone (Hacac) to give the classical trihydride OsH 3 (acac)(P i Pr 3) 2 (3). The protonation of 3 with triflic acid (HOTf) produces the release of H 2 and the formation of the unsaturated osmium(IV)-dihydride [OsH 2 (acac)(P i Pr 3) 2 ]OTf (4), which is also prepared starting from 2 via the intermediate OsH 2 Cl(acac)(P i Pr 3) 2 (5). Treatment of acetylacetone solutions of 5 with KOH affords Os(acac) 2 (P i Pr 3) 2 (6). In presence of 5 mol% of KOH, complexes 3-6 promote the coupling of benzylalcohol and aniline to give N-benzylideneaniline and H 2. Under the same conditions, complex 3 catalyzes a wide range of analogous couplings to afford a variety of imines, including aliphatic ones, with yields between 90 and 40% after 1-48 h. Complex 3 also catalyzes the dehydrogenation of cyclic amines. According to the amount of H 2 released by each gram of employed substrate, the amines have been classified into three classes: poor-(1,2,3,4-tetrahydroquinaldine, 2-methylindoline and 2,6dimethylpiperidine), moderate-(1,2,3,4-tetrahydroquinoline and 6-methyl-1,2,3,4-tetrahydroquinoline) and good-hydrogen donors (1,2,3,4-tetrahydroisoquinoline).
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