Osmium Catalysts for Acceptorless and Base-Free Dehydrogenation of Alcohols and Amines: Unusual Coordination Modes of a BPI Anion

A novel type of catalyst
precursors for the dehydrogenation of
hydrogen carriers based on organic liquids has been discovered. Complexes
OsH6(PiPr3)2 (1) and OsH­(OH)­(CO)­(PiPr3)2 (2) react with 1,3-bis­(6′-methyl-2′-pyridylimino)­isoindoline
(HBMePI) to give OsH3{κ2-Npy,Nimine-(BMePI)}­(PiPr3)2 (3) and OsH­{κ2-Npy,Nimine-(BMePI)}­(CO)­(PiPr3)2 (4). The unprecedented
κ2-Npy,Nimine coordination
mode of BMePI is thermodynamically preferred with Os­(IV) and Os­(II)
metal fragments and allows for preparation of BMePI-based dinuclear
metal cations. Treatment of OsH2Cl2(PiPr3)2 (5) with
0.5 equiv of HBMePI in the presence of KOtBu affords the chloride salt of the bis­(osmium­(IV)) dinuclear cation
[{OsH3(PiPr3)2}2{μ-(κ2-Npy,Nimine)2-BMePI}]+ (6). Related
homoleptic bis­(osmium­(II)) complexes have been also synthesized. Complex 4 reacts with the bis­(solvento) [OsH­(CO)­{κ1-O-[OCMe2]2}­(PiPr3)2]­BF4 to give [{OsH­(CO)­(PiPr3)2}2{μ-(κ2-Npy,Nimine)2-BMePI}]­BF4 (7), whereas the addition of 0.5 equiv of HBMePI
to {OsCl­(η6-C6H6)}2(μ–Cl)2 (8) affords [{OsCl­(η6-C6H6)}2{μ-(κ2-Npy,Nimine)2-BMePI}]Cl (9). The reactions of 4 with 8 and
{OsCl­(η6-p-cymene)}2(μ–Cl)2 (10) lead to the heteroleptic cations [(PiPr3)2(CO)­HOs­{μ-(κ2-Npy,Nimine)2-BMePI}­OsCl­(η6-arene)]+ (arene = C6H6 (11), p-cymene (12)). The electronic
structrure and electrochemical properties of the dinuclear complexes
were also studied. Complexes 3 and 4 are
efficient catalyst precursors for the acceptorless and base-free dehydrogenation
of secondary and primary alcohols and cyclic and lineal amines. The
primary alcohols afford aldehydes. The amount of H2 released
per gram of heterocycle depends upon the presence of a methyl group
adjacent to the nitrogen atom, the position of the nitrogen atom in
the heterocycle, and the size of the heterocycle.