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((PPr3)-Pr-i)(2) (1) and OsH(OH)(CO)((PPr3)-Pr-i)(2) (2) react with 1,3-bis(6'-methyl 2'-pyridylimino)isoindoline (HBMePI) to give OsH3{kappa(2)-N-py,N-imine-(BMePI)}((PPr3)-Pr-i)(2) (3) and OsH{kappa(2)-N-py,N-imine-(BMePI)}(CO)((PPr3)-Pr-i)(2) (4). The unprecedented kappa(2)-N-py,N-imine coordination mode of BMePI is thermodynamically preferred with Os(IV) and Os(II) metal fragments and allows for preparation of BMePI-based dinudear metal cations. Treatment of OsH2Cl2((PPr3)-Pr-i)(2) (5) with 0.5 equiv of HBMePI in the presence of (KOBu)-Bu-t affords the chloride salt of the bis(osmium(IV)) dinuclear cation [OsH3((PPr3)-Pr-i)(2)}(2){mu-(kappa(2)-N-py,N-imine)(2)-BMePI}](+) (6). Related homoleptic bis(osmium(II)) complexes have been also synthesized. Complex 4 reacts with the bis(solvento) [OsH(CO){kappa(1)-O-[OCMe2](2)}((PPr3)-Pr-i)(2)]BF4 to give [{OsH(CO)-((PPr3)-Pr-i)(2)}(2){mu-(kappa(2)-N-py,N-imine)(2)-BMePI}]BF4 (7), whereas the addition of 0.5 equiv of HBMePI to {OsCl(eta(6)-C6H6)}(2)(mu-Cl)(2) (8) affords [{OsCl(eta(6)-C6H6)}(2){mu-(kappa(2)-N-py,N-imine)(2)-BMePI}]Cl (9). The reactions of 4 with 8 and {OsCl(eta(6)-p-cymene)}(2)(mu-Cl)(2) (10) lead to the heteroleptic cations [((PPr3)-Pr-i)(2)(CO)HOs{mu(kappa(2)-N-py,N-imine)(2)-BMePI}OsCl(eta(6)-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 H-2 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.