Heterobimetallic Complexes of Rhenium and Zinc: Potential Catalysts for Homogeneous Syngas Conversion

6-(Diphenylphosphino)-2,2'-bipyridine (PNN) coordinates to rhenium carbonyls in both kappa(1)(P) and kappa(2)(N,N) modes; in the former, the free bpy moiety readily binds to zinc alkyls and halides. [Re(kappa(1)(P)-PNN)(CO)(5)][OTf] reacts with dialkylzinc reagents to form [Re(kappa(1)(P)-PNN center dot ZnR)(CO)(4)(mu(2)-C(O)R)][OTf] (R = Me, Et, Bn), in which an alkyl group has been transferred to a carbonyl carbon and the resulting monoalkyl Zn is bound both to the bpy nitrogens and the acyl oxygen. ZnCl2 binds readily to the bpy group in Re(kappa(1)(P)-PNN)(CO)(4)Me, and the resulting adduct undergoes facile migratory insertion, assisted by the Lewis acidic pendent Zn, to yield Re(kappa(1)(P)-PNN center dot ZnCl(mu(2)-Cl)(CO)(3)(mu(2)-C(O)Me), in which one of the chlorides occupies the sixth coordination site on Re. Migratory insertion is inhibited by THF or other ethers that can coordinate to ZnCl2. Migratory insertion is also observed for Re(kappa(1)(P)-PNN)(CO)(4)(CH2Ph) but not for Re(kappa(1)(P)-PNN)(CO)(4)(CH2OCH3); coordination of the methoxy oxygen to Zn appears to block its ability to coordinate to the carbonyl oxygen and facilitate migratory insertion. Intramolecular Lewis acid promoted hydride transfer from [(dmpe)(2)PtH] [PF6] to a carbonyl in [Re(kappa(1)(P)-PNN)(CO)(5)][OTf] results in formation of a Re-formyl species; additional hydride transfer leads to a novel Re-Zn-bonded product along with some formaldehyde.