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

Abstract

6-(Diphenylphosphino)-2,2′-bipyridine (PNN) coordinates to rhenium carbonyls in both κ^1(P) and κ^2(N,N) modes; in the former, the free bpy moiety readily binds to zinc alkyls and halides. [Re(κ^1(P)-PNN)(CO)_5][OTf] reacts with dialkylzinc reagents to form [Re(κ^1(P)-PNN·ZnR)(CO)_4(μ_(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. ZnCl_2 binds readily to the bpy group in Re(κ^1(P)-PNN)(CO)_4Me, and the resulting adduct undergoes facile migratory insertion, assisted by the Lewis acidic pendent Zn, to yield Re(κ^1(P)-PNN·ZnCl)(μ_(2-)Cl)(CO)_3(μ_(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 ZnCl_2. Migratory insertion is also observed for Re(κ1(P)-PNN)(CO)_4(CH_2Ph) but not for Re(κ^1(P)-PNN)(CO)_4(CH_2OCH_3); 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)_2PtH][PF_6] to a carbonyl in [Re(κ^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 formal dehyde.