Neutral Bis(imino)-1,4-dihydropyridinate and Cationic Bis(imino)pyridine σ-Alkylzinc(II) Complexes as Hydride Exchange Systems: Classic Organometallic Chemistry Meets Ligand-Centered, Biomimetic Reactivity

The 1,4-dihydropyridinate complex [(4-Bn-HBIP)Zn(Bn)], readily available through the highly selective reaction of a 2,6-bis(imino)pyridine ligand with dibenzylzinc, contains two distinct reactive centers. One of them is the σ-organometallic benzylzinc moiety, which reacts with weak protic acids (e.g., water and methanol) to release the free dihydropyridine ligand. In contrast, the reaction with p-tolualdehyde, a mild electrophile, does not involve the benzylzinc but the 1,4-dihydropyridinate fragment. Even a strong electrophile such as B(C6F5)3 selectively removes the hydrogen atom from the C4 position of the heterocyclic ring but leaves intact the organometallic fragment, to afford the ionic complex [(4-Bn-BIP)Zn(Bn)]+[HB(C6F5)3]−. The hydride donor capacity of the dihydropyridinate ligand is strongly reminiscent of the widespread pyridine-based cofactors (e.g., NADH/NAD+), one of the most common redox exchange molecules in biologic chemistry. In order to investigate the reversibility of the hydride exch...