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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

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journal contribution
posted on 30.05.2018, 21:14 by John J. Sandoval, Eleuterio Álvarez, Pilar Palma, Antonio Rodríguez-Delgado, Juan Cámpora
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 exchange, we developed an efficient methodology to prepare a family of alkylzinc cations [(4-R1-BIP)­Zn­(R)]+ (R1 = H, Bn; R = Bn, CH2SiMe3, neophyl) as salts of the inert tetraarylborate anion [BArF4] (ArF = 3,5-C6H3(CF3)2), on the basis of the reaction of zinc dialkyls ZnR2 with protonated ligands [H­(4-R1-BIP)]+[BArF4] (R1 = H, Bn). However, the reaction of cationic BIP-organozinc complexes with the hydridic reductant Na­[HBEt3] does not revert to the corresponding electroneutral dihydropyridinate derivatives but causes irreversible release of the tridentate BIP ligands, recovered as stable sodium complexes. The crystal structures of two representative members of the [(4-R1-BIP)­Zn­(R)]+ family show the Zn center in a flattened coordination environment, midway between tetrahedral and square planar, which leaves room for the coordination of additional ligands along the direction normal to the mean coordination plane. This relatively open geometry may be enhancing the Lewis acidity at the metal center, driving the selectivity of the reaction with hydride donors to the metal unit to the detriment of ligand-centered reactivity.