Ligand and metal-centred reactivity in 2,6-bis(imino)-1,4-dihydropyridinate Zn(II) alkyls: the dual behaviour of an intriguing type of complex

Abstract

Dihydropyridines, either free or metal-coordinated, are organic hydride transfer reductants that perform on the same premises as the natural redox cofactor NAD(P)⁺/NAD(P)H. 1-Bn and 1-Me are alkylzinc complexes containing dihydropyridinate-based pincer ligands that have been synthesized through different routes involving the addition of ZnR₂ (R = Bn, Me) to the 2,6-bis(imino)-pyridine and 2,6-bis(imino)-4-Bn-dihydropyridine (^iPrBIP and 4-Bn^iPrBIPH₂) ligands, respectively. The alkyls complexes 1-R react with fluorinated alcohols R^FOH (R^F = C₆F₅ or t-C₄F₉) yielding isolable fluoroalkoxides 2-F5 and 2-F9, in which the reactive 1,4-dihydropyridinate ligand remains unchanged. The crystal structure of 2-F5 shows the shortest Zn⋯F-C interaction reported so far, involving one of the o-F atoms of the C₆F₅ group. However, the mechanism of the alcoholysis reactions is not straightforward, as NMR monitoring revealed that acidic R^FOH first protonates the dihydropyridine nitrogen, releasing the dihydropyridine base 4-BniPrBIPH2 and a highly reactive Zn(R)(OR^F) species that re-captures the dihydropyridine in a subsequent step, eliminating the corresponding alkane (R-H). Depending on the mixing conditions, the pincer dihydropyridinate ligand may undergo aromatization to produce the new Zn(II) dialkoxides 3-F5 and 3-F9 stabilised by a neutral ^iPrBIP ligand [(4R-^iPrBIP)Zn(OR^F)₂]. These protonation and hydride transfer reactions illustrate the dual reactivity of the pincer 1,4-dihydropyridinate zinc entity.