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-BniPrBIPH2) ligands, respectively. The alkyls complexes 1-R react with fluorinated alcohols RFOH (RF = 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 RFOH first protonates the dihydropyridine nitrogen, releasing the dihydropyridine base 4-BniPrBIPH2 and a highly reactive Zn(R)(ORF) 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(ORF)2]. These protonation and hydride transfer reactions illustrate the dual reactivity of the pincer 1,4-dihydropyridinate zinc entity.