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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2023-07, Vol.52 (29), p.994-9951
• Main Authors: Delgado-Collado, J. M, Gallardo-Villagrán, M, Álvarez, E, Cámpora, J, Rodríguez-Delgado, A
• Format: Article
• Language: English
• Subjects: Alcohols; Alkanes; Crystal structure; Hydrides; Ligands; NMR; Nuclear magnetic resonance; Protonation; Reducing agents
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d3dt01492d

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 ZnR 2 (R = Bn, Me) to the 2,6-bis(imino)-pyridine and 2,6-bis(imino)-4-Bn-dihydropyridine ( iPr BIP and 4-Bn iPr BIPH 2 ) ligands, respectively. The alkyls complexes 1-R react with fluorinated alcohols R F OH (R F = C 6 F 5 or t -C 4 F 9 ) 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 C 6 F 5 group. However, the mechanism of the alcoholysis reactions is not straightforward, as NMR monitoring revealed that acidic R F OH first protonates the dihydropyridine nitrogen, releasing the dihydropyridine base 4-Bn iPr BIPH 2 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 iPr BIP ligand [(4R- iPr BIP)Zn(OR F ) 2 ]. These protonation and hydride transfer reactions illustrate the dual reactivity of the pincer 1,4-dihydropyridinate zinc entity. 2,6-Bis(imino)-dihydropyridinate ligands in mononuclear organo and alkoxo-zinc compounds. When complexation chemistry mimics biological systems.