Anion effects on construction of ZnII compounds with a chelating ligand bis(2-pyridylmethyl)amine and their catalytic activities

Three ZnII complexes containing bispicam ligands (bispicam = bis(2-pyridylmethyl)amine), [Zn(bispicam)2](NO3)2·2CH3OH 4A, [Zn(bispicam)(NO3)2] 4B, and [Zn(bispicam)2](OTf)2 6, were obtained, and their structures were determined by X-ray crystallography. Complexes of the general formulation [Zn(bispicam)2]X2 (X = Cl− (1), Br− (2), I− (3), NO3− (4A), ClO4− (5), and OTf− (6)) show fac geometric isomers (a) or enantiomers (c) and (d) according to anions. Moreover, complexes 4–6 could carry out the catalytic transesterification of a range of esters with methanol under the mild conditions. Importantly, the catalyst 4B with an unsaturated structure has shown better efficiency than the catalysts, 4A, 5, and 6, having saturated structures. To explain this reactivity difference, two different reaction mechanisms have been proposed (metal-based vs. amide N–H-based).