Switching from 4 + 1 to 4 + 2 zinc coordination number through the methyl group position on the pyridyl ligand in the geometric isomers bis[N-2-(4/6-methyl-pyridyl)salicylaldiminato-κ2N,O]zinc(II)

Abstract

The Schiff bases N-2-(4/6-methyl-pyridyl)salicylaldimine (HL) which are geometric isomers differing in the para- or ortho-position of the methyl group to the pyridyl nitrogen atom react with zinc(II) acetate to give the bis-ligand chelate complexes bis[N-2-(4/6-methyl-pyridyl)salicylaldiminato-κ2N,O]zinc(II) {4-methyl or para (1), 6-methyl or ortho (2)}. The zinc complexes feature an N2O2 chromophore from the two salicylaldiminato moieties with one or two additional weak Zn–N(pyridyl) contacts to give a 4+1 or 4+2 coordination in 1 or 2, respectively. The difference in metal coordination is traced to competitive alternative C–H⋯π interactions of the non-coordinated methyl-pyridyl ring in compound 1 which are absent in 2. Instead, the weakly zinc-coordinated methyl-pyridyl rings in 1 and 2 are at the same time engaged in π⋯π interactions. The latter compound features two C–H⋯O contacts. DFT calculation produces the similar structural features for 1 and 2. The excited state properties calculated by TDDFT reveal that the complexes have distinctive ligand–ligand (LL) and metal-to-ligand (ML) charge transfer bands. Hirshfeld surface analysis also discloses similar C–H⋯π and C–H⋯O interactions in crystals 1 and 2, respectively.