Anion-dependent synthesis, crystal structures and antibacterial activity of nickel(II) complexes derived from 1-(((2-(pyrrolidin-1-yl)ethyl)imino)methyl)naphthalen-2-ol

An anion-directed assembly strategy has been developed for the construction of nickel(II) Schiff base complexes with diverse coordination architectures. The Schiff base ligand 1-(((2-(pyrrolidin-1-yl)ethyl)imino)methyl)naphthalen-2-ol (HL) was employed for the preparation of four new nickel(II) complexes, namely [Ni 3 (µ 2 -η 1 :η 1 -OAc) 2 (µ 2 -η 2 :η 1 -OAc) 2 L 2 ] ( 1 ), [NiL(NCS)] ( 2 ), [Ni(HL) 2 ](NO 3 ) 2 ( 3 ), and [NiL(N 3 )] ( 4 ). All complexes were characterized by single-crystal X-ray diffraction, revealing that the auxiliary anions play a crucial role in directing the assembly process and determining the final coordination motifs. The antibacterial activities of HL and the complexes were evaluated against Gram-positive and Gram-negative bacterial strains. All nickel complexes exhibit improved antibacterial activity compared with the free ligand, and the trinuclear complex 1 shows the highest overall antibacterial efficiency among the investigated complexes. This work highlights the significant role of auxiliary anions in regulating nickel(II) Schiff base assembly and provides a useful approach for designing coordination compounds with tunable structures and biological properties.