Zinc– and Copper–Salicyaldimine complexes: Simultaneous observation of both metal–ligand coordination and weak CH⋯N contact about a single N-donor and the transmetallation reactions

Abstract

Two salicyaldimine ligands, HsalCl-3-py and HsalOMe-3-py, functionalized with the pyridyl group on the imine side and their Zn and Cu complexes with a general formula [ML2], where M ​= ​Zn, L ​= ​salCl-3-py (1), salOMe-3-py (2); M ​= ​Cu, L ​= ​salCl-3-py (3), salOMe-3-py (4), have been synthesized and characterized by nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy (MS), infrared (IR) spectroscopy, elemental analysis (EA), scanning electron microscope (SEM) and further by single-crystal X-ray diffraction analysis and X-ray powder diffraction (XRPD). The crystal structures of Zn– and Cu–salicyaldimine complexes exhibited that the M(II) center is coordinated by two N,O-chelated salicyaldimine ligands in a square-planar geometry, and two further pyridyl donating groups from two neighboring molecules at the axial positions, resulted in self-aggregation to form a two-dimensional (4,4)-sheet. Interestingly, apart from the strong M–N(pyridyl) coordination bond, the weak CH⋯N(pyridyl) hydrogen bonding interactions are simultaneously formed between the pyridyl N-donor and one pyridyl hydrogen of the equatorial salicyaldimine ligand. The electronic absorption spectra of the Zn– and Cu–salicyaldimine complexes in methanol solutions showed the appearance of a metal-to-ligand charge transfer absorption other than the intraligand charge transfer transitions after the complexation of azomethine (–CN–) with M(II) center. On the other hand, salicyaldimine ligands and Cu–salicyaldimine complexes showed no or weak fluorescence emissions. By contrast, Zn–salicyaldimine complexes emitted intense cyan and green light fluorescence in both solid-state and methanol solutions. Noteworthy, transmetallation of Zn–salicyaldimine complexes with Cu2+ ions would be achieved, producing corresponding Cu–salicyaldimine complexes. This implies the stronger binding ability of Cu2+ than that of Zn2+ to salicyaldimine ligands.