Exploring different equatorial donors in a series of five-coordinate Cu(II) complexes supported by rigid tetradentate ligands

Abstract

A series of tetradentate mixed-donor ligands has been synthesized containing an amine, thioether, or phosphine donor at the 8-position of a 2-(1,1-di(pyridin-2-yl)ethyl)quinoline backbone. Upon metalation with CuCl2, the preorganized N4, N3P, and N3S donor ligands enforce distorted trigonal bipyramidal coordination environments around the copper metal center, in which a chlorido donor occupies the remaining axial coordination site. The complexes were characterized by elemental analysis, high-resolution mass spectrometry, and X-ray crystallography. The non-pyridyl equatorial donor was the sole difference from among the tetradentate ligands, and is responsible for the structural and electronic variations across the series. Indeed, more intermediate geometries between the ideal trigonal bipyramidal and square pyramidal extremes are observed with the larger phosphorus and sulfur donors. Relative to the N4 derivative, the N3P and N3S systems have greater chemical stability upon redox cycling as observed in spectroelectrochemical experiments and lower inner-sphere reorganization energies for the CuII/I redox couples based on density functional theory calculations.