Unexpected coordination mode for a phenol/pyridine-containing tripodal ligand towards copper(II) ions: Solid state, solution and DFT Studies

Reaction between the tripodal ligand HBPAMFF, namely, 3-[N,N-(2-pyridylmethyl)(2-hydroxybenzyl)amino-methyl]5-methylsalicylaldehyde, and stoichiometric amounts of CuCl2 center dot 2H(2)O afforded a turquoise mono-nuclear complex (1), which crystallizes in the monoclinic system, space group P 21/c. The pentacoordinated copper site adopts a slightly distorted square pyramidal geometry with the apical position occupied by a phenolic oxygen. A combined experimental-theoretical analysis of the mid-infrared absorptions of 1 confirmed that both phenol groups present in HBPAMFF remain fully protonated upon complexation. Still in the solid state, X-band EPR spectroscopy showed a three g-values rhombic spectrum without a hyperfine unfolding. Solution studies were performed in acetonitrile (MeCN), in which the chloride ions remain mostly coordinated to copper, and dimethylsulfoxide (DMSO), with the replacement of a chloride ion by a solvent molecule in the coordination sphere of copper. Electronic spectroscopy and frozen solution EPR also confirm the different nature of the species present in MeCN and DMSO. This work shines a light on some structural aspects regarding