Pyridine-2,6-Dicarboxylic Acid Esters (pydicR2) as O,N,O-Pincer Ligands in CuII Complexes

Abstract

The pyridine-2,6-carboxylic esters pydicR2 with R = Me or Ph form the unprecedented mononuclear CuII complexes [Cu(pydicR2)Cl3]− in one-pot reactions starting from pyridine-2,6-carboxychloride pydicCl2, CuII chloride, and NEt3 in MeOH or PhOH solution under non-aqueous conditions. The triethylammonium salts (HNEt3)[Cu(pydicR2)Cl3] were isolated. The methyl derivative could be crystallized to allow a XRD structure determination. Both structures were optimized using DFT calculations in various surroundings ranging from gas phase and the non-coordinating solvent CH2Cl2 to the weakly coordinating acetone and well-coordinating solvents acetonitrile (MeCN) or dimethylformamide (DMF), while detailed calculation showed the charge distribution, dipole moments, and HOMO–LUMO gap energies changing upon solvation. According to these calculations, the ion pairs and the anionic CuII complexes were stable, which shows only Cu–Cl bond elongation and weakening of the charge transfer between the anionic complex and the cation as solvents become polar. Synthesis attempts in the presence of water yielded the CuII complexes [Cu(pydic)(OH2)2]n and [Cu(OH2)6][{Cu(pydic)}2(µ-Cl)2], which results from pydicCl2 hydrolysis. Alternatively, the new pydic(IPh)2 (IPh = 2-iodo-phenyl) ester ligand was synthesized and reacted with anhydrous CuCl2, which yields the new binuclear complex [{Cu(pydic(IPh)2)Cl}2(µ-Cl)2]. EPR spectroscopy of the solid compounds reveals typical axial spectra in line with the observed and DFT calculated geometries. Cyclic voltammetry and UV–vis absorption spectroscopy in solution are in line with un-dissociated complex species [Cu(pydicR2)Cl3]−.