Synthesis and electronic structure of a mononuclear copper(II) complex supported by tris(2-hydroxyliminopropyl)amine

Abstract

The blue-green, mononuclear copper complex [CuL(NO3)]NO3, (1), was synthesized by reacting copper(II) nitrate with L = tris(2-hydroxyliminopropyl)amine in methanol. The structure of 1 was determined by single-crystal X-ray diffraction, which revealed a copper(II) ion that exhibits a pseudo-octahedral coordination geometry. While one of the nitrate anions is coordinated in an η2 fashion, the other is non-coordinated and is involved in intra- and inter-molecular hydrogen bonds. The nitrate-promoted intermolecular hydrogen bonding is supported by the protonated ligand and leads to the formation of supramolecular undulating chains. The electronic structure of 1 was investigated using Electron Paramagnetic Resonance (EPR) measurements performed at cryogenic temperatures on both neat powders and frozen solutions. In the solid state we observe a quasi-axial S = 1/2 signal described by gz = 2.256, gy = 2.086, and gx = 2.048. These values were rationalized using coupled-perturbed (CP) Density Functional Theory (DFT) and CASSCF/NEVPT2 calculations. The computational investigation of 1 suggests an (x2 − y2)-type ground state and a g -tensor dominated by the (xy) → (x2 − y2) transition. EPR investigation of frozen solution samples revealed the presence of two similar spectral components indicating that upon dissolution the nitrate anion is displaced from the coordination sphere and 1 speciates into six- and five-coordinated complex-cations. The cyclic voltammograms of 1 revealed a quasi-reversible Cu(I)/Cu(II) redox couple, E1/2 = +0.26 V vs. SHE, and that 1 functions as a precursor for electrodeposited Cu(0)/Cu2O films.