Distinguishing between aquo and hydroxo coordination in molecular copper complexes by 1H and 17O ENDOR spectroscopy

Abstract

Aquo and hydroxo ligands play an essential role in the chemistry of many copper enzymes and small molecule catalysts. The formation of a series of copper complexes with H₂O and OH⁻ ligands in various positions, including [Cu(bpy)(OAc)(H₂O)_2,ax]⁺ (Cu-I), [Cu(bpy)(OH)_2,eq(H_xO)_2,ax] (Cu-III), [Cu(OH)_4,eq(H_xO)_2,ax]²⁻ (Cu-IV), [Cu(bpy)(H₂O)_2,eq(H₂O)_2,ax]²⁺ (Cu-V) and [Cu(bpy)₂(H₂O)_ax]²⁺ (Cu-VI), were investigated through Electron Paramagnetic Resonance (EPR) and UV-Vis spectroscopy in aqueous copper bipyridine solutions in the dependence of the pH and the copper-to-bipyridine ratio (bpy = 2,2′-bipyridine). ²H- and ¹⁷O-enrichment of the copper complexes allowed us to determine the ¹H and ¹⁷O nuclear hyperfine interactions of their H_xO ligands via Q-band Electron Nuclear Double Resonance (ENDOR) spectroscopy. These techniques gave direct insight into the metal–ligand covalencies and geometries and were further supported by Density Functional Theory (DFT) calculations. It is shown that ¹H and ¹⁷O ENDOR spectroscopy can aid in (1) determining the coordination position, thereby differentiating between equatorial and axial H_xO ligands and (2) distinguishing equatorial aqua and hydroxo ligands, particularly through their anisotropic dipolar components. We further studied the influence of trans coordinating ligands on the hyperfine parameters of aquo and hydroxo ligands, enabled through contrasting the coordination environments in the examined complexes, supported by quantum chemical computations.