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

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_<i>x</i>O)_2,ax] (Cu-III), [Cu(OH)_4,eq(H_<i>x</i>O)_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_<i>x</i>O ligands <i>via</i> 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_<i>x</i>O ligands and (2) distinguishing equatorial aqua and hydroxo ligands, particularly through their anisotropic dipolar components. We further studied the influence of <i>trans</i> 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.