Snapshots of a metamorphosing Cu(II) ground state in a galactose oxidase-inspired complex

The novel ligand 2,6-bis[S-(3,5-di-tert-butyl-2-hydroxyphenyl) sulfanylmethyl]pyridine (H₂L1) and its copper(II) complex Cu(L1), 1, were synthesized with the aim of constructing a model of the active site of the enzyme galactose oxidase (GOase). Cyclic voltammetry studies show that 1 undergoes ligand-based quasi-reversible oxidations (phenolate/phenoxyl) and reversible metal-based reduction [copper(II)/copper(I)] similar to those of GOase, but at potentials much higher and lower, respectively, than those found for the enzyme. At room temperature, spectrophotometric titrations show that 1 binds strongly to 1 equiv of pyridine. In frozen solutions (77 K), 1 quantitatively binds both pyridine and ethers (e.g., 1,4-dioxane) as assessed by X- and Q-band EPR spectroscopy. Profound shifts in the pattern of g values result, from rhombic (g₁ > g₂ > g₃) in toluene to either inverted axial patterns (g₁ = g₂ ≫ g₃) in the presence of ethers or a near-axial pattern (g₁ ≫ g₂ > g₃) in the presence of pyridine. Crystallographic analyses of the parent complex 1·MeCN, the dioxane-bridged dimer [(Cu(L1))₂((μ-1,4)-1,4-dioxane)] ·(Me₂CO)₂ (2), and the pyridine complex [Cu(L1)(pyridine)] (3) show that the pyridine and ether ligands bond to copper at a sixth octahedral position left vacant by the pentadentate NO ₂S₂ coordination mode of L1^2- and induce perturbations of its geometry. Hybrid DFT calculations based on the crystallographic coordinates combined with perturbation theory expressions for the g values of a d⁹ system correlate the results from EPR spectroscopy to the proportions of d_x2-y2 and d_z2 character in the relevant copper-centered unoccupied molecular orbital. The combination of spectroscopic, structural, and computational results for this set of copper(II) complexes provides a demonstrative example of the physical phenomena underlying rhombic EPR spectra of d⁹ systems.