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 NO2S2 coordination mode of L12- 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 dx2-y2
and dz2 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.