Solvent Effects on the Conversion of Dicopper(II) μ-η<sup>2</sup>:η<sup>2</sup>-Peroxo to Bis-μ-oxo Dicopper(III) Complexes: Direct Probing of the Solvent Interaction
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A new tridentate ligand, PYAN, is employed to investigate solvent influences for dioxygen reactivity with [Cu(PYAN)(MeCN)]B(C6F5)4 (1). Stopped-flow kinetic studies confirm that the adducts [{CuII(PYAN)}2(O2)][B(C6F5)4]2 (2Peroxo) and [{CuIII(PYAN)}2(O)2][B(C6F5)4]2 (2Oxo) are in rapid equilibrium. Thermodynamic parameters for the equilibrium between 2Peroxo and 2Oxo are as follows: THF, ΔH° ∼ −15.7 kJ/mol, ΔS° ∼ −83 J/K·mol; acetone, ΔH° ∼ −15.8 kJ/mol, ΔS° ∼ −76 J/K·mol. UV−visible absorption and resonance Raman spectroscopic signatures demonstrate that the equilibrium is highly solvent dependent; the mixture is mostly 2Peroxo in CH2Cl2, but there are significantly increasing quantities of 2Oxo along the series methylene chloride → diethyl ether → acetone → tetrahydrofuran (THF). Copper(II)−Neq stretches (239, 243, 244, and 246 cm-1 in CH2Cl2, Et2O, acetone, and THF, respectively) are identified for 2Peroxo, but they are not seen in 2Oxo, revealing for the first time direct evidence for solvent coordination in the more open 2Peroxo structure.
