Fast O2 Binding at Dicopper Complexes Containing Schiff-Base Dinucleating Ligands

Abstract

A new family of dicopper(I) complexes [CuI2RL](X)2(R = H, 1X, R =tBu, 2X and R = NO2, 3X, X = CF3SO3, ClO4, SbF6, or BArF, BArF = [B{3,5-(CF3)2C6H3}4]-), whereRL is a Schiff-base ligand containing two tridentate binding sites linked by a xylyl spacer, has been prepared and characterized, and its reaction with O2has been studied. The complexes were designed with the aim of reproducing structural aspects of the active site of type 3 dicopper proteins; they contain two three-coordinate copper sites and a rather flexible podand ligand backbone. The solid-state structures of 1ClO4, 2CF3SO3, 2ClO4, and 3BArF·CH3CN have been established by single-crystal X-ray diffraction analysis. 1ClO4adopts a polymeric structure in the solid state while 2CF3SO3, 2ClO4, and 3BArF-CH3CN are monomeric. The complexes have been studied in solution by means of1H and19F NMR spectroscopy, which put forward the presence of dynamic processes. 1-3BArF and 1-3CF3SO3in acetone react rapidly with O2to generate metaestable [CuIII2(μ-O)2(RL)]2+1-3(O2) and [CuIII2(μ-O)2(CF3SO3)(RL)]+1-3(O2)(CF3SO3) species, respectively, that have been characterized by UV-vis spectroscopy and resonance Raman analysis. Instead, reaction of 1-3BArF with O2in CH2Cl2results in intermolecular O2binding. DFT methods have been used to study the chemical identities and structural parameters of the O2adducts, and the relative stability of the CuIII2(μ-O)2form with respect to the CuII2(μ- η2:η2-O2) isomer. The reaction of 1X, X = CF3SO3and BArF, with O2in acetone has been studied by stopped-flow UV-vis exhibiting an unexpected very fast reaction rate (k = 3.82(4) ± 103M-1s-1, ΔH‡ = 4.9 ± 0.5 kJ·mol-1, ΔS‡ = -148 ± 5 J·K-1·mol-1), nearly 3 orders of magnitude faster than in the parent [CuI2(m-XYLMeAN)]2+. Thermal decomposition of 1-3(O2) does not result in aromatic hydroxylation. The mechanism and kinetics of O2 binding to 1X (X = CF3SO3and BArF) are discussed and compared with those associated with selected examples of reported models of O2-processing copper proteins. A synergistic role of the copper ions in O2binding and activation is clearly established from this analysis