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

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posted on 2007-06-11, 00:00 authored by Anna Company, Laura Gómez, Rubén Mas-Ballesté, Ivan V. Korendovych, Xavi Ribas, Albert Poater, Teodor Parella, Xavier Fontrodona, Jordi Benet-Buchholz, Miquel Solà, Lawrence Que,, Elena V. Rybak-Akimova, Miquel Costas
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]-), where RL is a Schiff-base ligand containing two tridentate binding sites linked by a xylyl spacer, has been prepared and characterized, and its reaction with O2 has 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. 1ClO4 adopts a polymeric structure in the solid state while 2CF3SO3, 2ClO4, and 3BArF·CH3CN are monomeric. The complexes have been studied in solution by means of 1H and 19F NMR spectroscopy, which put forward the presence of dynamic processes. 13BArF and 13CF3SO3 in acetone react rapidly with O2 to generate metaestable [CuIII2(μ-O)2(RL)]2+ 13(O2) and [CuIII2(μ-O)2(CF3SO3)(RL)]+ 13(O2)(CF3SO3) species, respectively, that have been characterized by UV−vis spectroscopy and resonance Raman analysis. Instead, reaction of 13BArF with O2 in CH2Cl2 results in intermolecular O2 binding. DFT methods have been used to study the chemical identities and structural parameters of the O2 adducts, and the relative stability of the CuIII2-O)2 form with respect to the CuII2(μ-η22-O2) isomer. The reaction of 1X, X = CF3SO3 and BArF, with O2 in acetone has been studied by stopped-flow UV-vis exhibiting an unexpected very fast reaction rate (k = 3.82(4) × 103 M-1 s-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 13(O2) does not result in aromatic hydroxylation. The mechanism and kinetics of O2 binding to 1X (X = CF3SO3 and 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 O2 binding and activation is clearly established from this analysis.

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