Fast O2 binding at dicopper complexes containing Schiff-base dinucleating ligands

A new family of dicopper (I) complexes [(Cu2L)-L-I-L-R](X)(2) (R = H, 1X, R = Bu-t, 2X and R = NO2, 3X, X = CF3SO3, ClO4, SbF6, or BArF, BArF) [B{3,5-(CF3)(2)C6H3 4](-)), where L-R is a Schiff-base ligand containing two tridentate binding sites linked by a xylyl spacer, has been prepared and characterized, and its reaction with O-2 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 1ClO(4), 2CF(3)SO(3), 2ClO(4), and 3BArF center dot CH3CN have been established by single-crystal X-ray diffraction analysis. 1ClO(4) adopts a polymeric structure in the solid state while 2CF(3)SO(3), 2ClO(4), and 3BArF, CH3CN are monomeric. The complexes have been studied in solution by means of H-1 and F-19 NMR spectroscopy, which put forward the presence of dynamic processes. 1-BArF and 1-3CF(3)SO(3) in acetone react rapidly with O2 to generate metaestable [Cu-III (2)(alpha-O)(2)(L-R)](2+) 1-3(O-2) and [Cu-2(III)(mu-O)(2)(CF3SO3)(L-R)](+) 1-3(O-2)(CF3SO3) species, respectively, that have been characterized by UV-vis spectroscopy and resonance Raman analysis. Instead, reaction of 1-3BArF with O-2 in CH2Cl2 results in intermolecular O-2 binding. DFT methods have been used to study the chemical identities and structural parameters of the O-2 adducts, and the relative stability of the Cu-2(III)(mu-O)(2) form with respect to the Cu-2(II)(mu-eta(2): eta(2)-O-2) isomer. The reaction of 1X, X = CF3SO3 and BArF, with O-2 in acetone has been studied by stopped-flow UV-vis exhibiting an unexpected very fast reaction rate (k) 3.82(4) x 10(3) M-1 s(-1), Delta H double dagger = 4.9 +/- 0.5 kJ center dot mol(-1), Delta S double dagger = -148 +/- 5 J center dot K-1 center dot mol(-1)), nearly 3 orders of magnitude faster than in the parent [Cu-2(I)(m-XYLMeAN)](2+). Thermal decomposition of 1-3(O-2) does not result in aromatic hydroxylation. The mechanism and kinetics of O-2 binding to 1X (X) CF3SO3 and BArF) are discussed and compared with those associated with selected examples of reported models of O-2-processing copper proteins. A synergistic role of the copper ions in O-2 binding and activation is clearly established from this analysis.