Anion- and water-facilitated oxidative carbon–carbon bond cleavage and diketonate carboxylation in Cu(II) chlorodiketonate complexes

Abstract

The O2-dependent carbon–carbon (CC) bond cleavage reactions of the mononuclear Cu(II) chlorodiketonate complexes [(6-Ph2TPA)Cu(PhC(O)CClC(O)Ph)]ClO4 (1-ClO4) and [(bpy)Cu(PhC(O)CClC(O)Ph)(ClO4)] (3-ClO4) have been further examined in terms of their anion and water dependence. The bpy-ligated Cu(II) chlorodiketonate complex 3-ClO4 is inherently more reactive with O2 than the 6-Ph2TPA-ligated analog 1-ClO4. Added chloride is needed to facilitate O2 reactivity for 1-ClO4 but not for 3-ClO4 at 25(1) °C. Evaluation of kobs for the reaction of 1-ClO4 with O2 under pseudo first-order conditions as a function of the amount of added chloride ion produced saturation type behavior. The bpy-ligated 3-ClO4 exhibits different behavior, with rate enhancement resulting from both the addition of chloride ion and water. Computational studies indicate that the presence of water lowers the barrier for O2 activation for 3-ClO4 by ∼12 kcal/mol whereas changing the anion from perchlorate to chloride has a smaller effect (lowering of the barrier by ∼3 kcal/mol). Notably, the effect of water for 3-ClO4 is of similar magnitude to the barrier-lowering chloride effect found in the O2 activation pathway for 1-ClO4. Thus, both systems involve lower energy O2 activation pathways available, albeit resulting from different ligand effects. Probing the effect of added benzoate anion, it was found that the chloro substituent in the diketonate moiety of 1-ClO4 and 3-ClO4 will undergo displacement upon treatment of each complex with tetrabutyl ammonium benzoate to give Cu(II) benzoyloxydiketonate complexes (4 and 5). Complexes 4 and 5 exhibit slow O2-dependent CC cleavage in the presence of added chloride ion. These results are discussed in the context of the chemistry identified for various divalent metal chlorodiketonate complexes, which have relevance to catalytic systems and metalloenzymes that mediate O2-dependent CC cleavage within diketonate substrates.