Oxidative C-N bond cleavage of (2-pyridylmethyl)amine-based tetradentate supporting ligands in ternary cobalt(ii)-carboxylate complexes

Three mononuclear cobalt(ii)-carboxylate complexes, [(TPA)Co-II(benzilate)](+) (1), [(TPA)Co-II(benzoate)](+) (2) and [(iso-BPMEN)Co-II(benzoate)](+) (3), of N4 ligands (TPA = tris(2-pyridylmethyl)amine and iso-BPMEN = N-1,N-1-dimethyl-N-2,N-2-bis((pyridin-2-yl)methyl)ethane-1,2-diamine) were isolated to investigate their reactivity toward dioxygen. Monodentate (eta(1)) binding of the carboxylates to the metal centre favours the five-coordinate cobalt(ii) complexes (1-3) for dioxygen activation. Complex 1 slowly reacts with dioxygen to enable the oxidative decarboxylation of the coordinated alpha-hydroxy acid (benzilate). Prolonged exposure of the reaction solution of 2 to dioxygen results in the formation of [(DPA)Co-III(picolinate)(benzoate)](+) (4) and [Co-III(BPCA)(2)](+) (5) (DPA = di(2-picolyl)amine and HBPCA = bis(2-pyridylcarbonyl)amide), whereas only [(DPEA)Co-III(picolinate)(benzoate)](+) (6) (DPEA = N-1,N-1-dimethyl-N-2-(pyridine-2-ylmethyl)-ethane-1,2-diamine) is isolated from the final oxidised solution of 3. The modified ligand DPA (or DPEA) is formed via the oxidative C-N bond cleavage of the supporting ligands. Further oxidation of the -CH2- moiety to -C(=O)- takes place in the transformation of DPA to HBPCA on the cobalt(ii) centre. Labelling experiments with O-18(2) confirm the incorporation of oxygen atoms from molecular oxygen into the oxidised products. Mixed labelling studies with O-16(2) and H2O18 strongly support the involvement of water in the C-N bond cleavage pathway. A comparison of the dioxygen reactivity of the cobalt complexes (1-3) with those of several other five-coordinate mononuclear complexes [(TPA)Co-II(X)](+/2+) (X = Cl, CH3CN, acetate, benzoylformate, salicylate and phenylpyruvate) establishes the role of the carboxylate co-ligands in the activation of dioxygen and subsequent oxidative cleavage of the supporting ligands by a metal-oxygen oxidant.