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)CoII(benzilate)]+ (1), [(TPA)CoII(benzoate)]+ (2) and [(iso-BPMEN)CoII(benzoate)]+ (3) of N4 ligands (TPA = tris(2-pyridylmethyl)amine, iso-BPMEN = N1,N1-dimethyl-N2,N2-bis((pyridin-2-yl)methyl)ethane-1,2-diamine) were isolated to investigate their reactivity toward dioxygen. Monodentate (η1) 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 exhibit oxidative decarboxylation of the coordinated -hydroxy acid (benzilate). A prolonged exposure of the reaction solution of 2 to dioxygen results in the formation of [(DPA)CoIII(picolinate)(benzoate)]+ (4) and [CoIII(BPCA)2]+ (5) (DPA = di(2-picolyl)amine and HBPCA = bis(2-pyridylcarbonyl)amide), whereas only [(DPEA)CoIII(picolinate)(benzoate)]+ (6) (DPEA = N1,N1-dimethyl-N2-(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 oxidative C-N bond cleavage of the supporting ligands. Further oxidation of the –CH2- moiety to –C(=O)- takes place in the transformation the DPA to HBPCA on cobalt(II) centre. Labelling experiments with 18O2 confirm the incorporation of oxygen atoms from molecular oxygen into the oxidised products. Mixed labelling studies with 16O2 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 several other five-coordinate mononuclear complexes [(TPA)CoII(X)]+, (X = Cl, CH3CN, acetate, benzoylformate, salicylate and phenylpyruvate) establishes the role of carboxylate co-ligands in the activation of dioxygen and subsequent oxidative cleavage of the supporting ligands by a metal-oxygen oxidant.