N3-Ligated nickel(ii) diketonate complexes: synthesis, characterization and evaluation of O2 reactivity

Abstract

Interest in O₂-dependent aliphatic carbon–carbon (C–C) bond cleavage reactions of first row divalent metal diketonate complexes stems from the desire to further understand the reaction pathways of enzymes such as DKE1 and to extract information to develop applications in organic synthesis. A recent report of O₂-dependent aliphatic C–C bond cleavage at ambient temperature in Ni(II) diketonate complexes supported by a tridentate nitrogen donor ligand [(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl (7-Cl; MBBP = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine) in the presence of NEt₃ spurred our interest in further examining the chemistry of such complexes. A series of new TERPY-ligated Ni(II) diketonate complexes of the general formula [(TERPY)Ni(R₂-1,3-diketonate)]ClO₄ (1: R = CH₃; 2: R = C(CH₃)₃; 3: R = Ph) was prepared under air and characterized using single crystal X-ray crystallography, elemental analysis, ¹H NMR, ESI-MS, FTIR, and UV-vis. Analysis of the reaction mixtures in which these complexes were generated using ¹H NMR and ESI-MS revealed the presence of both the desired diketonate complex and the bis-TERPY derivative [(TERPY)₂Ni](ClO₄)₂ (4). Through selective crystallization 1–3 were isolated in analytically pure form. Analysis of reaction mixtures leading to the formation of the MBBP analogs [(MBBP)Ni(R₂-1,3-diketonate)]X (X = ClO₄: 5: R = CH₃; 6: R = C(CH₃)₃; 7-ClO₄: R = Ph; X = Cl: 7-Cl: R = Ph) using ¹H NMR and ESI-MS revealed the presence of [(MBBP)₂Ni](ClO₄)₂ (8). Analysis of aerobic acetonitrile solutions of analytically pure 1–3, 5 and 6 containing NEt₃ and in some cases H₂O using ¹H NMR and UV-vis revealed evidence for the formation of additional bis-ligand complexes (4 and 8) but suggested no oxidative diketonate cleavage reactivity. Analysis of the organic products generated from 3, 7-ClO₄ and 7-Cl revealed unaltered dibenzoylmethane. Our results therefore indicate that N₃-ligated Ni(II) complexes of unsubstituted diketonate ligands do not exhibit O₂-dependent aliphatic C–C bond clevage at room temperature, including in the presence of NEt₃ and/or H₂O.