Bioinspired iron(II)-β-diketonate and iron(II)-α-hydroxy ketone complexes of a carbanionic N3C ligand: Oxidation of metal center vs C C bond cleavage of co-ligand with dioxygen

Light-induced O2-dependent aliphatic carbon-carbon (C-C) bond cleavage in bipyridine-ligated Co(II) chlorodiketonate complexes

Mononuclear bipyridine (bpy)-ligated Co(II) chlorodiketonate complexes [(bpy)2Co(R-PhC(O)C(Cl)C(O)R-Ph)]ClO4 (R = -H (8), -CH3 (9), and -OCH3 (10)), were prepared, characterized and investigated for O2-dependent aliphatic C-C bond cleavage reactivity. Complexes 8-10 have a distorted psuedo-octahedral geometry. 1H NMR spectra of 8-10 in CD3CN show signals for the coordinated diketonate moiety, and signals suggesting ligand exchange reactivity leading to the formation of a small amount of [(bpy)3Co](ClO4)2 (11) in solution. While 8-10 are air stable at room temperature, illumination at 350 nm results in oxidative cleavage reactivity within the diketonate moiety leading to the formation of 1,3-diphenylpropanetrione, benzoic acid, benzoic anhydride, and benzil. Illumination of 8 under 18O2 results in a high level of 18O incorporation (>80%) in the benzoate anion. The product mixture, high level of 18O incorporation, and additional mechanistic studies suggest a reaction sequence wherein light-induced reactivity leads to the formation of a triketone intermediate that undergoes either oxidative C-C bond cleavage or benzoyl migration promoted by a bipyridine-ligated Co(II) or Co(III) fragment.

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

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.