O2 activation at a trispyrazolylborato nickel(ii) malonato complex

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.

Manganese-promoted cleavage of acetylacetonate resembling the β-diketone cleaving dioxygenase (Dke1) reactivity

We here report a manganese-based oxidative cleavage of inactivated acetylacetonate, the mechanistic pathway of which resembles Dke1-catalyzed reactions of β-diketone and α-keto acid. This oxidative transformation proceeds through an acetylacetonate-pyruvate-oxalate pathway, which can be terminated at the stage of pyruvate through ligand/solvent variation. XRD, time-dependent GC-MS, and isotope-labeling studies suggested that our system represents the same cleaving specificity and dioxygenase-like reactivity of Dke1.

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.

The Behavior of Trispyrazolylborato-Metal(II)-Flavonolate Complexes as Functional Models for Bacterial Quercetinase-Assessment of the Metal Impact

A series of five compounds Tp^MesMFla (Tp^Mes = hydrotris(3-mesityl)pyrazolylborate; M = Mn, Fe, Co, Ni, Zn; Fla = 3-hydroxyflavonolate) has been synthesized as models for the 2,4-quercetin dioxygenase, QueD. The structures have been determined and the complexes proved to be isomorphous. Considering the structures more closely revealed that they differ in the degree of delocalization in the chelate ring formed through the binding of the two O donors of the flavonolate to the metal center, which is also supported by the results of UV-vis and IR spectroscopic investigations. The resulting trend (Zn/Fe > Co > Mn > Ni) is, however, not in line with the one that was found investigating the redox properties of the complexes by cyclic voltammetry (Zn > Fe > Ni > Co > Mn). Notably, from CV clear-cut information could be derived, as the complexes exhibited exceptionally well-behaved quasi-reversible redox transitions, indicating that the Tp ligand stabilizes the flavonolate radical formed in the oxidation process rather well. The fact that the rates, with which the complexes react with O₂ in DMF solution, correlate with the position of the flavonolate redox couples, suggest that these reactions proceed via the initial electron transfer from the flavonolate to O₂. After the O₂ reaction, salicylic acid was identified as one of the products, the formation of which can be explained by the hydrolysis of the depside that should form upon a dioxygenation similar to the QueD enzyme-catalyzed reaction. ¹⁸O labeling experiments confirmed the presence of O₂ derived O atoms. Mechanistic inferences based on the above results are discussed.

Nickel(ii) complexes of a 3N ligand as a model for diketone cleaving unusual nickel(ii)-dioxygenase enzymes

Diketone substrate bound nickel(ii) complexes of 2,6-bis(1-methylbenzimidazolyl)pyridine have been synthesized and characterized as relevant active site models for unusual diketone cleaving Ni(ii)-dependent enzymes Ni-ARD and DKDO. The average Ni-N_py/benzim bond distances (2.050-2.107 Å) of model complexes are almost identical to the Ni-N_His bond distances of Ni^II-ARD (2.02-2.19 Å). The reaction of these adducts with dioxygen exhibited C-C cleavage with the rate of k_O2, 5.24-73.71 × 10^-3 M^-1 s^-1. The phenyl substituted adduct regioselectively elicits 52% of benzoic acid as the major product.