Chun-Wai Tse

Nonheme Iron Mediated Oxidation of Light Alkanes with Oxone: Characterization of Reactive Oxoiron(IV) Ligand Cation Radical Intermediates by Spectroscopic Studies and DFT Calculations†

The oxidation of light alkanes that is catalyzed by heme and nonheme iron enzymes is widely proposed to involve highly reactive {Fe(V)=O} species or {Fe(IV)=O} ligand cation radicals. The identification of these high-valent iron species and the development of an iron-catalyzed oxidation of light alkanes under mild conditions are of vital importance. Herein, a combination of tridentate and bidentate ligands was used for the generation of highly reactive nonheme {Fe=O} species. A method that employs [Fe(III)(Me3tacn)(Cl-acac)Cl](+) as a catalyst in the presence of oxone was developed for the oxidation of hydrocarbons, including cyclohexane, propane, and ethane (Me3tacn=1,4,7-trimethyl-1,4,7-triazacyclononane; Cl-acac=3-chloro-acetylacetonate). The complex [Fe(III)(Tp)2](+) and oxone enabled stoichiometric oxidation of propane and ethane. ESI-MS, EPR and UV/Vis spectroscopy, (18)O labeling experiments, and DFT studies point to [Fe(IV)(Me3tacn)({Cl-acac}(.+))(O)](2+) as the catalytically active species.

The effects of chelating N4 ligand coordination on Co(II)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations

Here we describe the synthesis and X-ray crystal structures of a panel of cis-[CoII(N4)Cl2] complexes (N4 = tetradentate N atom donor ligand). We also examine the catalytic activities of these complexes in the photochemical and electrochemical reduction of CO2 to CO using [Ir(ppy)3] as the photosensitizer. Among the complexes studied, cis-[CoII(PDP)Cl2] (C1) (PDP = 1,1′-bis(2-pyridinylmethyl)-2,2′-bipyrrolidine) displayed the highest catalytic activity. This Co(II) complex was able to effectively mediate the reduction of CO2 to CO under either electrochemical or visible light photocatalytic conditions. For the electrocatalysis, C1 catalysed CO2 to CO with up to 96% Faraday efficiency at −1.70 V (vs. SCE, SCE = saturated calomel electrode). A selectivity of up to 95% for CO production was achieved in a photocatalytic CO2 reduction system by using C1 as the catalyst, Ir(ppy)3 as the photosensitizer and triethylamine as the electron donor. The Co(I) species in situ generated by the one electron reduction of cis-[CoII(PDP)Cl]+ is suggested to be directly responsible for CO2 activation. Ultrafast time (ns) resolved absorption spectroscopy revealed that the photoinduced electron transfer from the triplet excited state of Ir(ppy)3 to C1 is a key step in the generation of active Co(I) species. The electronic structure and redox properties of the Co(I) species, [CoI(N4)Cl], as well as its role in the catalytic reaction were investigated by DFT calculations. The presence of one chloride ligand cis to the CO2 coordination site neutralizes the positive charge on the Co(I) centre, therefore assisting the bound CO2 molecule in attracting protons. The reaction mechanism for CO2 reduction to CO catalysed by the recently reported [CoII(TPA)Cl]+ (TPA = tris(2-pyridylmethyl)amine) catalyst was also computed. Subtle modifications of the chelating N4 ligand from cis-[CoII(N4)Cl2] were found to have a profound effect on the efficiency of CO2 reduction by DFT calculations.

Anti-cancer iron(II) complexes of pentadentate N-donor ligands: cytotoxicity, transcriptomics analyses, and mechanisms of action.

Two cytotoxic iron(II) complexes [Fe(L)(CH3 CN)n ](ClO4 )2 (L=qpy for Fe-1 a, Py5 -OH for Fe-2 a) were synthesized. Both complexes are stable against spontaneous demetalation and oxidation in buffer solutions. Cyclic voltammetry measurements revealed the higher stability of Fe-2 a (+0.82 V vs Fc) against Fe(II) to Fe(III) oxidation than Fe-1 a (+0.57 V vs Fc). These two complexes display potent cytotoxicity at micromolar level against a panel of cancer cell lines (Fe-1 a=0.8-3.1 μM; Fe-2 a=0.6-3.4 μM), and induce apoptosis that involves caspase activation. Transcriptomic and Connectivity Map analyses revealed that the changes of gene expression induced by Fe-1 a and Fe-2 a are similar to that induced by ciclopirox, an antifungal compound whose mode of action involves formation of intracellular cytotoxic iron chelates. Both Fe-1 a and Fe-2 a caused cellular nuclear DNA damage, as revealed by Comet assay and H2 AX immunofluorescence experiments. The cytotoxicity is associated with production of reactive oxygen species (for Fe-1 a), cell cycle regulation, and stress kinase pathways. The relative contributions of these to the overall cytotoxic mechanism is significantly affected by the structure of penta-N-donor ligand.

CCDC 870633: Experimental Crystal Structure Determination

Related Article: Chun-Wai Tse, Toby Wai-Shan Chow, Zhen Guo, Hung Kay Lee, Jie-Sheng Huang, Chi-Ming Che|2014|Angew.Chem.,Int.Ed.|53|798|doi:10.1002/anie.201305153