High-Valent Cobalt-Difluoride in Oxidative Fluorination of Saturated Hydrocarbons

The heme paradigm where Fe=O acts as the C-H oxidant and Fe-OH rebounds with the formed carbon-centered radical guides the design of the prototypical synthetic hydroxylation catalyst. We are exploring methods to evolve beyond the metal-oxo oxidant and hydroxide rebound, to incorporate a wider array of functional group. We have demonstrated the application of CoII(OTf)2 (10 mol% catalyst; OTf=trimfluoromethanesulfonate) in combination with polydentate N-donor ligands (e. g. BPMEN=N,N'-dimethyl-N,N'-bis(pyrid-2-ylmethyl)ethane-1,2-diamine) and Selectfluor in the oxidative fluorination of saturated hydrocarbons in high yields. The addition of CsF to the reaction mixture induced near-quantitative yields of fluorinated saturated hydrocarbons (>90 % yield of fluorinated product). For 1-hydroxy, 1-acetyl, 1-carboxy-, and 1-acetamido-adamantane, we demonstrated selective fluorination at the 3-position. We propose two mechanisms for the CoII-catalyzed reaction: either (i) an N-radical, derived from Selectfluor, acted as the C-H oxidant followed by radical rebound with CoIII-F; or (ii) a CoIV-(F)2 species was the C-H oxidant followed by radical rebound with CoIII-F. Our combined spectroscopic, kinetic, and chemical trapping evidence suggested that an N-radical was not the active oxidant. We concluded that a CoIV-(F)2 species was the likely active oxidant and CoIII-F was the likely F-atom donor to a carbon centered radical producing a C-F bond.

Non-heme oxoiron complexes as active intermediates in the water oxidation process with hydrogen/oxygen atom transfer reactions

In this study, we explore the water oxidation process with the help of density functional theory. The formation of an oxygen-oxygen bond is crucial in the water oxidation process. Here, we report the formation of the oxygen-oxygen bond by the N5-coordinate oxoiron species with a higher oxidation state of Fe^IV and Fe^V. This bond formation is studied through the nucleophilic addition of water molecules and the transfer of the oxygen atom from meta-chloroperbenzoic acid (mCPBA). Our study reveals that the oxygen-oxygen bond formation by reacting mCPBA with Fe^VO requires less activation barrier (13.7 kcal mol^-1) than the other three pathways. This bond formation by the oxygen atom transfer (OAT) pathway is more favorable than that achieved by the hydrogen atom transfer (HAT) pathway. In both cases, the oxygen-oxygen bond formation occurs by interacting the σ*d_z²-2p_z molecular orbital of the iron-oxo intermediate with the 2p_x orbital of the oxygen atom. From this study, we understand that the oxygen-oxygen bond formation by Fe^IVO with the OAT process is also feasible (16 kcal mol^-1), suggesting that Fe^VO may not always be required for the water oxidation process by non-heme N5-oxoiron. After the oxygen-oxygen bond formation, the release of the dioxygen molecule occurs with the addition of the water molecule. The release of dioxygen requires a barrier of 7.0 kcal mol^-1. The oxygen-oxygen bond formation is found to be the rate-determining step.

Exploring the Potential of Water-Soluble Cu(II) Complexes with MPA–CdTe Quantum Dots for Photoinduced Electron Transfer

Three water-soluble copper complexes based on the amine/pyridine functionalities were investigated, along with quantum dots, as a catalyst–photosensitizer assembly, respectively, for fundamental understanding of photoinduced electron transfer. Luminescence quenching and lifetime measurements were performed to try and establish the actual process that leads to the quenching, such as electron transfer, energy transfer, or complex formation (static quenching). Cyclic voltammetry and dynamic light scattering experiments were also performed. Irrespective of the similar reduction potentials of the three complexes, very different photoluminescence properties were observed.

Reactivity and Mechanisms of Photoactivated Heterometallic [RuII NiII ] and [RuII NiII RuII ] Catalysts for Dihydrogen Generation from Water

Two heterometallic photocatalysts were designed and probed for water reduction. Both [(bpy)₂ Ru^II Ni^II (L¹ )](ClO₄ )₂ (1) and [(bpy)₂ Ru^II Ni^II (L² )₂ Ru^II (bpy)₂ ](ClO₄ )₂ (2) can generate the low-valent precursor involved in hydride formation prior to dihydrogen generation. However, while the bimetallic [Ru^II Ni^II ] (1) requires the presence of an external photosensitizer to trigger catalytic activity, the trimetallic [Ru^II Ni^II Ru^II ] (2) displays significant coupling between the catalytic and light-harvesting units to promote intramolecular multielectron transfer and perform photocatalysis at the Ni center. A concerted experimental and theoretical effort proposes mechanisms to explain why 1 is unable to achieve self-supported catalysis, while 2 is fully photocatalytic.

Synthesis, structure, and biological activity of bis(benzimidazole)amino thio- and selenoether nickel complexes

Four new nickel (II) complexes with bis(benzimidazole)thio- and selenoether-based ligands have been synthesized and characterized in the solid state by elemental analysis, IR, magnetic susceptibility and X-ray crystallography, and in solution by FAB⁺ mass spectrometry, UV-vis spectroscopy and cyclic voltammetry. Single-crystal X-ray diffraction analysis of the compounds revealed octahedral geometries for all nickel centers. Three of the four complexes are dimers with chloride bridges between the two Ni(II) ions. However, in solution all complexes have a monomeric formulation, based on mass spectrometry and osmometry measurements. The complexes were also screened for their cytotoxic activity on human cell lines (HeLa, SK-LU-1 and HEK-293), and compared with a related Cu(II) complex.

Synthesis of water-soluble Ni(II) complexes and their role in photo-induced electron transfer with MPA-CdTe quantum dots

Photocatalytic water splitting using solar energy for hydrogen production offers a promising alternative form of storable and clean energy for the future. To design an artificial photosynthesis system that is cost-effective and scalable, earth abundant elements must be used to develop each of the components of the assembly. To develop artificial photosynthetic systems, we need to couple a catalyst for proton reduction to a photosensitizer and understand the mechanism of photo-induced electron transfer from the photosensitizer to the catalyst that serves as the fundamental step for photocatalysis. Therefore, our work is focused on the study of light driven electron transfer kinetics from the quantum dot systems made with inorganic chalcogenides in the presence of Ni-based reduction catalysts. Herein, we report the synthesis and characterization of four Ni(II) complexes of tetradentate ligands with amine and pyridine functionalities (N2/Py2) and their interactions with CdTe quantum dots stabilized by 3-mercaptopropionic acid. The lifetime of the quantum dots was investigated in the presence of the Ni complexes and absorbance, emission and electrochemical measurements were performed to gain a deeper understanding of the photo-induced electron transfer process.

Influence of nitro substituents on the redox, electronic, and proton reduction catalytic behavior of phenolate-based [N2O3]-type cobalt(iii) complexes

We report on the synthesis, redox, electronic, and catalytic behavior of two new cobalt(iii) complexes, namely [CoIII(L1)MeOH] (1) and [CoIII(L2)MeOH] (2). These species contain nitro-rich, phenolate-based pentadentate ligands and present dramatically distinct properties associated with the position in which the -NO2 substituents are installed. Species 1 displays nitro-substituted phenolates, and exhibits irreversible redox response and negligible catalytic activity, whereas 2 has fuctionalized phenylene moieties, shows much improved redox reversibility and catalytic proton reduction activity at low overpotentials. A concerted experimental and theoretical approach sheds some light on these drastic differences.