Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex.

High-valent metal-oxo species are key intermediates for the oxygen atom transfer step in the catalytic cycles of many metalloenzymes. While the redox-active metal centers of such enzymes are typically supported by anionic amino acid side chains or porphyrin rings, peptide backbones might function as strong electron-donating ligands to stabilize high oxidation states. To test the feasibility of this idea in synthetic settings, we have prepared a nickel(II) complex of new amido multidentate ligand. The mononuclear nickel complex of this N5 ligand catalyzes epoxidation reactions of a wide range of olefins by using mCPBA as a terminal oxidant. Notably, a remarkably high catalytic efficiency and selectivity were observed for terminal olefin substrates. We found that protonation of the secondary coordination sphere serves as the entry point to the catalytic cycle, in which high-valent nickel species is subsequently formed to carry out oxo-transfer reactions. A conceptually parallel process might allow metalloenzymes to control the catalytic cycle in the primary coordination sphere by using proton switch in the secondary coordination sphere.

Non-Heme Iron Catalysts for Olefin Epoxidation: Conformationally Rigid Aryl-Aryl Junction To Support Amine/Imine Multidentate Ligands.

Atom-transfer chemistry represents an important class of reactions catalyzed by metalloenzymes. As a functional mimic of non-heme iron enzymes that deliver oxygen atoms to olefins, we have designed monoiron complexes supported by new N-donor chelates. These ligands take advantage of heme-like conformational rigidity of the π-conjugated molecular backbone, and synthetic flexibility of tethering non-heme donor groups for additional steric and electronic control. Iron complexes generated in situ can be used to carry out catalytic epoxidation of a wide range of olefin substrates by using mCPBA as a terminal oxidant. The fate of initial iron-peracid adduct and the involvement of iron-oxo species in this process were investigated further by mechanistic probes and isotope exchange studies. Our findings suggest that anilidopyridyl-derived [N,N]-bidentate motif could serve as a versatile structural platform to build non-heme ligands for catalytic oxidation chemistry.

Synthesis, Characterization, and Efficient Catalytic Activities of a Nickel(II) Porphyrin: Remarkable Solvent and Substrate Effects on Participation of Multiple Active Oxidants.

A new nickel(II) porphyrin complex, [NiII (porp)] (1), has been synthesized and characterized by 1 H NMR, 13 C NMR and mass spectrometry analysis. This NiII porphyrin complex 1 quantitatively catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Reactivity and Hammett studies, H218 O-exchange experiments, and the use of PPAA (peroxyphenylacetic acid) as a mechanistic probe suggested that participation of multiple active oxidants NiII -OOC(O)R 2, NiIV -Oxo 3, and NiIII -Oxo 4 within olefin epoxidation reactions by the nickel porphyrin complex is markedly affected by solvent polarity, concentration, and type of substrate. In aprotic solvent systems, such as toluene, CH2 Cl2 , and CH3 CN, multiple oxidants, NiII -(O)R 2, NiIV -Oxo 3, and NiIII -Oxo 4, operate simultaneously as the key active intermediates responsible for epoxidation reactions of easy-to-oxidize substrate cyclohexene, whereas NiIV -Oxo 3 and NiIII -Oxo 4 species become the common reactive oxidant for the difficult-to-oxidize substrate 1-octene. In a protic solvent system, a mixture of CH3 CN and H2 O (95:5), the NiII -OOC(O)R 2 undergoes heterolytic or homolytic O-O bond cleavage to afford NiIV -Oxo 3 and NiIII -Oxo 4 species by general acid catalysis prior to direct interaction between 2 and olefin, regardless of the type of substrate. In this case, only NiIV -Oxo 3 and NiIII -Oxo 4 species were the common reactive oxidant responsible for olefin epoxidation reactions.

Furan and Julolidine-Based "Turn-on" Fluorescence Chemosensor for Detection of F- in a Near-Perfect Aqueous Solution.

A new fluorescent sensor 1, containing furan and julolidine moieties linked through a Schiff-base, has been synthesized. Distinct "turn-on" fluorescence enhancement of 1 was observed upon the addition of F- in a near-perfect aqueous solution. The binding capabilities of 1 with F- were studied by using fluorescent spectroscopic techniques, ESI-mass analysis and NMR titration measurements. The detection limit for the analysis of F- was found to be 10.02 µM, which is below the WHO guideline (79 µM) for drinking water. Practically, the sensing ability of 1 for F- was successfully applied in real water samples. The sensing mechanism for F- was proposed to be the ICT mechanism via the hydrogen bonding, which was well explained by theoretical calculations.

Robust fluorescence sensing platform for detection of CD44 cells based on graphene oxide/gold nanoparticles.

Gold-coated graphene oxide hybrid material (GO/AuNPs) has exceptional physical and chemical properties like π-π stacking interaction and plays a role in quencher of fluorescence dye. Therefore, GO/AuNPs could enhance the signal-to-background ratio with fluorescence dye that was the point in this fluorescent biosensor. In this study, tetramethyl-6-carboxy-rhodamine (TAMRA)-labeled aptamers that specifically interact with the hyaluronic acid binding domain of CD44 were used as targets to investigate the applicability of the method. GO/AuNPs-TAMRA-aptamer complexes could detect CD44 target cancer cells within a concentration range of 1 × 10(1) to 1 × 10(7) CFU/mL. A linear relationship was observed between target cell concentration and relative fluorescence intensity. The more mounted up CD44 target cell concentrations, relative fluorescence intensity of GO/AuNPs-TAMRA-aptamer complexes was increased even more, which was superior to that of GO alone. Sensitivity of the detection system displayed a low detection limit of 1 × 10(1) CFU/mL. Additionally, this method is specific in that fluorescence is not much enhanced in CD44 negative cancer cell line. Thus, the fluorescence sensing based on GO/AuNPs could be developed to receptive and robust detection tool for various target molecules.