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Relevant academic research and scientific papers

Rapid atmospheric carbon dioxide fixation by nickel(ii) complexes: meridionally coordinated diazepane-based 3N ligands facilitate fixation

Octahedral complexes of the type [Ni(L)(H2O)3](ClO4)2(1and2), where L is the tridentate 3N ligand 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1,1), or 4-methyl-1-(N-methylimidazolyl)-1,4-diazacycloheptane (L2,2), have been isolated and characterized using elemental analysis, ESI-MS and electronic absorption spectroscopy. The DFT optimized structures of1and2reveal that the tridentate 3N ligands are coordinated meridionally constituting a distorted octahedral coordination geometry around nickel(ii). In methanol solution, the complexes, upon treatment with triethylamine, generate the reactive red colored low-spin square planar Ni-OH intermediate [Ni(L1/L2)(OH)]+(1aand2a), as characterized by ESI-MS and electronic absorption spectroscopy, and energy minimized structures. The latter when exposed to the atmosphere rapidly absorbs atmospheric CO2to produce the carbonate bridged dinickel(ii) complexes [Ni2(L1/L2)2(μ-CO3)(H2O)2](ClO4)2(3and4), as characterized by elemental analysis and the IR spectral feature (～1608 cm?1) characteristic of bridging carbonate. The single crystal X-ray structure of3reveals the presence of a dinickel(ii) core bridged by a carbonate anion in a symmetric mode. Both the Ni(ii) centers are identical to each other with each Ni(ii) possessing a distorted octahedral coordination geometry constituted by a meridionally coordinated 3N ligand, a carbonate ion and a water molecule. The decay kinetics of the red intermediates generated by1(kobs, 7.7 ± 0.1 × 10?5s?1) and2(kobs, 5.8 ± 0.3 × 10?4s?1) in basic methanol solution with atmospheric CO2has been determined by absorption spectroscopy. DFT studies illustrate that meridional coordination of the 3N ligand and the electron-releasing imidazole ring as in2facilitate fixation of CO2. The carbonate complex3efficiently catalyzes the conversion of styrene oxide into cyclic carbonate by absorbing atmospheric and pure CO2with excellent selectivity.

Regioselective oxidative carbon-oxygen bond cleavage catalysed by copper(II) complexes: A relevant model study for lytic polysaccharides monooxygenases activity

Lytic polysaccharide monooxygenases (LPMOs) are copper-containing monooxygenase and catalyzing the oxidative cleavage of recalcitrant polysaccharides using dioxygen. The copper(II) complexes [Cu(L1)(H2O)ClO4]ClO4 1, [L1 = 4-methyl-1-[(pyridin-2-yl-methyl)]-1,4-diazepane]; [Cu(L2)(H2O)ClO4]ClO4 2, [L2 = 4-methyl-1-[(2-(pyridine-2-yl)ethyl]-1,4-diazepane] and [Cu(L3)(H2O)ClO4]ClO4 3, [L3 = 1-(4-methoxy-3,5-dimethyl-pyridin-2-yl)methyl)-4-methyl-1,4-diazepane] have been synthesized and characterized as the novel models for LPMOs. The molecular structures exhibit distorted square pyramidal geometry (τ 0.183–0.388) as similar to LPMOs. The Cu–N (1.99–2.02 ?) bond distances of the model complexes are almost identical to those of native LPMOs enzyme (1.9–2.2 ?). The 1, 4-diazepane backbone and pyridine unit of complexes provide reasonable structural resemblances to ‘histidine brace’ and histidine residues of LPMOs respectively. The spectral, redox and kinetic studies were performed in water to mimic accurate enzymatic reaction conditions. The well-defined Cu(II)/Cu(I) reduction couples were observed around 8–112 mV versus NHE, which is lower than that of LPMOs. The electronic spectra of the complexes showed the d-d transitions around 600–635 nm and axial EPR parameter (g||, 2.28–2.29; A|| 160–168 × 10?4 cm?1), which are almost identical to that of LPMOs. The model complexes were catalyzed oxidative cleavage of model substrate p-nitrophenyl-β-D-glucopyranoside into p-nitrophenol and D-allose with a maximum yield up to 78.4% and TON, 300. The kinetics of reaction monitored by following the formation of an absorption band around at 400 nm corresponds to p-nitrophenol, showed the rate of 3.19–5.26 × 10?3 s?1. The oxidative cleavage reaction may occur via CuII-OOH intermediate, whose formation accompanied by an electronic spectral signature around 375 nm with the rate of 1.61–9.06 × 10?3 s?1. The CuII-OOH intermediate was characterized by spectral methods and its geometry was optimized by DFT calculations.

New copper(II) complexes as efficient catalysts for olefin aziridination: The effect of ligand steric hindrance on reactivity

Three new copper(II) complexes derived from the tridentate N3 ligands 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1), 1-(quinol-2-ylmethyl)-1,4-diazacycloheptane (L2), and 4-methyl-1-(quinol-2- ylmethyl)-1,4-diazacycloheptane (L3) have been prepared and examined as copper catalysts for olefin aziridination. In the X-ray crystal structures of the complexes [Cu(L2)Cl2] (2) and [Cu(L3)Cl2] (3) copper(II) adopts a trigonal-bipyramidal distorted square-based pyramidal geometry (TBDSBP) as seen from the values of the trigonality index τ (0.08 for 2 and 0.48 for 3). The enhanced trigonal distortion in 3 is due to the presence of an N-Me group, the lone pair orbital of which is not oriented exactly along the d x2_y2 orbital of copper(II). While [Cu(L1)(H2O)](ClO 4)2 (1) assumes a tetragonal geometry in solution, complexes 2 and 3 adopt a distorted tetragonal geometry, as revealed by UV/Vis and EPR spectral studies. The complexes undergo quasi-reversible Cu II/CuI redox behavior in methanol solution. The ability of the complexes to mediate nitrene transfer from PhINTs to olefins to form N-tosylaziridines has been studied. The complexes are found to be efficient catalysts (in 5 mol-% amounts) for the aziridination of the reactive olefin styrene, with yields varying from 80 to 90% (with respect to PhINTs). They exhibit significant catalytic nitrene transfer reactivity (yields of 30 to 60%) also towards the less reactive olefins cyclohexene and cyclooctene. A remarkable observation is the significantly accelerated rate of aziridination by 3, which is ascribed to the steric crowding around copper(II) imposed by the bulky quinolyl and N-Me groups of the tridentate ligand L3. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.