In this study, we report the electrocatalytic behavior of the neutral, monomeric Ni(II) complex of diacetyl-bis(N-4-methyl-3-thiosemicarbazonato), NiL 1 , for ligand-assisted metal-centered hydrogen evolution in acetonitrile (ACN) and dimethylformamide (DMF). Using foot-of-the-wave analysis (FOWA), NiL 1 displays a maximum turnover frequency (TOF) of 4200 and 1200 s −1 for acetic acid (CH 3 COOH) in ACN and DMF, whereas for trifluoroacetic acid (CF 3 COOH) the TOFs are 1300 and 120 s −1 in ACN and DMF, respectively. In ACN, the overpotentials are 0.53 and 0.67 V for CH 3 COOH and CF 3 COOH, respectively. In DMF, the overpotential is 0.85 V for CH 3 COOH. First-order dependence with respect to the catalyst is established. NiL 1 displays a minimum Faradaic efficiency of 87% from controlled potential electrolysis. Gas analysis from controlled potential electrolysis in both ACN and DMF using CH 3 COOH and CF 3 COOH confirms NiL 1 as an electrocatalyst to produce H 2 . In ACN, TONs of 48 and 24 were obtained for CH 3 COOH and CF 3 COOH, respectively in 4 h. In DMF, TONs of 13 and 3 were obtained for CH 3 COOH and CF 3 COOH, respectively. The H 2 evolution reaction was evaluated using deuterated acid, demonstrating an inverse kinetic isotope, which is consistent with formation of a metal hydride intermediate. A proposed ligand-assisted metal-centered mechanism for HER is supported by computational investigations. All catalytic intermediates in the proposed mechanism were structurally and energetically characterized using density functional theory (DFT), with the B3LYP/6-311g(d,p) and BP86/TZV/P in solution modeled via polarizable continuum model. The final step of catalysis involves the reaction of [HNi(L 1 •)] − with H + generating H 2 . The correctness of proposed mechanism was confirmed by location of corresponding transition state (TS) having single imaginary frequency (i1786 cm −1 ).

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The zinc­(II) complex of diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazonepyridine), ZnL1 (1), was prepared and evaluated as a precatalyst for the hydrogen evolution reaction (HER) under homogeneous conditions in acetonitrile. Complex 1 is protonated on the noncoordinating nitrogen of the hydrazonepyridine moiety to yield the active catalyst Zn­(HL1)­OAc (2) upon addition of acetic acid. Addition of methyl iodide to 1 yields the corresponding methylated derivative ZnL2I (3). In solution, partial dissociation of the coordinated iodide yields the cationic derivative 3′. Complexes 1–3 were characterized by 1H NMR, FT-IR, and UV–visible spectroscopies. The solid-state structures of 2 and 3 were determined by single crystal X-ray diffraction. HER studies conducted in acetonitrile with acetic acid as the proton source yield a turnover frequency (TOF) of 7700 s–1 for solutions of 1 at an overpotential of 1.27 V and a TOF of 6700 s–1 for solutions of 3 at an overpotential of 0.56 V. For both complexes, the required potential for catalysis, E cat/2, is larger than the thermodynamic reduction potential, E 1/2, indicative of a kinetic barrier attributed to intramolecular proton rearrangement. The effect is larger for solutions of 1 (+440 mV) than for solutions of 3 (+160 mV). Controlled potential coulometry studies were used to determine faradaic efficiencies of 71 and 89% for solutions of 1 and 3, respectively. For both catalysts, extensive cycling of potential under catalytic conditions results in the deposition of a film on the glassy carbon electrode surface that is active as an HER catalyst. Analysis of the film of 3 by X-ray photoelectron spectroscopy indicates the complex remains intact upon deposition. A proposed ligand-centered HER mechanism with 1 as a precatalyst to 2 is supported computationally using density functional theory (DFT). All catalytic intermediates in the mechanism were structurally and energetically characterized with the DFT/B3LYP/6-311g­(d,p) in solution phase using a polarizable continuum model (PCM). The thermodynamic feasibility of the mechanism is supported by calculation of equilibrium constants or reduction potentials for each proposed step.

Atmospheric particulate matters were collected on quartz fibre filters for 24 hours with a low volume sampler from January 2014 to March 2014 at the Southeast Asian mega city (Dhaka, Bangladesh). Particulate matters samples were analysed for eleven trace metals with inductively coupled plasma mass spectrometer (ICP-MS) at Cà Foscari University of Venice, Italy. Trace metals were extracted from filters with digestion method using a mixture of HNO3 and H2O2. The average concentration of the determined trace metals of As, Cd, Ni, Cu, Pb, Cr, Fe, Mn, Zn, Sband Se were 3.06, 6.28, 3.77, 11.98, 305.6, 9.2, 2057.0, 42.2, 303.3, 5.47 and 2.43 ng•m −3 , respectively. Arsenic concentration is much lower in the atmosphere of Dhaka, though Bangladesh has severe arsenic problem in the ground water. Lead and cadmium concentrations showed decreasing trend in Dhaka compared than previous measurements-but still they have very high levels compared than Europe and USA. There is very limited information for Mn, Sb and Se concentrations in Dhaka air. Correlation studies showed that several trace metals had potential joint sources of origin, e.g., manganese is highly correlated with iron (r 2 = 0.97) and nickel (r 2 = 0.84), copper (r 2 = 0.86); lead with arsenic (r 2 = 0.79) and antimony (r 2 = 0.78). Enrichment factors analysis was also done with the data base for the respective metals in earth crust and coal fly ash. As and Cu both have combined sources, whereas Cd, Pb and Zn were from coal fly ash.Trace metals concentrations in Dhaka city air were much higher than Europe and USA but comparable or slightly lower than other south Asian countries. This is the first extensive study for the eleven trace metals with ICP-MS in Dhaka,

Coenzyme B12 (adenosylcobalamin = AdoCbl)-dependent enzymes catalyze complex molecular transformations by employing radical chemistry. The initial step in the native catalytic cycle, upon substrate binding, involves homolytic cleavage of the Co–C bond of AdoCbl to form the Co­(II)/Ado• radical pair (RP). Formation of Co­(II)/Ado• is subsequently coupled with H atom abstraction from the substrate. Interestingly, these same RPs can be generated upon light absorption without presence of a substrate. Herein, the photochemistry associated with the mechanism of Co–C bond photocleavage inside the AdoCbl-dependent ethanolamine ammonia-lyase (EAL) was investigated using a combined time-dependent density functional theory and molecular mechanics (TD-DFT/MM) approach. Excited state potential energy surfaces (PESs), constructed as a function of axial bond lengths, were used to understand the photocleavage of the Co–C bond and to elucidate the mechanism of photodissociation for AdoCbl inside the enzyme. The S1 PES is characterized by two minima regions, namely, metal-to-ligand charge transfer (MLCT) and ligand field (LF) states, which are key minima regions along the reaction pathway. There are two possible routes for photolysis of AdoCbl inside EAL named Path A and Path B. Path B is slightly more energetically favorable than Path A and involves the elongation of the Co–Nax bond followed by the elongation of both axial bonds Co–C and Co–Nax. To further understand the effect of environment on the formation of RP, the photochemical data for AdoCbl-dependent EAL was also compared with base-on and base-off AdoCbl in solution.