Dinuclear nickel(II) complexes with ONO-pincer and coordinated aquo ligands as a robust homogeneous water oxidation cata
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Dinuclear nickel(II) complexes with ONO-pincer and coordinated aquo ligands as a robust homogeneous water oxidation catalyst A. Khurshid,a M. N. Zafar,a K. Javed,b N. Iqbal,c and M. N. Arshadd aDepartment
of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan. E-mail: [email protected] bDepartment of Chemistry, University of Gujrat, Gujrat, Pakistan cU. S.-Pakistan Centre for Advanced Studies in Energy, National University of Sciences and Technology, Sector H-12, Islamabad 44000, Pakistan dChemistry Department, Faculty of Science, King Abdul Aziz University, Jeddah 21589, Saudi Arabia A nickel(II) complex with the newly synthesized dicarboxamide ligand [H2LBZ][(CF3SO3)Cl] was explored as a water oxidation catalysis. All the synthesized compounds were characterized by IR and NMR spectroscopy, thermogravimetry, and single-crystal XRD analysis. The superior kinetics of the water oxidation reaction in the presence of the NiII catalyst was shown by an enhanced current of 4535 A (pH 8) or 8900 A (pH 10) at an overpotential of 186 mV in a cyclic voltammogram. Controlled potential electrolysis demonstrated that the catalyst was stable during the water oxidation experiment. These results are akin to those for other reported nickel catalysts. Key words: catalytic water oxidation, dicarboxamide ligands, nickel(II) complex, homogeneous catalysis, pincer ligands.
One of the most challenging tasks for researchers in the 21st century is the production of green and sustainable energy from renewable sources.1—5 Water is the only cheap and abundantly available source of electrons and protons for natural aerobic photosynthesis as well as artificial photosynthesis. The water oxidation is an attractive research area since it involves studies of a complicated mechanism of the rearrangement of different bonds like O—O bond formation and multiple proton-coupled electron transfer resulting in O2 evolution with the generation of four electrons and four protons6 as shown in Eq. (1). 2 H2O
O2 + 4 H+ + 4e–.
(1)
Utilization of water as a source of hydrogen and oxygen in energy production needs highly efficient water oxidation catalysts (WOCs).7 Transition metals capable of transforming into high oxidation states during the water oxidation catalytic cycle make them an ideal choice to serve as a central atom of the main core of the catalyst. Among transition metals, a Ni catalyst is a superior choice for the construction of molecular systems for water oxidation because cyclic voltammetry (CV) of the complex shows no crossover, the catalytic current and concentration exhibit a direct rela-
tionship, no catalytic response is given by free Ni ions at acidic or neutral pH, no current enhancement occurs during repeating CV cycles, and no current is observed at the working electrode at the end of electrolysis.8—10 Thus, an iron/nickel coordination polymer with carboxylate ligands for the water oxidation reaction was recently developed.11 This polymer demonstrated good catalytic activity with an onset potential E0 = 1.4 V. Spiccia
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