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Ghaderian, Abolfazl; Franke, Alicja; Gil-Sepulcre, Marcos; Benet-Buchholz, Jordi; Llobet, Antoni; Ivanovic-Burmazovic, Ivana and Gimbert-Surinach, Carolina (2020): A broad view on the complexity involved in water oxidation catalysis based on Ru-bpn complexes. In: Dalton Transactions, Vol. 49, No. 47: pp. 17375-17387

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A new Ru complex with the formula [Ru(bpn)(pic)(2)]Cl-2 (where bpn is 2,2 '-bi(1,10-phenanthroline) and pic stands for 4-picoline) (1Cl(2)) is synthesized to investigate the true nature of active species involved in the electrochemical and chemical water oxidation mediated by a class of N4 tetradentate equatorial ligands. Comprehensive electrochemical (by using cyclic voltammetry, differential pulse voltammetry, and controlled potential electrolysis), structural (X-ray diffraction analysis), spectroscopic (UV-vis, NMR, and resonance Raman), and kinetic studies are performed. 1(2+) undergoes a substitution reaction when it is chemically (by using NaIO4) or electrochemically oxidized to Ru-III, in which picoline is replaced by an hydroxido ligand to produce [Ru(bpn)(pic)(OH)](2+) (2(2+)). The former complex is in equilibrium with an oxo-bridged species {[Ru(bpn)(pic)](2)(mu-O)}(4+) (3(4+)) which is the major form of the complex in the Ru-III oxidation state. The dimer formation is the rate determining step of the overall oxidation process (k(dimer) = 1.35 M-1 s(-1)), which is in line with the electrochemical data at pH = 7 (k(dimer) = 1.4 M-1 s(-1)). 3(4+) can be reduced to [Ru(bpn)(pic)(OH2)](2+) (4(2+)), showing a sort of square mechanism. All species generated in situ at pH 7 have been thoroughly characterized by NMR, mass spectrometry, UV-Vis and electrochemical techniques. 1(2+) and 4(2+) are also characterized by single crystal X-ray diffraction analysis. Chemical oxidation of 1(2+) triggered by Ce-IV shows its capability to oxidize water to dioxygen.

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