Synthesis, characterization and heterogeneous catalysis for water oxidation of a di-manganese complex with 4′-(4-pyridyl)-2,2′:6′,2-terpyridine

A new Mn-III (mu-O)(2)Mn-VI dimer, [(OH2)(Py-terpy)Mn(mu-O)(2)Mn(Py-terpy)(OH2)](3+) (Py-terpy = 4'-(4-pyridyl)-2,2':6',2 ''-terpyridine) (1Py) with electron-withdrawing pyridyl (Py) groups was synthesized and characterized by UV-Vis and IR spectroscopic, X-ray crystallographic, magnetometric, and electrochemical techniques. 1Py is stable in an aqueous solution at room temperature, in contrast to the unstable Cl and Br derivatives with R-terpy (R = Cl or Br of electron-withdrawing groups). The maximum concentrations of 1Py adsorbed on saponite suggests that 1Py is adsorbed on saponite or mica as a pentavalence cation, like [1Py2H](5+) with protonation of Py groups on Py-terpy ligands. Water oxidation catalyses by 1Py were examined in homogenous solution and heterogeneous saponite or mica adsorbate systems. O-2 was evolved in a reaction of 1Py with a Ce(IV) oxidant in an aqueous solution. However, the maximum turnover number (TN) of 1Py for O-2 evolution was 0.63 for 4 days, showing that 1Py does not work as a catalyst for water oxidation in solution. The reactions of the 1Py/saponite or mica adsorbate with a Ce-IV oxidant in water catalytically produced the significant amount of O-2. The maximum TNs of 1Py on saponite and mica for O-2 evolution are 5.6 and 16 for 4 days, respectively. The kinetic analysis of O-2 evolution suggested that the unimolecular decomposition reaction of 1Py involving O-2 evolution competes with the cooperative catalysis by two equivalents of 1Py on saponite or mica. The k(2) values of 1Py adsorbates were lower than those of 1 adsorbates for saponite and mica systems, in contradiction to the magnitude relationship predicted from the E-1/2 values of 1Py and 1 based on our previous work (Yamazaki et al., 2012 [29]) The lower k(2) values could be explained most possibly by electrostatic repulsion between a pentavalent [1Py2H](5+) cation on saponite and mica, compared with a trivalent 1 cation. (C) 2012 Elsevier Ltd. All rights reserved.