High nuclearity heterometallic [Fe₇Ln₄] neutral coordination clusters with electrocatalytic activity for water oxidation

Three mixed metal iron-lanthanide isostructural clusters formulated, [Fe(7)Ln(4)(mu(4)-O)(3)(mu(3)-O)(mu(3)-OH)(2)(mu(2)-OH)(mdea)(6)(kappa(1)-OAc)(kappa(2)-OAc)(3)(mu-OAc)(6)(H2O)].nH(2)O where {n = 1.0, 2.5 and 0.5 for Ln = Nd (1), Pr (2), and Eu (3), respectively} are prepared by the reaction of FeCl24H(2)O and Ln(NO3)(3)6H(2)O with N-methyldiethanolamine (mdea) and CH3COO-. The core of these compounds is irregular tent-like clusters. In these structures, the central Fe is bridged to another Fe in the cluster ball by (mu(4)-O)(3)(mu(3)-O) and with bridged Ln(2) with two (mu(3)-OH) ions. All these clusters were investigated as electrocatalysts for water oxidation. The results illustrate that 1 is the most efficient material for water oxidation with overpotential of 309 mV and Tafel slope of 100 mV dec(-1) in neutral media. This is due to the larger electrochemical active surface area and more active catalytic sites of this cluster compared to Fe7Pr4 and Fe7Eu4 clusters. The electrocatalytic behavior of these clusters during bulk electrolysis is similar. In all 1-3, a yellow thin film is formed on the surface of electrode during water oxidation. The analyses show the presence of iron oxide and Ln oxide as the main components in this film. Long-term stability testing confirms that the film is active in the reaction, but its level activity depends on the nature of lanthanides present.