Water-soluble mono- and star-shaped hexanuclear functional organo-iron catalysts for nitrate and nitrite reduction in water: syntheses and electroanalytical study

Ten functional sandwich complexes [FeII(η5-C5H4CO2H)(η6-(arene)][PF6] and [Fe+(η5-C5H5)(η6-(C6(CH2CH2C6H4OH)6)][PF6] have been synthesized to provide water-soluble redox catalysts with a range of redox potentials for the FeII–FeI redox couple. The CpFe+ induced CH benzylic activation and the carboxylation of hexamethylbenzene, the CpFe+ induced one-pot polymethylation of mesitylene and hexamethylbenzene, and the one-pot CpFe+ induced hexa-p-methoxybenzylation of hexamethylbenzene have led to the desired functionalized complexes. These compounds catalyze the cathodic reduction of nitrate and nitrite in a basic aqueous medium on a Hg cathode. The rates of the rate-limiting steps of the redox-catalytic processes involving the homogeneous reduction of nitrate and nitrite by the ‘19-electron’ states of the redox catalysts have been measured using enhancement of the intensity of the monoelectronic cathodic wave FeII→FeI in the presence of the substrate by cyclic voltammetry, polarography and chronoamperometry. For six iron sandwich complexes without excessive bulk around the iron center, a linear Marcus-type correlation between the kinetics and the thermodynamics was found by plotting the logarithm of this rate constant versus the redox potential, which indicates that the homogeneous electron-transfer from the 19-electron catalyst to the substrate is rate-limiting. With four complexes in which chains of the arene ligand introduced more bulk around the iron center, the rates were found to be an order of magnitude lower, showing the crucial effect of the distance on the outer-sphere electron transfer. A small inner-sphere contribution cannot be discarded, however. When the redox catalysts are attached to the termini of the branches of hexa-branch star-shaped cores, the rate constants are close to those of unencumbered catalysts, i.e. no significant loss of activity is observed.