Molecular versus Silica-Supported Iridium Water Oxidation Catalysts

Abstract

A stringent comparison between two pairs of molecular/immobilized water oxidation catalysts ([Cp*Ir(Me-pica)Cl], 1, versus 1_SiO2, Me-pica=κ2-N-methyl-picolinamide; [Cp*Ir(pysa)NO3], 2, versus 2_SiO2, pysa=κ2-pyridine-2-sulfonamide]) reveals distinctive catalytic trends. While the molecular compound 1 exhibits a substantial higher activity than the analogous immobilized system 1_SiO2, under all the experimental conditions explored, the contrary is found with 2 that is far less active than its immobilized counterpart 2_SiO2. This is explained by the unique tendency of 2 to form dimeric complexes [Cp*Ir-(κ2-μ2-Hpysa)(κ2-μ2-pysa)IrCp*], 2 a, in phosphate buffered solution at pH 7, and [Cp*Ir-(κ2-μ2-Hpysa)2IrCp*], 2 b, in water. 2 a and 2 b have been completely characterized in solution by multinuclear and multidimensional NMR spectroscopy. They have been also isolated as single crystals and their structure in solid state determined by X-Ray diffractometry. 2 a and 2 b appear to be off-cycle species, whose formation is detrimental for water oxidation activity, as indicated by the observation of a long induction period when 2 a is used as catalytic precursor. In addition, TOF versus ΔE (E−E0=−RT/nF ln([IO4−]/[IO3−]) trends for the first two runs do not overlap for catalyst 2 and TOFmax is remarkably higher in the second run upon the addition of fresh NaIO4. In the immobilized system 2_SiO2 the detrimental associative processes are likely inhibited leading to an activity higher than that of 2.