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_SiO 2 , Me‐pica=κ 2 ‐N‐methyl‐picolinamide; [Cp * Ir(pysa)NO 3 ], 2 , versus 2_SiO 2 , pysa=κ 2 ‐pyridine‐2‐sulfonamide]) reveals distinctive catalytic trends. While the molecular compound 1 exhibits a substantial higher activity than the analogous immobilized system 1_SiO 2 , under all the experimental conditions explored, the contrary is found with 2 that is far less active than its immobilized counterpart 2_SiO 2 . This is explained by the unique tendency of 2 to form dimeric complexes [Cp * Ir‐(κ 2 ‐μ 2 ‐Hpysa)(κ 2 ‐μ 2 ‐pysa)IrCp * ], 2 a , in phosphate buffered solution at pH 7, and [Cp * Ir‐(κ 2 ‐μ 2 ‐Hpysa) 2 IrCp * ], 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−E 0 =−RT/nF ln([IO 4 − ]/[IO 3 − ]) trends for the first two runs do not overlap for catalyst 2 and TOF max is remarkably higher in the second run upon the addition of fresh NaIO 4 . In the immobilized system 2_SiO 2 the detrimental associative processes are likely inhibited leading to an activity higher than that of 2 .