Molecular versus Silica-Supported Iridium Water Oxidation Catalysts |
| |
Authors: | Caterina Trotta Dr Gabriel Menendez Rodriguez Dr Leonardo Tensi Dr Alexander V Yakimov Dr Luca Rocchigiani Prof Anna Donnadio Prof Cristiano Zuccaccia Prof Christophe Copéret Prof Alceo Macchioni |
| |
Institution: | 1. Department of Chemistry, Biology and Biotechnology and CIRCC, Università degli Studi di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy;2. Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir Prelog Weg 1–5, 8093 Zurich, Switzerland;3. Department of Pharmaceutical Sciences, Università degli Studi di Perugia, Via del Liceo 1, 06123 Perugia, Italy |
| |
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 . |
| |
Keywords: | amido-bridged dimers heterogeneous catalysis homogeneous catalysis iridium water oxidation |
|
|