Molecular and Heterogenized Cp*Ir Water Oxidation Catalysts Bearing Glyphosate and Glyphosine as Ancillary and Anchoring Ligands

Two hybrid materials (1_TiO2 and 2_TiO2) were designed and prepared by anchoring novel [Cp*Ir(κ3-O,N,O-glyphosate)] (1; glyphosate=N-(phosphonomethyl)glycine) and [Cp*Ir(κ3-O,N,O-glyphosine)] (2; glyphosine=N,N-bis(phosphonomethyl)glycine) complexes onto rutile TiO2. Characterization in solution (NMR spectroscopy) and solid state (X-Ray diffractometry) indicates that 1 and 2 are stabilized by the two arms of the glycine fragment, whereas they have (2) or can easily generate (1 and 2) a dandling phosphonate arm suitable for their anchoring on TiO2. As a matter of fact, they exhibit rather elongated Ir–OP (1: 2.1405 Å, 2: 2.142 Å) and short Ir–OC (1: 2.0784 Å, 2: 2.086 Å) and Ir–N (1: 2.155 Å, 2: 2.207 Å) bonds. NMR spectra of 2 show a dynamic process that exchanges the two phosphonate moieties, consistently with an easy –OP detachment from Ir. Both molecular and heterogenized species were tested as catalysts in water oxidation (WO) driven by NaIO4. 1 (TOF up to 96 min−1) and 2 (26 min−1) proved to be effective molecular catalysts; more importantly, 1_TiO2 and 2_TiO2 showed very high activity (TOF ∼200 min−1), which remained rather constant over seven successive runs, even if substantial leaching of the noble metal in solution occurred during the first catalytic tests. TON was very high and limited only by the amount of NaIO4 used. These results show the potentialities of mixed carboxylate/phosphonate ligands for the development of highly active water oxidation catalysts.