Carbene iridium complexes for efficient water oxidation: scope and mechanistic insights

Abstract

Iridium complexes of Cp* and mesoionic carbene ligands were synthesized and evaluated as potential water oxidation catalysts using cerium ammonium nitrate as a chemical oxidant. Performance was evaluated by turnover frequency at 50% conversion and by absolute turnover number, and the most promising precatalysts were subjected to further study. Molecular turnover frequencies varied from 190 to 451 per hour with a maximum turnover number of 38,000. While the rate of oxygen evolution depends linearly on iridium concentration, 10 concurrent spectroscopic and manometric monitoring of stoichiometric additions of oxidant suggests oxygen evolution occurs as two sequential first order reactions. Under the conditions herein, the oxygen evolving species appears to be well defined and molecular based on the kinetic effects of careful ligand design, reproducibility, and the absence of persistent dynamic light scattering signals. Outside of these conditions, the complex mechanism is highly dependent on reaction conditions. While confident characterization of the catalytically active species is difficult, especially under high-turnover conditions, this work indicates IrOx is not essential for the 15 formation of catalytically active water oxidation species.

Ligand Exchange and Redox Processes in Iridium Triazolylidene Complexes Relevant to Catalytic Water Oxidation

“…19 All electrochemical experiments were performed with a Bio-logic SP-150 potentiostat using a three-electrode cell composed of a glassy carbon disk (3 mm diameter, CH instruments) as working electrode, a platinum disk were titrated with NaOH (1 M) to the desired pH; for pH >1.5, aqueous solutions were prepared using different phosphate, acetate, or carbonate buffers at 0.1 M ionic strength and their detailed preparation is included in the supporting information. UV measurements were performed with a Cary 50-UV-Vis Agilent spectrophotometer.…”

“…[6][7][8][9][10][11][12][13][14][15][16][17][18] Depending on the ligand design, very high turnover numbers have been achieved. 19 Moreover, kinetic and mechanistic studies have provide increasingly compelling evidence that some complexes are precursors for homogeneous rather than heterogeneous [20][21][22] water oxidation catalysts and that the oxidation therefore occurs at an iridium center that is in a well-defined environment. 19,[23][24][25][26][27][28] This environment has remained elusive up to now despite various efforts to trap and isolate catalytically competent species.…”

“…19 Moreover, kinetic and mechanistic studies have provide increasingly compelling evidence that some complexes are precursors for homogeneous rather than heterogeneous [20][21][22] water oxidation catalysts and that the oxidation therefore occurs at an iridium center that is in a well-defined environment. 19,[23][24][25][26][27][28] This environment has remained elusive up to now despite various efforts to trap and isolate catalytically competent species. [29][30][31][32] In particular iridium cyclopentadienyl complexes [Ir(Cp*)(L,L)X] + containing a chelating N,N-, C,N-, or C,Cbidentate ligand motive afforded high catalytic activity (Cp* = = C 5 Me 5 -).…”

“…[29][30][31][32] In particular iridium cyclopentadienyl complexes [Ir(Cp*)(L,L)X] + containing a chelating N,N-, C,N-, or C,Cbidentate ligand motive afforded high catalytic activity (Cp* = = C 5 Me 5 -). [7][8][9]17,19,33,34 Postreaction analyses strongly indicate oxidative degradation of the Cp* ligand 27,32,33 rather than formation of IrO x nanoparticles as a heterogeneous catalyst phase. These reactivity pattern underline the relevance of initial steps of transformation at the iridium center as a concept to understand and improve catalytic activity and to reduce the catalyst activation barrier.…”

Application of 1,2,3-triazolylidenes as versatile NHC-type ligands: synthesis, properties, and application in catalysis and beyond

“…Turnover frequencies with this complex are improved to 450 h -1 (cf 280 h -1 for 44 and 190 h -1 for 45). 66 Kinetic studies and DLS analyses, paired with the strong dependence of the catalytic activity on the ligand setup 35 strongly suggest that the active catalyst is a well-defined molecular species.…”