Water Oxidation Catalyzed by Strong Carbene‐Type Donor‐Ligand Complexes of Iridium

Abstract

Think oxygen: Iridium(III) complexes containing an unusually bound chelating triazolylidene ligand (see picture) show excellent activity towards water oxidation, producing hundreds of milliliters or O2 per milligram of iridium. The active catalysts include either an ylide or an unusually bound pyridylidene as the chelating L group and are readily accessible by click chemistry.

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

“…For example, the iridium complexes 42 and 44 tolerate aqueous 1 M HCl for several days. 23,50 Under these conditions, no traces of triazolium salt are observed, demonstrating the high resistance of the M-C trz bond towards acidolysis. Hence, the C trz -Ir bond and also the iridium-75 pyridylidene bond are substantially more robust towards protonation than the C aryl -Ir bond.…”

“…9 The chemistry of these abnormal/mesoionic carbenes has predominantly focused on imidazol-4-ylidenes (H) and has led to new reactivity patterns. 10 A recent addition to this subclass of NHC ligands are 1,2,3-triazolylidenes (I), 11 which 50 result from formal substitution of the C(2)-R unit in imidazolylidenes by nitrogen. The triazolium precursors are readily available and convert smoothly to the abnormal/mesoionic carbene complexes upon metallation procedures similar to those established for Arduengo-type NHCs.…”

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

“…5 Over the last few years, iridium complexes have emerged as very powerful catalysts for this water oxidation process. [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.…”

“…[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.…”

Tunable single-site ruthenium catalysts for efficient water oxidation

“…11 Moreover, mononuclear iridium triazolylidene complexes have shown to be highly active in water oxidation, reaching turnover numbers (TONs) around 10,000. 12 Here we report on the expansion of this approach to monometallic ruthenium complexes containing an easily tunable chelating triazolylidene ligand functionalized with a pyridyl donor. Tailoring of the catalytic activity, still rare in water oxidation catalysis, is readily achieved by appropriate modification of the substituent at the triazolylidene nitrogens.…”