Iridium Water Oxidation Catalysts Based on Pyridine‐Carbene Alkyl‐Substituted Ligands

Abstract

Iridium complexes bearing pyridine triazolylidene ligands with variable steric hindrance, derived by the presence of an R group (R=H, Me, Et, nPr, iPr, Bu, and Oct) onto the N1‐nitrogen, have been synthesized, fully characterized and tested as water oxidation catalysts (WOCs), using chemical sacrificial oxidants (CAN and NaIO4) or in photocatalytic experiments ([Ru(bpy)3]Cl2 as phosensitizer and Na2S2O8 as an electron acceptor). The catalytic activity is barely affected by the nature of R when WO is driven by NaIO4 (1 min−1<TOF<14 min−1) or light/[Ru(bpy)3]Cl2/Na2S2O8 (TOF≈0.15 min−1); on the contrary, a remarkable effect is observed with CAN. Particularly, in the latter case, complexes with R=H and Me exhibit similar activity (10 min−1<TOF<20 min−1), whereas all other complexes (R=Et, Pr, i‐Pr, Bu, and Oct) are significantly more active and exhibit comparable TOFs in the range 40–130 min−1. Thus, a marked discontinuity in performances occurs when passing from R=Me to R=Et. This can be hardly explained solely based on the previously proposed hypothesis, suggesting that an increased R‐dimension might favor the association of iridium complexes leading to the formation of highly active multimetallic species. An additional, more specific effect has to be present. It appears plausible that the steric hindrance introduced in close proximity of the iridium center, when R≥Et, hampers transfer of the hydroperoxo or peroxo moiety from iridium active species to cerium, a process that could slow down kinetics of O2 evolution.

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands

“…The Ir-N (2.26 Å) and Ir-O (2.11-2.12 Å) distances are comparable to those found in other Ir(III)-Cp* amine [95] and phosphonate [96] complexes. The Ir-Cp* centroid distance (1.758 Å) is slightly lower than that found in similar compounds (1.77-1.82 Å) [46,52,55,95,96], indicating a relatively high electrophilicity of the metal center.…”

“…Nevertheless, the aspiration of establishing clear structure/properties correlations remains largely unaccomplished, as in situ structural modifications of the ligands often occur under the strongly oxidative reaction conditions necessary for WO. A particularly interesting case is that of Ir-Cp* complexes (Cp*= pentamethylcyclopentadienyl), which include some of the known most active molecular WOCs [1,[45][46][47][48][49][50][51][52][53][54][55][56][57][58][59]. The Cp* fragment undergoes major oxidative transformations under catalytic conditions; these modifications are likely necessary, in some cases, for precatalyst activation, but complicate the identification of the true active species and, consequently, the rationalization of ancillary ligand effects on activity [20,[60][61][62][63][64][65][66].…”

Ir- and Ru-doped layered double hydroxides as affordable heterogeneous catalysts for electrochemical water oxidation

“…This is the reason why water oxidation (WO) has become one of the most studied reactions, over the last decades, especially as far as the development of a catalyst (C) for its acceleration is concerned. [1][2][3][4][5][6] Significant, but still technologically unsatisfactory, 7 results have been achieved by using homogeneous, [8][9][10][11][12][13][14][15][16][17] heterogenized, [18][19][20][21][22][23][24]25 and heterogeneous [26][27][28][29][30][31][32][33] WOCs. The best performances are usually obtained by using noble-metal (e.g.…”

Iridium-Doped Nanosized Zn–Al Layered Double Hydroxides as Efficient Water Oxidation Catalysts

“…In WO driven by NaIO 4 as a sacrificial oxidant (SO), Ir-LDHs of a micrometric dimension (about 0.3–1 μm) exhibit excellent turnover frequency (TOF) up to 113 min –1 and turnover numbers (TONs) > 11,900, 21 which are comparable to those of some leading molecular WOCs. 5 , 7 , 10 Furthermore, these heterogeneous systems are characterized by high stability and recyclability. Over eight successive runs, no leaching of noble metal to the liquid phase is detected by inductively coupled plasma–optical emission spectrometry (ICP–OES) analysis and no significant loss of activity is observed.…”