Water oxidation (WO) is a central reaction in the photo‐ and electro‐synthesis of fuels. Iridium complexes have been successfully exploited as water oxidation catalysts (WOCs) with remarkable performances. Herein, we report a systematic study aimed at benchmarking well‐known Ir WOCs, when NaIO4 is used to drive the reaction. In particular, the following complexes were studied: cis‐[Ir(ppy)2(H2O)2]OTf (ppy=2‐phenylpyridine) (1), [Cp*Ir(H2O)3]NO3 (Cp*=1,2,3,4,5‐pentamethyl‐cyclopentadienyl anion) (2), [Cp*Ir(bzpy)Cl] (bzpy=2‐benzoylpyridine) (3), [Cp*IrCl2(Me2‐NHC)] (NHC=N‐heterocyclic carbene) (4), [Cp*Ir(pyalk)Cl] (pyalk=2‐pyridine‐isopropanoate) (5), [Cp*Ir(pic)NO3] (pic=2‐pyridine‐carboxylate) (6), [Cp*Ir{(P(O)(OH)2}3]Na (7), and mer‐[IrCl3(pic)(HOMe)]K (8). Their reactivity was compared with that of IrCl3⋅n H2O (9) and [Ir(OH)6]2− (10). Most measurements were performed in phosphate buffer (0.2 m), in which 2, 4, 5, 6, 7, and 10 showed very high activity (yield close to 100 %, turnover frequency up to 554 min−1 with 10, the highest ever observed for a WO‐driven by NaIO4). The found order of activity is: 10>2≈4>6>5>7>1>9>3>8. Clues concerning the molecular nature of the active species were obtained, whereas its exact nature remains doubtfully.

A class of iron(III) complexes containing diversely substituted cross-bridged cyclams (1-R,R′: 1-Me,Me, 1-Me,Et, 1-Et,Et, 1-Me,Bn, 1-Bn,Bn) has been prepared and characterized.(1-R,R′) complexes catalyze the oxidation of alcohols to ketones in green conditions, by using tert-butylhydroperoxide as the oxidant in aqueous solutions. They also exhibit catalytic [a]

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.

Layered double hydroxides (LDHs) are an ideal platform to host catalytic metal centers for water oxidation (WO) owing to the high accessibility of water to the interlayer region, which makes all centers potentially reachable and activated. Herein, we report the syntheses of three iridium-doped zinc–aluminum LDHs (Ir-LDHs) nanomaterials ( 1–3 , with about 80 nm of planar size and a thickness of 8 nm as derived by field emission scanning electron microscopy and powder X-ray diffraction studies, respectively), carried out in the confined aqueous environment of reverse micelles, through a very simple and versatile procedure. These materials exhibit excellent catalytic performances in WO driven by NaIO 4 at neutral pH and 25 °C, with an iridium content as low as 0.5 mol % (∼0.8 wt %), leading to quantitative oxygen yields (based on utilized NaIO 4 , turnover number up to ∼10,000). Nanomaterials 1–3 display the highest ever reported turnover frequency values (up to 402 min –1 ) for any heterogeneous and heterogenized catalyst, comparable only to those of the most efficient molecular iridium catalysts, tested under similar reaction conditions. The boost in activity can be traced to the increased surface area and pore volume (>5 times and 1 order of magnitude, respectively, higher than those of micrometric materials of size 0.3–1 μm) estimated for the nanosized particles, which guarantee higher noble metal accessibility. X-ray absorption spectroscopy (XAS) studies suggest that 1–3 nanomaterials, as-prepared and after catalysis, contain a mixture of isolated, single octahedral Ir(III) sites, with no evidence of Ir–Ir scattering from second-nearest neighbors, excluding the presence of IrO 2 nanoparticles. The combination of the results obtained from XAS, elemental analysis, and ionic chromatography strongly suggests that iridium is embedded in the brucite-like structure of LDHs, having four hydroxyls and two chlorides as first neighbors. These results demonstrate that nanometric LDHs can be successfully exploited to engineer efficient WOCs, minimizing the amount of iridium used, consistent with the principle of the noble-metal atom economy.