The complex [Cp*Ir(pica)Cl] (1; pica = picolinamidate = κ 2 -pyridine-2-carboxamide) was found to be an effective catalyst for both water oxidation to molecular oxygen and NAD + /NADH transformations, which are the key reactions of light-dependent natural photosynthesis. In particular, 1 exhibits high activity in water oxidation driven by CAN and NaIO 4 . With the former, the initial TOF exceeds that of [Cp*Ir(pic)Cl] (2; pic = picolinate = κ 2 -pyridine-2-carboxylate), which is the fastest iridium catalyst reported to date, whereas with NaIO 4 it compares well with the best catalysts. 1 exhibits top performances also in the hydrogenation of NAD + with HCOOK, leading to the regiospecific formation of 1,4-NADH (pH 7) with TOF = 143 h −1 , which is about 3 times higher than the previous highest value (54 h −1 ) reported for [Cp*Ir(4-(1H-pyrazol-1-yl-κN 2 )benzoic acid)(H 2 O)]SO 4 (3). The activity seems to be critically affected by the presence of the NH functionality, as indicated by its drop of about 1 order of magnitude when 2 (TOF = 17 h −1 ) was used as the catalyst instead of 1. 1 is also able to mediate the dehydrogenation of β-NADH, under slightly acidic conditions, as determined by NMR and GC measurements. Furthermore, an in-depth investigation carried out combining 1D, 2D, and diffusion NMR techniques indicate a remarkable speciation of β-NADH leading not only to the expected β-NAD + but also to α-NAD + , nicotinamide (NA), and 1,2,5,6-tetrahydronicotinamide (NAH 3 ). The formation of NAH 3 has been identified as the cause of the low TON values obtained with 1 and 3, because it consumes part of the produced H 2 .

The development of an efficient water oxidation catalyst is crucial in the framework of constructing an artificial photo­(electro)­synthetic apparatus for the production of solar fuels. Herein, new hydroxy–pyridine–carboxylate iridium complexes are reported exhibiting high activity in water oxidation with both cerium ammonium nitrate and NaIO4 as sacrificial oxidants. With the latter, the catalytic activity strongly depends on the pH and position of the OH-substituent in the pyridine ring, reaching a record turnover frequency of 458 min–1 and turnover number (>14 500) limited only by the amount of NaIO4. Kinetic experiments measuring O2 evolution paralleled by NMR studies on oxidative transformation with NaIO4 suggest that Cp* of the catalyst is readily degraded, whereas the hydroxy–pyridine–carboxylate ligands remain coordinated at iridium, tuning its activity.

The generation of solar fuels by means of a photosynthetic apparatus strongly relies on the development of an efficient water oxidation catalyst (WOC). Cerium ammonium nitrate (CAN) is the most commonly used sacrificial oxidant to explore the potentiality of WOCs. It is usually assumed that CAN has the unique role to oxidatively energize WOCs, making them capable to offer a low-energy reaction pathway to transform H2O to O-2. Herein, we show that CAN might have a much more relevant and direct role in WO, mainly related to the capture and liberation of O-O-containing molecular moieties

Activating molecular catalysts at the surface of metal oxides can be a promising strategy to overcome the sluggish interfacial kinetics and enhance the efficiencies for photo­(electro)­chemical (PEC) water oxidation. However, the physical association between inorganic semiconductors for PEC process and organometallic molecular catalysts for surface catalytic reactions generally remains a challenging problem. In the present work, Kläui-type organometallic precursor [Cp*Ir­{P­(O)­(OH)2}3]Na was first synthesized and subsequently successfully anchored onto BiVO4 nanopyramids grown on transparent conducting substrates through various procedures. Treating the resulting hybrid heteronanostructure with IO4 – induces a strong synergism between iridium atoms and BiVO4 nanocrystals that exhibits a 5.5 times enhancement in photocurrent density at 1.23 V vs reversible hydrogen electrode (RHE) for PEC water oxidation. This simple approach provides an effective alternative pathway for molecular catalysts anchoring on inorganic semiconductors for efficient renewable energy utilization.

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.