Isolating an Inner-Sphere Adduct of [RuIV(═O)(terpy)(bpy)]2+ and [CeIV(OH)(NO3)5]2- with the Oxo Bonded to the CeIV Center.

Water oxidation is a key to achieving sustainable energy cycles, for which higher-valent metal-oxo species often play a key role to accelerate the rate-limiting O-O bond formation. The present study undertook efforts to clarify one of the steps postulated for the water oxidation (WO) catalyzed by [RuII(terpy)(bpy)(OH2)]2+ (terpy = 2,2':6',6″-terpyridine, bpy = 2,2'-bipyridine). This study focuses on inner-sphere electron transfer for the CeIV-driven oxidation of the RuIV═O species into the RuV═O species. The approach to this step became possible by inventing a feasible method to isolate an air-stable RuIV═O powder sample in this work. Importantly, by mixing the thus-obtained RuIV═O sample with CAN (cerium ammonium nitrate), the inner-sphere adduct [RuIV(═O)(terpy)(bpy)][CeIV(NO3)5(OH)] was successfully isolated. The IR spectrum of the isolated adduct exhibits a strong band at 774 cm-1 attributable to the RuIV═O-CeIV stretching vibration, proving covalent bonding of the oxo to the CeIV center. Furthermore, the absorption spectrum of this greenish black powder shows a broad absorption band at 600 nm, suggesting a charge transfer transition from the π* orbital of RuIV═O to the 4f orbital of CeIV, as supported by TD-DFT calculations. The addition of one equivalent of CAN to the RuIV═O solution induces the spectral change due to formation of the 1:1 adduct identical to the isolated adduct. Our study provides a clue to the formation of an inner-sphere adduct having a RuIV═O-CeIV core in the CeIV-driven WO catalysis.

Improving singlet oxygen resistance during photochemical water oxidation by cobalt porphyrin catalysts.

Enabling the production of solar fuels on a global scale through artificial photosynthesis requires the development of water oxidation catalysts with significantly improved stability. The stability of photosystems is often reduced owing to attack by singlet oxygen, which is produced during light harvesting. Here, we report photochemical water oxidation by CoFPS, a fluorinated Co-porphyrin designed to resist attack by singlet oxygen. CoFPS exhibits significantly improved stability relative to its non-fluorinated analogue, as shown by a large increase in turnover numbers. This increased stability results from resistance of CoFPS to attack by singlet oxygen, the formation of which was monitored in situ by using 9,10-diphenylanthracene as a chemical probe. Dynamic light scattering (DLS) confirms that CoFPS remains homogeneous, proving its stability during water oxidation catalysis.

Comparison of primary oxidants for water-oxidation catalysis.

In this tutorial review, we compare chemical oxidants for driving water-oxidation catalysts, focusing on the advantages and disadvantages of each oxidant.

Precursor transformation during molecular oxidation catalysis with organometallic iridium complexes.

We present evidence for Cp* being a sacrificial placeholder ligand in the [Cp*Ir(III)(chelate)X] series of homogeneous oxidation catalysts. UV-vis and (1)H NMR profiles as well as MALDI-MS data show a rapid and irreversible loss of the Cp* ligand under reaction conditions, which likely proceeds through an intramolecular inner-sphere oxidation pathway reminiscent of the reductive in situ elimination of diolefin placeholder ligands in hydrogenation catalysis by [(diene)M(I)(L,L')](+) (M = Rh and Ir) precursors. When oxidatively stable chelate ligands are bound to the iridium in addition to the Cp*, the oxidized precursors yield homogeneous solutions with a characteristic blue color that remain active in both water- and CH-oxidation catalysis without further induction period. Electrophoresis suggests the presence of well-defined Ir-cations, and TEM-EDX, XPS, (17)O NMR, and resonance-Raman spectroscopy data are most consistent with the molecular identity of the blue species to be a bis-µ-oxo di-iridium(IV) coordination compound with two waters and one chelate ligand bound to each metal. DFT calculations give insight into the electronic structure of this catalyst resting state, and time-dependent simulations agree with the assignments of the experimental spectroscopic data. [(cod)Ir(I)(chelate)] precursors bearing the same chelate ligands are shown to be equally effective precatalysts for both water- and CH-oxidations using NaIO4 as chemical oxidant.

Sodium periodate as a primary oxidant for water-oxidation catalysts.

Sodium periodate was characterized as a primary chemical oxidant for the catalytic evolution of oxygen at neutral pH using a variety of water-oxidation catalysts. The visible spectra of solutions formed from Cp*Ir(bpy)SO(4) during oxygen-evolution catalysis were measured. NMR spectroscopy suggests that the catalyst remains molecular after several turnovers with sodium periodate. Two of our [Cp*Ir(bis-NHC)][PF(6)](2) complexes, along with other literature catalysts, such as the manganese terpyridyl dimer, Hill's cobalt polyoxometallate, and Meyer's blue dimer, were also tested for activity. Sodium periodate was found to function only for water-oxidation catalysts with low overpotentials. This specificity is attributed to the relatively low oxidizing capability of sodium periodate solutions relative to solutions of other common primary oxidants. Studying oxygen-evolution catalysis by using sodium periodate as a primary oxidant may, therefore, provide preliminary evidence that a given catalyst has a low overpotential.

Electrochemical properties and C-H bond oxidation activity of [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)].

[Ru(tpy)(pyalk)Cl]Cl (pyalk = 2-(2'-pyridyl)-2-propanol) was synthesized and characterized crystallographically and electrochemically. Upon dissolution in water and acetonitrile, [Ru(tpy)(pyalk)Cl]Cl was found to form [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)]+, respectively. The Ru(ii/iii) couple of [Ru(tpy)(pyalk)Cl]+ was found to be relatively low compared to that of other Ru complexes in acetonitrile, but the Ru(iii/iv) couple was not significantly different than other Ru complexes bearing anionic ligands. Pourbaix diagrams were generated for [Ru(tpy)(phpy)(OH2)]+ (phpy = 2-phenylpyridine) and [Ru(tpy)(pyalk)(OH)]+ in water, and it was found that [Ru(tpy)(pyalk)(OH)]+ has a lower Ru(ii/iii) potential than [Ru(tpy)(phpy)(OH2)]+ under neutral to alkaline pH. [Ru(tpy)(pyalk)(OH)]+ was found to catalyze C-H bond hydroxylation of secondary alkanes and epoxidation of alkenes using cerium(iv) ammonium nitrate as the primary oxidant.

Light-induced water oxidation catalyzed by an oxido-bridged triruthenium complex with a Ru-O-Ru-O-Ru motif.

A µ-oxido-bridged triruthenium complex (RuT(2+)), formed by air-oxidation of a previously reported monoruthenium water oxidation catalyst (WOC), serves as an efficient photochemical WOC with the turnover frequency (TOF) and turnover number (TON) 0.90 s(-1) and 610, respectively. The crystal structures of RuT(2+) and its one-electron oxidized RuT(3+) are also reported.

Mechanism of water oxidation by non-heme iron catalysts when driven with sodium periodate.

Iron tris(2-methylpyridyl)amine (TPA) and iron 1-(bis(2-methylpyridyl)amino)-2-methyl-2-propanoate (BPyA) salts are characterized as water oxidation catalysts (WOCs) using sodium periodate. Under the conditions used, these complexes serve as homogeneous WOCs as demonstrated via kinetic analysis and dynamic light scattering (DLS). The Fe(BPyA) salt serves as both a mononuclear and dinuclear catalyst, with the mononuclear form showing higher catalytic activity. Based on the H/D kinetic isotope effect and pH dependence, the rate determining step (RDS) in water oxidation (WO) by Fe(BPyA) is nucleophilic attack by water during O-O bond formation. In contrast, Fe(TPA) shows complex kinetic behavior due to the formation of multiple oxidation states of the complex in solution, each of which exhibits catalytic activity for WO. The RDS in WO by Fe(TPA) follows an equilibrium established between monomeric and dimeric forms of the catalyst. Under acidic conditions formation of the monomer is favored, which leads to an increase in the WO rate.

Wilkinson's iridium acetate trimer as a water-oxidation catalyst.

The catalytic water-oxidation activity of Wilkinson's iridium acetate trimer (1) has been characterized electrochemically and by using chemical oxidants. We show that 1 can function as an operationally homogeneous water-oxidation catalyst when driven with sodium periodate as a primary oxidant, but rapidly decomposes using Ce(IV) as a primary oxidant.