Antoni Llobet

Molecular Catalysts that Oxidize Water to Dioxygen

During the past four years we have witnessed a revolution in the field of water-oxidation catalysis, in which well-defined molecules are opening up entirely new possibilities for the design of more rugged and efficient catalysts. This revolution has been stimulated by two factors: the urgent need for clean and renewable fuel and the intrinsic human desire to mimic natures reactions, in this case the oxygen-evolving complex (OEC) of the photosystem II (PSII). Herein we give a short general overview of the established basis for the oxidation of water to dioxygen as well as presenting the new developments in the field. Furthermore, we describe the new avenues these developments are opening up with regard to catalyst design and performance, together with the new questions they pose, especially from a mechanistic perspective. Finally the challenges the field is facing are also discussed.

Oxygen−Oxygen Bond Formation Pathways Promoted by Ruthenium Complexes

The photoproduction of hydrogen from water and sunlight represents an attractive means of artificial energy conversion for a world still largely dependent on fossil fuels. A practical technology for producing sun-derived hydrogen remains an unachieved goal, however, and is dependent on developing a better understanding of the key reaction, the oxidation of water to dioxygen. The molecular complexity of this process is such that sophisticated transition metal complexes, which can access low-energy reaction pathways, are considered essential as catalysts. Complexes based on Mn, Co, Ir, and Ru have been described recently; a variety of ligands and nuclearities that comprise many complex topologies have been developed, but very few of them have been studied from a mechanistic perspective. One step in particular needs to be understood and better characterized for the transition-metal-catalyzed oxidation of water to dioxygen, namely, the circumstances under which the formation of O-O bonds can occur. Although there is a large body of work related to the formation of C-C bonds promoted by metal complexes, the analogous literature for O-O bond formation is practically nonexistent and just beginning to emerge. In this Account, we describe the sparse literature existing on this topic, focusing on the Ru-aqua complexes. These complexes are capable of reaching high oxidation states as a result of the sequential and simultaneous loss of protons and electrons. A solvent water molecule may or may not participate in the formation of the O-O bond; accordingly, the two main pathways are named (i) solvent water nucleophilic attack (WNA) and (ii) interaction of two M-O units (I2M). Most of the complexes described belong to the WNA class, including a variety of mononuclear and polynuclear complexes containing one or several Ru-O units. A common feature of these complexes is the generation of formal oxidation states as high as Ru(V) and Ru(VI), which render the oxygen atom of the Ru-O group highly electrophilic. On the other hand, only one symmetric dinuclear complex that undergoes an intramolecular O-O bond formation step has been described for the I2M class; it has a formal oxidation state of Ru(IV). A special section is devoted to Ru-OH(2) complexes that contain redox active ligands, such as the chelating quinone. These ligands are capable of undergoing reversible redox processes and thus generate a complex but fascinating electron-transfer process between the metal and the ligand. Despite the intrinsic experimental difficulties in determining reaction mechanisms, progress with these Ru complexes is now beginning to be reported. An understanding of recent successes, as well as pitfalls, is essential in the search for a practical water oxidation catalyst.

Water Oxidation at a Tetraruthenate Core Stabilized by Polyoxometalate Ligands: Experimental and Computational Evidence To Trace the Competent Intermediates

Converging UV-vis, EPR, rRaman, and DFT calculations highlight the evolution of [Ru(4)(H(2)O)(4)(mu-O)(4)(mu-OH)(2)(gamma-SiW(10)O(36))(2)](10-), 1, to high-valent intermediates. In analogy with the natural enzyme, five different oxidation states, generated from 1, have been found to power the catalytic cycle for water oxidation. A high electrophilic tetraruthenium(V)-hydroxo species is envisaged as the competent intermediate, undergoing nucleophilic attack by an external water molecule as a key step in the formation of a new O-O bond under catalytic conditions.

Molecular water oxidation mechanisms followed by transition metals: state of the art.

One clean alternative to fossil fuels would be to split water using sunlight. However, to achieve this goal, researchers still need to fully understand and control several key chemical reactions. One of them is the catalytic oxidation of water to molecular oxygen, which also occurs at the oxygen evolving center of photosystem II in green plants and algae. Despite its importance for biology and renewable energy, the mechanism of this reaction is not fully understood. Transition metal water oxidation catalysts in homogeneous media offer a superb platform for researchers to investigate and extract the crucial information to describe the different steps involved in this complex reaction accurately. The mechanistic information extracted at a molecular level allows researchers to understand both the factors that govern this reaction and the ones that derail the system to cause decomposition. As a result, rugged and efficient water oxidation catalysts with potential technological applications can be developed. In this Account, we discuss the current mechanistic understanding of the water oxidation reaction catalyzed by transition metals in the homogeneous phase, based on work developed in our laboratories and complemented by research from other groups. Rather than reviewing all of the catalysts described to date, we focus systematically on the several key elements and their rationale from molecules studied in homogeneous media. We organize these catalysts based on how the crucial oxygen-oxygen bond step takes place, whether via a water nucleophilic attack or via the interaction of two M-O units, rather than based on the nuclearity of the water oxidation catalysts. Furthermore we have used DFT methodology to characterize key intermediates and transition states. The combination of both theory and experiments has allowed us to get a complete view of the water oxidation cycle for the different catalysts studied. Finally, we also describe the various deactivation pathways for these catalysts.

Synthesis, Electronic Structure, and Magnetism of (Ni(6-Mes) 2 ) + :A Two-Coordinate Nickel(I) Complex Stabilized by Bulky N‑Heterocyclic Carbenes

The two-coordinate cationic Ni(I) bis-N-heterocyclic carbene complex [Ni(6-Mes)2]Br (1) [6-Mes =1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene] has been structurally characterized and displays a highly linear geometry with a C-Ni-C angle of 179.27(13)°. Density functional theory calculations revealed that the five occupied metal-based orbitals are split in an approximate 2:1:2 pattern. Significant magnetic anisotropy results from this orbital degeneracy, leading to single-ion magnet (SIM) behavior.

Ru complexes that can catalytically oxidize water to molecular dioxygen.

The main objective of this review is to give a general overview of the structure, electrochemistry (when available), and catalytic performance of the Ru complexes, which are capable of oxidizing water to molecular dioxygen, and to highlight their more relevant features. The description of the Ru catalysts is mainly divided into complexes that contain a Ru-O-Ru bridging group and those that do not. Finally a few conclusions are drawn from the global description of all of the catalysts presented here, and some guidelines for future catalyst design are given.

Oxygen-oxygen bond formation by the Ru-Hbpp water oxidation catalyst occurs solely via an intramolecular reaction pathway.

A thorough kinetics investigation of the Ru-Hbpp water oxidation catalyst has been carried out at temperatures in the range 10-40 degrees C. Four oxidative electron-transfer processes that take the catalyst from its initial II,II oxidation state up to the formal IV,IV oxidation state were kinetically characterized and the corresponding activation parameters determined. Once the IV,IV oxidation state is reached, two additional slower kinetic processes take place, corresponding to the formation of an intermediate that finally evolves oxygen and regenerates the initial Ru-Hbpp catalyst. These two kinetic processes were also fully characterized with respect to the evaluation of their rate constants and activation parameters. Furthermore, (18)O labeling experiments were performed with different degrees of labeled catalyst and solvent, and the (16)O(2)/(16)O(18)O/(18)O(2) isotopic distribution of the generated molecular oxygen was calculated. These results clearly point to the existence of a single intramolecular reaction pathway for the formation of the oxygen-oxygen bond in the case of the Ru-Hbpp catalyst.

Fenton reagents (1:1 FeIILx/HOOH) react via [LxFeIIOOH(BH+)] (1) as hydroxylases (RH → ROH), not as generators of free hydroxyl radicals (HO.)

The one-to-one combination of hydrogen peroxide and FeII(PA)2 [PAH = picolinic acid (2-carboxyl-pyridine)], a Fenton reagent, forms an adduct \( \{ {\text{F}}{{\text{e}}^{{\text{II}}}}{({\text{PA}})_2} + {\text{HOOH}}\xrightarrow[{{{({\text{py}})}_2}{\text{HOAc}}}]{{{\text{k}},{\text{2}} \times {\text{1}}{{\text{0}}^3}{{\text{M}}^{ - 1}}{{\text{s}}^{ - 1}}}}[{({\text{PA}})_2}^ - {\text{F}}{{\text{e}}^{{\text{II}}}}{\text{OOH}} + {\text{py}}{{\text{H}}^ + }](1)\} \) that is (py)2HOAc the primary reactant with (a) excess FeII(PA)2 to give two (PA)2FeIIIOH, (b) excess HOOH to give O2 plus two H2O, and (c) excess cyclohexane (C-C6H12) to give (c-C6H11)py (or c-C6H11OH in the absence of pyridine). The presence of a carbon-radical trap (PhSeSePh) with the organic substrate yields c-C6H11SePh with an efficiency of >90% (relative to HOOH). This Fenton reagent has reactivity ratios (kA/kB) [in comparison with free HO·] with c-C6H12/c-C6D12 (KIE) of 1.7 [versus 1.0], with l°/2°/3° carbon centers (per C-H, normalized) of 0.07/0.44/1.0 [0.41/0.50/1.0], and with c-C6H12/PhCH2CH3 of 2.0 [0.6]. In the presence of O2 (1 atm, 3.4 mM) 1 forms an adduct [1(O2), 5], which reacts with c-C6H12 to give c-C6H10(O) (KIE, 2.1), cyclohexene (c-C6H10) to give c-C6H8(O), and PhCH2CH3 to give PhC(O)CH3 (reactivity ratio for c-C6H12/PhCH2CH3, 0.6). Similar chemistry results for HOOH/O2 with FeII(bpy)22+ [reactivity ratio for c-C6H12/PhCH2CH3 (0.4)], FeII(OPPh3)42+ (1.0), and CuI(bpy) 2 + (1.0)]. All of this is compelling evidence that Fenton reagents do not produce (a) free hydroxyl radicals (HO·) or (b) free carbon radicals (R·), but (c) can exhibit catalytic turnovers with respect to HOOH and O2.

Facile C−H Bond Cleavage via a Proton-Coupled Electron Transfer Involving a C−H···CuII Interaction

The present study provides mechanistic details of a mild aromatic C-H activation effected by a copper(II) center ligated in a triazamacrocylic ligand, affording equimolar amounts of a Cu(III)-aryl species and Cu(I) species as reaction products. At low temperatures the Cu(II) complex 1 forms a three-center, three-electron C-H...Cu(II) interaction, identified by pulse electron paramagnetic resonance spectroscopy and supported by density functional theory calculations. C-H bond cleavage is coupled with copper oxidation, as a Cu(III)-aryl product 2 is formed. This reaction proceeds to completion at 273 K within minutes through either a copper disproportionation reaction or, alternatively, even faster with 1 equiv of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), quantitatively yielding 2. Kinetic studies of both reactions strongly implicate a rate-limiting proton-coupled electron transfer as the key C-H activation step, a mechanism that does not conform to the C-H activation mechanism in a Ni(II) analogue or to any previously proposed C-H activation mechanisms.

The cis‐[RuII(bpy)2(H2O)2]2+ Water‐Oxidation Catalyst Revisited

The only operating mechanism in the oxidation of water to dioxygen catalyzed by the mononuclear cis-[RuII(bpy)2(H2O)2]2+ complex when treated with excess CeIV was unambiguously established. Theoretical calculations together with 18O-labeling experiments (see plot) revealed that it is the nucleophilic attack of water on a Ru=O group.