Lluis Escriche

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.

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.

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.

Synthesis, Structure, and Reactivity of New Tetranuclear Ru-Hbpp-Based Water-Oxidation Catalysts

The preparation of three new octadentate tetranucleating ligands made out of two Ru-Hbpp-based units [where Hbpp is 3,5(bispyridyl)pyrazole], linked by a xylyl group attached at the pyrazolate moiety, of general formula (Hbpp)(2)-u-xyl (u = p, m, or o) is reported, together with its dinucleating counterpart substituted at the same position with a benzyl group, Hbpp-bz. All of these ligands have been characterized with the usual analytical and spectroscopic techniques. The corresponding tetranuclear ruthenium complexes of general formula {[Ru(2)(trpy)(2)(L)](2)(μ-(bpp)(2)-u-xyl)}(n+) [L = Cl or OAc, n = 4; L = (H(2)O)(2), n = 6] and their dinuclear homologues {[Ru(2)(trpy)(2)(L)](μ-bpp-bz)}(n+) [L = Cl or OAc, n = 2; L = (H(2)O)(2), n = 3] have also been prepared and thoroughly characterized both in solution and in the solid state. In solution, all of the complexes have been characterized spectroscopically by UV-vis and NMR and their redox properties investigated by means of cyclic voltammetry techniques. In the solid state, monocrystal X-ray diffraction analysis has been carried out for two dinuclear complexes {[Ru(2)(trpy)(2)(L)](μ-bpp-bz)}(2+) (L = Cl and OAc) and for the tetranuclear complex {[Ru(2)(trpy)(2)(μ-OAc)](2)(μ-(bpp)(2)-m-xyl)}(4+). The capacity of the tetranuclear aqua complexes {[Ru(2)(trpy)(2)(H(2)O)(2)](2)(μ-(bpp)(2)-u-xyl)}(6+) and the dinuclear homologue {[Ru(2)(trpy)(2)(H(2)O)(2)](μ-bpp-bz)}(3+) to act as water-oxidation catalysts has been evaluated using cerium(IV) as the chemical oxidant in pH = 1.0 triflic acid solutions. It is found that these complexes, besides generating significant amounts of dioxygen, also generate carbon dioxide. The relative ratio of [O(2)]/[CO(2)] is dependent not only on para, meta, or ortho substitution of the xylylic group but also on the concentration of the starting materials. With regard to the tetranuclear complexes, the one that contains the more sterically constrained ortho-substituted ligand generates the highest [O(2)]/[CO(2)] ratio.

Silver(I) ion-selective electrodes based on polythiamacrocycles

Some polythiamacrocycles having aromatic rings as macrocyclic components have been synthesised and tested as sensors in solid-state electrodes. High selectivity for Ag+, even over that for Hg2+, and excellent electrode properties have been found.

Perchlorate-selective MEMFETs and ISEs based on a new phosphadithiamacrocycle

Abstract A new phosphadithiamacrocycle has been synthesized and used as neutral carrier in ion-selective PVC membranes. These membranes have been applied to the development of perchlorate-selective MEMFETs and ISEs. Both devices have shown Nernstian response and a wide working pH range. The response and selectivity found for perchlorate ions is better than those of conventional Cl O 4 − electrodes based on hydrophobic cations as electroactive species.

Silver(I), mercury(II) and copper(I) complexes of acyclic and macrocyclic dithioether, metaxylyl based ligands

Abstract The compounds [HgL 1 (NO 3 ) χ ] (L 1 = 6-oxa-3,9-dithiabicyclo[9,3,1] pentadeca-l(15),11,13-triene) ( 1 ), [AgL 2 (NO 3 )} χ ] (L 2 = (1,3-bis(phenylthio)methyl) benzene) ( 2 ), [HgL 3 Cl 2 2 ] and [CuL 3 Cl} χ ] (L 3 = (1,3-Bis(ethylthio)methyl)benzene) ( 3 ) and ( 4 ) have been synthesized and their structures determined by X-ray crystallography. Compounds ( 1 ) and ( 2 ) have linear polymeric chains of alternating trigonal silver(I) cations and bridging ligands bonded through the sulfur atoms. In both structures the nitrate anion retains coordination to the silver atom. Compound ( 3 ) is a dimeric complex. Each mercury(II) cation is bound to two bridging chlorine atoms, one terminal chlorine atom and one sulfur atom from the ligand. Compound ( 4 ) is a threedimensional polymer. Each Cu 1 cation has a distorted tetrahedral environment defined by two sulfur atoms of different L 3 ligands and two bridging chlorine atoms. In each case the dithioether ligand acts as a bridging or monodentate ligand and does not chelate the metal.

exo-Dithio and monothio carborane derivatives: a mechanism for their partial degradation. Molecular structure of tetramethylammonium 7,8-(3′,6′,9′-trioxaundecane-1′, 11′-dithiolato-SS′)-7,8-dicarba-nido-undecaborate

Abstract The reaction of 1,2-dithiol-o-carborane, under basic conditions, with appropriate organic dihalogenated compounds results in the formation of macrocycles incorporating 7,8-dicarba-nido-undecaborate or 1,2- dicarba-closo-dodecaborane units. The synthesis of some non-cyclic dicarborane compounds containing only one sulfur per cage directly bonded to the cluster is described. On the basis of the different behavior of the dithiolato compounds versus the monothiolato, a mechanism that explains the formation of partially degraded and non-degraded compounds is proposed. The molecular structure of tetramethylammonium 7,8-(3′,6′,9′-trioxaundecane-1′,11′ -dithiolato-SS′)-dicarba-nido-undecaborate has been determined. It crystallizes in space group P21/c with 4 formula units per cell. The cell dimensions are a=14.058(7), b=7.536(2), c=22.980(9) A, β=99.41°.

Synthesis and characterization of cyclopalladated and non-cyclopalladated complexes of ligands containing the 1,3-bis(thiomethyl)benzene unit

Abstract The reactivity of [Pd(CH 3 CN) 2 )Cl 2 ] toward the ligands 1,3-bis(ethylthiomethyl)benzene (HL 1 ), 1,3-bis(benzylthiomethyl)benzene HL 2 ), 1,3-bis(phenylthiomethyl) benzene (HL 3 ) and dimethyl 2,2′-[1,3-phenylenebis(methylthio)]-dibenzoate (HL 4 ) has been investigated. In all cases the reaction of the palladium(II) salt with the corresponding ligand in acetonitrile at room temperature afforded the simple non-cyclopalladated complex. Analytical and spectroscopic data of these complexes suggest that the two sulfur atoms of the ligands and two chlorine atoms coordinate simultaneosly to the palladium ion defining monomeric species. Cyclopalladated complexes of ligands HL 1 -HL 4 have been obtained by refluzing their non-cyclopalladated derivatives in acetonitrile or by direct reaction of the starting materials in the same solvent at refluxing temperature. Different reaction times have been found depending on the ligand. Cyclopalladated complexes of ligands HL 1 , HL 2 and HL 4 have been characterized by X-ray diffraction methods. All of them display square-planar coordination around the palladium atom provided by the rigid S 2 C chelating moiety of the 1,3-bis(thiomethyl)benzene unit and the chlorine atom which is located in a trans position with respect to the PdC bond.

Simple sensor molecules for detection of silver(I) based on monothioethers

Polyvinyl chloride (PVC) membrane all solid-state electrodes based on the thioethers diethyl sulfide, ethyl phenyl sulfide or diphenyl sulfide have been developed exhibiting high selectivity for silver(I), with low detection limits and short response times.