Jordi García-Antón

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.

Influence of the self-organization of ionic liquids on the size of ruthenium nanoparticles: effect of the temperature and stirring

The size of ruthenium nanoparticles is governed by the degree of self-organization of the imidazolium based ionic liquid in which they are generated from (η4-1,5-cyclooctadiene)(η6-1,3,5-cyclooctatriene)ruthenium: the more structured the ionic liquid, the smaller the size.

Synthesis of New PdII Complexes Containing Thioether−Pyrazole Hemilabile Ligands − Structural Analysis by 1H and 13C NMR Spectroscopy and Crystal Structures of [PdCl2(bddo)] and [Pd(bddo)](BF4)2 [bddo = 1,8-Bis(3,5-dimethyl-1-pyrazolyl)-3,6-dithiaoctane]

Treatment of the ligands 1,6-bis(3,5-dimethyl-1-pyrazolyl)-2,5-dithiahexane (bddh), 1,7-bis(3,5-dimethyl-1-pyrazolyl)-2,6-dithiaheptane (bddhp), 1,8-bis(3,5-dimethyl-1-pyrazolyl)-3,6-dithiaoctane (bddo) and 1,9-bis(3,5-dimethyl-1-pyrazolyl)-3,7-dithianonane (bddn) with [PdCl2(CH3CN)2] produces [PdCl2(L)] or [Pd2Cl4(L)] complexes, depending on the stoichiometry. Treatment of the complexes [PdCl2(bddo)] and [PdCl2(bddn)] with AgBF4 gives [Pd(bddo)](BF4)2 and [Pd(bddn)](BF4)2. These PdII complexes have been characterised by elemental analyses, conductivity measurements, IR and 1H and 13C NMR spectroscopy where possible. The X-ray structures of the complexes [PdCl2(bddo)] and [Pd(bddo)](BF4)2 have been determined. In [PdCl2(bddo)] the thioether−pyrazole ligand is coordinated through the azine nitrogen atoms to the metal atom, which completes its coordination with two chloride ions in a trans disposition. In [Pd(bddo)](BF4)2 the metal atom is tetracoordinated by the two thioether sulfur atoms and the two azine nitrogen atoms of the pyrazole rings. Complexes [PdCl2(bddo)] and [PdCl2(bddn)] were obtained again when the complexes [Pd(bddo)](BF4)2 and [Pd(bddn)](BF4)2 were heated under reflux in a solution of Et4NCl in CH2Cl2/MeOH (1:1). (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Synthesis, X-ray crystal structure, and NMR characterisation of thiolate-bridged dinuclear Ni(II), Pd(II) and Pt(II) complexes of didentate ligands with NS-donor set

Abstract Thiolate-bridged dinuclear nickel(II), palladium(II) and platinum(II) complexes with N-(2-mercaptoethyl)-3,5-dimethylpyrazole (Hmed), [MCl(med)]2 (M=Ni (1), Pd (2), Pt (3)), have been synthesised and characterised by elemental analyses, conductivity, IR, electronic spectra and NMR spectroscopies. The crystal structure of 2 was determined by a single-crystal X-ray diffraction method. The structure consists of thiolate-bridged dinuclear units. Each Pd(II) atom is coordinated by a pyrazolic nitrogen, one chlorine and two bridging sulfur atoms. When the synthesis of complex 1 was carried out in acetonitrile and in the presence of oxygen, [NiCl3(Hdeds)] (4) was formed (deds=1,1′-(dithiodiethylene)bis(3,5-dimethylpyrazole)). The crystal structure of this complex was also determined by single-crystal X-ray diffraction method. The structure consists of nickel(II) ions coordinated by three chloride ions and one pyrazolic nitrogen atom. Ligand deds is the result of the oxidation of N-(2-mercaptoethyl)-3,5-dimethylpyrazole (Hmed).

Palladium catalytic systems with hybrid pyrazole ligands in C–C coupling reactions. Nanoparticles versus molecular complexes

This paper reports the comparison of the chemoselectivity of two different Pd catalytic systems, namely molecular and colloidal systems, in C–C coupling reactions. For this purpose, new hybrid pyrazole derived ligands containing alkylether, alkylthioether or alkylamino moieties have been synthesized and used to form Pd(II) complexes and to stabilize Pd nanoparticles (Pd NPs). With the aim of studying the coordination mode of the ligands and further to understand their role in catalysis, both types of Pd species were characterized by appropriate techniques. In C–C coupling reactions promoted by different Pd colloidal systems, several reports evidenced that active species are molecular catalysts leached from Pd NPs. The most important feature of this work relies on the differences observed in the output of C–C coupling reactions, depending on the colloidal or molecular nature of the catalyst employed. Thus, molecular systems carry out typical Suzuki–Miyaura cross-coupling, together with the dehalogenation of the substrate in different proportions. In contrast, Pd NPs catalyze either Suzuki–Miyaura or C–C homocoupling reactions depending on the haloderivative used. Interestingly, Pd NPs catalyze the quantitative dehalogenation of 4-iodotoluene. Differences observed in the chemoselectivity of these two catalytic systems support that reactions carried out with Pd NPs stabilized with the hybrid pyrazole ligands employed here take place on the surface of the colloids.

Design of new N,O hybrid pyrazole derived ligands and their use as stabilizers for the synthesis of Pd nanoparticles.

We describe the stabilization studies of new palladium nanoparticles (Pd NPs) with a family of hybrid ligands. For this purpose, two new N,O-hybrid pyrazole derived ligands, as well as other previously reported, have been used as NP stabilizing agents following an organometallic approach. A comparison with corresponding palladium complexes has been carried out. We have also studied the superstructures formed by the agglomeration of NPs. To evaluate the scope of the system, different parameters have been studied such as the structure of the ligand, the ligand/metal ratio, the nature of the solvent, the concentration and the reaction time. The colloidal materials resulting from the different syntheses were all characterized by IR, transmission electron microscopy techniques at low or high resolution (TEM and HR-TEM), and scanning electron microscopy (SEM-FEG). All these observations have allowed us to better understand the coordination modes of the different ligands onto the surface of the NPs.

Synthesis and Characterization of New Palladium(II) Complexes Containing N-Alkylamino-3,5-diphenylpyrazole Ligands. Crystal Structure of [PdCl(L2)](BF4) {L2 = Bis[2-(3,5-diphenyl-1-pyrazolyl)ethyl]ethylamine}

In this paper, the synthesis of two new N,N′,N-ligands, bis[2-(3,5-diphenyl-1-pyrazolyl)ethyl]amine (L1) and bis[2-(3,5-diphenyl-1-pyrazolyl)ethyl]ethylamine (L2) is reported. These ligands form complexes with the formula [PdCl(N,N′,N)]Cl when reacting with [PdCl2(CH3CN)2] in a 1:1 metal-to-ligand molar ratio. Treatment of these ligands with [PdCl2(CH3CN)2] in a 1:1 metal-to-ligand molar ratio in the presence of AgBF4 or NaBF4 gave [PdCl(N,N′,N)](BF4) complexes. These PdII complexes were characterized by elemental analyses, conductivity measurements, mass spectrometry, and IR, 1H, and 13C{1H} NMR spectroscopies. The X-ray structure of the complex [PdCl(L2)](BF4) has been determined. The metal atom is coordinated by two azine nitrogen atoms and one amine nitrogen atom of the aminopyrazole ligand. The distorted square planar coordination is completed by one chlorine atom. In this complex, intermolecular π–π stacking interactions are present.

Chemical, electrochemical and photochemical molecular water oxidation catalysts

Hydrogen release from the splitting of water by simply using sunlight as the only energy source is an old human dream that could finally become a reality. This process involves both the reduction and oxidation of water into hydrogen and oxygen, respectively. While the first process has been fairly overcome, the conversion of water into oxygen has been traditionally the bottleneck process hampering the development of a sustainable hydrogen production based on water splitting. Fortunately, a revolution in this field has occurred during the past decade, since many research groups have been conducting an intense research in this area. Thus, while molecular, well-characterized catalysts able to oxidize water were scarce just five years ago, now a wide range of transition metal based compounds has been reported as active catalysts for this transformation. This review reports the most prominent key advances in the field, covering either examples where the catalysis is triggered chemically, electrochemically or photochemically.

Study of new metallomacrocyclic Pd(II) complexes based on hybrid pyrazole sulfoxide/sulfone ligands and their contribution to supramolecular networks

Treatment of the pyrazole sulfoxide, 1,1′-(2,2′-sulfoneinylbis(ethanediyl))bis(3,5-dimethyl-1-pyrazole) (L1), and pyrazole sulfone, 1,1′-(2,2′-sulfoneonylbis(ethanediyl))bis(3,5-dimethyl-1-pyrazole) (L2) ligands, with [PdCl2(CH3CN)2] in a 1 : 1 M : L ratio, yields two kinds of complexes, monomer or dimer, depending on the solvent of the reaction. Monomeric complexes [PdCl2(L)] (L = L1 (1) or L2 (3)) are obtained when the solvent is acetonitrile, whereas when the reaction takes place in tetrahydrofuran, dimeric complexes [PdCl2(L)]2 (L = L1 (2) or L2 (4)) are obtained. Diffusion NMR studies have been performed to characterize a mixture of the monomeric/dimeric species in CD3CN solution. The solid-state structures of 3 and 4 have been determined by single-crystal X-ray diffraction methods. The extended structures allowed the study of the effects of the structure of the ligands on the topology and interpenetration form (two-dimensional layer polymers and three-dimensional networks). Finally, it has been observed that dimers are converted into the corresponding monomers under continuous acetonitrile reflux conditions, thus indicating that the latter are thermodynamically more stable than dimers.

Synthesis and Characterization of New N-Alkylamino-3,5-diphenylpyrazole Ligands and Reactivity Toward PdII and PtII. Study of the cis–trans Isomerization

In this paper, the synthesis and characterization of two new N-alkylaminopyrazole ligands, 1-[2-(ethylamino)ethyl]-3,5-diphenylpyrazole (dpea) and 1-[2-(octylamino)ethyl]-3,5-diphenylpyrazole (dpoa) are reported. The reaction of these ligands with [MCl2(CH3CN)2] (M = PdII, PtII) affords the following square planar complexes: cis-[MCl2(NN′)] (M = PdII: NN′ = dpea, 1; dpoa, 2; M = PtII: NN′ = dpea, 3; dpoa, 4). Reaction of [PdCl2(CH3CN)2] and dpea or dpoa in 1:2 M:NN′ molar ratio, in the presence of NaBF4, yields complexes [Pd(NN′)2](BF4)2 (NN′ = dpea, [5](BF4)2); dpoa, [6](BF4)2). The solid-state structures of complexes 1, 3, and [5](BF4)2 have been determined by single-crystal X-ray diffraction methods. In complexes 1 and 3, the dpea ligand is coordinated through the Npz and Namino atoms to the metallic centre, which completes its coordination with two chlorine atoms in a cis disposition. For complex [5](BF4)2, the crystal structure consists of cations involving a [Pd(Npz)2(Namino)2]2+ core with a cis disposition of the two dpea ligands in a square-planar geometry and BF4 – anions. Theoretical calculations were carried out to optimize the geometries of the cis and trans isomers of the [Pd(dpea)2]2+ cation and of the [Pd(dpea)2](BF4)2 complex. The results show that the trans isomer is the most stable for [Pd(dpea)2]2+, in contrast with the cis stereochemistry observed in the crystal structure of [Pd(dpea)2](BF4)2. The calculations also predict that in acetonitrile solution, the dissociation of this complex into the corresponding ions is thermodynamically favourable. The cis–trans isomerization process of [Pd(dpea)2]2+ in acetonitrile solution has been studied by NMR spectroscopy at different temperatures. These experimental results confirm that the trans isomer is the thermodynamically most stable form of the complexes [5](BF4)2 and [6](BF4)2.