Montserrat Rodríguez

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.

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.

A theoretical analysis of how geometrical distortions on Cu(μ-Cl)2Cu dimers influence their electronic and magnetic properties

Abstract A structural classification of dimers, containing the Cu(μ-Cl)2Cu core, based on data obtained from X-ray diffraction analysis reported in the literature has been performed. In these complexes, the local geometry around the copper metal center is generally a square pyramid, with a different degree of distortion towards a trigonal bipyramid. The global geometry of the dinuclear complex can be described in terms of the relative arrangement of the two square pyramids, giving rise to three types of geometries, termed: coplanar bases, parallel bases and perpendicular bases. Ideal models representing these geometries were defined and EH calculations were performed in each case, showing the different molecular orbitals involved in their corresponding frontier orbitals, together with their energy. EH calculations were also carried out for dimers embodying different type of structural distortions from the ideal models. The results obtained from those EH calculations have proven to be extremely useful for the proper interpretation and correlation of the magnetic data and dimer structure for those Cu(μ-Cl)2Cu complexes reported in the literature.

Multireversible Redox Processes in Pentanuclear Bis(Triple-Helical) Manganese Complexes Featuring an Oxo-Centered triangular {MnII2MnIII(μ3-O)}5+ or {MnIIMnIII2(μ3-O)}6+ Core Wrapped by Two {MnII2(bpp)3}−

A new pentanuclear bis(triple-helical) manganese complex has been isolated and characterized by X-ray diffraction in two oxidation states: [{Mn(II)(μ-bpp)(3)}(2)Mn(II)(2)Mn(III)(μ-O)](3+) (1(3+)) and [{Mn(II)(μ-bpp)(3)}(2)Mn(II)Mn(III)(2)(μ-O)](4+) (1(4+)). The structure consists of a central {Mn(3)(μ(3)-O)} core of Mn(II)(2)Mn(III) (1(3+)) or Mn(II)Mn(III)(2) ions (1(4+)) which is connected to two apical Mn(II) ions through six bpp(-) ligands. Both cations have a triple-stranded helicate configuration, and a pair of enantiomers is present in each crystal. The redox properties of 1(3+) have been investigated in CH(3)CN. A series of five distinct and reversible one-electron waves is observed in the -1.0 and +1.50 V potential range, assigned to the Mn(II)(4)Mn(III)/Mn(II)(5), Mn(II)(3)Mn(III)(2)/Mn(II)(4)Mn(III), Mn(II)(2)Mn(III)(3)/Mn(II)(3)Mn(III)(2), Mn(II)Mn(III)(4)/Mn(II)(2)Mn(III)(3), and Mn(III)(5)/Mn(II)Mn(III)(4) redox couples. The two first oxidation processes leading to Mn(II)(3)Mn(III)(2) (1(4+)) and Mn(II)(2)Mn(III)(3) (1(5+)) are related to the oxidation of the Mn(II) ions of the central core and the two higher oxidation waves, close in potential, are thus assigned to the oxidation of the two apical Mn(II) ions. The 1(4+) and 1(5+) oxidized species and the reduced Mn(4)(II) (1(2+)) species are quantitatively generated by bulk electrolyses demonstrating the high stability of the pentanuclear structure in four oxidation states (1(2+) to 1(5+)). The spectroscopic characteristics (X-band electron paramagnetic resonance, EPR, and UV-visible) of these species are also described as well as the magnetic properties of 1(3+) and 1(4+) in solid state. The powder X- and Q-band EPR signature of 1(3+) corresponds to an S = 5/2 spin state characterized by a small zero-field splitting parameter (|D| = 0.071 cm(-1)) attributed to the two apical Mn(II) ions. At 40 K, the magnetic behavior is consistent for 1(3+) with two apical S = 5/2 {Mn(II)(bpp)(3)}(-) and one S = 2 noninteracting spins (11.75 cm(3) K mol(-1)), and for 1(4+) with three S = 5/2 noninteracting spins (13.125 cm(3) K mol(-1)) suggesting that the {Mn(II)(2)Mn(III)(μ(3)-O)}(5+) and {Mn(II)Mn(III)(2)(μ(3)-O)}(6+) cores behave at low temperature like S = 2 and S = 5/2 spin centers, respectively. The thermal behavior below 40 K highlights the presence of intracomplex magnetic interactions between the two apical spins and the central core, which is antiferromagnetic for 1(3+) leading to an S(T) = 3 and ferromagnetic for 1(4+) giving thus an S(T) = 15/2 ground state.

Redox and catalytic properties of new polypyrrole modified electrodes functionalized by [Ru(bpea)(bpy)H2O]2+ complexes; bpea=N,N′-bis(2-pyridylmethyl)ethylamine, bpy=2,2′-bipyridine

Abstract New ruthenium(II) complexes containing one or two pyrrole-functionalized polypyridylic ligands have been prepared in order to study their electrochemical behaviour in heterogeneous phase, after anodic polymerization from CH2Cl2 solution on an electrode surface. Complexes containing one pyrrole unit have general formula [Ru(bpea-pyr)(bpy)(L)]2+ (bpea-pyr=N-[3-bis(2-pyridylmethyl)aminopropyl]pyrrole, bpy=2,2′-bipyridine, L=Cl, complex 3, or L=H2O, complex 1), whereas compounds having two pyrrole units correspond to [Ru(bpea-pyr)(bpy-pyr)(L)]2+ (bpy-pyr=4-methyl-4′-pyrrolylbutyl-2,2′-bipyridine, L=Cl, complex 4, or L=H2O, complex 2). Upon oxidative polymerization, all complexes form highly stable polypyrrolic films on a graphite disk electrode surface. An electrode modified with complex 2 polypyrrole coating film, C/poly-2, has been tested as heterogeneous catalyst for the oxidation of benzyl alcohol, showing a remarkably high efficiency and notably improving the results obtained with analogous complexes in homogeneous phase.

Synthesis, catalytic activity and redox properties of palladium(0) complexes with 15-membered triolefinic macrocyclic ligands containing one, two or three ferrocenyl groups

A series of 15-membered triolefinic macrocycles containing ferrocenyl groups and their palladium(0) complexes have been synthesized and characterized. Their catalytic activity has been demonstrated in Suzuki-type cross-coupling and in the Heck reaction. Their redox properties have been investigated by means of cyclic voltammetry.

Water‐Soluble Manganese Inorganic Polymers: The Role of Carborane Clusters and Producing Large Structural Adjustments from Minor Molecular Changes

The reaction of two different carboranylcarboxylate ligands, 1-CH3-2-CO2H-1,2-closo-C2B10H10 or 1-CO2H-1,2-closo-C2B10H11, with MnCO3 in water leads to polymeric compounds 1 a and 1 b. Both compounds have been characterized by analytical and spectroscopic techniques. Additionally, electrochemical techniques have also been used for compound 1 a. X-ray analysis revealed substantial differences between both compounds: whereas a six-coordinated Mn(II) compound with water molecules bridging two Mn(II) centers has been observed for 1 a, a square pyramidal geometry around each Mn(II) ion with terminal water molecules coordinated to each Mn(II) center has been found for 1 b. The observed differences have been attributed to the existence of different substituents, -CH3 or -H, on one of the carbon atoms of the carboranylcarboxylate ligand. The reaction of 1 a and 1 b with coordinating solvents, such as ethers or Lewis bases, leads to the formation of new compounds with low (mononuclear 4 a, 4 b; dinuclear 3 a, 3 b; and trinuclear 2 a) or high nuclearity (hybrid polymer, 5 a), due to breakage of the corresponding polymer. X-ray analysis shows that the structural core present in the polymeric materials is not maintained in the resulting compounds, with the exception of trinuclear compound 2 a. The magnetic properties of the compounds studied show weak antiferromagnetic coupling.

Reusable manganese compounds containing pyrazole-based ligands for olefin epoxidation reactions

We describe the synthesis of new manganese(ii) and manganese(iii) complexes containing the bidentate ligands 2-(3-pyrazolyl)pyridine, pypz-H, and 3(5)-(2-hydroxyphenyl)pyrazole, HOphpz-H, with formula [MnX2(pypz-H)2] (X = Cl(-), 1, CF3SO3(-), 2, OAc(-), 3 or NO3(-) (4)), [MnCl2(pypz-H)(H2O)2], 5, or [MnCl(Ophpz-H)2], 6. All the complexes have been characterized through analytical, spectroscopic and electrochemical techniques. Single X-ray structure analysis revealed a six-coordinated Mn(ii) ion in complexes 1-5, and a five-coordinated Mn(iii) ion in complex 6. Compound 5 is the first co-crystal of Mn(ii) containing Cl and H2O ligands together with bidentate nitrogen ligands. The catalytic activity of complexes 1-6 has been tested with regard to the epoxidation of styrene and, in the case of 1, 5 and 6, other alkenes have been epoxidized using peracetic acid as oxidant in different media, among which glycerol, a green solvent never used in epoxidation reactions using peracetic acid as oxidant. The catalysts show moderate to high conversions and selectivities towards the corresponding epoxides. For complexes 1, 5 and 6, a certain degree of cis→trans isomerization is observed in the case of cis-β-methylstyrene. These observations have been explained through computational calculations. The reutilization of catalysts 1 and 6 for the epoxidation of alkenes has been evaluated in [bmim] : acetonitrile mixture (bmim = 1-butyl-3-methylimidazolium), allowing the effective recyclability of the catalytic system and keeping high conversion and selectivity values up to 12 successive runs, in all cases.

Synthesis and Structure of Novel RuII–N≡C–Me Complexes and their Activity Towards Nitrile Hydrolysis: An Examination of Ligand Effects

The synthesis and isolation of new RuII–acetonitrile complexes, of general formula trans,fac-[Ru(bpea)(B)(MeCN)](BF4)2 (bpea = N,N-bis(2-pyridylmethyl)ethylamine; B = bpy, 2,2′-bipyridine, 4; B = dppe, 1,2-bis(diphenylphosphino)ethane, 5), together with a synthetic intermediate trans,fac-[Ru(NO3)(bpea)(dppe)](BF4), 6, are described. Ru(bpea)Cl3, 1, is used as the starting material for the synthesis of all complexes 2–6 presented in this paper, which are characterized by analytical, spectroscopic (IR, UV/Vis, 1D and 2D NMR), and electrochemical techniques (cyclic voltammetry). Furthermore, complexes 4, 5, and 6 have also been characterized in the solid state by single crystal X-ray diffraction analysis. Their structures show a distorted octahedral geometry where the bpea ligand binds in a facial mode, the bidentate ligands bpy and dppe bind in a chelate manner, and finally the MeCN or the NO3 – ligand occupy the sixth position of the octahedral Ru metal centre. The kinetics of the basic hydrolysis of the coordinated MeCN ligand for complexes 4 and 5 and for the related complex [Ru(phen)(MeCN)([9]aneS3)](BF4)2, 7, which contains the 1,4,7-trithiacyclonane ligand ([9]aneS3) and 1,10-phenanthroline (phen) is also described. Second-order rate constants for acetonitrile hydrolysis measured at 25°C of k = 1.01 × 10–3 M–1 s–1 for 4, 1.08 × 10–4 M–1 s–1 for 5, and 6.8 × 10–3 M–1 s–1 for 7, have been obtained through UV-vis spectroscopy. Activation parameters have also been determined over the temperature range 25.0–45.0°C and agree with a mechanism that involves an associative rate-determining step. Finally the electronic and steric influence of the auxiliary ligands on this reaction for the above and related complexes is discussed.

Polypyrrole-functionalized ruthenium carbene catalysts as efficient heterogeneous systems for olefin epoxidation

New Ru complexes containing the bpea-pyr ligand (bpea-pyr stands for N,N-bis(pyridin-2-ylmethyl)-3-(1H-pyrrol-1-yl)propan-1-amine), with the formula [RuCl2(bpea-pyr)(dmso)] (isomeric complexes 2a and 2b) or [Ru(CN-Me)(bpea-pyr)X)](n+) (CN-Me = 3-methyl-1-(pyridin-2-yl)-1H-imidazol-3-ium-2-ide; X = Cl, 3, or X = H2O, 4), have been prepared and fully characterized. Complexes 3 and 4 have been anchored onto an electrode surface through electropolymerization of the attached pyrrole group, yielding stable polypyrrole films. The electrochemical behaviour of 4, which displays a bielectronic Ru(IV/II) redox pair in solution, is dramatically affected by the electropolymerization process leading to the occurrence of two monoelectronic Ru(IV/III) and Ru(III/II) redox pairs in the heterogeneous system. A carbon felt modified electrode containing complex 4 (C-felt/poly-4) has been evaluated as a heterogeneous catalyst in the epoxidation of various olefin substrates using PhI(OAc)2 as an oxidant, displaying TON values of several thousands in all cases and good selectivity for the epoxide product.