Carmen E. Castillo

An efficient Ru(II) -Rh(III) -Ru(II) polypyridyl photocatalyst for visible-light-driven hydrogen production in aqueous solution.

The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single-component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen-evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy-conversion applications. Herein, we introduce a new ruthenium-rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H2 production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components.

Geometric isomerism in pentacoordinate Cu2+ complexes: equilibrium, kinetic, and density functional theory studies reveal the existence of equilibrium between square pyramidal and trigonal bipyramidal forms for a tren-derived ligand.

A ligand (L1) (bis(aminoethyl)[2-(4-quinolylmethyl)aminoethyl]amine) containing a 4-quinolylmethyl group attached to one of the terminal amino groups of tris(2-aminoethyl)amine (tren) has been prepared, and its protonation constants and stability constants for the formation of Cu(2+) complexes have been determined. Kinetic studies on the formation of Cu(2+) complexes in slightly acidic solutions and on the acid-promoted complex decomposition strongly suggest that the Cu(2+)-L1 complex exists in solution as a mixture of two species, one of them showing a trigonal bipyramidal (tbp) coordination environment with an absorption maximum at 890 nm in the electronic spectrum, and the other one being square pyramidal (sp) with a maximum at 660 nm. In acidic solution only a species with tbp geometry is formed, whereas in neutral and basic solutions a mixture of species with tbp and sp geometries is formed. The results of density functional theory (DFT) calculations indicate that these results can be rationalized by invoking the existence of an equilibrium of hydrolysis of the Cu-N bond with the amino group supporting the quinoline ring so that CuL1(2+) would be actually a mixture of tbp [CuL1(H(2)O)](2+) and sp [CuL1(H(2)O)(2)](2+). As there are many Cu(2+)-polyamine complexes with electronic spectra that show two overlapping bands at wavelengths close to those observed for the Cu(2+)-L1 complex, the existence of this kind of equilibrium between species with two different geometries can be quite common in the chemistry of these compounds. A correlation found between the position of the absorption maximum and the tau parameter measuring the distortion from the idealized tbp and sp geometries can be used to estimate the actual geometry in solution of this kind of complex.

Cobalt(III) tetraaza-macrocyclic complexes as efficient catalyst for photoinduced hydrogen production in water: Theoretical investigation of the electronic structure of the reduced species and mechanistic insight.

We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2](+) (1) (CR=2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [Ru(II)(bpy)3](2+) (Ru) as photosensitizer and ascorbate/ascorbic acid (HA(-)/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [Co(III)(CR)(H2O)2](3+) (2) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [Zn(II)(CR)Cl](+) (3) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H2-evolving activity. Turnover numbers versus catalyst (TONCat) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05 mM. Interestingly, the analogous nickel derivative, [Ni(II)(CR)Cl2] (4), when tested under the same experimental conditions was found to be fully inactive for H2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA(-)/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq=2.5×10(7) M(-1) s(-1)) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket=1.4×10(9) M(-1) s(-1)). The reduced catalyst can then enter into the cycle of hydrogen evolution.

Alternated bimetallic [Ru–M] (M = Fe2+, Zn2+) coordination polymers based on [Ru(bpy)3]2+ units connected to bis-terpyridine ligands: synthesis, electrochemistry and photophysics in solution or in thin film on electrodes

Two alternated bimetallic Ru–Fe and Ru–Zn coordination polymers, [{RuII(bpy)2(L2)MII}n]4n+ (M = Fe2+, Zn2+), were synthesized using the [Ru(bpy)2(L2)]2+ (bpy = 2,2′ bipyridine) complex as a building block, in which L2 is a bipyridine ligand substituted by two terpyridine sites. The [Ru(bpy)3]2+ like-subunits provide the assemblies with photoredox properties whereas the second metal allows the build-up of the polymer structure by coordination of the free terpyridine units, associated with additional redox activities. Thin robust films of these metallo supramolecular structures can be easily obtained as a coating on electrode surfaces (C, Pt, and ITO) by a simple electrochemical procedure based on an electroreductive precipitation adsorption process. The morphology of the films has been characterized by AFM. Electrochemical and photophysical properties of these coordination polymers were investigated in CH3CN solution as well as thin films deposited on an electrode. The Ru(II)–Zn(II) film, deposited on a transparent ITO electrode, displays luminescence properties. On the other hand, the Ru(II)–Fe(II) film exhibits electrochromic properties under continuous cycling over the FeII/FeIII and RuII/RuIII waves, shifting from reddish dark at the Fe(II)–Ru(II) reduced state to orange-yellow at the Fe(III)–Ru(II) state and to pale green at the more oxidized state Fe(III)–Ru(III). These oxidation processes can be also driven by visible light. Indeed, upon continuous irradiation of an CH3CN solution of the Ru–Fe polymer in the presence of a diazonium salt as a sacrificial electron acceptor, the fast quantitative one-electron oxidation of the Fe(II) centers followed by that of the Ru(II) ones occurs, despite the strong quenching of the luminescence of the Ru(II) moieties by the Fe(II) center. The photoinduced oxidation of the Fe(II) center is still efficient when the Ru(II)–Fe(II) polymer is electrodeposited as a thin film on ITO leading to the storage of an oxidative equivalent on an electrode.

Electro- and Photo-driven Reduction of CO2 by a trans-(Cl)-[Os(diimine)(CO)2Cl2] Precursor Catalyst: Influence of the Diimine Substituent and Activation Mode on CO/HCOO− Selectivity

A series of [OsII(NN)(CO)2Cl2] complexes where NN is a 2,2′‐bipyridine ligand substituted in the 4,4′ positions by H (C1), CH3 (C2), C(CH3)3 (C3), or C(O)OCH(CH3)2 (C4) has been studied as catalysts for the reduction of CO2. Electrocatalysis shows that the selectivity of the reaction can be switched toward the production of CO or HCOO− with an electron‐donating (C2, C3) or ‐withdrawing (C4) substituent, respectively. The electrocatalytic process is a result of the formation of an Os0‐bonded polymer, which was characterized by electrochemistry, UV/Visible and EPR spectroscopies. Photolysis of the complexes under CO2 in DMF+TEOA produces CO as a major product with a remarkably stable turnover frequency during 14 h of irradiation. Our results suggest that electrocatalysis and photocatalysis occur through two distinct processes, starting mainly from an OsI dimer precatalyst if the reduction is performed by an electrode and an OsI mononuclear species in case of a photoreduction process.