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Chromophore–catalyst assemblies are interesting benchmark molecules for photocatalysis. We have prepared two examples of these assemblies and characterized their behaviour as catalysts for the water oxidation reaction. In the bimetallic complexes [Ru(tpy)(4,4′-X2-bpy)(μ-CN)Ru(bda)(DMSO)](PF6) (X = –H (1), –OCH3 (2), tpy = 2,2′:6′,2′′-terpyridine, bpy = 2,2′-bipyridine, H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid and DMSO = (CH3)2SO), a Ru(II)-polypiridine chromophore {Ru(tpy)(4,4′-X2-bpy)} is linked by a cyanide bridge to a {Ru(bda)} water oxidation catalyst. Both complexes catalyze water oxidation both chemically, using Ce(IV) as the terminal oxidant, and electrochemically. Electrochemical and spectroelectrochemical studies, aided with (TD)DFT calculations, allowed us to study the different species involved in the catalytic cycle. For the RuIIRuIII, an intense MMCT (metal-to-metal charge transfer) band suggests a geometrical reorganization during the oxidation process. The wave associated with oxidation to the RuIIIRuIV species shows an increased current that suggests that it may be a catalyst for the water oxidation reaction. Further oxidation results in the RuIIIRuV redox state which is an active catalyst for the water oxidation reaction as demonstrated by the evolution of oxygen. [ABSTRACT FROM AUTHOR]

Upon MLCT photoexcitation, {(tpy)Ru} becomes the electron acceptor in the mixed valence {(tpyṖ−)RuIII−δ-NC-MII+δ} moiety, reversing its role as the electron donor in the ground-state mixed valence analogue. Photoinduced mixed valence interactions can be tuned to obtain extended lifetimes and higher emission quantum yields, beneficial in supramolecular energy conversion schemes. [ABSTRACT FROM AUTHOR]

In this work, we report the evolution of the properties of the inter-valence charge transfer (IVCT) transition in a family of cyanide-bridged ruthenium polypyridines of general formula [RuII(tpy)(bpy)(μ-CN)RuIII(bpy)2(L)]3/4+ (tpy = 2,2′,6′,2′′-terpyridine; bpy = 2,2′-bipyridine; L = Cl−, NCS−, 4-dimethylaminopyridine or acetonitrile). In these complexes, the redox potential difference between both ruthenium centers (ΔE) is systematically modified. A decrease in ΔE causes a red shift of the energy and an intensity enhancement of the observed IVCT transitions. For L = acetonitrile, the IVCT band becomes narrower and asymmetrical, and shows very little dependence on the nature of the solvent, suggesting a delocalized configuration, although a non-symmetrical one. Also, additional electronic transitions of low energy are clearly resolved in this complex. The observed variation in the properties of the IVCT transitions can be understood on the basis of DFT calculations, that point to increasing mixing between the dπ orbitals of both Ru ions. [ABSTRACT FROM AUTHOR]

Examines the synthesis of nitrosyl dinuclear compound. Photoinduced release of nitrogen oxide during irradiation; Observation of strong electronic coupling after the Hush model application; Difference between the optical absorption energy and redox potentials.

Abstract: We report here the synthesis and properties of a family of mixed-valence cyanide-bridged dinuclear complex ions trans-[(L′L4RuII(μ-NC)FeIII(CN)5]− (with L=pyridine or 4-dimethylaminopyridine (dmap) and L′=pyridine, 4-methoxypyridine (meopy) or 4-dimethylaminopyridine)) whose properties could be adjusted smoothly by changing the acceptor properties of the solvent and the σ donor properties of the L′ pyridine ligand. In solution these complexes exhibit an intense solvent-dependent MM′CT (RuII →FeIII) absorption in the near infrared region. Analysis of this band in different complexes and solvents suggests an enhanced interaction as the energies of the metal centers come closer. From this trend the anion trans-[(dmap)5Ru(μ-NC)Fe(CN)5]− (dmap=4-dimethylaminopyridine) in water is expected to belong to the class II-III, but its spectral properties indicates a ground state with Ru(III)–Fe(II) character. The stabilization of this electronic isomer is probably related to the better donor properties of the hexacyanoferrate(II) moiety and its stronger interaction with water. [Copyright &y& Elsevier]

The NIR donor-acceptor charge transfer (DACT) bands of the series of trinuclear complexes trans-[(NC)5FeII/III-(μ-CN)RuIIL4μ-NC)FeIII(CN)5]5/4- (L= pyridine, 4-tert-butylpyridine, and 4-methoxypyridine) are analyzed in terms of a simplified molecular orbital picture that reflects the interaction between the donor and acceptor fragments. The degree of electronic coupling between the fragments is estimated by a full fit of the DACT band profiles according to a three-state model inspired in the Mulliken-Hush formalism. The information is complemented with determinations performed on the asymmetric heterotrinuclear species trans-[(NC)5CoIII(μ-CN)RuII(py)4(μ-NC)FeIII(CN)5]4-, whose preparation is reported here for the first time. The analysis of the NIR spectra of the symmetric trans-[(NC)5FeIII-(μ-CN)RuIIL4(μ-NC)FeIII(CN)5]4- species reveals a low degree of mixing between the terminal acceptor fragments and the bridging moiety containing RuII, with H12 values between 1.0 × 10³ and 1.5 × 10³ cm-1. The reorganization energy contributions seem to be the same for the three species, even when the spectra were recorded in different media. This observation also applies for the CoIII-substituted compound. The computed potential energy surfaces (PES) of the ground state for these complexes show only one stationary point, suggesting that the FeII-RuIII-FeIII (or FeII-RuIII-CoIII) electronic isomers are not thermally accessible. One-electron reduction leads to asymmetric trans-[(NC)5FeII(μ-CN)RuIIL4(μ-NC)FeIII(CN)5]5- compounds with potentially two DACT bands involving the RuII and the FeII donor fragments. These species reveal a similar degree of electronic mixing but the PES shows three minima. We explore the role of the bridging fragment in the long-range thermally induced electron transfer between the distant iron centers. The results suggest that superexchange and hopping might become competitive paths, depending on the substituents in the bridging fragment. [ABSTRACT FROM AUTHOR]

In this article, we report the structural, spectroscopic, and electrochemical properties of the cyanide-bridged complex salts trans-[(NC)RuII(L)4(μ-CN)RuII(py)4Cl]PF6 and trans-[RuIIL)4{(μ-CN)-RuII(py)4Cl}2] (PF6)2 (L = pyridine or 4-methoxypyridine). The mixed-valence forms of these compounds show a variety of metal-to-metal charge-transfer bands, including one arising from charge transfer between the remote ruthenium units. The latter is more intense when L = 4-methoxypyridine and points to the role of the bridging ruthenium unit in promoting mixing between the du orbitals of the terminal fragments. [ABSTRACT FROM AUTHOR]

We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged mixed-valent compounds, trans-[RuIIL4{NCFeIII(CN)5}2]4-, trans-[RuIIL4{CNFeIII(CN)5}2]4-, and cis-[RuII(bpy)2{NCFeIII(CN)5}2]4- (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of complexes trans-[RuIIL4{NCFeIII(CN)5}2]4- (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups C2 and P21/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed configuration of the equatorial ligands. An intense band (ϵ ≈2000-9000 M-1 cm-1) is observed for all of the compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting long-range metal-metal interactions. These interactions are rationalized in terms of the energy matching between the pi-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived from compounds trans-[RuIIL4{NCFeIII(CN)5}2]4-,5-,6- were characterized in methanolic solution. The mixed-valent FeII-RuII-FeIII system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant Fe(II) to Fe(III) centers. [ABSTRACT FROM AUTHOR]