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Electron transfers in chemistry and biology

A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light.

Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.






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Figure 1


Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

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Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

Ru(II) polypyridine complexes: photophysics, photochemistry, eletrochemistry, and chemiluminescence

Great attention is currently paid to the synthesis of polynuclear transition metal complexes and the study of their photochemical, photophysical, and electrochemical properties. This interest is stimulated, in particular, by attempts to design and construct multicomponent systems (often called supramolecular species) capable of performing useful lightand/or redox-induced functions.1-16 A great deal of investigations on mononuclear transition metal complexes had previously shown that several families of these compounds are very interesting from the electrochemical, photochemical, and photophysical viewpoints.17-22 The metalligand interaction, in fact, is often (i) weak enough to allow the manifestation of intrinsic properties of metal and ligands (e.g., ligand-centered and metalcentered absorption bands and redox waves) and, at the same time, (ii) strong enough to cause the appearance of new properties, characteristic of the whole compound (e.g., metal-to-ligand or ligand-tometal charge-transfer bands). On passing from mononuclear to polynuclear transition metal complexes, the situation becomes even more interesting because in the latter (supramolecular) compounds one can find, besides properties related to each metal-based component, properties related to the structure and composition of the whole array. A suitable choice of the mononuclear building blocks and bridging ligands and an appropriate design of the (supramolecular) structure can in fact allow the occurrence of very interesting and potentially useful processes such as energy transfer along predetermined pathways, photoinduced charge separation, multielectron exchange at a predetermined potential, etc. The knowledge on the luminescence and redox properties of polynuclear transition metal complexes is rapidly accumulating, but it is disperse in a great number of journals. We have made an attempt to collect the available results, and we present them together with some fundamental introductory concepts and a few comments. One of the main problems, of course, was to delimit the field of this review. Using personal criteria which are related to our own research interests, we decided to consider only polynuclear transition metal complexes that can be defined as supramolecular species (section 2.2) and that are reported to exhibit luminescence. For such compounds only, the electrochemical properties have also been reviewed. Furthermore, we decided to include only classical (Werner-type) polynuclear transition metal compounds where the number of metal-based units is exactly known and the connection between the metal centers is provided only by bridging ligands. Thus, a number of interesting systems such as polymer-appended metal † In memoriam of Mauro Ciano. 759 Chem. Rev. 1996, 96, 759−833

Luminescent and Redox-Active Polynuclear Transition Metal Complexes.

Photophysics, photochemistry and solar energy conversion with tris(bipyridyl)ruthenium(II) and its analogues

The electronic spectrum of tris(2,2'-bipyridine)ruthenium(2+)

[(phen) 2 Mn IV (μ-O) 2 Mn III (phen) 2 ](PF 6 ) 3 •CH 3 CN cristallise dans le systeme orthorhombique groupe d'espace Pbcn. [(phen) 2 Mn IV (μ-O) 2 Mn IV (phen) 2 ](ClO 4 )•CH 3 CN cristallise dans le systeme monoclinique groupe d'espace P2 1 /c et sa structure est affinee jusqu'a R=0,035

Tetrakis(phenanthroline)di-.mu.-oxodimanganese(III,IV) tris(hexafluorophosphate).cntdot.acetonitrile and tetrakis(phenanthroline)di-.mu.-oxodimanganese(IV) tetraperchlorate.cntdot.acetonitrile: crystal structure analyses at 100 K, interpretation of disorder, and optical, magnetic, and electrochemical results

Ruthenium(II)-bipyrimidine complexes. Spectroscopic and electrochemical properties of a novel series of compounds

Water exchange on ruthenium(II) and ruthenium(III) was studied by /sup 17/O NMR and acetonitrile exchanges on ruthenium(II) was studied by /sup 1/H NMR spectroscopy as a function of temperature and pressure. The kinetic parameters reported for Ru(II) imply that the solvent exchange on Ru/sup 2+/ occurs via an interchange I mechanism for both H/sub 2/O and CH/sub 3/CN. The kinetic parameters are also reported for Ru(III). These data are conclusive for an associative interchange I/sub a/ mechanism for water exchange on Ru(H/sub 2/O)/sub 6//sup 3+/ but for an I mechanism on the deprotonated species Ru(H/sub 2/O)/sub 5/(OH)/sup 2+/. These mechanistic results for low-spin ruthenium solvates are compared to those of other di- and trivalent transition-metal ions. 39 references, 8 figures, 5 tables.

High-pressure NMR kinetics. Part 34. Variable-temperature and variable-pressure NMR kinetic study of solvent exchange on hexaaquaruthenium(3+) and -(2+) and hexakis(acetonitrile)ruthenium(2+)

Andreas Ludi

Papers

The electronic spectrum of tris(2,2'-bipyridine)ruthenium(2+)

Ruthenium(II)-bipyrimidine complexes. Spectroscopic and electrochemical properties of a novel series of compounds

High-pressure NMR kinetics. Part 34. Variable-temperature and variable-pressure NMR kinetic study of solvent exchange on hexaaquaruthenium(3+) and -(2+) and hexakis(acetonitrile)ruthenium(2+)

The electronic spectrum of tris(2,2'-bipyridine)iron(2+) and tris (2,2'-bipyridine)osmium(2+)