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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

The use of visible light sensitization as a means to initiate organic reactions is attractive due to the lack of visible light absorbance by organic compounds, reducing side reactions often associated with photochemical reactions conducted with high energy UV light. This tutorial review provides a historical overview of visible light photoredox catalysis in organic synthesis along with recent examples which underscore its vast potential to initiate organic transformations.

Visible light photoredox catalysis: applications in organic synthesis

▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

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Proton-Coupled Electron Transfer

Mixed phosphine 2,2'-bipyridine complexes of ruthenium

Rate constants for electron-transfer quenching of Ru(bpy)32+* (bpy is 2,2'-bipyridine) by a series of organic quenchers have been determined in acetonitrile (μ = 0.1 M) at 22 ± 2 °C. The reactions studied were based on three different series of structurally related quenchers having varying redox potentials. They include oxidative quenching both by a series of nitroaromatics (ArNCh) and by a series of bipyridinium ions (P2+) and reductive quenching by a series of aromatic amines (R^NAr). After corrections for diffusional effects, the quenching rate constant (kq') data fall into two classes both of which can be treated successfully using Marcus-Hush theory. For case 1, which includes the data for oxidative quenching by P2+ and reductive quenching by RaNAr, RT In kq varies as AG21/2 where | AGasI « /2. AG23 is the free energy change for electrontransfer quenching within an association complex between the quencher and excited state and is the vibrational contribution to the activation barrier to electron transfer. The experimental data are also consistent with Marcus-Hush theory over a more extended range in AG22 where the free energy dependence includes a quadratic term. For case II, which includes quenching by several of the nitroaromatics, RT In kq varies as AG23 and evidence is obtained from the remainder of the data for a transition in behavior from case 11 to case I. The microscopic distinction between the two cases lies in competitive electron transfer to give either groundor excited-state products following the electron-transfer quenching step. For case II, back-electron transfer (A32) to give the excited state, e.g., Ru(bpy)33+,ArNC>2— Ru(bpy)32+*,ArN02, is more rapid than electron transfer to give the ground state (A30), e.g., Ru(bpy)33+,ArN02~ Ru(bpy)32+,ArN02· For case I, electron transfer to give the ground state is more rapid. The different behaviors are understandable using electron-transfer theory when account is taken of the fact that k30 is a radiationless decay rate constant, and the electron-transfer process involved occurs in the abnormal free-energy region where — AG22 > . An appropriate kinetic treatment of the quenching rate data allows estimates to be made of redox potentials for couples involving the excited state. Formal reduction potentials in CH3CN (μ = 0.1 M) at 22 ± 2 °C are £(RuB33+-/2+*) = —0.81 ± 0.07 V and £(RuB32+‘/+) = +0.77 ± 0.07 V. Comparisons between groundand excited-state potentials show that the oxidizing and reducing properties of the Ru(bpyb2+ system are enhanced in the excited state by the excited-state energy, that the excited state is unstable with respect to disproportionation into Ru(bpy)3+ and Ru(bpy)33+, and that the excited state is thermodynamically capable of both oxidizing and reducing water at pH 7. A comparison between the estimated 0-0 energy of the excited state and the energy of emission suggests that there may be only slight differences in vibrational structure between the ground and excited states.

Estimation of excited-state redox potentials by electron-transfer quenching. Application of electron-transfer theory to excited-state redox processes

New luminescent complexes of Os(II) that contain either 2,2{prime}-bipyridine (bpy) or 1,10-phenanthroline (phen) as the chromophorib acceptor ligand have been prepared by a combination of established and new synthetic methods. Extensive use of Os(IV) and Os(III) precursors, e.g., Os{sup IV}(bpy)Cl{sub 4} and mer-Os{sup III}(PMe{sub 2}Ph){sub 3}Cl{sub 3} has led to the preparation of materials with ancillary ligands such as tertiary phosphines as preparative intermediates, including Os{sup III}(bpy)(PMe{sub 2}Ph)Cl{sub 3} and cis-Os{sup II}(phen)(diphosphine)Cl{sub 2}. Further substitution of chloro ligands into complexes such as these results in the formation of emissive complexes of Os(II). Another new synthetic route utilizes the versatile Os(II) precursor Os(bpy){sub 2}CO{sub 3}, which allows the facile preparation of dicationic, disubstituted species such as (Os(bpy){sub 2}(norbornadiene)){sup 2+}. Another general procedure, based on the control of solvent and temperature in the substitution chemistry of cis-Os(bpy){sub 2}Cl{sub 2}, has been further developed to produce a variety of new complexes of the types cis-(Os(bpy){sub 2}(L)Cl){sup +} and cis(Os(bpy){sub 2}(L){sub 2}){sup 2+}, where L is a phosphine, arsine, nitrogen, or olefin donor ligand. In a few cases, phosphine entering groups cause the cis geometry to be unfavorable and new trans-(Os(bpy){sub 2}(L){sub 2}){sup 2+} complexes have also been isolated. The resultant complexes comprise themore » largest family of transition-metal-based excited-state reagents with tunable photophysical and redox properties available. When possible, the new complexes have been characterized by UV-visible spectroscopy, emission spectroscopy, cyclic voltammetry, and {sup 31}P and/or {sup 1}H NMR spectroscopy.« less

Synthetic routes to new polypyridyl complexes of osmium(II)

Metal-to-ligand charge-transfer (MLCT) absorption bands for the complexes Ru(bpy)/sub 3//sup 2 +/, Os(bpy)/sub 3//sup 2 +/, Os(bpy)/sub 2/(py)/sub 2//sup 2 +/, Os(bpy)/sub 2/(CH/sub 3/CN)/sub 2//sup 2 +/, and Os(bpy)/sub 2/(1,2-(Ph/sub 2/P)/sub 2/C/sub 6/H/sub 4/)/sup 2 +/ (bpy is 2,2'-bipyridiene;py is pyridine) are solvent dependent. The dependence can be interpreted with use of dielectric continuum theory but the D/sub 3/ ions Ru(bpy)/sub 3//sup 2 +/ and Os(bpy)/sub 3//sup 2 +/ only if in the excited state the excited electron is localized on a single ligand rather than delocalized over all three. 37 references, 4 figures, 2 tables.

Solvent dependence of metal-to-ligand charge-transfer transitions. Evidence for initial electron localization in MLCT excited states of 2,2'-bipyridine complexes of ruthenium(II) and osmium(II)

New methods of preparing 2,2'-bipyridyl (bpy) complexes of rhenium are described. The procedures involve starting complexes of Re(IV), Re(III), Re(I) and result in new bpy complexes of the type fac-Re/sup III/(bpy)(P)Cl/sub 3/, trans,cis-(Re/sup III/(bpy)(P)/sub 2/Cl/sub 2/)/sup +/, and trans,cis-(Re(bpy)(P)/sub 2/(CO)/sub 2//sup +/, where P is a teritary phosphine. In addition, the complex cis-(Re(bpy)/sub 2/(CO)/sub 2/)/sup +/ has been prepared, which is a rare example of a bis(bipyridyl)rhenium species. Aspects of the NMR spectra, electronic spectra, and electrochemistry of the complexes are discussed, and special attention is given to the fact that the Re/sup I/(bpy) complexes luminescence in fluid solution at room temperature. They present a growing class of potentially exploitable metal-to-ligand charge-transfer (MLCT) excited-state species of Re(I) that have potential interest for use in photochemical energy conversion schemes. 21 references, 6 figures, 3 tables.

Synthetic routes to luminescent 2,2'-bipyridyl complexes of rhenium: preparation and spectral and redox properties of mono(bipyridyl) complexes of rhenium(III) and rhenium(I)

B. P. Sullivan

Papers

Mixed phosphine 2,2'-bipyridine complexes of ruthenium

Estimation of excited-state redox potentials by electron-transfer quenching. Application of electron-transfer theory to excited-state redox processes

Synthetic routes to new polypyridyl complexes of osmium(II)

Solvent dependence of metal-to-ligand charge-transfer transitions. Evidence for initial electron localization in MLCT excited states of 2,2'-bipyridine complexes of ruthenium(II) and osmium(II)

Synthetic routes to luminescent 2,2'-bipyridyl complexes of rhenium: preparation and spectral and redox properties of mono(bipyridyl) complexes of rhenium(III) and rhenium(I)