Polypyridyl

Elliott C M, Derr D L, Matyushov D V and Newton M D 1998 Direct experimental comparison of the theories of thermal and optical electron-transfer studies of a mixed-valence dinuclear iron polypyridyl complex J. Am. Chem. 

Covalent hydration and pseudobase formation in transition metal polypyridyl complexes reality or myth N. Serpone, G. Ponterini, M. A. Jamieson, F. Bolletta and M. Maestri, Coord. Chem. Rev.,... 

Moucheron C, Kirsch-De Mesmaeker A, Kelly JM (1998) Photophysics and Photochemistry of Metal Polypyridyl and Related Complexes with Nucleic Acids. 92 163-216... 

Nasr C, Hotchandani S, Kim WY, Schmehl RH, Kamat PV (1997) Photoelectrochemistry of composite semiconductor thin films. Photosensitization of Sn02/CdS coupled nanocrystal-Utes with a ruthenium polypyridyl complex. J Phys Chem B 101 7480-7487... 

Palanichamy, K., and Ontko, A.C. (2006) Synthesis, characterization, and aqueous chemistry of cytotoxic Au(III) polypyridyl complexes. Inorganica Chimica Acta, 359, 44. 

Another type of mixed ligand technetium or rhenium complexes in the above sense contains two or more monodentate ligands (Fig. 26b). The six-coordinated polypyridyl-thiolato complexes of rhenium(V) [ReO(terpy)(SR)2] + [186] are an example. Further representatives are a variety of mixed-ligand complexes, combining tetramethylthiourea with dimethyldithiocarbamate [TcO(tu)2-((CH3)2NCSS)]2+ [194],... 

Asymmetric ECH with [Rh(L)2(Cl)2]+ complexes containing chiral polypyridyl ligands has been attempted, in homogeneous media (L = (7)-(12)) and at carbon electrodes coated with polymer films prepared by electropolymerization of [Rh(13)2(Cl)2]+ -61 62 The latter catalytic system gave the best results in terms of turnover number (up to 4,750) and enantiomeric excess, (ee) when applied to the hydrogenation of acetophenone (ee 18%) and 2-butanone (ee 10%).62 Polymeric materials derived from the complexes [RhI(bpy)(COD)]+ 36 and [Pd(bpy)2]2+33have also been applied to the ECH reaction. 

Numerous metal complexes have been proven to be active electrocatalysts for C02 reduction.1,66-68 These catalysts can be conveniently grouped into three main families metal complexes with polypyridyl ligands, metal complexes with macrocyclic ligands, and metal complexes with phosphorus ligands. 

Many polypyridyl metallic complexes have been used successfully as electrocatalysts in homogeneous and heterogeneous systems and generally CO and 11 COO are the main products of reduction. Among these, the two complexes, [Re(bpy)(CO)3Cl] and [Ru(bpy)2(CO)X]" + (X = CO, Cl, H 11 2 or 1), have attracted much attention because of their characteristic... 

Several other polypyridyl metal complexes have been proposed as electrocatalysts for C02 reduction.100-108 For some of them HCOO- appears as the dominant product of reduction. It has been shown for instance that the complexes [Rhin(bpy)2Cl2]+ or [Rh n(bpy)2(CF3S03)2]+ catalyze the formation of HCOO- in MeCN (at —1.55 V vs. SCE) with a current efficiency of up to 80%.100,103 The electrocatalytic process occurs via the initially electrogenerated species [RhI(bpy)2]+, formed by two-electron reduction of the metal center, which is then reduced twice more (Rlr/Rn Rh°/Rh q. The source of protons is apparently the supporting electrolyte cation, Bu4N+ via the Hoffmann degradation (Equation (34)). 

Finally, it has been reported that carbon electrodes modified with thin polymeric films of polypyridyl metal complexes containing a dispersion of metal particles (Rh° or Pd°) can be used as electrocatalyst for reduction of C02 to hydrocarbons in MeCN. Apparently CH4 is the dominant reduction product (up to 18% of faradaic efficiency).123,124 It should be noted that the product distribution is reminiscent of a Fischer-Tropsch process since C2, C3, and C4 hydrocarbons are also formed. 

The main classes of metal complex that act as electrocatalysts for reduction of organic halides are polypyridyl, macrocyclic, and phosphine complexes. 

Alkyl, allyl, and aryl bromides are dehalogenated mainly with the formation of R R dimers in the presence of polypyridyl complexes of the metals of Group VIII. It has been demonstrated that the complexes [Co(bpy)3] + 203-204 [Ni(bpy)3]2+,205 and [Ni(phen)3]2+206 catalyze the reductive dimerization of allyl and alkyl bromides in organic 203 205 206 and aqueous micellar 204 solution. 

Run(Hedta)(NO+)]° and [Fen(Hedta)(NO )] have been shown to be effective electrocatalysts for the reduction of N02 in acidic aqueous media, to yield N20, N2, NH3OH+, or NH4 339,340 An element of selectivity is available by control of pH and applied potential. Steps involved in the typical six-electron reduction of nitrite to ammonia catalyzed by [Run(Hedta)(NO+)]° are summarized in Equations (67)-(69). The mechanisms by which nitrite is reduced appeared to be similar to those identified for Fe-porphyrin331 and Ru or Os-polypyridyl complexes.337... 

Among the metal complexes used in electrocatalytic oxidation of organic compounds, polypyridyl oxo-ruthenium complexes have attracted special attention,494"508 especially [RuIV(terpy)(bpy)0]2+.495 197,499,500,502,504 This high oxidation state is reached from the corresponding Run-aqua complex by sequential oxidation and proton loss (Equations (75) and (76)). 

By using various polypyridyl oxo complexes a relationship between redox potentials ( 1/2) of the complexes and the efficiency and the selectivity of the electrocatalytic oxidation of alcohols and diols has been established.506 Higher 1/2 gives higher reactivity. The best results, from the point of view of synthesis, were obtained with the complex /ra ,v-[Ru (terpy)(0)2(0I I2)]2 which is characterized by a high redox potential and a relatively high stability. 

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

Figure 8 Tuning of HOMO (t2g) and LUMO (tt ) orbital energy in various ruthenium polypyridyl...

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