Electrochemical models complexity

Within the past 10 years, various biomimetic Fe model complexes were prepared and their catalytic activities in the electrochemical reduction of protons to H2 were investigated (Scheme 57). 

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

The same happens for the last haemerythrin model complex [Fe2(Htpp-do)(C6H5C02)]3 + (Htppdo = V,V,A A+tetrakis(6-pivalamido-2-pyridyl-methyl)-l,3-diaminopropa-2-ol), in which one Fe(II) atom is heptacoordinate, and the other Fe(II) atom is hexacoordinate. At low temperature (—50°C), in acetone solution this complex reversibly reacts with dioxygen.37 The only electrochemical information is that, in MeCN solution, the original trication exhibits two one-electron oxidations ( Fe(II)Fe(II)/Fe(II)Fe(III) = +0.29 V VS. NHE Fe(II)Fe(III)/Fe(III)Fe(III) =... 

The rich spectroscopy and electrochemistry of the heme moiety yields a wealth of opportunities for the denovo heme protein design to evaluate the success of the heme binding site design. Combinations of these spectroscopic and electrochemical methods are elucidating the structure and function of de novo heme proteins and illustrating that they serve as excellent bioinorganic model complexes for simple cytochromes. 

The fact that the electrochemical oxidation of ds-[M2(cp)2(/r-SR)2(CO)4] and of [M2(/r-SR)2(CO)8] is reversible despite the amount of structural reorganization involved suggests that these changes require low activation energy. Extended Htlckel Molecular Orbital (EHMO) calculations on the model complex ds-13 (R = H) indicated that the Lowest Unoccupied Molecular Orbital (LUMO) was the level [47]. Weakening of the... 

These compounds, both the model complexes and in nature, engage in electron transfer, redncing to snccessively more ferrous oxidation states. Rednction of the oxidized forms of the model complexes occur at negative potentials ( -1.5 V). The electrochemical properties and Mossbaner data of these clusters are shown in Table 7. 

Transition metal-carbonyl-diimine complexes [Ru(E)(E ) (CO)2(a-diimine)] (E, E = halide, alkyl, benzyl, metal fragment a-diimine = 1, 4-diazabutadiene or 2,2 -bipyridine) are widely studied for their unconventional photochemical, photophysical, and electrochemical properties. These molecules have a great potential as luminophores, photosensitizers, and photoinitiators of radical reactions and represent a challenge to the understanding of excited-state dynamics. The near-UV/visible electronic spectroscopy of [Rn(X)(Me)(CO)2(/Pr-DAB)] (X = Cl or I iPr-DAB = A,A -di-isopropyl-l,4-diaza-l,3-butadiene) has been investigated throngh CASSCF/C ASPT2 and TD-DFT calculations on the model complexes [Ru(X)(Me)(CO)2(Me-DAB)] (X = Cl or I) (Table 2). 

Catalytic Processes. Catalytic processes lead to intramolecular and intermolecular C-C bond constructions which are usually directly analogous to the stoichiometric reactions. This topic was reviewed in 1983. Catalytic processes often lead to reduction rather than alkene regeneration this is more likely to happen with B12 as a catalyst than it is with a cohaloxime. Schef-fold pioneered the use of vitamin B12 as a catalyst for C-C bond formation, and Tada pioneered the use of model complexes such as cobaloximes. Several of the reactions described in the section on stoichiometric reactions have also been performed cat-aly tically, as mentioned in that section. Commonly used chemical reductants include Sodium Bomhydride and Zinc metal. Electrochemical reduction has also been used. A novel catalytic system with a Ru trisbipyridine unit covalently tethered to a B12 derivative has been used for photochemically driven catalytic reactions using triethanolamine as the reductant. A catalytic system using DODOH complexes can lead to reduction products or alkene regeneration depending upon the reaction conditions. These catalytic B12 and model complex systems all utilize a... 

Bond and Oldham 184,185) have pointed out that when a variety of conformations are possible for a complex ion in either or both oxidation states the simple electrochemical model... 

Cheng and Euler have prepared polyazines functionalized with [Ru(bpy)2]2+ moieties (Scheme 4.53).142 These polymers exhibit strong MLCT transitions assigned to the Ru complex. The authors prepared a number of model complexes for comparison of electrochemical and spectroscopic properties. 

The redox properties of the dimanganese model complexes are listed in Table IX. In general, three redox states can be accessed electrochemically. For oxo-bridged complexes, the Mn, Mn" Mn, and Mn " can be attained with the Mn2 /Mn" Mn couple at about 1-1.8 V vs. NHE and the Mn "Mn /MnJ" couple 0.6-0.9 V lower. The phenoxo-bridged complexes, on the other hand, access the Mn , Mn"Mn ", and MnJ states in the same potential range. This positive shift in potential when the oxo bridge... 

Is Fe -NO a kinetic trap A common mechanistic quandary in both NiR and NoR chemistries is the possible inhibition of catalysis due to formation of a stable ferrous nitrosyl. Electrochemical studies of heme model systems suggest that the potential needed to reduce these species are well out of the normal range of biological reductants. Is the Fe -NO state truly a kinetic trap, and if so, how is it avoided during biological catalysis There is most obviously a correlation between macrocyclic structure and the potential of the ferrous NO adduct, and therefore structural even for the isobacteriochlorin model complex would require a powerful reductant to generate a nitroxyl adduct. 

Sasaki, S. (1992). An ab-initio MO/SD-CI study of model complexes of intermediates in electrochemical reduction of CO2 catalyzed by NiCl2( cyclam). J. Am. Chem. Soc. 114, 2055-2062. 

Mononuclear thiolate complexes containing nickel sites in different oxidation state as [Ni(SC5H40)2] and [Ni(SC6H40)2] were described by KOckerling and Henkel (1993). With respect to the chemical as well as electrochemical properties of the nickel sites of various dehydrogenases, [Ni(SC6H40)2] is a relevant model complex of coordination number four. 

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