Rhodium complexes electrochemical reduction

Systems that fulfill these conditions are substituted or unsubstituted (2,2 -bipyridyl)(pentamethylcyclopentadienyl)rhodium complexes. Electrochemical reduction of these complexes at potentials between -680 mV and -840 mV vs. SCE leads to the formation of rhodium hydride complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolyses are indicating a very fast hydride transfer from the complex to NAD(P)+ under formation of the starting complex, as shown in the following reaction scheme [64] ... 

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

Electro-generated and regenerated bis(bipyridine)rhodium(I) complexes were able to catalyze the selective non-enzymatically coupled electrochemical generation of NADH from NAD . The direct cathodic reduction even at very negative working potentials leads to the formation of large amounts of enzymatically inactive NAD dimers, while the indirect electrochemical reduction via the rhodium complex acting as... 

There are two systems so far which fulfill these requirements tris(2,2 -bipyridyl) rhodium complexes [109, 110] and substituted or nonsubstituted (2,2 -bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes [111]. At potentials between —680 and —840 mV vs SCE, the electrochemical reduction of these complexes leads to the formation of rhodium hydride complexes. Hydride ions are transferred from the complex to NAD(P)+ under specific formation of 1,4-NAD(P)H and the initial complex. 

The same system was employed in the reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol using HLADH as well as 5-ADH from Rhodococcus sp. with high enantioselectivity [113]. With pentamethylcyclopentadienyl-4-ethoxy-methyl-2,2 -bipyridinechloro-rhodium(III) as mediator and HLADH as catalyst, after 5 h 70% of 4-phenyl-2-butanone was reduced to (S)-4-phenyl-2-butanol with 65% ee. Using 5-ADH. 76% of the ketone was converted to the (S)-alcohol after 5 h with 77% ee. Furthermore, this system has been applied in an electrochemical EMR with a polymer bound rhodium complex as mediator. 

Di(porphyrinato)rhodium(II) [Rh(Por)]2 described by OgoshP Wayland , Coil-man, and Kadish are the only known metalloporphyrin dimers containing one single metal-metal bond. Oxidative cleavage (Eq. 24) or thermal homolytic cleavage (Eq. 25) of the Rh-H bond of Rh(OEP)H can produce the [Rh(OEP)]2 dimer The Rh" dimer may also be formed by electrochemical reduction of some Rh" porphyrin complexes (See Sect. D III 4). 

As only guanine moieties in the close vicinity of the electrode surface can undergo direct electrooxidation, soluble redox mediators such as rhodium or ruthenium complexes are sometimes used to shuttle electrons from guanine residues in distant parts of DNA chains to the electrode [20]. In such a case, we cannot speak more about the reagent-less technique. Nevertheless, the electrochemical reduction and oxidation of nucleobases are irreversible and thus do not allow reusability of biosensors. 

A common class of mediators, which fulfill these requirements, are rhodium complexes (e.g., tris(2,2 -bipyridyl)- and substituted or nonsubstituted (2,2 -bipyridyl) (pentamethylcy-clopentadienyl)-rhodium complexes). This regeneration system has been efficiently applied in electroenzymatic reduction of pyruvate to D-lactate and the reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol [1]. In an electrochemical membrane reactor, NADH was... 

The electrochemical reduction of CO2 catalyzed by rhodium and iridium bpy complexes at ca. -1.35 V primarily yields formate [19]. 

Cobalt, Rhodium, and Iridium.- Dlcatlons of the type (Cp Co(n-arene)1(arene = PhH, mesltylene, hmb, etc.) have been synthesised electrochemical reduction of these complexes Is reversible and can be controlled to give either the corresponding... 

Very efficient reduction of NAD(P) with formate catalyzed by cationic rhodium complexes. /. Chem. Soc. Chem. Commun., 1150-1151 (d) Franke, M. and Steckhan, E. (1988) Tris(2,2 -bipyridyl-5-sulfonic add) rhodium(III), an improved redox catalyst for the light-induced and the electrochemically initiated enzymatic reduction of carbonyl compounds. Angeiv. Chem., Int. Ed., 27, 265-267 (e) Grammenudi, S., Franke, M., Vogde, F., and Steckhan, E. (1987) The rhodium complex of a tris(bipyridine) ligand - its electrochemical behavior and frmction as mediator for the regeneration of NADH from NAD. /. Ind. Phenom. Macrocycl. Chem., 5,695-707 (f) Hollmaim, F., Kleeb, A., Otto, K., and Schmid, A. (2005)... 

Recently, the use of pentamethylcyclopentadienyl(l,10-phenanthrohne-5,6-di-one)chloro rhodium(III) hexafluorophosphate [(Cp )Rhm(phend)Cl]PF6, 11 (Fig. 43.4) has been reported for the electrochemical NAD+ reduction. TONs between 7 and 453 h-1 have been achieved by varying pH, temperature and the complex concentrations [44]. This study reveals only preliminary results, so the mechanism of cofactor reduction is not explained however, due to the structural... 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

This system fulfills the four above-mentioned conditions, as the active species is a rhodium hydride which acts as efficient hydride transfer agent towards NAD+ and also NADP+. The regioselectivity of the NAD(P)+ reduction by these rhodium-hydride complexes to form almost exclusively the enzymatically active, 1,4-isomer has been explained in the case of the [Rh(III)H(terpy)2]2+ system by a complex formation with the cofactor[65]. The reduction potentials of the complexes mentioned here are less negative than - 900 mV vs SCE. The hydride transfer directly to the carbonyl compounds acting as substrates for the enzymes is always much slower than the transfer to the oxidized cofactors. Therefore, by proper selection of the concentrations of the mediator, the cofactor, the substrate, and the enzyme it is usually no problem to transfer the hydride to the cofactor selectively when the substrate is also present [66]. This is especially the case when the work is performed in the electrochemical enzyme membrane reactor. 

Fig. 17. Electroenzymatic reduction of 4-phenyI-2-butanone catalyzed by HLADH with in-situ indirect electrochemical regeneration of NADH using a Cp (2,2 -bipyridyl)aquo rhodium(III) complex as mediator...

The only electrochemically studied rhodium derivative, Rh(acac)3, exhibits an irreversible one-electron oxidation Ep = -1-1.70 V, vs. SCE, in MeCN solution) and an irreversible two-electron reduction Ep = —2.21 V, in THE solution) . The same situation holds for Ir(dik)3 complexes. At variance with the rhodium derivative, Ir(acac)3 shows a chemically reversible one-electron oxidation ( ° = -1-1.15 V, vs. SCE, in MeCN solution) " and an irreversible two-electron reduction Ep = —2.60 V, in THE solution). No crystallographic data of species containing the [Ir(acac)3]+ cation are available. ... 

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