Electrochemical osmium complexes

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Moderate enantioselectivity factors have also been found for electron transfer reactions between HRP or GO and resolved octahedral ruthenium or osmium complexes, respectively. In particular, the rate constants for the oxidation of GO(red) by electrochemically generated and enantiomers of [Os(4,4 - 2 ) ]3 + equal 1.68 x 106 and 2.34 x 106 M-1 s-1, respectively (25 °C, pH 7) (41). The spectral kinetic study of the HRP-catalyzed oxidation of and A isomers of the cyclo-ruthenated complex [Ru(phpy)(phen)2]PF6 (Pig. 21) by hydrogen peroxide has revealed similarities with the oxidation of planar chiral 2-methylferrocene carboxlic acid (211). In both cases the stereoseleci-vity factor is pH dependent and the highest factors are not observed at the highest rates. The kA/kA ratio for [Ru(phpy)(phen)2]PF6 is close to 1 at pH 5-6.5 but increases to 2.5 at pH around 8 (211). 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

A variety of techniques are used to characterize organometallic osmium complexes. Particularly important are X-ray crystallography, for a three-dimensional picture of the placement of atoms within a molecule, NMR spectroscopy ( H, C, P, and Os), for information on the structure and symmetry of ligands in diamagnetic complexes, and IR spectroscopy, for the identification of multiple bonds between osmium and a ligand or within a ligand. Electrochemical studies and photoelectron spectroscopy provide information on the oxidation state of osmium and the relative electron density of the complex. 

Polymers containing all metal backbones of Ru-Ru or Os-Os bonds have been prepared via the electrochemical reduction of ruthenium and osmium complexes containing /ram-chloride ligands.81,82 Scheme 2.6 shows the synthesis of polymers with their backbones comprised solely of metal-metal bonds. The polymers were prepared by reducing [Mn(/ran.s-Cl2)(bipyXCO)2] (M = Ru, Os), 33, to M° complexes and forming the polymer after the loss of the chloride ligands. In both cases, the polymers were selective for the reduction of carbon dioxide. 

Fig. 1. (A) Unfiltered electrochemical STM image, taken in 0.1 M KCIO4 electrolyte solution, of an Au(l 11) electrode modified wiih [Os(bpy)2[4-(aminomethyl)pyridine]Cl](PF6) using 3-mercapto-propionic acid and DCC. Tip bias = -50 mV, tuimeling current =1.0 NA, scan rate 5.1 Hz. (B) Structure and proposed binding mode of the covalently immobilized osmium complex.

Moderate enantioselectivity factors have also been found for electron transfer reactions between HRP or GO and resolved octahedral ruthenium or osmium complexes, respectively. In particular, the rate constants for the oxidation of GO(red) by electrochemically generated A and A enantiomers of [Os(4,4 -Me2bpy)3] equal 1.68 x 10 and 2.34 X 10 respectively (25 °C, pH 7) 41). The spectral kinetic... 

The electroactive labels most used in genosensing design are ferrocene and its derivates [24-27] (the reversible oxidation process of ferrocene can be detected by means of several electrochemical techniques), osmium complexes [28], platinum complexes [29], gold complexes [30, 31], and metallic [32-36] or semiconductor nanoparticles [37]. Among the last ones, gold nanoparticles are the most used, their detection can be carried out by means of the measurement of resistance or capacitance changes, usually after an amplification procedure with silver, or by means of the anodic stripping voltammetry of Au(lll) obtained after the nanoparticle oxidation Fig. 9.3. 

Electrochemical measurements on transition metal complexes are performed for a wide variety of reasons. In this paper we discuss the synthesis of new species by electrogeneration, the measurement of rates of chemical reaction by cyclic voltammetry and the parameterisation of redox potentials within the related series of complexes [OsX. j pyjj], where X = Cl, Br, I, py = pyridine and n = 0-6. A further goal realised by this work was to increase the limited number of Ej/2 values for mononuclear osmium complexes reported in the literature. ... 

The redox reaction between the polyelectrolytes in PEM films and specific molecules in solutions has been used to induce responsive film swelhng. To name a few, multilayer films containing a ferrocene-derivatized polyaUylamine hydrochloride (PAH-Fc) [185] and an osmium complex-derivatized polyaUylamine hydrochlorides (PAHOs) [186] can swell by 10% of initial film thickness upon oxidation of the Os(II). Multilayer capsules with layered anionic and cationic polyferrocenylsilanes to form multilayer capsules expand and increase their per-meabihty upon chemical oxidation of the ferrocene units [187]. Additionally, a poly(L-glutamic acid)/PAH multilayer film is reported to take up ferrocyanide ions from solutions and can expand and contract by 5-10% in response to electrochemical oxidation and reduction of the ferrocyanide species [188]. 

So far, it has been reported that redox polymers, such as polysiloxane or polyarrylamide on which ferrocene is introduced as a side chain, are effective mediators [15-25]. Other effective mediators include poly(vinyl pyridine) or poly(vinyl imidazole) on which osmium complex is introduced in the side chain. However, the structure-mediator fimction correlation has been poorly understood. Here, the glucose sensor was used as an example of enzyme sensors, and as shown in Table 1, sensors using enzymes other than glucose oxidase have also been actively studied. It is now possible to detect electrochemically materials in the body using an electrode with an enzyme and a polymer gel. 

Electrochemical reduction of ruthenium and osmium complexes containing /ra i-chloride ligands leads to metal-containing polymers in which metal-metal bonds make up the entire polymer backbone. Hence, reduction of [M (tran5-Cl2) (bipy)(CO)2l (M=Ru, Os) (77) to M complexes generated a polymeric film (78) after loss of the chloride ligands (Scheme 23). Both the ruthenium- and osmium-based coordination polymers were selective for the reduction of carbon dioxide to carbon monoxide and formate. 

Osmium(VI) hydrazido complexes can be generated by electrochemical oxidation of the corresponding osmium(V) hydrazido complexes (Section 5.6.5.3.1). The complex trans-[0s (tpy)(Cl)2(NN(CH2)40)] " (83) is able to oxidize benzyl alcohol to benzaldehyde. It also oxidizes PPhs to PPh30, and R2S to give R2SO the source of O atoms is presumably H2O in the solvent. 

Osmium(V) sulfilimido complexes can be generated by electrochemical or chemical oxidation of the corresponding osmium(IV) complexes (see Section 5.6.6.4.5). 

The one-electron reduction of NP is associated with an increase in the population of the antibonding FeNO orbital. Figure 6a shows the DFT computed LUMO of NP (58), and Fig. 6b shows the IR electrochemical response for the [OsII(CN)5NO]2 ion (59) upon one-electron reduction in acetonitrile. The spectral characterization of the osmium-nitrosyl reduced complex could be done successfully because of the inertness of the Os-L bonds (L = NO or cyanide). In contrast, NP rapidly releases a cyanide ligand upon reduction in acetonitrile (57b,57d). The strong decrease of both the vqn and v o stretching frequencies in [Osn(CN)5NO]3 is very noticeable, particularly uN(> This is as predicted from the LUMOs description, since the addition of electrons to [OsII(CN)5NO]2 must weaken the NO bond. 

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