Osmium complexes applications

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. 

Miscellaneous. Aside from the oxidation chemistry described, only a few catalytic applications are reported, including hydrogenation of olefins (114,115), a, [3-unsaturated carbonyl compounds (116), and carbon monoxide (117) and the water gas shift reaction (118). This is so owing to the kinetic inertness of osmium complexes. A 1% by weight osmium tetroxide solution is used as a biological stain, particulady for preparation of samples for electron microscopy. In the presence of pyridine or other heterocyclic amines it is used as a selective reagent for single-stranded or open-form B-DNA (119) (see Nucleic acids). Osmium tetroxide has also been used as an indicator for unsaturated fats in animal tissue. Osmium tetroxide has seen limited if controversial use in the treatment of arthritis (120,121). 

B. Serra, A.J. Reviejo, C. Parrado and J.M. Pingarron, Graphite-Teflon composite bienzyme electrodes for the determination of L-lactate application to food samples, Biosens. Bioelectron., 14(5) (1999) 505-513. A.A.J. Torriero, E. Salinas, F. Battaglini and J. Raba, Milk lactate determination with a rotating bioreactor based on an electron transfer mediated by osmium complexes incorporating a continuous-flow/ stopped-flow system, Anal. Chim. Acta, 498(1-2) (2003) 155-163. 

Application of computaional methods to the enantioselective dihydroxylations of alkenes by osmium complexes have been reviewed with a special focus on methods used to study the origin of high enantioselectivity. The use of a vast number of computational techniques such as QM, MM, Q2MM, QM/MM, molecular dynamics, and genetic algorithms has been enumerated.98... 

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. 

There are only very few applications of osmium complexes in radical chemistry. Gasanov and coworkers studied the efficiency of M3(CO)12 (M=Fe, Ru, Os) in Kharasch addition reactions in the presence and absence of DMF as a ligand and found that the efficiency of the catalyst decreases in the order Fe>Ru>Os [93]. DMF activates all three systems. Its role was attributed to the generation of... 

Another interesting application is the study of the kinetics of thermodynamically unfavourable oxidations of a series of iron, ruthenium and osmium complexes with 2,2 -bipyridyl or 1,10-phenanthroline (ML32 ") (equation 28). If E ° for is more positive than E° for the... 

In our application to molecular hysteresis accompanying a change in oxidation state, we have exploited linkage isomerization of ruthenium and osmium complexes (oxidation states 2+ and 3+). An early example in this circumscribed field is the linkage isomerization observed for the pentammine ruthenium complex with dimethyl sulfoxide when the oxidation state changes from 2+ (heteroligand attached at S) to 3+ (relocation to oxygen) (Fig. 3A) [13, 14]. The points of attachment have been confirmed by X-ray diffraction... 

Fig. 16. Resolution of inherently chiral fullerenes. Top Kinetic resolution based on the differential reactivity of an optically pure osmium complex towards the enantiomers of 1 2" C76. Hawkins et al. utilized this method also for the resolution of and D2 C 4, Bottom Separation of the enantiomers of H2-C76 by application of the retro-Bingel reaction to each of two optically pure, diastereoisomeric Cyg derivatives having enantiomeric carbon cores...

Another approach is based on the application of redox polders, e.g. osmium complex-modified poly(vinyl pyridine) (9-11) or ferrocene-modified poly(siloxanes) (12,13X crosslinked together with an enzyme on the top of the electrode. The electron transfer fi-om the active site of the polymer-entrapped enzyme to the electrode surfece occurs to a first polymer-bound mediator which has suflSdently approached the prosthetic group to attain a fast rate constant for the electron-tranrfer reaction. From this first mediator the redox equivalents are transported along the polymer chains by means of electron hopping between adjacent polymer-linked mediator molecules (Fig. 2). Extremely fast amperometric enzyme electrodes have been obtained with si ificantly decreased dependence fi-om the oxygen partial pressure. However, die redox polymer/enzyme/crosslinker mbcture has to applied either manually or by dipcoating procedures onto the electrode surface. 

A wide range of ruthenium and osmium complexes incorporating metallated pincer phosphines or pincer amines have been prepared and characterized and used in applications ranging from materials to catalysis. These will not be reviewed in great detail here—for information, readers are referred to the following citations. ... 

BattagUni F, Bartlett PN, Wang JH. Covalent attachment of osmium complexes to glucose oxidase and the application of the resulting modified enzyme in an enzyme switch responsive to glucose. Anal Chem 2000 72 502-509. 

A catalytic enantio- and diastereoselective dihydroxylation procedure without the assistance of a directing functional group (like the allylic alcohol group in the Sharpless epox-idation) has also been developed by K.B. Sharpless (E.N. Jacobsen, 1988 H.-L. Kwong, 1990 B.M. Kim, 1990 H. Waldmann, 1992). It uses osmium tetroxide as a catalytic oxidant (as little as 20 ppm to date) and two readily available cinchona alkaloid diastereomeis, namely the 4-chlorobenzoate esters or bulky aryl ethers of dihydroquinine and dihydroquinidine (cf. p. 290% as stereosteering reagents (structures of the Os complexes see R.M. Pearlstein, 1990). The transformation lacks the high asymmetric inductions of the Sharpless epoxidation, but it is broadly applicable and insensitive to air and water. Further improvements are to be expected. 

The application of organometallic complexes of the other group 8 elements, iron and osmium, in anticancer drug design has until recently been almost exclusively focused on iron, with the ferrocenyl derivative of tamoxifen (ferrocifen) being the most prominent example (104). Organometallic osmium compounds have been little explored in this respect. 

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