Osmium complexes properties

We have recently extended our interest to the analogous halfsandwich osmium-arene complexes and are exploring the chemical and biological properties of [Os(r 6-arene)(XY)Z]ra 1 complexes (Fig. 25) (105). Both the aqueous chemistry and the biological activity of osmium complexes have been little studied. Third-row transition metals are usually considered to be more inert than those of the first and second rows. Similar to the five orders of magnitude decrease in substitution rates of Pt(II) complexes compared to Pd(II), the [Os(ri6-arene)(L)X]"+ complexes were expected to display rather different kinetics than their Ru(II)-arene analogs. A few other reports on the anticancer activity of osmium-arene complexes have also appeared recently (106-108). 

NLO properties, 12, 771 from oxygenated ligands, 6, 842 with palladium, 8, 213 and Rh Cp complexes, 7, 160 trinuclear clusters, overview, 6, 835-871 Osmium complexes... 

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. 

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

Salts of [Os(bpy>3] are best prepared by the reaction of bpy with [Os(bpy)2Cl2] (102) or by prolonged reaction with other osmium complexes (56, 318, 621). The crystal structure of [Os(bpy)3][PF6]2 reveals identical molecular properties to the corresponding ruthenium complex (159). Studies of the and NMR spectra of [Os(bpy>3] + salts... 

The vinyl derivative 4-methyl-4 -vinyl-bpy is valuable for chemical electrode modification because its ruthenium and osmium complexes can be polymerized to generate electroactive films with variable properties.86 This unsymmetric ligand was prepared in 35% overall yield from 4,4 -dimethyl bipyridine by first lithiating one methyl group and quenching the anion with (chloromethyl)methyl ether, then reacting with potassium t-butoxide to effect elimination.87,88... 

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