Photochemical activation, osmium

The osmylation of arenes (Ar) with osmium tetroxide is a particularly informative system with which to illustrate the close interrelationship between the thermal and photochemical activation of electron-transfer oxidation. For example, a colorless solution of osmium tetroxide in n-hexane or dichlorometbane upon exposure to benzene turns yellow instantaneously. With durene an orange coloration develops and a clear bright red solution results from hexamethylbenzene. The quantitative effects of the dramatic color changes are illustrated in Figure 3 by the spectral shifts of the electronic absorption bands that accompany the variations in aromatic conjugation and substituents. The progressive bathochromic shift parallels the decrease in the arene ionization potentials (/F) in the order benzene 9.23 eV naphthalene... 

Several different coordination modes have been found in dinitrogen metal compounds (Fig. 2) and they are strongly influenced by the identity of the metal atom(s), their oxidation state(s), and the ligand environment. This results in different degrees of reduction of the N=N bond, which is often also referred to as activation . For monomeric complexes, the end-on coordination mode A is most frequently encountered. However, in very rare cases side-on coordination B has also been observed, eg, a metastable Tj -bound Nz hgand can be trapped and structurally characterized upon photochemical activation of [Os(NH3)5(r -N2)][PF6]2. The large overlap between the osmium d-orbitals with the 2p orbitals of the bound Nz confers stability to this species. 

Haber has already demonstrated that Os is also a promising heterogeneous catalyst for NH3 synthesis, but the number of molecular Os—N2 or Os— N2H4 complexes is rather limited. This is caused by the fact that osmium forms relatively labile N2 compounds, which are usually not a result of direct N2 activation, but formed (a) by intermolecular N-"N coupling between two osmium nitridos, (b) by (thermal/photochemical) activation of... 

Photochemical Activation. Coordinative unsaturated fragments may also be produced by photolytic reactions. In presence of UV-irradiation metal carbonyl compounds lose sequentially CO-ligands. Electron-deficient, solvent coordinated species produced in this way may combine with inactivated metal complexes via the formation of donor-acceptor metal-metal bonds. Iron, ruthenium, and osmium trinuclear carbonyl clusters may be prepared by this way ... 

Polymetallic complexes presenting directional energy migration are of much significance for the design of photochemical molecular devices. Large arrays of multiple photoactive and redox-active building blocks (of ruthenium- or osmium tris(bipyri-dine)-type for instance) have been constructed for such purposes [A. 10,8.25-8.27]. 

Photochemically and radiolytically activated oxidations and reductions - A variety of photochemically or radiolytically activated oxidations and reductions of osmium porphyrins were investigated [197]. The only reactions not already covered by Scheme 2 are the radiolytic formation of OsCl2(P) in aerated dichloromethane and the reduction of Os(OR)2(P) to Os(ROH)2(P) in the presence of isopropanol. 

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