Osmium complexes bipyridyl

The intense colors in 2,2/-bipyridyl complexes of iron(II), ruthenium(II), and osmium(II) are due to excitation of an electron from metal t2g orbitals to an empty, low-lying ir orbital of a conjugated 2,2 bipyridyl ligand. The photoexcitation of this MLCT excited state can lead to emission as the excited state collapses back to the ground state. However, not all complexes are... 

Osmium(VI) complexes biguanides, 568 binuclear oxo-bridged, 594 halo, 594 bipyridyl, 541 cyanides, 526 dtoxo, 581... 

ECPs including coordination complexes are also able to show electrocatalytic properties e.g., toward the oxidation of nitric oxide in the case of porphyrin functionalized polypyrroles containing various metallic centres [275], for the oxygen or hydrogen peroxide reduction in the case of cobalt-salen PEDOT [276] or iron-containing polysalen [245], or for oxidation of ascorbic acid in the case of osmium bipyridyl functionalized PPy [277]. 

Figure 17 (a) Dithienylethene-bridged dinuclear tris(bipyridyl)ruthenium(II) and osmium(II) complexes, (b) UV-vis spectral changes... 

Evidence has been obtained for an inner-sphere mechanism in the iso-electronic peroxodiphosphate (P208 ) ion oxidation of metal complexes. For a series of reactants of the type Fe(LL)a + and Fe(LLL)a + (where LL = bipyridyl or phenanthroline and LLL = terpyridyl), there is no parallel to the outer sphere path observed in the SaOs " oxidation of the same complexes. The mechanism is considered to involve the rate-determining partial dissociation of one of the ligands, with PaOg " (or P04 ) approaching the vacant position, followed by a subsequent fast oxidation reaction. An exception may be in the case of the complex [Ru"(phen)2pyCl]+. In the case of osmium(ii) complexes, however, no reaction takes place in the absence of catalysts since no prior dissociation occurs. 

Electrochemical indication makes use either of redox labels, which are bound chemically to DNA strands, or of redox active intercalators . The latter are flat molecules able to intercalate the windings of DNA helix. It has been shown, that the well known, commercially available fluorescence quenching agent dabcyl (p-methyl red) is useful also for hybridisation detection with heated electrodes [51]. It was found that osmium tetroxide in different forms is a most powerful redox label for hybridisation detection, especially at heated electrodes [42, 52-55]. OSO4 has been used as bipyridyl complex and also botmd to a heated gold electrode surface via thiol bridges. 

The electroreduction of some typically inorganic compoimds such as nitrogen oxides is catalysed by the presence of polymeric osmium complexes such as [Os(bipy)2(PVP)2oCl]Cl, where bipy denotes 2,2 -bipyridyl and PVP poly(4-vinylpyridine). This polymer modifies the reduction kinetics of nitrite relative to the reaction at a bare carbon electrode, and provides calibration graphs of slope 0.197 nA with detection limits of 0.1 pg/mL and excellent short-term reproducibility (RSD = 2.15% for n = 20). The sensor performance was found to scarcely change after 3 weeks of use in a flow system into which 240 standards and 30 meat extracts were injected [195]. 

The 1,2-dithienylethene unit has also been used to link together metal tris(bipyridyl) moieties including an unsymmetrical Ru(II)/Os(II) complex. In the open form luminescence from the MLCT state is observed with efficient energy transfer from ruthenium to osmium. However, the emission is quenched upon conversion to the closed form because of energy transfer to the photo-chromic 1,2-dithienylethene orbitals.55... 

Keywords Bipyridyl Energy transfer Osmium complexes Osmium photochemistry Osmium photophysics Photosubstitution Terpyridyl... 

In the first step, the precursor, typically a ruthenium or osmium bis(2,2,-bipyridyl) (bpy) complex, reacts with solvent (S) to produce a solvated complex. When solvents such as dry methanol and ethanol are used, only one chloride is exchanged and the species [Ru(bpy)2(PVP) Cl]+ is obtained as the sole product. The nature of the coordination sphere around the metal center can be determined by UV-visible (UV/Vis) spectroscopy (Xmax, 496 nm) and by its redox potential, (about 0.65 V (vs. SCE), depending on the electrolyte being used). By a systematic variation of the ratio of monomer units to redox-active centers, the loading of the polymer backbone ( n) can be varied systematically. (Here, n stands for the number of monomer units in the polymer per redox-active center, e.g. in a PVP-based, n = 10 polymer, there are 10 pyridine units for every redox center. 

A few substituted osmium(II) cyano complexes are known (for [Os(NO)(CN)s]2 see p. 546). Reaction of (NH4)2[OsC16] and 2,2 -bipyridyl followed by treatment with NaCN gives Os(C-N)2(bipy)2 2H20 in which the CN groups may be cis. Protonation is said to occur in perchloric acid to give 0s(CN)2(bipy)2 2HC104 ... 

Nitrogen is, after oxygen, the most frequently encountered donor atom in the coordination chemistry of osmium. There is a large and growing body of work on the ammine, pyridine, ethylenediamine and porphyrin complexes, but the most popular and rapidly growing field of study at the present time is that of the polypyridyls , the 2,2 -bipyridyl, 1,10-phenanthroline and 2,2,2,6 - terpyridyl complexes of the metal. 

As a tridentate conjugated ligand this would be expected, like 2,2 -bipyridyl and 1,10-phenanth-roline, to stabilize osmium(II) and this is indeed the case, with most of the reported complexes containing divalent osmium. The unsubstituted bis complex [Os(terpy)2]2+ is remarkably stable. It seems likely that terpyridyl complexes of osmium(II) are good candidates for further investigation as photosensitizers, having so far received less attention than the corresponding bipyridyl or phenanthroline species. 

The intense red-colored diamagnetic tris-bipyridyl and tris-phen-anthroline iron(II) complex cations are perhaps the most widely studied compounds of these ligands. It is now accepted that the absorption responsible for their characteristic deep color results from a Laporte-allowed transition of the t., tt type (7). This is supported by the fact that the intensity increases in cooling [578) and by measurement of the circular dichroism of the complexes [372). The corresponding complexes of ruthenium(II) (5 3) and osmium(II) [252) have very similar charge transfer spectra. 

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... 

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