Metallocomplexes

Metallocomplexes Metalloenzymes Metalloid peroxides Metalloimmunoassays Metallomesogens... 

Based on experimental results the method for identification of anthocyanin metallocomplexes was suggested. 

Al-Heterocycles, formation from olefins and acetylenes in a metallocomplex-catalyzed cycloaddition reaction and further transformations 98IZV816. 

The distance between the diagonal nitrogen atoms is approximately 4 A [ 12]. The distortion induced by the incorporated metal is negUgible and planar four-coordinated metallocomplexes are produced. Therefore, the metal-nitrogen dis-... 

Afanasyeva, V.A., Glinskaya, L.A., Klevtsova, R.E. and Mironov, I.V. (2005) Protonation of the acyclic tetraaza metallocomplex of gold(III) [Au (C9HigN4)] in aqueous solution. Synthesis and crystal and molecular structure of [Au(CgH2oN4)](H502) (C104)4. Joumal of Structural Chemistry, 46, 876. 

CN" Metallocomplexes Abiotic transformation in the presence of metals Towilletal. 1978... 

Metallocomplexes of functionalized fullerenes have been obtained, such as the mono- and di-epoxofullerenes [Ir(C6oO)(CO)Cl(AsPh3)2] and [Ir(C60O2)(CO)Cl(PPh3)2].31 32... 

One-electron reduction of metalloorganic complexes or coordination between a metal and an anion-radical ligand may expand an electron shell of the central metal atom. Sometimes, anion-radical metallocomplexes contrast in this regard with the cation-radical ones. Thus, the same metal-loporphyrins form cation-radicals with charges and unpaired electrons on ligands (Shinomura et al. 1981) and anion-radicals with charges and unpaired electrons on metals (Lexa et al. 1989). 

There is a large volume of literature describing one-electron oxidation of metallocomplexes. Such abundance is caused by the chemical nature of metallocomplexes in which the metallic center readily transforms into a higher state of oxidation. Kochi (1986) and Kaim (1987) have covered many of these problems in their reviews. Astruc (1995) has done a very important generalization in the field. 

One of the most important intricacies in the redox chemistry of organyl metallocomplexes is that both a metal and a ligand can be involved in oxidation. The following examples illustrate both the possibilities and manifest the corresponding consequences in chemical reactivity of the complexes. 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

In all other cases, inner-sphere mechanisms are at work. These mechanisms include addition and subsequent dissociation. Eor C—H dissociation, the so-called metallocomplex activation... 

Titration according to this scheme showed that the treatment of Co Salen with excess amounts of sodium resulted in nonquantitative formation of [(Co°Salen) 2Na ]. Thus, catalytic and, especially, kinetic investigations of such complexes have to take into account the presence of Co Salen or (Co Salen) in the samples studied. The described convenient method of quantitative electron transfer in solutions is good at determining low-valence metallocomplexes. 

The difference between the two reactions of Scheme 2.9 may also be considered in terms of the complete electron transfer in both cases. If the a-nitrostilbene anion-radical and metallocomplex cation-radical are formed as short-lived intermediates, then the dimerization of the former becomes doubtful. The dimerization under electrochemical conditions may be a result of increased concentration of reactive anion-radicals near the electrode. This concentration is simply much higher in the electrochemical reaction because all of the stuff is being formed at the electrode, and therefore, there is more dimerization. Such a difference between electrode and chemical reactions should be kept in mind. In special experiments, only 2% of the anion-radical of a-nitrostilbene were prepared after interruption of controlled-potential electrolysis at a platinum gauze electrode. The kept potential was just past the cathodic peak. The electrolysis was performed in the well-stirred solution of trani -a-nitrostilbene in AN. Both processes developed in this case, namely, trans-to-cis conversion and dimerization (Kraiya et al. 2004). The partial electrolysis of a-nitrostilbene resulted in redox-catalyzed equilibration of the neutral isomers. 

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

As a rule, however, the distance between the donor and the acceptor in such binuclear bridge metallocomplexes is not large. Only a few molecules of this type are known in which the electron transfer occurs over considerable distances, comparable with those for electron transfer between randomly arranged centres in vitreous matrices. Consider the results of research on electron tunneling over large distances in bridge systems. 

Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

There are anion radical metallocomplexes with complete spin retention at the anion radical ligand (Glockle et al. 2001), as well as those having an unpaired electron on a metal atom entirely, or those that share an unpaired electron with all parts of the complex (Kaim 1987 and references therein). 

Insertion of new ligands into metallocomplex systems may proceed reversibly. Being reduced in the framework of the complex, these ligands lose the ability to be coordinated and leave the coordination sphere as products. One important example of such ligand sliding is the catalytic transformation of C02 into CO. Rhenium, palladium, platinum, and nickel complexes were recommended to catalyze this process (Hawecker et al. 1986 Du Bois Meidaner 1987). The Ni(II) complex with 1,4,8,11-tetraazacyclotetradecane is preferential (Beley and co-authors 1984). 

There is some lack in data on anion radical stabilization as a result of binding into a metallocomplex. One specific case was described for bis(trimethylsilyl) diacetylene (Kaim 1988). When treated with potassium, this compound gives tetrakis(trimethylsily)butate-traene, probably owing to some transformations of the unstable anion radical. This anion... 

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