Quinones, complexes

The pieparatioii of quinone-metal complexes TMQ represents tetiamethyl quinone the methyl substituents of the quinone rings in the complexes are omitted 

As noted above, in all quinone complexes the ketonic C=0 stretching 

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DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

A single but noteworthy example of a [2 + 2 + 2 + l]-cycloaddition reaction was reported by Takats and Cooke in 1997. In this process, Fe(CO)4(7]2-C2H2) reacts with acetylene to give an iron-tropanone complex in 26% yield (Equation (44)). When the analogous reaction was tried with substituted alkynes under an atmosphere of CO, iron-quinone complexes were observed (Equation (45)).168... 

The valence tautomerism of cobalt-quinone complexes in non-aqueous solvents... 

Quercetinase, 44 281 Quinlol complexes, osmium, 37 270 1,5-Quinoid-7,8-dihydro-6//-L-biopterin, pro-tonated, oxo-Mo(IV) complex, 40 11-12 Quinone complexes... 

Redox chemistry of vanadium-catechol systems is complicated References 256, 497 and 499-508 discuss this subject in detail. In complexes, the metal centre may be in the +5, +4, +3 (and +2) formal oxidation state and quinones complex in three localized electronic forms ... 

The resolution of tris(catecholato)chromate(III) has been achieved by crystallization with L-[Co(en)3]3+ the diastereomeric salt isolated contained the L-[Cr(cat)3]3 ion.793 Comparison of the properties of this anion with the chromium(III) enterobactin complex suggested that the natural product stereospeeifically forms the L-cis complex with chromium(III) (190). The tris(catecholate) complex K3[Cr(Cat)3]-5H20 crystallizes in space group C2/c with a = 20.796, 6 = 15.847 and c = 12.273 A and jS = 91.84° the chelate rings are planar.794 Electrochemical and spectroscopic studies of this complex have also been undertaken.795 Recent molecular orbital calculations796 on quinone complexes are consistent with the ligand-centred redox chemistry generally proposed for these systems.788... 

In order to overcome the above limitations, several workers developed a chronometric assay that involves measuring the rate of loss of ascorbate in an o-DPO/phenolic substrate/ascorbate coupled system however, this is a cumbersome procedure. Other coupled assay systems have also been developed, some using quinone complexing agents such as Besthom s hydrazone (Pifferi and Baldassari, 1973 Espin et al., 1995). 

Some bridgehead amines [l,4-diazabicyclo-[2,2,2]octane (102), quinuclidine (103) and quinuclidine-3-ol] form 1 1 molecular complexes with quinones 104. Formation of 2 1 (amine/quinone) complexes was observed in solutions of DABCO (102) and chloranil (104, R = Cl). These tertiary amines are able to form complexes, while non-bridgehead amines (triethylamine, piperidine) cannot because of steric hindrance or nitrogen inversion183. Stable complexes may be predicted (by CNDO/2 calculations) for... 

The electrophilic reactions of co-ordinated 1,10-phenanthrolines are not always as simple as might be expected. Thus, the nitration of cobalt(m) 1,10-phenanthroline complexes yields 5-nitro-1,10-phenanthroline derivatives at low temperature, but prolonged reaction in hot solution leads to further reaction and oxidation of the ligand to give excellent yields of 1,10-phenanthroline-5,6-quinone complexes (Fig. 8-40). Even after the formation of the quinone, the complexes may exhibit further reaction. For example, reaction of the l,10-phenanthroline-5,6-quinone complexes with base results in the formation of a complex of 2,2 -bipyridine-3,3 -dicarboxylic acid (Fig. 8-41)... 

More detailed information on metal-quinone complexes is presented in Sec. 5.2. 

Additionally to the theoretical data and synthetic techniques for various metal complexes presented in Chaps. 2-A, we would like to pay special attention to three kinds of coordination compounds (complexes of phthalocyanines, quinones, and radioactive elements), whose syntheses, in our opinion, have been insufficiently generalized in monographs and textbooks on synthetic coordination chemistry. This choice is caused by the facts that phthalocyanines, as n-aromatic macrocyclic compounds, possess unusual thermal stability (nonstandard for organic and organometallic species) the quinone complexes have free-radical properties and coordination and organometallic compounds of radioactive elements are interesting at least for the reasons of necessity of special precautions in their syntheses and applications in the nuclear industry and nuclear medicine. So, this chapter is dedicated to the peculiarities of structure and properties and the main synthetic procedures for the complexes above. 

As far back as 1931, L. Michaelis suggested the formation of active forms (radical particles) of some ferments from quinones [132]. The coordination chemistry of catechols and semiquinones has developed dramatically over the past 20 years and, at present, extensive experimental data dedicated to element-organic o-semi-quinone complexes have been accumulated and reviewed in papers by Pierpont [125,133], Tuck [134], Abakumov [135,136], and Kabachnik [137]. In this respect, a review [125] which represents an excellent generalization of the latest achievements in this area should be especially noted. 

The change of metal oxidation number in quinone complexes can also be reached by electrochemical methods. For example, the electrochemical oxidation [(5.20), (5.21)] of [MIV(DBCat)3]2 (M = Mn, Tc, Re) yields products having different oxidation state of the central atom [171,172] ... 

Thus, metal-quinone complexes have a series of peculiarities, which allows us to put them into a special class of coordination compounds. As will be shown below, they can be synthesized by direct interaction between quinones and elemental metals (and also nonmetals) or from their salts or complexes. 

In case of oxidation of mercury by o-quinones, no stable Hg quinone complexes have been observed by direct oxidation of metallic Hg, with or without LiCl addition [209,210]. In the case of LiCl, o-quinones are reduced (5.29) by Hg to form Hg2Cl2 and Li-semiquinone complex 956 ... 

It will be shown below that tellurium-quinone complexes can also be obtained from Ph2Te2 [222]. The resulting products in the reactions above are presented in Table 5.9. 

Table 5.9 Synthetic Methods for Obtaining Metal-Quinone Complexes...

A series of organotin(IV) o-quinone complexes was prepared and characterized by Tuck et al. [224]. The primary process in the reaction of hexaphenylditin with 3,5-di-/-butyl-l, 2-benzoquinone, phenantrene-9,1 O-quinone, 1,2-naphthoquinone,... 

The comparable sequence for Ph4Sn, which was also used as a precursor of tin-quinone complexes and gives related products, is (5.37) ... 

Among other reported p-quinone adducts with N,N,-ethylene-/h.v(salicylidenimi-nate), there is a cobalt one. o-Quinone complexes with the same ligands have the composition 1 1 [Fe(salen)Q] (Q = 9,10-phenanthrenequinone and 1,2-naphtho-quinone) and [Co(salen)(py)]2Q [239]. 

Tien and co-workers [100, 238] observed a photopotential and photocurrent arising in the planar BLM containing bridging molecules with a porphyrin bound covalently to a quinone (see System 44 of Table 1). The size of this porphyrin-quinone complex was not large enough to span across the whole of the membrane, therefore the mechanism of the arising photoeffect is most likely to be similar to those discussed in Sect. 2.4 for other BLMs doped with photosensitizers. 

As shown by Nango et al. [243] for the porphyrin-quinone complex as an example, even when an electron travels via intramolecular transfer only a part of its way across the membrane, the observed overall rate of transmembrane electron transport can be notably increased as compared to transport via diffusion only. 

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