Catechol, complexes

Carboxylates, Oxalates, and Catecholates. Complexes of Th(IV) with mono-, di-, tri-, and polycarboxylates have been extensively studied. Monocarboxylates, RCOO , have been complexed with Th(IV), eg, Th(RCOO)4, where R = H, CH, CCl, or and M Th(HC02)4, ... 

Oxidation of 3,5-di-tert-butyl catechol to 3,5-di-tert-butyl-o-benzoquinone by 02 is catalyzed by the Ir111 catecholate complex (124).212 The authors suggest that formation of the dioxygen adduct, as in reaction Scheme 16, is an important step in the process. 

The stability constants of the mono- and bis-complexes between Cu(II) and catecholate were determined under anaerobic conditions and were found to be the same as reported earlier, i.e. log p1 = 13.64 (CuC) and log p2 24.92 (CuC2+) (36,39). A comparison of the speciation and oxidation rate as a function of pH clearly indicated that the mono-catechol complex is the main catalytic species, though the effect of other complexes could not be fully excluded. The rate of the oxidation reaction was half-order in [02] and showed rather complex concentration dependencies in [H2C]0, [Cu(II)]0 and pH. The experimental data were consistent with the following rate equation ... 

An interesting behaviour is shown by the V(IV)-catecholate complex [V(cat)3]2-, the distorted octahedral 06 coordination of which is shown in the Figure 10.17... 

The molecular structure of the oxygenated form of Ir(III)-phenantren-catecholate complex, [Ir(triphos)(Phensq)(C>2)]+, is shown in Figure 21. 

Several factors were found to affect dioxygen uptake by the metal catecholates. Of particular importance were (i) the coordination number of the metal, (ii) the relative basicity of both the catecholate ligand and the metal, (iii) the temperature, and (iv) the pressure of dioxygen. Under some circumstances, both the product containing reacted dioxygen and the unreacted starting catecholate complex existed in equilib-... 

Dihydrogen evolves from vanadium(II)-cysteine at pH6.0-9.5. This reduction is first order in V11 and independent of pH in the range 7.5-8.5. If cysteamine or cysteine methyl ester is used, dihydrogen is still evolved. The reaction with serine is 1000 times slower than with cysteine even though the half wave potentials are comparable.156 This reaction may be explained by a hydride pathway similar to that proposed for catechol complexes or alternatively Scheme 8. 

Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496 In fact complexes with 1 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat)2]2- and [V(cat)3]2 are not distinguishable by potentiometric studies. 

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... 

Catecholate complexes of the type Zrans-[0s(0)2L2]2 (H2L = dopa, dopamine, adrenaline, noradrenaline, or isoproterenol) are prepared from the reactions between Zrans-[0s(0)2(0H)4]2- and H2L. The complexes have been characterized using Raman, IR, and NMR (JH and 13C) spectroscopies, which indicate that the ligands are bound via the catecholate oxygens. These types of complexes are believed to be models for the staining of catecholamine rich sites in biological tissues... 

Evans and coworkers62 described the synthesis and structural study of a series of pentacoordinate bis(catecholate) complexes of silicon(IV). The crystal structure of the anion of [Et3NH][SiMe(3,5-dncat)2] [H2(3, 5-dncat) = 3, 5-dinitrocatechol] is reported (Figure 7). 

Tris-catecholate complexes were prepared from symmetrically and unsymmetrically substituted catechols (LH2) 4-chlorocatechol, 4,5-dichlorocatechol, 4-nitrocatechol, 3,4-dinitrocatechol and 4,5-dinitrocatechol202. All of these complexes are prepared in aqueous solution and are water-stable, down to ca pH 4. The 111 NMR signals for the free and complexed catechol moieties are well separated, and enable the determination of formation constants for each of the complexes, according to equation 46, where L is the catechol dianion. 

Chiral C2-symmetric boron bis(oxazolines) act as enantioselective catalysts in the reduction of ketones promoted by catecholborane.321 DFT calculations indicate that the stereochemical outcome is determined by such catalysts being able to bind both the ketone and borane reducing agent, activating the latter as a hydride donor, while also enhancing the electrophilicity of the carbonyl. X-ray structures of catalyst-catechol complexes are also reported. 

Van den Berg, C.M.G., 1984. Determination of the complexing capacity and conditional stability constants of complexes of copper (II) with natural organic ligands in seawater by CSV of copper-catechol complex ions. Mar. Chem., 15 1-18. 

The chelation of Pu(IV) and Am(III) by the LICAM series has been studied in detail at neutral pH by electrochemical and spectrophotometric procedures268). The Pu(IV) chelate of 3,4,3-LICAMS appears to be a tris (catecholate) complex, indicating that the full denticity of the ligand is not utilized in vivo. Investigation of the complexation of Pu(IV) by 3,4,3-LICAMC establishes a complexation involving the carboxylate para to the carbonyl. Spectroscopic evidence of the complexation of Am(III) by 3,4,3-LICAMS and 3,4,3-LICAMC was also obtained. Significant differences in the spectra of the two complexes were noted. The authors did not exclude complexation through the C-4 car-boxylates. 

Like the PCD ES complex, the BphC ES complex also has an unsymmetri-cally chelated catecholate [155], A similar bond length asymmetry was also deduced from EXAFS studies on the 2,3-CTD-catechol complex, which showed four O/N scatterers at 2.10 A and one scatterer at 1.93 A [158]. This short bond is comparable in length to one Fe—OcatechoMe bond in the crystal structure of a synthetic Fe(II)-catecholate complex, [Fe(6-Me3-tmpa) (H-dbc)]+ [164], The significant asymmetry in catechol binding in the synthetic complex stems from the presence of a didentate but monoanionic catecholate. The much weaker affinity of Fe(II) for catecholate explains why the monoanionic ionization state is favored in the Fe(II) complex, a situation that is also likely to apply in the ES complexes. 

Their capacity to form chelating units having different oxidation states explains a variety of formed coordination compounds [133-136], A series of novel preparative synthetic methods of o-semiquinolate (SQ) and catecholate complexes of transition (Mn, Re, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Ti, Hg), nontransition, and rare-earth metals in various oxidation states and ligand environments has been developed... 

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