Metal catechol complexes

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-... 

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. 

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... 

Geometry of Metal Catechol Complexes. The coordination geometries of [Fe(cat)3]3" and [Cr(cat)3]3" have been determined by single crystal studies of the salts K3[M(cat)3] 1.5 H20 (M = Cr, Fe) in order to explore the crystal field effect of chromic ion on the coordination geometry and, indirectly, to determine the coordination geometry of enterobactin itself (26). 

EXAFS studies and the aforementioned high-resolution crystallography indicate that the catechol snbstrate binds in an asymmetric fashion to the iron(II) center with Fe-Ocatechoiate bond lengths that differ by 0.2 0.4 A. These structural parameters are in excellent agreement with those reported for synthetic iron(II)-monoanionic catecholate complexes, and, on the basis of this comparison, it was proposed that the catechol binds to the iron(n) center as a monoanion. This notion was supported by subsequent UV-resonance Raman andUV-vis studies. " The monoanionic nature of the catechol substrate in extradiol dioxygenases is in sharp contrast with the dianionic catecholate character commonly found in iron(III) complexes. This difference can be rationahzed by the differing Lewis acidities of the metal centers in their divalent and trivalent oxidation states. 

In addition to preparing the model catecholate complexes of rhodium and chromium, the analogous enterobactin complexes were also prepared and their CD spectra recorded (62). From examination of molecular models it is apparent that either the A-cis or A-cis diastereomers of a metal enterobactin complex are structurally possible. In theory, these diastereomers should be separable by chromatographic techniques analogous to those used for the hydroxamates (vide supra) however, under a variety of conditions only one chromatographic fraction is obtained. We conclude that one isomer predominates to the exclusion of the other. 

The crystal structure of this isostructural series of catechol complexes consists of discrete [M(catechol) 4 ] dodecahedra, a hydrogen bonded network of 21 waters of crystallization and sodium ions, each of which is bonded to two catecholate oxygens and four water oxygens. Of the possible eight coordinate poly-hedra, only the cube and the dodecahedron allow the presence of the crystallographic 4 axis on which the metal ion sits. As depicted in Figure 5 and verified by the shape parameters in Table IV, the tetrakis(catecholato) complexes nearly display the ideal D2d molecular symmetry of the mmmm isomer of the trigonalfaced dodecahedron. 

Karakhanov s group has also been exploring poly(ethylene oxide)- and poly(alkene oxide)-copolymer-bound catalysts [99-102]. A notable aspect of this work is the design of polyethers like 39 that contain jS-cyclodextrins and calyx[4]- and calyx[6]arenes. Such polyethers couple the molecular recognition associated with these macrocycles with the catalytic activity of acac, phosphine, dipyridyl, and catechol ligands. Metals complexed to such ligands have been used in reactions like hydroformylation, Wacker oxidations, and arene hy-droxylation. 

In the present paper we would like to present the results of synthetic experiments on semiquinone and catechol complexes of copper, with characterization that may provide insights on the extent to which redox activity at both the metal and the quinone ligand contributes to the synthetic course of oxidation reactions with dioxygen. 

Fig. 3 (a)The coordinate vector for catechol coordination to a metal center, (b) The chelate plane defined by a metal ion and the coordinate vectors in a tris-catechol complex, (c). The approach angle for each chelate of the tris-catechol complex, (d) A M4L6 tetrahedral cluster (adaptedfrom Ref. [24]). (View this art in color at www.dekker.com.)... 

The racemization of dinuclear triple-stranded helicates has been studied. These complexes adopt either AA or AA configurations arising from the tris-bidentate coordinated metal centers. A mononuclear tris-catecholate complex ML3 racemizes via a Bailar twist mechanism. Linked tris-catecholate metal centers would be expected to racemize at the same rate as that of the mononuclear complex in the absence of mechanical coupling. However, when two tris-catecholate complexes are linked in a helicate, the racemization of the M2L3 structure (from AA to AA) slows by a factor of one hundred, while racemization of the four tris-catecholate metal centers in an M4L6 tetrahedron M4L6 is not observed. Although the components of these assemblies are labile, the chiral tetrahedral structure displays structural inertness. 

Mo, U, and V (following their preconcentration as metal complexes) and a wide variety of organic compounds including drugs such as codeine, herbicides such as atrazine, and vitamins such as fohc acid and riboflavin [Ij. The preconcentration step is crucial for AdSV analysis and its rapidity affects the selectivity and sensitivity of the method [8j. The extent of preconcentration and hence the detection limit depends on the length of time over which the adsorption is allowed to proceed. Consequently, insonation was found to significantly improve the sensitivity of the AdSV of a vanadium (V) catechol complex by increasing the rate at which the complex was deposited onto a mercury surface. Maximum responses can be achieved for an insonated system in less than one-tenth the time required for conventional AdSV under silent conditions. 

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