Coordination modes carboxylates

NMR measurements also provide information on the coordination of the ligands in the uranyl polymers. Solid-state I c-NMR confirms the coordination modes of the carboxylate ligands to the uranyl ion that is, both monodentate and bidentate carboxylate coordination modes are evident. The uranyl dicarboxyl ate polymers which possess two moles of coordinated DMSO exhibit two carbon-13 carbonyl resonances, one at about 175 ppm downfield from tetramethylsilane (TMS) and one at about 185 ppm. The polymers which possess only one mole of coordinated DMSO exhibit only the carbonyl peak near 185 ppm. Based on other known coordination compounds, the 175 ppm peak can be assigned to monodentate carboxylate and the 185 ppm peak to bidentate carboxylate. Thus, 7-coordination predominates in the polymers with either one or two moles of solvent coordinated to the uranyl ion, which is consistent with the infrared results reported elsewhere (5). 

Fig. 6 Three possible carboxylate coordination modes toTi02 surfaces (a) unidentate, (b) bidentate chelating, and (c) bridging.

The influence of the length of the peptide chain on the coordination mode in [RsSnfrV)] complexes has recently been studied. The Gly-Gly-Gly moiety proved to be long enough to shade the —I effect of the Bz group in its N-Bz derivatives [R3Sn(IV)] complexes of N-Bz-Gly-Gly-Gly were considered to be polymeric, with monodentate carboxylate and -C = O coordination similar to that in the corresponding complexes of N-acetylated peptides. ... 

IR spectroscopy is often used for distinguishing between unidentate and bidentate coordination of carboxylate (02CR) ligands. For monomeric carboxylate derivatives the separation between the symmetric and asymmetric C02 stretching bands, At = [t asym(C02) - t syJCOa)], provides a useful indication of the coordination mode complexes which exhibit values of greater than 200 cm-1 invariably possess unidentate coordination. Deacon, G. B. Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

Figure 21 Structures of digallane adducts illustrating the different coordination modes of carboxylic and triazenato ligands.

Figure 3. Coordination modes observed in the solid state of metal-carboxylate complexes.

In conclusion, ID coordination polymer [Zn(tmbdc)(dmso)2]-2(DMSO) 2 has been synthesized with 2,3,5,6-tetramethyl-l,4-benzenedicarboxylic acid in DMSO. The structure contains ID chains formed by octahedraly coordinated Zn ions chelated by the carboxyl groups of tmbdc, rather than the 2D (4,4) nets constructed from paddle-wheel SBU of pairs Zn ions as found in [Zn2(bdc)2(dmso)2]-5DMSO 1. Analysis of the structure reveals that the steric hindrance of die four methyl groups of tmbdc determines the coordination environments of the zinc ions and the coordination modes of the carboxyls, and thus the final structures of the coordination polymers. The result also shows that DMSO is a stronger ancillary ligand and is also easier to be included in the structures of coordination polymers, compared to DMF. DMSO can be a better solvent for the syntheses of porous coordination polymers. 

FIGURE 5. Coordination modes available to a carboxylate ligand when interacting with Mg +... 

Carboxylates exhibit diverse coordination modes (Fig. 9) from monodentate, where the ligand is part of a chelate ring (Fig. 9, top left), through symmetrical or asymmetrical bidentate, to bridging via syn and/or anti linkages. 

Another important structural aspect in terms of the reaction mechanisms of zinc enzymes concerns the coordination mode of water and carboxylates, which can bind in a monoden-tate or bidentate fashion to the Zn(II) center, as shown in Figure 4. 

Carboxylate anions are versatile ligands that are found in a variety of coordination modes including unidentate, chelate, and a number of bis(monodentate) bridging modes.1 Such versatility makes the chemistry of carboxylate complexes particularly interesting. It is now also apparent that the diverse catalytic activity shown by many metal carboxylates may be rationalized in terms of the chemistry of the carboxylato ligand.2 In general though, few rational syntheses of metal carboxylates are known.1 This... 

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