Carboxylate bonding modes

The four possible modes of carboxylate bonding which have been identified are purely ionic, unidentate, bridging bidentate and chelating bidentate, as illustrated in Figure 5.3. 

Fig. 22 a Schematic illustration of a predominantly one-handed helix induction in 66 upon complexation with chiral carboxylic acids, b Expected hydrogen-bonding modes... 

The carboxylates are a very important class of ligands with the following bonding modes. 

Several studies have addressed how car-boxylated Ru complexes interact with Ti02 surface [303, 378, 380-382, 385], In most vibrational studies, an asymmetric C—O stretch consistent with carboxylate binding modes (bidentate or bridging) has been reported [378, 380, 385]. Evidence for ester linkages and H-bonded network have also been reported [303, 376, 382]. Qu and Meyer [378] demonstrated that high surface proton concentrations favor carboxylic acid type linkage where low proton concentrations favor carboxylate type binding modes for complexes. Possible modes of coordination of the... 

The enhanced nucleophilicity of coordinated CO2 in complex [Fe(C02)(depe)2] allowed reaction with organotin chlorides. In the resulting carboxylate complexes 31, the CO2 unit links the two metals via a fi-ij C) if(P,0 ) bonding mode. Although the X-ray data of 31b exclude the contribution of a carbenoid form in the solid state, chemical shifts of IX-CO2 in the NMR spectra indicate a borderline case between oxycarbonyl and dioxycarbene character... 

Diverse coordination types have been reported for rare earth carboxylates with the most common being bidentate chelation, bidentate syn-syn bridging, and tridentate bridging with many examples being reported by Meyer and co-workers. Some of the bonding modes found in rare earth carboxylates are given in Fig. 1.2. 

As foretold in the introduction, ring formation via attack on a double bond in the endo-trig mode is not well exemplified. The palladium(II) catalyzed oxidative cyclization of o-aminostyrenes to indoles has been described (78JA5800). The treatment of o-methyl-selenocinnamates with bromine in pyridine gives excellent yields of benzoselenophene-2-carboxylates (Scheme 10a) (77BSF157). The base promoted conversion of dienoic thioamides to 2-aminothiophenes is another synthetically useful example of this type (Scheme 10b) (73RTC1331). 

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