Carboxylate complexes bonding

Polytopic macrocyclic receptors 1, 2 (Figure 10.1) are able to complex the zwitterionic form of the amino acids by a double non-covalent charge interaction [28,29]. The unsymmetrical benzocrown sulfonamide derivative, 2 which contains benzo-18-crown-6 and benzo-15-crown-5 moieties was used as a ditopic receptor for multiple molecular recognition of the amino acids, by combining two non-covalent interactions ammonium-crown hydrogen bonding and carboxylate- complexed Na+-benzo-15-crown-5 charge interactions [28,33]. 

Dimeric Cr(II) and Mo(II) carboxylate complexes, investigated by Green et al (119,120) show low-energy multiple ionization patterns of the [d4]2 system, assigned to metal-metal a, n and 8 bonding and antibonding orbitals. 

As unidentate ligands, carboxylates are expected to (i) lose the equivalence of the two carbon- oxygen bonds found in the anion and (ii) have one metal-oxygen distance considerably shorter than the next shortest M—O contact. Lithium acetate dihydrate exemplifies this14 with C—O distances of 133 and 122 pm and Li—O distances of 227 and 257 pm. Most examples of unidentate carboxylate complexes have this classical configuration of M(0—C) and C=0 respectively so certainly the presence of features (i) and (ii) unambiguously determine this mode of coordination. 

Differential reactivity of a C—H bonds has also been reported for Ni11 complexes of peptides and peptide Schiff bases115 in which it is the carboxyl-terminal bond that is the more reactive. However, jV-salicylideneglycyl-L-valinatocopper is found to react with aqueous formaldehyde to give serine-L-valine containing highly optically pure serine. In this case the reaction obviously occurs stereoselec-tively at the amino-terminal residue.115... 

The hydrocarboxylation can take place by insertion of the alkene into a metal-hydride bond followed by CO insertion and finally reaction of the acyl complex with solvent as illustrated in equation (36). Alternatively, a transition metal-carboxylate complex can be generated initially. Insertion of the alkene into the metal-carbon bond of this carboxylate complex followed by cleavage of the metal-carbon bond by solvent completes the addition, as shown in equation (37). Both sequences provide the same product. 

In certain other systems, there is compelling evidence for the insertion into a metal-caiboxylate complex (equation 37). For example, in the synthesis of a-methylene-y-lactones from alkynic alcohols,70,71 no double bond rearrangement to a butenolide occurs, a reaction shown to take place in the presence of transition metal hydrides. The source of the vinyl proton (deuterium) on the a-methylene group is indeed the alcohol function. Finally, palladium carboxylate complexes containing alkynic (equation 40) or vinyl tails (equation 41) can be isolated and the corresponding insertion reaction can be observed. 

Cleavage of Ru—CH3 bonds by acids has been used by Werner et al. for the selective introduction of small molecules at the ruthenium center. Addition of HBF4 to complex 33, in the presence of carbon monoxide or ethylene, allows the coordination of these molecules in complexes 48,49, and 50 (43). Complexes of type 33 give carboxylate complexes 34,35, and 38 on treatment with carboxylic acids (42,43). A bis(alkyl)ruthenium(II) complex (52) was also obtained by addition of PMe3 to the ethylene complex 51 (24,26). 

While acetates and other lower carboxylate complexes are prepared from the acids or alkali metal salts, for formates a different synthesis is the insertion reaction of C02 into M—H bonds, for example,... 

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