Imidazole complexes with metals

Imidazoles are amphoteric compounds with a basic, pyridine-type nitrogen (they are about 106 times more basic than oxazoles and 104 times more basic than thiazoles173), and (where the NH is unsubstituted) a weakly acidic, pyrrole-type amino nitrogen in the ring. In consequence, imidazoles readily form salts with acids and often form salts (or complexes) with metals. The sparingly soluble silver salts formed by imidazoles have been used by Giesemann et al.174 as intermediates in the synthesis of 1-triphenylmethylimidazoles. Normally, however, the salts formed with acids are more important in isolation and purification procedures. 

The imidazole ring, with a physiological pH value of 7.4, exists in the histidine building blocks of proteins as a free base and as a conjugate acid (pAT = 7.00, see p 166) because of a regulating acid-base equilibrium. Especially in enzymes, the ring can act as a Bronsted base or as a Bronsted acid as the occasion demands, i.e. it acts as a buffer. It is also able to form complexes with metal ions. Such properties are not found in any other proteinogenic amino acids [119]. 

Often, however, the imidazole nucleus of histidine (free and bound histidine in peptides and proteins) is involved in metal binding. The order of dissodation of functional groups in the molecule of histidine is evident from the following pK values pXj (COOH) = 1.80, pX2 (imidazohum) = 6.04, pXj (NH3+) = 9.33, p 4 (imidazole) = 14. This results in the following stages during the formation of histidine complexes with metals ... 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

Ru—C(carbene) bond distances are shorter than Ru—P bond lengths, but this can simply be explained by the difference in covalent radii between P and The variation of Ru—C(carbene) bond distances among ruthenium carbene complexes illustrates that nucleophilic carbene ligands are better donors when alkyl, instead of aryl, groups are present, with the exception of 6. This anomaly can be explained on the basis of large steric demands of the adamantyl groups on the imidazole framework which hinder the carbene lone pair overlap with metal orbitals. Comparison of the Ru—C(carbene) bond distances among the aryl-substituted carbenes show... 

Stability of Complexes of Imidazole with Metal Ions of Biological and Biomedical Interest ... 

Ylide formation, and hence X-H bond insertion, generally proceeds faster than C-H bond insertion or cyclopropanation [1176], 1,2-C-H insertion can, however, compete efficiently with X-H bond insertion [1177]. One problem occasionally encountered in transition metal-catalyzed X-H bond insertion is the deactivation of the (electrophilic) catalyst L M by the substrate RXH. The formation of the intermediate carbene complex requires nucleophilic addition of a carbene precursor (e.g. a diazocarbonyl compound) to the complex Lj,M. Other nucleophiles present in the reaction mixture can compete efficiently with the carbene precursor, or even lead to stable, catalytically inactive adducts L M-XR. For this reason carbene X-H bond insertion with substrates which might form a stable complex with the catalyst (e.g. amines, imidazole derivatives, thiols) often require larger amounts of catalyst and high reaction temperatures. 

The apparently first kinetic study of a metal-assisted electrophilic substitution in a Co(III) complex is recent. The bromination of Co(NH3)5imidH is complicated by the presence of different bromine species in solution (Brj, HOBr and Brj"). In addition, successive brominations of the coordinated imidazole occur. Rate data can be interpreted in terms of reaction of the conjugate base of the Co(III) complex with Brj, and a suggested mechanism for the first steps is (Rq = Co(NH3)5 ")... 

Ni" forms square-planar bis-complexes with the amidate anions of L-Val, L-Phe, and L-Pro. The structure of bis(Gly)-bis(imidazole)nickel(ii) has been reported and the configuration around the metal atom is cis-O(carboxyl), cis-N(amine), cis-N(imidazole). Tetra- and penta-peptide complexes of nickel(ii) consume oxygen in neutral solutions as the metal ion catalyses peptide oxidation to give a number of products, including amides of amino-acids and peptides, oxo-acids, and C02- ... 

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