Of coordinated imidazoles

Examples of reactions of coordinated imidazoles are discussed later (see Section 3.02.5.3.5), but some conception of the effects of such coordination can be obtained by observation of the H NMR spectra where complex formation usually causes deshielding <89BSF472>. The biological activity of metal complexes of imidazoles and benzimidazoles has been reviewed <85RCR676>. 

Though a large number of studies have been carried out on the PVIm complexation, quite a limited number of works have indicated the number of coordinating imidazole groups on a metal ion of the polymer complexes. The maximum coordinating number of imidazole units to a metal ion has been reported by Gregor as 2 for Ag+ complexes [50] and 4 for Cu2+ complexes [51]. The presence of 1 4 complexes for Cu2+-PVIm complexation has also been verified by an electronic spectroscopic measurement [51]. In these studies, the free imidazole concentration monomer basis needed for the complexation analyses has been calculated by the empirical modified Henderson Hasselbalch equation [50,51], The Gibbs-Donnan approach has recently been applied to the PVIm complexation equilibrium analyses of monovalent (Ag+) and divalent (Cu2+ and Cd2+) ions [49],... 

The and nmr spectra of [Co(NH3)5(ImH)] and the nmr spectra of d5-a-[Co(trien)(ImH)2] and cis-a [Co(trien)(ImH)2] have been recorded (ImH = imidazole). The values of coordinated imidazole determined from the dependence of the chemical shift on pH are 10.0,9.6, and 10.1, respectively. The C-2 proton is not exchanged with solvent deuterium under acidic or basic conditions, in marked contrast to the rapid exchange observed in free imidazole and 1-methyl imidazole. Lack of C-2 exchange should be useful for the identification of metal-coordinated histidines in proteins. 

FIGURE 21.18 (a) The Cn site of cytochrome oxidase. Copper ligands inclnde two histidine imidazole groups and two cysteine side chains from the protein, (b) The coordination of histidine imidazole ligands to the iron atom in the heme a center of cytochrome oxidase. 

In an attempt to study the electronic effects of the substituents in the four- and five-positions of the coordinated imidazol-2-ylidene on the bonding, and... 

Formation of a 2 1 L-histidine Au(III) complex is suggested on the basis of multiinstrumental techniques the N1 of the imidazole ring and the nitrogen of the a-amino group are likely involved in the coordination [66]. 

The crystal structure of the C-functionalized imidazole derivative of 1,5,9-triazcyclododecane (75) shows a five-coordinate zinc with four /V-donors from ligand and chloride in a distorted trigonal bipyramidal arrangement. The Zn—N imidazole bonds are the shortest at 2.025(3) A.678 Deprotonation of the imidazole group resulted in a bridging imidazolate to form dinuclear zinc complexes. The pATa of 10.3 varies from the pATa of bound water with similar ligands (as low as 7.3) and the complex is not catalytic for the hydrolysis of esters. 

The imidazole complex raras-[Ir(imid)2Cl4]- is stable for days in neutral aqueous solution, and for hours in the presence of added thiocyanate. Addition of silver nitrate precipitates the silver salt of the complex, with no indication of Ag+-catalyzed removal of coordinated chloride. Thus this iridium(III) complex is substitutionally much more inert than its much-studied (because (potentially) anti-tumor) ruthenium(III) analogue (96). 

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