Imidazole-metal bonding

The frequent presence of an imidazole-co-ordinated residue in an axial position, coupled with the suggestions that metal-imidazole bond rupture might play an important role in biological processes of coenzyme B12 and the cytochrome c system, indicates that a detailed characterization of the structure and dynamics of the imidazole-metal bond in such porphyrin complexes could have significant biological implications. La Mar and Walker present some preliminary data on the exchange of iV-methylimidazole (WMIm) on the bis-adduct of A MIm with [Fe(tpp)Cl] in CDCI3, and Kolski and Plane describe temperature-jump studies on the reaction between imidazole and the iron(m) derivative of ethylenediamine-substituted protoporphyrin IX in water. 

Fluorination of organometallics, e.g., Me3Sn-imidazoles, has been used to improve selectivity with the aromatic—metal bond being broken preferentially (86BSF930). 

The paramagnetic nature of Cu ion was utilised to characterise these polymeric receptors by studying the interaction between the Cu centres and imidazole groups using ESR spectroscopy [17]. Results from the ESR experiments indicate that Cu -imidazole coordination bonds formed during the assembly of the monomer and template are conserved upon polymerisation. Furthermore, analysis of the ESR data obtained for different polymers suggested a defined arrangement of metal ion sites in the templated materials, which is absent in the non-templated control polymers. 

Because of the importance of metal bonding to histidine in metalloproteins, there have been numerous studies of the pAg values of imidazoles tr-bound to transition metal ions. Usually the coordination decreases acid strength substantially (by 4-6 pA units with LM " systems) because of ff-donation from ligand to metal backbonding effects from metal to ligand are minimal... 

Other nitrile rubber epoxy adhesives have been described for rubber to rubber and rubber to metal bonds. Liquid carboxylated nitrile materials have been found useful in a number of applications. An adhesive comprised of carboxylated nitrile rubber, epoxy resins, imidazole, or tertiary amine and an organic peroxide has been described for bonding metals, paper, plastics, and inorganic materials. 

Silicon analogues of imidazole-2-ylidenes are stable carbenes that form adducts where the metal-silicon bond is relatively weaker than that between metal and carbon atoms. 

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... 

Bidentate NHC-Pd complexes have been tested as hydrogenation catalysts of cyclooctene under mild conditions (room temperature, 1 atm, ethanol). The complex 22 (Fig. 2.5), featuring abnormal carbene binding from the O carbon of the imidazole heterocycles, has stronger Pd-C jj, bonds and more nucleophilic metal centre than the bound normal carbene chelate 21. The different ligand properties are reflected in the superior activity of 22 in the hydrogenation of cyclooctene at 1-2 mol% loadings under mild conditions. The exact reasons for the reactivity difference in terms of elementary reaction steps are not clearly understood [19]. 

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