Evaluation of intramolecular equilibria in complexes formed between substituted imidazole ligands and nickel (II), copper (II) or zinc (II).
The metal ion-binding properties of imidazole-4-acetate (ImA-), 4(5)-aminoimidazole-5(4)-carboxamide (AImC), 2,2'biimidazole(BiIm) (I. Török et al., J. Inorg. Biochem. 71 (1998) 7-14), and bis (imidazol-2-yl)methane(BiImM) (K. Várnagy et al., J. Chem. Soc., Dalton Trans. (1994) 2939-2945) have been evaluated by using the recently published stability constants and by applying the recently established log K(ML)M versus pK(HL)H straight-line plots (L. E. Kapinos et al., Inorg. Chim. Acta 280 (1998) 50-56) which hold for simple imidazole-type ligands. The indicated analysis regarding the intramolecular equilibrium between a monodentatally imidazole-nitrogen-coordinated (open) species and a chelated isomer provides helpful insights, e.g., the formation degree of chelates is more favored if six-membered rings can be formed, as in the case with M(BiImM)2+ compared to M(BiIm)2+, though in both instances the formation degree of the chelates is large. The formation degree of chelates in the M(ImA)+ complexes increases in the series Zn(ImA)+ (87%)<Ni(ImA)+ (96%)<Cu(ImA)+ (99.5%). A carbonyl oxygen, if sterically favorably positioned as in the M (AImC)2+ complexes, may also participate well in chelate formation. In this way a carbonyl group can certainly be activated via metal ion coordination and become ready for further reactions. For Ni2+, Cu2+, and Zn2+ the formation degree of the chelated M(AImC)2+ isomers varies between about 30 and 75%. In all of the so-called 'open' species the metal ion is solely coordinated to a pyridine-like nitrogen of the imidazole residue. Some further observations of biological interest are indicated.
Sigel
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University of Basel, Institute of Inorganic Chemistry, Spitalstrasse 51, CH-4056 Basle, Switzerland. helmut.sigel@unibas.ch
Saha
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Saha
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Carloni
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Kapinos
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Griesser
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D002413:Cations, Divalent; D002614:Chelating Agents; D003300:Copper / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D015032:Zinc / Q000737:chemistry*
D002413:Cations, Divalent; D002614:Chelating Agents; D007093:Imidazoles; D009942:Organometallic Compounds; D003300:Copper; D009532:Nickel; D015032:Zinc; D010100:Oxygen