Imidazole, complex stability

Diamine chelate complexes are more stable than the monodentate amine heterocycles and, therefore, can be studied under physiological conditions. The imidazole complexes are unstable in aqueous solution and decompose rapidly to technetium oxide hydrate. Six-membered ring chelates are significantly less stable than five-membered ones. Lesser flexibility of the ligand, such as 1.2-diamino-cyclohexane, parallels somewhat lower stability of the complex [53] ... 

From the increase in pH and rate constant [with consequent increase in free (Im)] in going from experiment 1 to 4 in Table II, it is seen that the stability of the imidazole complexes is in the order Ni+2 > Cd+2 Ca+2. This is in agreement with the order of formation constants of these complexes (I), and with the finding (II) that there is no appreciable interaction between calcium ion and imidazole. Moreover, in a solution with an initial composition of 0.238M imidazole and 0.158M HC1, k was found to be 0.0977 min.-1 (Table I), so that the increase in k for this solution in the presence of 0.020M Ca(N03)2 (experiment 4 in Table II) amounts to only 3.6%. 

Solution equilibria have also been the subject of several studies. For example, the gross stability constants (/3 n = 1-5) of imidazole complexes with the Mn ions of Cu, Ni, Cd, Zn and Co have been determined the following order of stability was found Cu>Ni>Cd> Zn > Co.418... 

The pKa of the unusual C-bound imidazole complex [Ru(NH3)5imH2]3+ (25) (11.0) is of note, as, in this species, the ligand is formally the neutral C-2 deprotonated ylide, which is stabilized via metal coordination. In this case, comparison should be made with the pKa of protonated imidazole (7.0) and the observed value is in agreement with the poorer electron-... 

In the case of Ag+, only two imidazolyl groups participate in complex formation, and the overall stability constant is log p2 = 8.00, about one order of magnitude larger than the overall formation constant of the Ag/imidazole complex (log p2 = 6.84). Complexes of Cu2 + with poly 4(5)-vinyl-imidazole, poly(N-vinyl-imidazole), and... 

Accordingly, Hammett constants or combinations of the argnments a and b can be used to estimate both complex stabilities of substituted aromatics with donor site atoms other than C in biology/biochemistry. Therefore the connection between the rest of the protein and tyrosine phenolate, tryptophan indole or histidine imidazole moieties is taken to be one huge substituent (e.g. in the frequent cases where three histidine residues coordinate to one copper or zinc ion or with tyrosine residues in water photooxidation), possibly introducing charge effects by pH, phosphorylation ( P switch ) or by complexation of other metals like Ca on the outer periphery of the molecule. 

The proposed kinetic scheme for the f-BuOOH reactions depends upon the ability to form mono-nitrogen base complexes, PFe i(H2 0) (L) or PFe i(OH) (L), since the bis-ligand complexes, PFe (L)2, were shown to be unreactive with f-BuOOH. The observed lack of rate enhancement for the reaction in the presence of small concentrations of ImH is thus undoubtedly due to the overwhelming stability of the bis-imidazole complex over the monoimidazole complex K = (1-5) x lO M P2 = (5-8) x 10 compared to the bis-ammonia complex over... 

Fig. 4 Effect of solvent molecule size on complex stability between imidazole and cyclophane host. View thh art in color at WWW. dekker. com.)...

As mentioned above, His is located in a hydrophobic pocket of the myoglobin molecule. The electron density and, therefore, the oxidation state of the iron are regulated by protonation and deprotonation of the imidazole ring. With an increase in pH, there is an increase in basicity and, hence, an increase in binding of O2 (the Bohr effect cf. Fig. 12.14). A second histidine residue of myoglobin, His " (distal His), contributes to heme-02-complex stabilization by formation of a hydrogen bridge or ionic bond between N and O (cf. Formula 12.2). 

The Cu complex again oxidizes to Cu + in air. It is interesting fliat oxidation of Cu bound to poly(l-vinylimidazole) is characterized by higher enthalpy and lower entropy compared to the low-molecular-weight imidazole complex. These differences are attributed to the changes in polymer hgand conformation as a result of Cu oxidation to Cu and is evidenced by viscosity measurements. It has been noted that copper ion doping in poly(ihioether) stabilizes the oxidation state of Cu and results in the formation of a Cu+-Cu + system with a potential of 1V in aqueous solution. ... 

From the results discussed in Section 6.3 it is evident that the weak point in macrochelate formation of M(ATP) complexes is the coordination of N7 of the adenine residue. Indeed, by NMR shift experiments it has been shown for Zn " and Cd " that already the formation of the ternary M(ATP)(OH) complexes releases N7 [227]. Similar observations have been made for Cd(ATP)(NH3) [228]. For the ternary complexes of M(ATP)(imidazole) , where = Mn ", Co ", Zn " or Cd ", it is shown, based on a careful analysis of stability data, that the adenine moiety is released from the coordination sphere [228]. However, there is also evidence that in these M(ATP)(imidazole) complexes intramolecular stacking between the purine moiety and the imidazole ring occurs to some extent (see Table 14 [229-232]). 

Classic A/-heterocychc ligands, eg, bipyridyl (bipy), terpyridyl, imidazole, pyrazine, phenanthroline, piperazine (including alkyl- and aryl-substituted derivatives), and polypyrazol-l-yl-borates (bis, tris, and tetra), have all been found to coordinate Th(IV) chlorides, perchlorates, and nitrates. The tripodal hydrotris(pyrazolyl)borates, HBPz, have been used to stabilize organometaHic complexes (31). Bis-porphyrin Th(IV) "sandwich" complexes have been... 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

The catalytic activity of these new complexes (12,13, and 14) was tested by using the standard RCM substrate, diethyldiallylmalonate (Eq. (9)). Results are shown in Table Moreover, the thermal stability of the imidazol-2-ylidene bearing... 

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