Imidazole-free complex

Complex 123 is formed reversibly between 119 and 122 (Scheme 30) this has been proposed as an excellent method for purifying 2f/-imidazoles free from precursors. ... 

A) Frozen solution EPR spectra of Fe (TPP)(4-MeIm) (top) and Fe (TPP)(4-MeIm)2 (bottom) prepared by addition of 4-methylimidazolate anion (4-MeIm ) to a solution of Fe(TPP)(SbF6). The top spectrum is characteristic of a high-spin Fe" -porphyrin complex, with a resonance atg = 6(g = 2.7, 2.3, and 1.8 are due to formation of a small amount of Fe" (TPP)(4-MeIm)2 ). The bottom spectrum is characteristic of a low-spin ferric-porphyrin bis(imidazole)-type complex.(B) Frozen solution EPR spectrum of Cu"(ImH)4-+ with gjl = 2.06, A = 183 G, and gj = 2.256 (courtesy of Dr. J. A. Roe). This type of spectrum is typical of square-planar Cu complexes, except that the ligand hyperfine splitting of the gi feature is frequently unresolved, especially in copper proteins (for example, see Figure 5.20). (C) Simulated EPR spectrum of a typical organic free radical with no hyperfine interaction. 

Co(II) or Cu(II) histidine or imidazole complexes were immobilized in porous matrices (montmorillonite and MCM-41) via two methods (introduction of preformed complex or complex formation within the ion-exchanged host substances). It was found that immobilization in general and the latter method in particular increased catalytic activity and catalyst life time in the decomposition reactions of hydrogen peroxide relative to the matrix-free complexes. The immobilized materials were characterized by experimental and computational methods and the structures of the guest molecules inside the hosts were also investigated. 

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

Free carbenes based on 1,2,4-triazole are not as numerous as those based on imidazole (70ZN(B)1421, 95AGE1021, 97JA6668, 98JA9100). The carbene complex 169 (Ar = Ph, p-Tol) is prepared by the [3 + 2] cycloaddition route from [W(CO)j(C+=NC-HCOOEt)]- and aryldiazonium (930M3241). Oxidative decomplexation causes tautomerization of the 1,2,4-triazole ligand, the products being 170 (Ar= Ph, i-Tol). 

Since the successful exploration of silver(i) oxide usage as a multifunctional precursor for the synthesis of silver(i) A-heterocyclic carbene complexes, there has been an increasing number of reports related to silver(i) A-heterocyclic carbene chemistry. Silver(i) oxide can act as a weak base to deprotonate imidazolium salts, generating the free A-heterocyclic carbene ligands in situ, which then forms the silver(i) carbene complexes readily. This reaction can take place in the presence of air and moisture, and as a result, no special treatment in regard to the solvents has to be undertaken. More importantly, its basicity is rather specific toward the deprotonation at the G2 position of the imidazole moiety. Exploration of using silver(i) carbonate as a milder precursor in place of silver(i) oxide has also been pursued, but longer reaction times are usually required. 

The most common ligands are those derived from imidazole and benzimidazol (Scheme 54), followed by the (benz)thiazols. The free Wanzlick-Arduengo carbenes can be isolated and employed for the synthesis of the complexes, but often it is more convenient to prepare the carbenes in situ from the dimers or the corresponding onium salts, or to use carbene-transfer reactions.256-259... 

The simultaneous mutation of 2-imidazole-distamycin to distamycin at both the sites I and II led to a free energy change of -1.8 kcal/mol (AGg - 2 AG3). The NMR experiments showed that the relative populations of Dst Dst DNA and 2-ImD 2-ImD DNA are 50 1 giving an experimental free energy difference of -2.3 kcal/mol (AG7 - AG2). This indicates that the favorable van der Waals interactions between distamycin and DNA at sites I and II and the stacking interactions between the two distamycin molecules stabilize the 2 1 Dst DNA complex over the 2 1 2-ImD DNA complex. The major destabilization factor for the 2 1 2-ImD DNA complex is the lack of... 

The first group 13 (IIIA) element-carbene complex to be reported was an imi-dazol-2-ylidene-alane complex (Scheme 8.23). Based on NMR data, it was suggested that the imidazole fragment has an electronic structure that is intermediate between those of the free carbene and imidazolium ion. The use of an imidazol-2-... 

In contrast, LFMM/LFMD is much faster and provides a uniform treatment over the whole molecule (i.e., there are no link atoms). Moreover, proteins behave like giant ligands. They may be exquisitely complex and varied but, at the end of the day, the intrinsic bonding interaction between, say, Cu(II) and the N of imidazole is essentially the same whether the imidazole is a free ligand or happens to be part of a histidine which is, in turn, connected to a peptide backbone. Hence, if an LFMM FF can be constructed for small coordination complexes containing biologically relevant donors, then it should work for whole proteins. 

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%. 

Tables II and III demonstrate abundantly that in a given medium the rate constant and pH decrease (with accompanying decrease in the concentration of free imidazole or benzimidazole) with increase in concentration of metal ion. This is as expected, because there is greater complexation in a given medium when the concentration of the metal ion increases.

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