Zinc -cyclen complex

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

Some model Zn2+ complexes of the ethanol-pendant [12]aneN3 (34i) and ethanol-pendant cyclen (228) were reported. The former formed a dimeric alkoxide complex, while the latter (pifa 7.6) yielded a mononuclear hydroxide species. Some model Zn2+ complexes of carboxy-methyl and carboxyethyl and carboxypropyl pendant cyclen were prepared. All these zinc complexes exist in equilibrium between the CO2-bound and CO2 -unbound forms in acidic aqueous solution. Herr et al. (229) synthesized some zinc(II) complexes with tripodal peptide ligands with histidine side chains to mimic the zinc(II)-coordination sphere of CA. 

Fig. 7. Reaction mechanism for P— ester bond cleavage of bis(4-nitrophenyl) phosphate by alkoxide-pendent cyclen zinc(II) complex.

Zinc complexes of the cyclen ([12]aneN4 = 1,4,7,10-tetraazacyclododecane) ligand have been extensively studied in terms of phosphate ester hydrolysis reactivity. For example, a proposed binuclear Zn(II) hydroxide complex of cyclen was reported to enhance the rate of hydrolysis of ethyl(2,4-dinitrophenyl)-methylphosphonate and diethyl(2,4-dinitrophenyl) phosphate.223,224 It should be noted that the nuclearity of the zinc-cyclen complex in solution was not conclusively identified in this work. 

Fig. 3. Rate-pH profile for the second-order rate constants (k ) of 4-nitrophenyl acetate hydrolysis with alcohol-pendent cyclen zinc(II) complex at 25°C and 113] + [141 = 1 mM in 10% (v/v) CHjCN.

Fig. 4. An overall reaction mechanism for 4-nitrophenyl acetate hydrolysis catalyzed by alcohol-pendent cyclen zinc(II) complex 14.

C NMR studies suggest that 1,4,7,10-tetra-azacyclododecane-l,7-diacetic acid binds to zinc in a cis octahedral geometry—the two carboxylate oxygens are cis and the remaining four donors from the cyclen macrocycle. The formation constant was determined for the complex.733... 

While there have been a considerable number of structural models for these multinuclear zinc enzymes (49), there have only been a few functional models until now. Czamik et al. have reported phosphate hydrolysis with bis(Coni-cyclen) complexes 39 (50) and 40 (51). The flexible binuclear cobalt(III) complex 39 (1 mM) hydrolyzed bis(4-nitro-phenyl)phosphate (BNP-) (0.05 mM) at pH 7 and 25°C with a rate 3.2 times faster than the parent Coni-cyclen (2 mM). The more rigid complex 40 was designed to accommodate inorganic phosphate in the in-temuclear pocket and to prevent formation of an intramolecular ju.-oxo dinuclear complex. The dinuclear cobalt(III) complex 40 (1 mM) indeed hydrolyzed 4-nitrophenyl phosphate (NP2-) (0.025 mM) 10 times faster than Coni-cyclen (2 mM) at pH 7 and 25°C (see Scheme 10). The final product was postulated to be 41 on the basis of 31P NMR analysis. In 40, one cobalt(III) ion probably provides a nucleophilic water molecule, while the second cobalt(III) binds the phosphoryl group in the form of a four-membered ring (see 42). The reaction of the phosphomonoester NP2- can therefore profit from the special placement of the two metal ions. As expected from the weaker interaction of BNP- with cobalt(in), 40 did not show enhanced reactivity toward BNP-. However, in the absence of more quantitative data, a detailed reaction mechanism cannot be drawn. 

Scheme 1 Zinc(II) complexes with [12]aneN3 and cyclen.

Very recently, a new type of metallomicelle with a zinc(II) cyclen attached to a hexadecyl group, 11, has been synthesized (Scheme 6). An unprecedented clear picture of its catalytic behavior in a comicellar solution with Triton X-100 has been disclosed [28], Since the parent Zn2+—cyclen complex 2 showed discrete behavior in aqueous solution (i.e., no dissociation of the zinc(II) ion stable, discrete monomeric species) [7,13,14], 11 was anticipated to behave in an orderly way, and that turned out to be the case. 

Scheme 6 Lipophilic cyclen, its zinc(II) complex, and neutral surfactant Triton X-100.

Figure 11 Proposed reaction mechanism for TNP hydrolysis catalyzed by lipophilic and hydrophilic zinc(II)-cyclen complexes, lib and 2b.

Aoki, S. and Kimura, E. (2000) Highly selective recognition of thymidine mono- and diphosphate nucleotides in aqueous solution by ditopic receptors zinc(II)-bis(cyclen) complexes (cyclen = 1, 4, 7, 10-tetraazacyclododecane), J. Am. Chem. Soc. 122, 4542-4548. 

Stoichiometric complexes of the zinc(II)-cyclene monomer with tetraacetylriboflavin and creatinine were prepared and co-polymerised with ethylene glycol dimethyl acrylate. After exhaustive extraction of the material which removed impurities and the templates, a functional polymer was formed. Control polymers without template and with cobalt(II) instead of zinc(II) were also prepared. 

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