Zinc complexes formation rate constants

A number of important structural aspects of zinc complexes as found in enzymes are introduced in this section to serve as background information for the subsequent sections. Aquated Zn(II) ions exist as octahedral [Zn(H20)6] + complexes in aqueous solution. The coordinated water molecules are loosely bound to the Zn + metal center and exchange rapidly with water molecules in the second coordination sphere (see Figure 1) with a rate constant of ca 10 s at 25 °C extrapolated from complex-formation rate constants of Zn + ions with a series of nucleophiles. The mechanism of the water exchange reaction on Zn(II) was studied theoretically, from which it was concluded that the reaction follows a dissociative mechanism as outlined in Figure 2. ... 

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... 

Alkali, alkaline-earth, and rare-earth metal cations also catalyze electron transfer reactions. Thus, in the pair of Co -tetraphenylporphyrin complex with BQ, no redox reaction takes place, or it takes place too slowly to be determined. The metal cations promote this reaction. For example, in the presence of 80(0104)3, the corresponding rate constant of 2.7 X 10 M s was observed. BQ transforms into benzosemiquinone under these conditions (Fukuzumi and Ohkubo 2000). Zinc perchlorate accelerates the reaction between aromatic amines and quinones (Strizhakova et al. 1985). This reaction results in the formation of charge-transfer complexes [ArNHj Q ]. The complexes dissociate in polar solvents, giving ion-radicals ... 

ZnA+ (HA = amino acid) rapidly associates with the free base porphyrin to form a molecular complex (I) in which ZnA" " is weakly bound to the porphyrin plane. Then the water molecule coordina zincfll) in I dissociates, and zinc(ll) is incorporated into the porphyrin core. Thus the over-all rate constant of formation of zinc(ll) porphyrin in the presence of an amino acid is given by... 

Binuclear zinc complexes have been shown to mediate the hydrolysis of the activated peptide model substrate L-leucine- -nitroanilide (LNA, Fig. 32) with the reactions being followed spectroscopically by the formation of /7-nitroaniline ( max 400 nm).165-167 This model substrate has also been used in studies of ApAP.170 The hydrolysis of LNA mediated by the zinc complex [(bomp-)]Zn2(CH3C02)2]BPh4 (Fig. 33a H(bomp) 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol) in water/DMF (6 4) occurs via a reaction that is first-order in complex and substrate, with a second-order rate constant k = 2.3(1) x 10 3M 1 s-1 at 25 °C. At best, a yield of 65% for a single turnover reaction was obtained, indicating that... 

The reactivity of zinc complexes supported by a variety of tridentate amine donor ligands (Fig. 44) with diphenyl 4-nitrophenyl phosphate in 20% (v/v) acetonitrile-water, and with 2,4-dinitrophenyl diethyl phosphate in 1 % (v/v) in methanol-water, has been investigated. 5 For the former reaction, the second-order rate constant for the hydrolysis of 2,4-dinitrophenyl diethyl phosphate correlates linearly with the —AH value for the formation of the [(ligand)Zn(OH2)]2 + complex from free chelate ligand and aqueous zinc ion. This indicates that in this series of complexes, faster hydrolysis of 2,4-dinitrophenyl diethyl phosphate corresponds to weaker chelate... 

The formation constant for 1 1 complex formation between bipy and Zn " in DMSO has been determined the interesting feature from the viewpoint of this section is that the dissociation of [Zn(bipy)(DMSO)4] " is catalyzed by The zinc-ion-cat yzed aquation of cis-[Cr(H20)2L2] (HL = CH2(C02H)2) has been shown to obey a rate law of the type in equation (10), although other transition metal ions are more effective catalysts. 

The apparent second-order rate constants measured for substitution of inner-sphere H2O by other ligands invariably show little or no dependence on ligand structure. Thus, acetate, sulfate, and chloride ions all combine with the aquo-zinc complex with 2 3 X sec i at 25° (2), while complex formation... 

Copper(II) and zinc(II) are two of the more labile divalent metal ions and as a consequence the former is too labile for its water exchange rate to be determined by the NMR methods which utilize the paramagnetism of other divalent first-row transition metal ions, while the latter is diamagnetic and such NMR methods cannot be applied. However, it has been shown that water exchange rates and mechanisms can be deduced with reasonable reliability from simple ligand substitution studies, and this is one of the reasons for a recent variable-pressure spec-trophotometric SF study of the substitution of 2-chloro-l,10-phenanthroline on Cu(II) and Zn(II). The observed rate constants for the complexation reaction (kc) and the decomplexation reaction (k ) and their associated activation parameters for Cu(II) and Zn(II) are kc(298 K) = 1.1 x 10 and 1.1 x 10 dm mol" s", AH = 33.6 and 37.9 kJ mol", A5 = 3 and -2JK- mol", AV = 7.1 and 5.0 cm" mol", k 29S K) = 102 and 887 s", AH = 60.6 and 57.3 kJ mol", A5 = -3 and 4 J K" mol" and A V = 5.2 and 4.1 cm" mol". These data are consistent with the operation of an mechanism for the rate-determining first bond formation by 2-chloro-l,10-phenanthroline with the subsequent chelation step being faster [Eq. (18)]. For this mechanistic sequence (in which [M(H20)6 L-L] is an outer-sphere complex) it may be shown that the relationships in Eq. (19) apply. 

Ibrm a Waison-Crick hydrogen-bonding inieraction. The association conslant derived from a H NMR study is 3.1 X lO M in CDiCli. Upon addition of die quinone. the fluorescence of the zinc porphyrin was quenched and its decay profile became biphasic with two exponential components of lifetimes, x, —1.5 ns and rs = 0.94ns, respectively. Both lifetimes did not depend on the concentration of the quinone, while the relative contribution of the shorter component incieased as the quinone concentration w as increa.sed.. suggesting a porphyrin-quinone complex formation. The rate constant of forward ET from photoexcited porphyrin to quinone was estimated to be 4.2 X lO s... 

In a polarographic study, the rates of dissociation of some aminopolycarboxylato-zinc(n), -cadmium(ii), and -lead(ii) complexes have been measured, and n.m,r. has been used in an analogous study on the same metals. The kinetics of the dissociation of the cobalt(ii) complex of ethylenediamine monoacetate have also been measured polarographically, and the rate constants for the formation of the cobalt(n) complexes of anthranilate, salicylate, 5-sulphosalicylate, and ligand (5) have been compared with those of the respective nickel(n) analogues. 

Zinc chloride and (under some conditions) zinc nitrate solutions in aqueous DMSO, on the other hand, yield " two ultrasonic absorption maxima with the chloride this is taken as evidence for an octahedral-tetrahedral coordination change accompanying the addition of the third bound Cr, while with the nitrate the high-frequency relaxation (observable only at low-water-mole fractions) is attributed to outer-sphere complex formation. The rate constants (25 C) for solvent loss with the nitrate (Xh20 = 0.59) and the chloride (Xhjo = 0.039 and 0.904) are, respectively, 2.2 X 10 , 4.1 X 10 , and 3.3 x lO s. ... 

The mechanism of 1 1 complex formation between palladium(II) and catechol and 4-methylcatechol has been studied in acidic media, and the rate of 1 1 (and 1 2) complex formation between silver(II) and several diols is an order of magnitude higher in basic solution than in acidic. The kinetics of formation and dissociation of the complex between cop-per(II) and cryptand (2,2,1) in aqueous DMSO have been measured and the dissociation rate constant, in particular, found to be strongly dependent upon water concentration. The kinetics of the formation of the zinc(II) and mercury(II) complexes of 2-methyl-2-(2-pyridyl)thiazolidine have been measured, as they have for the metal exchange reaction between Cu " and the nitrilotriacetate complexes of cobalt(II) and lead(II). Two pathways are observed for ligand transfer between Ni(II), Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) and their dithiocarbamate complexes in DMSO the first involves dissociation of the ligand from the complex followed by substitution at the metal ion, while the second involves direct electrophilic attack by the metal ion on the dithiocarbamate complex. As expected, the relative importance of the pathways depends on the stability of the complex and the lability and electrophilic character of the metal ion. 

The influence of ZnCFO concentration (3,0 5,0 7,0 phr) on formation of properties complex of the unfilled rubber mixes and their vulcanizates on the basis of isoprene rubber of the following recipe, phr isoprene rubber - 100,0 sulfur - 1,0 di - (2-benzothiazolyl) -disulfide - 0,6 N, N -diphenylguanidine - 3,0 stearic acid - 1,0, was carried out in comparison with the known activator - zinc oxide (5,0 phr). The analysis of Rheometer data of sulfur vulcanization process of elastomeric compositions at 155°C (fig. 5) shows, that on crosslink density and cure rate, about what the constants of speed in the main period (k2) testify, they surpass the control composition with 5,0 phr of zinc oxide. Improvement of the complex of elastic - strong parameters of rubbers with ZnCFO as at normal test conditions, and after thermal air aging (tab. 1), probably, is caused by influence of the new activator on vulcanization network character. So, the percent part of polysulfide bonds (C-Sx-C) and amount of sulfur atoms appropriating to one crosslink (S atoms/crosslink) in vulcanizates with ZnCFO are decreased, the percent part of disulfide bonds (C-S2-C) is increased (fig. 62). 

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