Studies of Zinc Complexes

Zinc forms a number of complexes but they are not as numerous as those of the 3d transition metals. They are generally less stable than those of the preceding metals. The complexes may be cationic, anionic or neutral e.g. the complex with 8-hydroxyquinoline encountered above. 

The tetrahedral tetraamminezinc(II) ion can be precipitated by the large tetrafluoroborate(III) anion. The complex can be analysed by dissolving in a known excess of HCl, back titrating the excess with standard alkali. 

Recrystallise 3 g of NH4BF4 (Sec.4.4.1). Dissolve 2 g of zinc sulphate heptahydrate in the minimum volume of water. Add cone, ammonia until the precipitate initially formed has completely dissolved. Add a solution of the recrystallised fluoroborate to the zinc salt solution with stirring. Allow the precipitate to stand until crystallisation seems complete. Filter the complex precipitated under suction. Wash on the filter with small volumes of acetone. Dry by continued suction while pressing between filter paper. Weigh your product and calculate the % yield based on zinc. 

Weigh out accurately about 0.25 g of your preparation into a conical flask. Add 50.0 cm of 0.1 M HCl from a burette followed by a few drops of methyl red indicator. Titrate with standard 0.1 M NaOH until the colour changes to yellow. Repeat to obtain concordant results. Calculate the average % purity of your preparation. 

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Most studies of zinc complexes of amino acids have concentrated upon naturally occurring ligands, but some other amino acids have been investigated. Complexes with 3- and 4-aminobenzoic acids have been reported247,248 and a crystal structure of [ZnL2]-1.5H20 (HL = 4-aminobenzoic acid) has shown that the compound is polymeric, with the metal in a distorted N202 environment.248... 

Outside the scope of coverage of this contribution are studies of zinc complexes having peptide-based ligands260 262 and Zn(II)-complex-promoted reactions involving RNA or DNA hydrolysis.261 263 288... 

Parkhurst DA, Brown GE Jr, Parks GA, Waychunas GA (1984) Structural study of zinc complexes in aqueous chloride solutions by fluorescence EXAFS spectroscopy. Abstracts with Program Geol Soc Am AnnMtg 16 618... 

In this work, the results of study of zinc thiocyanate complexes anion-exchange extraction by non-symmetric QASes in toluene ai e discussed. The non-symmetric QASes have the common formula [(C,3H g03)N(CH3) (C,H Q3 J-X-, where C,3H3 03 - highly lipophilic substituent, (2, 3, 4-tn. s-dodecyloxy)benzyl. It was found that exchange... 

ANALYTICAL AND PHYSICAL TECHNIQUES FOR THE STUDY OF ZINC COORDINATION COMPLEXES... 

IR and Raman spectroscopy have been commonly used and, for example, the effects of pressure on the Raman spectrum of a zinc compound with a N2C12 coordination sphere around the metal, have been investigated.28 IR spectroscopy has been utilized in studies of the hydration of zinc in aqueous solution and in the hydrated perchlorate salt.29 Gas phase chemistry of zinc complexes has been studied with some gas phase electron diffraction structures including amide and dithiocarbamate compounds.30-32... 

Bhalla el al. synthesized and studied the zinc complex of bis(l -methyl-4,5-diphenylimidaz-2-oyl) carbinol. The ligand coordinates as bidentate resulting in a slightly distorted tetrahedral geometry, [ZnL2]2+. 

Blue luminescence of zinc complexes of pyridyl-containing complexes is an area of current interest.277 Design of blue luminescent materials is of relevance to display applications, as blue-light-emitting diodes, and to this end Che examined solution luminescence of zinc pyridylamine complexes.73,278 Che and co-workers studied the complex Zn40(7-azaindoyl)6 which has a blue emission at 433 nm in the solid state.279,280 In an attempt to improve on stability Wang et al. examined compounds with neutral 7-azaindole and an A-functionalized pyridyl derivative.281 In contrast with other metal complexes of the neutral 7-azaindole (32), Zn(7-azaindole)2(OAc)2 is a blue luminescent compound and a A-(2-pyridyl) 2-azaindole (33) and its complexes were also... 

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. ... 

Rather earlier but still recent studies include those of zinc complexes with glycine,270,273 alanine,272,274 /5-alanine,272 norleudne,271 phenylalanine,272 proline, methionine, serine,274 histidine,271 asparagine,276 adrenaline,277 noradrenaline,277 terizidone,278 S-ethyl-L-cysteine,279 5,5 -methylenebis(L-cysteine),280 ornithine, lysine, 2,4-diaminobutyric add and 2,3-diaminopropionic acid.281... 

The formation of zinc complexes with a further series of [14Jane-N4 macrocycles has been investigated.1127 The macrocycle [9]ane-l-0-4,7-N2 forms a zinc complex,1128 and the crown ether complex [ZnL][SbQ6]2 (L = 18-crown-6) has also been characterized.1129 Zinc complexes of a range of encrypting ligands have been studied.1130,1131... 

A number of studies concerning zinc complexes of porphyrins or related ligands have also been reported 1193-1196 cadmium complexes of a number of porphyrins have also been investigated,1197-1199 a number of NMR studies of zinc porphyrin complexes have been made1200,1201 and a number of donor-acceptor complexes of zinc tetrapyrrole species with 1,3,5-trinitrobenzene have been described.1202 Recent developments in metallo-phthalocyamine chemistry, including aspects of zinc complexes, have been reviewed.9-15,12026,1468 11... 

Copper, silver and gold - The paramagnetism of Cu11 has limited the use of NMR for the direct study of copper complexes, but information is readily obtained from studies of ligand nuclei. For example, the broadening of the NMR signal by copper in copper-zinc and copper-cobalt superoxide dismutases (SODs) has been used to determine the distance between the copper and the proton on bound formate (Sette et al., 1992). Also, broadening of the formate 13C NMR resonance reveals information about the orientation of the formate. 

The study of zinc-aqua complexes as synthetic carbonic anhydrase models has shown a low coordination number and a hydrophobic environment to be prerequisites for a low pK value of the aqua ligand, which is essential for efficient enzyme function. The pK value in the case of 13 is expected to be greater than 10.7, which is the value determined for pentacoordinate [Zn(tren)(H20)](C104)2, in which the water ligand is more strongly bound and thus expected to be more acidic (cf. the discussion of bond lengths, above). Thus, 13 is not expected to show carbonic anhydrase-like reactivity. However, in related tetraazamacrocyclic systems it has been shown that upon... 

Ward, R. L., and J. A. Happe 35C1 NMR studies of zinc adenosine diphosphate complexes. Biochem. Biophys. Res. Commun. 28, 785 (1967). 

The principal impetus to the study of zinc thiolates has undoubtedly been the search for structural models for the metal coordination in zinc metalloproteins such as the Zn(S-Cys)2(His)2 center in the transcription factor IIIA and other zinc fingers that feature in protein-DNA interactions (93, 94). The 2 1 complexes [M(SR)2l (M = Zn, Cd SR = TIPT, TEMT) were used as the principle precursors for the synthesis of such models for the Zn(S-Cys)2(His)2. Thus complexes of the types [Cd(TIPT)2(l-CH3-imid)2], [Zn(TIPT)2(bipy)], and [Zn(TEMT)2(l-CH3-imid)2l (imid = imidazole, bipy = bipyridine) were ssmthesized by addition of a nitrogeneous base to the precursor in MeCN. All of them have been characterized by X-ray crystallography and were found to have distorted tetrahedral geometries. It was noticed that the cobalt analogue binds a solvent molecule MeCN to form five-coordinate [Co(TIPT)2(bipy)(CH3CN)], whereas [Zn(TIPT)2(bipy)] does not 96,97). 

Stopped-flow fluorescence studies of ES complexes provided a direct comparison of the peptide binding aflBnities of the zinc and cadmium enzymes and, simultaneously, an explanation for the different roles of metals in peptide and ester hydrolysis (48). Cadmium carboxypeptidase binds the peptide Dns-(Gly)3-L-Phe as readily as does [(CPD)Zn] but catalyzes its hydrolysis at a rate that is reduced considerably (Figure 8). Initial rate studies of oligopeptides are in agreement with this observation. For all peptides examined, the catalytic rate constants of the cadmium enzyme are decreased markedly, but the association constants (1) (Km values) of the cadmium enzyme are identical to those of the zinc enzyme (48,51,57). However, in marked contrast, for all esters examined the catalytic rate constants of the cadmium enzyme are nearly the same as those of the zinc enzyme, but the association constants are decreased greatly. 

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