Zinc halides coordination

Zinc halide complexes with acetophenone have been structurally characterized and form monomeric or polymeric structural motifs.346 The bromide and iodide derivatives are monomeric and the chloride derivative is a coordination polymer with bridging chlorides. 

The presence of halides in the coordination sphere with aldehydes is important as zinc halides are typically used in organic synthesis as organic carbonyl activators. Large excesses of aldehydes and anhydrous zinc halides were necessary. Both monomeric and polymeric structures with halide bridges were observed. Tetrahedral geometries were observed for mixed ligand, aldehyde, and halide complexes.353... 

A number of zinc halide complexes, including bridging halides, have already been mentioned in the context of the other ligands systems discussed in this Chapter. Examples will be presented of catalytic systems and solution speciation, particularly in the presence of other coordinating ligands. There are now also a few zinc fluoride species that have been well characterized. 

In addition, the mechanism of the zinc-catalyzed [3+2] dipolar cycloaddition of azides and nitriles to form tetrazoles was examined <2003JA9983>. The energy barrier of the reaction is lowered by 5-6kcalmol 1 which corresponds to an acceleration of 3 1 orders of magnitude. The source of the catalytic activity seems to be the coordination of the Lewis acidic zinc halide to the nitrile, which is supported by model calculations. Also AICI3 was examined as another Lewis acid which catalyzes the reaction to a greater extent than ZnBr2-... 

Metal complexes of several zinc, cadmium and mercury salts with 2-, 3- and 4-cyanopyridine have been reported.495 In none of the complexes was cyanide coordination observed. Zinc halides react with 3- and 4-cyanopyridine, but not with 2-cyanopyridine, to give 1 2 complexes which are assigned a monomeric tetrahedral structure on the basis of IR evidence. The cadmium halides also form 1 2 complexes with all the cyanopyridines, except cadmium chloride, which reacts with 2-cyanopyridine to give a 1 1 complex. The former contain... 

The same coordination geometry was also inferred from the analysis of the lowest energy LMCT bands (see Ligand-to-Metal Charge Transfer) in the far-UV absorption spectra of Zu7- and Cd7-MT, and those of corresponding tetrahedral halide complexes (see Cadmium Inorganic Coordination Chemistry see Zinc Inorganic Coordination Chemistry) Further support came from... 

Zinc(I) complexes, 989 Zinc compounds industrial uses, 998 Zinc halides, 981 molten state coordination, 981 solution chemistry, 893... 

These data show that in the course of the formation of the zinc halides in dimethyl sulphoxide from the originally octahedral hexasolvate, the tetrahedral complex is formed as a result of the coordination of the second halide. This is the cause of the strikingly low KJK2 value and the strikingly high KJK value. In the case of the cadmium complexes, it appears that this transformation occurs during the coordination of the third ligand. 

The halogen quadrupole relaxation rates have been used to obtain information on the nature of the halide ion binding sites. Ward and Happe [456] assumed the chloride ions to coordinate directly to the zinc ions situated at the active sites of the enzyme. Lindman et al. [54 458 459] f on the other hand, reasoned that halide coordination to zinc has no observable influence on the halide ion quadrupole relaxation. This deduction was based mainly on results from studies with ligands believed or known to coordinate to the functional zinc ions. Thus it has been observed that neither oxyquinoline nor orthophen-anthroline influence the halogen relaxation rates [54 456 458 459]. While the failure of orthophenanthroline, a well known chelator of zinc ions, to affect halide ion relaxation was referred by Lindman... 

The nature of the halide binding sites in LADH have been in dispute. Ward and Happe (74) assumed that Cl did coordinate directly to the zinc atom at the active site. On the other hand Lindman et al. (73,76,77) concluded that halide coordination to the zinc, if it occurs, has no influence on the halide ion relaxation rate. Their conclusions were based on results from halogen NMR studies in the presence of ligands believed or known to coordinate to the zinc in the active site. Thus neither oxy-quinoline nor ortho-phenanthroline affect the chloride ion relaxation. 

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