Zinc n

The Leclanche dry cell with zinc replaced by magnesium offers considerable advantages. In the electromotive series, magnesium occupies a more basic position as compared to zinc ( n = -0.76 V / g = -2.38 V), and this means that the voltage of a magnesium cell is greater than that of a similar zinc cell. 

Limiting currents measured for a deposition reaction may be excessively high due to surface roughness formation near the limiting current. Rough deposits in the case of copper deposition have been mentioned several times in previous sections, since this reaction is one commonly used in limiting-current measurements. However, many other metals form dendritic or powdery deposits under limiting-current conditions, for example, zinc (N lb) and silver. Processes of electrolytic metal powder formation have been reviewed by Ibl (12). 

Recently Cavaleiro et al. described an easy synthetic approach to glycoporphyrins from zinc(n) protoporphyrin-IX dimethyl ester 4 and O-allyl carbohydrate acetonides 5A-E (D-ribose (A), D-galactose (B), D-glucose (C), and two isomeric derivatives (D) and (E) of D-fructose) by cross-metathesis (Scheme 2).12 Two equivalents of each carbohydrate and the Grubbs catalyst were used, giving the carbohydrate derivatives 6 in a range of 74% to 93% yields. 

Mancini, I. Guella, G. Debitus, C. Pietra, F. (1995) Novel naamidine-type alkaloids and mixed-ligand zinc(n) complexes from a calcareous sponge, Leucetta sp., of the Coral Sea. Helv. Chim. Acta, 78, 1178-84. 

Zinc N-phenylporphyrin (17) also brings about the living polymerization of PS, affording a polymer of controlled molecular weight with a narrow MWD. The polymerization with the initial mole ratio [PS]o/[17]q of 400 proceeded up to 88% conversion in 50 min at 25 °C, giving a polymer with Mn and Mw/Mn of 24,400 and 1.05, respectively. 

When the guest used is p-nitrophenylcholine carbonate (PNPCC) the Lewis acid zinc(n) activates the well-positioned carbonyl group in the P PCC Zn-cavitand towards reactions with external nucleophiles. The energy minimized structure of the PNPCC Zn-cavitand complex shows that cation-n interactions and C —O -Zn coordination bond occurs simultaneously. 

A monophosphine complex is formed when 3 is mixed with three equivalents of a zinc(ii)salphen complex and half an equivalent of Rh(acac)(CO)2 (acac = acetyl acetonate), whereas the assembly based on template 4 and the zinc(n)salphen complexes forms a bis-phosphine rhodium species. In the latter case, the bisphosphine rhodium complex is completely encapsulated by six salphen building blocks. This difference in mono- versus diphosphine ligation to the Rh -center and, to a lesser extent, the difference in electronic features (and thus donating properties of the phosphine) between template ligands 3 and 4, can be used to induce a different catalytic behavior. 

The rates of hydrolysis of amino acid esters or amides are often accelerated a million times or so by the addition of simple metal salts. Salts of nickel(n), copper(n), zinc(n) and cobalt(m) have proved to be particularly effective for this. The last ion is non-labile and reactions are sufficiently slow to allow both detailed mechanistic studies and the isolation of intermediates, whereas in the case of the other ions ligand exchange processes are sufficiently rapid that numerous solution species are often present. Over the past thirty years the interactions of metal ions with amino acid derivatives have been investigated intensively, and the interested reader is referred to the suggestions for further reading at the end of the book for more comprehensive treatments of this interesting and important area. 

In this reaction, the incoming alkoxide group is almost certainly co-ordinated to the metal centre in the key stages of the reaction. A particularly elegant demonstration of this is seen in the zinc(n) catalysed transesterification of 4-nitrophenyl picolinate with HO(CH2)2NH(CH2)2NH2 (Figs. 5-87 and 5-88). In the initial step of the reaction, an intermediate complex 5.38 may be isolated (Fig. 5-87). 

S-Donor ligands. As an extension of previous studies on related complexes of iron(n), cobalt(n), nickel(n), and zinc(n), the structure of Mn[SPPh2-N PPh2S]2 has been determined by X-ray methods. The metal atom is co-ordinated in an approximately tetrahedral manner by the four sulphur atoms. The two MnS2P2N rings adopt the twisted boat conformation with S and P atoms at the apices. The single-crystal electronic spectrum has been measured and interpreted.92... 

Raman spectral studies of the species MX(n" 2) (n = 2—4 M = Zn, Cd, or Hg X = Cl, Br, or I) in anhydrous tributyl phosphate have been reported.24 For the MX2 molecules, sufficient metal dihalide-solvent interaction exists to suggest bent X—M—X species with C2v rather than >ooh symmetry. The effect appears most marked for zinc(n) and least marked for mercury(n), which is in accord with the Lewis acidity sequence ZnX2 > CdX2 > HgX2. A similar analysis of the anionic MX3 complexes formed from a 1 1 mixture of LiX and MX2 again demonstrates solvent interaction, and a tetrahedral C3v species is indicated, rather than the planar structure found in the solid state. Studies involving the halogeno-complexes of zinc, cadmium, and mercury in DMSO and DMF have also been reported.25,26... 

A similarity between the location of zinc(n) in yeast alcohol dehydrogenase (YADH) and that of the metal in LADH has been established.240 NADH and substrate proton relaxation rates using the paramagnetic iodoacetamide analogue complex of YADH show the coenzyme nicotinamide moiety to be situated ca. 7 A from the zinc ion. This is virtually identical with the position of the metal in LADH determined by X-ray crystallography.241 The substrate resides between the NADH and the metal ion, i.e. it is co-ordinated to the zinc. 

The same workers have reported245 that in three-ligand systems of zinc(n) or lead(n) and orthophosphate, cysteine, and a carboxylate, mixed-ligand complexes predominate at physiological pH. Such complexes solubilize lead(n). 

A study of the mixed complexes formed by metal interactions with asparagine and glutamine has shown that quite a large proportion of asparagine in blood may be co-ordinated to zinc(n) and iron(n) as well as copper(n).246... 

The stability constants of zinc(n) complexes of uracil, thymine, and cytosine have been reported.249 At 45 °C in 0.1M-KNO3, 1 1 complexes are formed. The 2 1 ligand metal complexes formed between thiosalicylic acid and zinc(n), mercury(n), cadmium-(n) and lead(n) have been isolated, and formation of the 1 1 complexes in solution has been characterized by pH-titration.250 With mercury, the 2 1 complex has been assigned the structure (7), while the other metals form complexes of general structure (8). This is thought to be a consequence of the order S—M11 bond strength being Hg > Zn > Cd > Pb. 

C N.m.r. has been used to determine the binding sites of zinc(n), cadmium(n), mercury(n) and lead(n) to glutathione.251 All the metals bind to the potential co-ordi-... 

The 2 1 DL-penicillamine zinc(n) complex has been isolated and its structure designated as (9) using i.r. spectra.255... 

It has been reported that zinc ion inhibits the uptake of Fe2+ by apo-ferritin as a result of zinc(n) binding (i) on the surface of the protein at those sites thought usually to be occupied by Fe2+, and where catalytic hydrolysis of Fe2+ to FemOOH occurs, and (ii) within the protein, where it prevents the microcrystalline growth of FemOOH.274... 

A study of the hyperfine splitting of the e.s.r. spectrum of the cation radical of oxidized zinc(n) bacteriochlorin, together with selective deuteriation, has allowed assignment of the electron density to the atoms of the system.279 There is high spin density on the protons of the saturated rings of the bacteriochlorin with or without the zinc. A similar situation may pertain in metal-free bacteriochlorophyll. 

Although strictly not a dendritic system, Agar et al.[75] have reported the preparation of copper(n) phthalocyaninate substituted with eight 12-membered tetraaza macrocycles as well as its nickel(n), copper(n), cobalt(n), and zinc(n) complexes. Thus, the use of the l,4,7-tritosyl-l,4,7,10-tetraazacyclododecane offers a novel approach to the 1 — 3 branching pattern and a locus for metal ion encapsulation. 

A zinc(n) meso-meso linked porphyrin oligomer 57 exists in a nonhelical conformation in solution, but may adopt a dynamic helical conformation upon complexation with an achiral urea 58 through complementary hydrogen bonding interactions [115]. In the presence of the chiral diamine (S)-59, the 57-58 complex forms a predominantly one-handed helical conformation, thus showing a characteristic ICD in the absorption region of the porphyrin chromophore. This system may be used to sense the chirality of chiral diamines. 

A further method to induce chirality in the pyridoxamine-mediated transamination reactions was developed by Kuzuhara et al. [13]. They synthesized optically resolved pyridinophanes (21, 22) having a nonbranched ansa chain" between the 2 - and 5 -positions of pyridoxamine. With the five-carbon chain in 21 and 22, the two isomers do not interconvert readily. In the presence of zinc(n) in organic solvents such as methanol, tert-butanol, acetonitrile, and nitromethane, they observed stereoselective transamination between pyridinophanes and keto acids. The highest ee%s are 95 % for d-and L-leucine by reaction of the corresponding a-keto acid with (S)- and (R)- 22, respectively. On the basis of kinetic analysis of the transamination reactions, Kuzuhara et al. originally proposed a mechanism for the asymmetric induction through kinetically controlled stereoselective protonation to the carboanion attached to an octahedral Zn(n) chelate intermediate. However, they subsequently raised some questions about this proposal [14]. 

Zinc(n) complexes have also proved effective [70]. 

Although in some cases, copper catalysis has little effect on the stereochemistry, some asymmetric induction by chiral copper catalysts such as copper(i) complexes of aminotropone iminates (8) [79] or the chiral arylthiocopper compound (9) [80] has been achieved. Chiral zinc(n) complexes (8) also promote enantioselective conjugate addition [81]. 

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