Cobalt zinc dimer

Analogously, pyrazolyl-aluminate and -indate ligands have been prepared <75JCS(D)749) and their chelating properties evaluated with cobalt, nickel, copper and zinc. Gallyl derivatives of pyrazoles and indazoles have been extensively studied by Storr and Trotter e.g. 75CJC2944) who determined several X-ray structures of these compounds. These derivatives exist in the solid state as dimers, such as (212) and (288). A NMR study in acetone solution showed the existence of a slow equilibrium between the dimer (212) and two identical tautomers (289) and (290) (Section 4.04.1.5.1) (81JOM(215)157). 

Bied-Charreton (63) showed by XH NMR spectroscopy that a zinc porphyrin bearing a primary amino group attached via a flexible spacer to the meta position of a /neso-phenyl ring spontaneously dimerizes in solution via NH Zn coordination. The corresponding ortho derivative shows an equilibrium between the dimeric form and the intramolecu-larly coordinated monomer. ESR spectroscopy was used to demonstrate the same behavior in the corresponding cobalt porphyrin (64). 

There has been some uncertainty concerning the metal content of alkaline phosphatase and the role of zinc in the catalytic process. Early measurements by Plocke et al. (36, 50) showed that there were 2 g-atoms per dimer. The zinc requirement for enzymic activity was demonstrated by the inhibition of the enzyme with metal binding agents in accord with the order of the stability constants of their zinc complexes. It appears that in some cases (EDTA) zinc is removed from the enzyme and in other cases (CN) the ligand adds to the metalloprotein. A zinc-free inactive apoenzyme was formed by dialysis against 1,10-phenanthro-line. Complete activity was restored by zinc only zinc, cobalt, and possibly mercury produce active enzyme. 

Interest in these ligands stems from a desire to synthesize improved analogues of the cobalamines 100). With this in mind, 2,6-diacetylpyridine was condensed with N,N,N- f ra(aminopropyl)amine in the presence of nickel(II) or eopper(II) to afford the complex of 105, Cobalt(II) and zinc(II) have also been employed as templating agents in the synthesis of 105 101). The reaction of the nickel(II) complex of 105 with acetone results in a dimeric complex, 106101), by a process that is well established for primary amines 102). 

Selenourea is less stable than urea or thiourea, as it is sensitive to air and light, particularly in solution, and tends to dimerize in certain systems. In general, the donor properties of selenourea are similar to those of thiourea,2.3 and complexes with cobalt(II),4,5 palladium(II), platinum(n), zinc, cadmium, and mercury acceptors can be obtained by appropriate modification of the methods used for the preparation of the analogous thiourea derivatives. 

Benzylic and allylic halides dimerize (homo-couple) readily in the presence of a number of metal species. Synthetically useful yields of homo-coupled products have been obtained using chlorotris(tri-phenylphosphine)cobalt(I), nickel(O) complexes generated in situ, ° Te " species, VCb/LAH, CrCh/LAH and TiCb or TiCl4/LAH. Representative examples are given in equations (37) and (38). Treatment of 8-bromocrotonates with zinc(O) in DMSO leads to dimerization. ... 

To this end, the spin labeling technique was used to probe the active site structure of LADH in various solvent systems. The spin label, SL-1, alkylated cysteine 46 (12), an amino acid in the active site of LADH that normally serves as a ligand to the catalytic zinc ion. LADH is a dimer, and each monomeric subunit contains one firmly-bound catalytic zinc ion. The position of enzyme-bound SL-1 was estimated by the spin label-spin probe technique (14.15). Cobalt (II) was employed as the spin probe, and the catalytic zinc in the enzyme s active site was replaced by Co + according to the procedure of Sytkowski and Vallee (16). The EPR spectrum of enzyme-bound SL-1 was then measured before and after replacement of the active-site zinc by cobalt from the decrease in spectral amplitude the average nitroxide-metal distance was determined to be 4.8 1.5 A (17). 

For TIOA with hydrochloric acid the concentration-based equilibrium constant for salt formation" according to reaction (8.2-6) is 1.51 x 10 and the equilibrium constant for amine-hydrochloride salt dimerization" is 8.0 M Combination of these parameters and the ion-complex stability constants with experimental metal-distribution data allows determination of the equilibrium constants for reactions (8.2-5) or (8.2-7). This completes the description of the amine-metal extraction-phase equilibria. For cobalt(II) in acidic sodium chloride solutions the equilibrium constant" for reaction (8.2-7) with TIOA is 2.0 X 10 and that for coppeifll) is 370 The corresponding value for zinc" is 7.5 x 10 Af -In spile of these relative values, the order of selectivity of TIOA for extraction of the metals is Zn > Cu > Co because of the relative extent of chloride complex formation. For the same reason, zinc stripping is difficult in this system, and copper has a tendency to be reduced to cuprous, which also complexes and extracts extensively. 

Other Zinc Enzymes. Studies aimed at elucidating the mechanism of dimeric alcohol dehydrogenase enzymes have also been reported. Cobalt(n) can replace zinc(ii) at the two catalytic and non-catalytic sites in the liver enzyme, LADH. Reduction of pyridinealdehyde derivatives by the yeast enzyme has been examined. In acetonitrile solution, reduction by NADH analogues is catalysed by metal ions and mechanistic studies of the reaction have been carried out. Biomimetic studies of zinc-catalysed carbonyl reduction have also been reported. ... 

OPH catalyzes the hydrolysis of organophosphonates with P-0, P-F, P-S, and P-CN phosphoryl bonds such as those of coumaphos, sarin, VX, and paraoxon. Hydrolysis is facilitated by two metal atoms in the active site and results in the release of two protons (79). In the native dimeric form of the enzyme, zinc is coordinated to histidine residues in the active site. Reconstitution of the apo-entyme with cobalt or cadmium results in higher enzymatic activity than that observed in the zinc-metalloenzyme (20). 

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