Cobalt complexes phosphates

Stepwise Hydrolysis of a Cobalt (III) Phosphate Complex (X = CXi6H4(N02)2)... 

The absorbance of the Co(III) complex solution may be measured with higher sensitivity at 415-425 nm, or with lower sensitivity at 500-520 nm (a reagent blank, or water as reference). The reaction of cobalt with nitroso-R salt is usually done in a hot weakly acidic medium buffered with sodium acetate. The solution is then made sufficiently acidic with hydrochloric or nitric acid to decompose the nitroso-R salt complexes of other metals (e.g., Cu, Ni, Fe, and Mn), which are less stable than the cobalt(III) complex. Phosphate or fluoride masks iron (III), which has a yellow colour in hydrochloric acid medium. 

A cooperative effect of two mononuclear complexes has been observed in the hydrolysis of phosphate mono- or diesters, the reaction order being then two with respect to the metal (343-348). Hydrolysis of methyl para-nitrophenylphosphate diester doubly coordinated to a di-nuclear cobalt(III) complex was reported, and a crystalline compound of dimethylphosphate coordinated to the same cobalt complex was characterized [Co2 (l,4,7-triazacyclononane)2 (0H)2 02P(0CH3)2 ] ... 

Two different binuclear copperdi) complexes have been prepared recently, one with a bridging phenoxy ligand having two bis-benzi-midazole arms (12, Fig. 14), and the second having a bis-cyclen-naphthalene ligand (13, Fig. 15) (352, 353). Both of them show bimetallic cooperativity for the hydrolysis of phosphate diesters, contrary to studies with the dinuclear cobalt complex (354). The pseudo-first-order rate constants for hydrolysis of the para-nitrophenylphosphate ester of propylene glycol by bis-benzimidazole-based copper complexes... 

Preparations are listed in Table 47. Main interest lies in determining how metal ions catalyze the hydrolysis of ADP and ATP. ATP plays a crucial role in the energy metabolism of all living cells and divalent metal ions (Mg " ", Mn " and Ca ) play an important role in these phosphoryl transfer processes. Divalent metal ions such as Mg ", Ca, Zn " ", Cu and Mn " provide only modest in vitro catalysis and stronger, more specific coordination to phosphate units appears to be required by the enzyme. Co " (and Cr" ) complexes of ADP and ATP have been shown to mimic many of the biological functions of the Mg " " enzyme, and since the cobalt(III)-phosphate coordination remains intact, the specificity of alternative coordination sites, and the stereochemical requirements at phosphorus, have been elucidated in some cases. Often the Co" -enzyme species is biologically active and several enzymic functions of ATP have been examined in this manner. 

After that, the built-up Zinc-enzyme model (Fig. 6.2(b)) could effectively catalyze the hydrolysis of aryl phosphate. Another better hydrolyzing enzyme model is 4 nitrogen heterocyclic 12 silane cobalt complexes (Fig. 6.2(c)), which is one of the most effective acyl transferring catalysts. If the 4 nitrogen heterocyclic 12 silane cobalt complexes were covalently linked with -CD, the obtained enzyme model could increase 900-fold of the hydrolysis rate of PNPA (p-nitrophenol acetate). If the same group was conjugated onto the second hydroxyl group of f -CD, the obtained model could increase 2,900-3,700-fold of the hydrolysis rate of PNPA [21]. 

The effects of four cobalt(III) complexes [Co(trien)(OH)(OH2)] , [Co(en)2(OH)(OH)2]"-", [Co(dien)(OH)(OH2)2] ", and [Co(en)2(OH)(NH3)] -" on the rate of hydrolysis of bis(p-nitrophenyl)phosphate, p-nitrophenyl phosphate, and bis(2,4-dinitrophenyl)phosphate have been examined at 50 under neutral pH conditions. At pH 7 the cobalt-bound phosphodiester [Co(en)2(OH)[OP(0)-(0C6H4N02)2] is cleaved 10 times more rapidly (2.7 x 10 s" ) than the unbound phosphodiester, while the cobalt-bound phosphomonoester is cleaved only 10" times more rapidly. The reactivity of the cobalt complexes is [Co(trien)(OH)-... 

Efforts to produce a phosphate-selective ISE have been hindered by its diverse spe-ciation and lability in biological samples a recent review describes various potentiometric and amperometric approaches to this problem [22]. Of the potentiometric approaches, selectivity is most often achieved using inorganic or organometal-lic extracting agents such as organotin compounds in liquid-membrane ISEs, cobalt complexes or metallic cobalt in coated-wire and metallic electrodes, respectively. Nickel phosphate, silver phosphate, and mixtures of lead precipitates have also been used in phosphate ISEs. All of these sensors suffer from limited selectivity. Enzyme-based sensors for phosphate have also been a topic of research as yet, however, no commercial phosphate sensor exists [22]. 

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