Carbon reaction with aquo complexes

Equilibrium complexation constants for Cu reactions with natural organic matter and the details of Cu speciation are bound to remain somewhat uncertain, since the composition of the complexing molecules varies from site to site. What is not in dispute is that the fraction of dissolved copper present as free aquo Cu is probably very small in any natural water. In extremely pristine waters, hydroxide and carbonate complexes may dominate, but organic complexes usually dominate in waters containing more than a few tenths of a mg/L organic carbon. 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

In summary, oxygen reduces significantly the yield of immediate strand breaks following attack of hyaluronic acid by hydroxyl radicals, particularly at pH 7. This effect is mimicked quantitatively by the oxidants tetranitromethane and the copper(II)-aquo complex where the yield of strand breaks was reduced to 0.19 and 0.24 pmol J respectively. It is likely that these interact with some of the hyaluronic-acid free radicals which would otherwise be precursors of strand breakage. Candidates for such reactions may be carbon-centred a-hydroxy radicals which can be expected to be good reducing agents. An example... 

Preliminary results of the reaction between vanadium(iii)-tetrasulpho-phthalocyanine complex with oxygen have been reported these data were compared with those obtained for the corresponding reaction of the hexa-aquo complex ion. The oxidation of methyl ethyl ketone by oxygen in the presence of Mn"-phenanthroline complexes has been studied Mn " complexes were detected as intermediates in the reaction and the enolic form of the ketone hydroperoxide decomposed in a free-radical mechanism. In the oxidation of 1,3,5-trimethylcyclohexane, transition-metal [Cu", Co", Ni", and Fe"] laurates act as catalysts and whereas in the absence of these complexes there is pronounced hydroperoxide formation, this falls to a low stationary concentration in the presence of these species, the assumption being made that a metal-hydroperoxide complex is the initiator in the radical reaction. In the case of nickel, the presence of such hydroperoxides is considered to stabilise the Ni"02 complex. Ruthenium(i) chloride complexes in dimethylacetamide are active hydrogenation catalysts for olefinic substrates but in the presence of oxygen, the metal ion is oxidised to ruthenium(m), the reaction proceeding stoicheiometrically. Rhodium(i) carbonyl halides have also been shown to catalyse the oxidation of carbon monoxide to carbon dioxide under acidic conditions ... 

Reactivity of The Carbonate Radical. —Rate constants for the reaction of C63H radicals with several complexes have been measured. Of the labile aquo-ions Co +, Zn +, Ni +, Cu ", and Mn +, only Co (A =2.8x 10 M cm ), Cu + ( 4.5 X 10 M s 0> and Mn (A =1.5x 10 M s ) react at measurable rates the oxidation potential of the metal ion appears to be a governing factor. Two Co -macrocycle complexes, [Coi (2)] and [Co (3)] (see below), whose equatorial co-ordination sites are substitution inert and axial sites have labile HjO, react 200 times faster than Colq, and here the rate is probably substitution limited. ... 

The actual formation step is the reaction of carbon dioxide itself with the conjugate base of the penta-ammine-aquo-complex. For this process the second-order rate constant in Scheme 2) is 2.2 x 10 1 mol s at 25 °C the activation parameters are A/f = 15.3 0.9 kcal mol and LS = + 3.6 3,0 cal deg mol. The activation parameters for the reaction of [Co(NH3)5(OH2)] + with the neutral ligand glycine are = 27.6 kcal... 

Rhodium(III) tetra(/ -sulfonatophenyl)porphyrin [(TSPP)Rh] aquo and hydroxo complexes react with a series of alkenes in water to form /3-hydroxyalkyl coordination compounds. Addition reactions of (TSPP)Rh-OH to unactivated terminal alkenes CH2=CHR invariably occur with both kinetic and thermodynamic preferences to place rhodium on the terminal carbon to form (TSPP)Rh-CH2CH(OH)R complexes. Acrylic and styrenic alkenes initially react to place rhodium on the terminal carbon to form [Rh]-CH2CH(OH)R as the kinetically preferred isomer, but subsequently proceed to an equilibrium distribution of regioisomers where [Rh]-CH(CH2OH)R is the predominant thermodynamic product. Equilibrium constants for reactions of the diaquo rhodium(III) compound [(TSPP)Rhm(H20)2]3 in water with a series of terminal alkenes that form /9-hydroxyalkyl complexes were directly evaluated and used in deriving thermodynamic values for addition of the Rh-OH unit to alkenes. The AG° for reactions of the Rh-OH unit with alkenes in water was found to be approximately 3 kcalmol-1 less favourable than the comparable Rh-H reactions in water.100... 

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