ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The synthesis of two new polyamines containing 2-pyridyl and 6-methyl-(2-pyridyl) groups is described. The equilibria between H+ and Co2+ and the new ligand 1,9-di(2-pyridyl)-2,5,8-triazanonane (dptn) as well as the protonation of the hydroxo complexes of 1,6-di(2-pyridyl)-2,5-diazahexane-Co(II) (Co(dpdh) and 1-(6-methyl-2-pyridyl-6-(2-pyridyl)-2,5-diazahexane-Co(II) (Co(mdpdh)) have been studied in aqueous solution using the pH method. The coordination ability of the pyridine containing ligand dptn is compared with the chelating tendency of the analogous aliphatic amine (tetren). In spite of the lower basicity of the pyridine derivative the stability constants of its Co(II) complex is higher by a factor of thirty. The absorption spectra give evidence for a pseudooctahedral geometry of Co(dpdh) (H2O)2+2 and Co(dpdh)(H2O)(OH)+. Oxygen-uptake measurements indicate the formation of binuclear peroxo species. The potentiometric equilibrium data indicate the presence of dibridged species (dpdh)Co(O2, OH)Co(dpdh)3+ and (mdpdh)Co(O2, OH)Co-(mdpdh)3+. The kinetics of the rapid O2-uptake was measured over a wide pH range on a stopped-flow apparatus. For Co(dpdh)2+ and Co(mdpdh)2+ we found a second order rate constant independent of pH up to pH 9, but in more alkaline solutions it increases and reaches an upper limit around pH 12.3. The data could be fitted by a rate law of the form k1 = (k′1[H+] + k″1 KH) ([H+] + KH)-1. This variation with pH was explained by a rapid equilibrium Co(dpdh) (H2O)2+2 ⇌ Co(dpdh)(H2O)(OH)+ + H+(KH). The enhanced rate constants of the hydroxo species must arise from a rate determining H2O replacement by O2, dominated by Co-OH2 bond breaking and the expected ability of an OH- group to labilize neighboring H2O molecules. The protonation constant of the hydroxo complex obtained by equilibrium measurements (pKH = 11.19 ± 0.03) was in good agreement with that derived from kinetic data (11.12 ± 0.04). The hydrolysis of Co(dptn)(H2O)2+ influences the rate of O2-incorporation in a different way. In this system retardation occurs as a result of hydrolysis ascribed to the slower leaving of OH- compared to H2O. This was expected if a mechanism with rate determining H2O replacements by O2 holds.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19770600803
