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On Reactions of Oxygenated Cobalt(II) Complexes. XI. Note on the Mechanism of O 2 Uptake by Cobalt(II) Chelates with Tetra‐ and Pentadentate Amines
Author(s) -
Exnar Ivan,
Mäcke Helmut
Publication year - 1977
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19770600803
Subject(s) - chemistry , cobalt , protonation , chelation , pyridine , potentiometric titration , ligand (biochemistry) , medicinal chemistry , aqueous solution , stability constants of complexes , amine gas treating , equilibrium constant , inorganic chemistry , kinetics , reaction rate constant , stereochemistry , ion , organic chemistry , biochemistry , physics , receptor , quantum mechanics
The synthesis of two new polyamines containing 2‐pyridyl and 6‐methyl‐(2‐pyridyl) groups is described. The equilibria between H + and Co 2+ 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) (H 2 O) 2+ 2and Co(dpdh)(H 2 O)(OH) + . Oxygen‐uptake measurements indicate the formation of binuclear peroxo species. The potentiometric equilibrium data indicate the presence of dibridged species (dpdh)Co(O 2 , OH)Co(dpdh) 3+ and (mdpdh)Co(O 2 , OH)Co‐(mdpdh) 3+ . The kinetics of the rapid O 2 ‐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 k 1 = ( k ′ 1 [H + ] + k ″ 1 K H ) ([H + ] + K H ) −1 . This variation with pH was explained by a rapid equilibrium Co(dpdh) (H 2 O) 2+ 2⇌ Co(dpdh)(H 2 O)(OH) + + H + ( K H ). The enhanced rate constants of the hydroxo species must arise from a rate determining H 2 O replacement by O 2 , dominated by Co‐OH 2 bond breaking and the expected ability of an OH − group to labilize neighboring H 2 O molecules. The protonation constant of the hydroxo complex obtained by equilibrium measurements (p K H = 11.19 ± 0.03) was in good agreement with that derived from kinetic data (11.12 ± 0.04). The hydrolysis of Co(dptn)(H 2 O) 2+ influences the rate of O 2 ‐incorporation in a different way. In this system retardation occurs as a result of hydrolysis ascribed to the slower leaving of OH − compared to H 2 O. This was expected if a mechanism with rate determining H 2 O replacements by O 2 holds.