Metal Complexes with Macrocyclic Ligands,V. Formation and dissociation kinetics of the pentaco-ordinated Ni2+, Cu2+, Co2+ and Zn2+ complexes with 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane |
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Authors: | Liselotte Hertli Thomas A Kaden |
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Abstract: | The formation and dissociation kinetics of the pentaco-ordinated Cu2+, Ni2+, Co2+ and Zn2+ complexes with 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (4-MeCyclam-14) was studied by pH-stat techniques and spectrophotometrically. The rates of the reactions between 4-MeCyclam-14 and each of the four metal ions, although slower than normal complexations by a factor of 103?104, closely follow the order Cu2+ > Zn2+ > Co2+ > Ni2+, found for the rate of water exchange. This implies that beside water exchange an other constant factor plays an important role in the rate determing step. The dissociation of the pentaco-ordinated 4-MeCyclam-14 complexes is acid catalyzed. The limiting rate for acid dissociation is not reached even in 2.5M HNO3 in the case of Ni(4-MeCyclam-14)2+. From the formation and dissociation rates stability constants have been calculated, which do not show any macrocyclic effect. |
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