Alkaline earth complexes of 1,4,7,10-tetrakis(2-hydroxyethyl)- and 1,4,7,10-tetrakis(2-methoxyethyl)-1,4,7,10-tetraazacyclododecane: An equilibrium and kinetic study

The 1,4,7, 10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane compl exes [M(thec12)](2+), where M2+ = Mg2+, Ca2+, Sr2+, and Ba2+, are character ized by log(K/dm(3) mol(-1)) = 2.86 +/- 0.09, 7.41 +/- 0.04, 6.47 +/- 0.04, and 4.84 +/- 0.03 at 298.2 K in aqueous Et4NClO4 (I = 0.10 mol dm(-3)), wh ere fi is a potentiometrically determined stability constant. The analogous literature values for the 1,4,7, 10-tetrakis(2-methoxyethyl)- 1,4,7,10-tet raazacyclododecane complexes [M(tmec12)](2+) are 2.47, 5.47, 5.00, and 4.72 . The enantiomerization of eight-coordinate Delta- and Lambda-[M(thec12)](2 +) is characterized by k(298.2 K) = 2310 +/- 260, 582 +/- 17, and 445 +/- 5 s(-1), Delta H double dagger = 19.1 +/- 0.8, 33.3 +/- 0.5, and 43.9 +/- 0. 4 kJ mol(-1), and Delta S double dagger: = -117 +/- 4, -80.3 +/- 1.8, and - 47.0 +/- 1.3 J K-1 mol(-1) when M2+ = Mg2+, Ca2+, and Ba2+, respectively, i n methanol-C-12-d(4) as shown by C-13 NMR spectroscopy. For the enantiomeri zation of eight-coordinate Delta- and Lambda-[M(tmec12)](2+),- k(298.2 K) = 310 +/- 1 and 688 +/- 3 s(-1), Delta H double dagger = 54.0 +/- 0.2 and 39 .6 +/- 0.1 kJ mol(-1), and Delta S double dagger = -16.1 +/- 0.5 and -57.9 +/- 0.3 J K-1 mol(-1) when M2+ = Ca2+ and Ba2+, respectively. However, [Mg( tmec12)](2+) has a seven-coordinate structure where one of the methoxy grou ps is not coordinated and exchange of the methoxy groups between the coordi nated and free states is characterized by k(298.2 K) = 163 000 +/- 8000 s(- 1), Delta H double dagger = 35.8 +/- 0.4 kJ mol(-1), and Delta S double dag ger = -25.1 +/- 1.7 J K-1 mol(-1). The intermolecular exchange of thec12 an d tmec12 between the coordinated and free states is substantially slower th an the enantiomerizations in the first five complexes and the intramolecula r exchange process observed in [Mg(tmec12)](2+).