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

Abstract

The 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane complexes [M(thec12)]2+, where M2+ = Mg2+, Ca2+, Sr2+, and Ba2+, are characterized by log(K/dm3 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), where K is a potentiometrically determined stability constant. The analogous literature values for the 1,4,7,10-tetrakis(2-methoxyethyl)-1,4,7,10-tetraazacyclododecane 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++ = 19.1 +/- 0.8, 33.3 +/- 0.5, and 43.9 +/- 0.4 kJ mol-1, and delta S++ = -117 +/- 4, -80.3 +/- 1.8, and -47.0 +/- 1.3 J K-1 mol-1 when M2+ = Mg2+, Ca2+, and Ba2+, respectively, in methanol-12C-d4 as shown by 13C NMR spectroscopy. For the enantiomerization of eight-coordinate delta- and lambda-[M(tmec12)]2+, k(298.2 K) = 310 +/- 1 and 688 +/- 3 s-1, delta H++ = 54.0 +/- 0.2 and 39.6 +/- 0.1 kJ mol-1, and delta S++ = -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 groups is not coordinated and exchange of the methoxy groups between the coordinated and free states is characterized by k(298.2 K) = 163,000 +/- 8000 s-1, delta H++ = 35.8 +/- 0.4 kJ mol-1, and delta S++ = -25.1 +/- 1.7 J K-1 mol-1. The intermolecular exchange of thec12 and tmec12 between the coordinated and free states is substantially slower than the enantiomerizations in the first five complexes and the intramolecular exchange process observed in [Mg(tmec12)]2+.