Co-ordination chemistry and molecular mechanics study of the magnesium(II) and calcium(II) complexes of trisubstituted 1,4,7-triazacyclononane derivatives

摘要

The affinities of 1,4,7-tris(2-hydroxyalkyl)-1,4,7-triazacyclononane derivatives for Mg-II and Ca-II were found to differ greatly. Whereas 1,4,7-tris(2-hydroxyethyl)-(L-1), 1,4,7-tris(2-hydroxy-7-methylpropyl)-(L-2) and the unsymmetrical 1,4,7-tris(2-hydroxypropyl)-1,4,7-triazacyclononane derivative L-3b barely discriminated between Mg-II and Ca-II, the symmetrical isomer L-3a was more than 500 times more selective for Mg-II than for Ca-II. Similar selectivity differences were observed between the diastereomers of 1,4,7-tris(2-hydroxy-2-phenylethyl)-(L-4),,and 1,4,7-tris(2-hydroxydodecyl)-1,4,7-triazacyclononane (L-5). These selectivities were related to the structure of the magnesium complexes as shown by molecular mechanics (MMX) calculations. Only those ligands favoring the formation of a magnesium complex with a large twist angle between the planes of co-ordinating oxygens and ring nitrogens resulting in a small, tight cavity showed a large preference for Mg-II over Ca-II., The MMX calculations predicted large twist angle structures for phosphinate derivatives, and for a phosphonate monoester derivative, and these ligands were found to have a high selectivity for Mg-II. Similarly, the calculated preference for the smaller twist angle correctly predicted the lack of Mg-II/Ca-II selectivity for acetate and amide derivatives and for a phosphonate diester derivative. Equilibration of the complexes of the ligands in the presence of both Mg-II and Ca-II was slow, as shown for example by k(d,0)=2.1x10(-4) s(-1) for CaL3a. These dissociation rates were a factor of 100 times larger at the boundary of a two-phase (water-chloroform) system. [References: 19]