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

Abstract

The affinities of 1,4,7-tris(2-hydroxyalkyl)-1,4,7-triazacyclononane derivatives for MgII and CaII were found to differ greatly. Whereas 1,4,7-tris(2-hydroxyethyl)- (L1), 1,4,7-tris(2-hydroxy-2-methylpropyl)- (L2) and the unsymmetrical 1,4,7-tris(2-hydroxypropyl)-1,4,7-triazacyclononane derivative L3b barely discriminated between MgII and CaII, the symmetrical isomer L3a was more than 500 times more selective for MgII than for CaII. Similar selectivity differences were observed between the diastereomers of 1,4,7-tris(2-hydroxy-2-phenylethyl)- (L4) and 1,4,7-tris(2-hydroxydodecyl)-1,4,7-triazacyclononane (L5). 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 MgII over CaII. 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 MgII. Similarly, the calculated preference for the smaller twist angle correctly predicted the lack of MgII/CaII selectivity for acetate and amide derivatives and for a phosphonate diester derivative. Equilibration of the complexes of the ligands in the presence of both MgII and CaII was slow, as shown for example by kd,0 = 2.1 × 10–4 s–1 for CaL3a. These dissociation rates were a factor of 100 times larger at the boundary of a two-phase (water–chloroform) system.