Modelling metal complexes of calixarene esters and phosphine oxides using molecular mechanics

This paper focuses on the molecular modelling of a number of calixarene est er and phosphine oxide metal ion complexes. Monte Carlo conformational sear ches, in conjunction with the Merck Molecular Force Field, were carried out using Spartan SGI Version 5.0.1. running on Silicon Graphics O2 workstatio ns. In the case of the calix[4]arene tetraesters, the optimised models stro ngly suggest that the selectivity of these ligands is strongly related to t he eight-fold nature of the coordination with the Na+ ion, while coordinati on with the Li+ ion, for example, is merely three-fold. This feature of eig ht-fold coordination is also observed in the models of the complexes formed by the calix[4] arene tetraphosphine oxides with calcium. However, whereas the eight-fold coordination is unique to the model of the TPOL:Ca2+ comple x among the ions modelled, this mode of coordination occurs for TPOS with s odium and potassium, in addition to calcium. This concurs with the observat ion that calcium selectivity is obtained with ion selective electrodes base d on TPOL but not TPOS. Though the cavity in the calix[5]arenes PPOL and PP OLx and the calix[6]arene HPOL, in their uncomplexed form, are much larger than that of the corresponding calix[ 4] arenes, the pattern of selectivity is the same - the ligands are selective for calcium. The models of the com plexes of these larger calixarenes, such as PPOL:Ca2+, strongly suggest tha t the reason for this similarity is that four of the available phosphine ox ide groups complex with the calcium ion, and the others are forced away fro m the cavity region for steric reasons. The resulting eight-fold coordinati on, is therefore, similar to that of the calix[4]arenes studied.