Keywords Calixarene, Molecular mechanics, Monte Carlo simulation, Selectivity, Complexation, Potentiometry, PVC membrane ion-selective electrode, Spartan
Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
Abstract This paper focuses on the molecular modelling of a number of calixarene ester and phosphine oxide metal ion complexes. Monte Carlo conformational searches, in conjunction with the Merck Molecular Force Field, were carried out using Spartan SGI Version 5.0.1. running on Silicon Graphics O2 workstations. In the case of the calixarene tetraesters, the optimised models strongly suggest that the selectivity of these ligands is strongly related to the eight-fold nature of the coordination with the Na+ ion, while coordination with the Li+ ion, for example, is merely three-fold. This feature of eight-fold coordination is also observed in the models of the complexes formed by the calixarene tetraphosphine oxides with calcium. However, whereas the eight-fold coordination is unique to the model of the TPOL:Ca2+ complex among the ions modelled, this mode of coordination occurs for TPOS with sodium and potassium, in addition to calcium. This concurs with the observation that calcium selectivity is obtained with ion selective electrodes based on TPOL but not TPOS. Though the cavity in the calixarenes PPOL and PPOLx and the calixarene HPOL, in their uncomplexed form, are much larger than that of the corresponding calixarenes, the pattern of selectivity is the same – the ligands are selective for calcium. The models of the complexes of these larger calixarenes, such as PPOL:Ca2+, strongly suggest that the reason for this similarity is that four of the available phosphine oxide groups complex with the calcium ion, and the others are forced away from the cavity region for steric reasons. The resulting eight-fold coordination, is therefore, similar to that of the calixarenes studied.
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