Electronic properties of free and coordinating polythioether macrocycles. A spectroscopic and computational analysis

Abstract

Helium I photoelectron spectra of the polysulfur macrocycles 1, 4, 7-trithiacyclodecane ([10]ane S₃), 1, 4, 7, 10-tetrathiacyclododecane ([12]aneS₄), 1, 4, 8, 11-tetrathiacyclotetradecane ([14]aneS₄), 1,5, 9, 13-tetrathiacyclohexadecane ([16]aneS₄), 3, 3, 7, 7, 11.11, 15, 15-octamethyl-1, 5, 9, 13-tetrathiacyclohexadecane (Me₈[16]aneS₄) and 1, 4, 7, 10, 13, 16-hexathiacyclooctadecane ([18]aneS₆) are reported and correlated with the results of semi-empirical molecular orbital (MO) calculations. Where, available, published X-ray crystallographic data have been used to provide input geometries for the MO calculations; in addition, alternative low energy conformations have been generated by molecular mechanics. In order to probe the effects on the electronic properties of the macrocycles of changes in conformation resulting from coordination to a metal centre, MO calculations have also been carried out on conformations of [9]aneS₃, [12]aneS₄, [14]aneS₄and [16]aneS₄ taken from X-ray structures of their metal complexes. The energy required to transform the macrocycles from their free conformations is found to be reasonably small compared to typical metal–thioether bond energies. energies[14]aneS₄ and [16]aneS₄ both exhibit a number of different η⁴-chelating conformations of similar energies and electronic properties.