Metal binding studies with macrocyclic hexathioethers: Transition metal complexes of 22S6 and 20S6

Abstract

Dedication: This paper is dedicated to Professor Jerald S. Bradshaw upon his retirement, in recognition of the pioneering contributions by him and his BYU colleagues for the advancement of macrocyclic chemistry, and in particular, for their establishment and development of the International Symposium of Macrocyclic Chemistry.The two macrocyclic hexathioethers, 1,4,7,12,15,18-hexathiacyclodocosane (22S6) and 1,4,7,11,14,17-hexathiacycloeicosane (20S6) have been examined with respect to their complexation behavior towards a variety of transition metals ions. In all of these complexes, the two ligands generate relatively strong ligand field, but their field strengths fall between the smaller ring and stronger field hexathioether ligands (such as 18S6) and larger ring and weaker field ligands, such as 24S6. The stability of the complexes towards hydrolysis parallels their reduced field strengths with first row transition metal complexes of both ligands showing enhanced sensitivity to solvolysis reactions compared to the analogous 18S6 complexes. In general, the electrochemical behavior of the hexathioether complexes shows metal centered oxidations which occur at higher potentials than those complexes involving trithioethers. We suggest that the hexathioether ligand is less able to expand and contract during the reduction and oxidation of the metal center. The complex [Pd(22S6)] 2+ does not display the unusual spectroscopic and electrochemical properties observed in the analogous 18S6 complex.