Complexation and conformational analysis studies of 11-membered ring crown trithioethers

We wish to report the synthesis of a series of transition metal complexes with two isomeric 11-membered ring trithioethers, 1,4,7-trithiacycloundecane (11S3-147) and 1,4,8-trithiacycloundecane (11S3-148). With respect to Ni(II), Fe(II), Co(II), Co(III), and Ru(II), the two function as relatively strong field ligands, but their ligand field strengths fall between the stronger field and smaller ring trithioether ligands such as 1,4,7-trithiacyclononane(9S3) and the larger ring and weaker field ligand, 1,5,9-trithiacyclododecane(12S3). Complexes of both ligands with first row transition metals are much more sensitive to solvolysis reactions than the analogous 9S3 complexes. 13C NMR spectroscopy shows that there is little stereoisomer selectivity between the cis and trans diastereoisomers for the bis complexes of 11S3-147. The complex [Pd(11S3-147)2]2+ does not display the unusual spectroscopic and electrochemical properties observed in smaller ring Pd(II) complexes. Its absorption spectrum does not show any d–d transitions and an irreversible ligand-centered oxidation is observed. A conformational analysis study for both macrocycles showed that neither has a lowest energy conformation suitable for tridentate coordination. The effects of the lack of pre-organization dominate the complexation behavior of the two macrocyclic ligands.