Towards Multifunctional Materials based on Coordination Complexes of Sulfur-Containing Diarylazo and Cucurbit[6]uril Ligands

Abstract

In this thesis, the coordination chemistry of two families of ligands are explored, working towards the discovery of new multifunctional materials. The first project focuses on the coordination chemistry of sparsely reported redox active sulfur-bearing diarylazo ligands. Homoleptic complexes with ligands 2-5 were successfully prepared with Cu2+, Ni2+ and Co2+/3+, in addition to Co3+ heteroleptic complexes containing 7. Single crystals suitable for SCXRD were isolated for complexes 4e, 5a, 5a’ and 5c. The complexes were fully characterized and select complexes were further investigated by cyclic voltammetry, EPR spectroscopy and magnetic susceptibility studies. EPR measurements of 4c at 80 K and RT revealed noticeable differences in profiles suggesting the occurrence of structural and/or electronic changes with temperature. DC magnetic studies on 4c and 5a reveal the expected LT χMT values, however RT χMT values increase to 0.75 cm3Kmol-1 and 0.54 cm3Kmol-1, respectively. The second project explores the hydrothermal synthesis of Ln4-cubane sandwich-type structures from cucurbit[6]uril and isonicotinate together with suitable Ln3+ salts. A family of complexes with two structural topologies were isolated; [Gd4(µ3-OH)4(µ2-OH)4] (8a) and [Ln4(μ3- OH)4(μ2-OH)(µ2-IN)] (where Ln3+= Dy or Tb; 8b, 8c). The complexes were structurally and magnetically characterized. Structural analysis reveals 8a has a different structure when compared to the isostructural 8b and 8c. The 8-coordinate square antiprismatic coordination geometry for the oblate Ln3+ ions in 8b and 8c serves to increase the axial anisotropy. DC magnetic studies reveal the magnetic cores of the molecules are magnetically isolated with weak antiferromagnetic interactions in 8a. AC magnetic studies reveal slow relaxation of magnetization consistent with SMM properties in 8b and 8c. Modelling the magnetic data afforded effective energy barriers of 17.6 K (in zero DC-field) for 8b, and 11.1 K (5000 Oe) for 8c. Photoluminescence studies on 8a reveal transitions associated with the CB[6] ligand. Partial and full energy conversion from the ligand to the metal ion upon excitation, for 8b and 8c, respectively. Although DC magnetic studies on the Gd3+ have been previously reported, the Dy and Tb complexes are the first reported Ln4- CB[6] complexes to display slow relaxation of the magnetization consistent with SMM properties.