Ligand Design for Metal-Organic Frameworks and Single Molecule Magnets

Abstract

This thesis describes two projects in which ligand design has been employed for the synthesis of coordination compounds exhibiting interesting structural and magnetic properties. In Project 1 a flexible, polydentate 4,4'-bipyridine ligand (LI) was prepared and fully characterized. Its coordination chemistry with Cu2(OAc)4 afforded a new complex with stoichiometry [Cu4(LI)1.5(OAc)2(py)2(OH2)]n (III). Single crystal X-ray diffraction experiments revealed that (III) crystallized as a porous, 3-D MOF with a structurally unique trinodal 4,4,5-c net topology. Variable temperature powder X-ray diffraction and TGA experiments revealed that (III) became amorphous upon desolvation, but that the crystallinity of the complex was fully restored after resolvation, rendering it a new addition to the family of breathable MOFs. Project 2 presents a joint synthetic, experimental and theoretical approach towards the discovery of Ln-based SMMs. The synthesis and characterization of a new dual-compartmental macrocycle (LII), with an N3O2 cavity suitable for the preparation of seven-coordinate lanthanide complexes was achieved. Reaction of (LII) with LnCl3∙6H2O, (where Ln3+ = Dy, Tb and Er), in the presence of NaOH afforded three novel complexes with stoichiometry [Ln2Na2(LII)2(Cl)4(MeOH)]·xH2O (IVa-c). X-ray diffraction studies revealed that the complexes were isostructural, comprising two coordinated macrocycles linked via a bridging 2 chloride to afford a dimer. Both macrocycles of each dimer contained a Ln3+ ion with pseudo D5h geometry that is coordinated equatorially by the five donor atoms in the N3O2 pocket together with two axial chloride ligands, as well as a 6-coordinate Na+ ion, residing in the O3O22− pocket. All three complexes have been magnetically characterized. A frequency dependence to the out of phase component of the ac susceptibility data was observed for the Dy3+complex (IVa), consistent with SMM behaviour. The ac data was successfully modelled to a single component Debye equation and a fit of the temperature dependence of c to the Arrhenius equation afforded an effective energy barrier (Ueff) of 12.6 cm−1 and a pre-exponential factor, τ0 of 2.91 x 10−7 s for this complex. Unfortunately, no slow relaxation of the magnetization was observed for the Tb3+ and Er3+ derivatives (IVb) and (IVc). Comprehensive ab initio studies carried out on (IVa-c) shed important light on the relaxation dynamics in all three complexes, revealing that deviation from idealised D5h geometry results in less well isolated ground states and active quantum tunnelling mechanisms, further supporting the experimental observations of predominantly field induced SMM behaviour for (IVa), but no SMM properties for complexes (IVb and c).