Tetra- and Octa-nuclear 3d-4f Complexes of Macrocyclic Ligands as Potential Single-Molecule Magnets

Single-Molecule Magnets (SMMs) are complexes in which the individual molecules act as discrete magnetic domains. As such they are potential candidates for nano-scale data storage and processing. However, it can be difficult to tailor the structural and magnetic properties of an SMM because modifying a component may lead to a completely different product forming. To address this lack of predictability, in this project SMMs were prepared using macrocyclic ligands, as macrocycles offer predictable binding, as well as solubility, stability and tunability. Therefore, one hundred and nine 3d-4f complexes of [3 + 3] hexaimine macrocycles (L)6- were prepared by mixing 1,4-diformyl-2,3-dihydroxybenzene (H2LAld), the acetate salt of a first-row transition metal ion, the nitrate salt of a lanthanide(III) ion and the chosen diamine in a 3:3:1:3 ratio. The magnetic properties of selected complexe s were measured from 300 K down to 1.8 K to check for SMM behaviour. X-ray crystallography on nine of the macrocyclic complexes confirmed that the large lanthanide(III) ion is bound in the central O6 cavity of (LR)6- while the three smaller transition metal ions are bound in the outer N2O2 binding pockets. Three of the tetranuclear complexes, [ZnII3DyIII(LPr)(NO3)3(MeOH)3]•4H2O, ZnII3ErIII(LPr)(NO3)3•2MeOH•5H2O and ZnII3YbIII(LPr)(NO3)3•4MeOH, displayed field-induced SMM behaviour, where the out-of-phase component of ac susceptibility was significantly enhanced in the presence of a weak external dc field. Three of the other complexes displayed SMM behaviour in zero dc field. Overall, a remarkably robust template synthesis has been established, and one hundred and nine pure macrocyclic complexes prepared. Of those that have been magnetically characterised to date, three complexes displayed field-induced SMM behaviour and three complexes displayed SMM behaviour. Pleasingly, [ZnII3DyIII(LPr)] is the first reported example of an SMM containing a single lanthanide ion bound to a macrocyclic ligand in a non-sandwiched fashion. Also, ZnII3YbIII(LPr)(NO3)3•4MeOH is only the second reported example of a ytterbium(III)-based SMM. Successful production of the first examples of 3d-4f macrocyclic complexes to exhibit SMM behaviour in zero dc field is another highlight of this study