Designing magnetically interesting supramolecular architectures: towards Single-Chain Magnets
Single-Molecule Magnets (SMMs) are single molecules that can behave as magnets. Similarly, Single-Chain Magnet (SCM) are 1D polymers that can exhibit slow relaxation of the magnetisation. Over the last decade, interest in SCMs has increased rapidly, with the main driving force being the possibility of increasing the blocking temperature above that realised by SMMs, in order to meet this requirement for future application as memory devices. In this project, a range of magnetically interesting complexes previously studied in the Brooker research group have been prepared and combined with appropriate linkers for the construction of 1D-chains. This field, which represents a new research direction for this group, is explored herein and the results obtained using various linkers are presented. Chapter 1 presents a review of the field of SCM to date and introduces the most important concepts of magnetism. This is followed by an overview of the methods of crystallisation used to obtain SCMs, an introduction to Schiff base ligands and macrocycles and finishes with the aims of the study. In Chapter 2 the synthesis and magnetic characterisation of the 3d-4f tetrametallic macrocyclic SMM building blocks [CuII3TbIII(L1Et)(NO3)3(MeOH)]∙MeOH ([CuII3TbIII(L1Et)]), [CuII3TbIII(L1Pr)(NO3)2(MeOH)3](NO3) ([CuII3TbIII(L1Pr)]) and [CuII3TbIII(L1Bu)(NO3)2(DMF)(H2O)](NO3) ([CuII3TbIII(L1Pr)]), originally obtained by Dr. Humphrey Feltham during his PhD thesis, is presented, as they are the main building blocks used in this project. The assembly of the macrocyclic building block, [CuII3TbIII(L1Pr)], with K3[MIII(CN)6] linkers, where M = Fe, Cr or Co, results in a range of monomer, dimer and 1D chain architectures, all of which have been structurally and magnetically characterised, and are described in Chapter 3. In Chapter 4 the assembly of the macrocyclic building block, [CuII3TbIII(L1Pr)], with a (TBA)3[WV(CN)8] linker, which results in two different 1D chains depending on the crystallisation conditions, is presented. In these two chapters, a total of nine complexes using [CuII3TbIII(L1Pr)] were structurally characterised. Although no SCM behaviour was seen, metamagnet and SMM behaviour was observed. Following these results with N-donor linkers, a different approach was taken to deliberately link the 3d-4f macrocyclic building block via the Tb(III) ion by using an O-donor linker. In Chapter 5, the assembly of the macrocyclic SMM building block, [CuII3TbIII(L1Et)], with a Na2[tpa] linker (tpa2- terephthalate ion), which results in a structurally and magnetically characterised 1D chain architecture, is described. This is a proof of principle that use of an oxygen donor linker can successfully target linking of the macrocyclic Ising centres via the Tb(III) ions rather than via the Cu(II) ions, to give a 1D chain, and shows enhanced control in the design of the supramolecular architecture by using 3d-4f macrocyclic building blocks. Another strategy envisaged to enhanced the magnetic communication along these 1D-chain systems was to use a different macrocyclic building block, [NiII3LnIII(L1Pr)]. In Chapter 6, work on the [NiII3LnIII(L1Pr)] family, reproducing the synthesis of eleven macrocyclic complexes [NiII3LnIII(L1Pr)] (LnIII = CeIII, PrIII, NdIII, SmIII, EuIII, GdIII, HoIII, ErIII, TmIII, YbIII or LuIII) for magnetic characterisations, and attempts at synthesising 1D chains using the attempts [NiII3TbIII(L1Pr)] building block with a range of linkers, are detailed. Chapter 7 describes the magnetic characterisation of a different macrocyclic building block, [MnIII(L2)(NCS)2], and the results obtained when trying to synthesise 1D chains from it. The 1D architecture obtained with a terephthalate linker has been structurally and magnetically characterised. It exhibits slow relaxation of the magnetisation, associated with the SMM behaviour of the [MnIII(L2)(NCS)2] building block, and this has been quantified. In Chapter 8, work done with a third building block, [CoII2(L3)(H2O)(MeCN)5](BF4)4 ([CoII2(L3)]), is presented. Attempts at synthesising 1D chains from this noncyclic non-SMM building block are presented, as well as the [MII3(L3)3]6+ (MII = Fe2+ and Zn2+) triangles obtained when this bis-terdentate Schiff base ligand L3 was reacted with Fe(II) and Zn(II) tetrafluoroborate salts. Finally, conclusions and suggestions for future work are presented in Chapter 9. Overall in this thesis, thirty inorganic compounds have been made. Eighteen of them had been made before but were remade in order to use them as building blocks, for magnetic characterisation or for full characterisation. Twelve are original compounds amongst which seven are 1D chains. Amongst these 1D chains, no SCM behaviour was observed, but four of them show SMM behaviour associated with the building block employed, and for two of them the SMM behaviour has been quantified.
Advisor: Brooker, Sally
Degree Name: Doctor of Philosophy
Degree Discipline: Chemistry
Publisher: University of Otago
Keywords: Single Chain Magnet; Single Molecule Magnet; Molecular magnetism
Research Type: Thesis