Magnetostructural correlations in iron(II)- and nickel(II)-based cluster complexes

• Main Author: Piga, Francesco
• Published: University of Nottingham 2013
• Subjects: 538.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604299

This thesis presents a study of the magnetic properties of cluster complexes of nickel and iron, with a focus on the relationships between structure and magnetism. Chapter 1 gives an introduction to the concepts involved in molecular magnetism, with particular attention to single-molecule magnets (SMMs). The main types of SMMs are outlined, and the origin of magnetic anisotropy and its role in the single-molecule magnetism phenomena are discussed, as well as the potential applications of SMMs as data storage devices. The chapter also describes the experimental techniques employed for the characterisation of the complexes studied. Chapter 2 deals with three Ni(II)-based, cubane-like complexes with formula [Ni(hmp)(MeOH)CI]4 (complex 1), [Ni(hmp)(dmb)CI]4 (2) and [Ni(hmp)(dmp)Clh (3), where hmp- is the anion of 2- (hydroxymethyl)pyridine, dmb is 3,3-dimethylbutan-l -01 and dmp is 2,2-dimethylpropan-l -ol. A magnetostructural study was carried out on these complexes, employing X-ray crystallography, SQUID magnetometry, FDMRS and MCD spectroscopy.l MCD spectra allowed for determining the signs of superexchange interactions and magnetic anisotropy parameters, and simulations on FDMRS data yielded zerofield splitting parameters values consistent with those previously obtained for 1 and 2 from simulations of EPR data, corroborating the validity of the FDMRS-MCD approach. 2 ·1 In chapter 3, three Fe(II)-based, cubane-like complexes are presented, with formula [Fe(hmp)(MeOH)Cl]4 (complex 4), [Fe(hmp)(EtDH)Cl]4 (complex 5) and [Fe(hmp)(EtXan)]4 (complex 6). A study of the magnetic anisotropy and of the spin-spin coupling interactions in these complexes is described, which employed X-ray crystallography, Mossbauer spectroscopy and SQUID magnetometry. In addition, a detailed study was carried out on 4 and 5 by means of W-band EPR and MCD spectroscopies.2 Complexes 4-6 have been found to have a spin ground state S = 8 arising from ferromagnetic superexchange interactions, and a large positive anisotropy which has been estimated by means of simulations of the EPR and SQUID data. The fourth chapter deals with a heptanuclear, mixed-valence cluster complex with formula [FeII6FelII(hmp)12][BF4h (complex 7), which displays SMM behaviour. A preliminary study of the magnetic properties of this species is presented and correlated with its structural parameters. Two distinct relaxation processes have been detected in the solid state AC susceptibility measurements, while only one relaxation process is observed in solution as highlighted by ColeCole plots. Chapter 5 presents a series of compounds (8-17) that have been obtained serendipitously during the timeframe of this project. The preparation of these compounds is described, as well as a preliminary description of their structures. The sixth and final chapter draws final conclusions on the results presented in Chapters 2-5 and suggests desirable 3 ! :1 developments for the research outlined in this work, as well as the potential challenges and requirements for such developments. The crystallographic information files for complexes 3, 4, 5, 6 and 7 are included in electronic format on a DVD attached to the thesis. Two journal articles have also been published1,2 presenting the findings that we discuss here in chapter 2 and 3, and will be referred to in the text. References