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Title: Ultra-low temperature structural investigations of single-molecule magnets
Author: Farrell, Andrew Robert
ISNI:       0000 0004 2736 6715
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2013
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Previous studies of single-molecule magnets (SMMs) have interpreted their low-temperature (below 8 K) magnetic measurements on the basis of X-ray crystal structures measured at 100 K or even room temperature. The work presented herein describes the first structural measurements performed on SMMs in the temperature regime in which magnetisation studies are performed. The ultra-low temperature (2 K) crystal structure of the prototypical SMM [Mn12O12(O2CMe)16(H2O)4]⋅2MeCO2H⋅4H2O (Mn12-acetate, 1) was determined by using single-crystal X-ray and neutron diffraction. We have observed an additional hydrogen-bonding interaction in the disordered region of 1 that backs up the link between solvent disorder and disorder in the position of an acetate ligand. Two new desolvated species [Mn12O12(O2CMe)16(H2O)4]⋅4H2O (2) and [Mn12O12(O2CMe)16(H2O)4] (3) were formed from 1 by single-crystal to single-crystal (SCSC) transformations and lead to the conclusion that the acetic acid of crystallisation in 1 is responsible for positional disorder in an acetate ligand. Another new {Mn12} derivative [Mn12O12(O2CMe)16(CH3CH2OH)4]•CH3CH2OH (4) is formed by exposing the desolvated species to ethanol vapour. The structure of the SMM [Mn3O(Et-sao)3(ClO4)(MeOH)x(H2O)3-x] (5, Et-saoH2 = C6H4-1-CH(C2H5)N(OH)-2-OH) at 2 K is also reported from a combination of single-crystal neutron and X-ray diffraction experiments. There is linked disorder in the identity of the coordinated solvent (methanol or water) and in the position of the ethyl substituent in the Et-sao2- ligand. This leads to a number of possible isomers present in a single-crystal and is consistent with the observation of transverse anisotropy in 5 previously recorded by high-frequency- and high-field-electron paramagnetic resonance (HF-EPR) spectroscopy. The crystal structure of the triangle intermediate [Mn3O(O2CEt)6(py)3](ClO4) (6, py = pyridine) formed during the synthesis of the SMM [Mn4O3Cl4(O2CEt)3(py)3]⋅4MeCN (7) is reported for the first time. Additionally a related new compound [Mn4O3Cl7(O2CEt)3](C5H5NH)3 (8) that crystallises in the same space group as 7 is also reported. The structure of the SMM [Ni4(dea)4Cl4]⋅4MeCN⋅0.6H2O (9, deaH = (HOCH2CH2)2NH) is reported at 1.7 K by using single-crystal neutron diffraction. The isostructural bromide complex [Ni4(dea)4Br4]⋅6MeCN (12) was synthesised for the first time and its structure is reported at 30 K. Methanol solvates of each species are also reported (10 and 13) and crystallise in the lower symmetry P-1 space group. 13 displays similar low-temperature (sub-3 K) magnetic behaviour to that reported for 9, as well as an additional slow relaxation process at 4-6 K. Mass spectrometry is consistent with the formation of {Ni4} cubanes with N-substituted diethanolamine ligands, N-phenyl-deaH and triethanolamine (teaH); the former with chloride ligands and the latter with bromide ligands. Another new solvothermally-derived {Ni4} cubane species is described with a slightly different cubane core, [Ni4(dea)4Br2(H2O)2]Br2 (14). We attempted to investigate two species based on a {Ni10} supertetrahedron (16 and 17); unfortunately the single-crystal neutron diffraction experiment carried out on these compounds was unsuccessful. The structure of the SMM (C(NH2)3)8[Co4(cit)4]⋅4H2O (18, H4cit = (HO2CCH2)2C(OH)CO2H) was determined at 2 K using high-resolution neutron powder diffraction, single-crystal X-ray diffraction and single-crystal neutron diffraction. The former two measurements offer some evidence for disorder in the hydrogen bonding between the {Co4} clusters, guanidinium cations and solvent of crystallisation; however, the latter measurement does not support this. A series of inelastic and polarised neutron scattering measurements are presented for a deuterated sample of 18 (denoted 19) and allow observation of scattering related to magnetic excitations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry