Effect of high-pressure on molecular magnetism
View/ Open
Date
2010Author
Prescimone, Alessandro
Metadata
Abstract
The effect of pressure on a number of magnetically interesting compounds such as
single-molecule magnets and dimeric copper and manganese molecules has been
investigated to probe the validity of ambient magneto-structural correlations.
The first chapter is an introduction to the equipment and methodologies that have
been adopted to carry out the experimental high-pressure work.
The second chapter reports the first combined high-pressure single crystal X-ray
diffraction and high pressure magnetism study of four single-molecule magnets
(SMMs). At 1.5 GPa the structures [Mn6O2(Et-sao)6(O2CPh(Me)2)2(EtOH)6] (1) – an
SMM with a record effective anisotropy barrier of ~86 K – and [Mn6O2(Etsao)
6(O2C-naphth)2(EtOH)4(H2O)2] (2) both undergo significant structural distortions
of their metallic skeletons which has a direct effect upon the observed magnetic
response. Up to 1.5 GPa pressure the effect is to flatten the Mn-N-O-Mn torsion
angles weakening the magnetic exchange between the metal centres. In both
compounds one pairwise interaction switches from ferro- to antiferromagnetic, with
the Jahn-Teller (JT) axes compressing (on average) and re-aligning differently with
respect to the plane of the three metal centres. High pressure dc χMT plots display a
gradual decrease in the low temperature peak value and slope, simulations showing a
decrease in |J| with increasing pressure with a second antiferromagnetic J value
required to simulate the data. The “ground states” change from S = 12 to S = 11 for
1 and to S = 10 for 2. Magnetisation data for both 1 and 2 suggest a small decrease in
|D|, while out-of-phase (χM
//) ac data show a large decrease in the effective energy
barrier for magnetisation reversal. The third SMM is the complex
[Mn3(Hcht)2(bpy)4](ClO4)3·Et2O·2MeCN (3·Et2O·2MeCN) that at 0.16 GPa loses all
associated solvent in the crystal lattice, becoming 3. At higher pressures structural
distortions occur changing the distances between the metal centres and the bridging
oxygen atoms making |J| between the manganese ions weaker. No significant
variations are observed in the JT axis of the only MnIII present in the structure. Highpressure
dc χMT plots display a gradual decrease in the low temperature peak value
and slope. Simulations show a decrease in J with increasing pressure although the
ground state is preserved. Magnetisation data do not show any change in |D|. The fourth SMM, [(tacn)6Fe8O2(OH)12](ClO4)3.9Br4.1⋅6H2O, (4) is the largest inorganic
compound ever studied at high-pressure. Up to 2.0 GPa the conformation of the
complex remains largely unaffected, with the counter ions and water molecules
moving around to accommodate a compression of the unit cell volume. High
pressure magnetic susceptibility data collected up to 0.93 GPa confirm minimal
changes in the intra-molecular exchange interactions.
The third chapter focuses on three hydroxo-bridged CuII dimers:
[Cu2(OH)2(H2O)2(tmen)2](ClO4)2 (5), [Cu2(OH)2(tben)2](ClO4)2 (6) and
[Cu2(OH)2(bpy)2](BF4)2 (7) have been structurally determined up to 2.5, 0.9 and 4.7
GPa, respectively. 6 and 7 have never been reported before. Pressure imposes
important distortions in the structures of all three complexes, particularly on the bond
distances and angles between the metal centres and the bridging hydroxo groups. 5
undergoes a phase transition between 1.2 and 2.5 GPa caused by the loss of a
coordinated water molecule. This leads to a loss of symmetry and dramatic changes
in the molecular structure of the complex. The structural changes are manifested in
different magnetic behaviours of the complexes as seen in dc susceptibility
measurements up to ~0.9 GPa: J becomes less antiferromagnetic in 5 and 6 and more
ferromagnetic in 7.
The fourth chapter shows the compression of two oxo-bridged MnII/MnIII mixed
valence dimers: [Mn2O2(bpy)4](ClO4)3⋅3CH3CN, (8) has been squeezed up to 2.0
GPa whilst [Mn2O2(bpy)4](PF6)3⋅2CH3CN⋅1H2O, (9) could be measured
crystallographically up to 4.55 GPa. 9 has never been reported before, while 8 has
been reported in a different crystallographic space group. The application of pressure
imposes significant alterations in the structures of both complexes. In particular, in 8
the Mn-Mn separation is reduced by the contraction of some of the Mn-O bond
distances, 9 shows essentially analogous behaviour: the Mn-Mn distance and nearly
all the Mn-N bonds shrink significantly. The magnetic behaviour of the complexes
has been measured up to 0.87 GPa for 8 and 0.84 GPa for 9, but neither display any
significant differences with respect to their ambient data.