observed, decreasing in length by 0.075(6) Å for L ¼ ÀO 2 CPh(Me) 2 and 0.081(8) Å
for L ¼ naphthalene carboxylate, on average [169].
In all of the aforementioned complexes, the twisting or distortion of ligands
between metal centres within a molecular magnet results in a change in the magnetic
exchange between the metal centres. Another area of interest in the world of
molecular magnetism is the study of single-ion magnets (or SIMs). A significant
feature required for molecular magnets to be used for information storage is the
presence of giant axial magnetic anisotropy. The barrier to magnetic relaxation,
which would cause loss of data, is determined by the size of the spin ground state and
the size of the axial anisotropy. Here, the focus has been on achieving exquisite
control of the coordination environment around a single paramagnetic metal ion, to
generate a ligand field that leads to first-order spin-orbit coupling. In the SIM [Ni
(MeDABCO) 2 Cl 3 ]
+
(MeDABCO ¼ 1-methyl-4-aza-1-azoniabicyclo[2.2.2]
octanium), the application of pressure causes the giant axial magnetic anisotropy
to decrease as the symmetry about the Ni centre [170]. Here, as in the previous
examples, pressure is used to distort the metal centre to determine which parameters
control the magnetic anisotropy, rather than modifying the ligands around the metal
centre. Here, the Cl-Ni-Cl angles, in particular, were particularly prone to distortion,
with two of the three Cl-Ni-Cl angles increasing in size (the largest by % 5
), whilst
the third actually decreased by % 9
.
4.4.2 Spin-Crossover Complexes
Metal complexes particularly first-row transition metal complexes with a d
4 to d
7
configuration in an octahedral geometry are well-known to undergo transitions from
high- to low-spin states or vice versa. This field is dominated by Fe(II)-containing
spin-crossover complexes, where changes in spin state can be easily followed by
measuring magnetic susceptibility, and even performing Mössbauer spectroscopy,
as the change from a paramagnetic to diamagnetic species can be easily followed.
Transitions between these two spin states can be induced by temperature, pressure or
light. Pressure is particularly useful, as increasing pressure favours the low-spin
(smaller volume) spin state which may be inaccessible via any other route. In the
complex [{Fe(bpp)(NCS) 2 } 2 (4,4
0 -bipy)]Á2MeOH, (bpp ¼ 2,6-bis(pyrazol-3-yl)pyridine, 4,4
0 -bipy ¼ 4,4
0 -bipyridine), increasing pressure above 0.7 GPa induces a
spin-crossover transition, which is not thermally accessible. It does this without
causing a crystallographic phase transition (i.e. major structural change) in the
crystal [171]. Phase transitions involving a change in symmetry, or change in size
of the unit cell dimensions, however, are often observed. In the complex [FeII
(bapbpy)(NCS) 2 ] [172] (bapbpy ¼ 6,6-bis(amino-2-pyridyl)-2,2-bipyridine), a
‘stepped’ first-order transition from the high-spin to low-spin phase was observed
upon compression, where on increasing pressure, an intermediate phase (between a
fully high-spin and low-spin state) was observed between 0.4 and 1.1 GPa. This
phase was characterised by supercell reflections and tripling of the c-axis of the unit
cell due to the formation of a periodic [HS-LS-LS] structural motif. Interestingly, in
Crystallography Under High Pressures
185
for L ¼ naphthalene carboxylate, on average [169].
In all of the aforementioned complexes, the twisting or distortion of ligands
between metal centres within a molecular magnet results in a change in the magnetic
exchange between the metal centres. Another area of interest in the world of
molecular magnetism is the study of single-ion magnets (or SIMs). A significant
feature required for molecular magnets to be used for information storage is the
presence of giant axial magnetic anisotropy. The barrier to magnetic relaxation,
which would cause loss of data, is determined by the size of the spin ground state and
the size of the axial anisotropy. Here, the focus has been on achieving exquisite
control of the coordination environment around a single paramagnetic metal ion, to
generate a ligand field that leads to first-order spin-orbit coupling. In the SIM [Ni
(MeDABCO) 2 Cl 3 ]
+
(MeDABCO ¼ 1-methyl-4-aza-1-azoniabicyclo[2.2.2]
octanium), the application of pressure causes the giant axial magnetic anisotropy
to decrease as the symmetry about the Ni centre [170]. Here, as in the previous
examples, pressure is used to distort the metal centre to determine which parameters
control the magnetic anisotropy, rather than modifying the ligands around the metal
centre. Here, the Cl-Ni-Cl angles, in particular, were particularly prone to distortion,
with two of the three Cl-Ni-Cl angles increasing in size (the largest by % 5
), whilst
the third actually decreased by % 9
.
4.4.2 Spin-Crossover Complexes
Metal complexes particularly first-row transition metal complexes with a d
4 to d
7
configuration in an octahedral geometry are well-known to undergo transitions from
high- to low-spin states or vice versa. This field is dominated by Fe(II)-containing
spin-crossover complexes, where changes in spin state can be easily followed by
measuring magnetic susceptibility, and even performing Mössbauer spectroscopy,
as the change from a paramagnetic to diamagnetic species can be easily followed.
Transitions between these two spin states can be induced by temperature, pressure or
light. Pressure is particularly useful, as increasing pressure favours the low-spin
(smaller volume) spin state which may be inaccessible via any other route. In the
complex [{Fe(bpp)(NCS) 2 } 2 (4,4
0 -bipy)]Á2MeOH, (bpp ¼ 2,6-bis(pyrazol-3-yl)pyridine, 4,4
0 -bipy ¼ 4,4
0 -bipyridine), increasing pressure above 0.7 GPa induces a
spin-crossover transition, which is not thermally accessible. It does this without
causing a crystallographic phase transition (i.e. major structural change) in the
crystal [171]. Phase transitions involving a change in symmetry, or change in size
of the unit cell dimensions, however, are often observed. In the complex [FeII
(bapbpy)(NCS) 2 ] [172] (bapbpy ¼ 6,6-bis(amino-2-pyridyl)-2,2-bipyridine), a
‘stepped’ first-order transition from the high-spin to low-spin phase was observed
upon compression, where on increasing pressure, an intermediate phase (between a
fully high-spin and low-spin state) was observed between 0.4 and 1.1 GPa. This
phase was characterised by supercell reflections and tripling of the c-axis of the unit
cell due to the formation of a periodic [HS-LS-LS] structural motif. Interestingly, in
Crystallography Under High Pressures
185