Nevertheless, bond formation is not always favoured at pressure. In the
one-dimensional
Cu-(II)-containing
coordination
polymer
(Cu 2 L 2 (1-methylpiperazine) 2 ]n
(where
H 2 L
¼
1,1
0 -(1,3-phenylene)-bis
(4,4-dimethylpentane-1,3-dione)), pressure-induced Cu-N bond breaking/bond
forming occurs at a very low pressure of only 0.05 GPa, immediately on loading
the crystals inside the DAC. The phase change results in the depolymerisation of the
material through the cleavage and formation of axial Cu-N bonds, resulting in the
formation of a discrete dinuclear complex [161]. The breaking and subsequent
release of coordinated water ligands was also observed in the hydroxo-bridged Cu
(II) dimer [CuF 2 (H 2 O) 2 (pyz)] (1, pyz ¼ pyrazine) [162]. In this study, two phase
transitions were observed, with the second transition (occurring between 2.85 and
3.3 GPa), resulting in a loss in symmetry and ejection of one water molecule per
copper unit from two thirds of the chains, forcing them to dimerise through the
F-atoms to fill the vacant coordination sites. This study was performed in parallel
with high-pressure EPR, with the main focus of the paper being a Jahn-Teller
switching within the material, which we refer to in the next section.
4.4 Functional Materials at Pressure
4.4.1 Molecular Magnetic Materials
Structure/property relationships in a wide range of functional materials are usually
established by systematically modifying the chemical structure to produce a number
of related compounds, complexes or frameworks, whose physical properties can be
measured and observations correlated to the molecular or crystal structure. This
approach has been used successfully in molecular magnetic materials such as
dinuclear metal complexes, where the geometrical parameters between the metals
and associated bridging ligands can affect whether the metals interact ferro- or
antiferromagnetically [163]. A more recent approach has been to perform highpressure crystallographic experiments, in parallel with high-pressure magnetic measurements, to directly measure how changes in the intramolecular geometry and
intermolecular interactions affect magnetic behaviour. Some of this behaviour has
already been detailed in the review by Moggach et al. [143], though several other
examples have been observed since the review was published. One of the main
structural concepts that has been explored in molecular magnetic materials is the
Jahn-Teller distortion around the metal centre. This is exemplified by the Mn 12
complex Mn 12 O 12 (O 2 CCH 2
t Bu) 16 (H 2 O) 4 ÁCH 2 Cl 2 ÁMeNO 2 . The complex consists of
a cube of Mn(IV) 2 O 4 surrounded by eight Mn(III) metal sites that are bonded to the
ligands. As pressure is applied to this system, the compression of the Jahn-Teller
axes on each of the eight Mn(III) centres differs slightly with one of the centres
switching the Jahn-Teller axes above 1.5 GPa so that they lie normal to the plane
made of the two most compressible directions [164]. This transition was also
accompanied by a release of solvent from the crystal into the PTM on compression,
Crystallography Under High Pressures
183
one-dimensional
Cu-(II)-containing
coordination
polymer
(Cu 2 L 2 (1-methylpiperazine) 2 ]n
(where
H 2 L
¼
1,1
0 -(1,3-phenylene)-bis
(4,4-dimethylpentane-1,3-dione)), pressure-induced Cu-N bond breaking/bond
forming occurs at a very low pressure of only 0.05 GPa, immediately on loading
the crystals inside the DAC. The phase change results in the depolymerisation of the
material through the cleavage and formation of axial Cu-N bonds, resulting in the
formation of a discrete dinuclear complex [161]. The breaking and subsequent
release of coordinated water ligands was also observed in the hydroxo-bridged Cu
(II) dimer [CuF 2 (H 2 O) 2 (pyz)] (1, pyz ¼ pyrazine) [162]. In this study, two phase
transitions were observed, with the second transition (occurring between 2.85 and
3.3 GPa), resulting in a loss in symmetry and ejection of one water molecule per
copper unit from two thirds of the chains, forcing them to dimerise through the
F-atoms to fill the vacant coordination sites. This study was performed in parallel
with high-pressure EPR, with the main focus of the paper being a Jahn-Teller
switching within the material, which we refer to in the next section.
4.4 Functional Materials at Pressure
4.4.1 Molecular Magnetic Materials
Structure/property relationships in a wide range of functional materials are usually
established by systematically modifying the chemical structure to produce a number
of related compounds, complexes or frameworks, whose physical properties can be
measured and observations correlated to the molecular or crystal structure. This
approach has been used successfully in molecular magnetic materials such as
dinuclear metal complexes, where the geometrical parameters between the metals
and associated bridging ligands can affect whether the metals interact ferro- or
antiferromagnetically [163]. A more recent approach has been to perform highpressure crystallographic experiments, in parallel with high-pressure magnetic measurements, to directly measure how changes in the intramolecular geometry and
intermolecular interactions affect magnetic behaviour. Some of this behaviour has
already been detailed in the review by Moggach et al. [143], though several other
examples have been observed since the review was published. One of the main
structural concepts that has been explored in molecular magnetic materials is the
Jahn-Teller distortion around the metal centre. This is exemplified by the Mn 12
complex Mn 12 O 12 (O 2 CCH 2
t Bu) 16 (H 2 O) 4 ÁCH 2 Cl 2 ÁMeNO 2 . The complex consists of
a cube of Mn(IV) 2 O 4 surrounded by eight Mn(III) metal sites that are bonded to the
ligands. As pressure is applied to this system, the compression of the Jahn-Teller
axes on each of the eight Mn(III) centres differs slightly with one of the centres
switching the Jahn-Teller axes above 1.5 GPa so that they lie normal to the plane
made of the two most compressible directions [164]. This transition was also
accompanied by a release of solvent from the crystal into the PTM on compression,
Crystallography Under High Pressures
183