spectroscopic changes. The emergence of high-pressure diffraction studies,
conducted in parallel with other physical property measurements, such as luminescence, fluorescence, and magnetometry, has really opened up the possibility of
deriving structure/property relationships in a plethora of functional materials.
Here, we will give examples of recent studies that show changes in intramolecular
conformation, M-M, M-L, and intermolecular interaction distances, which can lead
to both the formation and breakage of bonds. Finally, we will give examples of how
these changes have been used effectively to develop structure/property relationships
in molecular magnetic materials, spin-crossover complexes and, finally, metalorganic frameworks and coordination polymers.
4.2 Intramolecular Conformational Changes
and Compressibility of M-M and M-L Bonds
The investigation of the coordination environment has been a source of rich
discovery from purely structural context as well as from a structure/property
perspective. The decrease in length of metal-metal bonds has been observed in
a handful of metal complexes. For example, in the linear chain compound
Co 2 (dpa) 4 Br 2 (dpa ¼ 2,2
0 -dypyridylamide anion), a reduction in unit cell volume
of 30% is observed at 13.6 GPa. Most of the compression is ‘taken up’ by the
compressibility of much softer intermolecular interactions that cause the redistribution of DCM (dichloromethane) solvent molecules during the compression.
However, a portion of the compression is due to a decrease in length of the
Co-Co and Co-Br bonds with each showing a reduction of 4% and 12%, respectively, at 11.8(2) GPa [148]. A remarkable study by Wu and co-workers of Mg
(dipnacnac), ((DipNCMe)2CH, Dip ¼ 2,6-diisopropylphenyl), a non-nuclear
attractor (NNA), indicated a small (yet significant) decrease in length of a
Mg-Mg bond in the complex. Despite the challenges of Mg(I) compounds being
very reactive and acting as very efficient reducing agents, the authors were able to
show that the central Mg-Mg bond decreases from 2.84 to 2.76 Å, a change of 3%,
whilst maintaining NNA behaviour to 1.9 GPa [149]. The compression of metalcontaining complexes has also been performed to help determine the nature of
metal-ligand interactions. Notably, the compressibility of U-U and U-C-H bonds
was studied in the diuranium(III) compound [UN
00 2]2(μ-η6:η6-C 6 H 6 ) (N
00
¼ N
(SiMe 3 ) 2 ), primarily to study agostic interactions of a low-coordinate U
III complex
[150]. In this study, which was supported by complementary QTAIM and NBO
analyses, one particular U-CH bond (3.022(3) Å at ambient pressure) indicated an
agostic interaction but in fact does not become agostic until 3.2 GPa, where the
U-CH bond decreased to 2.95(2) Å (Fig. 14).
The following examples are highlighted as they have demonstrated the changes in
spectroscopic properties that occur in metal complexes as the coordination environment is compressed. In particular, the compressibility of argentophilic interactions
Crystallography Under High Pressures
179
conducted in parallel with other physical property measurements, such as luminescence, fluorescence, and magnetometry, has really opened up the possibility of
deriving structure/property relationships in a plethora of functional materials.
Here, we will give examples of recent studies that show changes in intramolecular
conformation, M-M, M-L, and intermolecular interaction distances, which can lead
to both the formation and breakage of bonds. Finally, we will give examples of how
these changes have been used effectively to develop structure/property relationships
in molecular magnetic materials, spin-crossover complexes and, finally, metalorganic frameworks and coordination polymers.
4.2 Intramolecular Conformational Changes
and Compressibility of M-M and M-L Bonds
The investigation of the coordination environment has been a source of rich
discovery from purely structural context as well as from a structure/property
perspective. The decrease in length of metal-metal bonds has been observed in
a handful of metal complexes. For example, in the linear chain compound
Co 2 (dpa) 4 Br 2 (dpa ¼ 2,2
0 -dypyridylamide anion), a reduction in unit cell volume
of 30% is observed at 13.6 GPa. Most of the compression is ‘taken up’ by the
compressibility of much softer intermolecular interactions that cause the redistribution of DCM (dichloromethane) solvent molecules during the compression.
However, a portion of the compression is due to a decrease in length of the
Co-Co and Co-Br bonds with each showing a reduction of 4% and 12%, respectively, at 11.8(2) GPa [148]. A remarkable study by Wu and co-workers of Mg
(dipnacnac), ((DipNCMe)2CH, Dip ¼ 2,6-diisopropylphenyl), a non-nuclear
attractor (NNA), indicated a small (yet significant) decrease in length of a
Mg-Mg bond in the complex. Despite the challenges of Mg(I) compounds being
very reactive and acting as very efficient reducing agents, the authors were able to
show that the central Mg-Mg bond decreases from 2.84 to 2.76 Å, a change of 3%,
whilst maintaining NNA behaviour to 1.9 GPa [149]. The compression of metalcontaining complexes has also been performed to help determine the nature of
metal-ligand interactions. Notably, the compressibility of U-U and U-C-H bonds
was studied in the diuranium(III) compound [UN
00 2]2(μ-η6:η6-C 6 H 6 ) (N
00
¼ N
(SiMe 3 ) 2 ), primarily to study agostic interactions of a low-coordinate U
III complex
[150]. In this study, which was supported by complementary QTAIM and NBO
analyses, one particular U-CH bond (3.022(3) Å at ambient pressure) indicated an
agostic interaction but in fact does not become agostic until 3.2 GPa, where the
U-CH bond decreased to 2.95(2) Å (Fig. 14).
The following examples are highlighted as they have demonstrated the changes in
spectroscopic properties that occur in metal complexes as the coordination environment is compressed. In particular, the compressibility of argentophilic interactions
Crystallography Under High Pressures
179