23 research outputs found
MOFs Under Pressure: The Reversible Compression of a Single Crystal
The structural change and resilience of a single crystal
of a metal–organic
framework (MOF), ZnÂ(HO<sub>3</sub>PC<sub>4</sub>H<sub>8</sub>PO<sub>3</sub>H)·2H<sub>2</sub>O (ZAG-4), was investigated under high
pressures (0–9.9 GPa) using <i>in situ</i> single
crystal X-ray diffraction. Although the unit cell volume decreases
over 27%, the quality of the single crystal is retained and the unit
cell parameters revert to their original values after pressure has
been removed. This framework is considerably compressible with a bulk
modulus calculated at ∼11.7 GPa. The <i>b</i>-axis
also exhibits both positive and negative linear compressibility. Within
the applied pressures investigated, there was no discernible failure
or amorphization point for this compound. The alkyl chains in the
structure provide a spring-like cushion to stabilize the compression
of the system allowing for large distortions in the metal coordination
environment, without destruction of the material. This intriguing
observation only adds to the current speculation as to whether or
not MOFs may find a role as a new class of piezofunctional solid-state
materials for application as highly sensitive pressure sensors, shock
absorbing materials, pressure switches, or smart body armor
MOFs Under Pressure: The Reversible Compression of a Single Crystal
The structural change and resilience of a single crystal
of a metal–organic
framework (MOF), ZnÂ(HO<sub>3</sub>PC<sub>4</sub>H<sub>8</sub>PO<sub>3</sub>H)·2H<sub>2</sub>O (ZAG-4), was investigated under high
pressures (0–9.9 GPa) using <i>in situ</i> single
crystal X-ray diffraction. Although the unit cell volume decreases
over 27%, the quality of the single crystal is retained and the unit
cell parameters revert to their original values after pressure has
been removed. This framework is considerably compressible with a bulk
modulus calculated at ∼11.7 GPa. The <i>b</i>-axis
also exhibits both positive and negative linear compressibility. Within
the applied pressures investigated, there was no discernible failure
or amorphization point for this compound. The alkyl chains in the
structure provide a spring-like cushion to stabilize the compression
of the system allowing for large distortions in the metal coordination
environment, without destruction of the material. This intriguing
observation only adds to the current speculation as to whether or
not MOFs may find a role as a new class of piezofunctional solid-state
materials for application as highly sensitive pressure sensors, shock
absorbing materials, pressure switches, or smart body armor
MOFs Under Pressure: The Reversible Compression of a Single Crystal
The structural change and resilience of a single crystal
of a metal–organic
framework (MOF), ZnÂ(HO<sub>3</sub>PC<sub>4</sub>H<sub>8</sub>PO<sub>3</sub>H)·2H<sub>2</sub>O (ZAG-4), was investigated under high
pressures (0–9.9 GPa) using <i>in situ</i> single
crystal X-ray diffraction. Although the unit cell volume decreases
over 27%, the quality of the single crystal is retained and the unit
cell parameters revert to their original values after pressure has
been removed. This framework is considerably compressible with a bulk
modulus calculated at ∼11.7 GPa. The <i>b</i>-axis
also exhibits both positive and negative linear compressibility. Within
the applied pressures investigated, there was no discernible failure
or amorphization point for this compound. The alkyl chains in the
structure provide a spring-like cushion to stabilize the compression
of the system allowing for large distortions in the metal coordination
environment, without destruction of the material. This intriguing
observation only adds to the current speculation as to whether or
not MOFs may find a role as a new class of piezofunctional solid-state
materials for application as highly sensitive pressure sensors, shock
absorbing materials, pressure switches, or smart body armor
MOFs Under Pressure: The Reversible Compression of a Single Crystal
The structural change and resilience of a single crystal
of a metal–organic
framework (MOF), ZnÂ(HO<sub>3</sub>PC<sub>4</sub>H<sub>8</sub>PO<sub>3</sub>H)·2H<sub>2</sub>O (ZAG-4), was investigated under high
pressures (0–9.9 GPa) using <i>in situ</i> single
crystal X-ray diffraction. Although the unit cell volume decreases
over 27%, the quality of the single crystal is retained and the unit
cell parameters revert to their original values after pressure has
been removed. This framework is considerably compressible with a bulk
modulus calculated at ∼11.7 GPa. The <i>b</i>-axis
also exhibits both positive and negative linear compressibility. Within
the applied pressures investigated, there was no discernible failure
or amorphization point for this compound. The alkyl chains in the
structure provide a spring-like cushion to stabilize the compression
of the system allowing for large distortions in the metal coordination
environment, without destruction of the material. This intriguing
observation only adds to the current speculation as to whether or
not MOFs may find a role as a new class of piezofunctional solid-state
materials for application as highly sensitive pressure sensors, shock
absorbing materials, pressure switches, or smart body armor
Molecular Structures of Free-Base Corroles: Nonplanarity, Chirality, and Enantiomerization
The
molecular structures of free-base corroles are illustrative of a variety
of bonded and nonbonded interactions including aromaticity, intra-
as well as intermolecular hydrogen bonding, steric interactions among
multiple NH hydrogens within a congested central cavity, and the effects
of peripheral substituents. Against this backdrop, an X-ray structure
of 2,3,7,8,12,13,17,18-octabromo-5,10,15-trisÂ(pentafluorophenyl)Âcorrole,
H<sub>3</sub>[Br<sub>8</sub>TPFPCor], corresponding to a specific
tautomer, has been found to exhibit the strongest nonplanar distortions
observed to date for any free-base corrole structure. Two adjacent <i>N</i>-protonated pyrrole rings are tilted with respect to each
other by approximately 97.7°, while the remainder of the molecule
is comparatively planar. Dispersion-corrected DFT calculations were
undertaken to investigate to what extent the strong nonplanar distortions
can be attributed to steric effects of the peripheral substituents.
For <i>meso</i>-triphenylcorrole, DFT calculations revealed
nonplanar distortions that are only marginally less pronounced than
those found for H<sub>3</sub>(Br<sub>8</sub>TPFPCor). A survey of
X-ray structures of sterically unhindered corroles also uncovered
additional examples of rather strong nonplanar distortions. Detailed
potential energy calculations as a function of different saddling
dihedrals also emphasized the softness of the distortions. Because
of nonplanar distortions, free-base corrole structures are chiral.
For H<sub>3</sub>[Br<sub>8</sub>TPFPCor], DFT calculations led to
an estimate of 15 kcal/mol (0.67 eV) as the activation barrier for
enantiomerization of the free-base structures, which is significantly
higher than the barrier for NH tautomerism calculated for this molecule,
about 5 kcal/mol (0.2 eV). In summary, steric crowding of the internal
NH hydrogens appears to provide the main driving force for nonplanar
distortions of <i>meso</i>-triarylcorroles; the presence
of additional β-substituents adds marginally to this impetus
Undecaphenylcorroles
A first major study of undecaphenylcorrole (UPC) derivatives
is
presented. Three different Cu-UPC derivatives with different para
substituents X (X = CF<sub>3</sub>, H, CH<sub>3</sub>) on the β-aryl
groups were synthesized via Suzuki–Miyaura coupling of CuÂ[Br<sub>8</sub>TPC] and the appropriate arylboronic acid. A single-crystal
X-ray structure of the X = CF<sub>3</sub> complex revealed a distinctly
saddled macrocycle conformation with adjacent pyrrole rings tilted
by ∼60–66° relative to one another (within the dipyrromethane units), which is somewhat higher than that observed for β-unsubstituted
Cu-TPC derivatives but slightly lower than that observed for CuÂ[Br<sub>8</sub>TPC] (∼70°) derivatives.
Electrochemical and electronic absorption measurements afforded some
of the first comparative insights into meso versus β substituent
effects on the copper corrole
core. The Soret maxima of the Cu-UPC complexes (∼440–445
nm), however, are comparable to those of CuÂ[Br<sub>8</sub>TPC] derivatives
and are considerably red-shifted relative to Cu-TPC derivatives. Para
substituents
on the β-phenyl groups were found to tune the redox
potentials of copper corroles more effectively than those on <i>meso</i>-phenyl substituents, a somewhat surprising observation
given that neither the HOMO nor LUMO has significant amplitudes at
the β-pyrrolic
positions
Undecaphenylcorroles
A first major study of undecaphenylcorrole (UPC) derivatives
is
presented. Three different Cu-UPC derivatives with different para
substituents X (X = CF<sub>3</sub>, H, CH<sub>3</sub>) on the β-aryl
groups were synthesized via Suzuki–Miyaura coupling of CuÂ[Br<sub>8</sub>TPC] and the appropriate arylboronic acid. A single-crystal
X-ray structure of the X = CF<sub>3</sub> complex revealed a distinctly
saddled macrocycle conformation with adjacent pyrrole rings tilted
by ∼60–66° relative to one another (within the dipyrromethane units), which is somewhat higher than that observed for β-unsubstituted
Cu-TPC derivatives but slightly lower than that observed for CuÂ[Br<sub>8</sub>TPC] (∼70°) derivatives.
Electrochemical and electronic absorption measurements afforded some
of the first comparative insights into meso versus β substituent
effects on the copper corrole
core. The Soret maxima of the Cu-UPC complexes (∼440–445
nm), however, are comparable to those of CuÂ[Br<sub>8</sub>TPC] derivatives
and are considerably red-shifted relative to Cu-TPC derivatives. Para
substituents
on the β-phenyl groups were found to tune the redox
potentials of copper corroles more effectively than those on <i>meso</i>-phenyl substituents, a somewhat surprising observation
given that neither the HOMO nor LUMO has significant amplitudes at
the β-pyrrolic
positions
Molecular Paneling in the Rational Design of Calixarene Coordination Polymers
We
demonstrate the design of calixarene coordination polymers by
using versatile molecular panels comprising binuclear CoÂ(II) clusters
and <i>p</i>-carboxylatocalixÂ[4]Âarenes linked together with
4,4′-bipyridyls. By variation in the length of the 4,4′-bipyridyl,
conformational changes in the calixarene molecule have been observed
and the solvent-filled channels within the crystalline solid have
been adjusted
Enhancing Strategies for the Assembly of Metal–Organic Systems with Inherent Cavity-Containing Calix[4]arenes
<i>p</i>-CarboxylatocalixÂ[4]Âarenes have recently emerged
as useful building blocks in the assembly of both discrete and polymeric
coordination compounds. Steric effects of coligands used are now shown
to dramatically influence the assembly process, dictating the assembly
of one-dimensional (1D)-three-dimensional (3D) systems. Solvothermal
techniques have also been found to promote formation of 3D systems
with a sterically undemanding coligand
Enhancing Strategies for the Assembly of Metal–Organic Systems with Inherent Cavity-Containing Calix[4]arenes
<i>p</i>-CarboxylatocalixÂ[4]Âarenes have recently emerged
as useful building blocks in the assembly of both discrete and polymeric
coordination compounds. Steric effects of coligands used are now shown
to dramatically influence the assembly process, dictating the assembly
of one-dimensional (1D)-three-dimensional (3D) systems. Solvothermal
techniques have also been found to promote formation of 3D systems
with a sterically undemanding coligand