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), Zn­(HO3PC4H8PO3H)·2H2O (ZAG-4), was investigated under high pressures (0–9.9 GPa) using in situ 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 b-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), Zn­(HO₃PC₄H₈PO₃H)·2H₂O (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), Zn­(HO₃PC₄H₈PO₃H)·2H₂O (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), Zn­(HO₃PC₄H₈PO₃H)·2H₂O (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), Zn­(HO₃PC₄H₈PO₃H)·2H₂O (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-tris­(pentafluorophenyl)­corrole, H₃[Br₈TPFPCor], 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 H₃(Br₈TPFPCor). 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 H₃[Br₈TPFPCor], 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

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-tris­(pentafluorophenyl)­corrole, H₃[Br₈TPFPCor], 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 H₃(Br₈TPFPCor). 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 H₃[Br₈TPFPCor], 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 = CF₃, H, CH₃) on the β-aryl groups were synthesized via Suzuki–Miyaura coupling of Cu­[Br₈TPC] and the appropriate arylboronic acid. A single-crystal X-ray structure of the X = CF₃ 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 Cu­[Br₈TPC] (∼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 Cu­[Br₈TPC] 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

Steric and Electronic Effects of Carbene Substitution in Grubbs First-Generation Catalysts

The relative energies of the carbene species in a variety of metathesis reactions catalyzed by Grubbs first-generation catalyst (Cy3P)2Cl2RuCHC6H5 have been determined. High-quality equilibrium data are obtainable by 1H NMR methods. In the case of para-substituted aromatic derivatives (Cy3P)2Cl2RuCH(p-C6H4X), the carbenes are stabilized by donor substituents, and energies may be determined indirectly by use of either NMR data or Hammett correlations, because only electronic effects are operative. Four systems have been characterized by X-ray crystallography. The energies of alkyl-substituted carbenes (Cy3P)2Cl2RuCHR are almost entirely dependent on steric interactions

Undecaphenylcorroles

A first major study of undecaphenylcorrole (UPC) derivatives is presented. Three different Cu-UPC derivatives with different para substituents X (X = CF₃, H, CH₃) on the β-aryl groups were synthesized via Suzuki–Miyaura coupling of Cu­[Br₈TPC] and the appropriate arylboronic acid. A single-crystal X-ray structure of the X = CF₃ 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 Cu­[Br₈TPC] (∼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 Cu­[Br₈TPC] 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