Metal–organic frameworks (MOFs) NU-1000 and UiO-66 are herein exposed to two different gamma irradiation doses and dose rates and analyzed to determine the structural features that affect their stability in these environments. MOFs have shown promise for the capture and sensing of off-gases at civilian nuclear energy reprocessing sites, nuclear waste repositories, and nuclear accident locations. However, little is understood about the structural features of MOFs that contribute to their stability levels under the ionizing radiation conditions present at such sites. This study is the first of its kind to explore the structural features of MOFs that contribute to their radiolytic stability. Both NU-1000 and UiO-66 are MOFs that contain Zr metal-centers with the same metal absorption cross section. However, the two MOFs exhibit different linker connectivities, linker aromaticities, node densities, node connectivities, and interligand separations. In this study, NU-1000 and UiO-66 were exposed to high (423.3 Gy/min, 23 min, and 37 s) and low (0.78 Gy/min, 4320 min) dose rates of 60Co gamma irradiation. NU-1000 displayed insignificant radiation damage under both dose rates due to its high linker connectivity, low node density, and low node connectivity. However, low radiation dose rates caused considerable damage to UiO-66, a framework with lower aromaticity and smaller interligand separation. Results suggest that chronic, low-radiation environments are more detrimental to Zr MOF stability than acute, high-radiation conditions.
The performance of metal–organic frameworks (MOFs) under mechanical stress is an important consideration in the design, synthesis, and application of MOF materials in both fundamental and industrial settings. We herein demonstrate that the bulk modulus (K = −V dP/dV) of a 4,8-connected Zr-based MOF, NU-901, comprised of Zr6O8 nodes and tetratopic pyrene linkers, increases systematically upon postsynthetic installation of a structural organic linker, 2,6-naphthalenedicarboxylic acid (NDC), via solvent assisted linker incorporation. We calculated the bulk modulus, a measure of resistance to hydrostatic compression, of these modified NU-901 samples through in situ variable powder X-ray diffraction pressure measurements collected using a synchrotron source. As the amount of NDC incorporation into the NU-901 framework increased, the lattice strength of the framework also increased. This strategy of postsynthetic modification at the molecular level serves as a promising blueprint to tune the bulk mechanical properties of other MOFs by increasing the connnectivity of the secondary building unit. Furthermore, we envision this method may allow for structural reinforcement of other frameworks along one preferential axis or direction dependent on the desired application.
Gadolinium(III) nanoconjugate contrast agents (CAs) provide significant advantages over small-molecule complexes for magnetic resonance imaging (MRI), namely increased Gd(III) payload and enhanced proton relaxation efficiency (relaxivity, r 1 ). Previous research has demonstrated that both the structure and surface chemistry of the nanomaterial substantially influence contrast. We hypothesized that inserting Gd(III) complexes in the pores of a metal−organic framework (MOF) might offer a unique strategy to further explore the parameters of nanomaterial structure and composition, which influence relaxivity. Herein, we postsynthetically incorporate Gd(III) complexes into Zr-MOFs using solvent-assisted ligand incorporation (SALI). Through the study of Zr-based MOFs, NU-1000 (nano and micronsize particles) and NU-901, we investigated the impact of particle size and pore shape on proton relaxivity. The SALI-functionalized Gd nano NU-1000 hybrid material displayed the highest loading of the Gd(III) complex (1.9 ± 0.1 complexes per node) and exhibited the most enhanced proton relaxivity (r 1 of 26 ± 1 mM −1 s −1 at 1.4 T). Based on nuclear magnetic relaxation dispersion (NMRD) analysis, we can attribute the performance of Gd nano NU-1000 to the nanoscale size of the MOF particles and larger pore size that allows for rapid water exchange. We have demonstrated that SALI is a promising method for incorporating Gd(III) complexes into MOF materials and identified crucial design parameters for the preparation of next generation Gd(III)-functionalized MOF MRI contrast agents.
Protonation of C−M bonds and its microscopic reverse, metalation of C−H bonds, are fundamental steps in a variety of metal-catalyzed processes. As such, studies on protonation of C−M bonds can shed light on C−H activation. We present here studies on the rate of protodemetalation (PDM) of a suite of arylnickel(II) complexes with various acids that provide evidence for a concerted, cyclic transition state for the PDM of C−Ni bonds and demonstrate that five-, six-, and seven-membered transition states are particularly favorable. Our data show that while the rate of protodemetalation of arylnickel(II) complexes scales with acidity for many acids, several are faster than predicted by pK a . For example, while acetic acid and acetohydroxamic acid are much less acidic than HCl, they both protodemetalate arylnickel(II) complexes significantly faster than HCl. Our data also show how in the case of acetohydroxamic acid, a seven-membered cyclic transition state (CH 3 C(O)NHOH) can be more favorable than a six-membered transition state (CH 3 C(O)NHOH). Similarly, five-membered transition states, such as for pyrazole, are highly favorable as well. Comparison of transition state polarization (from density functional theory) compares these new nickel transition states to betterstudied precious-metal systems and demonstrates how the base can change the polarization of the transition state giving rise to opposing electronic preferences. Collectively, these studies suggest several new avenues for study in C−H activation as well as approaches to accelerate or slow protodemetalation in nickel catalysis.
Protonation of C–M bonds and its microscopic reverse, metalation of C–H bonds, are fundamental steps in a variety of metal-catalyzed processes. As such, studies on protonation of C–M bonds can shed light on C–H activation. We present here studies on the rate of protodemetalation (PDM) of a suite of arylnickel(II) complex-es with various acids that provide evidence for a concerted, cyclic transition state for the PDM of C–Ni bonds and demonstrate that five, six, and seven-membered transition states are particularly favorable. Our data show that while the rate of protodemetalation of arylnickel(II) complexes scales with acidity for many acids, several are faster than predicted by pKa. For example, while acetic acid and acetohydroxamic acid are much less acid-ic than HCl, they both protodemetalate arylnickel(II) complexes significantly faster than HCl. Our data also shows how in the case of acetohydroxamic acid, a seven-membered cyclic transition state (CH3C(O)NHOH) can be more favorable than a six-membered transition state (CH3C(O)NHOH). Similarly, five-membered transition states, such as for pyrazole, are highly favorable as well. Comparison of transition-state polarization (from den-sity functional theory) compares these new nickel transition states to better-studied precious metal systems and demonstrates how the base can change the polarization of the transition state giving rise to opposing electron-ic preferences. Collectively, these studies suggest several new avenues for study in C–H activation as well as approaches to accelerate or slow protodemetalation in nickel catalysis.
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