Synthesis, Characterization, and the Effect of Lewis Bases on the Nuclearity of Iron Alkoxide Complexes.

Inspired by the potential of alkoxides as weak-field ligands and their ability to bridge, we report herein a series of high-spin iron complexes supported by a bis-alkoxide framework PhDbf. A diiron complex [Fe2(PhDbf)2] (1a) is obtained upon metalation of the ligand, whereas addition of substituted pyridines affords five-coordinate mononuclear iron complexes [(R-Py)2Fe(PhDbf)] (2a-4a, R = H, p-tBu, p-CF3). The potential for nuclearity control of the metal complexes via auxiliary ligands is highlighted by the formation of asymmetric diiron species [(p-CF3-Py)Fe2(PhDbf)2] (5a) and [(m-CF3-Py)Fe2(PhDbf)2] (6a) with trifluoromethyl substituted pyridines, while electron-rich pyridines only produced monomeric species. Electronic properties analysis via UV-vis, electron paramagnetic resonance, 57Fe Mössbauer spectroscopy, and time-dependent density functional theory, along with redox capabilities of these complexes are reported to illustrate the effect of nuclearity on reactivity and the potential of these complexes to access higher oxidation states relevant in oxidative chemistry. Species 1a-5a, [(THF)2Fe(PhDbf)][PF6] (7), [PyFe(PhDbf)Cl] (2b), and [Py2Fe(PhDbf)][PF6] (2c) were characterized via SCXRD. Indirect evidence for the formation of dimeric Fe(III) species (1b, 5b, and 6b) is discussed.

Photoinduced Dynamic Ligation in Metal-Organic Frameworks.

Metal organic frameworks (MOFs), a class of porous crystalline materials consisting of metal-based nodes and organic linkers, have emerged as a promising platform for photocatalysis due to their ultrahigh functional surface area, customizable topologies, and tunable energetics. While interesting photochemistry has been reported, the related photoinduced structural dynamics of MOFs remains unclear. The consensus is that the coordination bonds between MOF nodes and linkers are considered static during photoexcitation, while the open-metal sites on the nodes are taken as the key active sites for catalysis. In this work, through a complementary time-resolved visible and infrared (IR) spectroscopic investigation, along with computational studies, we report for the first time light-induced structural bond dissociation (COO-M) and reformation in an iron-oxo framework, MIL-101(Fe). The probed excited state displayed ligand-to-metal charge transfer (LMCT) characteristics and exhibited a ca. 30 µs lifetime. The incredibly long excited-state lifetime led us to probe potential structural rearrangements that facilitated charge separation in MIL-101(Fe). By probing the vibrational fingerprints of the carboxylate linker upon LMCT photoexcitation, we observed the reversible transition of the carboxylate-Fe bond from a bidentate bridging mode to a monodentate mode, indicating the partial dissociation of the carboxylate ligand. Importantly, the bidentate configuration is recovered on the same time scale of the excited state lifetimes as probed via visible transient absorption spectroscopy. The elucidated photoinduced configurational dynamics provides a foundation for an in-depth understanding of MOF-based photocatalytic mechanisms.

Copper-Catalyzed Asymmetric Acylboration of 1,3-Butadienylboronate with Acyl Fluorides.

We report herein a Cu-catalyzed regio-, diastereo- and enantioselective acylboration of 1,3-butadienylboronate with acyl fluorides. Under the developed conditions, the reactions provide (Z)-ß,γ-unsaturated ketones bearing an α-tertiary stereocenter with high Z-selectivity and excellent enantioselectivities. While direct access to highly enantioenriched E-isomers was not successful, we showed that such molecules can be synthesized with excellent E-selectivity and optical purities via Pd-catalyzed alkene isomerization from the corresponding Z-isomers. The orthogonal chemical reactivities of the functional groups embedded in the ketone products allow for diverse chemoselective transformations, which provides a valuable platform for further derivatization.

Conformer-Specific Desorption in Propanol Ices Probed by Chirped-Pulse Millimeter-Wave Rotational Spectroscopy.

We present a new technique for the detection of molecules desorbed from an ice surface using broad-band millimeter-wave rotational spectroscopy. The approach permits interrogation of molecules that have undergone the slow warmup process of temperature-programmed desorption (TPD), analogous to the warmup phase of icy grains in the interstellar medium as they approach the central protostar. The detection is conformer- and isomer-specific and quantitative, as afforded by chirped-pulse rotational spectroscopy. To achieve this, we combine ice TPD with buffer gas cooling, followed by detection in the millimeter-wave regime. In this report we examine the TPD profiles of n- and i-propanol, the former of which may be in five different conformational isomeric forms, and which display distinct desorption profiles. The limited conformational isomerization and temperature-dependent relative yields of n-propanol conformers observed show that the desorption is highly conformer-specific.

Uncorrelated Lithium-Ion Hopping in a Dynamic Solvent-Anion Network.

Lithium batteries rely crucially on fast charge and mass transport of Li+ in the electrolyte. For liquid and polymer electrolytes with added lithium salts, Li+ couples to the counter-anion to form ionic clusters that produce inefficient Li+ transport and lead to Li dendrite formation. Quantification of Li+ transport in glycerol-salt electrolytes via NMR experiments and MD simulations reveals a surprising Li+-hopping mechanism. The Li+ transference number, measured by ion-specific electrophoretic NMR, can reach 0.7, and Li+ diffusion does not correlate with nearby ion motions, even at high salt concentration. Glycerol's high density of hydroxyl groups increases ion dissociation and slows anion diffusion, while the close proximity of hydroxyls and anions lowers local energy barriers, facilitating Li+ hopping. This system represents a bridge between liquid and inorganic solid electrolytes, thus motivating new molecular designs for liquid and polymer electrolytes to enable the uncorrelated Li+-hopping transport needed for fast-charging and all-solid-state batteries.

Adsorptive removal of iodate oxyanions from water using a Zr-based metal-organic framework.

A Zr6-based metal-organic framework (MOF), MOF-808, is investigated for the adsorptive removal of IO3- from aqueous solutions, due to its high surface area and abundance of open metal sites. The uptake kinetics, adsorption capacity and binding mode are studied, showing a maximum uptake capacity of 233 mg g-1, the highest reported by any material.

Aqueous-Phase Destruction of Nerve-Agent Simulants at Copper Single Atoms in UiO-66.

Metal-organic frameworks (MOFs) have shown great success in aqueous-phase hydrolysis of nerve agents, with some even showing promise in the gas phase. However, both aqueous-phase reactivity and gas-phase reactivity are hindered because of the binding of the hydrolyzed products to the MOF nodes in a stable, bridging configuration, which limits turnover. Single transition-metal atoms in MOFs have been a growing field of interest for catalytic applications, and single atoms have been proposed to prevent the unwanted bridged conformation and increase catalytic turnover. To date, there has been little experimental evidence to support the hypothesis. Herein, we report two copper single atom-modified UiO-66 MOFs for nerve-agent simulant degradation. Despite the capping of highly active Zr4+ nodes with fewer Lewis acidic Cun+ atoms, the reactivity of both CuMOFs approaches that of native UiO-66 under aqueous conditions. Computational studies reveal that the Cu coordination environment impairs product inhibition with respect to the native MOF.

Reversible Dissociation for Effective Storage of Diborane Gas within the UiO-66-NH2 Metal-Organic Framework.

There is an acute need for materials that can store the toxic and highly reactive diborane gas at room temperature. In this work, the interfacial chemistry leading to safe and reversible storage of diborane (B2H6) in the UiO-66-NH2 metal-organic framework (MOF) was investigated via in situ transmission infrared (IR) spectroscopy, temperature-programmed desorption (TPD), and electronic structure calculations. The infrared spectrum of B2H6 adsorbed within UiO-66-NH2 indicates hydrogen bonding with the µ3-OH groups of the MOF nodes and chemisorption at the -NH2 groups of the MOF linkers. The conversion of physisorbed to chemisorbed diborane, as observed through a spectroscopically unique intermediate species, occurred over a broad temperature regime from 80 to 410 K. During B2H6-TPD studies, both the weakly and strongly bound species were found to desorb exclusively as molecular B2H6. Infrared spectroscopic studies, performed during diborane adsorption and reaction, combined with electronic structure calculations, revealed that chemisorption occurred via a reversible dissociation reaction involving a "half-open" B2H6 intermediate and resulted in the formation of two NH2-bound BH3 units, which leave the MOF as B2H6 via recombinative desorption. The close spacing of -NH2 groups in the UiO-66-NH2 MOF is key to enabling high-temperature chemisorptive storage of B2H6, and the spatial arrangement of the amine groups has a significant effect on the dissociation energy profile. This work demonstrates that reversible dissociation of B2H6 on precisely engineered, nucleophile-rich materials represents a promising pathway to diborane stabilization and long-term storage.

Conformational Analysis of Fluoro-, Chloro-, and Proteo-Alkene Gly-Pro and Pro-Pro Isosteres to Mimic Collagen.

Collagen is the most abundant human protein, with the canonical sequence (Gly-Pro-Hyp)n in its triple helix region. Cis-trans isomerization of the Xaa-Pro amide has made two of these amide bonds the target of alkene replacement: the Gly-Pro and the Pro-Hyp positions. The conformations of Gly-Pro and Pro-Pro (as a Pro-Hyp model) fluoro-, chloro-, and proteo-alkene mimic models were investigated computationally to determine whether these alkenes can stabilize the polyproline type II (PPII) conformation of collagen. Second-order Møller-Plesset (MP2) calculations with various basis sets were used to perform the conformational analyses and locate stationary points. The calculation results predict that fluoro- and chloro-alkene mimics of Gly-Pro and Pro-Pro can participate in n→π* donation to stabilize PPII conformations, yet they are poor n→π* acceptors, shifting the global minima away from PPII conformations. For the proteo-alkene mimics, the lack of significant n→π* interactions and unstable PPII-like geometries explains their known destabilization of the triple helix in collagen-like peptides.

Prolonged Association between Water Molecules under Hydrophobic Nanoconfinement.

We present an investigation of the dynamics of water confined among rigid carbon rods and between parallel graphene sheets with molecular dynamics simulations. Diffusion coefficients, activation energy of diffusion, and residence-time correlation functions as a function of confinement geometry reveal a retardation of water dynamics under hydrophobic confinement compared to bulk water. In fact, water under various confinements possesses longer associations with its neighbors and exhibits diffusion dynamics characteristic of a lower temperature. Analysis of the residence-time correlation functions reveals long and short residence times, which we relate to the diffusion coefficient and activation energy of diffusion, respectively. Additional investigations reveal how the level of confining surface hydrophobicity affects water dynamics, further broadening our understanding of water diffusion inside diverse media. Overall, this study sheds light on the physical origin of retarded water dynamics under hydrophobic confinement and the close relationship between residence times and diffusion behavior.

Insight into Hydrogen Abstractions by Nitrate Radical: Structural, Solvent Effects, and Evidence for a Polar Transition State.

The role of a polarized transition state and solvent effects on nitrate radical reactions was examined with a previously under-reported class of substrates, ethers, for their atmospheric implications. Absolute rate constants for hydrogen abstraction from a series of alcohols, ethers, and alkanes by nitrate radical have been measured in acetonitrile, water, and mixtures of these two solvents. Across all of these classes of compounds, using a modified form of the Evans-Polanyi relationship, it is demonstrated that the observed structure/reactivity trends can be reconciled by considering the number of abstractable hydrogens, strength of the C-H bond, and ionization potential (IP) of the substrate. Hydrogen abstractions by nitrate radical occur with low selectivity and are characterized by an early transition state (α ≈ 0.3). The dependence of the rate constant on IP suggests a polar transition state with some degree (<10%) of charge transfer. These conclusions stand for reactions conducted in solution (CH3CN and H2O) as well as gas-phase values. Because of this polar transition state, the rate constants increase going from the gas phase to a polar solvent, and the magnitude of the increase is consistent with Kirkwood theory.

Outdoor dissolution and photodegradation of insensitive munitions formulations IMX-101 and IMX-104: Photolytic transformation pathway and mechanism study.

New munition compounds have been developed to replace traditional explosives to prevent unintended detonations. However, insensitive munitions (IM) can leave large proportion of unexploded charge in the field, where it is subjected to photodegradation and dissolution in precipitation. The photolytic reactions occurring on the surfaces of IMX-101 and IMX-104 formulations and the subsequent fate of photolytic products in the environment were thoroughly investigated. The constituents of IMX-101 and IMX-104 formulations dissolve sequentially under rainfall in the order of aqueous solubility: 3-nitro-1,2,4-triazol-5-one (NTO) > nitroguanidine (NQ) > 2,4-dinitroanisole (DNAN) > 1,3,5-hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). A linear relationship between DNAN dissolution and rainwater volume was observed (r2: 0.86-0.99). It was estimated that it would take 16-228 years to completely dissolve these formulation particles under natural environmental conditions in Oracle, AZ. We used LC/MS/MS and GC/MS to examine the dissolution samples from IMX-101 and 104 particles exposed to rainfall and sunlight and found six DNAN photo-transformation products including 2-methoxy-5-nitrophenol, 4-methoxy-3-nitrophenol, 4-methoxy-3-nitroaniline, 2-methoxy-5-nitroaniline, 2,4-dinitrophenol, and methoxy-dinitrophenol, which are in good agreement with computational modeling results of bond strengths. The main DNAN photodegradation pathways are therefore proposed. Predicted eco-toxicity values suggested that the parent compound DNAN, methoxy-nitrophenols, methoxy-nitroanilines and the other two products (2,4-dinitrophenol and methoxy-dinitrophenol) would be harmful to fish and daphnid. Our study provides improved insight about the rain dissolution and photochemical behavior of IM formulations under natural conditions, which helps to form target-oriented strategies to mitigate explosive contamination in military training sites.

Bifurcated Dihydrogen Bonding in the Uptake of Gas-Phase Diborane on Silica.

The interfacial chemistry of diborane (B2H6) with hydroxylated silica was investigated via in situ Fourier-transform infrared spectroscopy and temperature-programmed desorption. During exposure of silica to B2H6 under ultrahigh vacuum conditions, a decline in infrared band intensity assigned to excitation of the interfacial silanol O-H vibration at 3750 cm-1 and the associated appearance of a feature at 3687 cm-1 revealed hydrogen-bonding interactions between B2H6 and interfacial silanol groups. The IR spectrum for silica was completely recovered following desorption of the adsorbates, indicating that interactions between B2H6 and clean silica are reversible, in contrast to other reports on this system. During temperature-programmed desorption of diborane from silica, B2H6 was observed to desorb between 80 and 150 K, evidence for weak interactions between B2H6 and the surface. Electronic-structure calculations revealed that these interactions were due to bifurcated dihydrogen bonds between two terminal B-H groups of the adsorbate and interfacial silanol groups.

Semiconducting and Metallic [5,5] Fullertube Nanowires: Characterization of Pristine D5h(1)-C90 and D5d(1)-C100.

Although fullerenes were discovered nearly 35 years ago, scientists still struggle to isolate "single molecule" tubular fullerenes larger than C90. In similar fashion, there is a paucity of reports for pristine single-walled carbon nanotubes (SWNTs). In spite of Herculean efforts, the isolation and properties of pristine members of these carbonaceous classes remain largely unfulfilled. For example, the low abundance of spherical and tubular higher fullerenes in electric-arc extracts (<0.01-0.5%) and multiplicity of structural isomers remain a major challenge. Recently, a new isolation protocol for highly tubular fullerenes, also called f ullertubes, was reported. Herein, we describe spectroscopic characterization including 13C NMR, XPS, and Raman results for purified [5,5] fullertube family members, D5h-C90 and D5d-C100. In addition, DFT computational HOMO-LUMO gaps, polarizability indices, and electron density maps were also obtained. The Raman and 13C NMR results are consistent with semiconducting and metallic properties for D5h-C90 and D5d-C100, respectively. Our report suggests that short [5,5] fullertubes with aspect ratios of only ∼1.5-2 are metallic and could exhibit unique electronic properties.

Atomic resolution tracking of nerve-agent simulant decomposition and host metal-organic framework response in real space.

Gas capture and sequestration are valuable properties of metal-organic frameworks (MOFs) driving tremendous interest in their use as filtration materials for chemical warfare agents. Recently, the Zr-based MOF UiO-67 was shown to effectively adsorb and decompose the nerve-agent simulant, dimethyl methylphosphonate (DMMP). Understanding mechanisms of MOF-agent interaction is challenging due to the need to distinguish between the roles of the MOF framework and its particular sites for the activation and sequestration process. Here, we demonstrate the quantitative tracking of both framework and binding component structures using in situ X-ray total scattering measurements of UiO-67 under DMMP exposure, pair distribution function analysis, and theoretical calculations. The sorption and desorption of DMMP within the pores, association with linker-deficient Zr6 cores, and decomposition to irreversibly bound methyl methylphosphonate were directly observed and analyzed with atomic resolution.

Multichannel dynamics in the OH+ n-butane reaction revealed by crossed-beam slice imaging and quasiclassical trajectory calculations.

Multidimensional reactions present various channels that can exhibit very different dynamics and give products of varying subsequent reactivity. Here, we present a combination of experiment and theory to reveal the dynamics of hydrogen abstraction by OH radical at primary and secondary sites in n-butane at a collision energy of 8 kcal/mol. Crossed molecular beam slice imaging experiments unequivocally probe the secondary abstraction channel showing backward angular distributions with mild energy release to product translation, which are accurately captured by trajectory calculations using a specific-reaction-parameter Hamiltonian. Experiments containing both reaction channels indicate a less marked backward character in the angular distribution, whose origin is shown by trajectory calculations to appear as an evolution toward more sideways scattering from the secondary to primary channel. While the two channels have markedly different angular distributions, their energy release is largely comparable, showing ample energy release into the water product. The synergistic combination of crossed-beam imaging and trajectories opens the door to detailed reaction-dynamics studies of chemical reactions with ever-increasing complexity.

Metal-Organic Framework- and Polyoxometalate-Based Sorbents for the Uptake and Destruction of Chemical Warfare Agents.

The threat of chemical warfare agents (CWAs), assured by their ease of synthesis and effectiveness as a terrorizing weapon, will persist long after the once-tremendous stockpiles in the U.S. and elsewhere are finally destroyed. As such, soldier and civilian protection, battlefield decontamination, and environmental remediation from CWAs remain top national security priorities. New chemical approaches for the fast and complete destruction of CWAs have been an active field of research for many decades, and new technologies have generated immense interest. In particular, our research team and others have shown metal-organic frameworks (MOFs) and polyoxometalates (POMs) to be active for sequestering CWAs and even catalyzing the rapid hydrolysis of agents. In this Forum Article, we highlight recent advancements made in the understanding and evaluation of POMs and Zr-based MOFs as CWA decontamination materials. Specifically, our aim is to bridge the gap between controlled, solution-phase laboratory studies and real-world or battlefield-like conditions by examining agent-material interactions at the gas-solid interface utilizing a multimodal experimental and computational approach. Herein, we report our progress in addressing the following research goals: (1) elucidating molecular-level mechanisms of the adsorption, diffusion, and reaction of CWA and CWA simulants within a series of Zr-based MOFs, such as UiO-66, MOF-808, and NU-1000, and POMs, including Cs8Nb6O19 and (Et2NH2)8[(α-PW11O39Zr(µ-OH)(H2O))2]·7H2O, (2) probing the effects that common ambient gases, such as CO2, SO2, and NO2, have on the efficacy of the MOF and POM materials for CWA destruction, and (3) using CWA simulant results to develop hypotheses for live agent chemistry. Key hypotheses are then tested with targeted live agent studies. Overall, our collaborative effort has provided insight into the fundamental aspects of agent-material interactions and revealed strategies for new catalyst development.

Multimodal Characterization of Materials and Decontamination Processes for Chemical Warfare Protection.

This Review summarizes the recent progress made in the field of chemical threat reduction by utilizing new in situ analytical techniques and combinations thereof to study multifunctional materials designed for capture and decomposition of nerve gases and their simulants. The emphasis is on the use of in situ experiments that simulate realistic operating conditions (solid-gas interface, ambient pressures and temperatures, time-resolved measurements) and advanced synchrotron methods, such as in situ X-ray absorption and scattering methods, a combination thereof with other complementary measurements (e.g., XPS, Raman, DRIFTS, NMR), and theoretical modeling. The examples presented in this Review range from studies of the adsorption and decomposition of nerve agents and their simulants on Zr-based metal organic frameworks to Nb and Zr-based polyoxometalates and metal (hydro)oxide materials. The approaches employed in these studies ultimately demonstrate how advanced synchrotron-based in situ X-ray absorption spectroscopy and diffraction can be exploited to develop an atomic- level understanding of interfacial binding and reaction of chemical warfare agents, which impacts the development of novel filtration media and other protective materials.

Ab Initio and Quasiclassical Trajectory Study of the O(3P) + 2-Propanol Hydrogen Abstraction Reaction.

We present a theoretical study of the hydrogen abstraction reaction from 2-propanol by ground-state oxygen atoms. First, ab initio calculations are used to characterize the stationary points of the potential energy surface. Rotation around the C-C-O-H dihedral affords two conformers in 2-propanol, which gives rise to 13 hydrogen abstraction reaction pathways grouped into three channels, Cα, Cß, and O, depending on the abstraction site. Reaction at Cα exhibits the lowest barrier and largest exothermicity, followed by reaction at Cß, and at 2-propanol's oxygen atom. Additional ab initio calculations beyond the stationary points are employed to obtain a grid of energies with which a specific-reaction-parameters (SRP) PM6 semiemipirical Hamiltonian is derived for the title reaction. The SRP-PM6 model captures the energetics of the reaction with higher accuracy than some conventional first-principles methods but is efficient enough to allow for extensive reaction dynamics calculations. Quasiclassical trajectories are subsequently propagated with the SRP-PM6 Hamiltonian to obtain reaction dynamics properties that are compared to experiments. Product translational energy and angular distributions for reaction at Cα with the two conformers of 2-propanol are in good agreement with recent molecular-beam measurements, and they exhibit largely backward scattering with modest energy release to relative translation. Most of the energy is deposited into the organic product, substantiating a reaction mechanism dominated by rebound dynamics.

Self-Amplified Depolymerization of Oligo(thiourethanes) for the Release of COS/H2S.

Herein we report the self-amplified depolymerization of an aryl oligo(thiourethane) (OTU) for the release of COS/H2S. The OTU was synthesized via polyaddition of 4-isothiocyanatobenzyl alcohol and end-capped with an aryl azide. The aryl azide chain-end was reduced by tris(2-carboxyethyl)phosphine or H2S to the corresponding aniline, resulting in depolymerization (i.e., self-immolation) and the release of COS/H2S. Depolymerization was monitored by 1H NMR and UV-Vis spectroscopy, and the released COS was converted into H2S by the ubiquitous enzyme carbonic anhydrase in aqueous media.