Elucidating dissociation activation energies in host–guest assemblies featuring fast exchange dynamics

The ability to mediate the kinetic properties and dissociation activation energies (E$_{a}$) of bound guests by controlling the characteristics of "supramolecular lids" in host-guest molecular systems is essential for both their design and performance. While the synthesis of such systems is well advanced, the experimental quantification of their kinetic parameters, particularly in systems experiencing fast association and dissociation dynamics, has been very difficult or impossible with the established methods at hand. Here, we demonstrate the utility of the NMR-based guest exchange saturation transfer (GEST) approach for quantifying the dissociation exchange rates (k$_{out}$)) and activation energy (E$_{a, out}$) in host-guest systems featuring fast dissociation dynamics. Our assessment of the effect of different monovalent cations on the extracted E$_{a, out}$ in cucurbit[7]uril:guest systems with very fast k$_{out}$ highlights their role as "supramolecular lids" in mediating a guest\u27s dissociation E$_{a}$. We envision that GEST could be further extended to study kinetic parameters in other supramolecular systems characterized by fast kinetic properties and to design novel switchable host-guest assemblies

Metal-ligand cooperation facilitates bond activation and catalytic hydrogenation with zinc pincer complexes

We thank the European Research Council (ERC AdG 692775) for support of this research. M.R. acknowledges the Zuckerman STEM Leadership Program for a research fellowship. S.K. acknowledges the Sustainability and Energy Research Initiative (SAERI) Weizmann Institute of Science for a research fellowship. A. K. is thankful to the Feinberg Graduate School for the Senior Postdoctoral Fellowship.A series of PNP zinc pincer complexes capable of bond activation via aromatization/dearomatization metal-ligand cooperation (MLC) were prepared and characterized. Reversible heterolytic N-H and H-H bond activation by MLC is shown, in which hemilability of the phosphorus linkers plays a key role. Utilizing this zinc pincer system, base-free catalytic hydrogenation of imines and ketones under relatively mild conditions is demonstrated. A detailed mechanistic study supported by computation implicates the key role of MLC in facili-tating effective catalysis. This approach offers a new strategy for (de)hydrogenation and other catalytic transformations mediated by zinc and other main group metals.PostprintPeer reviewe

Hydrogenative depolymerization of nylons

This research was supported by the European Research Council (ERC AdG 692775). D. M. holds the Israel Matz Professorial Chair of Organic Chemistry. A. K. is thankful to the Planning and Budgeting Committee of Israel and Feinberg Graduate School for a (senior) postdoctoral fellowship. Y.-Q. Z. acknowledges the Sustainability and Energy ResearchInitiative (SAERI) foundation for a research fellowship. Computations were performed using HPC resources from GENCI-CINES (Grant 2019 AP010811227).The widespread crisis of plastic pollution demands discovery of new and sustainable approaches to degrade robust plastics such as nylons. Using a green and sustainable approach based on hydrogenation, in the presence of a ruthenium pincer catalyst at 150 oC and 70 bar H2, we report here the first example of hydrogenative depolymerization of conventional, widely used nylons, and polyamides in general. Un-der the same catalytic conditions, we also demonstrate the hydrogenation of a polyurethane to produce diol, diamine and methanol. Additionally, we demonstrate an example where monomers (and oligomers) obtained from the hydrogenation process can be dehydrogenated back to a poly(oligo)amide of approximately similar molecular weight, thus completing a closed loop cycle for recycling of poly-amides. Based on the experimental and DFT studies, we propose a catalytic cycle for the process that is facilitated by metal-ligand cooperativity. Overall, this unprecedented transformation, albeit at the proof of concept level, offers a new approach towards a cleaner route to recycling nylons.Publisher PDFPeer reviewe

The effect of rotational angle and experimental parameters on the diffraction patterns and micro-structural information obtained from q-space diffusion NMR: implication for diffusion in white matter fibers

Diffusion NMR may provide, under certain experimental conditions, micro-structural information about confined compartments totally non-invasively. The influence of the rotational angle, the pulse gradient length and the diffusion time on the diffusion diffraction patterns and q-space displacement distribution profiles was evaluated for ensembles of long cylinders having a diameter of 9 and 20 μm. It was found that the diffraction patterns are sensitive to the rotational angle (α) and are observed only when diffusion is measured nearly perpendicular to the long axis of the cylinders i.e., when α=90°±5° under our experimental conditions. More importantly, we also found that the structural information extracted from the displacement distribution profiles and from the diffraction patterns are very similar and in good agreement with the experimental values for cylinders of 20 μm or even 9 μm, when data is acquired with parameters that satisfy the short gradient pulse (SGP) approximation (i.e., δ→0) and the long diffusion time limit. Since these experimental conditions are hardly met in in vitro diffusion MRI of excised organs, and cannot be met in clinical MRI scanners, we evaluated the effect of the pulse gradient duration and the diffusion time on the structural information extracted from q-space diffusion MR experiments. Indeed it was found that, as expected, accurate structural information, and diffraction patterns are observed when Δ is large enough so that the spins reach the cylinders’ boundaries. In addition, it was found that large δ results in extraction of a compartment size, which is smaller than the real one. The relevance of these results to q-space MRI of neuronal tissues and fiber tracking is discussed

Unique Organization of Solvent Molecules Within the Hexameric Capsules of Pyrogallol[4]arene in Solution

The hexameric capsules of pyrogallol[4]­arene (<b>2b</b>) were prepared in nondeuterated solvents in the absence and presence of adamantane carboxylic acid (<b>3</b>). The small encapsulated molecules were shown to occupy different sites within the same capsule. In the presence of <b>3</b>, which are also encapsulated in the hexameric capsules, one observes yet another pair of signals for the encapsulated solvent molecules. Different NMR experiments enabled assignment of the different sites within the hexameric capsules of <b>2b</b>

Controlled selectivity through reversible inhibition of the catalyst: Stereodivergent semihydrogenation of alkynes

Catalytic semihydrogenation of internal alkynes using H2 is an attractive atom-economical route to various alkenes, and its stereocontrol has received widespread attention, both in homogeneous and heterogeneous catalysis. Herein, a novel strategy is introduced, whereby a poisoning catalytic thiol is employed as a reversible inhibitor of a ruthenium catalyst, resulting in the first controllable H2-based semihydrogenation of internal alkynes. Both (E)- and (Z)-alkenes were obtained efficiently and highly-selectively, under very mild conditions, using a single homogenous acridine-based ruthenium catalyst. Mechanistic studies indicate that the (Z)-alkene is the reaction intermediate leading to the (E)-alkene, and that addition of a catalytic amount of bidentate thiol impedes the Z-E isomerization step by forming stable ruthenium thiol(ate) complexes, while still allowing the main hydrogenation reaction to proceed. Thus, the absence or presence of catalytic thiol controls the stereoselectivity of this alkyne semihydrogenation, affording either the (E)-isomer as the final product, or halting the reaction at the (Z)-intermediate. The developed system, which is also applied to the controllable isomerization of a terminal alkene, demonstrates how selective metal catalysis can be achieved by reversible inhibition of the catalyst with a simple auxiliary additive

Three-dimensional water diffusion in impermeable cylindrical tubes: theory versus experiments

Characterizing diffusion of gases and liquids within pores is important in understanding numerous transport processes and affects a wide range of practical applications. Previous measurements of the pulsed gradient stimulated echo (PGSTE) signal attenuation, E(q), of water within nerves and impermeable cylindrical microcapillary tubes showed it to be exquisitely sensitive to the orientation of the applied wave vector, q, with respect to the tube axis in the high-q regime. Here, we provide a simple three-dimensional model to explain this angular dependence by decomposing the average propagator, which describes the net displacement of water molecules, into components parallel and perpendicular to the tube wall, in which axial diffusion is free and radial diffusion is restricted. The model faithfully predicts the experimental data, not only the observed diffraction peaks in E(q) when the diffusion gradients are approximately normal to the tube wall, but their sudden disappearance when the gradient orientation possesses a small axial component. The model also successfully predicts the dependence of E(q) on gradient pulse duration and on gradient strength as well as tube inner diameter. To account for the deviation from the narrow pulse approximation in the PGSTE sequence, we use Callaghan's matrix operator framework, which this study validates experimentally for the first time. We also show how to combine average propagators derived for classical one-dimensional and two-dimensional models of restricted diffusion (e.g. between plates, within cylinders) to construct composite three-dimensional models of diffusion in complex media containing pores (e.g. rectangular prisms and/or capped cylinders) having a distribution of orientations, sizes, and aspect ratios. This three-dimensional modeling framework should aid in describing diffusion in numerous biological systems and in a myriad of materials sciences applications