Engineering Frustrated Lewis Pair Active Sites in Porous Organic Scaffolds for Catalytic CO2 Hydrogenation.

Frustrated Lewis pairs (FLPs), featuring reactive combinations of Lewis acids and Lewis bases, have been utilized for myriad metal-free homogeneous catalytic processes. Immobilizing the active Lewis sites to a solid support, especially to porous scaffolds, has shown great potential to ameliorate FLP catalysis by circumventing some of its inherent drawbacks, such as poor product separation and catalyst recyclability. Nevertheless, designing immobilized Lewis pair active sites (LPASs) is challenging due to the requirement of placing the donor and acceptor centers in appropriate geometric arrangements while maintaining the necessary chemical environment to perform catalysis, and clear design rules have not yet been established. In this work, we formulate simple guidelines to build highly active LPASs for direct catalytic hydrogenation of CO2 through a large-scale screening of a diverse library of 25,000 immobilized FLPs. The library is built by introducing boron-containing acidic sites in the vicinity of the existing basic nitrogen sites of the organic linkers of metal-organic frameworks collected in a "top-down" fashion from the CoRE MOF 2019 database. The chemical and geometrical appropriateness of these LPASs for CO2 hydrogenation is determined by evaluating a series of simple descriptors representing the intrinsic strength (acidity and basicity) of the components and their spatial arrangement in the active sites. Analysis of the leading candidates enables the formulation of pragmatic and experimentally relevant design principles which constitute the starting point for further exploration of FLP-based catalysts for the reduction of CO2.

Mapping Active Site Geometry to Activity in Immobilized Frustrated Lewis Pair Catalysts.

The immobilization of molecular catalysts imposes spatial constraints on their active site. We reveal that in bifunctional catalysis such constraints can also be utilized as an appealing handle to boost intrinsic activity through judicious control of the active site geometry. To demonstrate this, we develop a pragmatic approach, based on nonlinear scaling relationships, to map the spatial arrangements of the acid-base components of frustrated Lewis pairs (FLPs) to their performance in the catalytic hydrogenation of CO2 . The resulting activity map shows that fixing the donor-acceptor centers at specific distances and locking them into appropriate orientations leads to an unforeseen many-fold increase in the catalytic activity of FLPs compared to their unconstrained counterparts.

Design of Frustrated Lewis Pair Catalysts for Direct Hydrogenation of CO2.

Despite recent progress in the chemistry of frustrated Lewis pairs (FLPs), direct FLP-catalyzed hydrogenation of CO2 remains elusive. From a near-infinite array of plausible Lewis pairs, it is challenging to identify individual combinations that are appropriate for catalyzing this reaction. To this end, we propose a mapping of the chemical composition of FLPs to their activity towards direct catalytic hydrogenation of CO2 into formate. The maps, built upon linear scaling relationships, pinpoint specific FLP combinations with the proper complementary acidity and basicity to optimally balance the energetics of the catalytic cycle. One such combination was experimentally validated to achieve hitherto unreported catalytic turnover for this transformation.

Explaining the Advantageous Impact of Tertiary versus Secondary Nitrogen Center on the Activity of PNP-Pincer Co(I)-Complexes for Catalytic Hydrogenation of CO2.

Pincer ligated coordination complexes of base metals have shown remarkable catalytic activity for hydrogenation/dehydrogenation of CO2 . The recently reported MeN[CH2 CH2 (i Pr2 )]2 Co(I)PNP-pincer complex was shown to exhibit substantially higher catalytic activity in comparison to the corresponding catalyst, HN[CH2 CH2 (i Pr2 )]2 Co(I)PNP, bearing a secondary nitrogen center on the pincer ligand. Here, we computationally investigate the mechanisms for hydrogenation of CO2 to formate catalyzed by these two Co-PNP complexes to explain how such a small structural difference could have a sizable impact on their catalytic activity. Plausible hydrogenation routes were examined in details and our findings provide solid support for the experimental observations. Our results reveal that such trends in catalytic activity could be explained from the lower activation barrier for the hydride transfer step upon changing the pincer nitrogen center from secondary to tertiary.

Delineating Conformation Control in the Photophysical Behaviour of a Molecular Donor-Acceptor-Donor Triad.

Mechanochromic luminescent materials, exhibiting a change in luminescence behavior under external stimuli have emerged as one of the promising candidates for upcoming efficient OLEDs. Recently mechanochromic luminescence was reported in a donor-acceptor-donor (D-A-D) triad featuring two phenothiazine units separated by a dibenzo[a,j]phenazine motif. The triad follows different emissive routes ranging from phosphorescence to TADF based on the conformational switching of the D units. In this article, we investigate such conformation-dependent photophysical behavior of this triad through theoretical calculations. By analyzing the nature of ground state, excited state and factors determining the reverse ISC crossing rates associated with the relative orientation of the D and A units, we delineate the effect of the conformational changes on their photophysical properties. Our findings reveal that axial orientation of both the donor groups enhances the overlap between HOMO and LUMO leading to a large singlet-triplet gap ( ΔEST ) which drives phosphorescence emission. On the contrary, the equatorial orientation of the donor groups minimizes ΔEST to facilitate rISC making the conformers TADF active. The role of several geometric factors affecting the photophysical properties of the conformers is also highlighted. Finally, we show how to regulate the population difference among the conformers by functionalizing the triad to harvest the maximum TADF efficiency.

Investigating Tetrel-Based Neutral Frustrated Lewis Pairs for Hydrogen Activation.

Tetrel Lewis acids are a prospective alternative to commonly employed neutral boranes in frustrated Lewis pair (FLP) chemistry. While cationic tetrylium Lewis acids, being isolobal and iso(valence)electronic, are a natural replacement to boranes, neutral tetrel Lewis acids allude as less trivial options due to the absence of a formally empty p orbital on the acceptor atom. Recently, a series of intramolecular geminal FLPs (C2F5)3E-CH2-P(tBu)2 (E = Si, Ge, Sn) featuring neutral tetrel atoms as acceptor sites has been reported for activation of small molecules including H2. In this work, through density functional theory computations, we elucidate the general mechanistic picture of H2 activation by this family of FLPs. Our findings reveal that the acceptor atom derives the required Lewis acidity utilizing the antibonding orbitals of its adjacent bonds with the individual contributions depending on the identity of the acceptor and the donor atoms. By varying the identity of the Lewis acid and Lewis base sites and attached substituents, we unravel their interplay on the energetics of the H2 activation. We find that switching the donor site from P to N significantly affects the synchronous nature of the bond breaking/formations along the reaction pathway, and as a result, N-bearing FLPs have a more favorable H2 activation profile than those with P. Our results are quantitatively discussed in detail within the framework of the activation-strain model of reactivity along with the energy-decomposition analysis method. Finally, the reductive elimination decomposition route pertinent to the plausible extension of the H2 activation to catalytic hydrogenation by these FLPs is also examined.

MOF Derived Co3O4@Co/NCNT Nanocomposite for Electrochemical Hydrogen Evolution, Flexible Zinc-Air Batteries, and Overall Water Splitting.

Toward the goal of clean and sustainable energy source, the development of a trifunctional electrocatalyst is a boon for energy storage and conversion devices such as regenerative fuel cells and metal-air batteries. MOF-derived semiconducting-metallic core-shell electrocatalyst Co3O4@Co/NCNT (NCNT = nitrogen-doped carbon nanotube), which was shown to catalyze oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is also found to be an active electrocatalyst for hydrogen evolution reaction (HER) with a low overpotential of 171 mV. Here, the HER activity of Co3O4@Co/NCNT is presented and is shown as highly efficient and robust trifunctional electrocatalyst. The detailed theoretical calculation has found N-center of Co-N4 moiety to be the H+ binding active site and thus proves Co3O4@Co/NCNT to be active for HER. Further, the ORR and OER bifunctionality of Co3O4@Co/NCNT helped in fabricating secondary Zn-air battery with high power density of 135 mW/cm2. Also, an all-solid-state flexible and wearable battery with Co3O4@Co/NCNT as cathode and electrodeposited Zn on carbon fiber cloth as anode was shown to withstand its performance even under stressed conditions. Finally, the material being trifunctional in nature was used both as an anode and cathode material for the electrolysis of water, which was powered by the Zn-air batteries with Co3O4@Co/NCNT as the cathode material. It is believed that the development of a trifunctional catalyst would help in wide commercialization of regenerative fuel cells.

Insights into Reaction Intermediates to Predict Synthetic Pathways for Shape-Controlled Metal Nanocrystals.

Understanding nucleation phenomena is crucial across all branches of physical and natural sciences. Colloidal nanocrystals are among the most versatile and tunable synthetic nanomaterials. While huge steps have been made in their synthetic development, synthesis by design is still impeded by the lack of knowledge of reaction mechanisms. Here, we report on the investigation of the reaction intermediates in high temperature syntheses of copper nanocrystals by a variety of techniques, including X-ray absorption at a synchrotron source using a customized in situ cell. We reveal unique insights into the chemical nature of the reaction intermediates and into their role in determining the final shape of the metal nanocrystals. Overall, this study highlights the importance of understanding the chemistry behind nucleation as a key parameter to predict synthetic pathways for shape-controlled nanocrystals.

Regulating Charge-Transfer in Conjugated Microporous Polymers for Photocatalytic Hydrogen Evolution.

Bandgap engineering in donor-acceptor conjugated microporous polymers (CMPs) is a potential way to increase the solar-energy harvesting towards photochemical water splitting. Here, the design and synthesis of a series of donor-acceptor CMPs [tetraphenylethylene (TPE) and 9-fluorenone (F) as the donor and the acceptor, respectively], F0.1 CMP, F0.5 CMP, and F2.0 CMP, are reported. These CMPs exhibited tunable bandgaps and photocatalytic hydrogen evolution from water. The donor-acceptor CMPs exhibited also intramolecular charge-transfer (ICT) absorption in the visible region (λmax =480 nm) and their bandgap was finely tuned from 2.8 to 2.1 eV by increasing the 9-fluorenone content. Interestingly, they also showed emissions in the 540-580 nm range assisted by the energy transfer from the other TPE segments (not involved in charge-transfer interactions), as evidenced from fluorescence lifetime decay analysis. By increasing the 9-fluorenone content the emission color of the polymer was also tuned from green to red. Photocatalytic activities of the donor-acceptor CMPs (F0.1 CMP, F0.5 CMP, and F2.0 CMP) are greatly enhanced compared to the 9-fluorenone free polymer (F0.0 CMP), which is essentially due to improved visible-light absorption and low bandgap of donor-acceptor CMPs. Among all the polymers F0.5 CMP with an optimum bandgap (2.3 eV) showed the highest H2 evolution under visible-light irradiation. Moreover, all polymers showed excellent dispersibility in organic solvents and easy coated on the solid substrates.

NHC-Coordinated Diphosphene-Stabilized Gold(I) Hydride and Its Reversible Conversion to Gold(I) Formate with CO2.

An NHC-coordinated diphosphene is employed as ligand for the synthesis of a hydrocarbon-soluble monomeric AuI hydride, which readily adds CO2 at room temperature yielding the corresponding AuI formate. The reversible reaction can be expedited by the addition of NHC, which induces ß-hydride shift and the removal of CO2 from equilibrium through the formation of an NHC-CO2 adduct. The AuI formate is alternatively formed by dehydrogenative coupling of the AuI hydride with formic acid (HCO2 H), thus in total establishing a reaction sequence for the AuI hydride mediated dehydrogenation of HCO2 H as chemical hydrogen storage material.

Mechanistic Insights into Hydrogen Activation by Frustrated N/Sn Lewis Pairs.

The mechanism of H2 activation by recently reported N/Sn Lewis pairs is unravelled using the representative iPr3 SnOTf/DABCO combination. Computations provide evidence for weak intermolecular associations between Lewis acid and Lewis base (LA/LB) in which the counteranion to cationic LA fragment plays a critical role. Two frustrated Lewis pairs (FLPs) are observed; an unprecedented counteranion-mediated noncovalent LA/LB association is characterised along with the usual FLP structure. Both the FLPs are shown to be capable of H2 activation through cooperative electron transfer processes involving the LA/LB centres. Overall, computed results are in good agreement with the experimental findings and account for the observed reactivity. Insights obtained in this study are fundamentally important for the rational design of Sn-based alternative FLP LAs. The present findings could also provide a general mechanistic framework for H2 activation by FLPs having an ion pair LA component.

Aqueous Phase Phosphorescence: Ambient Triplet Harvesting of Purely Organic Phosphors via Supramolecular Scaffolding.

Ambient solution and amorphous state room temperature phosphorescence (RTP) from purely organic chromophores is rarely achieved. Remarkable stabilization of triplet excitons is realized to obtain deep red phosphorescence in water and in amorphous film state under ambient conditions by a unique supramolecular hybrid assembly between inorganic laponite clay and heavy atom core substituted naphthalene diimide (NDI) phosphor. Structural rigidity and oxygen tolerance of the inorganic template along with controlled molecular organization via supramolecular scaffolding are envisaged to alleviate the unprecedented aqueous phase phosphorescence.

On the Mechanism of Frustrated Lewis Pair Catalysed Hydrogenation of Carbonyl Compounds.

We have explored the reaction mechanism of the metal-free B(C6 F5 )3 -catalysed hydrogenation of carbonyl compounds to the corresponding secondary alcohols by density functional theory calculations. Possible reaction routes have been investigated in detail and the results provide solid support for the mechanism proposed on the basis of experimental observations. The critical role of the ethereal solvent, as an active participant in the hydrogenation process, is highlighted with the ether-borane Lewis pair shown to be involved in the heterolytic activation of H2 . The feasiblity of an alternative direct hydrogenation route featuring carbonyl-borane-mediated H2 cleavage has also been examined. We have also investigated the moisture sensitivity of the catalyst and possible decomposition routes. We found that the catalyst shows appreciable water-tolerance and that even in the presence of moisture the hydrogenation proceeds through the same mechanism as that followed under anhydrous conditions.

Unique Approach to Copper(I) Silylene Chalcogenone Complexes.

Silylene-S-thione [PhC(NtBu)2Si(═S)N(SiMe3)2] (2) and silylene-Se-selone [PhC(NtBu)2Si(═Se)N(SiMe3)2] (3) compounds were prepared from the silylene [PhC(NtBu)2SiN(SiMe3)2] (1) with 1 equiv of 1/8 S8 and 1 equiv of Se powder, respectively, in high yields. Furthermore, compounds 2 and 3 reacted with CuCl and CuBr and yielded [{PhC(NtBu)2}Si(═S→CuX)N(SiMe3)2] (X = Cl (4), Br (5)) and [{PhC(NtBu)2}Si(═Se→CuX)N(SiMe3)2] (X = Cl (6), Br (7)), respectively. Complexes 4-7 can also be obtained from the direct reaction of sulfur and selenium with the corresponding silylene copper complexes [{PhC(NtBu)2}Si{N(SiMe3)2}]2Cu2X2 (X = Cl (8), Br (9)). The latter route avoids the preparation of the highly reactive silylene chalcogenones. For comparison purposes the silylene PhC(NtBu)2SiN(SiMe3)2 in 2 and 3 was replaced by NHC (1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) (10). The resulting products NHC═S (thione 11) and NHC═Se (selenone 12) react with CuBr and lead to the expected complexes (NHC═S→CuBr) (13) and (NHC═Se→CuBr) (14). However, unlike silylene complexes, 13 and 14 cannot be prepared by reacting NHC-CuBr (15) with chalcogens.

Sequence-specific recognition of DNA minor groove by an NIR-fluorescence switch-on probe and its potential applications.

In molecular biology, understanding the functional and structural aspects of DNA requires sequence-specific DNA binding probes. Especially, sequence-specific fluorescence probes offer the advantage of real-time monitoring of the conformational and structural reorganization of DNA in living cells. Herein, we designed a new class of D2A (one-donor-two-acceptor) near-infrared (NIR) fluorescence switch-on probe named quinone cyanine-dithiazole ( QCY-DT: ) based on the distinctive internal charge transfer (ICT) process for minor groove recognition of AT-rich DNA. Interestingly, QCY-DT: exhibited strong NIR-fluorescence enhancement in the presence of AT-rich DNA compared to GC-rich and single-stranded DNAs. We show sequence-specific minor groove recognition of QCY-DT: for DNA containing 5'-AATT-3' sequence over other variable (A/T)4 sequences and local nucleobase variation study around the 5'-X(AATT)Y-3' recognition sequence revealed that X = A and Y = T are the most preferable nucleobases. The live cell imaging studies confirmed mammalian cell permeability, low-toxicity and selective staining capacity of nuclear DNA without requiring RNase treatment. Further, Plasmodium falciparum with an AT-rich genome showed specific uptake with a reasonably low IC50 value (<4 µM). The ease of synthesis, large Stokes shift, sequence-specific DNA minor groove recognition with switch-on NIR-fluorescence, photostability and parasite staining with low IC50 make QCY-DT: a potential and commercially viable DNA probe.

Imidazolyl-Naphthalenediimide-Based Threading Intercalators of DNA.

Intercalation by threading is anticipated to feature in DNA-binding molecules for developing DNA-targeted diagnostics and therapeutics. We investigated the role of an imidazolyl moiety in threading intercalators of DNA by employing a number of imidazolyl-naphthalenediimide conjugates. Threading intercalation was studied by UV spectroscopy, competitive binding fluorescent dye displacement, circular dichroism, isothermal calorimetry, and computational analysis. NIm6 was found to be a strong candidate, with good half-life, as revealed by dissociation kinetic analysis. Computational studies supported intercalation of the naphthalene core between base pairs and binding of the imidazolyl moieties in the adjacent grooves (threading mechanism) through electrostatic and hydrogen-bonding interactions. The interaction of the positively charged imidazolium moieties with the negatively charged phosphate backbone of DNA contributed to the favorable enthalpy change, as revealed by the experimental and computational data. Threading intercalation by NIm6 caused significant retardation of DNA in an electrophoretic mobility shift assay. The biological significance of potent imidazolyl naphthalenediimide conjugates was demonstrated by the inhibition of topo- isomerase I activity and cytotoxicity against HeLa cells.

Theory-guided development of homogeneous catalysts for the reduction of CO2 to formate, formaldehyde, and methanol derivatives.

The stepwise catalytic reduction of carbon dioxide (CO2) to formic acid, formaldehyde, and methanol opens non-fossil pathways to important platform chemicals. The present article aims at identifying molecular control parameters to steer the selectivity to the three distinct reduction levels using organometallic catalysts of earth-abundant first-row metals. A linear scaling relationship was developed to map the intrinsic reactivity of 3d transition metal pincer complexes to their activity and selectivity in CO2 hydrosilylation. The hydride affinity of the catalysts was used as a descriptor to predict activity/selectivity trends in a composite volcano picture, and the outstanding properties of cobalt complexes bearing bis(phosphino)triazine PNP-type pincer ligands to reach the three reduction levels selectively under different reaction conditions could thus be rationalized. The implications of the composite volcano picture were successfully experimentally validated with selected catalysts, and the challenging intermediate level of formaldehyde could be accessed in over 80% yield with the cobalt complex 6. The results underpin the potential of tandem computational-experimental approaches to propel catalyst design for CO2-based chemical transformations.

Uncovering the Activity of Alkaline Earth Metal Hydrogenation Catalysis Through Molecular Volcano Plots.

Recent advances in alkaline earth (Ae) metal hydrogenation catalysis have broadened the spectrum of potential catalysts to include candidates from the main group, providing a sustainable alternative to the commonly used transition metals. Although Ae-amides have already been demonstrated to catalyze hydrogenation of imines and alkenes, a lucid understanding of how different metal/ligand combinations influence the catalytic activity is yet to be established. In this article, we use linear scaling relationships and molecular volcano plots to assess the potential of the Ae metal-based catalysts for the hydrogenation of alkenes. By analyzing combinations of eight metals (mono-, bi-, tri-, and tetravalent) and seven ligands, we delineate the impact of metal-ligand interplay on the hydrogenation activity. Our findings highlight that the catalytic activity is majorly determined by the charge and the size of the metal ions. While bivalent Ae metal cations delicately regulate the binding and the release of the reactants and the products, respectively, providing the right balance for this reaction, ligands play only a minor role in determining their catalytic activity. We show how volcano plots can be utilized for the rapid screening of prospective Ae catalysts to establish a guideline to achieve maximum activity in facilitating the hydrogenation process. SUPPLEMENTARY INFORMATION: The online version of this article at 10.1007/s11244-021-01480-7.

Recent Advances in Group 14 and 15 Lewis Acids for Frustrated Lewis Pair Chemistry.

Frustrated Lewis pairs (FLP) which rely on the cooperative action of Lewis acids and Lewis bases, played a prominent role in the advancement of main-group catalysis. While the early days of FLP chemistry witnessed the dominance of boranes, there is a growing body of reports on alternative Lewis acids derived from groups 14 and 15. This short review focuses on the discovery of such non-boron candidates reported since 2015.

Constructing and interpreting volcano plots and activity maps to navigate homogeneous catalyst landscapes.

Volcano plots and activity maps are powerful tools for studying homogeneous catalysis. Once constructed, they can be used to estimate and predict the performance of a catalyst from one or more descriptor variables. The relevance and utility of these tools has been demonstrated in several areas of catalysis, with recent applications to homogeneous catalysts having been pioneered by our research group. Both volcano plots and activity maps are built from linear free energy scaling relationships that connect the value of a descriptor variable(s) with the relative energies of other catalytic cycle intermediates/transition states. These relationships must be both constructed and postprocessed appropriately to obtain the resulting plots/maps; this process requires careful execution to obtain meaningful results. In this protocol, we provide a step-by-step guide to building volcano plots and activity maps using curated reaction profile data. The reaction profile data are obtained using density functional theory computations to model the catalytic cycle. In addition, we provide volcanic, a Python code that automates the steps of the process following data acquisition. Unlike the computation of individual reaction energy profiles, our tools lead to a holistic view of homogeneous catalyst performance that can be broadly applied for both explanatory and screening purposes.