Synergistic Steric and Electronic Effects on the Photoredox Catalysis by a Multivariate Library of Titania Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) that display photoredox activity are attractive materials for sustainable photocatalysis. The ability to tune both their pore sizes and electronic structures based solely on the choice of the building blocks makes them amenable for systematic studies based on physical organic and reticular chemistry principles with high degrees of synthetic control. Here, we present a library of eleven isoreticular and multivariate (MTV) photoredox-active MOFs, UCFMOF-n, and UCFMTV-n-x% with a formula Ti6O9[links]3, where the links are linear oligo-p-arylene dicarboxylates with n number of p-arylene rings and x mol% of multivariate links containing electron-donating groups (EDGs). The average and local structures of UCFMOFs were elucidated from advanced powder X-ray diffraction (XRD) and total scattering tools, consisting of parallel arrangements of one-dimensional (1D) [Ti6O9(CO2)6]∞ nanowires connected through the oligo-arylene links with the topology of the edge-2-transitive rod-packed hex net. Preparation of an MTV library of UCFMOFs with varying link sizes and amine EDG functionalization enabled us to study both their steric (pore size) and electronic (highest occupied molecular orbital-lowest unoccupied molecular orbital, HOMO-LUMO, gap) effects on the substrate adsorption and photoredox transformation of benzyl alcohol. The observed relationship between the substrate uptake and reaction kinetics with the molecular traits of the links indicates that longer links, as well as increased EDG functionalization, exhibit impressive photocatalytic rates, outperforming MIL-125 by almost 20-fold. Our studies relating photocatalytic activity with pore size and electronic functionalization demonstrate how these are important parameters to consider when designing new MOF photocatalysts.

Efficient Iridium Catalysts for Formic Acid Dehydrogenation: Investigating the Electronic Effect on the Elementary ß-Hydride Elimination and Hydrogen Formation Steps.

We report herein a series of Cp*Ir complexes containing a rigid 8-aminoquinolinesulfonamide moiety as highly efficient catalysts for the dehydrogenation of formic acid (FA). The complex [Cp*Ir(L)Cl] (HL = N-(quinolin-8-yl)benzenesulfonamide) displayed a high turnover frequency (TOF) of 2.97 × 104 h-1 and a good stability (>100 h) at 60 °C. Comparative studies of [Cp*Ir(L)Cl] with the rigid ligand and [Cp*Ir(L')Cl] (HL' = N-propylpypridine-2-sulfonamide) without the rigid aminoquinoline moiety demonstrated that the 8-aminoquinoline moiety could dramatically enhance the stability of the catalyst. The electron-donating ability of the N,N'-chelating ligand was tuned by functionalizing the phenyl group of the L ligand with OMe, Cl, and CF3 to have a systematical perturbation of the electronic structure of [Cp*Ir(L)Cl]. Experimental kinetic studies and density functional theory (DFT) calculations on this series of Cp*Ir complexes revealed that (i) the electron-donating groups enhance the hydrogen formation step while slowing down the ß-hydride elimination and (ii) the electron-withdrawing groups display the opposite effect on these reaction steps, which in turn leads to lower optimum pH for catalytic activity compared to the electron-donating groups.

Impact of Surface Modification on the Lithium, Sodium, and Potassium Intercalation Efficiency and Capacity of Few-Layer Graphene Electrodes.

In a conventional lithium-ion battery (LIB), graphite forms the negative electrode or anode. Although Na is considered one of the most attractive alternatives to Li, achieving reversible Na intercalation within graphitic materials under ambient conditions remains a challenge. More efficient carbonaceous anode materials are desired for developing advanced LIBs and beyond Li-ion battery technologies. We hypothesized that two-dimensional materials with distinct surface electronic properties create conditions for ion insertion into few-layer graphene (FLG) anodes. This is because modification of the electrode/electrolyte interface potentially modifies the energetics and mechanisms of ion intercalation in the thin bulk of FLG. Through first-principles calculations; we show that the electronic, structural, and thermodynamic properties of FLG anodes can be fine-tuned by a covalent heteroatom substitution at the uppermost layer of the FLG electrode, or by interfacing FLG with a single-side fluorinated graphene or a Janus-type hydrofluorographene monolayer. When suitably interfaced with the 2D surface modifier, FLG exhibits favorable thermodynamics for the Li+, Na+, and K+ intercalation. Remarkably, the reversible binding of Na within carbon layers becomes thermodynamically allowed, and a large storage capacity can be achieved for the Na intercalated modified FLG anodes. The origin of charge-transfer promoted electronic tunability of modified FLGs is rationalized by various theoretical methods.

Nernstian Li+ intercalation into few-layer graphene and its use for the determination of K+ co-intercalation processes.

Alkali ion intercalation is fundamental to battery technologies for a wide spectrum of potential applications that permeate our modern lifestyle, including portable electronics, electric vehicles, and the electric grid. In spite of its importance, the Nernstian nature of the charge transfer process describing lithiation of carbon has not been described previously. Here we use the ultrathin few-layer graphene (FLG) with micron-sized grains as a powerful platform for exploring intercalation and co-intercalation mechanisms of alkali ions with high versatility. Using voltammetric and chronoamperometric methods and bolstered by density functional theory (DFT) calculations, we show the kinetically facile co-intercalation of Li+ and K+ within an ultrathin FLG electrode. While changes in the solution concentration of Li+ lead to a displacement of the staging voltammetric signature with characteristic slopes ca. 54-58 mV per decade, modification of the K+/Li+ ratio in the electrolyte leads to distinct shifts in the voltammetric peaks for (de)intercalation, with a changing slope as low as ca. 30 mV per decade. Bulk ion diffusion coefficients in the carbon host, as measured using the potentiometric intermittent titration technique (PITT) were similarly sensitive to solution composition. DFT results showed that co-intercalation of Li+ and K+ within the same layer in FLG can form thermodynamically favorable systems. Calculated binding energies for co-intercalation systems increased with respect to the area of Li+-only domains and decreased with respect to the concentration of -K-Li- phases. While previous studies of co-intercalation on a graphitic anode typically focus on co-intercalation of solvents and one particular alkali ion, this is to the best of our knowledge the first study elucidating the intercalation behavior of two monovalent alkali ions. This study establishes ultrathin graphitic electrodes as an enabling electroanalytical platform to uncover thermodynamic and kinetic processes of ion intercalation with high versatility.

Gold-Catalyzed Cross-Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications.

Transition-metal-catalyzed cross-coupling reactions are central to many organic synthesis methodologies. Traditionally, Pd, Ni, Cu, and Fe catalysts are used to promote these reactions. Recently, many studies have showed that both homogeneous and heterogeneous Au catalysts can be used for activating selective cross-coupling reactions. Here, an overview of the past studies, current trends, and future directions in the field of gold-catalyzed coupling reactions is presented. Design strategies to accomplish selective homocoupling and cross-coupling reactions under both homogeneous and heterogeneous conditions, computational and experimental mechanistic studies, and their applications in diverse fields are critically reviewed. Specific topics covered are: oxidant-assisted and oxidant-free reactions; strain-assisted reactions; dual Au and photoredox catalysis; bimetallic synergistic reactions; mechanisms of reductive elimination processes; enzyme-mimicking Au chemistry; cluster and surface reactions; and plasmonic catalysis. In the relevant sections, theoretical and computational studies of AuI /AuIII chemistry are discussed and the predictions from the calculations are compared with the experimental observations to derive useful design strategies.

Stabilizing polymer electrolytes in high-voltage lithium batteries.

Electrochemical cells that utilize lithium and sodium anodes are under active study for their potential to enable high-energy batteries. Liquid and solid polymer electrolytes based on ether chemistry are among the most promising choices for rechargeable lithium and sodium batteries. However, uncontrolled anionic polymerization of these electrolytes at low anode potentials and oxidative degradation at working potentials of the most interesting cathode chemistries have led to a quite concession in the field that solid-state or flexible batteries based on polymer electrolytes can only be achieved in cells based on low- or moderate-voltage cathodes. Here, we show that cationic chain transfer agents can prevent degradation of ether electrolytes by arresting uncontrolled polymer growth at the anode. We also report that cathode electrolyte interphases composed of preformed anionic polymers and supramolecules provide a fundamental strategy for extending the high voltage stability of ether-based electrolytes to potentials well above conventionally accepted limits.

Criticality of Symmetry in Rational Design of Chalcogenide Perovskites.

Chalcogenide perovskites constitute an emerging class of promising photovoltaic materials that are stable and less toxic than popular lead-halide perovskites. Transition-metal and chalcogenide doping are the possible strategies for improving the photovoltaic properties of these materials via the band gap engineering. At the same time, doping can facilitate nonradiative charge-carrier recombination in these materials, adversely affecting their photovoltaic properties. We report a systematic study of electronic structure and nonadiabatic dynamics in transition-metal- and chalcogenide-doped barium-zirconium-sulfide-based perovskites. The potential of these doping strategies to modulate the performance of photovoltaic materials is explored. Through the detailed analysis of the factors affecting the dynamics, we illustrate how symmetry (both structural and orbital) and decoherence can be critical to furnishing the most favorable properties. The noted factors of symmetry and decoherence may provide new rational design principles for efficient photovoltaics.

Direct and Autocatalytic Reductive Elimination from Gold Complexes ([(Ph3 P)Au(Ar)(CF3 )(X)], X=F, Cl, Br, I): The Key Role of Halide Ligands.

Kinetic and thermodynamic preferences for the reductive elimination of Caryl -CF3 , Caryl -X, Caryl -P, and CF3 -X bonds and competitive phosphine dissociation from a series of AuIII complexes [(Ph3 P)Au(Ar)(CF3 )(X)] (1 X ; Ar=4-Me-C6 H4 ; X=F, Cl, Br, I) are studied computationally. Kinetically, the most favorable pathways were found to consist of an initial phosphine dissociation from complex 1 X , which furnished the respective three-coordinate AuIII complexes [Au(Ar)(CF3 )(X)] (2 X ). The computed enthalpy barriers for various reductive elimination reactions from complex 2 X by a direct (or uncatalyzed) mechanism showed that Caryl -CF3 bond formation was the most favorable fate for any X group. When the direct elimination was compared with an autocatalytic mechanism that proceeded through the formation of a mixed-valent binuclear AuIII -AuI intermediate, the preference for the formation of a Caryl -CF3 bond is dependent on the nature of the bridging halide atom and follows the order F>Cl>Br>I. Concomitantly, the selectivity for the formation of Caryl -X bonds for various X atoms follows the opposite trend. The preference for the direct and autocatalytic processes is controlled entirely by the nature of the halide ligand. The predicted mechanisms and product selectivity trends for various halides show excellent agreement with recent experimental observation. The selectivity of various reductive elimination pathways was rationalized by using molecular orbital theory and distortion-interaction model analyses. Attractive interactions between the AuI complex and complex 2X were found to reduce the activation barrier for Caryl -X elimination and critically control the selectivity of the product formation.

Topochemical Transformations of CaX2 (X=C, Si, Ge) to Form Free-Standing Two-Dimensional Materials.

Topochemical transformations of layered materials CaX2 (X=Si, Ge) are the method of choice for the high-yield synthesis of pristine, defect-free two-dimensional systems silicane and germanane, which have advanced electronic properties. Based on solid-state dispersion-corrected calculations, mechanisms for such transformations are elucidated that provide an in-depth understanding of phase transition in these layered materials. While formation of such layered materials is highly favorable for silicane and germanane, a barrier of 1.2 eV in the case of graphane precludes its synthesis from CaC2 topochemically. The energy penalty required for distorting linear acetylene into a trans-bent geometry accounts for this barrier. In contrast it is highly favorable in the heavier analogues, resulting in barrierless topochemical generation of silicane and germanane. Photochemical generation of the trans-bent structure of acetylene in its first excited state (S1 ) can directly generate graphane through a barrierless condensation. Unlike the buckled structure of silicene, the phase-h of CaSi2 with perfectly planar silicene layers exhibits the Dirac cones at the high symmetry points K and H. Interestingly, topochemical acidification of the cubic phase of calcium carbide is predicted to generate the previously elusive platonic hydrocarbon, tetrahedrane.

Mechanistic insights into the synergistic catalysis by Au(I), Ga(III), and counterions in the Nakamura reaction.

A computational study based on density functional theory (DFT) establishes the mechanisms for synergistic Au/Ga catalyzed addition of unactivated terminal alkynes to dicarbonyls, the Nakamura reaction. The role played by each of the metal catalysts and the counterion in the reaction has been elucidated. It has been shown that the triazole (TA) ligand could specifically activate the formation of a particular regioisomer through strong non-covalent interactions. Calculated regioselectivities and activation free energies are in excellent agreement with the experimental results. Observed regioselectivities were rationalized employing a distortion interaction analysis which suggests that the interaction between metal activated reactant fragments in the transition state geometries is a major factor that contributes to the overall barrier height and selectivity. Such enhanced preference for the reaction at the alkyl/aryl substituted carbon of alkynes was strongly influenced by the additional non-covalent interactions exerted by the TA ligand. Excellent agreement between the calculations using a homogeneous gold complex as the catalyst and experimentally observed kinetics and selectivity negates the role of in situ formed gold clusters in the Nakamura reaction.

Understanding the mechanisms of unusually fast H-H, C-H, and C-C bond reductive eliminations from gold(III) complexes.

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.

Analyte interactions with a new ditopic dansylamide-nitrobenzoxadiazole dyad: a combined photophysical, NMR, and theoretical (DFT) study.

We report herein the synthesis and photophysical studies on a new multicomponent chemosensor dyad comprising two fluorescing units, dansylamide (DANS) and nitrobenzoxadiazole (NBD). The system has been developed to investigate receptor-analyte binding interactions in the presence of both cations and anions in a single molecular system. A dimethyl amino (in the DANS unit) group is used as a receptor for cations, and acidic hydrogens of sulfonamide and the NBD group are used as receptors for anions. The system is characterized by conventional analytical techniques. The photophysical properties of this supramolecular system in the absence and presence of various metal ions and nonmetal ions as additives are investigated in an acetonitrile medium. Utility of this system in an aqueous medium has also been demonstrated. The absorption and fluorescence spectrum of the molecular system consists of a broad band typical of an intramolecular charge-transfer (ICT) transition. A low quantum yield and lifetime of the NBD moiety in the present dyad indicates photoinduced electron transfer (PET) between DANS and the NBD moiety. The fluorescence intensity of the system is found to decrease in the presence of fluoride and acetate anions; however, the quenching is found to be much higher for fluoride. This quenching behavior is attributed to the enhanced PET from the anion receptor to the fluorophore moiety. The mechanistic aspect of the fluoride ion signaling behavior has also been studied by infrared (IR) and (1)H NMR experiments. The hydrogen bonding interaction between the acidic NH protons of the DPN moiety and F(-) is found to be primarily responsible for the fluoride selective signaling behavior. While investigating the cation signaling behavior, contrary to anions, significant fluorescence enhancement has been observed only in the presence of transition-metal ions. This behavior is rationalized by considering the disruption of PET communication between DANS and the NBD moiety due to transition-metal ion binding. Theoretical (density functional theory) studies are also performed for the better understanding of the receptor-analyte interaction. Interestingly, negative cooperativity in binding is observed when the interaction of this system is studied in the presence of both Zn(2+) and F(-). Fluorescence microscopy studies also revealed that the newly developed fluorescent sensor system can be employed as an imaging probe in live cells.

Tip enhanced Raman spectroscopy (TERS) as a probe for the buckling distortion in silicene.

Silicene, the all-Si analogue of graphene, is symmetrically buckled in each of the six-membered units and this buckling is periodically translated across the surface. Raman spectra of silicene clusters were calculated using first principles DFT methods to explore the intrinsic buckling in silicene. The presence of metal clusters as a tip over the silicene units affects the intensity of the buckling modes which can be enhanced by increasing the number of atoms in the clusters. The favourable sites of chemisorption of metal clusters over the silicene surface are studied along with the resulting red shift in buckling frequency and chemical enhancement in the Raman intensity.

Pattern formation due to fluorination on graphene fragments: structures, hopping behavior, and magnetic properties.

Structures and mechanism of pattern formation for the radical fluorination on selected polyaromatic hydrocarbons (PAH) has been studied using density functional theory (DFT) methods. Our study reveals that the F(•) radical addition occurs preferentially at the edges of PAHs followed by the hopping of F(•) to the center due to the fluxional nature of C-F bond. F(•) migrates preferentially over the C-C bonds having a lower barrier than that over the aromatic π-cloud in cases of monofluorinated PAHs. Addition of a second F radical can stabilize the system, cooperatively. When two F(•) are added to the adjacent C atoms, it forms the minimum energy patterns. However, the addition of two fluorine radicals at the meta position of the same aromatic ring would lead to the stabilization of the triplet state compared to the singlet ground state. Therefore, depending on the sites of F(•) addition, these structures exhibit ferromagnetic/antiferromagnetic ground states. Considering the low barrier heights for the F(•) hopping, these systems are predicted to be in a dynamic equilibrium with their less stable ferromagnetic states. Our study also provides an atomistic understanding of the well-known rate determining state for the fluorine pattern formation in graphene and CNT.

Oxidation reactions of thymol: a pulse radiolysis and theoretical study.

The reactions of (•)OH and O(•-), with thymol, a monoterpene phenol and an antioxidant, were studied by pulse radiolysis technique and DFT calculations at B3LYP/6-31+G(d,p) level of theory. Thymol was found to efficiently scavenge OH radicals (k = 8.1 × 10(9) dm(3) mol(-1) s(-1)) to produce reducing adduct radicals, with an absorption maximum at 330 nm and oxidizing phenoxyl radicals, with absorption maxima at 390 and 410 nm. A major part of these adduct radicals was found to undergo water elimination, leading to phenoxyl radicals, and the process was catalyzed by OH(-) (or Na(2)HPO(4)). The rate of reaction of O(•-) with thymol was found to be comparatively low (k = 1.1 × 10(9) dm(3) mol(-1) s(-1)), producing H abstracted species of thymol as well as phenoxyl radicals. Further, these phenoxyl radicals of thymol were found to be repaired by ascorbate (k = 2.1 × 10(8) dm(3) mol(-1) s(-1)). To support the interpretation of the experimental results, DFT calculations were carried out. The transients (both adducts and H abstracted species) have been optimized in gas phase at B3LYP/6-31+G(d,p) level of calculation. The relative energy values and thermodynamic stability suggests that the ortho adduct (C6_OH adduct) to be most stable in the reaction of thymol with OH radicals, which favors the water elimination. However, theoretical calculations showed that C4 atom in thymol (para position) can also be the reaction center as it is the main contributor of HOMO. The absorption maxima (λ(max)) calculated from time-dependent density functional theory (TDDFT) for these transient species were close to those obtained experimentally. Finally, the redox potential value of thymol(•)/thymol couple (0.98 V vs NHE) obtained by cyclic voltammetry is less than those of physiologically important oxidants, which reveals the antioxidant capacity of thymol, by scavenging these oxidants. The repair of the phenoxyl radicals of thymol with ascorbate together with the redox potential value makes it a potent antioxidant with minimum pro-oxidant effects.

Ion interactions with a new ditopic naphthalimide-based receptor: a photophysical, NMR and theoretical (DFT) study.

A new multi-component chemosensor system comprising a naphthalimide moiety as fluorophore is designed and developed to investigate receptor-analyte binding interactions in the presence of metal and non-metal ions. A dimethylamino moiety is utilized as receptor for metal ions and a thiourea receptor, having acidic protons, for binding anions. The system is characterized by conventional analytical methods. The absorption and fluorescence spectra of the system consist of a broad band typical for an intramolecular charge transfer (ICT). The effects of various metal-ion additives on the spectral behavior of the present sensor system are examined in acetonitrile. It is found that among the metal ions studied, alkali/alkaline earth-metal ions and transition-metal ions modulate the absorption and fluorescence spectra of the system. As an additional feature, the anion signaling behavior of the system in acetonitrile is studied. A decrease in fluorescence efficiency of the system is observed upon addition of fluoride and acetate anions. Fluorescence quenching is most effective in the case of fluoride ions. This is attributed to the enhancement of the photoinduced electron transfer from the anion receptor to the fluorophore moiety. Hydrogen-bond interactions between the acidic NH protons of the thiourea moiety and the F(-) anions are primarily attributed to the fluoride-selective signaling behavior. Interestingly, a negative cooperativity for the binding event is observed when the interactions of the system are studied in the presence of both Zn(2+) and F(-) ions. NMR spectroscopy and theoretical calculations are also carried out to better understand the receptor-analyte binding.

Aminoindolines versus quinolines: mechanistic insights into the reaction between 2-aminobenzaldehydes and terminal alkynes in the presence of metals and secondary amines.

DFT computational studies in the cyclization of aminoalkyne (see structure), which is generated in situ by 2-aminobenzaldehydes and terminal alkynes in the presence of metals and secondary amines, has been investigated. The study revealed that the mode of cyclization (exo vs endo) depends on the protecting group on nitrogen, the oxidation state of copper, and substitution on alkyne.

Molecular Balances Based on Aliphatic CH-π and Lone-Pair-π Interactions.

CH···π and lone-pair···π interactions are estimated for a series of conformationally dynamic bicyclic N-aryliimides. On the basis of their strengths and mutual synergy/competition, the molecules prefer a folded/unfolded conformation. Calculations suggest strategies to selectively isolate the folded form by increasing the strength of the attractive CH···π interaction or removing the lone-pair···π repulsion. While the barrier for the folded ⇄ unfolded transformation is too large to conformationally lock the molecules in either of the conformers, the dynamics for hopping of the alkyl group across rings and tumbling over the rings are found to be facile in the folded conformation.