Transition Metal-Mimicking Relay Catalysis by a Low-Valent Phosphorus Compound.

Low-valent main group species have been evolving as powerful alternatives to transition metals over the years due to their advantages such as low toxicity and high abundance. However, the inability of main group elements to mimic the redox-switching property of transition metals often limits their role as catalysts. Here, we demonstrate the use of a low-valent phosphorus(I) compound as an efficient metal-free catalyst for the synthesis of biologically relevant γ-butyrolactones through dual activation under ambient reaction conditions. The highly nucleophilic phosphorus(I) center plays a key role in leading to this transformation. Extensive experimental and theoretical studies suggest that the phosphorus center exhibits facile switching between its reduced state [P(I)] and its oxidized state [P(III)] during this transformation, mimicking the behavior of transition metals.

Organocatalytic Dearomative Spirocyclization Reaction of Enone-Tethered α-and ß-Naphthols and Dearomatization Reaction of In Situ Generated Nitro-Olefin-Tethered α-Naphthols.

Herein, we report a catalytic dearomative spirocyclization reaction of new substrates having aryl/alkyl enone tethered α- and ß-naphthols and a dearomatization reaction of in situ generated nitro-olefin-tethered α-naphthols. The spirocarbocycles were obtained in moderate to good yields with high diastereoselectivities. A preliminary catalytic asymmetric variant was reported. A few applications such as hydrogenations and epoxidation reaction have also been demonstrated. Theoretical study has also been performed to understand high diastereoselectivity in the triethylamine catalyzed spirocyclization reaction.

Exploiting the Strained Ion-Pair Interactions of Sterically Hindered Pyridinium Salts Toward SN2 Glycosylation of Glycosyl Trichloroacetimidates.

We demonstrate here that strained and sterically hindered protonated 2,4,6-tri-tert-butylpyridinium (TTBPy) tetrafluoroborate, a crystalline, bench stable salt serves as a mild and efficient organocatalyst for the SN2 type displacement of glycosyl trichloroacetimidates toward the stereoselective synthesis of both α- and ß-glycosides. The strained ion-pair interactions between the sterically hindered pyridinium cation and the tetrafluoroborate anion infuse unusual reactivity to the ions resulting in the unique anion assisted activation of alcohol. This mild activation of alcohol facilitates the SN2 type displacement of glycosyl α-trichloroacetimidates into ß-glycosides in a highly diastereoselective manner. These unique interactions were established based on extensive infrared and 1H, 19F, 11B NMR studies and theoretical studies.

Chromium-Catalyzed Cross-Coupling of Methyl Ketones with Cyclic Ketones toward the Selective Synthesis of ß-Branched ß,γ-Unsaturated Ketones.

Chromium-catalyzed cross-coupling of methyl ketones with cyclic ketones to ß-branched ß,γ-unsaturated ketones are reported. Interestingly, single-crossed aldol condensation products are formed, even in reactions in which a mixture of products is possible. The reaction is highly chemoselective and regioselective. This catalytic route gives a unique opportunity to integrate the chemistry of the synthetic challenge cross-coupling reaction of ketones and the alkene migration reaction into a reaction pot.

One-Step Synthesis of 7-Bromobenzo[c]chromeno[4,3,2-gh]phenanthridines through a Sequential Bromination/Cyclization/Aromatization Reaction Using N-Bromosuccinimide (NBS).

Herein, metal- and oxidant-free synthesis of 7-bromobenzo[c]chromeno[4,3,2-gh]phenanthridines is reported using N-bromosuccinimide. Sequential regioselective bromination, intramolecular ring cyclization, and aromatization reactions occur in a single step through a successive radical-catalyzed pathway. The mechanistic pathway for the cyclization is supported by a DFT study. Selective bromination in the fully aromatic skeleton is accomplished without involving additional aromatic electrophilic ring bromination. As a synthetic application, the Suzuki coupling reaction of compound 5a with boronic acid is reported to get compound 8a. Aggregation-induced emission of one of the synthesized compounds (5h) is also investigated in THF/hexane solvent along with concentration-dependent emission spectroscopy.

Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes.

Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p-CF3 -C6 H4 ) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa -gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N-H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.

Radical Activation of N-H and O-H Bonds at Bismuth(II).

The development of unconventional strategies for the activation of ammonia (NH3) and water (H2O) is of capital importance for the advancement of sustainable chemical strategies. Herein we provide the synthesis and characterization of a radical equilibrium complex based on bismuth featuring an extremely weak Bi-O bond, which permits the in situ generation of reactive Bi(II) species. The ensuing organobismuth(II) engages with various amines and alcohols and exerts an unprecedented effect onto the X-H bond, leading to low BDFEX-H. As a result, radical activation of various N-H and O-H bonds─including ammonia and water─occurs in seconds at room temperature, delivering well-defined Bi(III)-amido and -alkoxy complexes. Moreover, we demonstrate that the resulting Bi(III)-N complexes engage in a unique reactivity pattern with the triad of H+, H-, and H• sources, thus providing alternative pathways for main group chemistry.

Origin of the Failure of Density Functional Theories in Predicting Inverted Singlet-Triplet Gaps.

Recent experimental and theoretical studies have shown several new organic molecules that violate Hund's rule and have the first singlet excited state lower in energy than the first triplet excited state. While many correlated single reference wave function methods have successfully predicted excited-state energetics of these low-lying states, conventional linear-response time-dependent density functional theory (TDDFT) fails to predict the correct excited-state energy ordering. In this article, we have explored the performance of combined DFT and wave function methods like doubles-corrected TDDFT and multiconfiguration pair-density functional theory for the calculation of inverted singlet-triplet gaps. We have also tested the performance of the excited-state DFT (eDFT) method for this problem. Our results have shown that it is possible to obtain inverted singlet-triplet gaps both by using doubles-corrected TDDFT with a proper choice of double-hybrid functionals or by using eDFT.

Are Heavy Pnictogen-π Interactions Really "π Interactions"?

The noncovalent interactions of heavy pnictogens with π-arenes play a fundamental role in fields like crystal engineering or catalysis. The strength of such bonds is based on an interplay between dispersion and donor/acceptor interactions, and is generally attributed to the presence of π-arenes. Computational studies of the interaction between the heavy pnictogens As, Sb and Bi and cyclohexane, in comparison with previous studies on the interaction between heavy pnictogens and benzene, show that this concept probably has to be revised. A thorough analysis of all the different energetic components that play a role in these systems, carried out with state-of-the-art computational methods, sheds light on how they influence one another and the effect that their interplay has on the overall system. Furthermore, the analysis of such interactions leads us to the unexpected finding that the presence of the pnictogen compounds strongly affects the conformational equilibrium of cyclohexane, reversing the relative stability of the chair and boat-twist conformers, and thus suggesting a possible application of tuneable dispersion energy donors to stabilise the desired conformation.

An NHC-Stabilised Phosphinidene for Catalytic Formylation: A DFT-Guided Approach.

In recent years, the applications of low-valent main group compounds have gained momentum in the field of catalysis. Owing to the accessibility of two lone pairs of electrons, NHC-stabilised phosphinidenes have been found to be excellent Lewis bases; however, they cannot yet be used as catalysts. Herein, an NHC-stabilised phosphinidene, 1,3-dimethyl-2-(phenylphosphanylidene)-2,3-dihydro-1H imidazole (1), for the activation of CO2 is reported.A closer inspection of the CO2 activation process by DFT calculations along with intrinsic bond orbital analysis shows that phosphinidene is associated with phenylsilane through a noncovalent π-π interaction between two phenyl rings which activates the Si-H bond facilitating hydride transfer to the CO2 molecule. Detailed DFT studies along with spectroscopic experiments were combined to understand the mechanism of CO2 activation and its catalytic reductive functionalisation leading to the formylation of a range of chemically inert primary amides under mild reaction conditions.

Polymorphism Dependent 9-Phosphoanthracene Derivative Exhibiting Thermally Activated Delayed Fluorescence: A Computational Investigation.

Polymorphs of anthracene derivatives exhibit diverse photophysical properties that can help to develop efficient organic-based photovoltaic devices. 10-Anthryl-9-phosphoanthracene (10-APA) shows different photophysical behaviors for the solid state due to its variety in crystalline arrangement. Herein, we investigate the ground and excited-state properties of the monomer and two different polymorphs of 10-APA from first-principles. Calculations reveal that strong spin-orbit coupling (SOC) between first excited singlet state (S1) and triplet manifolds at their S1-optimized geometries enabling the reverse intersystem crossing (RISC). The electron-vibration coupling (Huang-Rhys factor) in the excited state is the most relevant factor here. For both ISC and RISC, a similarity in Huang-Rhys factors for the molecular vibration along the π···π stacking at low-frequency region makes the rates effective. On the other side, the nonvanishing vibronic relaxation modes provide a relatively slower RISC rate in the red crystal. However, for the red crystal, small reorganization energy (λ) and large Huang-Rhys factor toward S1 → S0 conversion reduce nonradiative decay, leading to a prompt fluorescence. As the feasibility of S1 ↔ T1 conversion increases in the yellow dimer, it allows a delay in fluorescence emission, leading to thermally activated delayed fluorescence (TADF).

Evaluation of bismuth-based dispersion energy donors - synthesis, structure and theoretical study of 2-biphenylbismuth(iii) derivatives.

A series of 2-biphenyl bismuth(iii) compounds of the type (2-PhC6H4)3-nBiXn [n = 0 (1); n = 1, X = Cl (2), Br (3), I (4), Me (5); n = 2, X = Cl (6), Br (7), I (8)] has been synthesized and analyzed with focus on intramolecular London dispersion interactions. The library of the compounds was set up in order to investigate the Biπ arene interaction by systematic variation of X. The structural analysis in the solid state revealed that the triarylbismuth(iii) compound 1 shows an encapsulation of the metal atom but the distances between the bismuth atom and the phenyl centroids amount to values close to or larger than 4.0 Å, which is considered to be a rather week dispersion interaction. In the case of monomeric diorganobismuth(iii) compounds 2-5 the moderate crowding effectively hinders the formation of intermolecular donor-acceptor interactions, but allows for intramolecular dispersion-type interactions with the 2-biphenyl ligand. In contrast, the structures of the monoorganobismuth compounds 6-8 show the formation of Bi-XBi donor-acceptor bonds leading to the formation of 1D ribbons in the solid state. These coordination bonds are accompanied by intermolecular dispersion interactions with BiPhcentroid distances < 4.0 Å. In solution the diorganobismuth(iii) halides 2-4 show a broadening of their NMR signals (H-8, H-8' and H-9, H-9' protons of the 2-biphenyl ligand), which is a result of dynamic processes including ligand rotation. For further elucidation of these processes compounds 2, 4 and 7 were studied by temperature-dependent NMR spectroscopy. Electronic structure calculations at the density functional theory and DLPNO-coupled cluster level of theory were applied to investigate and quantify the intramolecular London dispersion interactions, in an attempt to distinguish between basic intramolecular interactions and packing effects and to shed light on the dynamic behavior in solution.

Remote Functionalization through Symmetric or Asymmetric Substitutions Control the Pathway of Intermolecular Singlet Fission.

Singlet fission (SF) produces two coupled triplet excitons from a high energy singlet excitation. The mechanism of SF in a variety of phenyl (-Ph) substituted pentacene is systematically studied through both ab initio and density functional theory calculations. Two classes of substitution to pentacene are considered, namely, symmetric configuration with four Ph groups (TPP) and an asymmetric configuration with two Ph groups (DPP). The positions of the singlet and triplet states are determined by calibrating the active space through state averaged complete active space self-consistent field (SA-CASSCF) calculations. The SF rates are computed based on restricted active space with single and double spin flip wave functions (RAS-SF and RAS-2SF), which are analyzed based on different intermolecular π-stacking patterns of TPP and DPP. The contribution of charge transfer (CT) state near the multiexciton (ME) state plays a significant role for SF efficiency. The role of excimer formation is supportive for ME generation [J. Am. Chem. Soc. 2016, 138, 617], and hence it is critically studied. The ME generation in TPP is a slower process and occurs through an excimer-mediated path with a large coupling between the first singlet excited state and ME state. On the other hand, DPP exhibits a relatively faster SF rate through the formation of a ME state via low-lying CT state, especially the slip-stacked dimers. The present computation elegantly demonstrates the crucial role of functional group substitution in the structure of SF active molecules in determining the efficiency of fission dynamics.

Visible light driven efficient metal free single atom catalyst supported on nanoporous carbon nitride for nitrogen fixation.

The production of ammonia (NH3), an important carbon-free chemical, through nitrogen (N2) fixation under mild conditions, is one of the most challenging and attractive chemical processes for industrial applications. However, most N2 fixation occurs through transition-metal based systems and examples of metal-free catalysts remain elusive. Herein, by means of first-principles computations, we demonstrate that dynamical as well as highly thermally stable (up to 800 K) single boron atom doped nanoporous carbon nitride materials, i.e. C2N monolayers, are a potential metal-free single atom catalyst for efficient N2 fixation under visible light absorption. Based on the B-N synergistic effect, N2 strongly binds to the B/C2N surface through end-on and side-on modes respectively. Our computation reveals that the single B atom doped C2N-concept catalyst could effectively reduce N2 to NH3 with a record low onset potential (0.18 eV) through enzymatic pathways and can sufficiently suppress the competing hydrogen evolution reactions. Multimodal binding of gas phase N2 molecules with selective stabilization of NxHx by proton-electron (H+ + e-) pairs leads to the highest catalytic performance of B/C2N. Moreover, deposition of single B atoms on C2N dramatically enhances the absorption of light in the visible and IR regions, rendering it a promising solar light-driven N2 to NH3 reduction (NRR) catalyst. The excellent formation energy of B doped C2N advocates its experimental synthesis.

Aggregation induced non-emissive-to-emissive switching of molecular platinum clusters.

We show here for the first time the Aggregation Induced Emission (AIE) mechanism and solvatochromic impact on Pt-SG (SG-deprotonated glutathione) nanoclusters. In this work, the AIE properties of Pt-SG clusters were investigated through computational and spectroscopic investigations. Computational data established that aggregation triggers a distinct change in the frontier molecular orbitals (FMOs) from metal d-orbital centered FMOs in the monomer to metal-thiolate and thiolate centered FMOs in the dimer improving the radiative decay process. Solvent dependent photoluminescence studies proved that a Lewis-acidic environment can significantly perturb the metal-thiolate and thiolate centered FMOs that are involved in the electronic transitions as predicted by our computational work. These semiconducting clusters exhibit a large Stokes shift and zero spectral overlap between absorption and emission which makes this Pt-SG cluster an excellent material for solar concentrators and solid-state light emitters. This AIE-OFF-ON emission was utilized to delineate a proof-of-concept sensor device that is sensitive to temperature and an acid/base.

Light-Induced Conformational Change of Uracil-Anchored Polythiophene-Regulating Thermo-Responsiveness.

Tuning the electronic structure of a π-conjugated polymer from the responsive side chains is generally done to get desired optoelectronic properties, and it would be very fruitful when light is used as an exciting tool that can also affect the backbone chain conformation. For this purpose, polythiophene- g-poly-[ N-(6-methyluracilyl)- N, N-dimethylamino chloride]ethyl methacrylate (PTDU) is synthesized. On exposure to diffuse sunlight, the uracil moieties of the grafted chains cause the absorption maximum of PTDU solution to show gradual blue shift of 87 nm and a gradual blue shift of 46 nm in the emission maximum, quenching its fluorescence with time. These effects occur specifically at the absorption range of polythiophene (PT) chromophore on direct exposure of light of different wavelengths, and the optimum wavelength is found to be 420 nm. Impedance study suggests a decrease in charge transfer resistance upon exposure because of conformational change of PTDU. Theoretical study indicates that on exposure to visible light, uracil moieties move toward the backbone to facilitate photoinduced electron transfer between the PT and the uracil, attributing to the variation in optoelectronic properties. Morphological and light-scattering studies exhibit a decrease in particle size because of coiling of the PT backbone and squeezing of the grafted chain on light exposure. The transparent orange-colored PTDU solution becomes hazy with a hike in emission intensity on addition of sodium halides and becomes reversibly transparent or hazy on heating or cooling. The screening of cationic centers of PTDU by varying halide anion concentration tunes the phase transition temperature. Thus, the light-induced variation in the backbone conformation is responsible for tuning the optoelectronic properties and regulates the thermos-responsiveness of the PTDU solution in the presence of halide ions.

Gauging the Nanotoxicity of h2D-C2N toward Single-Stranded DNA: An in Silico Molecular Simulation Approach.

Recent toxicological assessments of graphene, graphene oxides, and some other two-dimensional (2D) materials have shown them to be substantially toxic at the nanoscale, where they inhibit and eventually disrupt biological processes. These shortfalls of graphene and analogs have resulted in a quest for novel biocompatible 2D materials with minimum cytotoxicity. In this article, we demonstrate C2N (h2D-C2N), a newly synthesized 2D porous graphene analog, to be non-nanotoxic toward genetic materials from an "in-silico" point of view through sequence-dependent binding of different polynucleotide single-stranded DNA (ssDNA) onto it. The calculated binding energy of nucleobases and the free energy of binding of polynucleotides follow the common trait, cytosine > guanine > adenine > thymine, and are well within the limits of physisorption. Ab-initio simulations completely exclude the possibility of any chemical reaction, demonstrating purely noncovalent binding of nucleobases with C2N through a crucial interplay between hydrogen bonding and π-stacking interactions with the surface. Further, we show that the extent of distortion inflicted upon ssDNA by C2N is negligible. Analysis of the density of states of the nucleobase-C2N hybrids confirms minimum electronic perturbation of the bases after adsorption. Most importantly, we demonstrate the potency of C2N in nucleic acid transportation via reversible binding of ssDNA. The plausible use of C2N as a template for DNA repair is illustrated through an example of C2N-assisted complementary ssDNA winding.

Evidence of homo-FRET in quantum dot-dye heterostructured assembly.

With an aim to understand the intermolecular/particle interaction and the optical properties of the inorganic-organic hybrid nanostructured materials, Förster resonance energy transfer (FRET) between negatively charged CdS quantum dots (donor) and positively charged Oxazine 170 perchlorate (acceptor) has been investigated by employing steady-state and time-resolved fluorescence spectroscopy. Investigations revealed that size-dependent changes in the FRET efficiency of different QD-dye FRET pairs occurred mainly due to the electrostatic effects. Interestingly, the present study also reveals that at a higher concentration of dye molecules, aggregation occurs on the QD surface and the quenching of dye fluorescence occurs due to homo-FRET process. The homo-FRET process in this case has been established by exploiting steady-state fluorescence anisotropy measurements. The feasibility of aggregate formation and the homo-FRET interaction between the dye molecules has also been demonstrated through quantum mechanical calculations.

External electric field control: driving the reactivity of metal-free azide-alkyne click reactions.

Recent reports have suggested that an external electric field (EEF) can assist and even control product selectivity. In this work, we have shown that the barrier for the Huisgen reaction between alkyl (aryl) azide and cyclooctyne(biflurocyclooctyne) is reduced by ∼3-4 kcal mol-1 when an oriented EEF is applied along the reaction axis. As a consequence of their inherently polar transition-states (TSs), a parallel orientation of the EEF results in enhancement of the charge transfer (CT) between the fragments and concomitant increase in the dipole moment along the reaction axes. This leads to an increase in the reaction rate for moderate EEFs in the range of 0.3-0.5 V Å-1. Since highly polar and directional environments are omnipresent in biological environments, metal-free click reactions can be further accelerated for non-invasive imaging of live-cells. Conceptually, electric field control appears to be a novel tool (catalyst) to drive, and possibly even tune, the reactivity of organic molecules.

Visible-Light-Mediated Excited State Relaxation in Semi-Synthetic Genetic Alphabet: d5SICS and dNaM.

The excited state dynamics of an unnatural base pair (UBP) d5SICS/dNaM were investigated by accurate ab-initio calculations. Time-dependent density functional and high-level multireference calculations (MS-CASPT2) were performed to elucidate the excitation of this UBP and its excited state relaxation mechanism. After excitation to the bright state S2 (ππ*), it decays to the S1 state and then undergoes efficient intersystem crossing to the triplet manifold. The presence of sulfur atom in d5SICS leads to strong spin-orbit coupling (SOC) and a small energy gap that facilitates intersystem crossing from S1 (ns π*) to T2 (ππ*) followed by internal conversion to T1 state. Similarly in dNaM, the deactivation pathway follows analogous trends. CASPT2 calculations suggest that the S1 (ππ*) state is a dark state below the accessible S2 (ππ*) bright state. During the ultrafast deactivation, it exhibits bond length inversion. From S1 state, significant SOC leads the population transfer to T3 due to a smaller energy gap. Henceforth, fast internal conversion occurs from T3 to T2 followed by T1 . From time-dependent trajectory surface hopping dynamics, it is found that excited state relaxation occurs on a sub-picosecond timescale in d5SICS and dNaM. Our findings strongly suggest that there is enough energy available in triplet state of UBP to generate reactive oxygen species and induce phototoxicity with respect to cellular DNA.