Orchestrated Octuple C-H Activation: A Bottom-Up Topology Engineering Approach toward Stimuli-Responsive Double-Heptagon-Embedded Wavy Polycyclic Heteroaromatics.

Curiosity-driven innovations on the design and synthesis of nonplanar polycyclic aromatic/heteroaromatic compounds with new molecular topologies unfold exciting opportunities for harnessing their intriguing supramolecular properties and thereby the development of novel functional organic materials. This work presents such an innovative synthetic concept of a bottom-up molecular topology engineering through a unique orchestrated octuple C-H activation reaction, toward the rapid synthesis of a novel class of double heptagon-incorporated nitrogen-doped laterally-fused polycyclic compounds with rarely reported wavy structural configuration. The profound impact of the molecular wavy structures of these compounds on their properties is manifested by weak and tunable solid-state intermolecular interactions controlling the electronic properties of the materials, leading to reversibly switchable fluorochromism in the solid state and thin films with mechanical force and solvent vapors as external stimuli, thereby indicating their potential applicability in rewritable fluorescent optical recording media, security papers, mechanosensors, volatile organic compound (VOC) sensors etc.

Unveiling N-Fused Nitreniums as Potent Catalytic Photooxidants.

The first example of a hitherto-unknown facet of catalytic photooxidant capability of nitrenium cations is reported herein. The fundamental limitation of inability of the traditional and reported nitreniums to achieve the excited-state redox potential beyond +2.0 V (vs Ag/AgCl), the primary requirement for a powerful photooxidant, is addressed in this work by developing a structurally unique class of N-fused nitrenium cations, with the required structural engineering involving extensive π-conjugation through ring fusion at the nitrenium site, which enabled significant lowering of the LUMO energy and easy reduction at the excited state (excited-state redox potential up to +2.5 V vs Ag/AgCl), facilitated by effective delocalization/stabilization of the generated radical. This finding opens a new way to discover novel and useful (photo)catalytic properties of nitrenium cations beyond just Lewis acidity.

Directly Knitted Hierarchical Porous Organometallic Polymer-Based Self-Supported Single-Site Catalyst for CO2 Hydrogenation in Water.

In recent times, heterogenization of homogeneous molecular catalysts onto various porous solid support structures has attracted significant research focus as a method for combining the advantages of both homogeneous as well as heterogeneous catalysis. The design of highly efficient, structurally robust and reusable heterogenized single-site catalysts for the CO2 hydrogenation reaction is a critical challenge that needs to be accomplished to implement a sustainable and practical CO2 -looped renewable energy cycle. This study demonstrated a heterogenized catalyst [Ir-HCP-(B/TPM)] containing a molecular Ir-abnormal N-heterocyclic carbene (Ir-aNHC) catalyst self-supported by hierarchical porous hyper-crosslinked polymer (HCP), in catalytic hydrogenation of CO2 to inorganic formate (HCO2 - ) salt that is a prospective candidate for direct formate fuel cells (DFFC). By employing this unique and first approach of utilizing a directly knitted HCP-based organometallic single-site catalyst for CO2 -to-HCO2 - in aqueous medium, extremely high activity with a single-run turnover number (TON) up to 50816 was achieved which is the highest so far considering all the heterogeneous catalysts for this reaction in water. Additionally, the catalyst featured excellent reusability furnishing a cumulative TON of 285400 in 10 cycles with just 1.6 % loss in activity per cycle. Overall, the new catalyst displayed attributes that are important for developing tangible catalysts for practical applications.

Annulation-Induced Hidden Reactivity of the 1,2,4-Triazole Backbone.

Triazoles are an important class of compounds with widespread applications. Functionalization of the triazole backbone is thus of significant interest. In comparison to 1,2,3-triazoles, C-H activation-functionalization of the congeners 1,2,4-triazoles is surprisingly underdeveloped. Indeed, no such C-H activation-functionalization has been reported for 4-substituted 1,2,4-triazole cores. Furthermore, although denitrogenative ring-opening of 1,2,3-triazoles is well-explored, 1,2,4-triazole/triazolium substrates have not been known to exhibit N-N bond-cleaving ring-opening reactivity so far. In this work, we unveiled an unusual hidden reactivity of the 1,2,4-triazole backbone involving the elusive N-N bond-cleaving ring-opening reaction. This new reactivity was induced by a Satoh-Miura-type C-H activation-annulation at the 1,2,4-triazole motif appended with a pyridine directing group. This unique reaction allowed ready access to a novel class of unsymmetrically substituted 2,2'-dipyridylamines, with one pyridine ring fully-substituted with alkyl groups. The unsymmetrical 2,2'-dipyridylamines were utilized to access unsymmetrical boron-aza-dipyridylmethene fluorescent dyes. Empowered with desirable optical/physical properties such as large Stokes shifts and suitable hydrophobicity arising from optimal alkyl chain length at the fully-substituted pyridine-ring, these dyes were used for intracellular lipid droplet-selective imaging studies, which provided useful information toward designing suitable lipid droplet-selective imaging probes for biomedical applications.

Exploiting the NADP+/NADPH-like Hydride-Transfer Redox Cycle with Bis-Imidazolium-Embedded Heterohelicene for Electrocatalytic Hydrogen Evolution Reaction.

Generation of clean energy in a viable manner demands efficient and sustainable catalysts. One prospective method of clean energy generation is the electrochemical hydrogen evolution reaction (HER). Over the years, various transition metal-based complexes/polymeric organic materials were utilized in HER. However, the use of a redox-active small organic molecule as a catalyst for HER has not been explored well. The requirements of a strongly acidic solution, very high overpotential, and stability under acidic conditions pose several challenges for applying organic electrocatalysts for HER. Considering these challenges, herein, we demonstrated an NADP+-like organic system (NADP+ = nicotinamide adenine dinucleotide phosphate), a bis-imidazolium-fused heterohelicene, which acts as a catalyst for HER with mild acid (acetic acid) as a proton source at moderate overpotential. The unique structural backbone of this dicationic heterohelicene allowed to exploit the NADP+/NADPH-type (NADPH = reduced nicotinamide adenine dinucleotide phosphate) hydride transfer-based redox cycle efficiently under the applied conditions, where the NADPH-like hydride intermediate transfers the hydride to the proton of the mild acid to generate H2. The Faradaic efficiency and turnover number for the present HER were achieved up to 85 ± 5% and 50 ± 3, respectively. In addition, the maximum turnover frequency, TOFmax, value of 410 s-1 was observed, which is around 400 times that obtained for the existing reported NADP+-like organic compounds used as catalysts for HER. Thorough mechanistic studies were conducted experimentally and computationally to establish a plausible catalytic cycle. This advancement could help in designing efficient organic electrocatalysts for HER from a mild proton source.

Bis-Imidazolium-Embedded Heterohelicene: A Regenerable NADP+ Cofactor Analogue for Electrocatalytic CO2 Reduction.

Biomimetic NAD(P)H-type organic hydride donors have recently been advocated as potential candidates to act as metal-free catalysts for fuel-forming reactions such as the reduction of CO2 to formic acid and methanol, similar to the natural photosynthesis process of fixing CO2 into carbohydrates. Although these artificial synthetic organic hydrides are extensively used in organic reduction chemistry in a stoichiometric manner, translating them into catalysts has been challenging due to problems associated with the regeneration of these hydride species under applied reaction conditions. A recent discovery of the possibility of their regeneration under electrochemical conditions via a proton-coupled electron-transfer pathway triggered intense research to accomplish their catalytic use in electrochemical CO2 reduction reactions (eCO2RR). However, success is yet to be realized to term them as "true" catalysts, as the typical turnover numbers (TONs) of the eCO2RR processes on inert electrodes for the production of formic acid and/or methanol reported so far are still in the order of 10-3-10-2; thus, sub-stoichiometric only! Herein, we report a novel class of structurally engineered heterohelicene-based organic hydride donor with a proof-of-principle demonstration of catalytic electrochemical CO2 reduction reaction showing a significantly improved activity with more than stoichiometric turnover featuring a 100-1000-fold enhancement of the existing TON values. Mechanistic investigations suggested the critical role of the two cationic imidazolium motifs along with the extensive π-conjugation present in the backbone of the heterohelicene molecules in accessing and stabilizing various radical species involved in the generation and transfer of hydride, via multielectron-transfer steps in the electrochemical process.

Conformationally flexible heterohelicenes as stimuli-controlled soft molecular springs.

Structurally engineered molecules which can behave as stimuli-controlled mechanical nanomachines such as molecular shuttles, rotors, ratchets, and springs are important in several research areas, including molecular robotics, actuation, sensing, cargo transportation, etc. Helicenes, by virtue of their unique screw-type structures, were proposed as functional models for molecular springs; however, experimental realization has remained an elusive and unmet task until now, because of the lack of appropriate helicene molecules consisting of backbone-decorated dynamic architectures. Aiming to explore this unearthed direction, we present herein a novel class of modular flexible heterohelicenes with a stimuli (acid/base and light)-responsive core and peripheral modules. By applying pH (at core-embedded free imidazole sites) and light (at backbone-tethered dithienylethene units) stimuli, we demonstrate that these flexible heterohelicenes exhibit spring-like movement, with the reversible contraction/extension of the helical pitch. The uniquely functionalized structure of these molecules played a critical role in bestowing such capability, as revealed by crystallographic, spectroscopic and computational data. Careful assessment disclosed that the protonation/deprotonation-induced reversible generation and delocalization of positive charge throughout the π-conjugated helical rim switch the operative interactions between the π clouds of the terminal overlapping arene rings of the helicenes between repulsive and attractive, leading to extension/contraction of the helical pitch. On the other hand, in the case of the light stimulus, it was analyzed that the light-induced ring-closure of the photoactive dithienylethene units created a geometric distortion causing the helicenic wings to bend outward from the helicene rim, which resulted in extension of the helical pitch. The photo-assisted (or thermal) reverse ring-opening reaction converted the system to its original conformation, thus enabling the helicene molecule to display spring-like reversible extension/contraction motion. The new insights on the reversible dynamic features of this class of heterohelicenes under the influence of external stress would guide crucial design principles of helicene-based molecular springs for potential applications.

An all-aqueous and phosphine-free integrated amine-assisted CO2 capture and catalytic conversion to formic acid.

A phosphine-free Ir(III)-NHC-based efficient catalytic system is developed for integrated CO2 capture with tetramethylguanidine as a capturing agent and conversion to formate with H2 gas, conducting both the steps in water, affording product yield up to 85% and TON up to 19 171 in just 12 h. In the segment of "integrated CO2-capture and conversion to formate", this system represents not only the first phosphine-free module, but also one of the few best known homogeneous catalysts.

Metal-Terpyridine Assembled Functional Materials for Electrochromic, Catalytic and Environmental Applications.

Molecular assembly induced by metal-terpyridine-based coordinative interactions has become an emergent research topic due to its ease of synthesis and diverse applications. This article highlights recent significant developments in the metal-terpyridine-based supramolecular architectures. At first, the design aspect of the molecular building blocks has been described, followed by elaboration on how the ligand backbone plays an important role for achieving different dimensionalities of the resulting assemblies which exhibit a wide range of potential applications. After that, we discussed different synthetic approaches for constructing these assemblies, and finally, we focused on their significant developments in three specific areas, viz., electrochromic materials, catalysis and a new application in wastewater treatment. In the field of electrochromic materials, these assemblies made important advancements in various aspects like sub-second switching time (<1 s), low switching voltage (<1 V), increased switching stability (>10000 cycles), tuning of multiple colors by using multimetallic systems, fabrication of charge storing electrochromic devices, utilizing and storing solar energy etc. Similarly, the catalysis field witnessed application of the metal-terpyridine assemblies in C-H monohalogenation, heterogeneous Suzuki-Miyaura coupling, photocatalysis, reduction of carbon dioxide, etc. Finally, the environmental application of these coordination assemblies includes capturing Cr(VI) from waste water efficiently with high capture capacity, good recyclability, wide pH independency etc.

Water-soluble and reusable Ru-NHC catalyst for aqueous-phase transfer hydrogenation of quinolines with formic acid.

Water-soluble Ru-NHC complexes were synthesized and their catalytic activity was tested in the transfer hydrogenation of quinoline-type N-heteroarenes using a formic acid/sodium formate buffer solution. The unique multifunctional features of the designed ligand within the catalyst backbone endowed it with excellent durability, reusability and compatibility with a simple aqueous-phase operation. Thus, it was possible to reuse as little as 0.25 mol% of the catalyst for three consecutive catalytic runs to provide an overall turnover number of around 900. A mechanistic investigation suggested that hydride generation was the rate-limiting step, whereas hydride transfer was relatively facile. Furthermore, computational studies supported that the reaction pathway was dominated by 1,4-hydride insertion at the N-heteroarene substrates.

Cationic π-extended heteroaromatics via a catalytic C-H activation annulative alkyne-insertion sequence.

Cationic π-conjugated organic molecules have broad applications in materials science as next-generation organic materials. The annulative alkyne-insertion π-extension (AAIPEX) strategy has emerged as a promising synthetic approach for the rapid synthesis of cationic polycyclic heteroaromatic compounds (cPHACs) in a single step. The AAIPEX reaction provides a synthetic shortcut to achieve complex organic molecules from simple (hetero)arene templates and alkynes as π-extending partners, which would otherwise be difficult to achieve using traditional methods. In general, a step-economic AAIPEX protocol proceeds via C-H activation of unfunctionalized heteroarene templates, followed by alkyne insertion-annulation to furnish cPHACs. In this Feature Article, recent progress in the AAIPEX strategy to construct cPHACs is described along with brief illustrations of the resulting cPHACs in luminescence-related applications.

Tris-NHC-propagated self-supported polymer-based Pd catalysts for heterogeneous C-H functionalization.

Three-dimensionally propagated imidazolium-containing mesoporous coordination polymer and organic polymer-based platforms were successfully exploited to develop single-site heterogenized Pd-NHC catalysts for oxidative arene/heteroarene C-H functionalization reactions. The catalysts were efficient in directed arene halogenation, and nondirected arene and heteroarene arylation reactions. High catalytic activity, excellent heterogeneity and recyclability were offered by these systems making them promising candidates in the area of heterogeneous C-H functionalization, where efficient catalysts are still scarce.

Reduction of Carbon Dioxide with Ammonia-Borane under Ambient Conditions: Maneuvering a Catalytic Way.

In the development of alternatives to the traditional catalytic hydrogenation of CO2 with gaseous H2, employing nongaseous H2 storage compounds as potential reductants for catalytic transfer hydrogenation of CO2 is promising. Ammonia-borane, due to its high hydrogen storage capacity (19.6 wt %), has been used for catalytic transfer hydrogenation of several organic unsaturated compounds. However, a similar protocol involving catalytic transfer hydrogenation of less reactive CO2 with NH3BH3 is yet to be realized experimentally. Herein, we demonstrate the first catalytic CO2 transfer hydrogenation process for generating formate salt with NH3BH3 under ambient conditions (1 atm and 30 °C) employing a cationic "Ir(III)-abnormal NHC" catalyst via an electrophilic NH3BH3 activation route. It exhibited an initial turnover frequency of 686 h-1 and a high turnover number (TON) of ≈1300 in just 4 h. Most significantly, the catalyst was durable enough to maintain long-term activity, and upon only periodic recharging of NH3BH3, it furnished a total TON of >4200 in 10 h.

3D Metallo-organic coordination assembly-based anion-enriched supramolecular material for fast and efficient removal of Cr2O72.

Chromium(VI), especially dichromate (Cr2O72-) contamination in wastewater due to rapid industrialization with uncontrolled effluent management is still a serious concern which needs focused attention. Multiprong approaches are practiced such as chemical precipitation, reverse osmosis, ion-exchange, adsorption by granular activated carbon etc. to capture and separate this "Group A" human carcinogenic effluent from water. However, low capture capacity, non-reusability, poor selectivity, pH-limited performance are some major limitations of these techniques. Recently, metal organic frameworks (MOFs), metal organic cages (MOCs), porous organic polymers (POPs) or covalent organic frameworks (COFs), covalent organic networks (CONs) etc. emerged as new-generation materials to overcome such limitations. However, the development is still in initial stage and issues related to structural stability and integrity of many MOFs in water and in wide pH range, as well as reusability need to be addressed. At this juncture, herein we report a novel [Zn(terpyridine)2]2+-templated trisimidazolium-based highly cationic three-dimensional metal-organic coordination assembly (3D MOCA), serving as a new class of efficient, fast, robust and recyclable dichromate-removal material. Not only the highly cationic assembly is enriched with a high density of Br- anions, but its three-dimensional propagation and flexibility also exposes the exchangeable Br- ions for facile anion-metathesis with Cr2O72-. By virtue of the benefits of these attributes, the presented supramolecular material exhibits a high capture capacity (469 mg g-1), fast exchange kinetics (0.028 g mg-1 min-1), wide working pH range (pH 2-12) and reusability up to a minimum of 10 cycles without much loss of efficiency. Key mechanistic examinations highlight the evidences in favor of ion-exchange-based chemistry to be responsible for dichromate removal with the present material.

Emerging Implications of the Concept of Hydricity in Energy-Relevant Catalytic Processes.

The "hydricity" of a species refers to its hydride-donor ability. Similar to how the pKa is useful for determining the extent of dissociation of an acid, the hydricity plays a vital role in understanding hydride-transfer reactions. A large number of transition-metal-catalyzed processes involve the hydride-transfer reaction as a key step. Among these, two key reactions-proton reduction to evolve H2 and hydride transfer to CO2 to generate formate/formic acid-represent a promising solution to build a sustainable and fossil-fuel-free energy economy. Therefore, it is imperative to develop an in-depth relationship between the hydricity of transition-metal hydrides and its influencing factors, so that efficient and suitable hydride-transfer catalysts can be designed. Moreover, such profound knowledge can also help in improving existing catalysts, in terms of their efficiency and working mechanism. With this broad aim in mind, some important research has been explored in this area in recent times. This Minireview emphasizes the conceptual approaches developed thus far, to tune and apply the hydricity parameter of transition-metal hydrides for efficient H2 evolution and CO2 reduction/hydrogenation catalysis focusing on the guiding principles for future research in this direction.

Ester Hydrogenation with Bifunctional Metal-NHC Catalysts: Recent Advances.

Hydrogenation of ester to alcohol is an essential reaction in organic chemistry due to its importance in the production of a wide range of bulk and fine chemicals. There are a number of homogeneous and heterogeneous catalyst systems reported in the literature for this useful reaction. Mostly, phosphine-based bifunctional catalysts, owing to their ability to show metal-ligand cooperation during catalytic reactions, are extensively used in these reactions. However, phosphine-based catalysts are difficult to synthesize and are also highly air- and moisture-sensitive, restricting broad applications. In contrast, N-heterocyclic carbenes (NHCs) can be easily synthesized, and their steric and electronic attributes can be fine-tuned easily. In recent times, many phosphine ligands have been replaced by potent σ-donor NHCs, and the resulting bifunctional metal-ligand systems are proven to be very efficient in several important catalytic reactions. This mini-review focuses the recent advances mainly on bifunctional metal-NHC complexes utilized as (pre)catalysts in ester hydrogenation reactions.

Electrocatalytic CO2 Reduction with a Half-Sandwich Cobalt Catalyst: Selectivity towards CO.

We present herein a Cp*Co(III)-half-sandwich catalyst system for electrocatalytic CO2 reduction in aqueous acetonitrile solution. In addition to an electron-donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton-responsive pyridyl-benzimidazole-based N,N-bidentate ligand. Owing to the presence of a relatively electron-rich Co center, the reduced Co(I)-state was made prone to activate the electrophilic carbon center of CO2 . At the same time, the proton-responsive benzimidazole scaffold was susceptible to facilitate proton-transfer during the subsequent reduction of CO2 . The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H+ reduced product HCOOH, in contrast to the only known similar half-sandwich CpCo(III)-based CO2 -reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled-potential electrolysis.

A fast-switching electrochromic device with a surface-confined 3D metallo-organic coordination assembly.

Demonstrated herein is a fast (<1 s)-switching solid-state electrochromic device (t = 0.49 s for coloration and 0.90 s for bleaching), fabricated with a novel imidazolium-linked [Fe(terpyridine)2]2+ chromophore-based surface-confined three dimensional metallo-organic coordination assembly. The device also exhibits promising electrochromic attributes such as high coloration efficiency (η = 275 cm2 C-1), moderate operating voltage (from -2 V to +3.2 V) and transmittance contrast (ΔT = 40%), and high cycling stability (up to 4500 cycles).

Coordination Booster-Catalyst Assembly: Remote Osmium Outperforming Ruthenium in Boosting Catalytic Activity.

Presented herein is a set of bimetallic and trimetallic "coordination booster-catalyst" assemblies in which the coordination complexes [RuII (terpy)2 ] and [OsII (terpy)2 ] acted as boosters for enhancement of the catalytic activity of [RuII (NHC)(para-cymene)]-based catalytic site. The boosters accelerated the oxidative loss of para-cymene from the catalytic site to generate the active catalyst during the oxidation of alkenes and alkynes into corresponding aldehydes, ketones and diketones. It was found that the boosting efficiency of the [OsII (terpy)2 ] units was considerably higher than its congener [RuII (terpy)2 ] unit in these assemblies. Mechanistic studies were conducted to understand this unique improvement.

Orchestrated catalytic double rollover annulation: rapid access to N-enriched cationic and neutral PAHs.

Disclosed herein is a rhodium(iii)-catalyzed novel one-step back-to-back double rollover annulation on pyridine and pyrazine backbones leading to a structurally and optoelectronically diverse class of nicely decorated multi-ring-fused, extensively π-conjugated, N-enriched PAH molecules by virtue of orchestrated quadruple C-H activation events. Selected N-PAHs have been utilized as potential mitochondria and lysosome markers.