Multielectron Redox Afforded by a Pincer Ligand Promoting Kumada Cross-Coupling Reactions.

The redox-active nature of a pincer has been exploited to conduct C-C cross-coupling reactions under mild conditions. A nickel complex with a NNN pincer was dimeric in the solid state, and the structure displayed a Ni2 N2 diamond core. In the dimeric structure, both ligand backbones house an electron, in the iminosemiquinonate form, to keep the metal's oxidation state at +2. In the presence of an aryl Grignard reagent, only 3 mol % loading the nickel complex generates a Kumada cross-coupled product in good yield from a wide variety of aryl-X (X= I, Br, Cl) substrates. That the ligand-based radical remains responsible for promoting such a coupling reaction following a radical pathway is suggested by TEMPO quenching. Furthermore, a radical-clock experiment along with tracing product distribution unambiguously supported the radical's involvement through the catalytic cycle. A series of thorough mechanistic probation, including computational DFT analysis, disclosed the cooperative action of both redox-active pincer ligand and the metal centre to drive the reaction.

Covalent Organic Framework Catalyzed Amide Synthesis Directly from Alcohol Under Red Light Excitation.

The dehydrogenative coupling of alcohols and amines to form amide bonds is typically catalysed by homogeneous transition metal catalysts at high temperatures ranging from 130-140 °C. In our pursuit of an efficient and recyclable photocatalyst capable of conducting this transformation at room temperature, we report herein a COF-mediated dehydrogenative synthesis. The TTT-DHTD COF was strategically designed to incorporate a high density of functional units, specifically dithiophenedione, to trap photogenerated electrons and effectively facilitate hydrogen atom abstraction reactions. The photoactive TTT-DHTD COF, synthesized using solvothermal methods showed high crystallinity and moderate surface area, providing an ideal platform for heterogeneous amide synthesis. Light absorption by the COF across the entire visible range, narrow band gap, and valence band position make it well-suited for the efficient generation of excitons necessary for targeted dehydrogenation. Utilizing red light irradiation and employing extremely low loading of the COF, we have successfully prepared a wide range of amides, including challenging secondary amides, in good to excellent yields. The substrates' functional group tolerance, very mild reaction conditions, and the catalyst's significant recyclability represent substantial advancements over prior methodologies.

Super-Reducing Behavior of Benzo[b]phenothiazine Anion Under Visible-Light Photoredox Condition.

Herein we describe the anion of benzo[b]phenothiazine as a super reductant species upon excitation by visible light. In contrary to N-substituted phenothiazines or benzophenothiazines, this molecule holds extreme reducing power to promote single electron transfer-based reductive cleavage at a potential of -3.51 V vs SCE. As a proof, a plethora of aryl chloride substrates have been reductively cleaved to fabricate molecules of the class isoindolinone and oxindole. Moreover, an aryl-chloride bond has been homolytically cleaved to generate aryl radicals that have been utilized for C-C cross-coupling or C-P bond formation reactions. To prove its extreme reducing ability, some of the aryl fluoride bonds have been cleaved to generate aryl radicals. A detailed photophysical study including steady-state and time-resolved spectroscopic techniques explain the molecule's behavior upon light excitation, and that correlates with its reactivity pattern. Theoretical calculations disclose the benzophenothiazine anion to be slightly puckered at the ground state as the molecule is antiaromatic in nature. In contrast, the excited-state geometry is planar, which is also close to that of the intermediate after one electron transfer. Abating the antiaromaticity of the anionic species is partially responsible for its highly reducing behavior.

A 2-Anilidomethylpyridine Ligand Framework Showcasing Hydride Storage and Transfer Abilities in Its Aluminum Chemistry.

Dearomatized 1,4-dihydropyridyl motifs are significant in both chemistry and biology for their potential abilities to deliver the stored hydride, driven by rearomatization. Biological cofactors like nicotinamide adenine dinucleotide (NADH) and organic 'hydride sources' like Hantzsch esters are prime examples. An organoaluminum chemistry on a 2-anilidomethylpyridine framework is reported, where such hydride storage and transfer abilities are displayed by the ligand's pyridyl unit. The pyridylmethylaniline proligand (NN LH) is simultaneously deprotonated and 1,4-hydroaluminated by AlH3 (NMe2 Et) to [(NN Lde )AlH(NMe2 Et)] (1; NN Lde =hydride-inserted dearomatized version of NN L). A hydride abstraction by B(C6 F5 )3 rearomatizes the pyridyl moiety to give the cationic aluminum hydride [(NN L)AlH(NMe2 Et)][HB(C6 F5 )3 ] (6). Notably, such chemical non-innocence is priorly unseen in this established ligand class. The hydroalumination mechanism is investigated by isolating the intermediate [(NN L)AlH2 ] (2) and by control experiments, and is also analyzed by DFT calculation. The results advocate an intriguing 'self-promoting' pathway, which underlines alane's Lewis acid/Brønsted base duality. NMe2 Et carrying the alane also plays a crucial role. In contrast, the chemistry between NN LH and AlMe3 is much different, giving only [(NN L)AlMe2 ] (4) from the adduct [(NN LH)AlMe3 ] (3) by deprotonation but not a subsequent pyridyl dearomatization in the presence or absence of NMe2 Et. This divergence is also justified by DFT analyses.

DFT-Guided Mechanistic Insights into Chemodivergence: A Mixed Explicit-Implicit Solvent Description to Dictate the Chemoselectivity.

Herein we report a density functional theory (DFT)-guided mechanistic investigation of the nitrile reduction reaction, which exhibits a solvent-dependent chemodivergence. This study reveals an interesting mechanistic picture, highlighting the exact role of a protic solvent, isopropanol, in regulating the reaction outcome. The explicit solvent effect involving polar protic isopropanol favors imine metathesis by proton hopping through stepwise addition and elimination steps and thus produces a secondary amine as the final product. In contrast, the nonpolar solvent n-hexane is incapable of facilitating the proton migration and stops the solvent-assisted imine metathesis. As a result, only primary amines are obtained as the final product. This DFT study provides a recipe for the choice of solvents that can dictate chemoselectivity in product formation.

Pyrazole-Mediated C-H Functionalization of Arene and Heteroarenes for Aryl-(Hetero)aryl Cross-Coupling Reactions.

Herein we introduce a transition-metal-free protocol that involves a commercially available, inexpensive pyrazole molecule to conduct C-C cross-coupling reactions at room temperature via a radical pathway. Using this method, an aryldiazonium salt has been coupled to a wide range of arenes and heteroarenes including benzene, mesitylene, thiophene, furan, benzoxazole to result the corresponding biaryl products. The full reaction mechanism is elucidated along with the crystallographic probation of an active initiator species. A potassium-stabilized deprotonated pyrazole steers single-electron transfer to the substrate and behaves as an initiator for the reaction.

A Bottleable Imidazole-Based Radical as a Single Electron Transfer Reagent.

Reduction of 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1H-imidazol-3-ium chloride (1) resulted in the formation of the first structurally characterized imidazole-based radical 2. 2 was established as a single electron transfer reagent by treating it with an acceptor molecule tetracyanoethylene. Moreover, radical 2 was utilized as an organic electron donor in a number of organic transformations such as in activation of an aryl-halide bond, alkene hydrosilylation, and in catalytic reduction of CO2 to methoxyborane, all under ambient temperature and pressure.

Aggregation-Induced Enhanced Emission-Active Zinc(II) ß-Diketiminate Complexes Enabling High-Performance Solution-Processable OLEDs.

Earth-abundant and cheaper zinc-based organometallic molecules as luminophores are drawing significant research attention for solid-state lighting devices. In this paper, we report two air-stable zinc complexes, where the zinc is coordinated to two sterically encumbered ß-diketiminate ligands in a tetrahedral geometry. In such a geometry, eight phenyl/aryl rings from the ligand backbones are oriented in a propeller shape, augmenting the restricted rotation of the putative rings. Such an architecture harnesses aggregation-induced emission behavior with an excellent solid-state emission property. The rigidity of these molecules reduces the possibility of non-radiative transitions and makes them excellent fluorescence emitters. Both molecules exhibit electroluminescence (EL) in the yellowish-green region of the visible spectrum. We have utilized these molecules as emitters to fabricate multilayered organic light-emitting diode (OLED) devices. The emitter Zn-I in host m-MTDATA exhibits EL with a maximum external quantum efficiency of 4.4%. Among the handful of zinc-based OLEDs, the performance of this emitter is very commendable with power and current efficacies of 15.2 lm W-1 and 12.1 cd A-1, respectively, along with a brightness of 2426 cd m-2.

Homogeneous Nickel-Catalyzed Sustainable Synthesis of Quinoline and Quinoxaline under Aerobic Conditions.

Dehydrogenative coupling-based reactions have emerged as an efficient route toward the synthesis of a plethora of heterocyclic rings. Herein, we report an efficacious, nickel-catalyzed synthesis of two important heterocycles such as quinoline and quinoxaline. The catalyst is molecularly defined, is phosphine-free, and can operate at a mild reaction temperature of 80 °C. Both the heterocycles can be easily assembled via double dehydrogenative coupling, starting from 2-aminobenzyl alcohol/1-phenylethanol and diamine/diol, respectively, in a shorter span of reaction time. This environmentally benign synthetic protocol employing an inexpensive catalyst can rival many other transition-metal systems that have been developed for the fabrication of two putative heterocycles. Mechanistically, the dehydrogenation of secondary alcohol follows clean pseudo-first-order kinetics and exhibits a sizable kinetic isotope effect. Intriguingly, this catalyst provides an example of storing the trapped hydrogen in the ligand backbone, avoiding metal-hydride formation. Easy regeneration of the oxidized form of the catalyst under aerobic/O2 oxidation makes this protocol eco-friendly and easy to handle.

Highly Active Carbene Potassium Complexes for the Ring-Opening Polymerization of ε-Caprolactone.

Herein we report the synthesis of two complexes of potassium employing strongly nucleophilic carbenes, such as cyclic "(alkyl)(amino)carbene (cAAC) and abnormal N-heterocyclic carbene (aNHC). Both complexes are dimeric in the solid state and the two potassium centers are bridged by trimethylsilylamide. In these complexes, the carbene- - -K interaction is predominantly electrostatic in character, which has been probed thoroughly by NBO and AIM analyses. Indeed, the delocalization energy of the cAAC lone pair calculated from the second-order perturbation theory was only 5.21 kcal mol-1, supporting a very weak interaction. The solution-state behavior of these molecules, as inferred from NOESY spectra, hints that the weak carbene- - -K interaction is retained in nonpolar solvents, and the bond is not dissociated at least on the NMR time scale. We took advantage of such a weak interaction to develop highly effective ring-opening polymerization catalysts for ε-caprolactone and rac-lactide. The efficacy of these catalysts is prominent from a very high substrate/metal-initiator ratio as well as very low dispersity index of the obtained polymer chains, reflecting significant control over polymerization.

H2S Reactivity on Oxygen-Deficient Heterotrimetallic Cores: Cluster Fluxionality Simulates Dynamic Aspects of Surface Chemical Reactions.

Understanding the mechanistic aspects of heterogeneous reactions on supported metal catalysts is challenging due to several disparate factors, among which the dynamic nature of the surface is a major contributor. In this study, the dynamic aspect of a surface has been probed by choosing small metal clusters as illustrative models. Two anionic hetero-trimetallic clusters, namely, W2TcO6(-) and W2OsO6(-), were reacted with H2S gas to exhibit splitting of the gas molecule and complete oxygen-sulfur exchange in the metal core. During this atom-exchange process, the core exhibits remarkable fluxionality to augment a thiol proton migration from one metal center to another, as well as a rapid interchange of the terminal and bridging oxygens. The fluxional nature of the core is further evidenced by two oppositely oriented oxo groups working in concert to accomplish the proton transfer, upon introduction of sulfur inside the core. These fluxional processes in the small hetero-trimetallic cores closely resemble the dynamic nature of the surface in a heterogeneous reaction. Throughout the fluxional processes investigated in this study, two-state reactivity and multiple instances of spin crossover are observed in our computational studies. Interestingly, the neutral hetero-trimetallic cores can also undergo complete oxygen-sulfur exchange reaction with H2S. The investigated metal clusters are promising materials, since they not only can liberate dihydrogen from water (reported in J. Phys. Chem. A, 2014, 118, 11047) but also can completely strip the sulfur from environmentally hazardous H2S gas.

Hydroxyl migration in heterotrimetallic clusters: an assessment of fluxionality pathways.

Water splitting at the unsaturated metal center and subsequent hydroxyl migration are key steps toward successful H2 liberation from cheap and abundant water using transition metal cluster anions. In this report we initiate a theoretical study (DFT) to assess the efficacy of heterometallic cores instead of the widely studied and well established homometallic cores. To accomplish this goal, one tungsten center in W3O6(-) core has been replaced by different transition metals such as titanium, technetium, and osmium. Introduction of the heterometal makes the core asymmetric and electronically anisotropic. To evaluate the efficiency of these heterometallic cores, fluxionality pathways for hydroxyl migration have been studied in detail. We show that the cores W2TcO6(-) (2) and W2OsO6(-) (3) can exhibit fluxionality for hydroxyl migration and thus can potentially facilitate H2 liberation from H2O. Notably, a new class of low-energy structures generated upon oxide bridge opening process and subsequent structural rearrangement facilitates the hydroxyl migration event. To illustrate the heterometallic effect further, we show that previously inaccessible energy barriers for hydroxyl migration in a homometallic trimolybdenum core become energetically achievable when one of the metals is replaced by a 5d element osmium.

Photochemical pincer-catalyzed reductive cyclisation towards indolines and oxindoles.

An organophotocatalytic method has been described towards the synthesis of indolines and oxindoles starting from unusual α-chloro amide and N-(2-chlorophenyl)-N-alkyl methacrylamide substrates. This marks a notable improvement since the earlier syntheses utilized iridium and gold catalysts, and involved C-I or C-Br bond cleavage as the initial step. Our photocatalyst is a pincer ligand that can be easily deprotonated to make a very strong reducing agent. The reductive cleavage of the carbon-chloride bond, and subsequent 5-exo-trig ring cyclization, followed by hydrogen atom abstraction, prepare the desired heterocycles under very mild reaction conditions. An atom economic use of KOtBu has been shown to demonstrate the unusual trifunctional role of the latter.

Photocatalytic α-arylation of cyclic ketones by a thermally activated delayed fluorescence molecule.

α-Arylation of cyclic ketones via an organophotocatalytic route has been described utilizing PXZ-TRZ, a molecule displaying thermally activated delayed fluorescence (TADF). Using this route, a plethora of cyclic ketones including cyclohexanone, cyclopentanone and even cyclooctanone can be effectively arylated with many aryl iodides or bromides under mild conditions.

Catalytic acceptorless dehydrogenative borylation of styrenes enabled by a molecularly defined manganese complex.

In this study, we employed a 3d metal complex as a catalyst to synthesize alkenyl boronate esters through the dehydrogenative coupling of styrenes and pinacolborane. The process generates hydrogen gas as the sole byproduct without requiring an acceptor, rendering it environmentally friendly and atom-efficient. This methodology demonstrated exceptional selectivity for dehydrogenative borylation over direct hydroboration. Additionally, it exhibited a preference for borylating aromatic alkenes over aliphatic ones. Notably, derivatives of natural products and bioactive molecules successfully underwent diversification using this approach. The alkenyl boronate esters served as precursors for the synthesis of various pharmaceuticals and potential anticancer agents. Our research involved comprehensive experimental and computational studies to elucidate the reaction pathway, highlighting the B-H bond cleavage as the rate-determining step. The catalyst's success was attributed to the hemilability and metal-ligand bifunctionality of the ligand backbone.

Unexplored Facet of Pincer Ligands: Super-Reductant Behavior Applied to Transition-Metal-Free Catalysis.

Pincer ligands are well-established supporting ancillaries to afford robust coordination to metals across the periodic table. Despite their widespread use in developing homogeneous catalysts, the redox noninnocence of the ligand backbone is less utilized in steering catalytic transformations. This report showcases a trianionic, symmetric NNN-pincer to drive C-C cross-coupling reactions and heterocycle formation via C-H functionalization, without any coordination to transition metals. The starting substrates are aryl chlorides that can tease the limit of a catalyst's ability to promote a reductive cleavage at a much demanding potential of -2.90 V vs SCE. The reducing power of the simple trianionic ligand backbone has been tremendously amplified by shining visible light on it. The catalyst's success relies on its easy access to the one-electron oxidized iminosemiquinonate form that has been thoroughly characterized by X-band electron paramagnetic resonance spectroscopy through spectroelectrochemical experiments. The moderately long-lived excited-state lifetime (10.2 ns) and such a super-reductive ability dependent on the one-electron redox shuttle between the bisamido and iminosemiquinonato forms make this catalysis effective.

A Donor-Acceptor Cyclopropane by Intramolecular C(sp3)-H Activation at a Cyclic(alkyl)(amino)carbene Center and Its Fascinating Ring-Opening Chemistry.

Virtually irreversible intramolecular C-H activations are deleterious for aza-carbenes. A picolyl-tethered cyclic(alkyl)(amino)carbene (CAAC) isomerizes into a donor-acceptor cyclopropane in this manner but restores the CAAC status by retro-C-H activation in the presence of trapping agents like Se or CuCl. The same DA cyclopropane is readily hydrolyzed to a pyrrolidin-2-ol that acts as another picoCAAC precursor by undergoing 1,1-dehydration in the presence of Se or CuCl. The chemistry is distinct from the N-heterocyclic carbene analogue throughout.

A mononuclear Fe(III) single molecule magnet with a 3/2↔5/2 spin crossover.

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.

Redox noninnocence of the formazanate ligand applied to catalytic formation of α-ketoamides.

The formazan ligands have been investigated as redox-noninnocent backbones for a long time. Despite their well-established behaviour as redox reservoirs, the demonstration of catalytic efficiency governed by redox noninnocence remains elusive. We report an iron-formazanate molecule for efficiently preparing α-keto amides, where a crucial reductive cleavage of the substrate molecule is tightly regulated by the electron donation from the formazanate, in a reversible manner.

Bimodal photocatalytic behaviour of a zinc ß-diketiminate: application to trifluoromethylation reactions.

A photoactive zinc ß-diketiminate complex spans a wide redox window of 3.97 V at its excited state. Having a highly reducing excited-state potential, it generates an electrophilic trifluoromethyl radical by the reductive cleavage of triflyl chloride. This leads to trifluoromethylation of a set of arenes and heteroarenes. During the oxidative quenching of the photocatalyst, a ligand-centered radical cation is formed, which has been detected by spectroelectrochemical EPR measurement.