Photocatalytic decarboxylation of free carboxylic acids and their functionalization.

Visible light mediated decarboxylative functionalization of carboxylic acids and their derivatives has recently emerged as a novel and powerful toolkit for small molecule activation in diverse carbon-carbon and carbon-hetero bond forming reactions. Naturally abundant highly functionalized bench-stable carboxylic acid analogs have been employed as promising alternatives to non-trivial organometallic reagents for mild and eco-benign synthetic transformation with traceless CO2 by-products. In this highlight article, we focus on the development of various photodecarboxylative functionalization strategies along with intra/inter-molecular cyclization via concerted single electron transfer (SET) or energy transfer (ET) pathways. Moreover, widely explored carboxylic acids are systematically classified here into four categories; i.e., α-keto, aliphatic, α,ß-unsaturated, and aromatic analogs for a concise overview to the readership. The association of decarboxylative radical species with coupling partners to construct C-C and C-N/O/S/P/X bonds for each analogous acid has been presented in brief.

Naphthalene Diimide and Bis-heteroleptic Ru(II) Complex-Based Hybrid Molecule with 3-in-1 Functionalities.

Multipurpose applications of a newly developed homobimetallic Ru(II) complex, Ru-NDI[PF6]4, which incorporates 1,10-phenanthroline and triazole-pyridine ligands and linked via a (-CH2-)3 spacer to the reputed anion-π interacting NDI system, are described. Solution-state studies of the bimetallic complex, including EPR, PL, UV-vis, and NMR experiments, reveal two sequential one-electron transfers to the NDI unit, generating NDI·- and NDI2- in the presence of F- selectively. This process inhibits the primary electron transfer from Ru(II) to the NDI unit, thereby allowing the 3MLCT-based emission of the complex to be recovered, resulting in a corresponding ten-fold increase in luminescence intensity. DFT and TD-DFT computational studies further elucidate the experimentally observed absorption spectra of the complex. Secondly, CT-DNA binding studies with the complex are performed using various spectroscopic analyses such as UV-vis, PL, and CD. Comparative DNA binding studies employing EB and molecular docking reveal that the binding with CT-DNA occurs through both intercalative and groove binding modalities. Thirdly, the photocatalytic activities of the complex towards C-C, C-N, and C-O bond formation in organic cross-coupling reactions, including the amidation of α-keto acids to amines and the oxidation of alcohol to aldehydes, are also demonstrated.

A macrocycle-based new organometallic nano-vessel towards sustainable C2-selective arylation of free indole in water.

C2-selectivity of unsubstituted indole over facile C3-substitution is attempted by utilizing the π-cavity of a nano-vessel made up of a palladium complex of an amino-ether heteroditopic macrocycle. Functional group tolerance (cyano, nitro, halo, ester, etc.), a broad substrate scope and outstanding selectivities with excellent yields (80-93%) of the desired products have been achieved in 12 h by maintaining all sustainable conditions like aqueous medium, recyclable catalyst, one-pot reaction, no external additives, mild temperature, etc. Interestingly, we observed that electron-deficient indole derivatives underwent the present transformation with marginally superior reactivity in comparison with electron-rich indole derivatives. This approach establishes a green pathway for selective C-C coupling employing a π-cavitand as a nano-reactor.

Chalcogen Bonding in Selective Recognition and Liquid-Liquid Extraction of Perrhenate.

Efficient recognition and extraction of hazardous anionic pollutants from water medium is of great significance for environmental concerns, representing a challenging area of research in supramolecular chemistry. In this study, we present, for the first time, a comprehensive demonstration of the ability of chalcogen bonding (ChB) to recognize and remove the ReO4- from 100% water medium. The anion recognition ability is well elucidated through solution phase NMR and ITC studies, which clearly reveal the selective binding of ReO4- over other oxo-anions. Moreover, the selenoimidazolium scaffold effectively engages in Se•••O ChB interaction with ReO4- as confirmed by X-ray crystal structure and XPS analysis. More importantly, the binding of ReO4- with different prolongations of the σ-holes, along with Se•••Se chalcogen bonding interactions, lead to the formation of a 1D supramolecular assembly. Eventually, ChB receptor Se4Me-Br exhibits ~62% ReO4- extraction efficiency through precipitation as the extraction method. Furthermore, in efforts to enhance efficiency, a hydrophobic ChB receptor Se4Do-PF6 has been prepared, achieving an efficiency of up to ~93% at a very low concentration (~5ppm) by liquid-liquid extraction.

Supramolecular chemistry of liquid-liquid extraction.

Liquid-Liquid Extraction (LLE) is a venerable and widely used method for the separation of a targeted solute between two immiscible liquids. In recent years, this method has gained popularity in the supramolecular chemistry community due to the development of various types of synthetic receptors that effectively and selectively bind specific guests in an aqueous medium through different supramolecular interactions. This has eventually led to the development of state-of-the-art extraction technologies for the removal and purification of anions, cations, ion pairs, and small molecules from one liquid phase to another liquid phase, which is an industrially viable method. The focus of this perspective is to furnish a vivid picture of the current understanding of supramolecular interaction-based LLE chemistry. This will not only help to improve separation technology in the chemical, mining, nuclear waste treatment, and medicinal chemistry sectors but is also useful to address the purity issue of the extractable species, which is otherwise difficult. Thus, up-to-date knowledge on this subject will eventually provide opportunities to develop large-scale waste remediation processes and metallurgy applications that can address important real-life problems.

Selective recognition and extraction of iodide from pure water by a tripodal selenoimidazol(ium)-based chalcogen bonding receptor.

A selenium-based tripodal chalcogen bond (ChB) donor TPI-3Se is demonstrated for the recognition and extraction of I- from 100% water medium. NMR and ITC studies with the halides reveal that the ChB donor selectively binds with the large, weakly hydrated I-. Interestingly, I- crystallizes out selectively in the presence of other halides supporting the superiority of the selective recognition of I-. The X-ray structure of the ChB-iodide complex manifests both the µ1 and µ2 coordinated interactions, which is rare in the C-Se···I chalcogen bonding. Furthermore, to validate the selective I- binding potency of TPI-3Se in pure water, comparisons are made with its hydrogen and halogen bond donor analogs. The computational analysis also provides the mode of I- recognition by TPI-3Se. Importantly, this receptor is capable of extracting I- from pure water through selenium sigma-hole and I- interaction with a high degree of efficiency (∼70%).

Low Potential CO2 Reduction by Inert Fe(II)-Macrobicyclic Complex: A New Concept of Cavity Assisted CO2 Activation.

The advantage of a pre-organized π-cavity of Fe(II) complex of a newly developed macrobicycle cryptand is explored for CO2 reduction by overcoming the problem of high overpotential associated with the inert nature of the cryptate. Thus, a bipyridine-centered tritopic macrobicycle having a molecular π-cavity capable of forming Fe(II) complex as well as potential for CO2 encapsulation is synthesized. The inert Fe(II)-cryptate shows much lower potential in cyclic voltammetry than the Fe(II)-tris-dimethylbipyridine (Fe-MBP) core. Interestingly, this cryptate shows electrochemical CO2 reduction at a considerably lower potential than the Fe-MBP inert core. Therefore, this study represents that a well-structured π-cavity may generate a new series of molecular catalysts for the CO2 reduction reaction (CO2 RR), even with the inert metal complexes.

Merging Photocatalytic Doubly-Decarboxylative Csp 2 -Csp 2 Cross-Coupling for Stereo-Selective (E)-α,ß-Unsaturated Ketones Synthesis.

A photocatalytic domain of doubly decarboxylative Csp 2 -Csp 2 cross coupling reaction is disclosed. Merging iridium and palladium photocatalysis manifested carbon-carbon bonds in a tandem dual-radical pathway. Present catalytic platform efficiently cross-coupled α, ß-unsaturated acids and α-keto acids to afford a variety of α, ß-unsaturated ketones with excellent (E)-selectivity and functional group tolerance. Mechanistically, photocatalyst implicated through reductive quenching cycle whereas cross coupling proceeded over one electron oxidative pallado-cycle.

From construction to application of a new generation of interlocked molecules composed of heteroditopic wheels.

Over the last few decades, research on mechanically interlocked molecules has significantly evolved owing to their unique structural features and interesting properties. A substantial percentage of the reported works have focused on the synthetic strategies, leading to the preparation of functional MIMs for their applications in the chemical, materials, and biomedical sciences. Importantly, various macrocyclic wheels with specific heteroditopicity (including phenanthroline, amide, amine, oxy-ether, isophthalamide, calixarene and triazole) and threading axles (bipyridine, phenanthroline, pyridinium, triazolium, etc.) have been designed to synthesize targeted multifunctional mononuclear/multinuclear pseudorotaxanes, rotaxanes and catenanes. The structural uniqueness of these interlocked systems is advantageous owing to the presence of mechanical bonds with specific three-dimensional cavities. Furthermore, their multi-functionalities and preorganised structural entities exhibit a high potential for versatile applications, like switching, shuttling, dynamic properties, recognition and sensing. In this feature article, we describe some of the most recent advances in the construction and chemical behaviour of a new generation of interlocked molecules, primarily focusing on heteroditopic wheels and their applications in different directions of the modern research area. Furthermore, we outline the future prospects and significant perspectives of the new generation heteroditopic wheel based interlocked molecules in different emerging areas of science.

Base-Promoted Tandem Pathway for Keto-Amides: Visible Light-Mediated Room-Temperature Amidation Using Molecular Oxygen as an Oxidant.

Herein, we report metal- and photocatalyst-free room-temperature amidation for α-ketoamide synthesis from feedstock phenacyl bromides and amines using molecular oxygen as an oxidant as well as a source of oxygen in the amide segment. Visible light-mediated base-promoted one-pot sequential C-N/C═N/C═O bond formation takes place in a tandem manner to afford the desired product. Functional group tolerance (benzylic alcohol, keto, cyano, nitro, halo, etc.), a broad substrate scope, and gram-scale synthesis make this synthetic methodology more attractive. We have observed that electron-rich aromatic amines, aliphatic amines, and phenacyl bromide derivatives proceeded the present transformation with marginally superior reactivity in comparison to electron-deficient aromatic amines and phenacyl bromide derivatives. Moreover, several control experiments, in situ isolation of secondary amine and imine as key intermediates, and 18O-labeling experiments provide complete insight into the mechanism of the tandem pathway.

Pd-Catalyzed Tandem Pathway for Stereoselective Synthesis of (E)-1,3-Enyne from ß-Nitroalkenes by Using a Sacrificial Directing Group.

The involvement of nitroalkenes instead of minimal one alkyne motif for (E)-1,3-enynes synthesis through a palladium catalyzed stereoselective bond forming pathway at room temperature is presented. Implication of nitro group as a sacrificial directing group, formation of magical alkyne on a newly developed Csp 3 -Csp 3 bond with initial palladium-MBH adduct make this methodology distinctive. This protocol features an unprecedented sequential acetate addition, carbon-carbon bond formation, isomerization of double bond and nitromethane degradation in a tandem catalytic walk via dancing hybridization. Mechanistic understanding through identification of intermediates and computational calculations furnishes complete insight into the tandem catalytic pathway. Broad substrates scope and functional groups tolerance make this synthetic methodology magnificent and dynamic. This represents the first example of stereoselective 1,3-enyne synthesis exclusively from alkene substrates by introducing the concept of sacrificial directing group.

Development and Application of Ruthenium(II) and Iridium(III) Based Complexes for Anion Sensing.

Improvements in the design of receptors for the detection and quantification of anions are desirable and ongoing in the field of anion chemistry, and remarkable progress has been made in this direction. In this regard, the development of luminescent chemosensors for sensing anions is an imperative and demanding sub-area in supramolecular chemistry. This decade, in particular, witnessed advancements in chemosensors based on ruthenium and iridium complexes for anion sensing by virtue of their modular synthesis and rich chemical and photophysical properties, such as visible excitation wavelength, high quantum efficiency, high luminescence intensity, long lifetimes of phosphorescence, and large Stokes shifts, etc. Thus, this review aims to summarize the recent advances in the development of ruthenium(II) and iridium(III)-based complexes for their application as luminescent chemosensors for anion sensing. In addition, the focus was devoted to designing aspects of polypyridyl complexes of these two transition metals with different recognition motifs, which upon interacting with different inorganic anions, produces desirable quantifiable outputs.

Merging Photocatalytic C-O Cross-Coupling for α-Oxycarbonyl-ß-ketones: Esterification of Carboxylic Acids via a Decarboxylative Pathway.

Herein, the first merged photocatalytic pathway for the C-O cross-coupled esterification of carboxylic acids to α-oxycarbonyl-ß-ketones has been demonstrated. Decarboxylation of α,ß-unsaturated acids promotes the formation of the ß-ketone fragment of the desired product. Water as the source of oxygen for the ketone segment and aerial oxygen as an oxidant make the present synthetic methodology green and sustainable. This new C═O and C-O bond-forming methodology takes place in a cascade manner under a dual Ir/Pd-catalytic pathway, with the liberation of H2O and CO2 as the only byproducts.

Superiority of the Supramolecular Halogen Bond Receptor over Its H-Bond Analogue toward the Efficient Extraction of Perrhenate from Water.

A halogen bond-based water-soluble tetrapodal iodoimidazolium receptor, (L-I)(4Br), exhibited a high degree of efficiency (∼96%) in extracting ReO4- from 100% aqueous medium within a wide range of concentrations and of pH values along with excellent reusability. The solid-state X-ray diffraction study showed the trapping of ReO4- by (L-I)(4Br) via the Re-O····I halogen bonding interaction. XPS studies also suggested the interaction between I and ReO4- through polarization of the electron density of I atoms by ReO4-. (L-I)(4Br) is found to be capable of retaining its high extraction efficiency in the presence of competing anions such as F-, Cl-, I-, SO42-, H2PO4-, CO32-, NO3-, BF4-, ClO4-, Cr2O72-, and a mixture of these anions. Interestingly, (L-I)(4Br) was found to be superior in ReO4- extraction as compared to its hydrogen-bond donor analogue, (L-H)(4Br), as confirmed by a series of control experiments and theoretical calculations. Our synthesized dipodal and tripodal halogen bond donor receptors and their H-analogues validated the superiority of these classes of supramolecular halogen bond donor receptors over their hydrogen-bond analogues. (L-I)(4Br) also showed superior practical applicability in terms of the removal of ReO4- at anion concentrations as low as ∼100 ppm, which was a major shortcoming of (L-H)(4Br).

Development of fluorophoric [2]pseudorotaxanes and [2]rotaxane: selective sensing of Zn(II).

Fluorophoric [2]pseudorotaxanes {NiPR1(ClO4)2-NiPR3(ClO4)2} are synthesized by utilizing newly designed fluorophoric bidentate ligands (L1-L3) and a heteroditopic naphthalene containing macrocycle (NaphMC) with high yields via Ni(II) templation and π-π stacking interactions. Subsequently, a fluorophoric [2]rotaxane (NAPRTX) is established through a Cu(I) catalysed click reaction between an azide terminated pseudorotaxane, {NiPR4(ClO4)2}, which contains the newly designed fluorophoric ligand L4, and alkyne terminated bulky stopper units. All these fluorophoric [2]pseudorotaxanes and the [2]rotaxane were characterized using numerous techniques such as mass spectrometry, NMR, UV/Vis, PL, and elemental analysis, wherever applicable. Furthermore, to investigate the effect of the fluorophoric moieties, the coordinating ability of chelating units, and size and shape of the three dimensional cavity generated by the mechanical bond in the interlocked [2]rotaxane (NAPRTX), we have performed a sensing study of various metal ions. Thus, the interlocked [2]rotaxane is found to have potential as a selective fluorescent sensor for Zn(II) metal ions over other transition, alkali and alkaline earth metal ions, where the 2,2'-bipyridyl arylvinylene moiety of the axle acts as a fluorescence signalling unit.

NDI-integrated rotaxane/catenane and their interactions with anions.

Complexation of alkali and alkaline earth metal ions with the heteroditopic Phen-ester oxy-ether macrocyclic wheel (PhenMC) is established for the synthesis of interlocked molecular systems. The single crystal X-ray structure of Na-bound PhenMC confirms the hexacoordinated geometry around the Na ion in the macrocycle. Further, Ca-ion-bound PhenMC (Ca-PhenMC) is explored with a fluorophoric azide-terminated NDI (naphthalene diimide) axle (NDIAz) for the synthesis of fluorophoric [2]rotaxane (NDIROT) and [2]catenane (NDICAT) via Cu(I)-catalyzed cycloaddition reaction. Characterizations of these two new interlocked molecular systems are performed by ESI-MS, NMR, UV-vis and PL spectroscopic studies wherever applicable. Moreover, the new molecular systems are explored towards anion sensing applications via colorimetric, UV-vis-NIR, PL and other spectroscopic studies.

Room-Temperature Synthesis of 1,3,5-Tri(het)aryl Benzene from Nitroalkenes Using Pd(OAc)2: Complete Mechanistic and Theoretical Studies.

Herein, a room-temperature catalytic pathway for 1,3,5-tri(het)aryl derivatives from nitroalkenes using simple Pd(OAc)2 is presented. This newly developed C-C bond-forming methodology takes place in a cascade manner with the initial pallado-Morita-Baylis-Hillman (MBH) type adduct. The broad substrate scopes, functional group tolerance, and different aryl-substituted benzene derivatives make this methodology more attractive. Furthermore, the mechanistic understanding through isolation of intermediates and DFT studies of the catalytic cycle provide requisite insight into the methodology.

A Bis-heteroleptic Imidazolium-bipyridine Functionalized Iridium(III) Complex for Fluorescence Lifetime-based Recognition and Sensing of Phosphates.

A new IrIII based 3,3'-([2,2'-bipyridine]-5,5'-diylbis(methylene))bis(1-ethyl-1H-imidazol-3-ium) functionalized receptor Ir-1 has been synthesized for selective recognition, sensing and as a lifetime based sensor for H2 PO4 - and HP2 O7 3- in acetonitrile. An increase in the lifetime (τ) from 0.03543 µs to 0.2736 µs and 0.1323 µs in the presence of H2 PO4 - and HP2 O7 3- , respectively, among all other competitive anions establishes Ir-1 as a simple lifetime-based sensor. Furthermore, 13.7- and 8.5-fold enhancement in PL intensities of Ir-1 along with blue-shifting is seen with H2 PO4 - and HP2 O7 3- , respectively. High selectivity of Ir-1 for these two ions even in the presence of a large excess of other anions also displayed sensitive detection (LOD=0.035 µM for HP2 O7 3- and 0.040 µM for H2 PO4 - ). NMR data further suggest that the recognition of phosphates by Ir-1 is occurring through C-H⋅⋅⋅phosphate hydrogen bond (HB) interaction.

Influence of Triazole Substituents of Bis-Heteroleptic Ru(II) Probes toward Selective Sensing of Dihydrogen Phosphate.

A series of seven new bis-heteroleptic Ru(II) probes (1[PF6]2-7[PF6]2) along with two previously reported probes (8[PF6]2 and 9[PF6]2) containing a similar anion binding triazole unit (hydrogen bond donor) functionalized with various substituents are employed in a detailed comparative investigation for the development of superior selective probes for H2PO4-. Various solution- and solid-state studies, such as 1H-DOSY NMR, dynamic light scattering (DLS), single-crystal X-ray crystallography, and transmission electron microscopy (TEM), have established that the selective sensing of H2PO4- by this series of probes is primarily due to supramolecular aggregation driven enhancement of 3MLCT emission. Intestingly, 1[PF6]2 and 7[PF6]2, having an electron-deficient (π-acidic) aromatic pentafluorophenyl substituent are found to be superior probes for H2PO4- in comparison to the other aryl- and polyaromatic-substituted analogues (2[PF6]2-6[PF6]2, 8[PF6]2, and 9[PF6]2), in terms of a higher enhancement of the 3MLCT emission band, a greater binding constant, and a lower detection limit. The superiority of 1[PF6]2 and 7[PF6]2 could be due to better supramolecular aggregation properties in the cases of pentafluorophenyl analogues via both hydrogen bonding and anion-fluorine/anion-π noncovalent interactions.

Superiority of a polymeric scavenger over its hexapodal monomer towards efficient ReO4- removal in water.

A new imidazolium functionalized hexapodal polymeric receptor, [PHIm-Br], showed selective and efficient removal (>99%) of perrhenate (ReO4-), from 100% aqueous medium via solid-liquid extraction, which was 13% higher as compared to its monomeric analouge [HIm-Br]. Most importantly, [PHIm-Br] overcomes the drawback of [HIm-Br] in terms of removal of ReO4- at lower anion concentration of ∼100 ppm along with excellent radiation resistivity and reusability within a wide pH range, which implies its potential towards practical applications.