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.

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 (~5 ppm) by liquid-liquid extraction.

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.

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.

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.

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.

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.

One-Pot Dual C-C Coupling Reaction via Site Selective Cascade Formation by PdII -Cryptate of an Amino-Ether Heteroditopic Macrobicycle.

Selectivity of aryl iodo over ethynyl iodo toward the Suzuki cross coupling reaction is explored by utilizing a palladium complex of amino-ether heteroditopic macrobicycle. Subsequently, unreacted ethynyl iodide undergoes homocoupling reaction in the same catalytic atmosphere, thereby representing a cascade dual C-C coupling reaction. Furthermore, this approach is extended for novel one-pot synthesis of unsymmetrical 1,3-diynes.

Heteroditopic Macrobicyclic Molecular Vessels for Single Step Aerial Oxidative Transformation of Primary Alcohol Appended Cross Azobenzenes.

A series of oxy-ether tris-amino heteroditopic macrobicycles (L1-L4) with various cavity dimensions have been synthesized and explored for their Cu(II) catalyzed selective single step aerial oxidative cross-coupling of primary alcohol based anilines with several aromatic amines toward the formation of primary alcohol appended cross azobenzenes (POCABs). The beauty of this transformation is that the easily oxidizable benzyl/primary alcohol group remains unhampered during the course of this oxidation due to the protective oxy-ether pocket of this series of macrobicyclic vessels. Various dimensionalities of the molecular vessels have shown specific size complementary selection for substrates toward efficient syntheses of regioselective POCAB products. To establish the requirement of the three-dimensional cavity based additives, a particular catalytic reaction has been examined in the presence of macrobicycles (L2 and L3) versus macrocycles (MC1 and MC2) and tripodal acyclic (AC1 and AC2) analogous components, respectively. Subsequently, L1-L4 have been extensively utilized toward the syntheses of as many as 44 POCABs and are characterized by different spectroscopic techniques and single crystal X-ray diffraction studies.

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.

Discriminative Behavior of a Donor-Acceptor-Donor Triad toward Cyanide and Fluoride: Insights into the Mechanism of Naphthalene Diimide Reduction by Cyanide and Fluoride.

A new molecular donor-acceptor-donor (D-A-D) triad, comprised of an electron deficient 1,4,5,8-naphthalene tetracarboxylic diimide (NDI) unit covalently connected to two flanking photosensitizers, i.e., a bis-heteroleptic Ru(II) complex of 1,10-phenanthroline and pyridine triazole hybrid ligand, is described. The single crystal X-ray structure of the perchlorate salt of the triad demonstrates that the electron deficient NDI unit can act as a host for anions via anion-π interaction. Detailed solution-state studies indicate that fluoride selectively interacts with the D-A-D triad to form a dianionic NDI, NDI2-, via a radical anion, NDI•-. On the contrary, cyanide reduces the NDI moiety to NDI•-, as confirmed by UV-vis, NMR, and EPR spectroscopy. Further, femtosecond transient absorption spectroscopic studies reveal a low luminescence quantum yield of the D-A-D triad attributable to the photoinduced electron transfer (PET) process from the photoactive Ru(II) center to the NDI unit. Interestingly, the triad displays "OFF-ON" luminescence behavior in the presence of fluoride by restoring the Ru(II) to phenanthroline/pyridine-triazole-based MLCT emission, whereas cyanide fails to show a similar property due to a different redox process operational in the latter. The reduction of NDI in the presence of fluoride and cyanide in different polar solvents indicates that involvement of such deprotonated solvents in the electron transfer mechanism may not be operative in our present system. Low-temperature kinetic studies support the formation of a charge transfer associative transient species, which likely allows overcoming the thermodynamically uphill barrier for the direct electron transfer mechanism.

Supramolecular Self-Assembly Driven Selective Sensing of Phosphates.

A new bis-heteroleptic RuII complex (1[PF6]2) with iodotriazole as the anion binding group along with the attached pyrene moiety is developed to investigate anion sensing properties and the origin of its selectivity toward a particular class of anions. Selective sensing of phosphates over other anions in both the solution and solid states by 1[PF6]2 is clearly evident from the perturbation of the absorption band and a large degree of amplification of 3MLCT emission band in the presence of phosphates. Importantly, macroscopic investigation such as Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) indicated the formation of supramolecular architecture in the presence of dihydrogen phosphate via halogen bonding interaction and π-π stacking of pyrene moieties. Such macroscopic property is further corroborated by solution and solid state spectroscopic studies, e.g., 1H-DOSY NMR, single crystal X-ray crystallography, and solid state photoluminescence (PL) spectroscopy.

A heteroditopic macrocycle as organocatalytic nanoreactor for pyrroloacridinone synthesis in water.

A heteroditopic macrocycle is reported as an efficient organocatalytic nanoreactor for the synthesis of diversely functionalized pyrroloacridinones in aqueous medium. A library of compounds was synthesized in a one-step pathway utilizing 10 mol % of the nanoreactor following a sustainable methodology in water with high yields.

Polyamide-Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase.

Polyamide-polyamine hybrid macrobicycle L is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated L with the series of dianions from succinate (suc2-) through glutarate (glu2-), α-ketoglutarate (kglu2-), adipate (adi2-), pimelate (pim2-), suberate (sub2-), to azelate (aze2-) have shown adipate preference with association constant value of K = 4900 M-1 in a H2O/DMSO (50:50 v/v) binary solvent mixture. The binding constant increases from glu2- to adi2- and then continuously decreases with the length of the anion chain. Further, potentiometric studies suggest that hydrogen bonding between the guest anions and the amide/ammonium protons of the receptor also contributes to the stability of the associations along with electrostatic interactions. Negative-mode electrospray ionization of aqueous solutions of host-guest complexes shows clear evidence for the selective formation of 1:1 complexes. Single-crystal X-ray structures of complexes of the receptor with glutaric acid, α-ketoglutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid assist to understand the observed binding preferences. The solid-state structures reveal a size/shape complementarity between the host and the dicarboxylate anions, which is nicely reflected in the solution state binding studies.

Bis-Heteroleptic Ruthenium(II) Complex of Pendant Urea Functionalized Pyridyl Triazole and Phenathroline for Recognition, Sensing, and Extraction of Oxyanions.

A new bis-heteroleptic RuII complex based ditopic receptor, 1[PF6]2, having an anion binding triazole -CH unit and appended 4-fluorophenyl urea arm has been developed. 1H NMR and isothermal titration calorimetry (ITC) experiments showed binding of 1[PF6]2 toward oxyanions such as phosphates (e.g., H2PO4- and HP2O73-) and carboxylates (e.g., CH3CO2- and C6H5CO2-) anions selectively. 1H NMR studies showed that highly basic phosphate anions such as HP2O73-/H2PO4- are bound by both -CH and -NH units of complex 1[PF6]2. However, comparatively less basic CH3CO2-, C6H5CO2- anions interacted with the urea -NH protons only. Thermodynamic parameters obtained from ITC experiments suggested that binding of all the interacting anions with complex 1[PF6]2 are highly enthalpy and entropy driven processes. Importantly, complex 1[PF6]2 showed extraction of H2PO4-, CH3CO2-, and C6H5CO2- anions from aqueous solution via liquid-liquid extraction with efficiencies of 28%, 74%, and 80%, respectively. The influential role of the urea moiety in the course of extraction is demonstrated by comparison with a model complex, 2[PF6]2. Additionally, complex 1[PF6]2 is capable of selective sensing of phosphate anions among all investigated anions.

Competitive Transmetalation of First-Row Transition-Metal Ions between Trinuclear Triple-Stranded Side-by-Side Helicates.

A hybrid tris-bidentate neutral ligand (L) composed of a central 2,2'-bipyridine and two terminal triazolyl-pyridine chelating units connected by methylene spacers is employed to synthesize trinuclear triple-stranded side-by-side helicates of first-row transition-metal(II) ions. Three such new homometallic helicates L3M3(OTf)6 [ M = Cu2+ (4); Ni2+ (5); Co2+ (6)], along with our recently reported helicates L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), and L3Fe2Zn(OTf)6 (3) are taken into consideration for competitive formation and transmetalation studies. Single-crystal X-ray structures of L3Cu3(OTf)6 (4) and L3Ni3(OTf)6 (5) show the formation of trinuclear triple-stranded side-by-side helicates with alternating Λ and Δ chiralities at the metal ions as earlier observed in cases of L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), and L3Fe2Zn(OTf)6 (3). ESI-FTICR mass spectrometry and UV-vis spectroscopy studies show that helicates L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), L3Fe2Zn(OTf)6 (3), and L3Co3(OTf)6 (6) can easily be transmetalated to helicate L3Cu3(OTf)6 (4) in the presence of Cu(OTf)2. On the other hand, only a trace amount of heterometallic helicate L3Ni2Cu(OTf)6 forms even after several days, when Cu(OTf)2 is added to a the solution of homometallic helicate L3Ni3(OTf)6 (5). Further, we have demonstrated the formation of a heterometallic helicate L3Ni2Co(OTf)6 (7) from a 1:1:1 reaction mixture of L, Ni(OTf)2, and Co(OTf)2, which can also be prepared from homometallic helicate L3Co3(OTf)6 (6) by transmetalation with Ni(OTf)2.

Selective Sensing of Phosphates by a New Bis-heteroleptic RuII Complex through Halogen Bonding: A Superior Sensor over Its Hydrogen-Bonding Analogue.

The selective phosphate-sensing property of a bis-heteroleptic RuII complex, 1[PF6 ]2 , which has a halogen-bonding iodotriazole unit, is demonstrated and is shown to be superior to its hydrogen-bonding analogue, 2[PF6 ]2 . Complex 1[PF6 ]2 , exploiting halogen-bonding interactions, shows enhanced phosphate recognition in both acetonitrile and aqueous acetonitrile compared with its hydrogen-bonding analogue, owing to considerable amplification of the RuII -center-based metal-to-ligand charge transfer (MLCT) emission response and luminescence lifetime. Detailed solution-state studies reveal a higher association constant, lower limit of detection, and greater change in lifetime for complex 1 in the presence of phosphates compared with its hydrogen-bonding analogue, complex 2. The 1 H NMR titration study with H2 PO4- ascertains that the binding of H2 PO4- occurs exclusively through halogen-bonding or hydrogen-bonding interactions in complexes 1[PF6 ]2 and 2[PF6 ]2 , respectively. Importantly, the single-crystal X-ray structure confirms the first ever report on metal-assisted second-sphere recognition of H2 PO4- and H2 P2 O72- with 1 through a solitary C-I⋅⋅⋅anion halogen-bonding interaction.

A Highly Sensitive ESIPT-Based Ratiometric Fluorescence Sensor for Selective Detection of Al(3.).

An excited-state intramolecular proton transfer (ESIPT)-based highly sensitive ratiometric fluorescence sensor, 1H was developed for selective detection of aluminum (Al(3+)) in acetonitrile as well as in 90% aqueous system. Single-crystal X-ray diffraction analysis reveals almost planar and conjugated structure of 1H. Photophysical properties of the sensor as well as its selectivity toward Al(3+) are explored using UV-visible, steady-state, and time-resolved fluorescence spectroscopic studies. The bright cyan (λem = 445 nm) fluorescence of 1H in acetonitrile turns into deep blue (λem = 412 nm) with ∼2.3-fold enhancement in emission intensity, in the presence of parts per billion level Al(3+) (detection limit = 0.5 nM). Interestingly, the probe 1H exhibits increased selectivity toward Al(3+) in H2O/acetonitrile (9:1 v/v) solvent system with a change in fluorescence color from pale green to deep blue associated with ca. sixfold enhancement in emission intensity. Density functional theoretical (DFT) calculations provide the ground- and excited-state energy optimized structures and properties of the proposed aluminum complex [Al(1) (OH)]2(2+), which is in harmony with the solution-state experimental findings and also supports the occurrence of ESIPT process in 1H. The ESIPT mechanism was also ascertained by comparing the basic photophysical properties of 1H with a similar O-methylated analogue, 1'Me.

Unusual Recognition and Separation of Hydrated Metal Sulfates [M2(µ-SO4)2(H2O)n, M = Zn(II), Cd(II), Co(II), Mn(II)] by a Ditopic Receptor.

A ditopic receptor L1, having metal binding bis(2-picolyl) donor and anion binding urea group, is synthesized and explored toward metal sulfate recognition via formation of dinuclear assembly, (L1)2M2(SO4)2. Mass spectrometric analysis, (1)H-DOSY NMR, and crystal structure analysis reveal the existence of a dinuclear assembly of MSO4 with two units of L1. (1)H NMR study reveals significant downfield chemical shift of -NH protons of urea moiety of L1 selectively with metal sulfates (e.g., ZnSO4, CdSO4) due to second-sphere interactions of sulfate with the urea moiety. Variable-temperature (1)H NMR studies suggest the presence of intramolecular hydrogen bonding interaction toward metal sulfate recognition in solution state, whereas intermolecular H-bonding interactions are observed in solid state. In contrast, anions in their tetrabutylammonium salts fail to interact with the urea -NH probably due to poor acidity of the tertiary butyl urea group of L1. Metal sulfate binding selectivity in solution is further supported by isothermal titration calorimetric studies of L1 with different Zn salts in dimethyl sulfoxide (DMSO), where a binding affinity is observed for ZnSO4 (Ka = 1.23 × 10(6)), which is 30- to 50-fold higher than other Zn salts having other counteranions in DMSO. Sulfate salts of Cd(II)/Co(II) also exhibit binding constants in the order of ∼1 × 10(6) as in the case of ZnSO4. Positive role of the urea unit in the selectivity is confirmed by studying a model ligand L2, which is devoid of anion recognition urea unit. Structural characterization of four MSO4 [M = Zn(II), Cd(II), Co(II), Mn(II)] complexes of L1, that is, complex 1, [(L1)2(Zn)2(µ-SO4)2]; complex 2, [(L1)2(H2O)2(Cd)2(µ-SO4)2]; complex 3, [(L1)2(H2O)2(Co)2(µ-SO4)2]; and complex 4, [(L1)2(H2O)2(Mn)2(µ-SO4)2], reveal the formation of sulfate-bridged eight-membered crownlike binuclear complexes, similar to one of the concentration-dependent dimeric forms of MSO4 as observed in solid state. Finally, L1 is found to be highly efficient in removing ZnSO4 from both aqueous and semiaqueous medium as complex 1 in the presence of other competing Zn(II) salts via precipitation through crystallization. Powder X-ray diffraction analysis has also confirmed bulk purity of complex 1 obtained from the above competitive crystallization experiment.