Synthesis and Characterization of a Tripodal Amide Ligand and Its Binding with Anions of Different Dimensionality

Selective colorimetric sensing and perfect linear recognition of azide: Formation of Cu-azide-Cu cascade complex within the cavity of cryptand

Bis-trpn [tris(3-aminopropyl)amine], capped dicopper complex of bicyclic cryptand L, 1, became a potential selective colorimetric chemosensor for azide anion. Complex 1 is generating a space inside the cylindrical cavity which will be opt for perfect linear recognition of azide anion through as N₄-Cu⋯N₃^-⋯Cu-N₄ axle. Naked eye colorimetric and UV-Vis spectrometric investigations shows the complex 1 has the capability of selective sensing of azide anion. The association constant and limits of detection (LoD) of complex 1 towards azide are found to be 2.754 × 10³ M^-1 and 1.91 × 10^-6 M. To the best of our knowledge, this is the first example of selective colorimetric sensing of azide by a bis‑copper cryptate 1 via ideal linear orientation of N₄-Cu⋯N₃⋯Cu-N₄ axle inside the cylindrical shaped cavity.

C3V symmetric tripodal molecular pocket for linear anions: Selective colorimetric detection of azide through N4-Cu-π···hole interactions

Tris(3-amino propyl)amine (TRPN) based C₃v symmetric tripodal molecular pocket, L has been prepared and mono copper complex of L, (1) having Cu-N₄-π··· hole as recognizing elements, becoming a potential and selective colorimetric chemo sensor for perfect linear recognition of N₃^-, generate a Cu-NNN- π··· hole unit inside the tripodal pocket. Systematic spectrometric and naked-eye colorimetric studies reveal that, this chemo sensor is also colorimetrically recognizing the cyanide ion by its cavity via Cu-N₄-π···hole interactions; nevertheless, when azide anion is entering as a guest into the molecular pocket of 1 which is already hosted cyanide anion, then host displaces cyanide ion, subsequently azide is getting inside the cavity. The strength of the copper complex, 1 towards azide and cyanide are found to be 2.36 × 10³M^-1, and 1.87 × 10³ M^-1 respectively in 7:3 acetonitrile:water solvent medium. Further, the ability of cyanide displacement by azide in complex 1 is found to be 2.53 × 10³ M^-1. To the best of our knowledge, this is the first example of naked-eye detection of azide via cyanide displacement assay by a tripodal receptor.

A molecular phototropic system for cyanide: Detection and sunlight driven harvesting of cyanide with molecular sunflower

We are reporting a simple, easy to prepare, and conformation switchable first molecular phototropic system L, "(E)-2-(2,4-dinitrophenyl)-1-((pyren-8-yl)methylene)hydrazine, for cyanide harvesting. This molecular phototropic system behaves as a molecular sunflower in which the conformation of this molecular sunflower can be altered in response to the sunlight. This molecular flower can sense and bind the cyanide anion colorimetrically through its transition state. Further, upon exposure of this transition state cyanide complex 1, under sunlight, this system is capable to release the bound cyanide via -C=N- free rotation to reach its lower energy stable conformation. Similar behaviors were observed for acetate and fluoride with L. The strength of the phototropic system L towards cyanide, acetate and fluoride is found to be 4.5 × 10⁵, 1.53 × 10² and 6.09 × 10² M^-1.

Most Efficient Tris(3-Aminopropyl) Amine Based Electron Deficient Tripodal Receptor for Azide

Investigation on strength of the tris(2-amino ethyl) amine and tris (3-amino propyl) amine backboned tripodal receptors, L and L¹ (incorporated with tripodal C₃ν frame, thio urea-amide linkage and π-hole assisting functionality) which are premeditated to explore the prospect for a particular anion recognition are studied. UV-Vis, ¹H- NMR, and IR spectroscopy studies indicates that both the receptors sensing azide anion, colorimetrically and binds azide anion stronger than any other anions such as acetate, and cyanide. In particular the receptor L¹ shows the highest binding strength towards azide anion. To the best of our knowledge this is the first receptor showing highest binding ability with azide anion. We used Molecular Electrostatic Potential Surface analyses to support our spectroscopic findings. The association constant and limits of detection for receptor L¹ with azide is found to be 8.4X10⁵M^-1 and 3.16X10^-6 M respectively. The observed highest binding strength of L¹ with azide is, could be due to the cooperative effect of extended traditional hydrogen bonding via thiourea-amide functionality, anion-π interaction and C₃ν suitable framework.

Hydrogen bonding elements, π - hole functional moieties and C3v tripodal scaffold controlled turn-on cyanide and turn-off azide selective receptors

Here in, we are reporting electron deficient amide and sulfonamide based tripodal receptors L, L¹, L² and L³. Systematic studies show a strong selectivity towards cyanide and azide anions. Detailed UV-Visible and fluorescent spectrometric investigation shows the amide based tripodal receptors L and L³ acts as a colorimetric and turn-on fluorescent chemo-sensor for cyanide, and the sulfonamide based tripodal receptors L¹ and L² acts as a colorimetric and turn-off fluorescent chemo-sensor for azide. At the end we have successfully prepared tripodal receptors for a particular anion with judicious choice of recognition elements such as hydrogen bonding amide/sulfonamide moiety, electron deficient pentafluorophenyl functionality for anion-π interaction and the well defined C₃v symmetric tripodal backbone for perfect recognition.

Tuning the recognition properties of urea based receptor towards cyanide in aqueous medium: Effect of fluorine substitution on strength and selectivity of the anion receptor

In this report, we have successfully tuned the selectivity and sensitivity of an anion receptor L¹ by substituting electron withdrawing natured fluorine atoms directly on to the rim of the phenyl ring. Despite the fact that, we have two electron withdrawing natured nitro substituents on the other side of receptor L¹; two fluorine substitutions made dramatic change in the sensing ability as well as the selectivity of the receptor L¹ towards anions. The acetonitrile solution contains L¹ with tetrabutyl ammonium salts of fluoride, cyanide, acetate and dihydrogen phosphate results sudden color changes from yellow to brown; almost negligible spectral/color change for azide and bifluoride, where as there is no color change observed with any other anionic guests with L¹. Solution state binding studies of L¹ are carried out by UV-Vis spectrometry titration in 100% acetonitrile and it is found to be the receptor L¹ selectively binds cyanide, phosphate and fluoride stronger than acetate; it is also found that receptor binds fluoride 100 times stronger than that of the receptor L and L¹ has almost similar efficiency in binding towards acetate ion (AcO^-). The strength of the receptor L¹ towards fluoride, cyanide, acetate and dihydrogen phosphate bindings are found to be in the order of 1.271 × 10⁵ M^-1 > 1.245 × 10⁵ M^-1 > 1.368 × 10³ M^-1, 1.23 × 10³ M^-1 respectively. When we used aqueous environment (10% of water in acetonitrile) as testing solvent system, the receptor L¹ selectively sense cyanide ion. Over all, strength of the receptor is increased towards anions with an increasing the number of fluorine atom onto the receptors. Here in we have also prepared a reference compound L² in which the receptor molecule is substituted with only one fluoride atom. The acetonitrile solution of reference receptor L² with help of naked-eye colorimetric experiment and spectrometric ammonium salts of fluoride, cyanide, acetate and bifluoride results sudden color changes from faint-yellow to brown in color. Unlike receptor L¹, receptor L² does not recognize H₂PO₄^-, but instead of H₂PO₄^-, it recognizes bifluoride as evidenced from UV-Vis spectroscopic and naked-eye colorimetric studies.

Simple and highly electron deficient Schiff-base host for anions: First turn-on colorimetric bifluoride sensor

Here in, we have designed, synthesized and isolated sensor L, as a turn-on colorimetric chemo sensor for bifluoride ion. The acetonitrile solution contains L with tetrabutyl ammonium salts of bifluoride, cyanide and fluoride results sudden color changes from yellow to dark brown where as there is no color change observed with any other anionic guests with L. Solution state binding studies of L are carried out by UV-Vis spectrometry titration and the strengths of the chemosensor L towards bifluoride, cyanide and fluoride bindings are found to be 2.67 × 10⁵ M^-1, 4.78 × 10⁵ M^-1_, 4.45 × 10⁵ M^-1 respectively. The strength and sensitivity of the absorbance based assay with L for bifluoride ion, are found to be 2.67 × 10⁵ M^-1 and 0.7 μM respectively, which is the best (highest binding constant and lowest LOD) ever reported in the literature. In order to use this sensor in practical application, we also prepared a cassette which is fabricated with sensor L and we succeeded to sense bifluoride ion.

Electron-deficient tripodal amide based receptor: An exclusive turn-on fluorescent and colorimetric chemo sensor for cyanide ion

Here in, we have designed, synthesized and isolated sensor L, as an exclusive selective turn-on fluorescent chemo sensor for cyanide ion. The acetonitrile solution contains L with tetrabutyl ammonium cyanide, results sudden color change from colorless to yellowish-brown. Chemosensor L produced a strong fluorescence response with an enhancement of very high fluorescence intensity while addition of CN^- ion and the strength of the chemosensor L towards cyanide binding is found to be 3.9813×10⁴M^-1. In order to use this sensor in practical application, we also prepared a cassette which is fabricated with sensor L and we succeeded to sense cyanide ion.

Conformation and Visual Distinction between Urea and Thiourea Derivatives by an Acetate Ion and a Hexafluorosilicate Cocrystal of the Urea Derivative in the Detection of Water in Dimethylsulfoxide

Structures of different solvates and solute-solvent interactions of 4-(3-(4-nitrophenyl)urido)benzoate (L ₁ ) and methyl-4-(3-(4-nitrophenyl)thiourido)benzoate (L ₂ ) with different solvents are analyzed. The solution of L ₁ with tetrabutylammonium acetate (TBAA) in dimethylsulfoxide (DMSO) is colorless, but a similar solution of L ₂ with TBAA is orange. On the other hand, respective solutions of these urea and thiourea derivatives with tetrabutylammonium fluoride (TBAF) in DMSO are orange. Urea derivative L ₁ facilitates the reaction of TBAF with glass to form tetrabutylammonium hexafluorosilicate, which on further interaction with L ₁ forms cocrystal 2L ₁ ·(TBA)₂SiF₆. Reorganization of hydrogen-bonded self-assembly of 2L ₁ ·(TBA)₂SiF₆ in DMSO caused by water is established by a dynamic light scattering study. With an increase in the amount of water in the solution, visual color changes from orange to colorless, and the color changes are reversed upon the addition of a dehydrating agent such as molecular sieves. Solvates of L ₁ with DMSO, dimethylformamide (DMF), and dimethylacetamide are quasi-isostructural. The respective self-assembly of these solvates differs due to orientations of aromatic rings and the carbomethoxy group across the thioamide/amide bond. Significant differences in self-assemblies of the respective DMSO solvate of L ₁ and L ₂ are observed; self-assembly of the former has dimeric subassemblies as repeat units, whereas the latter has monomeric subassemblies. DMF solvates of L ₁ and dimethylacetamide of L ₁ are built by dimeric subassemblies to form self-assembled structures, but these subassemblies differ in the orientation of the carbomethoxy group across the urea units.

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 (suc^2-) through glutarate (glu^2-), α-ketoglutarate (kglu^2-), adipate (adi^2-), pimelate (pim^2-), suberate (sub^2-), to azelate (aze^2-) 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 glu^2- to adi^2- 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.

Anion complexation with cyanobenzoyl substituted first and second generation tripodal amide receptors: crystal structure and solution studies

Solid and solution state anion binding via a side cleft and pseudocapsular fashion by two cyanophenyl substituted first (L1) and second (L2) generation tripodal amides has been demonstrated along with a selective anion induced conformational change of L2 from open C_2v to folded C_3v symmetry.

Recent developments in anion induced capsular self-assemblies

This Feature Article covers recent developments in anion induced capsular self-assemblies, with particular focus on important reports from 2011 to 2013. Contemporary studies on the capsular binding of environmentally and biologically relevant anions in aqueous medium are described. Emerging reports of such systems reveal their potential utility towards various functional aspects like anion separation, CO2 fixation, hydrated halide recognition and anion transportation. This article also highlights potential applications of anion induced molecular capsules.

Dual modes of binding on the hexafluorosilicate anion by a C3v symmetric flexible tripodal amide ligand in solid state

A para-nitrophenyl functionalized C_3v symmetric flexible tripodal amide ligand, L, shows remarkable solvent dependent dual binding behaviour towards the octahedral hexafluorosilicate anion in solid state.

A highly selective colorimetric chemosensor for cobalt(ii) ions based on a tripodal amide ligand

A highly selective colorimetric chemosensor for cobalt(ii) ions was synthesized and structurally characterized by single crystal X-ray diffraction. The ligand enabled the detection of cobalt(ii) ions at a concentration of 10⁻⁵ mol L⁻¹ by the “naked-eye”.

Molecular Self Assembly: Solvent Guests Tune the Conformation of a Series of 2,6-Bis(2-anilinoethynyl)pyridine-Based Ureas

The conformations of 2,6-bis(2-anilinoethynyl)pyridine-based urea receptors were studied by single crystal X-ray diffraction methods and revealed a rich conformational flexibility influenced by solvents. Whereas receptor L(1) in DMSO prefers an "S" conformation, receptor L(1) crystallizes in an "O" conformation from DMSO/CH(3)OH binary solvent system, and a "W" conformation in the ternary solvent mixture DMSO/toluene/1,4-dioxane. In the case of L(2), the molecule adopts an "S" conformation where water molecules are sandwiched between two molecules of L(2) to form a dimer. Similar to L(2), L(3) also forms a dimer where water molecules are sandwiched between L(3) molecules, which are capped with two molecules of DMSO. Such a capping DMSO solvate is lacking in the case of L(2). Taken together, these results demonstrate that the conformation of 2,6-bis(2-anilinoethynyl) pyridine-based urea receptors can be dramatically manipulated and tuned by the choice of crystallization solvents.

Seven-coordinate anion complex with a tren-based urea: binding discrepancy of hydrogen sulfate in solid and solution states

Structural characterization of a hydrogen sulfate complex with a tren-based urea suggests that the anion is coordinated with six NH···O bonds (d(N···O) = 2.857 (3) to 3.092 (3) Å) and one OH···O bond (d(O···O) = 2.57 (2) Å) from three receptors; however, in solution the anion is bound within the pseudo-cavity of one receptor.

Anion complexation of a pentafluorophenyl-substituted tripodal urea receptor in solution and the solid state: selectivity toward phosphate

The binding and selectivity of halides (spherical) and oxyanions (tetrahedral) toward a recently reported pentafluorophenyl-substituted tripodal urea-based receptor L(1) are examined thoroughly in the solid state by single-crystal X-ray crystallography as well as in solution by multinuclear NMR techniques. Crystallographic results show proof of a fluoride encapsulation in the cavity of L(1) in complex [L(1)(F)][Bu(4)N], . Fluoride encapsulation inside the C(3v) symmetric cleft is observed via six hydrogen bonds to all six urea protons of the receptor. In case of complex crystallographic results show encapsulation of sulfate ion inside a supramolecular cage formed upon 1 : 2 (guest-host) complex formation between sulfate and L(1). Sulfate encapsulation is observed via fourteen hydrogen bonding interactions from all six urea moieties of two L(1) units. Our effort to isolate single crystal of halides/oxyanions complexes of L(2) always yield single crystals of free L(2) though literature shows anion binding with this receptor in solution. Solution state binding studies of L(1) are carried out by (1)H-NMR titration to calculate binding constants, which show the following anion binding sequence H(2)PO(4)(-) > SO(4)(2-)> CH(3)COO(-) > F(-) > Cl(-) >> Br(-) whereas there is no binding with I(-), NO(3)(-) and ClO(4)(-) guests. Comparison of phosphate and sulfate binding in L(1) and L(2), show higher binding with the pentafluorophenyl substituted receptor, L(1). Further (19)F and (31)P-NMR experiments in solution are also carried out to probe the binding of F(-) and H(2)PO(4)(-) with L(1), respectively. Extensive (1)H-NMR experiments in solution and crystallization in the presence of multiple anions are also undertaken to evaluate the selectivity of H(2)PO(4)(-) over other anions.

Sulfate ion encapsulation in caged supramolecular structures assembled by second-sphere coordination

A tripodal tris(3-pyridylurea) receptor (L) assembles with metal sulfate salts MSO(4) (M=Mn, Zn) to afford supramolecular cages [SO(4) subset L(2)] that encapsulate the SO(4)(2-) ion via multiple hydrogen bonds in a three-dimensional structure held by second-sphere coordination; (1)H NMR and negative-ion mode ESI-MS spectra reveal significantly strong sulfate binding in solution.