Cavity Of Cryptand Research Articles

Acceleration and deceleration of chirality inversion speeds in a dynamic helical metallocryptand by alkali metal ion binding

We report that the chirality inversion kinetics of a trinickel(II) cryptand can be controlled by guest recognition in the cryptand cavity. When the guest was absent, the nickel(II) cryptand underwent a dynamic interconversion between the P and M forms in solution, preferring the M form, with a half-life of t 1/2 = 4.99 min. The P / M equilibrium is reversed to P -favored by binding with an alkali metal ion in the cryptand cavity. The timescale of this M→P inversion kinetics was both notably accelerated and decelerated by the guest binding ( t 1/2 = 0.182 min for K + complex; 186 min for Cs + complex); thus, the equilibration rate constants differed by up to 1000-fold depending on the guest metal ions. This acceleration/deceleration can be explained in terms of the virtual binding constants at the transition state of the P / M chirality inversion; K + binding more stabilizes the transition state rather than the P and M forms to result in the acceleration.

A Carbazole-1,8-Disulfonamide-Derived Cryptand Receptor for Anion Recognition.

A novel cryptand-like anion receptor 1 was synthesized in reasonable yield by a one-step condensation reaction. The UV-vis spectroscopic titrations indicated that cryptand 1 bound AcO- in preference to other monovalent anions (including its competing F- and H2PO4-) in CH3CN, generating a 1:1 binding complex with Ka = 51,000 M-1. Moreover, the crystal structures revealed that the acetate ion was encapsulated inside the cryptand's cavity in a 1:1 manner, through multiple N-H···O hydrogen bonds (although having two different crystal forms).

Control of chirality inversion kinetics of triple-helical metallocryptands

Abstract Dynamic helical molecules, which can undergo a reversible chirality inversion between the P and M forms, are useful as a platform for switchable chiral functional molecules. The chirality inversion of these molecules has been extensively studied. However, it has mostly been discussed from the viewpoint of the equilibrated P/M ratios before and after the inversion; control of the response speeds or kinetic profiles has rarely been explored. In order to construct helical structures with controllable kinetic profiles, triple-helical metallocryptands, LM3, have been designed and synthesized. These molecules can undergo a relatively slow dynamic P/M chirality inversion (helicity inversion) to produce an equilibrated mixture. The P/M equilibration was accelerated or decelerated based on the following two strategies. One is based on the guest binding in the cryptand cavity. The P/M racemization kinetics of LNi3 was significantly decelerated by recognition of guanidinium ion in the cavity. The other strategy is based on the ligand exchange reactions at the octahedral cobalt(III) centers in LCo3(amine)6. The P/M chirality inversion speeds were controlled by changing the initial and entering amine ligands. In addition, a unique transient chirality inversion behavior was observed when chiral amine ligands were removed from the metallocryptand by the ligand exchange reaction with piperidine.

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.

Weak and Reversible Binding of Alkali Metal Ions (Na+/K+) by an Aza‐Oxa Cryptand

AbstractA laterally asymmetric cryptand incorporating amino N and ethereal O was utilized to bind the hard alkali metal ions (Na+ and K+) in aqueous medium. Single crystal X‐ray diffraction studies revealed that both Na+ and K+ ions were included inside the cryptand cavity. However, the trapped metal ions were weakly bound and could be released from the cavity of the cryptand. This cryptand had been used as a column chromatographic material for trapping Na+ and K+ ions from water as the trapped metal ion could be released back into water to extract the cryptand in its free state. Metal binding capability of the cryptand was evaluated using computational studies that revealed lower binding energy (B.E.) for the Na+ and K+ ions compared to first row transition metal ions.

Halocarbon Encapsulation via Halogen···π Interactions in a Bispyrazole-Based Cryptand

A novel bispyrazole-based expanded cryptand was synthesized via Schiff base condensation reaction having an internal cavity of 160 A3 with a hydrophobic nature. The cryptand features electron-rich multiple pyrazole rings for enhanced weak noncovalent interactions with the guest molecules. The host–guest capabilities of the cryptand were investigated for encapsulation of the most inactive halogen bond donor molecules (having small σ-hole size), namely, CH2Cl2, CHCl3, CCl4, C2HCl3, C2H4Cl2, and C2H4Br2. Analysis of crystal structures clearly revealed that halogen bonding (C–Cl/Br···π (pyrazole)) and hydrogen bonding (C–H···π(pyrazole)) interactions played a key role in stabilizing the halogenated guests inside the hydrophobic cavity of cryptand. At the same time, the cage is efficiently able to exclude hydrophilic solvent molecules, like, water and methanol, suggesting the hydrophobic nature of the cavity. Due to the comparably large σ-hole in C2H4Br2, it showed the strongest halogen bonding interaction wit...

High-yield preparation of new crown ether-based cryptands and improving complexation with paraquat, paraquat derivatives and diquat

Efficient syntheses of two novel cryptands 2a and 2b based on bis(m-phenylene)-32-crown-10 are reported. Unimolecular macrocyclization of 9 through copper(II)-mediated Eglinton coupling generated the diacetylene-containing cryptand 2a in 93% yield. Pd/C-catalyzed hydrogenation of 2a yielded the cryptand 2b (74%). The complexation behavior of a series of cryptand hosts (1a–2b) with dicationic guests (3–6) has been comprehensively investigated by NMR spectroscopy, mass spectrometry and X-ray analysis. It is found that all four cryptands binding with guests form 1:1 complexes both in solution and in the solid state, except the case of cryptand 1a/1b with 5 forming 2:1 complex. Moreover, it is demonstrated that cryptand 2a/2b not only binds guests more strongly than the corresponding BMP32C10, but also has greater interacting affinities than the cryptand 1a/1b. Meanwhile, the association constants of the N-methyl-substituted guests 4 and 5 with cryptands are higher than non-methyl-substituted guest 3. The X-ray analysis of crystals of these complexes revealed that the complexation is stabilized by π–π stacking interactions and CH⋯O hydrogen bonds between the cryptands and the guests. Additionally, the paraquat derivative 3 penetrated symmetrically through the cavity of the two same 29-membered rings of cryptand 2 in the solid state, while the threading of paraquat unit into the cavity of cryptand 1 was unsymmetrical both in crystal structures of 1a3 and 1b3.

Electron-transfer studies of a peroxide dianion.

A peroxide dianion (O2(2-)) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)⊂mBDCA-5t-H6](2-), stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)⊂mBDCA-5t-H6](2-) with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O2(2-) occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O2(2-) within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O2)⊂mBDCA-5t-H6](2-) as a chemical reagent.

Cavity Effect on Perrhenate Recognition by Polyammonium Cages

AbstractThe affinity of azacryptands towards perrhenate has been investigated by potentiometric, 1H NMR spectroscopic and ITC studies in aqueous solutions. The association constants could only be determined for the p‐xylyl and m‐xylyl azacryptands in the hexaprotonated form. The experimental results showed the outstanding affinity of the p‐xylyl cryptand for perrhenate, attributable to the geometric complementarity between the anion and the cavity of the receptor. Single crystals of the inclusion complex could also be obtained. The X‐ray diffraction studies indicate that the perrhenate anion is included in the cavity of the receptor and interacts with the ammonium groups by means of both direct and water‐mediated H‐bonding interactions. The receptors were also investigated in the presence of different anions (i.e. chloride, bromide, iodide, perchlorate and nitrate). The affinity trend demonstrates that the p‐xylyl cryptand cavity is more suitable to host large anionic guests such as perrhenate, perchlorate and iodide, despite their low density charge. On the contrary, nitrate, bromide and chloride fit better into the smaller cavity of the m‐xylyl cryptand. The higher affinity of the m‐xylyl cage towards chloride over that of perrhenate was also demonstrated by the crystal structure of the 1:1 adduct.

A Macrocyclic Ligand as Receptor and ZnII‐Complex Receptor for Anions in Water: Binding Properties and Crystal Structures

Binding properties of 24,29-dimethyl-6,7,15,16-tetraoxotetracyclo[19.5.5.0(5,8).0(14,17)]-1,4,9,13,18,21,24,29-octaazaenatriaconta-Δ(5,8),Δ(14,17)-diene ligand L towards Zn(II) and anions, such as the halide series and inorganic oxoanions (phosphate (Pi), sulfate, pyrophosphate (PPi), and others), were investigated in aqueous solution; in addition, the Zn(II)/L system was tested as a metal-ion-based receptor for the halide series. Ligand L is a cryptand receptor incorporating two squaramide functions in an over-structured chain that connects two opposite nitrogen atoms of the Me(2)[12]aneN(4) polyaza macrocyclic base. It binds Zn(II) to form mononuclear species in which the metal ion, coordinated by the Me(2)[12]aneN(4) moiety, lodges inside the three-dimensional cavity. Zn(II)-containing species are able to bind chloride and fluoride at the physiologically important pH value of 7.4; the anion is coordinated to the metal center but the squaramide units play the key role in stabilizing the anion through a hydrogen-bonding network; two crystal structures reported here clearly show this aspect. Free L is able to bind fluoride, chloride, bromide, sulfate, Pi, and PPi in aqueous solution. The halides are bound at acidic pH, whereas the oxoanions are bound in a wide range of pH values ranging from acidic to basic. The cryptand cavity, abundant in hydrogen-bonding sites at all pH values, allows excellent selectivity towards Pi to be achieved mainly at physiological pH 7.4. By joining amine and squaramide moieties and using this preorganized topology, it was possible, with preservation of the solubility of the receptor, to achieve a very wide pH range in which oxoanions can be bound. The good selectivity towards Pi allows its discrimination in a manner not easily obtainable with nonmetallic systems in aqueous environment.

ChemInform Abstract: Approaches to Improvement of Metal Ion Selectivity by Cryptands

Abstract Several approaches to improvement of metal ion selectivity by mixed oxygen–nitrogen donor cryptands are discussed. Because of the relatively rigid structures of cryptands, thermodynamic stabilities of the cryptate complexes strongly depend on the match of the cation size and cryptand cavity diameters. Symmetry is an important factor influencing the properties of cryptand complexation. One rule to improve metal ion selectivity by oxygen–nitrogen donor cryptands is to obtain the ligand that has the best size-match cavity with the cation while keeping a symmetric spherical-coordination array. High selectivity for a small cation can be obtained when the cryptand is able to form a number of six-membered chelate rings with the metal ion while the requirements of the high-symmetric donor atom array and size-matched cavity are met. On the other hand, introduction of benzene rings and other groups to cryptands usually decreases metal ion binding and selectivity.

Synthesis and crystal structure of 4,7,13,16,21,24-hexaoxa-1,10-diazoniabicyclo[8.8.8]hexacosane Bis(citrate) hydrate

A crystalline salt of 2.2.2-cryptand and citric acid, 4,7,13,16,21,24-hexaoxa-1,10-diazoniabicyclo[8.8.8]hexacosane bis(citrate) hydrate [H2(Crypt-222)]2+·2(C6H7O7)− · 1.65H2O, was synthesized and studied by single crystal X-ray diffraction. In the crystal structure of this salt, the 2.2.2-cryptand dication is somewhat disordered, and its H atoms at two protonated N atoms are oriented inside the cryptand cavity. Two independent citrate anions are essentially different, as different COOH groups in them are deprotonated. The geometric parameters (bond lengths, bond angles, etc.) of the molecular ions and water molecules are determined with a relatively high accuracy. The structural units form a three-dimensional system of intermolecular (interionic) hydrogen bonds.

On the relative stabilities of the alkali cations 222 cryptates in the gas phase and in water-methanol solution

The relative stabilities of the alkali [M subset 222]+ cryptates (M = Na, K, Rb and Cs) in the gas phase and in solution (80:20 v/v methanol:water mixture) at 298 K, are computed using a combination of ab initio quantum-chemical calculations (HF/6-31G and MP2/6-31+G*//HF/6-31+G*) and explicit-solvent Monte Carlo free-energy simulations. The results suggest that the relative stabilities of the cryptates in solution are due to a combination of steric effects (compression of large ions within the cryptand cavity), electronic effects (delocalization of the ionic charge onto the cryptand atoms) and solvent effects (dominantly the ionic dessolvation penalty). Thus, the relative stabilities in solution cannot be rationalized solely on the basis of a simple match or mismatch between the ionic radius and the cryptand cavity size as has been suggested previously. For example, although the [K subset 222]+ cryptate is found to be the most stable in solution, in agreement with experimental data, it is the [Na subset 222]+ cryptate that is the most stable in the gas phase. The present results provide further support to the notion that the solvent in which supramolecules are dissolved plays a key role in modulating molecular recognition processes.

Where Are the Protons in α‐[HxW12O40](8‐x)‐ (x = 2—4)

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A Perfect Linear Cu−NNN−Cu Unit Inside the Cryptand Cavity and Perchlorate Entrapment within the Channel Formed by the Cascade Complex

Dicopper complexes of bis-tren cryptand L1 having 1,4-xylyl spacers, becoming a potential receptor for perfect linear recognition of N3-, generate a Cu-NNN-Cu unit inside the cryptand cavity. Solid-state packing of this cascade complex shows the formation of a thorough channel which encapsulate perchlorate anion within the channel.

Recognition of Water−Acetonitrile−Water Cluster in a Tetraprotonated Picrate Salt of Octaaminocryptand

Octaamino cryptand L (1,4,11,14,17,24,29,36-octaazapentacyclo[12.12.12.2.6,92.19,22231,34]tetratetraconta-(43),7,9(44), 19(41),20,22(42),31(39),32,34(40)-nonaene, N(CH2CH2NHCH2-p-xylyl-H2NHCH2CH2)3N), in tetraprotonated form shows encapsulation of a water−acetonitrile−water cluster (1), whereas a molecule of water encapsulation inside the cryptand cavity is observed in the triprotonated state (2), with picrate as counteranions. These two compounds were examined crystallographically. Thermal analysis supports the crystallographic findings.

Where Are the Protons in α-[HxW12O40](8-x)- (x = 2−4)?

The equilibria and speciation of the proton cryptate polyoxometalate alpha-(H2)W(12)O(40)]6- (1) were examined by NMR following the phase-transfer cation metathesis of aqueous Na(6)1 with Q+Br-/CH(2)Cl(2), leading to the isolation of the (n-Bu)4N+ (Q+) salts Q(6)1 and alpha-Q5[(H3)W(12)O(40)](Q(5)2). Several groups report salts of the protonated anions H(x)1 (x = 1 and 2) with no consensus on proton numbers or locations. Reported herein, a combination of 1H and 183W NMR evidence, elemental analysis, acid titration measurements, and H/D isotopomer assignments establishes that in nonaqueous media the internal cryptand cavity of 1 reversibly accommodates only one more proton to form 2. Because an external proton must transfer across the close-packed tungsten oxide surface of 1, which should constitute a substantial activation barrier, it is significant that the transformation is instantaneous by 1H NMR (1 equiv of HBr in CH(3)CN), whereas the reverse process is slow (t1/2 approximately 17.4 h; 1 equiv of Q+OH-).

Counteranion-Controlled Water Cluster Recognition in a Protonated Octaamino Cryptand

Structural aspects of binding of water cluster and halides in the octaamino cryptand L (1,4,11,14,17,24,29,36-octaazapentacyclo[12.12.12.2.(6,9)2.(19,22)2(31,34)]tetratetraconta-6(43),7,9(44),19(41),20,22(42),31(39),32,34(40)-nonaene, N(CH2CH2NHCH2-p-xylyl-CH2NHCH2CH2)3N) in a protonated state were examined. Crystallographic results show binding of the acyclic quasiplanar water tetramer [H4L(H2O)4](I)4.2.57H2O (1) in a tetraprotonated cryptand L having an iodide counteranion, where two water molecules reside inside the two tren-based cavity, bridged by a third water molecule, and a fourth external water molecule is hydrogen bonded to the bridged water molecule. In the case of complexes [H6L(Br)][(Br)6H].4H2O.2HBr (2) and [H6L(Cl)][(Cl)6H].10.86H2O (3), a single bromide and chloride occupied, respectively, the inside of the cryptand cavity, where L is in a hexaprotonated state. Monotopic recognition of bromide/chloride was observed at the center of the cryptand cavity where halides show C-H...halide interactions instead of the N-H...halide interactions reported in the ditopic complexes of halides with the same cryptand, 5 and 6. Thermal analyses on 1-3 were carried out, and the data obtained distinctly differentiate water cluster complex 1 from the anion-encapsulated cryptates 2 and 3. This study represents the first example of anion-controlled cluster formation inside the cavity of a cryptand.

A novel field of ab initio studies: complexation of simple anions within neutral cryptands

We present here an ab initio study on anion guest - macrocycle host relationships. The complexation of different simple inorganic anions (F - , Cl - , BF 4 - and ClO 4 - ) with the neutral cryptand, the macrotricycle (H 3 BN) 4 [(CH 2 ) n ] 6 , (n=3-6), has been studied using ab initio Hartree-Fock calculations and density functional theory using the B3LYP hybrid functional. As the side-lengths of the pure cryptands increase with the oligomeric sidechain (n), the cryptand cavity increases as n 3 . F - is the most strongly preferred anion for all cryptands, only for the largest host cavity (n=6) complexes are formed exothermally for all anion guests. Morokuma-Kitaura decomposition has been performed to analyse the total interaction energies within the created supermolecules in physically relevant terms including the importance of treatment of the basis set superposition error. According to this scheme the polarisation/charge transfer parts differ in importance amongst the complexes in facilitating a strong binding of the anion guests, but the dominant balance being that of Coulombic attraction and Pauli repulsion. © 2004 Elsevier B.V. All rights reserved.

Attachment of an electron-withdrawing fluorophore to a cryptand for modulation of fluorescence signaling.

The laterally nonsymmetric aza cryptand synthesized by condensing tris(2-aminoethyl)amine (tren) with tris[2-[(3-(oxomethyl)phenyl)oxy]ethyl]mine readily forms mononuclear inclusion complexes with both transition- and main-group-metal ions. The fluorophore 7-nitrobenz-2-oxa-1,3-diazole is attached to one of the secondary amines, to give an integrated fluorophore-receptor configuration. The fluorophoric system does not show any appreciable emission when excited due to an efficient photoinduced intramolecular electron transfer (PET) from the nitrogen lone pair. When a metal ion enters the cavity, the PET is blocked, causing recovery of fluorescence; Cd(II) gives the highest quantum yield. The fluorophore, with pi-accepting ability, drastically alters the binding property of the cryptand. With perchlorate or tetrafluoroborate salts of Cd(II), the metal ion enters the cavity, causing recovery of fluorescence. However, in the presence of coordinating ions such as Cl-, N3-, and SCN-, the metal ion comes out of the cavity, causing PET to take place once again, and the fluorescence is lost. Thus, translocation of Cd(II) between the inside and outside of the cryptand cavity can lead to a reversible fluorescence on/off situation.