Ammonium Guests Research Articles

Kinetic-thermodynamic correlation of conformational changes in ammonium complexes of a flexible naphthocage.

Conformational changes in non-covalent complexes are of fundamental importance to many chemical and biological processes. Yet, these low-energy structural changes are usually fast and difficult to monitor, which poses challenges in their detailed kinetic understanding. The correlation between kinetics and thermodynamics of the conformational change of a model supramolecular system featuring a flexible naphthocage and quaternary ammonium guests is described in this work. Guest binding initially locks the host in two major conformations, which then equilibrates over time to the more stable conformer. The overall rate of the system to attain conformational equilibrium is found to inversely correlate with the thermodynamic stability of the host-guest complexes, and hence not only can the kinetic parameters of the conformational exchange be predicted from the easily obtainable thermodynamic data, but the kinetic profile can also be rationalized by using the structural properties of the different guests.

Binding Thermodynamics of Ferrocenyl Ammoniums by Sulfonated Calix[4]arenes

AbstractThe binding constants (Ka), standard molar enthalpy changes (ΔH°), and entropy changes (TΔS°) between two sulfonated calix[4]arene hosts including rigid p‐sulfonatocalix[4]arene (SC4A) and flexible p‐sulfonatothiacalix[4]arene (STC4A), and three ferrocenyl ammonium guests modified by different aliphatic chains were determined in aqueous solution (pH 7.0 and pH 2.0) by isothermal titration microcalorimetry (ITC). The gained data indicated that for the flexible host STC4A, the host‐guest binding affinities increased with the increase of length of the aliphatic chain of guests whereas the rigid host SC4A behaved almost oppositely. Compared with those at pH 2.0, the binding affinities between the hosts and the guests were much stronger at pH 7.0. This should be attributed to the higher π‐electron density and the extra negative charge of calixarene cavities owing to one of the phenolic hydroxyl groups is deprotonated at pH 7.0, thus affording stronger π⋯π, C−H⋯π and electrostatic interactions. Job plot and ITC confirmed that the molecular binding were stoichiometric 1 : 1 between hosts and guests. NMR experiments showed that the signals of the guest protons with addition of hosts underwent pronounced upfield shifts suggesting the effective binding between hosts and guests. In addition, the molecular selective binding behavior was discussed from the perspective of thermodynamics.

Catalytic Inhibition of Base-Mediated Reactivity by a Self-Assembled Metal-Ligand Host.

Spacious M4 L6 tetrahedra can act as catalytic inhibitors for base-mediated reactions. Upon adding only 5 % of a self-assembled Fe4 L6 cage complex, the conversion of the conjugate addition between ethylcyanoacetate and β-nitrostyrene catalyzed by proton sponge can be reduced from 83 % after 75 mins at ambient temperature to <1 % under identical conditions. The mechanism of the catalytic inhibition is unusual: the octacationic Fe4 L6 cage increases the acidity of exogenous water in the acetonitrile reaction solvent by favorably binding the conjugate acid of the basic catalyst. The inhibition only occurs for Fe4 L6 hosts with spacious internal cavities: minimal inhibition is seen with smaller tetrahedra or Fe2 L3 helicates. The surprising tendency of the cationic cage to preferentially bind protonated, cationic ammonium guests is quantified via the comprehensive modeling of spectrophotometric titration datasets.

Controlled Self-Assembly, Isomerism, and Guest Uptake/Release of Charge-Reversible Lanthanide-Organic Octahedral Cages.

Charge plays a crucial role in the function of molecular and supramolecular systems, but coordination hosts capable of orthogonal charge regulation remain elusive so far. In this study, we report the condition-dependent self-assembly of charge-reversible lanthanide-organic tetra-capped octahedral cages, i.e., [Ln6(H3L)4]6+ and [Ln6L4]6-, from a series of lanthanide ions (Ln3+; Ln = Lu, Yb, Eu) and a tritopic tetradentate acylhydrazone ligand (H6L) featuring multiple deprotonation states and propeller conformations. While direct self-assembly under basic conditions produced a mixture of various ΔxΛ6-x-[Ln6L4]6- (x = 0-6) stereoisomers, racemic Δ6- and Λ6-[Ln6L4]6- could be exclusively obtained from the first self-assembly of Δ6- and Λ6-[Ln6(H3L)4]6+ under neutral conditions followed by post-assembly deprotonation. Rich isomerism on the tetra-capped octahedral cages arising from the coupling between the metal-centered Δ/Λ chirality and the ligand conformations has been discussed based on X-ray single-crystal structures of the C3-symmetric Δ3Λ3-Ln6L4 and T-symmetric Δ6/Λ6-Ln6L4 complexes. Host-guest studies confirmed that positively charged rac-Δ6/Λ6-[Ln6(H3L)4]6+ could bind anionic sulfonates, and negatively charged rac-Δ6/Λ6-[Ln6L4]6- exhibited strong encapsulation ability toward ammonium guests, where acid/base-triggered guest uptake/release could be realized taking advantage of the charge reversibility of the cage. Moreover, photophysical studies revealed visible-light-sensitized and guest-encapsulation-enhanced NIR emissions on the rac-Δ6/Λ6-Yb6L4 cage. This work not only enriches the library of functional lanthanide-organic cages but also provides a promising candidate with charge reversibility for the development of smart supramolecular materials.

Solution- and gas-phase study of binding of ammonium and bisammonium hydrocarbons to oxacalix[4]arene carboxylate.

Oxacalixarenes represent a distinctive class of macrocyclic compounds, which are closely related to the parent calixarene family, offering binding motifs characteristic of calixarenes and crown ethers. Nevertheless, they still lack extensive characterization in terms of molecular recognition properties and the subsequent practical applicability. We present here the results of binding studies of an oxacalix[4]arene carboxylate macrocycle toward a variety of organic ammonium cationic species. Our results show that the substituents attached to the guest ammonium compound largely influence the binding strengths of the host. Furthermore, we show that the characteristic binding pattern changes upon transition from the gas phase to solution in terms of the governing intermolecular interactions. We identify the key factors affecting host-guest binding efficacy and suggest rules for the important molecular structural motifs of the interacting parts of ammonium guest species and the macrocycle to facilitate sensing of ammonium cations.

Confused-Prism[5]arene: a Conformationally Adaptive Host by Stereoselective Opening of the 1,4-Bridged Naphthalene Flap.

The confused-prism[5]arene macrocycle (c-PrS[5]Me ) shows conformational adaptive behavior in the presence of ammonium guests. Upon guest inclusion, the 1,4-bridged naphthalene flap reverses its planar chirality from pS to pR (with reference to the pS(pR)4 enantiomer). Stereoselective directional threading is also observed in the presence of directional axles, in which up/down stereoisomers of homochiral (pR)5 -c-PrS[5]Me pseudorotaxanes are formed.

Multicolor emission based on a N, N'-Disubstituted dihydrodibenzo [a, c] phenazine crown ether macrocycle.

Dynamic fluorophore 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) affords a new platform to produce diverse emission outputs. In this paper, a novel DPAC-containing crown ether macrocycle D-6 is synthesized and characterized. Host-guest interactions of D-6 with different ammonium guests produced a variety of fluorescence with hypsochromic shifts up to 130nm, which are found to be affected by choice of solvent or guest and host/guest stoichiometry. Formation of supramolecular complexes were confirmed by UV-vis titration, 1H NMR and HRMS spectroscopy.

Molecular Recognition in an Aqueous Medium Using Water-Soluble Prismarene Hosts.

The synthesis of water-soluble prism[n]arenes (n = 5 and 6) bearing anionic carboxylato groups on the rims is described. The binding properties of this novel class of water-soluble hosts are studied by nuclear magnetic resonance and calorimetry. The complexation of singly and doubly charged ammonium guests with the more rigid pentamer is enthalpically driven by secondary interactions, while the thermodynamic fingerprint for the larger hexamer reveals driving forces that strongly depend on the guest charge and/or size.

Conformation of Benzo-12-Crown-4 Complexes with Ammonium Ions Investigated by Cold Gas-Phase Spectroscopy.

In this study, we examined the conformation and intermolecular interactions of benzo-12-crown-4 (B12C4) complexes with NH4+, CH3NH3+ (MeNH3+), CH3CH2NH3+ (EtNH3+), and CH3CH2CH2NH3+ (PrNH3+) using cold gas-phase spectroscopy. All of the B12C4 complexes showed sharp vibronic features in the UV photodissociation spectra, and the position of the 0-0 band was close to that of the B12C4 complex with an isotropic K+ guest. This result suggests that the conformation of B12C4 is maintained despite oriented interactions with ammonium guests via anisotropic N-H···O interactions. Further, we measured the IR-UV double-resonance spectra of these complexes in the NH stretching region. In the IR-UV spectra of the EtNH3+ and PrNH3+ complexes, two distinct IR fingerprints were observed depending on the UV probe wavelength selected, indicating the existence of another (second) conformer for these complexes. Quantum chemical calculations clarified that the second conformer of the EtNH3+ and PrNH3+ complexes was partially stabilized by the C-H···π hydrogen bond. The conformation of B12C4 complexes with ammonium ions is strongly affected by the interaction between the alkyl chain of the ion guest and the benzene ring of the B12C4 host, although the main intermolecular interaction occurs between the NH3+ group and crown cavity through the N-H···O hydrogen bonds.

Chiral Molecular Cages Based on Cyclotriveratrylene and Sucrose Units Connected with p‐Phenylene Linkers

AbstractFour diastereomeric molecular cages based on cyclotriveratrylene (CTV) and sucrose scaffolds, connected via p‐phenylene linkers, were synthesized in a few steps synthesis. The absolute configuration of these diastereoisomers was determined by advanced NMR and ECD spectroscopies supported by DFT calculations. The X‐Ray crystal structure of P‐4 b cage gave an insight into its unique structure. The binding properties of P‐4 a and M‐4 a cages towards small ammonium guests (1‐methylpyridinium and 1,3‐dimethylimidazolium salts) were investigated.

AMOEBA binding free energies for the SAMPL7 TrimerTrip host-guest challenge.

As part of the SAMPL7 host-guest binding challenge, the AMOEBA force field was applied to calculate the absolute binding free energy for 16 charged organic ammonium guests to the TrimerTrip host, a recently reported acyclic cucurbituril-derived clip host structure with triptycene moieties at its termini. Here we report binding free energy calculations for this system using the AMOEBA polarizable atomic multipole force field and double annihilation free energy methodology. Conformational analysis of the host suggests three families of conformations that do not interconvert in solution on a time scale available to nanosecond molecular dynamics (MD) simulations. Two of these host conformers, referred to as the "indent" and "overlap" structures, are capable of binding guest molecules. As a result, the free energies of all 16 guests binding to both conformations were computed separately, and combined to produce values for comparison with experiment. Initial ranked results submitted as part of the SAMPL7 exercise had a mean unsigned error (MUE) from experimental binding data of 2.14kcal/mol. Subsequently, a rigorous umbrella sampling reference calculation was used to better determine the free energy difference between unligated "indent" and "overlap" host conformations. Revised binding values for the 16 guests pegged to this umbrella sampling reference reduced the MUE to 1.41kcal/mol, with a correlation coefficient (Pearson R) between calculated and experimental binding values of 0.832 and a rank correlation (Kendall τ) of 0.65. Overall, the AMOEBA results demonstrate no significant systematic error, suggesting the force field provides an accurate energetic description of the TrimerTrip host, and an appropriate balance of solvation and desolvation effects associated with guest binding.

Kinetic and Thermodynamic Behaviors of Pseudorotaxane Formation with C3v Macrocyclic BODIPY Trimers and the Remarkable Substituent Effect on Ring-Face Selectivity.

The thermodynamic and kinetic behaviors of the pseudorotaxane formation between the C3v macrocyclic BODIPY trimers and unsymmetrical secondary ammonium guests are investigated. We find a remarkable substituent effect of the BODIPY trimer on the ring-face selectivity during the threading. The difference in the small substituents (H or CH3) in the macrocyclic host molecules significantly modulated the thermodynamic and kinetic selectivity of the threading direction of the unsymmetrical ammonium ions.

Helic[1]triptycene[3]arene: Synthesis, Complexation, and Formation of [2]Rotaxane Shuttle.

A new macrocyclic arene, helic[1]triptycene[3]arene H, was conveniently synthesized in 37% yield by a one-pot reaction starting from 2,6-dimethoxyl-3,7-dihydroxymethyltriptycene. Macrocycle H showed fixed conformation in solution and could form 1:1 complexes with a series of neutral guests, secondary ammonium salts, and tertiary ammonium salts in both solution and solid states. The association constants between H and the neutral guests were between (1.23 ± 0.10) × 102 and (4.70 ± 0.47) × 103 M-1, while the association constants between H and the ammonium guests were between (1.35 ± 0.12) × 103 and (1.59 ± 0.14) × 105 M-1. Moreover, H showed bigger association constants with secondary ammonium salts than those with tertiary ammonium salts possibly because of the steric hindrance effect and multiple intermolecular interactions. The stimuli-responsive complexation between H and the ammonium salts could be controlled by the addition and removal of acids and bases as well. Based on the host-guest complexation between H and the secondary ammonium salt, [2]rotaxane was further synthesized, and its shuttling motion could be efficiently controlled by an acid and base.

Measuring the Stability of Supramolecular Complexes in the Proximity of Single-Walled Carbon Nanotubes.

Invited for this month's cover is Emilio M. Pérez from IMDEA Nanociencia in Madrid. The cover picture shows the supramolecular association of an ammonium guest to a crown-ether host which is covalently attached to a SWNT. In our manuscript, we show that the corresponding "KPOW!!" would be significantly louder for the binding of the free species in solution, in the absence of the SWNT. Read the full text of their Communication at 10.1002/open.202000018.

Entropy-Driven Cooperativity in the Guest Binding of an Octaphosphonate Bis-cavitand.

Uncommon entropy-driven cooperativity is reported in the guest binding of an octaphosphonate bis-cavitand. Isothermal titration calorimetry determined the thermodynamic parameters for the 1:2 host-guest binding of bis-cavitands with ammonium guests in methanol, ethanol, 2-propanol, and chloroform. Chloroform drove uncommon entropy-driven cooperative binding, whereas the alcohols resulted in enthalpy-driven noncooperative binding. 1 H NMR studies revealed that each cavity contained six water molecules in chloroform, which were liberated on guest binding. The enthalpy-entropy compensation relationship produced a large positive intrinsic entropy in chloroform, which implies that water desolvation causes a considerable entropic gain by paying an enthalpic penalty due to breaking the hydrogen-bonding networks of the water clusters.

Prismarenes: A New Class of Macrocyclic Hosts Obtained by Templation in a Thermodynamically Controlled Synthesis.

The novel title macrocycles, based on methylene-bridged 1,5-naphthalene units, have been obtained by template effect in a thermodynamically controlled synthesis. In detail, the prism[5]arene 1 or the prism[6]arene 3 was selectively removed from the equilibrium mixture by using the complementary ammonium-templating agent. When only the solvent 1,2-DCE was used, the 1,4-confused derivative 2 was obtained. The prism[5]arene here described shows a deep π-electron-rich aromatic cavity that exhibits a great affinity for the quaternary ammonium guests, originating from favorable cation···π and +NC–H···π interactions. This recognition motif is the basis of the templated synthesis of the prism[n]arenes here reported.

Synthesis and Host-Guest Properties of Acyclic Pillar[n]naphthalenes.

Here we report a new class of synthetic receptors, acyclic pillar[n]naphthalene (n = 2–4, Dimer, Trimer, and Tetramer) oligomers, which are made up of 2,3-diethoxynaphthalene units linked by methylene bridges at the 1- and 4-positions. They can be synthesized through a one-step condensation of 2,3-diethoxynaphthalene monomer and paraformaldehyde in the presence of BF3•(Et)2O catalyst. The crystal structure of Tetramer has an interesting pseudo-cycle shaped structure in the solid state. Their complexation behaviors toward several organic ammonium cations (1+-15+) and electron–deficient neutral guests (16–17), were examined by means of 1H NMR spectroscopy. Tetramer shows good host-guest properties toward the ammonium guests, giving association constants (Ka) in the magnitude of 102-104 M−1, which are comparable with those for some macrocyclic hosts.

Pillar[5]arene-Stabilized Silver Nanoclusters: Extraordinary Stability and Luminescence Enhancement Induced by Host-Guest Interactions.

Herein, we report the synthesis of a new class of functional silver nanoclusters (AgNCs) capped with pillar[5]arene (P5)-based host ligands. These NCs are readily prepared through direct synthesis or ligand exchange synthesis and are stable at room temperature for over 4 months. The pillar[5]arene-stabilized NCs (Ag29 (LA-P5)12 (TPP)2 ) endorse reversible host-guest interactions with neutral alkylamines and cationic quaternary ammonium guests. This results in the formation of spherical assemblies with unparalleled changes in their optical properties including an astonishing circa 2000-fold luminescence enhancement. This is the highest luminescence enhancement ratio reported so far for such atomically precise NCs. Our synthetic protocol paves the way for the preparation of a new generation of metal nanoclusters protected by macrocyclic ligands with molecular recognition and selectivity toward specific guests.

Pillar[5]arene‐Stabilized Silver Nanoclusters: Extraordinary Stability and Luminescence Enhancement Induced by Host–Guest Interactions

AbstractHerein, we report the synthesis of a new class of functional silver nanoclusters (AgNCs) capped with pillar[5]arene (P5)‐based host ligands. These NCs are readily prepared through direct synthesis or ligand exchange synthesis and are stable at room temperature for over 4 months. The pillar[5]arene‐stabilized NCs (Ag29(LA‐P5)12(TPP)2) endorse reversible host–guest interactions with neutral alkylamines and cationic quaternary ammonium guests. This results in the formation of spherical assemblies with unparalleled changes in their optical properties including an astonishing circa 2000‐fold luminescence enhancement. This is the highest luminescence enhancement ratio reported so far for such atomically precise NCs. Our synthetic protocol paves the way for the preparation of a new generation of metal nanoclusters protected by macrocyclic ligands with molecular recognition and selectivity toward specific guests.

Surface Engineering of Fluoropolymer Films via the Attachment of Crown Ether Derivatives Based on the Combination of Radiation-Induced Graft Polymerization and the Kabachnik-Fields Reaction.

In this manuscript, we present the successful attachment of crown ether moieties onto fluoropolymer surfaces, via the combination of radiation-induced graft polymerization and a subsequent surface Kabachnik–Fields three-component reaction. The obtained crown ether-tethered fluoropolymer films exhibited an ammonium cation capturing ability, owing to the host–guest interactions (i.e., hydrogen bonding) between the surface-anchored crown ethers and the guest ammonium cations.