VT-NMR Experiments Research Articles

Tri and hexaosmium clusters bearing saccharinate and diphosphine ligands: Synthesis, structure and fluxionality

The reactivity of saccharinate-bridged triosmium cluster [Os3(CO)10(μ-H)(μ-sac)] (1) with two diphosphines namely bis(diphenylphosphino)methane (dppm) and 1,1′-bis(diphenylphosphino)ferrocene (dppf) have been investigated. Cluster 1 reacts with dppm in refluxing toluene to yield triosmium [Os3(CO)9(κ1-dppm)(μ-H)(μ-sac)] (2), [Os3(CO)9(κ1-dppm)(μ-H)(μ-sac)] (3) and [Os3(CO)8(µ-dppm)(μ-H)(µ-sac)] (4) along with the hexaosmium [{Os3(CO)9(μ-H)(µ-sac)}2(κ2-dppm)] (5). The dppm ligand acts as a monodentate ligand in 2 and 3 by coordinating to a single osmium, while it bridges an osmium-osmium edge in 4 and connects to triosmium units in 6. A similar reaction with dppf furnishes [Os3(CO)9(κ1-dppf)(μ-H)(μ-sac)] (6), [Os3(CO)8(µ-dppf)(μ-H)(μ-sac)] (7) and [{Os3(CO)9(μ-H)(µ-sac)}2(κ2-dppf)] (8) which are dppf analogues of 3–5, respectively. Control experiments confirms that 3 is the precursor of 4 and 5, while 6 is the precursor of 7 and 8. All the products are formed via substitution of carbonyl by the diphosphine, and have been characterized by analytical and spectroscopic data. The molecular structures of 2 and 4–7 have been determined by X-ray crystallography. Cluster 7 shows fluxional behavior in solution at room temperature, and its fluxionality has been probed by VT NMR experiments.

A Triply Linked Porphyrin-Norcorrole Hybrid with Singlet Diradical Character.

Norcorrole Ni(II) complexes have recently received considerable attention because they are readily accessible antiaromatic molecules. Their high stability under ambient conditions and ease of synthesis have enabled the exploration of the intrinsic properties of antiaromatic molecules. Here, we report the synthesis and properties of meso-meso singly linked porphyrin-norcorrole hybrids and a triply linked porphyrin-norcorrole hybrid. The singly linked and triply linked porphyrin-norcorrole hybrids were fully characterized, including an X-ray structural analysis. Due to their orthogonal conformation, the singly linked hybrids maintain the individual electronic properties of their porphyrin and norcorrole subunits, while the triply linked hybrid shows a significantly smaller electrochemical HOMO-LUMO gap (0.45 eV) than that of Ni(II) dimesitylnorcorrole (1.08 eV). Furthermore, the triply linked hybrid exhibits singlet diradical characteristics, as confirmed by VT NMR, ESR, and SQUID experiments.

Reactivity of [Os3(CO)9(μ3-η1,κ1-C9H6N)(μ-H)] towards Ph3EH (E = Si, Ge, Sn): Synthesis and structure of triosmium clusters bearing a Ph3E ligand

The reactions of quinoline-capped triosmium cluster [Os3(CO)9(μ3-η1,κ1-C9H6N)(μ-H)] (1) with Ph3EH (E = Si, Ge, Sn) have been investigated. Cluster 1 reacts with Ph3EH in refluxing benzene to afford [Os3(CO)9(EPh3)(μ-η1,κ1-C9H6N)(µ-H)2] (2a, E = Si; 2b, E = Ge; 2c, E = Sn) and [Os3(CO)8(EPh3)(μ3-η1,κ1-C9H6N)(µ-H)2] (3a, E = Si; 3b, E = Ge; 3c, E = Sn). Control experiments confirm that 2 is a precursor of 3. Cluster 2 is formed via oxidative-addition of E–H bond to 1 whose decarbonylation leads to the formation of 3. The molecular structures of all the new clusters (except 3c) have been determined by X-ray crystallography. Both 2 and 3 exist as a pair of isomers in solution, and their fluxionality in solution has been probed by VT NMR experiments.

Stochastic Bullvalene Architecture Modulates Structural Rigidity in π-Rich Macromolecules.

The synthesis and processing of π-rich polymers found in novel electronics and textiles is difficult because chain stiffness leads to low solubility and high thermal transitions. The incorporation of "shape-shifting" molecular cages into π-rich backbone provides an ensemble of structural kinks to modulate chain architecture via a self-contained library of valence isomers. In this work, we report the synthesis and characterization of (bullvalene-co-phenylene)s that feature smaller persistence lengths than a prototypical rigid rod polymer, poly(p-phenylene). By varying the amount of bullvalene incorporation within a poly(p-phenylene) chain (0-50 %), we can tune thermal properties and solution-state conformation. These features are caused by stochastic bullvalene isomers within the polymer backbone that result in kinked architectures. Synthetically, bullvalene incorporation offers a facile method to decrease structural rigidity within π-rich materials without concomitant crystallization. VT NMR experiments confirm that these materials remain dynamic in solution, offering the opportunity for future stimuli-responsive applications.

Stochastic Bullvalene Architecture Modulates Structural Rigidity in π‐Rich Macromolecules

AbstractThe synthesis and processing of π‐rich polymers found in novel electronics and textiles is difficult because chain stiffness leads to low solubility and high thermal transitions. The incorporation of “shape‐shifting” molecular cages into π‐rich backbone provides an ensemble of structural kinks to modulate chain architecture via a self‐contained library of valence isomers. In this work, we report the synthesis and characterization of (bullvalene‐co‐phenylene)s that feature smaller persistence lengths than a prototypical rigid rod polymer, poly(p‐phenylene). By varying the amount of bullvalene incorporation within a poly(p‐phenylene) chain (0–50 %), we can tune thermal properties and solution‐state conformation. These features are caused by stochastic bullvalene isomers within the polymer backbone that result in kinked architectures. Synthetically, bullvalene incorporation offers a facile method to decrease structural rigidity within π‐rich materials without concomitant crystallization. VT NMR experiments confirm that these materials remain dynamic in solution, offering the opportunity for future stimuli‐responsive applications.

Atropisomerism about aryl-C(sp3) bonds: chemically driven rotational pathway in cannabidiol derivatives.

The conformational behaviour arising from the restricted C(sp2)-C(sp3) axis in ortho O-substituted naphthylcyclohexane and naphthylcyclohexene oxide derivatives of cannabidiol was examined by means of VT-NMR experiments and DFT calculations. Atropisomeric compounds with barriers in the range of 91.1 to 95.1 kJ mol-1 were obtained at 298 K. Two possible transition states (TS1 and TS2) were located, one is more stable depending on the chemical modification made on the monoterpene ring close to the pivot bond. Extended analysis of TS structures to previously reported phenyl derivatives bearing the same O-substituent led to similar rotational pathways according to the series: through TS1 in arylcylohexenes and TS2 in arylcyclohexanes. Likewise, conversion of arylcyclohexenes into both series affects the rotation speed by decelerating it, and the nature of the aryl ring seems to have a very minor effect on this phenomenon.

Synthesis and Stereochemical Analysis of Ten-membered Diallylic Nitrogen Cycle and Its Pt-Complex

Newly synthesized ten-membered diallylic nitrogen cycle, having E- and Z-alkene moieties, shows labile planar-chirality at ambient temperature. VT-NMR experiments revealed the detail of the stereochemical behavior of interconversion of the enantiomers and diastereomers. In sharp contrast, Pt-complex of the nitrogen cycle have more stable planar chirality, and the enantiomers of the Pt-complex were isolated by HPLC using a chiral stationary phase.

Bispidine Platform as a Tool for Studying Amide Configuration Stability.

In this work, the solution conformations of seventeen 3,7-diacyl bispidines were studied by means of NMR spectroscopy including VT NMR experiments. The acyl groups included alkyl, alkenyl, aryl, hetaryl, and ferrocene moieties. The presence of syn/anti-isomers and their ratios were estimated, and some reasons explaining experimental facts were formulated. In particular, all aliphatic and heterocyclic units in the acylic R(CO) fragments led to an increased content of the syn-form in DMSO-d6 solutions. In contrast, only the anti-form was detected in DMSO-d6 and CDCl3 in the case when R = Ph, ferrocenyl, (R)-myrtenyl. In the case of a chiral compound derived from the natural terpene myrtene, a new dynamic process was found in addition to the expected inversion around the amide N-C(O) bond. Here, rotation around the CO-C=C bond in the acylic R fragment was detected, and its energy was estimated. For this compound, ΔG for amide N-C(O) inversion was found to be equal to 15.0 ± 0.2 kcal/mol, and for the rotation around the N(CO)–C2′ bond, it was equal to 15.6 ± 0.3 kcal/mol. NMR analysis of the chiral bispidine-based bis-amide was conducted for the first time. Two X-ray structures are reported. For the first time, the unique syn-form was found in the crystal of an acyclic bispidine-based bis-amide. Quantum chemical calculations revealed the unexpected mechanism for amide bond inversion. It was found that the reaction does not proceed as direct N-C(O) bond inversion in the double-chair (CC) conformation but rather requires the conformational transformation into the chair–boat (CB) form first. The amide bond inversion in the latter requires less energy than in the CC form.

Allylnickel(II) complexes of bulky 5-substituted-2-iminopyrrolyl ligands

The optimized reaction between [Ni(COD)2] (COD = 1,5-cyclooctadiene) and ligand precursor 5-(2,4,6-triisopropylphenyl)-2-[N-(2,6-diisopropylphenyl)-formimino]-1H-pyrrole yielded the η3-cyclooctenyl-Ni(II) complex [Ni{κ2N,N’-5-(2,4,6-iPr3C6H2)-NC4H2-2-C(H) = N(2,6-iPr2C6H3)}(η3-C8H13)] 1. Subsequently, the η3-allyl complexes [Ni{κ2N,N’-5-R-NC4H2-2-C(H)=N(2,6-iPr2C6H3)}(η3-C3H5)] (R = 3,5-(CF3)2C6H3 (2a), 2,6-Me2C6H3 (2b), 2,4,6-iPr3C6H2 (2c) and CPh3 (2d)) were prepared in good yields via metathesis of [Ni(η3-C3H5)(µ-Br)]2 with the respective potassium 5-R-2-[N-(2,6-diisopropylphenyl)formimino]pyrrolyl salt (KLa-d). Complexes 1 and 2a-d were characterized by NMR spectroscopy, elemental analysis and complex 2d further analyzed by single crystal X-ray diffraction. Addition of excess pyridine to solutions of complexes 2a-d led to the observation of a fluxional process that, according to VT-NMR experiments, corresponds to a pyridine-assisted cis–trans isomerization process occurring in these complexes, via a η3-η1-η3 haptotropic shift of the allyl ligand, with ΔG‡ values in range of 9.5–17.3 kcal mol−1. Additionally, complexes 2a-d, when activated by B(C6F5)3, slowly catalyzed the isomerization of hex-1-ene to mixtures of internal olefins.

Role of Macrocyclic Conformational Steering in a Kinetic Route toward Bielschowskysin.

Macrocyclic furanobutenolide-derived cembranoids (FBCs) are the biosynthetic precursors to a wide variety of highly congested and oxygenated polycyclic (nor)diterpenes (e.g. plumarellide, verrillin, and bielschowskysin). These architecturally complex metabolites are thought to originate from site-selective oxidation of the macrocycle backbone and a series of intricate transannular reactions. Yet the development of a common biomimetic route has been hampered by a lack of synthetic methods for the pivotal furan dearomatization in a regio- and stereoselective manner. To address these shortcomings, a concise strategy of epoxidation followed by a kinetically controlled furan dearomatization is reported. The surprising switch of facial α:β-discrimination observed in the epoxidation of the most strained E-acerosolide versus E-deoxypukalide and E-bipinnatin J derived macrocycles has been rationalized by the variation of the 3D conformational landscape between macrocyclic scaffolds. A careful conformational analysis of these macrocycles by VT-NMR and NOESY experiments at low temperature was supported by DFT calculations to characterize these equilibrating macrocyclic conformers. The shift in conformational topology associated with a swing of the butenolide ring in E-deoxypukalide is in general agreement with the reversal of β-selectivity observed in the epoxidation. We also describe the downstream functionalization of FBC-macrocycles and how the C-7 epoxide configuration is retentively translated to the C-3 stereogenicity in dearomatized products under kinetic control to secure the requisite 3S,7S,8S configurations for the bielschowskysin synthesis. Unlike previously speculated, our results suggest that the most strained FBC-macrocycles bearing a E-(Δ7,8)-alkene moiety may stand as the true biosynthetic precursors to bielschowskysin and several other polycyclic natural products of this class.

Cyclic RGD and isoDGR Integrin Ligands Containing cis-2-amino-1-cyclopentanecarboxylic (cis-β-ACPC) Scaffolds.

Integrin ligands containing the tripeptide sequences Arg-Gly-Asp (RGD) and iso-Asp-Gly- Arg (isoDGR) were actively investigated as inhibitors of tumor angiogenesis and directing unit in tumor-targeting drug conjugates. Reported herein is the synthesis, of two RGD and one isoDGR cyclic peptidomimetics containing (1S,2R) and (1R,2S) cis-2-amino-1-cyclopentanecarboxylic acid (cis-β-ACPC), using a mixed solid phase/solution phase synthetic protocol. The three ligands were examined in vitro in competitive binding assays to the purified αvβ3 and α5β1 receptors using biotinylated vitronectin (αvβ3) and fibronectin (α5β1) as natural displaced ligands. The IC50 values of the ligands ranged from nanomolar (the two RGD ligands) to micromolar (the isoDGR ligand) with a pronounced selectivity for αvβ3 over α5β1. In vitro cell adhesion assays were also performed using the human skin melanoma cell line WM115 (rich in integrin αvβ3). The two RGD ligands showed IC50 values in the same micromolar range as the reference compound (cyclo[RGDfV]), while for the isoDGR derivative an IC50 value could not be measured for the cell adhesion assay. A conformational analysis of the free RGD and isoDGR ligands by NMR (VT-NMR and NOESY experiments) and computational studies (MC/EM and MD), followed by docking simulations performed in the αVβ3 integrin active site, provided a rationale for the behavior of these ligands toward the receptor.

NHC-stabilized copper(I) aryl complexes and their transmetalation reaction with aryl halides

We report herein the synthesis, spectroscopic and structural characterization of a series of NHC-stabilized copper aryl complexes (NHC = N-heterocyclic carbene) of the type [Cu(NHC)(Ar)] 1–5, 7–22 with various aryl ligands (4-Me-C6H4, 4-MeO-C6H4, 2,4,6-Me3C6H2, C6F5, 4-CF3-C6H4, (3,5-(CF3)2C6H3), 2,3,5,6-Me4-C6H, 2,6-iPr2-C6H3, C6Me5) and NHCs (IiPr, ItBu, IMes, IPr). Three different synthetic routes were used to obtain these complexes: (i) reaction of a copper(I) aryl complex with a free NHC; (ii) reaction of an NHC-stabilized copper(I) chloride with a Grignard reagent, which is one important step in Kumada-Tamao-Corriu type cross-coupling reactions; and (iii) reaction of an NHC-stabilized copper(I) alkoxide with an organoboronic ester, which represents an important step in Suzuki-Miyaura type cross-coupling reactions. The latter process was investigated in detail by VT-NMR experiments using [Cu(IPr)(OtBu)] and 4-CF3-C6H4Bpin as an example, revealing the formation of an adduct (A) as a reaction intermediate. Furthermore, the reactivity of the [Cu(NHC)(Ar)] complexes towards aryl halides was investigated.

Phenoxyamidine Zn and Al Complexes: Synthesis, Characterization, and Use in the Ring-Opening Polymerization of Lactide

Herein, we report the synthesis of new ditopic ligands, which consist of a phenoxy group and N,N,N′-trisubstituted amidines linked by a methylene spacer (L1–L4). Their coordination chemistry has been investigated with Zn(II) and Al(III). Alkane elimination route between the phenol-amidine proligands (L1H–L4H) and Et2Zn led to dinuclear complexes [(L1–L4)ZnEt]2 (1a–4a) in which the Zn centers are chelated by phenoxyamidine ligands and bridged through the oxygen atom of the phenoxy groups. Salt metathesis reaction between two equivalents of the sodium amidine phenate L1Na and ZnCl2 led to a bis-chelate chiral spiro-complex (L12Zn) 1a′. Analogous alkane elimination route between AlMe3 and the phenol-amidine proligands L1H–L4H allowed the preparation of the mononuclear complexes [(L1–L4)AlMe2] (1b–4b). The phenoxyamidine-Al and -Zn complexes have been characterized by NMR spectroscopy, elemental analysis, and/or high-resolution ESI-MS. The solid state structures of the proligands [L1H2][Br] and L2H as well as of six complexes have been established by single crystal X-ray diffraction analysis. Fluxional properties of the proligands L1H–L2H and of the complexes 1a and 2b have been investigated by VT NMR experiments. In the presence of an alcohol source, complexes 1a–4a and 1b–4b were used as initiators for the controlled ring-opening polymerization (ROP) of rac-lactide to afford atactic polylactic acid (PLA).

Variable-Temperature Multinuclear Solid-State NMR Study of Oxide Ion Dynamics in Fluorite-Type Bismuth Vanadate and Phosphate Solid Electrolytes

Ionic-conducting materials are crucial for the function of many advanced devices used in a variety of applications, such as fuel cells and gas separation membranes. Many different chemical controls, such as aliovalent doping, have been attempted to stabilise δ-Bi2O3, a material with exceptionally high oxide ion conductivity which is unfortunately only stable over a narrow temperature range. In this study, we employ a multinuclear, variable-temperature NMR spectroscopy approach to characterise and measure oxide ionic motion in the V- and P-substituted bismuth oxide materials Bi0.913V0.087O1.587, Bi0.852V0.148O1.648 and Bi0.852P0.148O1.648, previously shown to have excellent ionic conduction properties (Kuang et al., Chem. Mater. 2012, 24, 2162; Kuang et al., Angew. Chem. Int. Ed. 2012, 51, 690). Two main 17O NMR resonances are distinguished for each material, corresponding to O in the Bi–O and V–O/P–O sublattices. Using variable-temperature (VT) measurements ranging from room temperature to 923 K, the ionic motion experienced by these different sites has then been characterised, with coalescence of the two environments in the V-substituted materials clearly indicating a conduction mechanism facilitated by exchange between the two sublattices. The lack of this coalescence in the P-substituted material indicates a different mechanism, confirmed by 17O T1 (spin-lattice relaxation) NMR experiments to be driven purely by vacancy motion in the Bi–O sublattice. 51V and 31P VT-NMR experiments show high rates of tetrahedral rotation even at room temperature, increasing with heating. An additional VO4 environment appears in 17O and 51V NMR spectra of the more highly V-substituted Bi0.852V0.148O1.648, which we ascribe to differently distorted VO4 tetrahedral units that disrupt the overall ionic motion, consistent both with linewidth analysis of the 17O VT-NMR spectra and experimental results of Kuang et al. showing a lower oxide ionic conductivity in this material compared to Bi0.913V0.087O1.587 (Chem. Mater. 2012, 24, 2162). This study shows solid-state NMR is particularly well suited to understanding connections between local structural features and ionic mobility, and can quantify the evolution of oxide-ion dynamics with increasing temperature.

Reversible 1,1-hydroaluminations and C-H activation in reactions of a cyclic (alkyl)(amino) carbene with alane.

Varying the reaction ratio of cyclic (alkyl)(amino) carbene (cAACEt) with AlH3·NEtMe2 leads to the isolation of (cAACEtH)AlH2·NEtMe21 and (cAACEtH)2Al(μ-H)2AlH2·NEtMe22 and the first example of a monomeric dialkyl-aluminum hydride (cAACEtH)2AlH 3. VT and 1H-1H EXSY NMR experiments of 3 demonstrated the isomerization of the diastereomers of 3via the first instance of reversible hydride migration between the Al and the C center. In addition, heating solutions of 3 at 100 °C affords (cAACEtH)Al(CHC(Et)2CH2C(Me)2NC6H3(iPr)C(Me)CH2) 4 with loss of H2.

Piperazine-Based N4-Type 16-Membered Macroheterocycles and Their Nickel(II) Complexes

Square-planar diamagnetic nickel(II) complexes 5a and 5b containing 16-membered diamino-diimino ligands were prepared from the corresponding open-chain complexes 2a and 2b via condensation with o-phthalic dialdehyde in methanol. The solid-state structure of the starting complex 2b revealed the cisoid conformation of aryl groups compared to the transoid one found in the case of 2a. At the same time, the cisoid conformation is not retained in acetone solution: rather, the tert-Bu-substituted complex 2b was fully transformed into the trans form whereas its analogue 2aexhibits both cis and transforms in acetone solution. The cisoid conformation was also observed for the cyclic structures 5a and 5b by X-ray analysis and VT NMR experiments. The borohydride reduction of 5a with subsequent cyanide-assisted removal of nickel led to a new 16-membered tetraazamacrocycle 6. Its X-ray structure showed a cisoid conformation supported by two intramolecular hydrogen bonds that was also sustained in solution. VT NMR experiments revealed the degenerative interconvertion of a macrocycle with activation energy ca. 41.9±0.8 kJ/mol.

Design, Synthesis, and Conformational Analysis of Proposed β-Turn Mimics from Isoxazoline-Cyclopentane Aminols.

Constrained aminols from oxazanorbornene derivatives have the geometrical features to be used as β-turn inducers. Four different stereoisomers were prepared and spectroscopically characterized (MD calculations, NMR-titration and VT-NMR experiments). Temperature coefficients in DMSO are indicative for the existence of an intramolecular hydrogen bond. Chirooptical properties revealed a β-turn arrangement of all the synthesized compounds, where, depending on the absolute configuration of the cyclopentane spacer, they can be labeled as left- or right-handed turns.

Deformative transition of the Menschutkin reaction and helical atropisomers in a congested polyheterocyclic system.

A 4,7-phenanthroline polycyclic 1A designed for probing the limits of the Menschutkin reaction was synthesized in a six-step sequence. The rotational barrier of the phenyl ring nearby the N-methyl group in rac-2A was estimated to be ≫ 18.1 kcal/mol from VT-NMR experiments, making them a new type of helical atropisomer. The methylation rate constants of 9 and 1A with MeI was found to be 2.22 × 10(-4) and 9.62 × 10(-6) s(-1) mol(-1) L, respectively; thus, the formation rate of (P/M)-2A is one of the slowest rates ever reported for a Menschutkin reaction. The N-methyl protons in (P/M)-2A exhibit a significant upfield shift (Δδ 1.0 ppm) in its (1)H NMR, compared to those without a nearby phenyl, indicating a strong CH-π interaction is involved. Conformational flexibility in dipyridylethene 9 is clearly shown by its complexation with BH3 to form helical atropisomers (P,P/M,M)-10. The pKa values of the conjugate acids of 1A and 9 in acetonitrile were determined to be 4.65 and 5.07, respectively, which are much smaller compared to that of pyridine 14a (pKa = 12.33), implying that the basicity, nucleophilicity, and amine alkylation rates of 1A and 9 are markedly decreased by the severe steric hindrance of the flanking phenyl rings in the polyheterocycles.

Intramolecular C-H/O-H bond cleavage with water and alcohol using a phosphine-free ruthenium carbene NCN pincer complex.

Transition metal complexes that exhibit metal-ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine-free ruthenium NCN pincer complexes with a central N-heterocyclic carbene donor and methylpyridyl N-donors. Reaction with base generates a neutral, dearomatized alkoxo-amido complex, which has been structurally and spectroscopically characterized. The tert-butoxide ligand facilitates regioselective, intramolecular proton transfer through a CH/OH bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo-amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine-free metal-ligand cooperative water splitting.

A tetraguanidinium macrocycle for the recognition and cavity expansion of calix[4]arene tetraoxoanions

Molecular containers have raised an increased interest over the last decade. These supramolecular architectures have found applications in catalysis, molecular sensing or as insulators for key intermediates, among others. In this study, we describe the synthesis and binding properties of a tetraguanidinium macrocycle which forms robust complexes with diverse calix[4]arene tetraoxoanions through hydrogen bonding and electrostatic interactions. The binding behaviour and affinity strength of these constructs have been measured by NMR and isothermal titration calorimetry. Besides, VT NMR experiments show that this novel cyclic tetracation is able to stabilise the cone conformation of these calix[4]arenes. Preliminary NMR-binding experiments between a tetraguanidinium calix[4]arenate and quinolinium or isoquinolinium salts suggest an effective increase in the cavity volume of these supramolecular constructs.