Tetrathiafulvalene (TTF)‐Bridged Resorcin[4]arene Cavitands: Towards New Electrochemical Molecular Switches

We report the synthesis of novel resorcin[4]arene‐based cavitands featuring two extended bridges consisting of quinoxaline‐fused TTF (tetrathiafulvalene) moieties. In the neutral form, these cavitands were expected to adopt the vase form, whereas, upon oxidation, the open kite geometry should be preferred due to Coulombic repulsion between the two TTF radical cations (Scheme 2). The key step in the preparation of these novel molecular switches was the P(OEt)₃‐mediated coupling between a macrocyclic bis(1,3‐dithiol‐2‐thione) and 2 equiv. of a suitable 1,3‐dithiol‐2‐one. Following the successful application of this strategy to the preparation of mono‐TTF‐cavitand 3 (Scheme 3), the synthesis of the bis‐TTF derivatives 2 (Scheme 4) and 19 (Scheme 5) was pursued; however, the target compounds could not be isolated due to their insolubility. Upon decorating both the octol bowl and the TTF cavity rims with long alkyl chains, the soluble bis‐TTF cavitand 23 was finally obtained, besides a minor amount of the novel cage compound 25a featuring a highly distorted TTF bridge (Scheme 6). In contrast to 25a , the deep cavitand 23 undergoes reversible vase → kite switching upon lowering the temperature from 293 to 193 K (Fig. 1). Electrochemical studies by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) provided preliminary evidence for successful vase → kite switching of 23 induced by the oxidation of the TTF cavity walls.     

meso‐Tetrakis[4‐(heptyloxy)phenyl]porphyrin

The core of the novel title centrosymmetric porphyrin derivative, C₇₂H₈₆N₄O₄, with long flexible hexyloxy substituents, is almost planar, which is anticipated to facilitate π‐electron delocalization and lead to a significant deviation between the planes of the benzene rings and the molecular plane. The two N‐bound H atoms on the pyrrole rings are disordered and the occupancy factors refined to a ratio of 0.28 (2):0.72 (2).     

Synthesis of Methylene-Bridged Resorcin[4]arene Dimers

Methylene-bridged resorcin[4]arene dimers were synthesized by the Sc(OTf)₃-catalyzed hydroxymethylation of partially acetylated resorcin[4]arenes, and their preliminary complexation properties with tetraethylammonium ion were examined in CD₃OD by ¹H NMR spectroscopy. The dimers adopted a closed capsular conformation in this solvent and bound the guest molecule into their cavity.     

Tetrakis(bicyclo[2.2.2]oct‐2‐ene)‐Fused Calix[4]pyrrole

Tetrakis(bicyclo[2.2.2]oct‐2‐ene)‐fused calix[4]pyrrole, 5 , was obtained starting from (E)‐1,2‐bis(phenylsulfonyl)ethylene. This new calixpyrrole derivative is the prospective precursor of tetrabenzocalix[4]pyrrole, a potential ion‐pair receptor and an attractive species as a possible deep‐walled ‘molecular container’.     

Cleft‐Type Diamidinium Receptors for Dicarboxylate Binding in Protic Solvents

A series of potential cleft‐type receptors for dicarboxylate substrates were prepared by attachment of two phenylamidinium ions to either naphthalene or 1,1′‐binaphthalene scaffolds. Their synthesis (Schemes 1 – 4) involved the Pd⁰‐catalyzed cross‐coupling of aryl nitriles to the central scaffold, followed by transformation of the nitrile into amidinium groups using the Garigipati reaction. The 1,1′‐binaphthalene derivative (±)‐ 1 with phenylamidinium residues attached to the 6,6′‐positions in the major groove was found to be a highly efficient receptor for dicarboxylate guests, such as glutarate and isophthalates, even in competing protic solvents such as CD₃OD (Table 1). The van't Hoff analysis of variable‐temperature ¹H‐NMR (VT‐NMR) titrations (Table 2 and Fig. 3) and isothermal microcalorimetry (ITC; Table 3 and Fig. 4) revealed that complexation in MeOH is strongly entropically driven with an unfavorable enthalpic change, which partially compensates the entropic gain. These thermodynamic quantities are best explained by a particularly favorable solvation of the binding partners in the unbound state and the release of the MeOH molecules, which solvate the free ions into the bulk upon complexation. Receptor (±)‐ 1 binds flexible glutarate and rigid isophthalates with similar association strength. This lack in response to guest preorganization and reduced guest selectivity is explained with the non‐directionality of the coulombic charge‐charge interactions in the complexes.     

Thioether‐Functionalized Quinone‐Based Resorcin[4]arene Cavitands: Electroswitchable Molecular Actuators

The utility of molecular actuators in nanoelectronics requires activation of mechanical motion by electric charge at the interface with conductive surfaces. We functionalized redox‐active resorcin[4]arene‐quinone cavitands with thioethers as surface‐anchoring groups at the lower rim and investigated their propensity to act as electroswitchable actuators that can adopt two different conformations in response to changes in applied potential. Molecular design was assessed by DFT calculations and X‐ray analysis. Electronic properties were experimentally studied in solution and thin films electrochemically, as well as by X‐ray photoelectron spectroscopy on gold substrates. The redox interconversion between the oxidized (quinone, Q ) and the reduced (semiquinone, SQ ) state was monitored by UV‐Vis‐NIR spectroelectrochemistry and EPR spectroscopy. Reduction to the SQ state induces a conformational change, providing the basis for potential voltage‐controlled molecular actuating devices.     

NMR Investigations into the Vase‐Kite Conformational Switching of Resorcin[4]arene Cavitands

We report the detailed investigation of temperature‐ and pH‐triggered conformational switching of resorcin[4]arene cavitands 1 – 10 (Figs. 1, 8, and 9). Depending on the experimental conditions, these macrocycles adopt a vase conformation, featuring a deep cavity for potential guest inclusion, or two kite conformations (kite 1 and kite 2) with flat, extended surfaces (Schemes 1 and 2). The thermodynamic and kinetic parameters for the interconversion between these structures were determined by variable‐temperature NMR (VT‐NMR) spectroscopy (Figs. 2–7 and 10, and Tables 1 and 2). It was discovered that vase→kite switching of cavitands is strongly solvent‐dependent: it is controlled not only by solvent polarity but also by solvent size. Conformational interconversions similar to those of the parent structure 1 with four quinoxaline flaps are also observed when the octol base skeleton is differentially or incompletely bridged. Only octanitro derivative 2 was found to exist exclusively in the kite conformation under all experimental conditions. The detailed insight into the vase?kite conformational equilibrium gained in this investigation provides the basis for the design and construction of new, dynamic resorcin[4]arene cavitands that are switchable between bistable states featuring strongly different structures and functions.     

Spatially Directional Resorcin[4]arene Cavitand Glycoconjugates for Organic Catalysis

The synthesis of novel spatially directional multivalent resorcin[4]arene cavitand glycoconjugates (RCGs) and their ability to catalyze organic reactions is reported. The β‐d ‐glucopyranoside moieties on the upper rim of the “bowl”‐shaped resorcin[4]arene cavitand core are capable of multiple hydrogen‐bond interactions resulting in a pseudo‐cavity, which has been investigated for organic transformations in aqueous media. The RCGs have been demonstrated to catalyze thiazole formation, thiocyanation, copper(I)‐catalyzed azide alkyne cycloaddition (CuAAC), and Mannich reactions; they impart stereoselectivity in the three‐component Mannich reaction. Thermodynamic values obtained from ¹H diffusion‐ordered spectroscopy (DOSY) experiments suggest that the upper saccharide cavity of the RCG and not the resorcin[4]arene cavity is the site of the complexation event.     

硫(硒)杂多杯[4]芳烃合成及其络合性能研究

合成了下缘硫、硒杂杯[4]芳烃, 然后与杯芳烃片段2,6-二(溴甲基)-4-甲氧基-甲苯在NaH存在条件下缩合得到硫、硒杂多杯[4]芳烃5a和5b. 离子萃取实验表明硫、硒杂多杯[4]芳烃显著增加了对Ag＋和Hg2＋重金属离子的萃取效果.     

Self‐inclusion structure of 5,11,17,23‐tetrakis(azidomethyl)‐25,26,27,28‐tetrahydroxycalix[4]arene,and 5,11,17,23‐tetra‐tert‐butyl‐25,27‐bis(chloroacetoxy)‐26,28‐bis(2‐pyridylmethoxy)calix[4]arene

In the structures of the two title calix[4]arene derivatives, C₃₂H₂₈N₁₂O₄, (I), and C₆₀H₆₈Cl₂N₂O₆, (II), compound (I) adopts an open‐cone conformation in which there are four intramolecular O—H...O hydrogen bonds, while compound (II) adopts a distorted chalice conformation where the two pendant pyridyl rings, one of which is disordered, are almost mutually perpendicular, with an interplanar angle of 79.2 (2) or 71.4 (2)°. The dihedral angles between the virtual plane defined by the four bridging methylene C atoms and the phenol rings are 120.27 (7), 124.03 (6), 120.14 (8) and 128.25 (7)° for (I), and 95.99 (8), 135.93 (7), 97.21 (8) and 126.10 (8)° for (II). In the supramolecular structure of (I), pairs of molecules associate by self‐inclusion, where one azide group of the molecule is inserted into the cavity of the inversion‐related molecule, and the association is stabilized by weak intermolecular C—H...N hydrogen bonds and π(N₃)–π(aromatic) interactions. The molecular pairs are linked into a two‐dimensional network by a combination of weak intermolecular C—H...N contacts. Each network is further connected to its neighbor to produce a three‐dimensional framework via intersheet C—H...N hydrogen bonds. In the crystal packing of (II), the molecular components are linked into an infinite chain by intermolecular C—H...O hydrogen bonds. This study demonstrates the ability of calix[4]arene derivatives to form self‐inclusion structures.     

Functionalized and Partially or Differentially Bridged Resorcin[4]arene Cavitands: Synthesis and Solid‐State Structures

We report the synthesis and structural characterization of modified Cram‐type, resorcin[4]arene‐based cavitands. Two main loci on the cavitand backbone were selected for structural modification: the upper part (wall domain) and the lower part (legs). Synthesis of unsymmetrically bridged cavitands with different wall components (i.e., 7, 8 , and 14 – 18 ) was performed by stepwise bridging of the four couples of neighboring, H‐bonded OH‐groups of octol 1a (Schemes 1, 2, 4, and 5). Cavitands with modified legs (i.e., 20, 24, 27 , and 28 ), targeted for surface immobilization, were synthesized by short routes starting from suitable aldehyde starting materials incorporating either the fully preformed leg moieties or functional precursors to the final legs (Schemes 7–10). The new cavitand substitution patterns described in this paper should enable the construction of a wide variety of functional architectures in the future. X‐Ray crystallography afforded the characterization of cavitands 2c (Fig. 3) and 24 (Fig. 7) in the vase conformation, with 2c featuring a well‐ordered CH₂Cl₂ guest molecule in its cavity. A particular highlight is the X‐ray crystal‐structure determination of octanitro derivative 19 (Scheme 6), which, for the first time, shows a cavitand, lacking substituents in the ortho‐position to the two O‐atoms of the four resorcinol moieties, in the kite‐conformation (Fig. 5).     

Synthesis and Complexation Properties of Novel Biscalixarene Composed of Calix[4]arene and Thiacalix[4]arene Subunits

Biscalixarenes, as examples of higher order molecular architectures than normal calixarene derivatives, were studied extensively in the recent years due to they possess two calixarene cavities which have the potential for complexing with two guests in two…     

Chromophoric Water-Soluble Tetrakis(4-Carboxyphenylazo)-calix[4]arene: Binding of Arylammonium Ions and Benzene

The interaction of water-solubletetrakis(4-carboxyphenylazo)calix[4]arene with N,N,N trimethylarylammoniumiodides and benzene was investigated in order to correlate the effect ofanilinium salts on the binding properties of the cup-shaped hydrophobiccavity with the monolayer spreading behavior of a series of amphiphilicanalogs. Whereas multiple binding is observed for N,N,N-trimethylaniliniumiodide and benzene, NMR titration oftetrakis(4-carboxyphenylazo)calix[4]arene withN,N,N-trimethyl-N-(2-naphthyl)ammonium iodide indicates 2 : 1 complexationwithin the cavity of the calixarene with binding constants of K₁ = 8024 M^-1and K₂ = 3134 M^-1.     

New Efficient Synthetic Pathways to Tetrakis{p‐[(diethylphosphono)methyl]}calix[4]arene

Various operating conditions have been applied on tetrakis[p‐(halogenomethyl)]‐ and tetrakis[p‐(aminomethyl)]calix[4]arene derivatives to improve the synthesis of the 5,11,17,23‐tetrakis[(diethylphosphono)methyl]‐25,26,27,28‐tetrahydroxycalix[4]arene. Two new, high yield, synthetic pathways have been selected, involving, for the first one, the 25,26,27,28‐tetrahydroxy‐5,11,17,23‐tetrakis[(trimethylamino)methyl]calix[4]arene, tetraiodide, DMF, and 10 equiv. of triethyl phosphite ((EtO)₃P), and, for the other one, the 5,11,17,23‐tetra(bromomethyl)‐25,26,27,28‐tetrahydroxycalix[4]arene, CH₂Cl₂, and only 4 equiv. of (EtO)₂P.     

Controlling the Size and Morphology of Supramolecular Assemblies of Viologen–Resorcin[4]arene Cavitands

A novel class of self‐assembling nanoparticles is formed with viologen–resorcin[4]arene cavitands; the association model is strongly controlled by their hydrophobicity. Interestingly, the cavitand assemblies are designed through click chemistry to form self‐assembled noncovalently connected aggregates through counterion displacement. The iodide and benzoate ions are utilized as strongly polarizable counterions to induce cavitand self‐assembly. The counterion‐mediated decrease in hydrophilicity of the viologen–resorcin[4]arenes is the underlying trigger to induce particle formation. These particles can be used as nanocontainers and find their applications in delivery systems.