Guest Moieties Research Articles

The Chemistry of Vanadium−Carbon Bond Functionalities over an Oxo-Surface Defined by the Calix[4]arene Skeleton: The Redox Relationship between Vanadium(III) and Vanadium(IV) Assisted by Carbon−Oxygen Bond Cleavage

This report deals with the chemistry of V−C functionalities anchored to a quasi-planar O4 matrix represented here by the [p-But-calix[4]-(OMe)2(O)2]2- macrocycle. The starting material [p-But-calix[4]-(OMe)2(O)2V−Cl], 2, has been converted into the corresponding alkyl and aryl derivatives [p-But-calix[4]-(OMe)2(O)2V−R] (R = Me, 3; R = CH2Ph, 4; R = p-MeC6H4, 5) by conventional procedures. The structure of 5 reveals the self-assembling of the monomeric unit into a columnar arrangement with the p-tolyl substituent functioning as a guest moiety in a neighboring unit. Complexes 3−5 undergo migratory insertion reactions with CO and ButNC to the corresponding η2-acyls [p-But-calix[4]-(OMe)2(O)2V(η2-COR)] (R = Me, 6; R = CH2Ph, 7) and η2-iminoacyls [p-But-calix[4]-(OMe)2(O)2V{η2-C(NBut)R}] (R = Me, 8; R = CH2Ph, 9; R = p-MeC6H4, 10), the reaction with CO being strongly dependent on the R substituent. The structural model for the hexacoordinate intermediate leading to the inserted product is exemplified by the d2 high-spin [p-But-calix[4]-(OMe)2(O)2V(Cl)(CNBut], 11, and proves the presence of four available orbitals at the metal for driving the reactivity in the [p-But-calix[4]-(OMe)2(O)2V] fragment. The bonding mode of the organic functionalities at the metal and the conversion of the alkyl substituent into the η2-acyl have been inspected by extended Hückel calculations. The oxidative demethylation of 3 and 5 by a controlled amount of iodine opened an interesting synthetic route to the corresponding vanadium(IV) organometallic derivatives [p-But-calix[4]-(OMe)(O)3V−X] (X = Cl, 12; X = p-MeC6H4, 13). A structural report on 2, 5, and 9 is included.

Molecular recognition by natural macrocycles. I. d-tubocurarine as a host molecule for organic anions

The binding of 8-anilino-1-naphthalenesulfonate and 15 anions of substituted benzoic, aliphatic dicarboxylic, and N-acetyl-α-amino acids to a macrocyclic alkaloid d-tubocurarine in aqueous solution has been studied by fluorometry, conductometry, and 1H NMR. The binding constants vary from ca. 50 to 3300 M−1 depending on the guest structure, charge and hydrophobicity. The results of fluorescence and NMR studies show that the host-guest complexation of the anions of aromatic acids involves the formation of a salt bridge between the quaternary nitrogen of the alkaloid and the anionic group of the guest as well as hydrophobic/Van der Waals interactions between the guest and host aromatic moieties. The binding of dianions of aliphatic dicarboxylic acids most probably is purely electrostatic. In general, d-tubocurarine possesses binding ability comparable to that of synthetic cyclophanes. It binds enantiospecifically anions of N-acetyl-α-amino acids and discriminates between positional isomers of anions of hydroxy and carboxy substituted benzoic acids.

Molecular Recognition by Natural Macrocycles. I. d‐Tubocurarine as a Host Molecule for Organic Anions

The binding of 8-anilino-1-naphthalenesulfonate and 15 anions of substituted benzoic, aliphatic dicarboxylic, and N-acetyl-α-amino acids to a macrocyclic alkaloid d-tubocurarine in aqueous solution has been studied by fluorometry, conductometry, and 1H NMR. The binding constants vary from ca. 50 to 3300 M−1 depending on the guest structure, charge and hydrophobicity. The results of fluorescence and NMR studies show that the host-guest complexation of the anions of aromatic acids involves the formation of a salt bridge between the quaternary nitrogen of the alkaloid and the anionic group of the guest as well as hydrophobic/Van der Waals interactions between the guest and host aromatic moieties. The binding of dianions of aliphatic dicarboxylic acids most probably is purely electrostatic. In general, d-tubocurarine possesses binding ability comparable to that of synthetic cyclophanes. It binds enantiospecifically anions of N-acetyl-α-amino acids and discriminates between positional isomers of anions of hydroxy and carboxy substituted benzoic acids.

A self-included cyclomaltoheptaose derivative studied by NMR spectroscopy and molecular modelling

The 3D structure of 6-deoxy-6- l-tyrosinylamidocyclomaltoheptaose, a self-complexing β-cyclodextrin derivative, was determined by NMR and molecular modelling. The aminoacyl side-chain is included in the cavity and induces chemical-shift variations in the CD proton signals, allowing their complete assignment. Dipolar interactions between protons of the tyrosine ring and internal protons of the cyclodextrin were used to obtain distance constraints. Then 42 structures were calculated from 32 distance constraints — 21 shorter than 4 Å involve the host-guest interactions — using a simulated annealing procedure. Starting from one of the resulting structures, a 250-ps molecular dynamics simulation was carried out in a waterbox without constraint. The simulation data are in agreement with NMR data such as nOe and ring-current effects. The cyclodextrin part takes an elliptical shape, which tightly fits the aromatic moiety. As a consequence, the respective motion of the host and the guest moieties have the same amplitude and time scale: the self-inclusion complex shows only little flexibility.

Isomerization processes in inclusion compounds: Xylenes in deoxycholic acid

The isomerization processes induced by UV photons in inclusion compounds of the host-guest type are examined, with special attention to the photophysics of the energy transfer process between the host and the guest, as well as to the influence of the host molecular cavity symmetry and the guest molecular symmetry. In particular, the experimental study has been carried out on the isomerization processes ofp-,m-, ando-xylene inside the molecular cavities of deoxycholic acid. The results have been compared with those obtained by irradiating the xylenes in an inert solution of hexane. The main difference is the elimination of by-products when the photochemical process is carried out in the solid state inclusion compound; however, the high purity of the isomerization product corresponds to a decrease in its yield with respect to the reaction in solution, due to the energy transfer process from the host to the guest moiety.

Intercalation of ferrocenylalkylammonium cations into the layered lattice of VOPO4

Some ferrocenylalkylammonium iodides, [FeII(C5H5){C5H4(CH2)nNR2R′}]+I–(n= 1 or 2; R,R′= H or Me), benzyldimethyl- and benzyltrimethyl-ammonium iodides reacted with VvOPO4·H2O·EtOH suspended in ethanol to afford intercalation compounds of VOPO4·H2O with 0.45[FeII(C5H5)(C5H4CH2NHMe2)]+, 0.20[FeII(C5H5)(C5H4CH2 NMe3)]+, 0.43[FeII(C5H5)(C5H4CH2 CH2NH3)]+, 0.17[FeIII-(C5H5)(C5H4 CH2CH2NMe3)]2+, 0.54PhCH2NHMe2+ and 0.45PhCH2NMe3+. Ferrocenylalkylammonium chlorides, [FeII(C5H5)(C5H4CH2 NHMe2)]+Cl– and [FeII(C5H5)(C5H4CH2 CH2NH3)]+ Cl–, were also intercalated to give compounds of VOPO4·H2O with 0.29[FeIII(C5H5)(C5H4 CH2NHMe2+Cl–)]+ and 0.18[FeIII(C5H5)(C5H4 CH2CH2NH3+Cl–)]+. The intercalation compounds comprise layered VOPO4 moieties having both the vanadium-(IV) and -(V) states and the ferrocenylalkyl-, benzyldimethyl-, and benzyltrimethyl-ammonium cations as well as ferrocenium-substituted alkylammonium chlorides, [FeIII(C5H5)(C5H4CH2 NHMe2+Cl–)]+ and [FeIII(C5H5)(C5H4CH2 CH2NH3+Cl–)]+, in the interlayer space, producing VOPO4–interlayer spacings of 9.8–21.0 A. The molecular arrangements of these alkylammonium-substituted ferrocene and ferrocenium species in the interlayer space and the electronic states of the guest and host moieties are discussed on the basis of powder X-ray diffraction patterns, and IR, ESR and X-ray photoelectron spectra.

Dichroic dyes, and liquid crystalline side chain polymers

AbstractFunctional guest molecules incorporated into LC‐side‐chain polymers lead to polymeric materials which combine the physical and optical anisotropic properties of liquid crystals with the special functions of the guest components. This contribution focused on dichroic dyes, but the results are transferable to other guest moieties. The covalent fixation of the dye to the polymer backbone affords several advantages. Liquid crystalline materials with high dye concentrations were obtained opening the way to the preparation of highly ordered, optical uniaxial films with adjustable thickness and optical density. Thin well aligned LC‐polymer films can in principle be used as an active medium for reversible optical information storage and mixtures of dye‐containing copolymers in low molar mass liquid crystals can be used in displays. Current developments in the area of ferroelectric chiral smectic‐C side‐chain polymers will further enhance interest in this area.

Binding neutral guests to concave surfaces of molecular hosts. Directional association of water and methylene chloride with hosts containing only cyclic urea binding sites

This paper is concerned with studies of weak intermolecular interactions in molecular inclusion type systems involving uncharged host and guest entities. Three new complexes of synthetic organic ligands with water and methylene chloride have been characterized by single-crystal X-ray diffraction. The hosts are composed of three cyclic urea units whose carbonyl groups are held in convergent positions by bonding their attached nitrogens to one another through two (noncyclic ligand) or three (macrocyclic ligand) rigid spacer units. Conformational organization is further enforced by an aliphatic bridge between two of the phenylene spacers in the macrocyclic hosts and an additional dimerization of the open-chain ligand. The host species were found to be particularly suitable to interact with proton donating H2O and CH2Cl2 guest moieties, as their molecular surface contains appropriately sized polar cavities lined with the carbonyl functions. Association between the interacting components in these complexes is stabilized by O−H⋯O and C−H⋯O hydrogen bonds. In the corresponding crystal structures additional molecules of the solvent are located between units of the complex. The significance of preorganization of the host structure to an efficient guest binding is emphasized by an observation that no stable complexes of a similar but unbridged macrocyclic ligand could be crystallized from the same solvent. The structural features of the inclusion compounds are described in detail, and the host-guest interaction scheme is compared to that observed in complexes of 18-crown-6 with neutral guests.

A nuclear magnetic resonance study of molecular motion and ‘guest’ structure in the clathrate hydrates of (i-C5H11)4NF and (n-C4H9)4NF

The proton magnetic resonance linewidth and second moments between 77°k and 286°k for i-amyl groups of molecules included as guests in the clathrate hydrate of (i-C5H11)4NF have been interpreted in terms of simple motional models of these guest moieties. The results complement the reported details of crystal structure of this clathrate. For the analogous hydrate containing guest n-butyl groups, proton magnetic resonance second moments in the same temperature range have supported the disordered guest structure reported from a previous x-ray diffraction study. In addition, hysteresis has been found in the second moment curve of this clathrate beyond 248°k, and this has been ascribed to a phase transition.