Neutral Guest Molecules Research Articles

ChemInform Abstract: Complexation of Neutral Guest Molecules by Synthetic Macroheterocyclic Hosts

ChemInform Abstract: Complexation of Neutral Guest Molecules by Synthetic Macroheterocyclic Hosts

Novel Strategies for the Complexation of Neutral Guest Molecules by Synthetic Macrocyclic Hosts

Abstract Complexes of macrocyclic polyethers with neutral guests have been studied in solution. The first part, a systematic study of complexes of malononitrile with a variety of macrocyclic polyethers, allowed the evaluation of the thermo-dynamic parameters of complexation. The second part deals with the complexation of urea. Complexes with simple 18-membered macrocycles were prepared. Protonation of urea allowed the phase transfer of urea from aqueous to chloroform solution. Finally the complexation of urea by 2-carboxy-1,3-xylyl crown ethers is described.

Complexation of Neutral Molecules by Cyclophane Hosts

AbstractSince the discovery of the crown ethers by Pedersen twenty years ago, the chemistry of synthetic hosts for the selective complexation of organic and inorganic guests has seen an extraordinarily rapid development. This article discusses in particular the contributions provided by synthetic cyclophanes as hosts to the understanding of molecular complexation of neutral organic guest molecules in aqueous and organic solvents. In aqueous solution, cyclophanes form stoichiometric complexes with neutral aromatic guests which can approach enzyme‐substrate complexes in their stability. Efficient molecular complexation is also observed in organic environments. Here, as a result of large solvation effects, the strength of complexation is strongly dependent on the nature of the organic solvent. Electron donor‐acceptor interactions can contribute significantly to the stability of complexes formed between cyclophane hosts and aromatic guests. Force‐field calculations together with computer graphics are powerful tools in the design of water‐soluble, optically active hosts for chiral recognition of complexed racemic guests. Simple and selective functionalization of the cyclophane framework leads to stable, bioorganic catalysts. Like enzymes, these catalysts bind their substrates in a rapid equilibrium prior to the reaction steps. As a perspective, some fascinating research objectives in the field of molecular recognition and catalysis which can be targeted with designed cyclophane hosts are shown.

Does the calixarene cavity recognise the size of guest molecules? On the ‘hole-size selectivity’ in water-soluble calixarenes

The binding constants of three p-sulphonatocalix[n]arenes (n= 4,6, and 8) for neutral guest molecules were determined in an aqueous system; the results established that the calixarene cavity can recognise the size of guest molecules.

Complexation of neutral guest molecules by synthetic macroheterocyclic hosts

AbstractMacroheterocyclic molecules can be used to organize the binding sites required for selective molecular recognition and/or molecular catalysis. Although most work in the literature deals with recognition of ionic species, there is an increasing interest in the selective complexation of neutral molecules. Based on X‐ray analysis of solid complexes and NMR in solution, hydrogen bonding of host to guest and/or vice versa determines the structure, and very likely also the stability of these complexes between neutral host and guests. This paper describes an NMR method to determine the (weak) association constants of such complexes. Subsequently, two strategies are described for the enhancement of the thermodynamic stability, viz. the introduction of (intraannular) acidic groups in the molecular cavity and the use of metallomacrocyclic hosts in which a ‘soft’ cavity is combined with a ‘hard’ macrocyclic moiety. For both approaches novel synthetic methodology has been designed and for the first approach, the pKa values of acidic groups present in molecular cavities have been studied. Finally we have used the novel metallomacrocycles to organize two (or more) different metal centres in molecular cavities.

ChemInform Abstract: Complexes of Macrocyclic Polyethers and Neutral Guest Molecules: A Systematic Approach to the Complexation of Water Molecules by 2,6‐Pyridinium Crown Ethers.

AbstractpKa determinations of 2,6‐pyridinium crown ethers (I) and openchain analogues (IV) [obtained from (II)] in different solvent systems show that the acidity of these ligands depends on ring size and solvent.

Complexes of macrocyclic polyethers and neutral guest molecules: a systematic approach to the complexation of water molecules by 2,6-pyridinium crown ethers

Etude de la complexation de l'eau par des hotes charges positivement: pentaoxa [15]-, hexaoxa [18]- et heptaoxa [21] pyridinophaniums

大環状シクロファン包接体の構造

The crystallographic study of macrocyclic cyclophane inclusion compounds is reviewed. Cyclophanes such as [2.2.2], [6.6.6], [3.3.3.3] and [6.1.6.1] paracyclophanes and calixarenes with intramolecular hydrophobic cavities surrounded by benzene walls incorporate neutral guest molecules to give molecular inclusion compounds (cavitate) in solutions and in the solid state. Tri-o-thymotide forms chiral single crystals of cage type or channel type clathrate which have enantiomeric excess of guest molecules. [J. Cryst. Soc. Jpn. 28, 261 (1986) ] .

Photon-echo relaxation measurements with two dye-lasers application to pentacene- h14 and - d14 in p-terphenyl- h crystals at 1.5 K

Electronically controlled photon-echo relaxation measurements, using two nitrogen pumped dye-lasers, are reported for mixed crystals of pentacene- h 14 and - d 14 in p-terphenyl- h 14 at 1.5 K. In dilute mixed crystals (ca. 10 −8 M) the photon-echo lifetime is found to be exclusively determined by the fluorescence lifetime. Homogeneous linewidths of 7.1 and 5.9 MHz at 1.5 K are then calculated for the electronic origin of the 1B 2u ← 1A 1g transition of pentacene- h 14 and - d 14 respectively. The decrease in photon-echo lifetime at higher guest concentrations (ca. 10 −7 M) is ascribed to energy transfer between excited and neutral guest molecules.