Jos W. H. M. Uiterwijk

The number of ideal rings on the diamond lattice; application to crown ethers

Three independent computing methods are described for the calculation of the number of different conformations of rings of up to 26 atoms, superimposable on the diamond lattice. Our computations extend results published earlier and show them to be partly incorrect. As an application, the number of different conformations for ‘ideal’ crown ethers has been calculated (6-crown-2, 12-crown-4,18-crown-6, and partly, 24-crown-8) and a comparison has been made with structures determined by X-ray crystallography.

Preparation and X-ray structures of complexes of 18-membered crown ethers with polyfunctional guests: Urea and (O-alkyliso)uronium salts

The preparation and X-ray structure determinations of six complexes of urea and (O-n-butyliso)uronium salts with crown ethers are presented. Urea forms isostructural 5:1 adducts with 18-crown-6 (1) and aza-18-crown-6 (2), in which two urea molecules are each hydrogen bonded to two neighbouring hetero atoms of the macroring. The remaining urea molecules form two-dimensional layers alternating with crown ether layers. In both complexes the macroring has theg+g+a ag−a ag−a g−g−a ag+a ag+a conformation withCi symmetry. In the solid 1:1 complex of O-n-butylisouronium picrate with 18-crown-6 (3) two types of conformations of the macroring were observed: theg+g+a ag−a ag+a ag−g−ag−a ag+a conformation with approximateCm symmetry and to a lesser extent theg+g+a ag−a ag+a g+g+a ag−a ag+a conformation with approximateC2 symmetry. Both conformations allow the guest to form three hydrogen bonds to the macrocyclic host. Three complexes of 18-crown-6 and uronium salts have been prepared and characterized by X-ray crystallography. The 1:1 complexes with uronium nitrate (4) and uronium picrate (5) both exhibit the sameC2 conformation and the same hydrogen bonding scheme as in the least occupied form of the previous complex. A 1:2 complex with uroniump-toluenesulphonate (6) has a different hydrogen bonding scheme (two hydrogen bonds per cation to neighbouring oxygen atoms of the macroring) and a different conformation of the host molecule (theag+a ag−a ag+a ag−a ag+a ag−a conformation with almostD3d symmetry). An attempt to prepare a solid uronium nitrate complex with diaza-18-crown-6 in the same way as the 18-crown-6·uronium nitrate (1:1) complex did not yield the expected result. Instead X-ray analysis revealed that the uronium ion is dissociated, resulting in the nitrate salt of the diprotonated diaza crown ether (7).

Liquid-liquid phase transfer of uronium salts via complexation by (di)benzo crown ethers. X-Ray structure of the benzo-27-crown-9 uronium perchlorate (1:1) complex

The complexation of uronium perchlorate with crown ethers of different ringsizes (18–33 ring atoms) has been studied by using two-phase extraction experiments. Crown ethers with 27 or more ring atoms are the best hosts to transfer uronium salts from an aqueous phase to an organic phase. The amount of uronium perchlorate transferred was measured by coulometric titration of the stoichiometric amount of ammonia produced by enzymatic degradation of urea. The crystal and molecular structure of the 1:1 complex of uronium perchlorate with benzo-27-crown-9 has been determined by X-ray crystallography. The uronium cation is encapsulated in the crown ether cavity with all its hydrogen atoms bonded to the macrocyclic host. A 1:1 complex of uronium picrate with benzo-21-crown-7 was isolated and is assumed to be a perching complex.

Syntheses and crystal structures of ‘encapsulated’ guanidinium complexes of 27-membered macrocyclic ligands

X-Ray structures of complexes of 2,6-pyrido-27-crown-9 and 1,3-xylyl-27-crown-8 with guanidinium perchlorate show a complementary hydrogen bonding relationship between host and guest; in the former complex the pyridyl nitrogen is used preferentially as a hydrogen binding site; the structures found are compared with those of the protonated 27-membered macrocycle and with 2,6-pyrido-24-crown-8.

A molecular mechanics study of 18-crown-6 and its complexes with neutral organic molecules

The flexibility of 18-crown-6 has been studied by the molecular mechanics method (MM2). Calculations on 190 different potentially favourable (‘ideal’) conformations reveal that the potential energy surface of the polyether has many minima with only small energy differences. The order of the conformational energies depends mainly on the contribution of the electrostatic energy. If no electrostatic interactions are present, the most favourable conformations are the ones occuring in complexes of 18-crown-6, as observed by X-ray crystallography. However, for larger contributions of the electrostatic energy the conformation adopted by uncomplexed 18-crown-6 in the crystalline state becomes the dominating conformation. Also results are presented for calculations on six experimentally observed conformations of 18-crown-6. It is shown that the calculated geometries correspond closely with the experimental ones, notwithstanding the omission of the guest molecules in the calculations, indicating that the minima in the potential energy surface of the polyether have steep slopes. Finally, results are given for calculations on the hydrogen-bonded complexes of 18-crown-6 with urea and formamide, both using the MM2 and the MM2HB force field. The latter is a modified version of MM2, incorporating an empirical N–H ⋯ O hydrogen-bond potential. MM2HB proves to be far superior over MM2 for calculations on this type of complexes, as well as for the conformation adopted by the macrocycles as regarding the hydrogen-bond geometries.

Basicity and structure of 2,6-pyrido-crown ethers; the effect of ringsize, solvent interactions, and hydrogen bonding

The acidity of 2,6-pyridinium-crown ethers (1) varies with the ringsize [pKa 4.88(15-membered ring), 4.95 (18-), 4.16(21-), 3.95 (24-), 3.70(27-), 3.53(30-); H2O, 25.0 °C]; X-ray analyses of the 2,6-pyrido-18-crown-6·H2O·HClO4 complex and the free 2,6-pyrido-18-crown-6 point to specific intra-annular hydration.

Synthesis, X-ray crystal structure, and reactivity of ternary complexes of crown ethers, organic π-acceptors, and salts

Single-crystal X-ray analysis of a stable ternary complex of 2, 3-dichloro-5, 6-dicyano-p-benzoquinone (DDQ), dibenzo-18-crown-6 (2), and t-butylammonium perchlorate reveals a close contact between the anion and DDQ; in similar ternary complexes of crown ethers, tetracyanoethylene (TCNE), and halides a one-electron transfer is observed.

Crystal and molecular structure of benzo-27-crown-9 guanidinium perchlorate (1 : 1): an encapsulated complex

In the crystal structure of the complex of benzo-27-crown-9 and guanidinium perchlorate (1 : 1) a complementary binding relationship between host and guest is found; all guanidinium hydrogen atoms are involved in hydrogen bonding to oxygen atoms of the macrocycle.