Herman J. den Hertog

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).

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.

Spherands with functional groups in the outer sphere: synthesis from modified 1,1′:3′,I″-terphenyls

Two spherands (1a,c), functionalized in the outer sphere, have been synthesized by oxidative coupling of the dianions of terphenyls; these terphenyls are obtained by aldol condensation of 1,3-diarylpropanones with nitromalonaldehyde.

A general method for the determination of the kinetic stability of macrocyclic alkali-metal complexes with rates of decomplexation below 10–3 s–1

A general method has been developed for the determination of kinetic stabilities of macrocyclic alkali-metal complexes with rates of decomplexation (Kd) below 10–3 s–1, by use of radioactive isotopes. This method offers the possibility to study the influence of the solvent polarity and of the salt concentration in solution on the rate of decomplexation of macrocyclic metal complexes. Further advantages are the small amounts of ligand required for these determinations and the simplicity of the method. Furthermore, it is possible by this method to study the degenerate exchange of sodium for sodium and of rubidium for rubidium. By this method the kinetic stabilities of the sodium and rubidium complexes of calixspherands 1–4 were determined. Calixspherand 3 forms kinetically very stable complexes with sodium and rubidium cations in acetone and Me2SO in the presence of high concentrations of sodium cations in solution; half-life times of exchange are 855 (Na+) and 528 (Rb+) h in acetone and 352 (Na+) and 845 (Rb+) h in Me2SO, respectively. The results of this method were verified by an independent 1H NMR spectroscopic method.

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.