MOLECULAR RECOGNITION OF NEUTRAL MOLECULES BY METALLOCLEFTS - SYNTHESIS, X-RAY STRUCTURE, H-1-NMR SPECTROSCOPY, ELECTROCHEMISTRY, AND MOLECULAR MODELING

Metalloclefts 2 and 10a-b and metallomacrocycles 7a-c possessing a Lewis acidic uranyl cation were synthesized from the corresponding aldehydes. The complexation of these compounds with neutral molecules was assessed by H-1 NMR spectroscopy, X-ray structure determination, polarography, and molecular modelling. H-1 NMR titration experiments and the crystal structure of 10b.4-tert-butylpyridine indicate that the neutral guests coordinate with their Lewis basic site at the uranyl, and that pi-pi stacking of the aromatic rings of host and guest stabilizes the complexes. Free energies of complexation, for a series of hosts and guests, obtained from polarographic titration data, vary from 1.5 to greater-than-or-equal-to 6.3 kcal mol-1. Compared to the reference salophene-uranyl 11 the stabilities of the complexes are enhanced up to 1.7 kcal mol-1 for 2 and up to 1.6 kcal mol-1 for 10a-b; for the more rigid metallomacrocyles 7 increases up to 2.2 kcal mol-1 were found. The calculated and experimentally determined relative energies of complexation of 4-methylpyridine vs pyridine with 2, 10a-b, and 11 correspond quantitatively.