Molecular recognition of neutral molecules by metalloclefts: synthesis, x-ray structure, proton NMR spectroscopy, electrochemistry, and molecular modeling

Metalloclefts 2 and lOa-b and metoZZomacrocycles 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 NMR spectroscopy, X-ray structure determination, polarography, and molecular modeling.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 x-x 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 >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 lOa-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, lOa-b, and 11 correspond quantitatively.