MOLECULAR RECOGNITION OF POLAR NEUTRAL MOLECULES BY METALLOMACROCYCLES - SYNTHESIS, H-1-NMR SPECTROSCOPY, X-RAY STRUCTURE, ELECTROCHEMISTRY, AND ABINITIO CALCULATIONS

Metallomacrocycles 5 possessing an immobilized Lewis acidic uranyl group were synthesized by reaction of aldehydes 4 with cis-1,2-cyclohexanediamine in the presence of Ba2+ as a template cation and subsequent transmetallation with UO2(2+). These metallomacrocycles are soluble in organic solvents and the complexation with neutral molecules was investigated by polarography, H-1 NMR spectroscopy, solid-liquid and liquid-liquid extraction experiments, X-ray structure determinations, and ab initio calculations. Several solid complexes (6) of metallomacrocycles 5b-d with polar neutral molecules (formamide, acetamide, N-methylurea, hydroxyurea, urea, and DMSO) were isolated; a ring size selective complexation is observed. Polarography demonstrated a ring size affinity with the following stability order for the complexes in CH3CN: urea > N-methylurea > acetamide almost-equal-to formamide > acetone almost-equal-to 0. The stability constants of the 6b-urea and 6c-urea complexes in CDCl3 are according to H-1 NMR spectroscopy at least 10(8) M-1; the highest number ever achieved by a complex consisting of a neutral monometalloreceptor and a neutral molecule. The high stabilities were confirmed by solid-liquid and liquid-liquid extraction experiments. The crystal structures of the 6b-urea and 6d-urea complexes reveal that urea is encapsulated in the cavity and that the complexes are stabilized by coordination of the carbonyl oxygen of urea to the immobilized uranyl cation, multiple H-bond formation, and electrostatic interactions between urea nitrogens and ether oxygens. Ab initio calculations suggest that charge transfer determines the coordination between the uranyl cation and urea. The optimal coordination angle (C = O...M2+) is approximately 130-degrees, for both in-plane and perpendicular coordination.