MOLECULAR ARCHITECTURE .1. SODIUM, POTASSIUM, AND STRONTIUM COMPLEXES OF A HEXAAZAMACROCYCLE, AN 18-CROWN-6/TORAND ANALOG

Complexes of 4,5:15,16-dibenzo-3,6,14,17,23,24-hexaazatricyclo[17.3.1.1(8,12)]tetracosa-1(23),4,8-(24),9,11,15,19,21-octaene (2) with Sr(CF3SO3)2 and SrI2 have been prepared by templated 2 + 2 cyclocondensation reactions of pyridine-2,6-dicarboxaldehyde and benzene-1,2-diamine. Complexes 2.KClO4, 2.KI, and 2.KCF3SO3 were prepared by reaction of the corresponding Sr complexes with KF. Free ligand 2 was prepared by reaction of 2.KCF3SO3 with 18-crown-6 (1) and 2.NaCF3SO3 was prepared from 2. The X-ray crystal structure of 2.Sr(CF3SO3)2 was determined, showing two molecules per asymmetric unit. In both complexes Sr is nine-coordinate with Sr-N, distances ranging from 2.65-2.82 angstrom. The stability constants of 2.KCF3SO3 and 2.NaCF3SO3 in DMSO were determined by NMR and potentiometric methods as log K(s) = 3.7 and 3.1, respectively. Molecular mechanics calculations were performed on 2.SrCl2, 2, and a tetramethyl analogue (9). Computational and structural studies of the complexes of 2 indicated an optimum hole size corresponding to metal ionic radii of 1.4-1.55 angstrom. Spectroscopic, structural, and computational results demonstrated that 2 is not preorganized for complexation, and the minimum energy conformer in solution is elliptical rather than circular. The K/Na selectivities and binding strengths of 1, 2, and torand 3 are compared, concluding that guest encapsulation is not required for strong binding and that preorganization does not ensure selectivity.