UNTWISTING A HELICALLY COORDINATED HEXAAZAMACROCYCLE - DIVALENT MG, CA, SR, CD, HG, AND PB COMPLEXES OF (PYO)(2)[18]DIENEN(6) AND THE X-RAY CRYSTAL-STRUCTURE OF THE CD COMPLEX

The synthesis and structural characterization of divalent Mg, Ca, Sr, Cd, Hg, and Pb complexes of (pyo)(2)[18]-dieneN(6) (3,6,14,17,23,24-hexaazatricyclo[17,3,1.1(8,12)]tetracosa-1(23),8,10,12,(24),19,21-hexaene) are reported. The complexes were isolated as triflte salts and characterized by elemental analyses, IR and NMR spectral studies, and, in the case of the Cd complex, by an X-ray crystal structure. The proton NMR spectra of all the complexes are consistent with D-2 symmetry in which the ligand is hexacoordinated in a meridional wrap with the pyridine groups twisted relative to each other and the -CH2NHC2H4NHCH2- groups forming helical linkages connecting the pyridine groups. A comparison of the crystal structures of the Cd and Zn complexes reveals that as the size of the metal ion increases, the pyridine groups untwist and the helicity of the coordinated ligand decreases. The NMR spectra of all the complexes are interpreted in light of this effect, and molecular modeling is used to predict the structures of the Hg and Pb complexes, giving results consistent with the NMR studies and the untwisting effect. The results of the X-ray crystal structure of [Cd((pyo)(2)[18]dieneN(6))](CF3SO3)(2) are as follows: formula, CdC20N6)H26S2O6F6; monoclinic; I2/m space group, with a = 9.6945(6), b = 15.6656 (17), and c = 18.5115(11) Angstrom, beta, 91.392(5)degrees, and Z = 4, resulting in R = 0.0410, R(w) = 0.052, and GoF = 1.24.