UNTWISTING A HELICALLY COORDINATED HEXAAZAMACROCYCLE - DIVALENT MG, CA, SR, CD, HG, AND PB COMPLEXES OF (PYO)(2)[18]DIENEN(6) AND THE X-RAYCRYSTAL-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) (8,12)]tetracosa-1( 23),8,10,12,(24),19,21-hexaene) are reported. The complexes were isola ted as triflte salts and characterized by elemental analyses, IR and N MR spectral studies, and, in the case of the Cd complex, by an X-ray c rystal structure. The proton NMR spectra of all the complexes are cons istent with D-2 symmetry in which the ligand is hexacoordinated in a m eridional wrap with the pyridine groups twisted relative to each other and the -CH2NHC2H4NHCH2- groups forming helical linkages connecting t he pyridine groups. A comparison of the crystal structures of the Cd a nd Zn complexes reveals that as the size of the metal ion increases, t he pyridine groups untwist and the helicity of the coordinated ligand decreases. The NMR spectra of all the complexes are interpreted in lig ht of this effect, and molecular modeling is used to predict the struc tures of the Hg and Pb complexes, giving results consistent with the N MR 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: f ormula, CdC20N6)H26S2O6F6; monoclinic; I2/m space group, with a = 9.69 45(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.