SYNTHESIS, OPTICAL RESOLUTION, AND X-RAY CRYSTAL-STRUCTURE OF A HELICAL HEXAAZAMACROCYCLIC ZINC COMPLEX

The synthesis and structural characterization of the zinc complex of an 18-membered macrocyclic ligand containing four saturated amines and two pyridine groups are described. The compound is formulated as (3,6,14,17,23,24-hexaazatricyclo[17.3.1.1(8,12)]tetracosa-1 (23),8,10,12(24),19,21-hexaene)zinc(2+) trifluoromethanesulfonate, [Zn-(pyo2[18]dieneN6)](CF3SO3)2. The proton-NMR spectrum of the complex is consistent with D2 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. Well defined multiplets in the NMR spectra were analyzed by spin simulation, and the resulting dihedral angles were in agreement with those determined in the solid-state by X-ray crystallography. The complex was inert to both NH and metal ion exchange in aqueous solution. The right- and left- handed helical enantiomers were resolved by formation of diastereomers with (+)-tartrate. The CD spectra gave positive and negative Cotton effects at 262 nm corresponding to the pyridine pi to pi* transition. [Zn(pyo2[18]dieneN6)](CF3SO3)2 (ZnC20N6H26S2O6F6) crystallizes in the orthorhombic space group Imcb (Ibam) with Z = 4, a = 15.851(4) angstrom , b = 18.391(5) angstrom , c = 9.325(5) angstrom , and V = 2718.4(17) angstrom3. X-ray crystallographic results showed that the complex has 222 (D2) symmetry in the solid state, which is the same structure as found in solution.