FACTORS AFFECTING THE ASSEMBLY OF HELICAL AND NON-HELICAL ZINC(II) BIPYRIDYL METALLOMACROCYCLES

The assembly of a non-helical bimetallic [2 + 2] macrocyclic complex between two zinc(II) atoms and two bis(bipyridyl) compounds containing a 2,7-disubstituted pyrene linker, previously assigned by dynamic NMR spectroscopy, has been confirmed by X-ray crystallography. The crystal structure of bis{2,7-bis[3-(6'-methyl-2,2'-bipyridin-6-yl)-2-oxapropyl]pyrene}dizinc tetrakis(trifluoromethane-sulfonate) was determined by X-ray diffraction methods and refined to a residual of 0.092 for 1916 independent observed reflections: triclinic, space group P1BAR, a = 16.333(6), b = 18.934(5), c = 10.004(1) angstrom, alpha = 100.96(2), beta = 97.33(2), gamma = 11 0.71(2)-degrees. The two zinc(II) ions are bound to four nitrogens from two bipyridyl groups and two oxygen atoms from the ether links in a distorted-octahedral geometry. A non-helical geometry is observed and the two pyrene rings are offset by 3.84 angstrom and separated by 3.65 angstrom. A related compound, in which a 1,4-disubstituted benzene linker is used in place of pyrene, has also been prepared, and in solution this forms approximately equal amounts of helical and non-helical complexes with zinc(II). The relative stabilities of helical and non-helical complexes formed between 6,6'-disubstituted bis(bipyridyl) ligands connected by 2,7-disubstituted pyrene, 2,6-disubstituted naphthalene and 1,4-disubstituted benzene are discussed.