Homo- and heteropolynuclear helicates with a ‘2 + 3 + 2’-dentate compartmental ligand

An octadentate ligand L has been prepared which contains a sequence of bidentate (pyrazolyl-pyridine), terdentate [bis(pyrazolyl)pyridine] and bidentate (pyrazolyl-pyridine) binding sites separated by p-xylyl spacers. This forms a range of double helical complexes in which the two ligands define 4-, 6-, and 4-coordinate binding sites, and there is substantial -stacking between overlapping parallel areas of the ligands. In [Cu3L2][PF6]4 the sequence of oxidation states for the copper ions is +1, +2, +1 with the Cu(I) ions being four-coordinate at the terminal sites and Cu(II) being in the central six-coordinate site. In [Cu3(OAc)2L2][PF6]4 all copper centres are in oxidation state +2, with the terminal ions having an additional monodentate acetate ligand giving them a five-coordinate geometry. The 4 + 6 + 4 arrangement of coordination numbers means that reaction of L with a mixture of Fe(II) and Ag(I) results in high yield formation of [Ag2FeL2][BF4]4 in which Ag(I) ions occupy the terminal 4-coordinate sites and Fe(II) occupies the central pseudo-octahedral site. Reaction of L with Ag(I) produced a mixture of [Ag3L2][BF4]3(major product) and [Ag4L2][BF4]4(minor product). In [Ag3L2][BF4]3 the central Ag(I) ion is, unusually, in a pseudo-octahedral coordination environment from the two meridional, terdentate bis(pyrazolyl)pyridine donors. In [Ag4L2][BF4]4 in contrast the central 6-coordinate cavity is occupied by two Ag(I) ions separated by 2.85 . The terdentate chelating bis(pyrazolyl)pyridine units at the centre of the helicate are now substantially twisted such that each donates a bidentate pyrazolyl-pyridine to one Ag(I) centre and a monodentate pyrazole unit to the other. In solution, 1H NMR and mass spectroscopic evidence indicates that the fourth Ag(I) ion is lost and [Ag3L2][BF4]3 forms, unless a large excess of Ag(I) is present in which case traces of [Ag4L2][BF4]4 can be detected by mass spectrometry.