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

Abstract

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 π-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 Å. 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.