Manganese(Ⅱ), iron(Ⅱ), cobalt(Ⅱ), and copper(Ⅱ) complexes of an extended inherently chiral tris-bipyridyl cage

摘要

Manganese(Ⅱ), iron(Ⅱ), cobalt(Ⅱ), and copper(Ⅱ) derivatives of two inherently chiral, Tris(bipyridyl) cages (L and L) of type [ML]-(PF_6)_2(solvent)_n and [FeL'](ClO_4)_2 are reported, where L is the hexa-tertiary butyl-substituted derivative of L'. These products were obtained by using the free cage and metal template procedures; the latter involved the reductive amination of the respective Tris-dialdehyde precursor complexes of iron(Ⅱ), cobalt(Ⅱ), or nick-el(Ⅱ). Electrochemical, EPR, and NMR studies have been used to probe the nature of the individual complexes. X-ray structures of the manganese(Ⅱ), iron(Ⅱ), and copper(Ⅱ) complexes of L and the iron(Ⅱ) complex of L' are presented; these are compared with the previously reported structures of the corresponding nickel(Ⅱ) complex and metal-free cage (L). In each complex the metal cation occupies the cage's central cavity and is coordinated to six nitrogens from the three bipyridyl groups. The cations [MnL]~(2+) and [FeL]~(2+) are isostructural but both exhibit a different arrangement of the bound cage to that observed in the corresponding nickel(Ⅱ) and copper(Ⅱ) complexes. The latter have an exo-exo arrangement of the bridgehead nitrogen lone pairs, with the metal inducing a triple helical twist that extends ≈ 22 A along the axial length of each complex. In contrast, [MnL]~(2+) and [FeL]~(2+) have their terminal nitrogen lone pairs directed endo, causing a significant change in the configuration of the bound ligand. In [FeL']~(2+), the cage has both bridgehead nitrogen lone pairs orientated exo. Semiempirical calculations indicate that the observed endo-endo and exo-exo arrangements are of comparable energy.