Nickel(ii) and iron(ii) triple helicates assembled from expanded quaterpyridines incorporating flexible linkages

Abstract

In the present study the interaction of Fe(II) and Ni(II) with the related expanded quaterpyridines, 1,2-, 1,3- and 1,4-bis-(5′-methyl-[2,2′]bipyridinyl-5-ylmethoxy)benzene ligands (4–6 respectively), incorporating flexible, bis-aryl/methylene ether linkages in the bridges between the dipyridyl domains, was shown to predominantly result in the assembly of [M₂L₃]⁴⁺ complexes; although with 4 and 6 there was also evidence for the (minor) formation of the corresponding [M₄L₆]⁸⁺ species. Overall, this result contrasts with the behaviour of the essentially rigid ‘parent’ quaterpyridine 1 for which only tetrahedral [M₄L₆]⁸⁺ cage species were observed when reacted with various Fe(II) salts. It also contrasts with that observed for 2 and 3 incorporating essentially rigid substituted phenylene and biphenylene bridges between the dipyridyl domains where reaction with Fe(II) and Ni(II) yielded both [M₂L₃]⁴⁺ and [M₄L₆]⁸⁺ complex types, but in this case it was the latter species that was assigned as the thermodynamically favoured product type. The X-ray structures of the triple helicate complexes [H₂O⊂Ni₂(4)₃](PF₆)₄·THF·2.2H₂O, [Ni₂(6)₃](PF₆)₄·1.95MeCN·1.2THF·1.8H₂O, and the very unusual triple helicate PF₆ ⁻ inclusion complex, [(PF₆)⊂Ni₂(5)₃](PF₆)₃·1.75MeCN·5.25THF·0.25H₂O are reported.