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·.2H2O, [Ni2(6)3](PF6)4·195MeCN·1.THF·1.82O, and the very unusual triple helicate PF6− inclusion complex, [(PF6)⊂Ni2(5)3](PF6)3·1.75eCN·5.25TF·0.25H2O are reported.