New nickel(II) and iron(II) helicates and tetrahedra derived from expanded quaterpyridines

As an extension of prior studies involving the linear quaterpyridine ligand, 5,5'''-dimethyl-2,2': 5', 5 '': 2 '', 2'''-quaterpyridine 1, the synthesis of the related expanded quaterpyridine derivatives 2 and 3 incorporating dimethoxy-substituted 1,4-phenylene and tetramethoxy-substituted 4,4'-biphenylene bridges between pairs of 2,2'-bipyridyl groups has been carried out via double-Suzuki coupling reactions between 5-bromo-5'-methyl-2'-bipyridine and the appropriate di-pinacol-diboronic esters using microwave heating. Reaction of 2 and 3 with selected Fe(II) or Ni(II) salts yields a mixture of both [M2L3](4+) triple helicates and [M4L6](8+) tetrahedra, in particular cases the ratio of the products formed was shown to be dependent on the reaction conditions; the respective products are all sufficiently inert to allow their chromatographic separation and isolation. Longer reaction times and higher concentrations were found to favour tetrahedron formation. The X-ray structures of solvated [Ni-2(2)(3)](PF6)(4), [(PF6) subset of Fe-4(2)(6)](PF6)(7), [Fe-4(3)(6)](PF6)(8) and [Ni-4(3)(6)](PF6)(8) have been determined, while the structure of the parent Fe(II) cage in the series, [(PF6) subset of Fe-4(1)(6)](PF6)(7), was reported previously. The internal volumes of the Fe(II) tetrahedral cages have been calculated and increase from 102 angstrom(3) for [Fe-4(1)(6)](8+) to 227 angstrom(3) for [Fe-4(2)(6)](8+) to 417 angstrom(3) for [Fe-4(3)(6)](8+) and to an impressive 839 angstrom(3) for [Ni-4(3)(6)](8+). The corresponding void volume in the triple helicate [Ni-2(2)(3)](4+) is 29 angstrom(3).