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

Abstract

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 [M(2)L(3)](4+) triple helicates and [M(4)L(6)](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)](PF(6))(4), [(PF(6)) ⊂ Fe(4)(2)(6)](PF(6))(7), [Fe(4)(3)(6)](PF(6))(8) and [Ni(4)(3)(6)](PF(6))(8) have been determined, while the structure of the parent Fe(II) cage in the series, [(PF(6)) ⊂ Fe(4)(1)(6)](PF(6))(7), was reported previously. The internal volumes of the Fe(II) tetrahedral cages have been calculated and increase from 102 Å(3) for [Fe(4)(1)(6)](8+) to 227 Å(3) for [Fe(4)(2)(6)](8+) to 417 Å(3) for [Fe(4)(3)(6)](8+) and to an impressive 839 Å(3) for [Ni(4)(3)(6)](8+). The corresponding void volume in the triple helicate [Ni(2)(2)(3)](4+) is 29 Å(3).