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

摘要

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_2L_3]~(4+) triple helicates and [M_4L_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 ?~3 for [Fe_4(1)_6]~(8+) to 227 ?~3 for [Fe_4(2)_6]~(8+) to 417 ?~3 for [Fe_4(3)_6]~(8+) and to an impressive 839 ?~?3 for [Ni_(3)_]~ (8+)The corresponding void volume in the triple helicate [Ni_2(2) _3]~(4+) is 29 ?~3.