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 [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 [Ni₂(2)₃](PF₆)₄, [(PF₆) ⊂ Fe₄(2)₆](PF₆)₇, [Fe₄(3)₆](PF₆)₈ and [Ni₄(3)₆](PF₆)₈ have been determined, while the structure of the parent Fe(II) cage in the series, [(PF₆) ⊂ Fe₄(1)₆](PF₆)₇, was reported previously. The internal volumes of the Fe(II) tetrahedral cages have been calculated and increase from 102 ų for [Fe₄(1)₆]⁸⁺ to 227 ų for [Fe₄(2)₆]⁸⁺ to 417 ų for [Fe₄(3)₆]⁸⁺ and to an impressive 839 ų for [Ni₄(3)₆]⁸⁺. The corresponding void volume in the triple helicate [Ni₂(2)₃]⁴⁺ is 29 ų.