Iron(III) Metallacryptand and Metallacryptate Assemblies Derived from Aroylbis(N,N-diethylthioureas)

The reaction of isophthaloylbis(N,N-diethylthiourea), H₂L¹, with FeCl₃·6H₂O gives the dinuclear tris-complex [Fe₂(L¹)₃] (5), possessing a cryptand-like structure. A similar reaction with the ligand 2,6-dipicolinoylbis(N,N-diethylthiourea), H₂L², however, results in the formation of the anionic, mononuclear Fe(III) complex [Fe(L²)₂]^- (6), which could be isolated as its "Tl⁺ salt" by the subsequent addition of Tl(NO₃). A tighter view to the solid state structure of the obtained product, however, characterizes compound 6 as a one-dimensional coordination polymer, in which four-coordinate Tl⁺ ions connect the {[Fe(L²)₂]^-} units to infinite chains. When Fe³⁺ ions and Tl⁺ ions are added to H₂L² simultaneously in a one-pot reaction, a different product is obtained: a cationic trinuclear complex of the composition {M⊂[Fe₂(L²)₃]}⁺. It has been isolated as a PF₆^- salt and represents a {2}-metallacryptate with a nine-coordinate Tl⁺ ion in the central void. Structurally related products of the compositions {M⊂[Fe₂(L²)₃]}(PF₆) (M = Na⁺, K⁺, Rb⁺) (8(PF₆)) could be isolated from analogous reactions with alkaline salts instead of Tl(NO₃). {2}-Metallacryptates with larger central voids were synthesized with the ether-spaced aroylbis(N,N-diethylthiourea) H₂L³. The compounds {M⊂[Fe₂(L³)₃]}(PF₆) (M = K⁺, Rb⁺, Tl⁺ or Cs⁺) (9(PF₆)) were prepared by a similar protocol like those with H₂L² with the simultaneous addition of the metal ions to a solution of H₂L³. Due to the larger spacer between the aroylthiourea units, the coordination number of the central M⁺ ions is 12 by six carbonyl and six ether oxygen atoms. All products were characterized by elemental analysis, IR spectroscopy, and X-ray structure analysis. Cyclic voltammetric studies were carried out with the three representative complexes [Fe₂(L¹)₃], {K⊂[Fe₂(L²)₃]}(PF₆), and {K⊂[Fe₂(L³)₃]}(PF₆). The obtained voltammograms indicate the dependence of the redox properties of the oligonuclear systems on the conjugation in the organic backbones of the ligands.