Supramolecular assemblies prepared from an iron(ii) tripodal imidazole complex. A molecular scaffolding for the self assembly of icosahedral complexes of K+, Rb+, Cs+ and NH4+ cations

Abstract

The iron(II) complex of the 1 : 3 Schiff base condensate of tris(2-aminoethyl)amine (tren) with 2-imidazolecarboxaldehyde, H₃L¹, [FeH₃L¹](ClO₄)₂, was reacted with MClO₄ (M = K, Rb, Cs, and NH₄). The products were double salts of the formula {[FeH₃L¹](ClO₄)₂}·MClO₄. The complexes have been characterized by Elemental Analysis, X-ray crystallography, ESMS, IR and Mössbauer spectroscopy. The resultant complexes are all trigonal, crystallize in P and exhibit a hexameric structure in the extended lattice. The cations are located at the origin (0, 0, 0) and c/2 (0, 0, 0.5) on the c axis. The cations are twelve coordinate and exhibit a distorted icosahedral geometry formed by six bidentate perchlorate anions. The perchlorate anions are involved in extensive hydrogen bonding with the iron complexes, which are located at the vertices of the hexagon. Each perchlorate anion has five hydrogen bonds to three iron complexes. The iron complexes use the imidazole NH and imine NCH as donors to the perchlorate oxygen atoms. The imidazole NH is a trifurcated donor and the imine CH is a bifurcated donor. The formation of the double salt and the accompanying hydrogen bonding results in a shift in the spin equilibrium of the parent iron(II) complex from 63% high spin, HS, and 37% low spin, LS, at 295 K to pure LS in the double salts. Attempts to incorporate sodium or silver in the above salts failed due to their size and oxidizing strength, respectively. The analogous manganese complex, [MnH₃L¹](ClO₄)₂, failed to give these double salts under identical reaction conditions. The lack of reactivity of [MnH₃L¹](ClO₄)₂ does not appear to be due to its solubility or geometric considerations of the hydrogen bonding donor site, but instead to the electronic differences between the high spin [MnH₃L¹](ClO₄)₂ and the spin crossover [FeH₃L¹](ClO₄)₂.