Dinuclear helicate and tetranuclear cage assembly using appropriately designed ditopic triazole-azine ligands

Abstract

Four new, symmetrical, bis-bidentate ditopic Rdpt-type, Rat (R azine-triazole), ligands have been prepared, L^{npym-meta/para} (n = 2 or 4), which contain bidentate n-pyrimidine/triazole binding pockets connected through an appropriate aromatic spacer, meta/para-phenyl, to enable assembly into dinuclear helicates or tetranuclear tetrahedral cages, respectively. The 3 : 2 self assembly reactions of each L^{npym-meta/para} ligand with iron(II) tetrafluoroborate gave the desired complexes, as shown by X-ray crystal structure determinations of the pair of helicates [Fe^II₂(L^npym-meta)₃(BF₄)₄]·6CH₃CN, with n = 2 (1·6CH₃CN) or 4 (2·6CH₃CN), and the pair of Td cages [Fe^II₄(L^npym-para)₆(BF₄)₈]·xsolvent, with n = 2 (3·xsolvent) or 4 (4·xsolvent). Reversible Fe^II/III processes at E_m = 0.95 ± 0.05 V vs. 0.01 M AgNO₃/Ag in MeCN are a feature of 1–4, with little variation in the redox potential as a function of nuclearity, architecture or choice of n-pyrimidine isomer. In all four complexes the iron(II) centres are low spin, despite having employed the weakest field diazines, the 2- and 4-pyrimidines, in these ditopic Rat ligands. Nevertheless this is an exciting proof of principle that Rdpt-type ligands (previously used to generate about 50 spin crossover-active complexes) can be extended into ditopic forms that are suitable for supramolecular assembly of helicates and cages.