Dinuclear and hexanuclear iron(III) carboxylate clusters with a bis(bipyridine) ligand: supramolecular aggregation of [Fe2O2] units to give a [Fe6O6] ladder structure

Abstract

The use of the bis-bipyridine ligand L (1,2-bis(2,2′-bipyridyl-6-yl)ethane) in iron(III) chemistry has yielded a variety of new dinuclear and hexanuclear iron complexes, the latter containing a ladder-like supramolecular architecture. Treatment of L with two equivalents of HBF₄ yields [H₂L][BF₄]₂1, the doubly protonated ligand. The FeX₃/NaO₂CR/L (2∶4∶1) reaction system in MeCN gives [Fe₂OX₂(O₂CR)₂L] (X = Cl, R = Ph 2; X = Br, R = Ph 3; X = Cl, R = Me 4). The same FeCl₃/NaO₂CPh/L (2∶4∶1) reaction system but in a 1∶1 MeCN–water solution gives green [Fe₂OCl₂(O₂CPh)L(H₂O)₂]Cl 5, and in a water solution gives brown [Fe₆O₆(O₂CPh)₃L₃(H₂O)₂]Cl₃6. Treatment of a 1∶2 MeOH–acetone solution of the nitrate salt of 6 with an excess of NaNO₃ gives brown-red [Fe₂O(NO₃)₄L] 7. Reaction of [NEt₄]₂[Fe₂OCl₆] with L and NaNO₃ (2∶2∶1) in MeCN gives orange-brown Na₂[Fe₂OCl₆L] 8, whereas reaction with L and NaO₂CPh (1∶1∶1) in MeCN gives red-green [Fe₂OCl₂L₂]Cl₂9. Complexes 1, 2 and 5–7 were characterized by X-ray crystallography. The cation of 1 exists in a single-step conformation, with planar bpy units stabilized by the presence of intra-bpy hydrogen bonding and intermolecular hydrogen bonding with the anion. All the dinuclear complexes contain a [Fe₂O]⁴⁺ unit bridged by one L and either zero, one or two carboxylate groups. Complexes 2–4 are bridged by two carboxylate groups, with the two terminal halide ligands in a cis conformation. Complex 5 contains only one bridging benzoate group, and octahedral geometry at each Fe^III is completed with terminal Cl⁻ and H₂O ligands, with the Cl⁻ ligands in a trans conformation. Complex 6 can be described as three [Fe₂O₂(O₂CPh)L]⁺ units joined together to give a near-planar [Fe₆O₆]⁶⁺ ladder-like core. This chain of Fe₂ units is terminated by water molecules. The cation can be prepared as a Cl⁻, Br⁻, NO₃⁻, ClO₄⁻, PF₆⁻, or BF₄⁻ salt. Complex 7 contains four chelating NO₃⁻ ligands and no bridging carboxylate group. Variable-temperature magnetic susceptibility studies of 4 and 6 in the 5.00 to 300 K range reveal both complexes to have S = 0 ground states. The data for 4 were fitted by the appropriate theoretical expression for a Fe^III₂ complex and a molar fraction (p) of S = 5/2 paramagnetic impurity, giving J = −119 (1.5) cm⁻¹, g = 1.96(2) and p = 0.0102(2), with temperature-independent paramagnetism held constant at 500 × 10⁻⁶ cm³ mol⁻¹. The obtained J value is consistent with that predicted by a previously published magnetostructural relationship.