Dinuclear and hexanuclear iron(III) carboxylate clusters with a bis(bipyridine) ligand: supramolecular aggregation of [Fe_2O_2] units to give a [Fe6O_6] ladder structure

摘要

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 HBF_4 yields [H_2L][BF_4]_2 1, the doubly protonated ligand. The FeX_3/NaO_2CR/L (2:4:1) reaction system in MeCN gives [Fe_2OX_2(O_2CR_2)_2L] [X = Cl, R = Ph 2; X = Br, R = Ph 3; X = Cl, R = Me 4]. The same FeCl_3/NaO_2CPh/L (2:4:1) reaction system but in a 1:1 MeCN-water solution gives green [Fe_2OCl_2(O_2CPh)L(H_2O)_2]Cl 5, and in a water solution gives brown [Fe_6O_6(O_2CPh)_3L_3(H_2O)_2]Cl_3 6. Treatment of a 1:2 MeOH-acetone solution of the nitrate salt of 6 with an excess of NaNO_3 gives brown-red [Fe_2O(NO_3)_4L] 7. Reaction of [NEt_4]_2[Fe_2Ocl_6] with L and NaNO_3 (2:2:1) in MeCN gives orange-brown Na_2[Fe_2Ocl_6L] 8, whereas reaction with L and NaO_2CPh (1:1:1) in MeCN gives red-green [Fe_2Ocl_2L_2]Cl_2 9. 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 [Fe_2O]~(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 H_2O ligands, with the Cl~- ligands in a trans conformation. Complex 6 can be described as three [Fe_2O_2(O_2CPh)L]~+ units joined together to give a near-planar [Fe_6O_6]~(6+) ladder-like core. This chain of Fe_2 units is terminated by water molecules. The cation can be prepared as a Cl~-, Br~-, NO_3~-, ClO_4~-, PF_6~-, or BF_4~-- salt. Complex 7 contains four chelating NO_3~- 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_2~(III) 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 X 10~(-6) cm~3 mol~(-1). The obtained J value is consistent with that predicted by a previously published magnetostructural relationship.