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

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 hexanucl ear iron complexes, the latter containing a ladder-like supramolecular arch itecture. Treatment of L with two equivalents of HBF4 yields [H2L][BF4](2) 1, the doubly protonated ligand. The FeX3/NaO2CR/L (2 : 4 : 1) reaction sys tem in MeCN gives [Fe2OX2(O2CR)(2)L] (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, a nd in a water solution gives brown [Fe6O6(O2CPh)(3)L-3(H2O)(2)]Cl-3 6. Trea tment of a 1 : 2 MeOH-acetone solution of the nitrate salt of 6 with an exc ess of NaNO3 gives brown-red [Fe2O(NO3)(4)L] 7. Reaction of [NEt4](2)[Fe2OC l6] with L and NaNO3 (2 : 2 : 1) in MeCN gives orange-brown Na-2[Fe2OCl6L] 8, whereas reaction with L and NaO2CPh (1 : 1 : 1) in MeCN gives red-green [Fe2OCl2L2]Cl-2 9. Complexes 1, 2 and 5-7 were characterized by X-ray cryst allography. The cation of 1 exists in a single-step conformation, with plan ar bpy units stabilized by the presence of intra-bpy hydrogen bonding and i ntermolecular hydrogen bonding with the anion. All the dinuclear complexes contain a [Fe2O](4+) unit bridged by one L and either zero, one or two carb oxylate groups. Complexes 2-4 are bridged by two carboxylate groups, with t he two terminal halide ligands in a cis conformation. Complex 5 contains on ly one bridging benzoate group, and octahedral geometry at each Fe-III is c ompleted 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 c hain of Fe-2 units is terminated by water molecules. The cation can be prep ared as a Cl-, Br-, NO3-, ClO4-, PF6-, or BF4- salt. Complex 7 contains fou r chelating NO3- ligands and no bridging carboxylate group. Variable-temper ature 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 fit ted 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 parama gnetism held constant at 500 x 10(-6) cm(3) mol(-1). The obtained J value i s consistent with that predicted by a previously published magnetostructura l relationship.