A Novel Nonairon(III) Citrate Complex: A "Ferric Triple-Decker"

The novel nonairon(III) citrate complex [Fe₉O(cit)₈(H₂O)₃]^7- (3) [cit = C(O^-)(COO^-)(CH₂COO^-)₂] was prepared as its neocuproinium salt by reacting ferric nitrate, sodium citrate, and neocuproine (neo; 2,9-dimethyl-1,10-phenanthroline) in a 1:1:1 ratio in aqueous solution. The compound (Hneo)₇·3·neo·61H₂O crystallizes in the monoclinic space group P2₁/c with unit cell parameters of a = 21.910(3) Å, b = 25.788(5) Å, c = 39.653(6) Å, β = 99.47(1)°, V = 22 099(7) ų, and Z = 4. The anionic complex consists of three parallel triangular Fe₃ units, 5.6 Å apart, forming a slightly distorted trigonal prism. The outer two units are connected by three bridging tetradentate cit^4- ligands to the central one. and each of the terminal Fe₃ units is capped by another cit^4- ligand. The central Fe₃ unit has the oxo-bridged basic iron carboxylate structure, whereas the two terminal ones have a "voided cubane" structure with an {Fe₃O₄} core. Mössbauer spectra are composed of two subspectra ("a" and "b") with intensity ratio 2:1, reflecting the existence of two inequivalent trivalent iron sites. Subspectra "a" are assigned to the three equivalent iron atoms of the terminal Fe₃ units, and subspectra "b" to the three equivalent iron atoms of the central unit.