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 . 61H(2)O crystallizes in the monoclinic space group P2(1)/c with unit cell parameters of a = 21.910(3) Angstrom, b = 25.788(5) Angstrom, c = 39.653(6) Angstrom, beta = 99.47(1)degrees, V = 22 099(7) Angstrom(3). and Z = 4. The anionic complex consists of three parallel triangular Fel units, 5.6 Angstrom 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 Fe-3 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. Mossbauer 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 Fe? units, and subspectra "b'' to the three equivalent iron atoms of the central unit.