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

Abstract

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) Å3, 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 cit4- ligands to the central one, and each of the terminal Fe3 units is capped by another cit4- 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.