Synthesis, x-ray structural determination, and magnetic susceptibility, mössbauer, and EPR studies of (Ph4P)Fe2(Cat)4(H2O) 2·6H2O, a catecholato-bridged dimer of iron(III)

Reaction of Fe(OH) with 2 equiv of catechol (Cat) and 1 equiv of NaOH in aqueous solution produces violet crystals (monoclinic unit cell, space group Cc/c, with a = 22.544(9) A, b = 12.949(6) A, c = 22.459(9) A, β= 91.10(3)°, V = 6555(5) A, Z = 4, and R = 0.039) of (PhP)[Fe(Cat)(H O)]·6HO. X-ray crystallography reveals that the complex consists of an anionic binuclear iron(III) core in which each iron is coordinated by six oxygen atoms with the two FeO octahedra sharing a common edge. The complex is centrosymmetric with an inversion center in the middle of the FeO core. The Fe⋯Fe distance is 3.272(1) A, and the bridging Fe-O(1)-Fe angle is 106.2°. Bridging catechol ligands are coplanar, and the nonbridging catechol ligands and coordinated water molecules are located trans to the FeO plane. Infrared spectra of (PhP)[Fe(Cat)(H O)]· 6HO show characteristic C-O stretching frequencies at 1475 and 1432 cm, and the absorption spectrum in dimethyl sulfoxide is dominated by an intense broad peak at 18 350 cm (e = 3430 M cm). Variable temperature magnetic susceptibility data show that the two iron(III) sites are weakly antiferromagnetically coupled (J = -9.7 cm, g = 2.00, p(%monomer) = 5.2%). X- and Q-band EPR spectra are interpreted in terms of the strong exchange limit, and the resonances arise from an S = 1 spin multiplet (g = 1.95 ±0.01, |D| = 0.052 ± 0.003 cm, |E| = 0.013 ± 0.001 cm, and E/D = 0.25) and an S = 2 spin multiplet (g = 1.93 ± 0.02, |D| = 0.041 ± 0.002 cm, |E| = 0.0102 ± 0.0005 cm, and E/D = 0.25). Mossbauer spectra of the complex at 4.2 K in zero applied magnetic field were fitted with a single quadrupole split doublet with an isomer shift of 0.556 mm s and a quadrupole splitting of 0.898 mm s.