Synthesis, x-ray structural determination, and magnetic susceptibility, Mossbauer, and EPR studies of (Ph(4)P)(2)[Fe-2(Cat)(4)(H2O)(2)]center dot 6H(2)O, a catecholato-bridged dimer of iron(III)

Reaction of Fe(OH)(3) with 2 equiv of catechol (Cat) and 1 equiv of NaOH in aqueous solution produces violet crystals (monoclinic unit cell, space group C2/c, with a = 22.544(9) Angstrom, b = 12.949(6) Angstrom, c = 22.459(9) Angstrom, beta = 91.10(3)degrees, V = 6555(5) Angstrom(3), Z = 4, and R = 0.039) of (Ph(4)P)(2)[Fe-2(Cat)(4)(H2O)(2)]. 6H(2)O. 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 FeO6 octahedra sharing a common edge. The complex is centrosymmetric with an inversion center in the middle of the Fe2O2 core. The Fe ... Fe distance is 3.272(1) Angstrom, and the bridging FeO(1)-Fe angle is 106.2 degrees. Bridging catechol ligands are coplanar, and the nonbridging catechol ligands and coordinated water molecules are located trans to the Fe2O2 plane. Infrared spectra of (Ph(4)P)(2)[Fe-2(Cat)(4)(H2O)(2)]. 6H(2)O show characteristic C-O stretching frequencies at 1475 and 1432 cm(-1), and the absorption spectrum in dimethyl sulfoxide is dominated by an intense broad peak at 18 350 cm(-1) (epsilon(max) 3430 M(-1) cm(-1)). Variable temperature magnetic susceptibility data show that the two iron(III) sites are weakly antiferromagnetically coupled (J = -9.7 cm(-1), 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-T= 1 spin multiplet (g = 1.95 +/- 0.01, \D\ = 0.052 +/- 0.003 cm(-1), \E\ = 0.013 +/- 0.001 cm(-1), and E/D = 0.25) and an S-T = 2 spin multiplet (g = 1.93 +/- 0.02, \D\ = 0.041 +/- 0.002 cm(-1), \E\ = 0.0102 +/- 0.0005 cm(-1), 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(-1) and a quadrupole splitting of 0.898 mm s(-1).