Synthesis, X-ray Structural Determination, and Magnetic Susceptibility, Mössbauer, and EPR Studies of (Ph4P)2[Fe2(Cat)4(H2O)2]·6H2O, 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 C 2/ c, with a = 22.544(9) Å, b = 12.949(6) Å, c = 22.459(9) Å, β = 91.10(3)°, V = 6555(5) Å 3, Z = 4, and R = 0.039) of (Ph 4 P) 2 [Fe 2 (Cat) 4 (H 2 O) 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 FeO 6 octahedra sharing a common edge. The complex is centrosymmetric with an inversion center in the middle of the Fe 2 O 2 core. The Fe···Fe distance is 3.272(1) Å, 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 Fe 2 O 2 plane. Infrared spectra of (Ph 4 P) 2 [Fe 2 (Cat) 4 (H 2 O) 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 (ε 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). Mössbauer 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 .