MAGNETOCHEMISTRY OF THE TETRAHALOFERRATE(III) IONS .2. CRYSTAL-STRUCTURE AND MAGNETIC-ORDERING IN [4-BR(PY)H]3FE2CL1.3BR7.7 AND [4-CL(PY)H]3FE2BR9 - THE SUPEREXCHANGE PATHS IN THE A3FE2X9 SALTS

A series of neutron diffraction investigations at 25 K are reported on single crystals of stoichiometry (4-chloropyridinium)3Fe2Br9 and on (4-bromopyridinium)3Fe2Cl13Br7.7. The compound of stoichiometry (4-bromopyridinium)3Fe2Cl1.3Br7.7 is found to belong to the space group P21/n with four formula units in the unit cell. Structural analysis shows that the iron is present as the [FeX4]−ion (X = Cl, Br). The cell dimensions are a = 14.892 (3) Å, b = 14.184 (4) Å, c = 15.147 (4) Å, and β = 91.44 (2)°. The related compound (4-chloropyridinium)3Fe2Br9 is found to belong to the same space group with a = 14.856 (3) Å, b = 14.066 (3) Å, c = 15.216 (3) Å, and β = 92.15 (1)°. Both materials are isomorphous with the previously reported bis[4-chloropyridinium tetrachloroferrate(III)]-4-chloropyridinium chloride. Magnetic measurements on single crystals show that the S = 5/2 material (4-bromopyridinium)3Fe2Cl1.3Br7.7 orders as a canted antiferromagnet at 5.67 K, while (4-chloropyridinium)3Fe2Br9 orders in a similar fashion at 7.96 K. Magnetic susceptibility data are compared with the theoretical predictions for the simple cubic (sc) Heisenberg high-temperature series expansion model, and superexchange pathways for the isostructural series of monoclinic A3Fe2X9 salts are examined. © 1990, American Chemical Society. All rights reserved.