Novel Chiral Three-Dimensional Iron(III) Compound Exhibiting Magnetic Ordering at Tc = 40 K

Abstract

The preparation and crystal structure determination of the iron(III) compound of formula {(NH₄)₂[Fe₂O(ox)₂Cl₂]·2H₂O}_n (1) (ox = oxalate dianion) are reported here. Complex 1 crystallizes in the orthorhombic system, space group Fdd2, with a = 14.956(7) Å, b = 23.671(9) Å, c = 9.026(4) Å, and Z = 8. The structure of complex 1 consists of the chiral anionic three-dimensional network [Fe₂O(ox)₂Cl₂]^2- where the iron(III) ions are connected by single oxo and bisbidentate oxalato groups. The metal−metal separations through these bridging ligands are 3.384(2) and 5.496(2) Å, respectively. Ammonium cations and crystallization water molecules are located in the helical pseudohexagonal tunnels defined by iron atoms. The longest iron−iron distance in the pseudohexagonal tunnel is 15.778(2) Å whereas the shortest one is 8.734(2) Å. The iron atoms are hexacoordinated: a terminal chloro ligand and five oxygen atoms, that of the oxo group and four from two cis coordinated oxalate ligands, build a distorted octahedral environment around the metal atom. The Fe−O(oxo) bond distance [1.825(2) Å] is significantly shorter than the Fe(III)−O(ox) [average value 2.103(4) Å] and Fe(III)−Cl bond distances [2.314(2) Å]. Magnetic susceptibility measurements of 1 in the temperature range 2.0−300 K reveal the occurrence of a susceptibility maximum at 195 K and a transition toward a magnetically ordered state in the lower temperature region with T_c = 40 K. The strong antiferromagnetic coupling through the oxo bridge (J = −46.4 cm^-1, the Hamiltonian being H = −JS_A·S_B) accounts for the susceptibility maximum whereas a weak spin canting of ∼0.3° due to the antisymmetric magnetic exchange within the chiral three-dimensional network is responsible for the magnetic ordering. The values of coercive field (H_c) and remnant magnetization (M_r) obtained from the hysteresis loop of 1 at 5 K are 4000 G and 0.016 μ_B.