New Extended Magnetic Systems Based on Oxalate and Iron(III) Ions

A series of oxalate-bridged iron(III) complexes have been synthesized by the reaction of FeCl3 with oxalic acid (H2ox) and XCl, where X is a substituted univalent ammonium or an alkaline cation. We have obtained basically two different types of compounds by varying the nature and the shape of the counterion, with the dimensionality of the resulting product being strongly influenced by the counterion. Three-dimensional (3D) networks of oxo- and oxalato-bridged iron(III) ions of the general formula {X2[Fe2O(ox)2Cl2]·pH2O}n have been obtained for X = Li+ (1), Na+ (2), and K+ (3) with p = 4 and X = MeNH3+ (4), Me2NH2+ (5), and EtNH3+ (6) with p = 2. Similar 3D hydroxo- and oxalato-bridged iron(III) networks of the formula {X[Fe2(OH)(ox)2Cl2]·2H2O}n resulted for X = EtNH3+ (7a) and PrNH3+ (8). Compound 7a undergoes a solid-to-solid transformation, leading to a new species of the formula {(H3O)(EtNH3)[Fe2O(ox)2Cl2]·H2O}n (7b). Chainlike compounds of the formula {X2[Fe2(ox)2Cl4]·pH2O}n [X = Me2NH2+(9, p = 1), Me3NH+ (10, p = 2), and Me4N+ (11, p = 0)] have been obtained for the bulkier alkylammonium cations. Magnetic susceptibility measurements in the temperature range 1.9–295 K show the occurrence of weak ferromagnetic ordering due to spin canting in the 3D networks 1−8, with the value of the critical temperature (Tc) varying with the cation in the range 26 K (2) to 70 K (8) without significant structural modifications. The last three one-dimensional compounds exhibit the typical behavior of antiferromagnetically coupled chains of interacting spin sextets [J = −8.3 (9), −6.9 (10), and –8.4 (11) cm−1 with H = −J∑iSiSi+1].