Magnetic Studies on Hexaiodorhenate(IV) Salts of Univalent Cations. Spin Canting and Magnetic Ordering in K₂[ReI₆] with <i>T</i>_c = 24 K

The ionic salts of rhenium(IV) of formula (Cat)2[ReI6] with Cat = Li+ (1), Na+ (2), K+ (3), Rb+ (4), Cs+ (5), NH4+ (6), and AsPh4+ (7) [AsPh4+ = tetraphenylarsonium cation] have been synthesized, and the structures of two of them (namely, 3 and 6) were determined by single-crystal X-ray diffraction. 3 crystallizes in the monoclinic system, space group Pn, with a = 7.815(1) Å, b = 7.874(1) Å, c = 11.335(1) Å, β = 90.38(1)°, and Z = 2 whereas 6 crystallizes in the tetragonal system, space group P4/mnc, with a = 7.881(1) Å, b = 7.881(1) Å, c = 11.474(2) Å, and Z = 2. The structures of 3 and 6 are made up of discrete [ReI6]2- anions and K+ (3) or NH4+ (6) cations held together by electrostatic forces (3 and 6) and N−H···I hydrogen bonds (6). The rhenium(IV) cation in 3 and 6 is surrounded by six iodide ligands in an octahedral environment with the Re−I bond lengths varying in a very narrow range [2.704(3)−2.738(3) and 2.716(1)−2.722(2) Å for 3 and 6, respectively]. The [ReI6]2- anions in 6 describe a tetragonally distorted body-centered cubic structure. In 3, the arrangement of these units is similar but more distorted. The different arrangement of the anions in 3 and 6 accounts for the centrosymmetric (6) and non-centrosymmetric (3) structures observed. The magnetic properties of 1−7 were investigated in the temperature range 2.0−300 K. The magnetic behavior of 7 is that of a magnetically diluted Re(IV) complex with a large value of the zero-field splitting of the ground level (|2D| = 49.8 cm-1) whereas those of 1, 2, and 4−6 are typical of antiferromagnetically coupled systems with susceptibility maxima at 28 (1), 27 (2), 21 (4), 16 (5), and 20 K (6). In the case of compound 3, its magnetic behavior in the high-temperature range is parallel to that observed in the parent salts but below 24 K it is a weak ferromagnet with a canting angle of ca. 1.2°.