Structural and Magnetic Studies on Two-Dimensional Square Planar Lattice Magnets Composed of Organic Radical Cation Salts (Benzo[1,2-d:4,5-d′]Bis[1,3,2]dithiazolyl-2-zolium)·X (X = TlBr₄, TlI₄, and InI₄)

The magnetostructural correlation was investigated in three organic radical cation salts based on the monocation of benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazolyl (BBDTA), which is an S = 1/2 system, and the tetrahedral diamagnetic monoanions, TlBr4− (1), TlI4− (2), and InI4− (3), by analysis of their crystal structures and magnetic properties. X-ray diffraction studies showed that these three salts consisted of alternatively stacked layers of the diamagnetic anions layers and the organic cation layers in which BBDTA+ formed a square planar lattice, which is rare in organic radical crystals. 1 exhibited canted antiferromagnetism with an antiferromagnetic interaction in the organic cation layer below 11.6 K, whereas 2 and 3 showed metamagnetism with a ferromagnetic interaction in the organic cation layer below 8.9 and 11.3 K, respectively. Their intra- and interlayer magnetic coupling constants, 2J/kB and zJ′/kB, were estimated to be −11.2 and −4.1 K for 1, +11.4 and −3.7 K for 2, and +12.8 and −2.3 K for 3, respectively, using a two-dimensional square planar S = 1/2 Heisenberg model with mean field approximation. A consideration of the molecular orbitals showed that the differences in the intralayer magnetic interactions in 1 and, 2 or 3, originate in the molecular orientation of BBDTA+ in the organic cation layer. Overlap relation between the singly occupied molecular orbitals (SOMOs) of the nearest neighbor BBDTA+ pairs was found in 1, whereas an orthogonal relation was found in 2 and 3.