Synthesis and Properties of the Heterospin (S1 = S2=1/2) Radical-Ion Salt Bis(mesitylene)molybdenum(I) [1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazolidyl

Low-temperature interaction of [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (1) with MoMes(2) (Mes = mesitylene/1,3,5-trimethylbenzene) in tetrahydrofuran gave the heterospin (S-1 = S-2 = 1/2) radical-ion salt [MoMes(2)](+)[1](-) (2) whose structure was confirmed by single-crystal X-ray diffraction (XRD). The structure revealed alternating layers of the cations and anions with the Mes ligands perpendicular, and the anions tilted by 45 degrees, to the layer plane. At 300 K the effective magnetic moment of 2 is equal to 2.40 mu(B) (theoretically expected 2.45 mu(B)) and monotonically decreases with lowering of the temperature. In the temperature range 2-300 K, the molar magnetic susceptibility of 2 is well-described by the Curie Weiss law with parameters C and theta equal to 0.78 cm(3) K mol(-1) and -31.2 K, respectively. Overall, the magnetic behavior of 2 is similar to that of [CrTol(2)](+)[1](-) and [CrCp2*]+[1](-), i.e., changing the cation [MAr2](+) 3d atom M = Cr (Z = 24) with weak spin orbit coupling (SO C) to a 4d atom M = Mo (Z = 42) with stronger SOC does not affect macroscopic magnetic properties of the salts. For the XRD structure of salt 2, parameters of the Heisenberg spin-Hamiltonian were calculated using the broken-symmetry DFT and CASSCF approaches, and the complex 3D magnetic structure with both the ferromagnetic (FM) and antiferromagnetic (AF) exchange interactions was revealed with the latter as dominating. Salt 2 is thermally unstable and slowly loses the Mes ligands upon storage at ambient temperature. Under the same reaction conditions, interaction of 1 with MoTol(2) (Tol = toluene) proceeded with partial loss of the Tol ligands to afford diamagnetic product.