Heterospin π-heterocyclic radical-anion salt: Synthesis, structure, and magnetic properties of decamethylchromocenium [1,2,5]Thiadiazolo[3,4-c ][1,2,5]thiadiazolidyl

Decamethylchromocene, Cr^II(η⁵-C ₅(CH₃)₅)₂ (2), readily reduced [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (1) in a tetrahydrofuran solvent at ambient temperature with the formation of radical-anion salt [2] ⁺[1]^- (3) isolated in 97% yield. The heterospin salt 3 ([2]⁺, S = ³/₂; [1]^-, S = ¹/₂) was characterized by single-crystal X-ray diffraction as well as magnetic susceptibility measurements in the temperature range 2-300 K. The experimental data together with theoretical analysis of the salt-s magnetic structure within the CASSCF and spin-unrestricted broken-symmetry (BS) density functional theory (DFT) approaches revealed antiferromagnetic (AF) interactions in the crystalline 3: significant between anions [1]^-, weak between cations [2]⁺, and very weak between [1]^- and [2]⁺. Experimental temperature dependences of the magnetic susceptibility and the effective magnetic moment of 3 were very well reproduced in the assumption of the AF-coupled [1]^-⋯[1]^- (J₁ = -40 ± 9 cm^-1) and [2]⁺⋯ [2]⁺ (J₂ = -0.58 ± 0.03 cm^-1) pairs. The experimental J₁ value is in reasonable agreement with the value calculated using BS UB3LYP/6-31+G(d) (-61 cm^-1) and CASSCF(10,10)/6-31+G(d) (-15.3 cm^-1) approaches. The experimental J₂ value is also in agreement with that calculated using the BS DFT approach (-0.33 cm^-1).