Bisdithiazolyl Radical Spin Ladders

A series of four bisdithiazolyl radicals 1a–d (R1 = Pr, Bu, Pn, Hx; R2 = F) has been prepared and characterized by X-ray crystallography. The crystal structure of 1a (R1 = Pr) belongs to the tetragonal space group P4̅21m and consists of slipped π-stack arrays of undimerized radicals packed about 4̅ centers running along the z-direction, an arrangement identical to that found for 1 (R1 = Et; R2 = F). With increasing chain length of the R1 substituent, an isomorphous set 1b–d is generated. All three compounds crystallize in the P21/c space group and consist of pairs of radical π-stacks locked together by strong intermolecular F···S′ bridges to create spin ladder arrays. The slipped π-stack alignment of radicals produces close S···S′ interactions which serve as the “rungs” of a spin ladder, and the long chain alkyl substituents (R1) serve as buffers which separate the ladders from each other laterally. Variable temperature magnetic susceptibility measurements indicate that 1a behaves as an antiferromagnetically coupled Curie–Weiss paramagnet, the behavior of which may be modeled as a weakly coupled AFM chain. Stronger antiferromagnetic coupling is observed in 1b–d, such that the Curie–Weiss fit is no longer applicable. Analysis of the full data range (T = 2–300 K) is consistent with the Johnston strong-leg spin ladder model. The origin of the magnetic behavior across the series has been explored with broken-symmetry Density Functional Theory (DFT) calculations of individual pairwise exchange energies. These confirm that strong antiferromagnetic interactions are present within the ladder “legs” and “rungs”, with only very weak magnetic exchange between the ladders.