Seeking Hidden Magnetic Phenomena by Theoretical Means in a Thiooxoverdazyl Adduct

The oxidation of 1,5-dimethyl-3-(2'-pyridyl)-6-thiooxotetrazane (SvdH3py) by benzoquinone leads to a 1:1 adduct of 1,5-dimethyl-3-(2'-pyridyl)-6-thiooxoverdazyl radical (Svdpy) with hydroquinone (hq). The single-crystal X-ray diffraction of this adduct at room temperature (RT) shows that the radicals exhibit a slight curvature that leads to the formation of alternating head-to-tail (antiparallel) stacked 1D chains. Moreover, temperature-dependent X-ray measurements at 100, 200, and 303 K reveal that the lateral slippages between the radicals of the stacks |delta 1| and |delta 2| vary from 0.64 to 0.78 angstrom and 0.54 to 0.40 angstrom between 100 and 303 K. Despite the alternation of the inter-radical distances and lateral slippages, the magnetic susceptibility data can be fitted with excellent agreement using a regular one-dimensional antiferromagnetic chain model with J=-5.9 cm-1. Wavefunction-based calculations indicate an alternation of the magnetic interaction parameters correlated with the structural analysis at RT. Moreover, they demonstrate that the thermal slippage of the radicals induces a switching of the physical behavior, since the exchange interaction changes from antiferromagnetic (-0.9 cm-1) at 100 K to ferromagnetic (1.4 cm-1) at 303 K. The theoretical approach thus reveals a much richer magnetic behavior than the analysis of the magnetic susceptibility data and ultimately questions the relevance of a spin-coupled picture based on temperature-independent parameters.