A Magnetostructural Investigation of an Abrupt Spin Transition for 1‑Phenyl-3-trifluoromethyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl

1-Phenyl-3-trifluoromethyl-1,4-dihydro­benzo­[e]­[1,2,4]­triazin-4-yl is the first example of a hydrazyl radical that shows a reversible sharp spin transition fully completed within 5(1) K. The nominally first-order transition takes place at ca. 58(2) K and proceeds via subtle changes of intra- and interstack interactions between two similar structural phases. The low-temperature phase (5–60 K) is diamagnetic and has a singlet ground state (2Jexp = −166.8 cm–1, gsolid = 2.0042, ρ = 0.2%) stemming from a multicenter two-electron interaction. The high-temperature phase (60–300 K) is paramagnetic as a result of noninteracting S = 1/2 spins arising from weakly bound dimers.