Diiron Complexes of 3H-1,2,3,5-Dithiadiazolines and the Fate of the Parent Dithiadiazolyl: A Crystallographic and Computational Study

Low-temperature crystal structures of iron carbonyl complexes of diamagnetic 4-aryl-3H-1,2,3,5-dithiadiazolines, [Fe₂{S₂N(NH)CAr}(CO)₆] (Ar = 4-C₆H₄CF₃ (2), and 4-C₆H₄OMe (3)) are reported, alongside a new low-temperature structure of [Fe₂{S₂N(NH)CPh}(CO)₆] (1), in which the presence of an NH bond is established unambiguously. Diverse NH···N hydrogen bonding networks in 1, 2, and several variants of 3 (3a–c) are observed, with intermolecular distances refined to neutron-diffraction accuracy using Hirshfeld atom refinement with nonspherical atomic scattering factors, using a state-of-the-art method. To rationalize the formation of 1–3 from Fe₃(CO)₁₂ and 1,2,3,5-dithiadiazolyl radicals, i.e., the conspicuous absence of paramagnetic [Fe₂(S₂N₂CAr)(CO)₆] (1′–3′), the thermochemistry of the reactions, and properties of viable intermediates are profiled using density functional theory (DFT) methods. From these calculations, a clear electronic basis for the instability of 1′–3′ is revealed: unlike S₂N₂CAr^•, which are stable π radicals, their iron complexes are reactive σ radicals that abstract a hydrogen atom from dry toluene to form the observed diamagnetic complexes. Furthermore, the putative paramagnetic complexes are uniquely susceptible to hydrogen atom transfer and are thus probable intermediates in the syntheses of 1–3.