Thermally Induced Oxidation of [Fe II (tacn) 2 ](OTf) 2 (tacn = 1,4,7‐triazacyclononane)

We previously reported the spin‐crossover (SC) properties of [Fe II (tacn) 2 ](OTf) 2 ( 1 ) (tacn = 1,4,7‐triazacyclononane). Upon heating under dynamic vacuum, 1 undergoes oxidation to generate a low‐spin iron(III) complex. The oxidation of the iron center was found to be facilitated by initial oxidation of the ligand through loss of a H atom. The resulting complex was hypothesized to have the formulation [Fe III (tacn)(tacn‐H)](OTf) 2 ( 2 ) where tacn‐H is N ‐deprotonated tacn. The formulation was confirmed by ESI‐MS. The powder EPR spectrum of the oxidized product at 77 K reveals the formation of a low‐spin iron(III) species with rhombic spectrum ( g = 1.98, 2.10, 2.19). We have indirectly detected H 2 formation during the heating of 1 by reacting the headspace with HgO. Formation of water ( 1 H NMR in anhydrous [D 6 ]DMSO) and elemental mercury were observed. To further support this claim, we independently synthesized [Fe III (tacn) 2 ](OTf) 3 ( 3 ) and treated it with one equivalent of base yielding 2 . The structures of 3 were characterized by X‐ray crystallography. Compound 2 also exhibits a low‐spin iron(III) rhombic signal ( g = 1.97, 2.11, 2.23) in DMF at 77 K. Variable temperature magnetic susceptibility measurements indicate that 3 undergoes gradual spin increase from 2 to 400 K. DFT studies indicate that the deprotonated nitrogen in 2 forms a bond to iron(III) exhibiting double‐bond character (Fe–N, 1.807 Å).