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

We previously reported the spin-crossover (SC) properties of [FeII (tacn)2 ](OTf)2 ( 1 ) (tacn = 1,4,7-triazacyclononane) [ Eur. J. Inorg. Chem. 2013 , 2115]. 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 via loss of an H atom. The resulting complex was hypothesized to have the formulation [FeIII (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 H2 formation during the heating of 1 by reacting the headspace with HgO. Formation of water (1 HNMR in anhydrous d6 -DMSO) and elemental mercury were observed. To further support this claim, we independently synthesized [FeIII (tacn)2 ](OTf)3 ( 3 ) and treated it with one equiv base yielding 2 . The structures of 3 was 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).