Thermally Induced Oxidation of FeII(tacn)2(OTf)2 (tacn = 147-triazacyclononane)

摘要

We previously reported the spin-crossover (SC) properties of [Fe^II(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 [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 H2 formation during the heating of >1 by reacting the headspace with HgO. Formation of water (¹HNMR in anhydrous d6-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 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).