An Ultra-Stable Oxoiron(IV) Complex and Its Blue Conjugate Base.

Treatment of Fe() (), (where = 1,4,8-Mecyclam-11-CHC(O)NMe) with iodosylbenzene yielded the corresponding = 1 oxoiron(IV) complex Fe(O() () in nearly quantitative yield. The remarkably high stability of ( ≈ 5 days at 25 °C) facilitated its characterization by X-ray crystallography and a raft of spectroscopic techniques. Treatment of with strong base was found to generate a distinct, significantly less stable = 1 oxoiron(IV) complex, ( ~ 1.5 hrs. at 0 °C), which could be converted back to by addition of a strong acid; these observations indicate that and represent a conjugate acid-base pair. That can be formulated as Fe(O)() was further supported by ESI mass spectrometry, spectroscopic and electrochemical studies, and DFT calculations. The close structural similarity of and provided a unique opportunity to probe the influence of the donor to the Fe=O unit upon its reactivity in H-atom transfer (HAT) and O-atom transfer (OAT), and was found to display greater reactivity than in both OAT and HAT. While the greater OAT reactivity of is expected on the basis of its higher redox potential, its higher HAT reactivity does not follow the anti-electrophilic trend reported for a series of [Fe(O)(TMC)(X)] complexes (TMC = tetramethylcyclam) and thus appears to be inconsistent with the Two-State Reactivity rationale that is the prevailing explanation for the relative facility of oxoiron(IV) complexes to undergo HAT.