An ultra-stable oxoiron(iv) complex and its blue conjugate base

Treatment of [Fe^II(L)](OTf)₂ (4), (where L = 1,4,8-Me₃cyclam-11-CH₂C(O)NMe₂) with iodosylbenzene yielded the corresponding S = 1 oxoiron(iv) complex [Fe ^IV(O)(L)](OTf)₂ (5) in nearly quantitative yield. The remarkably high stability of 5 (t_1/2 ≈ 5 days at 25°C) facilitated its characterization by X-ray crystallography and a raft of spectroscopic techniques. Treatment of 5 with strong base was found to generate a distinct, significantly less stable S = 1 oxoiron(iv) complex, 6 (t _1/2 ∼ 1.5 h at 0°C), which could be converted back to 5 by addition of a strong acid; these observations indicate that 5 and 6 represent a conjugate acid-base pair. That 6 can be formulated as [Fe^IV(O)(L-H)] (OTf) was further supported by ESI mass spectrometry, spectroscopic and electrochemical studies, and DFT calculations. The close structural similarity of 5 and 6 provided a unique opportunity to probe the influence of the donor trans to the Fe^IVO unit upon its reactivity in H-atom transfer (HAT) and O-atom transfer (OAT), and 5 was found to display greater reactivity than 6 in both OAT and HAT. While the greater OAT reactivity of 5 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^IV(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.