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

Treatment of [Fe-II(L)](OTf)(2) (4), (where L = 1,4,8-Me(3)cyclam-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) approximate to 5 days at 25 degrees 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) similar to 1.5 h at 0 degrees 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-IV=O 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.