Structural and spectroscopic characterization of an Fe(VI) bis(imido) complex

Abstract

High-valent iron species are key intermediates in oxidative biological processes, but hexavalent complexes apart from the ferrate ion are exceedingly rare. Here, we report the synthesis and structural and spectroscopic characterization of a stable Fe(VI) complex (3) prepared by facile one-electron oxidation of an Fe(V) bis(imido) (2). Single-crystal x-ray diffraction of 2 and 3 revealed four-coordinate Fe centers with an unusual “seesaw” geometry. 57Fe Mössbauer, x-ray photoelectron, x-ray absorption, and electron-nuclear double resonance (ENDOR) spectroscopies, supported by electronic structure calculations, support a low-spin (S = 1/2) d3 Fe(V) configuration in 2 and a diamagnetic (S = 0) d2 Fe(VI) configuration in 3. Their shared seesaw geometry is electronically dictated by a balance of Fe-imido σ- and π-bonding interactions.

C−H Activation by an Iron‐Nitrido Bis‐Pocket Porphyrin Species

“…This is attributable to the higher negative charge carried by the nitrido ligand compared to the oxo ligand. Stabilization of the high‐valent Fe center by multiply bonded nitrogen‐donor ligands was recently shown by an Fe VI ‐bis(imido) complex bearing low Fe VI/V redox potential at −0.73 V (vs. Fc +/0 in THF) [32] . Experimentally, the reduction potential of 2 c could not be easily determined by the LFP method since prior addition of a strong reductant would directly reduce the Fe III ‐azido precursor 1 c .…”

A Pair of Cobalt(III/IV) Terminal Imido Complexes

“…Va riable-temperature and variable-field (VT-VF) SQUID measurements of amicrocrystalline sample of 1 revealed am agnetic moment (m eff )o f 4.28 m B at room temperature,t hus supporting the high-spin S = 3/2, Co II ,d 7 electronic structure assignment (Figure S6). Thus,t he room-temperature moment of 1 falls within the range of 4.3 m B to 4.8 m B ,which is typically observed for highspin cobalt(II) complexes with pseudotetrahedral or lower symmetry, [15] and is suggestive of aq uartet spin ground state for the Co II complex 1.U pon reduction, the axially bound chlorido ligand in 1 is removed, the N-anchor binds the cobalt ion (d(CoÀN1) = 2.245(2) ), and, consequently,p ulls it deeper into the electron-rich cavity of TIMMN mes .H owever, the cobalt atom in monovalent 2 is still located aremarkable 0.3306 (7) above the plane of the three carbene moieties (d oop ;Scheme 1a nd Figure 1).…”

“…Late transition metal imido complexes in high oxidation states are rare,w hich is somewhat surprising, given that electron-rich imido ligands are excellent candidates for the stabilization of an electron-deficient metal ion. So far, several high-valent, formal iron(IV), [5] iron(V), [6] and iron(VI) [7] imido species have been reported. Considerably fewer high-valent metal imido complexes of cobalt, [8] nickel, [9] and copper have been reported.…”

C-H Activation by Metal-Nitrido Species

“…Stabilization of the high-valent Fe center by multiply bonded nitrogen-donor ligands was recently shown by an Fe VI -bis(imido) complex bearing low Fe VI/V redox potential at À0.73 V (vs. Fc +/0 in THF). [32] Experimentally, the reduction potential of 2 c could not be easily determined by the LFP method since prior addition of a strong reductant would directly reduce the Fe IIIazido precursor 1 c. Nonetheless, addition of ferrocene did not lead to quenching of 2 c, suggesting that Fe porphyrin nitrido species are much less oxidizing than their isoelectronic Fe-oxo analogues (E red > 0.4 V vs. Fc +/0 for Cpd-I species). [33] Such a dramatic effect of a nitrogen ligand versus an oxygen ligand on the redox potential has also been reported recently that, the 1 e À reduction potential of a Ru VI -oxo(alkylimido) complex [Ru (2,6- .…”