Structural And Spectroscopic Characterization Of An Fe(Vi) Bis(Imido) Complex

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. Fe-57 Mossbauer, 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) d(3) Fe(V) configuration in 2 and a diamagnetic (S = 0) d(2) Fe(VI) configuration in 3. Their shared seesaw geometry is electronically dictated by a balance of Fe-imido sigma- and pi-bonding interactions.