Electronic Structure Contributions of Non-Heme Oxo-Iron(V) Complexes to the Reactivity.

Abstract

Oxo-iron(V) species have been implicated in the catalytic cycle of the Rieske dioxygenase. Their synthetic analog, [FeV(O)(OC(O)CH3)(PyNMe3)]2+ (1, PyNMe3 = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-trimethyl), derived from the O-O bond cleavage of its acetylperoxo iron(III) precursor, has been shown experimentally to perform regio- and stereoselective C-H and C═C bond functionalization. However, its structure-activity relation is poorly understood. Herein we present a detailed electronic-structure and spectroscopic analysis of complex 1 along with well-characterized oxo-iron(V) complexes, [FeV(O)(TAML)]- (2, TAML = tetraamido macrocyclic ligand), [FeV(O)(TMC)(NC(O)CH3)]+ (4, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and [FeV(O)(TMC)(NC(OH)CH3)]2+ (4-H+), using wave function-based multireference complete active-space self-consistent field calculations. Our results reveal that the x/ y anisotropy of the 57Fe A-matrix is not a reliable spectroscopic marker to identify oxo-iron(V) species and that the drastically different A x and A y values determined for complexes 1, 4, and 4-H+ have distinctive origins compared to complex 2, a genuine oxo-iron(V) species. Complex 1, in fact, has a dominant character of [FeIV(O···OC(O)CH3)2-•]2+, i.e., an SFe = 1 iron(IV) center antiferromagnetically coupled to an O-O σ* radical, where the O-O bond has not been completely broken. Complex 4 is best described as a triplet ferryl unit that strongly interacts with the trans acetylimidyl radical in an antiferromagnetic fashion, [FeIV(O)(•N═C(O-)CH3)]+. Complex 4-H+ features a similar electronic structure, [FeIV(O)(•N═C(OH)CH3)]2+. Owing to the remaining approximate half σ-bond in the O-O moiety, complex 1 can arrange two electron-accepting orbitals (α σ*O-O and β Fe-d xz) in such a way that both orbitals can simultaneously interact with the doubly occupied electron-donating orbitals (σC-H or πC-C). Hence, complex 1 can promote a concerted yet asynchronous two-electron oxidation of the C-H and C═C bonds, which nicely explains the stereospecificity observed for complex 1 and the related species.