Catalytic C−H Bond Amination from High-Spin Iron Imido Complexes

Abstract

Dipyrromethene ligand scaffolds were synthesized bearing large aryl (2,4,6-Ph₃C₆H₂, abbreviated Ar) or alkyl (^tBu, adamantyl) flanking groups to afford three new disubstituted ligands (^RL, 1,9-R₂-5-mesityldipyrromethene, R = aryl, alkyl). While high-spin (S = 2), four-coordinate iron complexes of the type (^RL)FeCl(solv) were obtained with the alkyl-substituted ligand varieties (for R = ^tBu, Ad and solv = THF, OEt₂), use of the sterically encumbered aryl-substituted ligand precluded binding of solvent and cleanly afforded a high-spin (S = 2), three-coordinate complex of the type (^ArL)FeCl. Reaction of (^AdL)FeCl(OEt₂) with alkyl azides resulted in the catalytic amination of C−H bonds or olefin aziridination at room temperature. Using a 5% catalyst loading, 12 turnovers were obtained for the amination of toluene as a substrate, while greater than 85% of alkyl azide was converted to the corresponding aziridine employing styrene as a substrate. A primary kinetic isotope effect of 12.8(5) was obtained for the reaction of (^AdL)FeCl(OEt₂) with adamantyl azide in an equimolar toluene/toluene-d₈ mixture, consistent with the amination proceeding through a hydrogen atom abstraction, radical rebound type mechanism. Reaction of p-^tBuC₆H₄N₃ with (^ArL)FeCl permitted isolation of a high-spin (S = 2) iron complex featuring a terminal imido ligand, (^ArL)FeCl(N(p-^tBuC₆H₄)), as determined by ¹H NMR, X-ray crystallography, and ⁵⁷Fe Mössbauer spectroscopy. The measured Fe−N_imide bond distance (1.768(2) Å) is the longest reported for Fe(imido) complexes in any geometry or spin state, and the disruption of the bond metrics within the imido aryl substituent suggests delocalization of a radical throughout the aryl ring. Zero-field ⁵⁷Fe Mössbauer parameters obtained for (^ArL)FeCl(N(p-^tBuC₆H₄)) suggest a Fe^III formulation and are nearly identical with those observed for a structurally similar, high-spin Fe^III complex bearing the same dipyrromethene framework. Theoretical analyses of (^ArL)FeCl(N(p-^tBuC₆H₄)) suggest a formulation for this reactive species to be a high-spin Fe^III center antiferromagnetically coupled to an imido-based radical (J = −673 cm⁻¹). The terminal imido complex was effective for delivering the nitrene moiety to both C−H bond substrates (42% yield) as well as styrene (76% yield). Furthermore, a primary kinetic isotope effect of 24(3) was obtained for the reaction of (^ArL)FeCl(N(p-^tBuC₆H₄)) with an equimolar toluene/toluene-d₈ mixture, consistent with the values obtained in the catalytic reaction. This commonality suggests the isolated high-spin Fe^III imido radical is a viable intermediate in the catalytic reaction pathway. Given the breadth of iron imido complexes spanning several oxidation states (Fe^II−Fe^V) and several spin states (S = 0 → ³/₂), we propose the unusual electronic structure of the described high-spin iron imido complexes contributes to the observed catalytic reactivity.