Person:

Iovan, Diana Alexandra

A sterically accessible tert-butyl-substituted dipyrrinato di-iron(II) complex [(tBuL)FeCl]2 possessing two bridging chloride atoms was synthesized from the previously reported solvento adduct. Upon treatment with aryl azides, the formation of high-spin FeIII species was confirmed by 57Fe Mössbauer spectroscopy. Crystallographic characterization revealed two possible oxidation products: (1) a terminal iron iminyl from aryl azides bearing ortho isopropyl substituents, (tBuL)FeCl(•NC6H3-2,6-iPr2); or (2) a bridging di-iron imido arising from reaction with 3,5-bis(trifluoromethyl)aryl azide, [(tBuL)FeCl]2(μ-NC6H3-3,5-(CF3)2). Similar to the previously reported (ArL)FeCl(•NC6H4-4-tBu), the monomeric iron imido is best described as a high-spin FeIII antiferromagnetically coupled to an iminyl radical, affording an S = 2 spin state as confirmed by SQUID magnetometry. The di-iron imido possesses an S = 0 ground state, arising from two high-spin FeIII centers weakly antiferromagnetically coupled through the bridging imido ligand. The terminal iron iminyl complex undergoes facile decomposition via intra- or intermolecular hydrogen-atom abstraction (HAA) from an imido aryl ortho isopropyl group, or from 1,4-cyclohexadiene, respectively. The bridging di-iron imido is a competent N-group transfer reagent to cyclic internal olefins as well as styrene. Although solid-state magnetometry indicates an antiferromagnetic interaction between the two iron centers (J = −108.7 cm−1) in [(tBuL)FeCl]2(μ-NC6H3-3,5-(CF3)2), we demonstrate that in solution the bridging imido can facilitate HAA as well as dissociate into a terminal iminyl species, which then can promote HAA. In situ monitoring reveals the di-iron bridging imido is a catalytically competent intermediate, one of several iron complexes observed in the amination of C–H bond substrates or styrene aziridination.

The dipyrrinato iron catalyst reacts with organic azides to generate a reactive, high-spin imido radical intermediate, distinct from nitrenoid or imido species commonly observed with low-spin transition metal complexes. The unique electronic structure of the putative group-transfer intermediate dictates the chemoselectivity for intermolecular nitrene transfer. The mechanism of nitrene group transfer was probed via amination and aziridination of para-substituted toluene and styrene substrates, respectively. The Hammett analysis of both catalytic amination and aziridination reactions indicate the rate of nitrene transfer is enhanced with functional groups capable of delocalizing spin. Intermolecular amination reactions with olefinic substrates bearing allylic C–H bonds give rise to exclusive allylic amination with no apparent aziridination products. Amination of substrates containing terminal olefins give rise exclusively to allylic C–H bond abstraction, C–N recombination occurring at the terminal C with transposition of the double bond. A similar reaction is observed with cis-β-methylstyrene where exclusive amination of the allylic position is observed with isomerization of the olefin to the trans-configuration. The high levels of chemoselectivity are attributed to the high-spin electronic configuration of the reactive imido radical intermediate, while the steric demands of the ligand enforce regioselective amination at the terminal position of linear α-olefins.