Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron(III) Complex

Abstract

Hydrogen atom transfer (HAT, eq 1) is an elementary chemical transformation that results in the net transfer of both a proton and an electron. 1,2 MetalÀoxo complexes are widely used to abstract hydrogen atoms from organic compounds through HAT, which leads to the metal hydroxide (Scheme 1). 3 In oxidations by cytochrome P450, the mechanism is generally accepted to be HAT to an ironÀoxo species (FedO) followed by radical rebound. 4 Other enzymatic systems such as soluble methane monooxygenase, 5 ribonucleotide reductases and other B 12 -dependent enzymes, 6 lipoxygenases, 7 isopenicillin-N synthase, 8 and TauD 9 also utilize mechanisms with key HAT steps.Considerable effort has been devoted to synthesizing and studying biomimetic oxoiron complexes 10 in order to help elucidate the enzymatic mechanisms and to develop homogeneous iron-based oxidation catalysts. Studies on heme ironÀoxo complexes in the 1980s by Balch, La Mar, and Groves pioneered this field. 11 More recently, Que, Nam, and co-workers have reported isolable non-heme oxoiron(IV) complexes that react with hydrocarbons via HAT. 12 Reactions proceeding by HAT mechanisms have also been studied for terminal oxo complexes of Mn, Ru, Cr, and V. 13À19 In general, the selectivity of nonenzymatic HAT reactions is thermodynamically controlled, and reaction rates follow a linear correlation with the bond dissociation enthalpy (BDE) of the XÀH bond being broken (the BellÀEvansÀPolanyi relation), 1,20 i.e., homolytically weaker substrate bonds react more rapidly.Imido (NR 2À ) ligands are isoelectronic to oxo (O 2À ) ligands (Scheme 1), and imido complexes are often proposed as intermediates in hydrocarbon amination mechanisms in which the imido species performs the cleavage of the CÀH bond that precedes CÀN bond formation. Imido complexes are also more versatile than oxo complexes, because there is an opportunity to tune the steric and electronic properties of the complex by changing the nitrogen substituent. However, the HAT reactivity of imido complexes (MdNR) has not been investigated in as much detail as that of their oxo counterparts. There has been a recent renaissance of activity in the synthesis of imido complexes of the late transition metals (groups 8À11), 21 and this activity has resulted in the isolation of late transition metal complexes Received: January 18, 2011 ABSTRACT: In the literature, ironÀoxo complexes have been isolated and their hydrogen atom transfer (HAT) reactions have been studied in detail. IronÀimido complexes have been isolated more recently, and the community needs experimental evaluations of the mechanism of HAT from late-metal imido species. We report a mechanistic study of HAT by an isolable iron(III) imido complex, L Me FeNAd (L Me = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl). HAT is preceded by binding of tert-butylpyridine ( t Bupy) to form a reactive fourcoordinate intermediate L Me Fe(NAd)( t Bupy), as shown by equilibrium and kinetic studies. In the HAT step, very la...

Kinetic and Mechanistic Studies of Carbon-to-Metal Hydrogen Atom Transfer Involving Os-Centered Radicals: Evidence for Tunneling

“…For example, a mechanistic study of HAT by an iron(III) imido complex revealed a strong dependence of the rate constants on the size and steric hindrance of the hydrocarbon substrate, e.g., the rate for CHD was about 90 times larger than for 1,4-dimethyl-1,4-cyclohexadiene, and 1,2,4,5-tetramethyl-1,4-cyclohexadiene did not undergo HAT although all three hydrocarbons have the same calculated BDE. 8 HAT was not directly observed in the time-resolved spectroscopy experiment with FLUO. However, when continuous photolysis (λ > 300 nm) of [(η 5 -i Pr 4 C 5 H)(CO) 2 Os] 2 was carried out in C 6 D 6 in the presence of FLUO, after 27 min of irradiation all of the starting material was decomposed as monitored by 1 H NMR.…”

“…5a An even higher KIE of 105 for the HAT step in the reaction of an iron(III) imido complex with CHD has been measured. 8 The origin of the KIE arises from different zero point energies (ZPEs) between C−H and C−D. ZPEs are dependent on the frequency of the bond stretch, which is in turn dependent on the reduced mass of the two connected atoms.…”

C-H bond amination by iron-imido/nitrene species

“…The three-coordinated iron(III)-imido complex [(nacnac)FeNAd] (nacnac = diketimine; Ad = adamantyl) (Chart 1(c)) reported by the Holland group [55][56][57][58] represents the first example of this type. The imido compound was prepared by the reaction of [(nacnac)Fe(N 2 ) Fe(nacnac)] with adamantyl azide in tetrahydrofuran.…”

“…Although the good stability of this three-coordinated complex at 45°C enabled its full characterization by crystallography, NMR, EXAFS, EPR, Mössbauer, and magnetic susceptibility, on addition of p-tert-butylpyridine (t-BuPy) to a solution of the compound, an intramolecular C-H activation reaction of the isopropyl groups on the nacnac ligand occurred and resulted in the formation of a unique high-spin iron(III)-adamantylamido complex (Scheme 9) [55]. The addition of hydrocarbons with weak C-H bonds, such as 1,4-dihydronaphthalene, 1,4-cyclohexadiene, indene, and cyclopentadiene, could prevent intramolecular attack on the diketiminate ligand and facilitate intermolecular HAA reactions, affording the iron(II)-amido compound [(nacnac) Fe(NHAd)(t-BuPy)] (Scheme 9) [56,57]. Both equilibrium and kinetic studies suggest that the four-coordinated imido intermediate (nacnac)Fe(NAd)(t-BuPy) is the HAA-performing species.…”

“…To clarify the factors effecting the magnified C-H activation reactivity of this four-coordinated species, theoretical calculations on the electronic structures of the three-and four-coordinated imido species as well as their HAA reactions toward 1,4-cyclohexadiene were performed [56]. The calculations revealed that although in (nacnac)Fe(NAd)(tScheme 9 Hydrogen atom abstraction reactivity of (nacnac)FeNAd.…”