Reaction of the {LMeFe} fragment with adamantyl azide (AdN3) in the presence of 4‐tert‐butylpyridine (tBuPy) gives an S=3/2 species that has been examined by 1H NMR, IR, EPR, and Mössbauer spectroscopies. DFT calculations point toward an identification as [LMeFeNAd]. This species abstracts hydrogen atoms from the ligand or external reagents only through its pyridine adduct (see scheme; Ar=2,6‐iPr2C6H3).

High-spin organometallic complexes have not received extensive mechanistic study, despite their potential importance as unsaturated intermediates in catalytic transformations. We have found that, with a suitably bulky bidentate ligand, three-coordinate, high-spin alkyl complexes of iron(II) are stable. They undergo isomerization and exchange reactions of the alkyl group through -hydride elimination and reinsertion, and the -hydride elimination step is rate-limiting. The alkyl complexes transfer a -hydrogen atom to CdC, CdN, and CdO double bonds and undergo deprotonation by Brønsted acids. The reversible -hydride elimination reactions can be used to explore relative M-C bond energies. Competition experiments and density functional calculations demonstrate an enthalpic preference for alkyl isomers with iron bound to the terminal carbon of the alkyl fragment. This preference arises from steric and electronic effects. The steric preference could be overcome with a phenyl substituent, which steers iron to the benzylic position. A Hammett correlation and density functional calculations suggest that the substituent effect is attributable to resonance stabilization of partial negative charge on the alkyl ligand.

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...

Hydrogen atom abstraction (HAA) from 1,4-cyclohexadiene (CD-H) by (dtbpe)Ni(NAr) to form a Ni I -amide, (dtbpe)Ni(NHAr), and cyclohexadienyl radical is calculated to be thermodynamically reasonable, ∆H HAA(dtbpe) ) -1.3 kcal/mol, dtbpe ) bis(di-tert-butylphosphino)ethane, Ar ) 2,6-diisopropylphenyl. However, radical rebound to form a metal-bound amine is highly endothermic (∆H reb(dtbpe) ) +25.1 kcal/ mol). Analysis of bond enthalpies indicates that weakening of the Ni-N bond (Ni-amide f Ni-amine) upon radical rebound is not compensated by the weak C-N bond formed. Hence, a ligand was sought that would enhance the metal-amine bond strength while diminishing the metal-amide bond strength. Reaction of (dfmpe)Ni(NAr) with CD-H was thus analyzed, dfmpe ) bis(di(trifluoromethyl)phosphino)ethane. While there is a small change in the thermodynamics of HAA (∆H HAA(dfmpe) ) -5.7 kcal/mol), there is a profound change in the rebound step (∆H reb(dfmpe) ) -7.8 kcal/mol) upon replacing dtbpe by dfmpe. Regeneration of the nitrene active species by reaction of ArN 3 with the metal-bound product is calculated to be highly exothermic, ∆H reg ) -36.7 kcal/mol. Two candidates for a precatalyst, (dfmpe)Ni(COD) and (dfmpe)Ni(bpy), COD ) 1,5-cyclooctadiene and bpy ) 2,2′-bipyridine, were calculated to undergo highly exothermic reactions with ArN 3 to form the nitrene active species. The calculated enthalpic barrier for HAA of CD-H by (dfmpe)Ni(NAr) is 21.3 kcal/mol. Hence, consideration of the computed thermodynamics and kinetics suggests that nickel-nitrenes with fluorinated phosphine supporting ligation are promising candidates for catalytic amination of C-H bonds.

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The first FeII and NiII complexes with bidentate pyrazolate ligands have been prepared (see scheme; M=FeII, NiII; R=tBu, Me; Ar=2,6‐iPr2C6H3; X=Cl, (μ‐Cl)2Li(thf)2). The coordination mode of the pyrazolate ligand η2 or μ‐(η1:η1) and the geometry of the resulting complex depends on the steric bulk of the diketiminate ligand, the d‐electron configuration of the metal center, the solvent, and the relative stabilities of the σ‐ and π‐type frontier orbitals.