Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex Page: 9,796
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ACS
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JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Selectivity and Mechanism of Hydrogen Atom Transfer by an
Isolable Imidoiron(lll) Complex
Ryan E. Cowley,t Nathan A. Eckert,t Sridhar Vaddadi,* Travis M. Figg,* Thomas R Cundari,*'* and
Patrick L. Holland*'t
tDepartment of Chemistry, University of Rochester, Rochester, New York, 14627, United States
tDepartment of Chemistry and Center for Advanced Scientific Computing and Modeling, University of North Texas,
Denton, Texas, 76203, United States
0 Supporting Informationg/JACS
org/JACSABSTRACT: In the literature, iron-oxo complexes have been kREL -1s-1
isolated and their hydrogen atom transfer (HAT) reactions have jH>YH H H
been studied in detail. Iron-imido complexes have been isolated
more recently, and the community needs experimental evaluations \Fe Bupy N p H H H H
of the mechanism of HAT from late-metal imido species. We report N N + H N N 1.6 1
a mechanistic study of HAT by an isolable iron(III) imido complex, Ad Ar AdHDD
LMeFeNAd (LMe = bulky fj-diketiminate ligand, 2,4-bis(2,6-diiso- H-Atom Transfer 1.1
propylphenylimido)pentyl; Ad = 1-adamantyl). HAT is preceded H-Atom Transfer
by binding of tert-butylpyridine (tBupy) to form a reactive four-
coordinate intermediate LMeFe(NAd)(tBupy), as shown by equilibrium and kinetic studies. In the HAT step, very large substrate
H/D kinetic isotope effects around 100 are consistent with C-H bond cleavage. The elementary HAT rate constant is increased by
electron-donating groups on the pyridine additive, and by a more polar medium. When combined with the faster rate of HAT from
indene versus cyclohexadiene, this trend is consistent with H+ transfer character in the HAT transition state. The increase in HAT
rate in the presence of tBupy may be explained by a combination of electronic (weaker Fe=N r-bonding) and thermodynamic
(more exothermic HAT) effects. Most importantly, HAT by these imido complexes has a strong dependence on the size of the
hydrocarbon substrate. This selectivity comes from steric hindrance by the spectator ligands, a strategy that has promise for
controlling the regioselectivity of these C-H bond activation reactions.m INTRODUCTION
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 Metal-oxo 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 iron-oxo species (Fe=O) followed
by radical rebound.4 Other enzymatic systems such as soluble
methane monooxygenase,s ribonucleotide reductases and other
B 12-dependent enzymes,6 lipoxygenases,7 isopenicillin-N
synthase,8 and TauD9 also utilize mechanisms with key HAT
steps.
A-H + B' - A' + B-H (1)
Considerable effort has been devoted to synthesizing and
studying biomimetic oxoiron complexes10 in order to help
elucidate the enzymatic mechanisms and to develop homoge-
neous iron-based oxidation catalysts. Studies on heme iron-oxo
complexes in the 1980s by Balch, La Mar, and Groves pioneered
this field." 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
ACS Publications a 2011 American Chemical Societymechanisms have also been studied for terminal oxo complexes
of Mn, Ru, Cr, and V.13-19 In general, the selectivity of non-
enzymatic HAT reactions is thermodynamically controlled, and
reaction rates follow a linear correlation with the bond dissocia-
tion enthalpy (BDE) of the X-H bond being broken (the
Bell-Evans-Polanyi relation),1,20 i.e., homolytically weaker
substrate bonds react more rapidly.
Imido (NR2-) ligands are isoelectronic to oxo (02-) ligands
(Scheme 1), and imido complexes are often proposed as inter-
mediates in hydrocarbon amination mechanisms in which the
imido species performs the cleavage of the C-H bond that
precedes C-N 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 (M=NR) 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 8-11),21 and this activity
has resulted in the isolation of late transition metal complexes
Received: January 18, 2011
Published: May 12, 2011dx.doi.org/10.1021/ja2005303 I J. Am. Chem. Soc. 2011, 133, 9796-9811
9796
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Cowley, Ryan E.; Eckert, Nathan A.; Vaddadi, Sridhar; Figg, Travis M.; Cundari, Thomas R., 1964- & Holland, Patrick L. Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex, article, May 12, 2011; [Washington, D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc107786/m1/1/?rotate=90: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.