Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex Page: 9,797
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Journal of the American Chemical Society
Scheme 1. Analogy between HAT by Oxo and Imido
Complexes
HAT by metal-oxo H
[M]n+=O + H-R > [M](n-1)+-O + 'R
HAT by metal-imido H
[M]n+=N + H-R " [M](n-1)+--N + 'R
R' R'
with terminal imido ligands for the first time.22-27 Surprisingly,
only a few of the isolated late-metal imido complexes perform
HAT reactions, which has limited chemists' ability to evaluate the
mechanism of HAT in imido systems. Warren' group demon-
strated stoichiometric HAT with a nickel(III) imido complex,27c
and Theopold's group showed that a Co-imido complex inserts
the imide into a ligand C-H bond upon heating to form an amine
complex.25d Betley's group has shown that an isolated iron-imido
complex is capable of C-H amination, presumably through a
HAT-based mechanism.22m Gallo's group reported an isolable RuwI
diimido complex that catalyzes hydrocarbon amination through
trappable radical intermediates. 23Che's group evaluated the effect
of imido substituents and Ru(VI) reduction potential for imido
transfer by a series of (porphyrinato)Ru(NR)2 complexes.23b
In the chemistry of iron and other first-row metals, there are
also reports of ligand C-H activation products that strongly
implicate HAT by a reactive (but unobserved) imido moiety. For
example, in 2003 Que's group observed ortho tosylamination of a
ligand phenyl group upon addition of PhI=NTs to an iron(II)
precursor and suggested an iron(WV) tosylimido intermediate.28
More recently, the groups of Power,22j Warren,29 Theopold,30
Borovik,3 Arnold, Betley, Peters,4 and Latour35 observed
amido complexes of Fe, Co, or Cu whose formation is most easily
rationalized as resulting from HAT by unobserved imido com-
plexes. Stephan's group also observed radical reactivity of a
putative Ni" arylimido intermediate, in which radical attack at
the para position of the arylimido moiety results in nick-
el-ketimido products.36 In a few especially exciting cases,29'33
ligand C-H activation leads to formation of a new N-C bond
following the HAT step, likely through a "radical rebound"
mechanism akin to the one used for hydroxylation by cyto-
chrome P450.4 However, the inability to isolate the presumed
imido intermediates in these reactions prevented detailed study
of the mechanisms.
We recently characterized the three-coordinate imidoiron(III)
complex LMeFe=NAd (1) (LMe = 2,4-bis(2,6-diisopropyl-
phenylimido)pentyl anion; Ad = 1-adamantyl) by EPR, NMR,
and Mossbauer spectroscopies and density functional theory
(DFT),37 and subsequently by crystallography, EXAFS, and
magnetic susceptibility.38 It is especially notable that 1 brings
about facile HAT reactions: our preliminary results suggested
that the active species in HAT reactions was L eFe(NAd)(tBupy)
(1-tBupy) (tBupy = 4-tert-butylpyridine), which derives from
weak coordination of 1 by added 4-tert-butylpyridine.37 Because
1 is the first isolated iron-imido complex that performs inter-
molecular HAT reactions, it offers a special opportunity to gain
insight into how these reactions proceed. In this study, we
investigate the kinetics and thermodynamics of hydrocarbon
HAT by 1-tBupy using experimental and computational anal-
ysis. This is one of the first systematic studies of HAT reactivity of
an isolated imido complex with any late transition metal.23b'39Iron-based imido complexes are especially exciting since there
Scheme 2. Intramolecular HAT by LMeFeNAd (1)
Keq
+ tBupyintramolecular HAT
(fast)
N tBupy
K Fe
N NAd
(1 .tBupy)
kintraputative HA T intermediate
IScheme 3. Intermolecular HAT by LMeFeNAd (1)
Fe=NAd + tBupy
(1)intermolecular HATI
LMeFe(NAd)(tBupy)
(1.tBupy)
(fast)
LMeFe(NHAd)(tBupy)
(2.tBupy)
+Keq
H, H
H(1-'Bupy)'
H H
Skinter
H H
N, tBupy
+ C .FewN 0H
(Ad
(2.tBupy)has been a surge in interest in the use of inexpensive and nontoxic
iron for catalysis.40
I RESULTS
Hydrogen Atom Transfer Reactions of 1 -tBupy. The
imidoiron(III) complex LMeFe=NAd (1) is produced upon
addition of 2 equiv of N3Ad to a THF solution of the formally
diiron(I) dinitrogen complex LMeFeNNFeLMe.3s In the presence
of tBupy, 1 exists in equilibrium with 1-tBupy (see below).dx.doi.org/10.1021/ja2005303 IJ. Am. Chem. Soc. 2011, 133, 9796-9811
9797
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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/m1/2/?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.