N-O bond homolysis of an iron(II) TEMPO complex yields an iron(III) Oxo intermediate

Article abstract of DOI:10.1021/ja211882e

The reaction of TEMPO with the iron(I) synthon PhB(MesIm) ₃Fe(COE) leads to formation of the κ¹-TEMPO complex PhB(MesIm)₃Fe(TEMPO). Structural and spectroscopic data establish the complex contains divalent iron bound to a nitroxido anion and is isoelectronic to an iron(II) peroxo complex. Thermolysis of the complex results in N-O bond homolysis, leading to the formation of an iron(III) oxo intermediate. The oxo intermediate is active in oxygen atom transfer reactions and can be trapped by the triphenylmethyl radical to give the iron(II) alkoxo complex PhB(MesIm) ₃Fe(OCPh₃).

Full text of DOI:10.1021/ja211882e

Communication
pubs.acs.org/JACS
N−O Bond Homolysis of an Iron(II) TEMPO Complex Yields an Iron(III)
Oxo Intermediate
Jeremy M. Smith,*,† Derick E. Mayberry,† Charles G. Margarit,† Jorg Sutter,‡ Haobin Wang,*,†
̈
Karsten Meyer,*,‡ and Ranko P. Bontchev§
†Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces New Mexico 88003, United States
‡Department of Chemistry and Pharmacy, Friedrich-Alexander-University Erlangen-Nuremberg, Egerlandstr. 1, 91058 Erlangen,
Germany
§Cabot Corporation, 5401 Venice Avenue N.E., Albuquerque, New Mexico 87113, United States
S
* Supporting Information
thermolysis of a high spin iron(II) peroxide analogue. The
ABSTRACT: The reaction of TEMPO with the iron(I)
synthon PhB(MesIm)3Fe(COE) leads to formation of the
κ1-TEMPO complex PhB(MesIm)3Fe(TEMPO). Struc-
tural and spectroscopic data establish the complex contains
divalent iron bound to a nitroxido anion and is
isoelectronic to an iron(II) peroxo complex. Thermolysis
of the complex results in N−O bond homolysis, leading to
the formation of an iron(III) oxo intermediate. The oxo
intermediate is active in oxygen atom transfer reactions
and can be trapped by the triphenylmethyl radical to give
the iron(II) alkoxo complex PhB(MesIm)3Fe(OCPh3).
iron(III) oxo intermediate, which is not stabilized by hydrogen
bonding or Lewis acids, can be trapped by a carbon-centered
radical to provide an iron(II) alkoxo complex and is also
capable of two electron oxygen atom transfer.
Reaction of the iron(I) synthon PhB(MesIm)3Fe(COE)11
with TEMPO (where PhB(MesIm)3 = phenyltris(1-mesityli-
midazol-2-ylidene)borate, COE = cis-cyclooctene, TEMPO =
2,2′,6,6′-tetramethylpiperidinyloxy radical) leads to the for-
mation of the yellow complex PhB(MesIm)3Fe(TEMPO) 1, a
rare example of a late transition metal TEMPO complex.12 The
solid state structure of 1 (Figure 1A) reveals a four-coordinate
onheme iron dioxygenases and their model complexes
N
have attracted interest as potential green oxidants.1
Oxoferryl species are proposed as the key oxidizing
intermediates in the oxidation mechanism of these species.
There is spectroscopic evidence for the [FeIVO] unit in a
number of metalloenzymes.2 In the case of model complexes,
the [FeIVO] unit has also been characterized by single-crystal
X-ray crystallography for S = 13 and S = 24 spin states.5
The reactivity of synthetic iron(IV) oxo complexes has been
extensively investigated.6 Depending on the supporting ligand,
these complexes oxidize a range of substrates by a number of
mechanisms. The hydroxylation of alkanes and oxidation of
alcohols in particular are proposed to involve initial hydrogen
atom abstraction by the iron(IV) oxo complex.5
Figure 1. (A) X-ray crystal structure of 1. Thermal ellipsoids shown at
50% probability, hydrogen atoms, and most of the tris(carbene)borate
ligand omitted for clarity. Selected bond lengths (Å) and angles (deg):
Fe(1)−O(1) 1.8710(1); Fe(1)−C(101) 2.099(2); Fe(1)−C(301)
2.135(2); Fe(1)−C(201) 2.139(2); O(1)−N(501) 1.449(2);
N(501)−O(1)−Fe(1) 114.8(1); C(101)−Fe(1)−C(301) 89.17(8);
C(101)−Fe(1)−C(201) 88.14(8); C(301)−Fe(1)−C(201) 87.69(8).
An isolable iron(III) oxo complex, [FeIIIH3buea(O)]2−, has
been reported, where H3buea is the tripodal tris[(N′-tert-
butylureaylato)-N-ethylene]aminato ligand.7 Spectroscopic and
computational investigations are consistent with a single Fe−O
σ-bond that is stabilized by a hydrogen bonding network.8 It is
possible that the hydrogen bonding network attenuates the
reactivity of the oxo ligand, which is reflected in the relatively
low thermodynamic driving force toward hydrogen atom
transfer.9 In addition to this complex, electrochemical and
spectroscopic measurements implicate the formation of iron-
(III) oxo species upon one-electron reduction of iron(IV) oxo
complexes. These iron(III) oxos can be stabilized by Lewis
acids.10
(B) Mossbauer spectrum of 1 recorded at 77 K. Spectral parameters: δ
̈
= 0.70 mm s−1 with |ΔEQ| = 1.42 mm s−1. (C) Variable temperature dc
susceptibility data of 1 under an applied field of 1 T. The red line is a
fit to the data, yielding the parameters g = 2.15, |D| = 8.5 cm−1.
Herein, we provide kinetic, reactivity, and computational
evidence for an iron(III) oxo species that is formed by the
Received: December 20, 2011
Published: March 27, 2012
© 2012 American Chemical Society
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Communication
iron atom bound to the oxygen atom of a κ1-TEMPO ligand.13
The long N−O bond length (1.499(2) Å) and pyramidalized
nitrogen atom (sum of angles = 334°) of TEMPO are most
consistent with the reduction of TEMPO to [TEMPO]−,14
while the Fe−C bond lengths and C−Fe−C bond angles are
similar to other high spin four-coordinate iron(II) tris-
(carbene)borate complexes.11,15 These structural data therefore
suggest that the iron(I) synthon reduces TEMPO by one
electron, yielding a high spin, four-coordinate iron(II) nitroxide
complex.
an iron(IV) oxo species and the 2,2′,6,6′-tetramethylpiperidinyl
anion is not thermodynamically favorable, with ΔG ≥ +44.7
kcal/mol in THF.
The optimized structure of PhB(MesIm)3FeO (S = 5/2,
the lowest energy spin state) reveals the complex to have an
approximately 3-fold symmetry (Figure 2A). The FeO bond
Mossbauer spectroscopic and SQUID magnetometry meas-
̈
urements confirm this oxidation state assignment. At 77 K, a
single quadrupole doublet is observed in the zero-field 57Fe
Mossbauer spectrum at δ = 0.70 mm s−1 with |ΔEQ| = 1.42 mm
̈
s−1 (Figure 1B). These spectral parameters are similar to those
observed for other high spin iron(II) tris(carbene)borate
complexes, e.g. for PhB(MesIm)3FeCl, δ = 0.66 mm s−1,
|ΔEQ| = 1.83 mm s−1.15 Between 50 and 300 K, the magnetic
moment of 1 is temperature-independent with μeff = 5.26 μB
(Figure 1C). Together, these data support the assignment of a
mononuclear high spin (S = 2) iron(II) center in complex 1.
The solid state structure of 1 is maintained in solution.
Thirteen paramagnetically shifted resonances are observed in
the 1H NMR spectrum (see Supporting Information).
Interestingly, the number and relative integrations of these
resonances suggest that the TEMPO ligand is locked in the
chair conformation that is observed in the solid state structure,
presumably a consequence of the highly congested environ-
ment near the metal center. The solution magnetic moment
(μeff = 4.7(3) μB), determined by the Evans’ method, is also
consistent with a high spin (S = 2) state.
The combined structural and spectroscopic data therefore
support the assignment of a divalent iron center bound to an
anionic TEMPO ligand. The TEMPO ligand in 1 may be
considered as an analogue of the peroxide ligand; thus, this
complex is a model for the [Fe−OOR] unit, an intermediate in
the activation of O2 by iron-containing metalloenzymes.16 It is
known that the O−O bond in these species is readily cleaved,
leading to the formation an oxidizing iron oxo intermedi-
ate.1,17,18
Thermolysis of 1 results in the formation of stoichiometric
amounts of 2,2′,6,6′-tetramethylpiperidine along with multiple
paramagnetic products. The rate at which 1 disappears was
monitored by 1H NMR in C6D6 (8−46 mM). The reaction was
found to be first order with respect to the iron complex (k1 =
(4.46
0.06) × 10−4 s−1 at 68 °C), consistent with an
intramolecular reaction. It is likely that thermolysis of 1 results
in homolysis of the N−O bond, similar to other examples
involving N−O bond cleavage in TEMPO.19,20 The resulting
aminyl radical then abstracts hydrogen atoms from adventitious
H• sources to yield 2,2′,6,6′-tetramethylpiperidine. Homolysis
of the N−O bond in 1 implicates the formation of an iron(III)
oxo intermediate, similar to other TEMPO deoxygenation
reactions that afford Re(V) and U(V) oxo complexes,
respectively.20
Figure 2. (A) DFT optimized structure of PhB(MesIm)3FeO (S =
5/2). Selected bond length (Å) and angle (deg): FeO 1.717; B
FeO 176.6. (B) Molecular orbital diagram for PhB(MesIm)3FeO
(S = 5/2). Tris(carbene)borate mesityl substituents and phenyl group
have been omitted for clarity.
This proposal is supported by electronic structure theory.
Specifically, density functional theory calculations with the
B3LYP exchange-correlation functional and SDD basis set
reveal that the formation of an iron(III) oxo complex by NO
bond homolysis of 1 is thermodynamically favorable in THF
solution. Thus, for the formation of PhB(MesIm)3FeO (S =
5/2) along with the 2,2′,6,6′-tetramethylpiperidinyl radical, ΔG
= −7.3 kcal/mol.21 By contrast, NO bond heterolysis to give
(1.717 Å) is longer than observed in iron(IV) oxo complexes,
which are in the range 1.639(5)−1.680(1) Å.3,4,22 However,
this bond is shorter than in the iron(III) complex [FeIIIH3buea-
(O)]2− 1.813(3) Å, which has a single σ-bond,8 and thus the
FeO bond order in PhB(MesIm)3FeO is likely to be
between 1 and 2. The frontier orbitals are calculated to be
similar to those reported for four-coordinate iron(III) imido
complexes (Figure 2B).23 Notably, the three highest occupied
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Communication
a
Scheme 1. Synthesis and reactivity of PhB(MesIm)3FeO
a
Inset: Molecular structure of complex 2, thermal ellipsoids shown at 50% probability. Selected bond lengths (Å) and angles (deg): Fe(1)−O(15)
1.814(1); Fe(1)−C(11) 2.118(1); Fe(1)−C(12) 2.118(1); Fe(1)−C(13) 2.098(1); O(1)−C(15) 1.388(2); Fe(1)−O(15)−C(15) 160.8(1);
C(11)−Fe(1)−C(12) 89.03(5); C(11)−Fe(1)−C(13) 88.11(5); C(12)−Fe(1)−C(13) 89.20(5).
orbitals show substantial FeO antibonding character,
suggesting an FeO bond order of 3/2. The spin density at
the oxo ligand (0.85) indicates that oxyl radical character also
likely contributes to the electronic structure.
supported by electronic structure theory, show that, upon
thermolysis, N−O bond homolysis occurs to produce an
iron(III) oxo intermediate, a reaction pathway that is analogous
to that of iron peroxides.
The iron(III) oxo intermediate can be trapped by the
triphenylmethyl radical. Thus, heating 1 in the presence of
Ph2CC6H5CPh3,24 a source of the triphenylmethyl radical,
results in quantitative formation of the alkoxo complex
PhB(MesIm)3FeOCPh3 2 along with stoichiometric 2,2′,6,6′-
tetramethylpiperidine. The complex was characterized in the
solid state by single crystal X-ray crystallography, revealing a
four-coordinate iron alkoxo complex (Scheme 1, inset). The 1H
NMR spectrum of 2 is consistent with the solid state structure;
eleven paramagnetically shifted resonances with appropriate
relative integrations are observed in the spectrum. The
magnetic moment of the complex, μeff = 4.7(3) BM, as
measured by Evans’ method, is consistent with high spin
iron(II).
The rate at which 1 decomposes is unchanged when
thermolysis is conducted in the presence of excess Ph2C
C6H5CPh3 (kobs = (7.1 0.3) × 10−4 s−1).25 These results are
highly suggestive of a mechanism that involves rate limiting
formation of an iron(III) oxo complex by NO bond
homolysis, followed by a rapid reaction with the triphenyl-
methyl radical to yield 2 (Scheme 1).
ASSOCIATED CONTENT
* Supporting Information
Full experimental and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
jesmith@nmsu.edu; haobin@nmsu.edu; karsten.meyer@
chemie.uni-erlangen.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.M.S. gratefully acknowledges funding from NMSU, DOE-BES
(DE-FG02-08ER15996) and the Camille and Henry Dreyfus
Foundation. H.W. acknowledges support from the NSF (CHE-
1012479). K.M. acknowledges funding from the Deutsche
Forschungsgemeinschaft (DFG) and the FAU Erlangen-
Nuremberg. The Bruker X8 X-ray diffractometer was purchased
via an NSF CRIF:MU award to the University of New Mexico,
CHE-0443580. We thank Eileen N. Duesler, Ismael Nieto,
Wallace K. Foster, Johannes Hohenberger, and Xinjiao Wang
for experimental assistance.
Finally, the iron(III) oxo intermediate is active in two-
electron oxygen atom transfer reactions. While no reaction is
observed with either cyclooctene or PPh3, the latter presumably
due to steric constraints,26 PMe2Ph is oxidized to the
corresponding phosphine oxide in 50% yield (Scheme 1).
In conclusion, structural and spectroscopic characterization
of a four-coordinate iron(II) TEMPO complex reveal it to be
isoelectronic to an iron(II) peroxide. Kinetic experiments,
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