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An FeVI Nitride There Is Plenty of Room at the Top!.

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Highlights
DOI: 10.1002/anie.200603056
High Oxidation States
An FeVI Nitride: There Is Plenty of Room at the Top!**
Paul J. Chirik*
Keywords:
coordination compounds · iron · Moessbauer
spectroscopy · nitrides · nitrogen fixation
When it comes to iron coordination
chemistry, FeVI is considered to be “top”
with respect to valency—the + 6 oxidation state is thought to be the highest
available.[1] Until early in 2006,[2] the
ferrate ion [FeVIO4]2 (with a variety of
different countercations) stood alone
atop the oxidation-state scale as the
only known FeVI species.[3] However, the
synthesis and characterization of an
iron(VI) nitride by Wieghardt and coworkers demonstrates that hexavalent
iron can be prepared in the laboratory
and in fact there is plenty of room at the
top.
Interest in high-oxidation-state iron
chemistry derives from the potential
intermediacy of such compounds during
the function of many metalloenzymes
and in other catalytic reactions.[4, 5] Lowmolecular-weight model compounds
have been synthetic targets for quite
some time in an attempt to gain insight
into both the spectroscopic features and
reactivity patterns of these otherwise
transient, yet important, species.
Given the role of iron in oxidation
chemistry, considerable attention has
been devoted to the preparation of
FeIV=O coordination compounds and
many successes have recently appeared.[6] Somewhat surprisingly, the
chemistry of related high-valent, terminal iron nitrido compounds remains
relatively underdeveloped. The paucity
of known molecular iron nitrides should
not be interpreted as a lack of utility but
should rather provide an appreciation
for the challenges associated with their
synthesis. These compounds are of considerable interest given their likely role
in both industrial[4] and biological[7]
nitrogen fixation schemes and their
potential to serve as efficient N-atom
transfer agents.
The traditional synthetic approach
to molecular iron nitrides has focused on
the photooxidation of iron azido compounds (Scheme 1). Both the expulsion
of the stable dinitrogen molecule and
the ease with which the FeN3 linkage
can be assembled make this route attractive. It is often the case, however,
that photochemical conversion of the
azido compound to the desired nitrido
species is accompanied by competing
Scheme 1. Photooxidation versus photoreduction of iron azido compounds.
photoreduction, a process that initially
generates the azine radical, which then
quickly decomposes to dinitrogen.
The first successful application of
the photooxidation method to prepare
iron nitrides was reported by Wagner
and Nakamoto.[8] Observation of substituted porphyrin (Por) iron nitrido compounds was achieved at 30 K by using
resonance Raman spectroscopy. Raising
the temperature yielded relatively common m-nitrido diiron compounds. While
at first glance [(Por)FeN] species appear
to be examples of FeV compounds, these
molecules are probably better described
as FeIV ions that are ligated by porphyrin
radical cations.[9]
To avoid complications from redoxand spectroscopically active ligands,
Wieghardt and co-workers introduced
saturated cyclam (1,4,8,11-tetraazacyclotetradecane) tetradentate chelates
into iron nitride chemistry. Photolysis
of low-spin trans-[(cyclam)Fe(N3)2]+ in a
frozen CH3CN matrix at 4.2 K yielded
29 % of an iron(II) species generated by
photoreduction and 54 % of the desired
iron nitride trans-[(cyclam)Fe(N)(N3)]+,
as determined by MAssbauer and EPR
spectroscopy (Scheme 2).[10]
[*] Dr. P. J. Chirik
Department of Chemistry and
Chemical Biology
Cornell University
Ithaca, NY 14853 (USA)
Fax: (+ 1) 607-255-4137
E-mail: pc92@cornell.edu
[**] In reference to Richard Feynman’s famous
lecture on meso- and nanoscale science.
For a transcript of the lecture, see: http://
www.zyvex.com/nanotech/feynman.html.
6956
Scheme 2. Synthesis of iron(V) nitrides by photolysis of ferric azides.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 6956 – 6959
Angewandte
Chemie
Conditions for generating the terminal iron(V) nitride are exquisitely sensitive. Performing the photolysis at higher
temperatures yields a localized m-nitrido
diiron compound, [(cyclam)(N3)FeIV-NFeIII(N3)(cyclam)]2+, a result of the combination of photooxidation and photoreduction products. Remarkably, isomeric, high-spin cis-[(cyclam)Fe(N3)2]+
does not undergo photooxidation to
yield the terminal iron nitride. Irradiation again yields a m-nitrido diiron complex arising from combination of both
photooxidation and photoreduction
products.[10]
Since these initial studies, a “secondgeneration” version of the cyclam ligand
has been introduced where an acetate
group is appended to one of the nitrogen
donors.[11] Photolysis of the low-spin
ferric azide at l > 420 nm resulted in
over 80 % conversion to the desired
iron(V) nitride (Scheme 2). Magnetic
studies and MAssbauer spectroscopy in
combination with DFT calculations established an S = 1/2 ground state, thus
indicating a low-spin FeV d3 ion. XAS
and EXAFS data established the Fe=N
bond length to be approximately
1.60 G.[12]
At this point it is useful to discuss the
indispensable role MAssbauer spectroscopy has played in cyclam-substituted
iron nitride chemistry. The synthesis of a
family of [(cyclam)FeLx]n+ compounds
where the oxidation states range between + 2 and + 5 has allowed systematic evaluation of the experimental zerofield isomer shifts (d) as a function of
oxidation level. Because covalent variations in this series of compounds are
minimal, d varies linearly with charge
density and is described by the relationship d = a{jY(0) j 2C}, where a accounts for the general electronic and
nuclear properties and C is a scaling
constant for the reference scale. As the
oxidation state increases, charge density
increases (a result of the lower d count
and less penetration) and because of the
negative sign, lower values of d result.[13]
Thus, compounds of relatively low oxidation states such as trans-[(cyclam)FeII(N3)2] have relatively high isomer
shifts (d = 0.55 mm s1) while high-valent compounds such as trans-[(cyclam)FeV(N)(N3)] have low isomer shifts (d =
0.04 mm s1). As we will see, it is this
near-linear relationship that allows preAngew. Chem. Int. Ed. 2006, 45, 6956 – 6959
diction of the MAssbauer isomer shift for
a rare oxidation state such as iron(VI)
(Figure 1). A similar correlation was
also noted in the X-ray absorption data
where the energies of the pre-edge
peaks and the edge energies vary consistently by approximately 1 eV per unit
change in the oxidation state.[2]
Cyclam complexes are not the only
examples of iron nitrides. Recently,
Betley and Peters have reported the
synthesis and characterization of a fourcoordinate iron(IV) nitride, [{PhB(PiPr)3}FeIVN]
(PhB(PiPr)3 = PhB [14]
(CH2PiPr2)3 ). In this case, the supporting tridentate phosphine ligand is
monoanionic by virtue of the apical
borate, a feature known to have a
profound impact on the electronic structure of the resulting metal complex.
Notably, the synthesis of [{PhB(PiPr)3}FeIVN] was achieved under thermal rather than photochemical conditions by salt metathesis of [{PhB(PiPr)3}FeIICl] with the N-atom transfer
agent Li(dbabh).[15] As was pioneered
by Mindiola and Cummins for the synthesis of chromium nitrides,[16] loss of
anthracene from the intermediate iron
amide yielded the desired nitride product (Scheme 3). DFT calculations on
[{PhB(PiPr)3}FeIVN] established a (xy)2(x2y2)2(z2)0 d-electron configuration
with a large HOMO–LUMO gap
(ca. 4 eV), consistent with the diamagnetic (S = 0) ground state observed experimentally. In an argon atmosphere or
when concentrated under vacuum,
[{PhB(PiPr)3}FeIVN] couples to form the
iron(I) dinitrogen complex [{{PhB(PiPr)3}FeI}2(m2-N2)], which represents a
unique example of a six-electron redox
process mediated by two iron centers. It
is also noteworthy that [{PhB(PiPr)3}FeIVN] can be treated with proton/electron equivalents to yield NH3.
In recent studies, the Wieghardt
laboratory has reported a subsequent
variation of the acetato-substituted cyclam ligand.[17] Replacing the amine
Figure 1. Correlation of MDssbauer isomer shifts (d) with oxidation state for a family of cyclam
iron coordination compounds.
Scheme 3. Thermal synthesis of the iron(IV) nitrido complex, [{PhB(PiPr)3}FeIVN].
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6957
Highlights
hydrogen atoms with methyl groups
yielded a high-spin, instead of a lowspin, ferric azido compound, [(Me3-cyclam-OAc)FeIII(N3)]+.[18] Importantly,
this substitution also produces a molecule that undergoes reversible one-electron oxidation to furnish a stable iron(IV) azido compound, [(Me3-cyclamOAc)FeIV(N3)]2+ (Scheme 4).
The stability of the electrochemically generated iron(IV) azide allows
photooxidation at 77 K with light of
wavelength 650 nm to furnish a new
major product with a zero-field MAssbauer
isomer
shift
of
d=
0.29 mm s1.[2] The low value of d is
as predicted for the + 6 oxidation state
according to Figure 1, thus supporting
assignment as the iron(VI) nitride
[(Me3-cyclam-OAc)FeVI(N)]2+.
This
compound joins [FeO4]2 as the only
known hexavalent iron compounds and
is the first to be synthesized in a
laboratory!
As was observed with the parent
cyclam complex, the photooxidation
procedure is sensitive to the nature of
the precursor. Irradiation of the highspin ferric azide [(Me3-cyclam-OAc)FeIII(N3)]+ did not yield an iron(V)
nitride, but rather only photoreduction
to iron(II) resulted. To date, only lowspin compounds have resulted in mononuclear iron nitride photooxidation
products.
Additional evidence for the formation of [(Me3-cyclam-OAc)FeVI(N)]2+
was provided by XAS and EXAFS. In
the X-ray absorption spectrum, an in-
tense pre-edge peak was observed,
which is consistent with a covalent
iron–ligand multiple bond. Likewise,
the EXAFS data yielded an FeN bond
length of 1.57 G, slightly contracted
from the values measured for related
iron(V) nitrides.
Much like MAssbauer spectroscopy,
DFT calculations have proven invaluable in the development of iron nitride
coordination chemistry. For [(Me3-cyclam-OAc)FeVI(N)]2+, the DFT-optimized structure predicts an octahedral
geometry with an FeN bond length of
1.53 G, in agreement with the experimental value. The calculations also reproduced the experimental infrared
stretch of the carbonyl group in the
acetate ligand as well as the zero-field
MAssbauer isomer shift.[2]
With a firm computational model in
hand, the nature of the FeN bond can
be understood and its bond order and
spectroscopic properties rationalized in
the context of the other iron(IV) and
(V) molecular nitrides. The FeN bond
in [(Me3-cyclam-OAc)FeVI(N)]2+ is best
described as a triple bond (bond order =
2.8 from DFT),[2] where two p bonds are
formed from the linear combination of
nitrogen px and py orbitals with two
cloverleaf iron d orbitals. As a consequence, the frontier dxz and dyz orbitals
(LUMO) are significantly destabilized
by the antibonding character of the p*
interaction with the nitrogen p orbitals.
The (dxy)2 electronic configuration of
[(Me3-cyclam-OAc)FeVI(N)]2+ produces
a short iron–nitrogen bond length and a
Scheme 4. Photolysis of an iron(IV) azide to yield an iron(VI) nitride.
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
high-frequency FeN vibration at
1064 cm1. This value is comparable to
the FeN stretch of 1034 cm1 reported
for [{PhB(PiPr)3}FeIVN], where population of nonbonding d orbitals (for example, (xy)2(x2y2)2) does not reduce
the iron–nitrogen bond order.[14] These
complexes stand in contrast to the
cyclam-ligated iron(V) nitrides in which
the 2E ground state results in population
of a degenerate antibonding d orbital
(xz, yz), thus reducing both the Fe=N
bond order (d(Fe=N) = 1.60 G) and the
vibrational frequency (850–950 cm1).
Taken together, the results from the
Wieghardt and Peters laboratories clearly demonstrate how careful design and
manipulation of the architecture surrounding the metal coordination sphere
can result in completely new compounds
in the mature field of iron coordination
chemistry. These fundamental studies
into the synthesis and nature of the Fe
N linkage are merely the beginning—
much more remains to be uncovered.
For example, harnessing the reactivity of
these molecules is just one new frontier
awaiting exploration. Is it also possible
that FeVIII compounds, much like RuO4
and OsO4 but with a combination of
oxido and nitrido ligands, are awaiting
discovery? Only time will tell but no
matter the outcome, one fact is true—
another entry into the rarefied air of the
FeVI oxidation state demonstrates that
there is plenty of room at the top!
Published online: October 9, 2006
[1] F. A. Cotton, G. Wilkinson, Advanced
Inorganic Chemistry, 5th ed., Wiley, New
York, 1988, pp. 711 – 723.
[2] J. F. Berry, E. Bill, E. Bothe, S. D.
George, B. Mienert, F. Neese, K Wieghardt, Science 2006, 312, 1937 – 1941.
[3] R. J. Audette, J. W. Quail, Inorg. Chem.
1972, 11, 1904 – 1908.
[4] G. Ertl, Chem. Rec. 2001, 1, 33 – 45.
[5] M. Costas, M. P. Mehn, M. P. Jensen, L.
Que, Jr., Chem. Rev. 2004, 104, 939 –
986.
[6] X. P. Shan, L. Que, J. Inorg. Biochem.
2006, 100, 421 – 433.
[7] J. B. Howard, D. C. Rees, Chem. Rev.
1996, 96, 2965 – 2982.
[8] a) W. D. Wagner, K. Nakamoto, J. Am.
Chem. Soc. 1988, 110, 4044 – 4045;
b) W. D. Wagner, K. Nakamoto, J. Am.
Chem. Soc. 1989, 111, 1590 – 1598.
Angew. Chem. Int. Ed. 2006, 45, 6956 – 6959
Angewandte
Chemie
[9] J. E. Falk, Porphyrins and Metalloporphyrins (Ed.: K. M. Smith), Elsevier,
Amsterdam, 1975.
[10] K. Meyer, E. Bill, B. Mienert, T. WeyhermTller, K. Wieghardt, J. Am. Chem.
Soc. 1999, 121, 4859 – 4876.
[11] C. A. Grapperhaus, B. Mienert, E. Bill,
T. WeyhermTller, K. Wieghardt, Inorg.
Chem. 2000, 39, 5306 – 5317.
[12] N. Aliaga-Alcalde, S. DeBeer George,
B. Mienert, E. Bill, K. Wieghardt, F.
Neese, Angew. Chem. 2005, 117, 2968;
Angew. Chem. Int. Ed. 2006, 45, 6956 – 6959
Angew. Chem. Int. Ed. 2005, 44, 2908 –
2912.
[13] a) P. GTtlich, R. Link, A. Trautwein,
M-ssbauer Spectroscopy and Transition
Metal Chemistry, Springer, Berlin, 1978;
b) this relationship is true for low-spin
compounds because electrons are systematically removed for the t2g set of
molecular orbitals.
[14] T. A. Betley, J. C. Peters, J. Am. Chem.
Soc. 2004, 126, 6252 – 6254.
[15] dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.
[16] D. J. Mindiola, C. C. Cummins, Angew.
Chem. 1998, 110, 983; Angew. Chem. Int.
Ed. 1998, 37, 945.
[17] J. F. Berry, E. Bill, E. Bothe, T. WeyhermTller, K. Wieghardt, J. Am. Chem. Soc.
2005, 127, 11 550.
[18] J. F. Berry, E. Bill, R. Garcia-Serres, F.
Neese, T. WeyhermTller, K. Wieghardt,
Inorg. Chem. 2006, 45, 2027 – 2037.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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