Transformation of an [Fe(η2-N2H 3)]+ species to π-delocalized [Fe2(μ- N2H2)]2+/+ complexes

Article abstract of DOI:10.1002/anie.201006299

Diiron diazenes: A monomeric Fe(η²²-N ₂H₃) species has been prepared, and exposure to oxygen yields a diiron complex that features five-coordinate iron centers and an activated bridging diazene ligand (see picture; C gray, N blue, Fe black, P red, O green). Structural, theoretical, and spectroscopic data for the redox pair [Fe₂(μ-N₂H₂)]^2+/+ are consistent with 4-center, 4-electron π-delocalized bonding across the Fe-NH-NH-Fe core that finds analogy in butadiene and the butadiene anion. Copyright

Full text of DOI:10.1002/anie.201006299

Communications
DOI: 10.1002/anie.201006299
Iron-Mediated N2 Fixation
Transformation of an [Fe(h2-N2H3)]+ Species to p-Delocalized
[Fe2(m-N2H2)]2+/+ Complexes**
Caroline T. Saouma, R. Adam Kinney, Brian M. Hoffman,* and Jonas C. Peters*
Several mechanisms have been proposed to describe the
reduction of N2 to NH3 at the cofactor of MoFe nitrogenase.[1]
Although experimental evidence is consistent with initial
coordination of N2 through a single metal center of the
cofactor,[2] recent DFT studies have pointed to plausible
diiron intermediates of the type Fe2(N2Hy) (y = 1–4) en route
to NH3 formation.[3] In this context, it is noteworthy that
diazene[4] and hydrazine[5] are readily reduced to NH3 by
nitrogenase under turnover conditions. Diiron model com-
plexes that feature the Fe2(N2Hy) core are therefore of timely
interest,[6–8] especially as a spectral reference point to aid in
the interpretation of ENDOR/ESEEM data that is being
obtained with the enzymatic system during catalysis.[1d]
Herein we describe the characterization of an [Fe(h2-
N2H3)]+ species that gives rise to a binuclear complex with an
[Fe2(m-N2H2)]2+ core upon exposure to O2. The latter complex
is unique in that combined structural, spectroscopic, and DFT
calculations suggest that the bridging “diazene” is best
formulated as N2H22À. While this level of diazene activation
has been observed in complexes of highly reducing early
transition metals[9] it is not well established for the later
transition metals, including iron.[7,10] One-electron reduction
of the [Fe2(m-N2H2)]2+ complex furnishes the EPR-active
mixed-valent [Fe2(m-N2H2)]+ complex, whose electronic struc-
ture characterization by combined EPR/ENDOR spectros-
copy is described.
Entry to this chemical manifold arises from the addition of
À
N2H4 to the iron alkyl precursor [(PhBP3)FeMe] (1; PhBP3
=
À
PhB(CH2PPh2)3 ) in the presence of a suitable trap. We have
previously reported that the room temperature reaction
between 1 and N2H4 quantitatively forms [{(PhBP3)Fe}2(m-
h1:h1-N2H4)(m-h2:h2-N2H2)], with concomitant loss of meth-
ane.[7b]
A
hydrazido
complex
of
the
type
“[(PhBP3)Fe(N2H3)]” is a plausible thermally unstable inter-
mediate to invoke, and a strong-field trapping ligand was
hence pursued. Addition of 1 equiv of N2H4 to 1 at À788C,
followed by addition of 1 equiv of CO, affords orange
[(PhBP3)Fe(h2-N2H3)(CO)] (2) in ca. 70% chemical yield
(Scheme 1). Several side reactions compete with
formation of 2, and the crude reaction mixtures invariably
contain
[{(PhBP3)Fe}2(m-h1:h1-N2H4)(m-h2:h2-N2H2)],[7b]
[(PhBP3)Fe(CO)2H] (see Supporting Information), and sev-
eral other unidentified species. The similar solubilities of
[(PhBP3)Fe(CO)2H] and 2 diminish the isolated yield of 2 in
analytically pure form.
The solid-state structure of 2 was obtained and indicates
that the N2H3 ligand coordinates h2 to the Fe center
À
À
(Scheme 1). The Fe N distances of 1.992(3) and 2.018(3) ꢀ
are as expected for coordination of sp3-hybridized nitrogen to
Fe, and are similar to those observed in the related six-
[*] C. T. Saouma,[+] Prof. J. C. Peters[+]
Department of Chemistry and Chemical Engineering
California Institute of Technology
1200 E. California Blvd, Pasadena, CA 91125 (USA)
E-mail: jpeters@caltech.edu
Homepage: http://jcpgroup.caltech.edu
R. A. Kinney, Prof. B. M. Hoffman
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
E-mail: bmh@northwestern.edu
Homepage: http://chemgroups.northwestern.edu/hoffman/
index.htm
[+] Former address: Department of Chemistry, Massachusetts Institute
of Technology, Cambridge, MA 02139 (USA)
[**] We acknowledge the NIH (GM-070757, J.C.P.; HL 13531, B.M.H.)
and the NSF (MCB0723330, B.M.H.). Funding for the Caltech NMR
facility has been provided in part by the NIH (RR 027690) and
funding for the MIT Department of Chemistry Instrumentation
Facility has been provided in part by the NSF (CHE-0234877). The
Betty and Gordon Moore Foundation supports the Molecular
Observatory at Caltech. C.T.S. is grateful for an NSF graduate
fellowship. Alec Durrell and Jens Kaiser provided guidance for
resonance Raman and XRD experiments, respectively.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006299.
Scheme 1.
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coordinate [Fe(h2-N2H4)] and [Fe(h2-N2H2)]
species.[8] The N1 N2 bond of 1.383(3) ꢀ is
À
À
shorter than expected for an N N single
bond, but consistent with that of a related
N2H3À complex of tungsten.[11]
The 15N NMR spectrum (À758C,
[D8]THF) of 2 shows a complicated signal
centered around d = 32 ppm, which was fit to
obtain chemical shifts and coupling constants
À
(see Supporting Information). The NH NH2
À
and NH NH2 chemical shifts are noted at d =
31.8 ppm and d = 32.2 ppm, respectively, with
1J(N,N) = 10 Hz. The 1H NMR spectrum
(À758C, [D8]THF) of 2 shows three distinct
protons for the hydrazido ligand that split
into doublets when samples of 2 are prepared
15
À
with N2H4. The NH NH2 chemical shift is
noted at d = 2.85 ppm (1J(N,H) = 56 Hz), and
À
the inequivalent NH NH2 protons appear at
d = 6.55 ppm (1J(N,H) = 86 Hz) and d =
1.88 ppm (1J(N,H) = 79 Hz). The NMR data
collectively indicates that the N2H3À ligand is
comprised of two sp3-hybridized nitrogen
atoms.
Figure 1. Displacement ellipsoid (50%) representations of the core atoms of 3 (left, top) and
4 (right, top), and an overlay of their core atoms (bottom, left; black, 3; gray, 4), and a
representation showing the twist of the Fe-N-N-Fe linkage of 4 (bottom, right).
geometry, with the approximate equatorial plane defined by
The orange hydrazido(À) complex 2 undergoes decay to two phosphorous and one nitrogen atom. The two Fe centers
the bridged blue diazene complex [{(PhBP3)Fe(CO)}2(m- are related by a 1338 rotation about the Fe–Fe vector. The
h1:h1-N2H2)] (3) in the presence of 0.5 equiv oxygen trans protons on the diazene were located in the difference
(Scheme 1). Other oxidants (e.g., Pb(OAc)4, [Cp2Fe+], p- map, and form a planar diazene. However, the Fe-N-N-Fe
quinone), acids (e.g., pyridinium, FeCl3, Sm(OTf)3), and bases linkage departs from planarity and features a 20.38 dihedral
À
(e.g., N2H4, nBuLi, tBuN = P(cyclo-NC4H8)) were canvassed angle (Figure 1). The average Fe N bond of 1.83 ꢀ in 3
À
but do not facilitate this transformation. The reaction is indicates the presence of p-bonding, while the elongated N N
solvent-dependent and proceeds in benzene but not in THF, bond of 1.362(4) ꢀ establishes a significantly activated
perhaps owing to hydrogen bond stabilization of 2 by THF diazene unit. This distance is closer to that expected for an
2
2
À
solvent (see Supporting Information).
N(sp ) N(sp ) single bond than that for a double bond (ca.
The 15N NMR spectrum of 3 (prepared from 15N-2) 1.41 ꢀ and 1.24 ꢀ, respectively).[7,15]
displays a broad doublet at 292 ppm, indicative of an sp2-
Complex 3 is intensely colored and displays a transition at
hybridized nitrogen atom. The diazene protons are magneti- 716 nm (e = 8500mÀ1 cmÀ1) that is presumably charge transfer
cally inequivalent, and the corresponding 1H{31P} NMR in nature by analogy to assignments made for similar bands
spectrum of 3 shows an AA’XX’ splitting pattern centered observed for related dinuclear [M(h1:h1-N2H2)M] com-
at d = 9.5 ppm. The chemical shifts of both the H and N atoms plexes.[10,16] The resonance Raman spectrum of 3 (633 nm
of the diazene ligand differ from those observed in the related excitation) contains an NN vibration at 1060 cmÀ1, which
[{(PhBP3)Fe}2(m-h1:h1-N2H2)(m-h2:h2-N2H2)] (15N NMR: d = shifts to 1032 cmÀ1 in samples of 15N-enriched 3 (calculated
1
407.5, 58.0 ppm; H NMR: d = 13.20, 4.16 ppm),[7b] and sug- shift for a diatomic harmonic oscillator: 1023 cmÀ1). In
gest that the extent of diazene activation in the two complexes addition, a second vibration is observed at 665 cmÀ1 (15N:
1
may be different. Simulation of the H{31P} spectrum of 3 651 cmÀ1), which is tentatively assigned as the ns(FeN)
1
gives the following coupling constants: J(N,H) = À71.0 Hz, vibration that couples with the NN vibration. Both of these
3
1
2J(N,H) = À2.1 Hz, J(H,H) = 14.8 Hz, and J(N,N) = 9.5 Hz. vibrations are distinct from those measured by Lehnert et al.
The magnitude of the three-bond HH coupling is consistent in an octahedral [Fe2(m-h1:h1-N2H2)] complex (n(NN) =
with a trans configuration, and can furthermore be used as a 1365 cmÀ1
;
nas(FeN) = 496 cmÀ1),[17] and consistent with
probe for the extent of NN activation.[12] For example, appreciably stronger Fe N and weaker N N bonds in 3.
À
À
3J(H,H) = 28.0 Hz for [{(CO)5Cr}2(trans-m-N2H2)],[13] which The combined structural, NMR, and vibrational data suggest
has an N N bond distance of 1.25 ꢀ, while 3J(H,H) = that the diazene bridge in 3 might better be regarded as a
[14]
À
9.4 Hz for [{(h5-C5Me4H)2ZrI}2(trans-m-N2H2)], which has an dianionic hydrazido, N2H22À, as in the left resonance form
[9b]
À
N N bond distance of 1.414(3) ꢀ. Hence, the observed shown in Figure 2B.
3J(H,H) coupling in 3 is most consistent with a single bond.
Cyclic voltammetry of 3 shows a reversible one-electron
The solid-state structure of 3 was obtained and its core reduction event at À1.54 V (vs. Fc/Fc+), and chemical treat-
atoms are shown in Figure 1 (see Supporting Information for ment of 3 with 1 equiv of Na/Hg in THF cleanly generates
complete structure). Both Fe centers have similar metrical the purple mixed-valence [Fe2(m-N2H2)]+ complex,
parameters, and adopt a distorted trigonal bipyramidal [{(PhBP3)Fe}2(m-h1:h1-N2H2)][Na(THF)6] (4). Crystals of 4
Angew. Chem. Int. Ed. 2011, 50, 3446 –3449
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Communications
3 and 4. The frontier orbitals of 3 are isolobal to those of
butadiene, and both the HOMO and LUMO are primarily
composed of the Fe-N-N-Fe p-system. The HOMO displays
À
À
Fe N p-bonding and N N p*-bonding character (Fig-
À
À
ure 2A). The LUMO features N N p-bonding, and Fe N
p*-bonding character. Population of the LUMO should
À
therefore result in a decrease in the N N bond distance and
À
an increase in the Fe N bond distance. However, the SOMO
À
of 4 has only minimal density on the N N bridge, and so the
actual change should be small, as observed. The observation
À
that the reduction of 3 to 4 yields a shortened N N distance in
the N2H2 ligand in the present case is consistent with the DFT
calculations. A similar result has been provided for a series of
[Mo2(m-N2)]6+/7+/8+ species where formal overall oxidation of
the complex leads to a more “activated” bridging N2 ligand.[19]
To further probe the electronic structure of the [Fe2(m-
N2H2)]+ core of 4, we turned to EPR/ENDOR spectroscopy.
Complex 4 is paramagnetic, with a rhombic S = 1= EPR signal
2
(9:1 THF/MeTHF; g = 2.125, 2.040, 2.020) that remains
essentially invariant from 77 K to 2 K. To test the model of
a symmetrical, p-delocalized Fe-N-N-Fe core, 35 GHz 15N
electron-nuclear double resonance (ENDOR) measurements
were performed at 2 K on 15N-4.[20] Figure 2C displays 15N
ENDOR spectra selected from a 2D field–frequency pattern
of ENDOR spectra (n+ manifold) collected across the EPR
envelope of 15N-4 (see Supporting Information). The 2D
pattern can be simulated with a single type of 15N, having a
nearly axial hyperfine coupling tensor, principal values,
A(15N) = + [6.7, 5.6, 17.8] MHz, isotropic coupling, aiso
(15N) = 10 MHz, and anisotropic coupling, T(15N) = + [À3.3,
À4.5, 7.8] MHz (signs have been determined by pulsed
ENDOR protocols; see Supporting Information).[21] The
absence of a Mims ENDOR response associated with a
second, more weakly coupled 15N nucleus (not shown),[22]
indicates that the two 15N atoms from the bridge are
magnetically equivalent and contribute equally to the
ENDOR response depicted in Figure 2C, as expected for a
delocalized [Fe2(m-N2H2)]+ ground state.
Figure 2. A) HOMO and LUMO of 3 (isocontour=0.04); see the
Supporting Information for computational details. B) Plausible reso-
nance contributors to the electronic structure of 3. C) 35 GHz Davies
15N pulsed ENDOR spectra (black traces; n+ manifold (n+ =nn +A/2))
from 15N-4 at the indicated g values. Simulations (red, a=Æ78; blue,
a=08 and 158) use hyperfine and g tensors given in the text (see the
Supporting Information); the ENDOR linewidth in the g3 simulation is
greater than that for g1, as would be required by a distribution of a,
see text.
The strong anisotropy of A requires that the spin density
on the two N atoms is of p character (A3 parallel to the p
orbital for each N).[23] The positive sign of A for 15N indicates
p
À
that the p spin density on the N N bridge, 1 (N), is negative
(see Supporting Information), with the anisotropic coupling
corresponding to 1p(N) ꢀ À0.05 spins per nitrogen. This small
negative spin density on N arises from polarization of doubly
occupied bonding core p orbitals by the large spin density on
Fe. The DFT computations on 4 give 1p(N) ꢀ À0.36 spins per
nitrogen, which is in satisfactory agreement with experiment
given that DFT is well known to overestimate the effects of
spin polarization.[24] This finding of rather low spin delocal-
ization onto the bridging nitrogens of 4 illustrates why it is
instructive to consider 3 in terms of the butadiene-like
resonance structure shown in Figure 2B. The butadiene
anion is the corresponding analogue to 4, and its SOMO is
minimally delocalized onto the central atoms. In 4, delocal-
ization would be decreased further due to the greater
electronegativity of N compared to that of Fe.
suitable for XRD were grown by vapor diffusion of cyclo-
pentane into a saturated THF solution of 4.
The geometry of the [Fe2(m-N2H2)]+ core of 4 is very
similar to that of the [Fe2(m-N2H2)]2+ core of 3, as shown by an
overlay of their core atoms (Figure 1). Upon reduction the
À
average Fe N distance increases by ca. 0.03 ꢀ to 1.88 ꢀ.
Consistent with this, the ns(FeN) stretch decreases from
665 cmÀ1 to 643 cmÀ1 (15N: 624 cmÀ1) upon reduction.[18] The
À
N N bond distance in 4 is found to exhibit a marginal
decrease to 1.342(3) ꢀ upon reduction. These observations
are collectively consistent with p-delocalization within the Fe-
N-N-Fe core, with the unpaired electron populating an orbital
À
that is predominantly Fe N antibonding in character.
DFT calculations (see Supporting Information) were
performed to further probe the electronic structures of both
The orientations of the 15N hyperfine tensors also are
informative. The observation of a single, very sharp 15N
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d) B. M. Hoffman, D. R. Dean, L. C. Seefeldt, Acc. Chem. Res.
2009, 42, 609 – 619.
[2] B. M. Barney, D. Lukoyanov, R. Y. Igarashi, M. Laryukhin, T. C.
Yang, D. R. Dean, B. M. Hoffman, L. C. Seefeldt, Biochemistry
2009, 48, 9094 – 9102.
ENDOR feature at g1 indicates the g1 axis is coincident with
À
the N N vector and normal to the spin-bearing p orbitals on
15Ni (i = 1, 2). These are expected to be primarily defined by
the Fei-Hi-Nj (j = 2, 1) planes (Figure 1, bottom right), and
thus lie essentially normal to the g1 axis. Indeed, the 2D
ENDOR pattern is satisfactorily simulated by taking the g3
axis to bisect the angle between the two Fei-Hi-Nj planes, 2a
ꢀ 148, and then orienting each 15Ni hyperfine tensor along the
normal to its plane, which corresponds closely to simply
rotating the hyperfine tensors of N1 and N2 around g1 by
equal and opposite angles, a ꢀ 78 (see Figure 2C, red trace).
The data does not define these rotations with precision;
not only is agreement with experiment at g2 improved with a
a = 158 (see the Supporting Information), but also the
observation of broad features in the ENDOR spectrum at
g3, in contrast to the narrow peak at g1, suggest that there is a
distribution of angles in the frozen solution, likely associated
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À
with torsions about the N N “single” bond of as little as a few
degrees. Overall, the 15N ENDOR results support that 4, at
2 K, contains a p-delocalized Fe-N-N-Fe core, as predicted by
DFT computations, with the p-orbital “twist” indicated by the
X-ray structure.
In summary, we have prepared an Fe(h2-N2H3) species,
and have shown that the coordinated hydrazido ligand is
converted to diazene in the presence of oxygen. The end-on
diazene ligands in the [Fe2(m-N2H2)]2+/+ cores of 3 and 4 are
best regarded as “N2H22À”, a bonding formulation previously
observed for diazene complexes of highly reducing early
transition metals. Combined structural, theoretical, and
spectroscopic data for the dinuclear complex 3 indicate the
presence of 4-center, 4-electron p-delocalized bonding across
the Fe-N-N-Fe diiron m-diazene core. This picture is consis-
tent with DFT studies, as well as a combined EPR/ENDOR
study of its one-electron reduced congener 4. This electronic
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[18] The presence of an additional CT band precluded our ability to
obtain strong resonance enhancement, and the NN stretch could
hence not be reliably located for 4.
À
structure, in which the HOMO is N N p-bonding, provides
access to stable diazene complexes in both the [Fe2(m-
N2H2)]2+/+ oxidation states. Whether such a fragment arises
in the reaction pathway by which nitrogenase reduces N2 to
2NH3 is being explored by detailed comparisons of the results
presented here with ENDOR results for nitrogenase inter-
mediates.[25]
¨
[19] J. J. Curley, T. R. Cook, S. Y. Reece, P. Muller, C. C. Cummins, J.
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CCDC 795818 (2), 795819 (3), 795820 (4), and 795821
([(PhBp3)Fe(CO)2H]) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
[20] The ENDOR response for a 15N nucleus (I = 1/2) in which A > nn
is given by the equation n = A/2 Æ nn. Mims and Davies 35 GHz
pulsed ENDOR measurements: A. Schweiger, G. Jeschke,
Principles of Pulse Electron Paramagnetic Resonance, Oxford
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Received: October 7, 2010
Revised: November 30, 2010
Published online: March 10, 2011
Keywords: diazene · ENDOR spectroscopy · hydrazido ligand ·
.
mixed-valence complexes · nitrogen fixation
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part of the spin-coupled catalytic [Fe7Mo] molybdenum–iron
cofactor.
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