Isolation and characterization of stable iron(I) sulfide complexes

Article abstract of DOI:10.1002/anie.201202211

The first examples of iron(I) sulfide complexes are presented, in contrast with the +2 and +3 oxidation states that are well-known in synthetic and biological systems. Spectroscopic and computational studies show a high-spin d⁷ configuration at the metal. Alkali metal cations play a key role in supporting the unusually low oxidation state. Copyright

Full text of DOI:10.1002/anie.201202211

Angewandte
Chemie
DOI: 10.1002/anie.201202211
Iron–Sulfur Clusters
Isolation and Characterization of Stable Iron(I) Sulfide Complexes**
Meghan M. Rodriguez, Bryan D. Stubbert, Christopher C. Scarborough, William W. Brennessel,
Eckhard Bill,* and Patrick L. Holland*
Iron–sulfur clusters are widespread in metalloproteins, where
they most often function to transfer electrons but also can act
as sites for catalysis.[1] In known iron–sulfide clusters, the iron
ions are in the + 2 and + 3 oxidation states.[2,3] Even in
synthetic chemistry, with a much broader range of supporting
ligands, the iron ions in iron sulfide complexes are always Fe2+
or Fe3+. Synthetic all-Fe2+ clusters using cyanide or N-
heterocyclic carbene ligands are a recent advance.[4] However,
Scheme 1. Synthesis of the diiron(II) sulfide complex 1-Me.
there are no reports of iron sulfide compounds in which iron
ions are reduced to the Fe1+ level.[5] Herein, we describe the
first examples of isolable iron(I) sulfide compounds, which
establishes that iron(I) is a feasible oxidation state in iron
sulfide chemistry.
ration. The products were isolated in 65% and 73% yield,
respectively.
A red solution of 1-Me in diethyl ether reacted with two
molar equivalents of potassium graphite (KC8) to give a color
change to green. The product, [KMeLMeFe]2(m-S) (2-Me)
(Scheme 2), was isolated in 62% yield and crystallographi-
The progenitor of the new compounds is the previously
reported m-sulfidodiiron(II) compound [{LMeFe}2(m-S)] (1-H),
LMe = HC[C(Me)N(2,6-iPr2C6H3)]2.[6] This molecule is the
only crystallographically characterized iron sulfide with
a three-coordinate iron atom. The work reported herein
used a close variant of this compound, [MeLMeFe]2(m-S) (1-
Me), MeLMe = MeC[C(Me)N(2,6-iPr2C6H3)]2, in which the
supporting ligand contains an additional methyl group.
[{MeLMeFe}2(m-S)] (1-Me) is spectroscopically similar to its
LMe analogue (1-H).
We also developed a novel organometallic route to the m-
sulfidodiiron(II) complexes. This strategy takes advantage of
rapid, clean b-hydride elimination from low-coordinate alkyl
complexes,[7] and the ability of low-coordinate iron(II)
hydride complexes to reductively eliminate H2 upon addition
of coordinating ligands.[8] Thus, LMeFe(iso-butyl) or
MeLMeFe(iso-butyl) were mixed with PMe3S and heated to
1008C in toluene overnight to give the diiron(II) sulfides 1-H
or 1-Me (Scheme 1 shows 1-Me). All of the byproducts PMe3,
H2, and isobutylene were conveniently removed by evapo-
Scheme 2. Synthesis of the diiron(I) sulfide complexes.
cally characterized. 1-Me can instead be reacted with excess
metallic sodium in THF to give [NaMeLMeFe]2(m-S) (3-Me) in
56% yield. Compounds 2-Me and 3-Me had similar 1H NMR
spectra, and had half-lives of ca. 80 hours at 608C in C6D6
(Supporting Information, Figures S6,S7).
X-ray diffraction studies showed the solid-state structures
of 2-Me and 3-Me (Figure 1 and Supporting Information).
[*] M. M. Rodriguez, Dr. B. D. Stubbert, Dr. W. W. Brennessel,
Prof. P. L. Holland
ꢀ
The Fe S bond lengths in 2-Me and 3-Me are 2.1745(13) ꢀ
ꢀ
Department of Chemistry, University of Rochester
Rochester, NY 14627 (USA)
E-mail: holland@chem.rochester.edu
Homepage: http://chem.rochester.edu/~plhgrp/
Prof. C. C. Scarborough
Department of Chemistry, Emory University (USA)
and 2.1957(3) ꢀ, respectively. These Fe S bond distances are
typical for m2-S atoms in diiron compounds (2.22(3) ꢀ).[9]
ꢀ
However, the Fe S distances in 2-Me and 3-Me are signifi-
cantly longer than the 2.102(2) ꢀ for a three-coordinate
iron(II) atom to
a
bridging sulfide in LMeFe(m-S)Fe-
(NCCH3)LMe [6]
.
The longer Fe S bonds suggest that the iron
ꢀ
Dr. E. Bill
is in a lower oxidation state, and charge counting in the
structure suggests a diiron(I) formulation. This hypothesis is
addressed below using spectroscopic and computational
evidence.
Max-Planck-Institut fꢀr Bioanorganische Chemie
45470 Mꢀlheim an der Ruhr (Germany)
E-mail: bill@mpi-muelheim.mpg.de
[**] Funding was provided by the National Institutes of Health (GM-
065313). P.L.H. was also supported by a Fulbright Scholar grant.
In [{KMeLMeFe}2(m-S)] (2-Me) and [{NaMeLMeFe}2(m-S)] (3-
Me), the m-sulfido bridges are linear (Fe-S-Fe angles of
179.70(4)8 and 1808, respectively). Linear sulfide bridges are
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202211.
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and sodium cations can exchange between the aryl rings on
the b-diketiminate ligands. However, the compounds are not
stable without the alkali metal cations (see below).
We next turned to spectroscopic studies to support the
oxidation state assignment as iron(I). The Mçssbauer spec-
trum of solid 2-Me showed a single quadrupole doublet for
the two equivalent iron atoms, with isomer shift d =
0.67 mmsꢀ1 and quadrupole splitting j DEQ j= 2.17 mmsꢀ1
that was temperature independent from 4.2 to 80 K (Fig-
ure 2a. The zero-field Mçssbauer spectrum of 3-Me was
Figure 1. Thermal-ellipsoid plot of [{KMeLMeFe}2(m-S)] (2-Me). Ellipsoids
are set at 50% probability; hydrogen atoms omitted for clarity. The
sodium analogue 3-Me has also been crystallographically character-
ized, and is shown in the Supporting Information.
uncommon, and the average Fe-S-Fe bond angle for diiron
complexes with a single bridging sulfur atom is 126(24)8.[9]
The next most linear Fe-S-Fe bond angle is 167.0(2)8, in a five-
coordinate iron complex with a bulky salen ligand.[10] Linear
sulfido bridges have been seen in complexes of other
transition metals, such as V, Mo, Co, Ni, and Cu.[11–14]
In the crystal structures of 2-Me and 3-Me, the alkali
metal cations are sandwiched between the aryl groups of the
b-diketiminate ligands, as found in formally iron(I) hydride
and dinitrogen complexes.[15] Geometric restraints from the
cation–p interactions may play a role in enforcing the linear
sulfide bridge, though there are literature examples of linear
sulfide bridges without such restraints.[11–13] The K S distances
ꢀ
in 2-Me of 2.932(2) ꢀ and 2.936(2) ꢀ are the shortest
known.[9] The next shortest K S bond is 3.039(2) ꢀ in
ꢀ
a compound where the K+ ion also has a cation–p inter-
action.[16] The Na S distance in 3-Me is 2.6994(7) ꢀ, which is
ꢀ
ꢀ
only slightly shorter than the average Na S bond of
2.9(2) ꢀ.[9] Other parameters from the crystal structures of
2-Me and 3-Me are similar, and thus no major structural
differences arise from the choice of alkali metal cation.
The ability to exchange the alkali metals was evaluated
Figure 2. a) Mçssbauer spectrum of 2-Me at 80 K (top) and at 4.2 K
with a 4 T field perpendicular to the gamma rays (bottom). The solid
lines are fits for d=0.67 mmsꢀ1 and jDEQ j =2.17 mmsꢀ1. The
magnetic simulation reveals S=0 at 4.2 K, Vzz =ꢀ2.17 mmsꢀ1, and
h=0.3. b) Solid-state variable-temperature magnetic susceptibility of
2-Me. The solid line is a fit where both iron centers have a spin state
of Si =3/2 and antiferromagnetic coupling with J=ꢀ123 cmꢀ1
(H=ꢀ2JS1·S2 +gmB(S1+S2)·B). The dashed line represents a 1.4%
paramagnetic impurity (PI) with S=5/2, which was necessary to
account for the offset below 50 K.
1
using NMR spectroscopy. The H NMR spectra of 2-Me and
3-Me were consistent with D2d or D2h symmetry in solution,
with seven paramagnetically shifted resonances. Mixing 2-Me
with 3-Me resulted in the growth of a third set of resonances
1
in the H NMR spectrum with a shift pattern similar to the
reactants (Supporting Information). We assign the new peaks
to the mixed-cation complex [{NaKMeLMeFe}2(m-S)] (4-Me).
This reaction reached an equilibrium in which all 3 species (2-
Me, 3-Me, 4-Me) were present, requiring 24 h in C6D6 and 6 h
in Et2O. To further support the exchange of cations, Na+ and
K+ sources (1 equiv of NaBArF4 or KOTf, where ArF indicates
3,5-bis(trifluoromethyl)phenyl and OTf indicates trifluoro-
methanesulfonate) were added to 2-Me and to 3-Me in Et2O
solution. When the alkali salts matched (for example, addition
of KOTf to 2-Me), no reaction was seen, but the mixed alkali-
metal experiments produced 4-Me, as shown by 1H NMR
spectroscopy. These results demonstrate that the potassium
similar, with an isomer shift d = 0.64 mmsꢀ1 and quadrupole
splitting of j DEQ j= 2.28 mmsꢀ1 at 80 K. For comparison, the
Mçssbauer spectrum of the diiron(II) sulfide complex 1-Me
had distinctly different parameters of d = 0.59 mmsꢀ1 and
j DEQ j= 0.89 mmsꢀ1. The increase in isomer shift upon
reduction supports the hypothesis that reduction has occurred
at the iron centers. The isomer shifts observed for 2-Me and 3-
2
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Chemie
Me also resemble those for thioether-supported iron(I)
complexes (d = 0.62–0.76 mmsꢀ1),[17] and a phosphine-sup-
ported iron(I) complex (d = 0.57 mmsꢀ1).[18]
Applied-field Mçssbauer measurements on 2-Me
revealed an energetically well-isolated diamagnetic (Stotal
=
0) ground state for the dimer, and a positive sign of the
electric field gradient with small asymmetry h = 0.3. Solid-
state magnetic susceptibility studies (Figure 2b) also indi-
cated antiferromagnetic coupling of two paramagnetic iron
subsites to give a regular spin ladder with an Stotal = 0 ground
state, as expected for strong exchange interaction that
dominates the single-ion zero-field splitting (zfs). The data
fit to a fundamental model where each iron(I) ion is high-spin
(SFe = 3/2) and J = ꢀ123 ꢁ 8 cmꢀ1 quantifies the antiferromag-
netic coupling. Interestingly, this system does not have the
strong first-order orbital moment that was observed for
a related mononuclear iron(I) complex.[19] We estimate that
D = (0 ꢁ 30) cmꢀ1; the simulations are not particularly sensi-
tive to zfs in such a dinuclear system where a spin singlet is the
ground state.
The spectroscopic studies were supplemented with calcu-
lations on the full molecule with density-functional theory
(DFT) using the crystallographic coordinates. The functional
and basis set were varied to find the best match to the
geometry, the Mçssbauer parameters and the J value.[15b] The
electronic structure description of 2-Me derived from the
best-fit (broken-symmetry calculations with TPSSh functional
and TZVP basis set) calculations showed two antiferromag-
netically coupled high-spin iron(I) centers with J = ꢀ170 cmꢀ1
(Figure 3). The b-diketiminates showed no compelling evi-
dence for “redox non-innocent” sharing of spin density from
the metals. There is slight p backbonding from the iron(I)
centers to the unoccupied b-diketiminate orbitals, suggesting
that the electronic properties of the supporting ligand may
play a role in stabilizing the low oxidation state of iron(I).
However, the interaction between the iron atoms and the
p system of the b-diketiminates is small.
The role of the alkali metal in stabilizing the low iron
oxidation state was evaluated experimentally by studying the
diiron(I) sulfide in the presence of solvents and additives that
have the ability to remove the alkali metal cation. In C6D6, 2-
Me and 3-Me were stable for about 5 days at 608C, whereas
under the same conditions in [D8]THF they were stable for
less than 2 h. This result suggests that THF may pull the alkali
metal cations away from the Fe/S core, destabilizing 2-Me and
3-Me. In a more muscular test of this hypothesis, the
potassium chelators [18]crown-6 or cryptand-222 were
added to solutions of 2-Me under argon or N2, which led to
immediate decomposition. Electrochemical reduction of 1-
Me in Et2O indicated a one-electron wave at ꢀ2.7 V (vs.
Fc+/0), thus also supporting the idea that two-electron
reduction is not possible without the alkali metal cations.
All of these results indicate that the cation plays a significant
role in stabilizing the iron(I) complexes, most likely by the
close interactions between cations and the negatively charged
core of the molecule.
Figure 3. a) d orbitals from the computational model of 2-Me. The Fe
character is shown for each orbital, and the S values give the overlap
between pairs of corresponding orbitals. This picture indicates two
high-spin d7 centers, with antiferromagnetic coupling mediated by
sulfur p orbitals. b) Spin-density plot for 2-Me; as there is little spin on
the supporting ligands, they are redox-innocent.
ism, and computations. The unprecedented stability of an iron
sulfide complex in this oxidation state is enabled by steric
contributions (bulky ligands that protect the Fe-S-Fe core)
and electronic contributions (especially close interactions
between cations and the anionic core).
Received: March 20, 2012
Published online: && &&, &&&&
Keywords: clusters · high-spin complexes · iron ·
.
Mçssbauer spectroscopy · sulfides
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Iron–Sulfur Clusters
The first examples of iron(I) sulfide
complexes are presented, in contrast with
the +2 and +3 oxidation states that are
well-known in synthetic and biological
systems. Spectroscopic and computa-
tional studies show a high-spin d7 con-
figuration at the metal. Alkali metal
cations play a key role in supporting the
unusually low oxidation state.
M. M. Rodriguez, B. D. Stubbert,
C. C. Scarborough, W. W. Brennessel,
E. Bill,* P. L. Holland*
&&&& — &&&&
Isolation and Characterization of Stable
Iron(I) Sulfide Complexes
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!