Diiron Bridged-Thiolate Complexes That Bind N2 at the FeIIFeII, FeIIFeI, and FeIFeI Redox States

Article abstract of DOI:10.1021/jacs.5b04738

All known nitrogenase cofactors are rich in both sulfur and iron and are presumed capable of binding and reducing N₂. Nonetheless, synthetic examples of transition metal model complexes that bind N₂ and also feature sulfur donor ligands remain scarce. We report herein an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. A new binucleating ligand scaffold is introduced that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N₂-Fe(μ-SAr)Fe-N₂) across at least three oxidation states (Fe^IIFe^II, Fe^IIFe^I, and Fe^IFe^I). The (N₂-Fe(μ-SAr)Fe-N₂) system undergoes reduction of the bound N₂ to produce NH₃ (～50% yield) and can efficiently catalyze the disproportionation of N₂H₄ to NH₃ and N₂. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Synthetic model complexes of these types are desirable to ultimately constrain hypotheses regarding Fe-mediated nitrogen fixation in synthetic and biological systems.

Full text of DOI:10.1021/jacs.5b04738

Communication
pubs.acs.org/JACS
Diiron Bridged-Thiolate Complexes That Bind N2 at the FeIIFeII, FeIIFeI,
and FeIFeI Redox States
Sidney E. Creutz and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
Scheme 1. Synthesis of Protected Ligand 4
ABSTRACT: All known nitrogenase cofactors are rich in
both sulfur and iron and are presumed capable of binding
and reducing N2. Nonetheless, synthetic examples of
transition metal model complexes that bind N2 and also
feature sulfur donor ligands remain scarce. We report
herein an unusual series of low-valent diiron complexes
featuring thiolate and dinitrogen ligands. A new binucleat-
ing ligand scaffold is introduced that supports an Fe(μ-
SAr)Fe diiron subunit that coordinates dinitrogen (N2-
Fe(μ-SAr)Fe-N2) across at least three oxidation states
(FeIIFeII, FeIIFeI, and FeIFeI). The (N2-Fe(μ-SAr)Fe-N2)
system undergoes reduction of the bound N2 to produce
NH3 (∼50% yield) and can efficiently catalyze the
Scheme 2. Synthesis of 6 and 7
disproportionation of N2H4 to NH3 and N2. The present
scaffold also supports dinitrogen binding concomitant with
hydride as a co-ligand. Synthetic model complexes of these
types are desirable to ultimately constrain hypotheses
regarding Fe-mediated nitrogen fixation in synthetic and
biological systems.
lthough biological nitrogen fixation mediated by the iron−
Amolybdenum cofactor (FeMoco) of MoFe-nitrogenase
enzymes has inspired a wealth of synthetic model studies,1−4 the
modeling field is marked by a sharp dichotomy between
functional and structural models of the FeMoco cluster. In the
crystallographically characterized state of the biological Fe7MoS9
cluster, the “belt” irons that are hypothesized to be likely initial
5
binding site(s) for N2 are in an FeS3C coordination environ-
ment consisting of three sulfides bridged to either one or two
additional metal centers (Fe or Mo) and the interstitial carbide
(C4−) ligand (Figure 1).6 In contrast, synthetic iron complexes
for which spectroscopically and/or structurally characterized N2
complexes are known are dominated by ligands composed
primarily of phosphorus and nitrogen donors.
model chemistry; reported examples of iron centers ligated to a
sulfur donor ligand of any kind (e.g., S2−, SR−, SR2) and at the
same time an N2 ligand are few in number, limited to several
Fe(N2)(thioether) derivatives.9 No examples of Fe(N2)
complexes involving anionic sulfur donors (sulfides or thiolates)
have ever been reported,10 despite numerous examples of
synthetic iron sulfide and iron thiolate complexes and clusters.11
This is perhaps not surprising: sulfides and thiolates/thioethers
typically act as weak-field ligands that do not give rise to the types
of low-spin and low-valent iron centers that are well-suited to
bind N2.8b,12 The few examples of low-valent, low-coordinate
diiron bridged-sulfide complexes (Fe(μ-S)Fe) that are known
have not yet been observed to bind N2.12
Sulfur-supported transition metal complexes that bind N2
remain very uncommon.8 This state of affairs is particularly
noteworthy for iron, especially in the context of nitrogenase
Herein we pursue a strategy to overcome these challenges via a
binucleating ligand scaffold designed with a mixed phosphine−
thiolate coordination environment that places iron in a trigonal
Figure 1. Representation of the nitrogenase iron−molybdenum
cofactor, highlighting one candidate Fe−S−Fe substrate binding site.7
Received: May 7, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b04738
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Journal of the American Chemical Society
Communication
Scheme 3. Synthesis of Fe−N2 Adducts {8}−, 9, and 10
Figure 3. CV of 9, measured in 0.4 M [TBA][PF6] in THF at 100 mV/s
and internally referenced to Fc/Fc+.
Table 1. Comparison of Selected Bond Lengths and
Spectroscopic Parameters for Complexes 8−12
Fe−P (Å, avg) Fe−S (Å, avg) Fe−S−Fe (deg) ν(NN) (cm−1
)
8
2.226
2.291
2.341
2.264
2.219
2.184
2.208
2.244
2.189
2.332
137.098(15)
135.52(5)
138.02(4)
136.562(15)
140.42(3)
2017, 1979
2070, 1983
2129
9
10
11
12
2036, 2093
2044, 1981
IR absorption features corresponding to the symmetric and
asymmetric stretches arising from the two chemically equivalent
N2 ligands.14 These shift from 1978 and 1928 cm−1 in the ion-
paired {Na(THF)x}+ salt {8}{Na(THF)x} (thin film deposited
from THF) to 2017 and 1979 cm−1 in {8}{Na(12-crown-4)2}.
Both salts of the formally diiron(I) anion 8 are deep green in
color and diamagnetic due to strong coupling.
Figure 2. Crystal structure of {8}{Na(12-crown-4)2}. The counter-
cation (Na(12-crown-4)2), solvent molecules, and hydrogen atoms are
omitted for clarity. Thermal ellipsoids shown at 50% probability.
Stepwise oxidation of 8 with FcPF6 followed by FcBArF4 gives
the mixed-valent N2-FeII(μ-SAr)FeI-N2 complex 9 and then the
cationic {N2-FeII(μ-SAr)FeII-N2}+ complex 10, respectively;
both complexes have also been crystallographically characterized
(Scheme 3; crystal structures are provided in the SI).
Electrochemical characterization of mixed-valent 9 by cyclic
voltammetry (CV, Figure 3) shows a reversible oxidation
(generating 10) at −1.3 V (vs Fc/Fc+) and reversible reductions
at −1.9 and −3.3 V. The first reduction (−1.9 V) gives the anion
8, while the second reduction apparently generates a more highly
reduced, dianionic {N2-FeI(μ-SAr)Fe0-N2}2− species that has not
been isolated.
Compounds 8, 9, and 10 have very similar overall solid-state
structures despite minor changes in the bond lengths of the
immediate iron coordination environment (Table 1), consistent
with the reversible CV data described above. The Fe−P bond
lengths within 8−10 show little variation, and the Fe−S bonds
are nearly symmetrical; only average bond lengths are therefore
shown in Table 1.
Studies on nitrogen reduction by FeMoco suggest that iron
hydride species may play an important mechanistic role and
access to EPR active models of such species can help to constrain
spectroscopic parameters (e.g., EPR/ENDOR) for potential
hydride intermediate assignments.15,16 Synthetic access to N2/
hydride species within the present iron thiolate-N2 model system
proved viable. When complex 6 is reduced with excess sodium
amalgam in benzene, a new hydride product, {(N2)FeII(μ-
SAr)FeIIN2(H)} (11), is produced cleanly (Scheme 4). Complex
11 is an orange-brown, diamagnetic solid featuring two
uncoupled 31P NMR resonances in a 1:1 ratio at 84 and 94
ppm. The 1H NMR spectrum of 11 shows a triplet at −13 ppm
that integrates to a single hydride and is coupled only to the more
downfield phosphorus resonance. These data allow the position
of the hydride to be assigned as trans to the thiolate ligand
between two phosphine ligands at one of the two iron centers.
Additionally, the IR spectrum of 11 shows two sharp and strong
geometry. This strategy affords a bridging Fe(μ-SAr)Fe moiety
with high-affinity Fe−N2 binding sites across three redox states.
The binucleating ligand of choice and its synthesis are shown
in Schemes 1 and 2. Monolithiation of thioether 1 followed by
reaction with chlorosilane electrophile 2 gives the diphosphine−
thioether product 3; a second lithiation and electrophile addition
affords the protected ligand 4 in good yield. Deprotection of the
isopropyl thioether with sodium naphthalenide provides the
thiolate ligand 5 which, when stirred with 2 equiv of FeCl2,
generates a metalated brown paramagnetic solid product
formulated as 6 (Scheme 2).
Treating 6 with 2 equiv of methyl Grignard followed by [PPN]
Cl (PPN = bis(triphenylphosphine)iminium) results in formal
loss of 2 equiv of methane and concomitant installation of the
Fe−Si bonds to provide an anionic diiron(II) dichloride
complex, 7, as its PPN salt (Scheme 2). Complex 7 is a
paramagnetic, bright red solid and has been crystallographically
characterized (see SI); its structure shows two trigonal
bipyramidal Fe−Cl sites within a bis(phosphine)silyl binding
pocket (axial chloride trans to axial silyl) that are symmetrically
bridged by the arylthiolate. The two SiP2FeCl subunits are
canted with respect to the central arene ring; the Cl−Fe−Fe−Cl
dihedral angle is 36°.
Entry to the desired series of diiron N2 adduct complexes was
next pursued. Treatment of 7 with NaBPh4 gives a putative
intermediate monochloride complex (with loss of NaCl and
[PPN][BPh4]) which, when followed by reduction with excess
sodium amalgam in THF, affords the anion {N2-FeI(μ-SAr)FeI-
N2}− (8) as a {Na(THF)x}+ salt (Scheme 3). Treatment of this
salt with 12-crown-4 sequesters the sodium countercation to give
{N2-FeI(μ-SAr)FeI-N2}{Na(12-crown-4)2} ({8}{Na(12-crown-
4)2}). The solid-state structure of {8}{Na(12-crown-4)2} shows
coordination of a terminally bound N2 ligand at each of the two
iron centers at the axial position trans to the silyl donor and cis to
the bridging arylthiolate linker (Figure 2). Anion 8 displays two
B
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Journal of the American Chemical Society
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Scheme 4. Synthetic Access to Hydrides 11 and 12
Figure 5. Crystal structure of 12 highlighting differing P−Fe−P angles
due to the presence of a hydride ligand on Fe1 between P1 and P2.
Countercation (Na(12-crown-4)2), solvent molecules, and hydrogen
atoms (including hydride, which was not crystallographically located)
omitted for clarity; thermal ellipsoids shown at 50% probability.
Scheme 5. NH3 Generation from N2 or N2H4
Figure 4. Spectroscopic characterization of 11. (a) Mossbauer spectrum
of microcrystalline 11 (80 K, suspended in boron nitride matrix).
Parameters for the displayed fit are shown. (b) CV measured in 0.4 M
[TBA][PF6] in THF at 100 mV/s and internally referenced to Fc/Fc+.
(c) IR spectrum of 11 as a thin film deposited from benzene solution.
thiolate donor as assigned to the solution structure of
diamagnetic 11. By contrast to the solid-state structures of 8−
10 that show symmetric or near-symmetric coordination of the
bridged-thiolate ligand (Table 1), the two iron sites of 14 have
distinct Fe−S bond lengths: an elongated Fe−S bond of
2.3744(7) Å to the hydride-bound Fe1 site and a shorter Fe−S
bond of 2.2893(7) Å at the Fe2 site. The difference presumably
reflects a trans influence from the hydride ligand at Fe1.
peaks at 2036 and 2096 cm−1, corresponding to two inequivalent
NN stretches (Figure 4). The Mossbauer spectrum of 11 also
indicates inequivalent iron centers with two quadrupole doublets
in a 1:1 ratio. Complex 11 has been crystallographically
characterized, but the hydride position could not be located
from the data and, as the molecule sits on a crystallographically
imposed 2-fold rotation axis (see SI), its position could not be
indirectly inferred.
Biomimetic reactivity of the Fe-(μ-SAr)-Fe subunit in the
present scaffold has been explored via the reduction or
decomposition of the nitrogenase substrates N2 and N2H4; in
both these cases cleavage of the N−N bond has been
demonstrated. For instance, treatment of 8{Na(12-crown-4)2}
To chemically confirm the presence of the hydride ligand, 11
was exposed to an atmosphere of CO2 in benzene. This reaction
quantitatively affords the product of CO2 insertion into the Fe−
H bond (13, Scheme 4). Bright red, paramagnetic diiron(II) 13
has been crystallographically characterized (see SI), showing a
bridged formate that is κ1 with respect to each Fe center. The
structure of 13 suggests that this diiron platform may be
interesting to pursue in the context of bimetallic CO2 reduction
with an excess (100 equiv) of KC8 and HBArF ·2Et2O in the
4
presence of an N2 atmosphere (Et2O, −78 °C, 2 h) produces 1.8
0.3 equiv of ammonia (NH3); this yield is comparable to that
achieved by the related monometallic silyl-anchored iron
complex, {[SiPiPr3]FeN2}{Na(12-crown-4)2} (Scheme 5).17
The comparatively low yield of NH3 production from 8{Na-
(12-crown-4)2} may reflect rapid generation of H2 instead. No
NH3 is produced when 8{Na(12-crown-4)2} is treated with acid
in the absence of added reductant.
catalysis. Treatment of 11 with 1 equiv of HBArF ·2Et2O cleanly
4
generates 10 via loss of H2.
The CV of hydride 11 shows reversible oxidation (−1.1 V) and
reduction (−2.0 V) events (Figure 4b). The anionic {(N2)FeII(μ-
SAr)FeIN2(H)}− reduction product, 12, can be isolated and
crystallographically characterized. This is achieved by stirring 11
(or 6) over sodium amalgam in THF followed by treatment with
12-crown-4 (Scheme 4). Complex 12 is a paramagnetic brown
solid with sharp, strong IR absorbance features at 2044 and 1981
cm−1 (shifted from 1999 and 1928 cm−1 prior to treatment with
12-crown-4). Its crystal structure (Figure 5) shows a wide P−
Fe−P angle at Fe1 (147.72(4)°) compared to that at Fe2
(113.06(3)°). This variation reflects the presence of a hydride
ligand at Fe1, apparently in a position trans to the bridged
By contrast to its modest N2-reducing capacity, the cationic
complex 10 serves as an effective precatalyst for hydrazine
disproportionation to NH3 and N2 with a turnover number
(TON) that is significantly higher than previously reported for
any iron complex.18 Reproducible yields of NH3 were only
achieved in the presence of an acid co-catalyst (Scheme 5). Thus,
treatment of 10 in THF with 1 equiv of [LutH][BArF ] and 50
4
equiv of N2H4 produced 29 equiv of NH3 during the course of 1 h
at room temperature (LutH = lutidinium). The TON appears to
be limited by catalyst decomposition; neither longer reaction
times nor higher concentrations of N2H4 resulted in higher yields
C
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(5) Lukoyanov, D.; Dikanov, S. A.; Yang, Z.-Y.; Barney, B. M.;
Samoilova, R. I.; Narasimhulu, K. V.; Dean, D. R.; Seefeldt, L. C.;
Hoffman, B. M. J. Am. Chem. Soc. 2011, 133, 11655.
of NH3. For comparison, a related monometallic iron complex of
a silyl-anchored bisphosphine thioether ligand, {[SiPiPr2SAd]-
FeN2}{BArF },9b produced <2 equiv of NH3 under the same
4
(6) (a) Einsle, O.; Tezcan, A.; Andrade, S. L. A.; Schmid, B.; Yoshida,
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reaction conditions, even with longer reaction times (8 h); this
comparison suggests the possibility that some degree of
bimetallic cooperativity and/or the presence of the bridging
thiolate as a proton shuttle may be important in facilitating the
N−N bond cleavage of hydrazines catalyzed by 10. Mechanistic
studies will be interesting in this context since hydrazine has been
suggested as a possible intermediate in dinitrogen reduction
where the N−N bond is cleaved at a late stage; it can be
converted to NH3 either by further reduction or by a
disproportionation pathway that also produces N2.19
To conclude, a paradox in inorganic synthesis is the dichotomy
between the sulfur-rich coordination environment of the iron
and molybdenum centers of the FeMoco, and the dearth of well-
defined N2 adducts for these metals (and all transition metals)
featuring sulfur donor ligands.20 The synthetic work described
here has provided the first examples10 of thiolate-ligated Fe−N2
species via a bimetallic Fe-(μ-SAr)-Fe subunit benefiting from a
combination of phosphine and silyl donors. This subunit
moreover shows that the N2 ligands are retained across at least
three redox states (FeIIFeII, FeIIFeI, FeIFeI) in the presence of the
thiolate donor. This is significant because formally low-valent
iron sites in the presence of S2− or SH− are plausible
intermediates of biological nitrogen fixation but are not well
represented in the synthetic literature. Synthetic access to
terminally bonded iron hydrides in the presence of the bridging
thiolate and N2 ligands has also been established.21 Finally, the
ability of the present scaffold to mediate the stoichiometric and
catalytic cleavage of N−N bonds has been briefly explored.
Ongoing work will further examine the reactivity patterns of
these (N2)Fe-(μ-SAr)-Fe(N2) subunits in the context of
nitrogen fixation and reduction catalysis (e.g., H+, CO2) more
generally.
(9) (a) Bart, S.; Lobkovsky, E.; Bill, E.; Wieghardt, K.; Chirik, P. J.
Inorg. Chem. 2007, 46, 7055. (b) Takaoka, A.; Mankad, N. P.; Peters, J.
C. J. Am. Chem. Soc. 2011, 133, 8440.
(10) We are aware of recent work from the Holland group at Yale,
where an Fe−N2 complex that features both thiolate and arene donors
has been characterized (with permission; personal communication).
(11) (a) Lee, S. C.; Lo, W.; Holm, R. H. Chem. Rev. 2014, 114, 3579.
(b) Rao, P. V.; Holm, R. H. Chem. Rev. 2004, 104, 527. (c) Malinak, S.
M.; Coucouvanis, D. Prog. Inorg. Chem. 2001, 49, 599.
(12) (a) Lane, R. W.; Ibers, J. A.; Frankel, R. B.; Papaeftymiou, G. C.;
Holm, R. H. J. Am. Chem. Soc. 1977, 99, 84. (b) Lee, S. C.; Holm, R. H.
Chem. Rev. 2004, 104, 1135. (c) Malianak, S. M.; Coucouvanis, D. Prog.
Inorg. Chem. 2001, 49, 599.
(13) (a) Anderson, J. S.; Peters, J. C. Angew. Chem., Int. Ed. 2014, 53,
5978. (b) Rodriguez, M. M.; Stubbert, B. D.; Scarborough, C. C.;
Brennessel, W. W.; Bill, E.; Holland, P. L. Angew. Chem., Int. Ed. 2012,
51, 8247.
(14) Rittle, J.; McCrory, C.; Peters, J. C. J. Am. Chem. Soc. 2014, 136,
13853.
(15) (a) Hoffman, B. M.; Lukoyanov, D.; Yang, Z.-Y.; Dean, D. R.;
Seefeldt, L. C. Chem. Rev. 2014, 114, 4041. (b) Hoffman, B. M.; Dean, D.
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ASSOCIATED CONTENT
* Supporting Information
Synthetic and spectroscopic details for new compounds; crystal
structures of 7, 9−11, and 13; details of NH3 production
experiments. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
jacs.5b04738.
■
S
AUTHOR INFORMATION
Corresponding Author
*jpeters@caltech.edu
Notes
■
(16) Kinney, R. A.; Saouma, C. T.; Peters, J. C.; Hoffman, B. M. J. Am.
Chem. Soc. 2012, 134, 12637.
The authors declare no competing financial interest.
(17) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84.
(18) (a) Chen, Y.; Zhou, Y.; Chen, P.; Tao, Y.; Li, Y.; Qu, J. J. Am.
Chem. Soc. 2008, 130, 15250. (b) Chang, Y.-H.; Chan, P.-M.; Tsai, Y.-F.;
Lee, G.-H.; Hsu, H.-F. Inorg. Chem. 2014, 53, 664. (c) Umehara, K.;
Kuwata, S.; Ikariya, T. J. Am. Chem. Soc. 2013, 135, 6754.
(19) Davis, L. C. Arch. Biochem. Biophys. 1980, 204, 270.
(20) Sellmann, D.; Sutter, J. Acc. Chem. Res. 1997, 30, 460.
(21) Diiron thiolate-bridged iron hydrides are also structurally relevant
to hydrogenases. See, for example: Wang, W.; Rauchfuss, T. B.; Zhu, L. J.
Am. Chem. Soc. 2014, 136, 5773.
ACKNOWLEDGMENTS
■
This work was supported by the NIH (GM 070757) and the
Gordon and Betty Moore Foundation. We thank Larry Henling
and Michael Takase for crystallographic assistance.
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