Proton-Coupled Reduction of an Iron Cyanide Complex to Methane and Ammonia

Article abstract of DOI:10.1002/anie.201606366

Nitrogenase enzymes mediate the six-electron reductive cleavage of cyanide to CH₄and NH₃. Herein we demonstrate for the first time the liberation of CH₄and NH₃from a well-defined iron cyanide coordination complex, [SiP^iPr₃]Fe(CN) (where [SiP^iPr₃] represents a tris(phosphine)silyl ligand), on exposure to proton and electron equivalents. [SiP^iPr₃]Fe(CN) additionally serves as a useful entry point to rare examples of terminally-bound Fe(CNH) and Fe(CNH₂) species that, in accord with preliminary mechanistic studies, are plausible intermediates of the cyanide reductive protonation to generate CH₄and NH₃. Comparative studies with a related [SiP^iPr₃]Fe(CNMe₂) complex suggests the possibility of multiple, competing mechanisms for cyanide activation and reduction.

Full text of DOI:10.1002/anie.201606366

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International Edition: DOI: 10.1002/anie.201606366
German Edition: DOI: 10.1002/ange.201606366
Nitrogen Fixation
Hot Paper
Proton-Coupled Reduction of an Iron Cyanide Complex to Methane
and Ammonia
Jonathan Rittle and Jonas C. Peters*
In addition to catalyzing N2-to-NH3 conversion under ambi-
ent conditions,[1] nitrogenase (N2-ase) enzymes facilitate the
multi-electron reduction of a wide range of other substrates,
such as azide, acetylene, carbon dioxide, and nitrous oxide.[2]
Cyanide (CNꢀ) is perhaps most noteworthy in this context as
it is isoelectronic with N2 and can be reduced in two-, four-, or
=
six-electron processes to furnish H2C NH, H3C-NH2, or CH4
and NH3, respectively.[3] To date there is scant synthetic
precedent for the overall reduction of CNꢀ to CH4 and NH3,
either stoichiometrically or catalytically, by well-defined
coordination complexes; beyond N2-ases, CNꢀ conversion to
CH4 and NH3 is thus far limited to extracted nitrogenase
cofactors, or via reductive electrolysis by Ni electrodes.[4,5]
We have had an ongoing interest in the study of single-site
iron complexes that mediate N2-to-NH3 conversion as func-
tional inorganic models of biological nitrogen fixation at the
iron rich cofactors of N2-ases.[6] We naturally became inter-
ested in whether such single-site precursors might function-
ally model the enzymatic reduction of CNꢀ to liberate CH4
and NH3. Such a process is likely to proceed through classic
organometallic intermediates[7] with appreciable Fe-to-C
covalency, and this possibility, in addition to the recent
discovery that N2-ases can also mediate Fischer–Tropsch type
CO reduction,[8] motivates functional organometallic model
studies.
Scheme 1. Synthesis of Fe(CNHx) complexes.
and S = 1/2 (meff = 2.0 mB), respectively, observed at room
temperature. Anionic 2 additionally displays a quasi-axial
EPR spectrum (gk = 2.23, g? ꢁ 2.05) in a 2-MeTHF glass at
20 K. The solid-state structures[10] reveal trigonal-bipyramidal
Fe geometries (t5 = 1.02 and 0.85 for 1 and 2, respectively; see
Figure 1 and the Supporting Information) with the CN ligand
positioned trans to the apical Si atom. These Fe–CN
complexes are nearly isostructural to the similarly-charged
Fe–N2 and Fe–CO complexes supported by the [SiPiPr
platform.[9]
]
3
Herein we report an Fe(CN) complex supported by
a tris(phosphine)silyl ligand ([SiPiPr3]Fe(CN), 1, Scheme 1)
that releases significant amounts of NH3 and CH4 on exposure
to excess acid and reductant. We also report the isolation and
structural characterization of terminal Fe(CNH) and Fe-
(CNH2) complexes that are plausible intermediates of the
overall reduction process.
Entry into this CNꢀ reduction system was made via the
reaction of [SiPiPr3]Fe(Cl)[9a] ([SiPiPr3] = [(2-iPr2PC6H4)3Si]ꢀ)
with a slight excess of either NaCN, K13CN, or KC15N to
furnish the three isotopomers of [SiPiPr3]Fe(CN) (1) as pink
solids. Compound 1 was found to display rich electrochemical
behavior (see the Supporting Information) and reduction
with Na(Hg) in the presence of 12-crown-4 cleanly afforded
the one-electron reduced salt {Na(12-crown-4)2}{[SiPiPr3]Fe-
(CN)} (2) as a dark brown solid. Complexes 1 and 2 are
paramagnetic with solution spin states of S = 1 (meff = 2.6 mB)
[*] Dr. J. Rittle, Prof. J. C. Peters
Division of Chemistry and Chemical Engineering
California Institute of Technology (USA)
E-mail: jpeters@caltech.edu
Figure 1. X-ray diffraction crystal structures of 1, 3, 4, and 5 with
thermal ellipsoids drawn at 50% probability. Hydrogen atoms (except-
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201606366.
ing the N-H’s), the BArF24 counteranion of 3 and co-crystallized solvent
molecules have been removed for clarity. Two independent molecules
of 4 are found in the unit cell and only one is shown.
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Fe(CN) 1 is readily protonated by the acid {H(OEt2)2}-
{BArF24} (BArF24 = (3,5-(CF3)2C6H3)4Bꢀ) to afford the cationic
hydrogen-isocyanide complex, {[SiPiPr3]Fe(CNH)}{BArF24} (3)
(Scheme 1). Compound 3 adopts an S = 1 spin state in
solution (meff = 2.6 mB), complicating the assignment of the
acid-derived proton by NMR techniques. Fortunately, this
proton is unambiguously located in the Fourier difference
map of the solid-state structure of 3 (Figure 1) as a component
Solid IR spectra of 4 (Figure 2A) also reveal strong absorp-
tions near 3100 cmꢀ1 that shift to 2300 cmꢀ1 in 4-d2 (prepared
from 2 and DOTf) which are assigned to N-H(D) stretching
frequencies engaged in strong hydrogen bonding interac-
tions.[11] Similar features are noted in the recently-described
[6c]
terminal Fe = NNH2 complex, {[SiPiPr3]Fe NNH2}{OTf},
=
consistent with their similar structures. While synthetic Fe
complexes have previously been shown to support terminal
CNH ligands or bridging CNH2 functionalities,[13] compound
4 is the first example of a terminal Fe(CNH2) complex.[14]
We next examined reductive cleavage of the coordinated
cyanide unit from Fe(CN) 1 (Scheme 2). After canvassing
ꢂ
of a C N-H ligand. A hydrogen bond is observed between the
isocyanide proton and a co-crystallized molecule of diethyl
ether (d(N···O): 2.639(3) ꢀ, ](O···H-N): 173(4)8). The KBr
IR spectrum of polycrystalline 3 displays very broad absor-
bances that span the range of 3100 to 1900 cmꢀ1 (Supporting
Information), a feature that may arise from the coupling of
ꢀ
ꢂ
N H and C N vibrational modes and a continuum of
hydrogen bonding interactions in the polycrystalline state.[11]
Overall, the bond metrics of the {Fe(CNH)} unit compare
well to those of the few previously characterized exam-
ples.[7a,12] By comparison, we have to date been unable to
directly characterize related {Fe(NNH)} species supported by
either tris(phosphino)silyl or tris(phosphino)borane ligands
despite their presumed intermediacy in N2-to-NH3 conversion
reactions.[6]
Scheme 2. Reductive protonolysis of Fe(CN) 1. Ar=2,5-Cl2-C6H3. Refer
to the Supporting Information for experimental details.
Further activation of the CN ligand was achieved via
double protonation of the anionic Fe(CN) 2. Combining
solutions of 2 with 2.5 equivalents of trifluoromethanesul-
fonic acid (HOTf) in thawing 2-MeTHF affords the cationic
iron aminocarbyne complex, {[SiPiPr3]Fe(CNH2)}{OTf} (4)
(Scheme 1). Aminocarbyne 4 displays averaged 3-fold sym-
metry in THF solution, as judged by its 1H NMR spectrum at
193 K; its EPR spectrum is quasi-axial (gk = 2.51, g? ꢁ 1.98,
Figure 2B) with a larger observed g-anisotropy than that of
Fe(CN) 2. The solid-state structure of 4 reveals a shortened
several conditions we found that stirred Et2O solutions
containing 1, 24 equiv of Cp*2Co and 24 equiv [2,5-Cl2-
PhNH3][OTf] maintained at ꢀ788C furnished an average of
0.33(4) equiv/Fe of NH3 upon warming to room temperature
overnight. Isotopically-enriched [15NH4][Cl] is the sole nitro-
1
gen-containing product detected by H and 15N NMR spec-
troscopy when 15N-1 is subjected to these conditions, hence
establishing that the NH3 is derived from the cyanide N-atom.
The use of 50 instead of 24 equivalents of both Cp*2Co and
[2,5-Cl2-PhNH3][OTf] does not increase the yield of NH3
(0.31(2) equiv/Fe). Control reactions that replace 1 with
[TBA][CN] as a soluble source of CN(ꢀ) do not generate
detectable quantities of NH3, implicating a likely role of the
[SiPiPr3]Fe platform in the activation of cyanide towards
cleavage.
ꢀ
Fe C bond (1.800(4) ꢀ) relative to those found in compounds
1–3 (1.97–1.91 ꢀ) and is indicative of significant Fe-to-C
multiple bond character. While the CNH2 hydrogen atoms
were not directly located in the Fourier difference map, their
presence is indicated by close contacts between the CN-
derived N atom, the triflate counteranion and a co-crystal-
lized molecule of THF (d(N···O) range from 2.68 ꢀꢀ3.20 ꢀ).
To ascertain the C-containing product(s) of these reac-
tions, the headspaces were analyzed by gas chromatography
Figure 2. Pertinent spectroscopic data for Fe(CNH2) 4 and related complexes. A) Solid IR spectra of 4 (black) and 4-d2 (gray). B) X-band EPR
spectra of frozen 2-MeTHF solutions of 6 (top), 4 (middle) and 2 (bottom) collected at 20 K. C) Zero field, 80 K 57Fe Mçssbauer spectra of solid 3
(top), 57Fe-enriched 4 contaminated with 24% 3’ collected as a frozen 2-MeTHF solution (middle) and 57Fe-enriched 6 (bottom) collected as
a frozen 2-MeTHF solution.
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ꢀ
complex in the reductive C N bond cleavage described
above. The neutral and diamagnetic dialkylaminocarbyne
complex [SiPiPr3]Fe(CNMe2) (5), can be prepared in a one pot
reaction via the sequential addition of Na(Hg) and MeOTf to
solutions of 1 (82% yield) (Figure 1). Fe(CNMe2) 5 is
isoelectronic to the previously reported iron siloxycarbyne
complex[9b] and accordingly displays a short Fe-C distance
(1.710(1) ꢀ), a long C-N distance (1.328(1) ꢀ), and a down-
field resonance (d = 279.6 ppm) in the 13C{1H} NMR spectrum
assigned to the carbyne carbon. Complex 5 is readily oxidized
by {Cp2Fe}{BArF24} to afford {[SiPiPr3]Fe(CNMe2)}{BArF
(6). The salient spectroscopic features of Fe(CNMe2)+
closely match those of the isoelectronic Fe(CNH2)+
}
6
4
24
(Figure 2). Compounds 5 and 6 are very stable in solution
and in the solid state when stored at room temperature under
an inert atmosphere.
Fe(CNMe2) 5 was found to react with proton and electron
equivalents to afford a mixture of CH4, Me2NH and small
amounts of Me3N. Whereas Fe(CN) 1 is reduced to CH4 and
NH3 with Cp*2Co and [2,5-Cl2-PhNH3][OTf] (vide supra),
exposure of Et2O solutions of 5 to these reagents only
furnishes small quantities of CH4 (ca. 0.01 equiv/Fe). Appa-
rently, more reactive proton and electron sources are required
Figure 3. A) Representative GC-FID chromatograms of sampled head-
spaces in the reaction of (dashed gray) 1 or (dashed black) [TBA][CN]
with 24 equiv Cp*2Co and 24 equiv [2,5-Cl2-PhNH3][OTf] in Et2O, and
(black) 5 with 10 equiv KC8 and 10 equiv HOTf. B) Mass spectrum of
CH4 produced from 1. C) Mass spectrum of CH4 produced from
compound 13C-1. D) Mass spectrum of CHxD4ꢀx produced from 1,
24 equiv Cp*2Co and 24 equiv partially-enriched DOTf.
ꢀ
for C N bond scission in the case of 5, and 0.47 equiv of CH4
are detected when 5 is exposed to 10 equiv of KC8 and HOTf
(Figure 3) and found to contain 0.41(8) equiv/Fe of CH4.
Exposure of 13C-1 to these reaction conditions furnishes 13CH4
as the dominant isotopomer detected by GC-MS (Figure 3C).
Finally, replacing [2,5-Cl2-PhNH3][OTf] with partially-
enriched DOTf as the proton source furnishes a mixture of
deuterated methane products with masses up to and including
20 (Figure 2D) as is expected for CD4. Very little CH4 is
detected (0.007 equiv) when [TBA][CN] is used in place of
1 in these reactions (Figure 2A). Taken together, these
analyses indicate that the [SiP3iPr]Fe fragment facilitates the
proton-coupled six-electron reduction of coordinated cyanide
to CH4 and NH3 in moderate overall yields.
(Scheme 3). Reactions that employ 13C-5 generate 13CH4 as
Scheme 3. Reductive protonolysis of Fe(CNMe2) 5. Refer to the Sup-
porting Information for experimental details.
We also studied the stoichiometric reactivity of the
Fe(CNH) and Fe(CNH2) complexes to assess the intermedi-
acy of these species in a cyanide cleavage process. In the
absence of additional proton or electron equivalents, Fe-
(CNH2) 4 is unstable in solution (t1/2 (293 K) = 24 min), decaying
the predominant isotopomer, confirming the carbyne carbon
as the source of this hydrocarbon. Analysis of the reaction
volatiles obtained when 15N-5 is exposed to these conditions
reveals two detectable 15N-containing products (see the
Supporting Information). Resonances assigned to the N-H
protons of [H15NMe3][Cl] and [H215NMe2][Cl] are present at
to
a mixture of Fe-containing products that include
{[SiPiPr3]Fe(CNH)}{OTf} (3’) and [SiPiPr3]Fe(OTf)[6c] as read-
ily-identified species. Independent reactions reveal that both
NH3 (0.09(2) equiv/Fe) and H2 (0.24 equiv/Fe) are released on
leaving solutions of 4 to stand overnight. Furthermore,
Fe(CNH) 3’ slowly converts to [SiPiPr3]Fe(OTf) in THF
solutions (t1/2(298 K) = 4 h), presumably with the loss of the
CNH ligand. A similar ligand displacement reaction was
observed for the hydrazine adduct, {[SiPiPr3]Fe(N2H4}{OTf}.[9a]
This ligand exchange is likely irreversible under the relevant
reaction conditions as free hydrogen isocyanide readily
converts to hydrogen cyanide.[7a] Non-productive decomposi-
tion of an Fe(CNHx) complex may therefore partially account
for the moderate yields (< 41%) of CH4 and NH3 formed
upon exposure of 1 to excess proton and electron equivalents.
Given the challenges arising from the thermal instability
of 4 and 3’ in solution, we pursued more robust Fe(CNR2)
species to model the intermediacy of a terminal Fe carbyne
1
d = 10.84 and 9.05 ppm, respectively, in the H NMR spec-
trum and display well-resolved coupling to adjacent 15 N- and
1
1H nuclei. Comparison of the features present in the H, 13C,
and 15 N NMR spectra to authentic samples of these ammo-
nium salts solidifies their assignments. [SiPiPr3]Fe(OTf) was
identified as the major Fe-containing product of these
reactions (see the Supporting Information).
The organic products of these reactions lend some insight
into possible pathways for Fe-mediated cyanide reduction.
Upon exposure of Fe(CNMe2) 5 to proton and electron
equivalents, dimethylamine is found to be the dominant
(90%) N-containing product. Its formation is consistent with
a mechanism whereby an early cleavage of the cyanide-
ꢂ
derived C N bond occurs and is followed by the formation of
CH4. Scheme 4 outlines scenarios by which such a process
might in principle proceed, invoking unusual terminal carbide
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Acknowledgements
This work was supported by the NIH (grant number GM
070757) and the NSF (GRFP to J.R.). We thank Lawrence
Henling and Michael Takase for assistance with XRD studies,
and Kathryn Perez and Nathan Dalleska for assistance with
GC experiments.
Keywords: carbyne ligands · cyanide cleavage · iron complexes ·
nitrogenase · nitrogen fixation
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carbyne carbon of 5 without C N bond rupture [Scheme 4,
paths (iv) and (v)]. Conceptually related crossover distal-to-
alternating mechanisms have been proposed for Fe-mediated
N2 activation,[6c] and these scenarios will warrant further
consideration in the present context of cyanide reduction. The
fact that different reaction conditions/reagents are required
ꢀ
for the activation of the cyanide-derived C N bond in
Fe(CNMe2) 5 and Fe(CN) 1 necessitates caution in mecha-
nistically relating the reaction profile of 5 to generate CH4
and HNMe2 (or NMe3) with that of 1 to generate CH4 and
ꢀ
NH3. Nonetheless, reactions pathways that bypass C N bond
cleavage, such as that represented by (v) of Scheme 4, do not
appear to be competent in the case of the proton-coupled
reduction of 1 as MeNH2 is not observed.
In conclusion, we have disclosed the synthesis of a trigonal
bipyramidal Fe(CN) species whose C N bond is cleaved upon
exposure to proton and electron equivalents. An unusual
Fe(CNH2) complex has been characterized and may serve as
a plausible intermediate en route to the CH4 and NH3
ꢂ
ꢂ
products derived from this 6-electron/proton C N cleavage
reaction. This work lends indirect support to the notion that
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serve to activate myriad substrates, including N2, CO and
cyanide. The catalytic reduction of cyanide by a synthetic
molecular system remains a worthwhile challenge, and the
complexes described here serve as promising starting points.
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Received: June 30, 2016
Published online: && &&, &&&&
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Nitrogen Fixation
Reductive cleavage of cyanide : A trigonal
bipyramidal iron cyanide complex is
shown to evolve methane and ammonia
upon exposure to proton and electron
equivalents at low temperature (see pic-
ture). Terminally bound Fe(CNH) and
Fe(CNH2) complexes are characterized as
possible intermediates in this CNꢀ
cleavage reaction.
J. Rittle, J. C. Peters*
&&&& — &&&&
Proton-Coupled Reduction of an Iron
Cyanide Complex to Methane and
Ammonia
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