Mononuclear Fe(I) and Fe(II) Acetylene Adducts and Their Reductive Protonation to Terminal Fe(IV) and Fe(V) Carbynes

Article abstract of DOI:10.1021/jacs.9b06987

The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as a substrate, little is known concerning its binding or activation in the iron-rich active site. "Fischer-Tropsch" type coupling of non-native C₁ substrates to higher-order C_≥2 products is also known for nitrogenase, though potential metal-carbon multiply bonded intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)silyl-iron center, (SiP₃)Fe, to give Fe(I) and Fe(II) side-on adducts, including S = 1/2 Fe^I(??²-HCCH); the latter is characterized by pulse EPR spectroscopy and DFT calculations. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP₃)FeCCH₃ and the paramagnetic cation S = 1/2 [(SiP₃)FeCCH₃]⁺. Both alkylcarbyne compounds possess short Fe-C triple bonds (approximately 1.7 ?) trans to the anchoring silane. Pulse EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon p-orbitals. Furthermore, isotropic coupling of the distal β-CH₃ protons with iron indicates hyperconjugation with the spin/hole character on the FeCCH₃ unit. The electronic structures of (SiP₃)FeCCH₃ and [(SiP₃)FeCCH₃]⁺ are discussed in comparison to previously characterized, but heterosubstituted, iron carbynes, as well as a hypothetical nitride species, (SiP₃)FeN. Such comparisons are germane to the consideration of formally high-valent, multiply bonded FeC and/or FeN intermediates in synthetic or biological catalysis by iron.

Full text of DOI:10.1021/jacs.9b06987

Subscriber access provided by Bibliothèque de l'Université Paris-Sud
Article
Mononuclear Fe(I) and Fe(II) Acetylene Adducts and their
Reductive Protonation to Terminal Fe(IV) and Fe(V) Carbynes
Cooper Citek, Paul H. Oyala, and Jonas C. Peters
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06987 • Publication Date (Web): 20 Aug 2019
Downloaded from pubs.acs.org on August 20, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 13
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
Mononuclear Fe(I) and Fe(II) Acetylene Adducts and their Reductive
Protonation to Terminal Fe(IV) and Fe(V) Carbynes
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Cooper Citek, Paul H. Oyala, and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United
States
ABSTRACT: The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable
ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as
a substrate, little is known concerning its binding or activation in the iron-rich active site. “Fischer-Tropsch” type coupling of non-
native C1 substrates to higher-order C≥2 products is also known for nitrogenase, though potential metal-carbon multiply-bonded
intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)silyl-iron center,
(SiP3)Fe, to give Fe(I) and Fe(II) side-on adducts, including S =1/2 FeI(η2-HCCH); the latter is characterized by pulse EPR
spectroscopy and DFT calculatoins. Reductive protonation reactions with these compounds converge at stable examples of unusual,
formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP3)Fe≡CCH3 and the paramagnetic cation S = 1/2
[(SiP3)Fe≡CCH3]+. Both alkylcarbyne compounds possess short Fe-C triple bonds (approx. 1.7 Å) trans to the anchoring silane. Pulse
EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon
p-orbitals. Furthermore, isotropic coupling of the distal β-CH3 protons with iron indicates hyperconjugation with the spin/hole
character on the Fe≡C-CH3 unit. The electronic structures of (SiP3)Fe≡CCH3 and [(SiP3)Fe≡CCH3]+ are discussed in comparison to
previously characterized, but hetero-substituted, iron carbynes, and also a hypothetical nitride species, (SiP3)Fe≡N. Such comparisons
are germane to the consideration of formally high-valent, multiply-bonded Fe≡C and/or Fe≡N intermediates in synthetic or biological
catalysis by iron.
INTRODUCTION
of acetylene reduction by nitrogenase, they are nevertheless
fascinating.
Interest in nominally high valent iron nitrides (N3-) and
imides (NR2-) has grown in the past 15-20 years,1 and has
stemmed from a desire to explore their electronic structures
and reactivity patterns, especially as they may pertain to
important synthetic and biological transformations. For
instance, the intermediacy of an Fe≡N species has been
considered in the context of a distal pathway for biological
nitrogen fixation by nitrogenase enzymes (Fe-N2 + 3 H+/e-
→ Fe≡N + NH3).2 Relatedly, such an intermediate nitride
has been recently characterized within a synthetic iron system
that catalyzes N2-to-NH3 conversion.3
Until recently, iron carbynes of any type had little synthetic
precedent.8,9,10,11,12 Reductive activation of CO and CN- at
iron recently led to examples of heteroatom-substituted (i.e.,
Fischer-type) iron carbynes.10,11 By contrast, the
alkylcarbynes featured herein (FeIV≡CCH3 and [FeV≡CCH3]+)
are unique in that they are not substituted by heteroatoms. In
a broader context, iron complexes featuring strong iron-to-
carbon multiple bonds are of continuing interest, especially as
they may pertain to intermediates in “Fischer-Tropsch” type
C-C coupling pathways exhibited by nitrogenase enzymes in
the presence of non-native CO, CO2, and CN- substrates.4
Herein, we describe the following:
Nitrogenase enzymes reduce a range of non-native
substrates as well, including for example azide (N3-), cyanide
(CN-), CO, CO2, and acetylene (C2H2).4 Indeed, acetylene
reduction to ethylene (C2H4) is the most convenient and hence
most common assay for reductive “nitrogenase” activity.5,6,7
It is perhaps surprising, therefore, that acetylene complexes of
iron, and their associated reactivity patterns, have not been
well-studied to date. Noting this gap, we undertook a study of
the reactivity of acetylene with an iron system supported by a
tripodal phosphine ligand, in part for comparison with related
studies we and others had undertaken in the context of N2
chemistry. As a result, we uncovered an unexpectedly rich
reactivity profile that links acetylene binding at iron(I) to the
ultimate generation of nominally high-valent (+4 and +5) iron
carbynes via reductive protonation steps. While these
transformations do not appear to model the reactivity profile
(i) The first complexes featuring C2H2 bound to a single iron
center are characterized, in the formal +1 and +2 oxidation
states (i.e., FeI(C2H2) and FeII(C2H2)+); the iron(I) derivative is
S = ½, and pulse EPR data confirm η2 side-on coordination.
(ii) Well-characterized examples of FeIV≡CCH3 and
FeV≡CCH3+ carbynes are presented, including their solid-state
X-ray crystal structures and 57Fe Mössbauer spectra. The
+
FeV≡CCH3 species is S = ½ and pulse EPR spectroscopy is
used to additionally map its electronic structure.
(iii) Mechanistic studies indicate the FeI(C2H2) species
undergoes initial C-H activation, followed by bimolecular H2
loss, to generate a terminal FeIIC≡CH, which can also be
+
independently prepared. The FeV≡CCH3 species is most
conveniently accessed by reductive protonation steps from
FeIIC≡CH.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 13
The EPR data provided for FeI(C2H2) and FeV≡CCH3+ could
solid state (see Figure 2) as a trigonal bipyramidal S = 1
1
2
3
4
5
6
7
8
be of value to mechanistic studies employing related EPR
techniques of nitrogenases with C2H2,13 or to studies of non-
native C1 substrates that lead to Fischer-Tropsch type C-C
couplings.4
iron(II) species featuring the SiP3 ligand. Complex 3 exhibits
a weak C≡C vibration (1902 cm-1) and a terminal C-H stretch
(3277 cm-1); isotopic labeling with 13C2-TMED-LiC≡CH (see
SI for preparation details) gives 13C2-2 (νC≡C = 1832 cm-1 and
νC-H = 3262 cm-1; ΔC≡C = 70 cm-1; ΔC-H = 15 cm-1; Figure S23).
Three other terminal iron-acetylide compounds have
previously been crystallographically characterized.14
RESULTS AND DISCUSSION
Mononuclear iron-acetylene adducts
Figure 1 outlines the entire reaction manifold discussed
herein, where a tris(phosphino)silyl-iron subunit (abbreviated
herein as “(SiP3)Fe” or “Fe”) is common to all of the
complexes described. To test the affinity of acetylene for an
iron(I) center, the previously reported complex (SiP3)FeIN2 (1)
was exposed to acetylene gas. Displacement of N2 by 1.2
equivalents of acetylene occurs slowly at room temperature in
THF to give an unobserved acetylene adduct complex,
(SiP3)FeI(η2-HCCH) 2 (see below for further characterization
at low T), that proceeds to the red, terminal acetylide complex
(SiP3)FeIICCH 3, along with liberation of 0.5 equivalents of
H2 (confirmed by GC).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Solid-state crystal structures of 3, 6, 7, and 8, with
thermal ellipsoids displayed at 50% probability. The
hydrogen atoms of (SiP3) and the counter-anions present in the
structures for 6 and 7 have been omitted for clarity. A DFT-
minimized structure of (SiP3)FeI(η2-C2H2) 2 is also shown,
with bond metrics, along with the calculated SOMO orbital
and Löwdin spin densities in the primary ligation sphere.
Figure 1. Scheme depicting the reaction chemistry discussed
herein, along with labeled formal oxidation states and
compound numbers.
The acetylide complex 3 can alternatively be prepared by
stirring a sodium acetylide slurry with (SiP3)FeIIOTf (4) in the
presence of tetramethyl ethylenediamine (TMEDA) in 84%
yield. The complex has been characterized in solution and the
ACS Paragon Plus Environment

Page 3 of 13
Journal of the American Chemical Society
A thermally stable adduct of acetylene is accessible at the
“FeIII(H)(CCH)” isomers might be more appropriate, we
undertook continuous wave (CW) and pulse EPR experiments
of both the natural abundance and deuteron-labeled
isotopologue FeI(η2-DCCD) at X-band frequency, coupled
with DFT studies. The X-band CW EPR spectrum, shown in
Figure 3a,b, exhibits a g-tensor with rhombic symmetry (g =
[2.114, 2.040, 2.007]) with coupling evident to three distinct
phosphorous nuclei. Simulation of the data indicates that one
of the P-donors is coupled more strongly than the other two
(│A(31P)│ = [183, 182, 214] MHz, Table 1). These simulated
coupling values, and especially the asymmetry with respect to
one donor, qualitatively agree with the DFT-predicted 31P
hyperfine tensors calculated from an optimized structure of 2
1
2
3
4
5
6
7
8
iron(II) state. Accordingly, the light-blue cation
[(SiP3)FeN2]BArF24 (5)15 (BArF24 = [B(3-5-(CF3)2-C6H3)4]-)
reacts with one equivalent of acetylene gas in fluorobenzene
at 0 °C to afford green [(SiP3)Fe(η2-HCCH)]BArF24 (6) (70%
isolated yield). The solid-state X-ray structure of the S = 1
complex shows the η2-HCCH ligand bound side-on in the
axial position opposite to the silyl anchor (Figure 2). As stated
in the introduction, complex 6 is to our knowledge the only
example (other than 2, vide infra) of a mononuclear iron
complex featuring C2H2 as a ligand.16 Mononuclear adducts of
acetylene are highly uncommon for first row metals in
general.17 Fe-to-acetylene backbonding in 6 is modest (C≡C
bond length of 1.25 Å; cf. 1.20 Å in free acetylene18) and the
Fe-P and Fe-Si bond are relatively long, in accord with its S =
1 ground state and side-on accommodation of the acetylene
ligand.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(│A(31P)│
=
[201, 203, 242] MHz; TPSS/def2-
TZVP/D3ZERO; see SI for details).
Complementary X-band Electron Nuclear Double
Resonance (ENDOR) and Hyperfine Sublevel Correlation
(HYSCORE) spectroscopic data allow for further resolution
of the coupling of the paramagnetic iron center with NMR
active nuclei. Accordingly, hyperfine coupling of the
1
acetylene H nuclei to the spin on iron can be extracted by
comparison of the data for FeI(η2-HCCH) with that for FeI(η2-
1
DCCD). Field-dependent HYSCORE (Figure 4) and H-2H
ENDOR (Figure 5) spectra show weak 1H hyperfine couplings
to two very similar acetylene-derived protons: 1H1 (│A(1H)│
= [9.4, 20.2, 17.6] MHz, with Euler rotation angles [α,β,γ] =
1
[33, 18, -5] relative to the g-tensor and H2(│A(1H)│= [8.2,
18.4, 16.8] MHz, with Euler rotation angles [α,β,γ] = [43, -22,
11]. These results are also corroborated by the appearance of
2H features in the HYSCORE data for FeI(η2-DCCD), which
are well simulated by scaling these hyperfine tensors by the
1
ratio of the H/2H gyromagnetic ratios (γ1H/γ2H = 6.514)
(Figures S33-35). These two protons exhibit extremely similar
isotropic hyperfine couplings, with aiso(1H1) = 15.7 MHz and
aiso(1H2) = 14.5 MHz, and are only differentiated by the
respective orientations of their hyperfine tensors relative to the
molecular g-tensor coordinate frame, which is resolved in the
1H-2H difference ENDOR data acquired near the low-field
edge of the EPR spectrum (see Figure 5, top trace). This
observation strongly disfavors assignment as the
“FeIII(H)(CCH)” isomer, as a hydride directly bound to an S =
1/2 iron center should exhibit much stronger coupling (aiso
≈
45 MHz).19,20
To distinguish between the FeI(η2-HCCH) and “FeCCH2”
isomers, we prepared the 13C-enriched derivative 13C2H2-2.
Consistent fits between the field-dependent 13C-12C difference
HYSCORE (Figure 6) and ENDOR spectra (See SI) give two
nearly identical, strongly coupled 13C nuclei 13C1(│A(13C)│ =
[17, 24, 8.6], aiso = 16.5 MHz; 13C2(│A(13C)│ = [24, 16.6,
10.2] aiso = 16.9 MHz) (Table 1), which also disfavors the
“FeCCH2” isomer, as such a species would be anticipated to
show highly inequivalent 13Cα and 13Cβ couplings, with
coupling to Cα being much larger. Together, these EPR data
are fully consistent with a structure for 2 formulated as an η2-
HCCH adduct of iron(I), akin to its one-electron oxidized
derivative 6.
Figure 3. (a) X-band CW-EPR spectra of (SiP3)Fe(η2-HCCH)
prepared in 2-MeTHF with natural abundance (top), 13C-
2
enriched (middle), and H-enriched acetylene (bottom) with
simulations using parameters in Table 1 overlaid in red. (b)
Derivative spectra of X-band CW-EPR for each isotopologue
with simulations overlaid in red. Acquisition parameters:
temperature = 77 K; MW frequency = 9.375 GHz; MW power
= 2 mW; modulation frequency = 100 kHz; modulation
amplitude = 0.1 mT; conversion time = 41 ms.
Because the acetylene adduct FeI(η2-HCCH) 2 cannot be
observed upon addition of C2H2 to FeIN2 1 at room
temperature (owing to slow ligand substitution kinetics), we
sought its characterization via the low temperature reduction
of [FeII(η2-HCCH)]+ 6. Thus, addition of stoichiometric
cobaltocene (Cp2Co) to a stirred solution of 6 in 2-MeTHF at
-125 °C rapidly generates a new orange species that exhibits
first order decay above -80 °C to generate the terminal
acetylide FeIICCH 3 (t1/2(-40°C) ≈ 14 min; monitored by in
situ UV-visible spectroscopy; Figure S54).
Table 1. Hyperfine coupling constants in MHz determined for
(SiP3)Fe(η2-HCCH)
Euler angles
Nucleus
A1
A2
A3
aiso
[α,β,γ]◦
31P1
31P2
31P3
1H1
1H2
183
30
12
9.4
8.2
183
28
0.5
20.2
18.4
214
40
17
17.6
16.8
193.3 [0, 0, 0]
32.7
9.8
[0, 0, 0]
[0, 0, 0]
To more thoroughly characterize FeI(η2-HCCH) 2, in
particular to assess whether alternative “FeCCH2” or
15.7
14.5
[33, 18, -5]
[43, -22, 11]
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 13
13C1
13C2
17
24
24
16.6
8.6
10.2
16.5
16.9
[0, 30, 0]
[0, -30, 0]
reversible activation of phenylacetylene in a structurally
related trisphosphine-borane TPBFe system, likely by
oxidative addition to formally Fe(II)-alkyne-
hydride/borohydride complex.23
1
2
3
4
5
6
7
8
a
Several other side-on adducts of alkynes and alkenes at
reduced Fe(I) centers have been reported by Holland and
coworkers.21 These complexes exhibit high-spin S = 3/2
ground state electronic structures and were described with
substantial π-backbonding from iron to the coordinated
unsaturated ligand. In a study of an α-70Val mutant nitrogenase
that is able to accommodate several substituted-acetylene
substrates, the S = 3/2 resting state EPR signal is, by contrast,
converted to an iron-localized rhombic S = 1/2 doublet in a
freeze-quenched experiment using propargyl alcohol
(HC≡CCH2OH) as the substrate.13 This doublet signal closely
matches that of a CO-bound/inhibited state. While 13C
coupling to the doublet signal was measured, it was not
possible to distinguish between possible binding modes of the
propargyl alcohol.
Alternatively, another reaction pathway could involve
eventual rearrangement of FeI(η2-HCCH) 2 or an intermediate
FeIII(H)(CCH) to the end-on “FeCCH2” isomer, considered
above, again followed by bimolecular H2 loss. However,
transformation by a unimolecular 1,2 hydrogen shift of an η2-
alkyne adduct to a terminal alkylidene/vinylidene24 is
generally thought to have a high barrier and is therefore
difficult without an exogenous acid/base catalysts or high
temperature.25 Berke and coworkers have reported that
chromatography on silylated silica accelerates the formation
of terminally stable vinylidene complexes from mixtures of
low-valent iron acetylene/alkyne and alkyene-hydride
complexes.22c,26 In the present FeI(η2-HCCH) 2 system,
deprotonation by solvent (THF) to produce [FeCCH]-,
followed by re-protonation to produce FeCCH2, seems highly
unlikely considering the low acidity expected for 2 in THF.
Finally, while we consider it to be unlikely, a bimolecular
reaction between FeI(η2-HCCH) and itself, or with
FeIII(H)(CCH), could in principle evolve H2.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. (top) X-band HYSCORE spectrum of natural
abundance (SiP3)Fe(η2-HCCH) acquired at 338.6 mT (g =
2.058). (bottom) Monochromatic representation of the
HYSCORE data with simulations using parameters in Table 1
overlaid: (red) H-1 (blue) H-2, (green) 31P-3. Acquisition
parameters: temperature = 20 K; microwave frequency =
9.751 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 138
ns, t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt)
= 1 ms).
1
1
Figure 5. Field-dependent X-band 1H-minus-2H Davies
ENDOR difference spectra of (SiP3)Fe(η2-HCCH) (black)
with simulations using parameters in Table 1 overlaid: (red)
The transformation of FeI(η2-HCCH) 2 to acetylide
FeIICCH 3 is interesting to consider further. The available data
is insufficient to determine the fate of 2 after its first-order
decay. However, two distinct paths seem most plausible to us.
In one scenario, which we favor, C-H oxidative addition of the
coordinated C2H2 ligand produces “FeIII(H)(CCH)”, one of the
structural isomers considered above. This intermediate then
loses half an equivalent of H2 in a bimolecular step. Relatedly,
we recently reported an example of bimolecular H2 loss from
1
1
1
total H simulation, (blue) H-1, (green) H-2. Acquisition
parameters: temperature = 15 K; MW frequency = 9.751 GHz;
MW pulse length (π/2, π) = 40 ns, 80 ns; τ = 260 ns; RF pulse
length = 15 µs; TRF = 2 μs; shot repetition time = 5 ms.
a
well-defined FeIII-hydride complex that features
a
sufficiently weak Fe-H bond.20 Additionally, the oxidative
addition pathway (Figure 7) is well precedented for a number
of previously characterized alkyne-hydride complexes of Co
and Fe.22 Of note to the present study, we previously reported
ACS Paragon Plus Environment

Page 5 of 13
Journal of the American Chemical Society
others we know of relate to [(SiP3)FeCNH2]+ and
[(SiP3)FeCNMe2]+.11,12
1
The solid-state crystal structure of 7 is shown in Figure 2.
As required for an S = 1/2 system, the complex deviates
slightly from three-fold symmetry (∠P-Fe-P angles 114, 117,
120°). Most noteworthy is a characteristically short Fe≡C-
CH3 bond of 1.70 Å and a Fe≡C-CH3 single bond of 1.42 Å.
The Fe-Si bond is long at 2.39 Å, consistent with its position
trans to the carbyne ligand. The triflate counter-anion exhibits
close contacts with the (SiP3)-isopropyl and carbyne C-H
positions.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The conversion of (SiP3)FeCCH to [(SiP3)FeCCH3]+ is a net
2-H+/1-e- transformation, requiring sacrificial oxidation of
half of the starting iron material, for a theoretical 50% yield.
Analysis of recovered material shows primarily (SiP3)FeOTf
4, indicating the net reducing equivalent is likely derived from
the acetylide moiety. The propensity for reaction
intermediates to cannibalize themselves en route to 7 is
consistent with the observed thermodynamic stability of the
terminal product.
It is also noted that protonation of 3 by HOTf does not
instead lead to its net oxidation (i.e., to form (SiP3)FeCCH+)
with concomitant loss of 0.5 equivalents of H2 (eq. 1),
especially as the cationic [(SiP3)FeCCH]+ is electrochemically
accessible (-0.78 V vs Fc in THF; Figure S58).29
Figure 6. (top) X-band 13C-12C difference HYSCORE
spectrum of (SiP3)Fe(η2-HCCH) acquired at 338.6 mT (g =
2.058). (bottom) Monochromatic representation of the
HYSCORE data with 13C simulations using parameters in
Table 1 overlaid: (blue) 13C1, (red) 13C2. Acquisition
parameters: temperature = 20 K; microwave frequency =
9.751 GHz; MW pulse length (π/2, π) = 8 ns, 16 ns; τ = 138
ns, t1 = t2 = 100 ns; Δt1 = Δt2 = 16 ns; shot repetition time (srt)
= 1 ms).
FeCCH + HOTf → [FeCCH]+ + 0.5 H2
(not observed)
eq. 1
Alternatively, protonation of acetylide 3 in ether by the
weaker, insoluble acid, imidazolium triflate, to our surprise
generates the neutral alkyl carbyne derivative, (SiP3)Fe≡CCH3
8 (25% isolated yield), the product of a net 2 H+ / 2 e-
reduction, again requiring some amount of sacrificial
oxidation of starting material. The neutral carbyne 8 is thus
more favorably synthesized in a heterogeneous mixture of
triethylammonium chloride and excess sodium metal to
balance the proton-coupled reduction (Figure 1). The
+
Fe≡CCH3 /Fe≡CCH3 redox couple between 7 and 8 occurs at
-1.00 V vs Fc in THF (Figure S59); chemical oxidation of 8
by ferrocenium (Cp2Fe+) or
a
strong acid, like
Figure 7. A qualitative reaction coordinate for the proposed
slow substitution of acetylene for N2 at (SiP3)FeN2, 1, leading
to intermediate (SiP3)Fe(η2-HCCH) 2, which subsequently
undergoes sp(C-H)-activation and H2 release to generate
isolable (SiP3)Fe(CCH), 3. We presume the H2 elimination
step occurs in a bimolecular fashion via two Fe(III)-hydride
species. See text for a discussion of other possible pathways.
[H(Et2O)][BArF24], cleanly generates 7.
Carbyne 8 is diamagnetic with resolved coupling by 1H, 31P,
and 13C NMR spectroscopy (Figures S10-16). Isotopic
labelling in 13C2-8 reveals a very downfield 13C resonance for
Cα at 348 ppm30,31 with coupling to Cβ (48 ppm; JC(α)-C(β) = 18
Hz) and three equivalent phosphines (31P 107.5 ppm; JC(α)-P
=
18 Hz). By 1H NMR spectroscopy, the terminal carbyne CH3
resonance at 2.12 ppm couples to Cα (11 Hz) and Cβ (127 Hz).
The primary ligation of 8 is contracted with respect to S = ½
[Fe≡CCH3]+ 7, with a Fe≡C bond of 1.68 Å, a 2.33 Å Fe-Si
bond, and Fe-P bonds between 2.24-2.26 Å (Figure 2),
presumably consistent with increased backbonding upon
reduction.
Terminal iron-carbyne complexes
The addition of triflic acid (HOTf) to the terminal Fe-
acetylide
3
in diethyl ether at -78°C precipitates
[(SiP3)Fe≡CCH3][OTf] (7) as a dark brown solid from
solution (40% yield). Use of [H(Et2O)2][BArF24] in place of
HOTf affords [(SiP3)Fe≡CCH3][BArF24] (Figure 1). The
alkylcarbyne cation 7 is a nominally iron(V) complex that, to
our knowledge, is the only species of its type, formal or
otherwise. The complex exhibits an S = 1/2 ground state (vide
infra) and is stable as a solid or in THF or acetonitrile solution
at room temperature for extended periods. As stated above,
there are a handful of previously characterized, terminal,
heteroatom-substituted carbynes of iron.8,9,10,11,12 There are no
prior examples of alkylcarbynes of iron, nor examples where
the carbyne is substituted instead by H or aryl for that matter.
Additionally, paramagnetic examples of carbyne/alkylidyne
complexes are rare,27,28 and, for first row metals, the only
(SiP3)Fe≡CCH3 8 is quite stable, even in solution at 130°C
over a 24 hour time period. Its stability renders it a reaction
sink in this system. Indeed, with its diagnostic NMR
resonances in hand, it can be identified as a minor side-product
of other reactions we have surveyed, including the
aforementioned reaction between (SiP3)FeOTf and excess
sodium acetylide (Figure 1).
The terminal stability of both alkylidyne complexes
inspired us to further explore their interconnection with the
acetylene complexes, described above, by reductive
protonation reactions (Figure 1). Interestingly, net hydrogen
atom transfer (HAT) reactions can convert the acetylene
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 13
adducts 2 and 6 to the carbynes 7 and 8. For example, mixing
aapprox. measurements in Å. bmeasured in mm s-1. cmeasured
[Fe(η2-HCCH)]+ 6 with TEMPO-H in 2-MeTHF generates
in ppm in C6D6. dref. 9. eref. 10.
1
2
+
Fe≡CCH3 7. At -100 °C, a relatively stable orange
intermediate 9 can be observed by UV-visible spectroscopy,
which then decays to 7 on warming (Figure S55). Our
tentative assignment of 9 as a cationic alkenyl complex
[(SiP3)Fe(CH=CH2)]+, if correct, would make it a structurally
interesting one. Whereas there is precedent for related,
substituted iron alkenyl derivatives,22g the Fe-CH=CH2
subunit is still, to our knowledge, distinct.
Cationic carbyne 7 can also be generated by the addition of
stoichiometric [H(Et2O)2][BArF24] to a 2-MeTHF solution of
(SiP3)Fe(η2-HCCH) 2 at -125 °C, presumably via 9 as an
intermediate (Figure 1). Under similar conditions, the
reaction of 2 with TEMPO-H generates the neutral carbyne 8,
along with (SiP3)FeCCH 2 as side product. There thus appears
to be a rich reactivity landscape connecting the nascent
acetylene adducts and terminal carbynes described herein.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Electronic structure of iron-carbyne complexes
The Mössbauer parameters for carbynes 7 and 8 compare
well with those of the previously reported and structurally
characterized iron carbynes featuring the same (SiP3)Fe
subunit: (SiP3)Fe(COSiMe3) and [(SiP3)Fe(CNMe2)]0/+ (Table
2). The isomer shifts are relatively close to 0 mm s-1 across
the series, reflecting strong covalency in the short Fe-C triple
+
bond, with Fe≡CCH3 7 and and Fe≡CCH3 8 having the
smallest
δ
values.2b,3,32,33,34
By
comparison,
[(SiP3)FeCNMe2]+ has an isomer shift of 0.19 mm s-1,
indicating less Fe≡C covalency, possibly consistent with a
[Fe=C=NMe2]+ resonance form and its slightly longer Fe-C
bond (1.74 Å). The measured quadrupole splittings for this
series of carbynes are rather narrow, especially by comparison
to the very large ΔEQ values reported for other formally high-
valent oxide and nitride Fe complexes in C3
symmetry.1k,2,3,33,34 Frontier orbital manifolds whereby the
electrons populate only orbitals of d(xy) and d(x2-y2)
parentage, as for the d4 and d3 configurations described for the
latter systems, is predicted to give large quadrupole splittings,
> 5 mm s-1, due to the strongly asymmetric, equatorially
disposed electric field gradient at Fe.32 By contrast, the
comparatively small ΔEQ values observed for the (SiP3)Fe-
carbyne series reflects the additional presence of substantial
electron density along the z-axis (parallel to the Fe≡C-R
vector). Inspection of the molecular orbitals of the carbyne
series shows that the nominally high-lying Fe-C σ*-
antibonding orbital is highly stabilized by mixing with Si(σ),35
resulting in a heavily mixed orbital of a1 symmetry
energetically below the filled, primarily nonbonding orbitals
of d(xy) and d(x2-y2) parentage (Figure 8). This observation
bears relevance to a contributing resonance structure we have
previously considered, with electron density being polarized
between a cationic R3Si+ anchor and a nominally reduced Fe
center (Figure 9).9,36
Figure 8. Simplified molecular orbital diagram for the metal-
ligand bonding of a (SiP3)Fe≡CR carbyne in C3v symmetry.
Approximate orbital energies and compositions shown for
(SiP3)FeCCH3 (TPSS/Def2-TZVP/D3ZERO). (Inset) DFT-
predicted spin density map for [(SiP3)FeCCH3]+ (isovalue:
0.005 e_/Å3).
Table 2. Mössbauer parameters of (SiP3)Fe carbynes
d
e
+ e
+
Fe≡CX OSiMe3
NMe2
1.71
NMe2
1.74
2.35
0.19
1.51
CH3
1.68
CH3
Fe≡Ca
Fe-Sia
δb
1.67
2.30
1.70
2.39
2.31
2.33
0.061
1.12
0.058
1.12
-0.029
0.72
0.0097
1.33
b
ΔEQ
c
13Cα
250.3
279.6
--
348.4
--
ACS Paragon Plus Environment

Page 7 of 13
Journal of the American Chemical Society
This predicted spin delocalization compares very well with
Figure 9. (top) Representative chemical depictions of the
related amino- and alkylcarbynes (SiP3)Fe(CNMe2)+ and
(SiP3)Fe(CCH3)+, along with formal iron valence assignments.
The true relative state of oxidation at iron is presumed to vary
little due to strong covalency. The primary bonding of the
carbyne to iron is indicated as x3, x2, or L, with backbonds
from iron into empty p-orbitals on carbon indicated by the
green arrows. Note that a half-arrow is used to indicate
hyperconjugation stabilization for the C-centered radical
Fe(IV) form of (SiP3)Fe(CCH3)+. (bottom) DFT-optimized
structure of the hypothetical molecule (SiP3)FeIV(N), with a
simplified MO diagram and lobal representations for the Si-
Fe-N bonding in C3v symmetry.
that for a terminal iron(V)-nitride reported by Smith and
coworkers.33 A follow-up study on this system39 invoked a
dynamic “pseudo Jahn-Teller” distortion for a C3-symmetric
iron center. Related electronic structure descriptions have
been offered for ostensibly low-valent S = 1/2 (SiP3)Fe(H2)
and TPBCo(H2) complexes.40 In Smith’s iron(V)-nitride
complex, vibronic coupling to a partially filled, degenerate
d(x2− y2)/d(xy) pair gives rise to significant g-anisotropy and
mixing with the unfilled d(xz)/d(yz) pair – termed “e−e
mixing” – yielding some spin delocalization onto the terminal
nitride p-orbitals.39 The electronic structure of [Fe≡CCH3]+ 7
can likely be understood by a similar model: a nearly
degenerate S = 1/2 ground state leads to large g-anistropy and
mixing of unoccupied d(xz)/d(yz) Fe-C π*-bond orbitals with
partially filled d(xy)/d(x2-y2) orbitals delocalizes modest spin
onto Cα of the paramagnetic carbyne (Figure 8).41
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 3. Hyperfine coupling constants in MHz determined for
[(SiP3)FeCCH3]+ and computed spin densities.
Nucleus
A1
A2
A3
aiso
Spin density
Fe
0.99
0.016
0.008
31P
18
14
25
19.0
0.015
13Cα
13Cβ
18
5.8
33
5.0
47
4.0
32.7
4.9
-0.067
-0.006
-0.004
-0.001
-0.0002
Figure 10. X-band CW-EPR spectrum of [(SiP3)FeCCH3]OTf
prepared in 7:1 2-MeTHF/MeCN. Acquisition parameters:
temperature = 10 K; MW frequency = 9.637 GHz; MW power
= 200 μW; modulation frequency = 100 kHz; modulation
amplitude = 0.4 mT; conversion time = 82 ms.
8.6
2.3
12.5
6.3
8.6
6.3
9.9
5.0
1H3C
As a direct probe of this hypothesis, pulse X-band
HYSCORE and ENDOR spectroscopy of 7 was undertaken
(Figures 11 and 12). From these complementary methods,
comparison of data from (13C2-7) and its natural abundance
isotopologue shows hyperfine couplings for the 13C nuclei of
Cα and Cβ with the spin on iron. Simulation of the data gives
an anisotropic hyperfine tensor for 13Cα│A(13C)│ = [18, 33,
47] MHz; aiso = 32.7 MHz; T = 14.5 MHz) (Table 3),
indicative of strong coupling to the 13Cα nuclear spin.
Coupling to 13Cβ is weaker and nearly isotropic (│A(13C)│ =
[5.8, 5.0, 4.0] MHz; aiso = 4.9 MHz; T = 0.9 MHz). Following
the analysis of Hoffman and coworkers,39 the 13Cα hyperfine
coupling tensor can be decomposed into isotropic and
anisotropic terms; the anisotropic component allows
estimation of spin density of approximately 0.06 electrons at
Cα in the 2p(x) and 2p(y) orbitals, orthogonal to the Fe≡C-R
To further probe the electronic structure of paramagnetic
[Fe≡CCH3]+ 7 we undertook CW and pulse EPR studies. The
doublet ground state of 7 can only be observed by EPR at very
low temperatures due to fast electronic relaxation and line
broadening at 77 K. At 10 K, an axial EPR spectrum
exhibiting significant anisotropy between g-parallel at 2.61
and g-perpendicular at 1.96-1.93 is resolved (Figure 10),
consistent with a largely iron-centered spin. Electron spin
inversion recovery experiments show a strong temperature
dependence for T1 spin relaxation (Figure S41). This behavior
is likely consistent with a low-lying, thermally populated
doublet excited state or efficient electronic relaxation through
coupling of low-energy vibrations (< 100 cm-1) with the bath.
If an Orbach mechanism of electronic relaxation from the
excited state is assumed,37 a small energy difference (Δ)
between the ground state and first excited state as low as Δ =
24 cm-1 can be roughly calculated from the temperature
dependence of T1.38
EPR analysis of a paramagnetic carbyne/alkylidyne has
previously been limited to a tungsten methylidyne cation
([(dmpe)2(Cl)WV≡CH]+; giso = 2.026; │A(138W)│ = 221 MHz;
│A(31P)│ = 149 MHz;│A(13C)│ = 34 MHz; dmpe =
bis(dimethylphosphino)ethane) and to examples with the
(SiP3)Fe-system.27c,12 Interestingly, [(SiP3)FeCNMe2]+ and
[Fe≡CCH3]+ 7 both exhibit nearly identical EPR signatures.11
Löwdin spin population and spin density analysis of 7 by DFT
(TPSS/def2-TZVP/D3ZERO) predicts the majority of
unpaired spin on iron (0.99 e-), with 0.07 e- of opposite spin
polarization on Cα (Figure 8) (cf. [(SiP3)FeCNMe2]+: 0.97 e-
on Fe, oppositely polarized to 0.03 e- on Cα and 0.01 e- on Nβ).
This prediction is in agreement with the large anisotropy
between g-parallel and g-perpendicular and the strong
temperature dependence observed for T1 (Figure S41).
bond vector.
This estimate agrees well with the
aforementioned Löwdin spin population analysis.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 13
GHz; MW pulse length (π/2, π) = 40 ns, 80 ns; τ = 190 ns; RF
pulse length = 15 µs; TRF = 2 μs; shot repetition time = 5 ms.
1
2
3
4
5
6
7
8
9
Interestingly, despite weak coupling to Cβ, proton hyperfine
coupling to the -CH3 substituent of [Fe≡CCH3]+ 7 is detected
by both ENDOR and HYSCORE. Comparison of 2H-7 with
its natural abundance isotopologue and simulation of the
ENDOR difference spectrum (Figure 12) provides two
1
hyperfine tensors for H1(CH3) (│A(1H)│ = [8.6, 12.5, 8.6]
MHz; aiso = 9.9 MHz; T = 1.3 MHz) and 1H2(CH3) (│A(1H)│
= [2.3, 6.3, 6.3] MHz; aiso = 5.0 MHz; T = 1.3 MHz).
Importantly, the isotropic coupling components for the
carbyne -CH3 substituent to the unpaired spin are correlated
with a “hyperconjugation” interaction between the methyl H-
atoms and the spin active Fe and Cα orbitals.42,43,44
A hyperconjugation effect indicates, to some extent,
delocalization of spin/hole or cationic character on Fe≡Cα to
the hydrogen s-orbitals on the terminal methyl (see Figure 9
for this and other chemical depictions). This exaggerated
resonance description is thus juxtaposed with the electronic
structures typically invoked in heteroatom-substituted
carbyne complexes, like our previously reported
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(SiP3)FeCOSiMe3,
(DPB)Fe(COSiMe3)2,
[(SiP3)FeCNMe2]0/+, and [(SiP3)FeCNH2]+ 20–23. In each of
these cases, the Fe-C and C-O/N bonding is qualified by the
free “lone pair” on the distal atom, capable of delocalizing
bonding across the Fe-C-O/N unit and decreasing the formal
valency at iron. The CNR2 moiety of the amino carbynes are
even planar (Σ∠ ≈ 360°), consistent with conjugation between
nitrogen and the Fe-C π-bond. However, hyperconjugation
and delocalization in [(SiP3)FeCCH3]+ should be appreciated
as a subtle effect, as the spectroscopic data and DFT
calculations determine a majority of unpaired spin to be
located on iron (Fe: +0.99 e- vs. Cα: -0.07 e-). Furthermore,
the strong similarity between the EPR data for
[(SiP3)FeCNMe2]+ and [(SiP3)FeCCH3]+, the comparably
modest calculated spin density across the amino carbyne (Fe:
+0.97 e- vs Cα: -0.03 e- and Nβ: -0.01 e-), and the close
congruence between the Mössbauer parameters for all of the
(SiP3)Fe carbynes, both alkyl- and heteroatom-substituted,
suggests that a “Fischer-like” resonance description (the L-
type carbyne donor resonance forms in Figure 9) may in fact
be of limited importance to the structural and electronic
stabilities of this series of Fe≡C triply bonded species.
Figure 11. (top) X-band HYSCORE spectrum of 13C2-
[(SiP3)FeCCH3]OTf measured at 280 mT (g = 2.48) (Bottom)
Monochromatic representation of the HYSCORE data with
simulations of hyperfine couplings overlaid: (green) using
simulations parameters in Table 3: (green) 31P, (red) 13Cα,
(blue) 13Cβ. Acquisition parameters: Acquisition parameters:
temperature = 6.5 K; microwave frequency = 9.715 GHz; MW
pulse length (π/2, π) = 8 ns, 16 ns; τ = 168 ns, t1 = t2 = 100 ns;
Δt1 = Δt2 = 16 ns; shot repetition time (srt) = 1 ms).
The latter point speaks to the potential relevance of other
hypothetical iron-ligand multiply-bonded species in the
(SiP3)Fe
platform,
namely
terminal
Fe≡C(H)
carbide/methylidyne or Fe≡N nitride complexes (Figure 9,
bottom), the latter possibly arising during the limited nitrogen
reduction catalysis mediated by (SiP3)FeN2;45 the former have
yet to be directly evidenced from cleavage of CO, CN-, or
C2H2 at iron. We have extensively explored the reactivity and
thermochemistry of the early stages of reductive protonation
on FeN2 compounds. In the trisphosphine-borane TPBFeN2,
the N-N bond is cleaved to yield NH3 and a spectroscopically
characterized [(TPB)FeIV≡N]+ nitride cation.3 Observation of
an iron-N2H4 hydrazine cation46 derived from (SiP3)FeN2
questions the accessibility of any (SiP3)Fe≡N species through
functionalization of dinitrogen. However, the stable bonding
in the [(SiP3)FeCCH3]+/0 complexes, and the aforementioned
implication for the electronic structures of the heteroatom-
substituted relatives, support the potential viability of high-
valent, multiply-bonded Fe≡C or Fe≡N intermediates in the
chemistry of this system, and structurally related synthetic or
biological iron sites.
Figure 12. (A) (black) X-band 13C-12C difference ENDOR
spectra of (13C2-7) and (12C2-7); (red) 13Cα simulated fit; (blue)
13Cβ simulated fit. (B) (black) X-band 1H-2H difference
ENDOR spectra of (1H3C-7) and (2H3C-7); (red) 1H3C
1
simulated fit 1; (blue) H3C simulated fit 2. Acquisition
parameters: temperature = 6.5 K; MW frequency = 9.715
ACS Paragon Plus Environment

Page 9 of 13
Journal of the American Chemical Society
CONCLUSION
To close, we have described the activation of acetylene gas
1
at a mononuclear iron site, along with reductive protonation
reactions that converge at stable and structurally unusual
2
3
4
Fe(IV) and Fe(V) alkylcarbyne complexes.
X-ray
crystallography, pulse X-band EPR, and other spectroscopies
have allowed us to characterize the first Fe(I) and Fe(II)
adducts of acetylene, and to discriminate their side-on binding
forms from alternative structural isomers (e.g., Fe=CCH2).
Characterization of their electronic structures, through
Mössbauer, DFT, and pulse EPR experiments, reveal strong
covalency in the Fe≡C triple bonds of the carbyne species and
moderate spin delocalization in the cationic S = 1/2 congeners.
Congruence between the structural, spectroscopic, and
computed parameters of the heteroatom-substituted iron
carbynes with those of the newly explored alkylcarbynes
described here establishes the viability of a highly covalent
Fe-to-C triple bond in trigonal pyramidal symmetry without a
requirement for heteroatom resonance stabilization. More
generally, the complexes and spectroscopic signatures
reported here may guide the ongoing investigation of reactive
nitrogenase intermediates with non-native substrates,
particularly acetylenic adduct species, or possible carbyne
intermediates of Fischer-Tropsch type C-C couplings with C1
substrates.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
Procedures and characterization data (PDF)
X-ray Data (CIF)
Computational models (inp)
AUTHOR INFORMATION
Corresponding Authors
jpeters@caltech.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the National Institutes of Health
(General Medical Sciences, grant GM070757). C.C.
acknowledges the NIH Ruth L. Kirschstein National Service
Fellowship for financial support. Additional support has been
provided by the Caltech EPR Facility, supported via NSF-
1531940, and the Dow Next Generation Educator Fund. We also
acknowledge the Beckman Institute for use of the its X-ray
facility and thank Larry M. Henling and Dr. David G.
VanderVelde for technical assistance with X-ray and NMR
experiments, respectively.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 13
References
1
2
3
4
5
6
7
8
9
1 For representative reviews and several lead references, see: (a)
Berry, J. F. Terminal Imido and Nitrido Complexes of the Late
Transition Metals. Comm. Inorg. Chem. 2009, 30, 28-66. (b)
Saouma, C. A.; Peters, J. C. M≡E and M=E Complexes of Iron
and Cobalt that Emphasize Three-Fold Symmetry (E = O, N,
NR). Coord. Chem. Rev. 2011, 255, 920-937. (c) Mondal, B.;
Roy, L.; Neese, F.; Ye, S. F. High-Valent Iron-Oxo and -Nitrido
Complexes: Bonding and Reactivity. Isr. J. Chem. 2016, 56,
763-772. (d) Wagner, W. D.; Nakamoto, K. Formation of
Nitridoiron(V) Porphyrins Detected by Resonance Raman
Spectroscopy. J. Am. Chem. Soc. 1988, 110, 4044-4045; (e)
Meyer, K.; Bill, E.; Mienert, B.; Weyhermüller, T.; Wieghardt,
K. Photolysis of cis- and trans-[FeIII(cyclam)(N3)2]+ Complexes:ꢀ
Spectroscopic Characterization of a Nitridoiron(V) Species. J.
Am. Chem. Soc. 1999, 121, 4859-4876; (f) Berry, J. F.; Bill, E.;
Bothe, E.; George, S. D.; Mienert, B.; Neese, F.; Wieghardt, K.
An Octahedral Coordination Complex of Iron(VI). Science
2006, 312, 1937-1941; (g) Aldrich, K. E.; Billow, B. S.;
Holmes, D.; Bemowski, R. D.; Odom, A. L. Weakly
7 Kim, C.-H.; Newton, W. E.; Dean, D. R. Role of the MoFe Protein
.Alpha.-Subunit Histidine-195 Residue in FeMo-Cofactor
Binding and Nitrogenase Catalysis. Biochemistry 1995, 34, 2798–
2808.
8
Fischer, E. O.; Schneider, J.; Neugebauer, D.
[(CO)3PPh3FeCNiPr2]+, a Novel Stable Carbyneiron Complex
Cation. Angew. Chem. Int. Ed. Engl. 1984, 23, 820–821.
9 Lee, Y.; Peters, J. C. Silylation of Iron-Bound Carbon Monoxide
Affords a Terminal Fe Carbyne. J. Am. Chem. Soc. 2011, 133,
4438–4446.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
10 Suess, D. L. M.; Peters, J. C. A CO-Derived Iron Dicarbyne That
Releases Olefin upon Hydrogenation. J. Am. Chem. Soc. 2013,
135, 12580–12583.
11
Rittle, J.; Peters, J. C. Proton-Coupled Reduction of an Iron
Cyanide Complex to Methane and Ammonia. Angew. Chem. Int.
Ed. 2016, 55, 12262–12265.
12
Rittle, J.; Peters, J. C. N–H Bond Dissociation Enthalpies and
Facile H Atom Transfers for Early Intermediates of Fe–N2 and
Fe–CN Reductions. J. Am. Chem. Soc. 2017, 139, 3161–3170.
13
Benton, P. M. C.; Laryukhin, M.; Mayer, S. M.; Hoffman, B.
Coordinating yet Ion Paired: Anion Effects on an Internal
Rearrangement. Organometallics 2017, 36, 1227–1237; (h)
Martinez, J. L.; Lin, H.-J.; Lee, W.-T.; Pink, M.; Chen, C.-H.;
Gao, X.; Dickie, D. A.; Smith, J. M. Cyanide Ligand Assembly
by Carbon Atom Transfer to an Iron Nitride. J. Am. Chem. Soc.
2017, 139, 14037–14040; (i) Bucinsky, L.; Breza, M.; Lee, W.-
T.; Hickey, A. K.; Dickie, D. A.; Nieto, I.; DeGayner, J. A.;
Harris, T. D.; Meyer, K.; Krzystek, J.; Ozarowski, A.; Nehrkorn,
J.; Schnegg, A.; Holldack, K.; Herber, R. H.; Telser, J.; Smith, J.
M. Spectroscopic and Computational Studies of Spin States of
Iron(IV) Nitrido and Imido Complexes. Inorg. Chem. 2017, 56,
4751–4768.
M.; Dean, D. R.; Seefeldt, L. C. Localization of a Substrate
Binding Site on the FeMo-Cofactor in Nitrogenase:ꢀ Trapping
Propargyl Alcohol with an α-70-Substituted MoFe Protein.
Biochemistry 2003, 42, 9102–9109.
14
For structurally characterized examples of Fe-CCH complexes
see: (a) Le Narvor, N.; Toupet, L.; Lapinte, C. Elemental Carbon
Chain Bridging Two Iron Centers: Syntheses and Spectroscopic
Properties of [Cp*(dppe)Fe-C4-FeCp*(dppe)]n+•n[PF6]-. X-Ray
Crystal Structure of the Mixed Valence Complex (n = 1). J. Am.
Chem. Soc. 1995, 117, 7129–7138. (b) Dahlenburg, L.; Weiβ, A.;
Bock, M.; Zahl, A. Ethinyl- Und Butadiinylkomplexe Des Eisens
2
Und
Rutheniums
Mitterminalen
Hauptgruppenelement-
(a) Betley, T. A.; Peters, J. C. A Tetrahedrally Coordinated
Substituenten, Cp* Fe(Ph2PCH(X)CH2PPh2) C ≡ CY (X = H,
PPh2; Y = H, PPh2, P(+)Ph2Me) Und Ru(Ph2PCH2PPh2)2(X) C ≡
CC ≡ CSiMe3 (X = Cl, C ≡ CC ≡ CSiMe3). J. Organomet. Chem.
1997, 541, 465–471. (c) Akita, M.; Terada, M.; Oyama, S.;
Morooka, Y. Preparation, Structure, and Divergent Fluxional
L3Fe−Nx Platform that Accommodates Terminal Nitride
(FeIV≡N) and Dinitrogen (FeI−N2−FeI) Ligands. J. Am. Chem.
Soc. 2004, 126, 6252-6254. (b) Hendrich, M. P.; Gunderson, W.;
Behan, R. K.; Green, M. T.; Mehn, M. P.; Betley, T. A.; Lu, C.
C.; Peters, J. C. On the Feasibility of N2 Fixation via a Single-Site
FeI/FeIV Cycle: Spectroscopic Studies of FeI(N2)FeI, FeIVN, and
Related Species. PNAS 2006, 103, 17107–17112.
Thompson, N. B.; Green, M. T.; Peters, J. C. Nitrogen Fixation
via a Terminal Fe(IV) Nitride. J. Am. Chem. Soc. 2017, 139,
15312–15315.
Behavior of Cationic Dinuclear Iron Acetylides [Fp* (C≡CR)]BF4
2
(R = H, Ph). Organometallics 1990, 9, 816–825.
15
3
Lee, Y.; Mankad, N. P.; Peters, J. C. Triggering N2 Uptake via
Redox-Induced Expulsion of Coordinated NH3 and N2 Silylation
at Trigonal Bipyramidal Iron. Nat. Chem. 2010, 2, 558–565.
16
4 (a) Lee, C. C.; Hu, Y.; Ribbe, M. W. Catalytic Reduction of CN−,
CO, and CO2 by Nitrogenase Cofactors in Lanthanide-Driven
Reactions. Angew. Chem. Int. Ed. 2015, 54, 1219–1222. (b) Lee,
C. C.; Hu, Y.; Ribbe, M. W. Vanadium Nitrogenase Reduces CO.
Science 2010, 329, 642–642. (c) Hu, Y.; Lee, C. C.; Ribbe, M. W.
Extending the Carbon Chain: Hydrocarbon Formation Catalyzed
by Vanadium/Molybdenum Nitrogenases. Science 2011, 333,
753–755. (d) Yang, Z.-Y.; Dean, D. R.; Seefeldt, L. C.
Molybdenum Nitrogenase Catalyzes the Reduction and Coupling
of CO to Form Hydrocarbons. J. Biol. Chem. 2011, 286, 19417–
19421. (e) Spatzal, T.; Perez, K. A.; Einsle, O.; Howard, J. B.;
Rees, D. C. Ligand Binding to the FeMo-Cofactor: Structures of
CO-Bound and Reactivated Nitrogenase. Science 2014, 345,
1620-1623.
Burgess, B. K.; Lowe, D. J. Mechanism of Molybdenum
Nitrogenase. Chem. Rev. 1996, 96, 2983–3012.
Dilworth, M. J.; Fisher, K.; Kim, C.-H.; Newton, W. E. Effects
on Substrate Reduction of Substitution of Histidine-195 by
Glutamine in the α-Subunit of the MoFe Protein of Azotobacter
Vinelandii Nitrogenase. Biochemistry 1998, 37, 17495–17505.
Bennett, M. A. Olefin and Acetylene Complexes of Transition
Metals. Chem. Rev. 1962, 62, 611–652.
17
For structurally characterized examples of mononuclear C2H2
complexes of Cr, Ni, and Cu, see: (a) Alt, H. G.; Han, J. S.;
Rogers, R. D.; Thewalt, U. Acetylenkomplexe des Wolframs.
Moleküllstrukturen
C5H4CMe2C13H9)W(CO)(HC2Ph)Me,
C5H4CMe2C13H8)W(CO)(C2Ph2)
von
(η5-
η1-
(η5:
und
(η5-
C5H5)Cr(CO)(C2H2)NO; ein Vergleich von alkinischen Vier- und
Zweielektronenliganden. J. Organomet. Chem. 1993, 459, 209–
217. (b) Munakata, M.; Kitagawa, S.; Kawada, I.; Maekawa, M.;
Shimono, H. Synthesis, Formation Constants and Structures of
Ternary Copper(I) Complexes with 1,10-Phenanthroline and
Alkynes. J. Chem. Soc., Dalton Trans. 1992, 14, 2225–2230. (c)
5
Pörschke,
K.
R.;
Tsay,
Y.-H.;
Krüger,
C.
Ethynebis(Triphenylphosphane)Nickel(0). Angew. Chem. Int. Ed.
1985, 24, 323–324. (d) Thompson, J. S.; Whitney, J. F. Copper(I)
Complexes with Unsaturated Small Molecules. Preparation and
Structural Characterization of Copper(I)-Di-2-Pyridylamine
Complexes with Olefins, Acetylene, and Carbon Monoxide.
Inorg. Chem. 1984, 23, 2813–2819. (e) Thompson, J. S.; Whitney,
6
ACS Paragon Plus Environment

Page 11 of 13
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
J. F. Preparation and Structural Characterization of Acetylene(2,2’-Dipyridylamine)Copper(I) Tetrafluoroborate. J. Am. Chem.
Soc. 1983, 105, 5488–5490.
18 Overend, J. The Equilibrium Bond Lengths in Acetylene and HCN. Trans. Faraday Soc. 1960, 56, 310–314.
19 Buscagan, T. M.; Oyala, P. H.; Peters, J. C. N2-to-NH3 Conversion by a Triphos–Iron Catalyst and Enhanced Turnover under
Photolysis. Angew. Chem. Int. Ed. 2017, 56, 6921–6926.
20
Gu, N. X.; Oyala, P. H.; Peters, J. C. An S = 1/2 Iron Complex Featuring N2, Thiolate, and Hydride Ligands: Reductive
Elimination of H2 and Relevant Thermochemical Fe–H Parameters. J. Am. Chem. Soc. 2018, 140, 6374–6382.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
21
(a) Stoian, S. A.; Yu, Y.; Smith, J. M.; Holland, P. L.; Bominaar, E. L.; Münck, E. Mössbauer, Electron Paramagnetic
Resonance, and Crystallographic Characterization of a High-Spin Fe(I) Diketiminate Complex with Orbital Degeneracy. Inorg.
Chem. 2005, 44, 4915–4922. (b) Yu, Y.; Smith, J. M.; Flaschenriem, C. J.; Holland, P. L. Binding Affinity of Alkynes and
Alkenes to Low-Coordinate Iron. Inorg. Chem. 2006, 45, 5742–5751. (c) Horitani, M.; Grubel, K.; McWilliams, S. F.; Stubbert,
B. D.; Mercado, B. Q.; Yu, Y.; Gurubasavaraj, P. M.; Lees, N. S.; Holland, P. L.; Hoffman, B. M. ENDOR Characterization of
an Iron–Alkene Complex Provides Insight into a Corresponding Organometallic Intermediate of Nitrogenase. Chem. Sci. 2017,
8, 5941–5948.
22 (a) Bianchini, Claudio.; Peruzzini, Maurizio.; Vacca, Alberto.; Zanobini, Fabrizio. Metal-Hydride AlkynylMetal-Vinylidene
Rearrangements Occurring in Both Solid State and Solution. Role of the 1-Alkyne Substituent in Determining the Relative
Stability of π-Alkyne, Hydride Alkynyl, and Vinylidene Forms at Cobalt. Organometallics 1991, 10, 3697–3707. (b) Bianchini,
Claudio.; Masi, Dante.; Meli, Andrea.; Peruzzini, Maurizio.; Ramirez, J. A.; Vacca, Alberto.; Zanobini, Fabrizio. Activation of
1-Alkynes at 16-Electron Rhodium Fragments. Some Examples of Thermodynamically Favored Rearrangements [M(π-
HC≡CR)]  [M(H)(C≡CR)]. Organometallics 1989, 8, 2179–2189. (c) Gauss, C.; Veghini, D.; Orama, O.; Berke, H. The
Reactions of Dicarbonylbis(Phosphorus Donor) Iron Fragments with Tertiary Propargyl Alcohols. J. Organomet. Chem. 1997,
541, 19–38. (d) Pombeiro, A. J. L.; Hills, A.; Hughes, D. L.; Richards, R. L. Synthesis and Crystal Structure of
[MoH3(C≡CBut)(Ph2PCH2CH2PPh2)2], a Trihydrido-Alkynyl Complex. J. Organomet. Chem. 1990, 398, C15–C18. (e)
Basallote, M. G.; Hughes, D. L.; Jiménez-Tenorio, M.; Leigh, G. J.; Vizcaíno, M. C. P.; Jiménez, P. V. Chemistry of Cobalt
Complexes with 1,2-Bis-(Diethylphosphino)Ethane: Hydrides, Carbon Disulfide Complexes, and C–H Cleavage in Activated
Alk-1-Ynes. Crystal Structure of [CoH(C≡CCO2Et)(Et2PCH2CH2PEt2)2][BPh4]. J. Chem. Soc., Dalton Trans. 1993, 0, 1841–
1847. (f) Ittel, S. D.; Tolman, C. A.; English, A. D.; Jesson, J. P. The Chemistry of 2-Naphthyl
Bis[Bis(Dimethylphosphino)Ethane] Hydride Complexes of Iron, Ruthenium, and Osmium. 2. Cleavage of sp and sp3 Carbon-
Hydrogen, Carbon-Oxygen, and Carbon-Halogen Bonds. Coupling of Carbon Dioxide and Acetonitrile. J. Am. Chem. Soc.
1978, 100, 7577–7585. (g) Field, L. D.; George, A. V.; Malouf, E. Y.; Hambley, T. W.; Turner, P. A Reductive Rearrangement
of Iron Acetylide Hydridecomplexes. Chem. Commun. 1997, 0, 133–134.
23
Fong, H.; Moret, M.-E.; Lee, Y.; Peters, J. C. Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron
Metallaboratrane. Organometallics 2013, 32, 3053–3062.
24 Bruce, M. I. Organometallic Chemistry of Vinylidene and Related Unsaturated Carbenes. Chem. Rev. 1991, 91, 197–257.
25
(a) Lomprey, J. R.; Selegue, J. P. Structural Characterization of Alkyne and Vinylidene Isomers of
[Ru(C2H2)(PMe2Ph)2(Cp)][BF4]. J. Am. Chem. Soc. 1992, 114, 5518–5523. (b) Consiglio, G.; Bangerter, F.; Darpin, C.;
Morandini, F.; Lucchini, V. Diastereomeric Equilibria and Barrier of Rotation in Cationic Iron Diphosphine Alkylidenecarbene
(Vinylidene) Complexes. Organometallics 1984, 3, 1446–1448. (c) Bullock, R. M. Rearrangement of a Metal (Η2-Alkyne)
Complex to a Metal Vinylidene and Subsequent Reaction of the Metal Vinylidene to Regenerate the Alkyne. J. Chem. Soc.,
Chem. Commun. 1989, 0, 165–167. (d) Fryzuk, M. D.; Huang, L.; McManus, N. T.; Paglia, P.; Rettig, S. J.; White, G. S.
Synthesis and Reactivity of the Iridium Vinylidene Ir:C=CH2[N(SiMe2CH2PPh2)2]. Formation of Carbon-Carbon Bonds via
Migratory Insertion of a Vinylidene Unit. Organometallics 1992, 11, 2979–2990. (e) Birdwhistell, K. R.; Burgmayer, S. J. N.;
Templeton, J. L. Tungsten Vinylidenes and Carbynes from Terminal Alkyne Reagents. J. Am. Chem. Soc. 1983, 105, 7789–
7790.
26 Gauss, C.; Veghini, D.; Berke, H. Acetylene/Vinylidene Rearrangements of Fe(CO)2L2(silylacetylene) Complexes (L =
Phosphorus Donor). Chem. Ber. 1997, 130, 183–194.
27 (a) Felixberger, J. K.; Kiprof, P.; Herdtweck, E.; Herrmann, W. A.; Jakobi, R.; Gütlich, P. Alkyl and Alkylidyne Complexes
of Rhenium. Angew. Chem. Int. Ed. Engl. 1989, 28, 334–337. (b) N.D.A. Lemos, M. A.; Pombeiro, A. J. L.; Hughes, D. L.;
Richards, R. L. Synthesis and X-Ray Crystal Structure of Trans-[MoF(≡CCH2tBu)(Ph2PCH2CH2PPh2)2][BF4], a Paramagnetic
Alkylidynefluorocomplex. J. Organomet. Chem. 1992, 434, C6–C9. (c) van der Eide, E. F.; Piers, W. E.; Parvez, M.; McDonald,
R. Synthesis and Characterization of Cationic Tungsten(V) Methylidynes. Inorg. Chem. 2007, 46, 14–21. (d) Leep, C. J.;
Kingsbury, K. B.; McElwee-White, Lisa. Photooxidation of (η-C5H5)[P(OMe)3]2Mo≡CPh in CHCl3. Intermediacy of a 17-
Electron Cationic Metal Carbyne. J. Am. Chem. Soc. 1988, 110, 7535–7536.
28
Paramagnetic carbenes are well known. See, for example, Dzik, W. I.; Zhang, X. P.; de Bruin, B. Redox Noninnocence of
Carbene Ligands: Carbene Radicals in (Catalytic) C−C Bond Formation. Inorg. Chem. 2011, 50, 9896–9903 and references
therein.
29
It appears that [FeCCH]+ can be generated via oxidation of 3 by [Cp2Fe][BArF24]; preliminary characterization data for
[(SiP3)FeCCH]+ is available in the Supporting Information.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 13
1
2
3
4
5
6
7
8
9
30
Clark, D. N.; Schrock, R. R. Multiple Metal-Carbon Bonds. 12. Tungsten and Molybdenum Neopentylidyne and Some
Tungsten Neopentylidene Complexes. J. Am. Chem. Soc. 1978, 100, 6774–6776.
31
Köhler, F. H.; Kalder, H. J.; Fischer, E. O. Die Metall-Kohlenstoff-Bindung in Carben- Und Carbin-Komplexen. Aussagen
Der 183W≡13C-Kopplungen. J. of Organom. Chem. 1976, 113, 11–22.
32 Gütlich, P.; Bill, E.; Trautwein, A. Mössbauer Spectroscopy and Transition Metal Chemistry: Fundamentals and Application;
Springer: Berlinꢀ; Heidelberg, 2011.
33
Scepaniak, J. J.; Vogel, C. S.; Khusniyarov, M. M.; Heinemann, F. W.; Meyer, K.; Smith, J. M. Synthesis, Structure, and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Reactivity of an Iron(V) Nitride. Science 2011, 331, 1049–1052.
34 Oliveira, F. T. de; Chanda, A.; Banerjee, D.; Shan, X.; Mondal, S.; Que, L.; Bominaar, E. L.; Münck, E.; Collins, T. J. Chemical
and Spectroscopic Evidence for an FeV-Oxo Complex. Science 2007, 315, 835–838.
35 Nance, P. J.; Thompson, N. B.; Oyala, P. H.; Peters, J. C. Zerovalent Rhodium and Iridium Silatranes Featuring Two-Center,
Three-Electron Polar σ Bonds. Angew. Chem. Int. Ed. 2019, 58, 6220–6224.
36
Takaoka, A.; Moret, M.-E.; Peters, J. C. A Ru(I) Metalloradical That Catalyzes Nitrene Coupling to Azoarenes from
Arylazides. J. Am. Chem. Soc. 2012, 134, 6695–6706.
37 (a) Orbach, R. Spin-lattice Relaxation in Rare-Earth Salts. Proc. R. Soc. Ser. A. 1961, 264, 458-484. (b) Orbach, R. Spin-
lattice Relaxation in Rare-Earth Salts: Field Dependence of the Two Phonon Processes. Proc. R. Soc. Ser. A. 1961, 264, 485-
495.
38 (a) Lorigan, G. A.; Britt, R. D. Electron Spin-Lattice Relaxation Studies of Different Forms of the S2 State Multiline EPR
Signal of the Photosystem II Oxygen-Evolving Complex. Photosynth. Res. 2000, 66, 189–198. (b) Su, J.-H.; Cox, N.; Ames,
W.; Pantazis, D. A.; Rapatskiy, L.; Lohmiller, T.; Kulik, L. V.; Dorlet, P.; Rutherford, A. W.; Neese, F.; Boussac, A.; Lubitz,
W.; Messinger, J. The Electronic Structures of the S2 States of the Oxygen-Evolving Complexes of Photosystem II in Plants
and Cyanobacteria in the Presence and Absence of Methanol. Biochim. Biophys. Acta, Bioenerg. 2011, 1807, 829–840.
39 Cutsail III, G. E.; Stein, B. W.; Subedi, D.; Smith, J. M.; Kirk, M. L.; Hoffman, B. M. EPR, ENDOR, and Electronic Structure
Studies of the Jahn–Teller Distortion in an FeV Nitride. J. Am. Chem. Soc. 2014, 136, 12323–12336.
40 (a) Gunderson, W. A.; Suess, D. L. M.; Fong, H.; Wang, X.; Hoffmann, C. M.; Cutsail III, G. E.; Peters, J. C.; Hoffman, B.
M. Free H2 Rotation vs Jahn–Teller Constraints in the Nonclassical Trigonal (TPB)Co–H2 Complex. J. Am. Chem. Soc. 2014,
136, 14998–15009. (b) Lee, Y.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C. A Nonclassical Dihydrogen Adduct of S = 1/2
Fe(I). J. Am. Chem. Soc. 2011, 133, 16366–16369.
41 We also note that a distortion in [Fe≡CCH3]+ 7 is reflected in the bonding metrics of the P3Fe plane, while the Si-Fe-C
vector remains approximately linear; in the aforementioned Fe(V)-nitride complex, the rigid tris(carbene) ligand cannot
undergo such a distortion and instead manifests a nonlinear B-Fe-N vector. For related discussions of Mn systems, see: (a)
Kropp, H.; King, A. E.; Khusniyarov, M. M.; Heinemann, F. W.; Lancaster, K. M.; DeBeer, S.; Bill, E.; Meyer, K.
Manganese Nitride Complexes in Oxidation States III, IV, and V: Synthesis and Electronic Structure. J. Am. Chem. Soc. 2012,
134, 15538–15544; (b) Ding, M.; Cutsail, G. E., III; Aravena, D.; Amoza, M.; Rouzières, M.; Dechambenoit, P.; Losovyj, Y.;
Pink, M.; Ruiz, E.; Clérac, R.; Smith, J. M. A Low Spin Manganese(IV) Nitride Single Molecule Magnet. Chem. Sci. 2016, 7,
6132–6140.
42 McConnell, H. M. Electron Densities in Semiquinones by Paramagnetic Resonance. J. Chem. Phys. 1956, 24, 632–632.
43 Heller, C.; McConnell, H. M. Radiation Damage in Organic Crystals. II. Electron Spin Resonance of (CO2H)CH2CH(CO2H)
in β‐Succinic Acid. J. Chem. Phys. 1960, 32, 1535–1539.
44 A through-space mechanism of spin delocalization is reflected by the anisotropic coupling components.
45 Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. A Synthetic Single-Site Fe Nitrogenase: Higher Turnover, Freeze-Quench
57Fe Mössbauer Data, and a Hydride Resting State. J. Am. Chem. Soc. 2016, 138, 5341–5350.
46 Rittle, J.; Peters, J. C. An Fe-N2 Complex that Generates Hydrazine and Ammonia via Fe=NNH2: Demonstrating a Hybrid
Distal to Alternating Pathway for N2 Reduction. J. Am. Chem. Soc. 2016, 138, 4243–4248.
ACS Paragon Plus Environment

Page 13 of 13
Journal of the American Chemical Society
1
2
3
4
TOC Graphic:
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment