Two-State Reactivity in Iron-Catalyzed Alkene Isomerization Confers σ-Base Resistance

Article abstract of DOI:10.1021/jacs.0c07300

A low-coordinate, high spin (S = 3/2) organometallic iron(I) complex is a catalyst for the isomerization of alkenes. A combination of experimental and computational mechanistic studies supports a mechanism in which alkene isomerization occurs by the allyl mechanism. Importantly, while substrate binding occurs on the S = 3/2 surface, oxidative addition to an η1-allyl intermediate only occurs on the S = 1/2 surface. Since this spin state change is only possible when the alkene substrate is bound, the catalyst has high immunity to typical σ-base poisons due to the antibonding interactions of the high spin state.

Full text of DOI:10.1021/jacs.0c07300

Subscriber access provided by Stony Brook University | University Libraries
Article
Two-State Reactivity in Iron-Catalyzed Alkene
Isomerization Confers #-Base Resistance
Sean Lutz, Anne K. Hickey, Yafei Gao, Chun-Hsing Chen, and Jeremy M. Smith
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07300 • Publication Date (Web): 12 Aug 2020
Downloaded from pubs.acs.org on August 12, 2020
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 10
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Two-State Reactivity in Iron-Catalyzed Alkene Isomerization Confers
-Base Resistance
Sean A. Lutz,† Anne K. Hickey,† Yafei Gao,† Chun-Hsing Chen and Jeremy M. Smith*
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
Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
ABSTRACT: A low-coordinate, high spin (S = 3/2) organometallic iron(I) complex is a catalyst for the isomerization of
alkenes. A combination of experimental and computational mechanistic studies supports a mechanism in which alkene
isomerization occurs by the allyl mechanism. Importantly, while substrate binding occurs on the S = 3/2 surface, oxidative
addition to an -allyl intermediate only occurs on the S = 1/2 surface. Since this spin state change is only possible when the
alkene substrate is bound, the catalyst has high immunity to typical -base poisons due to the antibonding interactions of
the high spin state.
Introduction
G‡
G‡
G
a)
b)
Driven in part by perceived advantages in cost and toxicity,
recent years have seen an explosion of interest in the
development of iron-based catalysts for transformations of
organic substrates.1,2 While substantial efforts have been
made towards developing iron catalysts that replicate the
reactivity of noble metal congeners, it has also been
recognized that new catalysts may benefit from the
intrinsic properties of iron. Most notably, the rich
landscape of redox and spin states associated with iron
complexes has the potential for accessing reactivity that is
largely unavailable to noble metal complexes.
G
Reaction Coordinate
Reaction Coordinate
Figure 1. Reaction coordinates for two-state reactivity
leading to lowered activation barriers. Red and blue curves
represent different spin states. (a) Spin state change occurs
before the transition state, providing a product with a
different spin state than reactant; (b) Transition state is on a
different spin state than reactant and product, leading to “spin
acceleration”.
Due to relatively small ligand field strengths, particularly
with low coordination numbers, iron complexes are often
high spin; moreover the changes in geometry, ligands,
and/or oxidation state that occur during the course of a
reaction may also be associated with changes in the metal
spin state. “Two-state reactivity” emerges when these spin
state changes lower the energy of the transition state
(Figure 1). While extensively investigated in the context
of high valent iron oxo chemistry,5 two-state reactivity has
also been proposed for low valent organometallic iron
complexes, with the prototypical example being the
transient 16-electron species 3Fe(CO)4. Although this
species has a triplet ground state, reactions involving
ligand binding require spin crossover to the low-lying
1Fe(CO)4 state.
alkene-isomerization-hydroboration,1
cycloadditions1 and C-C coupling.15
[2
+
2]
Due to differential occupancies of antibonding orbitals,
simple ligand field considerations lead to the expectation
that two spin states of the same d-electron count may
differ in their reactivity. Indeed, it has been shown that
both the redox potentials and activation energies for
electron transfer in a series of cytochrome P450 substrate
analogues are directly related to the spin state of
,
1
iron(III).1
We hypothesized that such spin-state
reactivity differences could be used to engineer catalyst
selectivity. Specifically, we anticipated that entry into the
catalytic cycle could be gated by substrates that convert
the catalyst spin state to that of the transition state for an
appropriate elementary step. For example, substrates that
convert a high spin catalyst to its low spin form are
expected to create the appropriate d-orbital vacancies
required for 2e- organometallic reactions.
Two-state reactivity has subsequently been proposed for
the reactions of a number of low-coordinate iron
complexes, including -hydride elimination from three-
coordinate Fe(II) alkyl complexes, oxidative addition by
four-coordinate Fe(0) complexes and C-H activation by
Fe(II) complexes.1 Moreover, multiple spin states have
been implicated in the reaction mechanisms of low valent
iron catalysts,11 including aldehyde hydrosilylation,12
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 10
Scheme 1.
1
2
3
4
5
6
7
8
tBu
Ph
tBu
N
N
KC8,
2.2.2-cryptand
N2
N
N
Ph
Ph
N
N
Ph
Ph
N
N
Ph
PhCCPh
-N2
B
Fe
Li
tBu
B
N
N
Fe
Ph
Ph
tBu
N
N
Ph
Ph
O
Cl
N
B
Fe
B
Fe
N
N
N
pentane
THF
THF
N
N
- LiCl, THF
K(2.2.2-cryptand)
K(2.2.2-cryptand)
3
1
2
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
2.0534(15) Å, similar to previously characterized high spin
iron(II) alkyl complexes.1 At 80 K, the 57Fe Mössbauer
spectrum of 1 shows a symmetric quadrupole doublet
centered at δ = 0.34 mm/s, with ΔEQ = 1.41 mm/s, similar
to three-coordinate high-spin iron(II) β-diketiminate20 and
iron(II) NHC alkyl complexes.21 The solution magnetic
moment, determined by Evans’ method (eff = 5.0(3) µB),
confirms the high spin formulation of complex 1.
Reducing 1 by 1 equiv. of KC8 in the presence of 2.2.2-
cryptand
provides
dark
red
[K(2.2.2-
t
cryptand)][Ph2B(tBuIm)2FeCH2 Bu(N2)] (2) in 60 % yield
following workup. The molecular structure of 2 shows a
four-coordinate iron center, with the bis(carbene)borate,
neopentyl and dinitrogen ligands completing the
coordination sphere (Figure 2). The iron-carbene
(2.117(4) and 2.113(4) Å) and iron-neopentyl (2.121(5) Å)
bond distances are longer than in 1, suggesting that iron
remains high spin after reduction.22
Figure 2. Molecular structures of 1 - 3. Thermal ellipsoids
are shown at 50% probability, ligand tert-butyl groups
represented as wireframes, and hydrogen atoms and the
[K(2.2.2-cryptand)]+ cation of 2 and 3 omitted for clarity.
Pink, black, orange and blue ellipsoids represent boron,
carbon, iron and nitrogen atoms, respectively.
While no resonances are observed between 200 and -60
1
ppm in the H NMR spectrum of 2, a strong N-N stretch at
1897 cm-1 confirms that the N2 ligand remains bound to
iron. This frequency is significantly lower than other four-
coordinate
[Li(Et2O)2][iPrPDIFe(CH2 Bu)(N2)] (NN
(PNP)Fe(N2) (PNP
butylphosphinomethyl)pyrrolide)
iron(I)
complexes
such
as
t
=
1948 cm-1),
In this work, we report a high spin (S = 3/2) iron(I)
complex that is a catalyst for a model organometallic
=
2,5-bis(di-tert-
(1964
cm-1),
reaction, namely alkene isomerization.
A combined
[(ICy)3Fe(N2)][BPh4] (ICy = 1,3-dicyclohexylimidazol-2-
experimental and computational investigation reveals the
involvement of two spin states, with the transition states
for substrate isomerization only accessible on the low spin
(S = 1/2) surface. Since the partially filled d-orbitals of the
high spin state result in weak ligand binding in the ground
state, this confers immunity against typical -basic catalyst
poisons. This work shows for the first time that the
attributes of two-state reactivity allow for new reaction
selectivity. We anticipate that the concepts described for
this model system will be applicable to other catalytic
reactions.
ylidene) (1967 cm-1), or PhB(AdIm)3FeN2 suggesting a
2
high degree of -backbonding in 2. A doublet centered at δ
= 0.52 mm/s (ΔEQ = 1.70 mm/s) is observed in the
Mössbauer spectrum of 2 (80 K), whose asymmetry is
likely due to the onset of slow relaxation in this non-
integer spin complex.24 While the differences in
coordination geometry between 1 and 2 hinder direct
comparison of the spectroscopic parameters, the greater
isomer shift in 2 relative to 1 is consistent with a high spin
iron(I) assignment.23c, In addition, these parameters are
25
comparable to high-spin iron(I) hydride complexes
supported by a -diketiminate ligand.2 Complex 2 is
therefore assigned as high-spin (S = 3/2) iron(I), which is
Results and Discussion
Synthesis and Characterization
Treatment
of
the
iron
chloride
precursor
also consistent with the solution magnetic moment (eff
4.2(3) B).
=
t
Ph2B(tBuIm)2FeCl(THF)1 with 1 equiv. of LiCH2 Bu in
pentane gives the yellow complex Ph2B(tBuIm)2FeCH2 Bu
t
Complex 2 reacts with one equivalent of PhCCPh to yield
(1) in 95 % yield (Scheme 1). The molecular structure of 1
shows a planar, three-coordinate iron center, with the sum
of angles about iron = 359.9° (Figure 2). The iron-carbene
bond distances of 2.1000(13) and 2.0936(14) Å are
consistent with a high spin (S = 2) configuration. The
distance to the alpha carbon of the neopentyl ligand is
t
purple
[K(2.2.2-cryptand)][Ph2B(tBuIm)2Fe(CH2 Bu)(η2-
PhCCPh)] (3) in 65 % isolated yield. The molecular
structure of 3 confirms the formulation of the product,
with PhCCPh replacing the dinitrogen ligand (Figure
2). The Fe-C bond to the neopentyl ligand is nearly parallel
with the acetylene ligand (torsion angle 166.6(4) °) and is
ACS Paragon Plus Environment

Page 3 of 10
Journal of the American Chemical Society
almost perpendicular to the C-Fe-C plane containing the
Scheme 2
1
2
3
4
5
6
7
8
donor atoms of the bis(carbene)borate ligand (torsion
angle 86.6(4) °). The C-C bond distance of 1.281(7) Å is
elongated from free PhCCPh due to strong backdonation
from the iron center.
tBu
N2
N
N
N
Ph
Ph
B
Fe
Fe(I), S = 3/2
0 kcal/mol
N
1
Unlike its precursor 2, the H NMR spectrum of complex 3
-N2
R
displays resonances between 35 and -10 ppm, although the
spin state is unchanged (S = 3/2), as determined by Evans’
method (µeff = 5.1(3) µB). The CC stretching frequency
presents at 1769 cm-1 (KBr), similar to CC stretches
observed for acetylene adducts of iron(I) β-diketiminate
complexes.2 The 57Fe Mössbauer spectrum of complex 3
at 80 K shows a symmetric doublet with an isomer shift δ =
0.42 mm/s and quadrupole splitting ΔEQ =1.99 mm/s. This
isomer shift is lower than that of complex 2, consistent
with a greater degree of -backbonding from iron into
tBu
N
N
Ph
9
B
Fe
H
Ph
N
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
H
N
Substitution
H
Oxidative addition
Fe(I), S = 3/2
2.1 kcal/mol
G‡ = 26.5 kcal/mol
tBu
N
tBu
N
N
N
Ph
Ph
N
N
Ph
H
B
Fe
B
Fe H
Fe(III), S = 1/2
better
-acid
diphenylacetylene.
The
increased
Ph
15.5 kcal/mol
N
N
backbonding increases the covalency between the ligand
Fe(I), S = 3/2
3.6 kcal/mol
and metal, which in turn decreases the isomer shift.2
Catalytic Alkene Isomerization
tBu
N
Reductive elimination
Fe-C rotation
N
N
Ph
Ph
Complex 2 catalyzes the isomerization of 1-hexene to 2-
hexene (10 mol % 2, 40 °C). Complete isomerization to 2-
hexene in a 4:1 trans:cis ratio occurs within 24 h, as
G‡ = 17.0 kcal/mol
B
Fe H
N
1
13
determined by H and C{1H} NMR spectroscopies.2 This
ratio is that expected based on the thermodynamic
stabilities of the two isomers, suggesting there is no kinetic
selectivity. While we have not attempted to optimize the
catalytic conditions, the mild reaction conditions are
notable. By contrast, the iron(II) complex 1 does not
catalyze 1-hexene isomerization.
Fe(III), S = 3/2
15.2 kcal.mol
Importantly, neopentane is not observed during catalysis.
In addition, no new deuterium-containing products are
observed when the reaction of 2 with D2C=C(D)CD3 is
monitored by 2H NMR spectroscopy. Thus, products
resulting from loss of the neopentyl ligand, e.g.
(H3C)3CCH2D or D2C=C(D)CH2C(CH3)3 are not observed
within the detection limits of the measurement. Together,
these observations suggest that the neopentyl ligand
remains bound to iron during catalysis.
Since iron nanoparticles have been previously implicated
in the catalytic isomerization of alkenes,
we have
conducted control experiments to test for nanoparticle
formation. While the limited solubility of elemental iron in
Computational Investigation
1
mercury is a complicating factor, it is notable that the
While the experimental data are most consistent with the
allyl mechanism for alkene isomerization, they do not
provide insight into the role of spin states. Since the
strength of the ligand field is expected to increase for 3-
allyl intermediates in the allyl mechanism, lower spin
states may be catalytically relevant. We therefore used
computational methods to better understand the
addition of elemental mercury does not impede the
catalytic activity of 2. Additionally, the insensitivity of the
catalyst to large excesses of typical poisons (see below) is
not consistent with nanoparticle catalysis.
Kinetics and Mechanistic Experiments
Kinetic analysis of the rate of 1-hexene isomerization
reveals a first-order dependency for 1-hexene. The kinetic
traces do not show induction periods, which is also
consistent with a molecular catalyst. An Eyring analysis of
the temperature dependent rate constants (30 °C – 60 °C)
gives activation parameters ΔH⧧ = 11.5 ± 0.5 kcal/mol and
ΔS⧧ = −31.5 ± 1.5 e.u. The magnitude of ΔS⧧ is notable,
suggesting a highly ordered transition state. These
activation parameters give ΔG⧧ = 21.6 ± 1.0 kcal/mol at 40
C.
isomerization
mechanism,
and
particularly
the
involvement of other spin states. For computational
expediency, these calculations were performed with 1-
butene as the substrate, however all other features of the
experimental system were retained. It is also important to
note that these calculations are calibrated to experimental
measurements.
While several possible mechanisms
were investigated,34 only the allyl mechanism is consistent
with experimental observations. The lowest energy
computed cycle is shown in Scheme 2, with the reaction
coordinate shown in Figure 3.
An isotope labeling experiment provides additional
2
mechanistic insight. At 40 °C, the rate of 1-hexene-3,3-D2
isomerization (kobs = 2.97
10 s-1) provides a kinetic
-5
isotope effect, kH/kD = 2.9. The KIE is temperature
independent, and thus tunneling does not play a role in the
reaction mechanism. The combined experimental results
are most consistent with alkene isomerization by 2
occurring by the allyl mechanism, akin to many other low
valent iron complexes.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 10
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
Figure 3. Calculated reaction coordinate for alkene isomerization by 2. Relative energies in kcal/mol. Orange and blue curves
represent S = 3/2 and S = 1/2 potential energy surfaces, respectively
Substitution of the dinitrogen ligand by the alkene
substrate provides entry into the catalytic cycle. This step
is calculated to be modestly uphill thermodynamically (G
= 2.1 kcal/mol). The alkene can bind in one of two
isoenergetic orientations, where the alkyl tail is either syn
or anti to the diphenylborate group. Despite the stronger
surface, and the computed activation free energy is in good
agreement with that determined experimentally, where
ΔG⧧ = 21.6 ± 1.0 kcal/mol. Although an optimized
transition state could not be located on the S = 3/2 surface,
linear synchronous transit (LST) and quadratic
synchronous transit (QST) analysis suggests the barrier is
at least 15 kcal/mol higher in energy. The need for a spin
state change can be understood by considering the orbital
requirements for oxidative addition. Specifically, two
electron oxidative addition requires an empty metal-based
orbital to create the new iron-hydride bond. However,
since all orbitals are at least partially occupied in the S =
4
ligand field, the iron(I) 2-alkene complex A is calculated
to have an S = 3/2 ground spin state. As described above,
the crystallographically characterized 2-alkyne complex 3
has a high spin (S = 3/2) ground state.
In accord with the allyl mechanism, oxidative addition of
the allylic C-H bond provides an iron(III) allyl hydride
complex. Here, the 1-allyl isomers 2B (S = 1/2) and 4B (S =
3/2) are found to be 15.5 kcal/mol and 19.7 kcal/mol
higher in energy than the dinitrogen complex 2,
respectively. Surprisingly, the 3-allyl complex 2B′ (S =
1/2) is significantly higher in energy, making it unlikely to
be mechanistically relevant. While we were unable to
optimize the 3-allyl complex on the S = 3/2 surface, this
species is expected to be even higher in energy. The
greater stability of the 1-allyl intermediates is likely the
result of destabilizing steric interactions in the 3-allyl
complex 2B′, where the iron center is seven-coordinate.
5
3/2 spin state, this reaction is “spin blocked”.
The computed structure of 2TS1 is “3-allyl like” (Figure 4).
4
2
Compared to A, in TS1 there is an increase in the C1-C2
distance and a decrease in the C2-C3 distance, such that
these bonds are of similar length, and intermediate
between single and double bond lengths. In addition, the
distances from iron to the carbon atoms of the allyl ligand
are similar to those in structurally characterized iron 3-
allyl complexes.
This highly ordered transition state
structure is in accord with the large magnitude of the
experimentally determined entropy of activation, where
ΔS⧧ = −31.5 ± 1.5 e.u. The allylic C-H bond is largely broken
A transition state for the oxidative addition step (2TS1)
was located on the S = 1/2 surface, with G‡ = 26.5
kcal/mol. This is the highest energy species on the reaction
2
in TS1, as characterized by the long C3-H distance (1.550
Å) and an Fe-H distance (1.540 Å) that is similar to
paramagnetic iron hydride complexes.
These bond
ACS Paragon Plus Environment

Page 5 of 10
Journal of the American Chemical Society
length changes are consistent with the experimentally
observed KIE, which shows that the rate depends on
cleavage of the allylic C-H bond.
1
2
3
4
5
6
7
8
9
The next step of the reaction mechanism involves rotation
about the Fe-allyl bond in 2B. Here, the lowest energy
structure is computed to be the S = 3/2 1-allyl
intermediate 4C, likely as a result of decreased steric
interactions upon rotation. However, the small energy
difference between the S = 3/2 (17.8 kcal/mol) and S = 1/2
(15.2 kcal/mol) surfaces suggests the states are highly
mixed. Following bond rotation, reductive elimination
provides 4D, in which the alkene ligand has been
isomerized. As with oxidative addition, reductive
elimination is only viable on the S = 1/2 surface, with the
transition state (2TS2) also having an “3-allyl like”
structure. The barrier to reductive elimination (G‡ = 17.0
kcal/mol) is lower than that for oxidative addition, likely
due to attenuated steric interactions in this step. The final
step of the mechanism involves alkene ligand substitution
to release the isomerized product, which is computed to be
thermoneutral.
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. Computed transition state 2TS1. Selected bond
lengths (Å): Fe-H 1.541; Fe-C1 2.120; Fe-C2 1.975; Fe-C3
2.119; Fe-C4 2.141; C1-C2 1.427; C2-C3 1.442; C3-H 1.550.
In summary, the computationally determined mechanism
is consistent with a reaction surface that traverses two
spin states (i.e., two-state reactivity). Importantly, this
mechanism is consistent with experimentally determined
data, including the magnitudes of G‡ and S‡ as well as
the observed kinetic isotope effect. An important insight
from these studies is that S = 1/2 transition states are
stabilized by the “3-allyl like” geometry of the alkene
substrate.
The computational analysis also provides insight into the
unexpected stability of the neopentyl ligand (see above).
While reductive elimination of neopentane to provide
Figure 5. The Hammett parameters of 4-substituted pyridines
linearly correlate with the calculated log Keq for 2 in THF.
2
Ph2B(tBuIm)2Fe(3-allyl) (S = 3/2) from B is calculated to
Binding Studies
be endergonic (G = -14.2 kcal/mol), the transition state is
calculated to be thermally inaccessible G‡
> 40
A key aspect of the proposed reaction mechanism is that
alkene binding only occurs on the S = 3/2 surface, whereas
oxidative addition/reductive elimination only occurs on
the S = 1/2 surface. Thus, the reaction steps are associated
with distinct spin states, each with its own chemical
properties. This presents an opportunity to harness two-
state reactivity for selective catalysis.
kcal/mol). Indeed, an LST calculation reveals that the
transition state for reductive elimination of neopentane is
~10 kcal/mol higher in energy than for the reductive
elimination of the isomerized alkene. Since the orbital
requirements for reductive elimination are the same as
those of oxidative addition (see above), this reaction can
only occur on the S = 1/2 surface. However, in contrast to
reductive elimination of the alkene, an “3-allyl-like”
transition state for the reductive elimination of
Since all iron-based orbitals in the S = 3/2 state are at least
partially occupied, we anticipate that neutral -donor
ligands will only weakly bind to the metal. High d-electron
counts have been shown to weaken the binding of
additional ligands in low coordinate 3d metal complexes.
To provide support of this hypothesis, the binding abilities
of a series of 4-substituted pyridines were evaluated
according the equilibrium constant Keq:
2
neopentane is inaccessible. While the 3-allyl complex B’
provides another entry point to the S = 1/2 surface, this
species is also thermally inaccessible. Finally, formation of
neopentane requires C(sp3)-H reductive elimination, which
is less favorable than C(sp2)-H that leads to alkene
isomerization. Therefore, although neopentane reductive
elimination is thermodynamically favorable, kinetic
barriers disfavor this process.
tBu
tBu
N
X
N
N
N
N
N
Ph
Ph
Keq
Ph
Ph
B
Fe
B
Fe
N2
N
N
N
N
N
N
X
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 10
where N2 binds approximately perpendicularly to the
1
2
3
4
5
6
trigonal plane about the iron center. The negative charge
t
of [Ph2B(tBuIm)2FeCH2 Bu] will enhance these weak -
acid and good -base properties by increasing the orbital
energies on the complex. In addition, the negative charge
will serve to electrostatically repel incoming bases, further
decreasing the ligand binding affinities.
7
8
9
Figure 6. Selected frontier orbitals of the three-coordinate
t
fragment [Ph2B(tBuIm)2FeCH2 Bu]-. (a) SOMO – 1; (b) SOMO –
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
4.
As demonstrated for the case of pyridine (X = H),
significant changes are observed in the UV-vis spectrum
when the base is titrated into a solution of 2 (Figure S27).
These spectroscopic changes allow Keq to be determined
according to a weak binding model, providing Keq = (5.4
-4
0.9) 10 :
py]
Keq[
2 - py
0
[
]
=
2]
[py]
N ]
0 + [
2
[
Keq
0
(1)
where [2]0 and [py]0 are the initial concentrations of 2 of
pyridine, respectively.34 This study reveals that the binding
of pyridine to 2 is significantly less favorable than N2
binding.
Figure 7. Catalytic isomerization of 1-hexene by 2 in the
presence of excess DMAP. No change in the first-order rate
constant is observed. Initial conditions: [2] = 50 mM; [1-
hexene] = 0.5 M in THF-d8.
There is a linear correlation between log Keq and the
Hammett parameter (p)40 for a series of 4-substituted
pyridines. The positive slope ( = +2.8) indicates that
Alkene Isomerization in the Presence of -Bases
The poor affinity for -bases suggests that 2 will be
catalytically active towards alkene isomerization in the
presence of -basic ligands. Gratifyingly, high conversions
to 2-hexene are observed in the presence of excess
triethylamine, N-methylpyrrole, 4-tert-butylpyridine and
pyridine, bases that are often catalyst poisons. Some
bases (e.g. butyronitrile, methylimidazole and 4-CF3-
pyridine) are observed to inhibit, although not completely
electron-withdrawing groups facilitate binding to
2
1
(Figure 5), counter to the typical expectations for ligand
binding. Remarkably, there is no spectroscopic evidence
that 4-dimethylaminopyridine (DMAP) binds to 2,
consistent with the Hammett plot (predicted Keq = 4.1  10-
6). This binding is not kinetically hindered, since reduction
of the adduct between 1 and DMAP, which can be
generated in situ from 1 and excess DMAP, provides 2 as
the only iron-containing product.
5
shut down catalysis. Of the bases investigated, only 4-
methylthiazole completely inhibits isomerization.
A similar binding study for iron(II) complex 1 reveals a
linear correlation between log (Keq) and the Hammett
parameter para (Figure S25,  = -2.0). Thus, more basic -
donor ligands bind more strongly to 1, as expected from
ligand field theory. Similar observations have been made
The ability of 2 to catalyze alkene isomerization in the
presence of excess base was further investigated with
DMAP, whose strong -donor abilities have been shown to
inhibit other homogeneous catalysts. Remarkably, there is
no change in the first-order rate constant (kobs) for the
isomerization of 1-hexene to 2-hexene in the presence of
up to 0.75 M DMAP (Figure 7).
for other low-coordinate transition metal complexes.38,
2
The unusual ligand binding affinities of 2 can be
rationalized by considering the electronic structure of the
hypothetical three-coordinate, trigonal planar complex
Conclusions
A combined experimental and computational investigation
of the mechanism of alkene isomerization by the high spin
(S = 3/2) iron(I) complex 2 is most consistent with the
reaction occurring by the allyl pathway. These studies
reveal that alkene binding occurs on the S = 3/2 surface,
whereas the transition states that lead to alkene
isomerization are on the S = 1/2 surface. Due to the high
spin configuration, the binding of -bases on the S = 3/2
surface is generally weak, and consequently 2 has good
immunity to these typical catalyst poisons.
[Ph2B(tBuIm)2FeCH2 Bu] , which is formed by loss of N2
t
from 2. Interactions between this metal fragment and the
added base will determine the stability of the resultant
complex. Notably, the SOMO – 1 is appropriately oriented
for a favorable -symmetry interaction with an additional
ligand (Figure 6a). However, since this orbital is half-filled,
this interaction will necessarily be weak. Furthermore, the
doubly occupied SOMO – 4 has the appropriate symmetry
for interaction with a -acidic ligand (Figure 6b), with the
lobes of this orbital oriented such that the -acidic ligand
will occupy the apical position of the resulting trigonal
pyramidal iron complex. This electronic structure is
consistent with the geometry observed for complex 2,
While binding of the alkene substrate is likely enhanced by
the -basic properties of 2, we anticipate that the concepts
described in this work can be extended to other catalytic
ACS Paragon Plus Environment

Page 7 of 10
Journal of the American Chemical Society
processes. Thus, transition-metal catalyzed reactions in
which substrate binding enables two-state reactivity may
allow for selectivity based on the reactivity properties of
the two spin states. Catalytic reactions that involve “spin
acceleration” are expected to be the most favorable in this
regard due to the different spin states of the reaction
intermediates and reaction transition states.
1
2
3
4
5
6
Perspective on the Structure and Mechanism of Cytochrome P450
Enzymes” Chem. Rev. 2005, 105, 2279-2328; (c) Shaik, S.; Cohen,
S.; Wang, Y.; Chen, H.; Kumar, D.; Thiel, W. “P450 Enzymes: Their
Structure, Reactivity, and Selectivity
- Modeled by QM/MM
Calculations” Chem. Rev. 2010, 110, 949-1017.
Leadbetter, N. “Enlightening Organometallic Chemistry: The
Photochemistry of Fe(CO)5 and the Reaction Chemistry of
Unsaturated Iron Carbonyl Fragments” Coord. Chem. Rev. 1999,
188, 35-70.
(a) Besora, M.; Carreón-Macedo, J.-L.; Cowan, A.J.; George,
M.W.; Harvey, J.N.; Portius, P.; Ronayne, K.L.; Sun, X.-Z.; Towrie, M.
“A Combined Theoretical and Experimental Study on the Role of
Spin States in the Chemistry of Fe(CO)5 Photoproducts” J. Am.
Chem. Soc. 2009, 131, 3583-3592; (b) Besora, M.; Carreón-
Macedo, J.-L.; Cimas, A.; Harvey, J.N. “Spin-State Changes and
Reactivity in Transition Metal Chemistry: Reactivity of Iron
Tetracarbonyl” Adv. Inorg. Chem. 2009, 61, 573-623.
(a) Bellows, S.M.; Cundari, T.R.; Holland, P.L. “Spin Crossover
during β‑Hydride Elimination in High-Spin Iron(II)− and
Cobalt(II)−Alkyl Complexes” Organometallics 2013, 32, 4741-
4751; (b) Takayanagi, T.; Saito, K. Suzuki, H.; Watabe, Y.; Fujihara,
T. “Computational Analysis of Two-State Reactivity in β‑Hydride
Elimination Mechanisms of Fe(II)− and Co(II)−Alkyl Complexes
Supported by β‑Diketiminate Ligand” Organometallics 2019, 38,
3582-3589
7
8
9
ASSOCIATED CONTENT
Supporting Information. Full experimental procedures,
computational details and crystallographic information. This
material is available free of charge via the Internet at
http://pubs.acs.org. CCDC 2014746-2014748 contains the
supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
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
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/structures.
AUTHOR INFORMATION
Corresponding Author
* Jeremy M. Smith: smith962@indiana.edu.
Author Contributions
† These authors contributed equally.
(a) Baker, M.V.; Field, L.D. “Reaction of sp2 C-Hbonds in
Unactivated Alkenes with Bis(diphosphine) Complexes of Iron” J.
Am. Chem. Soc. 1986, 108, 7433-7434; (b) Jones, W.D.; Foster, G.P.;
Putinas, J.M. “The Catalytic Activation and Functionalization of C-
H Bonds. Aldimine Formation by the Insertion of Isonitriles into
Aromatic C-H Bonds” J. Am. Chem. Soc. 1987, 109, 5047-5048; (c)
Macgregor, S.A.; Eisenstein, O.; Whittlesey, M.K.; Perutz, R.N. “A
Theoretical Study of [M(PH3)4] (M = Ru or Fe), Models for the
ACKNOWLEDGMENT
This material is based upon work supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences Energy Frontier Research Centers program under
Award Number DE-SC-0019466. Funding from IU and the NSF
(CHE-1112299 and CHE-1566258) is also acknowledged.
Highly Reactive d8 Intermediates [M(dmpe)2] (dmpe
=
Me2PCH2CH2PMe2). Zero Activation Energies for Addition of CO
and Oxidative Addition of H2” J. Chem. Soc., Dalton Trans. 1998,
291-300; (d) Harvey, J.N.; Poli, R. “Computational Study of the
Spin-Forbidden H2 Oxidative Addition to 16-Electron Fe(0)
Complexes” Dalton Trans. 2003, 4100-4106; (e) Hickey, A. K.;
Lutz, S. A.; Chen, C.-H.; Smith, J. M. “Two-State Reactivity in C−H
REFERENCES
1
(a) Plietker, B. Iron Catalysis in Organic Chemistry: Reactions
Activation by
a Four-Coordinate Iron(0) Complex” Chem.
and Applications; Wiley-VCH: New York, 2008 (b) Plietker, B. Iron
Catalysis. Fundamentals and Applications; Springer International
Publishing: Switzerland 2011; (c) Bauer, E. Iron Catalysis II;
Springer International Publishing: Switzerland 2015.
Commun. 2017, 53, 1245−1248.
1
(a) Kalman, S.E.; Petit, A.; Gunnoe, T.B.; Ess, D.H.; Cundari,
T.R.; Sabat, M. “Facile and Regioselective C−H Bond Activation of
Aromatic Substrates by an Fe(II) Complex Involving a Spin-
Forbidden Pathway” Organometallics 2013, 32, 1797-1806; (b)
Sun, Y.; Tang, H.; Chen, K.; Hu, L.; Yao, J.; Shaik, S.; Chen, H. “Two-
State Reactivity in Low-Valent Iron-Mediated C−H Activation and
the Implications for Other First-Row Transition Metals” J. Am.
Chem. Soc. 2016, 138, 3715-3730.
2 Selected reviews: (a) Bauer, I.; Knölker, H.-J. “Iron Catalysis in
Organic Synthesis” Chem. Rev. 2015, 115, 3170-3387; (b) Wei, D.;
Darcel, C. “Iron Catalysis in Reduction and Hydrometalation
Reactions” Chem. Rev. 2019, 119, 2550-2610.
(a) Chirik, P.J.; Wieghart, K. “Radical Ligands Confer Nobility
on Base-Metal Catalysts” Science 2010, 327, 794-795; (b)
Fürstner, A. “Iron Catalysis in Organic Synthesis:
11
Holland, P.L. “Distinctive Reaction Pathways at Base Metals
A Critical
in High-Spin Organometallic Catalysts” Acc. Chem. Res. 2015, 48,
1696-1702.
Assessment of What It Takes To Make This Base Metal
a
Multitasking Champion” ACS Cent. Sci. 2016, 2, 778-789; (c)
Arevalo, R.; Chirik, P.J. “Enabling Two-Electron Pathways with
Iron and Cobalt: From Ligand Design to Catalytic Applications” J.
Am. Chem. Soc. 2019, 141, 9106-9123.
12 (a) Yang, J.; Tilley, T.D.; “Efficient Hydrosilylation of Carbonyl
Compounds with the Simple Amide Catalyst [Fe{N(SiMe3)2}2]”
Angew. Chem. Int. Ed. 2010, 49, 10186-10188; (b) Fan, G.; Shang,
Z.; Li, R; Shafiei-Haghighi, S.; Peng, Q.; Findlater, M.; Xu, X.
“Mechanism of the Iron(0)-Catalyzed Hydrosilylation of
Aldehydes: A Combined DFT and Experimental Investigation”
Organometallics 2019, 38, 4105-4114.
Selected reviews: (a) Schröder, D.; Shaik, S.; Schwarz, H.
“Two-State Reactivity as
a New Concept in Organometallic
Chemistry” Acc. Chem. Res. 2000, 33, 139-145; (b) Dzik, W.I.;
Böhmer, W.; de Bruin, B. “Multiple Spin-State Scenarios in
Organometallic Reactivity” in Spin States in Biochemistry and
Inorganic Chemistry: Influence on Structure and Reactivity, Swart,
M.; Costas, M. (Eds.) John Wiley & Sons, 2016: Chichester.
1
Macaulay, C.M.; Gustafson, S.J.; Fuller, III, J.T.; Kwon, D.-H.;
Ogawa, T.; Ferguson, M.J.; McDonald, R.; Lumsden, M.D.; Bischof,
S.M.; Sydora, O.L.; Ess, D.H.; Stradiotto, M.; Turculet, L. “Alkene
Isomerization−Hydroboration Catalyzed by First-Row Transition-
Metal (Mn, Fe, Co, and Ni) N‑Phosphinoamidinate Complexes:
Origin of Reactivity and Selectivity” ACS Catal. 2018, 8, 9907-
9925.
5
(a) Shaik, S.; Hirao, H.; Kumar, D. “Reactivity of High-Valent
Iron–Oxo Species in Enzymes and Synthetic Reagents: A Tale of
Many States” Acc. Chem. Res. 2007, 40, 532-542; (b) Shaik, S.;
Kumar, D.; de Visser, S.P.; Altun, A.; Thiel, W. “Theoretical
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 10
1
2
1
Environment Using Cryoreduction” Inorg. Chem. 2013, 52,
7323−732.
Hu, L.; Chen, H. “Substrate-Dependent Two-State Reactivity
in Iron-Catalyzed Alkene [2+2] Cycloaddition Reactions” J. Am.
Chem. Soc. 2017, 139, 15564-15567.
Hedström, A.; Lindstedt, E.; Norrby, P.-O. “Ox the Oxidattion
State of Iron in Iron-Mediated C-C Couplings” J. Organomet. Chem.
2013, 748, 51-55.
3
4
5
6
7
8
9
26 Chiang, K. P.; Scarborough, C. C.; Horitani, M.; Lees, N. S.; Ding,
K.; Dugan, T. R.; Brennessel, W. W.; Bill, E.; Hoffman, B. M.;
Holland, P. L. “Characterization of the Fe-H bond in a Three-
Coordinate Terminal Hydride Complex of Iron(I)” Angew. Chem.
Int. Ed. 2012, 51, 3658–3662.
15
1
Fisher, M.T.; Sligar, S.G. “Control of Heme Protein Redox
27
Yu, Y.; Smith, J. M.; Flaschenriem, C. J.; Holland, P. L. “Binding
Potential and Reduction Rate: Linear Free Energy Relation
between Potential and Ferric Spin State Equilibrium” J. Am. Chem.
Soc. 1985, 107, 5018-5019.
Affinity of Alkynes and Alkenes to Low-Coordinate Iron” Inorg.
Chem. 2006, 45, 5742−5751.
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
2
1
Ye, S.; Bill, E.; Neese, F. “Electronic Structures of the
(a) Turner, J.W.; Schultz, F.A. “Coupled Electron-Transfer and
[Fe(N2)(SiPiPr3)]+1/0/–1
Electron
Transfer
Series:
A
Spin-Exchange Reactions” Coord. Chem. Rev. 2001, 219-221, 81-
97; (b) Schultz, F.A. “Structure-Reactivity Relationships in
Inorganic Electrochemistry” J. Solid State Electrochem. 2011, 15,
1833-1843.
Hickey, A. K.; Lee, W.-T.; Chen, C.-H.; Pink, M.; Smith, J. M. “A
Bidentate Ligand Stabilizes a Low-Coordinate Iron(0) Carbonyl
Counterintuitive Correlation between Isomer Shifts and Oxidation
States” Inorg. Chem. 2016, 55, 3468-3474
29
The trans:cis ratio was determined from the integrations of
the 13C resonances at the olefin chemical shifts for cis- and trans-
2-hexene. See SI for details.
1
Phua, P.-H.; Lefort, L.; Boogers, J.A.F.; Tristany, M.; de Vries,
J.G. “Soluble Iron Nanoparticles as Cheap and Environmentally
Benign Alene and Alkyne Hydrogenation Catalysts” Chem.
Commun. 2009, 3747-3749.
Complex” Organometallics 2016, 35, 3069−3073.
1
For example: (a) Fernández, I.; Trovitch, R. J.; Lobkovsky, E.;
Chirik, P. J. “Synthesis of Bis(imino)pyridine Iron Di- and
Monoalkyl Complexes: Stability Differences between FeCH2SiMe3
and FeCH2CMe3 Derivatives” Organometallics, 2008, 27, 109-118;
(b) Vela, J.; Cirera, J.; Smith, J. M.; Lachicotte, R. J.; Flaschenriem, C.
J.; Alvarez, S.; Holland, P. L. “Quantitative Geometric Descriptions
of the Belt Iron Atoms of the Iron−Molybdenum Cofactor of
Nitrogenase and Synthetic Iron(II) Model Complexes” Inorg.
Chem. 2007, 46, 60-71.
1
(a) Mørup, S.; Linderoth, S.; Jacobsen, J.; Holmblad, M.
“Temperature Dependence of the Magnetic Hyperfine Splitting of
Metastable Iron-Amalgam” Hyperfine Int. 1991, 69, 489-492; (b)
Linderoth, S.; Mørup, S. “Stability and Magnetic Properties of an
Iron-Mercury Alloy” J. Phys.: Condens. Matter 1992, 4, 8627-8634.
2
Sirokman, G.; Molnar, A.; Bartok, M., Journal of Labelled
2
Compounds and Radiopharmaceuticals, 1989, 27, 439-448
(a) Schroeder, M.A.; Wrighton, M.A. “Pentacarbonyliron(0)
Photocatalyzed Hydrogenation and Isomerization of Olefins” J.
Am. Chem. Soc. 1976, 98, 551-558; (b) Mitchener, J.C.; Wrighton,
M.A. “Photogeneration of Very Active Homogeneous Catalysts
Using Laser Light Excitation of Iron Carbonyl Precursors” J. Am.
Chem. Soc. 1981, 103, 975-977; (c) Perthuisot, C.; Jones, W.D. “C-H
(a) Andres, H.; Bominaar, E. L.; Smith, J. M.; Eckert, N. A.;
Holland, P. L.; Münck, E. “Planar Three-Coordinate High-Spin FeII
Complexes with Large Orbital Angular Momentum:ꢀ Mössbauer,
Electron Paramagnetic Resonance, and Electronic Structure
Studies” J. Am. Chem. Soc. 2002, 124, 3012-3025; (b) Cowley, R. E.;
Bill, E.; Neese, F.; Brennessel, W. W.; Holland, P. L. “Iron(II)
Complexes with Redox-Active Tetrazene (RNNNNR) Ligands”
Inorg. Chem. 2009, 48, 4828-4836; (c) Dugan, T. R.; Bill, E.;
MacLeod, K. C.; Brennessel, W. W.; Holland, P. L. “Synthesis,
Spectroscopy, and Hydrogen/Deuterium Exchange in High-Spin
Iron(II) Hydride Complexes” Inorg. Chem. 2014, 53, 2370-2380.
Activation of Olefins with
a Zerovalent Iron Complex –
Isomerization of 4-Phenyl-1-butene to 1-Phenyl-1-butene using
Fe(depe)2N2” New J. Chem. 1994, 18, 621-628; (d) Glascoe, E.A;
Sawyer, K.R.; Shanoski, J.E.; Harris, C.B. “The Influence of the
Metal Spin State in the Iron-Catalyzed Alkene Isomerization
Reaction Studied with Ultrafast Infrared Spectroscopy” J. Phys.
Chem. C 2007, 111, 8789-8795; (e) Sawyer, K.R.; Glascoe, E.A.;
Cahoon, J.F.; Schlegel, J.P.; Harris, C.B. “Mechanism for Iron-
Catalyzed Alkene Isomerization in Solution” Organometallics
2008, 27, 4370-4379; (f) Fürstener, A.; Majima, K.; Martín, R.;
Krause, H.; Kattnig, E.; Goddard, R.; Lehmann, C.W. “A Cheap Metal
for a “Noble” Task: Preparative and Mechanistic Aspects of
Cycloisomerization and Cycloaddition Reactions Catalyzed by
Low-Valent Iron Complexes” J. Am. Chem. Soc. 2008, 130, 1992-
2004; (g) Mayer, M.; Welther, A.; Jacobi von Wangelin, A.; “Iron-
Catalyzed Isomerizations of Olefins” ChemCatChem 2011, 3, 1567-
1571; (h) Russell, S.K.; Lobkovsky, E.; Chirik, P.J. “Iron-Catalyzed
Intermolecular [2 + 2] Cycloaddition” J. Am. Chem. Soc. 2011,
133, 8858-8861; (i) Liu, X.; Li, B.; Liu, Q. “Base-Metal-Catalyzed
Olefin Isomerization Reactions” Synthesis 2019, 51, 1293-1310
34 See SI for full details.
21
Liu, Y.; Wang, L.; Deng, L. “Three-Coordinate Iron(II)
Dialkenyl Compound with NHC Ligation: Synthesis, Structure, and
Reactivity” Organometallics 2015, 34, 4401-4407
22
In
the
absence
of
cryptand,
the
complex
formed,
t
{[K][Ph2B(tBuIm)2FeCH2 Bu(N2)]}2(THF)4
is
demonstrating that cryptand is not essential for N2 coordination
(NN = 1877 cm-1, KBr). However, since this complex can only be
isolated in only poor yield, complex 2 is more practical for use in
catalysis.
2
(a) Tondreau, A. M.; Milsmann, C.; Patrick, A. D.; Hoyt, H. M.;
Lobkovsky, E.; Wieghardt, K.; Chirik, P. J. “Synthesis and Electronic
Structure of Cationic, Neutral, and Anionic Bis(imino)pyridine
Iron Alkyl Complexes: Evaluation of Redox Activity in Single-
Component Ethylene Polymerization Catalysts” J. Am. Chem. Soc.
2010, 132, 15046−15059; (b) Kuriyama, S.; Arashiba, K.;
Nakajima, K.; Matsuo, Y.; Tanaka, H.; Ishii, K.; Yoshizawa, K.;
Nishibayashi, Y. “Catalytic Transformation of Dinitrogen into
Ammonia and Hydrazine by Iron-Dinitrogen Complexes Bearing
Pincer Ligand” Nat. Commun. 2016, 22, 12181; (c) Ouyang, Z.;
Cheng, J.; Li, L.; Bao, X.; Deng, L. “High-Spin Iron(I) and Iron(0)
Dinitrogen Complexes Supported by N-Heterocyclic Carbene
Ligands” Chem. Eur. J. 2016, 22, 14162 – 14165; (d) Fan, Y.; Chen,
J.; Gao, Y.; Min, S.; Deng, L. “Iron Dinitrogen Complexes Supported
by Tris(NHC)borate Ligand: Synthesis, Characterization, and
Reactivity Study” Acta Chim. Sinica 2018, 76, 445-452.
5
Selected examples: (a) Keogh, D.W.; Poli, R. “Spin State
Change in Organometallic Reactions. Experimental and MP2
Theoretical Studies of the Thermodynamics and Kinetics of the CO
and N2 Addition to Spin Triplet Cp*MoCl(PMe3)2” J. Am. Chem. Soc.
1997, 119, 2516-2523; (b) Carreón-Macedo, J.-L.; Harvey, J.N. “Do
Spin State Changes Matter in Organometallic Chemistry?
A
Computational Study” J. Am. Chem. Soc. 2004, 126, 5789-5797; (c)
Hardman, N.J.; Fang, X.; Scott, B.L.; Wright, R.J.; Martin, R.L.; Kubas,
G.J. “High-Spin Diimine Complexes of Iron(II) Reject Binding of
Carbon Monoxide: Theoretical Analysis of Thermodynamic
Factors Inhibiting or Favoring Spin-Crossover” Inorg. Chem. 2005,
44, 8306-8316; (d) Benito-Garagorri, D.; Gonçalo Alves, L.; Verios,
2
Blume, M. “Magnetic Relaxation and Asymmetric Quadrupole
Doublets in the Mössbauer Effect” Phys. Rev. Lett. 1965, 14, 96-98.
Davydov, R. M.; McLaughlin, M. P.; Bill, E.; Hoffman, B. M.;
Holland, P. L. “Generation of High-Spin Iron(I) in a Protein
25
ACS Paragon Plus Environment

Page 9 of 10
1
Journal of the American Chemical Society
2
3
4
5
6
7
8
9
L.F.; Standfest-Hauser, C.M.; Tanaka, S.; Mereitner, K.; Kirchner, K.
Reactions” Nature Chem. 2013, 5, 597-601; (b) Collins, K. D.;
Glorius, F. “Intermolecular Reaction Screening as a Tool for
Reaction Evaluation” Acc. Chem. Res. 2015, 48, 619-627.
Crabtree, R. H. “Deactivation in Homogeneous Transition
Metal Catalysis: Causes, Avoidance, and Cure” Chem. Rev. 2015,
115, 127−150.
“Kinetically Controlled Formation of Octahedral trans-Dicarbonyl
Iron(II) PNP Pincer Complexes: The Decisive Role of Spin-State
Changes” Organometallics 2010, 29, 4932-4942; (e) Watanabe, K.;
Nakatani, N.; Nakayama, A.; Higashi, M.; Hasegawa, J. “Spin-
Blocking Effect in CO and H2 Binding Reactions to Molybdenocene
and Tungstenocene:
A
Theoretical Study on the Reaction
45 The calculated redox potentials suggest that electron transfer
could be a competitive reaction pathway for some of these bases,
e.g. Dugan, T.R.; Bill, E.; MacLeod, K.C.; Christian, G.J.; Cowley, R.E.;
Brennessel, W.W.; Ye, S.; Neese, F.; Holland, P.L. “Reversible C–C
Bond Formation between Redox-Active Pyridine Ligands in Iron
Complexes” J. Am. Chem. Soc. 2012, 134, 20352-20364. See SI for
details.
Mechanism via the Minimum Energy Intersystem Crossing Point”
Inorg. Chem. 2016, 55, 8082-8090.
(a) Simon, F.E.; Lauher, J.W. “Synthesis and Molecular
Structure of (3-C3H5)Fe(CO)3AuP(C6H5)3 and the Molecular
Structure of (3-C3H5)Fe(CO)3Br. Two Compounds with Distinctly
Different Coordination Geometries” Inorg. Chem. 1980, 19, 2338-
2343; (b) Brunner, H.; Weber, H.; Bernal, I.; Reisner, G.M.
“Optically Active Transition-Metal Compounds. 80. Synthesis,
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
Stereochemistry,
Allylcarbonylnitrosyl(aminophosphine)iron
and
X-ray
Analysis
of
Complexes”
Organometallics 1984, 3, 163-170; (c) Yu, Y.-F; Gallucci, J.;
Wojcicki, A. “New Bimetallic, Phosphido-Bridged Iron Complexes
Derived from [Fe2(µ-CO)(µ-PPh2)(CO)5(PPh2)]2–“ J. Chem. Soc.,
Chem. Commun. 1984, 653-655; (d) Saburo, N.; Seiji, M.;
Toshikazu, T.; Keiji, I. “Preparation, Ligand Substitution, and
Structural Stability of (η3-Allyl)dicarbonylnitrosyliron Complexes”
Bull. Chem. Soc. Jpn. 1998, 78, 403-412; (e) Trovitch, R.J.;
Lobkovsky, E.; Bouwkamp, M.W.; Chirik, P.J. “Carbon−Oxygen
Bond Cleavage by Bis(imino)pyridine Iron Compounds: Catalyst
Deactivation Pathways and Observation of Acyl C−O Bond
Cleavage in Esters” Organometallics 2008, 27, 6264-6278; (f)
Fürstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.;
Lehmann, C.W. “Preparation, Structure, and Reactivity of
Nonstabilized Organoiron Compounds. Implications for Iron-
Catalyzed Cross Coupling Reactions” J. Am. Chem. Soc. 2008, 130,
8773-8787.
(a) Hamon, P.; Toupec, L. Hamon, J.-R.; Lapinte, C. “Novel
Diamagnetic and Paramagnetic Iron(II), Iron(III), and Iron(IV)
Classical and Nonclassical Hydrides. X-ray Crystal Structure of
[Fe(C5Me5)(dppe)D]PF6” Organometallics 1992, 11, 1429-1431;
(b) Chiang, K.P.; Scarborough, C.C.; Horitani, M.; Lees, N.S.; Ding,
K.; Dugan, T.R.; Brennessel, W.W.; Bill, E.; Hoffman, B.M.; Holland,
P.L. “Characterization of the Fe–H Bond in a Three-Coordinate
Terminal Hydride Complex of Iron(I)” Angew. Chem. Int. Ed. 2012,
51, 3658-3662; (c) 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
Lin, C.-Y.; Fettinger, J.C.; Power, P.P. “Reversible
Complexation of Lewis Bases to Low-Coordinate Fe(II), Co(II), and
Ni(II) Amides: Influence of the Metal, Donor Ligand, and Amide
Substituent on Binding Constants” Inorg. Chem. 2017, 56, 9892-
9902.
39
Connors, K. A. “Binding Constants: The Measurement of
Molecular Complex Stability,” John Wiley & Sons: New York, 1987.
40
Hansch, C.; Leo, A.; Taft, R. W. “A Survey of Hammett
Substituent Constants and Resonance and Field Parameters”
Chem Rev. 1991, 91, 165-196.
41
(a) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. “Mechanistic
Study of the Oxidative N-Dealkylation Reactions of Bis(μ-
oxo)dicopper Complexes” J. Am. Chem. Soc. 1996, 118, 11575–
11586 (b) Johnson, C. D. The Hammett Equation; Cambridge
University Press: Cambridge, U.K., 1973.
2
Chiang, K.P.; Barrett, P.M.; Ding, F.; Smith, J.M.; Kingsley, S.;
Brennessel, W.W.; Clark, M.M.; Lachicotte, R.J.; Holland, P.L.
“Ligand Dependence of Binding to Three-Coordinate Fe(II)
Complexes” Inorg. Chem. 2009, 48, 5106-5116.
43
The choice of -bases was generally informed by those used
for catalyst robustness screens, see (a) Collins, K. D.; Glorius, F. “A
Robustness Screen for the Rapid Assessment of Chemical
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 10
1
2
3
SYNOPSIS TOC.
Insert Table of Contents artwork here
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
10
ACS Paragon Plus Environment