Bimetallic Mechanism for Alkyne Cyclotrimerization with a Two-Coordinate Fe Precatalyst

Article abstract of DOI:10.1021/acscatal.0c01828

The two-coordinate compound (IPr)Fe[N(SiMe3)DIPP] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; DIPP = 2,6-diisopropylphenyl) catalyzes the cyclotrimerization of alkynes to arenes. Treatment of the Fe complex with 1 equiv of diphenylacetylene results in the formation of a bimetallic bridging alkyne complex, along with dissociation of IPr from Fe. At elevated temperatures, the bridging alkyne complex undergoes oxidative coupling to form a dimetallacyclopentadiene complex, formally by a one-electron oxidation at each metal center. Each complex catalyzes the cyclotrimerization of diphenylacetylene. Kinetic studies exhibit first-order dependence on the bimetallic complexes, providing further support for the presence of these species in the catalytic cycle. DFT calculations support the experimental mechanistic data and suggest that the catalytic cycle is completed by binding of an alkyne to the diene complex, followed by insertion to form a hexatriene species that then undergoes ring closure to form an inverse sandwich complex, [DIPP(Me3Si)N]Fe(η6-arene)Fe[N(SiMe3)DIPP]. The arene product is then displaced by alkyne to close the catalytic cycle.

Full text of DOI:10.1021/acscatal.0c01828

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Article
Bimetallic Mechanism for Alkyne Cyclotrimerization
with a Two-Coordinate Fe Precatalyst
Ryan Josef Witzke, Diptarka Hait, Khetpakorn Chakarawet, Martin Head-Gordon, and T. Don Tilley
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c01828 • Publication Date (Web): 18 Jun 2020
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ACS Catalysis
Bimetallic Mechanism for Alkyne Cyclotrimerization with a Two-
Coordinate Fe Precatalyst
1
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Ryan J. Witzke, Diptarka Hait, Khetpakorn Chakarawet, Martin Head-Gordon, and T. Don Tilley*
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Department of Chemistry, University of California, Berkeley, Berkeley, California, 94720, United States
KEYWORDS: Catalysis, Two-coordinate, Bimetallic, Mechanism, Alkyne cyclotrimerization.
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ABSTRACT: The two-coordinate compound (IPr)Fe[N(SiMe3)DIPP], (IPr = 1,3-di(2,6-di-iso-propylphenyl)imidazolin-2-ylidene)
(DIPP = 2,6-di-iso-propylphenyl) catalyzes the cyclotrimerization of alkynes to arenes. Treatment of the Fe complex with one
equivalent of diphenylacetylene results in formation of a bimetallic bridging alkyne complex, along with dissociation of IPr from Fe.
At elevated temperatures, the bridging alkyne complex undergoes oxidative coupling to form a dimetallacyclopentadiene complex,
formally by a one-electron oxidation at each metal center. Each complex catalyzes the cyclotrimerization of diphenylacetylene.
Kinetic studies exhibit first order dependence on the bimetallic complexes, providing further support for the presence of these species
in the catalytic cycle. DFT calculations support the experimental mechanistic data and suggest that the catalytic cycle is completed
by binding of an alkyne to the diene complex, followed by insertion to form a hexatriene species that then undergoes ring closure to
form an inverse sandwich complex, [DIPP(Me3Si)N]Fe(6-arene)Fe[N(SiMe3)DIPP]. The arene product is then displaced by alkyne
to close the catalytic cycle.
INTRODUCTION
seemed to represent an interesting case for study, since
mechanisms for this reaction type have received considerable
attention and are generally believed to proceed via two-electron
reductive couplings and metallocyclopentadiene intermediates
(Scheme 2).
First-row transition metal complexes attract increasing
attention as catalysts, since base metals are more abundant and
more cost effective relative to analogous second- and third-row
metal compounds.1 The development of catalysts with the
lighter metals poses an interesting challenge, given the different
properties expressed by such metal centers, including their
tendency toward high-spin configurations and one-electron
redox processes.2 In general, these characteristics necessitate
unique elementary transformations for substrate activations and
catalytic turnover, many of which are yet to be defined.
Scheme 1. Previous results showing the capacity of 1 as a catalyst
for alkyne cyclotrimerization.
R
R
R
1
2-5 mol %
(IPr)Fe[N(SiMe3)DIPP]
C6D6, 2-6 hours
R'
R'
R
R'
R'
R'
R
+
R
R'
R
Studies from this laboratory have focused on open-shell, two-
coordinate first-row metal complexes and establishment of
fundamental reaction steps that may be incorporated into
catalytic cycles.3–11 For example, the two-coordinate Ni
complex Ni[N(SiMe3)DIPP]2 (DIPP = 2,6-di-iso-propylphenyl)
was observed to catalyze C–C cross-coupling reactions and
undergo well-defined one-electron redox processes that account
for catalysis.10 Stoichiometric reactivity studies allowed
isolation of kinetically competent intermediates and definition
of a detailed mechanism that features a number of one-electron
redox steps involving Ni(I), Ni(II), and Ni(III). This mechanism
differs from that associated with palladium-catalyzed
couplings, which involve classic two-electron oxidative
additions. The higher propensity of nickel vs. palladium to
undergo one-electron transformations has considerable
precedent, but more research is needed to better define the
reaction steps generally available to first-row metal
complexes.10,12
R'
R = Me, R' = Me:
R = Me, R' = Ph:
R = iPr, R' = H:
100%
31%
80%
69%
10%
The history of alkyne cyclotrimerization dates back to 1948
when Reppe first reported the trimerization of acetylene to
benzene catalyzed by a Ni(II) complex.16 Since then, this
reaction has been significantly expanded upon both in terms of
catalysts and substrates.17,18 Its maturation has led to its use in
the synthesis of several natural products.19–30 Substituted
aromatic rings are commonly found in nature; thus, this reaction
should still have significant applications in the future.31
The viability of catalysis by open-shell two-coordinate
complexes has been demonstrated by only a small number of
additional
examples,
including
cyclotrimerizations
hydrosilations
of
of
olefins/ketones,6,13
alkynes/isocyanates,8,14 and reduction of dinitrogen to
ammonia,15 all of which lack significant mechanistic
information. The cyclotrimerization of alkynes, catalyzed by
the Fe(I) complex (IPr)Fe[N(SiMe3)DIPP]8 (1, Scheme 1) (IPr
=
1,3-di(2,6-di-iso-propylphenyl)imidazolin-2-ylidene),
1
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Page 2 of 9
Scheme 2. Mechanisms for alkyne coupling.
substrates proved unsuccessful due to rapid catalytic turnover
(Scheme 1).8
1
Two electron/metal monometallic alkyne coupling
2
3
4
5
6
7
8
9
In order to access a slower trimerization more amenable to
mechanistic study, the sterically demanding alkyne
diphenylacetylene was examined. For this substrate, the rate of
catalytic coupling by 1 was significantly reduced such that
elevated temperatures were required for formation of
hexaphenylbenzene (ca. 60 °C). More interestingly, the color of
the reaction mixture changed from red to orange/brown upon
addition of the alkyne to 1 in toluene. This suggested the
presence of a new alkyne-iron complex, and in pursuit of this 1
was treated with one equivalent of diphenylacetylene to afford
(General mechanism)
R
[M]
R
R
R
R
R
R
R
[M]
[M]
R
R
2
Two electron/metal bimetallic alkyne coupling
(Known bimetallic mechanisms)
R
R
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[M] [M]
R
R
R
R
[M]
R
[M]
the
bimetallic
bridged
bis(alkyne)
complex
{[DIPP(Me3Si)N]Fe(μ,η2-PhCCPh)}2 (2), along with unbound
IPr (eq 1). Due to the similar solubility properties of IPr and 2,
removal of IPr from the reaction mixture proved difficult;
however, this separation was effected by introduction of one
atmosphere of CO2 to remove IPr from the reaction mixture via
formation of the toluene-insoluble CO2-IPr adduct.37 Filtration,
followed by crystallization from minimal toluene cooled to -35
°C, afforded 2 in 41% yield.
R
R
R
R
[M] [M]
[M] [M]
R
R
R
2
2
R
R
R
[M]
[M]
[M]
[M]
R
R
Ph
Ph
R
R
Si
N
N
[M]
[M]
N
N
PhCCPh
Toluene
22 °C
N
Fe
1/2
Fe
N
(1)
R
Fe
N
Si
Si
Ph
One electron/metal bimetallic alkyne coupling
(This work)
Ph
R
1
2
R
R
2 [M]
[M]
[M]
R
[M]
[M]
Characterization of 2 by single crystal X-ray diffraction
(Figure 1) reveals an elongated alkyne C–C bond (1.316(6) Å)
relative to that of free diphenylacetylene (1.204(2) Å38).
Consistently, the coordinated alkyne is bent, with an average C-
C-Cphenyl bond angle of 142.7°. In the 13C{1H} NMR spectrum,
a resonance corresponding to the bridging alkyne carbons was
observed at 110 ppm, which is downfield relative to the
corresponding shift for free diphenylacetylene (89 ppm) and
upfield of that for stilbene (137 ppm). These observations can
be attributed to π-backbonding of Fe electron density into the
π* orbitals of the alkyne. Complex 2 is diamagnetic,
presumably due to antiferromagnetic coupling as a result of
either direct overlap of the metal molecular orbitals or
superexchange occurring through the alkyne π-system.
R
R
2
R
R
R
R
As shown in Scheme 2, the generally accepted mechanism
for alkyne cyclotrimerization involves coordination of two
alkynes to the metal, followed by coupling to form a
metallacyclopentadiene complex. The latter intermediate then
coordinates and inserts another alkyne to form the arene
product.17,32 The possibility of a two-electron coupling at Fe(I)
to produce an Fe(III) metallacyclopentadiene was particularly
interesting, since it would seem to represent a two-electron
process for a first-row metal. This contribution describes
experimental and computational work showing that this
coupling actually corresponds more closely to a one-electron
redox process at Fe(I), and the cooperative participation of two
metal centers results in formation of a diene intermediate
(Scheme 2). Other mechanisms involving two metals have been
suggested (Scheme 2), but all proceed through unsymmetrical
binding of the bimetallacyclopentadiene fragment resulting in a
two-electron oxidation at one metal while the other metal
coordinates to the π-system of the resulting diene.33–36 The
results presented here provide important mechanistic
information regarding fundamental steps that are possible for
first-row mediated transformations, with implications for how
new catalytic coupling processes may be designed.
RESULTS AND DISCUSSION
Synthesis and Characterization of Potential Catalytic
Intermediates. While complex 1 exhibits good catalytic
efficiency, the identity and structure of intermediates in this
system was unknown. Previous catalytic reactivity reported
from this laboratory employed 2-butyne, 1-phenyl-1-propyne,
and 3-methyl-1-butyne as substrates; however, isolation of
possible intermediates with stoichiometric quantities of these
Figure 1. Solid state structure of 2 shown with thermal ellipsoids
at 50% probability. Most of the phenyl groups and all hydrogen
atoms have been omitted for clarity.
The loss of IPr during complexation of
1 with
diphenylacetylene was unexpected given its well-established σ-
2
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ACS Catalysis
donor properties.39 The driving force for this ligand exchange is
presumed to be associated with the superior π-acidity of the
alkyne ligands that stabilize 2 in the formally low (+I) oxidation
state. In addition, the high steric demands of IPr may facilitate
its dissociation. We propose that an initial association of alkyne
to an open coordination site in 1 promotes rapid IPr
dissociation, followed by dimerization to form 2; however, it is
not known whether the dissociation and dimerization occur in
sequence or in a concerted fashion. Attempts to study the
kinetics of this process by UV-Vis spectroscopy were
unsuccessful due to the rapid rate of reaction.
μB and a similar connectivity was confirmed by X-ray
diffraction (Figure S5, see ESI).
1
2
3
4
5
6
7
8
Si
Et
Fe
N
N
N
N
EtCCEt
Toluene
22 °C
N
Fe
Et
N
1/2
(3)
Fe
N
Si
Si
Et
Et
1
4
57Fe Mössbauer data for 2 and 3. The zero-field 57Fe
Mössbauer spectrum of compound 2 displays a Lorentzian
doublet with an isomer shift of 0.388(2) mm/s and a quadrupole
splitting of 1.678(4) mm/s at 5 K, while that of 3 exhibits an
isomer shift of 0.483(1) mm/s and a quadrupole splitting of
2.183(3) mm/s (Figure 3). Even though, to the best of our
knowledge, no similar Fe compounds exist in the literature with
Mössbauer analysis, the isomer shifts of 2 and 3 are comparable
to those of other low-coordinate Fe complexes.8,41,42
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Heating complex 2 to 60 °C resulted in coupling of the
alkynes to form
a
dimetallacyclopentadiene complex,
{[DIPP(Me3Si)N]Fe}2(μ,κ2-C4Ph4) (3, in 46% isolated yield), in
which each of the Fe atoms have been oxidized by one electron
(formally to Fe(II), eq 2). Complex 3 has a temperature-
dependent magnetic moment (approximately 2.72 μB at room
temperature) on account of weak antiferromagnetic coupling
between the two Fe(II) centers (see ESI). Magnetic
susceptibility data for 2 were fitted to the Heisenberg-Dirac-van
―2퐽 푆 ⋅ 푆
2, where
퐽
is the
Vleck exchange Hamiltonian
1
coupling constant. Fitting to the experimental data gives
퐽 = ―70 cm -1
functional theory (DFT) estimate of
, which is in reasonable agreement with a density
퐽 = ―80 cm -1
for the
simpler model system {Ph(H3Si)]Fe}2(μ,κ2-C4Me4). The X-ray
structure reveals two distinct sets of dienyl C–C bond distances:
C1–C2 = C3–C4 = 1.399(2) Å and C2–C3 = 1.497(2) Å, which
correspond to double and single bonds, respectively (Figure 2).
A similar symmetric dimetallacyclopentadiene Ta complex was
recently reported, but was not studied as a catalyst for alkyne
cyclotrimerization.40
Ph
Ph
Si
Si
Ph
Ph
FeII
N
60 °C
Toluene
FeI
N
FeII
Ph
FeI
N
(2)
N
Si
Si
Ph
Ph
Ph
2
3
Figure 3. 57Fe Mössbauer spectra of 2 (upper) and 3 (lower) at 5
K. Red lines represent the respective compounds, blue lines are
minor impurity, and black lines are the total fit.
The lower isomer shift of 2 compared to 3 suggests that the
Fe centers in the alkyne complex possess less electron density,
which may be attributed to the alkyne’s ability to accept
electron density via π-backbonding. On the other hand, the
diene’s π* orbitals in compound 3 are not accessible to the Fe
for π-backbonding due to the geometrical constraint. It should
be noted, however, that a similar trend in the change in isomer
shifts was observed for the two-coordinate Fe(I/II) system, and
may be attributed to an increase in 4s-3d mixing for Fe(I)
complexes.42 The appearance of only one doublet in the
Mössbauer spectrum of 3 is consistent with symmetric
oxidative coupling of the alkynes, with each Fe being oxidized
by one electron.
Figure 2. Solid state structure of 3 shown with thermal ellipsoids
at 50% probability. Most of the phenyls, a molecule of diethyl
ether, and all hydrogen atoms have been omitted for clarity.
Catalytic Activity and Kinetics. Both complexes 2 and 3
catalyze the cyclotrimerization of diphenylacetylene at 70 °C in
the absence of IPr. A kinetic study was performed to probe the
mechanism of alkyne cyclotrimerization with this system using
the conditions shown in Scheme 3. Complex 4 was used
because the resulting product, hexaethylbenzene, was
Presumably, the large steric profile of the phenyl groups of
the alkyne ligand stabilizes complex 2, which then requires
elevated temperatures for coupling to form 3. In contrast,
treatment of 1 with one equivalent of 3-hexyne, at ambient
significantly
more
soluble
than
theproduct
temperature,
resulted
in
the
isolation
of
{[
DIPP(Me3Si)N]Fe}2(μ,κ2-C4Et4) (4), without observation of the
(hexaphenylbenzene) from catalysis with 2 or 3. The order in
[4] was determined by initial rate measurements using varying
concentrations of 4 while the concentration of 3-hexyne was
1
bis(alkyne) intermediate (by H NMR spectroscopy) (eq 3).
Analogous to 3, this complex has a magnetic moment of 2.87
3
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