Selective Conversion of CO2 into Isocyanate by Low-Coordinate Iron Complexes

Article abstract of DOI:10.1002/anie.201802357

Discovery of the mechanisms for selective transformations of CO₂ into organic compounds is a challenge. Herein, we describe the reaction of low-coordinate Fe silylamide complexes with CO₂ to give trimethylsilyl isocyanate and the corresponding Fe siloxide complex. Kinetic studies show that this is a two-stage reaction, and the presence of a single equivalent of THF influences the rates of both steps. Isolation of a thermally unstable intermediate provides mechanistic insight that explains both the effect of THF in this reaction, and the way in which the reaction achieves high selectivity for isocyanate formation.

Full text of DOI:10.1002/anie.201802357

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Accepted Article
Title: Selective Conversion of CO2 to Isocyanate by Low-Coordinate
Fe
Authors: Daniel Broere, Brandon Q Mercado, and Patrick L Holland
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Selective Conversion of CO2 to Isocyanate by Low–Coordinate Fe
Daniёl L. J. Broere,* Brandon Q. Mercado, and Patrick L. Holland*
Abstract: Discovery of the mechanisms for selective transformations
of CO2 into organic compounds is a challenge. Here, we describe the
reaction of low-coordinate Fe silylamide complexes with CO2 to give
trimethylsilyl isocyanate and the corresponding Fe siloxide complex.
Kinetic studies show that this is a two-stage reaction, and the
presence of a single equivalent of THF influences the rates of both
We recently reported that Fe[N(SiMe3)2]2[9] gives access to
low-coordinate Fe formazanate complex 1 (Scheme 2), and that
THF contamination of solvent or starting materials results in the
formation of complex 1•THF.[ 10 ] Herein, we show that these
complexes both react with CO2 to selectively give
trimethylsilylisocyanate (TMSNCO) and the corresponding
siloxide complex, and explain the unusually strong influence of
steps. Isolation of
a thermally unstable intermediate provides
mechanistic insight that explains both the effect of THF in this reaction, THF through kinetic studies and through the isolation of a key
and the way in which the reaction achieves high selectivity for
isocyanate formation.
intermediate.
Exposing solutions of either 1 or 1•THF in non-coordinating
solvents to a CO2 atmosphere (0.9 atm) at room temperature
results in the formation of 1 equiv of TMSNCO in >95%
spectroscopic yield with respect to the Fe silylamide complexes.
Removal of volatile components gives a crude product consisting
The high abundance, low toxicity and wide availability of CO2 has
sparked great interest towards its use as a C1 building block in
organic synthesis.[1] However, the thermodynamic stability of the
most oxidized form of carbon poses a challenge for this purpose
that is often overcome through the use of highly reactive
compounds such as Grignard reagents. Binding of CO2 to
transition metals can lower the activation energy, enabling
functionalization using less reactive reagents.[2] The development
of efficient methodologies that employ stoichiometric or catalytic
amounts of transition metals to utilize CO2 is greatly aided by
fundamental understanding of the underlying reaction
mechanisms.[2] This is especially important when reaction
products are similar in electronic structure to CO2, and hence also
activated by the metal. A prominent example of such a scenario
is the reaction of metal silylamides with CO2. A wide range of
metal silylamides have been reported to react with CO2 to give
of
a
95:5 mixture of
2
and
a
known homoleptic
bis(formazanate)iron(II) complex,[ 11 ] which is readily washed
away to give pure 2. Compounds 1 and 2 each decompose over
several days in benzene solution at ambient temperature to form
the homoleptic formazanate complex and other unidentified
species, explaining the small amount of homoleptic complex that
is formed in the reactions of 1 and 1•THF with CO2. The 1H NMR
spectrum of complex 2 in C6D6 at 298 K showed the number of
paramagnetically shifted resonances between +30 and -16 ppm
expected for idealized C2v symmetry (Figure S1). The magnetic
moment in C6D6 from the Evans method at room temperature was
6.1 ± 0.3 μB per dimer, which is lower than the spin-only
uncoupled value of 6.9 μB and suggests antiferromagnetic
coupling between high-spin Fe2+ ions (S = 2) in a dimer. The
,
isocyanates[3 4,5,6], carbodiimides[7] and metal isocyanato compl-
1
exes[ 8 ] together with the corresponding silyl ethers, metal
siloxides or unidentified species (Scheme 1). However, amide
additions with high selectivity towards the isocyanate under mild
conditions are rare.[4,6] Because the formed isocyanate is
isoelectronic with CO2, metals that strongly activate CO2 also
activate the isocyanate, which is subsequently converted to the
carbodiimide or metal isocyanate and silyl ether.
resonances in the H NMR spectrum of complex 2 in THF-d8 at
298 K are broader and appear at significantly different chemical
shifts, indicative of THF coordination that breaks up the dimer to
form a monomeric species. However, even in THF solvent a small
amount of dimeric 2 is observed at 298 K, which increases in
concentration with temperature (Figure S2), consistent with an
equilibrium between dimer 2 and a monomeric THF solvate in
solution. This is in stark contrast with compound 1, which fully
converted to 1•THF in the presence of just a single equiv of
THF.[10] A van’t Hoff analysis (Figure S4) of 2 in THF gave ∆H° =
-4.3 ± 0.2 kcal mol-1 and ∆S° = -16.9 ± 0.6 cal mol-1 K-1. The
negative reaction entropy is consistent with binding of two THF
molecules to form two molecules of monomeric THF-containing
complexes. The magnetic moment in THF-d8 at room temperature
from the Evans method is 4.9 ± 0.2 μB per monomeric unit, which
is consistent with a mononuclear high-spin Fe2+ (S = 2) complex.
The zero-field Mӧssbauer spectrum of a solid sample of 2 at
80 K showed a doublet with δ = 0.84 mm s-1 and |ΔEQ| = 1.45 mm
Scheme 1. Common reaction products from the reaction of metal silylamides
with CO2.
s-1 (Figure S5), consistent with a high spin Fe2+ configuration.[12]
A
frozen THF solution of 2 showed a mixture of two species with
comparable isomer shifts (δ = 0.82 and 0.84 mm s-1) but different
quadrupole splittings (|ΔEQ| = 1.22 and 0.86 mm s-1) (Figure S6).
The Mӧssbauer parameters of one of these signals are similar to
those for dimer 2, in agreement with an equilibrium with a
monomeric THF complex that has different Mӧssbauer
parameters.
Dr. D. L. J. Broere, Dr. B. Q. Mercado, Prof. Dr. P. L. Holland
Department of Chemistry,Yale University
225 Prospect St., New Haven, CT 06511 (USA)
E-mail: daniel.broere@yale.edu / patrick.holland@yale.edu
Supporting information for this article is given via a link at the end of
the document xxxxx
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(Figure 2, top), rapid consumption of 1 and formation of TMSNCO
are observed, but formation of 2 is slower and does not follow an
exponential time dependence. Accordingly, there is formation of
an intermediate species (3) at a similar rate to the disappearance
of 1. After the consumption of 1, the reaction enters a second
stage, in which the concentration of intermediate 3 decreases at
a similar rate as isocyanate and 2 are formed. These observations
indicate a two-stage mechanism starting with rapid conversion of
two molecules of 1 to one equiv of TMSNCO and intermediate 3.
The second stage involves a slower decay of 3 into one equiv of
2 and TMSNCO.
The consumption of the iron starting material during the
reaction of 1•THF with CO2 under identical conditions (Figure 2,
bottom) is significantly slower. Moreover, the rates of formation of
2 and TMSNCO are not as different as in the reaction of THF-free
1 with CO2. Although intermediate 3 is again formed in a short
stage with relatively fast formation of TMSNCO, 3 forms in a
significantly smaller amount before it goes on to the same
products. In addition,
Scheme 2. Reaction of compounds 1 and 1•THF to form complex 2 and an
equiv of TMSNCO.
Crystals of complex 2 grew from a benzene solution at ambient
temperature. X-ray crystallographic analysis revealed a dimeric
structure where half of the molecule is crystallographically related
to the other half by an inversion center (Figure 1). The
formazanate–Fe bond lengths (1.978(2) and 1.979(2) ꢀ) are
similar to the values for four-coordinate 1•THF (1.955(4) –
1.966(4) Å). One of the Fe–O bonds is in the same plane as the
formazanate ligand, and is significantly shorter than its
counterpart (1.954(1) vs 2.056(2) Å), which sticks out of the Fe
formazanate plane, similar to the THF ligand in the solid state
structure of 1•THF.[10] The short Fe–Fe distance of 2.9916(5) ꢀ in
2 is conducive to antiferromagnetic coupling of the two high-spin
Fe2+ centers, described above.
.
Figure 1. Displacement ellipsoid plots (50% probability) of complex 2. Hydrogen
atoms have been omitted for clarity.
Although reactions of either 1 or 1•THF with CO2 give 2 and
TMSNCO in high yield, monitoring each reaction by 1H NMR
spectroscopy in C6D6 revealed that the two reactions follow
distinctly different time courses. In the reaction of 1 with CO2
Figure 2. Plots of concentration of starting complex, intermediate and products
vs time for the reaction of 1 (top) and 1•THF (bottom) with CO2 in C6D6.
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a small amount (~3%) of bis(trimethylsilyl)carbodiimide is formed
in this reaction. Both complexes 1 and 1•THF reacted with
TMSNCO, forming 2 and bis(trimethylsilyl)carbodiimide. We
propose that in the reaction of 1•THF with CO2 a small amount of
carbodiimide is formed because 1•THF is not rapidly consumed,
enabling the side reaction of 1•THF with formed TMSNCO to give
the carbodiimide and 2. In contrast, the rapid consumption of 1
with CO2 to give 3 (which does not react with isocyanate) prevents
formation of the carbodiimide byproduct.
Intrigued by the marked influence of a single equivalent of
THF on the formation and decay of intermediate 3, we sought to
elucidate its structure despite its transitory appearance.
Performing the reaction of 1 with CO2 at 0 °C in toluene-d8 results
in full consumption of 1 after 4 h to give 3 in 76% spectroscopic
yield with only 23 % of 2. Using the poor solubility of 2 to our
advantage, we exposed a pentane solution of 1 to CO2 for 1 h at
ambient temperature and placed it at -40 °C for 4 h, resulting in
crystallization of all 2. Concentration of the supernatant and
cooling to -40 °C gives crystals of pure 3 in 54% isolated yield.
The zero-field Mӧssbauer spectrum of a solid sample of 3 at
80 K has a doublet with δ = 0.81 mm s-1 and |ΔEQ| = 1.51 mm s-1
(Figure 3), which are similar to those for solid 2. The chemical
shifts in the 1H NMR spectrum of 3 in C6D6 (Figure S8) are in the
same range as those of 2, but all resonances are split into 1:1
pairs, indicating a loss of symmetry. Although two chemically
inequivalent formazanate ligands are observed for 3 in C6D6
solution, the two corresponding Fe environments in a solid sample
Two crystallographically independent molecules are found in the
asymmetric unit that differ in the relative orientation of the two
formazanate ligands in the dinuclear complex (Figure S12). The
two independent molecules have very similar bond lengths and
angles around the Fe centers. Within the central six-membered
ferracycle the Fe1–O17 and Fe1–O18 distances are ~0.08 Å
longer than the Fe2–N17 and Fe2–O18. Similarly, the O18–Fe1–
O17 angle of 97.6(1)° is significantly smaller than the O18–Fe2–
N17 angle of 113.4(1)°. The difference electron density map did
not allow unambiguous assignment of O and N atoms within the
bridging carbamate ligand. Fortunately, the bond lengths within
the carbamate do show clear differences that enable this
distinction. The bonds to C17 show clear partial double bond
character for the O17–C17 (1.266(3) Å) and N17–C17 (1.313(3)
Å) bonds. The longer O27–C17 distance of 1.348(3) Å is in the
expected range for a C–OSiMe3 single bond.[13] A CCSD search
of O–SiMe3 and N–SiMe3 bonds where O or N is also bound to Fe
showed that these are between 1.599–1.709 Å and 1.690–1.802
Å with mean values of 1.650 Å and 1.733 Å, respectively. The
N17–Si27 bond distance of 1.776(2) Å is outside of the range for
reported O–Si bonds. In addition, it is significantly longer than the
Si17–O27 bond distance (1.686(2)), which is comparable to the
O–Si bond distances of O18 in 3 (1.660(2) Å) and O14 in 2
(1.649(1) Å), and within the range of O–Si bonds in the CCSD. A
different CCSD search revealed only a few examples of N-
substituted carbamate esters that bridge two metals.[14] The bond
lengths within the bridging N-substituted carbamate ester in 3 are
comparable to these examples. Notably, in most examples[15]
these were formed by reaction of an alkoxide ligand with
isocyanates, which is also the case for N-substituted carbamate
esters that are bound to a single metal center[16]. Interestingly,
complex 3 displays the opposite reaction, and 2 does not react
with excess TMSNCO.
of
3
are indistinguishable by Mӧssbauer spectroscopy,
suggesting that they have similar geometries.
Crystals suitable for X–ray diffraction were obtained by
cooling a solution of 3 in pentane to -40 °C. The solid state
structure (Figure 4) revealed a nonsymmetric dinuclear complex
with a bridging alkoxide and N-substituted carbamate anion, in
agreement with the IR and 1H NMR spectra.
Figure 4. Displacement ellipsoid plot (50% probability) of complex 3. Hydrogen
atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°)
Fe1–O18 1.957(1); Fe1–O17 1.974(2); Fe1–N11 1.987(2); Fe1–N41 1.989(2);
Fe2–N14 1.967(2); Fe2–N44 1.978(2); Fe2–N17 2.023(2); Fe2–O18 2.034(15);
O18–Fe1–O17 97.59(6); N11–Fe1–N41 90.13(8); N14–Fe2–N44 90.42(8);
N17–Fe2–O18 113.39(7).
Figure 3. Zero-field Mӧssbauer spectrum of a solid sample of complex 3 at 80
K. δ = 0.81 mm s-1 and |ΔEQ| = 1.51 mm s-1.
The IR spectrum of 3 shows two bands at 1351 and 1514
cm–1 that are not observed in the IR spectra of 1, 1•THF, or 2
(Figure S9). The infrared spectrum of a sample of 3 that was
prepared using isotopically labeled 13CO2 gas (Figure S11) had
these bands shifted to 1321 and 1468 cm–1, respectively. These
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values are in the expected range for the C–N and C–O stretching
frequency of a metal bound N-substituted carbamate anion.[17]
Uncoordinated N-substituted carbamate ester anions in solution
have been observed to convert into the respective isocyanate and
alkoxide,[ 18 ] in agreement with the observed reactivity of 3.
Notably, the rate of the conversion of 3 into 2 and TMSNCO is
much more rapid in the presence of THF (Figure S10), explaining
the lower buildup and faster decay of 3 in the reaction of 1•THF
with CO2.
during its activation, and also show how relative rates can be
manipulated to achieve high selectivity.
Acknowledgements
This work was supported by The Netherlands Organization for
Scientific Research (Rubicon Postdoctoral Fellowship 680‐50‐
1517 to D.L.J.B.) and the National Institutes of Health (Grant GM‐
065313 to P.L.H.).
Scheme 3 shows a mechanism that explains the difference
in reactivity between 1 and 1•THF with CO2, and is in agreement
with the experimental data. In this mechanism, coordination of
CO2 occurs to THF-free 1 and not to the THF-bound 1•THF, and
thus 1•THF reacts more slowly with CO2 than 1 because it is
strongly inhibited by THF. Moreover, this THF inhibition indicates
that CO2 coordination to iron is necessary for the reaction to
proceed. Attack of the silylamide on coordinated CO2, followed by
migration of the SiMe3 group, gives B which rapidly reacts to form
3 and ½ equiv TMSNCO. Both the formation of 2 from 3, and loss
of isocyanate from B, are more rapid in the presence of THF
through formation of D, which rapidly converts to 2. This
mechanistic model explains the faster decay and lower buildup of
3 in the reaction of 1•THF with CO2 compared to its THF-free
counterpart.
Keywords: carbon dioxide fixation • intermediate isolation • iron
• isocyanate • low-coordinate
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Entry for the Table of Contents
COMMUNICATION
Daniёl L. J. Broere,* Brandon Q.
Mercado, and Patrick L. Holland*
Page No. – Page No.
Selective Conversion of CO2 to
Isocyanate by Low–Coordinate Fe
Carba-mating with CO2: Low-coordinate Fe silylamide complexes react with CO2
to selectively give trimethylsilyl isocyanate and an Fe siloxide complex. Kinetic
studies and isolation of a thermally unstable carbamate intermediate provides
mechanistic insight that explains the selectivity and the drastic effect that a single
equivalent of THF has on this reaction.
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