N2 Silylation Catalyzed by a Bis(silylene)-Based [SiCSi] Pincer Hydrido Iron(II) Dinitrogen Complex

Article abstract of DOI:10.1021/acs.organomet.0c00025

The bis(silylene)-based SiC(sp³)Si pincer ligand N,N′-bis(LSi:)dipyrromethane [SiCH₂Si] (L1; L = PhC(NtBu)₂) with a C(sp³) atom anchor was synthesized, and its coordination chemistry to iron was studied. Two novel iron hydride complexes, [SiCHSi]Fe(H)(N₂)(PMe₃) (1) and [SiCHSi]Fe(H)(PMe₃)₂ (2), were synthesized in the reaction of L1 with Fe(PMe₃)₄ via C(sp³)-H bond activation under different inert atmospheres (N₂ and argon). To the best of our knowledge, 1 and 2 are the first examples of a bis(silylene)-based hydrido pincer iron complex produced through activation of a C(sp³)-H bond. At the same time 1 is also the first example of a TM dinitrogen complex supported by a bis(silylene) ligand. The interconversion between 1 and 2 was achieved and monitored by operando IR and ³¹P NMR spectra to understand the transformation from 1 to 2 from the viewpoint of kinetics. To our delight, 1 could effectively catalyze silylation of dinitrogen and gave the highest turnover number so far among all the Fe-catalyzed N₂ silylation systems at room temperature and under atmospheric dinitrogen.

Full text of DOI:10.1021/acs.organomet.0c00025

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Article
N2 Silylation Catalyzed by a Bis(silylene)-Based [SiCSi] Pincer
Hydrido Iron(II) Dinitrogen Complex
Shengyong Li, Yajie Wang, Wenjing Yang, Kai Li, Hongjian Sun, Xiaoyan Li,* Olaf Fuhr,
and Dieter Fenske
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ABSTRACT: The bis(silylene)-based SiC(sp3)Si pincer ligand
N,N′-bis(LSi:)dipyrromethane [SiCH2Si] (L1; L = PhC(NtBu)2)
with a C(sp3) atom anchor was synthesized, and its coordination
chemistry to iron was studied. Two novel iron hydride complexes,
[SiCHSi]Fe(H)(N2)(PMe3) (1) and [SiCHSi]Fe(H)(PMe3)2
(2), were synthesized in the reaction of L1 with Fe(PMe3)4 via
C(sp3)−H bond activation under different inert atmospheres (N2
and argon). To the best of our knowledge, 1 and 2 are the first
examples of a bis(silylene)-based hydrido pincer iron complex
produced through activation of a C(sp3)−H bond. At the same
time 1 is also the first example of a TM dinitrogen complex supported by a bis(silylene) ligand. The interconversion between 1 and 2
was achieved and monitored by operando IR and 31P NMR spectra to understand the transformation from 1 to 2 from the viewpoint
of kinetics. To our delight, 1 could effectively catalyze silylation of dinitrogen and gave the highest turnover number so far among all
the Fe-catalyzed N2 silylation systems at room temperature and under atmospheric dinitrogen.
reactivity of the pincer complexes and many significant
transformations have been exploited.25 Our group has been
interested in the exploration of the first bis(silylene)-based
SiC(sp3)Si pincer ligand bearing a central sp3-hybridized
carbon as an anchoring site and the construction of an iron-
based pincer-type dinitrogen complex via C(sp3)−H activa-
tion. To the best of our knowledge, SiC(sp3)Si pincer systems
involving the activation of the C(sp3)−H bond and bis-
silicon(II)-based multidentate-ligand-supported TM dinitro-
gen complexes have not been reported so far.
Designing TM dinitrogen complexes for the catalytic N2
fixation under mild reaction conditions is one of the most
important topics in chemistry.26 In 2003, Schrock and co-
workers have found the first successful example of the direct
and catalytic conversion of dinitrogen into ammonia with a
molybdenum catalyst that contains a tetradentate triamido-
amine ligand.27 Since then, extensive studies on the
molybdenum-catalyzed transformation of N2 into ammonia
and hydrazine under ambient conditions have been reported
by some groups.28 In consideration of the significance of iron
at the active sites of nitrogenase enzymes29 and in the
industrial Haber−Bosch process,30 the preparation of various
1. INTRODUCTION
Suitable ligand frameworks can play a decisive role in steering
the reactivity of transition-metal (TM) complexes.1 In recent
years, N-heterocyclic silylenes (NHSis),1f,2 heavier analogues
of N-heterocyclic carbenes (NHCs), have been employed in
the activation of small molecules,3 low-valent main-group
chemistry,4 and homogeneous catalysis,1f,5,6 due to the unique
σ-donating/π-accepting and tunable steric hindrance proper-
ties. Most notably, the TM complexes stabilized by a variety of
silylene ligands have been recognized as superior catalysts in
the Suzuki,7 Heck,8 Negishi,9 Kumada,9−11 and Sonogashira12
coupling reactions, the regioselective borylation of arenes,13,14
the cyclotrimerization of alkynes,15 the hydrosilyation of
carbonyl compounds16−18 or olefins,19,20 the hydrogenation
of olefins21 or various ketones,22 and the transfer semi-
hydrogenation of alkynes to (E)-olefins.23 Despite the fact that
multidentate donor ligands, especially tridentate pincer-like
ligands, have the advantages of simultaneously controlling the
electronic and spatial properties of TM centers, in comparison
to the rich structural diversity of NHC and phosphine ligands,
multidentate NHSi ligands remain limited. Up to now, only a
few studies involving bis(silylene) pincer ligands have been
reported. Driess and co-workers reported the synthesis and
catalytic application of the bis(silylene)phenyl pincer ligand A
(termed SiC(sp2)Si; Chart 1)12,13,24 and the bis(silylene)-
Received: January 13, 2020
pyridine pincer ligand B (termed SiNSi; Chart 1).14,17,18
A
large amount of work has clarified that the sp3-hybridized
carbon coordinated to the metal center highly influences the
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Chart 1. Tridentate (Pincer-Type) Silylene Ligands A, B and Silylene Iron N2 Complex C
Scheme 1. Synthesis of the SiC(sp3)Si Pincer-Type Ligand L1 and Its Isomeride L2
Fe-N2 complexes and the development of iron-catalyzed N2
fixation have attracted increasing attention recently. The
Peters, Nishibayashi, and Ashley groups have disclosed
catalytic nitrogen fixation systems by using iron dinitrogen
complexes bearing P3E ligands (E = B, C, Si), anionic PNP-
type pincer ligands, and diphosphine ligands, respectively.31−33
It is noteworthy that a very low reaction temperature such as
−78 °C is necessary to slow down the direct reaction between
the reducing reagents and proton sources in these reaction
systems.
bearing a central sp3-hybridized carbon as an anchoring site. By
the reaction of L1 with Fe(PMe3)4, two bis(silylene)-based
hydrido pincer-type iron(II) complexes were prepared via
C(sp3)−H activation. Notably, the hydrido iron(II) dinitrogen
complex 1 with a bis(silylene) ligand has been found to work
as an effective catalyst for nitrogen fixation with the highest
TON so far of all the Fe-based N2 silylation catalysts at room
temperature and under atmospheric dinitrogen (up to 74.4
equiv of N(SiMe3)3 per Fe center).35,36,38,40
2. RESULTS AND DISCUSSION
As an alternative method of nitrogen fixation, the catalytic
reduction of N2 into tris(trimethylsilyl)amine (N(SiMe3)3) as
an ammonia equivalent has been greatly developed. Since
Shiina’s precursory work, several TM complexes, including Mo,
W, Ti, V, Cr, Fe, and Co, have been found to catalyze the
transformation of N2 into silylamines.34 As far as the catalytic
system containing iron is concerned, in 2012, Nishibayashi
found the pioneering example of Fe-catalyzed silylation of N2
under ambient conditions, in which several simple iron
complexes such as ferrocenes and Fe(CO)5 worked as effective
catalysts.35 In 2015, Peters reported the formation of
N(SiMe3)3 by using a two-coordinated iron complex bearing
two cyclic (alkyl)(amine)carbene ligands as a catalyst.36 After a
while, the reductive silylation of N2 catalyzed by well-defined
Fe-N2 complexes with a tetradentate P4N2 ligand and a PSiP-
pincer ligand was disclosed by the Mock and Nishibayashi
groups.37,38 In 2018, Murray and Ashley have reported Fe-
catalyzed N2 silylation systems with triiron complexes housed
within a tris(β-diketiminate)cyclophane and Fe-(PP)2(N2)
respectively.39 More recently, the first silylene-supported TM
dinitrogen complex, C (Chart 1), which worked as a catalyst to
afford up to 47 equiv of silylamine based on the Fe atom of the
catalyst, was synthesized by Cui and co-workers.40 Until now,
the preparation and reactivity of TM-N2 complexes bearing a
bis(silylene) ligand have not been reported.
2.1. Synthesis of the SiC(sp3)Si Pincer-Type Ligand.
On the basis of our interest in the multidentate NHSi ligands,
we intended to synthesize the first bis(silylene)-based SiC(sp3)
Si pincer ligand, featuring a C(sp3) atom anchor, through the
salt metathesis reactions of bis(pyrrol-2-yl)methane as the
backbone (Scheme 1). To a mixture of bis(pyrrol-2-yl)-
methane and 2 equiv of the chlorosilylene in toluene was
dropwise added 2.1 equiv of lithium bis(trimethylsilyl)amide in
a toluene solution at −55 °C to afford the desired compound
L1. L1 was isolated as colorless lamellar crystals in 37% yield
1
from diethyl ether and fully characterized by H, 13C, 29Si
NMR spectroscopy and high-resolution mass spectrometry
(HRMS). The 1H NMR spectrum of L1 shows two singlets at
1.07 and 5.56 ppm, one for the tBu groups and one
corresponding to the CH2 linkage protons. In the 29Si NMR
spectrum, a singlet resonance signal was detected at −16.4
ppm, which is comparable to that observed for the SiC(sp2)Si
ligand A and SiNSi ligand B.17,24 Moreover, the molecular
structure of L1 was determined by a preliminary X-ray
diffraction analysis (see Figure S1 in the Supporting
Information). The two N-donor-stabilized silylene moieties
have a similar orientation to form a pre-organized pincer
“pocket”, which is conducive to interaction between the metal
center and the C(sp3)−H bond. However, the metric
parameters are not sufficient for discussion because of the
poor crystal quality.
Herein, we report the facile synthesis and characterization of
a novel bis(silylene)-based SiC(sp3)Si pincer ligand (L1)
B
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Scheme 2. Synthesis of the SiC(sp3)Si Pincer-Type Iron Complexes 1 and 2
To our surprise, in the reaction of dipyrromethane and
chlorosilylene, the novel silyl-silylene compound L2 could
always be obtained as a byproduct. L2 was crystallized from n-
pentane in 17% yield. The constitution and structure of L2 was
determined by spectroscopic methods and HRMS. The 29Si
NMR spectrum of L2 exhibits two singlets at −31.8 and 39.8
ppm for the silyl substituent and low-valent silicon center,
respectively. In the proposed pathway, the first selective
metathesis reaction of deprotonated dipyrromethane with 1
equiv of chlorosilylene was completed. Then intramolecular
nucleophilic attack of pyrrole anion toward the SiII atom of
amidinate-stabilized silylene took place and resulted in the
cleavage of the Si−N bond in silylene concomitant with the
formation of a SiII−SiIV bond to furnish L2. To our knowledge,
silyl-silylene is scarcely found and usually exists only as a
reactive intermediate.41
solution was revealed by the 29Si NMR spectrum of 1, which
shows a doublet (JP−Si = 38.7 Hz) at 72.3 ppm, highlighting the
equivalency of both silicon atoms. The 29Si NMR spectrum is
significantly shifted downfield in comparison to that of L1,
which is an indication of the reduced electron density on the
silicon atom.
Single crystals of 1 suitable for X-ray analysis were obtained
from Et2O at room temperature. 1 has a distorted-octahedral
geometry around the iron atom (Figure 1). The Fe1−H bond
2.2. Synthesis of [SiCSi]-Pincer Iron(II) Hydrides 1 and
2 via C(sp3)−H Activation. With bis(silylene)-based pincer
ligand L1 in hand, we investigated its coordination ability to
iron. Surprisingly, the reaction of pincer ligand L1 with
Fe(PMe3)4 in tetrahydrofuran (THF) under an atmospheric
pressure of nitrogen gas gave a deep red reaction solution, from
which complex 1 was obtained in 33% yield (Scheme 2). In
consideration of the reaction of N,N′-bis(diphenylphosphino)-
dipyrromethane (PCH2P) with Fe(PMe3)4 reported by our
group in 2014,42 N2 coordination in complex 1 fully illustrates
the unique properties of silylene ligands in comparison with
diphenylphosphine. Given that nitrogen coordination cannot
be achieved in the metalation of the bis-silicon(II)-based
pincer ligands SiC(sp2)Si (A)12,13,24 and SiNSi (B),14,17,18 we
speculated that a central sp3-hybridized carbon as an anchoring
site has a significant effect on the properties of the complexes
and facilitates the coordination of N2. To our knowledge, 1 is
not only the first example of bis(silylene)-based hydrido pincer
iron complex produced through activation of the C(sp3)−H
bond but also the first example of a TM dinitrogen complex
supported by a bis(silylene) ligand.
The infrared spectrum of 1 exhibits a strong νNN band at
2036 cm−1 assignable to the terminal dinitrogen ligand and a
typical νFe−H stretching band at 1893 cm−1. In comparison with
some other iron nitrogen complexes,31b,38 the slightly lower
N−N stretching frequency of 1 suggests that the bis(silylene)-
based [SiCSi]-pincer ligand L1 has good electron-donating
ability to enhance back-bonding from the iron center to the
coordinated dinitrogen ligand. In the 31P NMR spectrum of 1,
the signal of the PMe3 ligand appears at 24.8 ppm as a singlet.
In the 1H NMR spectrum of 1, the characteristic hydrido signal
as a doublet peak was found at −16.80 ppm with a JP−H
coupling constant of 25.2 Hz. The Cs symmetry of 1 in
Figure 1. Molecular structure of compound 1. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms except for Fe−H
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Fe1−H 1.42(5), Fe1−P1 2.189(1), Fe1−N7 1.798(3), Fe1−Si1
2.172(1), Fe1−Si2 2.171(1), Fe1−C25 2.186(3), N7−N8 1.120(5);
P1−Fe1−C25 178.9(1), H−Fe1−N7 171(2), Si1−Fe1−Si2
142.40(4), Si1−Fe1−H 76(2), Si2−Fe1−H 69(2), Si1−Fe1−N7
106.4(1), Si2−Fe1−N7 107.0(1), Si1−Fe1−C25 82.36(9).
length of 1.42(5) Å is similar to that of the disilylene iron
hydride [(Si,N)(Si,C)Fe(H)(PMe3)].43 The N7−N8
(1.120(5) Å) and Fe1−N7 (1.798(3) Å) bond lengths lie in
the range of those in the known terminal nitrogen Fe
complexes (1.101−1.154 and 1.759−1.857 Å, respec-
tively).32,33,37−40 Both Fe−Si bonds (2.17 Å) are shorter
than those in silylene→Fe(CO)4 complexes (:Si(NtBuCH)2,
2.196 Å; OtBu(NtBu)2CPh, 2.237(7) Å)44 and comparable to
those for FeSi bonds reported by Tobita45 and Driess.17
This result indicates the formation of significant back-bonding
from Fe to Si. The above observation fully elucidates that
bis(silylene)-based pincer ligand L1 could be considered as not
only a particularly strong σ-donor but also a π-acceptor.
The successful coordination of N2 in 1 prompted us to
explore the construction of the highly electron rich hydrido
NHSi iron complexes, which have great potential application in
catalytic systems. Treatment of L1 with Fe(PMe3)4 under an
argon atmosphere in THF at room temperature for 24 h gave a
deep red reaction solution, from which orange crystals 2 were
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isolated upon workup (30% yield) (Scheme 2). The structural
details of 2 in the solid state were established by single-crystal
X-ray diffraction analysis (Figure 2). The molecular structure
It is noteworthy that the 1H, 13C, 31P, and 29Si NMR spectra
of isolated crystals of complex 2 exhibit two sets of signals
related to two flexible isomers (see the Supporting Information
1
for more details). For instance, the H NMR spectrum of 2 in
acetone-d6 exhibits two signals at −13.06 and −15.50 ppm,
two resonances at 3.98 and 3.68 ppm, four singlets at 1.03,
1.09, 1.32, and 1.38 ppm with relative ratios of 1:1.3, 1:1.3, and
18:18:(18 × 1.3):(18 × 1.3), respectively. These signals were
unambiguously assigned to hydrido, the C(sp3)−H of the
dipyrromethane moiety, and the tBu group of silylene,
respectively. In the 31P NMR spectrum of 2, four doublets,
at 15.8, 19.7, 20.2, and 23.6 ppm, could be divided into two
groups (JP−P = 41.1 Hz, 23.0 Hz). The fluxional behavior of 2
in solution is also consistent with the information from infrared
(IR) spectra. The infrared spectrum of the pure crystals of 2
shows a weak absorption at 1890 cm−1 assignable to Fe−H.
However, two weak Fe−H bands appear at 1883 and 1841
cm−1 in a C6D6 solution. The result also shows that 2 has two
isomers in solution.
2.3. Mutual Transformation between 1 and 2 and
Kinetic Study. Complex 1 appears to be very stable in the
solid state as well as in solution under an N2 atmosphere.
Although complex 2 has a very robust structure in the solid
state under an argon atmosphere, it will dissociate the PMe3
ligand very slowly in solution. The signal of dissociated PMe3
could be detected at −61.7 ppm after 2 was dissolved in C6D6
for 20 h. The slight instability of 2 in solution can be
rationalized by the strong trans effect of the hydride ligand and
steric hindrance, as indicated by a space-filling model of 2 (see
Figure S2 in the Supporting Information). Encouraged by
these results, we have achieved the interconversion between 1
and 2 by the selection of suitable reaction conditions (Figure
3). Treatment of 1 with PMe3 (excess) in THF under an argon
atmosphere afforded 2 through the ligand replacement of N2
by PMe3. When a solution of 2 was exposed to N2 in THF at
Figure 2. Molecular structure of compound 2. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms except for Fe−H
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Fe1−H1 1.37(2), Fe1−P1 2.2097(7), Fe1−P2 2.1738(7), Fe1−Si1
2.1758(7), Fe1−Si2 2.1787(7), Fe1−C20 2.216(2); H1−Fe1−P1
174.7(9), Si1−Fe1−Si2 133.65(3), P2−Fe1−C20 174.59(6), H1−
Fe1−Si1 71.5(9), H1−Fe1−Si2 66.3(9), P1−Fe1−Si1 110.10(3),
P1−Fe1−Si2 110.42(3), C20−Fe1−Si2 79.85(6).
of 2 shows characteristics similar to those of 1. Owing to the
greater steric congestion around the iron center, 2 has a more
distorted hexacoordinate octahedral geometry with
[P2Si1C20Si2] as the equatorial plane. The axial Fe1−P1
bond (2.210 Å) features a slightly longer bond distance in
comparison to the Fe1−P2 bond (2.174 Å) situated at the
equatorial position, presumably due to the strong trans
influence of the hydrido H ligand. The Fe1−H1 bond of
1.37(2) Å is slightly shorter than that in 1.
Figure 3. (top) Mutual transformation between 1 and 2 under specific conditions. (bottom) 31P NMR spectra (left, 1 to 2; right, 2 to 1) recorded
for the corresponding reaction in THF at specific times.
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Figure 4. 3D-kinetic profile of the reaction of complex 1 with 30 equiv of PMe3 in 5 mL of THF at various temperatures: (a) 273 K; (b) 293 K; (c)
307 K. The initial concentration of 1 was 0.02338 mol L−1.
room temperature, 1 was obtained by ligand exchange. As
shown in Figure 3, we recorded the 31P NMR spectra of both
transformations at the given times to monitor the progress.
Remarkably, the reactions were fully completed at last, as
judged from a 31P NMR study.
Intrigued by the interesting equilibrium between 1 and 2, we
were curious about how it would affect the rate if the
concentration of PMe3 were to be changed. In the trans-
formation from 1 to 2, the kinetic data for different initial
concentrations of PMe3 at the early stage of the reaction were
recorded by the 31P NMR spectra (see Table S2 and Figure S3
in the Supporting Information). The plot of υ(2) against the
concentration of PMe3 showed that υ(2) apparently depended
on [PMe3]. According to the plot of the log υ(2) vs
log[PMe3], the reaction order with respect to the PMe3 was
estimated to be ca. 1.
In order to gain insight into the transformation of 1 into 2,
the reaction between 1 and PMe3 was monitored at various
temperatures using operando IR (Figure 4 and the Supporting
Information). As can be seen from the kinetic profiles of
relative absorbance, when 1 was mixed with 30 equiv of PMe3
at 273, 293, 296, 305, and 307 K, the absorbance of the
terminal dinitrogen ligand decreased gradually. The plot of the
integrated absorbance (ln A) of N2 against time suggests a
linear relationship, which confirms that it is a pseudo-first-
order reaction (see Figures S4−S8 in the Supporting
Information). The values of the rate constant kobs and t1/2 at
various temperatures are summarized in Table 1. The reaction
influence catalytic activity toward nitrogen fixation,36,37 we
further believe that SiCSi-pincer iron dinitrogen complex 1
could be an effective catalyst toward the silylation of N2.
2.4. Catalytic Nitrogen Fixation Using 1 as Catalyst.
The investigation of catalytic silylation of dinitrogen gas was
carried out by using complex 1 as a catalyst under ambient
reaction conditions. The results are summarized in Table 2.
Table 2. Catalytic Silylation of Molecular Dinitrogen Using
a
Complex 1
reductant
(M)
T
reductant/ClSiMe3
N(SiMe3)3
bc
,
b
entry
solvent
THF
(°C)
(equiv)
(equiv)
1
2
3
4
5
Li
Na
K
KC8
KC8
25
25
25
25
25
600
600
600
600
600
trace
1.3
2.9
10.5
4.7
THF
THF
THF
n-
pentane
6
7
8
9
10
11
12
13
14
KC8
KC8
KC8
KC8
KC8
KC8
KC8
KC8
KC8
KC8
KC8
toluene
Et2O
DME
25
25
25
25
50
80
25
25
25
25
25
600
600
600
600
600
600
600
600
600
1800
1800
5.3
6.5
6.6
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
20.8
19.9
19.7
trace
13.4
trace
40.3
74.4
d
e
Table 1. Pseudo-First-Order Rate Constants and Half-Lives
of the Reaction at Different Temperatures
f
15
16
g
entry
temp (K)
kobs (s−1
)
t1/2 (h)
a
3.7 × 10−6
2.1 × 10−5
3.4 × 10−5
1.3 × 10−4
1.7 × 10−4
51.7
9.3
5.7
1.5
1.1
Experiments performed over 20 h in 20 mL of solvent using 0.01
mmol of the catalyst under N2, unless otherwise stated. Yields are an
1
2
3
4
5
273
293
296
305
307
b
c
d
average. Based on the catalyst. Determined by GC. Using 0.01
e
f
mmol of free ligand L1. Using 0.01 mmol of 2 instead of 1. Using
0.01 mmol of [PCHP]Fe(H)(PMe3)41 instead of 1. Conducted for
g
150 h.
rate was strongly affected by temperature, and t1/2 increased to
51.7 h from 1.1 h. According to the Arrhenius equation, the
activation energy Ea for the ligand exchange in the reaction of 1
to 2 was determined to be ca. 77 kJ/mol. The comparatively
large Ea value provides evidence for strong coordination of the
N2 ligand in 1. This is consistent with the remarkable stability,
no matter whether in the solid state or in solution. Given that
the N2 binding affinity is a very important parameter that can
The reaction of N2 (1 atm) with KC8 (600 equiv based on 1)
as a strong reductant and Me3SiCl (600 equiv based on 1) as
the silylating reagent in the presence of 1 as catalyst in THF at
room temperature for 20 h gave 10.5 equiv of N(SiMe3)3
based on the iron atom (Table 2, entry 4). The N(SiMe3)3 was
confirmed as a product by gas chromatography (GC) and
HRMS. Using various alkali metals (Li, Na, K) in place of KC8,
a lower amount of N(SiMe3)3 was produced because of the
E
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weaker reducing ability or smaller specific surface area (Table
2, entries 1−3). It is worth noting that the solvent polarity has
a great influence on the yields (Table 2, entries 4−9). The
reaction in dioxane afforded the highest yield (20.8 equiv). In
comparison with the reaction in n-pentane, the yield was
increased by 4 times. Interestingly, the reaction temperature
has little influence on the yield (Table 2, entries 9−11). The
control experiments indicate that complex 1 is necessary for
the catalytic formation of N(SiMe3)3 (Table 2, entries 12). A
13.4 equiv amount of N(SiMe3)3 was produced when complex
2 was used as the catalyst (Table 2, entry 13). The poor
catalytic behavior of [PCHP]Fe(H)(PMe3)42 fully illustrated
the significant role of the bis(silylene)dipyrromethane ligand
(Table 2, entry 14). With a larger amount of KC8 and Me3SiCl,
the catalytic reactions gave 40.3 equiv (a 7% yield, based on
KC8) in 20 h and 74.4 equiv (a 13% yield, based on KC8) of
N(SiMe3)3 in 150 h per Fe atom. This represents a very high
TON in comparison to the Fe-based N2 silylation catalysts
reported to date (Table 2, entries 15 and 16).35−38,40 We
monitored the time profile of the catalytic reactions using 1 as
the catalyst (see the Supporting Information for the detailed
procedure). The result is shown in Figure 5. The catalytic
formation of N(SiMe3)3 proceeded rapidly and was almost
complete after 150 h.
respectively),39 a very low reaction temperature or an elevated
pressure must be used to increase yields. In all of the Fe-
catalyzed N2 silylation systems at room temperature and with
atmospheric dinitrogen, complex 1 gave the highest turnover
number to date.
3. CONCLUSIONS
In summary, the novel bis(silylene)-based SiC(sp3)Si pincer
ligand L1 bearing a central sp3-hybridized carbon as an
anchoring site was synthesized by a salt metathesis reaction of
chlorosilylene with deprotonated dipyrromethane. Meanwhile,
the rare silyl-silylene complex L2 was obtained as a byproduct.
By the reactions of L1 with Fe(PMe3)4 under different inert
atmospheres (N2 and argon), two novel iron hydride
complexes, [SiCHSi]Fe(H)(N2)(PMe3) (1) and [SiCHSi]Fe-
(H)(PMe3)2 (2), were synthesized via C(sp3)−H bond
activation. To our knowledge, 1 and 2 are the first examples
of bis(silylene)-based hydrido pincer iron complexes produced
through activation of the C(sp3)−H bond; at the same time, 1
is also the first example of a TM dinitrogen complex supported
by a bis(silylene) ligand. Mutual transformations between
complexes 1 and 2 in solution were achieved by the selection
of suitable reaction conditions. Operando IR was used to
monitor the whole reaction process to gain insight into the
transformation of 1 into 2. The activation energy Ea for the
ligand exchange in the reaction of 1 to 2 was determined to be
77 kJ/mol, which is comparatively large, providing an evidence
for robust coordination of the N2 ligand in 1. Remarkably, the
hydrido iron nitrogen complex 1 with a bis(silylene) ligand
could be an effective catalyst for nitrogen fixation. 1 presents
the highest turnover number to date of all the Fe-catalyzed N2
silylation systems at room temperature and under atmospheric
dinitrogen (1 atm). Kinetic studies demonstrated that the
reaction order with respect to the Fe was estimated to be ca. 1.
We believe that our work will provide some new strategies and
strongly promote the further development of N-heterocyclic
silylene chemistry and nitrogen fixation under mild conditions
catalyzed by transition-metal dinitrogen complexes. Detailed
mechanistic studies and further modifications of the ligand are
currently being explored in our laboratory.
Figure 5. Time profile of the formation of N(SiMe3)3 using 1 as
catalyst.
4. EXPERIMENTAL SECTION
A filtration test, whereby insoluble and soluble specimens
were separated by filtration and independently assayed for
catalytic activity, was conducted to exclude the possibility that
Fe nanoparticles work as true active species in the present
reaction system (see the Supporting Information for the
detailed procedure).
Encouraged by these experimental results, we were
interested in the investigation of the catalytic behavior from
the viewpoint of a kinetic study (see the Supporting
Information for more details). The kinetic data for different
initial concentrations of catalyst 1 at the early stage of the
reaction were calculated (Table S4 and Figure S10). The plot
of υ(N(SiMe3)3) against the concentration of Fe showed that
υ(N(SiMe3)3) apparently depended on [Fe] when complex 1
was used as a catalyst. According to a plot of log υ(N(SiMe3)3)
vs log [Fe], the reaction order with respect to the Fe was
estimated to be ca. 1. Unfortunately, no reactive intermediates
have yet been isolated from the stoichiometric reaction of 1
with KC8 and Me3SiCl.
4.1. General Considerations. Infrared spectra (4000−400 cm−1)
were recorded on a Bruker ALPHA FT-IR instrument by using Nujol
1
mulls between KBr disks. The H, 13C, 31P, and 29Si NMR spectra
were recorded with a Bruker 300 spectrometer. High-resolution mass
spectra (HRMS) were obtained using an electrospray ionization time-
of-flight (ESI-TOF) mass spectrometer. Melting points (mp) were
measured on a WRR instrument with samples sealed in capillaries.
Gas chromatography (GC) was performed with n-dodecane as an
internal standard. All experiments and manipulations were carried out
under a nitrogen atmosphere using standard Schlenk techniques,
unless otherwise noted. All solvents were dried by general methods
and freshly distilled before use. Chlorosilylene,46 Fe(PMe3)4,47 and
48
KC8 were prepared according to the reported procedures.
4.2. Synthesis of Pincer-Type Ligand L1 and Silyl-Silylene
L2. To a solution of dipyrromethane (1.68 g, 11.3 mmol) and
chlorosilylene (6.68 g, 22.7 mmol) in toluene (80 mL) was added a
solution of LiN(SiMe3)2 (3.98 g, 23.8 mmol) in toluene (70 mL) at
−55 °C under N2. After the mixture was stirred for 1 h, the resulting
solution was warmed to room temperature and stirred overnight. The
solvents were removed under vacuum, and the residue was extracted
with n-pentane and Et2O. The organic solutions were concentrated
slowly in vacuo and further cooled in the freezer at −20 °C.
Although Murray and Ashley have reported Fe-catalyzed N2
silylation systems with higher yields (83 and 121 equiv,
F
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L1 was obtained as colorless lamellar crystals from diethyl ether
(2.76 g, 37%). Mp: 69−70 °C. 1H NMR (300 MHz, C6D6, 298 K): δ
(ppm) 1.07 (s, 36H, NC(CH3)3), 5.56 (s, 2H, −CH2−), 6.62 (d, J =
18.0 Hz, 4H, arom, C−H), 6.90−6.96 (m, 8H, arom, C−H), 7.05−
7.07 (m, 2H, arom, C−H), 7.13 (br s, 2H, arom, C−H). 13C NMR
(75 MHz, C6D6, 298 K): δ (ppm) 15.3 (−CH2), 31.3 (NC(CH3)3),
53.0 (NC(CH3)3), 109.4 (Carom), 110.1 (Carom), 119.9 (Carom), 127.5
(Carom), 129.3 (Carom), 129.9 (Carom), 133.9 (Carom), 139.5 (Carom),
163.9 (NCN). 29Si NMR (59.59 MHz, C6D6, 298 K): δ (ppm) −16.4
(s). HRMS (ESI-TOF): 663.4026 [M + H]+; calcd for C39H55N6Si2,
663.4027. Anal. Calcd for C39H54N6Si2: C, 70.65; H, 8.21; N, 12.67.
Found: C, 70.29; H, 8.10; N, 12.48..
15.8 (d, J = 41.1 Hz, 1.3 × 1P, PMe3), 23.6 (d, J = 41.1 Hz, 1.3 × 1P,
PMe3). 13C NMR (75 MHz, acetone-d6, 298 K): one conformer: δ
(ppm) 12.9 (d, J = 15 Hz, PMe3), 17.4 (d, J = 15 Hz, PMe3), 30.4
(−FeCH−), 31.2 (NC(CH3)3), 31.5 (NC(CH3)3), 53.5 (NC-
(CH3)3), 53.9 (NC(CH3)3), 102.5 (Carom), 111.3 (Carom), 111.7
(Carom), 127.2 (Carom), 127.9 (Carom), 128.1 (Carom), 129.6 (Carom),
130.2 (Carom), 133.1 (Carom), 155.4 (Carom), 172.0 (NCN); another
conformer, δ (ppm) 24.8 (d, J = 15 Hz, PMe3), 26.2 (d, J = 15 Hz,
PMe3), 30.6 (-FeCH−), 31.9 (NC(CH3)3), 32.5 (NC(CH3)3), 54.1
(NC(CH3)3), 54.3 (NC(CH3)3), 103.3 (Carom), 111.6 (Carom), 112.1
(Carom), 127.6 (Carom), 127.8 (Carom), 128.2 (Carom), 130.0 (Carom),
130.9 (Carom), 133.3 (Carom), 156.1 (Carom), 172.1 (NCN). 29Si NMR
(59.59 MHz, acetone-d6, 298 K): one conformer, δ (ppm) 63.4 (dd, J
= 40.5, 20.3 Hz); another conformer, δ (ppm) 79.2 (dd, J = 41.1, 32.8
Hz). HRMS (ESI-TOF): 871.4230 [M + H]+; calcd for
C45H73FeN6P2Si2, 871.4260. Anal. Calcd for C45H72FeN6P2Si2: C,
62.05; H, 8.33; N, 9.65. Found: C, 62.27; H, 8.40; N, 9.75.
4.5. X-ray Crystal Structure Determinations. A Bruker Apex II
single crystal diffractometer employed Mo Kα radiation (λ = 0.71073
Å) or Ga Kα radiation (λ = 1.34143) and a CCD area detector. The
structure was solved using the charge-flipping algorithm, as
implemented in the program SUPERFLIP,49 and refined by full-
matrix least-squares techniques against F2 (SHELXL)50 through the
OLEX interface.51 All non-hydrogen atoms were refined anisotropi-
cally, and all hydrogen atoms except for those of the disordered
solvent molecules were placed using AFIX instructions. Appropriate
restraints or constraints were applied to the geometry and the atomic
displacement parameters of the atoms. CCDC 1846424 (1) and
1876056 (2) contain supplementary crystallographic data for this
paper. Copies of the data can be obtained free of charge on
application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.
(fax, (+44)1223-336-033; e-mail, deposit@ccdc.cam.ac.uk).
L2 was crystallized from n-pentane as a green virgulate crystal (1.26
1
g, 17%). Dec pt: >133 °C. H NMR (300 MHz, C6D6, 298 K): δ
(ppm) 1.08 (s, 18H, NC(CH3)3), 1.22 (s, 9H, NC(CH3)3), 1,43 (s,
9H, NC(CH3)3), 4.41 (dd, J = 33.9, 17.4 Hz, 2H, −CH2−), 6.31 (br
s, 2H, arom, C−H), 6.50 (t, J = 3.0 Hz, 2H, arom, C−H), 6.85−6.88
(m, 1H, arom, C−H), 6.94−7.03 (m, 4H, arom, C−H), 7.08−7.15
(m, 3H, arom, C−H), 7.48−7.50 (m, 2H, arom, C−H), 7.66 (br s,
2H, arom, C−H). 13C NMR (75 MHz, C6D6, 298 K): δ (ppm) 27.1
(−CH2−), 31.1 (NC(CH3)3), 53.6 (NC(CH3)3), 107.1 (Carom),
109.9 (Carom), 124.5 (Carom), 127.2 (Carom), 127.4 (Carom), 128.3
(Carom), 129.2 (Carom), 129.4 (Carom), 131.0 (Carom), 132.9 (Carom),
134.5 (Carom), 141.9 (Carom), 157.7 (N−C = N), 161.7 (NCN). 29Si
NMR (59.59 MHz, C6D6, 298 K): δ (ppm) 39.8 (s, Si:), −31.8 (s,
Si). HRMS (ESI-TOF): 663.4037 [M + H]+; calcd for C39H55N6Si2,
663.4021. Anal. Calcd for C39H54N6Si2: C, 70.65; H, 8.21; N, 12.67.
Found: C, 70.32; H, 8.12; N, 12.50.
4.3. Synthesis of SiCSi Pincer-Type Iron Dinitrogen
Complex 1. A solution of ligand L1 (2.17 g, 3.3 mmol) in THF
(75 mL) was added slowly to a solution of Fe(PMe3)4 (1.41 g, 3.9
mmol) in THF (75 mL) at −78 °C under N2. The reaction mixture
was warmed to room temperature and stirred for 24 h, resulting in a
dark red solution. The volatiles were removed in vacuo, and the
residue was extracted with pentane and diethyl ether. Complex 1
(0.89 g) was isolated as orange block crystals in 33% yield at room
temperature. Dec pt: >184 °C. IR (Nujol mull, KBr, cm−1): 2036
ν(NN), 1893 ν(Fe−H), 942 ρ(PMe3). 1H NMR (300 MHz, C6D6,
298 K): δ (ppm) −16.80 (d, J = 25.2 Hz, 1H, Fe−H), 1.09 (s, 18H,
NC(CH3)3), 1.21 (s, 18H, NC(CH3)3), 1.61 (d, J = 6.6 Hz, 9H,
PMe3), 4.14 (s, 1H, −FeCH−), 6.86−7.00 (m, 13H, arom, C−H),
7.20−7.26 (m, 3H, arom, C−H). 31P{1H} NMR (121 MHz, C6D6,
298 K): δ (ppm) 24.8 (s, PMe3). 13C NMR (75 MHz, C6D6, 298 K):
δ (ppm) 25.6 (d, J = 18.8 Hz, PMe3), 29.9 (-FeCH−), 31.3
(NC(CH3)3), 31.5 (NC(CH3)3), 53.5 (NC(CH3)3), 54.3 (NC-
(CH3)3), 105.6 (Carom), 113.2 (Carom), 114.4 (Carom), 126.9 (Carom),
129.5 (Carom), 129.7 (Carom), 132.4 (Carom), 157.0 (Carom), 171.4
(NCN). 29Si NMR (59.59 MHz, THF (D2O), 298 K): δ (ppm) 72.3
(d, J = 38.7 Hz). HRMS (ESI-TOF): 823.3906 [M + H]+; calcd for
C42H64FeN8PSi2, 823.3880. Anal. Calcd for C42H63FeN8PSi2: C,
61.29; H, 7.72; N, 13.62. Found: C, 61.51; H, 7.86; N, 13.51.
4.4. Synthesis of Electron-Rich NHSi Iron Hydride Complex
2. The synthesis of 2 proceeded in a fashion similar to that for the
synthesis of 1 except under an argon atmosphere, with L1 (1.82 g, 2.8
mmol) and Fe(PMe3)4 (1.19 g, 3.3 mmol). Complex 2 (0.72 g) was
isolated as orange-red block crystals in 30% yield at room
temperature. Dec pt: >167 °C. IR: in solid (Nujol mull, KBr,
cm−1), 1890 ν(Fe−H); in solution (C6D6), 1883 and 1841 ν(Fe−H).
1H NMR (300 MHz, acetone-d6, 298 K): one conformer, δ (ppm)
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00025.
X-ray crystallographic data, operando IR, kinetic study,
IR and NMR data, nitrogen fixation (PDF)
Accession Codes
CCDC 1846424 and 1876056 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Xiaoyan Li − School of Chemistry and Chemical Engineering,
Key Laboratory of Special Functional Aggregated Materials,
Ministry of Education, Shandong University, Jinan 250100,
People’s Republic of China; orcid.org/0000-0003-0997-
0380; Email: xli63@sdu.edu.cn
−13.06 (d, J = 49.5 Hz, 1H, Fe−H), 0.82 (d, J = 3.6 Hz, 9H, PMe3),
1.03 (s, 18H, NC(CH3)3), 1.09 (s, 18H, NC(CH3)3), 1.50 (d, J = 4.2
Hz, 9H, PMe3), 3.98 (s, 1H, −FeCH−); another conformer, δ (ppm)
−15.50 (dd, J = 27.6, 2.4 Hz, 1.3 × 1H, Fe−H), 1.32 (s, 1.3 × 18H,
NC(CH3)3), 1.34 (overlap, 1.3 × 9H, PMe3), 1.38 (s, 1.3 × 18H,
NC(CH3)3), 1.64 (d, J = 5.4 Hz, 1.3 × 9H, PMe3), 3.68 (d, J = 12.3
Hz, 1.3 × 1H, −FeCH−); aromatic area (16 + 1.3 × 16), 5.79 (br s,
2H), 6.08 (br s, 6H), 6.60 (d, J = 16.8 Hz, 4H), 7.22−7.32 (m, 2H),
7.51−7.61 (m, 18H), 7.80−7.87 (m, 5H). 31P{1H} NMR (121 MHz,
acetone-d6, 298 K): one conformer, δ (ppm) 19.7 (d, J = 23.0 Hz, 1P,
PMe3), 20.2 (d, J = 23.0 Hz, 1P, PMe3); another conformer, δ (ppm)
Authors
Shengyong Li − School of Chemistry and Chemical Engineering,
Key Laboratory of Special Functional Aggregated Materials,
Ministry of Education, Shandong University, Jinan 250100,
People’s Republic of China
Yajie Wang − School of Chemistry and Chemical Engineering,
Key Laboratory of Special Functional Aggregated Materials,
Ministry of Education, Shandong University, Jinan 250100,
People’s Republic of China
G
https://dx.doi.org/10.1021/acs.organomet.0c00025
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pubs.acs.org/Organometallics
Article
Homo- and Heterocoupling of Carbon Monoxide. J. Am. Chem. Soc.
2019, 141, 626−634.
Wenjing Yang − School of Chemistry and Chemical Engineering,
Key Laboratory of Special Functional Aggregated Materials,
Ministry of Education, Shandong University, Jinan 250100,
People’s Republic of China
Kai Li − School of Chemistry and Chemical Engineering, Key
Laboratory of Special Functional Aggregated Materials,
Ministry of Education, Shandong University, Jinan 250100,
People’s Republic of China
Hongjian Sun − School of Chemistry and Chemical Engineering,
Key Laboratory of Special Functional Aggregated Materials,
Ministry of Education, Shandong University, Jinan 250100,
People’s Republic of China; orcid.org/0000-0003-1237-
3771
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Olaf Fuhr − Institut fu
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Dieter Fenske − Institut fur Nanotechnologie (INT) und
Karlsruher Nano-Micro-Facility (KNMF), Karlsruher Institut
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSF China 21971151/21572119
and NSF Shandong ZR2019ZD46.
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