Cyclic (Alkyl)(amino)carbene Lanthanide Amides: Synthesis, Structure, and Catalytic Selective Hydrosilylation of Alkenes

Article abstract of DOI:10.1021/acs.inorgchem.1c01780

The first examples of cyclic (alkyl)(amino)carbene (CAAC) lanthanide (Ln) complexes were synthesized from the reaction of CAAC with Yb[N(SiMe3)2]2 and Eu[N(SiMe3)2]2(THF)2 (THF = tetrahydrofuran). The structures of (CAAC)Yb[N(SiMe3)2]2 (2) and (CAAC)Eu[N(SiMe3)2]2(THF) (3) were determined by X-ray diffraction analysis. Density functional theory calculations of 2 revealed the predominantly ionic bond between the Ln ion and CAAC. Complex 3 enabled catalytic hydrosilylation of aryl- and silylalkenes with primary and secondary silanes in high yields and Markovnikov selectivity.

Full text of DOI:10.1021/acs.inorgchem.1c01780

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Cyclic (Alkyl)(amino)carbene Lanthanide Amides: Synthesis,
Structure, and Catalytic Selective Hydrosilylation of Alkenes
Zexiong Pan, Jianying Zhang, Lulu Guo, Hao Yang, Jianfeng Li,* and Chunming Cui*
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ABSTRACT: The first examples of cyclic (alkyl)(amino)carbene (CAAC) lanthanide (Ln) complexes were synthesized from the
reaction of CAAC with Yb[N(SiMe3)2]2 and Eu[N(SiMe3)2]2(THF)2 (THF = tetrahydrofuran). The structures of
(CAAC)Yb[N(SiMe3)2]2 (2) and (CAAC)Eu[N(SiMe3)2]2(THF) (3) were determined by X-ray diffraction analysis. Density
functional theory calculations of 2 revealed the predominantly ionic bond between the Ln ion and CAAC. Complex 3 enabled
catalytic hydrosilylation of aryl- and silylalkenes with primary and secondary silanes in high yields and Markovnikov selectivity.
anthanide (Ln) amides have attracted a great deal
attention as catalysts for organic and polymer synthesis
hydrosilylated to generate the Markovnikov products in high
yields with high regioselectivity.
L
during the last 2 decades because of their unique activity and
selectivity.1 Many types of anionic spectator ligands have been
designed to tune their stability and reactivity.2 The readily
available homoleptic amides Ln[N(SiMe3)2]n (n = 2, 3) have
also been applied to some catalytic reactions.3 Recently, we
showed that neutral N-heterocyclic carbenes (NHCs) could
bring remarkable improvement in the activity and selectivity of
the lanthanide bis(amido) complex Ln[N(SiMe3)2]2.4 Trifo-
nov and co-workers also reported the synthesis and catalytic
behaviors of NHC complexes of Ln[N(SiMe3)2]2.5
Cyclic (alkyl)(amino)carbenes (CAACs) represent different
types of carbenes with great prospects developed in recent
years.6 Because of their unique electronic structures, CAACs,
as both strong σ donors and π acceptors, exhibited some
advantages in the stabilization of low-coordinated main-group
compounds and transition-metal complexes7,8 and in homoge-
neous catalysis.9 However, to the best of our knowledge, there
are no reports on CAAC lanthanide complexes.4,5,10 In this
paper, CAAC complexes of divalent ytterbium and europium
amides (2 and 3; Scheme 1) have been synthesized and
structurally characterized. The bonding between the Ln ions
and CAAC was studied by X-ray diffraction analysis and
density functional theory (DFT) calculations. Remarkably,
CAAC-EuII complex 3 was found to catalyze the hydro-
silylation of alkenes with primary and secondary silanes. The
aryl-, (arylsilyl)-, (alkylsilyl)-, and (hydrosilyl)alkenes were
The reaction of solvent-free Yb[N(SiMe3)2]2 with CAAC
(1; Scheme 1) yielded (CAAC)Yb[N(SiMe3)2]2 (2; 53%
yield) as dark-green crystals. It is noted that no reaction was
observed between Yb[N(SiMe3)2]2(THF)2 and 1. In contrast,
the reaction of 1 with Eu[N(SiMe3)2]2(THF)2 (EuN) yielded
(CAAC)Eu[N(SiMe3)2]2(THF) (3) as yellow crystals in 49%
yield. It is assumed that the reaction of Ln[N-
(SiMe3)2]2(THF)2 with 1 is very likely to go through a
nucleophilic substitution mechanism. The relatively small Yb
ion (1.02 and 1.17 Å for YbII and EuII in six coordination) in
Yb[N(SiMe3)2]2(THF)2 could not be easily accessed by bulky
CAAC.1a
1
The structure of 2 was characterized by H and 13C NMR
and X-ray diffraction analysis. In the 13C NMR spectrum of 2,
the characteristic resonance of the carbenic carbon appeared at
δ = 288.4 ppm. The molecular structure of 2 is depicted in
Figure 1. The Yb atom is tricoordinated and has a trigonal-
pyramidal geometry. The Yb center deviates slightly from the
plane N(3)−N(2)−C(1) (0.459 Å). The trigonal-pyramidal
geometry of 2, instead of a planar geometry, may optimize the
overall crystal packing by minimizing the intermolecular
contacts.11 The Yb(1)−C(1) bond length of 2.672(3) Å is
longer than that observed in the tricoordinated NHC-YbII
bis(amido) complex (IMes)Yb[N(SiMe3)2]2 [IMes = 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; 2.600(3) Å].4a
The Yb−N bond lengths [2.321(2) and 2.327(2) Å] in 2
are similar to those [2.317(3) and 2.323(2) Å] in (IMes)Yb-
[N(SiMe3)2]2. The angle of N(2)−Yb(1)−N(3) [116.97(8)°]
is slightly smaller than that [122.21(9)°] in (IMes)Yb[N-
Scheme 1. Synthesis of CAAC-LnII Bis(amido) Complexes
Received: June 12, 2021
https://doi.org/10.1021/acs.inorgchem.1c01780
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Figure 3. DFT-computed HOMO−5 and LUMO of complex 2.
Figure 1. ORTEP representation of the X-ray structure of 2.
indicate that the Yb−CCAAC bond can be represented by
LP(CCAAC) with 93% C 2s2p1.54 and 4% Yb 6s6p0.035d0.67
contributions with a Wiberg bond index of 0.132. The
primarily electrostatic interaction is expected, given that a
hard metal is interacting with a soft and polarizable carbene.13
The lowest unoccupied molecular orbital (LUMO) of 2 is
mainly the antibonding π orbital of the C(1)−N(1) bond in
the CAAC.
(SiMe3)2]2, showing a larger steric hindrance of CAAC than of
IMes.
Because of the paramagnetic nature of the divalent Eu ion,
no useful information could be obtained from the NMR
spectrum of 3. The structure of 3 was characterized by
elemental and X-ray diffraction analyses. The Eu atom of 3
features a distorted tetrahedral geometry (Figure 2). The Eu−
The UV−visible absorption spectra of complexes 2 and 3
(Figure 4) displayed intense absorptions at wavelengths
Figure 2. ORTEP representation of the X-ray structure of 3.
CCAAC distance [2.945(3) Å] is longer than the Yb−CCAAC
distance because of the larger EuII ion and higher coordination
number. In addition, this distance is much longer than those
found in the tri- and tetracoordinated NHC-EuII bis(amido)
complexes (2.722−2.808 Å).4c Both complexes 2 and 3 feature
longer Ln−CCAAC distances than the corresponding ones in
NHC-LnII bis(amido) complexes, indicating the relatively
weak Ln−CCAAC bonding. The Eu−N bond lengths of
2.460(3) and 2.479(3) Å in 3 are comparable to those
observed in tetracoordinated complexes (IMe)2Eu[N-
(SiMe3)2]2 and (IiPr)2Eu[N(SiMe3)2]2 [2.479(5)−
2.4859(11) Å].
To explore the nature of the Ln−CCAAC bond, DFT
calculations were undertaken using the PBE1PBE functional
for complex 2. The computationally optimized structure
(Table S4) of 2 was in very good agreement with the
crystallographically obtained structure. Natural bond orbital
analyses (Figure S1) supported that most of the positive charge
is located on the Yb center, portraying a predominantly ionic
bond.12 On the basis of analysis of the frontier molecular
orbital for 2, a lone pair (LP) of the carbene can be observed in
the highest occupied molecular orbital (HOMO)−5 (Figure
3), which donated to the highly electropositive metal center.
The natural localized molecular orbital (NLMO) calculations
Figure 4. UV−visible absorption spectra of 2 and 3 in n-hexane (1
mM).
shorter than 300 nm. The low-energy absorption of 2 (643
nm) was observed with a lower extinction coefficient (ε = 212
M−1 cm−1) in the visible region, which is in agreement with the
green color of the solution of 2. The absorption of 643 and 316
nm for complexes 2 and 3 could be assigned to a Laporte-
allowed 4f−5d transition of LnII ions.14
Metal-catalyzed hydrosilylation of alkenes represents the
most straightforward and atom-economic protocol for the
synthesis of various organosilanes, which have played
important roles in organic and materials chemistry.15 With
the CAAC lanthanide amides in hand, we investigated their
catalytic applications for hydrosilylation. In the first place, the
catalytic hydrosilylation of styrene (4a) with primary silane
PhSiH3 (5) was selected as the model reaction for optimization
of the conditions (Table 1, entries 1−6). A total of 1 mol % 2
(entry 1) could catalyze the hydrosilylation of 4a in good
conversion (72%) at room temperature in 0.5 h, but the anti-
Markovnikov product (7a′) and double hydrosilylation
product (7aa) were also yielded in about 6% selectivity
B
https://doi.org/10.1021/acs.inorgchem.1c01780
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a
a
Table 1. Optimization of Reaction Conditions
Table 2. EuII-Catalyzed Hydrosilylation with 5
b
loading
(%)
conv
b
7a/7a′/7aa or 8a/8a′/
yield
c
b
entry catalyst
(%)
8aa
(%)
1
2
3
4
5
6
7
8
2
3
3
EuN
1
3
3
2
1
1
72
>99
>99
40
0
49
94
0
94/4/2
99/0/1
99/0/1
99/0/1
55
87
87
30
0.5
0.5
0.5
0.5
1
d
e
f
93/0/7
91/0/9
37
73
f
1
a
0.01 mmol of catalyst, 0.4 mL of solvent, 1.0−2.0 mmol of 4a, and
1.2−2.4 mmol of silane (5 for entries 1−6 and 6 for entries 7 and 8),
b
23 °C, 0.5 h. The conversions of 4a and selectivity were determined
by gas chromatography (GC)−mass spectrometry (MS) with a crude
c
d
reaction mixture. Isolated yields of 7a and 8a. EuN = Eu[N-
e
f
(SiMe3)2]2(THF)2. THF as the solvent. 80 °C, 6 h.
besides the major Markovnikov product (7a). Under the same
reaction conditions, 3 (entry 2) could enable highly efficient
and selective hydrosilylation reaction, leading to the
quantitative production of 7a in 99% selectivity even in 0.5
mol % catalyst loading (entry 3). For comparison, EuN and 1
were also examined as catalysts. Although EuN displayed high
selectivity (99%), only 40% of 4a could be hydrosilylated
(entry 4), while CAAC was inactive for this hydrosilylation
(entry 5). 4a was hydrosilylated in moderate conversion (49%)
in THF (entry 6) probably because of the interactions of the
solvent molecules with Eu ion, which suppresses the
coordination−insertion process of alkenes. To our delight,
catalytic hydrosilylation of 4a with the secondary silane
Ph2SiH2 (6) was also carried out efficiently (94% conversion
and 91% selectivity) to yield the Markovnikov product 8a
(entry 7). In sharp contrast, 2 has no activity for the same
reaction (entry 8). Other silanes were tested in 4a hydro-
silylation (Table S1). n-C6H13SiH3 and PhMeSiH2 exhibited
moderate activity (62 and 70% conversions) and good-to-poor
selectivity (94 and 74%). Et2SiH2 was inactive with catalyst 3.
Under the optimized conditions with 5 as the silylation
reagent, a range of aryl- and silylalkenes (4a−4t) were
examined. As shown in Table 2, the secondary silane products
(7a−7t) were generated in good-to-high yields (62−95%) and
high Markovnikov selectivity (from 90 to >99%). The
regioselectivity of the arylalkenes is attributed to η3
coordination between the Ln ion and aromatic fragment,16
whereas that of silylalkenes might arise from polarization of the
Si−C bond, in which the α-C atom with higher electron
density leads to bonding with the Ln ion.17 The catalytic
protocol tolerated alkoxy (7f−7h), amino (7i), alkylsilyl (7m−
7p), arylsilyl (7q and 7r), and hydrosilyl (7s and 7t) groups.
Although the selective hydrosilylation of arylalkenes can be
realized by a number of metal complex catalysts, the reaction
for silylalkenes could only be achieved with a couple of metal
catalysts.18 Furthermore, because the alkenes containing two
functional groups, like Si−H bond and double bond, preferred
stepwise polymerization as monomers with transition-metal
catalysts, the hydrosilylation of hydrosilylalkene has not been
a
0.01 mmol of 3 and 0.4 mL of toluene. The percent refers to the
isolated yields. The percent in brackets refers to the regioselectivity
determined by GC−MS measurement of the crude reaction mixture.
b
c
1.0 mmol of alkene and 1.2 mmol of 5. 0.2 mmol of alkene and 0.24
d
mmol of 5. Yields determined by GC−MS measurement of the
crude product.
reported.19 The existence of the two different hydrosilyl groups
in product 7t was confirmed by the proton-coupled 29Si NMR
spectrum, in which one doublet peak and one triplet peak were
observed (Figure S22). Moreover, the bulky 1,1-disubstituted
alkenes (α-methyl 7k and α-phenyl 7l) can be smoothly
catalyzed by 3 to generate the Markovnikov products in 62 and
78% yields and excellent regioselectivity (>99%).
With the secondary silane 6 as the silylation reagent (Table
3), complex 3 enabled the hydrosilylation of aryl- and
silylalkenes (4a−i, 4l, 4q, and 4s), leading to the tertiary
silane products (8a−8l) in moderate-to-high yields (49−90%)
and high Markovnikov selectivity (from 90 to >99%). The
arylalkenes with OMe and NMe2 substituents on the phenyl
rings and silylalkene with a PhMeSiH substituent can be
hydrosilylated smoothly to give the regioselective products 8f−
8i and 8l) in good yields (61−85%). Furthermore, complex 3
also catalyzed the hydrosilylation of bulky α-phenylstyrene
with 6 (8j) in 49% yield and excellent regioselectivity (>99%).
For complex 3-catalyzed hydrosilylation (Tables 2 and 3), the
activities of the primary silane and α-alkenes were higher than
C
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a
Table 3. EuII-Catalyzed Hydrosilylation with 6
Experimental procedures, crystallographic data, DFT
calculations, and spectra (PDF)
Accession Codes
CCDC 2064197 and 2068390 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 Authors
■
Jianfeng Li − State Key Laboratory of Elemento-organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, China; orcid.org/0000-0003-0996-
1679; Email: lijf@nankai.edu.cn
Chunming Cui − State Key Laboratory of Elemento-organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, China; orcid.org/0000-0002-1493-
7596; Email: cmcui@nankai.edu.cn
Authors
a
Zexiong Pan − State Key Laboratory of Elemento-organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, China
Jianying Zhang − State Key Laboratory of Elemento-organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, China; orcid.org/0000-0001-6119-
2497
0.01 mmol of 3 and 0.4 mL of toluene. The percent refers to the
isolated yields. The percent in brackets refers to the regioselectivity
determined by GC−MS measurement of the crude reaction mixture.
b
c
1.0 mmol of alkene and 1.2 mmol of 6. 0.2 mmol of alkene and 0.24
mmol of 6.
those of the secondary silane and 1,1-disubstituted alkenes
because of the steric effects. However, catalyst 3 could not
catalyze the hydrosilylation of arylalkenes with halogen
substituents (4-fluorostyrene, 4-chlorostyrene, and 4-bromos-
tyrene) and alkylalkenes (1-hexene and cyclohexene). The
mechanism of the hydrosilylation is proposed as the typical σ-
bond metathesis/coordination−insertion catalytic cycle with a
lanthanide hydride intermediate.16,20 The product PhSiH2N-
(SiMe3)2 of the σ-bond metathesis reaction between amide 3
and 5 could be observed during the hydrosilylation in an NMR
tube. However, the possible CAAC-stabilized europium
hydride cannot be isolated from the stoichiometric reaction
of 3 with 5. It is probably attributed to the instability of the
hydride under the experimental conditions.4a
In summary, we have described the synthesis and character-
ization of the first examples of CAAC lanthanide complexes 2
and 3. The europium complex 3 enabled selective hydro-
silylation of aryl- and silylalkenes, giving Markovnikov products
in high yields. Remarkably, complex 3 enabled the catalytic
hydrosilylation of hydrosilylalkenes, which were more likely to
polymerize with transition-metal catalysts. The high activity of
3 could be attributed to the large ionic radius of the Eu ion for
alkene coordination as well as the stabilizing effect of the
CAAC ligand for catalytic active species. The present results
suggest the potential of the CAAC lanthanide amides in
hydroelementation reactions, which are currently in progress.
Moreover, optimization of the reaction conditions for low
catalyst loadings and functional group tolerance is also our
future target.
Lulu Guo − State Key Laboratory of Elemento-organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, China
Hao Yang − State Key Laboratory of Elemento-organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, China; orcid.org/0000-0003-4281-
2974
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01780
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grants 21890722, 22071123, and 21632006) for
financial support. Dedicated to the 100th Anniversary of
Chemistry at Nankai University.
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ASSOCIATED CONTENT
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