Homoleptic Divalent Dialkyl Lanthanide-Catalyzed Cross-Dehydrocoupling of Silanes and Amines

Article abstract of DOI:10.1021/acs.organomet.6b00138

The rare-earth bis(alkyl) compound Sm{C(SiHMe₂)₃}₂THF₂ (1b) is prepared by the reaction of samarium(II) iodide and 2 equiv of KC(SiHMe₂)₃. This synthesis is similar to that of previously re

Full text of DOI:10.1021/acs.organomet.6b00138

Article
pubs.acs.org/Organometallics
Homoleptic Divalent Dialkyl Lanthanide-Catalyzed Cross-
Dehydrocoupling of Silanes and Amines
Aradhana Pindwal, Arkady Ellern, and Aaron D. Sadow*
Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, Iowa 50011, United States
S
* Supporting Information
ABSTRACT: The rare-earth bis(alkyl) compound Sm{C-
(SiHMe2)3}2THF2 (1b) is prepared by the reaction of
samarium(II) iodide and 2 equiv of KC(SiHMe2)3. This
synthesis is similar to that of previously reported Yb{C-
(SiHMe2)3}2THF2 (1a), and compounds 1a,b are isostruc-
tural. Reactions of 1b and 1 or 2 equiv of B(C6F5)3 afford
SmC(SiHMe2)3HB(C6F5)3THF2 (2b) or Sm{HB-
(C6F5)3}2THF2 (3b), respectively, and 1,3-disilacyclobutane
{Me2Si-C(SiHMe2)2}2 as a byproduct. Bands from 2300 to
2400 cm−1 assigned to νBH in the IR spectra and highly
paramagnetically shifted signals in the 11B NMR spectra of 2b
and 3b provided evidence for Sm-coordinated HB(C6F5)3. Compounds 1a,b react with the bulky N-heterocyclic carbene (NHC)
1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to displace both THF ligands and give three-coordinate monoadducts Ln{C-
(SiHMe2)3}2ImtBu (Ln = Yb (4a), Sm (4b)). Complexes 4a,b catalyze cross-dehydrocoupling of organosilanes with primary and
secondary amines at room temperature to give silazanes and H2, whereas 1a,b are not effective catalysts under these conditions.
Second-order plots of ln{[Et2NH]/[Ph2SiH2]} vs time for 4a-catalyzed dehydrocoupling are linear and indicate first-order
dependences on silane and amine concentrations. However, changes in the experimental rate law with increased silane
concentration or decreased amine concentration reveal inhibition by silane. In addition, excess ImtBu or THF inhibit the reaction
rate. These data, along with the structures of 4a,b, suggest that the bulky carbene favors low coordination numbers, which is
important for accessing the catalytically active species.
investigate Ln{C(SiHMe2)3}2THF2 (Ln = Yb, Sm) as catalysts
for the dehydrocoupling of organosilanes and amines. In this
reaction’s initiation, Ln{C(SiHMe2)3}2L2 could undergo
protonolysis to give a rare-earth amide. During the cycle,
amide transfer to an organosilane through a σ-bond metathesis-
like step would provide the silazane product and a rare-earth
hydride. Protonolysis of this species would re-form the amide.
These steps might be facile with divalent bis(alkyl) rare-earth
compounds, or their zwitterionic monoalkyl derivatives, as
precatalysts and not require oxidation to initiate the process.
This dehydrocoupling pathway, initiated by Sm(II) alkyls,
would be in contrast with the aforementioned polymerization
initiation as well as the Cp*2SmTHF2-catalyzed hydroamina-
tion/cyclization of aminoalkenes, which is proposed to first
form a samarium(III) amide.14
Catalytic cross-dehydrocoupling of silanes with amines is an
excellent method for silazane preparation because hydrogen gas
is the only byproduct, the degree of amine silylation or silane
amination can, in principle, be controlled by the catalyst, and
stoichiometric salt byproducts are not formed as is the case in
halosilane amination. Silazanes are used extensively as bases,15
ligands,16 and silylating agents,17 including as protecting groups
INTRODUCTION
■
Rare-earth-element−carbon bonds react via insertion, σ-bond
metathesis, and protonolytic ligand substitution reactions that
are important elementary steps in catalytic processes, including
olefin polymerization,1 hydrosilylation,2 and dehydrocoupling.3
Trivalent rare-earth alkyls are typical catalysts for these
processes, whereas fewer catalytic processes involve divalent
rare-earth alkyl compounds.4 In the few catalytic chemistries
involving divalent lanthanides, the metal center is typically a
precatalyst that is activated by a one-electron oxidation: for
example, in the initiation of acrylate polymerizations.4b,5
Moreover, while homoleptic tris(alkyl) lanthanide compounds
are useful and versatile starting materials for rare-earth
organometallic chemistry,6 fewer homoleptic bis(alkyl)
ytterbium(II),7 europium(II),8 and samarium(II)4d,9 com-
pounds are known. Note that, in this series of aqueous
lanthanide ions, samarium(II) is the most reducing (−1.55 V vs
NHE) followed by ytterbium(II) (−1.15 V) and europium(II)
(−0.35 V).10
We recently synthesized homoleptic bis(alkyl) ytterbium(II)
species Yb{C(SiHMe2)3}2L2 (L2 = THF2, TMEDA), inves-
tigated their nonclassical M↼H−Si-containing structures, and
studied the reactivity of β-SiH groups with electrophiles.11−13
Because only a few divalent bis(alkyl) lanthanide compounds
have been employed in catalytic processes, we decided to
Received: February 18, 2016
© XXXX American Chemical Society
A
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in synthesis.18 F-element,19 yttrium,20 and group 2 complexes,21
along with Lewis acids22 and Lewis bases,23 catalyze SiH/NH
cross-coupling. In SiH/NH dehydrocoupling reactions cata-
lyzed by bis(disilazido)ytterbium(II) compounds, Cui and co-
workers observed that an N-heterocyclic carbene (NHC) ligand
is required for efficient conversion.4f The rate of cross-
dehydrocoupling of organosilanes and amines catalyzed by
bis(disilazido) Yb{N(SiMe3)2}2L (L = 1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene (ImiC3H7), 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene (ImMes)) is dramatically
increased in comparison to THF-coordinated analogues. A
potential advantage of alkyl-based starting materials vs silazides
is that the latter may participate in the N−Si bond forming
reaction, giving 1−2 equiv of byproduct per catalyst molecule.
Despite the reactivity of carbene-coordinated rare-earth
complexes24 and the importance of NHC ligands in
transition-metal-catalyzed processes,25 the catalytic chemistry
of carbene-coordinated rare-earth alkyl compounds is under-
developed. This limitation in the divalent series may be related
to the few available starting organolanthanide(II) materials. In
terms of homoleptic rare-earth alkyls, the series of trivalent
compounds Ln(CH2SiMe3)3ImiC3H7 (Ln = Er, Lu)26 and
Ln(CH2SiMe3)3ImDipp (Ln = Y, Lu; ImDipp = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene)27 have been prepared by
substituting THF with NHC. Only a handful of
organoytterbium(II) and organosamarium(II) alkyl carbene
compounds have been crystallographically characterized.28
These include the divalent metallocene adduct Cp*2Sm-
(ImMe4)2 (ImMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene), in
which two THF groups from Cp*2SmTHF2 are replaced with
ImMe4 (the monocarbene adduct Cp*2Sm(ImMe4) is formed
but was not crystallographically characterized).28c To the best
of our knowledge, N-heterocyclic carbene coordinated divalent
bis(alkyl)lanthanide complexes were previously unknown. This
fact, as well as the increase in catalytic activity affected by an
NHC ligand for Yb(II)-catalyzed amine−silane dehydrocou-
pling, motivated our preparation of “carbon-only coordination”
divalent rare-earth bis(alkyl) compounds.
otherwise specified) contained signals assigned on the basis of
their relative integrated ratio, chemical shifts, and line widths.
The last two properties are influenced by the nuclei’s
interaction with the paramagnetic center. The highly upfield,
broad resonance at −66.5 ppm (6 H, 342 Hz at half-height)
and the sharp singlet at −1.12 ppm (36 H, 32 Hz at half-height)
were assigned to the C(SiHMe2)3 ligand. The remaining signals
1
at 11.9 and 2.78 ppm were assigned to THF. The H NMR
spectrum of 1b was unchanged after its benzene-d6 solution was
heated at 80 °C for 80 h. 29Si or 13C spectra acquired either
through direct or indirect experiments did not contain signals,
likely the result of the paramagnetic Sm(II) center.
The infrared spectrum of 1b contained signals at 2107, 2062,
and 1867 cm−1 (KBr) attributed to silicon−hydrogen stretching
modes. The first was assigned to a two-center−two electron
(2c-2e) bonded SiH, while lower energy signals were assigned
to groups engaged in three-center−two-electron (3c-2e)
interactions with the samarium(II) center. Three comparable
stretching modes in ytterbium and calcium analogues (1a,
2101, 2065, and 1890 cm−1; 1c, 2107, 2066, and 1905 cm−1)
suggest that the paramagnetic samarium compound contains
structural features similar to those of the ytterbium and calcium
derivatives.11a
A single-crystal X-ray diffraction study confirms this idea,
showing that the Sm center in 1b adopts a distorted-
pseudotetrahedral geometry based on C1/C1# and O1/O1#
coordination (Figure 1). The ∠O1−Sm1−O1# angle of
89.24(7)° is acute, while the ∠C1−Sm1−C1# angle is larger
at 133.96(7)°. The Sm1−C1 interatomic distance of 2.733(2)
Å is ca. 0.14 Å longer than the corresponding distance in 1a, as
expected on the basis of the larger ionic radii of 7-coordinate
Sm2+ (1.22 Å) than 7-coordinate Yb2+ (1.08 Å).29 The Sm1−
C1 interatomic distance is significantly longer than those in
Here, we report the synthesis of a divalent samarium
bis(alkyl) compound, which is compared to Yb{C-
(SiHMe2)3}2THF2 in reactions of a Lewis acid and
coordination of carbene ligands. The NHC-coordinated
compounds are efficient precatalysts for cross-dehydrocoupling
of organosilanes with primary and secondary amines, and
kinetic studies provide a rationale for the enhanced activity of
NHC-coordinated divalent catalysts.
4d
related Sm(II) alkyl compounds, including Sm{C(SiMe3)3}2
4c
(2.58 Å) and (C5Me5)SmCH(SiMe3)2(μ-C5Me5)KTHF2
( 2 . 6 4 ( 1 ) Å ) , b u t s h o r t e r t h a n i n S m { C -
(SiMe3)2(SiMe2OMe)}2THF (2.787(5) and 2.845(5) Å).9
These examples are the only three crystallographically
characterized dialkyl samarium(II) compounds found in the
Cambridge Structural Database. The second notable feature of
1b is the conformation of C(SiHMe2)3 ligands, which each
contains one nonclassical Sm↼H−Si interaction, giving short
Sm1−H 1s and Sm1−Si1 interatomic distances of 2.64(2) and
3.303(5) Å, and Sm1−C1−Si1 and Sm1−H 1s-Si1 angles of
90.94(7) and 104(1)°. Note that the two C(SiHMe2)3 ligands
are related by a crystallographic C2 axis that bisects the C1−
Sm1−C1# angle.
Reactions with B(C6F5)3. The samarium compound 1b and
B(C6F5)3 react to transfer hydride from the C(SiHMe2)3 ligand
to the Lewis acid, giving zwitterionic hydridoborate com-
pounds. These reactions follow the pathway observed for
interactions of the ytterbium complex 1a and B(C6F5)3, despite
the more reducing nature of Sm vs Yb.11a Thus, 1b and 1 or 2
equiv of B(C6F5)3 give SmC(SiHMe2)3HB(C6F5)3THF2 (2b)
or Sm{HB(C6F5)3}2THF2 (3b), respectively (Scheme 1). The
RESULTS AND DISCUSSION
■
Synthesis of Sm{C(SiHMe2)3}2THF2. The homoleptic
bis(alkyl)samarium compound Sm{C(SiHMe2)3}2THF2 (1b)
is synthesized by the reaction of SmI2THF2 and 2 equiv of
KC(SiHMe2)3, which proceeds in THF over 12 h at room
temperature. The solution changes color from blue-green to
dark green upon mixing and then to black after 1 h. The
diamagnetic isostructural analogues Yb{C(SiHMe2)3}2THF2
(1a) and Ca{C(SiHMe2)3}2THF2 (1c) were previously
synthesized under similar conditions, but the synthesis of
those diamagnetic compounds does not show the dramatic
color changes observed for samarium.11a Sm{C-
(SiHMe2)3}2THF2 is isolated as black blocks by crystallization
from pentane at −40 °C.
1
The H NMR spectrum of 1b (all NMR spectra reported
were acquired in benzene-d6 at room temperature unless
B
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stretching modes, while no bands were detected in the region
associated with Si−H stretching modes (2100−1800 cm−1).
Direct comparisons of samarium and ytterbium structures by
NMR are complicated by paramagnetic effects. Nonetheless,
signals in the 11B and 19F NMR spectra of 2b and 3b indicate
that tris(perfluorophenyl)hydridoborate groups are closely
associated with the samarium center. The 11B NMR spectra
acquired in bromobenzene-d5 for 2b and 3b contained
paramagnetically shifted broad signals at −88 and −100 ppm,
respectively, whereas 11B NMR signals for diamagnetic
ytterbium analogues appeared at −20.8 (2a) and −26.4 ppm
(3a). Only two signals were observed in the 19F NMR spectrum
in the ratio of 1:2, assigned to the para and meta fluorines on
C6F5 groups for compounds 2b and 3b, in comparison to the
spectra for the Yb analogues that revealed three fluorine
signals.11a
Reaction with 1,3-Di-tert-butylimidazol-2-ylidene
(ImtBu). Compounds 1a,b and 1 equiv of 1,3-di-tert-
butylimidazol-2-ylidene (ImtBu) react almost instantaneously
at room temperature in benzene or pentane to displace the
THF ligands, affording Ln{C(SiHMe2)3}2ImtBu (Ln = Yb
(4a), Sm (4b)) quantitatively during in situ reactions (Scheme
2). Recrystallization from pentane at −40 °C affords red
crystals of Yb{C(SiHMe2)3}2ImtBu (4a) or dark red crystals of
Sm{C(SiHMe2)3}2ImtBu (4b) in moderate isolated yield.
Figure 1. Rendered thermal ellipsoid plot of Sm{C(SiHMe2)3}2THF2
(1b). Ellipsoids are plotted at 50% probability. Hydrogen atoms
bonded to silicon were located objectively in the Fourier difference
map and were refined isotropically. All other H atoms were placed in
idealized positions, were refined with relative isotropic displacement
coefficients of neighboring atoms, and were not plotted for clarity.
Significant interatomic distances (Å): Sm1−C1, 2.733(2); Sm1−Si1,
3.303(5); Sm1−Si2, 3.6443(6); Sm1−Si3, 3.5990(7); Sm1−H1s,
2.64(2); Sm1−H2s, 3.57(3); Sm1−H3s, 3.69(2). Significant intera-
tomic angles (deg): C1−Sm1−C1#, 133.96(7); O1−Sm1−C1,
108.57(5); O1−Sm1−O1, 89.24(7); Sm1−C1−Si1, 90.94(7); Sm1−
C1−Si2, 104.48(7); Sm1−C1−Si3, 102.49(7); Sm1−H1s−Si1,
104(1).
Scheme 2. Reactions of Ytterbium and Samarium Dialkyls
with ImtBu
byproduct of each hydride transfer is 0.5 equiv of the 1,3-
disilacyclobutane {Me2Si-C(SiHMe2)2}2, which is the head-to-
tail dimer of the silene Me2SiC(SiHMe2)2.
Scheme 1. Reactions of 1b and 1 or 2 equiv of B(C6F5)3
The 1H NMR spectrum of diamagnetic 4a contained a
doublet at 0.45 ppm (3JHH = 3.6 Hz, 36 H) and a septet at 4.86
ppm with silicon satellites (1JSiH = 144 Hz, 6 H) assigned to the
SiHMe2 moiety, as well as singlets at 1.43 and 6.33 ppm
assigned to the coordinated ImtBu ligand. These signals
remained sharp even in spectra acquired at low temperature
(∼200 K) in toluene-d8. In the 13C{1H} NMR spectrum, a
signal at 196.9 ppm for the carbene carbon appeared with a
chemical shift similar to that for other lanthanide tris(alkyl)
carbene adducts.26,27 Neither 15N NMR signals of ImtBu nor
the 29Si NMR of the SiHMe2 moieties are changed significantly
in 4a in comparison with starting materials.
The SiH regions of the infrared spectra for 4a and 4b were
distinct from each other as well as from those of 1a,b. In 4a, a
broad signal at 2058 cm−1 was poorly resolved from bands at
2083 and 2114 cm−1, with one low-energy band at 1871 cm−1.
In contrast, the spectrum of the samarium analogue contained
sharper signals at 2108, 2076, 2064, and 2044 cm−1 as well as
two lower energy bands at 1910 and 1796 cm−1. Note that the
IR spectra of 1a,b (described above) only contained three νSiH
bands, suggesting that compounds 1 and 4 have inequivalent
conformations. This idea is supported by single-crystal X-ray
diffraction analysis.
X-ray-quality crystals are not yet available for zwitterionic
compounds 2b and 3b. The infrared spectra (KBr) of
analytically pure 2b, easily isolated after pentane washes,
showed bands at 2389, 2306, and 2110 cm−1. In contrast, the
IR spectrum of YbC(SiHMe2)3HB(C6F5)3THF2 (2a, KBr)
contained bands at 2310 (νBH), 2074 (νSiH), and 1921 cm−1
(νSiH). The presence of only one SiH band at high energy in 2b
suggests that the C(SiHMe2)3 group lacks secondary
interactions present in 1b and 2a. While the change in IR
spectrum is surprising, elemental analysis shows that one
C(SiHMe2)3 is present in 2b, and the IR band at 2110 cm−1 is
not from the hydrocarbon-soluble {Me2Si-C(SiHMe2)2}2 that
is removed during workup. In addition, the IR spectrum of 3b
contained bands at 2388 and 2318 cm−1 assigned to B−H
C
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While compounds 4a,b (see Figures 2 and 3) both contain
three-coordinate distorted-trigonal-planar metal centers (C1−
Ln−C8 > 120°), the two species crystallize in inequivalent
space groups (4a, Pbca, Z = 8; 4b, P1, Z = 2) with distinctly
̅
inequivalent conformations of their alkyl ligands. In 4a, one
ligand forms two Yb↼H−Si interactions and the other ligand
contains only normal Si−H groups. The diagostic-like ligand in
4a contains two short Yb−H distances (Yb1−H2s, 2.42(3) Å;
Yb1−H3s, 2.49(3) Å) and two short Yb−Si distances (Yb1−
Si2, 3.118(8) Å; Yb1−Si3, 3.169(8) Å). In 4b, one alkyl
contains one Sm↼H−Si interaction, while the other alkyl
contains two secondary interactions. The short distances Sm1−
H2s (2.54(2) Å), Sm1−Si2 (3.2686(6) Å), Sm1−H4s (2.65(2)
Å), and Sm1−Si4 (3.2916(6) Å), acute angles for Sm1−C1−
Si2 (88.13(7)°) and Sm1−C8−Si4 (90.27(7)°), and coplanar
Sm−C and Si−H vectors (Sm1−C1−Si2−H2s, −3.4(9)°;
Sm1−C8−Si4−H4s, 9.9(9)°) are consistent with Sm↼H−Si.
A second SiH approaches the Sm center (Sm1−H1s, 2.86(2)
Å; Sm1−Si1, 3.2973(6) Å; Sm1−C1−Si1, 89.14(7)°), but the
Sm1−C1 and Si1−H1s vectors are not coplanar (Sm1−C1−
Si1−H1s, −36(1)°). The differences in alkyl ligand con-
formation may also be reflected in the distinct IR spectra of 4a
and 4b, suggesting that the Ln↼H−Si close contacts affect the
vibrational properties (i.e., Si−H force constants). It is
somewhat unexpected that the conformations of 4a and 4b
are inequivalent; however, it is also worth noting that the ionic
radii for Sm(II) and Yb(II) are different by 0.14 Å.29
Compound 4a is the first reported example of a crystallo-
graphically characterized compound with a single alkyl ligand
containing two agostic-like SiH groups interacting with a single
metal center. Diagostic features are not generally observed in
homoleptic compounds but have been reported for lanthani-
docene disilazide compounds.30 However, the bridging
tetramethyldisilazido ligands in {La{N(SiHMe2)2}3}2 contain
close contacts with both La centers.31 In addition, the reaction
of Y{N(SiHMe2)2}3THF2 and ImMe2 (1,3-dimethylimidazol-2-
ylidene) gives a carbene-coordinated, THF-free compound that
also shows Y↼H−Si close contacts.32 All three alkyl ligands in
Y{C(SiHMe2)3}3 contain two secondary interactions, as
characterized by low-temperature NMR studies,33 but a
solution to X-ray diffraction data was not found. In addition
to the unusual diagostic-like structure in 4a, it is also notable
that the other ligand does not contain any other secondary
interactions with the Yb center.
Figure 2. Rendered thermal ellipsoid plot of Yb{C(SiHMe2)3}2ImtBu
(4a) at 50% probability. H atoms bonded to Si were located in the
Fourier difference map and were included in the representation. All
other H atoms are not plotted for clarity. Significant interatomic
distances (Å): Yb1−C1, 2.627(3); Yb1−C8, 2.611(3); Yb1−C25,
2.605(3); Yb1−Si1, 3.8246(9); Yb1−Si2, 3.118(8); Yb1−Si3,
3.169(8); Yb1−H1s, 4.16(3); Yb1−H2s, 2.422(3); Yb−H3s,
2.49(3). Significant interatomic angles (deg): C1−Yb1−C8,
138.69(8); C1−Yb1−C25, 106.26(8); C25−Yb1−C8, 115.04(8);
Si1−C1−Yb1, 117.0(1); Si2−C1−Yb1, 87.1(1); Si3−C1−Yb1,
89.1(1).
As noted above, the central carbons of the C(SiHMe2)3
ligands, the carbene carbon, and the metal center are coplanar
in 4a,b. The angles between the alkyl ligands and the carbene
group deviate from 120° (C1−Yb1−C25 = 106.25(8)° and
C8−Yb1−C25 = 115.03(8)°; C8−Sm1−C15 = 113.99(5)° and
C1−Sm1−C15 = 114.29(5)°) suggesting that carbene−alkyl
interligand repulsions are less severe than alkyl−alkyl
interactions. The Yb1−C1 and Yb1−C8 distances (2.627(3)
and 2.611(3) Å, respectively) are slightly longer than in 1a
(2.596(4) Å).11a However, the Sm−C distances in 1b (Sm1−
C1, 2.733(2) Å) and 4b (Sm1−C1, 2.779(2) Å; Sm1−C8,
2.739(2) Å) are similar. This observation may relate to the
larger ionic radii of Sm(II) in comparison to Yb(II), which
reduces interligand interactions that could affect Ln−C
distances in the former complexes.
Crystals of 4a,b persist at room temperature for 2 days under
an inert atmosphere. When a solution of 4a in benzene-d6 was
heated at 80 °C for 1 day, the alkane HC(SiHMe2)3 was
formed at the expense of 4a. The remaining 1H NMR signals at
1.60 and 4.75 ppm were attributed to a product resulting from
transformation of the ImtBu ligand into a new, as yet
Figure 3. Rendered thermal ellipsoid plot of Sm{C(SiHMe2)3}2ImtBu
(4b) at 50% probability. H atoms bonded to Si were located in the
Fourier difference map and were included in the representation. All
other H atoms are not plotted for clarity. Significant interatomic
distances (Å): Sm1−C1, 2.779(2); Sm1−C8, 2.739(2); Sm1−C15,
2.780(3); Sm1−Si1, 3.2973(7); Sm1−H1s, 2.86(2); Sm1−Si2,
3.2686(8); Sm1−H2s, 2.53(3); Sm1−Si4, 3.2917(7); Sm1−H4s,
2.65(2). Significant angles (deg): C1−Sm1−C8, 131.65(8); C1−
Sm1−C15, 114.29(6); C8−Sm1−C15, 113.99(6); Sm1−C1−Si1,
89.14(8); Sm1−C1−Si2, 88.13(8); Sm1−C8−Si4, 90.27(8), Sm1−
C1−Si1−H1s, −37(1); Sm1−C1−Si2−H2s, 3(1); Sm1−C8−Si4−
H4s, 9.8(9).
D
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unidentified organic species bonded to ytterbium. The crystals
of 4a,b also decompose under vacuum to HC(SiHMe2)3 and
free carbene, and the color of the crystals fades from deep red
to pale orange during this decomposition process. Hence, these
compounds are used only from freshly recrystallized and
isolated material by carefully decanting the pentane solution
and briefly evaporating residual solvent under vacuum for short
amounts of time.
To better assess the relative stability of carbene vs THF
coordination to samarium(II), UV−vis spectra were measured
for 1b and 4b. The UV−vis spectrum of 1b in pentane revealed
peaks with λmax values of 389 (ε = 556 M−1 cm−1) and 439 nm
(ε = 518 M−1 cm−1) shown in Figure 4. The spectrum for
Yb center in this zwitterionic species. The integrated ratio of
these signals and the carbene resonances at 6.19 and 1.12 ppm
indicate that a 1:1 adduct is formed. Unfortunately, this
material has not yet proven isolable.
Cross-Dehydrocoupling of Organosilanes and Amines
Catalyzed by Ln{C(SiHMe2)3}2ImtBu. The divalent com-
pounds 4a,b are efficient precatalysts for cross-dehydrocoupling
of Si−H and N−H bonds to give Si−N bonds and H2. Primary
and secondary amines effectively couple with primary and
secondary silanes to afford the desired silazanes (Tables 1 and
Table 1. Ln{C(SiHMe2)3}2ImtBu-Catalyzed
Dehydrocoupling of PhSiH3 and Amines
Figure 4. UV−vis spectra of Sm{C(SiHMe2)3}2THF2 (1b) and
Sm{C(SiHMe2)3}2ImtBu (4b) in pentane (2.0 mM). Inset: titration of
4b with THF.
2), which are readily isolated in good yield by distillation. In the
absence of 4a,b as a catalyst, only starting materials are
observed. Compounds 1a,b, 2b, and 3b, as well as ImtBu, were
also tested for catalytic activity in the cross-dehydrocoupling of
PhMeSiH2 and tBuNH2, but only starting materials were
observed in H NMR spectra of the reaction mixtures after 1
day at room temperature or 60 °C.
compound 4b in pentane contained peaks at 405 nm (ε = 551
M−1 cm−1) and 492 nm (ε = 586 M−1 cm−1), and the lower
energy peaks are assigned to f → d transitions.34 Both
compounds have a tail through the visible region, accounting
for their dark black color. Upon addition of 2 equiv of THF to
4b, the peak at 492 nm diminishes and intensity of the broad
peak of 1b at 439 nm increases. An isosbestic point at 446 nm
was apparent in a series of spectra with 2.0, 2.5, 3.0, 3.5, and 4.0
equiv of THF (Figure 4, inset). Thus, the conversion of 4b to
1b upon addition of THF occurs without detectable buildup of
intermediates. This process is clearly reversible; however, more
than 8 equiv is required for the 4b signals to be overwhelmed
by 1b peaks.
1
In catalytic reactions, the consumption of organosilane and
formation of silazane product were evident from the new
3
downfield-shifted SiH resonances of HSi−NR′2. The JHH
coupling between SiH and NH provided characteristic splitting
patterns for SiH resonances, which appeared as doublets for the
silazide RH2SiNHR′ and as triplets in spectra of RHSi(NHR′)2.
Although reactions of PhSiH3 and iC3H7NH2 or tBuNH2 in a
1:1 ratio result in a mixture of these silazane and siladiazido
products, reactions with excess silane or excess amine provide
control over the product identity. For example, the 4a-catalyzed
reaction of PhSiH3 and 4 equiv of iC3H7NH2 affords the
silyltrisazido species PhSi(NHiC3H7)3. In addition, the amine’s
steric bulk affects the reaction rate in conversions with PhSiH3.
Thus, 2 equiv of tBuNH2 and PhSiH3 react to yield the
silyldiazido species PhHSi(NHtBu)2 after 15 h. The reaction
with the secondary amine Et2NH (3 equiv) also requires 15 h
to yield the silyldiazido species PhSiH(NEt2)2, but the reaction
of phenylsilane and (iC3H7)2NH did not result in any silazane
formation after 1 day at room temperature or 60 °C.
Only starting materials were observed in 1H NMR spectra of
mixtures containing the zwitterionic compound 2a or 3a and
ImtBu. In contrast, the reaction of 4a and B(C6F5)3 generates
YbC(SiHMe2)3{HB(C6F5)3}ImtBu and 0.5 equiv of the 1,3-
disilacyclobutane {Me2Si-C(SiHMe2)2}2. The 11B NMR spec-
trum of the reaction mixture contained a doublet at −21 ppm
(1JBH = 73.6 Hz) and provided good evidence for hydrogen
1
abstraction. The H NMR spectrum contained a multiplet at
1
4.75 ppm (3 H, JSiH = 192 Hz) assigned to the SiH and a
singlet at 0.35 ppm (18 H, SiMe2). This large SiH coupling
constant suggests that the SiH groups do not interact with the
E
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Article
spite of this limitation, second-order rate constants increase in a
series of experiments in which [4a] is increased.
A plot of observed second-order rate constants vs catalyst
concentration (i.e., [4a]) reveals a linear dependence, giving
the ternary rate law of eq 2
Table 2. Ln{C(SiHMe2)3}2ImtBu-Catalyzed
Dehydrocoupling of Secondary Silanes and Amines
−d[Ph2SiH2]/dt = k′obs[4a][Ph2SiH2][Et2NH]
(2)
with k′obs = (1.04 0.07) × 10−7 mM−2 s−1 (Figure 5). The
nonzero intercept indicates that a portion of the ytterbium
4a,b-catalyzed reactions of secondary silanes such as
PhMeSiH2 and Ph2SiH2 and either primary or secondary
amines afford monosilazane products, while the reaction of 2
equiv of silane with primary amines gives disilazanes (Table 2).
One such example is illustrated by the 4a-catalyzed reaction of
2 equiv of PhMeSiH2 and H2NiC3H7, which gives (PhMeH-
Si)2NiC3H7 as a 1.3:1 mixture of diastereomers after 4 h at
room temperature. Similar to the case for primary silanes, only
starting materials are observed in mixtures of the bulky
secondary amine (iC3H7)2NH and secondary silanes. Addi-
tionally, only starting materials are observed in mixtures of
tertiary silanes such as BnMe2SiH (Bn = CH2Ph), Et3SiH, and
(CH2CH)SiMe2H with primary or secondary amines after 1
day at room temperature or at 60 °C.
Figure 5. Plot of second-order rate constants versus catalyst
concentration [4a] for reaction of Et2NH (673 mM) and Ph2SiH2
(81 mM) at 298 K.
species is not active for catalysis, and given the formation of
noncoordinated NHC, at least one of the inactive species is
likely carbene-free. On the basis of this and the observation of
free NHC during the reaction, we hypothesized that additional
NHC might increase catalytic activity by disfavoring dissocia-
tion. Instead, excess ImtBu, or excess THF, inhibits the rate of
catalytic conversion. This inhibition likely results from the
ytterbium amide or ytterbium hydride catalytic intermediates
coordinating either THF or excess ImtBu, since 4a and excess
ImtBu do not afford detectable quantities of Yb{C-
(SiHMe2)3}2(ImtBu)2. An additional and unexpected inhibition
process was also observed in the decrease in the second-order
rate constant with increasing [Ph2SiH2] (Figure 6).
At low concentrations of amine, the concentration profiles
show rapid initial conversion followed by a slower second phase
as the reaction approaches completion. Thus, the experimental
ternary rate law is likely only valid for given concentration
regimes (and not valid at high [Ph2SiH2] or low [Et2NH]).
Within the conditions of Figure 5, and taking into account the
qualitative observations under the other conditions, a few
conclusions are apparent. First, the ternary rate law suggests
that interaction of one of the reagents (silane or amine) with
catalytic intermediates is reversible and precedes the turnover-
limiting step. Because higher concentrations of Ph2SiH2 inhibit
the catalytic reaction, possibly by formation of a Yb−Ph2SiH2
adduct as an off-cycle species, the reversible interaction of
catalyst and Ph2SiH2 is unlikely to be part of the catalytic cycle.
The reaction of Ph2SiH2 and Et2NH catalyzed by 4a yields
Ph2HSiNEt2 as the sole coupling product when a range of
silane to amine ratios (1:1 to 1:25) were used. Therefore, this
transformation was used as a representative conversion for
mechanistic studies. Concentration vs time profiles of Ph2SiH2
1
and Et2NH were measured by H NMR spectroscopy and
quantified by integration relative to a standard of known
concentration. Upon addition of catalyst 4a to mixtures of
Ph2SiH2 and Et2NH, free ImtBu and the alkane HC(SiHMe2)3,
resulting from protonolysis, were detected by 1H NMR
spectroscopy. The active catalytic species is part of this
mixture, and dehydrocoupling ensues. Four concentration
regimes were investigated, namely combinations of high and
low [Et2NH] and [Ph2SiH2]. Under conditions of low
[Ph2SiH2]ini (81 mM) and high [Et2NH]ini (673 mM), with
[4a] equal to 8−16 mM, second-order plots suggest first-order
dependence on each reagent. The data analysis is limited by the
faster initial consumption of ca. 5 mM of Et2NH during catalyst
initiation in comparison to that during dehydrocoupling. In
F
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Article
deactivation by Ph2SiH2 is reversible, because greater than 3
half-lives of conversion were observed under several conditions,
and an irreversible catalyst decomposition reaction would likely
be noticed with severely nonlinear kinetic plots. This
coordinatively unsaturated intermediate could be a ytterbium
hydride (e.g., B), which might form a [Yb]H(H-SiHEt2)
species related to those proposed for Ru35 but could also be a
ytterbium amide, such as C in Scheme 3. We favor the latter,
because if B or its inhibited forms were the catalyst resting
state, then second-order dependence on [Et2NH] would be
expected.
CONCLUSION
■
The structure and non-redox-based chemistry of ytterbium and
samarium compounds Ln{C(SiHMe2)3}2THF2 are very similar
in terms of structure and reactivity toward the Lewis acid
B(C6F5)3 and substitution of THF ligands by ImtBu. In
addition, the carbene adducts of both dialkyl rare-earth
compounds are similarly reactive in dehydrocoupling of amines
and organosilanes. However, there are a few notable differences
between the samarium and ytterbium compounds, which
appear in the IR spectra of LnC(SiHMe2)3HB(C6F5)3THF2
and the X-ray crystal structures of Ln{C(SiHMe2)3}2ImtBu.
Notably, the ytterbium compound 4a contains one diagostic-
like alkyl ligand and one alkyl ligand lacking secondary
interactions, whereas both alkyl ligands in the samarium
analogue contain 3c-2e Sm↼H−Si structures.
The LnC(SiHMe2)3HB(C6F5)3THF2 compounds do not
react with ImtBu under the accessible conditions, and
apparently the substitution of THF in the zwitterionic
compounds is more difficult than in the neutral 1a,b. An
NHC ligand coordinates to ytterbium in YbC(SiHMe2)3HB-
(C6F5)3ImtBu, which is formed from the reaction of 4a and
B(C6F5)3. Even so, the ImtBu ligand dissociates from 4a,b upon
addition of small amounts of THF or amines, as suggested by
the UV−vis titration, by the observation of free ImtBu in
catalytic reaction mixtures, or even under vacuum. The
coordination and substitution chemistry of Yb and Sm in
these alkyl compounds affects catalytic dehydrocoupling of
amines and silanes. Coordination of substrates and exogenous
ligands, including THF and ImtBu, inhibit catalysis, and not all
of the ytterbium precatalyst gives active catalytic sites, as shown
by kinetic studies. The free ImtBu present during catalysis
suggests that irreversible catalyst deactivation involves its
dissociation from the rare-earth center. On the basis of these
observations, we are currently investigating bidentate hemilabile
carbene ligands to access coordinative unsaturation but stabilize
lanthanide(II)−ligand interactions in these alkyl compounds.
Figure 6. Plot of observed second-order rate constant versus
[Ph2SiH2] at 298 K in benzene-d6. The curve is drawn to merely
guide the eye and does not represent the results of a least-squares
regression analysis.
Instead, reversible ytterbium(II)−amine coordination is
proposed as a key step for the conversion. This ytterbium
amine compound is unlikely to be a hydride, which would
rapidly react to give ytterbium amide and H2. The alternative is
therefore suggested in which the turnover-limiting step would
involve the interaction of a ytterbium amido amine compound
(e.g., A in Scheme 3) and Ph2SiH2. Second, formation of
Scheme 3. Proposed Catalytic Cycle Consistent with
Experimental Kinetic Observations
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under a dry argon atmosphere using standard Schlenk techniques or
under a nitrogen atmosphere in a glovebox unless otherwise indicated.
Water and oxygen were removed from benzene and pentane solvents
using an IT PureSolv system. Benzene-d6 was heated to reflux over
HC(SiHMe2)3, presumably as part of catalyst initiation, implies
that a ytterbium amido species is an intermediate. Note that
independent reactions of 4a and Et2NH also give HC-
(SiHMe2)3, although ytterbium amido products were not
isolable. Third, deactivation by excess ImtBu, THF, or even
Ph2SiH2 but not Et2NH suggests that catalysis involves a
coordinatively unsaturated species, which must interact with
Et2NH for the reaction to proceed. We propose that
Na/K alloy and vacuum-transferred. The compounds YbI2,
33
SmI2 THF2 ,36 KC(SiHMe2)3 ,
B(C6 F5 )3,37 and Yb{C-
11a
(SiHMe2)3}2THF2 were prepared following literature procedures.
1,3-Di-tert-butylimidazol-2-ylidene (ImtBu) was purchased from
Sigma-Aldrich and used as received. NMR spectroscopic data for
38
{Me2SiC(SiHMe2)2}2 and the catalytic products PhSi(NHiPr)3,
(PhSiH2)2NiPr, PhSiH(NHtBu)2, (PhSiH2)2NtBu, (PhMeSiH)2NiPr,
PhMeSiH(NHtBu), PhMeSiH(NEt2), Ph2SiH(NHiPr), Ph2SiH-
G
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Article
(NHtBu), Ph2HSiNEt2,4f and PhSiH(NEt2)219 match those previously
(benzene-d6, 150 MHz, 25 °C): δ 196.90 (C3N2H2tBu2), 118.58
(C3N2H2tBu2), 57.75 (CMe3), 31.16 (CMe3), 12.00 (YbC), 4.00
(SiMe2). 15N NMR (benzene-d6, 61 MHz, 25 °C): δ −168.8
(C3N2H2tBu2). 29Si NMR (benzene-d6, 119 MHz, 25 °C): δ −19.4
(SiHMe2). IR (KBr, cm−1): 2977 m, 2949 s, 2895 s, 2059 w (νSiH),
1871 m br (νSiH), 1463 w, 1243 s, 1023 s, 930 s br, 900 s br, 831 m br,
767 s, 730 s, 673 m br, 634 m br. Anal. Calcd for C25H63N2Si6Yb: C,
40.95; H, 8.66; N, 3.82. Found: C, 41.09; H, 8.73; N, 3.87. Mp: 181−
184 °C.
Sm{C(SiHMe2)3}2ImtBu (4b). 1,3-Di-tert-butylimidazol-2-ylidene
(ImtBu, 0.027 g, 0.15 mmol) was added to a 5 mL pentane solution of
Sm{C(SiHMe2)3}2THF2 (0.100 g, 0.15 mmol) at room temperature
to yield a dark red solution. The solution was stirred at room
temperature for 24 h. After 1 day, the solution was concentrated and
cooled to −40 °C to yield deep red platelike crystals of Sm{C-
(SiHMe2)3}2ImtBu (0.042 g, 0.060 mmol, 39.8%). 1H NMR (benzene-
d6, 600 MHz, 25 °C): δ 14.01 (br, 18 H, C3N2H2tBu2), −0.58 (s, 2 H,
C3N2H2tBu2), −2.84 (s, 36 H, SiMe2), −73.9 (s br, 6 H, SiHMe2). IR
(KBr, cm−1): 2977 m, 2949 s, 2897 s, 2108 (νSiH), 2076 s br (νSiH),
2064 s br (νSiH), 2044 s (νSiH), 1910 s br (νSiH), 1796 m br (νSiH),
1463 w, 1397 w, 1372 w br, 1239 s, 1200 m, 1011 w br, 963 s, 930 s br,
896 s br, 832 m br, 764 s, 731 s, 671 m br, 635 m br. Anal. Calcd for
C25H63N2Si6Sm: C, 42.25; H, 8.94; N, 3.94. Found: C, 42.09; H, 8.88;
N, 3.85. Mp: 154−157 °C.
reported.
1
1
1H, 13C{1H}, 11B, H−15N HMBC, 19F, and H−29Si HMBC NMR
spectra were collected on a Bruker DRX-400 spectrometer, a Bruker
Avance III-600 spectrometer, or an Agilent MR 400 spectrometer. 11B
NMR spectra were referenced to an external sample of BF3·Et2O. 15
N
chemical shifts were originally referenced to liquid ammonia and
recalculated to the CH3NO2 chemical shift scale by adding −381.9
ppm. Infrared spectra were measured on a Bruker Vertex 80
instrument. Elemental analyses were performed using a PerkinElmer
2400 Series II CHN/S instrument. X-ray diffraction data were
collected on a Bruker APEX II diffractometer. UV−vis spectra were
measured on an Agilent 8453 Diode Array UV−vis instrument.
Sm{C(SiHMe2)3}2THF2 (1b). KC(SiHMe2)3 (0.159 g, 0.697 mmol)
and SmI2THF2 (0.190 g, 0.346 mmol) were stirred in THF (5 mL) at
room temperature for 12 h. The resulting black solution was
evaporated to dryness under reduced pressure and extracted with
pentane (2 × 5 mL) to give Sm{C(SiHMe2)3}2THF2 (0.154 g, 0.228
mmol, 66.1%). The compound was allowed to crystallize in pentane at
1
−40 °C to yield black crystals of Sm{C(SiHMe2)3}2THF2. H NMR
(benzene-d6, 600 MHz, 25 °C): δ 11.93 (s br, 8 H, THF), 2.78 (s br, 8
H, THF), −1.12 (s, 36 H, SiMe2), −66.53 (s br, 6 H, SiHMe2). IR
(KBr, cm−1): 2950 s, 2893 m, 2106 s br (νSiH), 2062 s br (νSiH), 1867
m br (νSiH), 1458 w, 1419 w, 1246 s, 1029 s, 945 s br, 885 s br, 832 m
br, 764 s, 671 m br, 624 m br. Anal. Calcd for C24H58O2Si6Sm: C,
In-situ YbC(SiHMe2)3(HB(C6F5)3)ImtBu (5a). B(C6F5)3 (0.020 g,
0.039 mmol) was added to a benzene-d6 (0.4 mL) solution of
Yb{C(SiHMe2)3}2ImtBu (0.028 g, 0.039 mmol), resulting in a yellow
solution. Attempts to isolate material from larger scale reactions were
unsuccessful. 1H NMR (benzene-d6, 600 MHz, 25 °C): δ 6.19 (s, 2 H,
39.23; H, 8.68. Found: C, 39.02; H, 8.73. Magnetic susceptibility:
298 K
μeff
= 3.22 μB. Mp: 118−121 °C.
SmC(SiHMe2)3HB(C6F5)3THF2 (2b). B(C6F5)3 (0.053 g, 0.103
mmol) was added to a benzene (4 mL) solution of Sm{C-
(SiHMe2)3}2THF2 (0.070 g, 0.103 mmol) in small portions. The
resulting red mixture was stirred at room temperature for 30 min. The
solvent was evaporated under reduced pressure to give a yellow paste.
The residue was washed with pentane (3 × 5 mL), and the volatiles
were evaporated to dryness in vacuo to give SmC(SiHMe2)3HB-
1
C3N2H2tBu2), 4.75 (m, JSiH = 192 Hz, 3 H, SiHMe2), 1.12 (s, 18 H,
3
CMe3), 0.35 (d, JHH = 3.6 Hz, 18 H, SiMe2). 11B NMR (benzene-d6,
1
192 MHz, 25 °C): δ −21.1 (d, JBH = 75.1 Hz). 13C{1H} NMR
(benzene-d6, 150 MHz, 25 °C): δ 194.26 (C3N2H2tBu2), 149.73
(C6F5), 148.26 (C6F5), 140.41(C6F5), 128.77 (C6F5), 137.13 (C6F5),
118.66 (C3N2H2tBu2), 57.41 (CMe3), 30.67 (CMe3), 25.67 (YbC),
3.08 (SiMe2). 29Si NMR (benzene-d6, 119 MHz, 25 °C): δ −18.4
(SiHMe2). IR (KBr, cm−1): 2978 m, 2960 s, 2395 br (νBH), 2305 br
(νBH), 2107 m (νSiH), 1865 w br (νSiH), 1644 s, 1603 w, 1513 s, 1465 s,
1375 m, 1275 m, 1257 m, 1236 m, 1201 m, 1114 s, 969 s, 898 s, 838
m.
1
(C6F5)3THF2 as a red solid (0.081 g, 0.081 mmol, 78.9%). H NMR
(bromobenzene-d5, 600 MHz, 25 °C): δ −1.37 (br, 8 H, OCH2),
−3.24 (br, 8 H, CH2). 13C{1H} NMR (bromobenzene-d5, 150 MHz,
25 °C): δ 153.0 (br, C6F5), 135.9 (br, C6F5), 134.4 (br, C6F5). 11B
NMR (bromobenzene-d5, 192 MHz, 25 °C): δ −88.4 (br). 19F NMR
(bromobenzene-d5, 564 MHz, 25 °C): δ −157.08 (s, 3 F, p-F),
−161.29 (s, 6 F, m-F). IR (KBr, cm−1): 2964 m, 2899 w, 2389 m
(νBH), 2307 m br (νBH), 2110 m br (νSiH), 1645 m, 1603 w, 1515 s,
1466 s br, 1373 m, 1277 br, 1259 br, 1113 s br, 1089 s br, 1023 m br,
965 s br, 899 s br, 839 s br. Anal. Calcd for C33H38BF15O2Si3Sm: C,
39.75; H, 3.84. Found: C, 38.94; H, 3.17. Mp: 172−174 °C.
Sm{HB(C6F5)3}2THF2 (3b). B(C6F5)3 (0.194 g, 0.378 mmol) was
added to a benzene (4 mL) solution of Sm{C(SiHMe2)3}2THF2
(0.128 g, 0.189 mmol) in small portions. The resulting red mixture
was stirred at room temperature for 30 min. The solvent was
evaporated under reduced pressure to give a red paste. The residue
was washed with pentane (3 × 5 mL), and the volatiles were
evaporated to dryness in vacuo to give Sm{HB(C6F5)3}2THF2 as a red
solid (0.205 g, 0.145 mmol, 77.0%). 11B NMR (bromobenzene-d5, 192
MHz, 25 °C): δ −100.8 (br). 19F NMR (bromobenzene-d5, 564 MHz,
25 °C): δ −157.07 (br, 3 F, p-F), −161.45 (6 F, m-F). IR (KBr, cm−1):
2968 m, 2899 m, 2389 m (νBH), 2318 s br (νBH), 1647 s, 1604 w, 1517
s, 1467 s br, 1374 m, 1274 s br, 1114 s br, 1082 s br, 1018 m, 973 s,
961 s, 927 m, 842 s br, 766 s. Anal. Calcd for C44H18B2O2F30Sm: C,
40.02; H, 1.37. Found: C, 40.17; H, 1.35. Mp: 177−180 °C.
Catalytic Synthesis of PhSi(NHiC3H7)3 as a Representative
Procedure for the Reaction of Organosilanes and Amines. See
the Supporting Information for full characterization and spectra of
silazane products. Yb{C(SiHMe2)3}2ImtBu (0.030 g, 0.0409 mmol)
was dissolved in benzene (4 mL). PhSiH3 (0.088 g, 0.818 mmol) was
added to the solution, followed by excess iC3H7NH2 (0.193 g, 3.27
mmol, 4 equiv). The mixture immediately changed color from orange
to brown and effervesced, the vial was quickly capped, and the mixture
was stirred for 1 h. A brown precipitate formed after 1 h, and this is
attributed to catalyst decomposition. The solution was filtered to
remove precipitated catalytic material, and volatile materials were
removed under vacuum. The product was purified by vacuum
distillation to yield spectroscopically pure, colorless oil of PhSi-
(NHiC3H7)3 (0.141 g, 0.504 mmol, 62%). 1H NMR (benzene-d6, 600
3
3
MHz): δ 7.79 (d, 2 H, JHH = 7.0 Hz, o-C6H5), 7.30 (vt, 2 H, JHH
=
7.2 Hz, m-C6H5), 7.26 (vq, 1 H, 3JHH = 7.2 Hz, p-C6H5), 3.25 (mult, 3
3
3
H, CH), 1.07 (d, JHH = 6.3 Hz, 18 H, CH3), 0.65 (d, JHH = 9.8 Hz,
NH). 13C{1H} NMR (benzene-d6, 150 MHz): δ 140.26 (ipso-C6H5),
135.06 (o-C6H5), 129.49 (m-C6H5), 128.18 (p-C6H5), 42.90 (CH),
28.48 (CH3). 15N NMR (benzene-d6, 61 MHz): δ −331.84. 29Si NMR
(benzene-d6, 119 MHz): δ − 37.56.
Yb{C(SiHMe2)3}2ImtBu (4a). 1,3-Di-tert-butylimidazol-2-ylidene
(ImtBu, 0.028 g, 0.153 mmol) was added to a pentane solution (5
mL) of Yb{C(SiHMe2)3}2THF2 (0.106 g, 0.153 mmol) at room
temperature to yield a deep red solution. The solution was stirred at
room temperature for 30 min. The reaction mixture was concentrated
briefly under reduced pressure to a total volume of 4 mL and then
cooled to −40 °C to yield red needlelike crystals of Yb{C-
(SiHMe2)3}2ImtBu (0.0615 g, 0.084 mmol, 54.8%). 1H NMR
(benzene-d6, 600 MHz, 25 °C): δ 6.33 (s, 2 H, C3N2H2tBu2), 4.86
General Procedure for NMR Kinetics Measurements. The
reaction progress was monitored by single-scan acquisition of a series
of 1H NMR spectra at regular intervals on a Bruker DRX400
spectrometer. Hexamethylbenzene was used as a standard of accurately
known and constant concentration (0.010 M in benzene-d6). The
temperature in the NMR probe was preset for each experiment at 25
°C (verified for each experiment with a thermocouple placed in an
NMR tube in toluene in the probe). A range of silane to amine ratios
(1:1 to 1:25) was used to study the catalytic reaction of Ph2SiH2 and
3
1
(sept, JHH = 3.4 Hz, JSiH = 149 Hz, 6 H, SiHMe2), 1.43 (s, 18 H,
CMe3), 0.46 (d, JHH = 3.4 Hz, 36 H, SiMe2). 13C{1H} NMR
3
H
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Et2NH in benzene-d6. Kinetic measurements were studied at low
concentration of Ph2SiH2 (0.081 M) and high concentration of Et2NH
(0.673 M).
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Representative Example. The catalytic reaction of Ph2SiH2 and
Et2NH using 10 mol % of Yb{C(SiHMe2)3}2ImtBu as catalyst is
described. A 5 mL stock solution of C6Me6 internal standard (0.0081
g, 0.0499 mmol, 10.0 mM) in benzene-d6 was prepared using a 5 mL
volumetric flask. The stock solution (0.50 mL) was added by a 1 mL
glass syringe to a known amount of 4a (0.0052 g, 0.0071 mmol). The
resulting solution was transferred to a NMR tube and capped with a
rubber septum, and a 1H NMR spectrum was acquired. Neat
substrates Ph2SiH2 (0.013 g, 0.071 mmol) and Et2NH (0.052 g,
0.711 mmol) were added to the NMR tube by injecting through the
rubber septum. Then, the NMR tube was quickly placed in the NMR
probe. Single-scan spectra were acquired automatically at preset time
intervals at 25 °C. The concentration of silane and product at any
given time was determined by the integration of silane and product
resonances relative to the integration of the internal standard. Second-
order rate constants (kobs) were obtained by nonweighted linear least-
squares fits of the data to the second-order rate law: ln{[Et2NH]/
[Ph2SiH2]} = Δ0kobst + ln{[Et2NH]0/[Ph2SiH2]0}. The experimental
third-order rate constant k′obs was determined by measuring kobs for
several catalyst concentrations. A plot of second-order rate constants
vs catalyst concentrations gave a first-order dependence on catalyst
concentration with the overall rate law −d[Ph2SiH2]/dt =
k′obs[catalyst][Ph2SiH2][Et2NH].
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00138.
■
S
1
Spectra of new compounds, synthetic procedures, H
NMR spectra of catalysis products, and kinetics plots
(PDF)
Crystallographic information files for Sm{C-
(SiHMe2)3}2THF2 (1b), Yb{C(SiHMe2)3}2ImtBu (4a),
and Sm{C(SiHMe2)3}2ImtBu (4b) (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.D.S.: sadow@iastate.edu.
■
(13) Note that the polarity of SiH and CH bonds are reversed, and
the nature of their interactions with metal centers is likely distinct.
Therefore, M−H−Si structures in this paper will be referred to as
nonclassical, agostic-like, or as three-center−two-electron interactions.
(14) Gagne, M. R.; Nolan, S. P.; Marks, T. J. Organometallics 1990, 9,
1716−1718.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully thank the National Science Foundation
(CHE-1464774) for financial support. Dr. Sarah Cady is
thanked for valuable NMR assistance.
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Organic Synthesis. Wiley: New York, 1967.
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