Reversible addition of the Si-H bond of phenylsilane to the Sc=N bond of a scandium terminal imido complex

Article abstract of DOI:10.1021/om3010349

The facile and reversible addition of the Si-H bond of phenylsilane to the Sc=N bond of the scandium terminal imido complex [LSc=NDIPP(DMAP)] (1; L = [MeC(N(DIPP))CHC(Me)(NCH₂CH₂NMe)]^-, DIPP = 2,6-ⁱPr₂C₆H₃) is reported. The reaction gives the scandium anilido hydride [LSc(H)(N(DIPP)(SiH₂Ph))] (2), and a labeling experiment shows a rapid σ-bond metathesis between Sc-H of the formed scandium anilido hydride and Si-H of phenylsilane during the reaction. 2 was trapped by an insertion reaction with diphenylcarbodiimide, giving the stable scandium anilido amidinate [LSc(N(DIPP)(SiH ₂Ph))(κ²(N,N′)-PhNCHNPh)] (3). Furthermore, the scandium terminal imido complex can efficiently catalyze the hydrosilylation of N-benzylidenepropan-1-amine. The reaction was completed within 2 h at 50 C with 5 mol % of catalyst loading and highly selectively produced the monoaminosilane.

Full text of DOI:10.1021/om3010349

Communication
pubs.acs.org/Organometallics
Reversible Addition of the Si−H Bond of Phenylsilane to the ScN
Bond of a Scandium Terminal Imido Complex
Jiaxiang Chu, Erli Lu, Yaofeng Chen,* and Xuebing Leng
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: The facile and reversible addition of the Si−H
bond of phenylsilane to the ScN bond of the scandium
terminal imido complex [LScNDIPP(DMAP)] (1; L 
[MeC(N(DIPP))CHC(Me)(NCH2CH2NMe)]−, DIPP =
2,6-iPr2C6H3) is reported. The reaction gives the scandium
anilido hydride [LSc(H)(N(DIPP)(SiH2Ph))] (2), and a
labeling experiment shows a rapid σ-bond metathesis between
Sc−H of the formed scandium anilido hydride and Si−H of
phenylsilane during the reaction. 2 was trapped by an insertion
reaction with diphenylcarbodiimide, giving the stable scandium
anilido amidinate [LSc(N(DIPP)(SiH2Ph))(κ2(N,N′)-PhNCHNPh)] (3). Furthermore, the scandium terminal imido complex
can efficiently catalyze the hydrosilylation of N-benzylidenepropan-1-amine. The reaction was completed within 2 h at 50 °C
with 5 mol % of catalyst loading and highly selectively produced the monoaminosilane.
ver the last two decades, terminal imido complexes of
early transition metals have attracted intense interest and
The scandium terminal imido complex [LScNDIPP-
(DMAP)] (1; L  [MeC(N(DIPP))CHC(Me)-
(NCH2CH2NMe)]−, DIPP = 2,6-iPr2C6H3) was prepared as
we previously reported.10 1H NMR spectroscopic monitoring
of the reaction in C6D6 showed that 1 rapidly reacted with 1
equiv of PhSiH3 at room temperature to give the new complex
2 with concomitant release of DMAP. The reaction reached
equilibrium in 1 h, and 63% of 1 was converted into 2. The 1H
NMR spectrum of the reaction solution at 1 h clearly showed
O
have been extensively studied.1 These complexes showed
versatile reactivities toward organic molecules, including
cycloaddition,2 Wittig-type group transfer,2d,f,3 and C−H2a,3a,4
and Si−H5 bond activation, and have been applied as useful
catalysts for some catalytic reactions, such as olefin metathesis6
and alkyne (or alkene) hydroamination.2b,7 One exception is
those with rare-earth-metal ions. The rare-earth-metal terminal
imido species, once formed, can easily assemble into more
stable μ or μn (n = 3, 4) bimetallic or multimetallic species8 or
undergo C−H bond activation to give the amides.9 We
synthesized and structurally characterized the first rare-earth-
metal terminal imido complex, a scandium terminal imido
complex.10 The complex was prepared from a scandium anilido
methyl complex supported by a tridentate β-diketiminato ligand
via alkane elimination in the presence of the Lewis base 4-
(dimethylamino)pyridine (DMAP). Later, we synthesized a
DMAP-free scandium terminal imido complex by using a β-
diketiminato-based tetradentate ligand.11 Recently, Cui and co-
workers reported a CpPN-based scandium terminal imido
complex, and Piers and co-workers reported a bulky β-
diketiminato ligand supported scandium terminal imido
complex.12 Both complexes are synthesized from the
corresponding scandium anilido alkyl complexes via alkane
elimination in the presence of DMAP. We discovered that the
scandium terminal imido complexes are capable of activating
elemental selenium,11 undergoing cycloaddition with CO2 and
a series of unsaturated organic small molecules,13 and even
reacting with metal halides.14 Recently, we found that the
scandium terminal imido complex can activate phenylsilane and
catalyze imine hydrosilylation. Herein we describe these results.
2
two doublets at 5.61 and 5.55 ppm with a JHH coupling
constant of 9.6 Hz for two diastereotopic hydrogen atoms of
SiH2Ph, which are comparable to those reported for the Ti−
N−SiH2Ph unit in the titanium complex Cp*Ti{MeC(NiPr)2}-
(H){N(NMe2)SiH2Ph}.5c,d Therefore, an addition reaction of
the Si−H bond of PhSiH3 to the ScN double bond of 1 to
form the scandium anilido hydride 2, as shown in Scheme 1,
was suggested, and the formation of a Sc−H bond causes
dissociation of DMAP from the scandium center. The
transformation of 1 to 2 can be promoted by adding excess
phenylsilane. At room temperature, 75% or 80% of 1 was
converted into 2 in 1 h when 3 or 5 equiv of PhSiH3 was used,
respectively. On the other hand, the equilibrium shifted to the
left upon addition of DMAP. The equilibrium constant of the
reaction at room temperature is 3.04 according to the equation
Ke = ([2][DMAP])/([1][PhSiH3]). Attempts to isolate 2 were
unsuccessful; evaporation of the volatiles from the reaction
solution of 1 with 5 equiv of PhSiH3 under reduced pressure
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: October 31, 2012
Published: December 24, 2012
© 2012 American Chemical Society
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Organometallics
Communication
Scheme 1
gave a mixture of 1, 2, and DMAP, in which only about 67% of
1 was converted into 2.
2.171(4) Å).10 The amino side arm of the tridentate nitrogen
ligand is no longer coordinated to the scandium center, due to
steric congestion. The amidinato ligand displays a delocalized
electronic framework and coordinates to the scandium center
through the two nitrogen atoms with Sc−N bond lengths of
2.255(2) and 2.261(2) Å, respectively. The anilido ligand is
coordinated with a Sc−N6 bond distance of 2.066(2) Å and
Sc−N6−C40 angle of 118.2(1)°. H1S and H2S atoms of the
anilido ligand were located from the Fourier map. The long
Sc···H1S and Sc···H2S distances (>3.60 Å) and large Sc−N6−
Si1 angle (130.6(1)°) indicate there is no Sc···H−Si agostic
interaction.
Trapping 2 by a insertion reaction with diphenylcarbodii-
mide was studied. The reaction was first investigated on a small
scale in C6D6. The complex 1 and 5 equiv of PhSiH3 were
mixed in C6D6, the mixture was allowed to stand at room
temperature for 1 h, and then 1 equiv of diphenylcarbodiimide
1
was added to this mixture. H NMR spectroscopic monitoring
of the reaction showed that the in situ generated 2 was mostly
converted into the new complex 3 in 15 min. The reaction was
scaled up in toluene, and 3 was isolated in 59% yield as a yellow
solid. The complex 3 was characterized by NMR spectroscopy
(1H and 13C), elemental analysis, and single-crystal X-ray
diffraction analysis (Figure 1), confirming that 3 is a scandium
Deuterium-labeling experiments were performed to provide
1
additional evidence for the reaction shown in Scheme 1. H
NMR spectroscopic monitoring of the reaction of 1 with 5
equiv of PhSiD3 in C6D6 showed that the formed 2-D did not
display the SiH2Ph signal at 5.61 and 5.55 ppm as for 2. This
reaction solution was subsequently treated with 1 equiv of
diphenylcarbodiimide, and 3-D was generated. In the 1H NMR
spectrum of 3-D, the SiH2Ph signal (5.53 ppm) and the
−NCHN− signal (8.40 ppm) were not observed (Scheme 2).
These observations clearly support the reaction suggested in
Scheme 1. The measurement of the KIE value of the reaction of
1 with phenylsilane by 1H NMR spectroscopy is difficult, as the
reaction is rapid even at low reactant concentration (0.005
mol/L) and low molar ratio of phenylsilane to 1 (1:1), and the
reaction is reversible. 1H NMR spectroscopic monitoring of the
reaction of 1 with a mixture of 0.5 equiv of PhSiH3 and 0.5
equiv of PhSiD3 in C6D6 showed an H/D exchange between
PhSiH3 and PhSiD3, which gave their partially deuterated
isotopomers. The observed H/D exchange between PhSiH3
and PhSiD3 is due to the σ-bond metathesis between Sc−H(or
D) of the formed scandium anilido hydride 2 (or 2-D) and Si−
D (or H) of phenylsilane, as shown in Scheme 3. The
occurrence of this σ-bond metathesis is not unexpected, and
similar σ-bond metathesis between Sc−H and H−H or C−H
has been documented.15 As a consequence of the formation of
partially deuterated phenylsilane, incorporation of Si−D into 2
was observed. The molar ratio of scandium anilido hydrides
containing PhSiH2, PhSiHD, and PhSiD2 substituents was ca.
1:2:1 and did not change after 1 and phenylsilane were mixed
for 10 min, although their concentrations all increased as the
1,2-addition reaction between 1 and phenylsilane proceeded.
Therefore, the σ-bond metathesis between Sc−H(or D) of the
scandium anilido hydride and Si−D (or H) of phenylsilane is
very fast.
Figure 1. Molecular structure of 3 with ellipsoids set at the 30%
probability level. Isopropyl groups at the DIPP substituents, hydrogen
atoms (except the amidinate hydrogen atom and two hydrogen atoms
bonded to silicon), and the hexane molecules in the lattice have been
omitted for clarity. Selected bond lengths (Å) and angles (deg): Sc−
N1 = 2.156(2), Sc−N2 = 2.128(2), Sc−N4 = 2.255(2), Sc−N5 =
2.261(2), Sc−N6 = 2.066(2), N4−C23 = 1.331(3), N5−C23 =
1.308(3), Si−N6 = 1.7317(19); Sc−N4−C23 = 90.45(14), Sc−N5−
C23 = 90.76(14), Sc−N6−Si = 130.6(1), Sc−N6−C40 = 118.2(1),
N4−C23−N5 = 118.3(2).
anilido amidinate, as shown in Scheme 1. The 1H NMR
spectrum of 3 displays a −NCHN− signal of the amidinato
ligand at 8.40 ppm. In 3, the distances from the scandium
center to two nitrogen atoms of the tridentate nitrogen ligand
are 2.156(2) and 2.128(2) Å, respectively, which are slightly
shorter than those in 1 (2.232(5) and 2.198(5) Å) and the
corresponding scandium anilido methyl complex (2.206(3) and
In comparison to rare-earth-metal-catalyzed olefin hydro-
silylation, much fewer examples of rare-earth-metal-catalyzed
imine hydrosilylation have been reported. This is not surprising,
considering that the silanolysis of the Ln−N bond is more
difficult than that of the Ln−C bond. In the lanthanocene-
catalyzed imine hydrogenation, Marks and co-workers used
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Organometallics
Communication
Scheme 2
silanolysis of Sc−N bond of 4 with PhSiH3 gives the
hydrosilylation product and regenerates 2; (b) 4 releases the
hydrosilylation product and gives 1 via silyl abstraction, and
then 1 reacts with PhSiH3 to regenerate 2.
Scheme 3
In summary, the scandium terminal imido complex 1 is able
to activate the Si−H bond of phenylsilane in a 1,2-addition
fashion to give the scandium anilido hydride 2. The reaction is
facile and reversible. Apart from the phenylsilane elimination,
the scandium anilido hydride 2 also undergoes a rapid σ-bond
metathesis between the Sc−H and Si−H bonds with
phenylsilane. The scandium terminal imido complex 1
efficiently catalyzes the hydrosilylation of N-benzylidenepro-
pan-1-amine. The scope and mechanism of the scandium
terminal imido complex catalyzed imine hydrosilylation are
under investigation.
PhSiH3 as the promoter, and a hydrosilylation product was
detected in ca. 10% yield in the case of N-benzylideneaniline.16
Takaki and co-workers synthesized the rare divalent Yb
complexes Yb(η-Ph2CNAr)(HMPA),17a which catalyzed the
imine hydrosilylation to give a mixture of mono- and
diaminosilanes.17b Primary studies showed that 1 efficiently
catalyzes the hydrosilylation of N-benzylidenepropan-1-amine
with PhSiH3 (Scheme 4). When 5 mol % of 1 was used, the
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 4
Text, figures, a table, and a CIF file giving experimental details,
characterization data, and X-ray crystallographic data for 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yaofchen@mail.sioc.ac.cn. Fax: (+86)21-64166128.
■
reaction was complete within 12 h at room temperature or 2 h
at 50 °C. The monoaminosilane was obtained in nearly
quantitative yield, and the diaminosilane was not detected. A
proposed catalytic cycle for the scandium terminal imido
complex catalyzed imine hydrosilylation is outlined in Scheme
5. An addition reaction of the Si−H bond of PhSiH3 to the
ScN double bond of the scandium terminal imido complex 1
gives the scandium anilido hydride 2, and then an insertion of
the CN double bond of imine into the Sc−H bond of 2
generates the scandium anilido amide 4. The regeneration of 2
from 4 can be realized by two possible pathways: (a) a direct
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the State Key Basic Research &
Development Program (Grant No. 2012CB821600), the
National Natural Science Foundation of China (Grant Nos.
21072209, 21132002 and 21121062), and the Chinese
Academy of Sciences.
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Scheme 5
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