Silyl-pyridine-amine pincer-ligated iridium complexes for catalytic silane deuteration: Via room temperature C-D bond activation of benzene- d 6

Article abstract of DOI:10.1039/c8cc09178a

Iridium-hydrido complexes bearing a hemilabile silyl-pyridine-amine pincer ligand were synthesised. They were found to catalyse Si-H deuteration of trialkylsilanes with excess benzene-d₆ in 99-94% conversion at room temperature through C-D bond

Full text of DOI:10.1039/c8cc09178a

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Silyl–pyridine–amine pincer-ligated iridium
complexes for catalytic silane deuteration via
room temperature C–D bond activation of
benzene-d6†
Cite this: Chem. Commun., 2019,
5, 957
5
Received 19th November 2018,
Accepted 14th December 2018
Takashi Komuro,
Tomohiro Osawa, Ryuju Suzuki,
Daiki Mochizuki,
DOI: 10.1039/c8cc09178a
Hironori Higashi and Hiromi Tobita
*
rsc.li/chemcomm
Iridium–hydrido complexes bearing a hemilabile silyl–pyridine– reactions with C D that proceed efficiently have recently been
6
6
6
–8
amine pincer ligand were synthesised. They were found to catalyse reported.
These reactions, however, require heating at
Si–H deuteration of trialkylsilanes with excess benzene-d in 99–94% 70–80 1C for 490% conversion.
6
conversion at room temperature through C–D bond activation and
H/D exchange.
In this work, we designed a silyl–pyridine–amine (SiNN-type)
4
SiNN
pincer ligand with a lutidine backbone, abbreviated as ‘‘Lut
’’
11,12
(
Scheme 1).
Because coordination of the amine moiety in
complexes is weakened by the strong trans influence
SiNN
The design of supporting ligands for metal catalysts toward metal–Lut
activation and transformation of the inert C–H/C–D bonds of of the silyl ligand, Lut
SiNN
is expected to serve as a hemilabile
organic molecules is still a major challenge in coordination and ligand that generates a vacant reaction site at the metal centre for
1
synthetic chemistry. As candidates for such supporting ligands, accommodation and bond-activation of substrates under mild
1
3
pincer ligands involving a silyl ligand moiety have recently conditions (Scheme 1). Herein, we report the synthesis of
2
–4
SiNN
attracted considerable attention.
Strong s-donating and iridium–Lut
complexes and their high catalytic efficiency
trans-labilising properties of the silyl ligand are expected to be for deuteration of trialkylsilanes via C–D bond activation of
effective for generating an electron-rich and coordinatively-
unsaturated metal centre. These properties of the metal centre
would facilitate oxidative addition of C–H/C–D bonds to it. For pincer ligand Lut
C
6
D
6
at room temperature.
Iridium–hydrido complexes bearing the silyl–pyridine–amine
SiNN
were synthesised by reactions of a ligand
instance, Turculet et al. have reported that a PSiP-type pincer- precursor, 2-(di-tert-butylsilyl)methyl-6-[(diethylamino)methyl]-
SiNN
ligated iridium complex cleaved a C–D bond of benzene-d
pyridine (HLut
), with [IrCl(coe) ] (Scheme 2). Thus, treat-
6
2 2
3
SiNN
(
C D ) at room temperature. Nevertheless, examples of silyl ment of HLut
with [IrCl(coe) ] (0.4 equiv.) at 60 1C for 24 h
6
6
2 2
pincer complexes that serve as efficient catalysts for C–H/C–D in toluene afforded a mixture of a 16-electron iridium complex,
2
,4
SiNN
bond transformation are still very limited.
Metal catalysed bond activation of C
Ir(Lut
)(H)Cl (1), and an unidentified iridium complex A
SiNN
6
D
6
is known to be containing the Lut
and hydrido ligands (see the ESI† for
applicable for Si–H deuteration of hydrosilanes via H/D exchange NMR data of A). For the purpose of converting these products into
5
–8
that gives deuteriosilanes.
These products are useful for isolable compounds, the mixture was treated with 4-(dimethylamino)-
mechanistic studies of reactions involving Si–H bond activation pyridine (DMAP, 1.1 equiv. to Ir) at room temperature for 10 min
9
SiNN
and for isotope labelling of organic compounds. Catalytic silane to give a DMAP-coordinated complex Ir(Lut
)(H)(DMAP)Cl (2)
14
deuteration using C D is regarded as a safer and straightforward in 62% isolated yield (Scheme 2). DMAP-free complex 1 was
6
6
method compared with a conventional one using the reaction of eventually synthesised in 75% isolated yield by heating a 0.4 : 1
SiNN
chlorosilanes with LiAlD
is more readily available than D
still more commonly used for metal-catalysed deuteration of
4
. Furthermore, as a deuterium source, mixture of [IrCl(coe)
]
2 2
and HLut
in toluene at a higher
C D
6 6
2
gas, although the latter is temperature (70 1C) for a longer period (3 days). At this stage,
10
hydrosilanes. Only a few examples of catalytic silane deuteration
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai
9
80-8578, Japan. E-mail: tobita@m.tohoku.ac.jp
†
Electronic supplementary information (ESI) available: Synthetic and experimental
procedures, characterisation data, details of X-ray crystallographic analysis and NMR
0
SiNN
spectra. CCDC 1590105 (for 1), 1590106 (for 2) and 1879207 (for 2 ). For ESI and
Scheme 1 Silyl–pyridine–amine pincer ligand (Lut
) and its expected
crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc09178a
hemilability.
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almost unchanged from the corresponding bond distances of 1
1
5
(
avg. 2.30 and 2.00 Å, respectively). This observation indicates
that the strength of the Ir–N(amine) coordinate bond varies
flexibly depending on the steric environment around the metal
centre and possibly its electronic properties. The Ir–N(DMAP)
bond distance of 2 (2.212(4) Å) is longer than that of the
SiNN
Ir–N(pyridine ring of Lut
) bond by 0.19 Å, which is attributable
to the large trans influence of the hydrido ligand trans to DMAP.
DMAP complex 2 in solution shows dynamic behaviour
possibly caused by the dissociation of the amine coordinating
SiNN 16
1
moiety of the pincer ligand Lut
.
In the H NMR spectrum
of 2 in toluene-d at 300 K, two remarkably broadened signals
8
SiNN
for the methyl groups of NEt of Lut
appeared at 0.59 and
2
SiNN
ca. 1.1–1.6 ppm while two signals for the t-Bu groups on Si
remained inequivalent (see Fig. S2 in ESI†). Upon cooling the
Scheme 2 Synthesis of iridium–Lut
pincer complexes.
2
solution to 260 K, the broad signals for the NEt methyl groups
unidentified complex A mostly diminished. These results imply
that unidentified complex A is an intermediate for the formation
changed to two triplets (0.53 and 1.41 ppm). A 2D EXSY NMR
spectrum was measured at this temperature, which showed cross
peaks between these triplets (Fig. S3 in ESI†). These observations
indicate that the dynamic behaviour involves the exchange of the
two inequivalent ethyl groups on the amine nitrogen. This behaviour
is considered to occur through dissociation/coordination of
the amine nitrogen accompanied by nitrogen inversion (see
1
29
1
of 1. The H and Si{ H} NMR spectra of 1 and 2 in C D show
6
6
Ir–H signals at ꢀ25.58 (1) and ꢀ21.21 (2) ppm and Ir–Si signals at
5.2 (1) and 7.4 (2) ppm, respectively. Complexes 1 and 2 were also
found to be interconvertible: 1 reacted instantaneously with DMAP
1 equiv.) in C at room temperature to give 2 quantitatively,
while the reverse reaction proceeded by treatment of 2 with Lewis
acid BPh (1 equiv.) at room temperature accompanied by the
2
(
6 6
D
SiNN
Scheme S1 in the ESI†). We therefore can regard that Lut
3
serves as a hemilabile ligand.
formation of DMAPꢁBPh (Scheme 2).
3
The Ir–H hydrogens of complexes 1 and 2 were readily
Single crystal X-ray analysis revealed that complex 1 (Fig. 1(a))
3
deuterated in benzene-d6 (solvent, 1 ꢂ 10 equiv.) at room
SiNN
possesses a five-coordinate Ir centre with Lut
, hydrido and
1
temperature (Scheme 3(a)). H NMR monitoring of the reaction
chlorido ligands, where the Ir–H bond is tilted toward the silyl
of 1 or 2 with C D at room temperature showed that the
6
6
15
silicon (Si–Ir–H angle = avg. ca. 601). The position of this hydrido
hydrogen was also supported by a DFT optimised structure of 1
intensity of the Ir–H signal gradually decreased. The deuteration
reaction of coordinatively-unsaturated complex 1 was much
faster than that of 2: 97% deuteration of Ir–H of 1 was achieved
(
(
Si–Ir–H = 641, see the ESI†). On the other hand, complex 2
Fig. 1(b)) has a six-coordinate, octahedral geometry with a
2
in 1 h while that of 2 took 18 h. The H NMR spectrum of each of
SiNN
tridentate Lut
ligand in a meridional coordination mode.
the reaction mixtures of deuteration showed an Ir–D signal at
The Ir–N(amine) bond distance of 2 (2.386(4) Å) is considerably
ꢀ
25.2 (1-d) or ꢀ21.0 (2-d) ppm.
15
elongated in comparison with that of 1 (avg. 2.24 Å) by ca. 0.15 Å.
When the reaction of 1 or 2 with excess C D as a solvent was
6
6
SiNN
In contrast, the Ir–Si and Ir–N(pyridine ring of Lut
) bond
performed in the presence of a stoichiometric amount of
hydrosilane, not only Ir–H but also Si–H was deuterated via
activation of the C–D and Si–H bonds (Scheme 3(b)). Thus,
treatment of complex 1 or 2 with 1 equiv. of triethylsilane in
distances of 2 (2.2938(11) and 2.019(4) Å, respectively) remain
6
benzene-d at room temperature for 1 h (for 1) or 12 h (for 2) gave
Fig. 1 Crystal structures of complexes 1 (left, (a)) and 2 (right, (b)) (50%
probability ellipsoids for all non-hydrogen atoms). Hydrogen atoms except
the hydrido hydrogen are omitted for clarity. For 1, because there is no
significant difference between the structures of two crystallographically
independent molecules 1-A and 1-B, only the structure of 1-A is depicted.
For selected interatomic distances and angles, see the ESI.†
6 6
Scheme 3 (a) Deuteration of Ir–H of complexes 1 and 2 with excess C D
3
and (b) Si–H deuteration reactions of HSiEt in the presence of a stoichio-
metric amount of 1 or 2.
9
58 | Chem. Commun., 2019, 55, 957--960
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Table 1 Deuteration of hydrosilanes with C
ligated iridium complex 1 or 2
6
D
6
catalysed by SiNN pincer-
On the other hand, reactions of the siloxy-substituted tertiary
silane (HSiMe(OSiMe ) ) and the dihydrosilane (H SiEt ) in the
3 2 2 2
a
presence of 1 mol% 1 or 2 (entries 6, 8, 15 and 16) resulted in
low or undetectable deuterium incorporation. In the reaction of
aryl-substituted tertiary silane HSiMe Ph catalysed by 1 (entry 7),
2
scrambling of the Me and Ph substituents occurred competitively,
and deuterium incorporation in the resulting hydrosilanes was high
b
Entry Hydrosilane
Cat. Time (h) Deuterium incorporation (%D)
(
ca. 74–88%) in most cases (see the ESI†).
Nevertheless, the catalytic activity of 1 and 2 for the Si–H
deuteration of trialkylsilanes with C is significantly higher
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
HSiMe
HSiMe
HSiMe
2
2
2
Et
(i-Pr)
(t-Bu)
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
1
3
9
2
3
9
173
1
97
97
94
98
99
12
6 6
D
5–8
than that of previously-reported catalysts. On the basis of the
HSiEt
HSi(n-Pr)
HSiMe(OSiMe
3
observation of the dynamic behaviour of complex 2 in solution
3
SiNN
3
)
)
2
via dissociation of the amine nitrogen of Lut
(hemilability
d
HSiMe
SiEt
2
Ph
81 (50% NMR yield)
B0
95
85
96
95
97
94
SiNN
of Lut
Si–H deuteration shown in Scheme S2 (see the ESI†). In this
), we tentatively propose a mechanism for catalytic
H
2
2
HSiMe
HSiMe
HSiMe
HSiMe
2
2
2
2
Et
Et
(i-Pr)
(t-Bu)
7
c
SiNN
0
1
2
3
4
5
6
23
52
72
12
14
47
6
mechanism, the labile amine nitrogen of Lut
in catalytic
intermediates easily dissociates to generate enough space to
activate the C–D/Si–H bonds on iridium. This phenomenon
possibly contributes to the high catalytic activity of 1 and 2.
In conclusion, we have demonstrated that silyl–pyridine–
amine pincer-ligated iridium–hydrido complexes 1 and 2 served
as powerful catalysts for Si–H deuteration of trialkylsilanes
HSiEt
HSi(n-Pr)
HSiMe(OSiMe
SiEt
3
3
3
2
20
B0
H
2
2
a
Conditions: catalyst (3 mmol), C
6
D
6
(0.6 mL, 6.8 mmol; neat solvent)
(0.28–0.30 mmol). Deuterium incorporation (%D) of deuterated
silane DSiR was determined by comparison of the intensity ratio of the
H NMR signals of Si–H and selected C–Hs of other substituents R with activation and H/D exchange on iridium at room temperature.
b
and HSiR
3
6
with benzene-d . This deuteration proceeds via C–D/Si–H bond
3
1
3
the corresponding intensity ratio for the original hydrosilane HSiR .
The activation of the inert C–D bonds is considered to be
accelerated not only by the electron-rich Ir centre generated
by the strong s-donation of the silyl ligand, but also by the
c
d
1
mol% DMAP was added. Scrambling products H/DSiMe
3 2
, H/DSiMePh
and H/DSiPh were also formed (see the ESI).
3
facile generation of a vacant site on Ir due to the hemilability of
SiNN
deuterated silane DSiEt
together with 1-d or 2-d. The H NMR spectrum of the reaction
mixture showed a signal of DSiEt at 3.9 ppm.
3
with 95–97% deuterium incorporation the pincer ligand Lut
.
2
This work was supported by JSPS KAKENHI grant numbers:
JP16K05714, JP15H03782 and JP25410058 from the Japan Society
3
On the basis of the stoichiometric reactions summarised in for the Promotion of Science (JSPS). We are grateful to the Research
Scheme 3, we examined the catalytic Si–H deuteration of and Analytical Center for Giant Molecules, Tohoku University for
hydrosilanes using complex 1 or 2 as a catalyst and C D as a mass spectroscopic measurements and elemental analysis.
6 6
deuterium source (Table 1). In the presence of 1 mol% 16-electron
complex 1, treatment of trialkylsilanes HSiR3 (SiR3 = SiMe Et,
2
Conflicts of interest
SiMe
23 equiv.) at room temperature gave the corresponding deuterated
silanes DSiR with high deuterium incorporation (99–94% D) (see
2 2 3 3 6 6
(i-Pr), SiMe (t-Bu), SiEt and Si(n-Pr) ) with excess C D
(
There are no conflicts of interest to declare.
3
17
entries 1–5 and the ESI†). DMAP complex 2 is also catalytically
active for silane deuteration (entries 9 and 11–14), but the reaction
rates are slower than those of the corresponding reactions catalysed
Notes and references
1
For selected recent examples, see: A. Kumar, T. Zhou, T. J. Emge,
O. Mironov, R. J. Saxton, K. Krogh-Jespersen and A. S. Goldman,
J. Am. Chem. Soc., 2015, 137, 9894; C.-I. Lee, J. C. DeMott, C. J. Pell,
A. Christopher, J. Zhou, N. Bhuvanesh and O. V. Ozerov, Chem. Sci.,
by 1 under the same conditions. When the deuteration of HSiMe Et
2
catalysed by 2 was carried out in the presence of 1 mol% DMAP
2
015, 6, 6572; G. Choi, H. Tsurugi and K. Mashima, J. Am. Chem.
Soc., 2013, 135, 13149; G. Wang, L. Xu and P. Li, J. Am. Chem. Soc.,
015, 137, 8058.
(entry 10), the reaction was considerably slowed down. Even after a
longer reaction time of 23 h, compared with 7 h in the absence of
DMAP (entry 9) that led to 95% deuterium incorporation, only 85%
deuterium incorporation of DSiMe Et was attained. This implies
2
that the catalytically-active species is 16-electron complex 1, which
can be generated from 2 (a catalyst precursor) by dissociation of
2
2 L. Turculet, in Pincer and Pincer-Type Complexes: Applications in
Organic Synthesis and Catalysis, ed. K. J. Szab ´o and O. F. Wendt,
Wiley-VCH, Weinhein, 2014, pp. 149–187; P. Sangtrirutnugul and
T. D. Tilley, Organometallics, 2007, 26, 5557; H. Fang, Y.-K. Choe,
Y. Li and S. Shimada, Chem. Asian J., 2011, 6, 2512; J. Takaya, S. Ito,
H. Nomoto, N. Saito, N. Kirai and N. Iwasawa, Chem. Commun.,
DMAP. The reaction rate for the deuteration of HSiMe R (R = Et, i-Pr
2
2
015, 51, 17662.
or t-Bu) decreases as the trialkylsilane becomes bulkier in the
following order Et 4 i-Pr 4 t-Bu (entries 1–3, 9, 11 and 12).
A preparative-scale reaction was also examined by the use of 1.0 g
3
4
D. F. MacLean, R. McDonald, M. J. Ferguson, A. J. Caddell and
L. Turculet, Chem. Commun., 2008, 5146.
Ozerov et al. have recently reported iridium and rhodium catalysts
bearing
a silyl–amido–quinoline SiNN-type pincer ligand for
of HSi(n-Pr)
which gave DSi(n-Pr)
3
, C
6
D
6
(25 equiv.) and 0.1 mol% 2 at 28 1C for 19 h,
(98% D) in 76% isolated yield.
C–H borylation of terminal alkynes and arenes. See: C.-I. Lee, J. Zhou
and O. V. Ozerov, J. Am. Chem. Soc., 2013, 135, 3560; C.-I. Lee,
3
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N. A. Hirscher, J. Zhou, N. Bhuvanesh and O. V. Ozerov, Organometallics, 12 We have previously developed a lutidine-based SiNSi-type pincer
015, 34, 3099. ligand. See: T. Komuro and H. Tobita, Chem. Commun., 2010, 46, 1136.
M. D. Curtis, L. G. Bell and W. M. Butler, Organometallics, 1985, 4, 701; 13 For a related study on a hemilabile PSiN-type silyl pincer ligand, see:
2
5
D. H. Berry and L. J. Procopio, J. Am. Chem. Soc., 1989, 111, 4099; P. I.
Djurovich, P. J. Carroll and D. H. Berry, Organometallics, 1994, 13, 2551.
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6
7
8
14 Complex 2 was found to isomerise nearly quantitatively upon
heating at 60 1C for 3 days in toluene to a thermodynamically more
0
stable complex 2 through exchange of the positions of the DMAP
4
3, 10816.
and chlorido ligands of 2 (see the ESI† for the synthetic procedure
0
9
For deuterium labelling of unsaturated organic compounds with
and characterisation of 2 ).
deuteriosilanes, see: M. Rubio, J. Campos and E. Carmona, Org. 15 The average bond distance or angle for the two crystallographically
Lett., 2011, 13, 5236; J. D. Egbert and S. P. Nolan, Chem. Commun., independent molecules of 1.
2
012, 48, 2794; J. Campos, M. Rubio, A. C. Esqueda and E. Carmona, 16 Five-coordinate complex 1 was also found to show dynamic behaviour
1
J. Labelled Compd. Radiopharm., 2012, 55, 29.
in solution on the basis of variable-temperature H NMR measure-
1
0 For selected examples of catalytic silane deuteration using D
deuterium source, see: J. Campos, A. C. Esqueda, J. L o´ pez-Serrano,
L. S ´a nchez, F. P. Cossio, A. de Cozar, E. A´ lvarez, C. Maya and
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2
as a
ments (see the ESI†). However, in this case, exchange of the two Et
groups on the amine nitrogen, the two t-Bu groups on silicon and
each of the two methylene protons of the py-CH1
moieties occurs simultaneously. For instance, the H NMR spectrum
of a solution of 1 in toluene-d at 300 K shows a sharp triplet for the
Me groups of NEt at 1.11 ppm and a single broad signal for the t-Bu
2 2
-N and py-CH -Si
8
4
7, 9723; K. A. Smart, E. Mothes-Martin, T. Annaka, M. Grellier and
2
S. Sabo-Etienne, Adv. Synth. Catal., 2014, 356, 759; Y. Kratish,
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1 As a related supporting ligand containing an amine-substituted
lutidine backbone, Milstein et al. have developed a PNN-type
phosphine–pyridine–amine pincer ligand and demonstrated its
significant utility for metal catalysts. See: J. Zhang, G. Leitus,
groups at 1.30 ppm (see Fig. S1 in the ESI†). Upon cooling the solution
to 230 K, these H NMR signals split into two inequivalent triplets
1
1
(1.09 and 1.13 ppm for Me) and singlets (1.24 and 1.55 ppm for t-Bu),
respectively. This behaviour can be explained by migration of the
hydrido ligand in the NMR timescale between the upper and lower
sides of the SiNN plane.
1
2
29
Y. Ben-David and D. Milstein, J. Am. Chem. Soc., 2005, 127, 10840; 17 Deuterated silanes were identified by H, H and Si NMR spectro-
C. Gunanathan and D. Milstein, Chem. Rev., 2014, 114, 12024. scopy (see the ESI†).
960 | Chem. Commun., 2019, 55, 957--960
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