Unprecedent formation of methylsilylcarbonates from iridium-catalyzed reduction of CO2 with hydrosilanes

Article abstract of DOI:10.1039/d0ra00204f

The iridium complex [Ir(μ-CF₃SO₃)(κ²-NSi^Me2)₂]₂ (3) (NSi^Me2 = {4-methylpyridine-2-yloxy}dimethylsilyl) has been prepared by reaction of [Ir(μ-Cl)(κ²-NSi^Me2)₂]₂ (1) with two equivalents of AgCF₃SO₃. The solid structure of 3 evidenced its dinuclear nature, being a rare example of an iridium species with triflate groups acting as bridges. The 3-catalyzed reduction of CO₂ with HSiMe(OSiMe₃)₂ affords a mixture of the corresponding silylformate and methoxysilane together with the silylcarbonate CH₃OCO₂SiMe(OSiMe₃)₂ (4a). This is the first time that the formation of silylcarbonates has been observed from the catalytic reduction of CO₂ with silanes. Analogous behaviour has been observed when HSiMe₂Ph and HSiMePh₂ were used as reductants.

Full text of DOI:10.1039/d0ra00204f

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Unprecedent formation of methylsilylcarbonates
from iridium-catalyzed reduction of CO2 with
hydrosilanes†
Cite this: RSC Adv., 2020, 10, 9582
Jefferson Guzman, Pilar Garcıa-Orduna, Fernando J. Lahoz
´
´
˜
*
and Francisco J. Fernandez-Alvarez
´
The iridium complex [Ir(m-CF3SO3)(k2-NSiMe2)2]2 (3) (NSiMe2 ¼ {4-methylpyridine-2-yloxy}dimethylsilyl) has
been prepared by reaction of [Ir(m-Cl)(k2-NSiMe2)2]2 (1) with two equivalents of AgCF3SO3. The solid
structure of 3 evidenced its dinuclear nature, being a rare example of an iridium species with triflate
groups acting as bridges. The 3-catalyzed reduction of CO2 with HSiMe(OSiMe3)2 affords a mixture of
Received 8th January 2020
the
corresponding
silylformate
and
methoxysilane
together
with
the
silylcarbonate
Accepted 23rd February 2020
DOI: 10.1039/d0ra00204f
rsc.li/rsc-advances
CH3OCO2SiMe(OSiMe3)2 (4a). This is the first time that the formation of silylcarbonates has been
observed from the catalytic reduction of CO2 with silanes. Analogous behaviour has been observed
when HSiMe2Ph and HSiMePh2 were used as reductants.
a continuation of our studies on the chemistry of iridium
complexes with pyridine-2-yloxyI-silyl ligands, we have found
that the triate derivative [Ir(m-CF3SO3)(k2-NSiMe2)2]2 (3)
Introduction
The catalytic reaction of CO2 with silicon-hydrides has proven to
be an effective and thermodynamically favoured methodology
for its reduction to formate, formaldehyde, methanol or
methane (Scheme 1).1,2
However, to the best of our knowledge the formation of
methylsilylcarbonates, MeOCO2SiR3 (Fig. 1),3 as products of the
catalytic reduction of CO2 with silicon-hydrides has not been
reported so far.
On the other hand, hydrosiloxanes are obtained in large
scale as side products of the silicone industry and therefore,
the development of catalytic processes effective for the
selective reduction of CO2 based on hydrosiloxanes, instead
of hydrosilanes, as reductants is of great interest.4 In
this context, in recent years we have focused our research
on the application of iridium species as catalysts for the
reduction of CO2 with 1,1,1,3,5,5,5-heptamethyltrisiloxane,
HSiMe(OSiMe3)2.5,6
(Scheme 2) catalyzes the reaction of CO2 (3 bar) with
HSiMe(OSiMe3)2 to give a mixture of the corresponding
silylformate and methoxysilane along with a new compound
that has been characterized by multinuclear NMR spectros-
copy as CH3OCO2SiMe(OSiMe3)2 (4a). To the best of our
knowledge, this is the rst time that the formation of silyl-
carbonates from the catalytic reduction of CO2 with silanes
has been observed. The results from these studies are
described below.
Recently, we have described the synthesis and catalytic
behavior of the iridium(III) complexes [Ir(m-Cl)(k2-NSiMe2)2]2 (1)
and [Ir(CF3CO2)(k2-NSiMe2)2] (2) (NSiMe2 ¼ {4-methylpyridine-2-
yloxy}dimethylsilyl) (Scheme 2).6 Species 2 catalyzed the selec-
tive reduction of CO2, depending on the reaction conditions, to
silylformate or methoxysilane with HSiMe(OSiMe3)2.6 As
Scheme 1 Products from the catalytic reduction of CO2 with silicon-
hydrides reported so far.
´
´
´
´
´
Departamento de Quımica Inorganica, Instituto de Sıntesis Quımica y Catalisis
´
Homogenea (ISQCH), Universidad de Zaragoza, Facultad de Ciencias, 50009,
Zaragoza, Spain. E-mail: paco@unizar.es
† Electronic supplementary information (ESI) available: NMR spectra and
crystallographic information. CCDC 1972218. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/d0ra00204f
Fig. 1 Methylsilylcarbonates.
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1H and 13C{1H} NMR spectra of 3 in C6D6 show only one
pattern of resonances for the four NSiMe2 ligands, which agrees
with its high symmetry in solution. 29Si{1H} NMR spectra of 3
show a resonance at d 38.2 ppm, corresponding to the four
silicon atoms, which compares well to the value of d 39.7 ppm
reported for 2.6 In addition, a singlet resonance at d ꢀ76.97 ppm
in the 19F{1H} NMR spectra conrm the presence of the triate
ligand.
3-Catalyzed reduction of 13CO2 with silicon hydrides
Preliminary studies on the 3-catalyzed reaction of CO2 (3 bar)
with HSiMe(OSiMe3)2 at 298 K showed that 24 hours are
required to achieve the conversion of 94% of the starting
hydrosiloxane into a mixture of the corresponding silylformate
(53.0%) and methoxysilane (21.4%) along with a new compound
(25.6%) (TOF ¼ 3.9 hꢀ1). This behavior differs from that previ-
ously reported for 2, which under the same reaction conditions
catalyzed the full conversion of HSiMe(OSiMe3)2 into silylfor-
mate (93.0%) and methoxysilane (7.0%) in 3.5 h (TOF ¼ 28.6
hꢀ1).6 Therefore, complex 2 is not only more active but also
more selective than 3.
Scheme 2 Preparation of the catalytic precursors 2 and 3.
Results and discussion
Synthesis and characterization of the catalyst precursor
The reaction of complex 1 with two equivalents of AgCF3SO3
leads to the formation of [Ir(m-CF3SO3)(k2-NSiMe2)2]2 (3), which
has been isolated as a white solid in 89% yield. The solid
structure of complex 3 evidenced its dinuclear nature (Fig. 2). As
far as we know, 3 is the rst example of an iridium species with
triate groups acting as bridges. The molecule exhibits a crys-
tallographic inversion center, with half the molecule as the
independent part. Each metal atom shows an octahedral
geometry like that found for 1.6 Main difference concerns the
To determine the nature of the new species observed when 3
is used as catalyst for the hydrosilylation of CO2, we decided to
1
study this reaction using isotopically labeled 13CO2. Thus, H
and 13C NMR studies of the 3-catalyzed (1.0 mol%) reaction of
13CO2 (2.7 bar, 323 K, C6D6) with HSiMe(OSiMe3)2 were per-
formed. These experiments evidenced, since early reaction
stages, the presence of
a
mixture of 13CH3O–13CO2-
ꢀ
separation between the iridium atoms (6.0870(2) A) in 3,
signicantly longer than that observed for 1 with chlorine
SiMe(OSiMe3)2 (4a), together with the corresponding silylfor-
mate, H13CO2SiMe(OSiMe3)2 (5a), and the methoxysilane,
13CH3OSiMe(OSiMe3)2 (6a) (Table 1).
ꢀ
bridges (4.0718(12) A).
The most characteristic resonance in the 1H NMR spectra of
4a is a doublet of doublets centered at d 3.33 ppm (1JC–H ¼ 146.9
3
Hz; JC–H ¼ 4.1 Hz) corresponding to the 13CH3O protons
Table 1 Results from 3-catalyzed reduction of 13CO2 (2.7 bar) with
a
HSiMe(OSiMe3)2
Entry
T (K)
4ab
5ab
6ab
Conversionc
Time (h)
1
2
3
4
323
323
323
358
358
323
26.7
18.8
19.0
11.7
8.5
65.2
70.0
70.0
68.7
68.0
0
8.1
70
98
98
100
100
0
3
Fig. 2 Molecular structure of compound 3. For clarity hydrogen atoms
have been omitted. Primed atoms are related to unprimed ones
through 1 ꢀ x, 1 ꢀ y,ꢀz symmetry operation. Selected bond distances
11.2
11.0
19.6
23.5
0
12
24
24d
48d
24
ꢁ
ꢀ
(A) and angles ( ): Ir–Si(1), 2.2570(5); Ir–Si(2), 2.2615(5); Ir–O(3),
2.3653(12); Ir–O(50), 2.4331(13); Ir–N(1), 2.0590(15); Ir–N(2),
2.0583(15); Si(1)–Ir–Si(2), 91.68(5); Si(1)–Ir–O(3), 95.18(3); Si(1)–Ir–
O(50), 172.22(3); Si(1)–Ir–N(1), 81.57(4); Si(1)–Ir–N(2), 94.94(4); Si(2)–
Ir–O(3), 170.27(3); Si(2)–Ir–O(50), 93.26(3); Si(2)–Ir–N(1), 95.76(4);
Si(2)–Ir–N(2), 82.22(4); O(3)–Ir–O(50), 80.67(4); O(3)–Ir–N(1), 92.06(5);
O(3)–Ir–N(2), 90.32(5); O(50)–Ir–N(1), 91.95(5); O(50)–Ir–N(2), N(1)–Ir–
N(2), 175.94(6).
5
6e
0
a
b
General conditions: 3 (1.0 mol%) in 0.5 mL of dry C6D6. Calculated
by 1H NMR and expressed in mol%. Calculated by 1H NMR relative
c
d
to the starting hydrosiloxane and expressed in mol%. Aer 24 h at
323 K. e Control experiment without iridium catalyst (Fig. S24 see ESI).
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The formation of methoxy-silanes from the catalytic reduc-
tion of CO2 with hydrosilanes has been so far explained by
reaction of bis(silyl)acetals with the corresponding hydro-
silane (Scheme 3, Path B).6,7 Indeed, since it has been
observed that at 323 K only traces of 4a are transformed into
6a (Table 1 entry 3) it is reasonable to assume that under these
conditions 6a is formed by reactions of the bis(silyl)acetal
13CH2{OSiMe(OSiMe3)2}2 (7a) with HSiMe(OSiMe3)2. In
agreement with this we have observed the presence of traces
of 7a in the NMR spectra of this reaction (ESI†). However, aer
24 h of reaction 98% of the starting hydrosiloxane was
consumed and therefore, under these conditions, the forma-
tion of 6a at 358 K can only be explained by thermal decom-
position of 4a (Scheme 3, Path B).
Analogous behavior has been observed when HSiMe2Ph and
HSiMePh2 were used as reducing agents. Thus, the formation of
the mixtures of methyl-silyl-carbonate 13CH3O–13CO2SiR3 (SiR3
¼ SiMe2Ph, 4b; SiMePh2, 4c) and the corresponding silylformate
and methoxysilane from the 3-catalyzed reduction of 13CO2 with
HSiR3 (SiR3 ¼ SiMe2Ph, SiMePh2) was also observed (ESI†).
It should be mentioned, that the formation of the methyl-
silyl-carbonates was not observed along the 2-catalyzed reduc-
tion of CO2 with HSiMe(OSiMe3)2.6 In this context, even though
species 2 and 3 seem very similar, their reactivity against silanes
is very different. Thus, while complex 3 is stable in presence of
one equivalent of HSiMe(OSiMe3)2 at 323 K, the triuoroacetate
ligand in 2 is reduced to give the corresponding silylether
CF3CH2OSiR3.5d,6 Therefore, it is reasonable to think that the
stability of the triate ligand could play a role stabilizing reac-
tion intermediates, which could open new paths of reaction, no
accessible in 2-catalyzed CO2 hydrosilylation reactions.
Fig. 3 CH3O-region of the 1H NMR spectra in C6D6 of the 3-catalyzed
reaction of 13CO2 (2.7 bar) with HSiMe(OSiMe3)2 at 323 K.
(Fig. 3). This resonance shows a direct C–H bond correlation in
the 1H–13C HSQC spectra with a doublet resonance centered at
d 54.1 ppm (3JC–C ¼ 1.7 Hz), assigned to the 13CH3O carbon in
the 13C APT NMR spectra, and also a C–H bond correlation in
the 1H–13C HMBC spectra with a doublet resonance that
appeared centered at d 153.0 ppm (3JC–C ¼ 1.8 Hz), assigned to
the 13CO3 carbon in the 13C APT NMR spectra. These data
compare well with those reported for alkyl-silyl-carbonates3 and
support the structure proposed for 4a in Table 1 (see ESI†).
Under the above described reaction conditions the slow
consumption of HSiMe(OSiMe3)2 was observed (Table 1, entries
1
1 and 2). Thus, aer 12 h the reaction is complete and H and
At this point the question arises, how methylsilylcarbonates
are formed from the 3-catalyzed hydrosilylation of CO2. A
plausible mechanism proposal is shown in Scheme 4. The
formation of methylcarbonates has been explained by insertion
of CO2 into the Ir–O bond of iridium–OCH3 species.8 Therefore,
we propose that the reaction of an iridium-methoxy interme-
diate (A in Scheme 4) with CO2 could lead to the methylcar-
bonate species B, which by reaction with HSiR3 would afford the
corresponding methylsilylcarbonate and the Ir–H species C.
The next step, is the reaction of C with CO2 to give the iridium–
formate intermediate D, which according to our previous
results has been found to be a thermodynamically favoured
13C{1H} NMR spectra evidenced the consumption of all the
starting 13CO2 and of the 98% of HSiMe(OSiMe3)2 to give
a mixture of 4a (18.8 mol%), 5a (70.0 mol%) and 6a (11.2 mol%)
(Table 1, entry 2). Heating this sample for another 12 h at 323 K
did not evidence changes in the composition of the mixture
(Table 1, entry 3). Interestingly, increasing the temperature at
358 K the slow transformation of 4a into 6a occurs (Table 1
entries 4 and 5), in addition, the formation of 13CO2 was
observed. This outcome agrees with the known thermal
behavior of alkyl-silyl-carbonates, ROCO2SiR3,3d,e which ther-
mally decompose to give the corresponding methoxysilane and
CO2 (Scheme 3, Path A).
Scheme 3 Reaction paths for the formation of methoxysilanes from Scheme
4 Plausible mechanism proposal for the formation of
the 3-catalyzed hydrosilylation of CO2.
methyl-silyl-carbonates from the 3-catalyzed hydrosilylation of CO2.
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process.5d,6 Finally, the reaction of the iridium–formate D with programs, included in APEX2 package. The structure was solved
excess of silane could regenerate A closing the catalytic cycle.
with direct methods with SHELXS-2013 (ref. 11) and rened by
full-matrix least-squares renement on F2 with SHELXL-2018
(ref. 12) program, included in WingX package.13 The disor-
dered solvent region has been analyzed with SQUEEZE
program.14
Experimental
General considerations
All manipulations were carried out under an argon atmosphere
CCDC 1972218 contains the supplementary crystallographic
by Schlenk-type techniques or in a Glovebox MBraun Unilab. data for this paper.
Organic solvents were dried by standard procedures and
Crystal data compound 3. C34H48F6Ir2N4O10S2Si4$0.5(H2O);
distilled under argon prior to use or obtained oxygen- and M ¼ 1364.65; colourless prism 0.140 ꢂ 0.240 ꢂ 0.330 mm3;
ꢀ
ꢁ
water-free from a solvent purication system (Innovative Tech- triclinic P1, a ¼ 9.0468(5), b ¼ 12.0453(6), c ¼ 12.0619(7) A, a ¼
1
nologies). H, 13C, 29Si and 19F NMR spectra were obtained on 66.4720(10), b ¼ 88.5460(10), g ¼ 88.7690(10)ꢁ, V ¼ 1204.65(11)
a Bruker AV-300, AV-400 or AV-500 spectrometer. Chemical A ; Z ¼ 1; Dc ¼ 1.881 g cmꢀ3; m ¼ 5.783 mmꢀ1; Tmin/Tmax: 0.1676/
ꢀ3
shis (d), reported in ppm, are referenced to the residual solvent 0.5996; 23 425/5849 reections measured/unique (Rint
¼
peaks and coupling constants (J) are reported in Hz.
0.0179), number of data/restraint/parameters 5849/0/286, R1(F2)
Synthesis of 3. Toluene (10 mL) was added to a light- ¼ 0.0131 (5771 reections, I > 2s(I)) and wR(F2) ¼ 0.0324 (all
ꢀꢀ3
protected Schlenk containing [Ir(m-Cl)(k2-NSiMe2)2]2 (1) data), nal GoF ¼ 1.034, largest difference peak: 0.988 e A
.
(300 mg, 0.268 mmol) and silver triate (151 mg, 0.590 mmol).
The mixture was stirred at room temperature for 5 hours and
then ltered through Celite. Solvent was removed under
Conclusions
reduced pressure and the solid was washed with pentane (3 ꢂ 8
In conclusion, the use of triate, instead of triuoroacetate, as
ancillary ligand in the chemistry of Ir-(NSiMe2)2 species allows
the preparation of 3, which is a rare example of a dinuclear
iridium complex with triate ligands acting as bridges.
1H NMR studies of the 3-catalyzed reduction of CO2 with
hydrosilanes evidenced the unprecedent formation of methyl-
silylcarbonates as reaction products, together with the corre-
sponding silylformate and methoxysilane.
1
mL) to afford a white solid. Yield: 320 mg (89%). H NMR (300
MHz, C6D6, 298 K): d 8.73 (d, JH–H ¼ 6.2 Hz, 2H, py), 6.32 (s, 2H,
py), 6.02 (d, JH–H ¼ 6.2 Hz, 2H, py), 1.51 (s, 6H, Me-py), 0.72 (s,
6H, Si–Me), 0.41 (s, 6H, Si–Me). 13C NMR plus APT and 1H–13C
HSQC (75 MHz, C6D6, 298 K): d 168.7 (s, py), 152.6 (s, py),
148.7(s, py), 118.4 (s, py), 111.9 (s, py), 20.6 (s, CH3-py), 3.8 (s,
1
CH3–Si), 2.4 (s, CH3–Si). 29Si{1H} NMR plus H–29Si HMBC (60
MHz, C6D6, 298 K): d 38.2 (Ir–Si). 19F NMR (282 MHz, C6D6, 298
K): d ꢀ76.97 (s, CF3SO3). High resolution mass spectrometry
(ESI+): calc. m/z ¼ 525.1006; found m/z ¼ 525.1004 (M+–CF3SO3).
3-Catalyzed (1.0 mol%) reaction of 13CO2 with HSiR3. A
Young cap NMR tube was charged with 3 (2.83 mg, 0.0021
mmol), 0.42 mmol of the corresponding silane (114 mL,
HSiMe(OSiMe3)2; 64.4 mL, HSiMe2Ph; 83.7 mL, HSiMePh2) and
0.5 mL of C6D6. Argon gas was evacuated by three freeze-pump-
thaw cycles. Then the tube was pressurized with 13CO2 (2.7 bar),
heated at 323 K and monitored by NMR spectroscopy.
The results of this investigation show that the formation of
methoxysilanes during the catalytic reduction of CO2 with
silanes, which traditionally has been explained by the catalytic
reaction of bis(silyl)acetals with silanes, could also be conse-
quence of thermal decomposition of the corresponding
methylsilylcarbonate.
Conflicts of interest
There are no conicts to declare.
1
1
Selected data for 4a. H NMR plus HSQC H–13C (300 MHz,
1
3
C6D6, 298 K): d 3.33 (dd, JH–C ¼ 146.9 Hz, JH–C ¼ 4.1 Hz, 3H,
CH3OCO2). 13C{1H} plus HSQC and HMBC 1H–13C (75 MHz,
Acknowledgements
2
2
C6D6, 298 K): d 153.0 (d, JC–C ¼ 1.7 Hz, CO3), 54.1 (d, JC–C
¼
1.7 Hz, CH3O). Selected data for 4b: 1H NMR plus HSQC 1H–13C
We gratefully acknowledge MICINN/FEDER projects PGC2018-
099383-B-100 and the Regional Government of Aragon/FSE
1
3
(300 MHz, C6D6, 298 K): d 3.30 (dd, JH–C ¼ 147.1 Hz, JH–C
¼
4.1 Hz, 3H, CH3OCO2). 13C{1H} plus HSQC and HMBC H–13C
(75 MHz, C6D6, 298 K): d 154.1 (d, 2JC–C ¼ 1.7 Hz, CO3), 54.2 (d,
2JC–C ¼ 1.7 Hz, CH3O). Selected data for 4c: 1H NMR plus HSQC
1H–13C (300 MHz, C6D6, 298 K): d 3.23 (dd, 1JH–C ¼ 147.1 Hz, 3JH–
1
´
2014–2020 “Building Europe from Aragon” (group E42_17R) for
funding.
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