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

Abstract

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.

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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

Scheme 1 Products from the catalytic reduction of CO₂ with silicon-hydrides reported so far.

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.

Fig. 1 Methylsilylcarbonates.

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

Recently, we have described the synthesis and catalytic behavior of the iridium(III) complexes [Ir(μ-Cl)(κ²-NSi^Me2)₂]₂ (1) and [Ir(CF₃CO₂)(κ²-NSi^Me2)₂] (2) (NSi^Me2 = {4-methylpyridine-2-yloxy}dimethylsilyl) (Scheme 2).⁶ Species 2 catalyzed the selective reduction of CO₂, depending on the reaction conditions, to silylformate or methoxysilane with HSiMe(OSiMe₃)₂.⁶ As a continuation of our studies on the chemistry of iridium complexes with pyridine-2-yloxyI-silyl ligands, we have found that the triflate derivative [Ir(μ-CF₃SO₃)(κ²-NSi^Me2)₂]₂ (3) (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 spectroscopy as CH₃OCO₂SiMe(OSiMe₃)₂ (4a). To the best of our knowledge, this is the first time that the formation of silylcarbonates from the catalytic reduction of CO₂ with silanes has been observed. The results from these studies are described below.

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(μ-CF₃SO₃)(κ²-NSi^Me2)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 first example of an iridium species with triflate groups acting as bridges. The molecule exhibits a crystallographic 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 separation between the iridium atoms (6.0870(2) Å) in 3, significantly longer than that observed for 1 with chlorine bridges (4.0718(12) Å).

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 (Å) and angles (°): Ir–Si(1), 2.2570(5); Ir–Si(2), 2.2615(5); Ir–O(3), 2.3653(12); Ir–O(5′), 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(5′), 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(5′), 93.26(3); Si(2)–Ir–N(1), 95.76(4); Si(2)–Ir–N(2), 82.22(4); O(3)–Ir–O(5′), 80.67(4); O(3)–Ir–N(1), 92.06(5); O(3)–Ir–N(2), 90.32(5); O(5′)–Ir–N(1), 91.95(5); O(5′)–Ir–N(2), N(1)–Ir–N(2), 175.94(6).

¹H and ¹³C{¹H} NMR spectra of 3 in C₆D₆ show only one pattern of resonances for the four NSi^Me2 ligands, which agrees with its high symmetry in solution. ²⁹Si{¹H} NMR spectra of 3 show a resonance at δ 38.2 ppm, corresponding to the four silicon atoms, which compares well to the value of δ 39.7 ppm reported for 2.⁶ In addition, a singlet resonance at δ −76.97 ppm in the ¹⁹F{¹H} NMR spectra confirm the presence of the triflate 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 = 3.9 h⁻¹). This behavior differs from that previously reported for 2, which under the same reaction conditions catalyzed the full conversion of HSiMe(OSiMe₃)₂ into silylformate (93.0%) and methoxysilane (7.0%) in 3.5 h (TOF = 28.6 h⁻¹).⁶ Therefore, complex 2 is not only more active but also more selective than 3.

To determine the nature of the new species observed when 3 is used as catalyst for the hydrosilylation of CO₂, we decided to study this reaction using isotopically labeled ¹³CO₂. Thus, ¹H 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 performed. These experiments evidenced, since early reaction stages, the presence of a mixture of ¹³CH₃O–¹³CO₂SiMe(OSiMe₃)₂ (4a), together with the corresponding silylformate, H¹³CO₂SiMe(OSiMe₃)₂ (5a), and the methoxysilane, ¹³CH₃OSiMe(OSiMe₃)₂ (6a) (Table 1).

Table 1 Results from 3-catalyzed reduction of ¹³CO₂ (2.7 bar) with HSiMe(OSiMe₃)₂^a

Entry	T (K)	4a^b	5a^b	6a^b	Conversion^c	Time (h)
a General conditions: 3 (1.0 mol%) in 0.5 mL of dry C6D6.b Calculated by ^1H NMR and expressed in mol%.c Calculated by ^1H NMR relative to the starting hydrosiloxane and expressed in mol%.d After 24 h at 323 K.e Control experiment without iridium catalyst (Fig. S24 see ESI).
1	323	26.7	65.2	8.1	70	3
2	323	18.8	70.0	11.2	98	12
3	323	19.0	70.0	11.0	98	24
4	358	11.7	68.7	19.6	100	24^d
5	358	8.5	68.0	23.5	100	48^d
6^e	323	0	0	0	0	24

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The most characteristic resonance in the ¹H NMR spectra of 4a is a doublet of doublets centered at δ 3.33 ppm (¹J_C–H = 146.9 Hz; ³J_C–H = 4.1 Hz) corresponding to the ¹³CH₃O protons (Fig. 3). This resonance shows a direct C–H bond correlation in the ¹H–¹³C HSQC spectra with a doublet resonance centered at δ 54.1 ppm (³J_C–C = 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 δ 153.0 ppm (³J_C–C = 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†).

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.

Under the above described reaction conditions the slow consumption of HSiMe(OSiMe₃)₂ was observed (Table 1, entries 1 and 2). Thus, after 12 h the reaction is complete and ¹H and 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 thermally decompose to give the corresponding methoxysilane and CO₂ (Scheme 3, Path A).

Scheme 3 Reaction paths for the formation of methoxysilanes from the 3-catalyzed hydrosilylation of CO₂.

The formation of methoxy-silanes from the catalytic reduction of CO₂ with hydrosilanes has been so far explained by reaction of bis(silyl)acetals with the corresponding hydrosilane (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, after 24 h of reaction 98% of the starting hydrosiloxane was consumed and therefore, under these conditions, the formation of 6a at 358 K can only be explained by thermal decomposition 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 = SiMe₂Ph, 4b; SiMePh₂, 4c) and the corresponding silylformate and methoxysilane from the 3-catalyzed reduction of ¹³CO₂ with HSiR₃ (SiR₃ = 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 reduction 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 trifluoroacetate ligand in 2 is reduced to give the corresponding silylether CF₃CH₂OSiR₃.^5d,6 Therefore, it is reasonable to think that the stability of the triflate ligand could play a role stabilizing reaction intermediates, which could open new paths of reaction, no accessible in 2-catalyzed CO₂ hydrosilylation reactions.

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 intermediate (A in Scheme 4) with CO₂ could lead to the methylcarbonate 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 process.^5d,6 Finally, the reaction of the iridium–formate D with excess of silane could regenerate A closing the catalytic cycle.

Scheme 4 Plausible mechanism proposal for the formation of methyl-silyl-carbonates from the 3-catalyzed hydrosilylation of CO₂.

Experimental

General considerations

All manipulations were carried out under an argon atmosphere by Schlenk-type techniques or in a Glovebox MBraun Unilab. Organic solvents were dried by standard procedures and distilled under argon prior to use or obtained oxygen- and water-free from a solvent purification system (Innovative Technologies). ¹H, ¹³C, ²⁹Si and ¹⁹F NMR spectra were obtained on a Bruker AV-300, AV-400 or AV-500 spectrometer. Chemical shifts (δ), reported in ppm, are referenced to the residual solvent peaks and coupling constants (J) are reported in Hz.

Synthesis of 3. Toluene (10 mL) was added to a light-protected Schlenk containing [Ir(μ-Cl)(κ²-NSi^Me2)₂]₂ (1) (300 mg, 0.268 mmol) and silver triflate (151 mg, 0.590 mmol). The mixture was stirred at room temperature for 5 hours and then filtered through Celite. Solvent was removed under reduced pressure and the solid was washed with pentane (3 × 8 mL) to afford a white solid. Yield: 320 mg (89%). ¹H NMR (300 MHz, C₆D₆, 298 K): δ 8.73 (d, J_H–H = 6.2 Hz, 2H, py), 6.32 (s, 2H, py), 6.02 (d, J_H–H = 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): δ 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, CH₃–Si), 2.4 (s, CH₃–Si). ²⁹Si{¹H} NMR plus ¹H–²⁹Si HMBC (60 MHz, C₆D₆, 298 K): δ 38.2 (Ir–Si). ¹⁹F NMR (282 MHz, C₆D₆, 298 K): δ −76.97 (s, CF₃SO₃). High resolution mass spectrometry (ESI⁺): calc. m/z = 525.1006; found m/z = 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 μL, HSiMe(OSiMe₃)₂; 64.4 μL, HSiMe₂Ph; 83.7 μL, 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.

Selected data for 4a. ¹H NMR plus HSQC ¹H–¹³C (300 MHz, C₆D₆, 298 K): δ 3.33 (dd, ¹J_H–C = 146.9 Hz, ³J_H–C = 4.1 Hz, 3H, CH₃OCO₂). ¹³C{¹H} plus HSQC and HMBC ¹H–¹³C (75 MHz, C₆D₆, 298 K): δ 153.0 (d, ²J_C–C = 1.7 Hz, CO₃), 54.1 (d, ²J_C–C = 1.7 Hz, CH₃O). Selected data for 4b: ¹H NMR plus HSQC ¹H–¹³C (300 MHz, C₆D₆, 298 K): δ 3.30 (dd, ¹J_H–C = 147.1 Hz, ³J_H–C = 4.1 Hz, 3H, CH₃OCO₂). ¹³C{¹H} plus HSQC and HMBC ¹H–¹³C (75 MHz, C₆D₆, 298 K): δ 154.1 (d, ²J_C–C = 1.7 Hz, CO₃), 54.2 (d, ²J_C–C = 1.7 Hz, CH₃O). Selected data for 4c: ¹H NMR plus HSQC ¹H–¹³C (300 MHz, C₆D₆, 298 K): δ 3.23 (dd, ¹J_H–C = 147.1 Hz, ³J_H–C = 4.1 Hz, 3H, CH₃OCO₂). ¹³C{¹H} plus HSQC and HMBC ¹H–¹³C (75 MHz, C₆D₆, 298 K): δ 153.9 (d, ²J_C–C = 1.8 Hz, CO₃), 54.3 (d, ²J_C–C = 1.8 Hz, CH₃O).

Crystal structure determination of complex 3. Single crystal X-ray diffraction data were collected at 100(2) K with graphite-monochromated Mo Kα radiation (λ = 0.71072 Å) using narrow frame rotation (Δω = 0.3°) on a Bruker Smart APEX diffractometer. Measured intensities were integrated and corrected for absorption effects with SAINT+⁹ and SADABS¹⁰ programs, included in APEX2 package. The structure was solved with direct methods with SHELXS-2013 (ref. 11) and refined by full-matrix least-squares refinement on F² with SHELXL-2018 (ref. 12) program, included in WingX package.¹³ The disordered solvent region has been analyzed with SQUEEZE program.¹⁴

CCDC 1972218 contains the supplementary crystallographic data for this paper.

Crystal data compound 3. C₃₄H₄₈F₆Ir₂N₄O₁₀S₂Si₄·0.5(H₂O); M = 1364.65; colourless prism 0.140 × 0.240 × 0.330 mm³; triclinic P1̄, a = 9.0468(5), b = 12.0453(6), c = 12.0619(7) Å, α = 66.4720(10), β = 88.5460(10), γ = 88.7690(10)°, V = 1204.65(11) ų; Z = 1; D_c = 1.881 g cm⁻³; μ = 5.783 mm⁻¹; T_min/T_max: 0.1676/0.5996; 23 425/5849 reflections measured/unique (R_int = 0.0179), number of data/restraint/parameters 5849/0/286, R₁(F²) = 0.0131 (5771 reflections, I > 2σ(I)) and wR(F²) = 0.0324 (all data), final GoF = 1.034, largest difference peak: 0.988 e Å⁻³.

Conclusions

In conclusion, the use of triflate, instead of trifluoroacetate, as ancillary ligand in the chemistry of Ir-(NSi^Me2)₂ species allows the preparation of 3, which is a rare example of a dinuclear iridium complex with triflate ligands acting as bridges.

¹H NMR studies of the 3-catalyzed reduction of CO₂ with hydrosilanes evidenced the unprecedent formation of methylsilylcarbonates as reaction products, together with the corresponding silylformate and methoxysilane.

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 consequence of thermal decomposition of the corresponding methylsilylcarbonate.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge MICINN/FEDER projects PGC2018-099383-B-100 and the Regional Government of Aragon/FSE 2014–2020 “Building Europe from Aragón” (group E42_17R) for funding.

Notes and references

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Footnote

† 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

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