Selective Catalytic Frustrated Lewis Pair Hydrogenation of CO2 in the Presence of Silylhalides.

The frustrated Lewis pair (FLP) derived from 2,6-lutidine and B(C6 F5 )3 is shown to mediate the catalytic hydrogenation of CO2 using H2 as the reductant and a silylhalide as an oxophile. The nature of the products can be controlled with the judicious selection of the silylhalide and the solvent. In this fashion, this metal-free catalysis affords avenues to the selective formation of the disilylacetal (R3 SiOCH2 OSiR3 ), methoxysilane (R3 SiOCH3 ), methyliodide (CH3 I) and methane (CH4 ) under mild conditions. DFT studies illuminate the complexities of the mechanism and account for the observed selectivity.

The dramatic and continuous increase in the atmospheric CO2 level since the industrial revolution results from the extensive use of fossil fuels and is the major contributor to climate change. This has prompted the scientific community to target a variety of new technologies to reduce emissions or provide alternative energy sources as these offer the most promising avenues to address climate change. Nonetheless, other efforts targeting the capture or use of atmospheric CO2 have also garnered attention. One potential avenue to the use of atmospheric CO2 involves reduction via hydrogenation. For example, recent reviews have described the conversion of CO2 to methanol using homogeneous and heterogeneous transition metal‐based catalysts while other reports have demonstrated the production of longer chain fuels or olefins or higher alcohols. In addition to the above metal‐catalyzed processes, there have also been extensive efforts to employ main group reagents to mediate CO2 reduction processes. A number of studies have explored catalytic processes including both base‐mediated and frustrated Lewis pair (FLP) hydrosilylations and hydroborations of CO2 while others have probed aminations.

Despite the seminal finding in 2009 in which Ashley and O'Hare reported the FLP‐mediated reduction of CO2 to methanol (Scheme 1), albeit in low yield and at 160 °C for 6 days, the direct hydrogenation of CO2 mediated by a main group species has garnered limited attention. A collaborative effort with the Fontaine group described the stoichiometric reactions of the intramolecular FLP, 1‐BMes2‐2‐NMe2‐C6H4, with H2 and CO2 yielding formyl, acetal and methoxy‐borane derivatives (Scheme 1). This study suggested that judicious selection of the combination of the Lewis acid and the base could plausibly lead to catalytic H2/CO2 chemistry. More recently, Zhao et al. described the hydrogenation of CO2 in the presence of H2 and K2CO3 using B(C6F5)3 as the catalyst, affording effective turn‐over to K[HCO2] at comparatively high H2/CO2 pressures of 60 bar (Scheme 1). While the achievement of catalytic hydrogenation is impressive, the reduction was limited to the formation of formate product.

Scheme 1

Direct reactions of CO2/H2 mediated by main group reagents.

Pondering an FLP system that would effect reduction beyond formate, we recognized that in earlier studies methanol or methane were obtained exploited hydrosilanes or hydroboranes that provide both a reducing agent and an oxophile.[ , ] In contrast, use of H2 as the reducing agent in direct FLP hydrogenations of CO2 does not provide such an oxygen‐atom scavenger. Thus, we speculated that further hydrogenation of CO2 could be effected in the presence of a silylhalide. Herein, we report the FLP‐mediated catalytic hydrogenation of CO2 using H2 as the reducing agent performed in the presence of a silylhalide which acts as an oxophile. Judicious choices of the silylhalide and reaction solvent are shown to provide fine control over the nature of the products of catalysis.

The activation of H2 by 2,6‐lutidine/B(C6F5)3 (Scheme 2) and subsequent reaction with CO2 is known to afford the salt [C5H3Me2NH][HCO2B(C6F5)3]. This species was allowed to react with 1 equivalent of Et3SiI in CDCl3 resulting in the upfield shift of the formyl proton in the 1H NMR from 8.31 ppm to 8.17 ppm and the appearance of a 11B{1H} NMR signal at −0.1 ppm. These data affirm the formation of B(C6F5)3 adduct of silyl formate Et3SiOC(O)H and are consistent with the cleavage of the B−O bond in the formyl‐borate salt (Scheme 2). Recognizing that the silyl formate‐borane adduct will exist in an equilibrium with free borane, this implies that it should be accessible for further reaction.

Scheme 2

Control reactions.

We also queried the possibility of reduction of Et3SiI in the presence of excess base. To this end, Et3SiI and 2,6‐lutidine were combined under H2 (4 atm) in the presence of 10 mol % B(C6F5)3 in either CDCl3 or C6D6 and heated at 100 °C for 40 h (Scheme 2). In both cases no reduction of the silylhalide was observed. This suggested that the silylhalide could act as an oxophile in the presence of H2, for the hydrogenation of CO2, without the possibility of invoking a hydrosilylation mechanism.

Thus, targeting FLP hydrogenations of CO2, reactions of 10 equivalents of Lewis base and silylhalide were performed in C6D6 or CDCl3 solution of 10 mol % of B(C6F5)3. In these reactions the substituted pyridines, 2,4,6‐collidine and less basic 2,6‐lutidine were employed and the systems were pressurized with H2 (4 atm.) and 13CO2 (2 atm.) and heated to 100 °C for up to 60 h. The reactions were monitored by 1H NMR and 13C NMR spectroscopy. Initial reactions using Me3SiCl and 2,6‐lutidine in C6D6 or CDCl3 (Table 1, entry 1, 2) as the solvent, afforded [C5H3Me2NH][HCO2B(C6F5)3] as the major product as evidenced by the doublet resonance (1 J C–H=209 Hz) at 8.37 ppm in the 1H NMR spectrum and the doublet resonance in the 1H‐coupled 13C NMR at 169.5 ppm. The generally poor reactivity in the presence of Me3SiCl was attributed to the relatively strong Si−Cl bond and prompted the use of 2,6‐lutidine and Me3SiBr. This led to an 83 % yield of methoxysilane Me3SiO13CH3, after 40 h of heating in C6D6 (entry 3). In this case, the major product was identified by a 1H NMR resonance at 3.25 ppm as a doublet (1 J C–H=141 Hz), the corresponding 13C{1H} NMR signal is found at 49.9 ppm. Repetition of the experiment in CDCl3 also led to the selective production of Me3SiO13CH3 in 73 % yield after 60 h heating (entry 4). The combination of 2,6‐lutidine and Me3SiI generated 13CH4 in 76 % yield after 60 h (entry 5). As these reactions were done in a sealed J‐Young NMR tube, the methane was identified by 13C NMR spectroscopy as a pentet at −4.3 ppm (1 J C–H=126 Hz) and further confirmed by an HSQC experiment, revealing a correlation with the 1H signal at 0.19 ppm. Further improvement in the reactivity was seen with use of CDCl3 as the solvent as 13CH4 was produced in 85 % yield after 20 h at 100 °C (entry 6). Reactions with the more sterically hindered halosilane Et3SiI afforded the acetal (Et3SiO)2 13CH2 as the dominant product in 72 % yield after heating at 100 °C for 60 h (entry 7). This product exhibited a doublet at 5.06 ppm in the 1H NMR with a 1 J C–H of 162 Hz and a 13C{1H} NMR signal at 84.5 ppm. Interestingly, performance of the reaction in the more polar solvent CDCl3 (entry 8) afforded 13CH3I in 82 % yield as evidenced by the quartet resonance in the 13C NMR at −23.5 ppm with 1 J C–H=151 Hz, while the HSQC experiment revealed a correlation with the 1H signal at 2.16 ppm. Use of the more basic 2,4,6‐collidine resulted in a significant reduction in reactivity affording low yields of the acetal and methoxylsilane in C6D6 and CDCl3, respectively (entry 9, 10), likely due to slightly reduced reactivity for CO2 reduction though better H2‐activation reactivity is expected.

Table 1

CO2 hydrogenation in the presence of silylhalides.

Ent	Solv.	Silylhalide[a]	base[a]	t [h]	Major product	Yield[b]
1	C6D6	Me3SiCl	Lut	20	‐	<1 %
2	CDCl3	Me3SiCl	Lut	20	‐	<1 %
3	C6D6	Me3SiBr	Lut	40	MeOSiMe3	83 %
4	CDCl3	Me3SiBr	Lut	60	MeOSiMe3	73 %
5	C6D6	Me3SiI	Lut	60	13CH4	76 %
6	CDCl3	Me3SiI	Lut	20	13CH4	85 %
7	C6D6	Et3SiI	Lut	60	(Et3SiO)2 13CH2	72 %
8	CDCl3	Et3SiI	Lut	40	13CH3I	82 %
9	C6D6	Et3SiI	Col	40	(Et3SiO)2 13CH2	8 %
10	CDCl3	Et3SiI	Col	40	MeOSiEt3	9 %

[a] 0.05 mmol silylhalide and Lewis base were added; Lut=2,6‐lutidine; Col=2,4,6 collidine. [b] Yields are determined by 1H NMR spectroscopy using 10 μL toluene as internal standard.

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The above reactions demonstrate that simple tuning of the reaction conditions for FLP hydrogenation of CO2 provided variation of the major products. While lutidine was identified as the preferred base in the presence of the Lewis acid catalyst B(C6F5)3, the use of Me3SiBr produced Me3SiO13CH3, whereas Me3SiI afforded primarily 13CH4 as the CO2 reduction product. The acetal, (Et3SiO)2 13CH2, was formed preferentially when Et3SiI was employed in C6D6 solution. Perhaps most remarkably, however was the impact of the use of Et3SiI in CDCl3 which resulted in the formation of 13CH3I as the major product (Scheme 3).

Scheme 3

Summary of major products of CO2 reduction using the FLP catalyst B(C6F5)3/2,6‐lutidine.

Efforts to probe the reaction affording isotopically enriched methyl iodide prompted us to monitor the reaction of 13CO2 (2 atm) and D2 (2 atm) in the presence of 2,6‐lutidine, Et3SiI and 10 mol % B(C6F5)3 in CDCl3 at 100 °C. At this lower pressure and with the shorter reaction time of 24 h, the reaction was not complete. However, the NMR spectra revealed the formation of isotopologues of the acetal and methoxy species in 33 % yield and 21 % yield, respectively. The three isotopologues of the acetal, (Et3SiO)2 13CH2 and (Et3SiO)2 13CHD and (Et3SiO)2 13CD2 were formed in an approximately 1:4:1 ratio. The isotopologue (Et3SiO)2 13CHD exhibited a triplet in the 13C{1H} NMR spectrum at 84.0 ppm (1 J C–D=25 Hz) as well as a doublet at 5.03 ppm (1 J C–H=161 Hz) in the 1H NMR spectrum; while the (Et3SiO)2 13CD2 was found as a pentet in the 13C{1H} NMR spectrum at 83.6 ppm (1 J C–D=25 Hz). The four isotopologues of methoxy, Et3SiO13CH3, Et3SiO13CH2D, Et3SiO13CHD2 and Et3SiO13CD3 were generated in a 1:5:8:4 ratio, each of them was found in the 13C{1H} NMR spectrum at 50.8 ppm, 50.5 ppm, 50.2 ppm and 49.7 ppm as singlet, triplet, pentet and septet resonance with 1 J C–D=22 Hz, respectively. In addition, the NMR data showed the formation of H2 as a singlet at 4.63 ppm and HD as a triplet at 4.59 ppm (JH–D=43 Hz) and a triplet at 2.39 ppm (2 J H–D=2 Hz) adjacent the methyl resonance of 2,6‐lutidine, which is corresponding to the mono‐methyl‐deuterated 2,6‐lutidine. These data suggest that competitive to reaction with CO2, the product of initial activation of D2, [C5H3Me2ND][DB(C6F5)3], can evolve HD, generating a transient enamine, while tautomerization regenerates lutidine leading to H/D scrambling into the methyl groups of lutidine, the generation of HD and H2, and the generation of the isotopologues of the CO2 reduction products (Scheme 4). It is noteworthy that on prolonged reaction for 70 h, the above reaction gave 78 % yield of the expected isotopologues of methyl iodide, CH3I, CH2DI, CD2HI and CD3I in a 1:5:5:4 ratio. These species are observed in the 13C{1H} NMR spectrum at −23.39 ppm, −23.41 ppm, −23.44 ppm and −23.47 ppm as singlet, triplet, pentet and septet resonances, respectively. The deuterated species exhibited 1 J C–D values of 23 Hz.

Scheme 4

a) Deuteration of CO2 b) deuteration of lutidine, mediated by B(C6F5)3 under D2.

Mechanistically, the above reactivity indicates that the present hydrogenation of CO2 begins with the known FLP activation of H2 followed by the reaction with CO2 affording a formyl borate anion. Reaction with the silylhalide affords the silyl‐formate and frees the borane for further activation of H2. Hydrido‐borate attack of the silyl‐formate and reactions with the silylhalide affords the acetal and subsequently the methyloxy‐silane, although the dominance of these reactions depends on the nature of the silyl‐substituent, the halide and the solvent. In a non‐polar solvent, reaction of the methyloxy‐silane with the hydrido‐borate in the presence of the silylhalide affords methane and the disilylether. In contrast, a polar solvent favors attack by iodide, affording methyl iodide as the dominant product.

This view of the reactivity was further probed by extensive DFT calculations at the dispersion‐corrected PW6B95‐D3/def2‐QZVP + COSMO‐RS// TPSS‐D3/def2‐TZVP + COSMO level of theory in chloroform solution, using the typical substrates of 2,6‐lutidine (Lut), H2, CO2 and Me3SiI along with the Lewis‐acid B(C6F5)3 as the catalyst. The final PW6B95‐D3 free energies (in kcal mol−1, at 298 K and 1 M concentration) are discussed.

The activation of H2 by the separated FLP Lut/B(C6F5)3 (Figure 1 A) is −10.0 kcal mol−1 exergonic over a low free energy barrier of 15.9 kcal mol−1 (via TS1) giving the ion pair [LutH]+[HB(C6F5)3]− (A). In CHCl3 solution, the separated ions are 1.1 kcal mol−1 less stable at room temperature but are easily accessible and even more stable upon heating due to favorable entropic effects. In contrast, both CO2 and Me3SiI cannot be activated by the FLP, as the adduct LutCOOB(C6F5)3 and the separated ions of [LutSiMe3]+ and I−, are 11.5 and 5.1 kcal mol−1 endergonic, respectively (see Supporting Information). However, CO2 is easily reduced by A via hydride transfer from [HB(C6F5)3]− to the carbon with H‐bonding of [LutH]+ to oxygen and the formation of [LutH]+[HCOOB(C6F5)3]− (B) is −5.3 kcal mol−1 exergonic over a free energy barrier of only 18.9 kcal mol−1 (via TS2). Consistent with experiment, the reduction of Me3SiI with A to form Me3SiH, [LutH]I and regenerated B(C6F5)3 catalyst is 10.1 kcal mol−1 endergonic and thus thermodynamically prevented (see Supporting Information). On the other hand, the reaction between Me3SiI and B is −1.6 kcal mol−1 exergonic and proceeds easily over a low barrier of 14.3 kcal mol−1 (via TS3). This affords the neutral adduct Me3SiOCHOB(C6F5)3 (C) that still requires 3.9 kcal mol−1 to eliminate B(C6F5)3 and give Me3SiOCHO (D). Such trapping of B(C6F5)3 with D effectively increases the free energy barrier to the initial H2‐activation to 19.8 kcal mol−1 (via TS1), which is thus the rate‐limiting step for the formation of D. For comparison, the Lewis bases Lut, Col, Cl− and Br− also form stable B(C6F5) adducts that are −2.0, −4.7, −5.7 and −1.3 kcal mol−1 exergonic in CHCl3 solution (see Supporting Information), respectively. The higher affinity for Col and Cl− may further inhibit H2‐activation reactivity.

Figure 1

DFT‐computed free energy paths for: A) the lutidine/B(C6F5)3 FLP‐mediated H2 activation and further reduction of CO2 into HCOOSiMe3; B) further reduction into H2C(OSiMe3)2 and even H3COSiMe3; C) slower and kinetically competitive formation of CH3I and CH4.

Once intermediate D is formed (Figure 1 B), further reduction via silylium transfer from Me3SiI (via TS4) and subsequent hydride transfer from A (via TS5) to give the acetal H2C(OSiMe3)2 (E) proceeds quickly and is −13.3 kcal mol−1 exergonic. Further silylium transfer from Me3SiI to E (via TS6) and subsequent hydride transfer from A (via TS7) to give H3COSiMe3 (F), O(SiMe3)2 and [LutH]I is still possible over a slightly higher barrier of 20.3 kcal mol−1 (via TS6), but is −39.9 kcal mol−1 exergonic. Under moderate heating, both formation of E and F should be kinetically facile. The use of bulkier silanes such as Et3SiI may enhance the barrier to silylium transfer and thus slow formation of F, making selective acetal formation possible in less polar benzene solution (Table 1, entry 7).

Silylium transfer from Me3SiI to F to give the cation H3CO(SiMe3)2 + (G) and the I− anion (via TS8, Figure 1 C), is 10.5 kcal mol−1 endergonic over a low barrier of 16.5 kcal mol−1 and thus is kinetically feasible. Further nucleophilic iodide transfers from [LutH]I to G to give the experimentally observed CH3I and O(SiMe3)2 is −24.3 kcal mol−1 exergonic over a low barrier of 13.9 kcal mol−1 (via TS9). The overall formation of CH3I from F is thus −13.8 kcal mol−1 exergonic over a sizable barrier of 24.4 kcal mol−1, consistent with the moderate heating required experimentally. On the other hand, nucleophilic hydride transfer from A to G to give CH4, O(SiMe3)2 and regenerate B(C6F5)3 is −49.0 kcal mol−1 exergonic over a low barrier of 14.3 kcal mol−1 (via TS10). Coupled with the facile H2 activation, the overall formation of CH4 from F is thus −53.2 kcal mol−1 exergonic over a barrier of 24.8 kcal mol−1. This is thermodynamically more favorable but kinetically comparable with the formation of CH3I. Indeed, the use of Et3SiI and Me3SiI are found to favor iodide and hydride transfer affording CH3I and CH4, respectively.

In conclusion, we have achieved metal‐free catalytic hydrogenation of CO2 using H2 and a silylhalide as an oxophile in the presence of a FLP derived from lutidine and B(C6F5)3. The judicious selection of the steric demands and nature of the silylhalide and the solvent provides control of these catalytic reductions affording avenues to the selective formation of the methoxysilane, Me3SiO13CH3, the acetal (Et3SiO)2 13CH2, 13CH4 and 13CH3I. The complexities of the mechanisms involved have been detailed using DFT studies. We are continuing to explore the use of FLPs in reactions of interest.

Supporting Information available: Synthetic and spectral data, computational details and DFT‐computed energies and Cartesian coordinates are deposited.

Conflict of interest

The authors declare no conflict of interest.

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