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

Abstract The frustrated Lewis pair (FLP) derived from 2,6‐lutidine and B(C6F5)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 (R3SiOCH2OSiR3), methoxysilane (R3SiOCH3), methyliodide (CH3I) and methane (CH4) under mild conditions. DFT studies illuminate the complexities of the mechanism and account for the observed selectivity.

Abstract: The frustrated Lewis pair (FLP) derived from 2,6lutidine and B(C 6 F 5 ) 3 is shown to mediate the catalytic hydrogenation of CO 2 using H 2 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 (R 3 SiOCH 2 OSiR 3 ), methoxysilane (R 3 SiOCH 3 ), methyliodide (CH 3 I) and methane (CH 4 ) 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 CO 2 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 CO 2 have also garnered attention. One potential avenue to the use of atmospheric CO 2 involves reduction via hydrogenation. [1] For example, recent reviews have described the conversion of CO 2 to methanol using homogeneous and heterogeneous transition metal-based catalysts [2] while other reports have demonstrated the production of longer chain fuels [3] or olefins or higher alcohols. [4] In addition to the above metal-catalyzed processes, there have also been extensive efforts to employ main group reagents to mediate CO 2 reduction processes. A number of studies [5] have explored catalytic processes including both base-mediated and frustrated Lewis pair (FLP) hydrosilylations [6] and hydroborations [7] of CO 2 while others have probed aminations. [8] Despite the seminal finding in 2009 in which Ashley and OHare [9] reported the FLP-mediated reduction of CO 2 to methanol (Scheme 1), albeit in low yield and at 160 8C for 6 days, the direct hydrogenation of CO 2 mediated by a main group species has garnered limited attention. A collaborative effort with the Fontaine group [10] described the stoichiometric reactions of the intramolecular FLP, 1-BMes 2 -2-NMe 2 -C 6 H 4 , with H 2 and CO 2 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 H 2 /CO 2 chemistry. More recently, Zhao et al. [11] described the hydrogenation of CO 2 in the presence of H 2 and K 2 CO 3 using B(C 6 F 5 ) 3 as the catalyst, affording effective turn-over to K[HCO 2 ] at comparatively high H 2 /CO 2 pressures of 60 bar (Scheme 1). While the achievement of catalytic hydrogenation is impressive, the reduction was limited to the formation of formate product.
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. [6,7] In contrast, use of H 2 as the reducing agent in direct FLP hydrogenations of CO 2 does not provide such an oxygen-atom scavenger. Thus, we speculated that further hydrogenation of CO 2 could be effected in the presence of a silylhalide. Herein, we report the FLP-mediated catalytic hydrogenation of CO 2 using H 2 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 H 2 by 2,6-lutidine/B(C 6 F 5 ) 3 (Scheme 2) [12] and subsequent reaction with CO 2 is known to afford the salt [C 5 H 3 Me 2 NH][HCO 2 B(C 6 F 5 ) 3 ]. [13] This species was allowed to react with 1 equivalent of Et 3 SiI in CDCl 3 resulting in the upfield shift of the formyl proton in the 1 H NMR from 8.31 ppm to 8.17 ppm and the appearance of a 11 B{ 1 H} NMR signal at À0.1 ppm. These data affirm the formation of B(C 6 F 5 ) 3 adduct of silyl formate Et 3 SiOC-(O)H [6c] and are consistent with the cleavage of the BÀO 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.
We also queried the possibility of reduction of Et 3 SiI in the presence of excess base. To this end, Et 3 SiI and 2,6lutidine were combined under H 2 (4 atm) in the presence of 10 mol % B(C 6 F 5 ) 3 in either CDCl 3 or C 6 D 6 and heated at 100 8C 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 H 2 , for the hydrogenation of CO 2 , without the possibility of invoking a hydrosilylation mechanism.
Thus, targeting FLP hydrogenations of CO 2 , reactions of 10 equivalents of Lewis base and silylhalide were performed in C 6 D 6 or CDCl 3 solution of 10 mol % of B(C 6 F 5 ) 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 H 2 (4 atm.) and 13 CO 2 (2 atm.) and heated to 100 8C for up to 60 h. The reactions were monitored by 1 H NMR and 13 C NMR spectroscopy. Initial reactions using Me 3 SiCl and 2,6-lutidine in C 6 D 6 or CDCl 3 (Table 1, entry 1, 2) as the solvent, afforded [C 5 H 3 Me 2 NH][HCO 2 B(C 6 F 5 ) 3 ] [13] as the major product as evidenced by the doublet resonance ( 1 J C-H = 209 Hz) at 8.37 ppm in the 1 H NMR spectrum and the doublet resonance in the 1 H-coupled 13 C NMR at 169.5 ppm. The generally poor reactivity in the presence of Me 3 SiCl was attributed to the relatively strong Si À Cl bond and prompted the use of 2,6-lutidine and Me 3 SiBr. This led to an 83 % yield of methoxysilane Me 3 SiO 13 CH 3, after 40 h of heating in C 6 D 6 (entry 3). In this case, the major product was identified by a 1 H NMR resonance at 3.25 ppm as a doublet ( 1 J C-H = 141 Hz), the corresponding 13 C{ 1 H} NMR signal is found at 49.9 ppm. [6b] Repetition of the experiment in CDCl 3 also led to the selective production of Me 3 SiO 13 CH 3 in 73 % yield after 60 h heating (entry 4). The combination of 2,6-lutidine and Me 3 SiI generated 13 CH 4 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 13 C NMR spectroscopy as a pentet at À4.3 ppm ( 1 J C-H = 126 Hz) and further confirmed by an HSQC experiment, revealing a correlation with the 1 H signal at 0.19 ppm. [14] Further improvement in the reactivity was seen with use of CDCl 3 as the solvent as 13 CH 4 was produced in 85 % yield after 20 h at 100 8C (entry 6). Reactions with the more sterically hindered halosilane Et 3 SiI afforded the acetal (Et 3 SiO) 2 13 CH 2 as the dominant product in 72 % yield after heating at 100 8C for 60 h (entry 7). This product exhibited a doublet at 5.06 ppm in the 1 H NMR with a 1 J C-H of 162 Hz and a 13 C{ 1 H} NMR signal at 84.5 ppm. Interestingly, performance of the reaction in the more polar solvent CDCl 3 (entry 8) afforded 13 CH 3 I in 82 % yield as evidenced by the quartet resonance in the 13 C NMR at À23.5 ppm with 1 J C-H = 151 Hz, while the HSQC experiment revealed a correlation with the 1 H signal at 2.16 ppm. [15] Use of the more basic 2,4,6collidine resulted in a significant reduction in reactivity affording low yields of the acetal and methoxylsilane in C 6 D 6 and CDCl 3 , respectively (entry 9, 10), likely due to slightly reduced reactivity for CO 2 reduction though better H 2activation reactivity is expected.
The above reactions demonstrate that simple tuning of the reaction conditions for FLP hydrogenation of CO 2 provided variation of the major products. While lutidine was identified as the preferred base in the presence of the Lewis acid catalyst B(C 6 F 5 ) 3 , the use of Me 3 SiBr produced Me 3 SiO 13 CH 3 , whereas Me 3 SiI afforded primarily 13 CH 4 as the CO 2 reduc-Scheme 2. Control reactions.

Angewandte Chemie
Communications tion product. The acetal, (Et 3 SiO) 2 13 CH 2 , was formed preferentially when Et 3 SiI was employed in C 6 D 6 solution. Perhaps most remarkably, however was the impact of the use of Et 3 SiI in CDCl 3 which resulted in the formation of 13 CH 3 I as the major product (Scheme 3). [16] Efforts to probe the reaction affording isotopically enriched methyl iodide prompted us to monitor the reaction of 13 CO 2 (2 atm) and D 2 (2 atm) in the presence of 2,6-lutidine, Et 3 SiI and 10 mol % B(C 6 F 5 ) 3 in CDCl 3 at 100 8C. 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, (Et 3 SiO) 2 13 CH 2 and (Et 3 SiO) 2 13 CHD and (Et 3 SiO) 2 13 CD 2 were formed in an approximately 1:4:1 ratio. The isotopologue (Et 3 SiO) 2 13 CHD exhibited a triplet in the 13 C{ 1 H} 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 1 H NMR spectrum; while the (Et 3 SiO) 2 13 CD 2 was found as a pentet in the 13 C{ 1 H} NMR spectrum at 83.6 ppm ( 1 J C-D = 25 Hz). The four isotopologues of methoxy, Et 3 SiO 13 CH 3, Et 3 SiO 13 CH 2 D, Et 3 SiO 13 CHD 2 and Et 3 SiO 13 CD 3 were generated in a 1:5:8:4 ratio, each of them was found in the 13 C{ 1 H} 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 H 2 as a singlet at 4.63 ppm and HD as a triplet at 4.59 ppm (J H-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-methyldeuterated 2,6-lutidine. These data suggest that competitive to reaction with CO 2 , the product of initial activation of D 2 , [C 5 H 3 Me 2 ND][DB(C 6 F 5 ) 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 H 2 , and the generation of the isotopologues of the CO 2 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, CH 3 I, CH 2 DI, CD 2 HI and CD 3 I in a 1:5:5:4 ratio. These species are observed in the 13 C{ 1 H} NMR spectrum at À23.39 ppm, À23.41 ppm, À23.44 ppm and À23.47 ppm as singlet, triplet, pentet and septet resonances, respectively. The deuterated species exhibited 1 J C-D values of 23 Hz.
Mechanistically, the above reactivity indicates that the present hydrogenation of CO 2 begins with the known FLP activation of H 2 followed by the reaction with CO 2 affording a formyl borate anion. Reaction with the silylhalide affords the silyl-formate and frees the borane for further activation of H 2 . 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 methyloxysilane 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.
The activation of H 2 by the separated FLP Lut/B(C 6 F 5 ) 3 (Figure 1 A) is À10.0 kcal mol À1 exergonic over a low free energy barrier of 15.9 kcal mol À1 (via TS1) giving the ion pair [LutH] + [HB(C 6 F 5 ) 3 ] À (A). In CHCl 3 solution, the separated ions are 1.1 kcal mol À1 less stable at room temperature but are easily accessible and even more stable upon heating due to favorable entropic effects. In contrast, both CO 2 and Me 3 SiI cannot be activated by the FLP, as the adduct LutCOOB-(C 6 F 5 ) 3 and the separated ions of [LutSiMe 3 ] + and I À , are 11.5 and 5.1 kcal mol À1 endergonic, respectively (see Supporting Information). However, CO 2 is easily reduced by A via hydride transfer from [HB(C 6 F 5 ) 3 ] À to the carbon with Hbonding of [LutH] + to oxygen and the formation of [LutH] + -[HCOOB(C 6 F 5 ) 3 ] À (B) is À5.3 kcal mol À1 exergonic over a free energy barrier of only 18.9 kcal mol À1 (via TS2). Consistent with experiment, the reduction of Me 3 SiI with A to form Me 3 SiH, [LutH]I and regenerated B(C 6 F 5 ) 3 catalyst is 10.1 kcal mol À1 endergonic and thus thermodynamically prevented (see Supporting Information). On the other hand, the reaction between Me 3 SiI and B is À1.6 kcal mol À1 exergonic and proceeds easily over a low barrier of 14.3 kcal mol À1 (via TS3 À ). This affords the neutral adduct Me 3 SiOCHOB(C 6 F 5 ) 3 (C) that still requires 3.9 kcal mol À1 to eliminate B(C 6 F 5 ) 3 and give Me 3 SiOCHO (D). Such trapping of B(C 6 F 5 ) 3 with D effectively increases the free energy barrier to the initial H 2activation to 19.8 kcal mol À1 (via TS1), which is thus the ratelimiting step for the formation of D. For comparison, the Lewis bases Lut, Col, Cl À and Br À also form stable B(C 6 F 5 ) adducts that are À2.0, À4.7, À5.7 and À1.3 kcal mol À1 Scheme 3. Summary of major products of CO 2 reduction using the FLP catalyst B(C 6 F 5 ) 3 /2,6-lutidine. exergonic in CHCl 3 solution (see Supporting Information), respectively. The higher affinity for Col and Cl À may further inhibit H 2 -activation reactivity.
Once intermediate D is formed (Figure 1 B), further reduction via silylium transfer from Me 3 SiI (via TS4) and subsequent hydride transfer from A (via TS5) to give the acetal H 2 C(OSiMe 3 ) 2 (E) proceeds quickly and is À13.3 kcal mol À1 exergonic. Further silylium transfer from Me 3 SiI to E (via TS6) and subsequent hydride transfer from A (via TS7) to give H 3 COSiMe 3 (F), O(SiMe 3 ) 2 and [LutH]I is still possible over a slightly higher barrier of 20.3 kcal mol À1 (via TS6), but is À39.9 kcal mol À1 exergonic. Under moderate heating, both formation of E and F should be kinetically facile. The use of bulkier silanes such as Et 3 SiI 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 Me 3 SiI to F to give the cation H 3 CO(SiMe 3 ) 2 + (G + ) and the I À anion (via TS8, Figure 1 C), is 10.5 kcal mol À1 endergonic over a low barrier of 16.5 kcal mol À1 and thus is kinetically feasible. Further nucleophilic iodide transfers from [LutH]I to G + to give the experimentally observed CH 3 I and O(SiMe 3 ) 2 is À24.3 kcal mol À1 exergonic over a low barrier of 13.9 kcal mol À1 (via TS9 + ). The overall formation of CH 3 I from F is thus À13.8 kcal mol À1 exergonic over a sizable barrier of 24.4 kcal mol À1 , consistent with the moderate heating required experimentally. On the other hand, nucleophilic hydride transfer from A to G + to give CH 4 , O(SiMe 3 ) 2 and regenerate B(C 6 F 5 ) 3 is À49.0 kcal mol À1 exergonic over a low barrier of 14.3 kcal mol À1 (via TS10). Coupled with the facile H 2 activation, the overall formation of CH 4 from F is thus À53.2 kcal mol À1 exergonic over a barrier of 24.8 kcal mol À1 . This is thermodynamically more favorable but kinetically comparable with the formation of CH 3 I. Indeed, the use of Et 3 SiI and Me 3 SiI are found to favor iodide and hydride transfer affording CH 3 I and CH 4 , respectively.
In conclusion, we have achieved metal-free catalytic hydrogenation of CO 2 using H 2 and a silylhalide as an oxophile in the presence of a FLP derived from lutidine and B(C 6 F 5 ) 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, Me 3 SiO 13 CH 3, the acetal (Et 3 SiO) 2 13 CH 2 , 13 CH 4 and 13 CH 3 I. 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.