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Nickel-Catalyzed Hydrosilylation of CO2 in the Presence of Et3B for the Synthesis of Formic Acid and Related Formates ́ Lucero González-Sebastián, Marcos Flores-Alamo, and Juventino J. Garcıa* Facultad de Quı ́mica, Universidad Nacional Autónoma de México, Mexico City 04510, Mexico S Supporting Information *

ABSTRACT: The reaction of CO2 with Et3SiH catalyzed by the nickel complex [(dippe)Ni(μ-H)] 2 (1) afforded the reduction products Et3SiOCH2OSiEt3 (12%), Et3SiOCH3 (3%), and CO, which were characterized by standard spectroscopic methods. Part of the generated CO was found as the complex [(dippe)Ni(CO)]2 (2), which was characterized by single-crystal Xray diffraction. When the same reaction was carried out in the presence of a Lewis acid, such as Et3B, the hydrosilylation of CO2 efficiently proceeded to give the silyl formate (Et3SiOC(O)H) in high yields (85−89%), at 80 °C for 1 h. Further reactivity of the silyl formate to yield formic acid, formamides, and alkyl formates was also investigated.

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INTRODUCTION Even though carbon dioxide is a significant contributor to global climate change, it also can be envisaged as a potential C1 building block for fine and commodity chemicals.1 However, the activation of CO2 represents a challenge for chemists because of its thermodynamic and kinetic stability. Hence, research for new catalysts, particularly those that utilize cheap and earth-abundant elements, to fix and transform CO2 into useful products is a topic of growing interest. For example, a number of transition-metal catalysts are known to be effective in the direct hydrogenation of CO 2 to formic acid. 2 Nevertheless, this reaction is unfavorable thermodynamically (ΔG = +33 kJ mol−1),2a and even under high H2 and CO2 pressures,3 it exhibits low activities.4 In order to face such unfavorable thermodynamics, hydrosilanes, R3−xSiH1+x, can be used to reduce and functionalize CO2 by means of the formation of stable Si−O bonds. Subsequently, the reduction of CO2 with hydrosilanes allows the further synthesis of silyl formates,5 methoxysilanes,6 and methane.7 Several hydrosilanes potentially useful as reducing agents are readily available, easy to handle, nontoxic, and inexpensive.8 Among the mentioned CO2 reduction products, silyl formates are some of the most attractive compounds, since they can potentially be used as valuable synthons to a wide range of value-added chemicals derived from CO2.9 For instance, silyl formates can be easily converted into formic acid by simple hydrolysis;10 this reaction is a convenient approach toward formic acid production from CO2, and a variety of carbonyl compounds can be synthesized by reactions with the appropriate nucleophiles. Thus, the metalcatalyzed hydrosilylation of CO2 is emerging as an alternative methodology for catalytic CO2 fixation. To date, several reaction systems using transition metals, such as Ir,5c,6a,11 Ru,5a Zr,5d,12 Rh, and Cu,10a and organocatalysts6b have been © 2013 American Chemical Society

reported for the hydrosilylation of CO2. Nevertheless, to the best of our knowledge, a nickel-catalyzed CO2 hydrosilylation reaction has not yet been described in the literature. Herein we report the first hydrosilylation of CO2 using the nickel complex [(dippe)Ni(μ-H)]2 (1) as a catalytic precursor in combination with Et3B, to give silyl formate in high yields. Furthermore, we also disclose the intermediacy of silyl formate in the consecutive synthesis of several carbonyl compounds from CO2 via a tandem process, which consists of a hydrosilylation reaction followed by nucleophilic attack by alcohols/HBF4, water, or amines.

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RESULTS AND DISCUSSION Reaction of CO2 with Et3SiH Catalyzed by [(dippe)Ni(μ-H)]2 (1). The reactivity of CO2 and Et3SiH in the presence of catalytic amounts of [(dippe)Ni(μ-H)]2 was first assessed using a variety of solvents, at 80 °C and an atmospheric pressure of CO2; the main results for these experiments are summarized in Table 1. As shown in the above table, a moderate conversion of Et3SiH was observed on using THF as solvent, yielding Et3SiOSiEt3 (2s), Et3SiOCH2OSiEt3 (3s), Et3SiOCH3 (4s), and CO. The reduction products were identified by their characteristic signals in 1H NMR: 3s (O−CH2−O, 5.1 ppm, s) and 4s (O−CH3, 3.5 ppm, s) and by direct comparison with recently reported data.7b By determination of the corresponding 13C{1H} NMR spectrum, the presence of 3s was confirmed by a signature signal at 84.6 ppm. Compounds 3s and 4s are interesting, since these species has been proposed as intermediates toward the total reduction of CO2 to methane.7b Received: September 1, 2013 Published: November 4, 2013 7186

dx.doi.org/10.1021/om400876j | Organometallics 2013, 32, 7186−7194

Organometallics

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Table 1. Reaction of CO2 and Et3SiH Catalyzed by [(dippe)Ni(μ-H)]2a

entry

solvent

conversn of Si−H (%)

2s (%)

3s (%)

4s (%)

1 2 3 4

THF toluene dioxaneb MeCN

50 8 39 0

35 8 17 nd

12 nd nd nd

3 nd 2 nd

All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve using 5 mL of solvent. Reaction conditions: 0.01/1 of [(dippe)Ni(μH)]2 and Et3SiH, respectively, and 1 atm of CO2 were used. Conversions and yields were determined by GC-MS analysis. nd = not detected. b20% of Et3SiOCH2CH2OSiEt3 was observed. a

Worthy of mention is the reactivity found in dioxane (entry 3); here the ring opening of dioxane was involved to yield Et3SiOCH2CH2OSiEt3. No reaction was observed in acetonitrile, probably due to the coordination and/or oxidative addition of the MeCN to the nickel center, as previously reported.13 In order to find the appropriate conditions for CO2 reduction with Et3SiH, further studies focused on the use of THF, at 100 and 120 °C and reaction times of 22, 36, and 44 h; however, all results were very similar to those of entry 1 (Table 1). An increased load of [(dippe)Ni(μ-H)]2 (10% mol) was used with results in yield similar to those in entry 1 (conversion, 65%; 2s, 39%; 3s, 11%; 4s, 5%), but this amount of complex allowed us to monitor the reaction by 31P{1H} NMR; only one major signal was observed at 73.9 ppm, assigned to the stable complex [(dippe)Ni(CO)2] (2). Suitable crystals for X-ray studies of this compound were obtained by spontaneous crystallization from the reaction mixture. The corresponding ORTEP representation for 2 is depicted in Figure 1. In this case, CO2 was reduced to CO with Et3SiH, the last acting both as reducing agent and oxygen acceptor. We previously reported the CO2 reduction with [(dippe)Ni(μ-H)]2 to yield [(dippe)Ni(CO)2] as one of the reduction products together with [(dippe)Ni(CO3)] and dippe oxides as the oxidation products.14 The independent preparation of [(dippe)Ni(CO)2] revealed it to be a very stable compound and nonactive in the hydrosilylation reaction (see the Experimental Section). Consequently, the low conversion in the hydrosilylation process may be due to a deactivation of the catalyst by the formation of the rather stable compound 2. Concerning the solid-state structure of complex 2, this has a nickel center in a distorted -tetrahedral geometry, coordinated to two P atoms from dippe and two terminal CO ligands. The bite angle for the chelating dippe ligand of 90.72(4)° is similar to those for other dippeNi0 complexes,14a while the OC−Ni− CO angle of 113.22(16)° (average) is larger than that reported for [(dtbpe)Ni(CO)2] (108.1(3)° average).14b The P−Ni−CO angle 112.75° (average) is close to that reported for the complex [Ni(CO)(np3)] (112.7.0(9)° average)15 (np3 = tris(2diphenylphosphinoethyl)amine) and slightly larger than that reported for [(dtbpe)Ni(CO)2] (114.03 average).14b On the other hand, the Ni−CO bond distance in 2 (1.762 Å average) is close to the Ni−CO bond distance in [Ni(CO)(np3)] (1.74 Å),15 [(dtbpe)Ni(CO)2] (1.787 Å average),14b and {[(dippe)Ni(CO)]2(μ-dippe)} (1.759 (3) Å)14a but shorter than those reported for Ni(CO)4 (1.84 Å) and [(CO)3Ni(PPh2)2Ni(CO)3)] (1.8003(8) Å).16 The Ni−P (2.206 Å average) bond

Figure 1. ORTEP drawing (50% probability ellipsoids) for complex 2. Selected distances (Å) and angles (deg): C(15)−O(1) = 1.145(4), C(16)−O(2) = 1.153(4), C(16)−Ni(1) = 1.761(4), C(15)−Ni(1) = 1.764(4), Ni(1)−P(1) = 2.2042(10), Ni(1)−P(2) = 2.2079(10); O(1)−C(15)−Ni(1) = 178.7(3), O(2)−C(16)−Ni(1) = 177.9(3), C(16)−Ni(1)−C(15) = 113.22(16), C(16)−Ni(1)−P(1) = 113.33(12), C(15)−Ni(1)−P(1) = 112.52(12), C(16)−Ni(1)−P(2) = 112.19(12), C(15)−Ni(1)−P(2) = 112.96(13), P(1)−Ni(1)−P(2) = 90.72(4).

distances are in the range of closely related tetrahedral d10 nickel complexes such as [Ni(P(OCH2)3CCH3)3NO]BF4 (2.186 Å average),17 Ni(CO)(np3)] (2.215 Å average),15 [(dtbpe)Ni(CO)2] (2.23 Å average),14b and {[(dippe)Ni(CO)]2(μ-dippe)} (2.23 Å).14a Reaction of CO2, Et3SiH, and Et3B Catalyzed by 1. In order to improve the CO2 hydrosilylation, some experiments were carried out using a Lewis acid (Et3B) as an additive, aiming to polarize the Si−H and/or CO bonds and thus drive the reaction.7a,b,18 The reactivity of 1 toward the hydrosilylation reaction using CO2, Et3SiH, and Et3B was first studied in THF. In a typical reaction, [(dippe)Ni(μ-H)]2, Et3SiH, and Et3B were dissolved in 5 mL of THF and then a CO2 stream was bubbled at room temperature for 10 min. 7187


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Scheme 1

Table 2. Hydrosilylation of CO2 Catalyzed by [(dippe)Ni(μ-H)]2 in Various Solventsa

entry

solvent

conversn of Si−H (%)

2s (%)

5s (%)

6s (%)

1 2 3 4 5

THF toluene dioxane MeCN neat

96 21 87 0 0

1.1 nd 3.0 nd nd

90 11 76 nd nd

6 10 8 nd nd

All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve using 5 mL of solvent. Reaction conditions: 0.01/0.1/1 of [(dippe)Ni(μ-H)]2, Et3B, and Et3SiH, respectively, and 1 atm of CO2 were used. Conversions and yields were determined by GC-MS analysis. nd = not detected. a

Table 3. Hydrosilylation of CO2 Catalyzed by [(dippe)Ni(μ-H)]2 at Different Temperaturesa

entry

temp (°C)

conversn (%)

2s (%)

5s (%)

6s (%)

1 2 3

25 80 100

80 96 95

nd 1 1

80 90 75

nd 5.7 19

Reactions were carried out in a Schlenk flask equipped with a Rotaflo valve using 5 mL of solvent. Reaction conditions: 0.01/0.1/1 of [(dippe)Ni(μ-H)]2, Et3B, and Et3SiH, respectively, and 1 atm of CO2 were used. Conversions and yields were determined by GC-MS analysis. nd = not detected. a

Scheme 2. Hydrosilylation of CO2 Catalyzed by [(dippe)Ni(μ-H)]2 using 5 mol % of Et3B

OCH(O)H, located at 8.05 ppm. In addition, the 13C{1H} NMR spectrum of Et3Si−OC(O)H showed a key resonance at 161.4 ppm and the corresponding 13C NMR yielded a doublet for that signal with 1JC−H = 223 Hz. Compound 6s was also identified by 1H NMR with characteristic resonances at 2.3 ppm as a quadruplet and 1.05 ppm as a triplet for the OC(O)CH2CH3 moiety. In addition, the 29Si NMR spectrum for 5s presented a resonance at 25 ppm. The formation of 6s can be explained by considering the known reactivity of Et3B,

Then the reaction was monitored by GC-MS. It was found that Et3SiH was almost consumed, 97%, yielding the products 2s (1%), 6s (6%), and the silyl formate 5s (90%) (Scheme 1). It is worth mentioning that in a blank experiment hydrosilylation did not occur in the absence of [(dippe)Ni(μH)]2. To confirm the reaction products shown in Scheme 1, 1H and 13C NMR analyses were conducted after the reaction. The 1 H NMR spectrum for 5s showed a signal assigned to Et3Si− 7188


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Scheme 3. Optimized Conditions for the Hydrosilylation of CO2 Catalyzed by [(dippe)Ni(μ-H)]2

Scheme 4. Mechanistic Proposal for the CO2 Hydrosilylation Reaction

At 100 °C the conversion is higher (entry 3); however, the selectivity to 5s is lower than at 80 °C (entry 2, Table 3) with an increase in the yield of the alkylation product 6s. The use of lower amounts of Et3B (5 mol %) results in lower conversion and longer reaction time, as depicted in Scheme 2. Using the conditions from entry 2 of Table 3 as a starting point, the reaction time was improved to 1 h at 80 °C, resulting in 95% conversion of Et3SiH to yield 88% of silyl formate; this yields involves a TOF of 87.7 h−1 (Scheme 3). A mechanistic proposal for the CO2 hydrosilylation is depicted in Scheme 4, on the basis of the following considerations: first, Lewis acids are generally envisaged as activators of CO bonds through the formation of CO B adducts, thus polarizing the CO bond7b,18,20 and making the

since its use as an alkylating agent has long been reported in the literature.19 Additionally, the reaction mixture was studied by 11 B NMR spectroscopy, displaying two resonances at 76.6 and 56.1 ppm assigned to Et3B and Et2B−O−BEt2, respectively. The main results for the catalytic hydrosilylation of CO2 with different solvents in the presence of Et3B are shown in Table 2. As shown before, again, the best yield, according to Table 2, is for THF as solvent with a high conversion and selectivity toward the formation of silyl formate 5s. Both THF and dioxane were found to be good solvents for this reaction. The effect of temperature for this reaction was also assessed; a summary of the results is presented in Table 3. Of note, the reaction takes place at room temperature (25 °C) with good conversion and selectivity, just slightly lower than that at 80 °C. 7189


Organometallics

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Scheme 5. Synthesis of Formic Acid from Et3SiH, CO2, and H2Oa

a

Reaction conditions: (1) 1 (0.01 mol), Et3B (0.1 mmol) Et3SiH (1 mmol), CO2 (Patm= 1 atm CO2), THF (5 mL), 80 °C, 1 h; (2) H2O (0.2 mL).

Scheme 6. Synthesis of Potassium Formate

Table 4. Consecutive Synthesis of Formamides from CO2 and Et3SiH with Primary Aminesa

All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve. Reaction conditions: (1) 1 (0.01 mol), Et3B (0.1 mmol), Et3SiH (1 mmol), CO2 (Patm = 1 atm CO2), THF (5 mL), 80 °C, 1 h; (2) amine (1 mmol). Conversions and yields were determined by GC-MS analysis.

a

Further reactivity showed the silyl formate hydrolysis by addition of H2O, yielding formic acid (Scheme 5) and silanol as a byproduct. This reaction was monitored by 1H and 13C NMR and GCMS. The conversion of 5s after the addition of water was complete, as indicated by the absence of signals for silyl formate (1H 8.18 ppm, 13C 161.5 ppm, JC−H = 223.63 Hz) and the appearance of signals due to formic acid (1H 8.05, 13C 163.15 ppm, JC−H = 213.7 Hz). Additionally, the addition of potassium fluoride into the solution containing 5s yielded the potassium formate in quantitative amounts and Et3SiF as byproduct (Scheme 6). Finally, we turned out our attention to the reactivity of the silyl formate 5s, produced during the hydrosilylation of CO2, with a variety of nucleophilic reagents. The overall conversion of CO2 to formic acid derivatives was accomplished by simple addition of the selected nucleophilic reagent to the reaction solution once the CO2 hydrosilylation with 1/Et3B was complete (vide infra). In the case of primary and secondary amines the silyl formate could act as a “formyl synthon” to produce the formamides as

carbon atom even more electrophilic. Second, the ability of R3B species to abstract an alkide21 and hydride7a,22 fragment from neutral organometallic precursors is well-known.23 Furthermore, it has been shown that the tributylstannyl cation can be generated via hydride abstraction from Bu3SnH using B(C6F5)3.12,24 Therefore, it seems reasonable to propose the hydride abstraction from Et3SiH by Et3B. In this regard, the formation of silyl formate can be achieved by one of the two pathways depicted in Scheme 4. In one of them (center of Scheme 4) an initial step, involving the CO2 coordination to the nickel center, is carried out to give I; this step is followed by the coordination of Et3B to CO2 and the silane, to favor a nucleophlic attack of the hydride over CO2 yielding species II, which undergoes a reductive elimination and then releases the product 5s and regenerates the nickel catalyst. Alternatively, an oxidative addition of Et3SiH over nickel(0) can occur to yield I′, followed by insertion of the adduct CO2·BEt3 into the Ni−H bond, generating the nickel formate II; finally, a reductive elimination of intermediate II releases 5s. 7190


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Table 5. Consecutive Synthesis of Formamides from CO2 and Et3SiH with Secondary Aminesa

All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve. Reaction conditions: (1) 1 (0.01 mol), Et3B (0.1 mmol), Et3SiH (1 mmol), CO2 (Patm = 1 atm CO2), THF (5 mL), 80 °C, 1 h; (2) amine (1 mmol). Conversions and yields were determined by GC-MS analysis.

a

Table 6. Consecutive Synthesis of Alkyl Formates from CO2 and Et3SiH with Alcohol/HBF4a

All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve. Reaction conditions: (1) 1 (0.01 mol), Et3B (0.1 mmol), Et3SiH (1 mmol), CO2 (Patm = 1 atm CO2), THF (5 mL), 80 °C, 1 h; (2) alcohol (1 mmol), HBF4 (1 mmol), NaSO4 (2 mmol). Conversions and yields were determined by GC-MS analysis. a

shown on Tables 4 and 5. With primary amines, 5s gave the Nalkyl formamides with high conversions and good yields as expected, and silanol was obtained as a coproduct (Table 4).

Likewise, on using secondary amines, N,N-disubstituted formamide products were produced in moderate yields along with the byproducts 2s and the protected Et3Si carbamate 8s 7191


Organometallics

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General Procedure for the Reaction of CO2 with Et3SiH Catalyzed by [(dippe)Ni(μ-H)]2 with Different Solvents. A typical methodology was used as follows. A 25 mL Schlenk flask, equipped with a Rotaflo valve and an inner magnetic stirring bar, was loaded into the glovebox with a THF solution (5 mL) of [(dippe)Ni(μ-H)]2 (6.4 mg, 0.01 mmol) and Et3SiH (116 mg, 1 mmol). A color change from wine red to brown was immediately observed. The solution was stirred for 10 min, and then a CO2 stream was bubbled at room temperature for 10 min. Then the flask was closed and heated in an oil bath at 80 °C for 22 h. After this time, the heating was stopped and the flask was vented into the hood. An aliquot of the reaction mixture was immediately analyzed by GC-MS. This methodology was followed using toluene, dioxane, and acetonitrile instead of THF. [(dippe)Ni(CO)2] was isolated from the reaction using 10 mol % of [(dippe)Ni(μ-H)]2 (32.2 mg, 0.05 mmol) and Et3SiH (58 mg, 0.5 mmol) in 5 mL of THF. The reaction mixture was concentrated to dryness, giving a yellow residue, which was dried under vacuum for 6 h and then redissolved in CD3OD (0.8 mL). This residue was identified as [(dippe)Ni(CO)2] (2). Yield for complex 2: 99%. Anal. Calcd for C16H32NiO2P2 (2): C, 50.97; H, 8.55. Found: C, 50.77; H, 8.52. 31 P{1H} NMR (22 °C, 300 MHz, CD3OD): δ 73.9 (s, (iPr 2)2PCH2CH2P(i-Pr 2)2). 13 C{1H} NMR (22 °C, 300 MHz, CD3OD): δ 204.3 (t, CO, JC−P = 4.1 Hz), 25.8 (t, (CH(CH3)2), JC−P = 9.8 Hz), 23.8 (dd, (CH(CH3)2), JC−P = 8.6, 6.9 Hz), 19.6 (m, (CH2CH2)). 1H NMR (22 °C, 300 MHz, CD3OD): δ 2.0−1.82 (m, CH, 4H), 1.56−1.48 (m, CH2, 4H), 1.1−0.9 (m, CH2, 24H). Crystals of [(dippe)Ni(CO)2] suitable for X-ray diffraction studies were obtained by cooling the CD3OD solution to −30 °C and isolating the formed crystals by decantation. Reaction of CO2 with Et3SiH Catalyzed by [(dippe)Ni(μ-H)]2 in THF-d8. A NMR tube equipped with a J. Young valve was loaded in a glovebox with a THF-d8 solution (0.8 mL) of [(dippe)Ni(μ-H)]2 (3.2 mg, 0.005 mmol) and Et3SiH (58 mg, 0.5 mmol). This solution was bubbled with a CO2 stream at room temperature for 5 min. Then, the tube was closed and heated in an oil bath at 80 °C for 22 h. The sample was monitored by 1H and 13C{1H} NMR. Using multinuclear NMR the following assignations were made. Et3SiH: 1H NMR (22 °C, 300 MHz, THF-d8): δ 3.7 (septet, 3JH−H = 3.0 Hz, SiH), 0.99 (t, 3JH−H = 7.5 Hz, 9H, CH3CH2−), 0.61 (d quartet, 3 JH−H = 7.5 Hz, 3JH‑SiH = 3.3 Hz, 6H, CH3CH2−); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 8.6 (s, CH3CH2−); 3.3 (s, CH3CH2−); 29Si NMR (22 °C, 400 MHz, THF-d8) δ 0.99 (s, Et3SiH). (Et3Si)2O: 1H NMR (22 °C, 300 MHz, THF-d8) δ 0.8 (t, 3JH−H = 7.8 Hz, CH3CH2−); 0.4 (q, 3JH−H = 7.8 Hz, CH3CH2−); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 7.25 (s, CH3CH2−); 6.8 (s, CH3CH2−); 29Si NMR (22 °C, 400 MHz, THF-d8) δ 7.7 (s, (Et3Si)2O). (Et3Si)2CH2: 1 H NMR (22 °C, 300 MHz, THF-d8) δ 5.06 (t, 3JH−H = 7.8 Hz, (Et3Si)2CH2); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 84.6 (s, (Et3Si)2CH2) 6.9 (s, CH3CH2−), 5.3 (s, CH3CH2−). Reaction of CO2, Et3SiH, and Et3B Catalyzed by [(dippe)Ni(μH)]2. A 25 mL Schlenk flask equipped with a Rotaflo valve and a magnetic stirrer was charged in the glovebox with a THF (5 mL) solution of [(dippe)Ni(μ-H)]2 (6.4 mg, 0.01 mmol), Et3SiH (116 mg, 1 mmol), and Et3B (9.8 mg, 0.1 mmol). A color change from wine red to brown and effervescence were observed. The reaction mixture was stirred for 10 min, and then a CO2 stream was bubbled at room temperature for 10 min. After that the flask was closed, followed by heating in an oil bath at 80 °C for 22 h. Then, the flask was vented in a hood and analyzed by GC-MS to give the products (Et3Si)2O (2s), Et3Si−OCH(O)H (5s), and Et3Si−OCH(O)CH2CH3 (6s). The very same procedure was followed using toluene, dioxane, and acetonitrile instead of THF. Neat conditions: [(dippe)Ni(μ-H)]2 (19.2 mg, 0.03 mmol), Et3SiH (348 mg, 3 mmol), and Et3B (29.4 mg, 0.3 mmol) were used and, on mixing, a brown-orange solution was obtained. The solution was transferred to a Schlenk flask, and then a CO2 stream was bubbled at room temperature for 10 min, followed by heating in an oil bath to 80 °C for 22 h. Reaction of CO2, Et3SiH, and Et3B Catalyzed by [(dippe)Ni(μH)]2 in THF-d8. [(dippe)Ni(μ-H)]2 (3.2 mg, 0.005 mmol), Et3SiH (58 mg, 0.5 mmol), and Et3B (4.9 mg, 0.05 mmol) were mixed in 0.8 mL

(Table 5). The product 8s was fully identified by GC-MS, and additionally the 13C{1H} NMR spectrum showed a key signal at 154.7 ppm. Since the formylation of alcohols is of relevance in organic synthesis,26 we also explored the reactivity of 5s with a wide variety of alcohols and HBF4. This reaction is driven by the formation of the highly energetic Si−F bond (565 kJmol−1)25 and thus gave the alkyl formates in good yields. The main results of this transformation are summarized in Table 6, all with high conversions.

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CONCLUSIONS In the current study we report the first nickel-catalyzed CO2 hydrosilylation promoted by Et3B to selectively give silyl formate. An important aspect of this transformation is the strong dependence on the use of Et3B as an additive to enhance the desired reactivity. Also, this work provided relevant findings in the utilization of silyl formate as a building block in the synthesis of value-added products. The simple reaction of silyl formate with water produced formic acid and silanol in excellent yields. The hydrosilylation of CO2 was successfully applied to the reaction with either amines or alcohols/HBF4, which can be considered as a potentially useful method to afford a great variety of amides and alkyl formates in good yields. Last but not least, we have also described the reduction of CO2 by 1 generating CO and some intermediates proposed toward the reduction of CO2 to methane.

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise noted, all operations were carried out in a MBraun glovebox (99.99%) and was used without further purification. All reagents for the catalytic reactions were loaded in the glovebox using Schlenk flasks equipped with Rotaflo high-vacuum stopcocks and further loaded with CO2. The crude reaction mixtures for each catalytic run were immediately analyzed by GC-MS. GC-MS determinations were performed using an Agilent 5975C system equipped with a 30 m DB-5MS capillary (0−32 mm i.d.) column. 1H, 13 C{1H}, and 31P{1H} NMR spectra were recorded at room temperature on a 300 MHz Varian Unity spectrometer in THF-d8, CD3OD, or CD2Cl2, unless otherwise stated. 1H and 13C{1H} chemical shifts (δ, ppm) are reported relative to the residual proton resonance in the corresponding deuterated solvent. 31P{1H} NMR spectra were referenced to an external 85% H3PO4 solution. 29Si and 11 B NMR spectra were recorded at room temperature on a 400 MHz Varian Unity spectrometer in THF-d8, unless otherwise stated. 29Si NMR spectra were referenced to an external 85% Si(CH3)4 solution, and 11B NMR spectra were referenced to external 85% BF3. All airsensitive NMR samples were handled under an inert atmosphere using thin-wall (0.38 mm) Wilmad NMR tubes equipped with J. Young valves. Single-crystal X-ray diffraction determinations were performed on an Oxford Diffraction Gemini-Atlas diffractometer. 7192


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data collection and data integration. Data sets consisted of frames of intensity data collected with a frame width of 1° in ω, a counting time of 5.6 s/frame, and a crystal-to-detector distance of 55.00 mm. The double-pass method of scanning was used to exclude any noise. The collected frames were integrated by using an orientation matrix determined from the narrow frame scans. Final cell constants were determined by a global refinement; collected data were corrected for absorbance by using analytical numeric absorption correction30 with a multifaceted crystal model based on expressions upon the Laue symmetry using equivalent reflections. Structure solution and refinement were carried out with the program(s) SHELXS9731 and SHELXL97; ORTEP-3 for Windows32 was used for molecular graphics, and WinGX33 was used to prepare the material for publication. Full-matrix least-squares refinement was carried out by minimizing (Fo2 − Fc2)2. All non-hydrogen atoms were refined anisotropically. H atoms attached to C atoms were placed in geometrically idealized positions and refined as riding on their parent atoms, with C−H = 0.98−0.99 Å and Uiso(H) = 1.2[Ueq(C)] and Uiso(H) = 1.5[Ueq(C)] for methylene and methyl groups, respectively.

of THF-d8. This mixture was transferred to a NMR tube equipped with a J. Young valve, and a CO2 stream was bubbled at room temperature for 5 min, followed by heating in an oil bath at 80 °C. The formation of Et3Si−OCH(O)H (5s) was followed up by 1H, 13C{1H}, 13 C, 29Si. Characteristic signals for the silyl formate Et3Si−OCH(O)H (5s): 1 H NMR (22 °C, 300 MHz, THF-d8) δ 8.05 (s, Et3Si−OCH(O)H), 0.99 (t, 9H, CH3CH2−), 0.61 (d quartet, 6H, CH3CH2−); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 161.4 (s, Et3Si−OCH(O)H), 6.8 (s, CH3CH2−), 5.4 (s, CH3CH2−); 13C NMR (22 °C, 300 MHz, THF-d8) δ 161.4 (d, JC−H = 223.6 Hz, Et3Si−OCH(O)H); 29Si NMR (22 °C, 400 MHz, THF-d8) δ 24.97 4 (s, Et3Si−OCH(O)H). Procedure for the Synthesis of Formic Acid. Once the hydrosilylation reaction of CO2 with 1/Et3B was completed (vide supra), water (0.2 mL, 11 mmol) was added and the reaction mixture was vigorously stirred at room temperature for 20 min. The conversions and yields of the products were determined by GC-MS and 1H, 13C, and 13C{1H} NMR analyses. HOC(O)H: 1H NMR (22 °C, 300 MHz, THF-d8) δ 11.23 (s, HOC(O)H), 8.05 (s, HOC(O)H); 13 C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 163.1 (s, HOC(O)H); 13 C NMR (22 °C, 300 MHz, THF-d8) δ 163.1 (d, JC−H = 213.7 Hz, HOC(O)H). Et3SiOH: 1H NMR (22 °C, 300 MHz, THF-d8) δ 3.8 (m, Et3SiOH), 0.95 (t, 3JH−H = 7.5 Hz, 9H, CH3CH2−), 0.54 (q, 3JH−H = 7.5 Hz, CH3CH2−); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 6.3 (s, CH3CH2−), 6.2 (s, CH3CH2−). Procedure for the Synthesis of Potassium Formate. After completion of the hydrosilylation of CO2 with 1/Et3B described above, potassium fluoride (95 mg, 1 mmol) was added to the reaction solution of 5s and this mixture was vigorously stirred at room temperature for 15 min; then the solution was evaporated to dryness, yielding a yellow solid. The potassium formate was separated with methanol, and this was removed by evaporation to yield KOC(O)H as a white solid. KOC(O)H: 1H NMR (22 °C, 300 MHz, THF-d8) δ 8.05 (s, HOC(O)H); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 163.1 (s, HOC(O)H); IR 1609, 1388, 1358, 1218, 1136, 1060 cm−1. Typical Procedure for the Formylation of Amines. Following the hydrosilylation of CO2, the corresponding amine (1 mmol) was added to the reaction solution of 5s produced in situ. The Schlenk tube was sealed, and the reaction mixture was vigorously stirred at 80 °C. The conversions and yields of the products were determined by GC-MS. Procedure for the Synthesis of Alkyl Formates. After the hydrosilylation of CO2 described above, NaSO4 (284 mg, 2 mmol) and a mixture of the corresponding alcohol (1 mmol) with HBF4 (1 mmol) were added to the reaction solution of 5s. Then, the Schlenk tube was sealed and the reaction mixture was vigorously stirred at 80 °C. The conversions and yields of the products were determined by CG-MS. Attempts of the Hydrosilylation of CO2 with [(dippe)Ni(CO)2]. The complex [(dippe)Ni(CO)2] was independently prepared and tested as a potential catalyst precursor in the hydrosilylation reaction: a 25 mL Schlenk flask, equipped with a Rotaflo valve and a magnetic stirring bar, was charged in a glovebox with a THF solution (5 mL) of [(dippe)Ni(CO)2] (0.19 mg, 0.05 mmol) and Et3SiH (58 mg, 0.5 mmol). The reaction mixture was stirred for 10 min, and then a CO2 stream was bubbled at room temperature for 10 min. Then the Schlenk was closed and heated in an oil bath to 80 °C for 22 h. After this time the reaction mixture was cooled to room temperature and an aliquot was analyzed by GC-MS. No hydrosilylation products were observed. The 31P{1H} NMR spectrum of this sample confirmed the presence of unreacted [(dippe)Ni(CO)2] by a key signal at 73.9 ppm, confirming the high stability of the complex. X-ray Structure Determination. Colorless prisms of 2, suitable for X-ray diffraction studies, were obtained by cooling a solution of deuterated methanol to −30 °C. A crystal of compound 2 was mounted under LVAC FOMBLIN Y on glass fibers and immediately placed under a cold nitrogen stream on an Oxford Diffraction Gemini “A” diffractometer with a CCD area detector, with a radiation source of λCu Kα = 1.5418 Å using graphite-monochromated radiation. CrysAlisPro and CrysAlis RED software packages29 were used for

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AUTHOR INFORMATION

* Supporting Information S

Figures, a table, and a CIF file giving selected multinuclear NMR data and crystallographic data for complex 2. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*E-mail for J.J.G.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank PAPIIT-DGAPA-UNAM (IN-210613) and CONACYT (0178265) for the financial support for this work. L.G.S. also thanks CONACYT for a Ph.D. grant. We also thank Dr. Alma Arévalo for her technical assistance.

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REFERENCES

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