Catalytic Reduction of Carbon Dioxide Using Cationic

Dec 7, 2017 - Ethide abstraction from Et3M (M = Al and Ga), (2,6-Ph2C6H3)AlEt2, 1, and (2,6-Dipp2C6H3)GaEt2, 2 (Dipp = 2,6-iPr2C6H3), using the silyli...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Catalytic Reduction of Carbon Dioxide Using Cationic Organoaluminum and -Gallium Compounds Mahmoud Saleh,† Douglas R. Powell,‡ and Rudolf J. Wehmschulte*,† †

Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, United States Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019-5251, United States

‡

S Supporting Information *

ABSTRACT: Ethide abstraction from Et3M (M = Al and Ga), (2,6-Ph2C6H3)AlEt2, 1, and (2,6-Dipp2C6H3)GaEt2, 2 (Dipp = 2,6-iPr2C6H3), using the silylium ion [Et3Si][CHB11Cl11] afforded crystalline ion-like compounds [Et2M][CHB11Cl11] (M = Al, 3; Ga, 5) and [(2,6-Ph2C6H3)AlEt][CHB11Cl11], 4, and the likely solvent-separated ion pair [(2,6-Dipp2C6H3)GaEt][CHB11Cl11], 6. Crystalline compounds 3−5 feature cation···anion contacts in the solid state, and their solubility in low polarity benzene indicates that these contacts are maintained in solution. All compounds catalyze the reduction of CO2 with Et3SiH, but the activity of the gallium compound 5 is significantly lower due to its polymeric structure and the lower Lewis acidity of gallium. Whereas the reduction products from the reactions catalyzed by compounds 3−5 are mostly methane and toluene (from Friedel−Crafts alkylation of the benzene solvent), catalysis by 6 led mostly to Et3SiOCH3.

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INTRODUCTION While the reduction of CO2 with dihydrogen or hydrosilanes catalyzed by transition metal compounds has been known for decades,1−4 the use of main group metal catalysts or even metal-free catalysts is a rather recent development.5 Generally, the latter systems use hydrosilanes as the reducing agent due to the weaker and slightly polar Si−H and stronger Si−O bonds compared to H−H and H−O bonds. Furthermore, the byproduct water would deactivate most of these systems, whereas siloxane byproducts are usually well-tolerated. Early attempts include the frustrated Lewis Pair TMP/B(C6F5)3 (TMP = 2,2,6,6-tetramethylpiperidine),6 which reduces CO2 to methane using Et3SiH as the reducing agent. This was followed by the demonstration that strongly basic Nheterocyclic carbenes were active catalysts to selectively afford methoxysilanes MeOSiR3,7 although it was later shown that the active species was the adduct NHC−CO2.8 Other strong organic bases such as bicyclic guanidines9 and phosphazenes10 can also be used as catalysts. The use of strong Lewis acids such as the ion-like [Et2Al][CH6B11I6] was demonstrated in 2012,11 and recently, the tandem system Al(C6F5)3/B(C6F5)3 was applied in CO2 hydrosilylation.12 Interestingly, even simple salts such as CsF and Cs2CO3 catalyze the CO2 reduction with PhMe2SiH,13 and even the weak Lewis acid BPh3 when used in polar solvents was found to catalyze the hydrosilylation of CO2 with several hydrosilanes selectively to the formate esters R3SiOCH(O).14 While [Et2Al][CH6B11I6] was able to catalyze the CO2 hydrosilylation, its activity was rather low.11 We hypothesized that this may be due to relatively strong cation···anion contacts, and we set out to prepare the compounds [Et2Al][CHB11Cl11], © XXXX American Chemical Society

3, and [(2,6-Ph2C6H3)AlEt][CHB11Cl11], 4, both featuring the much less basic [CHB11Cl11]− counterion.15 For comparison, we also synthesized the gallium analogues [Et2Ga][CHB11Cl11], 5, and [(2,6-Dipp2C6H3)GaEt][CHB11Cl11], 6. Here, we report the synthesis and structural characterization of these compounds and their activity as catalysts in the hydrosilylation of CO2 with Et3SiH.

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RESULTS AND DISCUSSION Synthesis of Terphenylaluminum and -Gallium Compounds. The synthesis of terphenylaluminum (2,6-Ph2C6H3)AlEt2, 1, and terphenylgallium (2,6-Dipp2C6H3)GaEt2, 2 (Dipp = 2,6-iPr2C6H3), was achieved in analogy to reported and standard organometallic procedures as shown in eq 1.16−18 The new compounds were characterized by 1H and 13C NMR spectroscopy. Synthesis of Cationic Aluminum and Gallium Compounds. Compounds 1 and 2 along with Et3Al and Et3Ga were converted to the corresponding cationic species by ethide abstraction with the silylium salt [Et3Si][CHB11Cl11] according to eq 2. These reactions were performed in a small grease-free tube or a Schlenk flask. The use of the trityl salt [Ph3C][CHB11Cl11] also afforded the target compounds, but their purification was more tedious. Despite the small scale of the reactions (due to the limited availability of [CHB11Cl11]− compounds) and the high air and moisture sensitivity, all of these reactions are readily reproducible. The identity of the new ionic compounds has been established by NMR spectroscopy Received: September 19, 2017

A

DOI: 10.1021/acs.organomet.7b00716 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

(1.941 Å (avg.)) and two Al···Cl contacts (2.453 Å (avg.)) to the anion. These values are close to those observed for the related ion pairs [Et2Al][CHB11H5Cl6] (Al−C 1.928 Å (avg.), Al···Cl 2.435 Å (avg.))19 and [Et2Al][Me3NB12Cl11] (Al−C 1.923 Å (avg.), Al···Cl 2.430 Å (avg.)),20 although the slight lengthening of the Al···Cl contacts in 3 may reflect the lower basicity of the [CHB11Cl11]− anion.15,21 Just as in [Et2Al][Me3NB12Cl11], the Al center in 3 is coordinated to m- and pchlorine donors from the anion. The C−Al−C angle of 136.44(9)° is significantly wider than the tetrahedral angle but does not come close to the 180° expected for a true twocoordinate species. This value is also close to those reported for [Et2Al][CHB11H5Cl6] (133.8(4)°) and [Et2Al][Me3NB12Cl11] (127.7(4)°). [(2,6-Ph2C6H3)AlEt][CHB11Cl11], 4, is significantly less soluble than 3, and colorless block-shape crystals suitable for single crystal X-ray analysis were grown from a hot benzene solution. Its structure (Figure 2) bears several similarities to as well as by single crystal X-ray diffraction for compounds 3− 5. Compound 6 was isolated as an oil, and NMR data support the expected structure. In contrast to the synthesis of [Et2Al][CB11H6Cl6],15 compound 3 was synthesized using the silylium cation to facilitate the isolation of the product in good yield. The compound is highly soluble in benzene and the attempted isolation from this solvent was unsuccessful. The reaction of the silylium salt16 [Et3Si][CHB11Cl11], prepared in situ, with excess Et3Al in hexane proceeded after sonication for several hours. Compound 3 was obtained as a colorless precipitate that can be used as is after washing with hexane to remove unreacted Et3Al and the side product Et4Si. Surprisingly, 3 was sufficiently soluble in hot (105 °C, closed flask) hexane to allow for crystallization after slow cooling to room temperature. The high solubility in benzene and even in hot hexanes indicated that 3 maintains its tight ion pair solid state structure (Figure 1) in solution. Compound 3 crystallizes in the monoclinic space group P21/ n. The asymmetric unit consists of two independent molecules with very similar geometrical features so that the data for only one of those will be used in the discussion here. The Al center is in a distorted tetrahedral environment with two Al−C bonds

Figure 2. Structure of 4 (50% ellipsoids). Hydrogen atoms except of H(1A) have been omitted for clarity. Selected bond distances (Å) and angles (deg): Al(1)−C(1) 1.948(2), Al(1)−C(19) 1.939(2), Al(1)··· Cl(7) 2.4922(6), Al(1)···Cl(12) 2.5153(6), C(1)−Al(1)−C(19) 142.24(7), Cl(7)−Al(1)−Cl(12) 88.84(2).

Figure 1. Structure of one of the two independent molecules of 3 (50% ellipsoids). Hydrogen atoms except of H(1A) have been omitted for clarity. Selected bond distances (Å) and angles (deg): Al(1A)− C(2A) 1.942(2), Al(1A)−C(4A) 1.941(2), Al(1A)···Cl(7A) 2.4515(8), Al(1A)···Cl(12) 2.4543(9), C(2A)−Al(1A)−C(4A) 136.44(9), Cl(12)−Al(1A)−Cl(7A) 92.22(3).

that of 3 such as the distorted tetrahedral Al center and cation− anion contact involving the Al center and chlorine donors from the m-belt and the para position of the anion. In contrast, the Al···Cl contacts are longer than those in 3 (2.504 (avg.) vs 2.453 (avg.) Å), and the C−Al−C angle is wider (142.24(7)° vs 136.44(9)°) possibly indicating a weaker cation−anion interaction. The structure of 4 is also quite similar to that of [DcpAlEt][CH6B11Cl6] (Dcp = 2,6-(2,6-Cl2C6H3)2C6H3),17 which features a slightly bigger and electron-poor substituent as well as a more basic counterion (Table S1). Furthermore, the 1 H NMR shifts of the ethyl signals in both compounds almost match (1.13 (t) and 0.65 (q) in 4, 1.02 (t) and 0.64 (q) in [DcpAlEt][CH6B11Cl6]),17 an indication of similar electronic environments close to the Al center. The gallium compound [Et2Ga][CHB11Cl11], 5, was isolated in good yield (82%) as colorless crystals by layering hexanes on top of the benzene reaction mixture. Contrary to 3 and 4, compound 5 does not feature isolated ion pairs in the solid state, but its structure consists of one-dimensional [Et2Ga][CHB11Cl11] chains (Figure 3). The coordination environment at the gallium center may be described as distorted octahedral B

DOI: 10.1021/acs.organomet.7b00716 Organometallics XXXX, XXX, XXX−XXX

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should also be pointed out that the positons of the CH2 and CH3 signals are reversed with respect to those from 3 and 4 (see Figure S13). [Dipp*GaEt][CHB11Cl11], 6, could only be obtained as a viscous oil with a low solubility in benzene, but it is readily soluble in bromobenzene or benzene/bromobenzene mixtures. This is a common behavior for ionic compounds with large cations and anions, and such compounds have been described as liquid clathrates.24,25 While not every oil separating from benzene is a liquid clathrate, removal of the volatiles from the oil containing 6 results in a significant increase in viscosity suggesting that solvent (benzene) molecules were incorporated in that oil. Many [B(C6F5)4]− salts show this behavior26 as well as [2,6-Mes 2 C 6 H 3 GaBu][CHB 11 Cl 11 ], [Dipp*GaBu][CHB11Cl11], and [Dipp*GaBu][B(C6F5)4],16 and this oil formation can be viewed as an indication for the presence of solvent separated ions. The proposed identity of 6 is supported by its 1H NMR spectrum, which shows a 1:1 ratio of cation and anion signals and also displays a high-field shift of the ethyl signals. Catalytic Reduction of CO2. These experiments were conducted at the NMR scale using J. Young type NMR tubes loaded with solutions of catalysts 3−6 in ca. 0.5 mL of C6D6 or 1:2 v/v C6D5Br/C6D6 and ca. 20 equiv of Et3SiH. The reaction mixture was then exposed to CO2 (ca. 1.3 atm), heated to 80 °C, and the progress of the reaction was monitored by 1H and 13 C{1H} NMR spectroscopy. Table 1 shows the catalytic activities of 3−6 and the product distribution. The reductions proceed in a stepwise fashion (Scheme 1) similar to those catalyzed by other Lewis acids including [Et2Al][CH6B11I6]11 and [(2,6-Mes2C6H3O)2Al][CHB11Cl11],27 and the tandem system Al(C6F5)3/B(C6F5)3.12 Using strong Lewis acids 3 and 4 as well as weaker Lewis acid 5 in C6D6 solution, methane was formed in 20−30% yield based on the hydrogen from Et3SiH, but the main product was C6D5CH3. This product and the small amounts of (C6D5)2CH2 are formed by Lewis acid catalyzed Friedel−Crafts alkylation of C6D6 with the intermediates Et3SiOCH3 and (Et3SiO)2CH2. HD is thought to arise from the Lewis acid catalyzed coupling of Et3SiOD with Et3SiH. The activities of compounds 3, 4, and 6 in the CO2 reduction reactions are comparable in that Et3SiH is consumed within 8− 13 h. Contrary to these results, there is still some unreacted Et3SiH present after 1 week at 80 °C in case of the gallium compound 5. Interestingly, while the product distribution in the reactions catalyzed by 3−5 with mainly d5-toluene (C6D5CH3) and methane is similar to those reported for the

Figure 3. Structure of 5 depicting the zigzag polymeric motif.

with two gallium−carbon σ-bonds and four weak gallium··· chlorine coordinative bonds from two anions (Figure 4).

Figure 4. Structure of 5 (50% ellipsoids). Hydrogen atoms except of H(1A) have been omitted for clarity. Selected bond distances (Å) and angles (deg): Ga(1)−C(1) = 1.927(2), Ga(1)−C(1)# = 1.927(2), Ga(1)···Cl(6) = 2.824, Ga(1)···Cl(5) = 3.127, C(1)−Ga(1)−C(1)# = 158.85, Cl(5)−Ga(1)−Cl(6) = 75.99, Cl(6)−Ga(1)−Cl(6)# = 67.88, Cl(5)−Ga(1)−Cl(5)# = 140.76. Atoms denoted by # were generated by the transformation x, 1/2-y, 1-z.

While the Ga−C bond distance with a value of 1.927(2) Å is close to the ones reported for the cation [(2,6-Mes2C6H3)2Ga]+ (Mes = 2,4,6-Me3C6H2)22 with an average value of 1.914 Å, the Ga···Cl contacts are significantly longer (by at least 0.372 Å) than those in aluminum compounds 3 and 4 with values of 2.824 and 3.127 Å, but still well within the sum of the van der Waals radii (3.62 Å).23 Some of the lengthening may be attributed to the higher coordination number in 5 but most is likely due to the lower Lewis acidity of gallium versus aluminum. Contrary to 3 and 4, the coordination of the anion to the [Et2Ga]+ fragment involves chlorine donors solely from the meta-belt of the anions. The C−Ga−C angle (158.85°) approaches 180°, which may be interpreted either as a result of the octahedral coordination or the rather weak Ga···Cl interactions affording an almost linear coordination environment for a quasi-two-coordinate gallium center. Its good solubility in benzene suggests that the chains do not persist in solution, but the broadened 1H NMR signals for the ethyl group could be due to dynamic processes in solution. It

Table 1. Catalytic Reduction of CO2 with Et3SiHa and Catalysts 3−6, [Et2Al][CH6B11I6],11 and [(ArO)2Al][CHB11Cl11]27 productsb (μmol) [%]c catalyst

solvent

Et3SiH (μmol)

time (h)

3 4 4e 5 6 [Et2Al][CH6B11I6]g [(ArO)2Al][CHB11Cl11]h

C6D6 C6D6/C6D5Br C6D6 C6D6 C6D6/C6D5Br C6D6 C6D6

202 219 88 94 218 189 100

13 9.5 9.5 181f 8.25 36 49

C6D5CH3 28 24 9.5 12 0.6 16 20

[42] [33] [32] [45] [0.8] [25] [60]

(C6D5)2CH2 1.6 0.5 0.8 0.03 0.1 5.6 1

[1.6] [0.5] [1.8] [0.1] [0.1] [6] [2]

Et3SiOCH3

38 [52]

CH4 11 11 6.7 4.6 5.8 33 4

[21] [21] [30] [24] [11] [69] [16]

HD 36 21 20 12

[18] [9.5] [23] [16]

not given 14 [14]

conversion (%)d >99 >99 97 82.6 >99 >99 >99

Reactions were performed with ca. 5% catalyst loading at 80 °C. bDetermined by 1H NMR integration using the solvent signal as internal standard. Percent calculated per Si−H bond. dBased on consumed Si−H. eReaction performed with 7% catalyst loading. fReaction stopped at 181 h. g Reaction performed with 8% catalyst loading.11 hReaction performed with 3% catalyst loading at 83 °C; Ar = 2,6-Mes2C6H3.27 a c

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DOI: 10.1021/acs.organomet.7b00716 Organometallics XXXX, XXX, XXX−XXX

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Dipp*Li,32 and [Ph3C][CHB11Cl11]20 were prepared according to literature procedures. All other reagents were obtained from commercial suppliers and used as received. NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. 1H NMR chemical shift values were determined relative to the residual protons in C6D5H in benzene-d6 as internal reference (δ 7.16 ppm). 13C NMR spectra were referenced to the solvent signal (δ 128.39 ppm). 11B NMR spectra were referenced to an external solution of F3B·OEt2 in C6D6 and the chemical shifts are reproducible with an error of less than 1 ppm. Given the high sensitivity of the new compounds, the small scale of their syntheses, the formation of clear solutions for NMR analysis, the excellent agreement of the NMR spectra with the crystal structures, and the generally high purity of the compounds based on NMR spectroscopy, no attempts were made to obtain CHN analysis data. GC-MS spectra were collected on an HP G1800GCD gas chromatograph. Safety Notes. Caution: Et3Al and Et3Ga are pyrophoric and should be handled with care under an inert atmosphere. The NMR tubes for the CO2 reduction experiments should not be cooled much below −70 °C to avoid deposition of solid CO2. Et3Ga. Carefully dried GaF3 (40.52 g, 319.81 mmol) was slowly added through a solids addition funnel to a solution of EtMgBr (prepared in situ from the reaction of magnesium (26.42 g, 1.09 mol) and EtBr (82.0 mL, 120.54 g, 1.11 mol) in diethyl ether (350 mL) at 0 °C. The reaction mixture was then refluxed for 2 days. The ether was distilled off, and the reaction flask heated for 1 h at 120 °C to remove most of the solvent. The crude Et3Ga·etherate was distilled under vacuum (oil bath temperature gradually increased up to 145 °C) and collected into an ice-cooled flask. The product still contained a large amount of ether based on 1H NMR analysis. Toluene (ca. 40 mL) was added, and the etherate solution was fractionally distilled first under nitrogen to remove the ether and toluene, then Et3Ga distilled under slight vacuum (vacuum valve was partially open) and collected in an ice cooled flask (boiling point 35−40 °C). Et3Ga was obtained as a colorless liquid and contained 0.26 equiv of toluene and 0.02 equiv of Et2O based on NMR integration (Figure S1). The isolated product was used without further purification. Isolated yield: 13.45 g, 85.72 mmol, 26.8%. 1H NMR (C6D6, 400.13 MHz): δ = 1.18 (t, J = 8.06 Hz, CH2CH3, 9H), 0.46 (q, J = 8.03 Hz, CH2CH3, 6H). 13C{1H} NMR (C6D6, 100.61 MHz): δ = 10.38 (CH2CH3), 10.07 (CH2CH3).33 (2,6-Ph2C6H3)AlEt2, 1. A solution of Et2AlCl (1.05 g, 8.71 mmol) in hexane (20 mL) was added dropwise to a suspension of 2,6Ph2C6H3Li (2.00 g, 8.47 mmol) in hexanes (20 mL) at −78 °C. The reaction mixture was stirred at −78 °C for 1 h, allowed to warm up to room temperature, and stirred overnight followed by heating at 50 °C for an additional 5 h. The reaction mixture was filtered through a medium porosity frit lined with Celite while still warm. Concentration of the resulting solution to ca. 15 mL and cooling to −20 °C for 4 days afforded (2,6-Ph2C6H3)AlEt2 as a colorless crystalline solid. The mother liquor was removed by cannula, the crystals were washed with hexanes (2 mL) and dried under vacuum. Isolated yield: 2.24 g, 7.12 mmol, 81.7%. 1H NMR (C6D6, 400.13 MHz): δ = 7.44 (d, br, J = 7.4 Hz, overlapping o-H(Ph) and m-H(C6H3), 6H), 7.34 (t, J = 7.7 Hz, pH(C6H3), 1H), 7.19 (t, J = 7.6 Hz, partially obscured by C6D5H, mH(C6H5), 4H), 7.06 (t, J = 7.5 Hz, p-H(C6H5), 2H), 0.98 (t, J = 8.1 Hz, CH2CH3, 6H), 0.04 (q, J = 8.1 Hz, CH2CH3, 4H). 13C{1H} NMR (C6D6, 100.61 MHz): δ = 153.06 (i-C), 150.72 (o-C), 148.98 (iC(Ph)), 131.48 (m-C(Ph)), 129.65 (p-C), 128.15 (p-C(Ph), obscured by C6D6), 126.31(o-C(Ph)), 125.45 (m-C), 9.33 (CH2CH3), 3.30 (CH2CH3). (2,6-Dipp2C6H3)GaEt2, 2. A solution of Et2GaCl (prepared in situ from the reaction of GaCl3 (0.32 g, 1.82 mmol) with Et3Ga (0.56 g, 3.57 mmol) in hexanes (20 mL)) was added dropwise to a suspension of Dipp*Li (2.19 g, 5.41 mmol) in hexanes (20 mL) at −78 °C. The reaction mixture was stirred at −78 °C for 1 h, allowed to warm to room temperature, stirred overnight and heated at 40 °C for an additional hour. The reaction mixture was filtered through a medium porosity frit lined with Celite. Removal of the solvent under vacuum yielded an oily phase that solidified upon standing at room temperature for several hours. The solid was heat-dissolved in a

Scheme 1. Important Steps and Side Reactions in the Lewis Acid Catalyzed CO2 Reduction with Et3SiH

Lewis acids [Et2Al][CH6B11I6]11 and [(2,6-Mes2C6H3O)2Al][CHB11Cl11],27 the m-terphenyl gallium species 6 afforded mostly Et3SiOCH3. This type of selectivity is less common for metal catalysts,28 which was surprising because previous investigations have shown that strong Lewis acids catalyze the methylation of benzene with Et3SiOCH3.11 The strong Lewis acidities of 3 and 4 are indicated by the loss of the 3J(H−Si−CH2) coupling in Et3SiH in the 1H NMR spectra as soon as the silane was added. Furthermore, Et3SiH was subject to substituent scrambling, and Et4Si, Et2SiH2 and EtSiH3 were observed in the 1H and 13C{1H} NMR spectra. This effect was also reported when Et3SiH was exposed to other strong Lewis acids such as [(2,6-Mes 2 C 6 H 3 O) 2 Al][CHB11Cl 11]27 and Al(C 6F5) 3.29 Some scrambling also occurred when Et3SiH was reacted with compounds 5 and 6, but its rate was significantly slower due to the lower Lewis acidity of gallium. A similar scrambling was also observed for the primary siloxane product (Et3Si)2O, which is rapidly converted into Et4Si and various polysiloxanes denoted here for simplicity as (Et2SiO)n.

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CONCLUSION While the crystal structure of [Et2Al][CHB11Cl11], 3, closely resembles those of the previously reported compounds [Et2Al][CH6B11X6] (X = Cl and I), its catalytic activity is about an order of magnitude higher due to the lower basicity of the undecachlorinated counterion. Introduction of a moderately bulky m-terphenylsubstituent (compound 4) does not result in significant structural changes and only in a slight increase in catalytic activity. The lower Lewis acidity of the polymeric [Et2Ga][CHB11Cl11] (5) results in a much lower reactivity, but the likely solvent separated ion pair [Dipp*GaEt][CHB11Cl11] (6) is not only very reactive but also selective with the formation of Et3SiOCH3 as the main product as opposed to methane or toluene for the other complexes.

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EXPERIMENTAL SECTION

General Procedures. All work was performed under anaerobic and anhydrous conditions by using either modified Schlenk techniques or a Vacuum Atmospheres drybox. Solvents were freshly distilled under N2 from sodium, potassium, or sodium/potassium alloy and degassed twice prior to use or they were dispensed from a commercial solvent purification system. The compounds Et2GaCl,30 Ph2C6H3Li,31 D

DOI: 10.1021/acs.organomet.7b00716 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

washed with hexanes (2 × 0.5 mL) and dried under vacuum. Isolated yield: 214 mg, 329 μmol, 82.5% (based on trityl salt). 1H NMR (C6D6, 400.13 MHz): δ = 2.27 (s, CHB11Cl11, 1H), 0.73 (s, br, w1/2 = 28 Hz, CH2CH3, 4H), 0.12 (t, J = 7.5 Hz, CH2CH3, 6H). 13C{1H} NMR (C6D6, 100.61 MHz): δ = 48.25(CHB11Cl11), 8.30 (CH2CH3). 11B NMR (C6D6, 100.38 MHz): δ = −2.9 (p-B, 1B), −9.8 (o- or m-B, 5B), −12.2 (o- or m-B, 5B). [Dipp*GaEt][CHB11Cl11], 6. A small grease-free tube equipped with a Teflon valve and a small magnetic stirring bar was charged with [Ph3C][CHB11Cl11] (30 mg, 39 μmol), benzene (1.0 mL) and Et3SiH (15 mg, 0.13 mmol). The mixture was stirred for 10 h upon which the orange color of the trityl salt faded giving an oily yellowish phase. The colorless benzene layer was pipetted off, and the oily layer was washed with hexanes (2 × 0.5 mL) to yield the silylium salt. To this was added benzene (0.5 mL) and 3 (23 mg, 44 μmol). The reaction mixture was stirred for 2 days at room temperature giving a colorless solution above a colorless dense oily phase. The mother liquor was pipetted off, and the oily gallium compound (ca. 0.07 mL) was washed with benzene (3 × 0.3 mL) and dissolved in a C6D6/C6D5Br 2/1 mixture (0.5 mL). 1H NMR (C6D6, 400.13 MHz): δ = 7.38 (t, J = 7.8 Hz, pH(Dipp), 2H), 7.31 (t, J = 7.6 Hz, p-H(C6H3), 1H), 7.16 (d, partially obscured by C6D5H, p-H(Dipp), 4H, 4H), 7.16 (d, obscured by C6D5H, p-H(C6H3), 2H), 2.69 (s, CHB11Cl11, 1H), 2.59 (sept, J = 6.7 Hz, CH(CH3)2, 4H), 1.04 (d, J = 6.8 Hz, CH(CH3)2, 12H), 0.94 (d, J = 6.6 Hz, CH(CH3)2, 12H), 0.11 (m, overlapping CH2CH3, 5H). 13 C{1H} NMR (C6D6, 100.61 MHz): δ = 148.58 (i-C), 147.62 (quaternary C), 146.42 (quaternary C), 138.83 (quaternary C), 133.04 (p-C(C6H3)), 132.15 (p-C(Dipp)), 129.75 (m-C(C6H3)), 126.08 (pC(Dipp)), 47.53 (CHB11Cl11), 31.37 (CH(CH3)2), 25.91 (CH(CH3)2), 24.00 (CH(CH3)2), 15.97 (CH2CH3), 8.61 (CH2CH3). 11B NMR (C6D6, 100.38 MHz): δ = −1.6 (p-B, 1B), −9.2 (o- or m-B, 5B), −12.4 (o- or m-B, 5B). CO2 Reduction. The catalytic reduction reactions of CO2 using catalysts 3−6 were performed in J. Young style NMR tubes in C6D6 or 1:2 v/v C6D5Br/C6D6 solution (ca. 0.5 mL). The reaction mixture containing the catalyst (usually 4−11 μmol) and ca. 20 equiv of Et3SiH (88−219 μmol, 5% catalyst loading) was degassed by two freeze− pump−thaw cycles and filled with CO2 at 1 atm while maintaining two-thirds of the NMR tube at −70 °C. The progress of the reaction was monitored by 1H NMR spectroscopy, and the products were identified by their NMR chemical shifts and GC-MS data. The results are summarized in Table 1, and Figures S19−S22 show the 1H NMR spectra illustrating the progress of selected reactions. The amounts of CH4 and HD were estimated using NMR integration and the Henry constants (0.021 M/atm for CH4,34 0.0028 M/atm for H235) assuming a gas volume of 1.5 mL. X-ray Diffraction. Crystals of compounds 3−5 were grown as described above. The data were collected using a diffractometer with a Bruker APEX ccd area detector and graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 100(2) K. The data were corrected for absorption by the empirical method,36 and the structures were solved and refined using the SHELXS and SHELXL package.37 Crystal data and a summary of the data collection and refinement details are provided in Table S2.

minimum amount of hexanes, ca. 5 mL, and allowed to stand at room temperature for 1 day affording the Dipp*GaEt2 as a white crystalline solid. The mother liquor was pipetted off, and the crystals were washed with hexanes (2 × 1 mL) and dried under vacuum. Isolated yield: 1.21 g, 2.30 mmol, 43.0%. Contains ca. 3% of the hydrolysis product Dipp*H. 1H NMR (C6D6, 400.13 MHz): δ = 7.30−7.21 (m, pH(Dipp), m- and p-H(Ph), 5H), 7.15 (d, br, partially obscured by C6D5H, J = 4.48 Hz, m-H(Dipp), 4H), 3.07 (sept, J = 6.84 Hz, CH(CH3)2, 4H), 1.24 (d, J = 6.92 Hz, 12 H, CH(CH3)2), 1.06 (d, J = 6.92 Hz, 12 H, CH(CH3)2), 0.81 (t, J = 8.04 Hz, CH2CH3, 6H), 0.40 (q, J = 8.08, CH2CH3, 4H). 13C{1H} NMR (C6D6, 100.61 MHz): δ = 156.71 (i-C), 147.45, 146.03, 142.23 (o-C), 129.14 (m-C), 128.05 (pC(Dipp)), 127.44 (p-C), 123.76 (m-C(Dipp)), 30.95 (CH(CH3)2), 26.59, 22.99 (CH(CH3)2), 11.11 (CH2CH3), 9.29 (CH2CH3). [Et2Al][CHB11Cl11], 3. A small grease-free Schlenk tube equipped with a Teflon valve and a small magnetic stirring bar was charged with [Ph3C][CHB11Cl11] (150 mg, 196 μmol), benzene (1.0 mL), and Et3SiH (52 mg, 0.44 mmol), and the mixture was stirred for 10 h upon which the orange color of the trityl salt faded to give a dense yellowish oily phase. The colorless benzene upper layer was pipetted off, and the oily layer was washed with hexanes (2 × 0.5 mL) to afford the silylium salt. Hexanes (1.5 mL) and Et3Al (25 mg, 219 μmol) were added, and the reaction mixture was sonicated for 2 h to afford [Et2Al][CHB11Cl11] as a colorless solid. The solids were heat-dissolved in an oil bath at 105 °C. Allowing it to slowly cool down afforded the product as a crystalline solid containing well-shaped blocks. The mother liquor was pipetted off, and the crystals were washed with hexanes (0.5 mL) and dried under vacuum. Isolated yield: 72 mg, 119 μmol, 60.7% (based on trityl salt). 1H NMR (C6D6, 400.13 MHz): δ = 2.11 (s, CHB11Cl11, 1H), 0.73 (t, J = 8.2 Hz, CH2CH3, 6H), 0.12 (q, J = 8.2 Hz, CH2CH3, 4H). 13C{1H} NMR (C6D6, 100.61 MHz): δ = 49.0 (s, CHB11Cl11, 1H), 7.54 (CH2CH3), 5.75 (CH2CH3). 11B NMR (C6D6, 100.38 MHz): δ = −3.6 (p-B, 1B), −10.2 (o- or m-B, 5B), −12.2 (o- or m-B, 5B). [(2,6-Ph2C6H3)AlEt][CHB11Cl11], 4. A small grease-free tube equipped with a Teflon valve and a small magnetic stirring bar was charged with [Ph3C][CHB11Cl11] (200 mg, 261 μmol), benzene (1.0 mL), and Et3SiH (50 mg, 0.43 mmol). The mixture was stirred for 10 h upon which the orange color of the trityl salt faded giving a dense oily phase. The colorless benzene top layer was pipetted off, and the oily layer washed with hexanes (2 × 0.5 mL) to yield the silylium salt. Benzene (1.5 mL) and 2 (90 mg, 286 μmol) were added at room temperature. The reaction mixture was stirred for 2 h to afford a white precipitate. The solids were heat-dissolved in an oil bath at 100 °C and allowed to slowly cool down to give the product as a colorless crystalline solid. The mother liquor was pipetted off, and the crystals were washed with benzene (2 × 0.3 mL) and dried under vacuum. Isolated yield: 172 mg, 213 μmol, 81.6%. 1H NMR (C6D6, 400.13 MHz): δ = 7.27−7.22 (m, br, overlapping m- and p-H(C6H3), partially obscured by o-H(Ph), 3H), 7.22 (d, J = 7.2 Hz, o-H(Ph), 4H), 7.12 (t, J = 7.5 Hz, m-H(Ph), 4H), 7.00 (t, J = 7.4 Hz, p-H(Ph), 2H), 1.98 (s, CHB11Cl11, 1H), 1.13 (t, J = 8.1 Hz, CH2CH3, 3H), 0.65 (q, J = 8.1 Hz, CH2CH3, 2H). 13C{1H} NMR (C6D6, 100.61 MHz): δ = 151.78 (quaternary C), 147.06 (quaternary C), 142.15 (i-C), 132.07 (pC(C6H3) partially deuterated), 131.68 (m-C(Ph)), 129.79 (p-C(Ph)), 128.00 (o-C(Ph)), 127.00 (m-C(C6H3)), 49.15 (CHB11Cl11), 10.80 (CH2CH3), 9.23 (CH2CH3). 11B NMR (C6D6, 100.38 MHz): δ = −3.9 (p-B, 1B), −10.4 (o- or m-B, 5B), −12.5 (o- or m-B, 5B). [Et2Ga][CHB11Cl11], 5. A small Schlenk tube equipped with a small magnetic stirring bar was charged with [Ph3C][CHB11Cl11] (305 mg, 399 μmol), benzene (3.0 mL) and Et3SiH (110 mg, 0.95 mmol). The reaction mixture was stirred for 10 h upon which the orange color of the trityl salt faded giving a dark yelllow oily phase. The colorless benzene layer was pipetted off, and the oily layer washed with hexanes (2 × 1.0 mL) to yield the silylium salt. To that was added benzene (3.0 mL) and Et3Ga (72 mg, 459 μmol). The reaction mixture was stirred overnight affording a clear, colorless solution. The product was isolated by layering hexanes on top of the reaction mixture (2.0 mL) and allowing it to stand for 1 day inside the glovebox at room temperature. The mother liquor was pipetted off, and the crystals

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00716. Multinuclear NMR spectra for Et3Ga and compounds 1− 6, reduction of CO2 catalyzed by compounds 3−6, and an X-ray table (PDF) Accession Codes

CCDC 1576118−1576120 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailE

DOI: 10.1021/acs.organomet.7b00716 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

(24) Atwood, J. L., Anionic and Cationic Organoaluminum Compounds. In Coordination Chemistry of Aluminum; Robinson, G. H., Ed.; VCH: New York, 1993; pp 197−232. (25) Pereira, J. F. B.; Flores, L. A.; Wang, H.; Rogers, R. D. Chem. Eur. J. 2014, 20, 15482−15492. (26) Chen, E. Y. X.; Lancaster, S. J. Weakly Coordinating Anions: Highly Fluorinated Borates. In Comprehensive Inorganic Chemistry II, 2nd ed.; Poeppelmeier, K., Ed.; Elsevier: Amsterdam, 2013; pp 707− 754 and references therein. (27) Wehmschulte, R. J.; Saleh, M.; Powell, D. R. Organometallics 2013, 32, 6812−6819. (28) Metsänen, T. T.; Oestreich, M. Organometallics 2015, 34, 543− 546. (29) Chen, J.; Chen, E. Y. X. Angew. Chem., Int. Ed. 2015, 54, 6842− 6846. (30) Beachley, O. T., Jr.; Rosenblum, D. B.; Churchill, M. R.; Churchill, D. G.; Krajkowski, L. M. Organometallics 1999, 18, 2543− 2549. (31) Crittendon, R. C.; Beck, B. C.; Su, J.; Li, X.-W.; Robinson, G. H. Organometallics 1999, 18, 156−160. (32) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 2150−2152. (33) Shatunov, V. V.; Korlyukov, A. A.; Lebedev, A. V.; Sheludyakov, V. D.; Kozyrkin, B. I.; Orlov, V. Y. J. Organomet. Chem. 2011, 696, 2238−2251. (34) Darwish, N. A.; Gasem, K. A. M.; Robinson, R. L., Jr J. Chem. Eng. Data 1994, 39, 781−784. (35) Zhou, Z.; Cheng, Z.; Yang, D.; Zhou, X.; Yuan, W. J. Chem. Eng. Data 2006, 51, 972−976. (36) Sheldrick, G. M. SADABS. Program for Empirical Absorption Correction of Area Detector Data; University of Göttingen: Göttingen, Germany, 2001. (37) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

ing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: rwehmsch@fit.edu. ORCID

Rudolf J. Wehmschulte: 0000-0003-0719-0620 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Petroleum Research Fund administered by the American Chemical Society (PRF No. 52856-ND3), the 2013 NASA/KSC Chief Technologist Center Innovation Fund, and the Florida Institute of Technology.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00716 Organometallics XXXX, XXX, XXX−XXX