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Research Article Cite This: ACS Catal. 2019, 9, 301−314

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Controlling Selectivity in the Hydroboration of Carbon Dioxide to the Formic Acid, Formaldehyde, and Methanol Oxidation Levels Matthew R. Espinosa, David J. Charboneau, Andre ́ Garcia de Oliveira, and Nilay Hazari* Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States

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S Supporting Information *

ABSTRACT: The factors that influence the selectivity of pincer supported group 10 transition metal hydride catalysts for CO2 hydroboration are investigated. We demonstrate that selective CO2 reduction to either the two-electron boryl formate reduction product, the four-electron bis(boryl)acetal reduction product, or the six-electron methoxy borane reduction product can be achieved by varying either the identity and concentration of the organoborane reductant, the nature and loading of the catalyst, or the presence of a Lewis acid cocatalyst. In fact, using one specific catalyst, (tBuPCP)NiH (tBuPCP = 2,6-C6H3(CH2PtBu2)2), we can selectively form either the two-, four-, or six-electron CO2 reduction products by changing either the nature of the reductant or the reaction conditions. Additionally, we show that Lewis acid cocatalysts can be used to alter the selectivity of CO2 hydroboration, which is a new method to control the selectivity of this type of hydroboration reaction. All of our results on selectivity are consistent with CO2 hydroboration being a tandem reaction, in which it is possible to either trap the kinetic two- or fourelectron reduction products or form the thermodynamic six-electron reduction product. We also explore the formation of offcycle κ2-borohydride species through the reaction of the transition metal hydride with the borane reductant and show that this can impact selectivity. Overall, our work provides detailed guidelines for designing even more active and selective catalysts for CO2 hydroboration and may also be relevant for the improvement of catalysts for related reactions such as CO2 hydrosilylation. KEYWORDS: hydroboration, carbon dioxide reduction, pincer complexes, mechanism, transition metal catalysis

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INTRODUCTION

carboxylated borane products can have value in synthetic chemistry in their own right. Some of the most active and selective catalysts for the hydroboration of CO2 are pincer supported group 10 complexes,5 although catalysts based on main group,6 group 6,7 group 8,4b,8 group 9,9 and group 114a metals and frustrated Lewis pairs10 are known. The ability to control the level of CO2 reduction is also significantly better using group 10 catalysts compared with uncatalyzed reactions between boranes such as NaBH4 and CO2.11 For example, group 10 catalysts have selectively generated products at the formic acid, formaldehyde, and methanol oxidation state levels in CO2 hydroboration reactions. In seminal work, Guan et al. demonstrated high selectivity for CO2 hydroboration to a methoxy borane product using a (tBuPOCOP)NiH (tBuPOCOP = 2,6-C6H3(OPtBu2)2) catalyst and catechol borane (HBCat) as the reductant (Figure 1a).5a An equivalent of bis(benzo1,3,2-dioxoborolanyl)oxide ((BCat)2O) is produced in this reaction as a byproduct. In contrast, our group showed that, using pinacol borane (HBPin) as the reductant and (CyPSiP)-

CO2 is a sustainable C1 feedstock for the synthesis of fine and commodity chemicals, which could potentially replace our current petrochemical derived C1 sources.1 In particular, there is interest in the generation of products such as formic acid, formaldehyde, and methanol from CO2, as they are currently synthesized from nonrenewable feedstocks.2 The direct hydrogenation of CO2 to formic acid, formaldehyde, and methanol is the most atom efficient route to these products, but the best conditions for these reactions are harsh and we currently lack the ability to sustainably generate H2 on a large scale.2 An alternative approach is to use boranes as the reductant, which results in systems that operate under milder conditions.3 Although the use of boranes generates stoichiometric waste products (after treatment of the carboxylated borane product with an acid to generate free formic acid, formaldehyde, or methanol), mechanistic insight gained by studying these systems may be translatable to catalysts that utilize H2, as some of the elementary steps such as CO2 insertion into a metal hydride are similar. Another benefit of using boranes as the reductant is that in some cases the products of these reactions can transfer formyl or methylene groups to organic compounds such as amines.4 Thus, the © XXXX American Chemical Society

Received: September 27, 2018 Revised: November 18, 2018

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Figure 1. Examples of CO2 hydroboration with different group 10 supported pincer complexes to (a) the methanol oxidation level, (b) the formic acid oxidation level, and (c) the formaldehyde oxidation level.

Figure 2. Proposed pathway for CO2 hydroboration using pincer supported group 10 metal hydride catalysts. Depending on the relative rates of the elementary reactions, products at the formic acid (boryl formate), formaldehyde (bis(boryl)acetal), and methanol (methoxy borane) oxidation levels are observed. When methoxy borane is produced, an equivalent of bis(boryl)oxide is also formed.

PdH (CyPSiP = Si(Me)(2-PCy2-C6H4)2) as the catalyst, the boryl formate product could be formed in high yield (Figure 1b).5d More recently, Turculet and co-workers reported the selective formation of the formaldehyde-bound borane product, bis(boryl)acetal, using HBPin as the reductant and (iPrPSiPInd)NiH (iPrPSiPInd = Si(Me)(2-PiPr2-3-MeIndole)2) as the catalyst (Figure 1c).5e In these reactions, the catalyst (in terms of the identity of the metal center, the steric bulk on the phosphine substituents, and the nature of the donor trans to the hydride), the reductant, and the concentrations of the

reagents are all being changed. As a result, it is not clear which factors are important to give high selectivity to a particular product, and it is not possible to predict the outcome of a reaction using even a slightly different system. Largely on the basis of computational studies, Guan et al. proposed that CO2 hydroboration to methoxy borane involves a tandem reaction with three sequential catalytic cycles (Figure 2).12 In the first cycle (cycle I), CO2 is reduced to the formic acid level, in the second cycle (cycle II), it is reduced to the formaldehyde level, and in the third cycle (cycle III), it is 302

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molecular buried volume calculation (VBur%, Table 1; see the Supporting Information for details).14 To examine electronic effects, we synthesized two literature variants of (tBuPCP)NiH,13d which contained either an electron donating methoxy substituent or an electron withdrawing iodo substituent in the para position of the phenyl group of the tBuPCP ligand. For the catalytic reactions described in this work, the performance of these substituted systems is similar to (tBuPCP)NiH, and the results are described in the Supporting Information. The catalytic activity of each complex described in Figure 3 for CO2 reduction using HBPin was evaluated under standard conditions for CO 2 hydroboration (Table 1 and the Supporting Information).5d Regardless of the nature of the phosphine substituents or the metal center, high selectivity to boryl formate (cycle I product in Figure 2) was observed in all cases. In fact, these are some of the most active catalysts for CO2 reduction to boryl formate.4a,5d,e,8a Minimal amounts of either bis(boryl)acetal (cycle II product) or methoxy borane (cycle III product) were observed, and the mass balance with regard to the initial HBPin starting material was greater than 80% in all cases. However, there was a clear correlation between the steric bulk of the ancillary ligand and the time required for complete consumption of HBPin. Specifically, as the steric bulk of the ligand increased, the rate of reaction decreased. This is best illustrated by comparing reactions catalyzed by (tBuPCP)NiH and (iPrPCP)NiH (Table 1, entries 4 and 6). Whereas all of the HBPin had been consumed in less than 10 min using (iPrPCP)NiH as the catalyst, the reaction was still not complete after 10 days using (tBuPCP)NiH as the catalyst. As described in Figure 2, we propose that, to obtain the boryl formate product, the reaction only involves two steps: (i) CO2 insertion into the metal hydride and (ii) transmetalation of the metal-bound formate with HBPin to regenerate the metal hydride and release the boryl formate product. Transmetalation is postulated to be the turnover limiting step, as NMR spectroscopy indicates that the metal formate is the catalyst resting state (see the Supporting Information) and previous work indicates that CO2 insertion into complexes of the type (RPCP)NiH occurs in milliseconds at room temperature.13d The rate of transmetalation is presumably slower for more sterically bulky catalysts, which accounts for the increased reaction times. The Turculet group’s Ni system, (iPrPSiPInd)NiH, capable of reducing CO2 to bis(boryl)acetal with HBPin as the reductant, features a very high trans influence central Si donor in the pincer ligand (Figure 1),5e which may be one explanation for the different selectivity observed when using RPCP supported catalysts. To rigorously compare the effect of a C central donor with a Si central donor, it would be necessary to synthesize complexes with a central Si atom instead of a central C atom with the rest of the RPCP framework remaining exactly the same. In this case, it is not chemically feasible to synthesize such a ligand. Therefore, to gain some understanding of the effects of the Si trans donor on catalytic selectivity, we prepared (RPSiP)MH (M = Ni or Pd; R = iPr15 or Cy13b), which are broadly analogous with (RPCP)MH (M = Ni or Pd; R = iPr or Cy) apart from the identity of the central donor (Figure 3). Consistent with our previous results, catalysis with (RPSiP)PdH (R = iPr or Cy) gave high selectivity to the boryl formate product (Table 1, entries 7 and 8).5d In contrast, although catalysis with (RPSiP)NiH (R = iPr or Cy) gave relatively high selectivity to the boryl formate product, we also observed the formation of bis(boryl)acetal and methoxy

reduced to the methanol level. An equivalent of borane reductant is required for each cycle, so reduction to the methanol level needs 3 equiv of borane. If this mechanism is correct, the relative rates of a number of different elementary reactions, including CO2 or borane insertion, transmetalation, and β-alkoxy elimination, control the selectivity of the reaction, but the factors that are important in determining the selectivity are not clear. In preliminary work, which focused on stoichiometric reactions, Guan et al. suggested that, with his Ni system, more sterically bulky boranes give products at the formic acid level, whereas less sterically bulky boranes lead to reduction to the methanol level.5b,c However, this hypothesis has not been evaluated in catalysis, and the results described in Figure 1 suggest that the situation is more complicated, and a combination of many factors, including the concentration of the borane, affect selectivity. Here, we perform a systematic study to understand how changing the properties of both the catalyst and the reductant affects the selectivity of CO2 hydroboration. Using pincer supported group 10 catalysts, we find correlations between catalyst and borane structure and the level and rates of catalytic CO2 reduction. We also demonstrate that the selectivity of the reaction can be altered by changing the reaction conditions or using a Lewis acid cocatalyst. For instance, a reaction that gives only reduction to the formic acid oxidation level in the absence of a Lewis acid cocatalyst gives reduction all the way to the methanol oxidation level in the presence of a Lewis acid cocatalyst. The structure−activity studies performed in this work provide both new methods for controlling the selectivity of CO2 hydroboration and fundamental information, which will enable the rational design of even more active and selective catalysts.

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RESULTS AND DISCUSSION Effect of the Metal, Ancillary Ligand, and Reductant Concentration on CO2 Reduction Using Pinacol Borane (HBPin). Previous work on CO2 hydroboration demonstrated that the extent of CO2 reduction varies when using different catalysts, even with the same reductant.4b,5,8a−c For example, as shown in Figure 1, using HBPin as the reductant, our group’s Pd catalyst reduces CO2 to the formate level, while the Turculet group’s Ni catalyst reduces it to the formaldehyde level. However, along with the change in the identity of the metal, there are also significant differences in the ancillary ligand, which makes it difficult to understand the relative importance of each factor in the change in selectivity. To rigorously probe the effect of the metal center and the steric bulk of the ancillary ligand on the selectivity of CO2 reduction, we synthesized a series of complexes of the type (RPCP)MH (RPCP = 2,6-C6H3(CH2PR2)2) (R = tBu, Cy, iPr; M = Pd, Ni) using literature procedures (Figure 3).13 The steric bulk around each metal complex was quantified using the Salerno

Figure 3. Catalysts that were evaluated for CO2 hydroboration in this work. In this series, the metal center, the steric properties of the ancillary ligand, and the nature of the central trans donor have been systematically varied. 303

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Table 1. Hydroboration of CO2 Using HBPin as the Reductant with Various RPCP and RPSiP Supported Catalystsa,b,c

a

Reaction conditions: [catalyst] = 0.0007 M, [HBPin] = 0.07 M, 1 atm of CO2 (5 equiv with respect to HBPin), C6D6, RT. bResults are the average of two runs and are based on 1H NMR integrations against a naphthalene internal standard. c2 equiv of HBPin is required to make product B, and 3 equiv is required to make product C. d% Buried volume (VBur%) was calculated using the coordinates from crystal structures previously described in the literature (see the Supporting Information). Entries denoted with an asterisk were calculated using a DFT generated structure (see the Supporting Information). eTime until full consumption of HBPin. fBis(boryl)oxide is formed concomitantly with methoxy borane.

Figure 4. A kinetic preference for CO2 insertion over boryl formate insertion into a metal hydride is one explanation for the selective formation of boryl formate products. Under this scenario, there is no equilibration between metal formate (formed through CO2 insertion in the metal hydride) and metal acetal (formed through boryl formate insertion into the metal hydride) products. Reduction to the formaldehyde and methanol level occurs when boryl formate insertion is competitive with CO2 insertion.

Table 2. Hydroboration of CO2 Using HBPin as the Reductant and (tBuPCP)PdH as the Catalyst at Varied CO2 Concentrationsa,b,c

a

Reaction conditions: [catalyst] = 0.0007 M, [HBPin] = 0.07 M, 1−0.15 atm of CO2 (5−0.5 equiv with respect to HBPin), C6D6, RT. bResults are the average of two runs and are based on 1H NMR integrations against a naphthalene internal standard. c2 equiv of HBPin is required to make product B, and 3 equiv is required to make product C. dTime until full consumption of HBPin. eBis(boryl)oxide is formed concomitantly with methoxy borane.

borane products, indicating that reduction to the formaldehyde and methanol levels had occurred (Table 1, entries 9 and 10).

These results show that Ni is more likely to give reduction past the formic acid level than Pd and that the RPSiP ligand leads to 304

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Table 3. Hydroboration of CO2 Using HBPin as the Reductant and (tBuPCP)PdH or (iPrPSiP)NiH as the Catalyst at Varied Borane Concentrationsa,b,c

Reaction conditions: [catalyst] = X M (X = 0.0007 M − 0.05 M), [HBPin] = Y M (Y = 0.035 M − 5 M), 1 atm of CO2 (5 equiv with respect to HBPin), C6D6, RT. bResults are the average of two runs and are based on 1H NMR integrations against a naphthalene internal standard. c2 equiv of HBPin is required to make product B, and 3 equiv is required to make product C. dTime that the 1H NMR spectrum was recorded. e Bis(boryl)oxide is formed concomitantly with methoxy borane. a

greater reduction of CO2 compared to the RPCP ligand. It is noteworthy, however, that under our reaction conditions, which are more dilute than Turculet et al., (iPrPSiP)NiH gave reduction primarily to boryl formate, as opposed to bis(boryl)acetal, even though it has a similar structure to (iPrPSiPInd)NiH. On the basis of the mechanism proposed in Figure 2, one explanation for the selective formation of the boryl formate product using our catalysts is related to kinetic factors.16 Specifically, once 1 equiv of the boryl formate product is formed, there is a competition between CO2 insertion into the metal hydride to form a metal formate and boryl formate insertion into the metal hydride to form a metal acetal, which will lead to more reduced products (Figure 4). The boryl formate product will be kinetically preferred if (i) the rate of CO2 insertion into the metal hydride is much faster than boryl formate insertion into the metal hydride and (ii) the rate of transmetalation of the metal formate is faster than the rate of equilibration between the metal formate (derived from CO2 insertion into the metal hydride) and the metal acetal (derived from boryl formate insertion into the metal hydride). In principle, the kinetic preference for CO2 insertion into the metal hydride compared to boryl formate insertion can be changed by decreasing the number of equivalents of CO2 with respect to HBPin. To probe this hypothesis, a series of catalytic experiments were performed under the conditions described in Table 1 but using decreasing equivalents of CO2 (Table 2). In the limiting case, there was an excess of HBPin and CO2 was the limiting reagent. For all reactions performed with an excess or 1 equiv of CO2 to HBPin, the boryl formate product was selectively formed. However, in agreement with our hypothesis, in the reaction where CO2 was the limiting reagent (Table 2, entry 4), predominantly bis(boryl)acetal and methoxy borane products were observed. This reaction required a longer time for complete consumption of HBPin (8 days), which is consistent with the lower concentration of CO2 slowing down the initial rate of CO2 insertion in cycle I (Figure 2). Although these results indicate that cycle II and

cycle III products are accessible with our RPCP supported catalysts, under the conditions utilized in Table 1, they are not observed because CO2 insertion outcompetes boryl formate insertion, confining catalysis to cycle I. This is supported by the following observation, which clearly demonstrates that the boryl formate product is a kinetic product: if more HBPin is added to a solution containing the boryl formate product derived from CO2, HBPin, and catalyst (in the absence of CO2), further reduction to bis(boryl)acetal or methoxy borane products is observed (see the Supporting Information). Guided by the hypothesis presented in Figure 4, another potential strategy to promote formation of cycle II and cycle III products is to reduce the amount of solvent used in catalysis, while keeping all other factors constant. This would effectively increase the concentration of intermediates such as boryl formate, while the concentration of CO2 remains constant, as it is governed by the solubility of CO2 in the solvent at a given temperature and pressure, not the amount of solvent. As a result, the rate of boryl formate insertion is expected to be enhanced relative to CO2 insertion, leading to the formation of cycle II and III products. To explore this theory, we performed a series of reactions using higher concentrations of HBPin compared to the concentration utilized in Table 1. Consistent with our hypothesis, when (tBuPCP)PdH was used as the catalyst, reactivity beyond cycle I was observed with increasing concentrations of HBPin (Table 3, entries 1−4). These results conclusively show that altering the concentration of reductant in catalysis can lead to more reduced products in CO2 hydroboration. Our earlier results suggest that RPSiP ligands promote reactivity beyond cycle I more readily than RPCP ligands. Consistent with this proposal when (iPrPSiP)NiH was used as the catalyst with varying concentrations of HBPin as the reductant, significant reactivity beyond the boryl formate product was observed at lower concentrations of HBPin than with (tBuPCP)PdH (Table 3, entries 5−9). In fact, at 1 M of HBPin, only cycle II and III products were observed. The ratio 305

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Figure 5. (a) Solid state structure of 1. Thermal ellipsoids shown at 30% probability. Selected hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)−C(19) 2.002(2), Pd(1)−P(1) 2.3058(6), Pd(1)−P(2) 2.3108(6), Pd(1)−O(1) 2.1711(16), O(1)− C(1) 1.219(3), O(2)−C(1) 1.290(3), O(2)−B(1) 1.498(3), O(3)−B(1) 1.448(3), O(3)−C(2) 1.331(3), O(4)−C(2) 1.192(3), C(19)−Pd(1)− O(1) 175.25(8), C(19)−Pd(1)−P(1) 82.41(7), O(1)−Pd(1)−P(1) 96.65(5), C(19)−Pd(1)−P(2) 83.27(7), O(1)−Pd(1)−P(2) 97.15(5), P(1)−Pd(1)−P(2) 164.59(2), O(1)−C(1)−O(2) 123.3(2), O(4)−C(2)−O(3) 122.6(3). (b) Lewis structure of 1 illustrating the adduct between a Pd formate complex and triformatoborane. (c) Catalysis with complex 1.

Scheme 1. Proposed Decomposition of the Boryl Formate Product Derived from HBPin and CO2 under a Vacuuma

a

Formation of triformatoborane is inferred by the stoichiometry of the reaction and its presence in the isolated crystals of structure 1, while B2Pin3 was characterized by NMR spectroscopy and X-ray crystallography (see the Supporting Information).

Lewis Acid Effects on Selectivity of CO2 Reduction. To understand the fate of the catalyst after CO2 reduction, we attempted to isolate a metal complex at the completion of the reaction. Using 5 mol % (tBuPCP)PdH as the catalyst, we fully converted HBPin and CO2 to boryl formate and removed the solvent in vacuo. After separation of the organic products (see the Supporting Information), the metal containing product was recrystallized from toluene layered with pentane and crystals suitable for X-ray diffraction were obtained. We crystallographically characterized (tBuPCP)Pd{OC(O)H}[B{OC(O)H}3] (1), which is an adduct between triformatoborane and (tBuPCP)Pd{OC(O)H} (Figure 5a,b). 1H and 31P NMR spectroscopy indicated that 1 was also the major metal containing species present in the sample before recrystallization. The formation of (tBuPCP)Pd{OC(O)H} is unsurprising, as it is known that this complex rapidly forms from CO2 insertion into (tBuPCP)PdH.13b However, the formation of triformatoborane is unexpected, as there was no evidence that this compound was directly formed during catalysis. We

of bis(boryl)acetal to methoxy borane, however, remained approximately constant, indicating that the concentration of HBPin does not impact the selectivity between cycles II and III. In contrast, the ratio of bis(boryl)acetal to methoxy borane could be altered by changing the catalyst loading. Specifically, when the (iPrPSiP)NiH loading was decreased from 1 to 0.1 mol %, the ratio of bis(boryl)acetal to methoxy borane increased from approximately 1.3:1 to 2.3:1 (Table 3, entries 9 and 10). At the lower catalyst loading, bis(boryl)acetal precipitated out of solution, which presumably provides a driving force for its formation. Our observations also provide an understanding of the high selectivity for bis(boryl)acetal observed by Turculet et al. In their case, the iPrPSiPInd ancillary ligand, the choice of Ni as the metal, and the high concentration of HBPin promote reduction of CO2 past the formic acid level, while the low catalyst loading causes the bis(boryl)acetal product to precipitate out of solution. The combination of these factors results in a system that gives high selectivity to the cycle II product. 306

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Table 4. Hydroboration of CO2 Using HBPin as the Reductant with Various RPCP and RPSiP Supported Catalysts in the Presence of Lewis Acid Co-Catalystsa,b,c

a

Reaction conditions (C6D6): [catalyst] = 0.0007 M, [HBPin] = 0.07 M, [B(OPh)3] = 0.007 M, 1 atm of CO2 (5 equiv with respect to HBPin), C6D6, RT. bResults are the average of two runs and are based on 1H NMR integrations against a naphthalene internal standard. c2 equiv of HBPin is required to make product B, and 3 equiv is required to make product C. d% Buried volume (VBur%) was calculated using the coordinates from crystal structures previously described in the literature (see the Supporting Information). Entries denoted with an asterisk were calculated using a DFT generated structure (see the Supporting Information). eTime until full consumption of HBPin. fBis(boryl)oxide is formed concomitantly with methoxy borane.

base adduct in an analogous fashion to the adduct between triformatoborane and (tBuPCP)Pd{OC(O)H}. The Lewis acid/Lewis base interaction would result in a more electrophilic carbon center at the boryl formate, which could increase the rate of boryl formate insertion, promoting reduction to cycle II and III products. In contrast, we have recently demonstrated that Lewis acids do not promote CO2 insertion into pincer supported group 10 hydrides.13d The role of Lewis acids in catalysis was rigorously examined by introducing triphenoxy borane, a neutral Lewis acid similar to triformatoborane, into catalytic CO2 hydroboration reactions using the full series of catalysts previously evaluated (Table 4). In all cases, the addition of 10 mol % triphenoxy borane changed the selectivity from the boryl formate species to bis(boryl)acetal and methoxy borane compounds, with almost no boryl formate product observed upon HBPin consumption. Similar results were also obtained using the slightly weaker trimethoxy borane as the Lewis acid (see the Supporting Information). Perhaps the most striking result in the RPCP family of complexes is when (tBuPCP)NiH is used as the catalyst (Table 4, entry 4). In this case, the reaction selectively forms methoxy borane, with almost no bis(boryl)acetal formed. Thus, the addition of the Lewis acid completely changes the selectivity f rom a two-electron reduction to the formic acid level to a six-electron reduction to the methanol level. The selectivity of CO2 reduction is also affected by the amount of Lewis acid that is present. Smaller amounts of Lewis acid (1−2 mol %) give predominantly boryl formate, and a loading of 5% is required to see a significant change in selectivity toward the bis(boryl)acetal and methoxy borane products (see the Supporting Information). The presence of a Lewis acid also affects the selectivity of RPSiP systems (Table 4, entries 7−10). Using (RPSiP)MH (M = Ni or Pd; R = iPr and Cy) as the catalysts, no boryl formate product is observed and the

propose that this species results from decomposition of the boryl formate product, which is known to be unstable, after catalysis has finished.4a,5a,8b Consistent with this hypothesis, when a vacuum was applied to the boryl formate derived from HBPin and CO2, the formation of the known compound B2Pin317 was observed by NMR spectroscopy and X-ray crystallography (Scheme 1 and the Supporting Information). The other decomposition product from this process is presumably triformatoborane, which balances the stoichiometry of the proposed decomposition pathway. Once generated, the triformatoborane can act as a Lewis acid and coordinate to the Lewis basic carbonyl oxygen in (tBuPCP)Pd{OC(O)H}. Lewis acid adducts of this nature have previously been reported for other group 10 pincer supported metal formates.5d,18 To confirm that the observation of 1 was an artifact of our isolation conditions and it was not the resting state during catalysis, the catalytic activity of 1 for CO2 reduction using HBPin as the reductant was tested under the conditions described in Table 1. Rather than selectively forming the boryl formate product, as was observed for (tBuPCP)PdH, significant amounts of both the bis(boryl)acetal and methoxy borane products were observed (Figure 5c). This indicates that 1 is not the catalyst resting state with (tBuPCP)PdH but suggests that the triformatoborane plays a role in changing the selectivity of CO2 reduction. In agreement with this hypothesis, a control experiment using an isolated sample of (tBuPCP)Pd{OC(O)H} as the catalyst gave only the boryl formate product, as observed with (tBuPCP)PdH (see the Supporting Information). In Figure 4, we propose that reduction beyond cycle I requires the relative rate of CO2 insertion into a metal hydride to be decreased compared to the rate of boryl formate insertion. Here, we suggest that triformatoborane and boryl formate form a Lewis acid/Lewis 307

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Table 5. Hydroboration of CO2 Using HBCat as the Reductant with Various RPCP and RPSiP Supported Catalystsa,b,c

a

Reaction conditions (C6D6): [catalyst] = 0.0007 M, [HBCat] = 0.07 M, 1 atm of CO2 (5 equiv with respect to HBCat), C6D6, RT. bResults are the average of two runs and are based on 1H NMR integrations against a naphthalene internal standard. c2 equiv of HBCat is required to make product B, and 3 equiv is required to make product C. d% Buried volume (VBur%) was calculated using the coordinates from crystal structures previously described in the literature (see the Supporting Information). Entries denoted with an asterisk were calculated using a DFT generated structure (see the Supporting Information). eBis(boryl)oxide is formed concomitantly with methoxy borane.

Figure 6. (a) Solid state structure of 2. Thermal ellipsoids shown at 30% probability. Selected hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): B(1)−O(1) 1.558(6), O(1)−C(1) 1.251(5), C(1)−O(2) 1.264(5), O(2)−B(2) 1.569(5), B(1)−O(1)−C(1) 119.4(3), O(1)−C(1)−O(2) 121.4(4), C(1)−O(2)−B(2) 118.7(3). (b) Lewis structure of 2. (c) CO2 hydroboration reaction with 9-BBN catalyzed by (CyPSiP)PdH.

reaction predominantly produces methoxy borane, with only a small amount of bis(boryl)acetal generated. The use of a Lewis acid cocatalyst changes other aspects of the reaction in addition to selectivity. For example, with the exception of catalysis using (tBuPCP)NiH, the reaction is significantly slower when a Lewis acid is present, and it takes longer for all of the HBPin to be fully consumed. A potential

explanation for the decrease in rate is that the Lewis acid forms a stable off-cycle intermediate with the metal formate, which resembles the isolated structure 1. In this case, the (tBuPCP)NiH may be too sterically bulky to form the off-cycle species. An alternative possibility is that using a Lewis acid changes the turnover limiting step from transmetalation in cycle I to a slower step in cycles II and III. Additionally, the mass balance 308

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reaction is adjusted to account for the formation of 2, then 84% of the borane is in the form of the boryl formate tetramer, while 9% is in the form of methoxy borane. This leads to a much higher borane mass balance of 93% (Figure 6c). The formation of 2 is proposed to occur due to the accumulation of boryl formate in solution. The boryl formate product then precipitates out of solution as a tetramer before any further reaction can occur. Only the two most sterically bulky catalysts, (tBuPCP)NiH and (tBuPCP)PdH, did not form a precipitate under our standard conditions using 9-BBN as the reductant. We propose that these systems do not form a precipitate because they are too sterically bulky to form an offcycle κ2-borohydride resting state with 9-BBN (Scheme 2).

is poorer in reactions that involve a Lewis acid. We suggest that this is because free formaldehyde, which is highly unstable, is generated when CO2 reduction to the methanol level occurs. This formaldehyde undergoes side reactions with the borane and reduces the mass balance.8c In agreement with this proposal, analysis of other systems in the literature shows that systems for CO2 hydroboration that generate products at the methanol level generally give lower mass balance.4b,5a Overall, the use of a Lewis acid cocatalyst represents a new strategy for changing the selectivity of CO2 hydroboration reactions. It is likely that it can be generalized to CO2 hydroboration reactions that are catalyzed by frustrated Lewis pairs or other transition metal catalysts, as well as potentially CO2 hydrosilylation reactions.19 CO2 Reduction Using Different Boranes. Our earlier results using HBPin highlight how the metal, ancillary ligand, concentration of reductant, and Lewis acid cocatalyst impact the selectivity and rate of CO2 reduction. To further understand the factors that are important in determining the selectivity of CO2 hydroboration, we changed the reductant to HBCat, a less sterically bulky and more electron withdrawing borane (Table 5). In this case, all catalysts rapidly and selectively reduced CO2 to the methoxy borane level. The only difference between the systems was the mass balance. For our best catalyst, (tBuPCP)PdH, almost all of the HBCat was converted to the methoxy borane product, while for most other systems mass balance ranged between 50 and 75%. In fact, the yield obtained with (tBuPCP)PdH is unusually high compared to other systems that perform CO2 hydroboration selectively to the methanol level.4b,5a We propose there are two reasons why HBCat promotes CO2 reduction beyond boryl formate, whereas HBPin only results in reduction to boryl formate (without a Lewis acid). First, because HBCat is less sterically bulky than HBPin, it is more likely to undergo insertion into a metal hydride, and as a consequence, the relative rate of boryl formate insertion compared to CO2 insertion is higher for HBCat. Second, because HBCat is less electron rich than HBPin, the boryl formate species is more electrophilic and thus more likely to undergo insertion. It is also noteworthy that, using HBCat as the reductant, methoxy borane is formed selectively over bis(boryl)acetyl. This issue is addressed in a subsequent section. Nevertheless, our results unequivocally demonstrate that the nature of the borane reductant plays a crucial role in determining the extent of CO2 reduction. The influence of the borane on the selectivity of CO2 reduction was further probed using borabicyclo[3.3.1]nonane (9-BBN) as the reductant. Using the majority of our catalysts, low mass balance of the borane was observed by 1H NMR spectroscopy when the same procedure described in Tables 1 and 5 was performed (see the Supporting Information). For example, when (CyPSiP)PdH was used as the catalyst, we only observed a 9% yield of the methoxy borane product after complete conversion of the 9-BBN starting material, with no other products present in solution. However, in this reaction, colorless crystals, which were suitable for X-ray diffraction, precipitated out of the reaction mixture. The solid-state structure of the precipitate indicated that a tetramer, 2, in which four boryl formate monomer units are combined in a 16-membered ring, had formed (Figure 6a,b). The 1H NMR spectrum of the precipitate from the catalytic reaction in d6DMSO contained a peak at 8.31 ppm, which is also consistent with the formation of a boryl formate containing product (see the Supporting Information). If the yield from the catalytic

Scheme 2. Equilibrium between Metal Hydride and a κ2Borohydride Resting Statea

a

As the catalyst steric bulk decreases, Keq becomes larger and catalysis slows down.

Off-cycle κ2-borohydride complexes have previously been shown to inhibit catalytic hydroboration by Guan et al.5c Similarly, in our case, we suggest the formation of the off-cycle κ2-borohydride species slows down the rate of catalysis with less sterically bulky systems by sequestering the catalyst in an inactive form (see the Supporting Information). This allows the boryl formate product to accumulate and then precipitate as 2. Evidence for the formation of κ2-borohydride complexes with less steric bulky catalysts was obtained using 1H NMR spectroscopy (see the Supporting Information). There is also a correlation between the selectivity of the reaction and the equilibrium constant (Keq) between the metal hydride and 9BBN and the κ2-borohydride species (Table 6). Catalysts that have a greater tendency to form the κ2-borohydride species give higher selectivity to the boryl formate product. The decrease in the rate of the reaction as the steric bulk of the catalyst is increased with 9-BBN is the opposite to the trend observed when using HBPin (without a Lewis acid). In the case of HBPin, the borane is too sterically bulky to interact with any of the metal hydrides studied in this work and form a κ2-borohydride complex (see the Supporting Information). As a consequence, the rates of catalytic reactions are governed by turnover limiting transmetalation, which is faster with less sterically bulky systems.13d In the case of HBCat, all pincer supported metal hydride catalysts evaluated in this work form κ2-borohydride complexes, so in reactions using HBCat as the reductant the formation of these off-cycle species is likely not a major factor in the observed differences between catalysts (see the Supporting Information). In catalytic reactions where the tetramer 2 forms, the selectivity with 9-BBN cannot be directly compared to reactions with HBCat and HBPin because it is presumably governed by precipitation of the boryl formate product. The results, however, using 9-BBN as a reductant and (tBuPCP)NiH or (tBuPCP)PdH as the catalyst, where the solution remains homogeneous, can be directly compared to analogous experiments with HBCat and HBPin (Table 6). In the case of (tBuPCP)PdH, when 9-BBN is used as the reductant, 309

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ACS Catalysis Table 6. Hydroboration of CO2 Using 9-BBN as the Reductant with Various (tBuPCP)MH Catalystsa,b,c

a

Reaction conditions (C6D6): [catalyst] = 0.0007 M, [9-BBN] = 0.07 M, 1 atm of CO2 (5 equiv with respect to 9-BBN), C6D6, RT. bResults are the average of two runs and are based on 1H NMR integrations against a naphthalene internal standard. c2 equiv of 9-BBN is required to make product B, and 3 equiv is required to make product C. d% Buried volume (VBur%) was calculated using the coordinates from crystal structures previously described in the literature (see the Supporting Information). eKeq is calculated using 1H NMR relative integrations and describes the equilibrium between metal hydride, 9-BBN, and the κ2 off-cycle borohydride species (see the Supporting Information for details). An asterisk denotes only the κ2-borohydride species was observed by 1H NMR spectroscopy. fIn the form of the boryl formate tetramer 2. gBis(boryl)oxide is formed concomitantly with methoxy borane. h10 mol % B(OPh)3 was added to the reaction.

Figure 7. Reaction of 9-BBN with a mixture of the bis(boryl)acetal and methoxy borane products derived from CO2 and 9-BBN in the presence of (tBuPCP)NiH. During the reaction, the bis(boryl)acetal product converts to the methoxy borane product.

insertion of the boryl formate intermediate into a metal hydride over CO2 insertion into a metal hydride (see Figure 4). It is, however, more complicated to understand and predict whether the four-electron bis(boryl)acetal reduction product or the six-electron reduction methoxy borane product is predominantly formed. Furthermore, the selectivity between the two products is influenced by the presence or absence of a Lewis acid cocatalyst. Initially, it is important to note that, on the basis of the mechanism proposed in Figure 2, the formation of the bis(boryl)acetal product is not a requirement in the formation of the methoxy borane product. This stands in contrast to the two-electron boryl formate product, which in our mechanism is an intermediate in the formation of either the four- or six-electron reduction products. To test whether the bis(boryl)acetal product could be converted into the methoxy borane product, we started with a reaction mixture containing a combination of the bis(boryl)acetal and methoxy borane products derived from CO2 and 9-BBN (Figure 7). This mixture was generated using (tBuPCP)NiH as the catalyst, which was also present in the sample. CO2 was removed from the solution, and additional 9-BBN was added. Over time, the ratio of the bis(boryl)acetal product to the methoxy borane product changed from 3.4 to 0.2. An analysis of the mass balance indicated that most of the 9-BBN added to the reaction mixture was either unreacted or had been incorporated into the methoxy borane product. Given that there is no CO2 present in this reaction to directly form the methoxy borane product, this experiment establishes that bis(boryl)acetal can be converted into methoxy borane under

methoxy borane is the major product, but there is some bis(boryl)acetyl. In contrast, the corresponding reaction with HBCat, which is less sterically bulky than 9-BBN, gives exclusively the methoxy borane product, while the reaction with the sterically bulky HBPin gives exclusively the boryl formate product. The results with (tBuPCP)NiH are even more striking. Using 9-BBN as the reductant, the major product is the bis(boryl)acetal species, whereas with HBPin the boryl formate product is selectively formed, and with HBCat only the methoxy borane product is generated. Essentially, when (tBuPCP)NiH is used as the catalyst, selective formation of products at either the formic acid, formaldehyde, or methanol oxidation levels can be achieved by changing the nature of the borane reductant. Our results with HBPin indicate that a Lewis acid can be used to change the selectivity of CO2 reduction from boryl formate products to bis(boryl)acetal and methoxy borane products (vide supra). The addition of 10 mol % trimethoxy borane to the reaction of CO2 with 9-BBN catalyzed by (tBuPCP)NiH also changes the selectivity. In this case, the methoxy borane product is formed in 87% yield, with minimal amounts of bis(boryl)acetal (6%). This again demonstrates that Lewis acids can be used to promote the formation of more reduced products in CO2 hydroboration reactions and that this may be a general strategy. Investigating Selectivity for Bis(borylacetal) versus Methoxy Borane Products. Our earlier results highlight that the key to facilitating either four- or six-electron reduction of CO2 rather than two-electron reduction is to promote the 310

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with the formation of the bis(boryl)acetal product, its chemical shift is similar to the resonances associated with the bis(boryl)acetal products in the 1H NMR spectra of related reactions using HBPin and 9-BBN. At lower temperature, this peak sharpens. We suggest that the fluxionality occurs because at room temperature the bis(boryl)acetal product is in exchange with the metal acetal through the reversible reaction described in Scheme 3, but at low temperature, this process does not occur. Furthermore, when the same reaction was repeated using isotopically labeled 13CO2, the singlet at 5.27 ppm was replaced by a doublet, due to 13C splitting (see the Supporting Information). An intense peak is also observed at 82.14 ppm in the 13C{1H} NMR spectrum, which an HSQC experiment demonstrates correlates to the peak at 5.27 ppm, and is the expected chemical shift for the methylene carbon of a bis(boryl)acetal product. At the end of the reaction, when all of the HBCat had been consumed, the peak at 5.27 ppm in the 1 H NMR spectrum had completely disappeared and only peaks associated with methoxy borane were present. Thus, we propose that in reactions involving HBCat there is a small steady state concentration of the bis(boryl)acetal product during the reaction, which is all eventually converted to the thermodynamic product. Our studies indicate that the presence of Lewis acid cocatalyst promotes the formation of the six-electron methoxy borane reduction product, regardless of whether the two- or four-electron product is favored in the absence of a Lewis acid. In agreement with this proposal, when triphenoxy borane was added to a combination of the bis(boryl)acetal and methoxy borane products derived from CO2 and 9-BBN (Figure 7) in the presence of (tBuPCP)NiH, much faster conversion to methoxy borane was observed than when no Lewis acid was present (see the Supporting Information). As described above, our rationale for the two-electron product being further reduced is that the Lewis acid activates the boryl formate product for insertion into a metal hydride. At this stage, however, the reasons why the Lewis acid promotes the further reduction of the four-electron product to the six-electron product are not clear. One potential explanation for this observation is that the Lewis acid increases the rate of β-alkoxy elimination from the metal acetal intermediate to form the thermodynamically preferred methoxy borane product, which in turn decreases the relative amount of the metal acetal which undergoes transmetalation to form the bis(boryl)acetal product. Alternatively, the Lewis acid could speed up the rate of interconversion between the bis(boryl)acetal product and the metal acetal intermediate. This would make it harder to trap the kinetic bis(boryl)acetal product and promote the formation of more methoxy borane product. From our catalytic data, it is clear that, whatever the role of the Lewis acid is, it causes more methoxy borane product to be formed, but the origins of this effect remain unclear. In the future, we intend to perform computational work to fully probe the Lewis acid effect on selectivity.

the reaction conditions. A similar result was obtained when HBPin was used as the reductant, confirming that this observation is not unique to 9-BBN (see the Supporting Information). On the basis of our mechanism, it also suggests that transmetalation of the unobserved metal acetal intermediate is reversible and that bis(boryl)acetal can re-enter the catalytic cycle by reacting with a metal hydride to form a metal acetal and an equivalent of free borane (Scheme 3). Finally, Scheme 3. Proposed Equilibrium of Bis(boryl)acetal and Metal Acetal Which Allows Bis(boryl)acetal to Re-Enter Catalysis in the Presence of a Metal Hydride

from this experiment, we can conclude that bis(boryl)acetal, like boryl formate, is a kinetic product in CO2 hydroboration, while methoxy borane is the thermodynamic product. Our control experiment, described in Figure 7, indicates that the essentially complete conversion of bis(boryl)acetal to methoxy borane is possible under our reaction conditions, but in catalytic reactions with 9-BBN and (tBuPCP)NiH, high conversion to bis(boryl)acetal is observed (in the absence of a Lewis acid cocatalyst). We suggest that the major reason for this difference is the presence of CO2. In catalytic reactions, the presence of CO2 sequesters the metal hydride in the form of a metal formate (vide supra). This shifts the equilibrium described in Scheme 3 by removing the metal hydride, which in turn causes an increase in the amount of bis(boryl)acetal product. Furthermore, in the experiment described in Figure 7, 9 days were required for the conversion of bis(boryl)acetal to methoxy borane, which shows that the rate of this process is slow when (tBuPCP)NiH is used as the catalyst. This conversion is likely even slower in the presence of CO2, which lowers the concentration of the metal hydride catalyst. Moreover, the elementary steps involved in the conversion of bis(boryl)acetal to methoxy borane are likely slower for (tBuPCP)NiH, as it is the most sterically bulky catalyst studied in this work. As a consequence, (tBuPCP)NiH gives more bis(boryl)acetal with 9-BBN than the less sterically bulky (tBuPCP)PdH, which is able to more readily convert any bis(boryl)acetal that is formed to methoxy borane (see the Supporting Information). The precipitation of the bis(boryl)acetal product out of solution, as observed in the system designed by Turculet et al. (Figure 1c), also prevents its conversion to methoxy borane via the equilibrium in Scheme 3 and is an alternative strategy to obtain high selectivity. The mechanistic pathway described above for 9-BBN can also account for our catalytic observations with HBCat, even though in this case we only observe methoxy borane products at the completion of the reaction (Table 5). The low steric bulk of HBCat means that an equilibrium between bis(boryl)acetal and the metal hydride and the metal acetal and HBCat (Scheme 3) is likely to be rapidly established and the reaction can readily proceed to the thermodynamically favored methoxy borane product. Consistent with this hypothesis, when a reaction between HBCat and CO2 catalyzed by (iPrPSiP)NiH was monitored in situ using 1H NMR spectroscopy, a broad peak at approximately 5.27 ppm was observed during the reaction (see the Supporting Information). Although we cannot unequivocally confirm that this peak is associated

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CONCLUSIONS In this work, we have demonstrated selective CO2 reduction to boryl formate, bis(boryl)acetal, or methoxy borane products through variation of either the identity and concentration of the reductant, the nature and loading of the catalyst, or the presence of a Lewis acid cocatalyst. Initial reduction of CO2 leads to the two-electron boryl formate reduction product, which is a kinetic product. There are two methods to 311

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ACS Catalysis selectively form the boryl formate product. The first method involves increasing the favorability of CO2 insertion into a metal hydride compared to a competing boryl formate insertion, which leads to further reduction. This can be achieved by using a sterically bulky and electron rich borane such as HBPin and increasing the relative concentration of CO2 compared to the borane. The second method is to use reaction conditions which enable the boryl formate product to precipitate out of solution before further reaction occurs, as we observe in some cases using 9-BBN as the reductant. Our studies also provide guidance on how to selectively form bis(boryl)acetal or methoxy borane products. To selectively form the kinetic bis(boryl)acetal product in homogeneous reactions, a combination of reductant and catalyst is required that promotes boryl formate insertion but slows down the equilibration of the bis(boryl)acetal product with a metal acetal intermediate to allow the bis(boryl)acetal product to be isolated in high quantities. Although we do observe selectivity to bis(boryl)acetal as a consequence of these factors, it is unlikely that the results from this work will allow us to make predictions about whether other systems would also promote this selectivity. An easier method to selectively form the bis(boryl)acetal product is to use a very high concentration of the borane to promote the precipitation of the four-electron reduction product. Our results suggest that this is how Turculet et al. were able to selectively form bis(boryl)acetal using HBPin as the reductant and (iPrPSiPInd)NiH as the catalyst.5e We have demonstrated that this strategy can also be used to promote selective formation of bis(boryl)acetal with other catalysts, provided similar reaction conditions are utilized. Finally, in the absence of a Lewis acid, methoxy borane can be selectively formed by using a borane that is electron withdrawing and not sterically demanding, for example, HBCat. This allows the reaction to be under thermodynamic control, which results in the formation of methoxy borane. In contrast, in the presence of Lewis acid, many different catalysts will give selectivity to methoxy borane. Taken together, our results explain many previous observations in the literature and provide guidance on how to design the next generation of catalysts for CO2 hydroboration. It is particularly noteworthy that using one catalyst, (tBuPCP)NiH, we can selectively form either the two-, four-, or six-electron CO2 reduction product by changing either the nature of the reductant or the reaction conditions. However, at this stage, many of our explanations for selectivity are based on phenomenological observations and studies that explicitly explore the structures and relative energies of key transition states and intermediates at a molecular level or measure kinetics may provide even more understanding and provide further support to our work. Nevertheless, it is likely that many of our empirical observations will be generalizable and have significance for the improvement of catalysts for other processes such as the hydrosilylation of CO2 and the reduction of heterocumulenes and polar double bonds related to CO2.

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Crystallographic structure file for compound 2 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nilay Hazari: 0000-0001-8337-198X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.H. acknowledges support from the National Science Foundation through grant CHE-1150826 and the Camille and Henry Dreyfus Foundation. We are grateful to Jessica Heimann and Hee Won Suh for assistance with synthesis and Megan Mohadjer Beromi, Dr. David Balcells, and Dr. Ainara Nova for DFT calculations to determine the buried volume of some complexes.

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REFERENCES

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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/acscatal.8b03894. Full compound characterization, experimental procedures, and kinetics data (PDF) Crystallographic structure file for compound 1 (CIF) 312

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ACS Catalysis

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DOI: 10.1021/acscatal.8b03894 ACS Catal. 2019, 9, 301−314

Research Article

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DOI: 10.1021/acscatal.8b03894 ACS Catal. 2019, 9, 301−314