A Hydride-Shuttle Mechanism for the Catalytic Hydroboration of CO2

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

A Hydride-Shuttle Mechanism for the Catalytic Hydroboration of CO2 Longfei Li,† Huajie Zhu,‡ Li Liu,†,§ Datong Song,*,∥ and Ming Lei*,† †

State Key Laboratory of Chemical Resource Engineering, College of Science, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ Chinese Center for Chirality, College of Pharmacy, Hebei University, Baoding 071002, Hebei, People’s Republic of China § Key Laboratory of Carbon Fiber and Functional Polymers, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ∥ Davenport Chemical Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *

ABSTRACT: Herein we report our investigation into the mechanism of CO2 reduction by HBpin catalyzed by [Ru(CO)H(L)(PPh3)2] (2; L is the 4,5-diazafluorenyl ligand with a Bpin functional group at the 9-position) through computational studies using the model complex [Ru(CO)H(L)(PMe3)2] (A1). The reaction consists of four stages: (1) CO2 insertion into the C−B bond of A1 to form A4, (2) the reduction of A4 by HBpin to afford HCOOBpin (P2) and regenerate A1, (3) the reduction of P2 by HBpin to HCHO (P5), and (4) the reduction of P5 to CH3OBpin (P6). We found that Lewis adduct formation plays a key role in all stages of the mechanism, in that it forms more relaxed rings in the key transition states and makes the hydride more hydridic. Oftentimes, the hydride and Bpin moieties can transfer within the Lewis adducts in a concerted manner in our proposed hydride-shuttle mechanism. The energy spans for all stages of our proposed mechanism are within the range of 15.7−22.6 kcal/mol in terms of Gibbs free energy. In contrast, the direct hydroboration and σ-bond metathesis mechanisms proposed in the literature have extremely high energy barriers because of the highly strained four-membered rings in the transition states and the unactivated hydride in HBpin.

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Scheme 1. Typical Transformations of CO211

INTRODUCTION

The concentration of carbon dioxide (CO2) in the atmosphere is increasing associated with the intensive consumption of fossil fuels.1,2 The use of CO2 as a nontoxic, abundant, and renewable C1 resource for value-added chemicals or fuels has attracted considerable attention.3−5 However, CO2 is quite inert and its conversion into other chemicals is highly kinetically and thermodynamically unfavorable.6,7 The present industrial use of CO2 takes up only 0.6% of the anthropogenic CO2 emissions.8,9 Novel chemistry enabling an economically viable CO2 capture, utilization, and storage strategy has been viewed as a “Holy Grail” and will inevitably require substantial research.10 With the electron deficiency of its carbonyl carbon, CO2 is known for insertion into the σ-bond of nucleophiles and oxidative cycloaddition with unsaturated compounds, as shown in Scheme 1.11 Lin and Marder preformed a DFT study on CO2 insertion into a Cu−C bond, in which the nucleophilic attack of the aryl ligand on the electron-deficient carbon of CO2 provides a new C−C bond.12 Substituents on the aryl group modify its nucleophilicity, affecting the CO2 insertion barrier. Shoji et al. reported that CO2 could react with the C−B bond © XXXX American Chemical Society

of the two-coordinate borinium complex, resulting in an unusual deoxygenation reaction.13 In the past decade, the hydroboration of CO2 has emerged as a promising strategy for CO2 fixation.14−18 In 2005, Sadighi suggested that the carbene-supported Cu(I) boryl complex could catalyze the reduction of CO2 with pinB−Bpin (pin = pinacolato) as the reducing agent.19 In 2010, Guan et al. reported that a PCP-pincer nickel complex could catalyze the hydroboration of CO2, in which CO2 reacted with HBcat (cat = catecholato) to yield CH3OBcat and catBOBcat at room temperature with high TOFs.20 In 2014, Sabo-Etienne and coReceived: November 14, 2017

A

DOI: 10.1021/acs.inorgchem.7b02887 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Hydroboration of CO2 Catalyzed by 225

workers reported the observation of free formaldehyde in the reduction of CO2 by borane catalyzed by the ruthenium polyhydride complex [Ru(H)2(H2)(PCy3)2].21 In 2012, the Song group demonstrated the reversible insertion of CO2 into the C−H bond of [Ru(CO)H(daf)(PPh3)2] (1; daf = 4,5-diazafluorenyl), featuring an actor ligand and spectator metal combination.22 In 2013, they further demonstrated that the reactivity of the actor diazafluorenyl ligand toward CO2 can be tuned using various spectator metal fragments.23 The applications of complexes with actor ligands24 are further exemplified by a recent report on 1-catalyzed hydroboration of CO2, which affords CH3OBpin (P6) and pinBOBpin (P4).25 Complex 1 first undergoes C−H borylation to give [Ru(CO)H(L)(PPh3)2] (2; L is the 4,5-diazafluorenyl ligand with a Bpin functional group at the 9-position) with the loss of dihydrogen, as shown in Scheme 2. Interestingly, CO2 inserts into the Csp2−B bond of 2 rather than the Ru−H bond. The resulting boryl ester 3 can be further reduced by HBpin in solution at 110 °C to form P6 and P4 and regenerate 2. Through experimental and DFT studies, Song et al. proposed a mechanism for CO2 insertion into the C−B bond, which includes the Csp2−Csp and B−O bond formations and C−B bond cleavage. However, the key mechanisms for the reduction of boryl ester 3 by HBpin remain unclear. In 2017, Sabet-Sarvestani et al.26 explored the mechanism computationally for a related transformation involving the metal-free system (Scheme 3),27 where the direct hydroboration of the carbonyl group by HBpin was proposed (i.e., with highly strained four-membered-ring transition states), albeit with inhibitively high energy barriers (32−47 kcal/mol). The corresponding C−C cleavage step for product release from the catalyst has an even higher energy barrier (about 50 kcal/ mol), suggesting that the computed pathway is kinetically incompetent in catalysis. An understanding of the reaction mechanism can potentially lead to more efficient strategies and catalysts for CO2 reduction. How does HBpin undergo effective hydroboration of CO2? It is conceivable that the boryl ester intermediate resulting from CO2 insertion into the C−B bond can coordinate to threecoordinate boron centers via the carbonyl oxygen atom to form Lewis adducts. Such adduct formations can potentially bring multiple molecules into close proximity in the transition states, which in turn leads to less strained rings in the transition states. Could the key hydride transfer, C−C cleavage, and C−O cleavage steps in the CO2 reduction processes occur more readily when Lewis adduct formation is involved? In order to answer these questions, we began a systematic study of the

Scheme 3. Direct Hydroboration Mechanism Proposed by Sabet-Sarvestani et al.26

hydroboration of CO2 using the model compound [Ru(CO)(H)L(PMe3)2] (A1) as the catalyst (where PMe3 is used instead of PPh3 to save computation time).

2. COMPUTATIONAL METHODS In accordance with previous computational studies for transition-metal complexes,28−32 all calculations in this study were carried out by the DFT method with the ωB97X-D functional33 using the Gaussian 09 program.34 Geometries were optimized in benzene solution using basis set system BS-I, where a double-ζ basis set (Lanl2dz)35 with the effective core potentials of Hay and Wadt was employed for Ru and 631G* basis sets were used for all other atoms. Single-point calculations were performed to present better electronic energy with the large basis set system BS-II, using the BS-I optimized geometries. In BS-II, Lanl2dz was used for Ru and 6-31++G** for all other atoms. The solvent effect was evaluated using the SMD (solution model based on density) solvation model.36,37 Thermal correction and entropy contribution to the Gibbs free energy were taken from the frequency calculations with the ωB97X-D/BS-I. B

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Figure 1. Four mechanistic stages of A1-catalyzed hydroboration of CO2.

Figure 2. Insertion of CO2 into the C−B bond of A1 to afford A4. The energies are relative Gibbs free energies (ΔG in kcal/mol) and potential energies (ΔE in kcal/mol). It should be noted that such thermal corrections based on the ideal gas-phase model inevitably overestimate entropy contributions to free energies for reactions in solvent, especially for reactions involving component changes, because the suppressing effect of the solvent on the transitional freedoms of the substrates is ignored.38 The present transformation involves a multicomponent change; thus, the entropy overestimations must be taken into account. In this study, translational movement was evaluated using the method presented by Whitesides et al.39 All transition states were confirmed to exhibit only one imaginary frequency in a Hessian analysis. The energies discussed below are relative to those of A1, unless otherwise stated. The details of relative energy calculations are shown in the Supporting Information. The natural bond orbital (NBO) calculations were performed using the NBO 3.1 program,40 as implemented in the Gaussian 09 package. The Cartesian coordinates of all optimized structures are presented in the Supporting Information.

A4 undergoes C−C bond cleavage to produce HCOOBpin (P2) and regenerate A1, which is discussed in section 3.2. In stage 3, P2 is further reduced to HCHO (P5) via the intermediate (pinBO)2CH2 (P3) accompanied by the formation of P4, which is discussed in section 3.3. In stage 4, P5 is further reduced to the final product P6, which is discussed in section 3.4. Compounds A4, P2−P4, and P6 in this reaction sequence were observed experimentally,25 which corroborates the proposed mechanism. In this work, a ball is used to represent the [Ru(CO)(H)(daf)(PMe3)2] moiety for clarity. 3.1. Insertion of CO2 into the C−B Bond of A1: Stage 1. The insertion of CO2 into the C−B bond of A1 proceeds in a stepwise manner, as shown in Figure 2. The negatively charged carbon backbone of the L− ligand in A1, with an NBO charge of −0.58, preferentially attacks the electron-deficient carbon atom of CO2 to achieve the C−C formation. The energy barrier for the process is 11.2 kcal/mol, and the resulting intermediate A2 lies 7.3 kcal/mol in Gibbs free energy above A1. Due to the existence of a vacant 2p orbital and the low electronegativity, the B atom in A2 can bond to the O atom of the CO2-derived carboxylate group to form intermediate A3, featuring a four-membered ring; A3 is 5.4 kcal/mol lower than

3. RESULTS AND DISCUSSION According to the experiment, the proposed mechanism for the A1-catalyzed hydroboration of CO2 is shown in Figure 1, which includes four stages. In stage 1, CO2 inserts into the C−B bond of A1 to form A4, which is discussed in section 3.1. In stage 2, C

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Figure 3. Hydride-shuttle mechanism for A4-catalyed reduction of A4′ to P1. The energies are relative Gibbs free energies (ΔG in kcal/mol) and potential energies (ΔE in kcal/mol). The notion of A4′ (which is the same species as A4) is to distinguish it from the catalyst A4.

A2. The C−B bond elongates from 1.512 Å in A1 to 1.771 Å in A3, bringing about the possibility for the cleavage of the C−B bond, which goes through the four-membered transition state TSA3-4 with an extremely low energy barrier (0.7 kcal/mol) to form A4. Although it has an entropy penalty, the insertion of CO2 into the C−B bond of A1 to form A4 is exergonic by 13.9 kcal/mol, which is associated with the substantial p−p stabilization in the B−O bond. The CO2-inserted product A4, with both the Lewis acidic B site and the Lewis basic O site (i.e., carbonyl oxygen), can dimerize through coordination. The experimentally unobserved insertion of CO2 into the Ru−H bond was found to be kinetically and thermodynamically unfavorable through our computational studies (see the Supporting Information). 3.2. Reduction of A4 To Afford HCOOBpin (P2) and Regenerate A1: Stage 2. The reduction of A4 by HBpin to form P2 and A1 (stage 2 in Figure 1), an apparent σ-bond metathesis process, actually consists of two catalytic cycles: HBpin addition across the C−O double bond of A4 to form P1 catalyzed by another molecule of A4 (Figure 3) and elimination of P2 from P1 catalyzed by HBpin (Figure 5). In the HBpin addition cycle (Figure 3), two A4 molecules first dimerize to form B1 through coordination (note: we define the substrate A4 as A4′ in Figure 3 to distinguish it from the catalyst A4). B1 can then coordinate to an HBpin molecule to form B2. Due to the substantial entropy penalty, B2 lies 7.4 kcal/mol above A4 in Gibbs free energy but 14.1 kcal/mol below A4 in potential energy. The coordination activates the B−H bond, as evidenced by an elongation from 1.191 Å (in free HBpin) to 1.214 Å and a change in the NBO charge from −0.09 (in free HBpin) to −0.14 in B2. In the subsequent step, the two Bpin moieties and hydride transfer to form B3 through the transition state TSB23, with an energy barrier of 12.8 kcal/mol. B3 can be viewed as an adduct of P1 and A4 through coordination, and therefore, the subsequent dissociation of P1 regenerates the catalyst A4. In brief, the addition of HBpin across the C−O double bond of A4 is exergonic by 8.0 kcal/mol and can be catalyzed by another molecule of A4 with an energy span of 20.2 kcal/mol. In contrast, the direct addition of HBpin across the C−O double bond of A4 is considered in Figure 4, where the hydride transfers from the HBpin moiety to the carbonyl carbon atom

Figure 4. Direct hydroboration mechanism for the addition of HBpin across the C−O double bond of A4. The energies are relative Gibbs free energies (ΔG in kcal/mol) and potential energies (ΔE in kcal/ mol).

of A4 unassisted. An extremely high energy barrier of 35.0 kcal/ mol is predicted, because of the highly strained four-membered ring in the transition state TSA5. Direct σ-bond metathesis for the reduction of A4 by HBpin to provide P2 and A1 (see the Supporting Information) has an even higher energy barrier. These suggest that the uncatalyzed pathways are kinetically incompetent, while our hydride-shuttle mechanism is more favorable with a lower energy barrier due to the lower ring strain in the transition state. As shown in Figure 5, catalyzed by HBpin, P1 can release a molecule of P2 and regenerate the starting species A1. First, P1 coordinates with HBpin via the carbanion ligand backbone to form the adduct B4, which is uphill by 5.4 kcal/mol due to the entropy penalty. Then, the hydride on the HBpin moiety of B4 can migrate to the B atom of an adjacent OBpin moiety to form B5 through the transition state TSB4-5, with an energy barrier of 11.7 kcal/mol. Note that although the Gibbs free energy of TSB4-5 appears slightly lower than that of B5, the potential energies show the reverse trend. It should be noted that the C− D

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the Lewis adduct of P3 and A4. The catalytic species A4 is then regenerated after the dissociation of P3 from C3. The reduction of P2 to P3 by HBpin is exergonic by 27.7 kcal/mol due to the formation of a strong B−O bond. As shown in Figure 7, with the assistance of A4, P3 can lose 1 equiv of pinBOBpin to generate P5. First, A4 binds the

Figure 5. Elimination of P2 from P1 catalyzed by HBpin. The energies are relative Gibbs free energies (ΔG in kcal/mol) and potential energies (ΔE in kcal/mol).

Figure 7. A4-catalyzed elimination of pinBOBpin from P3 to produce P5. The energies are relative Gibbs free energies (ΔG in kcal/mol) and potential energies (ΔE in kcal/mol).

C bond linking the red and black fragments in Figure 5 becomes weaker with a length increase from 1.528 Å (in B4) to 1.582 Å (in B5). Subsequently, HBpin and P2 both dissociate from B5 to regenerate the starting species A1, completing stage 2 of the mechanism. The product P2 will be used in stage 3. 3.3. Reduction of P2 to P5: Stage 3. The reduction of P2 into P5 (stage 3 in Figure 1) consists of two catalytic cycles: the hydroboration of P2 to form P3 and the elimination of P4 from P3 to form P5. Both are catalyzed by A4. As shown in Figure 6, the pathway for the addition of HBpin across the C−O double bond of P2 is similar to that depicted in Figure 3. First, A4 coordinates to the substrate P2 to form C1, which then binds with a molecule of HBpin to form C2. The subsequent hydroboration of the C−O double bond proceeds through the transition state TSC2-3, with a Gibbs free energy span of 22.6 kcal/mol, affording intermediate C3, which can be viewed as

substrate P3 to form the cyclic intermediate C4 through two dative bonds, which allows for the exchange of the Bpin fragment and the cleavage of a C−O bond, affording C5. Finally, the dissociation of pinBOBpin (P4) and P5 from C5 regenerates A4. The free energy span is 21.8 kcal/mol for the A4-catalyzed transformation of P3 into P4 and P5. The resulting P5 will be used in stage 4. 3.4. Reduction of P5 to P6: Stage 4. As shown in Figure 8, catalyzed by A4, the C−O double bond in P5 undergoes hydroboration to afford the final product P6. First, A4 binds P5 and HBpin molecules to form the adduct D1, bringing the two reactants into close proximity. This step is endergonic by 13.2 kcal/mol because of the entropy penalty. The subsequent hydride transfer is accompanied by the Bpin fragment exchange

Figure 6. Hydride-shuttle mechanism of A4-catalyzed reduction of P2 to P3. The energies are relative Gibbs free energies (ΔG in kcal/mol) and potential energies (ΔE in kcal/mol). E

DOI: 10.1021/acs.inorgchem.7b02887 Inorg. Chem. XXXX, XXX, XXX−XXX

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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.inorgchem.7b02887. Optimized geometries and energies of all stationary points along the reaction pathways and imaginary frequencies of transition states (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for M.L.: [email protected]. *E-mail for D.S.: [email protected]. ORCID

Datong Song: 0000-0001-6622-5980 Ming Lei: 0000-0001-5765-9664

Figure 8. Hydride-shuttle mechanism of A4-catalyzed reduction of P5 to P6. The energies are relative Gibbs free energies (ΔG in kcal/mol) and potential energies (ΔE in kcal/mol).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21672018, 2161101308, and 21373023), Beijing Municipal Natural Science Foundation (Grant No. 2162029), the Fundamental Research Funds for the Central Universities of China (Grant No. PYCC1708), and the China Scholarship Council (No. 201606880007). We thank the Special Program for Applied Research on Supercomputation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501.

via the cyclic transition state TSD1-2, resulting in the formation of P6 and the regeneration of the catalytic species A4. The free energy span for the A4-catalyzed reduction of P5 to P6 is 15.7 kcal/mol, which is narrower than those of stages 2 and 3. Moreover, stage 4 is extremely exergonic by 50.3 kcal/mol due to the formation of a strong B−O bond.

4. CONCLUSIONS The mechanism for the hydroboration of CO2 catalyzed by compounds 2 and 3 has been investigated using the corresponding model compounds A1 and A4 through computational studies. The proposed hydride-shuttle mechanism consists of four stages: (1) the insertion of CO2 into the C−B bond of A1 to form A4, (2) the reduction of A4 to P2 to regenerate A1, (3) the reduction of P2 to P5, and (4) the reduction of P5 to P6. In stage 1, the important intermediate A3 was located in the current study; this intermediate was alluded to by Song et al., but they were unable to locate it on the energy surface.25 The apparent σ-bond metathesis reaction (i.e., of H−B and C−C bonds) in stage 2 was found to proceed through two consecutive catalytic processes involving the formation of Lewis adducts. Our computed free energy barrier for the C−C bond cleavage step is only 17.1 kcal/mol, much lower than that of the direct metathesis via a four-memberedring transition state. Analogously, for all the hydroborations of C−O double bonds by HBpin involved in stages 2−4, the direct concerted pathways were found to be incompetent under the experimental conditions; in contrast, our hydride-shuttle mechanisms involving Lewis adduct formations have much lower energy barriers and are thermally achievable under the experimental conditions. The free energy spans are 20.2, 22.6, and 15.7 kcal/mol for stages 2−4, respectively. Similarly, the C−O bond cleavage process in stage 3 (i.e., the decomposition of P3 to form P4 and P5) has an energy barrier of 21.8 kcal/ mol when it is catalyzed by A4 through Lewis adduct formation. Finally, the formation of strong B−O bonds (i.e., with substantial p−p interactions) is the main contributor to the thermodynamic driving force of the overall reaction.

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