Understanding the Individual and Combined Effects of Solvent and

Jun 21, 2019 - Understanding the Individual and Combined Effects of Solvent and Lewis Acid on CO2 Insertion into a Metal Hydride ...
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Cite This: J. Am. Chem. Soc. 2019, 141, 10520−10529

Understanding the Individual and Combined Effects of Solvent and Lewis Acid on CO2 Insertion into a Metal Hydride Jessica E. Heimann,† Wesley H. Bernskoetter,‡ and Nilay Hazari*,† †

Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States

‡

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S Supporting Information *

ABSTRACT: The insertion of CO2 into a metal hydride bond to form a metal formate is a key elementary step in many catalytic cycles for CO2 conversion. Similarly, the microscopic reverse reaction, the decarboxylation of a metal formate to form a metal hydride and CO2, is important in both organic synthesis and strategies for hydrogen storage using organic liquids. There are however few experimental studies probing the mechanism of these reactions and identifying the effects of specific variables such as Lewis acid (LA) additives or solvent, which have been shown to significantly impact catalytic performance. In this study, we use a rapid mixing stopped-flow instrument to study the kinetics of CO2 insertion into the cationic ruthenium hydride [Ru(tpy)bpy)H]PF6 (tpy = 2,2′:6′,2″-terpyridine, bpy = 2,2′-bipyridine) in various solvents, both in the presence and in the absence of a LA. We show that LAs can increase the observed rate of this reaction and determine the first quantitative trends for the rate enhancement observed for CO2 insertion in the presence of cationic LAs, Li+ ≫ Na+ > K+ > Rb+. Furthermore, we show that the rate enhancement observed with LAs is solvent dependent. Specifically, as the acceptor number (AN) of the solvent increases, the effect of the LA becomes smaller. Last, we demonstrate that there is a significant solvent effect on CO2 insertion in the absence of a LA. Although the AN of the solvent has been previously used to predict the rate of CO2 insertion, this work shows that the best model for the rate of insertion is based on the Dimroth−Reichardt ET(30) value of the solvent, a parameter that better accounts for specific solute/ solvent interactions.

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INTRODUCTION Carbon dioxide (CO2) is one of the most attractive feedstocks for the sustainable synthesis of both carbon-based commodity chemicals and fuels.1 The abundance, low cost, and nontoxic nature of CO2 motivate its use; however, its high kinetic and thermodynamic stability makes its conversion into more valuable products difficult. One promising approach for overcoming the kinetic challenges associated with CO2 conversion is the utilization of homogeneous transition metal catalysts. In fact, numerous molecular catalysts for both thermal and electrochemical CO2 reduction have been described.1 In many of these processes, for example, the catalytic hydrogenation of CO2 to formic acid or methanol2 or the electrochemical reduction of CO2 to formic acid,3 the insertion of CO2 into a metal hydride bond to form a metal formate is postulated to be a crucial elementary step. The microscopic reverse reaction, the decarboxylation of a metal formate to generate a metal hydride and CO2, is also proposed as a key step in catalytic processes relevant to organic synthesis4 and chemical hydrogen storage in organic liquids.5 Despite the prevalence and importance of CO2 insertion into metal hydrides in catalysis, studies measuring the rates of this transformation are rare,3a,6 and there is even less work exploring how the reaction solvent as well as additives that © 2019 American Chemical Society

are often present in catalysis influence the kinetics of insertion.7,8 Over the last five years, a number of studies on both catalytic CO2 hydrogenation and formic acid dehydrogenation have demonstrated that LA cocatalysts can dramatically increase both turnover number and turnover frequency.1i,2i,5c,9 A proposed explanation for this effect is that the LA stabilizes the rate-determining transition state (TS) for the insertion of CO2 into metal hydrides (or the decarboxylation of metal formates).5c,d Primarily on the basis of computational studies, the general pathway for CO2 insertion into a metal hydride is proposed to involve two steps: (i) nucleophilic attack of the metal hydride on the electrophilic CO2 to form an H-bound formate intermediate and (ii) rearrangement of the H-bound formate to generate the O-bound formate product (Scheme 1).2b,10 Reactions in which the first step is rate-determining are considered to be outersphere insertions,10a,b,e and LAs are proposed to enhance the rate of these insertions by stabilizing the negative charge on the carboxylate group.5c,d,6g Recently, using a pincer supported iridium hydride, we were able to demonstrate the first quantitative rate enhancement for CO2 Received: May 14, 2019 Published: June 21, 2019 10520

DOI: 10.1021/jacs.9b05192 J. Am. Chem. Soc. 2019, 141, 10520−10529

Article

Journal of the American Chemical Society Scheme 1. Pathway for CO2 Insertion into Transition Metal Hydrides

Figure 1. Representative stopped-flow data for CO2 insertion into 1 in the presence of a LA. Reaction conditions: [1] = 0.1 mM, [CO2] = 130 mM, [LiNTf2] = 5 mM, acetonitrile, room temperature.

insertion in the presence of a LA.6g However, due to ion pairing effects in the nonpolar solvents in which the iridium hydride was stable, we were unable to determine any general trends about the LA effect on insertion. Furthermore, as this complex was only stable in nonpolar solvents that minimally solubilized many salts, the LA effect could not be measured in different solvents with the previous system. As a result, although we obtained clear experimental evidence for a LA enhancement, no information was provided on how to modify the LA and solvent to vary the rate of insertion. A detailed understanding of how these parameters affect the rate of insertion is required to provide guidance for the rational design of improved catalysts for reactions that involve CO2 insertion or the microscopic reverse process of decarboxylation. In this work, we use a stopped-flow instrument to measure the rate of outersphere CO2 insertion into the cationic ruthenium hydride [Ru(tpy)bpy)H]PF6 (tpy = 2,2′:6′,2″terpyridine, bpy = 2,2′-bipyridine) (1) in various solvents, both in the presence and in the absence of a LA. We show that LAs enhance the rate of CO2 insertion, but the magnitude depends on the identity of the solvent. We also compare the scale of the LA enhancement for alkali metal cations and demonstrate that the rate increases in the order Rb+ < K+ < Na+ ≪ Li+. Finally, on the basis of the measured rates in the absence of a LA, we develop a new method for predicting the rate of CO2 insertion

into a metal hydride, particularly in hydrogen-bond donor (HBD) solvents. Specifically, we show that correlating the rate of insertion in a particular solvent to the corresponding Dimroth−Reichardt ET(30) parameter11 better predicts the observed rate constant for CO2 insertion than the commonly used acceptor number (AN)12 of the solvent.6b−d,g This work represents the first quantitative study describing general LA trends for CO2 insertion and will assist in the design of improved catalysts for CO2 conversion and related processes.

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RESULTS AND DISCUSSION Lewis Acid Effect and General Lewis Acid Trends. The cationic ruthenium hydride [Ru(tpy)bpy)H]PF6 (1) was our system of choice to study trends in LA effects on CO2 insertion, as it is known to undergo insertion in several polar solvents.6b−d This criterion was essential to reduce or eliminate the effects of ion pairing. Although ion pairing is somewhat dependent on the identity of the anion, alkali cation salts are typically highly dissociated, with the cation generally coordinated by solvent molecules, in solvents such as DMSO or acetonitrile.13 This implies that the use of these types of solvents will allow for the direct comparison of the effect of different LAs on the rate of CO2 insertion. Additionally, previous DFT calculations on 1 show that CO2 insertion proceeds via a rate-determining H-bound formate intermediate 10521

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Table 1. Effect of LAs on the Rate of CO2 Insertion into 1 in Acetonitrile

Figure 2. Plot of kobs/[CO2] versus [LiNTf2] for CO2 insertion into 1 in acetonitrile. Reaction conditions: [1] = 0.1 mM, [CO2] = 130 mM, [LiNTf2] ranges from 1 to 10 mM, room temperature.

characteristic of an outersphere CO2 insertion reaction (Scheme 1),14 suggesting that LAs should impact the rate of insertion.6g Initially, we measured the kinetics of CO2 insertion into 1 to form [Ru(tpy)bpy)(OC(O)H)]PF 6 (2) in acetonitrile in the absence of a LA using a rapid mixing stopped-flow instrument equipped with a UV−vis detector. The reaction was monitored spectroscopically from 345 to 560 nm using a ≥20-fold excess of CO2. Globally fitting the data to a single exponential showed that the reaction is first order in [1]. Previous work demonstrated that the reaction is first order in [CO2],6b,d giving the overall rate law of kNoLA[1][CO2] in the absence of a LA. Subsequently, we measured the kinetics of CO2 insertion into 1 in the presence of a LA, specifically LiNTf2 (Figure 1). The addition of 50 equiv of LiNTf2 resulted in the observed rate of CO2 insertion increasing by a factor of approximately three. Varying the concentration of LiNTf2 indicated that there is a first-order dependence in [LiNTf2] with a nonzero intercept (Figure 2). This indicates that the rate law is kNoLA[1][CO2] + kLA[1][CO2][LA] in the presence of LiNTf2, with both terms being kinetically relevant. This is in sharp contrast to our previous results with (iPrPNHP)IrH3 (iPrPNHP = HN{CH2CH2(PiPr2)}2). In that case, the addition of 20 equiv of a cationic LA resulted in a rate increase of approximately 2 orders of magnitude in THF.6g The rate law for CO2 insertion into the iridium hydride in the presence of a LA could therefore be approximated as kLA[Ir][CO2][LA], as the LA-assisted pathway generated the corresponding formate orders of magnitude faster than the unassisted pathway. In the present study, this simplification is not valid, as both the unassisted and the LA-assisted pathways for CO2 insertion into 1 produce 2 at comparable rates. We propose that this difference may be in part due to solvent effects, as the effect of cationic LAs on the rate of insertion into (iPrPNHP)IrH3 was measured in THF, whereas the present study explores the effect in acetonitrile. This is discussed in the following section. To conclusively establish that the effect of LiNTf2 on CO2 insertion was related to the presence of the Lewis acidic Li+ and not the triflimide anion, we measured the rate of CO2 insertion in the presence of other Li+ salts (Table 1, entries 4 and 5). When 50 equiv of LiOTf or LiBPh4·3DME was used as an additive, the rate constant kLA was identical to that observed with LiNTf2, consistent with our hypotheses that the rate enhancement is due to Li+ and that ion pairing is not significant in acetonitrile. Our ability to measure the rate constant kLA without ion pairing enabled us to evaluate the effect of a series of alkali metal salts on CO2 insertion (Table 1, entries 6−14). Significant differences in the rate of CO2

entry

additive

kLA (M−2 s−1) at rt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

none (nBu)4NPF6 LiNTf2b LiOTfc LiBPh4·3DMEd NaBArF4e NaOTf NaBPh4 NaNTf2 KPF6 KBPh4 KNTf2 KB(C6F5)4 RbBPh4 B(OiPr)3

1.0(1) × 10−2a 7.9(8) × 10−2 3.1 ± 0.3 3.0 ± 0.3 3.0 ± 0.3 4.0(4) × 10−1 4.0(4) × 10−1 3.7(4) × 10−1 3.4(3) × 10−1 2.5(3) × 10−1 2.1(2) × 10−1 2.0(2) × 10−1 1.9(2) × 10−1 1.0(1) × 10−2 7.9(8) × 10−2

Entry 1 lists the value of kNoLA (M−1 s−1), the elementary rate constant for the unassisted pathway of CO2 insertion into 1. As noted in the text, this pathway is kinetically relevant even in the presence of a LA. For the full derivation of kNoLA and kLA values, please see the Supporting Information. bNTf2 = bis(trifluoromethane)sulfonamide. c OTf = trifluoromethanesulfonate. d LiBPh4·3DME = lithium tetraphenylborate tris(1,2-dimethoxyethane). eNaBArF4 = sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. a

insertion were observed when the cation was changed. For example, the rate constant kLA measured in the presence of a series of Na+ salts was approximately 1 order of magnitude smaller than that observed with Li+ salts, and that measured in the presence of K+ salts was slightly lower still. This suggest that smaller, more Lewis acidic cations provide more stabilization to the TS than larger, less acidic ones (Li+ ≫ Na+ > K+ > Rb+). In fact, the LA effect with the largest cation Rb+ is almost negligible, as the observed rate constants in the absence and presence of Rb+ are within error of one another. As demonstrated for Li+ salts, the identity of the anion had no significant effect on the rate constant for Na+ and K+ salts, suggesting that purely a cation effect is being measured. To confirm that the observed change in rate was in fact an effect of the LA rather than a change in the ionic strength of the solution, the rate of CO2 insertion into 1 was also measured in the presence of various amounts of (nBu)4NPF6 (see the Supporting Information). The rate constant attributed to the presence of (nBu)4N+ is significantly lower than those observed in the presence of Li+, Na+, or K+ cations (Table 1, entry 2), indicating that the presence of the LA (as opposed to the increase in ionic strength) is essential for the rate enhancement. Further support that the rate of CO2 insertion is increased with a LA was obtained when the neutral species B(OiPr)3 was used as an additive (Table 1, entry 15). In this case, the magnitude of the LA effect was on the same order as that measured for (nBu)4NPF6. However, as this additive does not increase the ionic strength of the solution, we attribute the small rate enhancement to the presence of the weakly Lewis acidic boron. The activation parameters for CO2 insertion in the absence of a LA as well as in the presence of the cationic LA LiNTf2 or the neutral LA B(OiPr)3 were determined to gain further insight into the role of the LA (Table 2). Due to decomposition that was observed when 1 was exposed to light over a wide range of wavelengths in acetonitrile (i.e., 10522

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50 equiv of LiNTf2. Interestingly, there is no kinetic isotope effect associated with kLA, as the ratio of kLA for insertion into the hydride and deuteride is 1.0 ± 0.1 in the presence of LiNTf2 (see the Supporting Information). The different isotope effects measured for the two pathways support the notion that the LA in some way alters the rate-determining TS of the assisted pathway as compared to the unassisted pathway, although it is important to note that there is a small overlap in the error between the two measured isotope effects. Table 1 represents the first time that trends in the effect of different LAs on the rate of CO2 insertion into a metal hydride have been elucidated. We were, however, limited to studying alkali metal cations in this work, as all di- and trivalent cationic LAs as well as several neutral boron-based LAs evaluated caused the decomposition of 1 prior to CO2 insertion (see the Supporting Information). We propose that the decomposition observed in many of these cases stems from a strong LA promoting ligand loss from the ruthenium complex, as the polypyridyl ligands interact more strongly with the Lewis acidic center than the ruthenium (Scheme 3). The chelation of polypyridyl ligands such as 2,2′-bpy or 1,10-phenanthroline to strongly Lewis acidic metal centers has been previously observed.16 To directly probe our hypothesis, we monitored the 1H NMR signals of 2,2′-bpy in acetonitrile-d3 as 1 equiv of either (nBu)4 NPF6 or Zn(OTf)2 was added (see the Supporting Information). In the control experiment with (nBu)4NPF6, no shift in the 2,2′-bpy peaks was seen, whereas in the presence of Zn(OTf)2, new sets of aromatic peaks were observed. This result confirms that divalent (and presumably more Lewis acidic trivalent) cations interact with the polypyridyl ligands, potentially causing the decomposition of 1 observed in the presence of strong LAs. The solvent may also be a factor in the observed decomposition. The ability of acetonitrile to act as a ligand perhaps facilitates the loss of the polypyridyl ligands from 1, as the solvent molecules are able to stabilize the now coordinatively unsaturated ruthenium center. Evidence for the formation of [Ru(NCMe)6][(PF6)2] in reactions between 1 and divalent and trivalent cationic LAs is provided in the Supporting Information. It should be noted, however, that a similar decomposition process is observed when 1 is treated with Zn(OTf)2 in iPrOH-d8. Solvent Dependence of Observed Lewis Acid Effect. As noted previously, the increased rate of CO2 insertion in the presence of a LA is proposed to stem from the stabilization of charge in the rate-determining TS. To investigate whether the solvent impacts the stabilization provided by the LA, we measured the rate of CO2 insertion into 1 in the presence of LiNTf2 in a variety of different solvents (Table 3 and Supporting Information). The salt LiNTf2 was chosen due to its stability and solubility in a wide range of solvents. To compare the LA effect across solvents, the rate of product

Table 2. Activation Parameters for CO2 Insertion into 1 in i PrOH entry

rate constanta

ΔH⧧ (kcal mol−1)

ΔS⧧ (cal mol−1 K−1)

ΔG⧧298K (kcal mol−1)

1 2 3

kNoLA kLA (LiNTf2) kLA (B(OiPr)3)

9.1 ± 0.3 10.2 ± 0.3 8.4 ± 0.4

−33 ± 2 −16 ± 2 −27 ± 2

18.9 ± 0.3 15.0 ± 0.3 16.5 ± 0.3

Units are M−1 s−1 for entry 1 and M−2 s−1 for entries 2 and 3.

a

when a diode array detector was used), the activation parameters were determined from data obtained in iPrOH using single wavelength fitting at 550 nm (see the Supporting Information and the following section for information on solvent effects). The activation parameters corresponding to kNoLA (Table 2, entry 1) are strikingly different from those measured for kLA in the presence of LiNTf2 (Table 2, entry 2). While the enthalpy of activation is slightly higher (∼1 kcal mol−1) for kLA, the entropy is significantly less negative for the Li+-assisted pathway. The difference in ΔS⧧ for the two pathways (17 cal mol−1 K−1) is approximately equal to the translational contribution of entropy for a molecule of iPrOH (see the Supporting Information for calculation).15 This suggests that the coordination number of the LA decreases from [(LA)(iPrOH)n] in the reactant state to [(LA)(iPrOH)n−1] in the rate-determining TS for the LA-assisted pathway (Scheme 2). The release of a solvent molecule not only sterically facilitates the interaction between the LA and the carboxylate group in the TS, but also offers an explanation for the observed trend in the entropy of activation for the two rate constants. Furthermore, the activation energies observed for B(OiPr)3 suggest that the effect of the LAs on the rate of insertion is not simply an artifact of solvent reorganization effects. For example, the difference in entropy between the Li+assisted and unassisted pathways could be interpreted as simply due to solvent reorganization in the ground state that occurs in the presence of LiNTf2, resulting in a more ordered ground state and consequently a smaller barrier of activation. The neutral LA B(OiPr)3, however, does not promote solvent reorganization in the same way that a charged species does, yet there is still a slight rate enhancement observed in the presence of B(OiPr)3. This observation supports our hypothesis that the role of the LA is to stabilize the rate-determining TS as opposed to destabilize the ground state. We also synthesized and performed kinetic studies with [Ru(tpy)(bpy)D]PF6 (1-d1). Comparing the rate of insertion into 1 and 1-d1 in the absence of a LA revealed an inverse kinetic isotope effect of 0.86 ± 0.09 in acetonitrile (see the Supporting Information). This is in agreement with the inverse isotope effects measured for CO2 insertion into other transition metal hydrides.6a,g We then measured the rate of insertion into 1 and 1-d1 in the presence of a LA, specifically

Scheme 2. Proposed Rate-Determining TS for (a) Unassisted and (b) LA-Assisted CO2 Insertion Pathway in iPrOH

10523

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Journal of the American Chemical Society Scheme 3. Proposed Decomposition of 1 in the Presence of Strong LAs

Table 3. Effect of Solvent on the LA Effect for CO2 Insertion into 1 in the Presence of LiNTf2 entry 1 2 3 4 5 6 7

solvent b

NMF DMSO MeOH EtOH acetone i PrOH acetonitrile

AN

kNoLA (M−1 s−1) at rt

kLA (M−2 s−1) at rt

ratio (kLA term/kNoLA term)a

32.1 19.3 41.3 37.1 12.5 33.5 18.9

1.2 ± 0.1 4.2(4) × 10−2 2.5 ± 0.3 3.4(3) × 10−1 2.8(3) × 10−3 9.7(10) × 10−2 1.0(1) × 10−2

3.9 ± 0.4 2.9(3) × 10−1 49 ± 5 23 ± 2 1.8(2) × 10−1 35 ± 4 4.3 ± 0.4

0.0065 0.013 0.039 0.12 0.17 0.58 1.1

kLA term is defined as the rate of product formation (M s−1) from the kLA term of the rate law under a standard set of conditions, while kNoLA term is defined as the rate of product formation (M s−1) from the kNoLA term of the rate law under the same conditions. This allows for comparison independent of molecularity. See the Supporting Information for calculations. bNMF = N-methylformamide. a

Figure 3. Relationship between the ratio of the rate of product formation (M s−1) from the kLA term and kNoLA term of the rate law for CO2 insertion into 1 (see Table 3) and the solvent AN12 for (a) all and (b) selected solvents.

formation (in M s−1) was calculated for both terms in the rate law under a standard set of conditions (see the Supporting Information), and the rate of product formation from the LAassisted pathway (the kLA[1][CO2][LA] term) was compared to that from the unassisted pathway (the kNoLA[1][CO2] term). This methodology corrects for the different molecularity of the two terms by comparing rates as opposed to rate constants and, furthermore, accounts for differences in the rate of CO2 insertion in different solvents in the absence of a LA (see the Supporting Information for more information), which is discussed in more detail in the next section. As shown in Table 3, the effect of the LA on the rate of insertion is heavily dependent on the solvent. For example, in NMF, the amount of product generated via the LA-assisted pathway is negligible as compared to that generated from the unassisted pathway. On the other hand, in acetonitrile, the two pathways produce 2 at approximately the same rate. Comparing the kLA term/kNoLA term ratio with different solvent parameters (Figure 3 and Supporting Information) provides insight into the interplay of these two effects. An inverse correlation between the solvent acceptor number (AN) and the kLA term/kNoLA term ratio is present for a specific subset of solvents: acetonitrile, iPrOH, EtOH, and MeOH (Figure 3b). This relationship is perhaps unsurprising, as AN is a measure of electrophilicity or Lewis acidity.12 The inverse correlation suggests that a solvent with a higher AN can better fulfill the role that a LA would play in the rate-determining TS, thus negating the effect of an additive. In

other words, CO2 insertion in a more Lewis acidic solvent exhibits a weaker effect from a LA additive. It is also important to note that the solvent is typically present in a concentration that is orders of magnitude higher than the additive. Thus, even if the solvent is the weaker LA (as compared to the cation present) in absolute terms, the vast difference in concentrations promotes TS stabilization by solvent molecules. If the inverse correlation described above proves general, it may have implications for the choice of reactions conditions and additives for processes such as CO2 hydrogenation or formic acid dehydrogenation. The correlation implies that even if a catalyst is prone to decomposition in solvents with high ANs, an enhancement in the rate of CO2 insertion (or decarboxylation) and potentially improved catalytic performance may be possible in solvents with lower ANs by adding a LA additive. Take, for example, the iridium complex (iPrPNHP)IrH3 discussed in the previous section. Because of the instability of the complex, CO2 insertion could not be studied in solvents with high ANs, and, as a result, changing the solvent was not an effective method to drastically accelerate this reaction.6g However, by using a LA additive in THF (a solvent with a low AN that, on the basis of the correlation observed above, should allow for a more dramatic LA effect), significant rate enhancements are obtained. While the inverse relationship presented above holds true for the alcoholic solvents and acetonitrile, the LA effect observed in acetone, DMSO, and NMF is not as large as predicted by 10524

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Figure 4. (a) Resonance structures for DMSO, acetone, and NMF in which the oxygen has a full negative charge. (b) Proposed O−···Li+ interactions that sequester the LA during CO2 insertion.

Figure 5. Plot of the observed rate constant kobs (s−1) versus [DMSO] showing an exponential decrease in kobs as more DMSO is titrated into solution, consistent with DMSO sequestering the Li+ and reducing the effective concentration of the LA. Reaction conditions: [1] = 0.1 mM, [CO2] = 130 mM, [LiNTf2] = 10 mM, [DMSO] ranges from 10 to 500 mM, room temperature.

the correlation. There is a common link between the three solvents that do not follow the trend. Specifically, they all have a resonance structure in which the oxygen has a full negative charge (Figure 4a). On the basis of this observation, we propose that certain solvents such as acetone and DMSO interact with the LA (here Li+) so strongly that the cations do not impact the rate of insertion as much as predicted under our model. Essentially, the Li+ is sequestered by solvent molecules via O−···Li+ interactions and thus does not stabilize the ratedetermining TS (Figure 4b). To directly probe this hypothesis, we monitored how the rate of CO2 insertion into 1 in the presence of LiNTf2 in acetonitrile changed as increasing amounts of DMSO (1, 2, 4, 8, and 50 equiv relative to [Li+]) were titrated into the solution. As predicted by our hypothesis, the observed rate of insertion decreased as more DMSO was added (Figure 5). In fact, at 50 equiv of DMSO relative to [Li+], the LA effect was completely shut down, and the observed rate of insertion matched that measured in the absence of a LA in acetonitrile. Although this experiment supports our hypothesis, the interaction between the solvent and the LA additive in the stabilization of the rate-determining TS is complex on a molecular level, and further work is undoubtedly needed to fully understand the nuances of this relationship. Solvent Effect in the Absence of a Lewis Acid. The solvent effect for CO2 insertion into 1 has been previously explored for a few solvents.6b−d Here, we sought to extend these studies and understand the effect of solvent for the full range of solvents investigated in this work. The measured rate constants for CO2 insertion into 1 in the absence of a LA in acetone, acetonitrile, EtOH, and MeOH match those previously reported in the literature.6b,d As shown in Table 4, the rate of CO2 insertion increases by almost 3 orders of magnitude between acetone and MeOH. Previous work has demonstrated that the rate of CO2 insertion into 1 in acetone, DMF, acetonitrile, EtOH, and MeOH correlates well with the AN of the solvent,6b,c and we have also shown that the rate of CO2 insertion into a different system, a pincer-supported nickel hydride, is similarly correlated to the solvent AN.6g AN is empirically determined on the basis of the 31P NMR shifts of triethylphosphine oxide in different solvents and, as noted in the previous section, represents a measure of the electrophilicity or Lewis acidity of a solvent.12 Upon broadening the scope of CO2 insertion into 1 to include nitromethane,

DMSO, iPrOH, and NMF, we however see that a different solvent parameter, specifically the Dimroth−Reichardt ET(30) value,11 is a better predictor of the rate of this reaction (Figure 6). The Dimroth−Reichardt ET(30) value of a solvent is defined as the molar electronic transition energy (in kcal mol−1 at 25 °C and 1 bar pressure) of the lowest energy intramolecular charge-transfer π−π* absorption band of a negatively solvatochromic pyridinium N-phenolate betaine dye in that particular solvent.11b This value has been measured for over 350 solvents as well as numerous binary solvent mixtures,11b making a scale based on the Dimroth−Reichardt ET(30) parameter among the most comprehensive and useful for solvent comparisons. As shown in Figure 6a, although the AN of the solvent provides a reasonable explanation for the solvent effect observed for CO2 insertion into 1, the rates measured in several solvents are significantly displaced from the regression line. For example, NMF has a lower AN than EtOH; however, the rate of CO2 insertion in NMF is more than 3 times faster than that measured in EtOH. Figure 6b shows that a model based on the Dimroth−Reichardt ET(30) value accounts for this trend and better predicts the rate of CO2 insertion into 1 in the solvents investigated in this work. It is particularly noteworthy to look at the linear relationship between the rate of insertion and ET(30) value for the hydrogen-bond donor (HBD) solvents (i.e., all solvents in this work with the exception of acetone and DMSO). Kamlet et al. previously demonstrated that the ANs for non-HBD or very weak HBD solvents can be linearly correlated with the solvatochromatic parameter π* developed by Kamlet, Abboud, and Taft.17 This relationship, however, does not hold for solvents that can hydrogen bond with triethylphosphine oxide (used in the determination of AN), as these data points are significantly displaced from the regression line.17 Furthermore, it has been proposed that this deviation is proportional to the perturbation energy caused by hydrogen bonding between the solvent and the oxygen of triethylphosphine oxide.18 Specifically, such a hydrogen bond leads to less electron density on the phosphorus atom and consequently a more downfield 31P NMR shift and a higher AN value. The empirical manner in which AN is determined, therefore, does not differentiate between the capability of a solvent to accept electron density or act as a hydrogen-bond donor. On the basis of this, we hypothesize that while the AN of the solvent 10525

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Journal of the American Chemical Society Table 4. Effect of Solvent on the Rate of CO2 Insertion into 1 in the Absence of a LA entry

solvent

ET(30) (kcal mol−1)

AN

kNoLA (M−1 s−1) at rt

rate increase

1 2 3 4 5 6 7 8

acetone acetonitrile nitromethane DMSO i PrOH EtOH NMF MeOH

42.2 45.6 46.3 45.1 48.4 51.9 54.1 55.4

12.5 18.9 20.5 19.3 33.5 37.1 32.1 41.3

2.8(3) × 10−3 1.0(1) × 10−2 3.8(4) × 10−2 4.2(4) × 10−2 9.7(10) × 10−2 3.4(3) × 10−1 1.2 ± 0.1 2.5 ± 0.3

3.6 14 15 35 120 430 890

Figure 6. Relationship between the second-order rate constant k1 for CO2 insertion into 1 in various solvents and the (a) AN12 or (b) Dimroth− Reichardt ET(30) value of the solvent.11

Scheme 4. CO2 Insertion into (tBuPCP)Ni(OH)

predicts the rate of CO2 insertion into transition metal hydrides well for non-HBD solvents (where specific solute/ solvent interactions on a microscopic level are negligible), a parameter such as ET(30) that considers a solvent as a discontinuum of individual molecules may better represent solute/solvent interactions in HBD solvents and, therefore, better predict interactions between the rate-determining TS and the solvent in the reaction of interest here. To investigate the generality of this hypothesis, we can apply our model to another system in which solute/solvent interactions are potentially important, specifically a pincersupported nickel hydroxide that can in principle participate in hydrogen bonding. The insertion of CO2 into (tBuPCP)Ni(OH) (tBuPCP = 2,6-C6H3(CH2PtBu2)2) has been studied in several solvents (Scheme 4), and it has been previously demonstrated that certain parameters such as AN or pKHB (a measure of the hydrogen-bond acceptor strength of a solvent)19 can predict the rate of this reaction reasonably well for a selected group of solvents.20 A number of outliers are, however, present in both models and prevent any definitive conclusions from being drawn. Re-evaluating the results using our current hypothesis shows that a model based on the Dimroth−Reichardt ET(30) value of the solvent best represents the entire body of data (Figure 7), which provides additional support to our current hypothesis. In general, we recommend that researchers optimize the solvent for CO2 insertion (or decarboxylation) by considering the Dimroth− Reichardt ET(30) value rather than AN, especially when using

Figure 7. Relationship between the second-order rate constant k1 for CO2 insertion into (tBuPCP)Ni(OH) in various solvents and the Dimroth−Reichardt ET(30) value of the solvent.11 For comparison, the coefficient of determination for the correlation between k1 and the AN or pKHB of the solvent is 0.7289 and 0.2829, respectively. Data are from ref 20.

HBD solvents or solvents/solutes in which specific interactions on the microscopic level govern their behavior.

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CONCLUSIONS In this work, we have used stopped-flow kinetics to determine both the solvent and the LA effect for CO2 insertion into a cationic ruthenium hydride. We have provided the first quantitative evidence that smaller, more Lewis acidic cations offer more stabilization to the rate-determining TS than do larger, less acidic ones. The activation parameters as well as the kinetic isotope effect measured here support our hypothesis that the LA stabilizes the incipient negative charge on the carboxylate group and thus directly impacts the ratedetermining TS of the assisted pathway as compared to the unassisted pathway. We have also explored the solvent dependence of the LA effect and demonstrated that CO2 insertion in a more Lewis acidic solvent (higher AN) exhibits a weaker effect from a LA additive as long as the solvent does 10526

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Journal of the American Chemical Society not sequester the LA via O−···LA+ interactions. Last, on the basis of the rates measured in eight different solvents, we propose a new model, one based on the Dimroth−Reichardt ET(30) parameter, for predicting the rate of CO2 insertion into a transition metal hydride, especially in HBD solvents where specific solute/solvent interactions are not negligible. The results presented here provide insight into how to optimize catalytic reactions involving CO2 insertion into a transition metal hydride or the microscopic reverse decarboxylation reaction as an elementary step. Specifically, they suggest that for reactions performed in solvents with low ANs, the rate of CO2 insertion can be increased by using a LA additive, with smaller, more highly charged LAs such as Li+ being the most effective. For reactions performed in solvents with high ANs, the rate of CO2 insertion likely cannot be enhanced by using a LA, but can be increased by moving to a solvent with a higher Dimroth−Reichardt ET(30) value. Ongoing research in our group aims to explore and apply these guiding principles to improve catalytic reactions.

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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/jacs.9b05192. Full experimental procedures, kinetics data, and additional evidence for the decomposition of [Ru(tpy)(bpy)H]PF6 (1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Wesley H. Bernskoetter: 0000-0003-0738-5946 Nilay Hazari: 0000-0001-8337-198X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.H. and W.H.B. acknowledge support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Catalysis Science Program, under award DESC0018222. We also thank Dr. James Mayer for allowing us to use the stopped-flow instruments in his laboratory and Nicholas E. Smith for his assistance with collecting and interpreting the crystallographic data.

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REFERENCES

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