Carbon Dioxide Insertion into Group 9 and 10 MetalâElement Ï Bonds

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Carbon Dioxide Insertion into Group 9 and 10 Metal−Element σ Bonds Nilay Hazari* and Jessica E. Heimann Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States ABSTRACT: Carbon dioxide (CO2) is an appealing feedstock for the sustainable preparation of a variety of carbon-based commodity chemicals because of its high abundance, low cost, and nontoxicity. The high kinetic and thermodynamic stability of CO2, however, means that there are currently only a limited number of practical catalytic systems for the conversion of CO2 into more valuable chemicals, and continued research in this area is required. One promising approach for the eventual transformation of CO2 is to initially insert the molecule into transition-metal−element σ bonds such as M−H, M−OR, M−NR2, and M−CR3 bonds to form products of the type M−OC(O)E (E = H, OR, NR2, or CR3). CO2 insertion has been demonstrated in numerous stoichiometric reactions involving transition-metal complexes, but in cases where insertion results in the formation of strong M−O bonds, the products are often too stable to undergo further transformations. Group 9 and 10 transition-metal complexes (M = Ni, Pd, Pt, Co, Rh, or Ir) form relatively weak M−O bonds, and as a consequence, a number of group 9 and 10 transition-metal catalysts in which CO2 insertion is proposed as an elementary step in catalysis have been developed. In this Award Article, we summarize group 9 and 10 transition-metal complexes in which CO2 insertion into a metal−element σ bond to form a M−OC(O)E-type product has been observed. Mechanistic similarities and differences are highlighted by comparing CO2 insertion reactions in different types of group 9 and 10 metal−element σ bonds, and a general trend for predicting the ratedetermining step of the insertion process is described based on the nucleophilicity of the element in the σ bond. Although we focus on stoichiometric reactivity, the relevance of CO2 insertion to catalytic reactions is also emphasized throughout the paper.

1. INTRODUCTION Carbon dioxide (CO2) is an attractive target as a chemical feedstock because of its low cost, nontoxic nature, and relatively high abundance.1−10 The most significant industrial-scale chemicals currently prepared from CO2 are urea, salicylic acid, inorganic carbonates, and polycarbonates, which together utilize only a small fraction of annual anthropogenic CO2 production.1 Additionally, CO2 is relatively rare as a reagent in synthetic chemistry.11−16 Full exploitation of CO2 as a feedstock is limited by its kinetic and thermodynamic stability, which poses a major scientific challenge for developing CO2-based synthetic routes to more valuable chemicals. Steady but slow progress on an academic level has been made in overcoming these problems through the development of catalysts for photo-, electro-, and thermochemical CO2 reduction into products such as carbon monoxide (CO), formic acid, methanol (MeOH), fuels, and acrylic acid.1−10 However, in general, the yields, selectivities, and turnover numbers of these reactions are significantly lower than those required for industrial use, and as a consequence, very few systems with practical applications have been developed. Furthermore, reaction mechanisms are often poorly understood, which suggests that fundamental studies exploring the reactivity of CO2 could provide guidelines for the design of new systems that facilitate the conversion of this ubiquitous gas to value-added chemicals. One promising approach for the conversion of CO2 into valueadded chemicals is the use of transition-metal catalysts.1−10 CO2 © XXXX American Chemical Society

can bind to many different transition-metal centers, and in some cases, coordination of CO2 weakens the strong CO double bonds and facilitates transformation.17−19 This strategy, however, often requires the use of harsh reagents after CO2 coordination and is thus unlikely to lead to widespread use in catalytic reactions. Another method for the activation and subsequent transformation of CO2 involves the insertion of CO2 into metal−element σ bonds (e.g., M−H, M−OR, M−NR2, and M−CR3), which is generally controlled by thermodynamic factors (eq 1).17,18

In particular, CO2 insertion into late-transition-metal−element σ bonds is attractive because of the relative weakness of the M−O bonds that are formed, making further reactions at the metal more facile. For example, comparing the bond dissociation energies for the Ni−H bond in a representative nickel hydride and the Ni−O bond in the corresponding nickel formate that forms after CO2 insertion indicates that the enthalpic driving force for this reaction is on the order of 30 kcal mol−1.20 In contrast, insertion of CO2 into the Ti−H bond of an earlytransition-metal hydride to generate a Ti−O bond in the Received: September 8, 2017

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DOI: 10.1021/acs.inorgchem.7b02315 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Examples of high-value products that are potentially accessible through catalytic cycles involving CO2 insertion into a metal−element σ bond.

2. CO2 INSERTION INTO METAL HYDRIDE BONDS The insertion of CO2 into late-transition-metal hydrides has been more extensively studied than CO2 insertion into any other type of metal−element σ bond. This is likely due to the fact that CO2 insertion into a metal hydride is proposed to be a crucial elementary step in catalytic cycles for the hydrogenation of CO2 to formic acid or MeOH and the hydrosilylation or hydroboration of CO2.9,23−25 Additionally, the microscopic reverse reactiondecarboxylation of a metal formate to form a metal hydride and CO2is postulated to be an important reaction in the catalytic dehydrogenation of formic acid and MeOH.26 To date, all stoichiometric insertion reactions between CO2 and metal hydrides have led to the formation of metal formates [M−OC(O)H], and there are no known examples of insertion to form the hydroxycarbonyl [M−C(O)OH] isomer even though the microscopic reverse of this processdecarboxylation of a hydroxycarbonylis proposed to be a key step in the water−gas shift reaction (CO + H2O → CO2 + H2).27 Based predominantly on computational studies, two general mechanisms have been proposed for CO2 insertion into latetransition-metal hydrides.28−35 The most common pathway involves two steps: (i) nucleophilic attack of the metal hydride on CO2 to form an H-bound formate and (ii) rearrangement of the H-bound formate to generate the observed O-bound formate product (Scheme 1a). Throughout this paper, we have adopted the convention that reactions in which the first step is ratedetermining are considered to be outer-sphere because there is no direct interaction between CO2 and the metal center in this transition state. On the other hand, reactions in which the second step is rate-determining are inner-sphere.36 The second common mechanism involves a single-step concerted pathway (Scheme 1b). The transition state for this pathway resembles the transition state for the second step in stepwise CO2 insertion: rearrangement of the H-bound formate to the O-bound formate product. Thus, the single-step concerted pathway is also referred to as an inner-sphere process. However, in some cases where a concerted pathway has been proposed, it is not clear if a stepwise pathway was computationally explored, and it is plausible that these reactions

CO2 insertion product has a much larger driving force of 110 kcal mol−1,20 highlighting the appeal of using late transition metals for CO2 conversion chemistry.21 The insertion of CO2 into late-transition-metal−element σ bonds is proposed to be a crucial reaction in several academic examples of catalytic CO2 conversion. For example, CO2 insertion into late-transition-metal hydride bonds is postulated to be a key step in the catalytic cycles for CO2 hydrogenation to formic acid and MeOH, which are valuable commodity chemicals (Figure 1).9 Similarly, CO2 insertion into M−O bonds is important in the synthesis of polycarbonates from CO2 and epoxides,22 and insertion into M−C bonds is important in the synthesis of carboxylic acids from CO2 and aryl halides or pseudohalides.16 The development of efficient and stable catalysts for these “holy grail” reactions would likely result in improved synthetic routes compared to those practiced at present; however, a large amount of further research is required to achieve this goal. Given the central role of CO2 insertion in the proposed catalytic cycles, understanding this elementary reaction could have a potentially transformative impact on both the pharmaceutical and commodity chemical industries. In this Award Article, we review examples of stoichiometric CO2 insertion reactions into group 9 and 10 metal−element σ bonds (M−E, where M = Ni, Pd, Pt, Co, Rh, or Ir and E = H, OR, NR2, or CR3). Mechanistic similarities and differences as a function of the metal−element σ bond are discussed, with particular emphasis placed on understanding the differences between inner-sphere and outer-sphere pathways, as described below. The paper is divided into sections based on the type of metal− element σ bond into which CO2 insertion occurs, with a concluding section that not only describes a trend for predicting the ratedetermining step of the insertion process based on the nucleophilicity of the element in the σ bond but also highlights areas where we suggest further work could be valuable. Although stoichiometric CO2 insertion is the focus of this paper, the relevance of these reactions to catalysis is also highlighted, and some of the general mechanistic trends we outline should provide guidance for the optimization of catalytic reactions involving CO2 insertion. B


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Scheme 1. Common Mechanisms for CO2 Insertion into Metal Hydrides Proposed on the Basis of Computational Studies

t

t

a benzene solution of ( BuPNPyrP)Co(H) [4; BuPNPyrP = 2,6-NC5H3(CH2PtBu2)2]40 generates the formate complex

also involve a stepwise mechanism. Guided by the computational studies performed to date, it appears that coordinatively unsaturated complexes insert CO2 via a concerted or stepwise inner-sphere process, whereas coordinatively saturated complexes insert via an outer-sphere process. Nevertheless, further work is still required to establish the generality of this observation, and as we describe below, there are examples in which other pathways for CO2 insertion into metal hydrides have been proposed. 2.1. Cobalt. The first example of CO2 insertion into a transitionmetal hydride was reported in 1968. Ikeda and co-workers demonstrated that treating a benzene or tetrahydrofuran (THF) solution of (PPh3)3Co(N2)(H) (1a) and excess PPh3 with CO2 at room temperature generates the formate compound (PPh3)3Co{OC(O)H} (2a; eq 2).37 Compound 2a also forms

t

( BuPNPyrP)Co{OC(O)H} (5; eq 4).41 Furthermore, while

exploring the catalytic hydrogenation of CO2 to formate, Linehan et al. reported that, at −40 °C, 1H NMR spectroscopy indicates that when (dmpe)2Co(H) (6; dmpe = Me2PCH2CH2PMe2) is placed under a mixture of CO2 and H2, CO2 initially inserts into 6 to form (dmpe)2Co{OC(O)H} (7).42 When the system is warmed to room temperature, 7 reacts with H2 to generate [(dmpe)2Co(H)2]+ (8) and formate (Scheme 2). Using density functional theory (DFT), Appel and co-workers investigated CO2 insertion into 6 and reported that the kinetic barrier for a novel associative mechanism is slightly lower (1.6 kcal mol−1) than the barrier for the traditional stepwise outer-sphere pathway.43 The similar barriers calculated for the two pathways suggest that both mechanisms are likely operative. In the associative pathway, CO2 initially binds to the metal through the carbon atom, which causes an increase in the formal oxidation state from cobalt(I) to cobalt(III) (Scheme 3). Subsequent intramolecular hydride transfer from the cobalt center to the carbon atom of CO2 reduces the metal center and generates an H-bound formate. Finally, there is a low-energy pathway for rearrangement of the H-bound formate to the O-bound formate product. It is possible that this new associative mechanism is also relevant for the insertion of CO2 into complexes such as 1a, where ligand loss is proposed to facilitate CO2 insertion. With the goal of ultimately designing improved catalysts for CO2 hydrogenation to formate, several computational studies have investigated CO2 insertion into cobalt hydrides. Yang and co-workers explored a series of cobalt complexes supported with acylmethylpyridinol ligands (Figure 2a).44 The turnover-limiting step in their proposed catalytic cycle is CO2 insertion, which was calculated to proceed via a stepwise outer-sphere pathway. Similarly, Ye and co-workers studied a series of complexes of the form [(XP3)Co(H)2]n+ [XP3 = X(2-PPh2-C6H4)3; X = P, n = 1; X = C− or Si−, n = 0], where X was modeled as either a neutral phosphorus donor or an anionic carbon or silicon donor (Figure 2b).45 For all three complexes, the turnover-limiting step in a hypothetical catalytic cycle for CO2 hydrogenation is the first step of CO2 insertion via a stepwise outer-sphere pathway, often referred to as hydride delivery. Compared to the complex with the neutral phosphorus donor, the presence of an anionic carbon or silicon donor significantly reduces the barrier for this process,

through the reaction of (PPh3)3Co(H)3 (3a) with CO2 in the presence of excess PPh3 (eq 3).38 In both cases, excess PPh3 was needed to limit the formation of dinuclear side products of the form [(PPh3)3Co(CO)]x (in which the authors propose that x is most likely 2, but these byproducts were not fully characterized). Subsequently, Gallo et al. demonstrated that the related compounds (L)3Co(N2)(H) [L = PPhEt2 (1b) or PPh2Et (1c)] and (L)3Co(H)3 [L = PPhEt2 (3b) or PPh2Et (3c)] undergo analogous CO2 insertion reactions.39 They also showed that there is no reaction between CO2 and the complexes (L)4Co(H) [L = P(OMe)3, P(OEt)3, P(OPh)3, P(OnBu)3, P(OPh)2Ph, or P(OEt)2Ph], even at elevated temperature.39 On the basis of these results, the authors suggested that the first step in CO2 insertion with 1a−1c is dissociation of N2 and coordination of CO2. It was not until 2014 that another example of CO2 insertion into a cobalt hydride was reported. As part of a study investigating hydrosilylation of CO2 with PhSiH3, Chirik and co-workers showed that the addition of 1−3 equiv of CO2 to C


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Scheme 2. CO2 Insertion into 6 To Form 7, Which Releases Formate When Warmed to Room Temperature in the Presence of H2

2.2. Rhodium. In 1972, Komiya and Yamamoto explored the reaction of (PPh3)4Rh(H) with CO2.46 Although a metal formate product was expected based on analogous chemistry with (PPh3)4Ru(H)2,46 an ill-defined dimeric complex proposed to have CO2 coordinated to the rhodium centers forms instead. The observation of coordination rather than insertion is a rare example of a late-transition-metal hydride undergoing a reaction other than insertion with CO2 but remains speculative given the lack of detailed characterization. The first example of CO2 insertion into a Rh−H bond was not reported until 1988 when Kaska and co-workers showed that the reaction of t t ( BuPCP)Rh(H2) [9a; BuPCP = 2,6-C6H3(CH2PtBu2)2] with t CO2 generates ( BuPCP)Rh(H)(κ2-O2CH) (10) as the kinetic product (Scheme 4a).47 The formally rhodium(I) complex 9a ist proposed to be in equilibrium with the rhodium(III) dihydride ( BuPCP)Rh(H)2 (9b), which undergoes CO2 insertion. t Complex 10 is unstable and decomposes to ( BuPCP)Rh(H)(OH) (11) and CO. Under certain conditions such as high pressures of CO2 or long reaction times in alkane solvents, there is a further reaction of 11 with CO (generated in the formation t of 11) to form ( BuPCP)Rh(CO) (12) and H2O. In contrast, int 19963 Milstein et al. showed that insertion of CO2 into t 3 ( BuPCsp P)Rh(H2) [13; BuPCsp P = HC(CH2CH2PtBu2)2] t 3 quantitatively forms ( BuPCsp P)Rh(H)(κ2-O2CH) (14), which does not decompose or react further (Scheme 4b). 48 The difference in reactivity between 10 and 14 was computationally investigated by Fujita et al., who suggested that the formation of a Rh−CO species could only occur after the initial reductive elimination of formic acid from the metal formate, followed by decarbonylation of the released formic acid by rhodium(I).49 They examined the barriers for the reductive elimination of formic acid using DFT and concluded that the t 3 saturated BuPCsp P ligand increases the electron density at the rhodium center and consequently disfavors reductive elimination, making 14 less prone to further reactions.

Scheme 3. Associative Pathway Proposed by Appel et al. for CO2 Insertion into 6

Figure 2. (a) (Acylmethylpyridinol)cobalt complexes studied by Yang et al. (b) [(XP3)Co(H)2]n+ [XP3 = X(2-PPh2-C6H4)3; X = P, n = 1; X = C− or Si−, n = 0] complexes studied by Ye et al.

most likely because of the stronger trans influence of C− and Si− compared to phosphorus and the increased electron density on the metal with the anionic donors. These results suggest that lowering the barrier for CO2 insertion into cobalt hydrides is crucial for designing improved cobalt systems for CO2 hydrogenation to formate, and research focused on modifying ancillary ligands or optimizing conditions to achieve this goal is likely to be valuable.

Scheme 4. (a) CO2 Insertion into 9 and Subsequent Decomposition and (b) CO2 Insertion into 13

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In the first examples of CO2 insertion into cationic metal hydrides, Tsai and Nicholas reported that [(PMe2Ph)3Rh(solv)(H)2]BF4 [solv = THF (15a) or H2O (15b)] reacts with CO2 in wet THF to form a mixture of [(PMe2Ph)2Rh(solv)(H) (κ2-O2CH)]BF4 (16) and another species that could not be unambiguously identified but was suggested to be either [(PMe 2 Ph) 3 Rh(solv)(H){OC(O)H}]BF 4 (1 7a) or [(PMe2Ph)2Rh(solv)2(H){OC(O)H}]BF4 (17b) (eq 5).50 The κ1-formate product 17a or 17b is only detectable at CO2 pressures greater than approximately 14 atm. Insertion into the Rh−H bond of 15b is faster than insertion into the corresponding bond in 15a. The same products 16 and 17a or 17b also form upon exposure of 15a or 15b to CO2 in dry THF, albeit at a much slower rate. On the basis of the accelerating effect seen with H2O, the authors propose that a hydrogen bond between a coordinated H2O molecule and one of the oxygen atoms of CO2 stabilizes the transition state. Leitner et al. also explored the insertion of CO2 into phosphine-supported rhodium hydrides, specifically {Ph2P(CH2)3PPh2}2Rh(H) (18) and {Ph2P(CH2)2PPh2}2Rh(H).51 Whereas 18 quantitatively converts to [{Ph2P(CH2)3PPh2}2Rh]HCO2 (19) in under 30 min at room temperature (1 atm of CO2 in dimethyl sulfoxide; eq 6), no

ancillary ligands, three computational studies have investigated CO2 insertion into rhodium hydrides. Leitner et al. used (PH3)2Rh(H) as a model system to study CO2 insertion as part of the catalytic cycle for the hydrogenation of CO2 to formic acid.53 The computations indicated that CO2 insertion into the gas phase is rapid and has a low barrier. The insertion pathway and rate-determining transition state proposed by Leitner et al. are similar to those found by Musashi and Sakaki for the insertion of CO2 into the two different isomers of cis-[(PH3)3Rh(H)2]+ (23a and 23b) and cis-[(PH3)2Rh(H2O)(H)2]+ (24) to form κ2-formate complexes (Scheme 6).54 The reaction pathway proposed involves the initial coordination of CO2 to the vacant coordination site followed by nucleophilic attack of the M−H σ bond on CO2 and rearrangement of the formate ligand. The nucleophilic attack on CO2 is proposed to be the ratedetermining step in the process. The activation energy for the reaction was compared when the ligand trans to the vacant site was changed from hydrogen to PH3 to OH2. These studies indicate that insertion into the RhIII−H bond to generate a κ2-formate has the highest activation barrier of 53.8 kcal mol−1 when a second hydride is trans to the vacant site. The barrier is lowered to 41.7 and 24.0 kcal mol−1 when the trans hydride is replaced with PH3 and OH2, respectively, suggesting that the transition state for the insertion is more stable in the latter cases because of the lower trans influence of PH3 and OH2 compared to hydrogen. Subsequently, Ni and Dang explored the electronic effect of substituents of a diphosphine ligand on the catalytic efficiency of the rhodium-based catalyst [(Me2PCH2XCH2PMe2)2Rh]+ (X = CH2, NMe, or CF2; Figure 3).55 They reported that, as part of the catalytic cycle for CO2 hydrogenation, CO2 inserts into a Rh−H bond by either (i) a direct outer-sphere hydride transfer from the metal to the carbon or (ii) an associative pathway where CO2 first coordinates to the rhodium center and is then nucleophilically attacked by the M−H σ bond, with both pathways being within a few kilocalories per mole of each other regardless of the identity of X. These computational studies suggest that there is more variation in the pathway for CO2 insertion into rhodium hydride bonds compared to other late-transition-metal hydrides and that the general mechanisms shown in Scheme 1 are less likely to apply. As a consequence, different trends in the ligand and solvent effects on the rate of CO2 insertion are likely, and optimization of catalytic reactions involving CO2 insertion may require different conditions. 2.3. Iridium. The insertion of CO2 into Ir−H bonds has received significant attention in the past 10 years, primarily as a result of Nozaki and co-workers’ seminal report that (iPrPNPyrP)Ir(H)3 [25; iPrPNPyrP = 2,6-NC5H3(CH2PiPr2)2] is a highly active catalyst for the hydrogenation of CO2 to formate.56 Under 1 atm of CO2 at room temperature, 25 is in equilibrium with the dihydrido formate species (iPrPNPyrP)Ir(H)2{OC(O)H} (26; Scheme 7a). CO2 insertion is proposed to be the first step in catalysis. In a computational study using a model iridium trihydride, (MePNPyrP)Ir(H)3 [MePNPyrP = 2,6-NC5H3(CH2PMe2)2], our group showed that the insertion of CO2 into one of the two trans hydrides is thermodynamically unfavorable by 3.1 kcal mol−1,30 consistent with the equilibrium reported by Nozaki et al. In general, our calculations indicated that CO2 insertion into coordinatively saturated iridium hydrides is thermodynamically unfavorable, although CO2 insertion could be made more thermodynamically favorable by modifying the ligand trans to the hydride. Specifically, a stronger trans influence ligand both weakens the Ir−H bond and increases the nucleophilicity of the

formate forms upon treatment of {Ph2P(CH2)2PPh2}2Rh(H) with CO2. In 2016, Milstein et al. probed the reactivity of CO2 t with the pincer-supported rhodium hydride ( BuPNPyrP)Rh(H) (20), in which the anionic RPCP ligand of 9 studied by Kaska and co-workers was replaced with a neutral RPNP ligand.52 They found that 20 reacts instantly and reversibly with 1 equiv or more t of CO2 in THF at room temperature to generate ( BuPNPyrP)1 Rh{OC(O)H} (21). This κ -formate complex is not stable in solution, however, and irreversibly forms a dearomatized Rh−CO complex 22 within 5 h at room temperature (Scheme 5). Scheme 5. Reductive Cleavage of CO2 Following Insertion into 20

To thoroughly examine the effect of changing specific variables such as the ligand trans to the hydride or the substituents on the E


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Scheme 6. Proposed Pathway for CO2 Insertion into the Two Isomers of 23a and 23b and 24

4.9 kcal mol−1, whereas insertion into Hb (the hydride that is anti to the ligand N−H) is uphill by 2.8 kcal mol−1 because hydrogen bonding is no longer possible. The reaction proceeds via an outer-sphere pathway. In 2015, Ahlquist et al. revisited the mechanism of CO2 hydrogenation using Nozaki’s catalyst with a focus on investigating the possibility of a second CO2 insertion into an Ir−H bond as part of the catalytic reaction.33 The authors proposed that, after the first CO2 insertion and the subsequent loss of formate, the catalyst reacts with hydroxide present under catalytic conditions to generate a six-coordinate dihydridohydroxoiridium species, 29, which then undergoes a second CO2 insertion into one of the trans Ir−H bonds (Scheme 8). DFT calculations performed in this work suggest not only that the insertion of a second CO2 molecule is energetically favorable but also that modification of the equatorial ligand trans to the pincer nitrogen atom to a more σ- or π-donating ligand decreases the activation energy of CO2 insertion. However, calculations exploring the relative favorability of insertion into the hydroxide ligand compared with the hydride were not performed, and there is no experimental support for this unusual pathway at this stage. As part of the development of electrocatalysts for the reduction of CO2 to formate, Brookhart and co-workers explored the insertion of CO2 into the five-coordinate iridium dihydrides

Figure 3. Rhodium-based CO2 hydrogenation catalysts studied by Ni and Dang.

Scheme 7. (a) Observed Equilibrium between 25 and 26 and (b) CO2 Insertion into the Ir−Ha Bond of 27

t

t

t

( BuPCP)Ir(H)2 (30a) and ( BuPOCOP)Ir(H)2 [30b; BuPOCOP = 2,6-C6H3(OPtBu2)2] in THF.57 Insertion into the Ir−H bond rapidly generates the corresponding κ2-formate complexes 31a and 31b, respectively (eq 7), which are stabilized by chelation of

hydride, making the insertion of CO2 more thermodynamically and kinetically favorable. Nevertheless, in the first example of CO2 insertion into an Ir−H bond that led to an isolable formate product, a secondary coordination sphere interaction was used to selectively stabilize the product. The coordinatively saturated complex (iPrPNHP)Ir(H)3 [27; iPrPNHP = HN(C2H4PiPr2)2] inserts CO2 in less than 5 min at room temperature to generate the stable formate product (iPrPNHP)Ir(H)2{OC(O)H} (28; Scheme 7b). The product is significantly stabilized by a N−H···O hydrogen bond between the ligand and the coordinated formate (Scheme 7b). DFT calculations revealed that CO2 insertion into Ha (the hydride that is syn to the ligand N−H) is downhill by

the formate moiety. The formate ligand cannot bind in a κ2 manner in CO2 insertion reactions into six-coordinate iridium trihydride complexes because of the lack of open coordination

Scheme 8. Selected Proposed Elementary Steps in CO2 Hydrogenation to Formate Including Two CO2 Insertion Reactions As Calculated by Ahlquist et al.

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bipyridine interacts with an oxygen atom from the CO2 molecule (Scheme 9)are both relatively low energy. However, the

sites, which may explain why CO2 insertion into six-coordinate systems is generally more difficult thermodynamically.30 Milstein et al. studied the reactivity of a 7:3 equilibrium mixture of t the four-coordinate monohydride ( BuPNPyrP)Ir(H) (32) and t t five-coordinate dihydride ( BuPNPyrP*)Ir(H)2 (33; BuPNPyrP* = deprotonated

t

Scheme 9. Ligand-Assisted Mechanism for CO2 Insertion Proposed by Zhao et al.a

PNPyrP ligand) with CO2 (Figure 4).58 In this

Bu

a

Figure 4. Complexes 32 and 33 studied by Milstein et al.

The ligand is depicted as partially deprotonated because the reaction is generally performed under basic conditions.

case, no formate products consistent of the CO2 insertion form, t and only the four-coordinate iridium carbonyl ( BuPNPyrP)Ir(CO) is generated. A relatively complex mechanism involving the initial binding of CO2 to the iridium center in 32 was proposed. We compared the mechanism of CO2 insertion for four-, five-, and six-coordinate iridium hydrides computationally.32 DFT calculations show that CO2 insertion into the model fivecoordinate iridium dihydride (MePCP)Ir(H)2 [34; MePCP = 2,6-C6H3(CH2PMe2)2; Figure 5a] proceeds via the following

ligand-assisted mechanism is preferred. In general, ligandassisted pathways for CO2 insertion have not been explored in detail, and this may represent an efficient strategy for promoting insertion when high kinetic barriers are involved. 2.4. Nickel. In 1987, Donald and Marcetta Darensbourg and co-workers reported the first example of CO2 insertion into a nickel hydride.60 They demonstrated that trans-(PCy3)2Ni(Ph)(H) (36) rapidly inserts CO2 at low temperature to form trans(PCy3)2Ni(Ph){OC(O)H} (37; eq 8). In contrast, the reaction

of the related compound trans-(PCy3)2Ni(Me)(H) and CO2 generates a mixture of compounds, one of which is postulated to be trans-(PCy3)2Ni(Me){OC(O)H}, although this product was not fully characterized. The difference in the reactivity between 36 and trans-(PCy3)2Ni(Me)(H) is most likely related to the instability of trans-(PCy3)2Ni(Me)(H) as opposed to intrinsic differences in their reactivity with CO2. In related work, Marcetta Darensbourg et al. showed that a family of complexes of the form trans-(PCy3)2Ni(X)(H) [X = SPh, S(p-tolyl), OC(O)Me, OC(O)H, OC(O)Ph, OC(O)CF3, or OPh] with different X-type ligands trans to the hydride do not react with CO2.61 This lack of reactivity suggests that the ligand trans to the hydride needs to be a strong trans donor such as an alkyl or aryl ligand to promote CO2 insertion into the nickel hydride. A number of nickel hydrides supported by pincer ligands (38−49) insert CO2 to generate formate products (Figure 6).29,31,62−69 Although no quantitative kinetic experiments directly comparing the systems have been performed, the rate of insertion into nickel hydrides supported by RPNP ligands (38−41) appears to be slower than the corresponding reactions with systems supported by RPCP (42 and 43), RPOCOP (44−47), RPSiP (48), or RPBP (49) systems. For example, the reaction between 38 and 1 atm of CO2 in C6D6 takes 10 days at room temperature to fully form the formate product.64 In contrast, the reactions of 42−49 reach complete conversion in minutes at room temperature.29,31,62,63,65,67,69 Preliminary investigations suggest that there is a solvent effect on the rate of insertion, with more polar solvents such as acetonitrile (ACN) resulting in a faster reaction compared to less polar solvents such as benzene.64 Computational studies comparing CO2 insertion as a function of the pincer ligand indicate that the reaction becomes more

Figure 5. Model complexes (a) 34 and (b) 35 studied by Bernskoetter and Hazari and (c) half-sandwich complexes studied by Zhao et al.

four-step mechanism: (i) CO2 weakly coordinates to the iridium center through an oxygen atom, forming an octahedral intermediate; (ii) the hydride cis to the weakly bound CO2 interacts with the carbon to form the rate-determining fourcentered transition state; (iii) a κ1-formate is generated; (iv) the product rearranges to give the lower-energy κ2-formate. The model four-coordinate iridium(I) monohydride (CNPyrC)Ir(H) [35; CNPyrC = 2,6-NC5H3(NCH2CH2N(Me)C)2; Figure 5b], on the other hand, inserts CO2 via a concerted mechanism with a single four-centered transition state to form the preferred κ1-formate product. Notably, the pathways proposed for both the five-coordinate dihydride and four-coordinate monohydride are different from the traditional outer-sphere pathway proposed for six-coordinate trihydrides (Scheme 1a). In another computational study, Zhao et al. explored the insertion of CO2 into half-sandwich complexes of the type [Cp*M{6,6′-(OH)2-bpy}(H)]2+ (M = Co, Rh, or Ir; Cp* = C5Me5; bpy = 2,2′-bipyridine; Figure 5c).59 The authors analyzed four possible mechanisms for hydride transfer: two inner-sphere pathways in which a four-centered transition state is rate-determining and two outer-sphere pathways. The calculations indicate that the two inner-sphere mechanismsone involving Cp* ring slippage and the other bipyridine rotation and partial dissociationare unlikely due to the energetic cost of breaking conjugation. The two outer-sphere pathwaysone involving direct hydride transfer and the other a ligand-assisted mechanism in which one of the pendant hydroxyl groups on the G


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insertion into (CyPSiP)Pd(H) (CyPSiP = Si(Me)(2-PCy2C6H4)2) (52) to form (CyPSiP)Pd{OC(O)H} (53; eq 10).31,72

Figure 6. Pincer-supported nickel hydrides that react with CO2 to form nickel formates (Ni{OC(O)H}).

Both Johansson and Wendt’s work exploring insertion into a R PCP system and our work exploring insertion into a RPSiP system indicate that CO2 insertion into a palladium hydride is a significantly more facile process than CO2 insertion into Pd−C bonds (e.g., methyl or allyl) with the same ancillary ligand.71,73 This trend is likely general to other metals. In a computational study, CO2 insertion into pincer-supported nickel and palladium hydrides was compared.31 Insertion into nickel hydrides is more thermodynamically favored than insertion into the corresponding palladium hydrides, and likely, as a consequence, the kinetic barriers for insertions into nickel systems are lower. 2.6. Platinum. There are few examples of CO2 insertion into a platinum hydride bond. In 1977, Immirzi and Musco reported that the reaction of trans-(PCy3)2Pt(H)2 (54a) with a saturated solution of CO2 in benzene generates trans-(PCy3)2Pt(H){OC(O)H} (55a; eq 11).74 The reaction is reversible, and 54a is regenerated from 55a when the formate species is placed under a stream of N2. Although 55a was characterized by X-ray crystallography, crystallization under an atmosphere of CO2 was required to obtain the crystals. Subsequently, Paonesea and Trogler demonstrated that trans-(PEt3)2Pt(H)2 (54b) undergoes analogous chemistry to form trans-(PEt3)2Pt(H){OC(O)H} (55b) upon exposure to CO2 (eq 11).75 In 2012, Mitton and Turculet demonstrated that a species they proposed to be a platinum(0)

thermodynamically favorable as the ligand trans to the hydride becomes a stronger donor.31,34 Therefore, insertion into a nickel hydride supported by a RPNP ligand is less favorable than insertion into a complex with an ancillary RPSiP ligand. This is consistent with experimental results suggesting that spontaneous decarboxylation of nickel formates is more likely to occur at elevated temperatures for complexes with ancillary RPNP ligands compared to pincer ligands with stronger donors trans to the formate.68 Presumably, as a result of the increased thermodynamic driving force, calculations also show that the kinetic barrier for CO2 insertion decreases as the ligand trans to the hydride becomes a stronger donor. The proposed mechanism involves either a stepwise pathway with an inner-sphere rate-determining transition state or a concerted pathway.28,29,31,34 As the trans effect of the donor ligand opposite to the nickel hydride increases, there is less interaction between CO2 and the nickel center in the rate-determining transition state, and the transition state starts to become more outer-sphere. These results demonstrate that, by controlling the identity of the ligand trans to the hydride, it is possible to tune both the kinetics and thermodynamics for CO2 insertion, which may be important for the design of catalysts. 2.5. Palladium. The insertion of CO2 into a palladium hydride was first proposed by Sakimoto and co-workers.70 They showed that the treatment of trans-(PMe3)2Pd(Cl)(H) with CO2 at −40 °C results in a product that gives a signal in the 1 H NMR spectrum at approximately 9.7 ppm, consistent with the formation of a palladium formate. They, however, did not fully characterize this species, and its identity still remains uncertain. In 2007, Johansson and Wendt demonstrated that treatment t of the pincer-supported palladium hydride ( BuPCP)Pd(H) (50) t with CO2 results in the rapid formation of ( BuPCP)Pd{OC(O)H} 71 (51; eq 9). Subsequently, in two simultaneous independent reports, Mitton and Turculet and our group demonstrated CO2

complex with a bridging silane72 [CyPSi-(μ-H)P]Pt, which was later shown to be (CyPSiP)Pt(H) (56),76 reacts with CO2 to generate (CyPSiP)Pt{OC(O)H} (57; Scheme 10). Complex 57 spontaneously decomposes back to 56 if it is not kept under an atmosphere of CO2. It can, however, be stabilized through an interaction with a Lewis acid such as B(C6F5)3 to form

Scheme 10. Reversible CO2 Insertion into 56 with Trapping by a Lewis Acid

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Scheme 11. General Mechanism for CO2 Insertion into Metal Hydroxides or Alkoxides Proposed on the Basis of Computational Studies

Figure 7. Examples of group 9 metal hydroxides that react with CO2 to form metal carbonates (M{OC(O)O−}) under basic conditions.

[(CyPSiP)Pt]+[{OC(O)H}B(C6F5)3]− (58). The lack of examples of CO2 insertion into a platinum hydride and the reversibility of the known reactions are consistent with the results from iridium, suggesting that CO2 insertion into third-row transition metals is thermodynamically disfavored compared to that into first- and second-row transition metals.77

in which the C−O bond is formed (Scheme 11).81−83 This process is normally barrierless. Subsequently, in the rate-determining step, the zwitterion rearranges to form a new M−O bond and fully cleaves the metal alkoxide or hydroxide bond to form the bicarbonate product. 3.1. Cobalt, Rhodium, and Iridium. In the first work exploring CO2 insertion into group 9 metal hydroxides, Harris and co-workers demonstrated that treating [(NH3)5M(OH)]2+ [M = Co (59), Rh (60), or Ir (61)] with CO2 in basic aqueous media generates carbonate complexes of the type [(NH3)5 M{OC(O)O}]+ (M = Co, Rh, or Ir; Figure 7).84,85 CO2 insertion into the M−OH bond was proposed to initially generate bicarbonate species, which are deprotonated to form the carbonate products. The second-order rate constant for CO2 insertion increases from cobalt(III) to rhodium(III) to iridium(III). Furthermore, the rate constants are fairly consistent across a pH range from 7 to 9. This work was later extended to show that other cationic cobalt(III) complexes including [(tren)Co(OH2)(OH)]2+ [62; tren = N(CH2CH2NH2)3], [(tren)Co(OH)2]+ (63), cis-[(en)2Co(OH2)(OH)]2+ (64; en = NH2CH2CH2NH2), trans-[(cyclam)Co(OH2)(OH)]2+ [65; cyclam = (NHCH2CH2NHCH2CH2CH2)2], trans-[(cyclam)Co(OH)2]+ (66), cis-[(cyclam)Co(OH2)(OH)]2+ (67), and cis-[(cyclam)Co(OH)2]+ (68) also react with CO2 to form the corresponding carbonate species (Figure 7).86,87 Interestingly, after CO2 insertion to generate the κ1-carbonate species, the cis-cyclam complexes rearrange to give H2O and cis-[(cyclam)Co(CO3)]+ with a κ2-carbonate group.87 Several rhodium(III) complexes such as trans-[(en)2Rh(OH2)(OH)]2+ (69), trans-(en)2Rh(OCO2)(OH)

3. CO2 INSERTION INTO METAL ALKOXIDE AND HYDROXIDE BONDS Copolymerization of CO2 with cyclic ethers such as epoxides could provide a sustainable method for the synthesis of polycarbonates, which are produced on a large industrial scale.2,22 Although many synthetic methods for polycarbonates still use phosgene as a starting material,78 there has been significant growth in the field of CO2-based synthetic routes within the past 15 years, particularly with the development of the Asahi−Kasei process, a large-scale synthetic method that generates an aromatic polycarbonate using CO2 as a feedstock.79 When these copolymerization reactions are facilitated by homogeneous transitionmetal catalysts, it is commonly proposed that CO2 insertion into a metal alkoxide bond is a crucial step, and as a consequence, there is significant interest in this type of reaction. Researchers are also studying the related CO2 insertion into metal hydroxides because it is postulated to be a key step in the reduction of CO2 to CO by the enzyme CO dehydrogenase.80 Computational investigations suggest that the general pathway for CO2 insertion into metal alkoxides and hydroxides follows a pathway in which the lone pair on the alkoxide or hydroxide initially acts as a nucleophile and attacks CO2 to form a zwitterionic intermediate I


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(70), and trans-[(en)2Rh(X)(OH)]+ (71; X = Cl, Br, or I) were also investigated and shown to exhibit analogous reactivity (Figure 7).88,89 Walton and co-workers in 2002 demonstrated that the cationic cobalt(II) complexes [(TCT)Co(OR)]+ [72; R = Et or Ph; TCT = 1,3,5-{N(CH)3Ph}3C6H9] reversibly react with CO2 in coordinating solvents such as MeOH and ACN (Scheme 12).90

complexes are dissolved in ethanol, both quantitatively convert to the corresponding bicarbonate complexes (75a or 75b).91 Otsuka and co-workers demonstrated that the related compound (PCy3)2Rh(CO)(OH) also inserts CO2 to form the corresponding κ2-formate complex.92 Like cobalt and rhodium, there have been several studies exploring the insertion of CO2 into iridium hydroxides or alkoxides. As described above, Palmer and Harris investigated CO2 insertion into 6185 and Flynn and Vaska that into 74b (eq 12).91 Subsequently, Newman and Bergman demonstrated that (Cp*)Ir(PPh3)(H){OC(O)OEt} forms through CO2 insertion into the Ir−O bond of (Cp*)Ir(PPh3)(H)(OEt).93 The product is thermally unstable both in solution and as a solid and was only characterized by 1H NMR and IR spectroscopy. Interestingly, in this work, no insertion into the Ir−H bond was observed.93 In a more recent example, Nolan and co-workers studied the reactivity of a series of NHC-supported iridium complexes (IiPr)Ir(cod)(OR) [cod = 1,5-cyclooctadiene; IiPr = 1,3-bis(isopropyl)imidazol-2-ylidene; R = H (76a), Me (76b), or Ph (76c)] with CO2.94 Exposure of a benzene solution of 76a to 1 atm of CO2 generates the carbonate-bridged complex {(IiPr)Ir(cod)}2(μ-κ1:κ2-CO3) (78; Scheme 13a). Complex 78 has one 16-electron, four-coordinate square-planar iridium(I) center and one 18-electron, five-coordinate pseudo-trigonalbipyramidal iridium(I) center. It presumably forms via the initial insertion of CO2 into 76a to generate (IiPr)Ir(cod){OC(O)OH} (77a), followed by an acid−base reaction between 77a and another 1 equiv of 76a to form 78 and H2O (Scheme 13a). Exposure of 76b and 76c to CO2 rapidly generates the corresponding monomeric insertion products (IiPr)Ir(cod){OC(O)OR} [R = Me (77b) or Ph (77c); Scheme 13b].94 No dinuclear species are observed. Similarly, the treatment of (IPr)Ir(cod)(OH) [79; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2ylidene] with CO2 forms only the monomeric bicarbonate species (IPr)Ir(cod){OC(O)OH} (80; Scheme 13c), suggesting that the size of the NHC ligand impacts the formation of dinuclear species.83 Poater and co-workers performed DFT calculations to understand the pathway of insertion for these NHC-supported iridium complexes.82 Insertion proceeds via the mechanism shown in Scheme 11, with initial nucleophilic attack

Scheme 12. CO2 Insertion into 72

On the basis of IR and UV−vis spectroscopy, the authors proposed the octahedral κ2-bicarbonate complex [(TCT)Co(solv) (κ2-O2COR)]+ (73; solv = MeOH, EtOH, or ACN) as the insertion product. Attempts to isolate this product, however, were unsuccessful, and a structure with a monodentate κ1-bicarbonate cannot be excluded. Furthermore, Flynn and Vaska showed that trans-(PPh3)2M(CO)(OH) [M = Rh (74a) or Ir (74b)] reacts reversibly with 1 atm of CO2 at 25 °C in ethanol to generate trans-(PPh3)2M(CO){OC(O)OH} [M = Rh (75a) or Ir (75b); eq 12].91 Complex 74a fully converts

to 75a within minutes at room temperature, while 75b is in a 2:1 equilibrium with 74b under the same conditions. Kinetically, the reaction with 74b is also slower. It was proposed that CO2 directly coordinates to the metal center to form (PPh3)2M(CO)(OH)(CO2) when CO2 reacts with the metal hydroxides in the solid phase; however, when these CO2

Scheme 13. (a) CO2 Insertion into 76a, (b) 76b or 76c, and (c) 79

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insert CO2 to form exclusively monomeric bicarbonate species, presumably because bicarbonate products do not readily undergo acid−base chemistry. For example, Immirzi and Musco demonstrated that insertion of CO2 into 54a in MeOH generates trans-(PCy3)2Pt(H){OC(O)OMe}.74 The first step in this reaction is proposed to be protonolysis of one of the Pt−H ligands in 54a by MeOH to generate H2 and trans-(PCy3)2Pt(H)(OMe) (88) as an intermediate (Figure 9). CO2 insertion into 88 is then highly selective for the Pt−OMe bond, as opposed to the Pt−H bond. Similarly, Lock et al. demonstrated that the insertion of CO2 into trans-(PEt3)2Pd(Me)2 in the presence of water, MeOH, EtOH, or nBuOH results in the formation of complexes of the type trans-(PEt3)2Pd(Me){OC(O)OR} (R = H, Me, Et, or nBu) after the generation of trans-(PEt3)2Pd(Me)(OR) (89) as an unobserved intermediate (Figure 9).99 In this case, CO2 insertion is selective for the Pd−OR bond as opposed to the Pd−Me bond. The first example of CO2 insertion into an isolated group 10 alkoxide was described by Wendt and co-workers.100 They demonstrated that the pincer-supported complex (PhPCP)Pt(OMe) (90; PhPCP = 2,6-C6H3(CH2PPh2)2) undergoes rapid insertion of CO2 to form (PhPCP)Pt{OC(O)OMe} in almost quantitative yield (Figure 9). Pincer-supported systems have also been used to study CO2 insertion into nickel and palladium hydroxides. Holm and co-workers studied CO2 insertion into anionic complexes of the type (RNNPyrN)Ni(OH)− [RNNPyrN = 2,6-NC5H3(C(O)NAr)22−; Ar = R2C6H3; R = Me (91a), Et (91b), iPr (91c), iBu (91d), or Ph (91e)] to form monomeric bicarbonates (Figure 9).80,101,102 They also prepared a nickel hydroxide complex with a macrocyclic ligand appended to the NNPyrN pincer scaffold 92, which also cleanly forms a bicarbonate upon exposure to CO2 (Figure 9). In fact, these anionic pincer-supported nickel hydroxides are so reactive with CO2 that, in the case of 91a, the bicarbonate product forms upon exposure of the complex to air. The kinetics of the reaction of 91a−91e with CO2 were measured using a stopped-flow instrument with a UV−vis detector.101,102 The reactions follow a second-order rate law (rate = k[Ni− OH][CO2]), with the rates decreasing in the order R = Me > macro > Et > iPr > iBu > Ph. This order shows that more sterically open systems insert faster. An extrapolated rate constant of 9.5 × 105 M−1 s−1 in N,N-dimethylformamide at 298 K was measured for 91a. This rate constant is similar to kcat/KM for the enzyme carbonic anhydrase, which catalyzes the rapid interconversion of CO2 and water to HCO3− and H+.101 The mechanism of the reaction was calculated using DFT and proposed to follow the stepwise pathway outlined in Scheme 11 with a ratedetermining four-centered transition state. The related palladium hydroxide (MeNNPyrN)Pd(OH)− (93) was also shown to rapidly product. Changing the pincer insert CO2 to form a bicarbonate t ligand from MeNNPyrN to BuPCP does not affect the outcome of t the reaction because ( BuPCP)M(OH) (94; M = Ni or Pd) reacts

of CO2 on the hydroxide or alkoxide to form a zwitterionic intermediate, followed by a rate-determining rearrangement to generate a monomeric bicarbonate species. In the case of the hydroxide complex 76a, the formation of a dinuclear carbonate species is facile after generation of the bicarbonate. The calculated barriers for CO2 insertion into (IiPr)Ir(cod)(OR) follow the order R = Me < H < Ph, which indicates that an increase in the electron density at the metal center increases the rate of insertion. 3.2. Nickel, Palladium, and Platinum. Nickel is in the active site of CO dehydrogenase.95 Perhaps, as a consequence, there has been significantly more research into CO2 insertion reactions for group 10 metal hydroxides and alkoxides compared to group 9 systems. In general, insertion reactions between monomeric group 10 metal hydroxides and CO2 can be separated into two classes: (i) reactions that generate dimeric carbonate products via the mechanism described above for iridium and (ii) reactions that generate monomeric bicarbonate products. Systems with sterically bulky ancillary ligands are more likely to form monomeric products. In 1984, Randaccio and co-workers demonstrated that insertion into bis(phosphine)-supported platinum hydroxide complexes forms dimeric carbonate products, regardless of whether the two phosphine ligands are cis, 81, or trans, 82, to each other in the starting material (Figure 8).96

Figure 8. Examples of platinum hydroxide complexes that insert CO2 to form dimeric carbonate products.

When the reaction was monitored using IR spectroscopy, there was some evidence for the formation of a monomeric bicarbonate intermediate. Similarly, Ruiz et al. demonstrated that CO2 insertion into palladium hydroxide complexes with relatively small bidentate nitrogen-containing ligands 83 also forms dinuclear carbonate products (Figure 8).97 In contrast, when Piers and co-workers explored CO2 insertion into platinum hydroxide complexes with a large bidentate nitrogen ligand, 84, only a monomeric bicarbonate product was observed (Figure 9).98 This reaction is reversible, and depending on the temperature, the ratio of the starting hydroxide to bicarbonate product could be changed. In all three of these examples from Randaccio, Ruiz, and Piers, CO2 selectively inserts into a metal hydroxide bond as opposed to the metal alkyl or aryl bonds also present in the starting materials. Piers et al. also demonstrated that CO2 insertion into a platinum bis(hydroxide) species with a sterically large bidentate nitrogen ligand, 85, results in the formation of bis(bicarbonate) species 87a at low temperature (235 K, a κ2-carbonate species 87b forms, most likely through an acid−base reaction between the bicarbonate and hydroxide ligands in 86. This reaction demonstrates the facile nature of CO2 insertion into M−OH bonds because a rapid reaction is observed even at very low temperature. Whereas hydroxide complexes that insert CO2 can form either monomeric or dinuclear products, alkoxide complexes generally

t

rapidly with CO2 to form ( BuPCP)M{OC(O)OH} (Figure 9).71,81 DFT studies on the model compound (MePCP)Ni(OH) suggest that insertion occurs via the standard stepwise pathway.81 Dinuclear nickel and palladium complexes with bridging hydroxide ligands also insert CO2. For example, both Kitajima et al. and Santana et al. demonstrated that one of the bridging hydroxide ligands in dimeric nickel complexes supported by facially coordinating tridentate nitrogen ligands (95) react with CO2 to form complexes with a symmetric bridging carbonate ligand (96) and water (eq 13).103,104 No studies were performed to elucidate the mechanism of these reactions, but a likely K


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Figure 9. Examples of group 10 hydroxide or alkoxide complexes that insert CO2 to form monomeric bicarbonate complexes.

Scheme 14. Double Insertion of CO2 into 85

pathway involves the initial opening of one of the bridging hydroxide ligands to form a terminal hydroxide that reacts with CO2 in an analogous fashion to monomeric terminal hydroxides. Santana et al. demonstrated that performing the reaction in a strongly coordinating solvent such as ACN leads to coordination of the solvent to one of the nickel centers in the product and causes the bridging carbonate ligand to bind in an asymmetric manner.104 Similarly, Carmona et al. formed products with an asymmetric dinuclear carbonate core, 98, through the reaction of nickel complexes with bridging hydroxide ligands 97 with CO2 in the presence of an equivalent of PMe3 (eq 14).105 In related palladium chemistry, Roundhill and co-workers showed that the reaction of CO2 with a dinuclear palladium complex with L


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Compound 101 reacts with CO2 at room temperature to form trans-(PPh3)2Ir(CO){OC(O)NHAr} (102; eq 16). Furthermore, as part of their studies on NHC-supported iridium complexes, Nolan and co-workers investigated the insertion of CO2 into (IiPr)Ir(cod)(NHR) [R = CH2Ph (103a) or p-BrC6H4 (103b)] to form (IiPr)Ir(cod){OC(O)NHR}) [R = CH2Ph (104a) or p-BrC6H4 (104b); eq 17].94 Although both 104a and

bridging hydroxide ligands 99 in the presence of water forms 2 equiv of the bis(bicarbonate) palladium monomers 100 (eq 15).106 At this stage, further work is required to fully understand the mechanism by which CO2 inserts into complexes with bridging hydroxide ligands as well as the exact role that the solvent plays.

4. CO2 INSERTION INTO METAL AMIDE BONDS Although the reaction of amines with CO2 to generate carbamic acids is a well-studied process107 because it is utilized industrially for the scrubbing of CO2 from gaseous waste streams,108 there has been comparatively little work on the reaction of transitionmetal amides with CO2 to form carbamate complexes. One potential application of the insertion of CO2 into transitionmetal amides is in the synthesis of ureas and carbamates from CO2 and amines.109 Urea is produced from ammonia and CO2 on an industrial scale using the Bosch−Mesier process, but high temperatures (150−250 °C) and pressures (50−250 atm) are required.109,110 The first step of the processthe uncatalyzed formation of ammonium carbamate from CO2 and ammoniais exothermic and, thus, disfavored by the high temperatures needed to promote the second step, the endothermic decomposition of ammonium carbamate to generate urea and water, resulting in the use of high pressures to compensate. There is still room for improvement in the synthesis of urea, and transitionmetal-based catalysts could potentially significantly alter the landscape. Additionally, transition-metal-based catalysts may be valuable for the synthesis of functionalized ureas and carbamates for the fine chemical industry through the formation of metal amide intermediates.109 To date, DFT investigations suggest that the general pathway for CO2 insertion into metal amides follows a mechanism analogous to that proposed for metal hydroxides or alkoxides (Scheme 15).81,82,111 Initially, the lone pair on the amide acts as a nucleophile and attacks CO2 to form a zwitterionic intermediate in which the C−N bond forms. This process is normally barrierless. Subsequently, in the ratedetermining step, the zwitterion rearranges to fully cleave the metal amide bond and form the carbamate product. The barriers for insertion into metal amide bonds are typically lower than the corresponding barriers for insertion into metal hydroxides or alkoxides; however, only a limited amount of data are currently available, making it difficult to draw definitive conclusions. 4.1. Cobalt, Rhodium, and Iridium. In 1993, Ahmed et al. reported the first example of CO2 insertion into a group 9 amide, specifically trans-(PPh3)2Ir(CO)(NHAr) (101; Ar = p-MeC6H4).112

104b were characterized by multinuclear NMR spectroscopy, only 104a could be isolated because 104b reverts to 103b under vacuum or when purged with argon. Computational work by both Nolan et al.83 and Poater et al.82 suggests that the ratedetermining transition state for CO2 insertion into complexes of the type (IiPr)Ir(cod)(NHR) is analogous to that shown in Scheme 15. Perhaps the most extensive study on CO2 insertion into group 9 amides was by Crabtree et al., who explored the insertion of CO2 into (PPh3)2Ir(H)3(2-NH2C5NH4) (105) to form the carbamato complex 107 (Scheme 16).111 Because a net loss Scheme 16. Proposed Mechanism for Generation of the Cyclometalated Carbamato Species 107

of H2 must occur to generate the observed insertion product, the authors proposed a pathway in which a cyclometalated iridium amide intermediate 106 initially forms, followed by CO2 insertion to give 107. To further probe this pathway, a series of cyclometalated iridium amides 108−115 were synthesized and treated with CO2 (Figure 10). Upon exposure to 1 atm of CO2 at room temperature, complex 108, which contains PCy3 ancillary ligands, and 109, which has an N-methyl rather than an N−H group, react instantaneously to generate the corresponding insertion products. Whereas 108 is fully converted to the carbamato complex, the reaction of 109 with CO2 results in a 9:1 mixture of the product and starting material. Compound 110 in which the pyridine group was replaced with a pyrimidine also

Scheme 15. General Mechanism for CO2 Insertion into Metal Amides Proposed on the Basis of DFT Calculations

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Figure 10. Iridium amide complexes studied by Crabtree et al.

insertion into the metal amide bond. DFT calculations on 121 suggest that the reaction follows the general pathway for CO2 insertion into a metal amide outlined in Scheme 15. To date, the only example of CO2 insertion into a disubstituted group 10 amide was reported by Cámpora and co-workers (eq 18).117

reacts with CO2; however, this reaction is much slower than those with 105, 108, and 109. When the anionic X-type amide ligand was modified to a neutral amine L-type ligand as in 111, no CO2 insertion occurs. Similarly, no reaction with CO2 occurs with the iridium trihydride 112, because this complex cannot lose H2 to cyclometallate, or 113, which is likely too sterically constrained to cyclometallate. Last, despite the structural similarity of the N-phenyl analogue 114 and the 2-hydroxypyridyl-supported species 115 with the other complexes studied, there is no reaction with CO2. Computations performed as part of this study suggest that insertion into the Ir−NH bond of 106 is more likely than insertion into the Ir−H bond of 105, consistent with the experimental results, and that insertion into the Ir−NH bond of 106 likely proceeds via a stepwise pathway analogous to the one shown in Scheme 15. 4.2. Nickel, Palladium, and Platinum. The first example of CO2 insertion into a group 10 amide bond was reported by Roundhill and co-workers.106 They showed that the reaction of CO2 with trans-(PCy3)2Pt(Ph)(NH2) (116) in a nonpolar solvent such as benzene generates a precipitate, which was characterized as the N-bound carbamate complex trans(PCy3)2Pt(Ph){NHC(O)OH} (117; Scheme 17). When this

They showed that insertion into (dippe)Ni(Me)(NC4H8) (124; dippe = iPr2PCH2CH2PiPr2) rapidly generates the carbamate product 125. Insertion is selective into the amide bond, and there is no evidence for insertion into the Ni−Me bond.

5. CO2 INSERTION INTO METAL−CARBON BONDS The catalytic formation of C−C bonds from CO2 is a reaction of considerable interest because it could be used to generate synthetically valuable chemicals such as carboxylic acids.16,118 In many proposed catalytic cycles for the formation of carboxylic acids from CO2, the insertion of CO2 into a M−C bond is the elementary step that directly results in the generation of a new C−C bond. Nevertheless, examples of CO2 insertion into welldefined late-transition-metal−carbon bonds are relatively rare, and there are many examples of this step being proposed in catalysis without any direct evidence. The exact mechanism for CO2 insertion into M−C bonds varies depending on the nature of the carbon-based ligand. For example, CO2 insertion into a metal−allyl bond proceeds via a different pathway than insertion into a metal−alkyl bond. However, in most cases, the ratedetermining step in these processes is the initial formation of the C−C bond via nucleophilic attack of the carbon ligand on CO2. Subsequent rearrangement of a zwitterionic intermediate to form a metal carboxylate product is typically relatively low in energy. Thus, these reactions can generally be classified as outer-sphere processes (except where noted below) because there is no interaction between CO2 and the metal in the rate-determining transition state. 5.1. Cobalt, Rhodium, and Iridium. In a seminal study, Darensbourg et al. explored the insertion of CO2 into rhodium(I) alkyl and aryl complexes.119 They showed that CO2 insertion into (PPh3)3Rh(Ph) (126) to form (PPh3)3Rh{OC(O)Ph} (127) is slower than insertion into (PMe3)3Rh(Ph) (128) to form (PMe3)3Rh{OC(O)Ph} (129) (Scheme 18a,b). Specifically, the

Scheme 17. CO2 Insertion into 116 Generating 117, Which Rearranges to 118

species is dissolved in a polar solvent such as dichloromethane, the O-bound carbamate complex trans-(PCy3)2Pt(Ph){OC(O)NH2} (118) forms. If the initial reaction between CO2 and 116 is performed in a polar solvent, the sole product is 118, but 117 is observed as an intermediate. These results are consistent with a mechanism that involves the initial nucleophilic attack of the amide on CO2, followed by ligand rearrangement. There is no evidence for insertion into the Pt−Ph bond. Subsequently, there have been a number of reports of CO2 insertion into pincersupported group 10 amides 119−123 (Figure 11).81,113−116 This includes systems with different donors trans to the amide and different substituents on the amide. However, at this stage, there are no quantitative studies assessing how changes in the metal, amide substituents, or ancillary ligand affect the rate of N


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Figure 11. Examples of group 10 amido complexes supported by pincer ligands that insert CO2 to form carbamate complexes.

Scheme 18. CO2 Insertion into (a) 126, (b) 128, (c) 131, and (d) 134

supported rhodium(I) ethyl complexes were analyzed to determine the factors that control CO2 insertion.120 The computational screening compared 26 neutral and 12 anionic complexes in a two-step mechanism, where CO2 first interacts with the square-planar starting material to potentially form an adduct and then inserts into the Rh−alkyl bond to generate the product via a three-membered transition state (Scheme 19). Of the 38 complexes investigated, 30 adopt a η1(C) binding mode (Figure 12a) when initially interacting with CO2. Only one complex adopts a η2(CO) binding mode (Figure 12b), while the remaining seven exhibit negligible interaction between the starting material and CO2. Although this was proposed as a potential first step in the insertion of CO2 into the Rh−alkyl bond, the authors noted that the ability to bind CO2 (i.e., the calculated association energy of CO2 with the starting material) does not correlate with the corresponding activation energy for CO2 insertion. Rather, it was determined that the most important factors in determining the barrier for CO2 insertion are the electron density on the metal center and alkyl chain. Specifically, the anionic complexes exhibit lower barriers than the neutral ones, most likely because of the increased electron density on the metal. As suggested above, the donor atom trans to the insertion site (here the alkyl chain) significantly influences the reactivity of the complex, with a stronger trans donor resulting in a lower barrier to insertion. In this study, the barriers for CO2 insertion based on the identity of the trans donor atom increase in the order C− ≈ B− < Si− < N < C < O. Finally, in an observation parallel to that made by Darensbourg et al., Leitner and co-workers calculated that, by tuning the electron-donating strength of the atoms cis to the alkyl

complete conversion of 126 into 127 requires 24 h under 20 atm of CO2, whereas the complete conversion of 128 to 129 requires 3 h under 10 atm of CO2. Although 127 is indefinitely stable, the initially formed 129 converts into the κ2-acetate complex (PMe3)2Rh{κ2-O2CPh} (130) with concomitant release of PMe3. Additionally, 129 is only stable under an atmosphere of CO2 and reverts back to 128 when placed under vacuum. Interestingly, there are no products consistent with insertion when the related rhodium(I) alkyl complex (PPh3)3Rh(Me) is placed under an atmosphere of CO2. In contrast to the lack of insertion observed with (PPh3)3Rh(Me), slow conversion of (PMe3)3Rh(Me) (131) to the κ1-acetate complex (PMe3)3Rh{OC(O)Me} (132) occurs under 1 atm of CO2 at room temperature (Scheme 18c). Complex 132 subsequently rearranges to (PMe3)3Rh{κ2-O2CMe} (133). When one of the PPh3 ligands in (PPh3)3Rh(Me) is replaced with a CO ligand, the resulting complex (PPh3)2Rh(CO)(Me) (134) slowly reacts with CO2 (14 atm) to form (PPh3)2Rh(CO){OC(O)Me} (135; Scheme 18d). However, because of the slow rate of reaction with CO2, there is also significant background decomposition of 134 to 136. On the basis of the reactivity trends observed, Darensbourg and co-workers concluded that increased electron density at the rhodium center correlates to a faster rate of CO2 insertion. Their work also demonstrates that both the stability and reactivity of these complexes can be dramatically tuned by modifying the phosphine or carbonyl ligand, which could be a general design principle for promoting catalysis in a range of systems. Similar reactivity trends were elucidated in a computational study by Leitner and co-workers in which 38 different pincerO


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Scheme 19. Generic Scheme for CO2 Insertion into Pincer-Supported Rhodium(I) Ethyl Complexes Studied by Leitner et al.

after which PCy3 can displace one oxygen donor of the acetate or benzoate group to form the κ1 product. To our knowledge, there are no examples of CO2 insertion into a Ir−C bond. In 2014, Nolan et al. probed the insertion of CO2 into the Ir−C bond of the NHC-supported iridium complexes (IiPr)Ir(cod)(R) [R = Ph (142), CH2NO2 (143), CCPh (144), or C6F5 (145)] and found that all of these complexes are unreactive toward CO2 (eq 19).94 To probe whether this lack of reactivity is due to thermodynamic or kinetic factors, decarboxylation of (IiPr)Ir(cod){OC(O)Ph} (146) was attempted. Neither applying vacuum nor heating at 50 °C for 3 h led to the generation of 142 (eq 20). That result, coupled with

Figure 12. (a) η1(C) and (b) η2(CO) binding modes.

chain, the reactivity can be further adjusted, with more donating substituents generally correlating to lower barriers for insertion. In a more recent example, Iwasawa et al. explored the insertion of CO2 into Rh−alkyl and Rh−aryl bonds as part of a mechanistic study on the rhodium(I)-catalyzed carboxylation of aromatic compounds.121 They demonstrated that (dcype)(PCy3)Rh(Me) (137; dcype = Cy2PCH2CH2PCy2) reacts with benzene at 85 °C to form (dcype)(PCy3)Rh(Ph) (138), which, when exposed to 1 atm CO2 at 85 °C, generates a 1:2 mixture of (dcype)Rh (κ2-OCOPh) (139) and (dcype)(PCy3)Rh(κ1-OCOPh) (140) (Scheme 20a). However, when 138 is exposed to 1 atm of CO2 in the presence of PCy3, only the κ1-benzoate 140 forms (Scheme 20b). Similarly, 137 inserts CO2 in the presence of PCy3 to give the κ1-acetate (dcype)Rh(κ1−OCOMe)(PCy3) (141; Scheme 20c); however, this insertion proceeds with a slower rate than the insertion into 138. Interestingly, when 137 is exposed to 1 atm of CO2 in benzene at 85 °C, only the κ1- and κ2-acetate complexes are observed, indicating that carboxylation is faster than C−H activation with benzene to give 138. On the basis of kinetic analysis, the authors proposed that carboxylation of both 137 and 138 proceeds via a 14-electron three-coordinate complex after PCy3 dissociates from the metal center. This active species reacts with CO2 to generate the κ2-insertion product,

the observation of no CO2 scrambling when 146 is placed under 1 atm of 13CO2, suggests the presence of a significant kinetic barrier to decarboxylation of 146 and, hence, carboxylation of 142. This is further supported by the computational work by Nolan et al.83 and Poater et al.,82 both of which calculate high kinetic barriers for CO2 insertion into 142. The kinetic barriers

Scheme 20. (a) Formation of 138 in Benzene and CO2 Insertion To Give a 1:2 Mixture of 139 and 140, (b) CO2 Insertion into 138 in the Presence of PCy3, and (c) CO2 Insertion into 137 in the Presence of PCy3

P


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calculated for CO2 insertion into (IiPr)Ir(cod)(R) follow the order 145 > 143 > 142 > 144, which corresponds to the ability of the R group to withdraw electron density, again supporting the hypothesis that increased electron density at the metal results in a lower reaction barrier.82 5.2. Nickel, Palladium, and Platinum. To our knowledge, there are no examples of CO2 insertion into a Pt−C bond. This is in stark contrast to the insertion of CO2 into Ni−C and Pd−C bonds, which are remarkably well-studied reactions. In particular, the insertion of CO2 into nickel and palladium complexes containing allyl ligands is one of the most well-studied C−C bond-forming reactions involving CO2, likely because of the fact that a number of catalytic cycles are proposed to involve the insertion of CO2 into a Pd−allyl bond to form a palladium carboxylate as a key elementary step. These include systems for the coupling of CO2 with butadiene to form lactones,122−126 carboxylation of allylstannanes127−129 and allenes,73,130 and carboxylative coupling of allylstannanes with allyl halides.131 In foundational work in 1979, Santi and Marchi described a series of stoichiometric reactions between CO2 and complexes containing a Pd−allyl bond that generate free carboxylic acids (after an acidic workup) but did not isolate any well-defined palladium carboxylates.132 Subsequently, Behr and Von Ilsemann reported similar results with dinuclear palladium complexes containing chelating allyl ligands.133 In 1980, Jolly et al. described that the reaction of (2-R-allyl)2Pd(L) (147; R = H or Me; L = PMe3 or PCy3) with CO2 generates palladium carboxylate containing complexes 148 (eq 21).134 The products were not isolated but characterized in situ using NMR spectroscopy. Building on this work, we examined CO2 insertion into (2-R-allyl)2Pd(L) (R = H or Me; L = PMe3, PEt3, PPh3, or IPr).135 In the case of complexes containing 2-methylallyl ligands, the carboxylate products proposed by Jolly et al. were isolated and fully characterized (eq 21). In contrast, for systems containing an unsubstituted allyl

Although not as well studied as palladium, there are also several reports of CO2 insertion into Ni−allyl bonds. In 1977, Tsay and co-workers reported that (η3-2-methylallyl)2Ni (151) reacts quantitatively with CO2 in the presence of phosphines to give carboxylate-containing product 153 (Scheme 21).137 Scheme 21. CO2 Insertion into Complex 151 in the Presence of Different Ancillary Ligands

We showed that this reaction proceeds via the initial coordination of phosphine to give complexes of the type (2-methylallyl)2Ni(PR3) (152a or 152b; R = Me, Et, Cy, or Ph), which subsequently insert CO2.138 We also extended these reactions to systems containing NHC ligands such as IPr. In a fashion similar to that of palladium systems, complexes of the type (2-methylallyl)2Ni(PR3) generally contain both η1- and η3-allyl ligands that are in rapid exchange. However, the facile reaction of CO2 with the nickel t analogue of Wendt’s ( BuPCP)Pd(η1-allyl) complex to form a carboxylate product suggests that it is the η1 ligand that inserts CO2 (eq 22).29 In related work, Tsay and co-workers demonstrated that octadienediyl complexes 154, which contain one η1-allyl and one η3-allyl ligand, also insert CO2 to form multinuclear carboxylate species 155 (eq 23).137 This type of reaction is likely

relevant to the formation of lactones from two molecules of butadiene and CO2.122−126 Several computational studies have been performed by investigating the mechanism of CO2 insertion into group 10 complexes containing an allyl ligand.29,135,136 The reactions are proposed to proceed via a stepwise pathway involving the initial nucleophilic attack of the terminal olefin of an η1-allyl ligand on electrophilic CO2 (Scheme 22). The first transition state is zwitterionic with a positive charge on the allyl ligand and a negative charge on the carboxylate. This transition state leads to a zwitterionic intermediate with a formal negative charge on the carboxylate group and a positive charge on the palladium. There is no direct interaction between CO2 and palladium in the first step. In the second step, ligand rearrangement generates the observed metal carboxylate product. Depending on the ancillary ligand utilized, either the first or second step can be ratedetermining. From this postulated mechanism, several conclusions can be made that are consistent with the experimental results: (i) Given that η3-allyl ligands are electrophilic, CO2 insertion reactions are only observed in systems that contain a nucleophilic η1-allyl ligand or can readily access an η1-allyl ligand from an η3-allyl ligand. (ii) C−C bond formation occurs between the nucleophilic terminal carbon of the η1-allyl ligand and electrophilic

ligand, we only observed decomposition of the starting material and did not see any evidence for CO2 insertion. Complexes of the type (2-R-allyl)2Pd(L) contain both η1- and η3-allyl ligands that are in rapid exchange, and as a result, it was not possible to experimentally determine which allyl ligand was undergoing CO2 insertion. Using a pincer-supported system, Wendt and co-workers reported CO2 insertion into well-defined η1-allylpalladium complexes 149 (eq 22).128,136 When they utilized an

η1-allyl ligand with substituents in the 1 position, the final product 150 was consistent with C−C bond formation occurring at the terminal position of the η1-allyl ligand. We later obtained similar results using a CyPSiP-supported palladium η1-prenyl complex.73 Q


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Scheme 22. General Mechanism for CO2 Insertion into Metal−Allyl Bonds Proposed on the Basis of Experimental and Computational Studies

CO2, which explains the regiochemistry that is observed in the carboxylation of allenes.130 (iii) When the first step is ratedetermining, electron-donating substituents on the 2 position of the η1-allyl ligand lower the barrier to insertion by stabilizing the positive charge on the allyl ligand in the transition state. Bridging allyl ligands have different properties compared with η1- or η3-allyl ligands.139,140 Nevertheless, it has been demonstrated that palladium(I) dimers with bridging allyl ligands 156 insert CO2 to form complexes with one bridging carboxylate and one bridging allyl ligand 157 (eq 24).141,142 Kinetic studies reveal

t

( BuPCP)Pd(Me) (160a) is treated with 1 atm of CO2 at 80 °C, there is complete conversion to the κ1-acetate complex t Bu ( PCP)Pd{OC(O)Me} in 2 days (Figure 13). We reported

that the rate of reaction increases when a more electron-donating substituent is present on the 2 position of the allyl ligand or when the ancillary ligand is a better σ donor. Detailed experimental and computational investigations suggest that there are two pathways by which CO2 insertion can occur. In the first pathway, which only occurs in the presence of weakly coordinating ligands such as THF or ACN, the palladium(I) dimer disproportionates into a palladium(II) monomer with one η1- and one η3-allyl ligand and a palladium(0) species. The palladium(II) species then inserts CO2 and is trapped by the palladium(0) species to give the dinuclear product. In the second pathway, the dimer reacts directly with CO2 through nucleophilic attack of the bridging allyl ligand on electrophilic CO2, followed by ligand rearrangement to give the product. The insertion of CO2 into the bridging allyl ligand is proposed to be the first step in the catalytic carboxylation of allylstannanes and allylboranes by palladium(I) bridging allyl dimers.141 Examples of CO2 insertion into group 10 alkyl bonds are rare, and to date, elevated temperatures are almost always required. Furthermore, these reactions are generally highly sensitive to moisture, which can lead to competing reactions. For instance, while studying CO2 insertion into (L2)Pd(Me)2 (158; L2 = TMEDA or dppe), Pushkar and Wendt did not observe any of the desired acetate-containing complexes.143 Instead, they observed bicarbonate products of the form (L2)Pd(Me){OC(O)OH} (159; eq 25). These products are most likely generated through the reaction of CO2 with trace amounts of H2O to generate carbonic acid, which protonates one of the methyl ligands in (L2)Pd(Me)2 to form methane with concomitant coordination of the resulting bicarbonate ion. An alternative pathway involving the direct protonation of a methyl ligand by H2O to form a palladium hydroxide, which subsequently undergoes CO2 insertion (see section 4), cannot be excluded. Similarly, Rosenthal and co-workers reported analogous chemistry caused by trace amounts of H2O using a palladium dimethyl compound supported by a bidentate NHC ligand (eq 25).144 The first example of CO2 insertion into a group 10 alkyl complex was reported by Wendt et al.145 They showed that when

Figure 13. Examples of nickel and palladium methyl complexes supported by pincer ligands that insert CO2 to form acetate complexes. t

that the related compound ( BuPCP)Ni(Me) (160b) also forms t the corresponding κ1-acetate complex ( BuPCP)Ni{OC(O)Me} upon exposure to 1 atm of CO2, but the reaction requires heating to 150 °C, and the yield of the acetate complex is only approximately 75% (Figure 13).29 This suggests that CO2 insertion into the Ni−Me bond is more kinetically difficult than insertion t intot the Pd−Me bond. Changing the pincer ligand from BuPCP 3 to BuPOCsp OP did not lead to an appreciable difference in the t 3 rate of reaction. Insertion of CO2 into ( BuPOCsp OP)Ni(Me) (161) only occurs at 150 °C, with a 50% yield of the κ1-acetate product (Figure 13).67 Jordan and co-workers reported the only example of CO2 insertion into a group 10 methyl complex at room temperature.146 They demonstrated that the monomeric anionic palladium dimethyl complex 163, which likely is in equilibrium with the dimer 162, reacts with 1 equiv of CO2 to form the acetate complex 164 (Scheme 23). The rate of the reaction was enhanced by the presence of Li+ counterions, which are held close to the metal center through interactions with the ligand. Three computational studies have been performed exploring the mechanism of CO2 insertion into group 10 methyl bonds.29,136,146 They all propose that CO2 insertion occurs via an SE2 mechanism, which is shown generically in Scheme 24. In the first and rate-determining step, the carbon atom of the methyl group nucleophilically attacks CO2 to form a zwitterionic intermediate. The subsequent rearrangement of the zwitterionic intermediate to the final O-bound product is a low-energy process. Kinetic studies are consistent with the proposed mechanism and show a second-order rate law, which is dependent on the concentration of both CO2 and the methyl complex.136,146 R


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Scheme 23. Anionic Palladium Methyl Complex 145 That Undergoes CO2 Insertion at Room Temperature

Scheme 24. General Mechanism for CO2 Insertion into Metal−Methyl Bonds Proposed on the Basis of the Computational Studies

Additionally, Jordan’s observation that Li+ increases the rate of CO2 insertion is in agreement with this mechanism because the cation is able to stabilize the negative charge that develops on the carboxylate group in the rate-determining transition state.146 Overall, this mechanism is analogous to the outer-sphere stepwise pathway for CO2 insertion into metal hydrides. However, when reactions proceed via this pathway, the barriers are, in general, higher for CO2 insertion into metal−methyl bonds because they are less nucleophilic than the corresponding metal hydride. There are no examples of CO2 insertion into group 10 aryl or vinyl complexes to form metal carboxylates. This is surprising given that CO2 insertion into these types of M−C bonds has been proposed as a key step in the carboxylation of aryl and vinyl halides and pseudohalides (eq 26).147−149 In the case of

proposed to be important in catalytic carboxylation reactions, but metal-containing products from stoichiometric reactions have never been isolated. This limits our ability to perform mechanistic studies, which could be valuable for the design of improved catalysts and the discovery of new catalytic transformations featuring this elementary step. On a fundamental level, it is worth noting that, to date, all CO2 insertion reactions involving group 9 and 10 metal−element σ bonds have led to products containing a M−O bond and there are no examples of abnormal insertion reactions to form M−C bonds. The selectivity of insertion is presumably governed by thermodynamic considerations, and a system will need to be biased, for example, through the use of secondary coordination sphere interactions or the formation of especially strong element−O bonds, to observe the products of an abnormal insertion. From the mechanism of CO2 insertion reactions into group 9 and 10 metal−element σ bonds that have been studied so far, it can be generalized that these reactions typically involve two steps: (i) the initial nucleophilic attack of an electrophilic ligand on CO2, followed by (ii) rearrangement to give a carboxylate product. For systems with a highly nucleophilic ligand such as an amide or hydroxide ligand, the first step is generally a low-energy process, and the second step is rate-determining. These reactions are therefore almost always inner-sphere (Figure 14). For systems containing a ligand with intermediate nucleophilicity such as a hydride or allyl ligand, both steps can be similar in energy, and thus depending on the system, either the first or second step can be rate-determining. Accordingly, these reactions can be either inner- or outer-sphere (Figure 14). Finally, for systems with a low

carboxylation of aryl halides using nickel catalysts, it has been suggested that the formation of nickel(I) aryl species is required to facilitate CO2 insertion,150 but experimental support for this hypothesis is lacking. Similarly, nickel(I) alkyl and benzyl species have been postulated to undergo CO2 insertion in coupling reactions involving alkyl and benzyl halides, respectively, but there is little direct evidence to support this hypothesis.151,152 We suggest that a more detailed understanding of the CO2 insertion reactions that are likely occurring in these catalytic systems will lead to the development of more efficient catalysts and perhaps new transformations.

6. CONCLUSIONS, FUTURE DIRECTIONS, AND OUTLOOK The large number of reactions summarized in this Award Article highlights that there are many examples of CO2 insertion into group 9 and 10 metal−element σ bonds. In general, CO2 insertion reactions are more thermodynamically favorable for first- and second-row transition metals compared to third-row metals, and as a result, there are many more examples for the lighter elements. Furthermore, there are additional CO2 insertion reactions that are likely catalytically relevant that have not been demonstrated in stoichiometric reactions. For example, CO2 insertion reactions into group 10 aryl and benzyl bonds are

Figure 14. Predicted inner- or outer-sphere CO2 insertion into an M−E σ bond based on the nucleophilicity of E.

nucleophilicity ligand such as an alkyl ligand, the first step is typically rate-determining, and the reaction is always outersphere (Figure 14). The barrier for CO2 insertion is typically lower for systems that proceed via an inner-sphere versus an outer-sphere transition state, and consequently, in complexes containing, for example, an alkyl and a hydroxide ligand, insertion into the hydroxide is kinetically favored. It should be noted that S


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Biographies

the mechanisms for CO2 insertion into group 9 and 10 metal− element σ bonds are not distinct from those observed for earlier transition metals, but there are subtle differences in both the nature and identity of the rate-determining step as a result of the reduced oxophilicity and higher electronegativity of the later transition metals. In most cases, the majority of mechanistic information about CO2 insertion into group 9 and 10 metal−element σ bonds has been obtained from computational studies, and there is often little experimental evidence to support the proposed pathways. For example, at this stage, there is no experimental parameter, such as the magnitude of a solvent effect, that can be used to determine if a reaction proceeds via an inner- or outer-sphere pathway. Understanding the mechanistic pathway of CO2 insertion is important because strategies for optimizing innersphere reactions will probably be different from those required for outer-sphere reactions. However, even though the two mechanistic pathways are now relatively well established, there is still a distinct lack of information about how to optimize the rates of CO2 insertion for either pathway. This is because there are very few systems where the effects of the ancillary ligands, solvents, or additives that may be present in catalysis have been thoroughly investigated. It is plausible that certain additives such as Lewis acids may promote outer-sphere CO2 insertion reactions but be essentially inert in inner-sphere reactions because of the structure of the rate-determining transition state. We suggest that detailed kinetic studies examining how to increase the rates of both inner- and outer-sphere reactions will be valuable and will aid in both catalyst development and optimization of the conditions. Overall, it is clear that CO2 insertion reactions are and will continue to be an important elementary reaction in the conversion of CO2 into more valuable products. There are already significant potential applications of this reaction in both synthetic organic chemistry and the preparation of commodity chemicals, and it is likely that new catalytic transformations involving this elementary step will also be discovered in the near future. Therefore, we anticipate that there will be continued research into CO2 insertion in the next decade, with emphasis shifting away from finding examples of CO2 insertion reactions to understanding the pathway for insertion on a more detailed level. An important goal will be to translate this mechanistic information into the design of improved and new systems for the catalytic conversion of CO2 into compounds such as MeOH, carboxylic acids, and polycarbonates.

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Nilay Hazari is currently a Professor of Chemistry at Yale University. He received a B.S. (2002), majoring in chemistry, and a M.S. (2003) in inorganic chemistry working with Professor Leslie D. Field at the University of Sydney. He then completed a D. Phil in inorganic chemistry (2006) at the University of Oxford as a Rhodes Scholar. His doctoral supervisor was Professor Jennifer C. Green. He finished his formal education by working for 3 years (2006−2009) as a postdoctoral scholar under the supervision of Professor John E. Bercaw and Dr. Jay A. Labinger at the California Institute of Technology. In 2009, he started his independent career at Yale. His primary research focus is the mechanism-based design of homogeneous transition-metal catalysts for the synthesis of fine and commodity chemicals. He has received a number of awards including the American Chemical Society Harry Gray Award for Creative Work in Inorganic Chemistry by a Young Investigator (2017), the Arthur Greer Memorial Prize for Outstanding Scholarship by Junior Faculty Members in the Social Sciences and Sciences at Yale University (2015), the Camille and Henry Dreyfus Teacher Scholar Award (2014), and the National Science Foundation Career Award (2012). In 2013, he was named an Alfred P. Sloan Research Fellow and, in 2012, an Organometallics Fellow (from the American Chemical Society journal Organometallics).

Jessica Heimann received her B.S. (2015) in Chemistry from Rice University in Houston, TX. She is currently a Ph.D. candidate in Chemistry at Yale University, where she studies the insertion of carbon dioxide into transition-metal complexes under the supervision of Dr. Nilay Hazari. Jessica is a John Gamble Kirkwood Fellow.

AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

ACKNOWLEDGMENTS We thank our co-workers and collaborators for their assistance with our own studies of CO2 insertion reactions. We are especially grateful to Professors Wesley Bernskoetter and Sven Schneider for long-standing collaborations. N.H. acknowledges

ORCID

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

The authors declare no competing financial interest. T


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of CO2 into a Co−H bond to form a Co−O bond is approximately 36 kcal mol−1, whereas the corresponding values for CO2 insertion into Sc−H and V−H bonds (to generate Sc−O and V−O bonds) are approximately 110 and 102 kcal mol−1, respectively. (22) Lu, X.-B.; Ren, W.-M.; Wu, G.-P. CO2 Copolymers from Epoxides: Catalyst Activity, Product Selectivity, and Stereochemistry Control. Acc. Chem. Res. 2012, 45, 1721. (23) Fernandez-Alvarez, F. J.; Aitani, A. M.; Oro, L. A. Homogeneous Catalytic Reduction of CO2 with Hydrosilanes. Catal. Sci. Technol. 2014, 4, 611. (24) Chakraborty, S.; Bhattacharya, P.; Dai, H.; Guan, H. Nickel and Iron Pincer Complexes as Catalysts for the Reduction of Carbonyl Compounds. Acc. Chem. Res. 2015, 48, 1995. (25) Bontemps, S. Boron-Mediated Activation of Carbon Dioxide. Coord. Chem. Rev. 2016, 308, 117. (26) Bernskoetter, W. H.; Hazari, N. Reversible Hydrogenation of Carbon Dioxide to Formic Acid and Methanol: Lewis Acid Enhancement of Base Metal Catalysts. Acc. Chem. Res. 2017, 50, 1049. (27) Ford, P. C. The Water Gas Shift Reaction: Homogeneous Catalysis by Ruthenium and Other Metal Carbonyls. Acc. Chem. Res. 1981, 14, 31. (28) Huang, F.; Zhang, C.; Jiang, J.; Wang, Z.-X.; Guan, H. How Does the Nickel Pincer Complex Catalyze the Conversion of CO2 to a Methanol Derivative? A Computational Mechanistic Study. Inorg. Chem. 2011, 50, 3816. (29) Schmeier, T. J.; Hazari, N.; Incarvito, C. D.; Raskatov, J. R. Exploring the Reactions of CO2 with PCP Supported Nickel Complexes. Chem. Commun. 2011, 47, 1824. (30) Schmeier, T. J.; Dobereiner, G. E.; Crabtree, R. H.; Hazari, N. Secondary Coordination Sphere Interactions Facilitate the Insertion Step in an Iridium(III) CO2 Reduction Catalyst. J. Am. Chem. Soc. 2011, 133, 9274. (31) Suh, H.-W.; Schmeier, T. J.; Hazari, N.; Kemp, R. A.; Takase, M. K. Experimental and Computational Studies of the Reaction of Carbon Dioxide with Pincer Supported Nickel and Palladium Hydrides. Organometallics 2012, 31, 8225. (32) Bernskoetter, W. H.; Hazari, N. A Computational Investigation of the Insertion of Carbon Dioxide into Four and Five Coordinate Iridium Hydrides. Eur. J. Inorg. Chem. 2013, 2013, 4032. (33) Osadchuk, I.; Tamm, T.; Ahlquist, M. S. G. Theoretical Investigation of a Parallel Catalytic Cycle in CO2 Hydrogenation by (PNP)IrH3. Organometallics 2015, 34, 4932. (34) Ríos, P.; Rodríguez, A.; López-Serrano, J. Mechanistic Studies on the Selective Reduction of CO2 to the Aldehyde Level by a Bis(phosphino)boryl (PBP)-Supported Nickel Complex. ACS Catal. 2016, 6, 5715. (35) Ma, Q.-Q.; Liu, T.; Adhikary, A.; Zhang, J.; Krause, J. A.; Guan, H. Using CS2 to Probe the Mechanistic Details of Decarboxylation of Bis(phosphinite)-Ligated Nickel Pincer Formate Complexes. Organometallics 2016, 35, 4077. (36) Both our group and others have previously referred to outersphere processes as those in which there is no interaction between the metal center and CO2 when the CO2 molecule first interacts with the metal complex. However, this is true in the vast majority of CO2 insertion reactions into group 9 or 10 metal−element σ bonds. Therefore, we suggest that differentiating between the inner- and outersphere pathways based on the nature of the rate-determining step is more informative and have adopted this convention throughout this Award Article. (37) Pu, L. S.; Yamamoto, A.; Ikeda, S. A Carbon Dioxide Insertion Reaction into the Co-H Bond of Nitrogentris(triphenylphosphine) Cobalt Hydride. J. Am. Chem. Soc. 1968, 90, 3896. (38) Yamamoto, A.; Kitazume, S.; Pu, L. S.; Ikeda, S. Synthesis and Properties of Hydridodinitrogentris (triphenylphosphine) Cobalt (I) and the Related Phosphine-Cobalt Complexes. J. Am. Chem. Soc. 1971, 93, 371. (39) Bianco, V. D.; Doronzo, S.; Gallo, N. The Reactivity of Orgoanophosphorus Cobalt Hydride-Complexes with Carbon Dioxide. Inorg. Nucl. Chem. Lett. 1981, 17, 75.

support from the National Science Foundation through Grant CHE-1150826.

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