Ligand and solvent effects on CO₂ insertion into group 10 metal alkyl bonds

Abstract

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (^RPBP)M(CH₃) (^RPBP = B(NCH₂PR₂)₂C₆H₄⁻; R = Cy or ^tBu; M = Ni or Pd) to generate κ¹-acetate complexes of the form (^RPBP)M{OC(O)CH₃} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual ^RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (^RPBP)M(CH₃) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using (^tBuPBP)Pd(CH₃). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd) proceeds via an S_E2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.

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Introduction

Carbon dioxide (CO₂) is an attractive target as a chemical feedstock due to its low cost, non-toxic nature, and abundance.¹ Although there are several chemicals currently industrially prepared from CO₂, such as urea and carbonates,² only a small fraction of our available CO₂ is used as a source of carbon for either commodity^1h or fine³ chemical synthesis. The full exploitation of CO₂ as a feedstock is limited by its kinetic and thermodynamic stability, which poses a scientific challenge for developing synthetic routes to make more valuable chemicals from CO₂. Utilizing homogeneous transition metal catalysts is one attractive approach for solving kinetic problems associated with the conversion of CO₂ into more valuable chemicals and a variety of systems have been developed for the thermal, electrochemical, and photochemical conversion of CO₂.^1,3 An elementary step which is proposed in many of these catalytic reactions is CO₂ insertion into an M–E σ-bond (for example E = H, OR, NR₂, or CR₃) to form a product with a M–O bond (eqn (1)).⁴ To date, the majority of catalysts feature late transition metals,^1,3 likely due to the relative weakness of the M–O bonds that are formed,⁴ which makes cleavage of the M–O bond and subsequent transformation more facile.

The crucial role of CO₂ insertion into late transition metal element σ-bonds in catalysis means that understanding the pathway for insertion is important for rationally designing improved catalytic systems.⁴ In particular, understanding the impact of the ancillary ligand, solvent, and additives that are commonly used in catalysis, such as Lewis acids, is valuable for the development of optimized catalysts and reaction conditions. Although the influence of these factors on CO₂ insertion into metal hydrides has been studied,⁵ there is limited information on how these variables affect CO₂ insertion into metal alkyl bonds.^6–13 These reactions are particularly important because they can result in the generation of products containing C–C bonds from CO₂, which has been identified as a high priority research target by the United States National Academies of Science.² For example, group 10 catalysts have been used for the reductive carboxylation of a variety of alkyl halides and pseudo halides.^3h,i In these reactions, C–C bonds are proposed to form between CO₂ and the alkyl electrophile via the insertion of CO₂ into a metal alkyl bond.

A major limitation in studying CO₂ insertion into group 10 metal alkyl bonds is the paucity of systems that are stable and react under mild conditions.¹⁴ In fact, the majority of well-defined examples of CO₂ insertion reactions into group 10 metal alkyls require elevated temperatures, long reaction times, and do not give quantitative yields of products, which makes it difficult to perform kinetic studies (Fig. 1a). Due to the need for high temperatures, there has been a focus on complexes supported by pincer ligands, which often result in complexes with high thermal stability. For example, Wendt and co-workers demonstrated that treatment of (^tBuPCP)Pd(CH₃) (^tBuPCP = 2,6-C₆H₃(CH₂P^tBu₂)₂) with 4 atm of CO₂ at 80 °C results in complete conversion to the κ¹-acetate complex (^tBuPCP)Pd{OC(O)CH₃}, but the reaction takes two days.¹⁵ Similarly, we reported that the reaction of 1 atm of CO₂ with the related compound (^tBuPCP)Ni(CH₃) requires heating to 150 °C to form the acetate complex (^tBuPCP)Ni{OC(O)CH₃}, but only approximately 75% of the acetate product is formed.¹⁶ Changing the pincer ligand from ^tBuPCP to a system containing an sp³-hybridized carbon donor or replacing one of the phosphine arms of the pincer ligand with a hemi-labile nitrogen donor only leads to small changes in the rate of reaction.¹⁷ A notable example of a group 10 methyl complex that does not contain a pincer ligand and inserts CO₂ at room temperature is an unusual anionic palladium dimethyl complex, with a pendant Lewis acid, prepared by Jordan and co-workers.⁷ Although this system inserts CO₂ rapidly it requires careful handling as it is unstable at room temperature. Overall, in order to gain detailed information on the factors that influence the rate of CO₂ insertion there is a need for stable alkyl complexes that cleanly react with CO₂ under mild conditions.

Fig. 1 (a) Previous examples of pincer-supported group 10 methyl complexes that undergo CO₂ insertion to form κ¹-acetate complexes. (b) ^RPBP supported palladium complexes studied in this work, which undergo CO₂ insertion under milder conditions.

In this work, we describe the insertion of CO₂ into palladium and nickel methyl complexes supported by ^RPBP (^RPBP = B(NCH₂PR₂)₂C₆H₄⁻; R = Cy or ^tBu) pincer ligands, which contain a strongly donating central boryl donor (Fig. 1b). As a result of the strong trans-influence of the boryl donor, these complexes insert CO₂ at significantly milder conditions (room temperature) than previously reported pincer systems. This enables us to measure the kinetics of CO₂ insertion and study the effect of modulating the pincer ligand and the solvent on CO₂ insertion. We demonstrate that the Dimroth–Reichardt E_T(30) parameter¹⁸ for a solvent correlates with the rate of CO₂ insertion and show that Lewis acids, which are often present in catalysis, do not promote CO₂ insertion. Our experimental studies are complemented by computational investigations to understand the pathway for CO₂ insertion and identify the structure of key transition states and intermediates. Overall, our work is one of the first quantitative investigations of the impact of the ancillary ligand, solvent, and LAs on CO₂ insertion into metal alkyl bonds and will assist in the development of improved and new catalysts for CO₂ utilization.

Results and discussion

Synthesis of ^RPBP supported nickel and palladium methyl complexes and reactivity with CO₂

Previous studies on CO₂ insertion into pincer supported group 10 metal hydrides demonstrate that the rate of CO₂ insertion increases as the trans-influence of the donor opposite to the hydride ligand increases.¹⁹ As a result, nickel and palladium hydride complexes supported by ^CyPSiP (^CyPSiP = Si(Me)(2-PCy₂-C₆H₄)₂) ligands, which contain a strongly donating silyl ligand trans to the hydride insert CO₂ more rapidly than the corresponding species containing ^RPCP ligands. We hypothesized that a similar trend may also occur for nickel and palladium methyl species. Unfortunately, the previously described complexes (^CyPSiP)M(CH₃) (M = Ni or Pd) undergo an intramolecular ligand rearrangement,²⁰ as does (^tBuPSiP)Pd(CH₃) (^tBuPSiP = Si(Me)(2-P^tBu₂-C₆H₄)₂, see ESI†) which complicates studies of CO₂ insertion. Therefore, we prepared nickel and palladium methyl complexes supported by ^RPBP ligands (^RPBP = B(NCH₂PR₂)₂C₆H₄; R = Cy or ^tBu) that feature a boryl donor, which has previously been shown to exert a large trans-influence,²¹ opposite the methyl ligand.

(^CyPBP)Ni(CH₃) was synthesized via an analogous route to that previously described for (^tBuPBP)Ni(CH₃).^21h Initially, NiCl₂·DME was treated with ^CyPB^HP to generate (^CyPBP)NiCl in 68% yield (Scheme 1). The subsequent reaction of (^CyPBP)NiCl with MeMgCl generates (^CyPBP)Ni(CH₃) in 26% yield. The major cause of the low yield of (^CyPBP)Ni(CH₃) is the need to perform sequential recrystallizations to remove magnesium salts. Specifically, considerable care is required to remove MgCl₂ by-products from the Grignard reagent, which cause the methyl species to convert back to the chloride complex. (^CyPBP)Ni(CH₃) was characterized in the solid-state using X-ray crystallography (Fig. 2) and has an analogous distorted square planar geometry to (^tBuPBP)Ni(CH₃).^21h The P(1)–Ni(1)–P(2) bond angle is 158.67(10)°, which is similar to that observed in (^tBuPBP)Ni(CH₃) (157.58(2)°). The B(1)–Ni(1)–C(1) bond angle is almost linear (179.1(4)°), which presumably results in the boryl ligand exerting the maximum possible trans-influence on the methyl ligand. The Ni(1)–B(1) bond length in (^CyPBP)Ni(CH₃) is 1.927(9) Å, which is comparable to other species with a Ni–boryl bond.^21h,22 The nickel methyl bond length is 2.019(9) Å, which is significantly shorter than the nickel methyl bond length in (^tBuPBP)Ni(CH₃), which is 2.059(2) Å. The longer bond length in (^tBuPBP)Ni(CH₃) is likely due to steric factors, as the larger steric properties of the tert-butyl substituents in (^tBuPBP)Ni(CH₃) may cause the nickel methyl bond to elongate. The increased steric demand of the ^tBuPBP ligand was confirmed using buried volume calculations (SambVca 2.1 (ref. 23)) using the crystal structures of (^RPBP)Ni(CH₃), which show that the buried volume of (^tBuPBP)Ni(CH₃) is 92.1% compared to 89.3% for (^CyPBP)Ni(CH₃) (see ESI†).

Scheme 1 Synthesis of (^CyPBP)Ni(CH₃).

Fig. 2 Solid-state structure of (^CyPBP)Ni(CH₃) with thermal ellipsoids at 30% probability. Hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (°): Ni(1)–B(1) 1.927(9), Ni(1)–C(1) 2.019(9), Ni(1)–P(1) 2.148(2), Ni(1)–P(2) 2.138(2), B(1)–Ni(1)–C(1) 179.1(4), B(1)–Ni(1)–P(1) 80.1(3), C(1)–Ni(1)–P(1) 100.6(3), B(1)–Ni(1)–P(2) 78.7(3), C(1)–Ni(1)–P(2) 100.6(3), P(1)–Ni(1)–P(2) 158.67(10).

Palladium chloride complexes supported by ^RPBP (R = Cy or ^tBu) ligands were synthesized through the reaction of the protonated ^RPB^HP ligand with commercially available [(η³-allyl)PdCl]₂ in yields of 69% and 63%, respectively. This is different to a previous route utilized to metallate the ^tBuPBP ligand, which used the more expensive Pd(COD)Cl₂ as the palladium source.^21k Treatment of (^RPBP)PdCl with MeLi results in the formation of the (^RPBP)Pd(CH₃) in yields of 25% for (^CyPBP)Pd(CH₃) and 60% for (^tBuPBP)Pd(CH₃) (Scheme 2). In an analogous fashion to (^CyPBP)Ni(CH₃), it is important to remove alkali halide by-products as they can cause the palladium methyl species to be converted back to the palladium chloride precursor and the multiple recrystallizations required for (^CyPBP)Pd(CH₃) lowers the yield. (^tBuPBP)Pd(CH₃) was characterized using X-ray crystallography (Fig. 3). The geometry around palladium is distorted square planar. The P(1)–Pd(1)–P(2) bond angle is 153.74(3)°, which is a greater deviation from linearity than that observed in a related complex with a ^tBuPCP ligand. Specifically in (^tBuPCP)PdCl P–Pd–P is 165.79(2)°.²⁴ As a consequence, the fourth coordination site in pincer complexes supported by ^tBuPBP is likely more sterically accessible than in species supported by ^tBuPCP ligands. The Pd(1)–B(1) bond length in (^tBuPBP)Pd(CH₃) is 2.027(4) Å, which is significantly longer than the Pd–B bond length in the only other crystallographically characterized palladium complex with a ^tBuPBP ligand, (^tBuPBP)PdCl (Pd–B is 1.972(4) Å). This is consistent with the methyl ligand exerting a significantly stronger trans-influence than the chloride ligand.

Scheme 2 Synthesis of (^RPBP)Pd(CH₃) (R = Cy or ^tBu).

Fig. 3 Solid-state structure of (^tBuPBP)Pd(CH₃) with thermal ellipsoids at 30% probability. Hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (°): Pd(1)–B(1) 2.027(4), Pd(1)–C(1) 2.185(4), Pd(1)–P(1) 2.3030(8), Pd(1)–P(2) 2.3035(8), B(1)–Pd(1)–C(1) 178.09(15), B(1)–Pd(1)–P(1) 77.36(12), C(1)–Pd(1)–P(1) 103.49(10), B(1)–Pd(1)–P(2) 77.33(12), C(1)–Pd(1)–P(2) 102.02(10), P(1)–Pd(1)–P(2) 153.74(3).

We treated complexes of the type (^RPBP)M(CH₃) (M = Ni or Pd, R = Cy or ^tBu) with 1 atm of CO₂ in C₆D₆ at room temperature (Scheme 3). In the cases of (^RPBP)Pd(CH₃) (R = Cy or ^tBu) NMR spectroscopy indicated clean and essentially quantitative CO₂ insertion to the corresponding acetate complexes at room temperature. The reactions took 1 and 6 hours to reach completion for (^CyPBP)Pd(CH₃) and (^tBuPBP)Pd(CH₃), respectively. These are the most rapid insertions of CO₂ into pincer-supported group 10 metal alkyl complexes observed to date. The acetate products were isolated and (^tBuPBP)Pd{OC(O)CH₃} was characterized using X-ray crystallography (Fig. 4). The solid-state structure of (^tBuPBP)Pd{OC(O)CH₃} has the same distorted square planar geometry as (^tBuPBP)Pd(CH₃) and confirms κ¹-binding of the acetate. As expected, the Pd(1)–B(1) bond length (1.975(2) Å) in (^tBuPBP)Pd{OC(O)CH₃} is significantly shorter than the Pd–B bond length in (^tBuPBP)Pd(CH₃) (2.027(4) Å) due to the weaker trans-influence of the acetate ligand. Further, the increased Pd–O bond length in (^tBuPBP)Pd{OC(O)CH₃} (2.1861(14) Å) in comparison to (^tBuPCP)Pd{OC(O)CH₃} (2.1279(19) Å)¹⁵ is consistent with the increased trans-influence of the ^tBuPBP ligand in comparison to the related ^tBuPCP ligand. In contrast, the insertion of CO₂ into (^CyPBP)Ni(CH₃) was not clean. Although NMR spectroscopy suggested that (^CyPBP)Ni{OC(O)CH₃}, which was not isolated, was the major product, free ligand and several unidentified decomposition products were present (see ESI†). Additionally, it took approximately 4 days at room temperature for all of the (^CyPBP)Ni(CH₃) to disappear. Overall, from these initial qualitative examples of CO₂ insertion into (^CyPBP)M(CH₃) (M = Ni or Pd) and (^tBuPBP)Pd(CH₃) we can make the following conclusions: (i) the insertion of CO₂ into (^CyPBP)Pd(CH₃) is faster than the corresponding insertion into (^CyPBP)Ni(CH₃). This is in agreement with results for CO₂ insertion into (^tBuPCP)M(CH₃) (M = Ni or Pd)^15,16 but stands in contrast to computational studies of CO₂ insertion into (^tBuPCP)MH (M = Ni or Pd), where insertion is proposed to be faster for nickel.¹⁹ (ii) The insertion of CO₂ into (^tBuPBP)Pd(CH₃) is significantly faster than the corresponding insertion into (^tBuPCP)Pd(CH₃).¹⁵ This suggests that the rate of CO₂ insertion can be increased by having a stronger trans-influence donor opposite the methyl, similar to what has been observed for CO₂ insertion into group 10 metal hydrides.¹⁹ (iii) The faster insertion of CO₂ into (^CyPBP)Pd(CH₃) compared to (^tBuPBP)Pd(CH₃) suggests that steric factors are important.

Scheme 3 Generation of (^RPBP)M{OC(O)CH₃} (M = Ni, R = Cy; M = Pd, R = Cy or ^tBu) through CO₂ insertion into (^RPBP)M(CH₃).

Fig. 4 Solid-state structure of (^tBuPBP)Pd{OC(O)CH₃} with thermal ellipsoids at 30% probability. Hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (°): Pd(1)–B(1) 1.975(2), Pd(1)–O(1) 2.1861(14), Pd(1)–P(1) 2.3256(5), Pd(1)–P(2) 2.3336(5), B(1)–Pd(1)–O(1) 174.99(7), B(1)–Pd(1)–P(1) 77.32(7), O(1)–Pd(1)–P(1) 100.17(4), B(1)–Pd(1)–P(2) 79.00(7), O(1)–Pd(1)–P(2) 103.05(4), P(1)–Pd(1)–P(2) 155.805(18).

In contrast to the quantitative or relatively clean insertion of CO₂ observed for (^CyPBP)Pd(CH₃), (^tBuPBP)Pd(CH₃), and (^CyPBP)Ni(CH₃), insertion into (^tBuPBP)Ni(CH₃) was not clean and was significantly slower than for the other methyl compounds. After 10 days at room temperature, approximately 30% of (^tBuPBP)Ni(CH₃) was still observed, along with 50% of a product we propose to be (^tBuPBP)Ni{OC(O)CH₃} on the basis of its similar ³¹P NMR chemical shift to (^CyPBP)Ni{OC(O)CH₃}, and 20% of unidentified decomposition products (Scheme 4). Attempts to independently synthesize (^tBuPBP)Ni{OC(O)CH₃} via reactions between silver acetate and (^tBuPBP)NiCl or sodium acetate and (^tBuPBP)Ni(OTf) were unsuccessful. Although, initially a new resonance is observed in the ³¹P NMR spectra of these reactions with the same chemical shift as the proposed insertion product with CO₂ (84.9 ppm), this resonance disappears over time, indicating that (^tBuPBP)Ni{OC(O)CH₃} is likely unstable in solution. This decomposition prohibited isolation and full characterization of (^tBuPBP)Ni{OC(O)CH₃}. Nevertheless, since the rate of CO₂ insertion into (^tBuPBP)Ni(CH₃) is presumably independent of the decomposition of the corresponding insertion product, we can make qualitative conclusions about the rate of insertion. We hypothesize that the slower rate of CO₂ insertion is related to the increased steric bulk of (^tBuPBP)Ni(CH₃), which makes it harder for CO₂ to interact with the complex. When (^tBuPBP)Ni(CH₃) was treated with CO₂ at higher temperatures even larger amounts of decomposition products were observed. Control experiments indicate that (^tBuPBP)Ni(CH₃) is not stable even when heated under an N₂ atmosphere. Although we were unable to identify all of the decomposition products of (^tBuPBP)Ni(CH₃), an unusual dinuclear species (^tBuPB^MeP)₂Ni₂(μ-N₂) (^tBuPB^MeP = MeB(NCH₂P^tBu₂)₂C₆H₄), containing two nickel(0) centers and a bridging dinitrogen ligand crystallized out of solution (Fig. 5). In this complex, the nickel methyl ligand has undergone reductive coupling with the boryl ligand to form a new C–B bond. The boron atom no longer coordinates to the nickel center and the two phosphorus donors of the new bidentate ^tBuPB^MeP ligands do not coordinate to the same nickel center but instead coordinate to two different nickel atoms. The geometry around the nickel centers is distorted trigonal planar and the dinitrogen ligand coordinates in an end-on fashion. The N–N bond distance of 1.145(8) Å is slightly elongated compared to that of free N₂ (1.098 Å), consistent with weak activation of N₂ and what has been observed in related nickel dinitrogen complexes.²⁵ The decomposition of (^tBuPBP)Ni(CH₃) into (^tBuPB^MeP)₂Ni₂(μ-N₂) demonstrates a new reductive pathway for decomposition of complexes supported by ^RPBP ligands. However, this is likely a particular problem for the ^tBuPBP ligand when it is coordinated to Ni because of steric constraints associated with the bulky tert-butyl substituents and small size of nickel.

Scheme 4 CO₂ insertion into (^tBuPBP)Ni(CH₃).

Fig. 5 a) Decomposition of (^tBuPBP)Ni(CH₃) under N₂ at 60 °C. (b) Solid-state structure of (^tBuPB^MeP)₂Ni₂(μ-N₂) with thermal ellipsoids at 30% probability. The model is near the crystallographic inversion center; half of the structure is generated by symmetry. Hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (°): Ni(1)–N(1) 1.854(4), Ni(1)–P(1) 2.293(11), Ni(1)–P(2) 2.164(11), N(1)–N(1A) 1.145(8), P(1)–Ni(1)–P(2) 122.0(5), N(1)–Ni(1)–P(1) 118.9, N(1)–Ni(1)–P(2) 110.8(3), Ni(1)–N(1)–N(1A) 178.7(5).

Kinetic studies of CO₂ insertion into (^tBuPBP)Pd(CH₃). It has traditionally been challenging to perform kinetics experiments on CO₂ insertion into group 10 alkyl complexes either because of the instability of the complexes¹⁴ or the harsh conditions required for insertion.^16,17 In contrast, the insertion of CO₂ into (^tBuPBP)Pd(CH₃) under mild conditions enables kinetics experiments to be performed. Initially, ¹H or ³¹P NMR spectroscopy was used to monitor the reaction of (^tBuPBP)Pd(CH₃) with CO₂ as a function of time (Fig. 6a). It was possible to measure the concentrations of both the (^tBuPBP)Pd(CH₃) starting material and the (^tBuPBP)Pd{OC(O)CH₃} product, which enabled us to confirm the reaction is clean and quantitative. An excess of CO₂ was used to ensure that the reactions were performed under pseudo-first order conditions with the concentration of CO₂ measured using quantitative ¹³C NMR spectroscopy (see SI).²⁶ Initial experiments in C₆D₆ indicate that the reaction is first order in both [(^tBuPBP)Pd(CH₃)] and [CO₂], demonstrating that the overall rate law is k₁[(^tBuPBP)Pd(CH₃)][CO₂] (Fig. 6b and 7). The k_obs values obtained from a plot of ln([(^tBuPBP)Pd(CH₃)]) versus time were divided by the concentration of CO₂ to obtain values of k₁ (see ESI†). Monitoring the rate of the reaction in C₆D₆ at various temperatures allowed for the activation parameters to be determined through Eyring analysis (see ESI†). The enthalpy of activation, ΔH^‡, 10.9 ± 1.1 kcal mol⁻¹, for insertion into (^tBuPBP)Pd(CH₃) is significantly lower than that for (^tBuPCP)Pd(CH₃) (ΔH^‡ = 17.4 ± 1.7 kcal mol⁻¹),²⁶ which is the most closely related system for which an experimental activation energy for CO₂ insertion has been measured. This is consistent with the hypothesis that the strong trans-influence of the ^tBuPBP ligand weakens the palladium methyl bond and destabilizes the ground state. The entropies of activation, ΔS^‡, for (^tBuPBP)Pd(CH₃) and (^tBuPCP)Pd(CH₃) are within error of each other at −32.2 ± 3.2 and −28.2 ± 4.54 cal mol⁻¹ K⁻¹, respectively. The similar negative values suggest that the nature of the rate-limiting transition state in both reactions are analogous and is in agreement with two molecules combining to form one molecule in the transition state (vide infra). The overall difference in ΔG^‡ at 298 K between (^tBuPBP)Pd(CH₃) (20.5 ± 2.1 kcal mol⁻¹) and (^tBuPCP)Pd(CH₃) (25.8 ± 2.6 kcal mol⁻¹) is consistent with the much faster rate of insertion observed for the ^tBuPBP system.

Fig. 6 Representative traces for the insertion of CO₂ into (^tBuPBP)Pd(CH₃) at 40 °C in C₆D₆ with 1 atm of CO₂ showing (a) the concentrations of (^tBuPBP)Pd(CH₃) and (^tBuPBP)Pd{OC(O)CH₃} as a function of time and (b) the ln of the concentration of (^tBuPBP)Pd(CH₃) as a function of time.

Fig. 7 Reaction order in [CO₂] of the insertion of CO₂ into (^tBuPBP)Pd(CH₃) in C₆D₆ at 40 °C.

In CO₂ insertion reactions into metal hydrides, it has been demonstrated that the solvent can have a major influence on the rate of insertion.⁵ To date there are no studies exploring how the rate of CO₂ insertion into metal alkyls varies as the solvent is changed. We determined the rate of CO₂ insertion (k₁) into (^tBuPBP)Pd(CH₃) in toluene-d₈, benzene-d₆, 1,4-dioxane, THF, and glyme (Table 1). There was only a small solvent effect with approximately a 1.9-fold difference in k₁ between the slowest (toluene-d₈) and fastest (glyme) solvent (Table 1). This compares with a 30-fold increase in the rate of CO₂ insertion into an iridium hydride in a similar set of solvents.^5a We propose that the smaller impact of solvent in this case is because there is less build-up of charge in the rate-determining transition state for CO₂ insertion (vide infra) compared to the corresponding transition state for metal hydrides. To determine if the variation in the rate of insertion correlated with any solvent parameters, ln(k₁) was plotted against the dielectric constant, Gutmann Acceptor Number (AN),²⁷ or Dimroth–Reichardt E_T(30) parameter¹⁸ of the solvent. No significant correlation was observed between the rate of CO₂ insertion and the dielectric constant or AN of a solvent (see ESI†). In contrast, an excellent correlation was observed between the E_T(30) value of a solvent and the rate of CO₂ insertion (Fig. 8), suggesting that E_T(30) is a good predictor of the rate of CO₂ insertion. A similar relationship was observed for CO₂ insertion into metal hydrides and hydroxides,^5b raising the possibility that this correlation is general across a variety of distinct metal-element σ-bonds. It is likely that E_T(30) is a better predictor of CO₂ insertion rates because it explicitly considers specific solute/solvent interactions, which are not accounted for when the solvent is modelled using either dielectric constant or AN.^5b Unfortunately, we were unable to extend the correlation between the rate of CO₂ insertion into (^tBuPBP)Pd(CH₃) and E_T(30) to more polar solvents. This is because these solvents led to the decomposition of (^tBuPBP)Pd(CH₃) or resulted in rates that were too fast to measure using NMR spectroscopy. For example, CO₂ insertion into (^tBuPBP)Pd(CH₃) is complete in less than 10 minutes at room temperature in pyridine and benzonitrile, which is too fast to obtain reliable kinetics by NMR spectroscopy. Nevertheless, given the slow rates of CO₂ insertion previously observed into pincer-supported group 10 alkyls, the completion of reactions in less than 10 minutes at room temperature is unprecedented.

Table 1 Effect of solvent on rate of CO₂ insertion into (^tBuPBP)Pd(CH₃) at 40 °C and 1 atm of CO₂

Entry	Solvent	k 1 (M^−1 s^−1 × 10^−2)	Rate enhancement
a These values are the average of two trials and the errors are ±10%.
1	Toluene-d8	1.1	1
2	Benzene-d6	1.3	1.2
3	1,4-Dioxane	1.5	1.4
4	Tetrahydrofuran	1.8	1.6
5	Glyme	2.1	1.9

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Fig. 8 Plot of ln(k₁) for the insertion of CO₂ into (^tBuPBP)Pd(CH₃) versus the Dimroth–Reichardt E_T(30) solvent parameter for various solvents at 40 °C and 1 atm of CO₂.

Lewis acids have been proposed to increase the rate of CO₂ insertion into a variety of different metal-element σ-bonds, but these effects are often dependent on the pathway of insertion and the reaction conditions.^5,7,8,9b,28 As a result, it is difficult to predict when a Lewis acid will increase the rate of a CO₂ insertion reaction. We evaluated the impact of a variety of different Lewis acids on the rate of CO₂ insertion into (^tBuPCP)Pd(CH₃) using THF as the solvent (Table 2).²⁹ Although there is an increase in rate when the reaction was performed in the presence of 20 equivalents of a variety of Lewis acids, a control experiment using 20 equivalents of (ⁿBu₄)PF₆, which does not contain a Lewis acid, gives the same enhancement. This indicates that there is no increase in rate caused by Lewis acids and instead it is likely the case that increasing the ionic strength of the solution results in a slight enhancement in the rate of CO₂ insertion. Consistent with this hypothesis, all of the different Lewis acids evaluated give approximately the same increase in the rate of CO₂ insertion. At this stage, it is unclear why no Lewis acid effect is observed in the insertion of CO₂ into (^tBuPBP)Pd(CH₃), given that Li⁺ has been demonstrated to increase the rate of CO₂ insertion into (CO)₅W(CH₃).⁸ However, in the next section, DFT calculations are performed to understand the exact nature of the transition states involved in CO₂ insertion into (^tBuPBP)Pd(CH₃).

Table 2 Effect of Lewis acids on rate of CO₂ insertion into (^tBuPBP)Pd(CH₃) in THF at 40 °C and 1 atm of CO₂

Entry	Additive (20 equiv.)	k obs (s^−1 × 10^−3)	Rate enhancement
a NTf = triflimide. b These values are the average of two trials and the errors are ±10%.
1	None	1.6	1
2	(NBu4)PF6	2.3	1.4
3	LiNTf2^a	2.2	1.4
4	NaNTf2^a	2.1	1.3
5	KNTf2^a	2.3	1.4
6	LiPF6	2.4	1.5
7	LiBPh4·3DME	2.5	1.6
8	LiOTf	2.1	1.3

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Computational studies of CO₂ insertion into (^RPBP)Pd(CH₃) and related species. Previous studies investigating CO₂ insertion into PCP-supported palladium and nickel methyl complexes have proposed two plausible pathways for insertion: (i) a 1,2-insertion (inner sphere) or (ii) an S_E2 pathway (outer sphere).^16,17c,26 In the 1,2-insertion pathway, the O-bound acetate product is formed in one step and CO₂ interacts with the metal in the key transition state (Fig. 9a). This means that both the C–C and M–O bonds are formed at the same time. In contrast, the S_E2 pathway involves two steps and starts with electrophilic attack of CO₂ on the coordinated methyl, without any direct interaction between CO₂ and the metal (Fig. 9b). This results in the formation of the new C–C bond. In the second step, the intermediate containing an acetate-metal ion-pair rearranges via TS2 to form the O-bound acetate adduct. Here, we used DFT methods to understand the pathway for CO₂ insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd). We also independently calculated CO₂ insertion into (^tBuPCP)Pd(CH₃) in order to directly compare it with the ^tBuPBP systems.

Fig. 9 Two plausible mechanisms for CO₂ insertion into pincer-supported group 10 methyls: (a) 1,2-insertion and (b) S_E2.

Our calculations indicate that CO₂ insertion into (^tBuPBP)Pd(CH₃) proceeds via an S_E2 pathway (Fig. 10, S43, S44† & Table 3). The rate-limiting step is the initial attack of CO₂ on the palladium methyl group (TS1_Out), which has a computed barrier of 19.0 kcal mol⁻¹ (298 K). This is within error of the activation barrier measured experimentally (vide supra). Although there is no interaction between CO₂ and the metal in TS1_Out, we note that there are two C–H…OCO interactions between the tert-butyl substituents of the ^tBuPBP ligand and CO₂ (2.43 and 2.74 Å, respectively), which provide charge stabilization. This may explain why the addition of Lewis acids to the system does not result in a rate enhancement (vide supra). The second step of the S_E2 pathway has a very small barrier and involves rotation of the acetate-palladium ion pair to form the O-bound acetate product (Fig. 10). In contrast, the barrier for the alternative 1,2-insertion is significantly higher at 25.1 kcal mol⁻¹ (TS1_Inn, Fig. 10 and S45†). The high barrier is likely caused by the bulky tert-butyl substituents of the ^tBuPBP ligand, which make it difficult for CO₂ to approach palladium without steric repulsion. In order to investigate this hypothesis, we calculated CO₂ insertion into (^MePBP)Pd(CH₃), where the tert-butyl substituents on the ^tBuPBP ligand have been replaced with methyl substituents. For CO₂ insertion into (^MePBP)Pd(CH₃) the 1,2-insertion pathway is preferred by 3.1 kcal mol⁻¹ over the S_E2 mechanism (Fig. 10, S46 and S47†). Thus, the size of the pincer ligand has a decisive role in promoting either a 1,2-insertion (small phosphine substituents) or an S_E2 pathway (large phosphine substituents). Similar ligand effects were previously observed for NHC–Cu complexes.³⁰

Fig. 10 Top: computed free energy profile (B3LYP-D3/def2-TZVPPD[CPCM]//B3LYP-D3/def2-SVP[CPCM], 298 K) for CO₂ insertion into (^tBuPBP)Pd(CH₃), bottom: optimized transition state structures for CO₂ insertion into (^RPBP)Pd(CH₃), with R = ^tBu or Me. All energies are in kcal mol⁻¹.

Table 3 Calculated free energy barriers for inner and outer sphere C–CO₂ bond formation with different pincer-supported palladium and nickel complexes (kcal mol⁻¹, B3LYP-D3/def2-TZVPPD[CPCM]//B3LYP-D3/def2-SVP[CPCM], 298 K)^a

Complex	TS1_Out (kcal mol^−1)	TS1_Inn (kcal mol^−1)
a For comparison, results for the ωB97XD functional are given in parenthesis, showing the same mechanistic trends.
(^tBuPBP)Pd(CH3)	19.0 (23.9)	25.1 (30.6)
(^MePBP)Pd(CH3)	22.5 (23.4)	19.4 (21.0)
(^tBuPBP)Ni(CH3)	24.5 (27.8)	31.9 (33.7)
(^tBuPCP)Pd(CH3)	24.2 (29.2)	36.7 (41.1)
(^MePCP)Pd(CH3)	26.8 (30.0)	25.0 (27.8)

---

The ^tBuPCP-supported complex, (^tBuPCP)Pd(CH₃), also shows a preference for the S_E2 pathway, with a computed barrier of 24.2 kcal mol⁻¹ (TS_Out, Fig. S48 and S49†). Consistent with previous results,²⁶ the 1,2-insertion mechanism is unlikely for (^tBuPCP)Pd(CH₃) due to the high barrier of 36.7 kcal mol⁻¹ (TS_Inn, Fig. S50†).³¹ Thus, both (^tBuPBP)Pd(CH₃) and (^tBuPCP)Pd(CH₃) show a clear preference for an S_E2 mechanism. In the rate limiting transition state (TS_Out), (^tBuPBP)Pd(CH₃) displays a longer C–CO₂ bond (by 0.09 Å) and a smaller CO₂ angle (by 1.7°) compared to (^tBuPCP)Pd(CH₃), which indicates an earlier TS. The lower barrier for (^tBuPBP)Pd(CH₃) (by 5.2 kcal mol⁻¹) is in agreement with our kinetic studies, showing that CO₂ insertion is much faster for (^tBuPBP)Pd(CH₃). The decrease in the barrier for CO₂ insertion is also matched by the greater thermodynamic driving force for CO₂ insertion into (^tBuPBP)Pd(CH₃) compared with (^tBuPCP)Pd(CH₃) (−23.3 kcal mol⁻¹versus −15.8 kcal mol⁻¹). These results highlight how a donor atom with strong trans-influence opposite to the methyl can lower the barrier for insertion and suggest that the enhancement is due to destabilization of the ground state as opposed to a kinetic effect associated with stabilization of the transition state.

The slower qualitative rate of CO₂ insertion into (^tBuPBP)Ni(CH₃) compared to (^tBuPBP)Pd(CH₃) (vide supra) is surprising given that CO₂ insertion into nickel hydrides is typically faster than palladium hydrides.¹⁹ Calculations on CO₂ insertion into (^tBuPBP)Ni(CH₃) confirm our experimental results as the calculated barrier is 24.5 kcal mol⁻¹ at 298 K, more than 5 kcal mol⁻¹ higher than for (^tBuPBP)Pd(CH₃) (B3LYP-D3, Table 3). The lowest energy pathway for CO₂ insertion into (^tBuPBP)Ni(CH₃) also follows an S_E2 mechanism (Fig. S51 and 52†). Geometrical analysis of the rate-determining transition states for CO₂ insertion into (^tBuPBP)Ni(CH₃) and (^tBuPBP)Pd(CH₃) shows that the Ni–P bonds are shorter by around 0.15 Å compared with the corresponding Pd–P bonds due to the smaller atomic radius of nickel. We propose that the increased steric congestion around nickel results in the higher barrier for CO₂ insertion relative to the palladium system.³² This is in agreement with previous results showing that CO₂ insertion reactions into nickel hydrides are highly sensitive to steric factors.^5a It is likely, however, that there is no inherent electronic reason for CO₂ insertion into nickel methyl complexes to be slower than palladium methyl complexes, and if a nickel complex could be prepared where steric factors were not relevant,³³ we expect to see comparable rates of insertion.

Conclusions

In this work, we have demonstrated that nickel and palladium methyl complexes supported by ^RPBP ligands undergo CO₂ insertion under significantly milder conditions than any previously reported pincer-ligated group 10 alkyl species. This is because of the strong trans-influence of the central boryl donor in ^RPBP type ligands, which destabilizes the M–CH₃ bond. Qualitatively, we show that CO₂ insertion into (^RPBP)Pd(CH₃) is faster than CO₂ insertion into (^RPBP)Ni(CH₃), which is likely due to steric factors. In contrast, we propose that steric factors are less significant in CO₂ insertion into group 10 hydrides, and as a result nickel hydrides typically undergo faster insertion than palladium hydrides.¹⁹ Given the rapid rate of insertion into ^RPBP-ligated complexes, we were able to complete a rare kinetic study on CO₂ insertion into a metal alkyl complex. Our results demonstrate that the solvent parameter that is best for predicting the rate of CO₂ insertion is the Dimroth–Reichardt E_T(30) parameter. This is analogous to what we have observed for transition metal hydrides,^5b but the dependence of the insertion rate on the solvent is significantly less pronounced in the case of the metal alkyl complex. We hypothesize that this is because there is less charge build-up in the transition state for carbon dioxide insertion into a metal alkyl complex compared to related metal hydrides. Consistent with there being only a small build-up of charge in the transition state, we did not observe any enhancement from Lewis acids in CO₂ insertion reactions into (^tBuPBP)Pd(CH₃). DFT calculations indicate that the proposed mechanism for insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd) follows an S_E2 pathway, but this is dependent on steric factors and compounds with less bulky substituents on the phosphine donors are more likely to undergo insertion via a 1,2-insertion mechanism. Overall, our results indicate that the fastest systems for CO₂ insertion into a group 10 methyl will have a strong trans-influence donor opposite the methyl, be able to undergo insertion in solvents with high E_T(30) values, and not have sterically bulky ancillary ligands that prevent the approach of CO₂. Future research in our group will aim to extend these principles to other metal-element σ-bonds and apply them to improve catalytic reactions.

Data availability

Additional information about selected experiments, NMR spectra, and other details are availabe via the internet.

Author contributions

Project conception: AJD, MRE, DJC, NH; experimental work: AJD, MRE, DJC; X-ray crystallography: BQM; computational analysis: LjP, KHH. All authors contributed to writing the manuscript.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

NH acknowledges support from National Science Foundation through Grant CHE-1953708. We thank Dr Hee Won Suh for preliminary work on this project and Dr Fabian Menges for assistance obtaining high resolution mass spectrometry data with equipment funded by NSF MRI Grant CHE-1828190. KHH and LjP thank the Research Council of Norway (Grant No. 300769), the Tromsø Research Foundation (Grant No. TFS2016KHH), NordForsk (Grant No. 85378) and Sigma2 - the National Infrastructure for High Performance Computing and Data Storage in Norway (No. nn9330k).

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31. We also calculated CO₂ insertion into (^MePCP)Pd(CH₃), where the tert-butyl substituents on the ^tBuPCP ligand have been replaced with methyl substituents. The calculations indicate that for (^MePCP)Pd(CH₃), the 1,2-insertion pathway is preferred over the S_E2 mechanism by 1.8 kcal mol⁻¹ (Table 3, Fig. S53 and S54†). This is consistent with the hypothesis that the steric properties of the pincer ligand play a large role in determining the pathway for CO₂ insertion.
32. We note that other computational protocols (ωB97XD, PBE0-D3BJ) provide the same reactivity trends as B3LYP-D3 (Table S3†).
33. Our results do not explain Wendt's observation (see ref. 17b and 17c) that CO₂ insertion into (^tBuPC^iPrN)Ni(CH₃) (^tBuPC^iPrN = 1-(3-((ditert-butylphosphino)methyl)phenyl)-N,N-diisopropyl-methanamine)) is faster than insertion into (^tBuPC^MeN)Ni(CH₃) (^tBuPC^MeN = 1-(3-((ditert-butylphosphino)methyl)phenyl)-N,N-methyl-methanamine)). In this case, ligand hemilability may play a role and it is possible that the more sterically bulky ^tBuPC^iPrN is more likely to decoordinate.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2107492–2107496 and 2108774. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc06346d

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