# Sodium Acrylate Synthesis Through CO 2 and Ethylene Coupling Catalyzed by Pd: Solvent Effects and PdBr 2 as a Surprisingly Suitable Precursor

Kana Kunihiro, Audrey Marty, Ludovic Chahen, Lionel Magna, Juan Asensio

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Kana Kunihiro, Audrey Marty, Ludovic Chahen, Lionel Magna, Juan Asensio. Sodium Acrylate Synthesis Through CO 2 and Ethylene Coupling Catalyzed by Pd: Solvent Effects and PdBr 2 as a Surprisingly Suitable Precursor. ChemCatChem, 2024, 16 (21), pp.e202401074. 10.1002/cctc.202401074. hal-04886276

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# Sodium Acrylate Synthesis Through CO₂ and Ethylene Coupling Catalyzed by Pd: Solvent Effects and PdBr₂ as a Surprisingly Suitable Precursor

Kana Y. Kunihiro,[a] Audrey Marty,[a] Ludovic Chahen,[a] Lionel Magna,*[a] and Juan M. Asensio*[a]

Pd-catalyzed carboxylation of ethylene into sodium acrylate is an industrially relevant reaction that has gained increasing attention in the last years. By this process, CO₂ can be transformed into a value-added molecule in a single, high atom economy step. However, there is still some room for improvement since the productivity of the reference catalyst is rather low. A key aspect to achieve this goal concerns the good understanding of reaction mechanism. Herein, we combine experimental and DFT studies to gain some knowledge on the reactivity and catalytic performance of the Pd/dicyclohexylphos-

phinoethane (dcpe) system. First, we were able to understand the influence of the solvent on the reaction, and we demonstrated that DMF was the best candidate due to the more efficient formation of the Pd-metallalactone. Moreover, we have demonstrated that highly available and less costly PdBr₂ can be a suitable precursor, as Pd¹¹ is reduced *in situ* in DMF in the presence of an excess of NaOtBu. Finally, the role of different monophosphine additives has been elucidated, which permitted us to rationally improve the catalytic performance.

## Introduction

CO₂ conversion into value-added products is a research topic that has attracted increasing attention of researchers working in both homogeneous and heterogeneous catalysis.[1] CO₂ can be considered as a readily available building block for the synthesis of different industrially relevant chemicals. Most of the efforts have focused on CO₂ hydrogenation to different molecules such as CO, CH₄, CH₃OH, etc.[2] However, direct carboxylation remains a challenge due to the low efficiency of the catalytic systems reported up to date, the low atom economy as sometimes the reaction requires the use of stoichiometric reductants, or the difficulties associated with catalyst recycling.[3]

In this context, an interesting transformation that has been addressed in the last years is the catalytic carboxylation of double bonds. More specifically, ethylene carboxylation to produce acrylic acid. This reaction is not thermodynamically favorable, and it must be performed in the presence of H₂,[4] haloalkanes to produce the acrylic ester derivative,[5] or a silane which can be further hydrolyzed to the carboxylic acid.[6] However, another possibility consists in performing the reaction in the presence of a base (i.e. sodium alkoxide or sodium

phenoxide), leading to the formation of the corresponding acrylate salt. This reaction possesses a strong industrial interest since: i) sodium acrylate is a high value-added product that can be used in the synthesis of commodity polymers ("super-absorbent" poly-acrylates),[7] ii) the reaction presents a high atom economy, and the only by-product is the conjugated acid of the base which can be regenerated,[8] iii) the catalyst may be recycled and reused. The first efficient catalytic system was reported by Limbach and co-workers, and it was based on the Ni/dicyclohexylphosphinoethane (dcpe) system,[9] although other diphosphine ligands can be used in the presence of Zn as reductant.[10] Metal complexes based on Rh[11] or Ru[12] have also shown their capacity to catalyze this transformation, but the Pd/dcpe system has undoubtedly arisen as the most efficient alternative in terms of catalytic activity and recyclability.[13]

Schaub *et al.* have notably improved the efficiency of the Pd/dcpe catalytic system in terms of solvent, base, and reaction conditions.[14] The most promising results were obtained at 145 °C under 10 bar of ethylene and 20 bar of CO₂ in high boiling point solvents such as anisole or cyclohexylpyrrolidone (CHP). Catalytic performance is maximized upon utilization of sodium alkoxides as bases, and the highest TONs were obtained in the presence of sodium tert-butoxide. Moreover, the authors were able to recycle the catalytic system at least for a second run. However, there is still some margin for improvement since CHP is a costly solvent and its utilization at industrial scale could be difficult to achieve.

Concerning the reaction mechanism, experimental evidence and theoretical calculations permit researchers to picture a plausible reaction pathway.[15] The first reaction step is the oxidative coupling between ethylene and CO₂ into the Pd⁰ center to generate the Pd¹¹-metallalactone, which was proposed to be the rate-determining step for the global process in the

<footnote>[a] K. Y. Kunihiro, A. Marty, L. Chahen, L. Magna, J. M. Asensio
IFP Energies Nouvelles, Rond-point de l'échangeur de Solaize. BP3, 69360
Solaize, France
E-mail: juan-manuel.asensio-revert@ifpen.fr
lionel.magna@ifpen.fr</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202401074</footnote>

<footnote>© 2024 The Authors. ChemCatChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_3.png" /></figure>

Pd/dcpe system.[16] However, metallalactone cleavage was not trivial and several reaction pathways were considered by Liu and co-workers in the Pd/dcpe/NaOtBu system using THF as model solvent.[17] In their case, ring-opening of the lactone to yield the Pd-alkyl zwitterionic complex stabilized by C–H agostic interaction was preferred over direct deprotonation by the base (Scheme 1).

In this work, we have studied the influence of the solvent and the metallic precursor on the catalytic carboxylation of ethylene in the presence of NaOtBu. Meanwhile, DFT studies confirmed that the rate-determining step for the process is the oxidative coupling, and that the better performances in DMF can be explained by the lowering of the kinetic barrier in this solvent. Surprisingly, PdBr₂ was found to be a suitable precursor for the reaction in DMF. We have demonstrated by NMR spectroscopy that Pd¹¹ can be easily reduced at room temperature in the presence of a base and without addition of a zero-valent metallic reductant. We have also investigated the role of three different monophosphines as additives which allowed us to improve the catalytic activity.

<figure><img src="image_4.png" /><figcaption>Scheme 1. Reaction mechanism proposed by Liu *et al*.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Reaction profile when using Anisole or DMF as solvents. Each experimental value was obtained from independent experiments.</figcaption></figure>

# Results and Discussion

## Solvent Effects: Anisole vs. DMF

We first performed a solvent screening under the conditions described by Schaub *et al.*[14b,c] ([Pd(PPh₃)₄], 1.1 eq. of dcpe, 200 eq. NaOtBu, see Comment S1 and Table S1). In our hands, the highest TONs were achieved in Anisole or in DMF, with values of 27 and 87 respectively (entries 1 and 6 in Table S1), whereas the catalytic system displayed either very low or no activities in other solvents. Hence, we focused our investigation on the results obtained in Anisole and DMF.

Intrigued by the solvent effect, we decided to investigate the reaction profile in both solvents. As shown in Figure 1, sodium acrylate formation continued increasing after 20 h in Anisole. The increase of TONs from 27 to 51 after 70 h suggests that the active species is still present at long reaction times. On the other hand, the reaction seems to be considerably faster in DMF, and a TON of 34 was reached after only 3 h. A TON of 87 was obtained after 20 h, whereas increasing the reaction time did not significantly improve the productivity of the catalyst. Formation of Pd black after 20 h of reaction was not observed neither in DMF nor in Anisole (Figure S1), which may suggest that the differences on the reaction profiles are not linked to the relative decomposition rate of the active species in both solvents.

³¹P{¹H} NMR experiments were conducted aiming at unveiling the nature of the complexes formed in both solvents when reacting Pd(PPh₃)₄ with one equivalent of dcpe ligand. In DMF, we observed the immediate formation of a yellow precipitate at room temperature. The orange supernatant was characterized by ³¹P{¹H} NMR spectroscopy, revealing the presence of free PPh₃ with some traces of the corresponding phosphine oxide and a singlet at 39.4 ppm. When the mixture was heated at 110 °C, the yellow precipitate was partially redissolved, and the NMR spectrum showed an increase of intensity of the peak at 39.4 ppm, corresponding to the P atom of the chelating ligand that we attribute to the complex Pd语言(PPh₃) (1) (Scheme 2a, see also Figure S2). [(dcpe)₂Pd(0)] complexes are expected to resonate above 20 ppm in the ³¹P{¹H} NMR.[18] The signal of coordinated PPh₃ could not be found, very likely because this ligand is in a fast exchange with free PPh₃. This fact is supported by the appearance of a broad resonance at 2 ppm, which is positioned between those of free PPh₃ (ca. -5 ppm) and Pd(PPh₃)₄ (broad signal at ca. 16 ppm). In the case of Anisole, we did not observe the formation of a precipitate, and the resulting orange solution contained a mixture of species as observed in the ³¹P{¹H} NMR spectrum (Figure S3). Complex 1 was the major product, but it was accompanied with the bis-chelating complex Pd语言 dcpe)₂ (2) appearing as a singlet at 33.6 ppm,[19] (complex 2 can be easily prepared upon addition of 2.5 eq. of dcpe to Pd(PPh₃)₄ in tol-d₈, see Scheme 2b, see also Figure S4). The mixture was heated at 110 °C inside the spectrometer for 5 min, leading to clean formation of complex 1. This equilibrium is reversible, and the starting mixture can be recovered after coming back at room temperature (Figure S5).
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Scheme 2.** ³¹P[{¹H} NMR experiments upon solubilizing Pd(PPh₃)₄ in (a) DMF or (b) Anisole in the presence of 1 equivalent of dcpe. Distribution of products in the reaction of Pd(PPh₃)₄ with dcpe under a CO₂/C₂H₄ atmosphere in (c) DMF and (d) Anisole.</figcaption></figure>

However, after heating the Anisole solution at 110 °C for 2 h, we observed the complete decomposition of both Pd complexes in the ³¹P{¹H} NMR spectrum (Figure S6), leading to a mixture of species above 40 ppm that could not be identified. It should be noted that NMR experiments were performed in the absence of an ethylene/CO₂ atmosphere, which presumably can prevent decomposition of the active species upon formation of the Pd-metallalactone. Hence, we examined the reactivity of Pd(PPh₃)₄ in the presence of 1 eq. of dcpe under 20 bar CO₂ and 10 bar C₂H₄, after heating the DMF or Anisole solutions at 145 °C for 5 h. Interestingly, the rather clean formation of the Pd-metallalactone 3 was observed in the ³¹P{¹H} NMR spectrum in DMF, appearing as two doublets at 58 and 67 ppm (Figure S7). The metallalactone complex was already isolated by Limbach and co-workers in THF after reaction of (dcpe)Pd(C₂H₅)₂ with acrylic acid.[13] This major product was accompanied with some minor Pd(II) complexes around 50 ppm, and an unidentified species at 47 ppm. On the other hand, lactone was formed

in a considerably lower ratio in anisole, and it was accompanied by complexes 1 and 2, together with several unknown species above 40 ppm which are the decomposition signature observed after heating complex 1 at 110 °C in the absence of C₂H₄/CO₂ atmosphere (Figures S6 and S8).

Based on these observations, we can propose that DMF afforded the best results because the formation of the metallalactone through oxidative coupling is more efficient in this solvent. A difference on the gases solubility does not seem as a plausible explanation since a very similar value of CO₂ molar fraction (χ<sub>CO₂</sub>) in the liquid phase was found in the literature for DMF (χ<sub>CO₂</sub> = 0.36; at 9.17 MPa and 395 K)[20] and anisole (χ<sub>CO₂</sub> = 0.37; at 9.28 MPa and 393 K).[21] To understand if the kinetic barrier for the rate-determining step was influenced by the solvent, we turned to DFT modelling of the reaction mechanism.

## Mechanistic Investigations by DFT

Reaction mechanism has been extensively studied by Liu and co-workers, who determined that the global rate-determining step was oxidative coupling with a barrier of ca. 120 kJ mol⁻¹ in the case of Pd/dcpe system in THF, when starting from (dcpe)Pd(C₂H₄) (structure A, Figure 2).[16,17] Regarding deprotonation, the authors proposed that the most likely pathway included a ring-opening step to generate a Pd-alkyl complex with an agostic C–H interaction at the α position (structure H in Scheme 3), which is then deprotonated by NaOtBu. The global barrier for this step starting from structure D was of ca. 109 kJ mol⁻¹ in their case.

We thus decided to model by DFT the reaction mechanism for sodium acrylate formation in DMF. We first performed a screening of functionals on the deprotonation of the metallalactone (Figure S9). Hence, we determined the ΔG⁺ after optimization of both the starting lactone (D) and the transition step (F) using different functionals, and ωB97XD was selected for our study (see Experimental section for further details).

The first step of the mechanism would consist in the endergonic shift of PPh₃ by ethylene to give complex A (Figure 2). This step is possesses a ΔG of 17.2 kJ mol⁻¹. Then, the oxidative coupling of ethylene and CO₂ elapsing through the transition state B would lead to the Pd-lactone C. The kinetic barrier for this step was of 120.3 kJ mol⁻¹ which is of the same order of magnitude than that reported in the literature in THF. Nevertheless, if we consider the stabilization given by the PPh₃ ligand in complex 1, the barrier for the oxidative coupling increases from 120.3 to 137.5 kJ mol⁻¹. Subsequently, the metallalactone C coordinates one molecule of NaOtBu leading to complex D with a κ–O,O’–Na bond, in an exergonic process with a ΔG of –57.4 kJ mol⁻¹.

In the second part of the mechanism, complex D can evolve by different pathways prior to deprotonation (Scheme 3): (I) ring-opening of the lactone (D→H) as previously described,[17] (II) isomerization of the complex to generate an interaction between the base and the metallic center while maintaining the chelating κ–O,O’–Na coordination (D→E), or (III) isomer-
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 2.** Energy span for the catalytic synthesis of sodium acrylate from ethylene and CO₂, modeled by DFT at the ωB97XD/def2-TZVP(C, H, P, O, Na)-LANL2DZ(Pd) level. Solvation in DMF was considered using the SMD model. The free energy (black) and enthalpy (blue) values in kJ·mol⁻¹ are displayed for each intermediate. The structure of the dcpe ligand has been omitted for clarity.</figcaption></figure>

ization after cleavage of one Na–O bond leading to complex J. Paths I and II were already investigated by Liu and co-workers[17] but, to the best of our knowledge, path III is described for the first time in this work.

In our DFT conditions, Path II was the most favorable mechanism for deprotonating the metallalactone. The barrier for this reaction (D→F) was of 91.2 kJ·mol⁻¹, which is considerably lower than the value described in the literature (127.9 kJ·mol⁻¹ in THF). On the other hand, Path I which was described as the most probable mechanism by Liu *et al*. (ΔG⁺ of 108.7 kJ·mol⁻¹), was also slightly lower in our case with a barrier of 94.4 kJ·mol⁻¹ (D→I). Path III is less favorable and displays a considerably higher barrier of 117.2 kJ·mol⁻¹ (D→K). However, we must indicate that the three pathways are in principle accessible as they must overcome a lower energy barrier than that of the rate-determining oxidative coupling.

Another interesting observation concerns the fact that, in the last step of acrylate exchange by ethylene, complex G is 11.1 kJ·mol⁻¹ more stable than complex A (see Figure 2). Indeed, one would expect that, as the concentration of sodium acrylate increases, the G↔A equilibrium shifts towards the formation of the Pd-acrylate complex. Hence, the kinetic barrier of the reaction would progressively increase with the TON, which can explain the fact that full conversion of the base was never observed (the overall barrier can be defined as the energetic span from the TOF-determining intermediate, TDI=G, to the TOF-determining transition state, TDTI=B[22]).

The solvent effect on the rate-determining step was then evaluated. Complexes 1, A, B and C were optimized in Anisole

using a SMD solvation model (complexes 1-An, A-An, B-An and C-An). The comparison of the free energy span for oxidative coupling in DMF and Anisole is displayed in Figure 3 (see also Figure S10 for the enthalpy span). Interestingly, the barrier for oxidative coupling is higher in Anisole, with a ΔG⁺ of

<figure><img src="image_10.png" /><figcaption>**Figure 3.** Free energy span for the rate-determining oxidative coupling, modeled by DFT at the ωB97XD/def2-TZVP(C, H, P, O, Na)-LANL2DZ(Pd) level. Solvation in DMF and Anisole was considered using the SMD model. The structure of the dcpe ligand has been omitted for clarity.</figcaption></figure>
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Scheme 3.** The three different pathways for lactone deprotonation modeled by DFT at the ωB97XD/def2-TZVP(C, H, P, O, Na)-LANL2DZ(Pd) level. The free energy values in kJ mol⁻¹ are displayed for each complex. The structure of the *dcpe* ligand has been omitted for clarity.</figcaption></figure>

150.0 kJ mol⁻¹ (1-An→B-An), 12.5 kJ mol⁻¹ above the barrier found in DMF. Additionally, reaction is thermodynamically less favorable in this solvent. After close analysis of both transition states (Figure S11), one can observe that the most important differences arise from the C₂-eth-Pd-C₂CO₂ angles (85.9° in DMF vs. 80.1° in Anisole) and the C₁-eth-C₂CO₂ distances (1.94 Å in DMF vs. 1.89 Å in Anisole). Hence, in Anisole, CO₂ and ethylene need to be approximated in a higher extent to reach the transition state, with the concomitant increase in the energy required for this step. On the other hand, oxidative coupling is more favorable in DMF, which is consistent with the higher ratios of palladalactone observed by NMR spectroscopy after treatment of Pd(PPh₃)₄/dcpe system with C₂H₄/CO₂.

## Screening of Pd Precursors

We decided then to modify the nature of the catalytic system (new unexplored Pd precursors and ligands, see Table S2 for further information). We confirmed the specificity of the

reaction respect to *dcpe* ligand. Indeed, the use of similar phosphines in terms of sterical hindrance and/or chelating angle afforded poor values of TON. Other ligands such as bis-NHC were completely unactive or gave traces of sodium acrylate. Hence, we decided to focus on *dcpe* and to study the influence of the Pd precursor on the catalytic performances. The most relevant results are summarized in Table 1.

As mentioned above, complex 1 is formed in the presence of an excess of free PPh₃ which is in a fast equilibrium with its coordinated counterpart. Since this equilibrium may hinder the formation of the metallalactone due to competition with ethylene and CO₂ coordination, we decided to explore other Pd precursors that do not carry phosphine ligands. We observed a TON of 20 when starting from commercial Pd(dba)₂ under the same reaction conditions (entry 1). Complex Pd(0)(tBuDAB)(η²-MA), which was synthesized as described by Elsevier and co-workers,²³ was slightly more active with TON values of 35 (entry 2).

Surprisingly, PdBr₂ was a suitable precursor for the reaction and a TON of 75 was found under the same reaction conditions used for Pd(PPh₃)₄. This result contrasts with the observations made in the literature in which the presence of a Zn reductant was necessary when carrying out the reaction from Pd(II) salts in THF or anisole solution.¹⁴ᵃ The reaction profile was compared for three Pd precursors in DMF: Pd(PPh₃)₄, PdBr₂ and Pd(dba)₂ (Figure S12). The very similar profiles obtained for PdBr₂ and Pd(PPh₃)₄ seem to indicate that generation of the Pd(0) active species from PdBr₂ must be fast, and its concentration in the reaction medium should be of the same order of magnitude than in the case of Pd(PPh₃)₄.

<table><caption>**Table 1.** Selected examples of the Pd precursor screening in catalytic ethylene carboxylation.ᵃ</caption><thead><tr><th>Entry</th><th>Pd precursor</th><th>TON</th></tr></thead><tbody><tr><td>1</td><td>Pd(dba)<sub>2</sub></td><td>20</td></tr><tr><td>2</td><td>Pd(0)(tBuDAB)(η<sup>2</sup>-MA)</td><td>35</td></tr><tr><td>3</td><td>PdBr<sub>2</sub></td><td>75</td></tr></tbody></table>

[a] A 100 mL stainless steel autoclave was charged under inert atmosphere at r.t. with a solution of Pd precursor (0.1 mmol) and *dcpe* ligand (0.11 mmol) in 10 mL of the solvent, followed by the base (NaOtBu, 1.0 M, 20 mL). The autoclave was then pressurized with CO₂ (20 bar) and stirred at 1500 rpm for 10 min, followed by injection of ethylene (10 bar). After stirring at 145 °C for 20 h, the autoclave was cooled to r.t., the pressure was released, and the crude reaction mixture was transferred into a glass bottle. The residues in the reactor were recovered using 30 mL of deionized water and transferred into the bottle. The DMF/water mixture was weighted. Then, an aliquot of ca. 1 mL was weighted and dried under vacuum. The TON was determined by NMR after addition of the standard solution.
<figure><img src="image_13.png" /></figure>

Pd(II) Reduction

With these results in hand, NMR experiments were conducted to elucidate the reduction mechanism of the Pd(II) precursor. PdBr₂ was reacted with one equivalent of *dcpe* ligand in DMF, leading to the clean formation of the complex (*dcpe*)PdBr₂ (3), appearing as a singlet at 96 ppm in the ³¹P{¹H} NMR spectrum (Scheme 4a and Figure S13). Complex 3 was very stable in DMF, and even upon heating at 140 °C for 20 h neither formation of metallic Pd nor evolution of this species was observed by ³¹P {¹H} NMR spectroscopy. The addition of different amounts of NaOtBu, namely 1, 2, 10, 50 or 200 eq., led to the appearance of a new species at 49 ppm that we tentatively attribute to a chelating (*dcpe*)Pd(0)(DMF) complex 4 (Scheme 4b and Fig- ure S14). The ¹H-³¹P HMBC spectrum of complex 4 revealed the presence of a correlated resonance at {8.3,49} ppm which suggests the interaction with a solvent molecule (Figure S15). Generation of 4 was clean in the presence of at least 10 eq. of NaOtBu, although this species is accompanied by the formation of a by-product at ca. 75 ppm in the presence of 50 eq. or more of base. Complex 4 was not stable at high temperatures and formation of metallic Pd was observed after heating at 140 °C for several hours, and this process was faster upon increasing the concentration of the base.

Addition of 4 equivalents of PPh₃ to a DMF solution of complex 4 obtained after reaction of 3 with 10 eq. of NaOtBu led to the appearance of a yellow precipitate (Scheme 4c). The

supernatant was analyzed by ³¹P{¹H} NMR spectroscopy, revealing the presence of some traces of unreacted complex and free PPh₃ (Figure S16). More interestingly, the yellow precipitate could be filtered and redissolved in anisole, showing the formation of complex 1, together with ca. 0.2 equivalents of the bis-diphosphine complex 3 (Figure S17) in the ³¹P{¹H} NMR spectrum. This mixture was very similar to the one observed in the direct reaction of Pd(PPh₃)₄ with *dcpe* in anisole (Scheme 2b), confirming the reduction to Pd(0) of complex 3 in the presence of the base. Again, a broad resonance around 4 ppm was observed that we attribute to PPh₃ in a fast equilibrium between its free and coordinated forms. Reduction from 3 to 1 can also be made in a single step after addition of 1 eq. PPh₃ and 200 eq. of NaOtBu (Scheme 4d). Under these conditions the yellowish solution turned to deep orange, with the formation of a yellow precipitate. The orange supernatant was analyzed by ³¹P{¹H} NMR spectroscopy, revealing the presence of a broad signal at 39 ppm corresponding to complex 1 (Figure S18). Aiming at understanding the stability of this solution under catalytic conditions, the mixture was heated at 140 °C for 20 h. This led to the generation of important amounts of Pd black, together with the appearance of some traces of unknown species as observed in the ³¹P{¹H} NMR (Figure S19) spectrum, suggesting that complex 1 is not thermally stable in the presence of the base and in the absence of an excess of PPh₃ and/or atmosphere of C₂H₄/CO₂.

Summarizing, we can assume that reduction of Pd(II) to Pd(0) is a fast process in DMF after addition of an excess of NaOtBu. However, the Pd(0) species are not stable in solution at high temperatures in the absence of an ethylene/CO₂ atmosphere. Regarding the reduction mechanism, we were not able to determine the structure of the byproducts. Therefore, proposing a reaction pathway seems difficult at this stage. However, reduction of diphenphine-Pd(OAc)₂ complexes in DMF has been previously described in the literature by Amatore and co-workers.[24] In that case, it was proposed that a nucleophilic attack of the base to the phosphine ligand may be a reasonable mechanism to promote Pd reduction, which happened at ca. 50 °C. In our conditions, the excess of the base would accelerate the reaction at room temperature. Following this hypothesis we must also assume the formation of phosphonium-alkoxide by-products and the generation of some Pd(0) clusters or nanoparticles due to the stoichiometry of the reaction. Indeed, the phosphonium-alkoxide by-product may correspond to one of the residual signals around 75 ppm in the ³¹P{¹H} NMR spectrum after reduction.

The Role of the Stabilizing Monophosphine

Schaub and co-workers already described that addition of 4 eq. of PPh₃ can have a positive impact in the reaction when using (*dcpe*)Pd(acrylate) complex G as pre-catalyst, very likely because the phosphine helps to stabilize the active species.[14b] Hence, intrigued by the potential role of the monophosphine in the reaction, we decided to perform some catalytic experiments starting from PdBr₂ in the presence of different PR₃ as additives

<figure><img src="image_14.png" /><figcaption>**Scheme 4.** ³¹P{¹H} NMR experiments to rationalize the reduction mechanism of PdBr₂ in the presence of 1 equivalent of *dcpe*. (a) after heating at 140 °C for 20 h, (b) after addition of NaOtBu to a solution of complex 3, (c) after addition of 4 eq. of PPh₃ to a solution of complex 4 obtained upon reduction in the presence NaOtBu, (d) after addition 200 eq. of NaOtBu and 1 eq. of PPh₃.</figcaption></figure>
<figure><img src="image_15.png" /></figure>

(Table 2). Interestingly, addition of 1 eq. of the PPh₃ respect to Pd led to an increase of the TON from 75 to 93 (entries 1–2). We attribute this behavior to the stabilizing effect provided by the monophosphine ligand, which can coordinate to the Pd center preventing decomposition at high temperatures even in the presence of a protecting C₂H₄/CO₂ atmosphere. Interestingly, P(C₆FH₄)₃ afforded higher values of TON of 103 (entry 3), whereas stronger σ-donor ligands such as PtBu₃ did not enable to increase the TON, (TON = 66, entry 4).

An optimum must be found to maximize the stabilizing effect of the monophosphine without reducing the catalytic activity due to competition with substrates coordination. It was observed that the addition of 2 eq. of PPh₃ did not lead to significant changes in the TON (entry 5), while the utilization of 4 eq. decreased the TON to 44 (entry 6). Similarly, the TON could be increased to 122 after addition of 2 eq. of P(C₆FH₄)₃, but the presence of 4 eq. was detrimental and a TON of 82 was found (entries 7–8). Finally, addition of 2 eq. of PtBu₃ did not seem to have a significant effect on the catalytic activity (entry 9).

It is well known that the combination of strong σ-donor ligands such as *dcpe* with π-acceptor ligands helps to stabilize Pd(0) species.[23] Moreover, it has been observed that the π-acceptor ligand is preferentially decoordinated over the σ-donor in Pd-catalyzed C–H arylation,[25] which in our case would be even more favored due to chelating effect of *dcpe*. However,

the electronic features of P(C₆FH₄)₃ are not significantly different respect to PPh₃, so the π-acceptor capacity may not be sufficient to understand the boosting effect observed.[26] We decided to evaluate the thermodynamic stability of the three (dcpe)Pd-monophosphine complexes by DFT.

Coordination of a PPh₃ molecule to complex G containing sodium acrylate is thermodynamically favored, with a ΔG of –6.1 kJmol⁻¹ (Figure 4). As discussed above, PPh₃ in complex 1 must be then displaced by an ethylene molecule, which displays a ΔG of 17.2 kJmol⁻¹, and an overall barrier of 137.5 kJmol⁻¹ for the oxidative coupling (1→B). PPh₃ can compete with ethylene coordination hindering the reaction. P(C₆FH₄)₃ coordination to complex G and liberation of sodium acrylate is however endergonic with a small ΔG of 4.2 kJmol⁻¹. Hence, we can assume that complex G and complex L (dcpe)Pd[P(C₆FH₄)₃] are in equilibrium. More interestingly, this monophosphine leads to a flatter span and it can be more easily shifted by ethylene with a ΔG of 6.9 kJmol⁻¹, and an overall barrier of 131.4 kJmol⁻¹ for the oxidative coupling (G→B). On the other hand, PtBu₃ coordination to complex G to generate complex M (dcpe)Pd(PtBu₃) is more endergonic with a ΔG of 22.3 kJmol⁻¹. The barrier for the oxidative coupling in this case would be the same as in the presence of P(C₆FH₄)₃, (G→B) namely 131.4 kJmol⁻¹. We can thus assume that this phosphine is a bad stabilizer for the reaction. Hence, we expect that PtBu₃ would have no significant effect on the catalytic activity as observed experimentally.

A final comment must be done regarding the possible role of the monophosphines in the generation of the active species. It is not excluded that, in the reduction process, the nucleophilic attack of the base did not take place on the dcpe ligand but on the monophosphine. In this respect, one would expect that the reactivity order for the generation of the Pd(0) active species will follow the trend P(C₆FH₄)₃ > PPh₃ > PtBu₃. However, this hypothesis seems unlikely as the complex 3 can

<table><caption>**Table 2.** Addition of a monophosphine additive in catalytic ethylene carboxylation.[a]</caption><thead><tr><th>Entry</th><th>PR3 (mmol)</th><th>TON</th></tr></thead><tbody><tr><td>1</td><td>–</td><td>75</td></tr><tr><td>2</td><td>PPh<sub>3</sub> (0.1 mmol)</td><td>93</td></tr><tr><td>3</td><td>P(C<sub>6</sub>FH<sub>4</sub>)<sub>3</sub> (0.1 mmol)</td><td>103</td></tr><tr><td>4</td><td>PtBu<sub>3</sub> (0.1 mmol)</td><td>66</td></tr><tr><td>5</td><td>PPh<sub>3</sub> (0.2 mmol)</td><td>96</td></tr><tr><td>6</td><td>PPh<sub>3</sub> (0.4 mmol)</td><td>44</td></tr><tr><td>7</td><td>P(C<sub>6</sub>FH<sub>4</sub>)<sub>3</sub> (0.2 mmol)</td><td>122</td></tr><tr><td>8</td><td>P(C<sub>6</sub>FH<sub>4</sub>)<sub>3</sub> (0.4 mmol)</td><td>82</td></tr><tr><td>9</td><td>PtBu<sub>3</sub> (0.2 mmol)</td><td>60</td></tr></tbody></table>

[a] A 100 mL stainless steel autoclave was charged under inert atmosphere at r.t. with a solution of Pd precursor (0.1 mmol), *dcpe* ligand (0.11 mmol) and the stabilizing phosphine in 10 mL of the solvent, followed by the base (NaOtBu, 1.0 M, 20 mL). The autoclave was then pressurized with CO₂ (20 bar) and stirred at 1500 rpm for 10 min, followed by injection of ethylene (10 bar). After stirring at 145 °C for 20 h, the autoclave was cooled to r.t., the pressure was released, and the crude reaction mixture was transferred into a glass bottle. The residues in the reactor were recovered using 30 mL of deionized water and transferred into the bottle. The DMF/water mixture was weighted. Then, an aliquot of ca. 1 mL was weighted and dried under vacuum. The TON was determined by NMR after addition of the standard solution.

<figure><img src="image_16.png" /><figcaption>**Figure 4.** DFT span for monophosphine coordination to complex G followed by phosphine displacement by an ethylene molecule.</figcaption></figure>
<figure><img src="image_17.png" /></figure>

be reduced at room temperature in the presence of NaOtBu
without the need of a monophosphine.

Conclusions

In conclusion, we have observed that the nature of the solvent
has a strong impact on the catalytic activity of the Pd/dcpe
system for the sodium acrylate synthesis through ethylene and
CO₂ coupling. DMF arises as the best candidate for the
reference system Pd(PPh₃)₄/dcpe, allowing us to reach a TON of
75. NMR experiments demonstrated that the good perform-
ances in such solvent are linked to the more efficient formation
of the Pd-metalallactone through oxidative coupling of
ethylene and CO₂, which was also confirmed by DFT. PdBr₂ was
surprisingly a suitable precursor for the reaction, and we were
able to reach very similar TONs than with Pd(PPh₃)₄, which was
the most widely employed precursor up to date. We have
observed that Pd(II) can be easily reduced at room temperature
in DMF in the presence of an excess of NaOtBu. This constitutes
a major advantage from an industrial point of view, since PdBr₂
is a more stable and less costly precursor than Pd(0) complexes.
Moreover, utilization of PdBr₂ as precursor enables to tune the
nature and amount of the monophosphine additive, contrary to
Pd(PPh₃)₄. The best results for the Pd/dcpe/NaOtBu system in
DMF were obtained in the presence of 2 eq. of P(C₆FH₄)₃, which
allowed us to reach a TON of 122. As suggested by DFT, this
behavior can be attributed to the stabilization of the presum-
ably sensitive Pd⁰ active species by the monophosphine ligand
without increasing the kinetic barrier for the rate-determining
oxidative coupling. This effect will be even more important at
the beginning of the reaction when sodium acrylate is still not
present in the medium, as this reaction product can also act as
stabilizing ligand. Our future work will be devoted to the
evaluation of the recyclability of the catalytic system, which is
an important criterion for the industrial application.

**Experimental Section**

General Procedures

All air- and moisture-sensitive manipulations were performed using
standard vacuum line, Schlenk and cannula techniques or by using
an MBraun Ar-atmosphere glovebox. Solvents were dried by using
an MBraun SPS solvent purification system, degassed and stored
over molecular sieves. CHP, DMF, NMP and Anisole were dried by
distillation and stored with molecular sieves. A Karl-Fischer titration
was systematically performed in all the cases. ¹H, ¹³C and ³¹P NMR
spectra were recorded on an Advance 300 MHz spectrometer. ¹H
and ¹³C NMR chemical shifts are reported in ppm relative to SiMe₄
as internal standard. Elemental analyses were performed at
Mikrolab Kolbe.

Pd(PPh₃)₄, Pd(Br)₂, Pd(dba)₂, dcpe, PPh₃, PtBu₃, P(C₆FH₄)₃ were purchased from Sigma-Aldrich and ((+)-1,2-Bis((2S,5S)-2,5-diphenylphospholano)ethane) was obtained from Strem Chemicals and used as received. P(CH₂CH₂PCy₂)₃ (PPR₃),[12,27] 3,3'-dimethyl-1,1'-methylene diimidazolium dibromide,[23] 3,3'-dimethyl-1,1'-ethylene diimidazolium dibromide,[28] 1,4-Di-tert-butyl-1,4-diaza-1,3-buta-

diene (tBuDAB),[29] tert-butyl-1,4-diaza-1,3-butadiene (tBuDAB palladium(0) maleic anhydride {Pd⁰(tBuDAB)(η²-ma)},[30] and (3,3'-dimethyl-1,1'-methylene diimidazolyl-2-ydene) palladium(0) maleic anhydride [{Pd⁰(CH₂-bis-NHC)(η²-ma)}, entry 7 in Table S2][23] were prepared according to the literature procedures. Imidazolium salt 3,3'-dimethyl-1,1'-ethylene diimidazolium dibromide was prepared by a modified procedure, upon using 1,2-dibromoethane instead of 1,2-dichloromethane.[28]

**Synthesis 3,3'-dimethyl-1,1'-ethylene diimidazolyl-2-ydene palladium(0) Maleic Anhydride [Pd⁰(CH₂CH₂-bis-NHC)(η²-ma)] (entry 8 in Table S2)**

In a dried Schlenk flask was added dried and degassed DMSO (9.0 mL) to a mixture of Pd⁰(tBuDAB)(η²-ma) (1 equiv., 0.648 mmol) and 3,3'-dimethyl-1,1'-ethylene diimidazolium dibromide salt (1.2 equiv., 0.781 mmol). Then, a KOtBu solution in DMSO (0.33 M, 0.315 mmol) was added dropwise while stirring the reaction mixture and removing the *in situ* generated tBuOH over time by applying vacuum. After stirring for 45 min at r.t., 5 mL of DCM followed by Et₂O (10 mL), and acetonitrile (10 mL) were added to the reaction mixture. A grey precipitate was formed and filtered through cannula filtration. The solid was then washed with Et₂O (3×5 mL). Yield = 49%. ¹H NMR (300 MHz, dmso-d₆, 25 °C): 7.28 (2H, d, ³Jₘₕ = 1.7 Hz, CH), 7.27 (2H, d, ³Jₘₕ = 1.7 Hz, CH), 4.60 (2H, m, CH₂), 4.50 (2H, m, CH₂), 3.61 (6H, s, -CH₃), 3.44 (2H, s, ma). ¹³C NMR (300 MHz, dmso-d₆, 25 °C, ppm): 183.0 (NCN), 174.6 (CO), 122.8 (NCH=CHN), 122.4 (NCH=CHN), 48.8 (CH₂), 38.2 (-CH₃), 37.5 (CH alkene). C₃₀H₂₉N₂O₃Pd (394.69): calcd.: C 42.5, H 4.05, N 14.19; found: C 40.41, H 4.29, N 13.14.

Catalytic Experiments

The catalytic reactions were carried out in a stainless-steel multi-
reactor unit (6 reactors of 100 mL in parallel) equipped with
mechanical stirring. Gases were purchased from Air Liquide. The
solutions were introduced in the reactor through a syringe that is
previously purged under vacuum. TON values are reported as the
average of at least two catalytic reactions. The differences in TON
for catalytic tests under similar reaction conditions were always
below 5%.

Catalytic Tests in N,N-Dimethylformamide (DMF)

A 100 mL stainless steel autoclave was charged under inert
atmosphere at room temperature with a solution containing the Pd
precursor (0.1 mmol) and dcpe (0.11 mmol) in 10 mL of solvent,
followed by a suspension containing the base in DMF (NaOtBu,
1.0 M, 20 mL). The solutions were introduced in the reactor upon
injection with a purged syringe. Note that NaOtBu was previously
kept under stirring in DMF overnight to allow the formation of a
fine suspension. The autoclave was then pressurized with CO₂
(20 bar) and stirred at 1500 rpm for 10 min, followed by injection of
ethylene (10 bar). After stirring at 145 °C for 20 h, the autoclave was
cooled to r.t., the pressure was released, and the crude reaction
mixture was transferred into a glass bottle. The residues in the
reactor were recovered using 30 mL of deionized water and
transferred into the bottle. The DMF/water mixture was weighted.
Then, an aliquot of ca. 1 mL was weighted and dried under vacuum.
After addition of a standard solution containing dioxane in D₂O
(30 mM), the TON was determined by integration in the ¹H NMR.
<figure><img src="image_18.png" /></figure>

## Catalytic Tests in Anisole

The procedure was similar to the one described in DMF. However, at the end of the reaction, the crude reaction mixture was transferred into a glass bottle. The residues in the reactor were recovered using 30 mL of deionized water then transferred into the bottle leading to a biphasic crude mixture. From the aqueous phase (lower phase) containing the sodium acrylate product, an aliquot was collected (1 mL), filtered on a 0.2 µm PTFE filter, then dried under vacuum. After addition of a standard solution containing dioxane in D₂O (30 mM), the TON was determined by integration in the ¹H NMR.

## Catalytic Tests in DMF in the Presence of Monophosphines

The procedure was similar to the one described in DMF but the solution containing the Pd precursor and dcpe also contained 0.1, 0.2 or 0.4 mmol of the corresponding monophosphine.

## NMR-Scale Experiments

### NMR-Scale Synthesis of Complex Pd⁰oven dcpe)PPh₃ (1)

Inside the glovebox, a dried J.-Young NMR tube was charged with 0.25 mL of Pd(PPh₃)₄ precursor solution in DMF ([Pd] = 2.0 mmol/L), and 0.25 mL of a dcpe solution in DMF ([dcpe] = 2.0 mmol/L). After homogenization of the mixture, ³¹P{¹H} NMR experiments were recorded at 110 °C. ³¹P{¹H} NMR (121 MHz, DMF, 25 °C, ppm): δ 39 (s).

### NMR-Scale Synthesis of Complex Pd⁰oven dcpe)₂ (2)

Inside the glovebox, a dried J.-Young NMR tube was charged with 0.25 mL of Pd(PPh₃)₄ precursor solution in toluene ([Pd] = 2.0 mmol/L), and 0.25 mL of a dcpe solution in toluene ([dcpe] = 4.0 mmol/L). After homogenization of the mixture, ³¹P{¹H} NMR experiments were recorded at 110 °C. ³¹P{¹H} NMR (121 MHz, toluene, 25 °C, ppm): δ 33 (s).

### NMR-Scale Synthesis of Complex Pd¹¹oven dcpe)Br₂ (3)

Inside the glovebox, a dried J.-Young NMR tube was charged with 0.25 mL of a PdBr₂ solution in DMF ([Pd] = 2.0 mmol/L), and 0.25 mL of a dcpe solution in DMF ([dcpe] = 2.0 mmol/L). After homogenization of the mixture, ³¹P{¹H} NMR experiments were recorded at room temperature ³¹P{¹H} NMR (121 MHz, DMF, 25 °C, ppm): δ 96.75 (s).

### NMR-Scale Synthesis of Complex Pd⁰oven dcpe) (DMF) (4)

Inside the glovebox, a dried J.-Young NMR tube was charged with 0.25 mL of a PdBr₂ solution in DMF ([Pd] = 2.0 mmol/L), and 0.25 mL of a dcpe solution in DMF ([dcpe] = 2.0 mmol/L). Then, 10 equiv. of NaOtBu were added into the mixture. After 1 h reaction time at r.t., ³¹P{¹H} NMR experiments showed the formation of complex 4. ³¹P{¹H} NMR (121 MHz, DMF, 25 °C, ppm): δ 49 (s).

## Formation of Metallalactone C Under CO₂/C₂H₄ Atmosphere

### Synthesis of Pd¹¹oven dcpe)(CH₂CH₂COO) (C)

Under inert atmosphere, a 100 mL reactor was charged with 30 mL of a Pd(PPh₃)₄/dcpe solution in DMF or anisole ([Pd] = 2.0 mmol/L,

[dcpe] = 2.0 mmol/L). Then, the reactor was pressurized with 20 bar CO₂ and 10 bar C₂H₄, and the mixture was heated at 145 °C. After 5 h reaction, the reactor was cooled down to room temperature, followed by depressurization and a purge with N₂. An aliquot was taken for analysis. ³¹P{¹H} NMR experiments showed the formation of the palladalactone complex C. ³¹P{¹H} NMR (121 MHz, DMF, 25 °C, ppm): δ 58.5 (d), 67.3 (d).

Additionally, we noticed the formation of important amounts of OPPh₃ either in Anisole (6%) or in DMF (25%). Indeed, Pd(II) complexes are active catalysts for the aerobic oxidation of PPh₃ at mild conditions.[31] Since our experimental set-up makes it difficult to work under strict inert atmosphere during depressurization, we attribute this fact to an oxygen leak during this process.

## Computational Details

All energy calculations were performed with the Gaussian 16 program.[32] We selected different kinds of functionals with and without dispersion corrections to investigate the deprotonation of the metallalactone. Both B3LYP and B3PW91 are widely used in computational chemistry and do not include dispersion corrections.[33] Hence, we decided to include empirical Grimme's dispersion with Becke-Johnson damping (D3BJ). These functionals are named as B3LYP-GD3BJ and B3PW91-GD3BJ. M06 and ωB97XD are hybrid functionals that include dispersion corrections.[34] Three functionals that include dispersion correction, namely ωB97XD, B3LYP-GD3BJ and M06, performed similarly. ωB97XD was chosen for the rest of our study as this functional presented the best compromise between accuracy and computational time.

Single-point energy calculations were done using the TZVP[35] basis set for P, C, F, Na, O and H, and LANL2DZ[36] including effective core potential (ECP)[37] for Pd, after the structures had been preoptimized using the 6-31 g* basis set. LANL2DZ has been previously used for DFT calculations in the analogous Pd-catalyzed alkene alkoxycarbonylation.[38] Solvation was considered upon using the SMD implicit solvation model in DMF and Anisole.[39] Transition states were obtained through energy scans and then refined by carrying out single-point energy calculations (Berny algorithm). Harmonic vibrational frequency calculations have been employed to confirm that the structures have been properly optimized. All the relative energies throughout the manuscript are given in kJ mol⁻¹ and refer to Gibbs free energies or enthalpies at 298.15 K as given by Gaussian 16 program.

## Acknowledgements

The authors thank IFP Energies Nouvelles for financial support.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.
<figure><img src="image_19.png" /></figure>

**Keywords:** Homogeneous catalysis · Pd complexes · Phosphines · CO₂ conversion · Sodium acrylate

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Manuscript received: June 24, 2024

Revised manuscript received: July 22, 2024

Accepted manuscript online: July 22, 2024

Version of record online: September 6, 2024