# Direct Synthesis of Polyesterether from Ethylene Glycol

Garima Saini, Pavel Kulyabin, Angus Mcluskie, Niklas von Wolff, Amit Kumar

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Garima Saini, Pavel Kulyabin, Angus Mcluskie, Niklas von Wolff, Amit Kumar. Direct Synthesis of Polyesterether from Ethylene Glycol. ACS Sustainable Chemistry & Engineering, In press, 10.1021/acssuschemeng.5c00886. [https://doi.org/10.1021/acssuschemeng.5c00886](https://doi.org/10.1021/acssuschemeng.5c00886)

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# Direct Synthesis of Polyesterether from Ethylene Glycol

Garima Saini, Pavel Kulyabin, Angus McLuskie, Niklas von Wolff,* and Amit Kumar*

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Cite This: https://doi.org/10.1021/acssuschemeng.5c00886

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## ACCESS

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Supporting Information

**ABSTRACT:** We report here a method for making polyesterether from ethylene glycol. The reaction is catalyzed by a ruthenium complex and liberates H₂ gas and H₂O as byproducts. Mechanistic studies conducted through experiments and DFT computations suggest that the chain growth of the polymerization process involves both dehydrogenation and dehydration pathways stemming from a hemiacetal intermediate, leading to the formation of esters and ethers, respectively. Investigations into the polymerization of other diols have also been conducted, showing that diols with a lower number of carbons between the alcohol groups (propylene glycol, glycerol, and 1,3-propanediol) lead to the formation of polyesterether whereas α,ω-diols containing a higher number of carbons (1,6-hexanediol and 1,10-decanediol) lead to the formation of polyester.

**KEYWORDS:** polyesterether, dehydrogenation, ethylene glycol, hydrogen-borrowing, ruthenium

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## INTRODUCTION

Aliphatic copolyesterethers have attracted significant interest recently due to their excellent biocompatibility, biodegradability and applications in areas such as tissue engineering and packaging.¹,² The conventional methods for their synthesis involve ring-opening polymerization of epoxides with lactones,³,⁴ lactides,⁵⁻⁸ or anhydrides (Figure 1).⁹,¹⁰ However, these methods often involve multiple steps of polymerization and purification, which make their large-scale application challenging. A few other methods based on melt polymerization of polyethylene glycol with diacids or diesters,¹¹,¹² hydroesterificative polymerization (coupling of enol ethers with CO),¹³ and reduction of polyesters¹⁴ have also been reported in the past (Figure 1). Most of these processes involve monomers that require multiple steps to be synthesized and are derived from fossil fuels. As such, there is a need to develop new methods for the synthesis of aliphatic copolyesterethers from renewable, inexpensive, and commercially available feedstock.

Catalytic dehydrogenation is an atom-economic approach for the synthesis of organic compounds, especially those containing carbonyl compounds such as aldehydes, ketones, esters, and amides.¹⁵ The concept of acceptorless dehydrogenative catalysis has also been utilized for the synthesis of various polymers.¹⁵ For example, Milstein¹⁶ reported the synthesis of polyamides from the dehydrogenative coupling of diols and diamines. Recently, we,¹⁷⁻¹⁹ Robertson,²⁰ and Liu²¹ have independently reported the synthesis of polyureas from the dehydrogenative coupling of diamines and methanol/diformamides. We have also reported the synthesis of polyethylenimines from the coupling of diols and diamines using the hydrogen-borrowing approach.²² Along this line, Robertson reported the synthesis of high molar mass

polyesters from the dehydrogenative coupling of diols with an alkyl spacer of six carbons or more (e.g., hexane diol) using Milstein's RuPNN catalyst.²³ Smaller diols, such as ethylene glycol or propylene glycol, did not result in the formation of polyester, presumably due to the formation of a metallacycle with ruthenium that could block the active site. Recently, Milstein has reported the use of ethylene glycol as a liquid organic hydrogen carrier, where ethylene glycol was dehydrogenated to form a "polyester," which could be hydrogenated back to ethylene glycol.²⁴ However, limited emphasis was given to the isolation and chemical and physical characterization of the formed polymers.

1,2-Diols, such as ethylene glycol, propylene glycol, and glycerol, can be sourced from biomass and are attractive feedstocks for making renewable polymers.²⁵⁻²⁸ We envisioned that their dehydrogenative/dehydrative coupling could lead to the formation of polyester, polyether, or polyesterether. To test this hypothesis, we studied the polymerization of ethylene glycol using bifunctional ruthenium catalysts known for the dehydrogenation of alcohols and hydrogenation of carbonyl compounds, as reported herein.

Received: January 29, 2025

Revised: March 26, 2025

Accepted: March 27, 2025

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<figure><img src="image_11.png" /><figcaption>**Figure 1.** Known methods for the synthesis of polyesterethers and the method disclosed herein.</figcaption></figure>

## EXPERIMENTAL SECTION

**General Method for Polymerization of Ethylene Glycol under Closed Conditions.** A 100 mL ampule equipped with a J. Young's valve was charged with precatalyst (e.g., **Ru-5**; 8.3 mg, 0.01 mmol, 1 mol %) and base (e.g., KO'tBu, 2.2 mg, 0.02 mmol, 2 mol %). THF (2 mL) and ethylene glycol (0.11 mL, 2.0 mmol) were added, and the flask was sealed under an argon atmosphere before heating to the desired temperature (e.g., 150 °C) for the desired length of time (e.g., 24 h) with stirring. After this period, the reaction vessel was allowed to cool to room temperature, and the amount of gas evolved (presumably H₂) during the reaction was measured by syringe and analyzed by GC-TCD (Gas Chromatography-Thermal Conductivity Detector). The solvent was removed by rotary evaporator. Water (10 mL) was then added to the residue, followed by vacuum filtration to remove any solid as the desired product is soluble in water. The aqueous solution containing the product was concentrated using a rotary evaporator to remove the water. It was then analyzed by different characterization techniques like NMR and IR spectroscopy, GPC, TGA, and DSC, further details of which are given in the Supporting Information.

## RESULTS AND DISCUSSION

We began our investigation by studying the self-coupling of ethylene glycol in the presence of various ruthenium catalysts known for their catalytic dehydrogenation of alcohols. In a pilot experiment, 2 mmol of ethylene glycol was refluxed in 2 mL of THF in a sealed system in the presence of 1 mol % Ru-MACHO complex (**Ru-1**) and 2 mol % KO'tBu at 150 °C for 24 h (Table 1, entry 1). At the end of the reaction, 24 mL of gas was collected in the overhead space of the Young's flask. Analysis of the gas by GC-TCD (Gas Chromatography-Thermal Conductivity Detector) confirmed that the released gas was mainly H₂ with a small component of CO (<1%). Conversion of ethylene glycol was

found to be 63%, as determined by ¹H NMR spectroscopy of the crude reaction mixture in D₂O using ethylene carbonate as an internal standard. The product was isolated by extraction in water and characterized by NMR, IR spectroscopy, and GPC (Gel Permeation Chromatography). The GPC analysis confirmed the product to be a polymer with a molar mass M~n~ = 38,020 Da and a dispersity of 1.3. Analysis by NMR spectroscopy revealed that the polymer contains both ester and ether linkages in a ratio of 3.6:1, suggesting that the polymer belongs to the class of polyesterethers (Figure 2A,B). This was further confirmed by IR spectroscopy analysis, which showed the presence of ester (1768 cm⁻¹) and ether linkages (1087–1201 cm⁻¹) in addition to the presence of the O–H (3377 cm⁻¹) and aliphatic C–H (2980 cm⁻¹) stretches (Figure 2D). The HSQC (multiplicity-edited) spectrum (Figure 2C) shows the correlation between the hydrogens and carbons of the polymer and also suggested (see Figure S9) the presence of a hemiacetal-type intermediate, which was further confirmed by the ESI-MS study (Figure 2E) that showed the presence of various random oligoesterethers with molar masses up to 400 Da.

To achieve a higher conversion of ethylene glycol, we studied various ruthenium catalysts (**Ru-2–Ru-5**) under identical reaction conditions. Complexes **Ru-2**, **Ru-3**, and **Ru-4** resulted in lower conversion rates of ethylene glycol, whereas a higher conversion (89%) was obtained with **Ru-5** complex (Table 1, entries 2–5). The polymer obtained in this case was characterized as a polyesterether with a molar mass of 34,450 Da and an ester:ether ratio of 4.6:1 (entry 5). Using **Ru-5**, we further varied reaction conditions (temperature, catalytic loading, solvent, and time) to achieve a higher conversion of ethylene glycol. Performing the reaction at a lower temperature (100 °C) and with lower complex loading (0.5 mol %) resulted in lower conversion of ethylene glycol (20%, and 47%, respectively), as mentioned in Table 1 (entries 6 and 7). Conducting the reaction in water led to a very low conversion of ethylene glycol (10%, entry 8), whereas performing the reaction in diglyme resulted in a higher conversion of ethylene glycol (83%, entry 9). The polyesterether formed in diglyme was found to have a molar mass of 31,200 Da and an ester:ether ratio of 4.2:1. A higher conversion of 91% was achieved when DME (dimethoxyethane) was used as solvent, although the molar mass of the resulting polyesterether was found to be lower (11,770 Da, entry 10) compared to that obtained in diglyme. Increasing the reaction time to 72 h slightly increased the conversion (95%) and the molar mass (24,320 Da, entry 11). Performing the reaction under an open flow of argon in diglyme solvent did not have much effect on the conversion of ethylene glycol or the characteristics of the formed polyesterether in terms of molar mass or ester:ether ratio (entries 9 and 12). Extending the reaction for a longer time (48 h) in THF led to a higher conversion of ethylene glycol (94%) and the formation of a polyesterether with a higher molar mass (48,940 Da, entry 13) compared to the reaction conducted for 24 h (ethylene glycol conversion: 89%, polymer molar mass: 34,450 Da, entry 5). Using a higher base loading (10 mol %) led to the formation of a polyesterether with a relatively lower molar mass (24,010 Da, entry 14) compared to the example where 2 mol % base was used (entry 5). The use of different bases K₂CO₃, KOH, and KBH₄ led to the formation of polyesterethers with lower molar masses (entries 16–18) compared to those produced using KO'tBu base (entry 5). Performing the reaction solely in the presence of KO'tBu (2 mol %) without using any metal complex (entry 15)
<table><caption>Table 1. Optimization of Catalytic Conditions for the Coupling of Ethylene Glycol<sup>i</sup></caption><thead><tr><th>Entry no.</th><th>Precatalyst</th><th>Base</th><th>Solvent</th><th>Time (h)</th><th>Gas released (mL)<sup>a</sup></th><th>Conversion (%)<sup>b</sup></th><th>Molar mass, M<sub>n</sub></th><th>D</th><th>Ester:Ether<sup>c</sup></th></tr></thead><tbody><tr><td>1</td><td>Ru-1</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>24</td><td>63</td><td>38,020</td><td>1.3</td><td>3.6:1</td></tr><tr><td>2</td><td>Ru-2</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>19</td><td>37</td><td>34,640</td><td>1.3</td><td>4.2:1</td></tr><tr><td>3</td><td>Ru-3</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>3</td><td>14</td><td>-</td><td>-</td><td>1.8:1</td></tr><tr><td>4</td><td>Ru-4</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>10</td><td>22</td><td>-</td><td>-</td><td>-</td></tr><tr><td>5</td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>49</td><td>89</td><td>34,450</td><td>1.3</td><td>4.6:1</td></tr><tr><td>6<sup>d</sup></td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>10</td><td>20</td><td>-</td><td>-</td><td>-</td></tr><tr><td>7<sup>e</sup></td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>22</td><td>47</td><td>15,640</td><td>1.5</td><td>3.8:1</td></tr><tr><td>8</td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>H<sub>2</sub>O</td><td>24</td><td>4</td><td>10</td><td>-</td><td>-</td><td>-</td></tr><tr><td>9</td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>Diglyme</td><td>24</td><td>45</td><td>83</td><td>31,200</td><td>1.3</td><td>4.2:1</td></tr><tr><td>10</td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>DME</td><td>24</td><td>56</td><td>91</td><td>11,770</td><td>2.6</td><td>5.6:1</td></tr><tr><td>11</td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>DME</td><td>72</td><td>66</td><td>95</td><td>24,320</td><td>1.5</td><td>4.8:1</td></tr><tr><td>12<sup>f</sup></td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>Diglyme</td><td>24</td><td>-</td><td>81</td><td>32,680</td><td>1.4</td><td>4.2:1</td></tr><tr><td>13</td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>48</td><td>60</td><td>94</td><td>48,940</td><td>1.4</td><td>4.2:1</td></tr><tr><td>14<sup>g</sup></td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>43</td><td>79</td><td>24,010</td><td>1.6</td><td>3.6:1</td></tr><tr><td>15</td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>1</td><td>1</td><td>-</td><td>-</td><td>-</td></tr><tr><td>16</td><td>Ru-5</td><td>K<sub>2</sub>CO<sub>3</sub></td><td>THF</td><td>24</td><td>39</td><td>87</td><td>29,130</td><td>1.5</td><td>3.0:1</td></tr><tr><td>17</td><td>Ru-5</td><td>KOH</td><td>THF</td><td>24</td><td>34</td><td>84</td><td>33,370</td><td>1.4</td><td>2.8:1</td></tr><tr><td>18</td><td>Ru-5</td><td>KBH<sub>4</sub></td><td>THF</td><td>24</td><td>21</td><td>55</td><td>17,370</td><td>1.6</td><td>3.4:1</td></tr><tr><td>19</td><td>Ru-5</td><td>-</td><td>THF</td><td>24</td><td>3</td><td>7</td><td>-</td><td>-</td><td>-</td></tr><tr><td>20<sup>h</sup></td><td>Ru-5</td><td>KO<sup>b</sup>Bu</td><td>THF</td><td>24</td><td>46</td><td>83</td><td>55,830</td><td>1.2</td><td>5.6:1</td></tr></tbody></table>

a

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In all cases, released gas was identified to be mainly H₂ with ≤1% of CO using GC-TCD analysis. <sup>b</sup>Conversion was determined by ¹H NMR spectroscopy of the crude reaction mixture, using ethylene carbonate as an internal standard and D₂O as solvent. <sup>c</sup>Ester:ether ratio was estimated by ¹H NMR spectroscopy (see Supporting Information for more details). <sup>d</sup>Reaction conducted at 100 °C. <sup>e</sup>0.5 mol % catalyst was used. <sup>f</sup>Reaction performed under open flow of argon. <sup>g</sup>10 mol % KO<sup>b</sup>Bu was used. <sup>h</sup>Reaction with non-anhydrous ethylene glycol. <sup>i</sup>Reaction conditions: ethylene glycol: 2 mmol, solvent: 2 mL, catalyst (1 mol %), base (2 mol %).

or solely in the presence of complex 5 without using any base (entry 19) did not lead to the formation of any polymer, suggesting the crucial role of both complex 5 and the base. Additionally, conducting the reaction with non-anhydrous ethylene glycol led to the formation of less ether compared to ester, indicating that the presence of water is unfavorable for ether formation (entry 20). Similarly, when the reaction was conducted in the presence of molecular sieves (see the Supporting Information), the ester:ether ratio changed to 3.6:1 from 4.6:1 (entry 5), suggesting that the removal of water can promote the etherification process.

To get an idea of the ether chain length, the saponification of a formed polyesterether (M<sub>n</sub> = 35,950 Da, D = 1.2) was carried out using KOH as the base and water as the solvent at 150 °C for 24 h. After the completion of the reaction, the resulting reaction mixture was extracted with diethyl ether. Analysis of the reaction mixture revealed a combination of polyethylene glycol (confirmed by NMR spectroscopy) with M<sub>n</sub> = 1550 Da (D = 2.1) and M<sub>n</sub> = 22,660 Da (D = 1.5). Additionally, ethylene glycol and a higher molecular weight polyethylene glycol [M<sub>n</sub> = 55,580 Da (D = 1.2)] were also observed (see Section S1.17).

Having developed the catalytic conditions for the formation of polyesterethers, we turned our attention to understanding the mechanism of the process. The dehydrogenative coupling of alcohols to esters is well studied for a number of ruthenium complexes, including Ru-1–Ru-5.29–32 Based on the reported mechanisms, we suggest that the process begins with the catalytic dehydrogenation of ethylene glycol (1) to glycolaldehyde (2) which reacts with another molecule of ethylene glycol to form a hemiacetal (3) that subsequently undergoes dehydrogenation in the presence of the ruthenium catalyst to form an ester (4, Figure 3). However, the formation of ethers from alcohols using bifunctional transition-metal complexes that operate via Noyori-type metal–ligand cooperativity33 (e.g., Ru-1–Ru-5) is intriguing and has not been reported before. In general, alcohol-to-ether transformation is achieved using acid-based catalysts.34 We suggest, in this case, that a possible pathway could operate via the dehydration of hemiacetal 3 to form an enol ether (5), which could tautomerize to the aldehyde ether (6). Conversion of a hemiacetal intermediate to an ether has also been proposed earlier by Beller during the hydrogenation of esters to ethers catalyzed by a ruthenium/Triphosph-based catalyst.35 We propose that the formation of a polyether
<figure><img src="image_13.png" /><figcaption>**Figure 2.** ¹H NMR (A), ¹³C{¹H} NMR (B), HSQC NMR (C), and IR(D) spectra, as well as ESI-MS data (E) for the polyesterether.</figcaption></figure>

sequence from the aldehyde ether intermediate (6) could proceed via the hydrogenation of the aldehyde group to form diol intermediate 7 that could then react with glycolaldehyde 2 to form intermediate 8 upon dehydration (Figure 3). The enol intermediate 8 could tautomerize to form the aldehyde

intermediate 9, and the continuation of this process could lead to the formation of polyether.

To further investigate this, we also performed DFT calculations (see the Supporting Information for more information and Figure 4). Under the basic dehydrogenative
<figure><img src="image_14.png" /><figcaption>**Figure 3.** Proposed pathway for the synthesis of polyester and polyether from ethylene glycol.</figcaption></figure>

conditions, precatalyst **Ru-5** could in situ generate amido-
complex **IntA**. We consider the most stable isomer of **IntA**
.square pyramidal with one hydride in the apical position; see
the Supporting Information) as a reference point (ΔG = 0.0
kcal/mol at 423.15 K). The reaction with ethylene glycol follows
the commonly observed pattern for ruthenium-based alcohol
dehydrogenation catalysis, with the alkoxide as off-cycle
intermediate (**IntB**, -5.0 kcal/mol) and H₂-elimination as the
rate-limiting transition state (TSA·C = 25.9 kcal/mol, see Figure
4). Importantly, the dual bis-dentate ligand system allows for
isomerization, and while **IntA** is most stable in trans-N-P
configuration, the most stable intermediate (TDI) is the cis-
dihydride species **IntC** (−5.8 kcal/mol, after the second H₂
elimination step). This is also corroborated by experiments,
showing a cis-dihydride as a resting state during dehydrogenative
coupling (see discussion below and Section S1.16). Hence, in
contrast to the Ru-acridine-ligand system known for ethylene
glycol dehydrogenation,²⁴ where an open coordination site is
available at ruthenium, in the present case, a hydride trans to the
N—N bidentate ligand remains bound along the catalytic cycle,
as observed for other pincer-ruthenium systems.³⁶,³⁷ After C—
O bond formation between ethylene glycol **1** and glycol
aldehyde **2** (TS$_{C-A}$ = 21.8 kcal/mol), the hemiacetal-bound
species **IntD** is formed (+5.1 kcal/mol). Thus, **IntD** represents
the branching point for (poly)ester vs (polyester)ether
formation (Figure 4B). While acetal dehydrogenation and H₂
elimination provide free ester **4** (0.0 kcal/mol), **IntD** could also
undergo dehydration to release enol **5** together with **IntA** (10.7
kcal/mol). While we were not able to locate a transition state for
this step, metal-assisted C—O bond breakage during repeated
optimization attempts is consistent with a kinetically feasible,
relatively flat PES³⁸ (see Section S1.16). Nevertheless, the
formation of **5** from **IntD** is 10.7 kcal/mol less favored than ester

formation. Tautomerization of 5 to aldehydic ether 6 (+3.3 kcal/mol), e.g., via sigma-complexes IntE (13.8 kcal/mol) or IntF (8.7 kcal/mol), commonly found in ruthenium-catalyzed C—C double bond isomerization³⁹⁻⁴² could serve as the driving force for (polyester)ether formation. Indeed, isomerization of alkenes via chain walking and olefin transposition mechanisms has been reported in the past using similar types of bifunctional catalysts that are also known for the dehydrogenation of alcohols.³⁹⁻⁴²

Thus, the relative propensity of ether bond formation vs ester
formation will depend on the free energy of H₂-release vs H₂O-
formation (affected, e.g., by temperature, solubility, mass
transfer between the liquid and gas phase²⁹ and the isomer-
ization of the C—C double bond for possible enol
tautomerization to the aldehyde). With a simple statistical
redistribution of the C—C double bond, longer-chain diols are
thus expected to give lower yields of (polyester)ether. Indeed,
while 1,2-diols and 1,3-diols led to the formation of
polyesterethers (Table 2, entries 1–4), higher diols such as
1,6-hexanediol and 1,10-decanediol led to the formation of a
polyester (Table 2, entries 5 and 6, see the Supporting
Information). Additionally, 1,4-cyclohexanediol under the
reaction conditions of Table 2 (entry 1) led to the formation
of 4-hydroxycyclohexan-1-one, and the formation of a polymer
was not observed in this case (entry 7). Similarly, performing the
dehydrogenation of 1-phenylethanol under the reaction
conditions of Table 2 (entry 1) led to the formation of
acetophenone as the only product with a 45% yield. The lack of
ether formation in this case further confirms the hypothesis of
the need for an enol-ether type intermediate to form ether. To
further confirm this hypothesis of C≡C double bond
isomerization, we studied the reactivity of 3-butenol and 4-
hexenol in the presence of Ru-5 (1 mol %) precatalyst and
KO¹Bu (2 mol %; Figure 5B). Interestingly, alcohols, aldehydes,
<figure><img src="image_15.png" /><figcaption>**Figure 4.** (A) Proposed reaction mechanism for ethylene glycol dehydrogenation to ester promoted by precatalyst **Ru-5**. (B) Possible bifurcation of reaction pathway toward ether intermediate **6**. Free energies were calculated at 423.15 K and at 1 M concentration of solutes in kcal/mol (see the Supporting Information for details).</figcaption></figure>

and esters with varying positions of C=C were observed, suggesting the possibility of alkene isomerization as hypothesized in Figure 5A. The crucial role of the tautomerization step (only possible with diols) was also addressed by control experiments with n-hexanol or n-propanol instead of ethylene glycol. Under the catalytic conditions described in Table 1 (entry 5), n-hexanol/n-propanol led to the formation of hexanal/propanal and hexyl hexanoate/propyl propanoate, and no ether was formed in this case. This is further supported by the lack of observation of any alkene groups in the NMR spectra of the formed polyesterethers (Table 1 and see Section S1.9) as the enol-ether intermediate (5) can easily tautomerize to form the aldehyde-ether intermediate (6). To probe whether an aldehyde group could be hydrogenated under the reaction conditions, we performed the polymerization of ethylene glycol (as per the conditions of Table 1, entry 5) in the presence of hexanal (1 mmol). Interestingly, 59% of hexanal was hydrogenated to hexanol in addition to the formation of

polyesterether, confirming our hypothesis that the aldehyde group can be hydrogenated to alcohol under these reaction conditions.

To understand the nature of the catalytically active species, we monitored the catalytic reaction by NMR spectroscopy. A ¹H NMR spectrum taken after 1 h of reaction time under the catalytic conditions described in Table 1, entry 5 (1 mol % complex 5, 2 mol % KOtBu, THF, 150 °C) showed a doublet of doublets centered at δ −6.47 ppm, suggestive of a hydride species that is both cis (²J<sub>PH</sub> = 27.7 Hz) and trans (²J<sub>PH</sub> = 120.5 Hz) to phosphorus (see Figure S114). The ³¹P{¹H} NMR spectrum (see Figure S115) showed the presence of two ruthenium bis-phosphine complexes with both phosphorus atoms being cis to each other (δ 45.1, 37.3, ²J<sub>PP</sub> = 25.2 Hz; and δ 54.5, 21.8, ²J<sub>PP</sub> = 14.0 Hz). Based on this, we speculate the formation of a ruthenium hydrido alkoxide complex **Ru-6** (IntB, Figure 6). Interestingly, leaving the reaction mixture for a further 24 h at room temperature resulted in the disappearance of these
The raw research data supporting this publication can be accessed at https://doi.org/10.17630/15c7c187-a8d6-42db-abea-c5df539e14e6.

## Accession Codes

All computed structures, including energies and thermochemistry, are available free of charge via the ioChem-BD online repository at the following link: 10.19061/iochem-bd-6-439.

## ■ AUTHOR INFORMATION

### Corresponding Authors

Amit Kumar - EaStCHEM, School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, U.K.; orcid.org/0000-0002-8175-8221; Email: ak336@st-andrews.ac.uk

Niklas von Wolff - Sorbonne Université, Institut Parisien de Chimie Moléculaire, IPCM, F-75005 Paris, France;
Email: niklas.von-wolff@cnrs.fr

### Authors

Garima Saini - EaStCHEM, School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, U.K.; orcid.org/0009-0005-5929-2874

Pavel Kulyabin - EaStCHEM, School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, U.K.; orcid.org/0000-0003-2548-2117

Angus McLuskie - EaStCHEM, School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, U.K.

Complete contact information is available at:
https://pubs.acs.org/10.1021/acssuschemeng.5c00886

## Notes

The authors declare no competing financial interest.

## ■ ACKNOWLEDGMENTS

This research is funded by a UKRI Future Leaders Fellowship (MR/W007460/1) and an EPSRC grant (EP/Y005449/1). We thank Johnson Matthey, Cambridge (Dr Antonio Zanotti-Gerosa and Dr Damian Grainger) for their generous donation of ruthenium complexes **Ru-2-Ru-5** used in this study. Computations were performed using HPC resources from GENCI-CINES (AD010812061R3).

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