Titanium-mediated coupling of CO₂ and ethylene to form acrylate: mechanistic insights from Cp*₂Ti complexes

Abstract

The coupling of CO₂ and ethylene to form acrylate is a promising route for CO₂ utilization, yet known catalytic systems are limited by low activity and harsh reaction conditions. These limitations are closely linked to the difficulty of metallalactone intermediate formation in late transition metal systems. Here, we investigate titanium-mediated coupling of CO₂ and ethylene to form acrylate using Cp*₂Ti complexes and elucidate the elementary steps that enable acrylate formation under mild conditions. The oxophilic titanium promotes CO₂/ethylene coupling with a low activation barrier, and the resulting Ti(IV) metallalactone is thermodynamically stabilized by strong Ti–O interactions. Subsequent conversion to acrylate is readily achieved through deprotonation by an alkoxide base, while the canonical β-hydride elimination pathway remains unfavorable. Despite the kinetic accessibility of acrylate formation, mechanistic studies reveal that catalytic turnover is limited by competing Ti(II)/Ti(IV) comproportionation. These findings outline the operative steps and current limitations of titanium-mediated acrylate synthesis and provide a basis for the design of early transition metal catalysts for carboxylation chemistry.

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Introduction

CO₂ utilization has emerged as a key strategy for mitigating environmental threats and harnessing it as a sustainable carbon source.^1–4 Among various approaches, carboxylation using CO₂ is attractive, as organic carboxylic acids find widespread use in the chemical industry.^5–7 In particular, acrylic acid is an important feedstock for polymers, coatings, adhesives and absorbent materials.^8–12 Since the first Ni-catalyzed synthesis of acrylate from CO₂ and ethylene in 2012,¹³ research has predominantly focused on Ni and Pd systems, exploring various ligands, bases, solvents, and additives (Fig. 1A).^9–24 Despite notable progress, catalytic systems still exhibit limited activity and require harsh conditions, with the best reported performance corresponding to 570 turnovers at 140 °C and 30 bar.²⁴ These limitations are attributed to slow kinetics rather than catalyst deactivation or side reactions.^20,25 Recent studies have extended to group 8 and 9 metals,^26–32 yet these have not overcome such limitations, motivating exploration of alternative metals beyond late transition metals.

Fig. 1 (A) Representative catalysts for the coupling of ethylene and CO₂ to form acrylate. (B) Conceptual comparison of the metallalactone formation pathway for group 10 metals (Ni, Pd) and titanium. Free energies for Ni/Pd systems are taken from ref. 13, 25, 33–35.

A fundamental challenge in developing efficient carboxylation catalysts lies in the formation of a metallalactone intermediate via oxidative coupling of ethylene and CO₂, which is both kinetically and thermodynamically demanding (Fig. 1B).^{13,25,33–35} In d¹⁰ late metals such as Ni⁰ and Pd⁰ complexes, fully occupied d orbitals disfavor CO₂ coordination, leading to high-energy transition states for coupling between ethylene and CO₂. This intrinsic electronic property accounts for the harsh conditions required for Ni and Pd catalysts. In contrast, early transition metals may present distinct opportunities. Their vacant d orbitals can facilitate CO₂ binding, lowering the kinetic barrier to metallalactone formation (Fig. 1B). In addition, the oxophilic Ti^IV forms strong Ti–O bonds, enhancing the thermodynamic stability of metallalactone species (Fig. 1B).

Literature precedents have shown that early transition metals can indeed form metallalactone species.^36–42 However, prior studies primarily focused on their synthesis, while their formation energetics and acrylate-relevant reactivity remain underexplored. Among early transition metals, titanium stands out as an appealing platform owing to its strong oxophilicity and versatile redox behaviour, which enable diverse chemistry in small-molecule activation and cross-coupling reactions,^43–46 suggesting its potential to mediate carboxylation chemistry.

In this study, we investigate titanium-mediated coupling of ethylene and CO₂ to form titanalactone and its reactivity toward acrylate synthesis. Using Cp*₂Ti complexes, we elucidate the kinetic and thermodynamic landscape of elementary steps and competing pathways, and identify the factors limiting catalytic turnover, thereby providing a mechanistic insight into titanium-mediated ethylene carboxylation.

Results and discussion

Kinetic and thermodynamic analysis of titanalactone formation

Earlier reports have described the formation of metallalactone complexes across several early transition metals, including Ti,³⁶ V,³⁷ Zr,³⁸ Mo,^39–41 and W.^39,40,42 These reports demonstrate that early transition metals are capable of mediating the coupling of ethylene and CO₂ under stoichiometric conditions. However, the kinetic and thermodynamic aspects of metallalactone formation and its reactivity toward acrylate production have not been examined. Among the reported systems, the titanalactone is particularly notable, as it can be generated at low temperature,³⁶ suggesting relatively favorable kinetics. This compound was first reported in 1985 by Cohen and Bercaw, who observed the reaction of Cp*₂Ti(C₂H₄) (1) with 1 equiv. of CO₂ at −78 °C to afford Cp*₂Ti(C₂H₄CO₂) (2).³⁶ We therefore first revisited the formation of 2.

Addition of CO₂ (50 equiv.) to 1 in toluene at room temperature resulted in an immediate color change from green to red (Fig. 2A). Product 2 was isolated in 85% yield and identified by its characteristic ¹H NMR triplets at δ 3.41 and 1.14 (³J_HH = 7.9 Hz). Given the rapid reaction at room temperature, the kinetics of formation of 2 were monitored at low temperatures by UV-Vis spectroscopy (Fig. 2B). Upon exposure of 1 to CO₂ (50 equiv.) at 0 °C, complete conversion was achieved within minutes, with an observed rate constant (k_obs) of 0.78 s⁻¹. Eyring analysis across the temperature range from −60 to 0 °C afforded a low activation enthalpy (ΔH^‡_{298 K}= +4.7 kcal mol⁻¹) and a free energy of ΔG^‡_{298 K} = +15.3–17.1 kcal mol⁻¹ (Fig. 2C and S19). The range arises from the uncertainty in the concentration of dissolved CO₂ under the reaction conditions, with the upper bound (+17.1 kcal mol⁻¹) corresponding to the apparent value from Eyring analysis without entropy correction, while the lower bound (+15.3 kcal mol⁻¹) was obtained by applying the maximum entropic correction (TΔS = 1.8 kcal mol⁻¹) under the assumption of complete CO₂ dissolution. The DFT-computed barrier of +15.4 kcal mol⁻¹ (vide infra, Fig. 4A, Path a) falls within this experimentally bracketed range, supporting a low kinetic barrier for C₂H₄–CO₂ coupling at a titanium center. Notably, this rate places the Ti system in a fundamentally different kinetic regime from late transition metal analogues. Ni and Pd systems exhibit hour-scale metallalactone formation kinetics.^13,22,25 Consistent with these slow kinetics, DFT studies have proposed activation barriers on the order of ∼30 kcal mol⁻¹ for metallalactone formation at nickel and palladium centers.^{13,25,33–35} These results show that metallalactone formation at the titanium center is an exceptionally fast process, in contrast to Ni and Pd systems, where this step is often kinetically limiting.

Fig. 2 (A) Reaction of 1 with CO₂ to form 2. (B) UV-Vis spectral changes from 1 (black) to 2 (red) in THF at −40 °C. The inset shows the absorbance at 410 nm. (C) Eyring plots constructed using k_obs values measured at 0, −20, −40 and −60 °C.

Compound 2 remained robust upon isolation, showing no indication of decarboxylation back to 1 over several weeks. The crystal structure of 2 provides insight into its stability (Fig. 3A and Table 1). The Ti–O bond (2.016(4) Å) is 0.133 Å shorter than the Ti–C bond (2.149(5) Å), whereas the analogous Pd and Ni complexes exhibit smaller M–O/M–C differences (Δd = −0.008 Å for Pd and −0.050 Å for Ni).^17,22 This pronounced Ti–O contraction reflects strong oxophilicity of titanium. The density functional theory (DFT)-optimized geometry of 2 also reproduces this structural feature, with Ti–O and Ti–C bond lengths of 1.964 and 2.223 Å, respectively. Wiberg bond index analysis further indicates that both the Ti–O and Ti–C interactions in 2 are stronger than the corresponding bonds in the Ni analogue, reflecting strong covalent interactions in the Ti complex, with a particularly large difference for the M–O interaction (0.557 for Ti vs. 0.276 for Ni). Natural population analysis reveals a more electropositive metal center in the Ti complex compared with the Ni congener, consistent with a more strongly polarized metal–oxygen interaction. These analyses are consistent with the pronounced oxophilicity of titanium and strong Ti–O interaction, providing thermodynamic stabilization to the metallalactone framework in 2.

Fig. 3 (A) Structural representation of 2 with ellipsoids shown at 50% probability. (B) Wiberg bond indices (blue) and natural population analysis charges (orange) for the metallalactone fragment of compound 2 and (NHC-P)Ni(C₂H₄CO₂).

Table 1 Comparison of bond distances and orders in metallalactone compounds

	Ti	Ni
a Distances from X-ray diffraction data; values for the Ni compound are taken from ref. 22.b Difference between dM–O and dM–C. Δd = dM–O − dM–C.c Wiberg bond indices and NPA charges for both compounds were computed at the TPSS/6-311g(d,p) level of theory.
Bond distances^a
dM–O (Å)	2.016(4)	1.880(3)
dM–C (Å)	2.149(5)	1.930(7)
Δd ^b (Å)	−0.133	−0.050
Wiberg bond indices^c
M–O	0.557	0.276
M–C	0.701	0.553
Natural population analysis charges^c
M	+1.382	+0.825
O	−0.719	−0.802
C	−0.676	−0.705

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Computed energy profiles support the advantage of titanium in metallalactone formation relative to Ni and Pd systems. Oxidative coupling between 1 and CO₂ proceeds via a five-membered transition state, which is strongly exergonic (ΔG = −16.3 kcal mol⁻¹) with a low activation barrier (ΔG^‡ = +15.4 kcal mol⁻¹) under standard conditions (Fig. 4A, Path a). No stable CO₂-bound intermediate is found along the reaction coordinate; instead, CO₂ coordination directly leads to the titanalactone product. In contrast, CO₂ binding to Ni(NHC-P)(C₂H₄) is unfavorable (ΔG = +4.4 kcal mol⁻¹), and the product formation is only slightly exergonic (ΔG = −3.7 kcal mol⁻¹, Table S5), with a higher barrier (ΔG^‡ = +17.9 kcal mol⁻¹, Table S5). The Pd(dmpe) analogue also shows unfavorable CO₂ binding (ΔG = +8.5 kcal mol⁻¹), slightly endergonic palladalactone formation (ΔG = +0.4 kcal mol⁻¹), and a substantially higher barrier (ΔG^‡ = +24.4 kcal mol⁻¹, Table S6). Computational results are consistent with the experimentally observed reactivity. The corresponding Ni and Pd systems require harsh conditions to shift the equilibrium and overcome slow kinetics,^22,25 whereas the titanium system enables thermodynamically favorable metallalactone formation with a low kinetic barrier.

Fig. 4 (A) Calculated free energies for titanalactone (Path a, black), ethylene dissociation followed by Cp* C–H activation (Path b, red) and ethylene cyclization (Path c, blue). (B) Yield of 2 with varying amounts of CO₂ and C₂H₄.

Stoichiometric effects on titanalactone formation

Although the formation of titanalactone 2 proceeds efficiently, the conversion is not quantitative under stoichiometric conditions, indicating the presence of competing pathways. To identify side reactions that limit lactone formation, the conversion of 1 to 2 was monitored by ¹H NMR spectroscopy. CO₂ addition to 1 led to the disappearance of 1 and the formation of 2 along with free C₂H₄ (δ 5.25), indicating the partial dissociation of C₂H₄ from 1 (Fig. S21). Low-intensity aliphatic signals (δ 0.8–2.0) also appeared, one matching the known Cp* C–H activation product 3 (Fig. S21).⁴⁷ These indicate that ethylene dissociation from 1 and subsequent C–H activation occur in parallel with the productive CO₂ insertion pathway.

To minimize side reactions associated with C₂H₄ dissociation, the effects of CO₂ and C₂H₄ equivalents on the yield of 2 were investigated (Fig. 4B). Reaction of 1 with 1 equiv. of CO₂ gave 2 in 60% yield, which gradually increased to 85% with higher CO₂ loading. Notably, the addition of a small amount of ethylene (5 equiv.) improved the yield to 98%, consistent with suppression of ethylene dissociation. In contrast, excess ethylene (∼50 equiv.) led to a decrease in the yield of 2, presumably due to the formation of the titanacyclopentane species Cp*₂Ti(C₄H₈) (4).⁴⁸ Compound 1 can form 4 in equilibrium under C₂H₄,⁴⁸ and therefore, excess C₂H₄ shifts the equilibrium toward 4 and hinders the formation of 2.

DFT calculations support the experimentally observed energy relationships for lactone formation and side reactions (Fig. 4A). Ethylene dissociation from 1 generates Cp*₂Ti, which can subsequently undergo C–H activation of the Cp* ligand (Fig. 4A, Path b).⁴⁷ The formation of the S = 1 decamethyltitanocene is endergonic (ΔG = +7.8 kcal mol⁻¹), while the corresponding S = 0 excited state lies 9.6 kcal mol⁻¹ higher, consistent with its experimentally observed paramagnetic nature.⁴⁷ The ethylene dissociation and subsequent C–H activation proceed with the overall barrier reaching ΔG^‡ = +28.8 kcal mol⁻¹. Alternatively, coordination of an additional C₂H₄ can initiate cyclization to form 4 (Fig. 4A, Path c).⁴⁸ This process is slightly endergonic (ΔG = +4.1 kcal mol⁻¹) and proceeds through a moderate activation barrier (ΔG^‡ = +25.3 kcal mol⁻¹), in reasonable agreement with the experimentally determined thermodynamics.⁴⁸ Taken together, both pathways may be energetically accessible under CO₂-deficient conditions but are expected to be kinetically suppressed in a CO₂-rich environment. While low C₂H₄ loading prevents C₂H₄ dissociation from 1, under C₂H₄-rich conditions, 1 can be predominantly converted to 4, suppressing productive titanalactone formation. Careful control of both substrates is therefore important to maximize the yield of 2 while minimizing competing side products.

Reactivity of titanalactone toward acrylate formation

The reactivity of titanalactone species 2 toward acrylate formation was investigated through potential β-hydride elimination or direct deprotonation pathways (Fig. 5).^13,34,49 β-Hydride elimination from metallalactones requires prior ring opening to adopt a geometry in which the β-hydrogen is positioned syn-coplanar with the metal center. This conformational rearrangement incurs a significant energetic penalty (Fig. 5A).^33,50 In the case of titanium, this pathway is expected to be even more challenging due to the robust Ti–O interaction and the d⁰ electronic configuration of the titanium center. Indeed, thermolysis of 2 up to 150 °C resulted only in decomposition without any detectable acrylate formation.

Fig. 5 Conversion of metallalactone to acrylate via (A) β-hydride elimination, (B) electrophile-assisted β-hydride elimination, and (C) direct deprotonation.

To further evaluate whether β-hydride elimination could be accessible, we explored the effect of electrophiles, inspired by previous studies on nickel and palladalactone systems in which an electrophile promotes M–O bond cleavage and subsequent β-hydride elimination.^50–54 Although the addition of NaBAr^F₄ or MeI to 2 did not result in any observable elimination or speciation, treatment with the stronger electrophile MeOTf led to O-methylation to afford the cationic methylated lactone complex [Cp*₂Ti(C₂H₄CO₂Me)][OTf] (5) (Fig. 5B). Compound 5 retains the characteristic lactone triplets at δ 3.25 and 1.42 (³J_HH = 7.84 Hz) in the ¹H NMR spectrum, along with new methyl resonances observed at δ 3.88 in ¹H NMR and at δ 57.28 in the ¹³C NMR spectra. The solid-state structure of 5 confirms methylation at the distal oxygen atom and reveals a slightly elongated yet persistent Ti–O interaction (average Ti–O = 2.083 Å in 5 vs. 2.016(4) Å in 2) (Fig. 6A). Although the O-methylation formally neutralizes the anionic oxygen in the carboxylate moiety and would be expected to promote Ti–O bond cleavage, the retention of Ti–O interaction shows the strong oxophilicity of the titanium center. Consistent with this structural observation, the IR spectrum of 5 exhibits a red-shifted carbonyl stretching frequency at 1612 cm⁻¹ relative to 2 (1670 cm⁻¹), which reflects weakening of the C=O bond upon electron donation to the titanium center and supports the presence of Ti–O interaction even after neutralization of the carboxylate.

Fig. 6 (A) Structural representation of 5. (B) ¹H NMR spectra showing methyl acrylate and PMA formation from 5. (C) The concentration changes of 5, methyl acrylate and PMA. (D) Eyring plots constructed using k_obs values measured at 40, 50 and 60 °C.

A trace amount of methyl acrylate was detected in the NMR spectrum of 5, indicating that β-hydride elimination from 5 is accessible (Fig. 6B). Upon heating 5 at 60 °C, however, only a small amount (0.5%) of methyl acrylate accumulated over 24 h, and the yield did not increase further. Instead, formation of poly(methyl acrylate) (PMA) was observed (Fig. 6B), demonstrating that the produced methyl acrylate is consumed more rapidly by polymerization than it is formed. As a result, methyl acrylate remains at a low steady-state concentration (0.5%), and 5 follows pseudo-first-order decay. The conversion of 5 is slow, with a k_obs of 1.4 × 10⁻⁶ s⁻¹ at 60 °C (Fig. 6C). Eyring analysis over the range of 40–60 °C affords a high activation barrier (ΔG^‡) of +28.2 kcal mol⁻¹ (Fig. S28). These results indicate that the promotional effect of the electrophile on β-hydride elimination is limited in titanium, which contrasts with the Ni system, where MeOTf readily cleaves the Ni–O bond and releases methyl acrylate in high yield.⁵² Although methylation weakens the Ti–O bond, the residual Ti–O coordination may remain sufficiently strong to disfavor β-hydride elimination.

To circumvent the intrinsic limitations associated with β-hydride elimination in the titanalactone system, a direct deprotonation pathway was explored as an alternative strategy for acrylate formation (Fig. 5C). Treating 2 with 1 equiv. of NaO^tBu in THF resulted in a rapid color change from orange to dark green within 30 min. The ¹H NMR spectrum showed complete disappearance of lactone signals, accompanied by the appearance of broad resonances in the acrylate region (δ 4.8–6.2). Although the putative Ti^II–acrylate species A could not be isolated, its transient formation is supported by D₂O quenching experiments, which afforded the free sodium acrylate product in 24% yield. A series of bases with varying basicity were evaluated (Table 2). Acrylate formation was found to be highly sensitive to base strength, with only strongly basic alkoxides (pK_a of 37–40)^55–59 being effective in promoting productive deprotonation. Increasing the loading of NaO^tBu from 1 to 10 or 20 equivalents led to a gradual improvement in acrylate yield, reaching up to 40%, demonstrating that direct deprotonation is a viable pathway for acrylate generation.

Table 2 Base screening for deprotonation of 2 to produce acrylate

Entry	Base	Additive	pKa ^a	Yield^b (%)
a  pKa of the conjugate acid in MeCN. Values are taken from ref. 55–59.b Yield determined by ^1H NMR analysis using 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt as an internal standard.c NaBAr^F4 was added as an additive.
1	NEt3	NaBAr^F4	18.8	—
2	DBU	NaBAr^F4	24.3	—
3	NaOPh-2-F	—	25.4	—
4	^tBuP1(pyrr)3 ^c	NaBAr^F4	28.4	—
5	Verkade's base^c	NaBAr^F4	33.6	—
6	NaOMe	—	∼37	11
7	NaO^iPr	—	∼39	11
8	NaO^tBu (1 equiv.)	—	∼40	24
9	NaO^tBu (10 equiv.)	—	∼40	39
10	NaO^tBu (20 equiv.)	—	∼40	40

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Recently, Pasha et al. proposed an alternative pathway, in which direct proton transfer from the β-carbon to the carbonyl oxygen is promoted by a Lewis acid/base pair (LiI/NEt₃).⁶⁰ While this mechanism is viable in the nickel system with relatively weak bases (NEt₃, pK_a of 18), analogous Lewis acid/weak base combinations, NaBAr^F₄ with amine and phosphazene bases spanning pK_a of 18–34, afforded no detectable acrylate (Table 2, entries 1, 2 and 4). The sharp pK_a threshold for productive reactivity (pK_a ≥ 37) is therefore more consistent with direct C–H cleavage by a strong external base than with base-assisted enolization.

To evaluate the feasibility of β-hydride elimination and deprotonation pathways, reaction coordinates were calculated for each proposed mechanism (Fig. 7). For the direct β-hydride elimination pathway (Fig. 5A), the titanalactone moiety first undergoes ring opening to locate the β-hydrogen in proximity to the titanium center. This structural reorganization requires Ti–O bond dissociation and is highly endergonic (ΔG = +34.9 kcal mol⁻¹). The subsequent β-hydride elimination step involves an additional barrier of 14.8 kcal mol⁻¹, resulting in an overall barrier of +49.7 kcal mol⁻¹ (Fig. 7, Path A). Such a prohibitively high barrier strongly indicates that direct β-hydride elimination from 2 is kinetically inaccessible.

Fig. 7 Calculated free-energy profiles for acrylate formation from CO₂ and ethylene by Cp*₂Ti via direct β-hydride elimination (Path A, blue), MeOTf-promoted β-hydride elimination (Path B, red) and direct deprotonation by NaO^tBu (Path C, black).

Meanwhile, the methylation of 2 with MeOTf is computed to be thermodynamically favorable (ΔG = −6.7 kcal mol⁻¹) and kinetically accessible (ΔG^‡ = +13.7 kcal mol⁻¹), consistent with the experimentally observed rapid formation of 5 (Fig. 7, Path B). Methylation weakens the Ti–O bond and enables ring opening to form a syn-coplanar Ti–C–C–H arrangement (ΔG = +16.9 kcal mol⁻¹, Fig. 7, Path B). Consequently, β-hydride elimination from 5 leading to methyl acrylate proceeds with a computed activation barrier of +35.1 kcal mol⁻¹, which is substantially lower than that of the non-methylated pathway (Path A, ΔG^‡ = +49.7 kcal mol⁻¹). Nevertheless, the overall kinetic barrier remains relatively high, and the thermodynamic driving force of methyl acrylate formation is modest (ΔG = −4.3 kcal mol⁻¹), in good agreement with the experimentally observed sluggish conversion of 5.

The DFT-computed reaction profile for NaO^tBu-assisted deprotonation of 2 was also calculated (Fig. 7, Path C). Coordination of the distal oxygen atom of the titanalactone to the sodium cation in the NaO^tBu-THF adduct stabilizes intermediate 2 by 1.9 kcal mol⁻¹. The thermodynamics of the subsequent deprotonation step is uphill by 15.2 kcal mol⁻¹, consistent with the experimentally observed untrappability of the putative intermediate A. This endergonic step can be driven forward by coupling with the exergonic release of sodium acrylate, accompanied by ethylene coordination (ΔG = −14.2 kcal mol⁻¹). The free-energy profile further indicates that NaO^tBu-assisted deprotonation proceeds with a barrier (ΔG^‡ = +19.9 kcal mol⁻¹) higher than that for titanalactone formation (ΔG^‡ = +15.4 kcal mol⁻¹) (Fig. 7). This suggests that deprotonation is readily accessible at room temperature but is still slower than the titanalactone formation step. These results are in good agreement with the experimentally observed relative rates of each elementary step. Along Path C, the overall thermodynamics of the catalytic sequence is −16.4 kcal mol⁻¹, supporting the feasibility of the catalytic reaction.

Based on the experimental observations and DFT analysis suggesting that deprotonation of 2 enables the generation of acrylate, we examined whether this pathway could support catalytic turnover. Under mild conditions (25 °C, 1 bar C₂H₄, 5 bar CO₂), catalytic acrylate formation was observed with a TON of 2.1 (Table 3, entry 4). Although this value is far lower than those for reported Ni and Pd catalysts, the observation of any turnover under such mild conditions is mechanistically informative. The Ti center is fundamentally capable of operating catalytically under mild conditions, which is in notable contrast to late transition metal systems.^13–24 Variation of reaction parameters revealed that the limited TON is not dictated by reaction conditions. Neither increasing pressure nor elevating temperature improved performance (Table 3, entries 5–9). This indicates an intrinsic limitation within the Ti system, or a competing process, rather than a simple dependence of kinetics on external parameters.

Table 3 Proof-of-concept catalysis using 2^a

Entry	2 (mmol)	NaO^tBu (mmol)	PC2H4 (bar)	PCO2 (bar)	Temp. (°C)	TON^b
a Standard conditions: NaO^tBu (2.0 mmol), 2 (0.1 mmol), 10 mL THF, 15 h.b TON determined by ^1H NMR analysis using 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt as an internal standard.
1	0.1	2.0	1	1	25	1.7
2	0	0.2	1	1	25	0
3	0.1	0	1	1	25	0
4	0.1	2.0	1	5	25	2.1
5	0.1	2.0	1	30	25	1.7
6	0.1	2.0	5	5	25	0.5
7	0.1	2.0	30	1	25	0.6
8	0.1	2.0	1	5	80	1.4
9	0.1	2.0	1	5	120	1.2

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Mechanistic investigation of the side reaction in the deprotonation of 2

To identify the factor responsible for limiting catalytic turnover, we investigated the reaction of 2 with NaO^tBu. Upon addition of NaO^tBu, compound 2 completely disappeared within 1 h in the ¹H NMR spectrum, accompanied by the formation of paramagnetic species, as indicated by magnetic susceptibility measurement using the Evans method.⁶¹ For clear observation and the characterization of the paramagnetic species, the reaction was monitored with EPR spectroscopy. The EPR study at 298 K revealed two distinct signals at g_iso = 1.983 and 1.978 that grew over the course of the reaction (Fig. 8A). The significant deviation of the g_iso value from 2.0023 indicates titanium-centered paramagnetic species. Consistent with this assignment, UV-Vis monitoring also showed d–d transitions at 615 nm (ε = 39 M⁻¹ cm⁻¹) and 779 nm (ε = 35 M⁻¹ cm⁻¹) (Fig. 8B).

Fig. 8 (A) EPR spectrum of the reaction mixture of 2 with NaO^tBu (10 equiv.) in THF with simulated contributions from 6 and 7. (B) UV-Vis spectral changes during the reaction of 2 with 10 equiv. of NaO^tBu over 1 h (top), and the corresponding extinction coefficients of the reaction mixture compared with those of 6 and 7. (C) Generation of 6 and 7 via comproportionation of 2 with the Ti^II-acrylate species A.

We assumed that two paramagnetic species arise from a redox process involving the Ti^IV-lactone (2) and the Ti^II-acrylate species A generated upon deprotonation of 2 (Fig. 8C). To test this hypothesis, we independently synthesized the Ti^III-lactone [Na(THF)₂][Cp*₂Ti(C₂H₄CO₂)] (6) and the Ti^III-acrylate Cp*₂Ti(O₂CCH=CH₂) (7) (Fig. 9A and B). Compound 6 was prepared by reducing 2 with sodium naphthalenide, while 7 was prepared by salt metathesis of Cp*₂Ti(OTf)⁶² with tetrabutylammonium acrylate. The EPR spectra of 6 and 7 show signals at g = 1.983 and 1.977, respectively, matching those observed during the deprotonation of 2 with NaO^tBu (Fig. 8A). The UV-Vis spectra of 6 and 7 also reproduce the d–d bands observed in the reaction mixture (Fig. 8B). Furthermore, the reaction mixture spectrum is well described by the sum of the authentic spectra of 6 and 7 in a 1 : 1 ratio, indicating quantitative (>98%) formation of these two species relative to the initial titanium. This suggests that A undergoes rapid comproportionation with 2 immediately upon its formation (2 + A → 6 + 7), preventing its accumulation. Low-temperature UV-Vis monitoring of the reaction (−40 to 0 °C) showed only the formation of 6 and 7, with no other detectable intermediates (Fig. S31).

Fig. 9 (A) Solid-state structure of 6, (B) solid-state structure of 7, (C) cyclic voltammograms of 2 (red) and 7 (black).

The driving force of the proposed comproportionation reaction was evaluated by electrochemical analysis. The cyclic voltammogram shows a reversible Ti^III/IV couple for 2 at an E_1/2 of −2.15 V and an irreversible Ti^II/III couple for 7 at an E_pc of −3.23 V vs. Fc/Fc⁺ (Fig. 9C), corresponding to favorable thermodynamics (ΔG = −24.9 kcal mol⁻¹) for comproportionation between 2 and A to form 6 and 7. In addition, a potential alternative pathway involving single-electron transfer (SET) from NaO^tBu to 2 is implausible due to the far more negative Ti^III/IV couple (−2.15 V) compared to the ^tBuO˙/^tBuO⁻ couple (−0.4 V vs. Fc/Fc⁺).⁶³ Furthermore, acrylate formation does not proceed when the Ti^III species 6 is treated with NaO^tBu.

Complexes 6 and 7 were detected not only under stoichiometric deprotonation conditions but also in reaction mixtures obtained after catalytic acrylate formation. This observation indicates that comproportionation is not merely a pathway under stoichiometric conditions, but also an operative process under the catalytic conditions. While the accumulation of 6 and 7 suggests that these species constitute the dominant titanium reservoirs after turnover, they are best regarded as off-cycle species formed through a thermodynamically favoured redox equilibrium rather than as productive resting states of the catalytic cycle.

Accordingly, we investigated the effect of external oxidants on reoxidization of Ti^III-lactone (6) to Ti^IV (2) (Table 4). In the absence of an oxidant, the yield was limited to 39%. The use of [Cp₂Co][PF₆] (E_1/2 = −1.33 V)⁶⁴ resulted in a significant increase in acrylate yield under stoichiometric deprotonation conditions (Table 4, entries 2–5). Increasing the amount of [Cp₂Co][PF₆] up to 0.5 equiv. led to a gradual enhancement of acrylate formation, reaching 72%. Further addition beyond 0.5 equiv. did not improve the outcome, consistent with the generation of 0.5 equiv. of 6 via comproportionation. In contrast, stronger oxidants, such as [C₇H₇][BF₄] and AgOTf, did not improve acrylate formation. These oxidants likely engage in competing chemical or redox interactions with NaO^tBu or other Ti species rather than reoxidizing 6. Thus, careful selection of a suitable oxidant capable of selectively oxidizing Ti^III without interfering with other components of the reaction mixture is essential for productive acrylate formation.

Table 4 Additive effect for deprotonation of 2 to produce acrylate^a

Entry	Additive	Equivalent^a	E1/2 (vs. Fc/Fc^+)^52	Yield^b (%)
a Amount of oxidant relative to compound 2.b Yield determined by ^1H NMR analysis using 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt as an internal standard.
1	—	—	—	39
2	[Cp2Co][PF6]	0.1	−1.33	53
3	[Cp2Co][PF6]	0.25	−1.33	64
4	[Cp2Co][PF6]	0.5	−1.33	72
5	[Cp2Co][PF6]	1	−1.33	72
6	[C7H7][BF4]	1	−0.65	15
7	AgOTf	1	+0.18	25

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Although this oxidant effect is clear under stoichiometric conditions, catalytic attempts with [Cp₂Co][PF₆] afforded a TON of 2.0, showing no improvement over base-only conditions. Control experiments indicate that [Cp₂Co][PF₆] reacts readily with both NaO^tBu and 1 (Fig. S30), suggesting that the oxidant would be intercepted by the alkoxide base and Ti(II) species rather than productively reoxidizing 6. Despite these limitations, the oxidation experiment demonstrates that suppressing the accumulation of Ti(III) species enables acrylate formation in high yield.

The observations described above can be rationalized by the mechanistic picture summarized in Fig. 10. Deprotonation of the titanalactone (2) generates the reduced intermediate A, which can proceed along the productive pathway to release acrylate and regenerate 1 or be diverted into a thermodynamically favored off-cycle sink via comproportionation to form 6 and 7 (Fig. 10). This thermodynamic stabilization accounts for the accumulation of Ti^III species and the resulting suppression of turnover.

Fig. 10 Mechanistic overview highlighting catalytic and off-cycle pathways.

Conclusions

This study establishes a mechanistic understanding of CO₂/ethylene coupling mediated by titanium complexes. Kinetic and thermodynamic analyses reveal that titanalactone formation is both rapid and exergonic, demonstrating inherent advantages of titanium over late transition metals. The strong Ti–O interaction stabilizes the titanalactone intermediate but inhibits subsequent β-hydride elimination. Instead, deprotonation with alkoxide bases provides a viable alternative pathway. A proof-of-concept catalytic experiment under mild conditions supports this mechanistic picture.

Notably, we identified comproportionation between Ti^II and Ti^IV intermediates as a key off-cycle pathway that limits turnover, which has not been recognized in previous carboxylation systems. Rather than elementary steps, redox compatibility across the catalytic intermediates therefore emerges as a key challenge in titanium-based carboxylation catalysis. Although reoxidation restores high-yield acrylate formation under stoichiometric conditions, achieving efficient catalytic turnover will require an oxidant or redox strategy compatible with the full set of catalytic intermediates. These findings define both the opportunities and the current mechanistic constraints of titanium-mediated CO₂/ethylene coupling and provide a basis for the future development of early transition metal systems for carboxylation chemistry.

Author contributions

C. Y. designed and supervised the project. A. K., N. S. and M. L. designed, conducted, and analysed the results of experiments. Y. K. H assisted with the high-pressure reactions and contributed to early discussions of the project. J. S. performed and analysed the EPR and computational studies and contributed to discussion of the project. C. Y., J. S., and A. K. wrote the manuscript with input from all coauthors.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information: characterization data (NMR, EPR, UV–Vis, IR, CV and crystallography), kinetic data, and computational details. See DOI: https://doi.org/10.1039/d6qi00548a.

CCDC 2463344–2463347 (for 2, 5, 6 and 7) contain the supplementary crystallographic data for this paper.^65a–d

Acknowledgements

This research was supported by the National Research Foundation of Korea (RS-2025-00514821, RS-2022-NR068725, RS-2025-02215028) and the Settlement Research Fund of UNIST (Ulsan National Institute of Science & Technology). We thank the UNIST Office of Research Facilities and Training (ResFacT) for support in using the equipment. The EPR measurements were supported by the Institute for Basic Science (IBS-R010-D1) in Korea. Experiments at PLS-II were supported in part by MSIT and POSTECH.

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