# Tetrameric self-assembling of water-lean solvents enables carbamate anhydride-based CO 2 capture chemistry

Julien Leclaire, David Heldebrant, Katarzyna Grubel, Jean Septavaux, Marc Hennebelle, Eric Walter, Ying Chen, Jose Leobardo Bañuelos, Difan Zhang, Manh-Thuong Nguyen, et al.

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## To cite this version:

Julien Leclaire, David Heldebrant, Katarzyna Grubel, Jean Septavaux, Marc Hennebelle, et al. Tetrameric self-assembling of water-lean solvents enables carbamate anhydride-based CO 2 capture chemistry. Nature Chemistry, 2024, 16 (7), pp.1160-1168. 10.1038/s41557-024-01495-z. hal-04957050

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# Tetrameric self-assembling of water-lean solvents enables carbamate anhydride-based CO₂ capture chemistry

Julien Leclaire,<fnref n="1" />* David J. Heldebrant,<fnref n="2" />,<fnref n="3" />* Katarzyna Grubel,<fnref n="2" /> Jean Septavaux,<fnref n="1" /> Marc Hennenbelle,<fnref n="1" /> Eric Walter,<fnref n="2" /> Ying Chen,<fnref n="2" /> Jose Leobardo Bañuelos,<fnref n="4" /> Difan Zhang,<fnref n="2" /> Manh Thuong Nguyen,<fnref n="2" /> Debmalya Ray,<fnref n="2" /> Sarah I. Allec,<fnref n="2" /> Deepika Malhotra,<fnref n="2" /> Wontae Joo,<fnref n="2" /> Jaelynne King.<fnref n="2" />

* = corresponding author, Julien Leclaire. E-mail: julien.leclaire@univ-lyon1.fr
and David J. Heldebrant. E-mail: david.heldebrant@pnnl.gov

## Abstract

Carbon capture, utilization, and storage is a key yet cost-intensive technology for the fight against climate change. Single component water-lean solvents have emerged as promising materials for post-combustion CO₂ capture, but little is known regarding their mechanism of action. Here we present a combined experimental and modeling study of single-component water-lean solvents, and we find that CO₂ capture is accompanied by the self-assembly of reverse micelle-like tetrameric clusters in solution. This spontaneous aggregation leads to stepwise cooperative capture phenomena with highly contrasting mechanistic and thermodynamic features. The emergence of well-defined supramolecular architectures displaying a H-bonded internal core, reminiscent of enzymatic active sites, enables the formation of CO₂-containing molecular species such as carbamic acid, carbamic-anhydride, and alkoxy carbamic anhydrides. This system extends the scope of adducts and mechanisms observed during carbon capture. It opens the way to materials with a higher CO₂ storage capacity and provides a means for carbamates to potentially act as initiators for future oligomerization or polymerization of CO₂.

## 1 Introduction

In its latest report, the International Energy Agency confirmed that CO₂ capture has a critical role in greenhouse gas mitigation and the clean energy transition.<fnref n="1" /> Solvent-based technologies are the most mature option for point source CO₂ capture, with many commercial offerings available.<fnref n="2" /> Yet, their substantial cost hampers deployment and thermal regeneration of solvent would consume energy at the gigatonne scale. This high energy penalty is inherent to the CO₂ absorption thermodynamics of aqueous amines A. Despite decades of research and calls for change,<fnref n="3" />,<fnref n="4" /> scrubbing is unlikely to advance with current half-loaded ammonium carbamate A(0)⁺A(1)<fnref n="1" />⁻ and ammonium bicarbonate A(0)⁺W(1)⁻ adduct pairs (see Figure 1A & 1E for notation).

Confining amines within nanoscopic sites in a solid material (either pre-synthesized⁵,⁶ or assembled during capture⁷) has been the only strategy achieving cooperative and full loading⁸⁻¹⁰ (into carbamic acid A(1), Fig. 1B). Although carbamic acid was postulated to exist in moderately polar aprotic liquid media,¹¹⁻¹⁴ unambiguous evidence of its formation in solution are scarce.¹⁰,¹⁵,¹⁶ The recent observation of pyrocarbonates M<fnref n="2" />⁺W(2)<fnref n="2" />⁻ (M = Pb, Sr) proved that the theoretical one CO₂ per binding site limit can be overcome at extreme temperatures and pressures.¹⁷

<footnote n="1">¹ Université Claude Bernard Lyon 1; Villeurbanne, 69100, France</footnote>

<footnote n="2">² Pacific Northwest National Laboratory; Richland, WA 99352, United States of America</footnote>

<footnote n="3">³ Washington State University; Pullman, WA 99163, United States of America</footnote>

<footnote n="4">⁴ University of Texas El Paso; El Paso, TX 79968, United States of America</footnote>
Biological systems cooperatively manage oxygen uptake using hemoglobin,¹⁸ though no equivalent molecule exists to absorb CO₂. Instead, CO₂ is transported as water-bound (bicarbonate) or a dissolved gas and converted into original adducts such as N-carboxybiotin **B**(1)⁻¹⁹ and carboxyphosphate **C**(1)⁻²⁰ or carbamoylphosphate **C**(1)N.²¹ These activated species, key intermediates for the biosynthesis of the building blocks of life (**D**(1)), are stabilized by a non-covalent hydrogen bonding network within the shielded active sites of enzymes. Through this confinement strategy, nature has extended the portfolio of capture reactions and products far beyond humanity's current achievements.

## 24 Result and Discussion

In this work, we explore the ability of a neat water-lean solvent to self-assemble into clusters with a shielded reactive site that enables CO₂ capture adduct formation under mild conditions (T≤ 313 K, PCO₂ < 15 bars, Fig. 1B). Single component water-lean solvents, such as N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (EEMPA, **E**), have emerged as promising for post-combustion CO₂ capture. EEMPA has the current highest solvent energy efficiency²² and lowest capture costs²³,²⁴ compared to aqueous amines²⁵ or two-component formulations in the peer reviewed literature.²⁶⁻³² EEMPA and its diamine analogues were initially designed to form a stable intramolecularly H-bonded carbamic acid and adopt a folded hairpin structure.³³ Until now, no experimental data could support the formation of a carbamic acid under processing conditions (pCO₂* < 0.15 bar). Here we posit an alternative hypothesis, where the properties of EEMPA are directly related to its *intermolecular* self-assembly rather than *intramolecular* folding (Fig. 1B) after chemically fixing CO₂. This is supported by the recent observation that water-lean alkoxyguanidines³⁴⁻³⁷ display a heterogeneous structure of aggregated ions upon loading (Fig. 1F). This spontaneous nano-structure formation may provide confined (re)active sites, likely explaining how **E** successfully performs integrated capture and conversion of CO₂ into fuels and chemicals including methanol³⁸ and methane.³⁹

## 41 NMR-based identification of molecular and supramolecular speciation

To elucidate the molecular features of CO₂-capture by **E** and identify the supramolecular interactions during self-aggregation, quantitative ¹H and ¹³C NMR spectra were recorded on a series of samples of neat **E** mixed with increasing molar ratios, x₀, of CO₂. Two sets of signals in slow exchange at the NMR timescale were observed. The signals correspond to free ammonium/amine and carbamic acid/carbamate pairs (respectively noted **E**(0)⁽⁺⁾ and **E**(1)⁽⁻⁾, see Supplementary Table 1 and 2 for nomenclature), the former converting to the latter throughout CO₂ absorption (Fig. 2A and Supplementary Figs. 12-17). Accurately quantifying both species produced the loading, α, i.e. the molar fraction of CO₂ effectively bound by **E**. Complemented at low x₀ values by vapor-liquid equilibrium (VLE, Supplementary Fig. 2) measurements,²² this data (Supplementary Fig. 5 and table 6) allowed us to plot the CO₂ binding isotherm with its sigmoidal profile typical of cooperative systems (Fig. 2B and Supplementary Figs. 10 and 11) ⁷,⁸ Initial partial pressures of up to 70 bars CO₂ (5 eq. of CO₂ vs **E**) were required to reach a final α of almost 1, well above the 0.5 CO₂ per EEMPA in unpressurized flue gas.¹⁴ Under the studied experimental conditions, slow CO₂ absorption by **E**, whose viscosity increases slightly, eventually leads to equilibrium pressure values no higher than 15 bars (Supplementary Figs. 6 and 47) and water content no higher than 700 ppm (a H₂O/EEMPA molar ratio < 1/120). During loading, NMR peaks of both **E**(0)⁽⁺⁾ and **E**(1)⁽⁻⁾ underwent noticeable chemical shift perturbations (CSPs, see Supplementary Figs. 22-25) due to fast protonation and self-aggregation phenomena. CSP mapping⁴⁰ (Fig. 2A) and monitoring⁴¹ (Fig. 2C) confirmed that protonation occurred first on the most basic site (the secondary nitrogen atom of **E**(0), see Supplementary Figs. 1 and 4), as expected, then unexpectedly on the least basic site (the carbamic oxygen of **E**(1), Fig. 2C, insert). Together, these observations can be translated into a two-stage molecular scenario (Fig. 2D). Below partial pressures in CO₂ of 1 bar, neat **E**(0) yields an
equimolecular mixture of charged ammonium $E(0)^+$ and carbamate $E(1)^-$, which convert into pure carbamic acid $E(1)$ upon gentle pressurization ($p^*_{f}(CO_2) < 20$ bars, Supplementary Fig. 7). A convergent set of evidence (strong shielding of the carbonyl carbon$^{42}$ and strong deshielding of the hydroxyl proton$^{11}$ peaks in $^{13}$C and $^{1}$H NMR, respectively (Supplementary Figs. 15 and 17) supports the presence of this elusive adduct. The data indicate that carbamic acid formation begins at unexpectedly low loading values (around 0.3, Fig. 2C, insert). Experimental data, which encompass CSPs, $\Delta\delta(i)$, chemical loading values, $\alpha$, and molar ratios, $x_0$, were fitted with a Matlab in-house script (Supplementary Fig. 3). Data processing provided the equilibrium binding constants of the two-stage covalent process and values of the chemical shifts $\delta(i)$ of the individual $E(0)$, $E(0)^+$, $E(1)^-$, and $E(1)$ adducts (Supplementary Figs. 26-27). Interestingly, a conventional dimeric model (Fig. 2D) could not be reliably fitted to the experimental data (in particular the $^{13}$C CSP of the carbonyl group of $E(1)^-$, Fig. 2C insert) in the absence or presence of an additional $E(1)-E(1)$ dimerization equilibrium. Of all scenarios involving higher aggregates, the tetrameric model provided the best match with the full set of experimental data (Figs. 2B and C, Supplementary Figs. 10-11, 22-25 and table 7).

## Thermodynamic analysis of self-assembly and CO₂ absorption

The full picture of the covalent adduct and non-covalent cluster populations, or chemical speciation, could be simulated after parameter adjustment through fitting the NMR data. Figs. 2F and 2E show the speciation and Gibbs' free energies of CO₂ absorption for each successive tetramer $E^4(\alpha)$ respectively. This information, derived from a thermodynamic model, is fully consistent with the experimental observations. Simulations (Fig. 2E, yellow curve), in agreement with NMR data (Fig. 2C), show that carbamic acid is a key component of the cluster $E^4(0.75)$ that emerges as loading reaches 0.3. Although EEMPA displays clustering similarly to hemoglobin, its stepwise CO₂ capture is negatively cooperative (Fig. 2E), with a drop in Gibbs' free energy of ~7 kJ.mol⁻¹ between the first, second, and third binding steps. The free energy reduction explains why pressurization is needed, despite the thermodynamic stabilization provided by tetrameric self-assembly. Energies and structures of each covalent species, $E(0)$, $E(0)^+$, $E(1)^-$ and $E(1)$, as well as of their dimeric and tetrameric clusters were assessed by density functional theory (DFT) calculations with an implicit model of the EEMPA-CO₂ medium.$^{43}$ This approach provides a good estimation of energies, which can be further refined by explicitly taking into account the molecules surrounding the computed structures. This preliminary campaign was complemented with classical molecular dynamics (cMD) simulations. Thermodynamic data of the non-covalent (dimerization and tetramerization) and covalent processes (CO₂ capture) were obtained from the DFT-computed energies and compared to experimental measurements to assess the modelling results. Although sizeable differences between computed and experimental enthalpies and entropies are expected, leading to large deviations between Gibbs's free energies, good agreement was found for covalent capture of CO₂ by the tetramers (Fig. 2E, and Supplementary Fig. 44).

Computed enthalpies of CO₂ absorption by tetramers at low loading ($\alpha = 0.07-0.22$) perfectly match values derived from prior VLE measurements on EEMPA (-79 vs -75 kJ.mol⁻¹),$^{22}$ further supporting our tetrameric model (Supplementary Figs. 32 and 41). The computed entropies of CO₂ capture by the tetramers (Supplementary Figs. 33 and 42) match the order of magnitude of values of aqueous amines.$^{44,45}$ With the exception of the first capture step (200 J.K⁻¹.mol⁻¹), these entropies are relatively constant along the loading process (~150 J.mol⁻¹.K⁻¹)). This agrees with the first capture reaction of the gaseous reactant being accompanied by the tetramerization of $E$, while subsequent absorption steps only involve the loss of translational and rotational freedom of the gaseous reactant. We experienced the limits of the DFT method while assessing the entropy of all non-covalent pairing processes ($2E -> E^2$ and $2E^2->E^4$, Supplementary Figs. 33, 36 and 39). Though the calculations confirmed that higher aggregates are systematically enthalpically favored (Supplementary Figs. 32, 35 and 38) regardless of loading, enthalpy values were compensated for by overestimating computed
entropies, leading to low positive Gibb's free energies of tetramerization.⁴⁶ This predicted
113
endergonicity for dimerization and tetramerization (Supplementary Figs. 34, 37 and 40) is imputed
114
to the limitation inherent to the solvent model. In fact, cMD simulations (1 µS trajectory) of the
115
**E/CO₂** system at 298K and 0.25 loading showed the co-existence of the **E**⁴**(0.25)** and **E**⁴**(0.50)
116
tetramers (defined by dominant hydrogen bonds, Supplementary Fig. 46) in proportions (18% and
117
22% of the whole system respectively), that qualitatively agree with the NMR data-derived Matlab
118
model. Complementary evidence supporting the existence of CO₂-rich tetramers such as **E**⁴**(0.75)
119
and **E**⁴**(1)** were provided by FT-IR and WAXS spectroscopies (*vide supra*). Static DFT calculations
120
confirmed that non-covalent clustering acts as a genuine thermodynamic driving force, stabilizing
121
covalent adducts enthalpically by around 15 kJ.mol⁻¹ compared to the isolated species
122
(Supplementary Figs. 32 and 35). Calculations revealed that combining unloaded **E**(0), partially
123
loaded **E**(0)⁺**E**(1)⁻, and fully loaded **E**(1) into clusters opens the door to a broad range of absorption
124
enthalpies during CO₂ capture, (with values decreasing between from -80 to -20 kJ.mol⁻¹ along the
125
**E**⁴**(0) – **E**⁴**(0.75)** series, Fig. 2E), far beyond those observed on solid absorbents⁹. Consequently,
126
clustering empowered by water-lean solvents may allow chemists to choose the thermodynamic
127
features of the capture and release cycle on demand by setting the loading range, selecting the active
128
tetrameric species.
129

130 **Structural analysis of the E⁴ clusters**

Given the level of agreement with experimental data, DFT modelling could be exploited to gain
131
insights about the non-covalent interactions (Figs. 3C, 3D and 4C) that govern self-assembly and the
132
structural features of the tetrameric clusters (Figs. 3A, 4A and Supplementary Fig. 45). The self-
133
assembly of subunits **E**(0), **E**(0)⁺, **E**(1)⁻ and **E**(1) is driven by a network of hydrogen bonds between
134
the amine, ammonium, carbamate, and carbamic acid polar head groups that form the tetrad at the
135
core of the re(active) site. Low-loading tetramers **E**⁴**(0)** and **E**⁴**(0.25) result from packing unfolded
136
**E**(0) (**E**(0)⁺ and **E**(1)⁻) into cylindrical bundle-like tertiary structures, where the basic moieties of the
137
(re)active site are buried and poorly accessible. In **E**⁴**(0.5), the **E**(0)⁺ and **E**(1)⁻ subunit chains
138
individually fold into turns, with the polyether moieties gathered in one hemisphere (Fig. 3A). As a
139
result, the tertiary structure of **E**⁴**(0.5) is a half ovoid, exposing the square planar H-bonded (re)active
140
site (Figs. 3B and C). Incorporation of an additional CO₂ molecule leads to dramatic structural
141
changes, both locally and globally (Figs. 3A and D). While carbamic acid formation is accompanied
142
by H-bonding reorganization, the polyether chains remain folded into turns as in **E**⁴**(0.5). This
143
induces a conformational change of the polar group tetrad from square planar to tetrahedral (Fig.
144
3B), affecting the orientation of the side chains. Consequently, **E**⁴**(0.75) adopts a star-shaped tertiary
145
structure with spaced side chains roughly pointing toward the vertices of a cube, shielding the active
146
site from the solution. In **E**⁴**(1), gathered pairs of ether chains (experimentally confirmed by NMR,
147
see Supplementary Fig. 17) yield a flattened figure eight global structure (Figs. 3A & 3D).
148

FT-IR experimentally confirmed the presence of EEMPA clusters at high loading. While the
149
conversion of ammonium carbamates to carbamic acid dimers is generally accompanied by a marked
150
increase in the stretching frequency of the carbonyl signal,⁴⁷,⁴⁸ the observed decrease (Supplementary
151
Fig. 28) agrees with reports about higher carboxylic aggregates (Supplementary Fig. 29).¹³ Wide
152
angle X-ray scattering (WAXS) analysis provided a second set of experimental evidence for the
153
tetrameric clusters and information about their morphological features in pressurized and
154
unpressurized conditions (Fig. 3E). The Q region exhibits a CO₂ loading-dependent structure factor
155
fit using the Teubner-Strey model.⁴⁹ ⁵⁰ The model qualitatively describes the segregation between the
156
polar reactive moieties and ethoxyethyl and morpholinopropane arms encountered in the tetramers
157
particularly well. The presence of this polar core provides the required electron density contrast and
158
enables the observation of molecular scale phase segregation at CO₂ loading above 0.6. The lack of
159
phase segregation at lower CO₂ loading may be ascribed to the solution composition, which includes
160
several tetramers (**E**⁴**(0.25) to **E**⁴**(0.75)) with differing morphologies poorly adapted to regular and
161
dense packing. WAXS was used to probe the dimensions of these micelle-like clusters and their
polar core. The measured cluster dimensions, including tetrad correlation length (ξ) of 5.8 Å and
periodicity between adjacent tetrads (d) of ~11.8 Å, quantitatively match with the DFT results
(Supplementary Figs. 30-31 and 45). The WAXS analysis also shows a cubic bicontinuous phase
based on the head-to-hip packing of CO₂-saturated tetramers (Figs. 3C and 3D). The WAXS data
provides additional experimental evidence for the formation of tetramers, agreeing with the NMR-
based speciation and DFT modeling.

169 Anhydride-based Mechanism of CO₂ capture

170 The molecular mechanism leading to the formation of carbamic acid-containing-clusters was
171 explored by a coupled experimental (NMR of neat pressurized samples) and theoretical approach
172 (DFT calculations). In the classical zwitterion model, the first CO₂ addition proceeds via a carbamic
173 acid intermediate, which converts into ammonium carbamate upon deprotonation by a second
174 amine.⁵² In this framework, carbamic acid can only be produced from a neutral amine precursor. In
175 our system, carbamic acid-containing species E⁴(0.75) and E⁴(1) arise from E⁴(0.5) via a stepwise
176 CO₂ absorption. We expected that forming carbamic acid from the E⁴(0.5) ammonium carbamate
177 tetrad would first require an energetically uphill proton transfer from the nitrogen atom of one
178 ammonium group to a neighboring carbamate oxygen (Fig. 4A, eq (1)). The free amine center of the
179 unstable intermediate E⁴(0.5)* would then bind to a third CO₂ molecule (Fig. 4A, eq (2)), generating
180 the zwitterion E⁴(0.75)⁺ that would relax into E⁴(0.75) (Fig. 4A, eq (3)).

181 We exploited the viscosity increase that accompanies CO₂ uptake by E⁴(0.5) to slow down the decay
182 of the elusive intermediates involved in the formation of E⁴(0.75) and E⁴(1). In practice, an EEMPA
183 sample was overpressurized with CO₂ (α = 0.8-1 range) until equilibrium was attained and then
184 transferred into an NMR tube with a headspace under 1 bar of CO₂. q¹³C spectra were immediately
185 recorded over time once the transfer was complete. The monitored phenomenon monitored is
186 governed by slow mass transfer within the viscous medium. It corresponds to the stripping of
187 E⁴(0.75) back into E⁴(0.5) and gaseous CO₂ through the intermediate(s) species. Examining the stack
188 of spectra recorded over time in reverse order (from end to beginning of the experiment, Fig. 4B,
189 bottom to top) provides a sequence of snapshots of the intermediate states encountered during the
190 E⁴(0.5) + CO₂ -> E⁴(0.75) absorption step (eq (4), see Figs. 4C and E). While the NMR signals of
191 the aliphatic backbone for both E(0)⁽⁺⁾ and E(1)⁽⁻⁾ match those recorded at the same loadings at
192 equilibrium (Supplementary Figs.47-48 vs Supplementary Figs. 12-15), the carbamate signal
193 displays a shouldered peak at high loadings (Fig. 4B and Supplementary Fig. 49). Relative
194 integration of the signals corresponding to CO₂-bearing vs CO₂-free species in this loading range
195 indicated that the CO₂-bearing species is bound to more than one CO₂-molecule on average.

196 To identify this carbamate-like intermediate species, we attempted *in situ* trapping via alkylation
197 with trimethylsilyldiazomethane (TMSCHN₂), previously employed for carbamate to urethane
198 conversions.⁵³ To our surprise, traces of the methyl carbamic anhydride of EEMPA, noted E(2)Me,
199 could be directly detected by ¹³C NMR in the crude mixture (with 0.5 eq of TMSCHN₂,
200 Supplementary Figs. 52 and 53). To confirm this observation and isolate the intermediate, CO₂-free
201 EEMPA was pre-mixed with 1eq. of TMSCHN₂ and pressurized with CO₂. Remarkably, E(2)Me
202 formed in up to 37 % conversion from the crude mixture. An analytically pure (99.5 %) sample was
203 recovered in a 24% isolated yield after column chromatography and characterized by mass
204 spectrometry (Supplementary Figs. 63-65) and NMR spectroscopy (Fig. 4C and Supplementary Figs.
205 ex situ from E with ditertbutyldicarbonate.⁵⁴⁻⁵⁷ Thus, we could compare the ¹³C NMR pattern of the
206 intermediate trapped *in situ* with the spectra of these derivatives and unambiguously confirm the
207 identity of E(2)Me. The most characteristic feature is the set of two doublets around 150 ppm in ¹³C
208 NMR observed for both E(2)Me and E(2)⁺Bu (Supplementary Figs. 55, 58, 61 and 66). This pattern
209
is indicative of a non-symmetrical bis(carbonyl) system split given the bulkiness of the capping end group (tBu vs Me), explaining the slight difference in splitting patterns observed between the adducts (Fig. 4D).

DFT calculations confirmed that the anhydride intermediate is strongly stabilized within the reactive site of the tetrameric reverse micelle-like clusters and favored over the conventional zwitterion intermediate (Fig. 4A, C, and E, see Supplementary Figs. 50-51 for more detail). The enthalpic cost of proton transfer from the ammonium carbamate to the amine-carbamic acid is rather high (Figs. 4A and 4C, eq. (1); more than 45 kJ.mol⁻¹), whereas the anhydride pathway follows a downhill energetic trajectory (Figs. 4C and 4E, eq. (4); -14 kJ.mol⁻¹) from E⁴(0.5). The unique stereoelectronic features of the tetrameric cluster obviously favor this alternative reaction pathway, as carbamic anhydride formation from a traditional ammonium carbamate dimer such a E²(0.5) is thermodynamically strongly disfavored (by +56 kJ.mol⁻¹). Electrostatic potential surface (ESP, Fig. 4E) mapping of E⁴(0.5)'s internal cavity additionally reveals an electron-poor cleft reminiscent of hydrolases's oxyanion hole.⁵⁸,⁵⁹ In these confined sites, poorly reactive moieties (alcohols and amides) are activated by a network of hydrogen bonds, which also stabilizes electron-rich intermediates formed. The same phenomenon is believed to be at work here, to both activate the nucleophilicity of the carbamate and the electrophilicity of CO₂ and stabilize the carbamic anhydride. Experimentally, this intermediate species instantaneously forms at room temperature. The rate limiting step is its conversion into the carbamic acid-containing tetrad E⁴(0.75). H-bond pairing between the members of the E(1)E(1)E(1)E(0)⁺ tetrad of E⁴(0.75) seems particularly efficient, as the resulting ESP is much less electrodeficient than in E⁴(0.5). This complementarity is perturbed in the carbamic anhydride-containing intermediate tetrad E⁴(1)*. As a result, the fourth CO₂ uptake is both kinetically and thermodynamically less favorable than the third, (-14 vs -9 kJ.mol⁻¹). Remarkably, TMSCHN₂ can selectively methylate and abstract the anhydride from this highly complex system as well as displace the cascade of reversible CO₂ absorption events (Figs. 4C and 4E). This assertion is supported by the fact that premixing TMSCHN₂ and unloaded EEMPA leads to the formation of significant amounts of E(2)Me upon exposure to CO₂ in standard temperature and pressure conditions. With this rather simple reactive system, two CO₂ molecules can be linearly bound to a single nitrogen center for the first time, doubling the amount of greenhouse gas that may be captured or sequestered into a carbon-based material. In agreement with recent reports,⁶⁰,⁶¹ the two urethane analogues E(2)Me and E(2)ₜBu display moderate stability (conversion into E(1)R within days at room temperature and pressure). Continued studies of structure-reactivity relationships and stability are underway, where we anticipate making the next generation of CO₂-rich urethanes with increased robustness. Ultimately, the reactive system identified here may have unlocked the principles needed to oligomerize and store several carbon dioxide molecules on the same backbone. If the recently discovered pyrocarbonate is the strict carbon-analogue of pyrophosphate, anhydride E(2)⁻ can be viewed as an analogue of ADP, one of the universal molecules used to transport and store energy by nature. Capture within clustered water lean solvent may provide a conceptual framework to build a sustainable carbon value chain inspired by cellular metabolism, an intriguing approach to effectively mitigating CO₂ emissions.

## Conclusion

The quest for an ideal CO₂ capture solvent has spanned almost a century, beginning with patenting the amine scrubbing process and intensifying with the threat of global warming.⁶² Single component water-lean solvents have recently emerged as promising, displaying the highest energy efficiencies and lowest operational costs. However, it has been challenging to determine what molecular and mechanistic features gave rise to the advantageous properties and performance of EEMPA and its
257 analogues. It appears that combining a central basic 2° amine site with two flexible, mildly polar side
258 chains enables EEMPA to behave as a proto-surfactant, forming micelle-like ionic clusters after CO2
259 binding. These stable supramolecular aggregates not only explain EEMPA's unusual physical
260 properties (low viscosity and high conductivity) but also enable different covalent capture chemistry
261 to take place within shielded and well-structured nano-environments. These are highly reminiscent of
262 enzymatic active sites, with structural and physical features that stabilize intermediates and catalyze
263 numerous reactions in exceptionally mild conditions. A

264 ‘double-tailed surfactant’-like secondary amine backbone seems to be a general pre-requisite for self-
265 assembly into reverse micelle clusters among water-lean solvents and opens the door to exciting
266 reactivity, such as carbamic anhydride formation. We have gathered preliminary evidence of self-
267 assembly and anhydride formation for a series of secondary-tertiary diamine analogues of EEMPA.
268 The reactivity of EEMPA and CO2 can be conceptually depicted using a dynamic combinatorial
269 framework.⁶³ Simple constituents such as E, CO2, and protons reversibly and covalently connect to
270 yield a collection of capture products (E(0), E(0)⁺, E(1), E(1)⁻), themselves the components of higher
271 tetrameric aggregates. These original proto-protein architectures may enable a shift beyond the
272 canonical mono and bimolecular capture adducts, in terms of both thermodynamics and kinetics.
273 Tetramerization opens the door for stepwise CO2 capture, with each step displaying highly
274 contrasting binding constants and absorption enthalpies beyond the values obtained with aqueous and
275 MOF-appended absorbents. While EEMPA displays negative cooperativity, we believe it is just an
276 example in a series toward positively cooperative liquid absorbents. Kinetically speaking, the
277 shielded and H-bonded reactive core enables the formation of intriguing intermediates such as the
278 CO2-enriched anhydride E(2)⁻. The way these pseudo-active sites modulate reactivity is illustrated by
279 the alkylation process, which selectively proceeds on the unusual anhydride with respect to the
280 carbamate. The spontaneous clustering of water-lean solvents dramatically expands the scope of
281 thermodynamic and kinetic capture features but may serve as an original medium for the integrated
282 transformation of unexpected capture intermediates such as E(2)⁻ into different end-products.

283 CO2 anhydrides represent an emerging class of CO2-storage products, with higher mass content and
284 similarities to the phosphate-rich biomolecules that control cellular energy storage. A next step is to
285 extend the proof of feasibility to higher CO2 content and obtain an adenosine triphosphate analogue.
286 This would pave the way toward a CO2-only oligomerization process, with solvent acting as an
287 initiator. Ultimately, these structures provide a wealth of valuable information on CO2 reactivity in
288 an unconventional, yet simple, medium. This may be the base for the next generation of impactful
289 carbon capture utilization and storage technologies.
290

291 Acknowledgements

292 FR authors were supported by the LABEX iMUST of the University of Lyon (ANR-10-LABX-
293 0064), created within the « Plan France 2030 » set up by the French government and managed by the
294 French National Research Agency (ANR) and by the Region Auvergne-Rhone Alpes (Pack
295 Ambition Recherche 2019). US authors acknowledge the U.S. Department of Energy, Office of
296 Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division,
297 Understanding and Control of Reactive Separations, FWP 75428. Data in this publication was
298 obtained using Pacific Northwest National Laboratory Catalysis Science NMR Facility. This
299 research used resources of the National Energy Research Scientific Computing Center (NERSC), a
300 U.S. Department of Energy Office of Science User Facility located at Lawrence Berkeley National
301 Laboratory, operated under Contract No. DE-AC02-05CH11231. PNNL is operated by Battelle for
302 the US Department of Energy under Contract DE-AC05-76RL01830. The authors thank the CRMN
303 (UCBL) for assistance with NMR analyses, Beth Mundy for help technical editing, and Phillip
304 Koech for fruitful discussions.
305

305
306 Author information

307 Jean Septavaux

308 Present address: Secoya Technologies srl, Fond des Mes 4, 1348 Ottignies-Louvain-la-Neuve,

309 Belgium

310 Ray Debmalya

311 Present address: Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, United

312 States of America

313

314 Author affiliation

315 Université Claude Bernard Lyon 1; Villeurbanne, 69100, France

316 Julien Leclaire, Jean Septavaux, Marc Hennebelle

317

318 Pacific Northwest National Laboratory; Richland, WA 99352, United States of America

319 David. J. Heldebrant, Katarzyna Grubel, Eric Walter, Ying Chen, Difan Zhang, Manh Thuong

320 Ngyuen, Debmalya Ray, Sarah Allec, Deepika Malhotra, Wontae Joo, Jaelynne King

321

322 Washington State University; Pullman, WA99163, United States of America

323 David. J. Heldebrant

324

325 University of Texas El Paso; El Paso, TX 79968, United States of America

326 Jose Leobardo Bañuelos

327

328 Author contributions:

329 JL and DJH jointly conceived the study, secured funding and administered the project. JL, JS, DJH,

330 KG, JLB, DZ, MTN EW designed the experimental and theoretical methodologies. MH, JS, KG, DZ,

331 JLB, EW, MTN, DR, SA, DM, WJ, JK performed experiments and interpreted data. Visualization

332 and construction of figures were performed by JL, DJH, DZ, MTN and JLB. Writing was led by JL,

333 DJH with review and editing by all other authors.

334

335 Corresponding author

336 Correspondence to david.heldebrant@pnnl.gov and Julien.leclaire@univ-lyon1.fr

337 Ethics declarations

338 Competing interests: Authors declare that they have no competing interests.

339

340
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482 Chemistry. Zenodo [10.5281/zenodo.10649309](https://doi.org/10.5281/zenodo.10649309)

483

484
485 **Methods**

486 All reagents were purchased from commercial sources and used as received. Regular liquid state
487 NMR spectra were recorded on either 500 MHz Varian iNOVA, 500 MHz Bruker Avance NEO or
488 600 MHz Bruker NEO equipped with the Cryo Probe using standard pulse sequences. High-pressure
489 liquid state NMR spectra were recorded on 500 MHz Varian iNOVA spectrometer, using in-house
490 manufactured PEEK NMR tubes connected to commercial Parr reactor and Teledyne ISCO pump.
491 ¹³C MAS-NMR spectra were collected on a 600 MHz Bruker Avance III spectrometer using 5 mm
492 zirconia rotors spinning at 3–5 kHz and a home-built custom HX probe and an in-house-developed
493 WHiMS rotor system. Mass Spectrometry analysis was performed using an Q Exactive Plus mass
494 spectrometer (Thermo Scientific, San Jose, CA) outfitted with a Heated ESI source and on a ToF-
495 SIMS 5 (IONTOF, Münster, Germany). High- and low-pressure IR spectra were collected on Nicolet
496 iS10 Thermo Scientific instrument using high pressure demountable transmission liquid Harrick cell,
497 and OMNIC 9 software. Wide-angle x-ray scattering experiments were carried out on a Xenocs
498 Xeuss 2.0 SAXS/WAXS system employing a monochromated Cu Kα (λ<sub>avg</sub>= 1.54189 Å) source and
499 an effective Q range of 0.1- 2.3 Å<sup>-1</sup>. EPR measurements were performed on a Bruker ELEXSYS
500 E580 spectrometer equipped with an SHQE resonator.

501
502 The Supplementary Information contains details of syntheses, spectroscopic analyses and
503 computational studies.

504

505

506 **Data availability statement**

507 The data supporting the findings of this study are available within the paper and its Supplementary
508 Information and data files. Should any raw data files be needed in another format they are available
509 from the corresponding author upon reasonable request

510

511 **Code availability**

512

513 The Matlab script as well as the dataset used for the parametric fitting are available as supplementary
514 data files. They have been uploaded to, and are accessible from
515 https://doi.org/10.5281/zenodo.10649310 and reference 64.

516

Figures
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Conventional vs unconventional CO₂-binding adducts with increasing CO₂/amine A stoichiometry. (A) Conventional adducts from aqueous CO₂ capture: ammonium carbamate A(0)⁺A(1), ammonium carbonate A(0)⁺W(1)⁻ and metal percarbonate M²⁺W(2)²⁻ vs (B) adducts observed water-lean medium such as MOF [reproduced from ref 4 with permission of the Royal Society of Chemistry]⁹ and water-lean solvent (green, this work) including anhydride A(2)⁻ and carbamic acid A(1). (C) Biological (urea-based : B(1)⁻ and phosphate-based C(1)⁻ and C(1)N) capture intermediates and (D) their final transformation products. (E) Structure of EEMPA and associated notation and (F) aggregation domains within CO₂-loaded water-lean solvents. ³⁴</figcaption></figure>

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526
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Tetrameric model based on EEMPA-CO₂ constituents. (A) Chemical shift perturbation mapping from q¹³C NMR analyses on E(0)⁺ and E(1)⁻ (colour code reflects CSP amplitude). (B) and (C): fitting of the α and Δδ(i) values with increasing CO₂/EEMPA initial molar ratio x₀ by the dimeric E² and tetrameric E⁴ models (dot: experimental; line: model; R²α, R²δ and R²CO are RMSD values for α, δ(aliphatic) and δ(CO) respectively). (D) The two stage loading process of E(0) by CO₂ yielding adducts E(0)⁺, E(1)⁻ and E(1). (E) CO₂-binding Gibbs' free energies (filled bars), entropies (empty bars) and enthalpies (sum) obtained from DFT and thermodynamic model (circles) respectively, and (F) the resulting speciation (x(i): molar fraction vs α: CO₂ loading; vertical dotted</figcaption></figure>
536

line: loading selected for AIMD modelling) with schematic representation of the tetrameric clusters

537

(bottom). (G) Notation relating oligomers, monomeric constituents and equilibrium constants.

538

<figure><img src="image_5.png" /></figure>

539

540

<caption>Figure 3. Structural analysis of the E⁴ tetrameric clusters and of their packing in solution. (A)</caption>

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DFT-derived van der Waals surface of dominant tetramers and schematic tertiary structure, (B)
simplified developed representation of the internal nitrogen-based tetrads, of (C-D) their H-bond
network and of the tetramer packing modes (distances in nanometers, A = H-bond acceptor, D = H-
bond Donor). (E) Stack of WAXS spectra recorded at increasing loading and (F) evolution of
periodicity and correlation length with loading. Data are presented as mean values. For details on
error bar calculation see material and method section of the supplementary material.

547
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Mechanism of carbamic acid formation involving anhydride intermediate within E⁴ clusters. (A) Zwitterion-mediated hypothetical carbamate to carbamic acid conversion pathway (path 1). (B) q¹³C NMR carbonyl signal evolution upon CO₂ loading above ambient pressure (bottom to top, * marks the additional CO signal from the anhydride) and (C) DFT derived enthalpies of E² and E⁴ clusters involved in the zwitterion (path 1) and anhydride mediated (path 2) pathways. (D) TMSCHN₂ derivatization of ¹³C-labelled E(2)⁻ into E(2)Me (snapshot of the carbonyl region of the ¹³C NMR spectrum) and (E) proposed mechanistic pathway.</figcaption></figure>

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<figure><img src="image_7.png" /></figure>