Enhanced CO₂/Epoxide Cycloaddition Catalyzed by Pyridine-Substituted Triazole-Quaternary Ammonium Bromide

Abstract

A series of ionic quaternary ammonium bromides featuring triazole moieties, QAS-trzBn ₄ , QAS-trzPic ₄ , and QAS-trzBn ₂ Pic ₂ , were synthesized via Cu-catalyzed azide–alkyne cycloaddition (CuAAC) between propargyl-based ammonium bromide and benzyl- or 2-picolylazide. X-ray crystallographic analyses of QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ revealed strong interactions between Br^– ions and both triazolyl H and methylene H atoms (⁺NCH₂), as evidenced by short Br^–···H contacts ranging from 2.68 to 3.00 Å. The catalytic activities of these compounds as bifunctional, single-component catalysts for the CO₂/epoxide cycloaddition were evaluated under both atmospheric and elevated CO₂ pressures. Notably, catalysts containing pyridyl-triazole groups exhibited superior catalytic performances compared with the benzyl-triazole-based catalyst, QAS-trzBn ₄ . A substrate scope study using QAS-trzPic ₄ under 20 atm of CO₂ at 100 °C revealed that electron-deficient epoxide substrates were more active, yielding good to excellent conversions (88–100%) to cyclic carbonates within 6 h. Computational studies identified key binding modes in pyridine-substituted systems that position both the epoxide and CO₂ in close proximity. In particular, the QAS-trzPic ₄ -CO ₂ -epoxide complex is more stabilized than its benzyl derivative, QAS-trzBn ₄ -CO ₂ -epoxide, due to favorable interactions of CO₂ with the pyridyl substituents.

Introduction

Carbon capture and utilization technologies have gained increasing interest due to their potential to mitigate CO₂ emissions while transforming this abundant C1 feedstock into value-added chemicals. This approach aligns with the principles of sustainable chemistry by enabling the conversion of CO₂, which has often been treated as waste, into useful products. Among the various chemicals that can be derived from CO₂, such as alcohols, formic acid, urea, and CO, cyclic carbonates stand out due to their broad applications in green solvents, lithium–ion battery electrolytes, and polymer precursors. −

Traditionally, CO₂/epoxide conversion to cyclic carbonates has relied on binary catalytic systems that pair a Lewis-acidic activator with a nucleophilic cocatalyst. ,, The Lewis acidic site, either a metal center or an organic hydrogen-bond donor, activates epoxides, while halide counterions usually from tetrabutylammonium salts (TBA⁺X^–; X = Cl, Br, I) promote epoxide’s ring-opening. , Within these catalyst frameworks, metal-free organocatalytic systems featuring HBDs, such as ureas, phenols, and amide-containing macrocycles, combined with TBA⁺X^– constitute a major class. Meanwhile, bifunctional, single-component catalysts integrate activation and nucleophilic functions within one structure, allowing easy handling and reduced chemical waste. − In addition, when immobilized, they enable more efficient reuse in heterogeneous platforms. ,

Quaternary ammonium halides have previously been investigated as effective bifunctional single-component catalysts for the CO₂/epoxide cycloaddition. In 2002, Caló and colleagues demonstrated that NBu₄ ⁺X^– could catalyze this type of reaction alone. In particular, a 1:1 wt % mixture of tetrabutylammonium bromide (TBABr) and tetrabutylammonium iodide (TBAI) converted styrene oxide (SO) and CO₂ (1 atm) into styrene carbonate (SC) with an 83% yield at 120 °C in 4 h. Zhang and co-workers further explored the catalytic conversion of propylene oxide using 1.6 mol % of TBABr or its hydroxyl-functionalized derivative, hydroxyethyltributylammonium bromide (HETBAB), under 2.0 MPa CO₂ at 125 °C. With the catalyst TBABr or HETBAB, 74% and 96% conversions of propylene oxide to propylene carbonate with >99% selectivity were achieved at 1 h, respectively. The presence of Lewis acid sites, i.e., hydroxyl functional groups, promotes epoxide activation and enhanced coupling activity. − Recently, our group also reported ionic, triazole-based polymers as efficient and reusable single-component heterogeneous catalysts for CO₂/epoxide cycloaddition.

Building on this concept, triazole-based compounds present a promising catalyst platform for CO₂/epoxide cycloaddition due to their structural tunability and ability to incorporate functional groups via click chemistry. In particular, we are interested in introducing pyridine into the catalyst framework to enhance CO₂-capturing ability. Meanwhile, the quaternary ammonium-triazole moiety provides several advantages: (i) the Lewis basic triazole N atoms contribute to CO₂ adsorption, , (ii) triazolyl H atoms may function as Lewis acid sites to activate epoxides, and (iii) the quaternary ammonium moiety creates a sterically hindered cationic environment that allows weak interactions with halide counterions, potentially increasing their nucleophilicity and overall catalytic efficiency.

In this study, we present a new class of quaternary ammonium salts featuring 1,2,3-triazole functional groups with either benzyl (QAS-trzBn ₄ ) or 2-picolyl (QAS-trzPic ₄ ) substituents. Additionally, we demonstrate a synthetic pathway for a dual-substituted variant, QAS-trzBn ₂ Pic ₂ . These novel QAS-trz catalysts were evaluated as bifunctional, single-component catalysts for CO₂/epoxide cycloaddition with a particular focus on understanding the impact of triazole substituents on catalytic performance.

Experimental Section

Materials, Methods, and Instrumentation

All air-sensitive reactions were carried out in dry glassware under a N₂ atmosphere in a glovebox or using standard Schlenk techniques. All reagents, including propargyl bromide (>97% stabilized in MgO) and tripropargylamine, were purchased from TCI or Sigma-Aldrich and used without further purification. Reagent-grade solvents were purchased from LabScan. PhCH₂N₃ and pyCH₂N₃ were prepared according to the literature. , Tetrapropargylammonium bromide (TPABr) and the triazole-based compounds, (1-R-1H-1,2,3-triazol-4-yl)­methanol [R = PhCH₂ (1-Bn), 2-pyCH₂ (1-Pic), 1-benzyl-1H-1,2,3-triazole-4-carbaldehyde (2-Bn), and N,N-bis((1-benzyl-1H-1,2,3-triazol-4yl)­methyl)­prop-2-yn-1-amine (3-Bn), were synthesized following the literature methods.

¹H (400 MHz) and ¹³C­{¹H} (100 MHz) NMR spectra were acquired in deuterated solvents at room temperature by using a Bruker-Ascend 400 high-resolution nuclear magnetic resonance spectrometer with chemical shifts referenced to residual solvent peaks. Fourier transform-infrared (FT-IR) spectra were collected on a Bruker model Alpha spectrometer from solid samples. High resolution mass spectra (HRMS) of 4-Bn, 4-Pic, QAS-trzR ₄ (R = Bn, 2-Pic), and QAS-trzBn ₂ Pic ₂ in CH₃CN were obtained in positive-ion mode on a Bruker micrOTOF II. Elemental analyses were performed using the LECO CHN analyzer model CNH-828. Product yields and percent substrate conversions obtained from catalytic experiments were analyzed by ¹H NMR spectroscopy.

Synthesis and Characterization

1-(Pyridin-2-ylmethyl)-1H-1,2,3-triazole-4-carbaldehyde (2-Pic)

A mixture of 1-Pic (7.16 g, 37.6 mmol) and excess MnO₂ (49.1 g, 564.6 mmol) in 150 mL of CH₃CN was refluxed at 90 °C for 2 days. After it was cooled to room temperature, the suspension was filtered through silica/Celite to remove MnO₂. The precipitates were washed with 50 mL of EtOAc and 50 mL of CH₂Cl₂. The combined filtrates were dried in vacuo to yield 2-Pic as a yellow liquid in 97% yield (6.86 g, 36.5 mmol). ¹H NMR (400 MHz, CDCl₃): δ 10.14 (s, 1H, CHO), 8.61 (m, 1H, pyH), 8.33 (s, 1H, trzH), 7.73 (t J = 8 Hz, 1H, PyH), 7.30 (d J = 4 Hz, 2H, PyH), 5.71 (s, 2H, CH ₂). ¹³C­{¹H} NMR (100 MHz, CDCl₃): δ 185.0 (CHO), 153.1, 150.1, 147.9, 137.5, 126.2, 123.8, 122.7 (aromatic Cs), 55.7 (pyCH₂). ESI-MS (m/z): calcd for C₉H₈N₄O, [M]: 188.07; found, 211.0595 [M + Na]⁺. FT-IR (cm^–1): 1686 (CO).

N,N-Bis­((1-(pyridin-2-ylmethyl)-1H-1,2,3-triazol-4yl)­methyl)­prop-2-yn-1-amine (3-Pic)

To a 50 mL CH₂Cl₂ solution of 2-Pic (6.86 g, 36.5 mmol) was added propargylamine (0.96 g, 17.4 mmol). The reaction solution was stirred for 24 h at room temperature, after which Na­[BH­(OAc)₃] (11.0 g, 52.1 mmol) was added into the reaction mixture. After another 24 h of stirring, a 1 M aqueous NaOH solution (50 mL) was added, and the reaction mixture was vigorously stirred for 20 min. The product was extracted by CH₂Cl₂ (3 × 50 mL), upon which the combined CH₂Cl₂ extracts were dried in anhydrous Na₂SO₄, filtered, and removed in vacuo to afford the spectroscopically clean product, 3-Pic, obtained as a yellow oil in 90% yield (6.27 g, 15.7 mmol). The product was immediately used in the next step without purification. 1H NMR (400 MHz, CDCl₃): δ 8.49 (m, 2H pyH), 7.67 (s, 2H, trzH), 7.59 (dt J = 8, 4 Hz, 2H, pyH), 7.18 (m, 2H, pyH), 7.08 (d J = 4 Hz, 2H, pyH), 5.56 (s, 4H, CH ₂py), 5.26 (s, 4H, NCH ₂trz), 3.78 (s, 2H, NCH ₂C), 2.06 (s, 1H, CH). 13C­{1H} NMR (125 MHz, CDCl₃): δ 154.4, 149.7, 144.8, 137.3, 123.8, 123.3, 122.2 (aromatic Cs), 78.4 (CCH), 73.8 (CCH), 55.4, 47.8, 41.9 (CH₂).

Dipropargyl-bis­(4-R-1H-1,2,3-triazolylmethyl)­ammonium bromide [R = PhCH₂ (4-Bn), 2-pyCH₂ (4-Pic)]

To a 20 mL CH₂Cl₂ solution of 3-Bn or 3-Pic was added excess propargyl bromide. The yellow solution was stirred at room temperature for 4 days, upon which white precipitates were formed. The solid was filtered, washed with CH₂Cl₂, and dried under vacuum, resulting in a spectroscopically clean product.

4-Bn: 3-Bn (1.5 g, 3.7 mmol) and propargyl bromide (1.79 g, 15.1 mmol) were used as substrates. A white solid identified as 4-Bn was obtained in a 95% yield (1.9 g, 3.7 mmol). 1H NMR (400 MHz, DMSO-d ₆): δ 8.57 (s, 2H, trzH), 7.43–7.34 (m, 10H, PhH), 5.70 (s, 4H, CH ₂Ph), 4.75 (s, 4H CH2trz), 4.30 (s, 4H, + NCH ₂C), 4.07 (s, 2H, CH). ¹³C­{¹H} NMR (100 MHz, DMSO-d ₆): δ 135.6, 134.4, 129.3, 129.0, 128.5, 128.3 (aromatic Cs), 84.2, 71.7 (CCH), 53.7, 53.3, 49.2 (CH₂). ESI-MS (m/z): calcd for C₂₆H₂₆N₇, [M⁺] 436.2244; found, 436.2303 [M⁺]. FT-IR (cm^–1): 3296 (w, Ar–H), 3154 (m, C–H), 2121 (w, CC).

4-Pic: 3-Pic (6.27 g, 15.7 mmol) and propargyl bromide (7.47 g, 62.8 mmol) were used as substrates. The product 4-Pic was obtained as a yellow solid in 70% yield (5.70 g, 11.0 mmol).1H NMR (400 MHz, DMSO-d ₆): δ 8.63 (s, 2H, trzH), 8.54 (m, 2H, pyH), 7.85 (m, 2H, pyH), 7.38 (m, 4H, pyH), 5.85 (s, 4H, CH2py), 4.79 (s, 4H, + NCH ₂trz), 4.32 (d J = 4 Hz, 4H, + NCH ₂C⁾ 4.12 (t J = 4 Hz, 2H, CH). ¹³C­{¹H} NMR (100 MHz, DMSO-d ₆): δ 154.9, 149.3, 137.9, 134.6, 130.7, 130.4, 123.9, 122.9 (aromatic Cs), 84.6, 72.0 (CCH), 55.0, 54.0, 49.5 (CH₂). ESI-MS (m/z): calcd for C₂₄H₂₄N₉, [M⁺] 438.215; found, 438.217 [M⁺]. FT-IR (cm^–1): 3296 (w, Ar–H), 3140 (str, C–H) 2122 (w, CC).

Symmetrical QAS-trzR₄ [R = CH₂Ph (Bn) and 2-CH₂py (Pic)]

To a mixture of TPABr (0.10 g, 0.40 mmol) and RN₃ (R = CH₂Ph or 2-CH₂py, 1.7 mmol) was added 15 mL of CH₃OH, followed by CuSO₄·5H₂O (24.9 mg, 0.10 mmol) and ascorbic acid (30.3 mg, 0.17 mmol) at room temperature. The reaction mixture was stirred for 24 h, upon which it became a homogeneous solution. The solvent was dried under vacuum, after which 30 mL of CH₂Cl₂ was added. To this solution, EDTA (0.41 g, 1.4 mmol) in 20 mL of 10% aqueous NH₄OH solution was added, and the mixture was stirred at room temperature for 8 h. The product was extracted with CH₂Cl₂ and dried by using anhydrous Na₂SO₄. Filtration, solvent evaporation, and crystallization afforded the desired product.

QAS-trzBn₄

RN₃ = PhCH₂N₃ (0.23 g). After the addition of EDTA, the homogeneous aqueous solution was extracted with 3 × 60 mL CH₂Cl₂. A layer diffusion of Et₂O onto a CH₂Cl₂ solution of the crude product at room temperature afforded an off-white crystalline solid in 65% yield (0.20 g, 0.26 mmol). 1H NMR (400 MHz, CDCl₃): δ 8.64 (s, 4H, trzH), 7.32–7.23 (m, 20H, PhH), 5.62 (s, 8H, trzCH ₂Ph), 4.43 (s, 8H, + NCH ₂). 13C­{1H} NMR (100 MHz, CDCl₃): δ 134.9, 134.1, 129.8, 129.1, 128.8, 128.3 (ArC), 54.5 (CH₂Ph), 52.8 (+N–CH₂). HRMS (m/z): calcd for C₄₀H₄₀N₁₃, [M+] 702.3524; found, 702.3596 [M⁺]. X-ray fluorescence (XRF) (wt %): Br 100%. Anal. Calcd for C₄₀H₄₀N₁₃Br: C, 61.37; H, 5.15; N, 23.27. Found: C, 61.50; H, 5.16; N, 23.45.

QAS-trzPic₄

RN₃ = 2-pyCH₂N₃ (0.24 g). After stirring with the EDTA solution, the addition of 30 mL of CH₂Cl₂ resulted in white precipitates, which were filtered, washed with 20 mL of CH₂Cl₂, and dried under reduced pressure. In addition, the combined CH₂Cl₂ filtrate was concentrated and layered with Et₂O at room temperature to induce more product precipitation. The product was obtained as a white solid in a 67% yield (0.21 g, 0.27 mmol). 1H NMR (400 MHz, DMSO-d ₆): δ 8.81 (s, 4H, trzH), 8.54 (m, 4H, pyH), 7.84 (dt J = 8, 4 Hz, 4H, pyH), 7.41 (d J = 8 Hz, 4H, pyH) 7.36 (m, 4H, pyH), 5.85 (s, 8H, trzCH ₂py), 4.61 (s, 8H, +NCH ₂). ¹³C­{¹H} NMR (100 MHz, DMSO-d ₆): δ 154.5, 149.5, 137.5, 134.9, 130.6, 123.4, 122.5 (ArC) 54.6 (trzCH₂py), and 52.1 (+NCH₂). ESI-MS (m/z): calcd for C₃₆H₃₆N₁₇, [M+] 706.3334; found, 706.3395 [M+]. XRF (wt %): Br 100%. Anal. Calcd for C₃₆H₃₆N₁₇Br: C, 54.96; H, 4.61; N, 30.27. Found: C, 54.91; H, 4.58; N, 30.32.

Dual-Substituted QAS-trzBn₂Pic₂

The CuAAC reaction was employed for the synthesis using either (i) 4-Bn and 2-pyCH₂N₃ or (ii) 4-Pic and PhCH₂N₃ as reactants. The procedure followed that of QAS-trzR ₄ , utilizing CuSO₄·5H₂O (60.4 mg, 0.24 mmol) and ascorbic acid (46 mg, 0.26 mmol) as the catalyst system. During the workup, a solution of EDTA (0.9 g, 3.4 mmol) in 20 mL of a 10% aqueous NH₄OH solution was added to sequester any residual copper. Crystallization by a layer diffusion of Et₂O onto a CH₃CN solution of the product at room temperature afforded an off-white crystalline solid after several days.

• (i) 4-Bn (0.50 g, 0.96 mmol) and 2-pyCH₂N₃ (0.26 g, 1.96 mmol) were stirred in 60 mL CH₃OH and 9 mL DMSO. Yield = 43% (0.32 g, 0.41 mmol).
• (ii) 4-Pic (0.50 g, 0.96 mmol) and PhCH₂N₃ (0.26 g, 1.96 mmol) were stirred in 30 mL CH₃OH. Yield = 87% (0.66 g, 0.84 mmol).

1H NMR (400 MHz, CD₃OD): δ 8.87 (s, 2H, trzH), 8.77 (s, 2H, trzH), 8.49 (m, 2H, pyH), 7.86 (m, 2H, pyH), 7.48 (d J = 8 Hz, 2H, pyH), 7.40–7.33 (m, 12H, pyH, and PhH) 5.82 (s, 4H, trzCH ₂py), 5.69 (s, 4H, trzCH ₂Ph), 4.59 (s, 8H, + NCH2). 13C­{1H} NMR (100 MHz, CD3OD): δ 155.4, 150.6, 139.2, 136.8, 136.7, 136.4, 131.9, 131.1, 130.1, 129.7, 129.4, 124.9, 124.2 (ArC), 56.1 (trzCH₂py), 55.1 (trzCH₂Ph), 53.5, 53.4 (+NCH2). ESI-MS (m/z): calcd for C₃₈H₃₈N₁₅, [M+]: 704.3429; found, 704.3494 [M⁺]. XRF (wt %): Br 100%. Anal. Calcd for C₃₈H₃₈N₁₅Br: C, 57.50; H, 4.95; N, 26.48. Found: C, 57.62; H, 4.94; N, 26.59.

X-ray Crystallography

Crystal data for 4-Pic, QAS-trzBn ₄ , and QAS-trzBn ₂ Pic ₂ are shown in Table . Single crystals of 4-Pic were obtained via a layer diffusion of Et₂O into a CH₃OH solution of the product at room temperature to afford colorless crystalline solids after several days. Meanwhile, X-ray quality crystals of QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ were grown by the slow diffusion of Et₂O into concentrated solutions of the ion compounds in CH₂Cl₂ (for QAS-trzBn ₄ ) and CH₃CN (for QAS-trzBn ₂ Pic ₂ ) at room temperature.

1. Crystallographic Data for 4-Pic, QAS-trzBn ₄ , and QAS-trzBn ₂ Pic ₂ .

	4-pic	QAS-trzBn4	QAS-trzBn2Pic2
formula	C24H24BrN9	C40H40BrN13	C38H38BrN15
formula wt	518.43	782.76	784.74
crystal system	monoclinic	monoclinic	monoclinic
space group	P21/c	C 2/c	P21/c
crystal size/mm	0.15 × 0.13 × 0.052	0.12 × 0.098 × 0.051	0.098 × 0.094 × 0.049
a/Å	6.06650(10)	51.117(2)	12.3902(12)
b/Å	21.2650(4)	15.3307(5)	29.425(3)
c/Å	18.2518(3)	10.0195(4)	10.4638(10)
α/deg	90	90	90
β/deg	98.051(2)	94.423(3)	98.241(5)
γ/deg	90	90	90
V/Å3	2331.35(7)	7828.4(5)	3775.6(6)
Z	4	8	4
D c/mg m–3	1.477	1.328	1.381
μ/mm–1	2.665	1.806	1.890
F(000)	1064	3248	1624
θ range/deg	3.209–68.479	3.010–68.538	3.004–68.368
reflns. collected	4249	30261	54416
reflns. unique	3844	3662	6922
parameters	308	549	549
goodness-of-fit on F 2	1.035	1.037	1.029
R 1, wR 2 [I > 2σ(I)]	0.0350, 0.0876	0.0727, 0.1759	0.0506, 0.1373
R all, wR 2 [all data]	0.0385, 0.0900	0.1375, 0.2074	0.0667, 0.1490
max, min Δρ/e Å–3	0.44, −0.39	0.65, −0.43	0.24, −0.48

X-ray crystallographic data of 4-Pic were collected on a Rigaku SuperNova diffractometer with a HyPix3000 detector and CuK_α radiation (λ_Cu = 1.54184 Å) at 150 K. The data were reduced, scaled, and integrated using CrysAlisPro (Rigaku, 1.171.40.81a, 2020). An absorption correction was applied as implemented in SCALE3 ABSPACK (Rigaku, 1.171.40.81a, 2020). Meanwhile, data of QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ were obtained on Bruker D8 Venture CMOS PHOTON I diffractometer with a microfocus detector and CuK_α radiation (λ_Cu = 1.54178 Å) at 298 K. Data reduction was performed using SAINT. All three structures were solved, and the space group was determined by ShelXT using intrinsic phasing and refined by full matrix least-squares minimization on F ² using ShelXL (version 2018/3) with OLEX² used as a graphical interface. All non-hydrogen atoms were refined anisotropically, while hydrogen atom positions were calculated geometrically and refined using a riding model. In the case of 4-Pic, the structure was found to be twinned and was refined as a two-component twin in a 71:29 ratio. Disorder of two of the phenyl groups was found in QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ , which were modeled in two parts with appropriate restraints. All pictures were created using OLEX². Crystallographic data for 4-Pic, QAS-trzBn ₄ , and QAS-trzBn ₂ Pic ₂ have been deposited with the Cambridge Crystallographic Data Center, CCDC 2428915, 2429633, and 2429633.

General Procedure for CO₂/Epoxide Cycloaddition

1 atm CO2: To a round-bottom flask with a magnetic stir bar was added epoxide (12.8 mmol) and QAS-trzR ₄ (50 mg, 0.50 mol %). The balloon was pressurized with 1 bar CO₂, and the reaction mixture was heated at 100 °C for a given time, after which the flask was immersed into an ice bath, and CO₂ was carefully released. The conversions to cyclic carbonates were determined using ¹H NMR spectroscopy (400 MHz, CDCl₃).

10 atm CO ₂ : To a stainless-steel autoclave equipped with a magnetic stir bar was added epoxide (25.6 mmol) and QAS-trzR ₄ (100 mg, 0.50 mol %). Then, the autoclave was sealed, pressurized with 10 atm CO₂, and heated at 100 °C. After a given time, it was immersed into an ice bath, and CO₂ was carefully released. The conversions to cyclic carbonates were determined using ¹H NMR spectroscopy (600 MHz, CDCl₃).

Results and Discussion

Symmetrical QAS-trzBn ₄ and QAS-trzPic ₄

The click reaction between TPABr and 4.4 equiv of ArCH₂N₃ (Ar = Ph or 2-py), catalyzed by CuSO₄·5H₂O and ascorbic acid in CH₃OH at room temperature, afforded ionic, symmetrical tetra­(triazolyl)­ammonium bromides, QAS-trzBn ₄ and QAS-trzPic ₄ , as a white solid (Scheme ). Substituting benzyl with 2-picolyl groups significantly impacted the solubility of the ionic compounds. Specifically, QAS-trzBn ₄ dissolves in a broad range of organic solvents, including CHCl₃, CH₃CN, and CH₃OH, whereas QAS-trzPic ₄ , is soluble only in highly polar solvents such as DMF and DMSO, exhibiting low solubility in CH₃OH, CH₃CN, and CH₂Cl₂. This property may result from extensive intermolecular dipole–dipole interactions between the pyridine N atom and electropositive ⁺NCH ₂, as suggested by the crystal structure of its related variant QAS-trzBn ₂ Pic ₂ (Figure ).

1. Synthesis of QAS-trzR ₄ (R = Bn, Pic).

2.

Crystal structures of (a) the asymmetric unit of QAS-trzBn ₄ , showing a short H···Br^– contact. (b) The asymmetric unit of QAS-trzBn ₂ Pic ₂ , showing short H···Br^– contacts. (c) Two QAS-trzBn ₂ Pic ₂ molecules, displaying H···Br^– and H···N_py contacts. Non-H atoms are represented as thermal ellipsoids at the 30% thermal probability level. For clarity, all H atoms except those involved in short contacts are omitted.

The low solubility of QAS-trzPic ₄ facilitated its precipitation as a pure white solid (67% yield) during workup by adding CH₂Cl₂ after the EDTA treatment. On the other hand, QAS-trzBn ₄ was purified by slow Et₂O diffusion into a concentrated CH₂Cl₂ solution at room temperature, yielding crystalline, colorless solids in 65% yield. Notably, despite the presence of SO₄ ^2– during catalysis, XRF and elemental analyses confirmed that the Br^– ion was the sole counterion, consistent with proposed molecular formulas. The ¹H NMR spectra of QAS-trzBn ₄ (CDCl₃) and QAS-trzPic ₄ (DMSO-d ₆) each display three characteristic singlet resonances at δ 8.71, 5.55, 4.63 and δ 8.81, 5.85, 4.61, which are assignable to trzH, trzCH ₂Ph, and ⁺NCH ₂, respectively (Figures S20 and S24).

Dual-Substituted QAS-trzBn₂Pic₂

In addition to all identical substituents on the quaternary ammonium compounds, the dual-substituted derivative QAS-trzBn ₂ Pic ₂ could also be synthesized from the ionic dipropargylbis­(triazolylmethyl)­ammonium bromide precursors 4-Bn and 4-Pic, whose synthesis is shown in Scheme . It should be mentioned that the neutral benzyl-substituted propargylbis­(triazolylmethyl)­amine compound 3-Bn and its precursors 1-Bn and 2-Bn have previously been reported by our group. Accordingly, the synthetic methods of related picolyl-substituted compounds 1-Pic, 2-Pic, and 3-Pic, followed those of their benzyl-substituted counterparts.

2. Synthesis of n-Pic ( n = 2–4) and 4-Bn .

The Cu-catalyzed click reaction between equimolar amounts of 2-pyCH₂N₃ and propargyl alcohol in the presence of 25 mol % CuSO₄·5H₂O and 45 mol % ascorbic acid in CH₃OH at room temperature afforded the triazole product 1-Pic as a yellow solid in 67% yield. Subsequent selective alcohol oxidation using excess MnO₂ in refluxing CH₃CN, followed by purification via flash column chromatography, yielded the aldehyde product 2-Pic as a yellow oil in 97% yield. The formation of the aldehyde functional group (CHO) was confirmed by a characteristic singlet resonance in the ¹H NMR spectrum at δ 10.14 (in CDCl₃, Figure S4) and a strong absorption band at 1686 cm^–1 in the FT-IR spectrum, corresponding to CO stretching (Figure S7). Reductive amination of 2-Pic with ca. 0.5 equiv of propargylamine in CH₂Cl₂, followed by the addition of 3 equiv of Na­[BH­(OAc)₃], afforded the neutral amine product 3-Pic as a yellow oil in 90% yield. Similar to its amine-triazole analogue 3-Bn, the picolyl-substituted compound 3-Pic was found to be unstable, converting within several days into intractable mixtures, as evidenced by ¹H NMR spectroscopy. Notably, even when spectra were acquired immediately after isolation, the ¹³C­{¹H} and 2D ¹H–¹³C HMBC NMR data already revealed impurities and signs of decomposition (Figures S9 and S10). Consequently, both 3-Bn and 3-Pic compounds were immediately reacted with excess propargyl bromide in CH₂Cl₂ at room temperature, which resulted in white precipitates, characterized as 4-Bn and 4-Pic, in 95% and 70% yields, respectively (pathway (b), Scheme ). The ¹H NMR spectra of 4-Bn and 4-Pic in DMSO-d ₆ displayed a characteristic singlet resonance at δ 8.57 and 8.63, assignable to trzH, along with an alkynyl ¹H peak at δ 4.07 and 4.12 (Figures S11 and S14). In contrast to their neutral amine analogues, the ionic compounds 4-Bn and 4-Pic are thermally robust and air stable.

X-ray quality crystals of 4-Pic were grown from layering Et₂O into a concentrated CH₃OH solution of 4-Pic to allow slow diffusion at room temperature. As shown in Figure , the crystal structure of 4-Pic features the quaternary ammonium nitrogen (N5) in a distorted tetrahedral geometry, with bond angles ranging from 104.2(2)° to 113.0(2)°. The asymmetric unit cell of the crystal structure indicates significant intermolecular electrostatic interactions of the Br^– ion with CCH (2.640 Å) and trzH atoms (2.875 and 2.987 Å). These values were notably shorter than the sum of the van der Waals radii for Br and H atoms (3.05 Å). , In contrast, ionic interactions between the quaternary ammonium center (N5) and the Br^– ion appear negligible, with a substantial distance of 6.246 Å observed.

1.

Crystal structures of 4-Pic. C, N, and Br atoms are depicted as thermal ellipsoids at the 30% probability level. (a) The asymmetrical unit of 4-Pic. (b) Packing diagram displaying short H···Br^– and H···N_trz contacts. All H atoms except those involved in short contacts are omitted for clarity.

The dual-substituted, ionic quaternary ammonium species–triazole species containing both benzyl and 2-picolyl substituents, namely, QAS-trzBn ₂ Pic ₂ , was synthesized via CuAAC between 2-pyCH₂N₃ and 4-Bn or, alternatively, PhCH₂N₃ and 4-Pic, as shown in Scheme . The reaction between 4-Pic and 2 equiv of PhCH₂N₃ in CH₃OH catalyzed by CuSO₄·5H₂O and ascorbic acid at room temperature afforded QAS-trzBn ₂ Pic ₂ as a dark orange solid in 87% yield. Alternatively, the CuAAC reaction between 4-Bn and 2 equiv of 2-pyCH₂N₃ in a 3:1 mixture of CH₃OH/DMSO solvents under similar conditions afforded the click product in a lower yield of 43%. The reduced yield can be attributed to the limited solubility of 4-Bn in CH₃OH, which necessitated the addition of a small amount of DMSO to achieve a homogeneous reaction mixture. However, the remaining DMSO in the product made its isolation challenging, ultimately contributing to the lower yield.

3. Synthesis of QAS-trzBn ₂ Pic ₂ .

The ¹H NMR spectrum of QAS-trzBn ₂ Pic ₂ in CD₃OD displays two singlet resonances at δ 8.87 and 8.77, corresponding to two types of trzH with benzyl and 2-picolyl substituents. In addition, three singlet resonances at δ 5.82, 5.69, and 4.59 can be assigned to trzCH ₂-py, trzCH ₂-Ph, and ⁺N–CH ₂, respectively (Figure S22). Elemental analysis, ESI-MS, and XRF data are consistent with the expected product.

Single crystals of QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ , grown from layering Et₂O into CH₂Cl₂ and CH₃CN solutions of the respective ionic compounds. Both crystallized in the monoclinic P2₁/c space group. Figure illustrates both crystal structures, revealing the Lewis acidic nature of ⁺N–CH ₂, with significant interactions with pyridine N donors (2.62 Å for QAS-trzBn ₂ Pic ₂ ) and Br^– ions (2.68 and 2.93 Å for QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ , respectively).

Catalytic CO₂/Epoxide Cycloaddition

The catalytic performance of the ionic QAS-trz compounds as bifunctional, single-component catalysts was evaluated using epichlorohydrin (ECH) as the substrate under 1 atm of CO₂ at 100 °C. With 50 mg of QAS-trz catalysts (0.50 mol % Br^–), the picolyl-containing derivatives, QAS-trzBn ₂ Pic ₂ and QAS-trzPic ₄ , exhibited high catalytic activity, achieving 98% and 93%, conversion with 95% and 91% selectivity to the cyclic carbonate product after 12 h, respectively (entries 2 and 3, Table ). In contrast, the benzyl analogue QAS-trzBn ₄ showed much lower activity, reaching only 39% conversion and 69% selectivity even after 24 h (entry 1). Using TBABr (0.5 mol %) as a control afforded 30% conversion and 73% selectivity at 12 h (entry 4). ¹H NMR analysis identified 1-bromo-3-chloropropan-2-ol as the major byproduct, consistent with Br^–-promoted epoxide ring opening, followed by protonation (i.e., no CO₂ incorporation). Collectively, these results suggest that pyridyl substitution in the QAS-trz framework enhances the catalytic efficiency and favors CO₂ incorporation under atmospheric CO₂ conditions. Interestingly, in our previous work, the heterogeneous QAS-trz polymer catalyst linked with p-Xylene (QAP-1), which is structurally related to QAS-trzBn ₄ , mediated CO₂/ECH cycloaddition, achieving 98% conversion and 82% selectivity in 24 h at 1 atm CO₂ and 120 °C using 0.59 mol % I^– as the nucleophile. The higher apparent catalytic activity of QAP-1 is plausibly due to the stronger nucleophilic iodide and its higher nitrogen content, which enhances the CO₂ adsorption at the polymer surface.

2. Catalytic Activity for CO₂/ ECH Cycloaddition .

entry	catalyst	conversion (%)	selectivity (%)
1	QAS-trzBn4	39	69
2	QAS-trzBn2Pic2	98	95
3	QAS-trzPic4	93	91
4	TBABr	30	73

^aConditions: catalyst (0.5 mol % Br^–), ECH (1.0 mL, 12.8 mmol), 1 atm CO₂, 100 °C, 12 h.

^b% Conversion and % selectivity were determined using ¹H NMR spectroscopy.

^cReaction time = 24 h.

^d20.6 mg of TBABr (0.5 mol % Br^–) was used as a catalyst.

Given its performance and straightforward synthesis, QAS-trzPic ₄ was selected for studies at elevated CO₂ pressures. At 10 and 20 atm CO₂ (0.5 mol % QAS-trzPic ₄ ,100 °C), ECH was efficiently converted to the cyclic carbonate, reaching 84% in 2 h and >99% conversions in 6 h, corresponding to TOF = 84 and 33 h^–1, respectively (entry 1, Table ). The styrene oxide/CO₂ cycloaddition reaction proceeded with exclusive formation of SC, with conversion increasing from 24% at 10 atm of CO₂ to 88% at 20 atm of CO₂ after 6 h at 100 °C (entry 2). In contrast, the more electron-rich propylene oxide resulted in lower reactivity, affording only 45% conversion to cyclic propylene carbonate under identical conditions at 20 atm of CO₂ (entry 3). Both allyl glycidyl ether and ethylene glycol diglycidyl ether underwent cycloaddition with CO₂, yielding high conversions of 83% and 97%, respectively, to the corresponding cyclic carbonate products (entries 4 and 5), whereas benzyl glycidyl ether was less reactive (12%, entry 6). Notably, with benzyl glycidyl ether, the QAS-trzPic ₄ catalyst remained in the solid phase throughout the reaction, which likely contributes to the low conversion. To test this, the same catalytic reactions (benzyl glycidyl ether, 20 atm of CO₂, 100 °C) were run using QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ . Both catalysts became completely soluble during the catalysis and gave 60% and 74% conversion, respectively (entry 6). TOF benchmarking against other single-component homogeneous organocatalysts places QAS-trzPic ₄ among the more active quaternary ammonium catalyst systems, despite the absence of strong H-bond donor groups such as hydroxyl, phenol, and carboxyl (Table ).

3. Substrate Scope with QAS-trzR ₄ .

^aConditions: QAS-trzPic ₄ (0.5 mol % Br^–, 0064 mmol), epoxide (12.8 mmol), 10–20 atm CO₂, 100 °C, 2–6 h.

^b% conversion and % selectivity were determined using ¹H NMR spectroscopy.

^cThe QAS-trzPic ₄ catalyst remained in the solid phase at the end of the reaction.

^dCatalyst = QAS-trzBn ₄ .

^eCatalyst = QAS-trzBn ₂ Pic ₂ .

4. Summary of Catalytic Conditions and TOF of Selected Single-Component Organocatalysts for Cycloaddition between CO₂ and Epoxide.

Computational Studies and Proposed Mechanism

Computational studies were conducted to evaluate the binding of CO₂ and styrene oxide (ep) with QAS-trzPic ₄ (1a) and QAS-trzBn ₄ (1d), as shown in Figure . We found key binding modes that position both epoxide and CO₂ in close proximity (∼2.8 Å). This is expected to facilitate the CO₂/epoxide cycloaddition, where O_ep acts as a nucleophile attacking CO₂ (Scheme ). For 1d, the binding mode 1d-ep-CO ₂ -B involves interactions of the triazole N atom in 1d with CO₂, i.e., N_trz···CO₂, along with ⁺NCH₂···O_ep, trzH···O₂C, and o-ArH···O₂C interactions. The binding is endergonic by 5.70 kcal/mol. For picolyl-substituted compound 1a, the corresponding binding mode of 1a-ep-CO ₂ -B is also endergonic by 7.53 kcal/mol. However, in the presence of the pyridine groups in 1a, an alternative binding mode was located. In particular, we observe the acid–base interactions between CO₂ and the pyridine substituents of 1a, N_py···CO₂, along with stabilization from ⁺NCH₂···O_ep interactions, which yielded a more stabilized complex 1a-ep-CO ₂ -A (2.41 kcal/mol). This data suggests that pyridine plays a distinctive role in stabilizing CO₂ in 1a-ep-CO ₂ -A, making such complexes more thermodynamically favorable than the related complexes with the N_trz···CO₂ interactions. Improved CO₂/epoxide stabilization in the presence of pyridine substituents is attributed to the observed higher catalytic activity of QAS-trzPic ₄ than QAS-trzBn ₄ for CO₂/ECH cycloaddition at 1 atm of CO₂.

3.

Binding modes and associated relative binding free energies of CO₂ and ep (ep = styrene oxide) to QAS-trzPic ₄ (1a) and QAS-trzBn ₄ (1d). The relative binding free energies (ΔG) are shown in kcal/mol. Selected bond distances are shown in Å.

4. Proposed Mechanism for Catalytic CO₂/Epoxide Cycloaddition by QAS-trzPic ₄ .

Based on computational results, the proposed mechanism for CO₂/epoxide cycloaddition catalyzed by QAS-trzPic ₄ is illustrated in Scheme . In the presence of CO₂ and epoxide substrates, the acidic H atoms of the triazole ring and the methylene groups surrounded by a quaternary ammonium nitrogen and aromatic triazolyl ring (⁺NCH ₂trz) interact with the O atoms of CO₂ and the epoxide (A). Meanwhile, the pyridine N donor, acting as a Lewis base, interacts with the electrophilic carbon of CO₂. The reaction proceeds via epoxide ring opening by Br^–, followed by the nucleophilic attack of the resulting alkoxide on CO₂. The stabilized carbonate intermediate (B) underwent intramolecular ring cyclization to yield the cyclic carbonate product and regenerate the homogeneous catalyst QAS-trzPic ₄ .

Conclusion

In this study, a new class of symmetrical and dual-substituted quaternary ammonium bromide featuring triazole functional groups, QAS-trzBn ₄ , QAS-trzPic ₄ , and QAS-trzBn ₂ Pic ₂ , were successfully synthesized. Despite the presence of competing anions such as SO₄ ^2– and OH^– during the synthesis, XRF and elemental analyses confirmed that Br^– was the sole counterion in the final products. X-ray crystallographic analysis of QAS-trzBn ₄ and QAS-trzBn ₂ Pic ₂ revealed strong interactions between Br^– and N_py with H atoms of both trzH and ⁺NCH₂, suggesting their significant Lewis acidic nature. Catalytic studies showed that picolyl-substituted compounds QAS-trzPic ₄ and QAS-trzBn ₂ Pic ₂ outperform QAS-trzBn ₄ as bifunctional, single-component homogeneous catalysts. Both QAS-trzPic ₄ and QAS-trzBn ₂ Pic ₂ efficiently promoted the CO₂/ECH cycloaddition, affording the cyclic carbonate product at 1 atm of CO₂, 100 °C. In addition, QAS-trzPic ₄ remains highly efficient across a range of epoxides under elevated CO₂ pressures. Computational studies further indicate that pyridine substituents play a crucial role in stabilizing the QAS-trzPic ₄ -CO ₂ -epoxide complex, likely contributing to its enhanced catalytic efficiency. Based on these insights, a catalytic mechanism was proposed, highlighting trzH and ⁺NCH₂ as key Lewis acidic sites responsible for the binding of CO₂ and epoxide activation. This study demonstrates the significant impact of triazolyl substituents on improving the catalytic performance in CO₂/epoxide cycloaddition. Notably, pyridyl-substituted triazole functional groups enable CO₂ stabilization, while the ⁺NCH₂ Lewis acid sites facilitate epoxide activation, as supported by computational and single-crystal analyses.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c04751.

• ¹H, ¹³C­{¹H} NMR spectra, HRMS, and FT-IR spectra of ligand substrates and quaternary ammonium-triazole salts (QAS-trzBn ₄ , QAS-trzBn ₂ Pic ₂ , and QAS-trzPic ₄ ); elemental analysis and XRF results of QAS-trzBn ₄ , QAS-trzBn ₂ Pic ₂ , and QAS-trzPic ₄ ; ¹H NMR spectra of the cycloaddition reactions under 1 and 10 atm CO₂ pressures and various epoxide substrates; and computational details including methodology and parameters used in DFT calculations (PDF)

• DpPyBr-150K 2 (CIF)

• QAS-trzBn₄ (CIF)

• QAS-trzBn₂Pic₂ (CIF)

Declaration of generative AI and AI-assisted technologies in the writing process During the preparation of this work, the author(s) used ChatGPT-4o in order to improve the readability of the article. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

The authors declare no competing financial interest.