Pyridine-Chelated Imidazo[1,5- a ]Pyridine N -Heterocyclic Carbene Nickel(II) Complexes for Acrylate Synthesis from Ethylene and CO 2

: Nickel(II) dichloride complexes with a pyridine-chelated imidazo[1,5- a ]pyridin-3-ylidene py-ImPy ligand were developed as novel catalyst precursors for acrylate synthesis reaction from ethylene and carbon dioxide (CO 2 ), a highly promising sustainable process in terms of carbon capture and utilization (CCU). Two types of ImPy salts were prepared as new C,N-bidentate ligand precursors; py-ImPy salts ( 3 , 4a – 4e ) having a pyridine group at C(5) on ImPy and a N -picolyl-ImPy salt ( 10 ) having a picolyl group at N atom on ImPy. Nickel(II) complexes such as py-ImPyNi(II)Cl 2 ( 7 , 8a – 8e ) and N -picolyl-ImPyNi(II)Cl 2 ( 12 ) were synthesized via transmetalation protocol from silver(I) complexes, py-ImPyAgCl ( 5 , 6a – 6e ) and N -picolyl-ImPyAgCl ( 11 ). X-ray di ﬀ raction analysis of nickel(II) complexes ( 7 , 8b , 12 ) showed a monomeric distorted tetrahedral geometry and a six-membered chelate ring structure. py-ImPy ligands formed a more planar six-membered chelate with the nickel center than did N -picolyl-ImPy ligand. py-ImPyNi(II)Cl 2 complexes ( 8a – 8e ) with tert -butyl substituents exhibited noticeable catalytic activity in acrylate synthesis from ethylene and CO 2 (up to 108% acrylate). Interestingly, the use of additional additives including monodentate phosphines increased catalytic activity up to 845% acrylate (TON 8). research: pyridine chelated ImPy Ni(II) complexes for acrylate synthesis from ethylene from (I) complex (DME)NiCl 2 and 2 Cl (135 X-ray quality were obtained by the liquid di ﬀ usion Elemental analysis (%) calcd. for C 24 H 25 Cl 2 N 3 Ni: N, 8.66. Found: C, 59.54; H, 5.398; N, 8.27;UV-Vis (CH 2 Cl 2 411; µ e C 30 H 37 Cl 2 3 2 Cl λ (nm) 409; e ﬀ 2.86 B.M


Introduction
The synthesis of acrylic acid derivatives through the C-H carboxylation of ethylene with carbon dioxide (CO 2 ) has received much attention lately in the area of carbon capture and utilization (CCU) [1][2][3][4][5][6][7][8][9][10][11], as the acrylate products are value-added chemicals for superabsorbent polymers, adhesives, and coatings. This new acrylate synthetic route could be superior to the existing industrial process (two-stage oxidation of propylene) by utilizing less expensive feedstock (ethylene vs. propylene) and a sustainable carbon source (CO 2 ) [12].
Pyridine ligands are known as active ligands for nickel promoted coupling reactions using ethylene and CO 2 [13,17,[91][92][93], which inspired us to develop mononuclear Ni(II) complexes containing a pyridine-chelated ImPy for catalytic acrylate synthesis from CO 2 /ethylene (Figure 1b). We herein present novel mononuclear Ni(II) complexes with a rigid six-membered ring, imposed by a special pyridine-chelated bidentate ImPy ligand, providing catalytic activity in the acrylate formation from ethylene and CO 2 .

Synthesis and Structural Analysis of Ag(I) Complexes
Synthesized ligand precursors were converted to silver (Ag) complexes that could serve useful transmetallating agents for Ni complexes (Scheme 1). Ag(I) complexes (5, 6a-6e, 11) were prepared from the imidazopyridinium salts (3, 4a-4e, 10) by reaction with Ag(I) oxide in good yields (81%-96%) (Scheme 1). After the disappearance of peaks related to the acidic imidazopyridinium proton of 3, 4a-4e, and 10 were confirmed in 1 H NMR, Ag complexes were purified by washing with hexane or recrystallization from CH 2 Cl 2 and hexanes. The molecular structure of an Ag complex 5 was characterized by X-ray crystallography ( Figure 2). A single crystal for X-ray diffraction was prepared by slow recrystallization from dichloromethane layered with hexane. The crystal structure exhibited a mononuclear Ag(I) complex with a linear geometry, which is attributed to the unique steric effect at C(5) on the ImPy. Similar structural features were observed in other previously reported Ag(I) chloride complexes with ImPy [98]. Scheme 1. Synthesis of ligand precursors, Ag complexes, and Ni complexes.

Synthesis and Structural Analysis of Ag(I) Complexes
Synthesized ligand precursors were converted to silver (Ag) complexes that could serve useful transmetallating agents for Ni complexes (Scheme 1). Ag(I) complexes (5, 6a-6e, 11) were prepared from the imidazopyridinium salts (3, 4a-4e, 10) by reaction with Ag(I) oxide in good yields (81%-96%) (Scheme 1). After the disappearance of peaks related to the acidic imidazopyridinium proton of 3, 4a-4e, and 10 were confirmed in 1 H NMR, Ag complexes were purified by washing with hexane or recrystallization from CH2Cl2 and hexanes. The molecular structure of an Ag complex 5 was characterized by X-ray crystallography ( Figure 2). A single crystal for X-ray diffraction was prepared by slow recrystallization from dichloromethane layered with hexane. The crystal structure exhibited a mononuclear Ag(I) complex with a linear geometry, which is attributed to the unique steric effect at C(5) on the ImPy. Similar structural features were observed in other previously reported Ag(I) chloride complexes with ImPy [98].

Synthesis and Structural Analysis of Nickel(II) Complexes
ImPy Ag(I) complexes (5, 6a-6e, 11) were employed for subsequent transmetalation with (DME)NiCl 2 (DME as dimethoxyethane) to afford Ni(II) complexes (7, 8a-8e, 12) in up to 99% yield (Scheme 1). All complexes were analyzed by elemental analysis and UV spectroscopy. Ni complexes could not be analyzed by NMR spectroscopy owing to their paramagnetic properties. Magnetic susceptibility in the solid-state was measured by magnetic susceptibility balance. Effective magnetic moments (µ eff = 2.56-3.09 BM, 297 K) were found to be similar to the spin-only value (2.87 BM, S = 1) [99], which demonstrates that the Ni(II) complexes are paramagnetic species, high-spin d 8 ions with two unpaired electrons.
Structural characterization of 7, 8b, and 12 were established by X-ray crystallography. Single crystals of 7, 8b, and 12 suitable for X-ray diffraction analysis were grown through the slow diffusion of n-hexane into a saturated solution of dichloromethane. Three complexes were observed as desired monomeric Ni(II) complexes with four-coordinated species (Figures 3-5). The Ni(II) center displays a distorted tetrahedral geometry. The most remarkable structural feature of 7, 8b, and 12 is the coordination of the carbene carbon and pyridine nitrogen (N) to the Ni(II) center, displaying a bidentate C, N chelating mode that yields a six-membered nickelacycle.

Nickel Mediated Acrylate Synthesis from Ethylene Using CO 2
Newly prepared ligand precursors (3, 4a-4e, 10) were tested in the "in-situ" Ni(II)-mediated C-H carboxylation of ethylene using CO 2 , which was modified from Vogt's reaction conditions (Scheme S1). Interestingly, the activities (up to 50% acrylate) were observed only for the py-ImPy R ligand salts (3, 4a-4e) bearing the pyridine chelating group at the C(5) position. On the other hand, no acrylate product was observed when non-pyridine bidentate ImPy ligand precursors were used, for example, bis-ImPy, OMe-ImPy, and O = P-ImPy. Moreover, no acrylate was detected when typical monodentate NHC ligands were used such as ImPy·HCl or IPr·HCl. Although the ligand screening using the in situ generation method might not accurately reflect the inherent ability of the ligands to promote the reaction, it could provide a reasonable starting point to develop as novel carbene ligands for the C-H carboxylation reaction.

Optimization of Reaction Conditions Using the Combination 8b and PCy 3
To optimize the reaction conditions for acrylate synthesis using the combination of 8b and PCy 3 , we have systematically varied several parameters, including the base and the solvent ( Table 2). The role of Et 3 N in the lithium acrylate synthesis helps to remove HI generated during the reaction, which is known to be similar to the role in Ni Heck-type reaction [32]. Therefore, several weak bases to abstract HI, Catalysts 2020, 10, 758 8 of 20 such as diisopropylethylamine (DIPEA), pyridine (py), K 2 CO 3 , Cs 2 CO 3 , 1,4-diazabicyclo [2.2.2] octane (DABCO), and tetramethylethylenediamine (TMEDA), were screened (entries 2-7). Among them, only organic bases, such as DIPEA and pyridine, showed reactivities, but these were significantly lower than that of Et 3 N (entries 2-3). Strong alkoxide bases such as sodium 2-fluorophenoxide (2-F-PhONa), sodium 2,6-dimethylphenoxide (2,6-Me-PhONa) and t-BuOK did not produce any acrylate (entries 8-12) [33,34]. In addition, the sodium iodide (NaI) did not produce the product (entry 13). Therefore, Et 3 N/LiI was chosen for the next catalysis experiments. Subsequently, we investigated the effect of solvents on the acrylate synthesis (entries [14][15][16][17][18][19][20][21]. First, we screened weakly coordinating solvents such as toluene, anisole, benzene, and 2-chlorotoluene (2-Cl-Tol), as they are effective solvents reported in the Vogt s system [32,42]. Although these solvents were suitable in acrylate synthesis (54%-799% acrylate), PhCl is still the most effective solvent. Next, screening of other solvents such as THF, CH 2 Cl 2 , DMF and 1,4-dioxane did not give any acrylate, suggesting that more strongly coordinating solvents could hinder the weakly binding substrate from coordinating to the metal center [32]. The reaction performed without Zn did not proceed, confirming the importance of Zn as a reducing agent (entry 22). Et 3 N base and weakly coordinating solvents that were favored in the Vogt conditions, are also effective in this system [32]. significantly lower than that of Et3N (entries 2-3). Strong alkoxide bases such as sodium 2fluorophenoxide (2-F-PhONa), sodium 2,6-dimethylphenoxide (2,6-Me-PhONa) and t-BuOK did not produce any acrylate (entries 8-12) [33,34]. In addition, the sodium iodide (NaI) did not produce the product (entry 13). Therefore, Et3N/LiI was chosen for the next catalysis experiments. Subsequently, we investigated the effect of solvents on the acrylate synthesis (entries [14][15][16][17][18][19][20][21]. First, we screened weakly coordinating solvents such as toluene, anisole, benzene, and 2-chlorotoluene (2-Cl-Tol), as they are effective solvents reported in the Vogt′s system [32,42]. Although these solvents were suitable in acrylate synthesis (54%-799% acrylate), PhCl is still the most effective solvent. Next, screening of other solvents such as THF, CH2Cl2, DMF and 1,4-dioxane did not give any acrylate, suggesting that more strongly coordinating solvents could hinder the weakly binding substrate from coordinating to the metal center [32]. The reaction performed without Zn did not proceed, confirming the importance of Zn as a reducing agent (entry 22). Et3N base and weakly coordinating solvents that were favored in the Vogt conditions, are also effective in this system [32].

General Remarks
All air-and moisture-sensitive reactions were performed under an argon atmosphere either using Schlenk techniques or a glove box. All reactions involving the formation of acrylate from Catalysts 2020, 10, 758 9 of 20 ethylene and CO 2 were carried out in 100 mL stainless steel autoclaves (Hanwoul Engineering Co., Gunpo-si, Republic of Korea). Nuclear magnetic resonance (NMR) (JEOL, Tokyo, Japan) spectra were recorded on a JEOL 400 spectrometer, operated at 400 MHz for 1 H NMR and at 100 MHz for 13 C NMR. Chemical shifts (ppm) for 1 H were referenced to the residual solvent peak (CDCl 3 = δ 7.26 ppm, CD 2 Cl 2 = δ 5.32 ppm, CD 3 OD = δ 3.31 ppm, (CD 3 ) 2 SO = 2.50 ppm, D 2 O = δ 4.79 ppm). Multiplicities were recorded as s (singlet), d (doublet), t (triplet), q (quartet), sept (septet), or m (multiplet). Chemical shifts (ppm) for 13 C were referenced relative to the residual solvent peak (CD 2 Cl 2 = δ 53.84 ppm, CD 3 OD = δ 49.00 ppm, (CD 3 ) 2 SO = 39.52 ppm). High-resolution mass spectra (HRMS) were recorded on a JEOL JMS-700 MStation mass spectrometer (JEOL, Tokyo, Japan). Elemental analyses were carried out with an UNICUBE Elemental Analyzer (Elementar, Langenselbold, Germany). The magnetic susceptibilities of nickel complexes were measured in the solid state using a magnetic susceptibility balance (Sherwood Scientific, Cambridge, UK). Diamagnetic corrections were ignored. UV/vis measurements of nickel complexes were carried out in CH 2 Cl 2 solution using a Perkin-Elmer UV/VIS NIR Spectrometer Lambda 950 (Perkin Elmer, Shelton, USA). Analytical thin layer chromatography (TLC) (Merck KGaA, Darmstadt, Germany) was performed with Merck pre-coated silica gel 60Ǻ (F254) glass plates and visualization on TLC was achieved by UV light. Flash chromatography was performed with 230-400 Mesh 60Ǻ Silica Gel purchased from Merck Inc.

Synthesis of Ligand Precursors. General Procedure
Aldehyde (1 equiv), aniline (1-1.05 equiv) and ethanol or methanol (0.1-0.2 M) were added to a Schlenk flask equipped with a magnetic stirrer and sealed with a rubber septum tightened with a cable tie. The mixture was stirred at 90 • C for 24-48 h. The solvent was removed under reduced pressure. If needed, the crude was purified by basic column chromatography. Then, imine derivatives were transferred to a Schlenk tube equipped with a Teflon-valve. The solvent was removed by blowing nitrogen gas and using vacuum. Subsequently, chloromethyl ethyl ether (20 equiv) was added and stirred at 90 • C. After cooling to room temperature, the volatiles were removed by rotary evaporation and recrystallization with dichloromethane and hexane. The crude solids were purified through flash column chromatography and recrystallization with dichloromethane and hexane.

Synthesis of Ag(I) Complexes. General Procedure
In the absence of light, ligand precursors (1 equiv) and Ag 2 O (2 equiv) in CH 2 Cl 2 (0.084 M) were stirred in a Schlenk flask at room temperature for 23 h. The crude solution was filtered through a Celite pad with CH 2 Cl 2 . The volatiles were evaporated in vacuo and washed with distilled hexane (if needed, the crude products were recrystallized using CH 2 Cl 2 with hexane).

Synthesis of Ni(II) Complexes. General Procedure
Ag (I) complexes (1 equiv) were added to a 250 mL Teflon-valve Schlenk flask containing (DME)NiCl 2 (1 equiv) in a glove box. The flask was removed from the glove box and dichloromethane was added. The resulting solution was stirred at 60°C for 8 h. After cooling to room temperature, the solution was filtered through a glass frit containing Celite under argon atmosphere. The solvent was removed under reduced pressure. The crude mixture was transferred to a vial with distilled CH 2 Cl 2 and recrystallized from CH 2 Cl 2 /hexane or washed with hexane. Ni(II) complexes (7, 8a-8e) do not decompose easily in air without moisture. In a low-humidity environment, they can be quickly filtered using celite under normal atmosphere and the solvent can be removed using a rotary evaporator.

Synthesis of Ni(II) complex 12:
Following the general procedure of Ni(II) complexes, a product 12 (172 mg, 90% yield) was obtained as a light-brown powder from Ag (I) complex 11 (198 mg, 0.42 mmol), (DME)NiCl 2 (92 mg, 0.42 mmol), and CH 2 Cl 2 (140 mL). X-ray quality crystal was obtained by liquid diffusion of n-hexane into aturated CH 2 Cl 2 at room temperature. UV-Vis (CH 2 Cl 2 ): λ(nm) 490 Elemental analysis (%) calcd. for C 22  3.6. General Procedure for the Synthesis of Lithium Acrylate Using Ethylene and CO 2 Lithium acrylate was synthesized following a modified version of a previously reported procedure. 7b Inside a glove box, Ni(II) complex (0.05 mmol), LiI (1.25 mmol) and Zn (2.50 mmol) were added into a 4 mL screw-cap vial equipped with a magnetic stir bar. The vial was removed from the glove box and charged with PhCl (2 mL) via a syringe under argon atmosphere. The vial was then stirred for 3 min, and Et 3 N (0.35 mL) was injected via a syringe. Under argon atmosphere, the vial was transferred to a 100 mL stainless steel autoclave and was punctured with a flat-cut needle (18 G, 0.6 cm). The autoclave was immediately closed and purged with ethylene gas (10 bar) for 10 min without stirring. The autoclave was pressurized with ethylene (25 bar) and then with CO 2 (5 bar) at room temperature. The autoclave was heated to 60 • C in an oil bath for 12 h. After cooling to room temperature, the pressure was released. D 2 O (1 mL) with sodium 3-(trimethylsilyl)-2,2,3,3-d 4 -propionate (0.070 mmol), was added to the reaction mixture as an internal standard. After vigorous stirring for 15 min and manual shaking for about 15 min, the D 2 O layer was separated from organic phase by centrifugation and filtration. The D 2 O layer was washed with ether (2 mL). The amount of acrylate was determined by 1 H NMR of the D 2 O layer.

Conclusions
In conclusion, a series of new pyridine-chelated ImPy ligand precursors (3, 4a-4e, 10) were prepared, and their catalytic efficiencies for Ni mediated acrylate synthesis from ethylene and CO 2 were investigated. Additionally, py-ImPy H Ni(II)Cl 2 complexes (3), py-ImPy t-Bu Ni(II)Cl 2 complexes (4a-4e) and N-picolyl-ImPyNi(II)Cl 2 (12) were synthesized from corresponding Ag complexes (5, 6a-6e, 11) through Ag transmetalation protocol and characterized by single-crystal X-ray crystallography. X-ray structures demonstrated that the six-membered chelate rings with a fused pyridine of ImPy were more rigid and planar than those with labile picolyl units at the N atom of ImPy. Ni(II) complexes (7, 8a-8e) with strong chelates yielded up to 108% acrylate (TON 1). Catalytic activities of complex 8b was further improved (TON up to 8.4) by the addition of monodentate phosphine. Currently, we are exploring other NHC ligands in the catalytic acrylate synthesis.