Abstract
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 (CO2), 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)Cl2 (7, 8a–8e) and N-picolyl-ImPyNi(II)Cl2 (12) were synthesized via transmetalation protocol from silver(I) complexes, py-ImPyAgCl (5, 6a–6e) and N-picolyl-ImPyAgCl (11). X-ray diffraction 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)Cl2 complexes (8a–8e) with tert-butyl substituents exhibited noticeable catalytic activity in acrylate synthesis from ethylene and CO2 (up to 108% acrylate). Interestingly, the use of additional additives including monodentate phosphines increased catalytic activity up to 845% acrylate (TON 8).
Highlights
The synthesis of acrylic acid derivatives through the C–H carboxylation of ethylene with carbon dioxide (CO2 ) 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
Turning over the proposed catalytic cycle was impeded by the high energy barrier for β-hydride elimination of stable five-membered nickelalactones and the endergonic thermodynamic feature of the overall reaction
There has been one example where NHC ligands are used in the moderate yields (67%–85%), through iminopyridine formation from 2,2′-bipyridine-6-carbaldehyde synthesis of acrylate from CO2 /ethylene [47]
Summary
The synthesis of acrylic acid derivatives through the C–H carboxylation of ethylene with carbon dioxide (CO2 ) 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 (CO2 ) [12]. Catalysts 2020, 10, 758 electron-rich Ni(0) complexes ligated with 2,20 -bipyridine (bpy), bis(dicyclohexylphosphino)ethane (DCPE), 1,8-diazabicyclo(5,4,0)undec-7-ene (DBU), and pyridyl-phosphine ligands These stoichiometric reaction results inspired researchers to develop a catalytic version; the development of catalytic processes turned out to be highly challenging [21,22,23,24,25,26,27]. Turning over the proposed catalytic cycle was impeded by the high energy barrier for β-hydride elimination of stable five-membered nickelalactones and the endergonic thermodynamic feature of the overall reaction
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