Dalton Transactions p. 7984 - 7994 (2021)
Update date:2022-08-16
Topics:
Mayilmurugan, Ramasamy
Muthuramalingam, Sethuraman
Velusamy, Marappan
The development of molecular catalysts for the activation and conversion of atmospheric carbon dioxide (CO2) into a value-added product is a great challenge. A series of nickel(ii) complexes, [Ni(L)(CH3CN)3](BPh4)2,1-4of diazepane based ligands, 4-methyl-1-[(pyridin-2-yl-methyl)]-1,4-diazepane (L1), 4-methyl-1-[2-(pyridine-2-yl)ethyl]-1,4-diazepane (L2), 4-methyl-1-[(quinoline-2-yl)-methyl]-1,4-diazepane (L3) and 1-[(4-methoxy-3,5-dimethyl-pyridin-2-yl)methyl]-4-methyl-1,4-diazepane (L4), have been synthesized and characterized as catalysts for the activation of atmospheric CO2. The single-crystal X-ray structure of1shows a distorted octahedral geometry with acis-β configuration around the NiN6coordination sphere. All the complexes are used as catalysts for the conversion of atmospheric CO2and epoxides into cyclic carbonates at 1 atmosphere (atm) pressure and in the presence of Et3N. Catalyst4was found to be the most efficient catalyst and showed a 31% formation of cyclic carbonates with a TON of 620 under 1 atm air as the CO2source. This yield was enhanced to 94% with a TON of 1880 under 1 atm pure CO2gas and it is the highest catalytic efficiency known for nickel(ii)-based catalysts. Catalyst4enabled the transformation of a wide range of epoxides (eight examples) into corresponding cyclic carbonates with excellent selectivity (>99%) and yields of 59-94% and 11-31% under pure CO2and atmospheric CO2, respectively. The catalytic efficiency is strongly influenced by the electronic nature of the complexes. The CO2fixation reactions without an epoxide substrate led to the formation of the carbonate bridged dinuclear nickel(ii) complexes [(LNiII)2CO3](BPh4)21a-4a, which are speculated as catalytically active intermediates. The formation of these species was accompanied by the formation of new absorption bands around 592-681 nm and was further confirmed by the ESI-MS and IR spectral studies. The molecular structures of these carbonate-bridged key intermediates were determined by X-ray analysis. The structures contain two Ni2+-centers bridgedviaa carbonate ion that originated from CO2. Distorted square pyramidal geometries are adopted around each Ni(ii) center. All these results support that CO2fixation reactions occurviaCO2-bound nickel key intermediates.
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