Computational Exploration of the Efficacy of Fe(II)PNN Versus Fe(II)NNN Pincer Complexes in the Hydrogenation of Carbon Dioxide to Methanol

Abstract

The efficacy of the computationally designed Fe(II) phosphorus−nitrogen−nitrogen (PNN) and Fe(II) nitrogen−nitrogen−nitrogen (NNN) pincer complexes in the hydrogenation of carbon dioxide to methanol in an aqueous medium at 298.15 K is thoroughly studied. The conversion of CO2 to methanol follows a cascade catalytic cycle comprising both metal–ligand cooperative (MLC) and noncooperative (NC) mechanistic pathways. Both of the catalysts follow the NC pathway for the formic acid formation and the MLC pathway for the formaldehyde formation. However, methanol formation by the Fe(II)PNN complex follows the NC pathway and by the Fe(II)NNN complex follows the MLC pathway. The formaldehyde formation step is crucial in methanol formation, and this step requires the −NH proton. The computationally designed catalysts exhibit better catalytic performance in the hydrogenation of CO2 to methanol. The −NH proton on the pincer complexes is a prerequisite for the hydrogenation of CO2 to methanol. Fe(II)NNN and Fe(II)PNN pincer complexes are found to be equally efficient in CO2 to formaldehyde formation. However, the Fe(II)PNN complex is more energy efficient than the Fe(II)NNN complex in the reduction of formaldehyde to methanol. These catalysts could be promising candidates to provide a low-cost, environmentally benign, and catalytically efficient protocol to convert CO2 into methanol.