Palladium-anchored donor-flexible pyridylidene amide (PYA) electrocatalysts for CO2 reduction

Abstract

The conversion of CO₂ into CO as a substitute for processing fossil fuels to produce hydrocarbons is a sustainable, carbon neutral energy technology. However, the electrochemical reduction of CO₂ into a synthesis gas (CO and H₂) at a commercial scale requires an efficient electrocatalyst. In this perspective, a series of six new palladium complexes with the general formula [Pd(L)(Y)]Y, where L is a donor-flexible PYA, N²,N⁶-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, N²,N⁶-bis(1-butylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, or N²,N⁶-bis(1-benzylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, and Y = OAc or Cl⁻, were utilized as active electrocatalysts for the conversion of CO₂ into a synthesis gas. These palladium(II) pincer complexes were synthesized from their respective H-PYA proligands using 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) or sodium acetate as a base. All the compounds were successfully characterized by various physical methods of analysis, such as proton and carbon NMR, FTIR, CHN, and single-crystal XRD. The redox chemistry of palladium complexes toward carbon dioxide activation suggested an evident CO₂ interaction with each Pd(II) catalyst. [Pd(N²,N⁶-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide)(Cl)]Cl showed the best electrocatalytic activity for CO₂ reduction into a synthesis gas under the acidic condition of trifluoracetic acid (TFA) with a minimum overpotential of 0.40 V, a maximum turnover frequency (TOF) of 101 s⁻¹, and 58% FE of CO. This pincer scaffold could be stereochemically tuned with the exploration of earth abundant first row transition metals for further improvements in the CO₂ reduction chemistry.