Effects of Pyridine Ligands on the Structure and Reactivity of Cu₃O⁺ Clusters toward CO₂

Understanding the ligand effects of organic ligand-protected copper clusters toward CO₂ is a prerequisite for improving their rational design for CO₂ capture and conversion. In this work, the regulatory roles of pyridine ligands on the structure of Cu₃O⁺ in Cu₃OPy_n⁺ (n = 0-3) clusters and their reactivity toward CO₂ were investigated through a combination of mass spectrometry and density functional theory (DFT) calculations. Experimental results reveal that pyridine ligands significantly modulate CO₂ adsorption capacity and rate: the Cu₃O⁺ cluster adsorbs up to three CO₂ molecules; Cu₃OPy⁺ and Cu₃OPy₂⁺ adsorb a maximum of two and one, respectively; and Cu₃OPy₃⁺ shows no adsorption. Notably, the pyridine-ligated Cu₃OPy_1-2⁺ clusters exhibit reactivity approximately 2 orders of magnitude higher than the bare Cu₃O⁺ cation, with Cu₃OPy⁺ being the most reactive one among all four Cu₃OPy_0-3⁺ clusters. DFT calculations indicate that the ligand-induced enhanced reactivity originates from the increased polarity, enlarged effective collision cross section (CCS), and added vibrational degrees of freedom (DOF) rather than CO₂ adsorption energy or charge transfer. These conclusions were further supported by reactions between CuPy_0-3⁺ clusters and CO₂. This study revealed the ligand effects on the structures and reactivity of copper-based clusters toward CO₂ at the atomic level, providing useful insights for the design of efficient ligand-protected cluster catalysts for CO₂ capture and conversion.