Revealing Structural Transformations of Ligand Detachment in Cu Clusters: Insights from Ion Mobility Spectrometry

Ligands play a pivotal role in stabilizing clusters and tuning their properties. However, during the exploration of the cluster's intrinsic properties, ligands often experience partial detachment, which imparts unique characteristics and enhances their functional flexibility. While previous studies have largely focused on macroscopic changes in composition and size, the atomic-level structural evolution of clusters upon ligand loss remains insufficiently explored, with direct experimental evidence being scarce. In this study, by utilizing Cu₆HL₆ and its fragment Cu₄L₄ (L = 2-mercaptobenzimidazole) clusters as pertinent model systems, we systematically investigated the structure transformation of the stepwise removal of one and two ligands, leveraging collision induced dissociation (CID) with traveling wave ion mobility-mass spectrometry (TWIM-MS) as a powerful analytical approach. Based on a detailed analysis of the collision cross-section combined with <i>ab initio</i> molecular dynamics (AIMD), it was found that the detachment of a single ligand can lead to a significant distortion of the Cu₆ core structure. Further ligand loss subsequently induced a reorganization of the Cu₆ core, resulting in a reshaped configuration. Moreover, for a smaller Cu₄L₄ cluster, it also experienced distortion upon the loss of a single ligand. Interestingly, this process resulted in the formation of two distinct Cu₄L₃ isomers, which could be attributed to the repulsion between the ligands of charged Cu₄L₃ in the gas phase. While it was generally assumed that ligand removal has little impact on the nanocluster structure, our results offer new atomic-level insights into the structural dynamics associated with ligand detachment.