On solvent-mediated stability of palladium clusters: Fundamental insights into the role of solvent structure and environment

Understanding the stability of small transition-metal clusters in solution is critical for their catalytic applications, yet the role of the solvent environment remains insufficiently resolved. In this study, classical molecular dynamics simulations are used to examine the solvation, structural organization, and agglomeration behavior of palladium clusters (Pd3 and Pd4) in water, N-methyl-2-pyrrolidone, and their solution. The results reveal two distinct stabilization mechanisms: water forms hydrogen-bonded solvation shells that are relatively diffuse and dynamic, facilitating rapid cluster agglomeration, whereas N-methyl-2-pyrrolidone stabilizes the clusters through strong coordination of its carbonyl oxygen, generating dense and well-defined solvation layers that inhibit aggregation. In their mixed solvents, N-methyl-2-pyrrolidone preferentially occupies the first solvation shell, displacing water to outer regions where it interacts primarily with N-methyl-2-pyrrolidone rather than the cluster. This redistribution produces a more stable solvation environment than the pure solvents. Cluster size further influences this behavior. Pd4 accommodates a denser coordinating shell and shows greater resistance to aggregation than Pd3. Solvent composition is shown to provide a means of tuning cluster evolution, with water promoting agglomeration and fragmentation processes and N-methyl-2-pyrrolidone retarding these effects, thereby enabling control over cluster size and stability. These findings provide molecular insight into the solvent-mediated stabilization of metal clusters and establish a framework for rational design of solvent environments to control their size.