Solvation and stabilization of palladium nanoparticles in phosphonium-based ionic liquids: a combined infrared spectroscopic and density functional theory study

Analysis of infrared spectra of palladium nanoparticles (NPs) immersed in the tri-tert-butyl-R-phosphonium-based ionic liquids (ILs) demonstrates that both cations and anions of the ILs interact with the NPs. According to quantum-chemical simulations of these interactions, the binding energy of anions to the Pd6 cluster, taken as a minimal-size model of the NPs, increases from ∼6 to ∼27 kcal mol−1 in the order [PF6]− ≈ [BF4]− < [Tf2N]− < [OTf]− < [Br]− ≪ [TFA]−. In contrast, the binding energy for all types of the [But3PR]+ cations slightly varies at about ∼22 kcal mol−1 only moderately depending on the choice of the R moiety (n-pentyl, 2-hydroxyethyl, 2-methoxyethyl, 2-ethoxy-2-oxoethyl). As a result, the energies of interaction between a Pd6 cluster and various ion pairs, formed by the abovementioned counter-ions, follow the order found for the anions and vary from ∼24 to ∼47 kcal mol−1. These values are smaller than the energy of addition of a Pd atom to a Pdn cluster (∼58 kcal mol−1), which suggests kinetic stabilization of the NPs in phosphonium-based ILs rather than thermodynamic stabilization. The results are qualitatively similar to the trends found earlier for interactions between palladium clusters and components of imidazolium-based ILs, in spite of much larger contributions of the London dispersion forces to the binding of the [But3PR]+ cations to the cluster (up to 80%) relative to the case of 1-R-3-methylimidazolium cations (up to 40%).