Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

The results of our combined experimental (FTIR, XPS, HAADF-STEM, EXAFS) and density functional theory study reveal that Pd ions in zeolites, previously identified as Pd⁺³ and Pd⁺⁴, are in fact present as super electrophilic Pd⁺² species (ion pairs with the negatively charged framework oxygens). In this contribution we re-assign the spectroscopic signatures of these species, discuss the unusual coordination environment of “naked” (ligand-free) Pd⁺² in SSZ-13, and their complexes with CO and NO. With CO, non-classical, highly positive [Pd(CO)₂]²⁺ ions are formed with the zeolite framework acting as a weakly coordinating anion (ion pairs). Non-classical carbonyl complexes also form with Pt⁺² and Ag⁺ in SSZ-13. The Pd⁺²(CO)₂ complex is remarkably stable in zeolite cages even in the presence of water. Dicarbonyl and nitrosyl Pd⁺² complexes, in turn, serve as precursors to the synthesis of previously inaccessible Pd⁺²-carbonyl-olefin [Pd(CO)(C₂H₄)] and -nitrosyl-olefin [Pd(NO)(C₂H₄)] complexes. Overall, we provide novel insight into the interactions of divalent metals with the zeolite framework, and show the new chemistry of Pd/SSZ-13 system with implications for adsorption and catalysis.