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.