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

We provide the first observation and characterization of super electrophilic metal cations on a solid support. For Pd/SSZ-13, the results of our combined experimental (Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, high-angle annular dark-field scanning transmission electron microscopy) 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 (charge-transfer complex/ion pair with the negatively charged framework oxygens). In this contribution, we reassign the spectroscopic signatures of these species, discuss the unusual coordination environment of “naked” (ligand-free) super electrophilic Pd⁺² in SSZ-13, and their complexes with CO and NO. With CO, nonclassical, highly positive [Pd(CO)₂]²⁺ ions are formed with the zeolite framework acting as a weakly coordinating anion (ion pairs). Nonclassical 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 Pd⁺²–nitrosyl–olefin [Pd(NO)(C₂H₄)] complexes. Overall, we show that the zeolite framework can stabilize super electrophilic metal (Pd) cations and show the new chemistry of the Pd/SSZ-13 system with implications for adsorption and catalysis.