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

Abstract

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.