LAUSR

Abstract Redox chemistry of nickel centers in high-silica ZSM-5 and BEA zeolites was investigated by a combined use of temperature-programmed reduction (TPR), electron paramagnetic resonance (EPR), and infrared (FTIR) spectroscopic techniques supported by quantum chemistry methods. The isolated nickel and nickel-oxo centers were identified and quantified by means of EPR using CO and NO probes and by IR using CO as a probe molecule. The nickel-oxo species gave rise to the nickel(I) centers upon reduction in CO. Their interaction with N2 under low temperature led to the formation of nickel(I)–dinitrogen adducts. Attribution of the IR bands and identification of the structures of dinitrogen adducts were based on the coadsorption experiments of 14N2 and 15N2 isotopes and complementary density functional theory (DFT) calculations. Because of the presence of a mixed ligand (14N215N2) adduct, a structural dichotomy of possible formation of the geminal Ni(I)–(N2)2 and two coupled vicinal mono-adducts N2–Ni(I)⋅⋅⋅Ni(I)–N2 was resolved. The isolated Ni(I)–N2 and geminal Ni(I)–(N2)2 adducts were identified with confidence. A mechanism of N2 bonding was accounted for by natural orbitals for chemical valence (NOCV) population analysis. Charge flow molecular channels due to the σ donation and π back-donation effects were established in a quantitative way. This allowed for unraveling the molecular background of the observed bathochromic shift of the stretching vibrations of all dinitrogen adducts.