Interaction of diatomic molecules with nickel ions inside the channels of high silica zeolites : an EPR and DFT study

Interaction of CO, NO, and O2 diatomics with NiII and NiI ions dispersed in ZSM-5 zeolite was investigated by electron paramagnetic resonance (EPR) spectroscopy and density functional theory (DFT) modelling. The resulting adducts NiI-CO, NiII-NO, and NiI-O2 were identified based on g-tensor parameters, obtained by computer fitting of the powder EPR spectra, and next ascertained by parallel relativistic DFT calculations of the corresponding g-tensor values. The structures of the NiI-CO, NiII-NO, and NiI-O2 complexes were obtained by geometry optimization with the Kohn- -Sham method. Binding of the diatomics was discussed in terms of the spin-pairing and electron density transfer events. Interaction of CO with NiI cations led to the pronounced change in the coordination and electronic structure of the NiI center, however, no redox processes were observed in agreement with the “innocent” nature of CO as a ligand. On the contrary, strong electron and spin density redistribution was observed upon NO and O2 interaction (“non-innocent ligands”) leading to the formation of the bound nitrosonium NOδ+ and superoxo O2 – species, respectively.