Ammonia-modified Co(ii) sites in zeolites: IR spectroscopy and spin-resolved charge transfer analysis of NO adsorption complexes

Abstract

IR spectroscopic studies and quantum chemical modeling (aided by the analysis of charge transfer processes between co-adsorbed ammonia and the Co(II)–NO adduct) evidence that donor ammonia molecules, ligated to extraframework Co²⁺ centers in zeolites, vitally affect the strength of the N–O bond. Calculations indicate that versatility of ammine nitrosyl complexes, differing in the number of NH₃ ligands as well as in the geometry and electronic structure of the Co–N–O unit (showing variable activation of NO) may co-exist in zeolite frameworks. However, only combined analysis of experimental and calculation results points to the adducts with three or five NH₃ coligands as decisive. The novel finding concerning the interpretation of discussed IR spectra is the assignment of the most down-shifted bands at 1600–1615 cm⁻¹ to the N–O stretch in the singlet [Co(NH₃)₃(NO)]²⁺ adduct, in place of tentative ascription to pentaammine adducts. Theory indicates also that the Co(II) center (with manifold of close-lying electronic and spin states) acts as a tunable electron donor where the spin state may open or close specific channels transferring electron density from the donor ligands (treated as the part of environment) to the NO molecule.