Intermediates of Carbon Monoxide Oxidation on Praseodymium Monoxide Molecules: Insights from Matrix-Isolation IR Spectroscopy and Quantum-Chemical Calculations

Identifying
reaction intermediates in gas-phase investigations
will provide understanding for the related catalysts in fundamental
aspects including bonding interactions of the reaction species, oxidation
states (OSs) of the anchored atoms, and reaction mechanisms. Herein,
carbon monoxide (CO) oxidation by praseodymium monoxide (PrO) molecules
has been investigated as a model reaction in solid argon using matrix-isolation
IR spectroscopy and quantum-chemical calculations. Two reaction intermediates,
OPr­(η1-CO) and OPr­(η2-CO), have
been trapped and characterized in argon matrixes. The intermediate
OPr­(η2-CO) shows an extremely low C–O stretching
band at 1624.5 cm–1. Quantum-chemistry studies indicate
that the bonding in OPr­(η1-CO) is described as “donor–acceptor”
interactions conforming to the Dewar–Chatt–Duncanson
motif. However, the bonding in OPr­(η2-CO) results
evidently from a combination of dominant ionic forces and normal Lewis
“acid–base” interactions. The electron density
of the singly occupied bonding orbital is strongly polarized to the
CO fragment in OPr­(η2-CO). Electronic structure analysis
suggests that the two captured species exhibit Pr­(III) OSs. Besides,
the pathways of CO oxidation have been discussed.