CO adsorption on a mixed-valence ruthenium metal-organic framework studied by UHV-FTIR spectroscopy and DFT calculations

The mixed-valence metal-organic framework [Ru₃ ^II,III(btc)₂Cl_1.5] (Ru-MOF) was synthesized by the controlled SBU approach and characterized by combined powder XRD, XPS, and FTIR methods. The interaction of CO molecules with Ru-MOF was studied by a novel instrumentation for ultra-high-vacuum (UHV) FTIR spectroscopy. The high-quality IR data demonstrate the presence of two different CO species within the framework: a strongly bonded CO showing a low-lying band at 2137 cm^-1 and a second CO species at 2171 cm^-1 with a lower binding energy. It was found that these IR bands cannot be assigned in a straightforward manner to CO molecules adsorbed on the coordinatively unsaturated Ru^II site (CUS) and Ru^III site connected to an additional Cl^- ion for charge compensation. The accurate DFT calculations reveal that the structural and electronic properties of the mixed-valence Ru-MOF are much more complex than expected. One of the Cl^- counterions could be transferred to a neighboring paddle-wheel, forming an anionic SBU blocked by two Cl^- counterions, whereas the other positively charged paddle-wheel with a Ru₂^II,III dimer exposes two "free" CUS, which can bind two CO molecules with different frequencies and binding energies.