CO Adsorption on a Mixed-Valence Ruthenium Metal–Organic Framework Studied by UHV-FTIR Spectroscopy and DFT Calculations

The mixed-valence metal–organic framework [Ru3II,III(btc)2Cl1.5] (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 RuII site (CUS) and RuIII 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...