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

Abstract

The mixed-valence metal−organic framework [Ru 3 II,III -(btc) 2 Cl 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 Ru 2 II,III dimer exposes two "free" CUS, which can bind two CO molecules with different frequencies and binding energies.

Adsorbate Interactions in Metal Organic Frameworks Studied by Vibrational Spectroscopy

“…Noei et al, in a recent study combining IR absorption spectroscopy and Density Functional Theory (DFT), showed that CO adsorbed at different sites in a mixed valence ruthenium (II,III) analogue to HKUST-1 (Cu 3 btc 2 ) can be assigned, and the binding strength determined. [41] Gaseous CO stretch mode is located at 2143 cm À1 , and adsorption of CO on metal cations via electrostatic interaction results in a blue shift of the CO stretch mode with respect to the gas phase. This blue shift is caused by the polarization of the CO molecule.…”

Ruthenium Metal–Organic Frameworks with Different Defect Types: Influence on Porosity, Sorption, and Catalytic Properties

“…As these sites were created by the incorporation of the 5‐OH‐ip linkers in 1 a – 1 d , we selected 1 c and 1 d as two representative samples that both exhibit high degrees of 5‐OH‐ip incorporation but quite different contents of Ru 2+ and Ru δ + . As shown in Figure , on CO 2 dosing for both the parent Ru‐MOF and Ru‐DEMOF 1 c , an intense and broad IR band appears at 2336 cm −1 , which is assigned to the asymmetric stretching mode ν as (COO) of physisorbed CO 2 binding linearly at various Ru sites . The weak band at 2273 cm −1 indicates the presence of a minority CO 2 species originating probably from the adsorption of a small amount of 13 CO 2 with an expected isotopic shift of 1.03.…”

Multi‐Spectroscopic Interrogation of the Spatial Linker Distribution in Defect‐Engineered Metal–Organic Framework Crystals: The [Cu3(btc)2−x(cydc)x] Showcase

“…showed that [Cu 3 (btc) 2 ] (BTC=1,3,5‐benzenetricarboxylate) can be doped with different concentrations of low‐coordinating linkers, such as 5‐cyano‐1,3‐benzenedicarboxylate (CYDC), which leads to a particular variety of MTV‐MOF in which additional Cu + sites and mesopores are formed as compared with the parent, undoped [Cu 3 (btc) 2 ] material. This has also been shown to be the case for its ruthenium analogue, with which very active olefin hydrogenation and dimerization catalysts could be synthesized …”

“…Consequently, the redox properties of the coordinatively unsaturated sites (CUSs), if already present, can be altered (e.g., reduced oxidation states) . Representative examples of this strategy for the HKUST‐1 topology (a MOF consisting of copper with 1,3,5‐benzenetricarboxylate linkers) have been reported by Baiker and Fischer and their co‐workers, in which the presence of mixed‐valence paddle‐wheel units with Cu 2+ /Cu + centers were observed. Indeed, Fang et al.…”