Stability and reactivity of grafted Cr(CO)₃ species on MOF linkers: A Computational study

The possibility to modulate Cr(CO)₃ properties by grafting it onto metal-organic framework (MOF) linkers of different natures has been investigated using density functional methods. MOF linkers were modeled using clusters constituted by benzene rings doubly substituted in the para position. The effect of the electron-donor or electron-acceptor nature of benzene substituents on the stability of the (η⁶-arene)Cr(CO)₃ adduct and on the shift of the CO bands has been considered. Different electron-donor (-NH₂, -CH₃, -OH, -COONa) and electron-acceptor (-F, -COOH, -CN, -CF₃) substituents have been used and the results compared with the bare benzene. C₆H ₄(COOZnOH)₂ and C₆H₄(Zn ₄O₁₃C₆H₅)² clusters have also been adopted as models of the MOF-5 benzene rings. The possibility of modulating the stability and the reactivity of Cr(CO)₃ species by grafting them to MOFs with different organic linkers was verified. In particular, this study indicates that electron-acceptor (e.g., C ₆H₄(COOH)₂) substituted MOF linkers facilitate the substitution of CO by incoming molecules, whereas the use of electron-donor ones (e.g., C₆H₄(OH)₂) would improve the stability of the Cr(CO)₃ adduct and the ring acidity. Furthermore, an almost linear dependence of the Cr(CO)₃ binding energies on the calculated structural and vibrational features of the tricarbonyl was found, suggesting that the stability of the Cr(CO)₃ adduct can be inferred experimentally from vibrational and diffraction data. In the end, on the basis of the results obtained, it was possible to successfully explain the experimental shift of the CO IR stretching features of grafted Cr(CO) ₃ on the UiO-66, CPO-27-Ni, and MOF-5 aromatic linkers and on the benzene rings of poly(ethylstyrene-co-divinylbenzene). The sign of the Δṽ_co shift with respect to C₆H ₆Cr(CO)₃ has been found to be strongly dependent on higher/lower electron density on the ring.