Incorporation of metallocenes into the channel structured Metal–Organic Frameworks MIL-53(Al) and MIL-47(V)

Abstract

A selection of metallocene inclusion compounds with channel structured MOFs (MOF = Metal–Organic Framework) were obtained via solvent-fee adsorption of the metallocenes from the gas-phase. The adsorbate structures ferrocene_0.5@MIL-53(Al) (MIL-53(Al) = [Al(OH)(bdc)]_n with bdc = 1,4-terephthalate), ferrocene_0.25@MIL-47(V) (MIL-47(V) = [V(O)(bdc)]_n), cobaltocene_0.25@MIL-53(Al), cobaltocene_0.5@MIL-47(V), 1-formylferrocene_0.33@MIL-53(Al), 1,1′dimethylferrocene_0.33@MIL-53(Al), 1,1′-diformylferrocene_0.5@MIL-53(Al) were determined from powder X-ray diffraction data and were analyzed concerning the packing and orientation of the guest species. The packing of the ferrocene guest molecules inside MIL-47(V) is significantly different compared to MIL-53(Al) due to the lower breathing effect and weaker hydrogen bonds between the guest molecules and the host network in the case of MIL-47(V). The orientation of the metallocene molecule is also influenced by the substituents (CH₃ and CHO) at the cyclopentadienyl ring and the interaction with the bridging OH group of MIL-53(Al). The inclusion of redox active cobaltocene into MIL-47(V) leads to the formation of a charge transfer compound with a negatively charged framework. The reduction of the vanadium centers is stoichiometric. The resulting material is a mixed valence compound with a V³⁺/V⁴⁺ ratio of 1 : 1. The new compounds were characterized via thermal gravimetric analysis, infrared spectroscopy, solid state NMR, and differential pulse voltammetry. Both systems are 1D-channel pore structures. The metallocene adsorbate induced breathing effect of MIL-53(Al) is more pronounced compared to MIL-47(V), this can be explained by the different bridging groups between the MO₆ clusters.