Site-Dependent Substitutions in Mixed-Metal Metal-Organic Frameworks: A Case Study and Guidelines for Analogous Systems

Complex architectures are often found among metal-organic framework (MOF) compounds. The mixed-metal approach to this type of material offers an additional degree of structural complexity, and potential tunability of their properties, which remains largely unexplored. We present an in-depth investigation of the crystal chemistry of mixed-metal MOFs based on succinate linkers (C4H4O4) and having the general formula (M'M1-x(x))(5)(OH)(2)(C4H4O4)(4) with M'/M = Mn/Co, Fe/Co, and Mn/Fe. The distribution of the metallic elements over three crystallographic sites throughout the different substitutions is finely characterized by resonant contrast diffraction (RCD) experiments corroborated by neutron diffraction (ND) measurements. We observe a size-effect in the filling of the oxygen octahedra, leading to the existence of compounds in which a partial order of the cations over the different metallic sites exists for some compositions of the Co/Mn solid solution. This points out the existence of complex structural phenomena potentially able to influence the physical behavior of such phases and that might, so far, have been overlooked in MOFs. In order to facilitate future studies on mixed-metal MOFs, we consider the possibility of using conventional single-crystal X-ray diffraction (SCXRD) to locate cations of close electronic densities in such cases. The comparison with the results from dedicated measurements based on synchrotron (RCD) and neutron (ND) radiations indicates guidelines for the use of laboratory SCXRD to address mixed-metal MOFs where metal distribution is fundamental to tuning physical properties.