Using Predefined M₃(μ₃-O) Clusters as Building Blocks for an Isostructural Series of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have attracted much attention in the past decade owing to their unprecedented internal surface areas, tunable topologies, designable surfaces, and various potential applications. One bottleneck in the field regarding MOF synthesis is controlling the metal-containing secondary building unit (SBU) incorporated into the structure. In this work we report the synthesis and characterization of five trimeric [M₃(μ₃-O)(CH₃CO₂)₆]^x clusters (where M = Fe³⁺, Cr³⁺, Fe³⁺/Cr³⁺, Fe³⁺/Co²⁺, or Fe³⁺/Ni²⁺ and x = +1 or 0). The monocarboxylate capping ligand, acetate in this case, readily undergoes exchange with several difunctional counterparts, including 1,4-benzenedicarboxylic acid (H₂-BDC) and biphenyl-4,4'-dicarboxylic acid (H₂-BPDC), for the formation of an isostructural series of MOFs, several of which are newly reported (for M = Fe³⁺/Cr³⁺, Fe³⁺/Co²⁺, and Fe³⁺/Ni²⁺) and show excellent CO₂ adsorption properties. In this report, a host of techniques including NMR, ICP, and ESI-MS are used to probe the ligand exchange process and composition of the SBUs, and XAS is used to monitor the Fe³⁺ and Cr³⁺ environment throughout the reactions, giving strong evidence that the clusters stay intact throughout the MOF synthesis. This work reveals that predefined SBUs is an effective means to create metal-substituted analogues of known frameworks. Further, CO adsorption and in situ IR are used to probe accessibility of the metals after solvent removal. We show for the first time that the incorporation of the neutral clusters, containing weaker Lewis acids like Ni²⁺ and Co²⁺, can promote the formation of open metal sites in the MOF frameworks, structural features known to enhance the binding energy of small guest molecules like CO₂.