Trapping of Photoluminescent Kinetic Intermediates During High-Concentration Synthesis of Non-Emissive Metal-Organic Frameworks

Abstract

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes with potential utility in gas separations, drug delivery, sensing, and catalysis. Small variations in MOF synthesis conditions can lead to a range of accessible frameworks with divergent chemical or photophysical properties. New meth-ods to controllably access phases with tailored properties would broaden the scope of MOFs that can be reliably prepared for specific applications. Herein, we demonstrate that simply increasing the reaction concentration during the solvother-mal synthesis of M2(dobdc) (M = Mg, Mn, Ni; dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate), also known as MOF-74, unexpectedly leads to trapping of a kinetic intermediate termed CORN-MOF-1 (CORN = Cornell University). In-depth spec-troscopic, crystallographic, and computational studies support that CORN-MOF-1 has a similar structure to M2(dobdc) but with partially protonated linkers and charge-balancing or coordinated formate groups in the pores. The resultant vari-ation in linker spacings causes CORN-MOF-1 (Mg) to be strongly photoluminescent in the solid state, whereas H4dobdc and Mg2(dobdc) are weakly emissive due to excimer formation. In addition, CORN-MOF-1 variants can be converted into high-quality samples of the thermodynamic M2(dobdc) phases by heating in N,N-dimethylformamide (DMF). Overall, our findings support that high-concentration synthesis provides a straightforward method to identify new kinetic MOF phas-es with different properties from known materials and to produce highly porous samples of MOFs, paving the way for the discovery and gram-scale synthesis of framework materials.

Simplifying the Synthesis of Metal–Organic Frameworks

“…An interesting question is why Zr-MOFs tend to form the same phase at low and high linker concentrations, , whereas M 2 (dobdc) materials undergo a phase transition to a new kinetic product at high reaction concentrations. , We propose that Zr- and Hf-MOFs are especially well-suited for high-concentration synthesis because their M 6 nodes are robust and well-defined and, thus, strongly direct linker coordination into specific structures, even at high concentrations. In contrast, the metal centers that form the backbone of M 2 (dobdc) materials can engage in many different coordination environments with the linkers during MOF self-assembly.…”

“…These frameworks are promising for gas storage and separations but are often difficult to produce on a gram scale, as they are typically synthesized under dilute (∼0.01 M) conditions in DMF/alcohol mixtures. We began by evaluating the synthesis of Mg 2 (dobdc) at linker concentrations varying from 0.01 to 1.50 M with stirring . PXRD analysis of the resulting materials confirmed that phase-pure Mg 2 (dobdc) was obtained at concentrations up to 0.05 M, but at 0.08 M, the MOF was contaminated with a previously unreported crystalline phase that we term CORN-MOF-1 (Mg).…”

“…Encouraged by our success, we set out to explore the high-concentration synthesis of other frameworks. One target family of MOFs is the MOF-74, CPO-27, or M 2 (dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd; dobdc 4– = 2,5-dioxido-1,4-benzenedicarboxylate) series . These frameworks are promising for gas storage and separations but are often difficult to produce on a gram scale, as they are typically synthesized under dilute (∼0.01 M) conditions in DMF/alcohol mixtures.…”

“…As discussed in Section , the attempted synthesis of Mg 2 (dobdc) at >0.1 M linker concentrations leads instead to a new framework, CORN-MOF-1 (Mg), that can subsequently be converted to Mg 2 (dobdc) by heating in DMF . This two-step route reproducibly enables the synthesis of high-quality Mg 2 (dobdc) (Langmuir surface area = 2060 m 2 /g) on ∼5 g scale and thus became our preferred approach to prepare this MOF.…”

“…As such, unintended topologies may be isolated under nonideal conditions. Nonetheless, our findings demonstrate that such kinetic phases may still serve as scalable intermediates to the desired frameworks . Overall, high-concentration synthesis represents a straightforward method to synthesize frameworks on multigram scale with reduced solvent use.…”