Kinetic Control of Interpenetration in Fe–Biphenyl-4,4′-dicarboxylate Metal–Organic Frameworks by Coordination and Oxidation Modulation

Abstract

Phase control in the self-assembly of metal-organic frameworks (MOFs) is often a case of trial and error; judicious control over a number of synthetic variables is required to select the desired topology and control features such as interpenetration and defectivity. Herein, we present a comprehensive investigation of self-assembly in the Fe-biphenyl-4,4′-dicarboxylate system, demonstrating that coordination modulation can reliably tune between the kinetic product, non-interpenetrated MIL-88D(Fe), and the thermodynamic product, twofold interpenetrated MIL-126(Fe). Density functional theory simulations reveal that correlated disorder of the terminal anions on the metal clusters results in hydrogen-bonding between adjacent nets in the interpenetrated phase and is the thermodynamic driving force for its formation. Coordination modulation slows self-assembly and therefore selects the thermodynamic product MIL-126(Fe), while offering fine control over defectivity, inducing mesoporosity, but electron microscopy shows MIL-88D(Fe) persists in many samples despite not being evident by diffraction. Interpenetration control is also demonstrated using the 2,2′bipyridine-5,5′-dicarboxylate linker; it is energetically prohibitive for it to adopt the twisted conformation required to form the interpenetrated phase, although multiple alternative phases are identified due to additional coordination of Fe cations to its N-donors. Finally, we introduce oxidation modulation-the use of metal precursors in different oxidation states to that found in the final MOF-to kinetically control self-assembly. Combining coordination and oxidation modulation allows the synthesis of pristine MIL-126(Fe) with BET surface areas close to the predicted maximum for the first time, suggesting that combining the two may be a powerful methodology for the controlled self-assembly of high-valent MOFs. linkers used in this study. b) Packing structure of the twofold interpenetrated MIL-126(Fe), with the distinct nets coloured red and blue. Redrawn from CCDC deposition MIBMER. 14 c) Packing structures, viewed down the c axis, of open and closed MIL-88D(Fe), generated from simulated structures (not to scale), C: grey; O: red; Fe: orange spheres; H atoms removed for clarity.

Exploring and expanding the Fe-terephthalate metal–organic framework phase space by coordination and oxidation modulation

“…Our previous work with Fe-BPDC MOFs indicated increased modulator content resulted in isolation of the thermodynamic product over the kinetic one, likely by inhibiting rapid nucleation of the kinetic phase through coordinative competition in precursor solutions. 34 Here, increased acetic acid initially favours MIL-88B(Fe), the kinetic product, likely as acetic acid competes with Clfor coordination to Fe, disfavouring initial formation of [FeCl4]and thus MOF-235(Fe) at shorter reaction times. As the reaction proceeds, the thermodynamic product, MOF-235(Fe) is the result.…”

“…H atoms removed for clarity. We have previously shown 34 that control over phase space in Fe-MOFs connected by the extended biphenyl-4,4′-dicarboxylate (bpdc) linker can be exerted by modulated self-assembly. 35 Using both coordination modulation, the addition of monotopic modulators that mimic the organic ligands, and oxidation modulation, utilising metal starting materials in different oxidation states to those in the product, it is possible to exert kinetic control over self-assembly and select either the non-interpenetrated MIL-88D(Fe) kinetic product or the two-fold interpenetrated MIL-126(Fe) polymorph that is the thermodynamic product.…”

“…35 Using both coordination modulation, the addition of monotopic modulators that mimic the organic ligands, and oxidation modulation, utilising metal starting materials in different oxidation states to those in the product, it is possible to exert kinetic control over self-assembly and select either the non-interpenetrated MIL-88D(Fe) kinetic product or the two-fold interpenetrated MIL-126(Fe) polymorph that is the thermodynamic product. 34 Coordination modulation has also been used to control the physical properties of Fe-MOFs, such as particle morphology [36][37][38] and size. 39,40 Herein, we apply coordination and oxidation modulation to the synthesis of Fe-terephthalate MOFs, a much more complex system, allowing mapping of the phase space and simple, reproducible isolation of individual phases.…”

Assessing Crystallisation Kinetics of Zr Metal–Organic Frameworks through Turbidity Measurements to Inform Rapid Microwave‐Assisted Synthesis

“…Our analogous work with Fe MOFs has shown that modulation can induce formation of the thermodynamic over the kinetic product, which may explain the low temperature synthesis of MIL‐140A in this case. Microwave heating has also previously been proposed to selectively produce kinetic phases rather than thermodynamic phases; synthesis of MIL‐53(Cr) and MIL‐101(Cr), both formed from chromium precursors and terephthalic acid, produces mixed phases via conventional heating, while microwave heating selectively produces phase‐pure MIL‐101(Cr), the kinetically favoured product, most likely due to the faster kinetics of nucleation and crystallisation …”

Exploring and Expanding the Fe-Terephthalate Metal-Organic Framework Phase Space by Coordination and Oxidation Modulation

“…35 Using both coordination modulation, the addition of monotopic modulators that mimic the organic ligands, and oxidation modulation, utilising metal starting materials in different oxidation states to those in the product, it is possible to exert kinetic control over self-assembly and select either the non-interpenetrated MIL-88D(Fe) kinetic product or the two-fold interpenetrated MIL-126(Fe) polymorph that is the thermodynamic product. 34 Coordination modulation has also been used to control the physical properties of Fe-MOFs, such as particle morphology [36][37][38] and size. 39,40 Herein, we apply coordination and oxidation modulation to the synthesis of Fe-terephthalate MOFs, a much more complex system, allowing mapping of the phase space and simple, reproducible isolation of individual phases.…”