Soft Mode Metal-Linker Dynamics in Carboxylate MOFs Evidenced by Variable-Temperature Infrared Spectroscopy

Abstract

Through comprehensive analysis of carboxylatebased metal−organic frameworks (MOFs), we present general evidence that challenges the common perception of MOF metallinker bonds being static. Structural dynamics in MOFs, however, typically refers to the "breathing" behavior of cavities, where pores open and close in response to guest molecules, and to the transient binding of guest molecules, but dynamic bonding would explain important MOF phenomena in catalysis, postsynthetic exchange, negative thermal expansion, and crystal growth. Here, we demonstrate, through use of variable-temperature diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS) aided by ab initio plane wave density functional theory, that similar evidence for melting behavior in zeolitic imidazolate frameworks (ZIFs), i.e., reversible metal-linker bonding, driven by specific vibrational modes, can be observed for carboxylate MOFs by monitoring the redshifts of carboxylate stretches coupled to anharmonic metal-carboxylate oscillators. To demonstrate the generality of these findings, we investigate a wide class of carboxylate MOFs that includes iconic examples with diverse structures and metal-linker chemistry. As the very vibrations invoked in ZIF melting but heretofore unobserved for carboxylate MOFs, these metal-linker dynamics resemble the ubiquitous soft modes that trigger important phase transitions in diverse classes of materials while offering a fundamentally new perspective for the design of next-generation metal−organic materials.

Capture of toxic gases in MOFs: SO2, H2S, NH3 and NOx

“…For example, can the polysulphide formation be a consequence of reversible metal-ligand bonding upon the adsorption of H 2 S? Recently Brozek and co-workers 73 A variety of MOFs, as well as classic materials such as zeolites, activated carbon and metal oxides, have been studied for the adsorption of these toxic gases, where some of these are summarized in Table 3. Noteworthy, the comparison of materials for H 2 S capture is sometimes challenging due to the difference in the experimental conditions (e.g., ue gas concentration).…”

“…111 These host-guest charge transfer results were experimentally determined by EPR and corroborated by bond valence sum (BVS) calculations, nding as well that the charge transfer between adsorbed NH 3 molecules and the V 4+ centre can only occur when the loading of NH 3 is sufficiently high so that a predominant occupancy of the N site, which is located close to the metal chain, is reached to initiate the redox reaction. Finally, such mechanisms could also be investigated by the approach of metal-ligand dynamics as recently proposed by Brozek and co-workers, 73 to fully understand the interaction of NH 3 and the hydroxo functional group in the MFM-300(M) family.…”

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

“…The prominence of absorption tails at higher temperatures therefore cannot be attributed to a wellordered structural change. We reasoned that the dynamic metal-linker bonding of MUV-10(Ca), as detailed previously, 60 could give rise to the absorption tail by creating mid-gap orbitals that arise from disorder around the MOF node. A thermally induced shift of the bonding equilibrium from "tight" to "loose" binding is also consistent with the reversibility evidenced in Figure 6.…”

“…In a recent study, variable-temperature IR data provided evidence for dynamic metal-linker bonding in MUV-10(Ca) and other common MOFs. 60 At higher temperatures, the metalcarboxylate linkages thermally populate a "loose" configuration that may involve an array of disordered geometries. The energetic degeneracy of the loose geometries could create a high density of mid-gap orbitals that would produce low-energy absorption, akin to Urbach tailing.…”

Recent Advances in Functional Carbon Quantum Dots for Antitumour

“…105 The intensity distribution of the original light source can be calculated by Fourier transform of the interference function on a computer. 106 Compared with traditional spectrometers, FTIR has the following advantages: fast scanning speed, high resolution, high sensitivity and wide spectral range. 107 Zhang et al prepared layered double hydroxide-combined CQDs (LDH-carbon quantum dots) capable of adsorbing methyl blue.…”