Nanoscale Cu(II) MOFs Formed via Microemulsion: Vibrational Mode Characterization Performed using a Combined FTIR, Synchrotron Far-IR, and Periodic DFT Approach

Abstract

Two Cu­(II) metal–organic frameworks (MOFs) were prepared on the nanoscale at room temperature using a microemulsion method, namely, [Cu3(BTC)2(H2O)3] (BTC = benzene-1,3,5-tricarboxylate), known as HKUST-1 (1), and [Cu2(OH)­(BTC)­(H2O)]·2H2O (2). Thermochemical and gas sorption properties of the microporous topologies were characterized by mid- and far-infrared vibrational spectroscopy, supported by periodic density functional theory calculations. The mid-infrared profile of 1 appeared altered in response to gas sorption under variable temperature and pressure conditions. Vibrational mode analysis indicated the most sensitive infrared peaks were associated with the internal vibrations of organic linker moieties indirectly coupled to the Cu­(II)–gas coordination site, activated by a lowered symmetry induced by guest interactions. Synchrotron far-infrared spectroscopy was shown to be a useful diagnostic for the microstructure of 1 and 2 where different temperature dependences were displayed in the low-frequency region. The loss of residual water during the activation of 2 at elevated temperature coincides with peaks indicative of free paddle-wheel moieties emerging in the far-IR spectra. As demonstrated for both materials 1 and 2, vibrational mode analysis was effective in screening MOF materials for their propensity toward gas uptake and, inversely, the diffusion of guest species such as adsorbed water from the microporous environments.

A high-efficient electrochemical synthesis of a low-nuclearity copper-cluster-based metal–organic framework for the size-selective oxidation of alcohols

“…S4†) of the COOH group in the free ligand H 4 L indicates the successful coordination of the organic ligands H 4 L with the Cu( ii ) centres. 37 Similarly, the PXRD of H-2 is consistent with the simulation, indicating the synthesis of highly crystalline H-2 . H-1 , H-2 , and H-1e have been successfully synthesized under the conditions of solvothermal and electrochemical synthesis (Fig.…”

Vibrational mode analysis of hydrogen-bonded organic frameworks (HOFs): synchrotron infrared studies

“…The observed rigidity of the T d subunits therefore constrains the vibrational degrees-of-freedom of the system, which results in the distinctive far-IR profiles for each framework where peaks associated with T d hindered rotational and deformation are populated. In addition, previous studies 36 have shown that, beyond acting as a diagnostic between HOF structures, the far-IR region is also highly sensitive to the changes in the (i) physical environment (temperature and pressure), (ii) morphology and pore/channel architecture, and (iii) solvent environment for prototype MOF systems. Similar experiments would be of high relevance to the HOF materials featured in this study, with particular focus toward the characterisation of water-of-hydration network within the channels through variable temperature and solvent substitution studies.…”

“…In coordination polymers and molecular crystals, these low frequency vibrational modes have been shown to be highly sensitive to the morphology of the network. 36,52,53 For example, it is observed that band profiles are altered with (i) change of phase, (ii) change of temperature and pressure conditions, and (iii) through the effect of retained solvent -in this case waterwhich can all act to perturb the equilibrium frequency of a given vibrational mode.…”

“…35 However, the far-IR is a particularly useful region to investigate physicochemical properties of molecular crystals and three dimensional materials. 36,37 Here, lattice (phonon) modes generate prominent absorption features at low frequency, which are often diagnostic to a specific structure. For porous materials, it has been shown that the topology of pores and channels under changing temperature, pressure, solvent, and light exposure can be examined with high sensitivity in the far-IR, as crystal symmetry and morphology is often altered in response to these stimuli.…”

“…38,39 In the specific case of MOFs and HOFs, such lattice modes are often coupled to low-lying intramolecular modes arising from the deformation and torsional motion of organic linker and aromatic spacer units. 36 As well as information on the neat framework as synthesized or after activation, IR studies have also provided insight toward host-guest interactions, [40][41][42] where the presence of solvent or guest molecule -typically bound to specific sites on the pore surface -acts to perturb the equilibrium vibrational and lattice frequencies of the host framework.…”