Postsynthetic Ligand Exchange Research Articles

Enhancing CO2 Separation Ability of a Metal–Organic Framework by Post‐Synthetic Ligand Exchange with Flexible Aliphatic Carboxylates

A series of porous metal-organic frameworks having flexible carboxylic acid pendants in their pores (UiO-66-ADn: n=4, 6, 8, and 10, where n denotes the number of carbons in a pendant) has been synthesized by post-synthetic ligand exchange of terephthalate in UiO-66 with a series of alkanedioic acids (HO2 C(CH2 )n-2 CO2 H). NMR, IR, PXRD, TEM, and mass spectral data have suggested that a terephthalate linker in UiO-66 was substituted by two alkanedioate moieties, resulting in free carboxyl pendants in the pores. When post-synthetically modified UiO-66 was partially digested by adjusting the amount of added HF/sample, NMR spectra indicated that the ratio of alkanedioic acid/terephthalic acid was increased with smaller amounts of acid, implying that the ligand substitution proceeded from the outer layer of the particles. Gas sorption studies indicated that the surface areas and the pore volumes of all UiO-66-ADns were decreased compared to those of UiO-66, and that the CO2 adsorption capacities of UiO-66-ADn (n=4, 8) were similar to that of UiO-66. In the case of UiO-66-AD6, the CO2 uptake capacity was 34 % higher at 298 K and 58 % higher at 323 K compared to those of UiO-66. It was elucidated by thermodynamic calculations that the introduction of flexible carboxyl pendants of appropriate length has two effects: 1) it increases the interaction enthalpy between the host framework and CO2 molecules, and 2) it mitigates the entropy loss upon CO2 adsorption due to the formation of multiple configurations for the interactions between carboxyl groups and CO2 molecules. The ideal adsorption solution theory (IAST) selectivity for CO2 adsorption over that of CH4 was enhanced for all of the UiO-66-ADns compared to that of UiO-66 at 298 K. In particular, UiO-66-AD6 showed the most strongly enhanced CO2 uptake capacity and significantly increased selectivity for CO2 adsorption over that of CH4 at ambient temperature, suggesting that it is a promising material for sequestering CO2 from landfill gas.

Ligand Induced Circular Dichroism and Circularly Polarized Luminescence in CdSe Quantum Dots

Chiral thiol capping ligands L- and D-cysteines induced modular chiroptical properties in achiral cadmium selenide quantum dots (CdSe QDs). Cys-CdSe prepared from achiral oleic acid capped CdSe by postsynthetic ligand exchange displayed size-dependent electronic circular dichroism (CD) and circularly polarized luminescence (CPL). Opposite CPL signals were measured for the CdSe QDs capped with D- and L-cysteine. The CD profile and CD anisotropy varied with size of CdSe nanocrystals with largest anisotropy observed for CdSe nanoparticles of 4.4 nm. Magic angle spinning solid state NMR (MAS ssNMR) experiments suggested bidentate interaction between cysteine and the surface of CdSe. Time Dependent Density Functional Theory (TDDFT) calculations verified that attachment of L- and D-cysteine to the surface of model (CdSe)13 nanoclusters induces measurable opposite CD signals for the exitonic band of the nanocluster. The origin of the induced chirality is consistent with the hybridization of highest occupied CdSe molecular orbitals with those of the chiral ligand.

Tandem Postsynthetic Metal Ion and Ligand Exchange in Zeolitic Imidazolate Frameworks

Herein, we report a general postsynthetic exchange (PSE) approach to introduce a redox-active transition metal, specifically Mn(II), into "inert" zeolitic imidazolate frameworks (ZIFs), a subclass of metal-organic frameworks (MOFs). It is shown that metal ion PSE occurs in ZIF-71 (RHO topology) and ZIF-8 (SOD topology) under ambient conditions. The metal exchanged ZIFs are the first porous, Mn(II)-based ZIFs and a rare example of ZIFs with two transition metal centers in a single lattice. Exchanged materials are characterized by scanning electron microscopy-energy dispersed X-ray spectroscopy (SEM-EDX), aerosol time-of-flight mass spectrometry (ATOFMS), X-ray fluorescence spectroscopy (XRF), and Brunauer-Emmett-Teller (BET) surface area analysis. In addition, stepwise "tandem" PSE strategies are described to exchange of metal ions and organic linkers consecutively in ZIFs. These findings are important for probing the chemical dynamics of ZIFs, despite their high crystallinity and robustness, and inspire the more widespread use of PSE to prepare multimetallic and multifunctional MOFs.

Achiral CdSe quantum dots exhibit optical activity in the visible region upon post-synthetic ligand exchange with d- or l-cysteine

Semiconductor cadmium selenide (CdSe) quantum dots (QDs) exhibited mirror-image circular dichroism (CD) spectra in the visible region (350-570 nm) after replacing the trioctylphosphine oxide/oleic acid ligands on achiral nanocrystals with D- and L-cysteines. Chiroptical properties of cysteine-capped CdSe QDs depend on their size and can be fine-tuned by changing the radius of QDs.

Postsynthetic Ligand and Cation Exchange in Robust Metal–Organic Frameworks

Postsynthetic ligand and metal ion exchange (PSE) processes are shown to readily occur in several "inert" metal-organic frameworks (MOFs), including zeolitic imidazolate frameworks (ZIFs). Ligand exchange can occur between solid samples, as was demonstrated under relatively mild conditions with two robust, topologically distinct MOFs, MIL-53(Al) and MIL-68(In). Interestingly, ligand PSE is not observed with MIL-101(Cr), which is attributed to the kinetic inertness of the Cr(III) ion. In addition to ligand exchange, metal ion (cation) PSE was also studied between intact MOF microcrystalline particles. Metal ion transfer between MIL-53(Al) and MIL-53(Fe) was readily observed. These PSE reactions were monitored and the products characterized by a number of techniques, including aerosol time-of-flight mass spectrometry, which permits single-particle compositional analysis. To show the potential synthetic utility of this approach, the PSE process was used to prepare the first Ti(IV) analogue of the robust UiO-66(Zr) framework. Finally, experiments to rule out mechanisms other than PSE (i.e., aggregation, dissolution/recrystallization) were performed. The results demonstrate that PSE, of either ligands or cations, is common even with highly robust MOFs such as UiO-66(Zr), MILs, and ZIFs. Furthermore, it is shown that PSE is useful in preparing novel materials that cannot be obtained via other synthetic methods.

Postsynthetic ligand exchange as a route to functionalization of ‘inert’ metal–organic frameworks

Herein, we report that the exchange of ligands from an intact metal–organic framework (MOF) can be exploited as a means to introduce functionalized ligands into MOFs under mild conditions. It is shown that ligand exchange can occur with ‘inert’ Zr(IV)-based UiO-66 MOFs in a solvent dependent manner. We call this process postsynthetic exchange (PSE) and show that it provides access to MOFs that are not readily prepared in high quality by solvothermal methods. It was found that ligand exchange can occur between UiO-66 MOFs as monitored by aerosol time-of-flight mass spectrometry (ATOFMS). ATOFMS was used to analyze the chemical composition of microcrystalline MOFs on the single particle level, providing information not available through bulk analysis. PSE is an important postsynthetic approach to the modification of MOFs, and the ligand exchange revealed by ATOFMS requires a re-evaluation of the assumed ‘stability’ of even the most robust MOFs.

Colloidal nanostructures as building blocks for macroscopic thermoelectric materials with electron-crystal phonon-glass properties

AbstractWe demonstrate the shape- and size-controlled synthesis of colloidal ∼10 nm bismuth telluride nanoparticles stabilized by organic ligands in solution. Post-synthetic ligand exchange with oleic acid allows for a quick and simple ligand removal by consecutive washing with basic ammonia solution. Mild spark plasma sintering yields a macroscopic nanostructured bulk solid with nanograins unaltered in size and shape. We present the full thermoelectric characterization with an emphasis on the thermal properties of this material. It will be shown that thus prepared nanostructured bulk solids possess significantly altered physical properties typical for materials with high surface-to-volume-ratios. These alterations have the potential to lead to improved thermoelectric performances benefiting from their phonon-glass electron-crystal behavior.