Rachel C. Klet

A Hafnium-Based Metal–Organic Framework as an Efficient and Multifunctional Catalyst for Facile CO2 Fixation and Regioselective and Enantioretentive Epoxide Activation

Porous heterogeneous catalysts play a pivotal role in the chemical industry. Herein a new Hf-based metal-organic framework (Hf-NU-1000) incorporating Hf6 clusters is reported. It demonstrates high catalytic efficiency for the activation of epoxides, facilitating the quantitative chemical fixation of CO2 into five-membered cyclic carbonates under ambient conditions, rendering this material an excellent catalyst. As a multifunctional catalyst, Hf-NU-1000 is also efficient for other epoxide activations, leading to the regioselective and enantioretentive formation of 1,2-bifuctionalized systems via solvolytic nucleophilic ring opening.

Metal–Organic Framework Thin Films as Platforms for Atomic Layer Deposition of Cobalt Ions To Enable Electrocatalytic Water Oxidation

Thin films of the metal-organic framework (MOF) NU-1000 were grown on conducting glass substrates. The films uniformly cover the conducting glass substrates and are composed of free-standing sub-micrometer rods. Subsequently, atomic layer deposition (ALD) was utilized to deposit Co(2+) ions throughout the entire MOF film via self-limiting surface-mediated reaction chemistry. The Co ions bind at aqua and hydroxo sites lining the channels of NU-1000, resulting in three-dimensional arrays of separated Co ions in the MOF thin film. The Co-modified MOF thin films demonstrate promising electrocatalytic activity for water oxidation.

Evaluation of Brønsted acidity and proton topology in Zr- and Hf-based metal–organic frameworks using potentiometric acid–base titration

Potentiometric acid–base titration is introduced as a method to evaluate pKa values (Bronsted acidity) of protons present in the nodes of water stable Zr6- and Hf6-based metal–organic frameworks (MOFs), including UiO-type MOFs, NU-1000, and MOF-808. pKa values were determined for the three typical types of protons present in these MOFs: μ3-OH, M–OH2, and M–OH (M = Zr, Hf). Additionally, the data was used to quantify defect sites resulting from either a surfeit or shortage of linkers in the MOFs and to provide information about the true proton topology of each material.

Single-Site Organozirconium Catalyst Embedded in a Metal–Organic Framework

A structurally well-defined mesoporous Hf-based metal-organic framework (Hf-NU-1000) is employed as a well-defined scaffold for a highly electrophilic single-site d(0) Zr-benzyl catalytic center. This new material Hf-NU-1000-ZrBn is fully characterized by a variety of spectroscopic techniques and DFT computation. Hf-NU-1000-ZrBn is found to be a promising single-component catalyst (i.e., not requiring a catalyst/activator) for ethylene and stereoregular 1-hexene polymerization.

Probing the correlations between the defects in metal–organic frameworks and their catalytic activity by an epoxide ring-opening reaction

Seven Zr/Hf-based MOFs with different degrees of defects were obtained by modulating the synthetic conditions. The number of missing linkers in these MOFs was calculated based on potentiometric acid-base titration. The number of defects was found to correlate quantitatively with the catalytic activity of UiO-type MOFs for an acid-catalyzed epoxide ring-opening reaction. More importantly, we were able to identify a MOF with inherent defective Zr6 nodes, which showed great activity and regio-selectivity for the epoxide ring-opening reaction.

Synthesis of nanocrystals of Zr-based metal–organic frameworks with csq-net: significant enhancement in the degradation of a nerve agent simulant

The synthesis of nano-sized particles of NU-1000 (length from 75 nm to 1200 nm) and PCN-222/MOF-545 (length from 350 nm to 900 nm) is reported. The catalytic hydrolysis of methyl paraoxon was investigated as a function of NU-1000 crystallite size and a significant enhancement in the rate was observed for the nano-sized crystals compared to microcrystals.

Functionalized defects through solvent-assisted linker exchange: Synthesis, characterization, and partial postsynthesis elaboration of a metal-organic framework containing free carboxylic acid moieties

Intentional incorporation of defect sites functionalized with free carboxylic acid groups was achieved in a paddlewheel-based metal-organic framework (MOF) of rht topology, NU-125. Solvent-assisted linker exchange (SALE) performed on a mixed-linker derivative of NU-125 containing isophthalate (IPA) linkers (NU-125-IPA) led to the selective replacement of the IPA linkers in the framework with a conjugate base of trimesic acid (H3BTC). Only two of the three carboxylic acid moieties offered by H3BTC coordinate to the Cu2 centers in the MOF, yielding a rare example of a MOF decorated with free -COOH groups. The presence of the -COOH groups was confirmed by diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS); moreover, these groups were found to be available for postsynthesis elaboration (selective monoester formation). This work constitutes an example of the use of SALE to obtain otherwise challenging-to-synthesize MOFs. The resulting MOF, in turn, can serve as a platform for accomplishing selective organic transformations, in this case, exclusive monoesterification of trimesic acid.

Research Update: A hafnium-based metal-organic framework as a catalyst for regioselective ring-opening of epoxides with a mild hydride source

Reaction of styrene oxide with sodium cyanoborohydride and a catalytic amount of Hf-NU-1000 yields the anti-Markovnikov product, 2-phenylethanol, with over 98% regioselectivity. On the other hand, propylene oxide is ring opened in a Markovnikov fashion to form 2-propanol with 95% regioselectivity. Both styrene oxide and propylene oxide failed to react with sodium cyanoborohydride without the addition of Hf-NU-1000 indicative of the crucial role of Hf-NU-1000 as a catalyst in this reaction. To the best of our knowledge, this is the first report of the use of a metal-organic framework material as a catalyst for ring-opening of epoxides with hydrides.