Isostructural Synthesis of Porous Metal–Organic Nanotubes

Employment of semirigid double-hinged di-1,2,4-triazoles has led to the synthesis of an isostructural series of metal–organic nanotubes (MONTs). The ditriazole ligands adopt a syn conformation between rigid metal chains while an appropriate anion choice provides a “capping” of the metal ions, leading to MONT formation. This approach of utilizing a variety of both semirigid ligands and metals is the first general methodology to prepare this class of 1D nanomaterial. The local geometry at the metal center depends on the metal ion employed, with Cu­(I) centers adopting a tetrahedral geometry, Ag­(I) centers adopting a seesaw geometry, and Cu­(II) centers adopting a square-pyramidal geometry upon MONT synthesis. The pore size of the MONTs is adjusted by changing the central portion of the double-hinged ligand, allowing for a predictable method to control the pore width of the MONT. The adsorption properties of MONTs as a function of pore size revealed selective uptake of CO₂ and CH₄, with copper MONTs exhibiting the highest uptake. In the case of the silver MONTs, an increase in pore width improves both gas uptake and selectivity

Effects of Solvation on the Framework of a Breathing Copper MOF Employing a Semirigid Linker

A semirigid di-1,2,4-triazole ligand leads to formation of the MOF [Cu₂(<b>L</b>)₂(SO₄)­(Br)₂]·<i>x</i>H₂O (<b>1</b>). The framework structure of <b>1</b> flexes reversibly upon removal or addition of water to form semihydrated ([Cu₂(<b>L</b>)₂(SO₄)­(Br)₂]·4H₂O) and dehydrated ([Cu₂(<b>L</b>)₂(SO₄)­(Br)₂]·0H₂O) MOFs, <b>1′</b> and <b>1″</b>, respectively. Single-crystal X-ray analysis demonstrated that the 2-butene subunit of the ligand rotates between two positions for <b>1</b> and <b>1′</b>, causing a change in the solvent-accessible volume in the framework. This double hinge within the semirigid ligand is a built-in breathing mechanism and suggests a novel approach for general synthesis of breathing MOFs

Synthesis of Fully Aliphatic Aziridines with a Macrocyclic Tetracarbene Iron Catalyst

A second-generation aziridination catalyst supported by a borate-based dianionic macrocyclic tetracarbene ligand has been synthesized. The new macrocyclic tetracarbene iron­(II) complex catalyzed the aziridination of alkyl azides and aliphatic alkenes showcasing the first fully aliphatic version of this C₂ + N₁ reaction. High isolated yields were obtained when no functional groups were present on the organic azides and alkenes, while modest yields were achieved when nonprotic functional groups were included. Even multiple functional groups can be added to the azide and alkene fragments to produce the most complex aziridines yet synthesized by this C₂ + N₁ catalytic reaction. The catalyst generated higher yields for aziridination with aryl azides and alkenes than the previously reported catalyst, [(^Me,EtTC^<i>Ph</i>)­Fe­(NCCH₃)₂]­(PF₆)₂. The contrast is particularly apparent with functionalized aryl azides where the second-generation catalyst now provides practical yields for synthetic chemistry. Finally, catalytic intramolecular aziridination was investigated since many natural products with aziridines feature bicyclic tertiary aziridines. For five- and six-membered rings, the bicyclic aziridines were formed catalytically, in contrast to previously studied catalyzed and uncatalyzed reactions

Effects of Solvation on the Framework of a Breathing Copper MOF Employing a Semirigid Linker

A semirigid di-1,2,4-triazole ligand leads to formation of the MOF [Cu₂(<b>L</b>)₂(SO₄)­(Br)₂]·<i>x</i>H₂O (<b>1</b>). The framework structure of <b>1</b> flexes reversibly upon removal or addition of water to form semihydrated ([Cu₂(<b>L</b>)₂(SO₄)­(Br)₂]·4H₂O) and dehydrated ([Cu₂(<b>L</b>)₂(SO₄)­(Br)₂]·0H₂O) MOFs, <b>1′</b> and <b>1″</b>, respectively. Single-crystal X-ray analysis demonstrated that the 2-butene subunit of the ligand rotates between two positions for <b>1</b> and <b>1′</b>, causing a change in the solvent-accessible volume in the framework. This double hinge within the semirigid ligand is a built-in breathing mechanism and suggests a novel approach for general synthesis of breathing MOFs

A Chromium(II) Tetracarbene Complex Allows Unprecedented Oxidative Group Transfer

Multiple distinct oxidative group transfer reactions to low valent chromium were examined. Six new chromium complexes were prepared from a highly electronically unsaturated Cr­(II) square planar complex that was supported by a macrocyclic tetracarbene ligand. This complex’s reactivity with Me₃NO and disparate azides was investigated. The reaction with Me₃NO generated a highly stable Cr­(IV)-oxo complex. Less bulky organic azides such as <i>p</i>-tolyl and <i>n</i>-octyl azides gave rise to metallotetrazenes, while more sterically demanding mesityl and adamantyl azides generated Cr­(IV)-imido complexes. The reaction of the square planar Cr­(II) complex with TMS-azide yielded the first linearly bridging nitrido chromium species. Reductive group transfer was explored for a Cr­(IV)-imido complex, and organic products, such as aziridines, were formed after addition. Cr­(IV) imidos and oxos are quite rare, while tetrazenes and bridging nitridos are virtually unknown. This is the most detailed study on oxidative group transfer reactions using chromium based complexes on a single auxiliary ligand to date

Rotating Phenyl Rings as a Guest-Dependent Switch in Two-Dimensional Metal–Organic Frameworks

A semirigid bis­(1,2,4-triazole) ligand binds in a syn conformation between copper­(I) chains to form a series of two-dimensional metal–organic frameworks that display a topology of fused one-dimensional metal–organic nanotubes. These anisotropic frameworks undergo two different transformations in the solid state as a function of solvation. The 2D sheet layers can expand or contract, or, more remarkably, the phenyl rings can rotate between two distinct positions. Rotation of the phenyl rings allows for the adjustment of the tube size, depending on the guest molecules present. This “gate” effect along the 1D tubes has been characterized through single-crystal X-ray diffraction. The transformations can also be followed by powder X-ray diffraction (PXRD) and solid-state ¹³C cross-polarization magic-angle-spinning (CP-MAS) NMR. Whereas PXRD cannot differentiate between transformations, solid-state ¹³C CP-MAS NMR can be employed to directly monitor phenyl rotation as a function of solvation, suggesting that this spectroscopic method is a powerful approach for monitoring breathing in this novel class of frameworks. Finally, simulations show that rotation of the phenyl ring from a parallel orientation to a perpendicular orientation occurs at the cost of framework–framework energy and that this energetic cost is offset by stronger framework–solvent interactions

Rotating Phenyl Rings as a Guest-Dependent Switch in Two-Dimensional Metal–Organic Frameworks

A semirigid bis­(1,2,4-triazole) ligand binds in a syn conformation between copper­(I) chains to form a series of two-dimensional metal–organic frameworks that display a topology of fused one-dimensional metal–organic nanotubes. These anisotropic frameworks undergo two different transformations in the solid state as a function of solvation. The 2D sheet layers can expand or contract, or, more remarkably, the phenyl rings can rotate between two distinct positions. Rotation of the phenyl rings allows for the adjustment of the tube size, depending on the guest molecules present. This “gate” effect along the 1D tubes has been characterized through single-crystal X-ray diffraction. The transformations can also be followed by powder X-ray diffraction (PXRD) and solid-state ¹³C cross-polarization magic-angle-spinning (CP-MAS) NMR. Whereas PXRD cannot differentiate between transformations, solid-state ¹³C CP-MAS NMR can be employed to directly monitor phenyl rotation as a function of solvation, suggesting that this spectroscopic method is a powerful approach for monitoring breathing in this novel class of frameworks. Finally, simulations show that rotation of the phenyl ring from a parallel orientation to a perpendicular orientation occurs at the cost of framework–framework energy and that this energetic cost is offset by stronger framework–solvent interactions

Stability of N-Heterocyclic Carbene Monolayers under Continuous Voltammetric Interrogation

N-Heterocyclic carbenes (NHCs) are promising monolayer-forming ligands that can overcome limitations of thiol-based monolayers in terms of stability, surface functionality, and reactivity across a variety of transition-metal surfaces. Recent publications have reported the ability of NHCs to support biomolecular receptors on gold substrates for sensing applications and improved tolerance to prolonged biofluid exposure relative to thiols. However, important questions remain regarding the stability of these monolayers when subjected to voltage perturbations, which is needed for applications with electrochemical platforms. Here, we investigate the ability of two NHCs, 1,3-diisopropylbenzimidazole and 5-(ethoxycarbonyl)-1,3-diisopropylbenzimidazole, to form monolayers via self-assembly from methanolic solutions of their trifluoromethanesulfonate salts. We compare the electrochemical behavior of the resulting monolayers relative to that of benchmark mercaptohexanol monolayers in phosphate-buffered saline. Within the −0.15 to 0.25 V vs Ag|AgCl voltage window, NHC monolayers are stable on gold surfaces, wherein they electrochemically perform like thiol-based monolayers and undergo similar reorganization kinetics, displaying long-term stability under incubation in buffered media and under continuous voltammetric interrogation. At negative voltages, NHC monolayers cathodically desorb from the electrode surface at lower bias (−0.1 V) than thiol-based monolayers (−0.5 V). At voltages more positive than 0.25 V, NHC monolayers anodically desorb from electrode surfaces at similar voltages to thiol-based monolayers. These results highlight new limitations to NHC monolayer stability imposed by electrochemical interrogation of the underlying gold electrodes. Our results serve as a framework for future optimization of NHC monolayers on gold for electrochemical applications, as well as structure–functionality studies of NHCs on gold.</p

Unmasking the Electrochemical Stability of N-Heterocyclic Carbene Monolayers on Gold

N-heterocyclic carbene (NHC) monolayers are transforming electrocatalysis and biosensor design via their increased performance and stability. Despite their increasing use in electrochemical systems, the integrity of the NHC monolayer during voltage perturbations remains largely unknown. Herein, we deploy surface-enhanced Raman spectroscopy (SERS) to measure the stability of two model NHCs on gold in ambient conditions as a function of applied potential and under continuous voltammetric interrogation. Our results illustrate that NHC monolayers exhibit electrochemical stability over a wide voltage window (-1 V to 0.5 V vs Ag|AgCl), but they are found to degrade at strongly reducing ( 0.5V) potentials. We also address NHC monolayer stability under continuous voltammetric interrogation between 0.2 V and -0.5 V, a commonly used voltage window for sensing, showing they are stable for up to 43 hours. However, we additionally find that modifications of the backbone NHC structure can lead to significantly shorter operational lifetimes. While these results highlight the potential of NHC architectures for electrode functionalization, they also reveal potential pitfalls that have not been fully appreciated in electrochemical applications of NHCs.</p

Solvent-Assisted Control of Metal–Organic Nanotube Size and Morphology

While intensive studies have focused on the synthesis and characterization of new metal–organic nanotube (MONT) structures, the lack of size and morphology control remains an obstacle in broadening applications for this class of materials. Herein, we demonstrate control of MONT crystallite size and morphology by tuning polarity and the protic/aprotic nature of solvents, including dimethylformamide, N-methyl-2-pyrrolidone, ethanol, and 2-methyltetrahydrofuran, for the isostructural syntheses of two MONTs. Through a combination of transmission electron microscopy, powder X-ray diffraction, and selected area electron diffraction, we find that MONT crystallite sizes can be tuned while maintaining control over the relative dispersity without significantly altering the underlying crystal structure