N‐Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons

Stable N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C₆H₄)(IPr)] and [(IPr)(C₆H₄)₂(IPr)] ( 4 and 5 , respectively; IPr=C{N(2,6‐iPr₂C₆H₃)}₂CHCH), are reported. In a nickel‐catalyzed double carbenylation of 1,4‐Br₂C₆H₄ and 4,4′‐Br₂(C₆H₄)₂ with IPr ( 1 ), [(IPr)(C₆H₄)(IPr)](Br)₂ ( 2 ) and [(IPr)(C₆H₄)₂(IPr)](Br)₂ ( 3 ) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC₈. Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet?triplet energy gap ΔE_S?T of 10.7 kcal mol^?1, whereas 4 features more quinoidal character with a rather large ΔE_S?T of 25.6 kcal mol^?1. In view of the low ΔE_S?T, 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character.     

Ultrafast Light‐Driven Substrate Expulsion from the Active Site of a Photoswitchable Catalyst

The photoswitchable piperidine general base catalyst is a prototype structure for light control of catalysis. Its azobenzene moiety moves sterically shielding groups to either protect or expose the active site, thereby changing the basicity and hydrogen‐bonding affinity of the compound. The reversible switching dynamics of the catalyst is probed in the infrared spectral range by monitoring hydrogen bond (HB) formation between its active site and methanol (MeOH) as HB donor. Steady‐state infrared (IR) and ultrafast IR and UV/Vis spectroscopies are used to uncover ultrafast expulsion of MeOH from the active site within a few picoseconds. Thus, the force generated by the azobenzene moiety even in the final phase of its isomerization is sufficient to break a strong HB within 3 ps and to shut down access to the active site.     

Crystal‐to‐Gel Transformation Stimulated by a Solid‐State E→Z Photoisomerization

The molecule (E)‐(5‐(3‐anthracen‐9‐yl‐allylidene)‐2,2‐dimethyl‐[1,3] dioxane‐4,6‐dione) (E‐ AYAD ) undergoes E→Z photoisomerization. In the solid state, this photoisomerization process can initiate a physical transformation of the crystal that is accompanied by a large volume expansion (ca. 10 times), loss of crystallinity, and growth of large pores. This physical change requires approximately 10 % conversion of the E isomer to the Z isomer and results in a gel‐like solid with decreased stiffness that still retains its mechanical integrity. The induced porosity allows the expanding gel to engulf superparamagnetic nanoparticles from the surrounding liquid. The trapped superparamagnetic nanoparticles impart a magnetic susceptibility to the gel, allowing it to be moved by a magnetic field. The photoinduced phase transition, starting with a compact crystalline solid instead of a dilute solution, provides a new route for in situ production of functional porous materials.     

An Expedient Stereoselective Synthesis of Spirocyclopropyl Oxindoles from Indolin‐2‐One/N‐Protected Indolin‐2‐Ones and Bromonitroalkenes

A direct and much simpler way was developed for the diastereoselective synthesis of spiro cyclopropan‐1,3′‐oxindoles from indolin‐2‐one/N‐protected indolin‐2‐ones and bromonitroalkene. The fused restrained cyclopropanes were obtained with high diastereomeric ratios (upto >99:1) and in reasonable to high isolated chemical yields (upto 94%).     

σ‐Noninnocence: Masked Phenyl‐Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of ?2 at a transition‐metal center. For a series of formal high‐valent Ni^IV complexes, aryl–CF₃ bond‐forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015 , 137, 8034–8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal Ni^IV centers are better described as approaching a +II oxidation state, originating from highly covalent metal–ligand bonds, a phenomenon attributable to σ‐noninnocence. A direct consequence is that the elimination of aryl–CF₃ products occurs in an essentially redox‐neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF₃ group is transferred to an electrophilic aryl group. The uncovered role of σ‐noninnocence in metal–ligand bonding, and of an essentially redox‐neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.     

CAAC‐Based Thiele and Schlenk Hydrocarbons

Diradicals have been of tremendous interest for over a century ever since the first reports of p‐ and m‐phenylene‐bridged diphenylmethylradicals in 1904 by Thiele and 1915 by Schlenk. Reported here are the first examples of cyclic(alkyl)(amino)carbene (CAAC) analogues of Thiele's hydrocarbon, a Kekulé diradical, and Schlenk's hydrocarbon, a non‐Kekulé diradical, without using CAAC as a precursor. The CAAC analogue of Thiele's hydrocarbon has a singlet ground state, whereas the CAAC analogue of Schlenk's hydrocarbon contains two unpaired electrons. The latter forms a dimer, by an intermolecular double head‐to‐tail dimerization. This straightforward synthetic methodology is modular and can be extended for the generation of redox‐active organic compounds.     

Medium‐Sized‐Ring Analogues of Dibenzodiazepines by a Conformationally Induced Smiles Ring Expansion

Analogues of dibenzodiazepines, in which the seven‐membered nitrogen heterocycle is replaced by a 9–12‐membered ring, were made by an unactivated Smiles rearrangement of five‐ to eight‐membered heterocyclic anthranilamides. The conformational preference of the tertiary amide in the starting material leads to intramolecular migration of a range of aryl rings, even those lacking electron‐withdrawing activating groups, and provides a method for n →n +4 ring expansion. The medium‐ring products adopt a chiral ground state with an intramolecular, transannular hydrogen bond. The rate of interconversion of their enantiomeric conformers depends on solvent polarity. Ring size and adjacent steric hindrance modulate this hidden hydrophilicity, thus making this scaffold a good candidate for drug development.     

Self‐Generation of Surface Roughness by Low‐Surface‐Energy Alkyl Chains for Highly Stable Superhydrophobic/Superoleophilic MOFs with Multiple Functionalities

We transformed the hydrophilic metal–organic framework (MOF) UiO‐67 into hydrophobic UiO‐67‐R s (R=alkyl) by introducing alkyl chains into organic linkers, which not only protected hydrophilic Zr₆O₈ clusters to make the MOF interspace superoleophilic, but also led to a rough crystal surface beneficial for superhydrophobicity. The UiO‐67‐R s displayed high acid, base, and water stability, and long alkyl chains offered better hydrophobicity. Good hydrophobicity/oleophilicity were also possible with mixed‐ligand MOFs containing metal‐binding ligands. Thus, a (super)hydrophobic MOF catalyst loaded with Pd centers efficiently catalyzed Sonogashira reactions in water at ambient temperature. Studies of the hydrophobic effects of the coordination interspace and the outer surface suggest a simple de novo strategy for the synthesis of superhydrophobic MOFs that combine surface roughness and low surface energy. Such MOFs have potential for environmentally friendly catalysis and water purification.     

Fine Tuning of MOF‐505 Analogues To Reduce Low‐Pressure Methane Uptake and Enhance Methane Working Capacity

We present a crystal engineering strategy to fine tune the pore chemistry and CH₄‐storage performance of a family of isomorphic MOFs based upon PCN‐14. These MOFs exhibit similar pore size, pore surface, and surface area (around 3000 m² g⁻¹) and were prepared with the goal to enhance CH₄ working capacity. [Cu₂(L2)(H₂O)₂]_n (NJU‐Bai 41: NJU‐Bai for Nanjing University Bai's group), [Cu₂(L3)(H₂O)₂]_n (NJU‐Bai 42), and [Cu₂(L4)(DMF)₂]_n (NJU‐Bai 43) were prepared and we observed that the CH₄ volumetric working capacity and volumetric uptake values are influenced by subtle changes in structure and chemistry. In particular, the CH₄ working capacity of NJU‐Bai 43 reaches 198 cm³ (STP: 273.15 K, 1 atm) cm⁻³ at 298 K and 65 bar, which is amongst the highest reported for MOFs under these conditions and is much higher than the corresponding value for PCN‐14 (157 cm³ (STP) cm⁻³).