Shear-induced crystallization of dyes in the amorphous state is an effective strategy for generating higher energy emission after mechanical perturbation—a rare phenomenon in mechanoresponsive materials. Recently, we reported that a β-diketone with a 3,4,5-trimethoxy-substituted phenyl ring formed a stable supercooled liquid (SCL) phase after melting and cooling in air. To tune the lifetime of β-diketones in the SCL phase, a series of dyes with 3,4,5-trimethoxy-substituted phenyl rings were synthesized. Derivatives with naphthyl and phenyl rings were prepared in order to modulate crystallization through arene interactions. Additionally, dyes were substituted with alkoxy chains of varying length to promote crystallization through increased van der Waals interactions. Video screening in conjunction with differential scanning calorimetry and X-ray diffraction studies indicated that naphthyl-substituted derivatives exhibited increased melted state lifetimes and that increasing the alkoxy chain length can induce crystallization. Analysis of molecular packing of single crystals of PH, PC1, PC3, and PC5 revealed that the central para-substituted methoxy group of the trimethoxy-substituted ring was forced out of the molecular plane because of steric interactions with neighboring methoxy groups. The stabilities of the SCLs were generally correlated with the torsion angles of the para methoxy groups, where derivatives with smaller angles exhibited faster rates of crystallization. Mechanical perturbation of the SCL phases resulted in shear-induced crystallization of PH, PC1, PC3, and NC6 derivatives. In some cases, traditional mechanochromic luminescence with a crystalline-to-amorphous phase transition was also observed, which indicates that some trimethoxy-substituted β-diketones exhibit more than one type of mechanoresponsive luminescence.

Luminescent β-diketones (bdks) and difluoroboron coordinated complexes (BF₂bdks) exhibit many environment-sensitive properties, such as solvatochromism, viscochromism, aggregation-induced emission (AIE), and thermal and mechanochromic luminescence (ML). In a previous study, an azepane-substituted bdk ligand (L1) and boron dye (D1) showed noteworthy luminescence properties but low quantum yields (Φ, L1: 0.26; D1: 0.02) due to free intramolecular bond rotation and twisted intramolecular charge transfer state formation with associated nonradiative decay. Thus, in order to improve the quantum yields, an azepane-substituted bdk ligand (L2) and boron complex (D2) with restricted C–C bond rotation were synthesized, and various luminescence properties were investigated. Restricting bond rotation blue-shifted absorptions and emissions, increased lifetimes, and greatly improved quantum yields (Φ, L2: 0.47; D2: 0.83). Excited state density functional theory calculations displayed twisted geometries for L1 and D1 but more planar geometries for L2 and D2. All compounds showed red-shifted emissions in more polar solvents. For viscochromism, L1 and D1 exhibited higher emission intensity in more viscous media. However, L2 and D2 did not show dramatic viscochromism, substantiating the relation between viscosity sensitivity and intramolecular bond twisting. Additionally, while both ligands showed quenched emission upon aggregation, the dyes exhibited AIE regardless of bond restriction. Thermal and ML studies showed a more dramatic emission shift for L2 than L1 between thermally annealed and melt-quenched states. In summary, the quantum yields of the azepane-substituted bdk ligand and boron dye were successfully improved by restricting the intramolecular C–C bond rotation, making various luminescence properties more promising for environment-sensitive applications.

Recent synthetic efforts have uncovered several bond activation pathways mediated by beryllium. Having the highest charge density and electronegativity, the chemistry of beryllium often diverges from that of its heavier alkaline earth metal congeners. Herein, we report the synthesis of a new carbodicarbene beryllacycle (2). Compound 2 converts to 3 via an unprecedented cyclic(alkyl)(amino) carbene (CAAC)-promoted ring expansion reaction (RER). While CAAC activates a carbon–beryllium bond, N-heterocyclic carbene (NHC) coordinates to beryllium to give the tetracoordinate complex 4, which contains the longest ^carbeneC–Be bond to date at 1.856(4) Å. All of the compounds were fully characterized by X-ray crystallography, Fourier transform infrared spectroscopy, and ¹H, ¹³C, and ⁹Be NMR spectroscopy. The ring expansion mechanism was modeled with both NHC and CAAC using density functional theory calculations. While the activation energy for the observed beryllium ring expansion with CAAC was found to be 14 kJ mol^–1, the energy barrier for the hypothetical NHC RER is significantly higher (199.1 kJ mol^–1).

The first examples of N‐heterocyclic carbene (NHC) and cyclic(alkyl)(amino) carbene (CAAC) stabilized borepinium and borafluorenium heterocycles are reported herein. The optical properties of the heterocyclic borenium cations were tuned by varying the Lewis base and by changing the number of atoms in the ring. More importantly, functionalizing the cationic boron ring system in the NHC‐borafluorenium cation affords a temperature‐sensitive molecule with reversible colorimetric “turn off/turn on” properties in solution. Notably, this is the first report of thermochromism in these cationic species. This property, which is mediated by an intermolecular boron–oxygen bond equilibrium, was examined in detail by X‐ray crystallography, variable temperature‐UV/Vis absorption spectroscopy (VT‐UV/Vis), and density functional theory (DFT).

Studies of catalytic benzene alkenylation using different diimine ligated Rh(I) acetate complexes and Cu(OAc)₂ as the oxidant revealed statistically identical results in terms of activity and product selectivity. Under ethylene pressure, two representative diimine ligated rhodium(I) acetate complexes were demonstrated to exchange the diimine ligand with ethylene rapidly to form [Rh(μ-OAc)(η²-C₂H₄)₂]₂ and free diimine. Thus, it was concluded that diimine ligands are not likely coordinated to the active Rh catalysts under catalytic conditions. At 150 °C under catalytic conditions using commercial Cu(OAc)₂ as the oxidant, [Rh(μ-OAc)(η²-C₂H₄)₂]₂ undergoes rapid decomposition to form catalytically inactive and insoluble Rh species, followed by gradual dissolution of the insoluble Rh to form the soluble Rh, which is active for styrene production. Thus, the observed induction period under some conditions is likely due to the formation of insoluble Rh (rapid), followed by redissolution of the Rh (slow). The Rh decomposition process can be suppressed and the catalytically active Rh species maintained by using soluble Cu(II) oxidants or Cu(OAc)₂ that has been preheated. In such cases, an induction period is not observed.

In the past two decades, the organometallic chemistry of the alkaline earth elements has experienced a renaissance due in part to developments in ligand stabilization strategies. In order to expand the scope of redox chemistry known for magnesium and beryllium, we have synthesized a set of reduced magnesium and beryllium complexes and compared their resulting structural and electronic properties. The carbene-coordinated alkaline earth–halides, (^Et2CAAC)MgBr₂ (1), (SIPr)MgBr₂ (2), (^Et2CAAC)BeCl₂ (3), and (SIPr)BeCl₂ (4) [^Et2CAAC = diethyl cyclic(alkyl)(amino) carbene; SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene] were combined with an α-diimine [2,2-bipyridine (bpy) or bis(2,6-diisopropylphenyl)-1,4-diazabutadiene (^DippDAB)] and the appropriate stoichiometric amount of potassium graphite to form singly- and doubly-reduced compounds (^Et2CAAC)MgBr(^DippDAB) (5), (^Et2CAAC)MgBr(bpy) (6), (^Et2CAAC)Mg(^DippDAB) (7), (^Et2CAAC)Be(bpy) (8), and (SIPr)Be(bpy) (9). The doubly-reduced compounds 7–9 exhibit substantial π-bonding interactions across the diimine core, metal center, and π-acidic carbene. Each complex was fully characterized by UV–vis, FT-IR, X-ray crystallography, ¹H, ¹³C, and ⁹Be NMR, or EPR where applicable. We use these compounds to highlight the differences in the organometallic chemistry of the lightest alkaline earth metals, magnesium and beryllium, in an otherwise identical chemical environment.

A family of cobalt(II) and zinc(II) compounds with the general formula (4-subC₆H₄NH₂)₂MX₂, (M = Co, Zn; X = Cl, Br; sub = CH₃, Cl) has been prepared and the compounds characterized by single-crystal X-ray diffraction. The eight compounds crystallize in the monoclinic space group I2/a and are isocoordinate, but show different crystal packing for the 4-methyl and 4-chloroaniline complexes as a result of halogen bonding in the 4-chloroaniline family. The cobalt complexes have also been studied via variable temperature magnetic susceptibility measurements. All four Co(II) compounds exhibit antiferromagnetic exchange (maxima in χ are observed) and have been fitted with a 1D-chain model (cobalt chloride complexes, J ∼ −3 K) or a 2D-model (cobalt bromide complexes, J ∼ −4 K). Analysis of potential superexchange pathways is provided.

The dihapto-coordination of benzene to the π-basic fragment {TpW(NO)(PMe₃)} (Tp = hydridotris(pyrazolyl)-borate) enhances the basicity of the arene ligand to the point that it can be protonated with a mild Brønsted acid (diphenylammonium triflate; pK_a ∼ 1). The resulting η²-benzenium complex reacts with a wide range of nucleophiles including protected enolates, cyanide, amines, methoxide, and aromatic nucleophiles to form 5-substituted 3,4-η²-1,3-cyclohexadiene complexes in good yield (42–70%). These coordinated dienes were successfully taken through a second protonation and nucleophilic addition with a similar scope of nucleophiles (54–80%). The resulting cis-3,4- and cis-3,6-disubstituted η²-cyclohexene complexes were prepared with high regio- and stereocontrol, as governed by the asymmetric nature of π-allyl intermediates. In some cases, a diene linkage isomerization from 3,4-η² to 1,2-η² could be effected with a redox catalyst, and reactions of the latter species led to cis-3,5-disubstituted cyclohexene products exclusively. Oxidative decomplexation afforded the free cyclohexene products in moderate yield (37–68%). Additionally, when a single enantiomer of the chiral dearomatization agent was used, the elaborated cyclohexenes were able to be synthesized in enantioenriched forms (86–90% enantiomeric excess). Full characterization of 40 new compounds is provided that includes two-dimensional NMR, IR, electrochemical and in some cases crystallographic data.

Carbenes are known to activate carbon dioxide to form zwitterionic adducts. Their inherent metal‐free redox activity remains understudied. Herein, we demonstrate that zwitterionic adducts of carbon dioxide formed with cyclic(alkyl)(amino) carbenes are not only redox active, but they can mediate the stoichiometric reductive disproportionation of carbon dioxide to carbon monoxide and carbonate. Infrared spectroelectrochemical experiments show that the reaction proceeds through an intermediate radical anion formed by one‐electron reduction, ultimately generating a ketene product and carbonate in the absence of additional organic or inorganic reagents.

The long‐sought carbene–bismuthinidene, (CAAC)Bi(Ph), has been synthesized. Notably, this represents both the first example of a carbene‐stabilized subvalent bismuth complex and the extension of the carbene‐pnictinidene concept to a non‐toxic metallic element (Bi). The bonding has been investigated by single‐crystal X‐ray diffraction studies and DFT calculations. This report also highlights the hitherto unknown reducing and ligand transfer capability of a beryllium(0) complex.