Recently, we have witnessed significant progress with regard to the synthesis of molecular alkaline earth metal reagents and catalysts. To provide new precursors for light alkaline earth metal chemistry, molecular aryloxide and alkoxide complexes of beryllium and magnesium are reported. The reaction of beryllium chloride dietherate with two equivalents of 1,3-diisopropyl-4,5-dimethylimidizol-2-ylidine (sIPr) results in the formation of a bis(N-heterocyclic carbene) (NHC) beryllium dichloride complex, (sIPr)₂BeCl₂ (1). Compound 1 reacts with lithium diisopropylphenoxide (LiODipp) or sodium ethoxide (NaOEt) to form the terminal aryloxide (sIPr)Be(ODipp)₂ (2) and alkoxide dimer [(sIPr)Be(OEt)Cl]₂ (3), respectively. Compounds 2 and 3 represent the first beryllium alkoxide and aryloxide species supported by NHCs. Structurally related dimers of magnesium, [(sIPr)Mg(OEt)Brl]₂ (4) and [(sIPr)Mg(OEt)Me]₂ (5), were also prepared. Compounds 1-5 were characterized by single crystal X-ray diffraction studies, ¹H, ¹³C, and ⁹Be NMR spectroscopy where applicable.

Herein we report the synthesis and characterization of dinuclear magnesium–hydride complexes, [(IⁱPr^Me2)Mg(μ-H)(HMDS)]₂ [4, IⁱPr^Me2 = N,N′-diisopropyl-2,3-dimethylimidazol-2-ylidine, HMDS = bis(trimethylsilyl)amide] and [(IⁱPr^Me2)Mg(μ-H)(ASCP)]₂ (5, ASCP = 2,2,5,5-tetramethyl-2,5-disila-1-azacyclopent-1-yl). Compounds 4 and 5 exhibit the lowest nuclearity of any carbene–magnesium hydride complex to date.

The preparation of the complex MoTp(NO)(DMAP)(4,5-η²-(2-trifluoromethyl)pyridine) (DMAP = 4-(dimethylamino)pyridine; Tp = tris(pyrazolyl)borate) is described. The CF₃ substituent is found to preclude κ-N coordination, allowing for direct coordination without protection of the nitrogen. The dihapto-coordinate complex can be isolated as a single diastereomer, methylated, and reacted with a range of nucleophiles. Oxidative decomplexation affords the free dihydropyridines in good yield (75–90%). As a demonstration of synthetic utility, a series of novel bridgehead CF₃-substituted isoquinuclidines was prepared from these decomplexed dihydropyridines.

An oxidant-initiated, substitution process for dihapto-coordinated ligands is described for the {MoTp(NO)(DMAP)} system. Complexes of the form MoTp(NO)(DMAP)(η²-alkene), MoTp(NO)(DMAP)(η²-ketone), and MoTp(NO)(DMAP)(η²-arene) (where Tp = hydridotris(pyrazolyl)borate and DMAP = 4-(dimethylamino)pyridine) undergo an alkene-to-ketone exchange that is catalyzed by the addition of <0.1 equiv of a metallocene oxidant (ferrocenium, permethylferrocenium, or cobaltocenium). A similar acceleration was observed in the presence of the H-bond donor hexafluoroisopropanol (HFIP). From experimental observations, a radical chain propagation mechanism is proposed that is dependent on the equilibrium between dihapto-coordinated (C, O-η²) and monocoordinated (κ-O) isomers and the differing redox characteristics of these two isomeric forms. This concept was then applied to the search of sodium-free reduction conditions for the conversion of MoTp(NO)(DMAP)(I) to various molybdenum(0) complexes of unsaturated ligands, including MoTp(NO)(DMAP)(η²-naphthalene) and MoTp(NO)(DMAP)(α-pinene).

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).