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.

Emissive β‐diketones (bdks) and difluoroboron complexes (BF₂bdks) show multi‐stimuli responsive luminescence in both solution and the solid state. A series of bdk ligands and boron coordinated dyes were synthesized with different cyclic amine substituents in the 4‐position to explore ring size effects on various luminescent properties, including solvatochromism, viscochromism, aggregation‐induced emission (AIE), mechanochromic luminescence (ML) and halochromism. Red‐shifted absorption and emission were observed in CH₂Cl₂ for both bdk ligands and boron dyes with increasing substituent ring size. The compounds displayed bathochromic emission in more polar solvents, and higher fluorescence intensity in more viscous media. The AIE compounds exhibited enhanced emission when aggregated. For solid‐state properties, a large emission wavelength shift was shown for the piperidine substituted bdk after melt quenching on weighing paper. Large blue‐shifted emissions were observed in all the boron dye spin cast films after trifluoroacetic acid vapor annealing, and the original emissions were partially recovered after triethylamine vapor treatment.

Substantial progress has been made in the coordination chemistry of main-group elements with neutral donor ligands, largely ushered in by the development of stable N-heterocyclic carbenes (NHCs). There is growing interest in the synthesis of well-defined coordination compounds containing s-block metals; however, examples of molecular compounds containing “normal” NHCs bound to magnesium remain relatively understudied. We report that NHCs react with magnesium halides, MgX₂ (X = Cl, Br, I), to afford (IPr)MgCl₂(THF) (1), [(IPr)MgCl₂]₂ (2), (sIPr)₂MgCl₂ (3), (sIPr)₂MgBr₂ (4), and (sIPr)₂MgI₂ (5), where IPr is 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidine and sIPr is 1,3-diisopropyl-4,5-dimethylimidizol-2-ylidine. Using the IPr ligand, weak NHC coordination and dynamic interaction with THF are observed in solution. In contrast, the coordination of two sIPr ligands results in higher purity, enhanced stability, and no observation of THF coordination. Dual carbene complexation with commercially available MgnBu₂ produced terminal dialkyl (sIPr)₂Mg(nBu)₂ (6). The reaction of methylmagnesium bromide with 2 equiv of sIPr afforded the first structurally characterized example of a terminal Grignard reagent which is stabilized by NHCs, (sIPr)₂Mg(Me)Br (7). Notably, compounds 3–7 represent the first members of a new class of compounds where two untethered NHCs are bound to a single Mg center. These experiments suggest that two less sterically demanding NHCs can have superior stabilizing properties compared to one bulky NHC. The structural identity of each compound was confirmed using single-crystal X-ray diffraction studies, and the bonding in these complexes was investigated by density functional theory.

The first examples of carbodicarbene (CDC)-s-block complexes have been synthesized. The addition of base or reducing agent to a CDC–beryllium (chloride)(hexamethyldisilazide) adduct results in the unprecedented activation of a pendant C(sp³)–H bond and cyclization of the CDC to form a five-membered beryllium metallacycle. This also represents the first example of chemical activation of a CDC which transforms the ligand from monodentate neutral to chelating anionic.

Alkyl and alkenyl arenes are used in a wide range of products. However, the synthesis of 1-phenylalkanes or their alkenyl variants from arenes and alkenes is not accessible with current commercial acid-based catalytic processes. Here, it is reported that an air-stable Rh(I) complex, (5-FP)Rh(TFA)(η²-C₂H₄) (5-FP = 1,2-bis(N-7-azaindolyl)benzene; TFA = trifluoroacetate), serves as a catalyst precursor for the oxidative conversion of arenes and alkenes to alkenyl
arenes that are precursors to 1-phenylalkanes upon hydrogenation. It has been demonstrated that coordination of the 5-FP ligand enhances catalyst longevity compared to unligated Rh(I) catalyst precursors, and the 5-FP-ligated catalyst permits in situ recycling of the Cu(II) oxidant using air. The 5-FP ligand provides a Rh catalyst that can maintain activity for arene alkenylation over at least 2 weeks in reactions at 150 °C that involve multiple Cu(II) regeneration steps using air. Conditions to achieve >13 000 catalytic turnovers with an 8:1 linear:branched (L:B) ratio have been demonstrated. In addition, the catalyst is active under aerobic conditions using air as the sole oxidant. At 80 °C, an 18:1 L:B ratio of alkenyl arenes has been observed, but the reaction rate is substantially reduced compared to 150 °C. Quantum mechanics (QM) calculations compare two predicted reaction pathways with the experimental data, showing that an oxidative addition/reductive elimination pathway is energetically favored over a pathway that involves C−H activation by concerted metalation−deprotonation. In addition, our QM computations are consistent with the observed selectivity (11:1) for linear alkenyl arene products.

Cyclic(alkyl)(amino) carbene (CAAC)-stabilized complexes of phosphorus, one of the lightest group 15 elements, are well-established and can often be obtained in high yields. In contrast, analogous CAAC compounds of bismuth, the heaviest nonradioactive member of group 15, are unknown. Indeed, reactivity increases as you descend the group, and as a result there are only a few examples of N-heterocyclic carbene (NHC)-bismuth complexes. Moreover, activated bismuth compounds often readily extrude bismuth metal, making isolation of stable complexes highly challenging. We report that CAACs react with phenylbismuth dichloride (PhBiCl₂) to afford ^Et2CAAC-Bi(Ph)Cl₂ and ^CyCAAC-Bi(Ph)Cl₂. Significantly, these complexes represent the first structurally characterized examples of CAAC-coordination to bismuth. The CAAC-stabilized bismuth compounds can also be obtained from air-stable salts, [^Et2CAAC-H]₂²⁺ [Cl₂(Ph)Bi(μ-Cl₂)Bi(Ph)Cl₂]²⁻ and [^CyCAAC-H]₂²⁺ [Cl₂(Ph)Bi(μ-Cl₂)Bi(Ph)Cl₂]²⁻, by deprotonation with potassium bis(trimethylsilyl)amide, K[N(SiMe₃)₂]. The electronic effects of the ligand on the bismuth center were investigated by comparing the CAAC-Bi(Ph)Cl₂ complexes to the NHC analogues, SIPr-Bi(Ph)Cl₂(THF) and IPr-Bi(Ph)Cl₂ (SIPr = 1,3- bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). Interestingly, the “normal” IPr-Bi(Ph)Cl₂ slowly isomerizes to the “abnormal” carbene complex, Cl2(Ph)Bi-IPr-H, at −37 °C. In the solid-state, the CAAC-, NHC-, and abnormal NHC-bismuth compounds exhibit Bi atomic centers in unique coordination environments. The complexes were fully characterized by NMR, elemental analysis, and single crystal X-ray diffraction studies. In addition, the bonding was probed by natural bond orbital (NBO) calculations.