Because of their rigidity, polycyclic aromatic hydrocarbons (PAHs) have become a significant building block in molecular materials chemistry. Fusion or doping of boron into PAHs is known to improve the optoelectronic properties by reducing the LUMO energy level. Herein, we report a comprehensive study on the syntheses, structures, and photophysical properties of a new class of fused N‐heterocyclic boranes (NHBs), pyrene‐ and benzene‐linked in a “Janus‐type” fashion (2–4, 6–9, and 11). Remarkably, these examples of fused NHBs display fluorescent properties, and collectively their emission spans the visible spectrum. The pyrene‐fused NHBs all display blue fluorescence, as the excitations are dominated by the pyrene core. In notable contrast, the emission properties of the benzene‐fused analogues are highly tunable and are dependent on the electronics of the NHB fragments (i.e., the functional group directly bound to the boron atoms). Pyrene‐fused 2–4 and 11 represent the only molecules in which the K‐region of pyrene is functionalized with NHB units, and while they exhibit distorted (twisted or stair‐stepped) pyrene cores, benzene‐fused 6–9 are planar. The electronic structure and optical properties of these materials were probed by computational studies, including an evaluation of aromaticity, electronic transitions, and molecular orbitals.

Earth-abundant transition-metal catalysts capable of reducing CO₂ to useful products have been gaining attention to meet increasing energy demands and address concerns of rising CO₂ emissions. Group 6 molecular compounds remain underexplored in this context relative to other transition metals. Here, we present a molecular chromium complex with a 2,2′-bipyridine-based ligand capable of selectively transforming CO₂ into CO with phenol as a sacrificial proton donor at turnover frequencies of 5.7 ± 0.1 s^–1 with a high Faradaic efficiency (96 ± 8%) and a low overpotential (110 mV). To achieve the reported catalytic activity, the parent Cr(III) species is reduced by two electron equivalents, suggesting an approximate d⁵ active species configuration. Although previous results have suggested that low-valent species from the Cr/Mo/W triad are nonprivileged for CO₂ reduction in synthetic molecular systems, the results presented here suggest that reactivity analogous to late transition metals is possible with early transition metals.

Oxidative addition and reductive elimination reactions are central steps in many catalytic processes, and controlling the energetics of reaction intermediates is key to enabling efficient catalysis. A series of oxidative addition and reductive elimination reactions using (^RPNP)RhX complexes (R = tert-butyl, isopropyl, mesityl, phenyl; X = Cl, I) was studied to deduce the effect of the size of the phosphine substituents. Using (^RPNP)RhCl as the starting material, oxidative addition of MeI was observed to produce (^RPNP)Rh(Me)(I)Cl, which was followed by reductive elimination of MeCl to form (^RPNP)RhI. The thermodynamics and kinetics vary with the identity of the substituent R on phosphorus of the PNP ligand. The presence of large steric bulk (e.g., R = tert-butyl, mesityl) on the phosphine favors Rh(I) in comparison to the presence of two smaller substituents (e.g., R = isopropyl, phenyl). An Eyring plot for the oxidative addition of MeI to (^tBuPNP)RhCl in THF-d₈ is consistent with a polar two-step reaction pathway, and the formation of [(^tBuPNP)Rh(Me)I]I is also consistent with this mechanism. DFT calculations show that the steric bulk affects the reaction energies of addition reactions which generate six-coordinate complexes by tens of kcal mol^–1. The ligand steric bulk is calculated to have a reduced effect (a few kcal mol^–1) on S_N2 addition barriers, which only require access to one side of the square plane.

A method for the resolution of η²-alkene-complex isomers of the type MoTp(NO)(DMAP)(η²-alkene) and WTp(NO)(PMe₃)(η²-alkene) (where Tp = hydridotris(pyrazolyl)borate and DMAP = 4-(dimethylamino)pyridine) has been explored. Alkene and polyene compounds form as a mixture of kinetically trapped isomers. For both types of complexes, it was found that addition of either a fluorinated alcohol or one-electron oxidant reduces the number of isomers in solution. Accelerated ligand exchange was also observed, although these reactions were accompanied by significant decomposition.

Formal [5 + 1] cycloadditions between aryl-substituted vinylcyclopropanes and nitrenoid precursors are reported. The method, which employs Rh₂(esp)₂ as a catalyst, leads to the highly regioselective formation of substituted tetrahydropyridines. Preliminary mechanistic studies support a stepwise, polar mechanism enabled by the previously observed Lewis acidity of Rh-nitrenoids. Overall, this work expands the application of nitrene-transfer cycloaddition, a relatively underexplored approach to heterocycle synthesis, to the formation of six-membered rings.