Although the activation of elemental sulfur by main group compounds is well-documented in the literature, the products of such reactions are often heterocyclic in nature. However, the isolation and characterization of sulfur catenates (i.e., acyclic sulfur chains) is significantly less common. In this study, we report the activation of elemental sulfur by the 9-CAAC-9-borafluorene radical (1) and anion (2) (CAAC = (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene) to form boron–sulfur catenates (3–6). From the isolation of the octasulfide-bridged compound 3, a sulfur extrusion reaction using 1,3,4,5-tetramethylimidazol-2-ylidene (IMe₄) was used to decrease the sulfide chain length from eight to seven (4). Bonding analysis of compounds 3–6 was performed using density functional theory, which elucidated the nature of the sulfur−sulfur bonding observed within these compounds. We also report the synthesis of a series of borafluorene-chalcogenide species (7–9), via diphenyl dichalcogenide activation, which portray characteristics described by an internal heavy atom effect. Compounds 7–9 each exhibit blue fluorescence, with the lowest energy emissive process (S₂ → S₀) at 436 nm (7 and 8) and 431 nm (9). The S₁ → S₀ emission is not observed experimentally due to a Laporte forbidden transition. Density functional theory was employed to investigate the frontier molecular orbitals and absorption and emission profiles of compounds 7–9.

Development of earth-abundant catalysts for the reduction of dioxygen (ORR) is essential for the development of alternative industrial processes and energy sources. Here, we report a transition metal-free dicationic organocatalyst (Ph₂Phen²⁺) for the ORR. The ORR performance of this compound was studied in acetonitrile solution under both electrochemical conditions and spectrochemical conditions, using halogenated acetic acid derivatives spanning a pK_a range of 12.65 to 20.3. Interestingly, it was found that under electrochemical conditions, a kinetically relevant peroxo dimer species forms with all acids. However, under spectrochemical conditions, strong acids diminish the kinetic contribution of this dimer to the observed rate due to lower catalyst concentrations, whereas weaker acids were still rate-limited by the dimer equilibrium. Together, these results provide insight into the mechanisms of ORR by organic-based, metal-free catalysts, suggesting that balancing redox activity and unsaturated character of these molecules with respect to the pK_a of intermediates can enable reaction tuning analogous to transition metal-based systems.

Homogeneous earth abundant transition-metal electrocatalysts capable of carbon dioxide (CO₂) reduction to generate value-added chemical products are a possible strategy to minimize rising anthropogenic CO₂ emissions. Previously, it was determined that Cr-centered bipyridine-based N₂O₂ complexes for CO₂ reduction are kinetically limited by a proton-transfer step during C–OH bond cleavage. Therefore, it was hypothesized that the inclusion of pendent relay groups in the secondary coordination sphere of these molecular catalysts could increase their catalytic activity. Here, it is shown that the introduction of a pendent methoxy group favorably impacts a pre-equilibrium protonation prior to the catalytic resting state, resulting in a significant increase in catalytic activity without a loss of product selectivity for generating carbon monoxide (CO) from CO₂. Interestingly, combining the pendent methoxy group with a cationic acid causes a positive shift of the catalytic reduction potential of the system, while maintaining increased activity and quantitative selectivity. This work suggests that tuning the secondary coordination sphere with respect to cationic proton sources can result in activity improvements by modifying the kinetic and thermodynamic aspects of proton transfer in the catalytic cycle.

A series of Pt–Sb complexes with two or three L-type quinoline side arms were prepared and studied. Two ligands, tri(8-quinolinyl)stibane (SbQ₃, Q = 8-quinolinyl, 1) and 8,8′-(phenylstibanediyl)diquinoline (SbQ₂Ph, 2), were used to synthesize the Pt^II–Sb^III complexes (SbQ₃)PtCl₂ (3) and (SbQ₂Ph)PtCl₂ (4). Chloride abstraction with AgOAc provided the bis-acetate complexes (SbQ₃)Pt(OAc)₂ (5) and (SbQ₂Ph)Pt(OAc)₂ (6). To better understand the electronic effects of the Sb moiety, analogous bis-chloride complexes were oxidized to an overall formal oxidation state of +7 (i.e., Pt + Sb formal oxidation states = 7) using dichloro(phenyl)-λ³-iodane (PhICl₂) and 3,4,5,6-tetrachloro-1,2-dibenzoquinone (o-chloranil) as two-electron oxidants. Depending on the oxidant, different conformational changes occur within the coordination sphere of Pt as confirmed by single-crystal X-ray diffraction and NMR spectroscopy. In addition, the nature of Pt–Sb interactions was evaluated via molecular and localized orbital calculations.

The complex sits in a general position. Each Ni^II ion has an N₄Cl₂ coordination sphere. Weak hydrogen bonding exists between three of the amino groups and the chloride ions of an adjacent mol­ecule. Chains of mol­ecules, linked by the hydrogen bonding and short Cl⋯Cl contacts, are well separated by the 3-meth­oxy­aniline ligands.

Designing molecules that can undergo late-stage modifications resulting in specific optical properties is useful for developing structure-function trends in materials, which ultimately advance optoelectronic applications. Herein, we report a series of fused diborepinium ions stabilized by carbene and carbone ligands (diamino-N-heterocyclic carbenes, cyclic(alkyl)(amino) carbenes, carbodicarbenes, and carbodiphosphoranes), including a detailed bonding analysis. These are the first structurally confirmed examples of diborepin dications and we detail how distortions in the core of the pentacyclic fused system impact aromaticity, stability, and their light-emitting properties. Using the same fused diborepin scaffold, coordinating ligands were used to dramatically shift the emission profile, which exhibit colors ranging from blue to red (350-650 nm). Notably, these diborepinium ions access expanded regions of the visible spectrum compared to known examples of borepins, with quantum yields up to 60%. Carbones were determined to be superior stabilizing ligands, resulting in improved stability in the solution- and solid-states. Density functional theory was used to provide insight into the bonding as well as the specific transitions that result in the observed photophysical properties.

Pyridine-alkoxide (pyalk) ligands that support transition metals have been studied for their use in electrocatalytic applications. Herein, we used the pyalk proligands diphenyl(pyridin-2-yl)methanol ([H]^PhPyalk, L1), 1-(pyren-1-yl)-1-(pyridin-2-yl)ethan-1-ol ([H]^PyrPyalk, L2), 1-(pyridine-2-yl)-1-(thiophen-2-yl)ethan-1-ol ([H]^ThioPyalk, L3), and 1-(ferrocenyl)-1-(pyridin-2-yl)ethan-1-ol ([H]^FePyalk, L4) to synthesize Cu^II complexes that vary in nuclearity and secondary coordination sphere. Also, the proligand 1-(ferrocenyl)-1-(5-methoxy-pyridin-2-yl)ethan-1-ol ([H]^FeOMePyalk, L5) was synthesized with a methoxy substituted pyridine; however, the isolation of a Cu^II complex ligated by L5 was not possible. Under variable reaction conditions, the pyalk ligands reacted with Cu^II precursors and formed either mononuclear or dinuclear Cu^II complexes depending on the amount of ligand added. The resulting complexes were characterized by single crystal X-ray diffraction, elemental analysis, and cyclic voltammetry.

The effects of fixing the redox mediator (RM) reduction potential relative to a series of Cr-centered complexes capable of the reduction of CO₂ to CO are disclosed. The greatest co-electrocatalytic activity enhancement is observed when the reduction potentials of the catalyst and RM are identical, implying that controlling the speciation of the Cr complex relative to RM activation is essential for improving catalytic performance. In all cases, the potential where co-catalytic activity is observed matches the reduction potential of the RM, regardless of the relative reduction potential of the Cr complex.