Reactions of 9-carbene-9-borafluorene monoanion (1) with elemental selenium and selenium-containing reagents are reported. When compound 1 is reacted with grey selenium in THF, various boryl-substituted selenides and diselenides are produced (2–6), including molecules resulting from migration of the carbene ligand Dipp group (Dipp = 2,6-diisopropylphenyl). However, when a similar reaction between 1 and grey selenium is performed in toluene in the presence of 18-crown-6, boryl-substituted selenide 7 is obtained as the sole boron-containing product. As compound 7 is the monomeric variant of organoselenide 3, 18-crown-6 promotes both product selectivity and solubility in a nonpolar solvent. Diselenide 5, which features a trans-bent B–Se–Se–B core, was directly isolated via reaction of 1 with Se₂Cl₂ in THF. Computational modeling suggests that the formation of 5 proceeds via a radical mechanism. This was supported by an experiment demonstrating that the CAAC-borafluorene radical also reacts with SeCl₂ to yield 5 [CAAC = (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene]. Energy decomposition analysis of 5 indicates a covalent borafluorene–diselenide bond (ΔE_int, −168.9 kcal mol^–1). All of the new compounds were fully characterized via single-crystal X-ray diffraction and multinuclear nuclear magnetic resonance (¹H, ¹³C, ¹¹B, and ⁷⁷Se).

Quinine (an antimalarial) and aspirin (a nonsteroidal anti-inflammatory drug) were combined into a new drug–drug salt, quininium aspirinate, C20H25N2O2 + C9H7O4 , by liquid-assisted grinding using stoichiometric amounts of the reactants in a 1:1 molar ratio, and water, EtOH, toluene, or heptane as additives. A tetrahydrofuran (THF) solution of the mechanochemical product prepared using EtOH as additive led to a single crystal of the same material obtained by mechanochemistry, which was used for crystal structure determination at 100 K. Powder X-ray diffraction ruled out crystallographic phase transitions in the 100– 295 K interval. Neat mechanical treatment (in a mortar and pestle, or in a ball mill at 20 or 30 Hz milling frequencies) gave rise to an amorphous phase, as shown by powder X-ray diffraction; however, FT–IR spectroscopy unambiguously indicates that a mechanochemical reaction has occurred. Neat milling the reactants at 10 and 15 Hz led to incomplete reactions. Thermogravimetry and differential scanning calorimetry indicate that the amorphous and crystalline mechanochemical products form glasses/supercooled liquids before melting, and do not recrystallize upon cooling. However, the amorphous material obtained by neat grinding crystallizes upon storage into the salt reported. The mechanochemical synthesis, crystal structure analysis, Hirshfeld surfaces, powder X-ray diffraction, thermogravimetry, differential scanning calorimetry, FT–IR spectroscopy, and aqueous solubility of quininium aspirinate are herein reported.

Generally, cobalt–N₂O₂ complexes show selectivity for hydrogen peroxide during electrochemical dioxygen (O₂) reduction. We recently reported a Co(III)–N₂O₂ complex with a 2,2′-bipyridine-based ligand backbone which showed alternative selectivity: H₂O was observed as the primary reduction product from O₂ (71 ± 5%) with decamethylferrocene as a chemical reductant and acetic acid as a proton donor in methanol solution. We hypothesized that the key selectivity difference in this case arises in part from increased favorability of protonation at the distal O position of the key intermediate Co(III)–hydroperoxide species. To interrogate this hypothesis, we have prepared a new Co(III) compound that contains pendent −OMe groups poised to direct protonation toward the proximal O atom of this hydroperoxo intermediate. Mechanistic studies in acetonitrile (MeCN) solution reveal two regimes are possible in the catalytic response, dependent on added acid strength and the presence of the pendent proton donor relay. In the presence of stronger acids, the activity of the complex containing pendent relays becomes O₂ dependent, implying a shift to Co(III)–superoxide protonation as the rate-determining step. Interestingly, the inclusion of the relay results in primarily H₂O₂ production in MeCN, despite minimal difference between the standard reduction potentials of the three complexes tested. EPR spectroscopic studies indicate the formation of Co(III)–superoxide species in the presence of exogenous base, with greater O₂ reactivity observed in the presence of the pendent −OMe groups.

Reactions are described for complexes of the form WTp(NO)(PMe₃)(η²-arene) and various amines, where the arene is benzene or benzene with an electron-withdrawing substituent (CF₃, SO₂Ph, SO₂Me). The arene complex is first protonated to form an η²-arenium species, which then selectively adds the amine. The resulting η²-5-amino-1,3-cyclohexadiene complexes can then be subjected to the same sequence with a second nucleophile to form 3-aminocyclohexene complexes, where up to three stereocenters originate from the arene carbons. Alternatively, 1,3-cyclohexadiene complexes containing an ester group at the 5 position (also prepared from an arene) can be treated with acid followed by an amine to form trisubstituted 3-aminocyclohexenes. When the amine is primary, ring closure can occur to form a cis-fused bicyclic γ lactam. Highly functionalized cyclohexenes can be liberated from the tungsten through oxidative decomplexation. The potential utility of this methodology is demonstrated in the synthesis of the alkaloid γ-lycorane. An enantioenriched synthesis of a lactam precursor to γ-lycorane is also described. This compound is prepared from an enantioenriched version of the tungsten benzene complex. Regio- and stereochemical assignments for the reported compounds are supported by detailed 2D NMR analysis and 13 molecular structure determinations (SC-XRD).

The attachment of molecular catalysts to conductive supports for the preparation of solid-state anodes is important for the development of devices for electrocatalytic water oxidation. The preparation and characterization of three molecular cyclopentadienyl iridium(III) complexes, Cp*Ir(1-pyrenyl(2-pyridyl)ethanolate-κO,κN)Cl (1) (Cp* = pentamethylcyclopentadienyl), Cp*Ir(diphenyl(2-pyridyl)methanolate-κO,κN)Cl (2), and [Cp*Ir(4-(1-pyrenyl)-2,2′-bipyridine)Cl]Cl (3), as precursors for electrochemical water oxidation catalysts, are reported. These complexes contain aromatic groups that can be attached via noncovalent π-stacking to ordered mesoporous carbon (OMC). The resulting iridium-based OMC materials (Ir-1, Ir-2, and Ir-3) were tested for electrocatalytic water oxidation leading to turnover frequencies (TOFs) of 0.9–1.6 s⁻¹ at an overpotential of 300 mV under acidic conditions. The stability of the materials is demonstrated by electrochemical cycling and X-ray absorption spectroscopy analysis before and after catalysis. Theoretical studies on the interactions between the molecular complexes and the OMC support provide insight onto the noncovalent binding and are in agreement with the experimental loadings.

The 2-phosphaethynolate (OCP) anion has found versatile applications across the periodic table but remains underexplored in group 2 chemistry due to challenges in isolating thermally stable complexes. By rationally modifying their coordination environments using 1,3-dialkyl-substituted N-heterocyclic carbenes (NHCs), we have now isolated and characterized thermally stable, structurally diverse, and hydrocarbon soluble magnesium phosphaethynolate complexes (2, 4^Me, and 8–10), including the novel phosphaethynolate Grignard reagent (2^iPr). The methylmagnesium phosphaethynolate and magnesium diphosphaethynolate complexes readily activate dioxane with subsequent H-atom abstraction to form [(NHC)MgX(μ-OEt)]₂ [X = Me (3) or OCP (8 and 9)] complexes. Their reactivities increased with the Lewis acidity of the Mg²⁺ cation and may be attenuated by Lewis base saturation or a slight increase in carbene sterics. Solvent effects were also investigated and led to the surreptitious isolation of an ether-free sodium phosphaethynolate (NHC)₃Na(OCP) (6), which is soluble in aromatic hydrocarbons and can be independently prepared by the reaction of NHC and [Na(dioxane)₂][OCP] in toluene. Under forcing conditions (105 °C, 3 days), the magnesium diphosphaethynolate complex (NHC)₃Mg(OCP)₂ (10) decomposes to a mixture of organophosphorus complexes, among which a thermal decarbonylation product [(NHC)₂P^I][OCP] (11) was isolated.

A series of olefin-coordinated Rh^I and Ir^I complexes bearing “capping arene” ligands (5-^XFP and 6-^XFP, see below) of the general formulas (FP)M(olefin)X, [(FP)M(olefin)₂][M(olefin)₂X₂], and [(FP)M(olefin)₂]BF₄ (FP = “capping arene” ligands, X = halide or pseudohalide, olefin = ethylene, cyclooctene, (olefin)₂ = (C₂H₄)₂ or cyclooctadiene) were synthesized and characterized. Single-crystal X-ray diffraction studies revealed structural differences that are a function of the identity of the capping arene ligand and the metal. For 5-^XFP ligands (5-^XFP = 1,2-bis(N-7-azaindolyl)-benzene and derivatives with substituents on the arene moiety), the coordination to both Rh and Ir gives rise to complexes that are best described as 16-electron and square planar. For 6-^XFP ligands (6-^XFP = 8,8′-(1,2-phenylene)diquinoline and derivatives with substituents on the arene moiety), the structures of Rh and Ir complexes are better considered as 18-electron and trigonal bipyramidal due to an η²-C,C interaction between the metal center and the arene group of the capping arene ligand. Variable-temperature ¹H NMR spectroscopy studies of ethylene rotation demonstrated that the Ir complexes possess higher activation barriers to rotation in comparison to Rh complexes and the 6-^XFP complexes tend to give ethylene higher rotational barriers in comparison to 5-^XFP complexes for complexes of the type (FP)Rh(η²-C₂H₄)Cl. DFT calculations are consistent with enhanced Rh to ethylene π-back-donation for Rh complexes ligated by the 6-^XFP ligands in comparison to the 5-^XFP ligands.

We report a trinuclear copper(II) complex, [(DAM)Cu₃(μ³-O)][Cl]₄ (1, DAM = dodecaaza macrotetracycle), as a homogeneous electrocatalyst for water oxidation to dioxygen in phosphate-buffered solutions at pH 7.0, 8.1, and 11.5. Electrocatalytic water oxidation at pH 7 occurs at an overpotential of 550 mV with a turnover frequency of ∼19 s^–1 at 1.5 V vs NHE. Controlled potential electrolysis (CPE) experiments at pH 11.5 over 3 h at 1.2 V and at pH 8.1 for 40 min at 1.37 V vs NHE confirm the evolution of dioxygen with Faradaic efficiencies of 81% and 45%, respectively. Rinse tests conducted after CPE studies provide evidence for the homogeneous nature of the catalysis. The linear dependence of the current density on the catalyst concentration indicates a likely first-order dependence on the Cu precatalyst 1, while kinetic isotope studies (H₂O versus D₂O) point to involvement of a proton in or preceding the rate-determining step. Rotating ring-disk electrode measurements at pH 8.1 and 11.2 show no evidence of H₂O₂ formation and support selectivity to form dioxygen. Freeze-quench electron paramagnetic resonance studies during electrolysis provide evidence for the formation of a molecular copper intermediate. Experimental and computational studies support a key role of the phosphate as an acceptor base. Moreover, density functional theory calculations highlight the importance of second-sphere interactions and the role of the nitrogen-based ligands to facilitate proton transfer processes.

We show how to improve the yield of MeX from CH₄ activation catalysts from 12% to 90% through the use of “capping arene” ligands. Four (FP)Rh^III(Me)(TFA)₂ {FP = “capping arene” ligands, including 8,8′-(1,2-phenylene)diquinoline (6-FP), 8,8′-(1,2-naphthalene)diquinoline (6-^NPFP), 1,2-bis(N-7-azaindolyl)benzene (5-FP), and 1,2-bis(N-7-azaindolyl)naphthalene (5-^NPFP)} complexes. These complexes and (dpe)Rh^III(Me)(TFA)₂ (dpe = 1,2-di-2-pyridylethane) were synthesized and tested for their performance in reductive elimination of MeX (X = TFA or halide). The FP ligands were used with the goal of blocking a coordination site to destabilize the Rh^III complexes and facilitate MeX reductive elimination. On the basis of single-crystal X-ray diffraction studies, the 6-FP and 6-^NPFP ligated Rh complexes have Rh–arene distances shorter than those of the 5-FP and 5-^NPFP Rh complexes; thus, it is expected that the Rh–arene interactions are weaker for the 5-FP complexes than for the 6-FP complexes. Consistent with our hypothesis, the 5-FP and 5-^NPFP Rh^III complexes demonstrate improved performance (from 12% to ∼60% yield) in the reductive elimination of MeX. The reductive elimination of MeX from (FP)Rh^III(Me)(TFA)₂ can be further improved by the use of chemical oxidants. For example, the addition of 2 equiv of AgOTf leads to 87(2)% yield of MeTFA and can be achieved in CD₃CN at 90 °C using (5-FP)Rh(Me)(TFA)₂.