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.

We report a combined experimental and computational study focused on the mechanism of oxidative conversion of benzene and ethylene to styrene using [(η²-C₂H₄)₂Rh(μ-OAc)]₂ as the catalyst precursor in the presence of Cu(OPiv)₂ (OPiv = pivalate). Using [(η²-C₂H₄)₂Rh(μ-OAc)]₂ as the catalyst precursor, ∼411 turnovers of styrene are observed after 1 h, giving an apparent turnover frequency of ∼0.11 s^–1 (calculated assuming the binuclear structure is maintained in the active catalyst). We identify the catalyst resting state to be [(η²-C₂H₄)₂Rh^I(μ-OPiv)₂]₂(μ-Cu), which is a heterotrinuclear molecular complex in which a central Cu^II atom bridges two Rh moieties. At high Rh concentration in the presence of Cu(OPiv)₂ and pivalic acid (HOPiv), the trinuclear complex [(η²-C₂H₄)₂Rh^I(μ-OPiv)₂]₂(μ-Cu) converts to the binuclear Rh(II) complex [(HOPiv)Rh^II(μ-OPiv)₂]₂, which has been identified by ¹H NMR spectroscopy and single crystal X-ray diffraction. The binuclear Rh(II) [(HOPiv)Rh^II(μ-OPiv)₂]₂ is not a catalyst for styrene production, but under catalytic conditions [(HOPiv)Rh^II(μ-OPiv)₂]₂ can be partially converted to the active catalyst, the Rh–Cu–Rh complex [(η²-C₂H₄)₂Rh^I(μ-OPiv)₂]₂(μ-Cu), following an induction period of ∼6 h. Using quantum chemical calculations, we sampled possible mononuclear and binuclear Rh species, finding that the binuclear Rh(II) [(HOPiv)Rh^II(μ-OPiv)₂]₂ paddle-wheel is a low energy global minimum, which is consistent with experimental observations that [(HOPiv)Rh^II(μ-OPiv)₂]₂ is not a catalyst for styrene formation. Further, we investigated the mechanism of styrene production starting from [(η²-C₂H₄)₂Rh^I(μ-OAc)₂]₂(μ-Cu), [(η²-C₂H₄)₂Rh(μ-OAc)]₂, and (η²-C₂H₄)₂Rh(κ²-OAc). For all reaction pathways studied, the predicted activation barriers for styrene formation from [(η²-C₂H₄)₂Rh(μ-OAc)]₂ and (η²-C₂H₄)₂Rh(κ²-OAc) are too high compared to experimental kinetics. In contrast, the overall activation barrier for styrene formation predicted by DFT from the Rh–Cu–Rh complex [(η²-C₂H₄)₂Rh^I(μ-OPiv)₂]₂(μ-Cu) is in agreement with experimentally determined rates of catalysis. Based on these results, we conclude that incorporation of Cu(II) into the active Rh–Cu–Rh catalyst reduces the activation barrier for benzene C–H activation, O–H reductive elimination, and ethylene insertion into the Rh–Ph bond.

Two‐electron reduction of carbene‐supported 9‐bromo‐9‐borafluorenes with excess KC₈, Na, or Li‐naphthalenide affords six N‐heterocyclic carbene (NHC)‐ or cyclic(alkyl)(amino) carbene (CAAC)‐stabilized borafluorene anions ( 3‐8 )−the first isolated and structurally authenticated examples of the elusive 9‐carbene‐9‐borafluorene monoanion. The electronic structure, bonding, and aromaticity of the boracyclic anions were comprehensively investigated via computational studies. Compounds  5  and  8  react with metal halides via salt elimination to give new B−E (E = Au, Se, Ge)‐containing materials ( 9 ‐ 12 ). Upon reaction with diketones, the carbene ligand cleanly dissociates from  5  or  8  to yield new B−O containing spirocycles ( 13 ‐ 14 ) that cannot be easily obtained using "normal" valent borafluorene compounds. Collectively, these results support the notion that carbene‐stabilized monoanionic borafluorenes may serve as a new platform for the one‐step construction of higher‐value boracyclic materials.