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)2RhI(μ-OPiv)2]2(μ-Cu), which is a heterotrinuclear molecular complex in which a central CuII 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)2RhI(μ-OPiv)2]2(μ-Cu) converts to the binuclear Rh(II) complex [(HOPiv)RhII(μ-OPiv)2]2, which has been identified by 1H NMR spectroscopy and single crystal X-ray diffraction. The binuclear Rh(II) [(HOPiv)RhII(μ-OPiv)2]2 is not a catalyst for styrene production, but under catalytic conditions [(HOPiv)RhII(μ-OPiv)2]2 can be partially converted to the active catalyst, the Rh–Cu–Rh complex [(η2-C2H4)2RhI(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)RhII(μ-OPiv)2]2 paddle-wheel is a low energy global minimum, which is consistent with experimental observations that [(HOPiv)RhII(μ-OPiv)2]2 is not a catalyst for styrene formation. Further, we investigated the mechanism of styrene production starting from [(η2-C2H4)2RhI(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)2RhI(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.
Publications
2021
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.
The synthesis and thermal redox chemistry of the first antimony (Sb)– and bismuth (Bi)–phosphaketene adducts are described. When diphenylpnictogen chloride [Ph2PnCl (Pn = Sb or Bi)] is reacted with sodium 2-phosphaethynolate [Na[OCP]·(dioxane)x], tetraphenyldipnictogen (Ph2Pn–PnPh2) compounds are produced, and an insoluble precipitate forms from solution. In contrast, when the N-heterocyclic carbene adduct (NHC)–PnPh2Cl is combined with [Na[OCP]·(dioxane)x], Sb– and Bi–phosphaketene complexes are isolated. Thus, NHC serves as an essential mediator for the reaction. Immediately after the formation of an intermediary pnictogen–phosphaketene NHC adduct [NHC–PnPh2(PCO)], the NHC ligand transfers from the Pn center to the phosphaketene carbon atom, forming NHC–C(O)P-PnPh2 [Pn = Sb (3) or Bi (4)]. In the solid state, 3 and 4 are dimeric with short intermolecular Pn–Pn interactions. When compounds 3 and 4 are heated in THF at 90 and 70 °C, respectively, the pnictogen center PnIII is thermally reduced to PnII to form tetraphenyldipnictines (Ph2Pn–PnPh2) and an unusual bis-carbene-supported OCP salt, [(NHC)2OCP][OCP] (5). The formation of compound 5 and Ph2Pn–PnPh2 from 3 or 4 is unique in comparison to the known thermal reactivity for group 14 carbene–phosphaketene complexes, further highlighting the diverse reactivity of [OCP]− with main-group elements. All new compounds have been fully characterized by single-crystal X-ray diffraction, multinuclear NMR spectroscopy (1H, 13C, and 31P), infrared spectroscopy, and elemental analysis (1, 2, and 5). The electronic structure of 5 and the mechanism of formation were investigated using density functional theory (DFT).
The elastic moduli of amorphous and crystalline atomic layer-deposited Hf1-xZrxO2 (HZO, x = 0, 0.31, 0.46, 0.79, 1) films prepared with TaN electrodes on silicon substrates were investigated using picosecond acoustic measurements. The moduli of the amorphous films were observed to increase between 211 ± 6 GPa for pure HfO2 and 302 ± 9 GPa for pure ZrO2. In the crystalline films, it was found that the moduli increased upon increasing the zirconium composition from 248 ± 6 GPa for monoclinic HfO2 to 267 ± 9 GPa for tetragonal ZrO2. Positive deviations from this increase were observed for the Hf0.69Zr0.31O2 and Hf0.54Zr0.46O2 compositions, which were measured to have moduli of 264 ± 8 GPa and 274 ± 8 GPa, respectively. These two compositions contained the largest fractions of the ferroelectric orthorhombic phase, as assessed from polarization and diffraction data. The biaxial stress states of the crystalline films were characterized through sin2(ψ) x-ray diffraction analysis. The in-plane stresses were all found to be tensile and observed to increase with the increasing zirconium composition, between 2.54 ± 0.6 GPa for pure HfO2 and 5.22 ± 0.5 GPa for pure ZrO2. The stresses are consistent with large thermal expansion mismatches between the HZO films and silicon substrates. These results demonstrate a device-scale means to quantify biaxial stress for investigation on its effect on the ferroelectric properties of hafnia-based materials.
A family of Ni(II) halide complexes of substituted aniline derivatives were prepared and studied via single-crystal X-ray diffraction and variable temperature magnetic measurements: [Ni(4-Mean)4Cl2] 1, [Ni(4-Clan)2Cl2(MeOH)2] 2, [Ni(4-Clan)2Br2(EtOH)2] 3, [Ni(4-Clan)4Br2] (4-Clan) 4, [Ni(4-Clan)2Br2(H2O)2] 5, [Ni(4-MeOan)2Br2(H2O)2] 6, [Ni(3-MeOan)4Cl2] 7, [Ni(3-MeOan)4Br2] 8, [Ni(4-MeOan)4Cl2] 9, [Ni(4-MeOan)4(H2O)2](Br)2 10, and [(4-MeOan)2(DMSO)4Ni](Br)2 11 (4-Mean = 4-methylaniline, 4-Clan = 4-chloroaniline, 3-MeOan = 3-methoxyaniline, 4-MeOan = 4-methoxyaniline). All complexes are six-coordinate, filling the coordination sphere with a combination of halide ions, aniline-based ligands and/or solvent molecules. The complexes demonstrate variable stability once removed from the mother liquor with loss of coordinated solvent molecules being common. Analysis of the magnetic properties of the compounds shows the presence of single-ion anisotropy, weak interactions, or a combination thereof.
The mono- and dianions of CO2 (i.e., CO2− and CO22−) have been studied for decades as both fundamentally important oxycarbanions (anions containing only C and O atoms) and as critical species in CO2 reduction and fixation chemistry. However, CO2 anions are highly unstable and difficult to study. As such, examples of stable compounds containing these ions are extremely limited; the unadulterated alkali salts of CO2 (i.e., MCO2, M2CO2, M = alkali metal) decompose rapidly above 15 K, for example. Herein we report the chemical reduction of a cyclic (alkyl)(amino) carbene (CAAC) adduct of CO2 at room temperature by alkali metals, which results in the formation of CAAC-stabilized alkali CO2− and CO22− clusters. One-electron reduction of CAAC–CO2 adduct (1) with lithium, sodium or potassium metal yields stable monoanionic radicals [M(CAAC–CO2)]n (M = Li, Na, K, 2–4) analogous to the alkali CO2− radical, and two-electron alkali metal reduction affords dianionic clusters of the general formula [M2(CAAC–CO2)]n (5–8) with reduced CO2 units which are structurally analogous to the carbonite anion CO22−. It is notable that crystalline clusters of these alkali–CO2 salts may also be isolated via the “one-pot” reaction of free CO2 with free CAAC followed by the addition of alkali metals – a process which does not occur in the absence of carbene. Each of the products 2–8 was investigated using a combination of experimental and theoretical methods.
Organometallic gold complexes are used in a range of catalytic reactions, and they often serve as catalyst precursors that mediate C–C bond formation. In this study, we investigate C–C coupling to form ethane from various phosphine-ligated gem-digold(I) methyl complexes including [Au2(μ-CH3)(PMe2Ar′)2][NTf2], [Au2(μ-CH3)(XPhos)2][NTf2], and [Au2(μ-CH3)(tBuXPhos)2][NTf2] {Ar′ = C6H3-2,6-(C6H3-2,6-Me)2, C6H3-2,6-(C6H2-2,4,6-Me)2, C6H3-2,6-(C6H3-2,6-iPr)2, or C6H3-2,6-(C6H2-2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl; tBuXPhos = 2-di-tert-butylphosphino-2′,4′,6′-triisopropylbiphenyl; NTf2 = bis(trifluoromethyl sulfonylimide)}. The gem-digold methyl complexes are synthesized through reaction between Au(CH3)L and Au(L)(NTf2) {L = phosphines listed above}. For [Au2(μ-CH3)(XPhos)2][NTf2] and [Au2(μ-CH3)(tBuXPhos)2][NTf2], solid-state X-ray structures have been elucidated. The rate of ethane formation from [Au2(μ-CH3)(PMe2Ar′)2][NTf2] increases as the steric bulk of the phosphine substituent Ar′ decreases. Monitoring the rate of ethane elimination reactions by multinuclear NMR spectroscopy provides evidence for a second-order dependence on the gem-digold methyl complexes. Using experimental and computational evidence, it is proposed that the mechanism of C–C coupling likely involves (1) cleavage of [Au2(μ-CH3)(PMe2Ar′)2][NTf2] to form Au(PR2Ar′)(NTf2) and Au(CH3)(PMe2Ar′), (2) phosphine migration from a second equivalent of [Au2(μ-CH3)(PMe2Ar′)2][NTf2] aided by binding of the Lewis acidic [Au(PMe2Ar′)]+, formed in step 1, to produce [Au2(CH3)(PMe2Ar′)][NTf2] and [Au2(PMe2Ar′)]+, and (3) recombination of [Au2(CH3)(PMe2Ar′)][NTf2] and Au(CH3)(PMe2Ar′) to eliminate ethane.