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).
Publications
2021
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)210, and [(4-MeOan)2(DMSO)4Ni](Br)211 (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.
A common feature of d‐ and p‐block elements is that they participate in multiple bonding. In contrast, the synthesis of compounds containing homo‐ or hetero‐nuclear multiple bonds involving s‐block elements is extremely rare. Herein, we report the synthesis, molecular structure, and computational analysis of a beryllium imido (Be=N) complex (2), which was prepared via oxidation of a molecular Be(0) precursor (1) with trimethylsilyl azide Me3SiN3 (TMS-N3 ). Notably, compound 2 features the shortest known Be=N bond (1.464 Å) to date . This represents the first compound with an s‐block metal−nitrogen multiple bond. All compounds were characterized experimentally with multi‐nuclear NMR spectroscopy ( 1H, 13C, 9Be) and single‐crystal X‐ray diffraction studies. The bonding situation was analyzed with density functional theory (DFT) calculations, which supports the existence of π ‐bonding between beryllium and nitrogen.
Electrochemical methods are coupled with chemical oxidant-based analytical strategies to evaluate the oxidative reactivity of the platinum(II) diimine complex (bpy)PtII(CH3)2 (bpy = 2,2′-bipyridine) in coordinating and noncoordinating media. Through this mechanistic analysis, we show that the one-electron oxidation of (bpy)PtII(CH3)2 generates a highly reactive, 15-electron PtIII radical cation and identify three reaction pathways that can follow this oxidation: radical–substrate dimerization, radical–radical dimerization, and oxidative disproportionation. Axial ligation of the initially generated [(bpy)PtIII(CH3)2]•+ species is a critical driver of this reactivity, gating the available mechanistic pathways and tuning the competition between radical–radical dimerization and oxidative disproportionation.
We report a Co-based complex for the reduction of O2 to H2O utilizing decamethylferrocene as chemical reductant and acetic acid as a proton donor in methanol solution. Despite structural similarities to previously reported Co(N2O2) complexes capable of catalytic O2 reduction, this system shows selectivity for the four-electron/four-proton reduction product, H2O, instead of the two-electron/two-proton reduction product, H2O2. Mechanistic studies show that the overall rate law is analogous to previous examples, suggesting that the key selectivity difference arises in part from increased favorability of protonation at the distal O position of the key intermediate Co(III)-hydroperoxide, instead of the proximal one. Interestingly, no product selectivity dependence is observed with respect to the presence of pyridine, which is proposed to bind trans to O2 during catalysis.
While pnictaalkenes are well‐established for the light group 15 elements, they become more reactive and exceptionally rare as the group is descended. Herein, we report a combined experimental and theoretical study on the first examples of carbodicarbene (CDC)‐stabilized bismuth complexes, which feature low‐coordinate cationic bismuth centers with C=Bi multiple bond character. Monocations [(CDC)Bi(Ph)Cl][SbF6] (8) and [(CDC)BiBr2(THF)2][SbF6] (11), dications [(CDC)Bi(Ph)][SbF6]2 (9) and [(CDC)BiBr(THF)3][NTf2]2 (12), and trication [(CDC)2Bi][NTf2]3 (13) have been synthesized via sequential halide abstractions from (CDC)Bi(Ph)Cl2 (7) and (CDC)BiBr3 (10). Notably, the dications and trication exhibit C⇉Bi double dative bonds, and thus represent unprecedented bismaalkene cations. In addition, the synthesis of these species highlights a unique non‐reductive route to C–Bi π‐bonding character. The CDC‐[Bi] complexes (7‐13) were compared with related NHC‐[Bi] complexes (1, 3‐6) and show substantially different structural properties. Indeed, the CDC ligand has a remarkable influence on the overall stability of the resulting low‐coordinate Bi complexes, suggesting that CDC is a superior ligand to NHC in heavy pnictogen chemistry. All compounds have been characterized by multiple analytical methods including 1H and 13C NMR, X‐ray crystallography, elemental analysis, and UV‐Vis spectroscopy. In addition, the bonding situation was analyzed with modern charge and energy decomposition analysis.