Lewis Superacidic Heavy Pnictaalkene Cations: Comparative Assessment of Carbodicarbene-Stibenium and Carbodicarbene-Bismuthenium Ions.

We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and -bismuthenium ions using the Gutmann-Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr3 and [(pyCDC)PnBr2][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr3 (1) with one or two equivalents of AgNTf2 (NTf2 = bis(trifluoromethanesulfonyl)imide) or AgSbF6 gives stibaalkene mono- and dications of the form [(CDC)SbBr3-n][A]n (2-4; n = 1,2; A = NTf2 or SbF6). The stibaalkene trication [(CDC)2Sb][NTf2]3 (5) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6-9. With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene (pyCDC) was used as a ligand to access [(pyCDC)PnBr2][Br] (10, 12) and trications [(pyCDC)Pn][NTf2]3 (Pn = Sb (11), Bi (13)), forgoing the need for a second CDC as used in the synthesis of 5. The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC-Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf2]2 (4) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF3, and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF5 (85.1 kcal/mol), which suggest that these cations are Lewis superacids.

Air-Stable Thermoluminescent Carbodicarbene-Borafluorenium Ions.

Borenium ions, originally synthesized as fundamentally important laboratory curiosities, have attracted significant attention due to their applications in catalysis and frustrated Lewis pair chemistry. However, investigations of the materials properties of these types of compounds are exceptionally rare. Herein, we report the synthesis, molecular structures, and optical properties of a new class of air-stable borenium ions, stabilized by the strongly donating carbodicarbene (CDC) ligand (2, 3, 6). Notably, CDC-borafluorenium ions exhibit thermoluminescence in solution, a result of a twisted intramolecular charge transfer process. The temperature responsiveness, which is observable by the naked eye, is assessed over a 20 to -60 °C range. Significantly, compound 2 emits white light at lower temperatures. In the solid state, these borocations exhibit increased quantum yields due to aggregation-induced emission. CDC-borafluorenium ions with two different counteranions (Br-, BPh4-) were investigated to evaluate the effect of anion size on the solution and solid-state optical properties. In addition, CDCs containing both symmetrical and unsymmetrical N-heterocycles (bis(1-isopropyl-3-methylbenzimidazol-2-ylidene)methane and bis(1,3-dimethyl-1,3-dihydro-2H-benzo[d]imidazol-2-ylidene)methane) were tested to understand the implications of free rotation about the CDC ligand carbon-carbon bonds. The experimental work is complemented by a comprehensive theoretical analysis of the excited-state dynamics.

Indirect Access to Carbene Adducts of Bismuth- and Antimony-Substituted Phosphaketene and Their Unusual Thermal Transformation to Dipnictines and [(NHC)2OCP][OCP].

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).

s-Block Multiple Bonds: Isolation of a Beryllium Imido Complex.

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 Be0 precursor (1) with trimethylsilyl azide Me3 SiN3 (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 (1 H, 13 C, 9 Be) 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.

Carbodicarbene Bismaalkene Cations: Unravelling the Complexities of Carbene versus Carbone in Heavy Pnictogen Chemistry.

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)2 Bi][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. 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.

A Stable, Crystalline Beryllium Radical Cation.

The alkaline-earth elements (Be, Mg, Ca, Sr, and Ba) strongly favor the formation of diamagnetic compounds in the +2 oxidation state. Herein we report a paramagnetic beryllium radical cation, [(CAAC)2Be]+• (2) [CAAC = cyclic (alkyl)(amino)carbene], prepared by oxidation of a zero-valent beryllium complex with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). Compound 2 was characterized by EPR spectroscopy, elemental analysis, X-ray crystallography, and DFT calculations. Notably, the isolation of 2 represents the first s-block charged radical and the first crystalline beryllium radical.

Low-nuclearity magnesium hydride complexes stabilized by N-heterocyclic carbenes.

Herein we report the synthesis and characterization of dinuclear magnesium-hydride complexes, [(IiPrMe2)Mg(µ-H)(HMDS)]2 [4, IiPrMe2 = N,N'-diisopropyl-2,3-dimethylimidazol-2-ylidine, HMDS = bis(trimethylsilyl)amide] and [(IiPrMe2)Mg(µ-H)(ASCP)]2 (5, ASCP = 2,2,5,5-tetramethyl-2,5-disila-1-azacyclopent-1-yl). Compounds 4 and 5 exhibit the lowest nuclearity of any carbene-magnesium hydride complex to date.

Cyclic(alkyl)(amino) Carbene-Promoted Ring Expansion of a Carbodicarbene Beryllacycle.

Recent synthetic efforts have uncovered several bond activation pathways mediated by beryllium. Having the highest charge density and electronegativity, the chemistry of beryllium often diverges from that of its heavier alkaline earth metal congeners. Herein, we report the synthesis of a new carbodicarbene beryllacycle (2). Compound 2 converts to 3 via an unprecedented cyclic(alkyl)(amino) carbene (CAAC)-promoted ring expansion reaction (RER). While CAAC activates a carbon-beryllium bond, N-heterocyclic carbene (NHC) coordinates to beryllium to give the tetracoordinate complex 4, which contains the longest carbeneC-Be bond to date at 1.856(4) Å. All of the compounds were fully characterized by X-ray crystallography, Fourier transform infrared spectroscopy, and 1H, 13C, and 9Be NMR spectroscopy. The ring expansion mechanism was modeled with both NHC and CAAC using density functional theory calculations. While the activation energy for the observed beryllium ring expansion with CAAC was found to be 14 kJ mol-1, the energy barrier for the hypothetical NHC RER is significantly higher (199.1 kJ mol-1).

Stepwise Reduction at Magnesium and Beryllium: Cooperative Effects of Carbenes with Redox Non-Innocent α-Diimines.

In the past two decades, the organometallic chemistry of the alkaline earth elements has experienced a renaissance due in part to developments in ligand stabilization strategies. In order to expand the scope of redox chemistry known for magnesium and beryllium, we have synthesized a set of reduced magnesium and beryllium complexes and compared their resulting structural and electronic properties. The carbene-coordinated alkaline earth-halides, (Et2CAAC)MgBr2 (1), (SIPr)MgBr2 (2), (Et2CAAC)BeCl2 (3), and (SIPr)BeCl2 (4) [Et2CAAC = diethyl cyclic(alkyl)(amino) carbene; SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene] were combined with an α-diimine [2,2-bipyridine (bpy) or bis(2,6-diisopropylphenyl)-1,4-diazabutadiene (DippDAB)] and the appropriate stoichiometric amount of potassium graphite to form singly- and doubly-reduced compounds (Et2CAAC)MgBr(DippDAB) (5), (Et2CAAC)MgBr(bpy) (6), (Et2CAAC)Mg(DippDAB) (7), (Et2CAAC)Be(bpy) (8), and (SIPr)Be(bpy) (9). The doubly-reduced compounds 7-9 exhibit substantial π-bonding interactions across the diimine core, metal center, and π-acidic carbene. Each complex was fully characterized by UV-vis, FT-IR, X-ray crystallography, 1H, 13C, and 9Be NMR, or EPR where applicable. We use these compounds to highlight the differences in the organometallic chemistry of the lightest alkaline earth metals, magnesium and beryllium, in an otherwise identical chemical environment.

s-Block carbodicarbene chemistry: C(sp3)-H activation and cyclization mediated by a beryllium center.

The first examples of carbodicarbene (CDC)-s-block complexes have been synthesized. The addition of base or reducing agent to a CDC-beryllium (chloride)(hexamethyldisilazide) adduct results in the unprecedented activation of a pendant C(sp3)-H bond and cyclization of the CDC to form a five-membered beryllium metallacycle. This also represents the first example of chemical activation of a CDC which transforms the ligand from monodentate neutral to chelating anionic.