1,2-Insertion reactions of alkynes into Ge-C bonds of arylbromogermylene.

1,2-Insertion reactions of alkynes into the Ge-C bonds in dibromodigermenes afford stable crystalline bromovinylgermylenes. In contrast to previously reported Lewis-base-supported vinylgermylenes, the bromovinylgermylene obtained from reaction of the bromogermylene with 3-hexyne via such an 1,2-insertion is a donor-free monomer. A feasible reaction mechanism, proposed on the basis of the observed experimental results in combination with theoretical calculations, suggests that the [1+2]-cycloadduct and the insertion product are the kinetic and thermodynamic product, respectively.

Reversible Isomerizations between 1,4-Digermabenzenes and 1,4-Digerma-Dewar-benzenes: Air-Stable Activators for Small Molecules.

The first examples of stable, crystalline, and air-sensitive 1,4-digermabenzenes were isolated. These species photochemically isomerize into the corresponding air-stable digerma-Dewar-benzenes. More importantly, alkyl-substituted Dewar-type-1,4-digermabenzenes can be considered as reversible "air-stable activators" for small molecules such as dihydrogen, carbon dioxide, and acetylene at room temperature. The regeneration of these activators can be accomplished via a thermal retro-isomerization that affords the corresponding 1,4-digermabenzenes.

Facile insertion of ethylene into a group 14 element-carbon bond: effects of the HOMO-LUMO energy gap on reactivity.

The diarylstannylenes, Sn(AriPr4)2 and Sn(AriPr6)2, (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2, AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2), undergo a facile migratory insertion reaction with ethylene at 60 °C to afford the alkyl aryl stannylenes AriPr4SnCH2CH2AriPr4 and AriPr6SnCH2CH2AriPr6 which were characterized via1H, 13C and 119Sn NMR, UV-vis and IR spectroscopy, as well as by X-ray crystallography. Quantum mechanical calculations were performed, and two potential mechanisms were identified, with a migratory insertion reaction pathyway being energetically preferred.

Synthesis and Structural Properties of Axially Chiral Binaphthothiophene Dicarboxylic Acid.

Axially chiral binaphthothiophene dicarboxylic acid was prepared as a novel functionalized chiral dicarboxylic acid. The crystal structures of both the racemic form and its salt with chiral diamine revealed the intramolecular S···O interactions (chalcogen bonds) between the sulfur in the naphthothiophene rings and the oxygen of the carboxy groups. The negative-positive and the positive-negative Cotton effects from longer to shorter wavelengths were observed for (R)- and (S)-enantiomers, respectively, in the circular dichroism (CD) spectra.

Activation of Small Molecules by Compounds that Contain Triple Bonds Between Heavier Group-14 Elements.

Dimetallynes (RE≡ER; E=Si, Ge, Sn, Pb), i.e., systems that contain triple bonds between heavier group-14 elements represent the heavier homologues of alkynes. The high reactivity of disilynes, digermynes, distannynes, and diplumbynes can be interpreted in terms of their unique bonding situation, which includes a trans-bent structure, a small HOMO-LUMO gap, a partial diradical character, their electronic nature, and interactions between bulky substituents. Relatively recent developments in the area of sterically demanding substituents have permitted isolating some stable compounds with a triple bond between heavier group-14 elements. The structural features of these triple-bonded compounds and their reactivity toward small molecules have been investigated using experimental and theoretical approaches. This review focuses on recent developments regarding the reactivity of stable compounds with a triple bond between heavier group-14 elements toward small molecules, where the results were examined using quantum chemical methods. A comprehensive review on carbon chemistry is beyond the scope of this focus review.

The selective formation of a 1,2-disilabenzene from the reaction of a disilyne with phenylacetylene.

A stable 3,5-diphenyl-1,2-disilabenzene was selectively synthesized by the reaction between the isolable disilyne TbbSi[triple bond, length as m-dash]SiTbb (Tbb = 2,6-[CH(SiMe3)2]2-4-t-Bu-phenyl) with phenylacetylenes. Its molecular structure and physical properties were examined and compared to those of the 1,2-disilabenzene that was obtained from the reaction between TbbSi[triple bond, length as m-dash]SiTbb and acetylene. Moreover, a plausible formation mechanism for this reaction is discussed.

Sigmatropic Rearrangements of Hypervalent-Iodine-Tethered Intermediates for the Synthesis of Biaryls.

Metal-free dehydrogenative couplings of aryliodanes with phenols to afford 2-hydroxy-2'-iodobiaryls have been developed. This reaction proceeds through ligand exchange on the hypervalent iodine atom followed by a [3,3] sigmatropic rearrangement and with complete regioselectivity. This coupling, in combination with in situ oxidation by mCPBA, enables the convenient conversion of iodoarenes into desirable biaryls. The obtained biaryls have convertible iodo and hydroxy groups in close proximity, and are thus synthetically useful, as exemplified by the controlled syntheses of π-extended furans and a substituted [5]helicene. DFT calculations clearly revealed that the rearrangement is sigmatropic, with C-C bond formation and I-O bond cleavage proceeding in a concerted manner. Acetic acid, which was found to be the best solvent for this protocol, renders the iodine atom more cationic and thus accelerates the sigmatropic rearrangement.

Regioselective Cyclotrimerization of Terminal Alkynes Using a Digermyne.

The catalytic activation of small neutral molecules followed by the formation of C-C bonds is a highly important method to increase the complexity and/or value of simple starting materials. Reported is an isolable digermyne, a compound with a Ge≡Ge bond, which acts as a precatalyst for the cyclotrimerization of terminal arylacetylenes to afford the corresponding 1,2,4-triarylbenzenes with absolute regioselectivity. The results demonstrate that bespoke main-group-element compounds can catalytically activate and transform small neutral organic molecules and induce the formation of C-C bonds.

A Redox-Active Bis(ferrocenyl)germylene and Its Reactivity.

Bis(ferrocenyl)germylene Fc*2 Ge: [2; Fc*=2,5-bis(3,5-di-tert-butylphenyl)-1-ferrocenyl] was isolated in the form of red crystals from the reaction of the sterically demanding ferrocenyl lithium dimer (Fc*Li)2 and GeI2 . Bis(ferrocenyl)germylene 2 exhibits extraordinary thermal stability in hydrocarbon solution and the solid state, as well as stable redox behavior. Moreover, it undergoes a ligand-redistribution reaction with GeCl2 ⋅(dioxane) to afford the corresponding chlorogermylene, which was isolated upon coordination with PBu3 .

Reversible addition of terminal alkenes to digermynes.

Stable digermynes with sterically demanding Bbt (Bbt = 2,6-[CH(SiMe3)2]2-4-[C(SiMe3)3]-C6H2) or Tbb (Tbb = 4-tBu-2,6-[CH(SiMe3)2]2-C6H2) groups underwent [2+2] cycloadditions with terminal alkenes to give the corresponding 1,2-digermacyclobutenes. In the case of the Bbt-substituted digermyne, the reaction was reversible at room temperature, i.e., the 1,2-digermacyclobutene (Ge(ii) species) is susceptible to a facile reductive elimination that affords the corresponding digermyne (Ge(i) species) and the alkene.

Preparation, Structural Determination, and Characterization of Electronic Properties of [5,6]- and [6,6]-Carbosilylated Sc3 N@Ih -C80.

Photochemical carbosilylation of Sc3 N@Ih -C80 with silirane 1 afforded two corresponding [5,6]-adducts, 2 and 3, and a [6,6]-adduct, 4. The structural and electronic properties of these products were characterized by means of spectroscopic, electrochemical, and theoretical methods. The structure of 2 was disclosed by means of single-crystal X-ray crystallographic analysis. Thermal isomerization of 3 to 2 was observed, whereas that of 2 to 3 proceeded less efficiently at 100 °C. Upon heating under the same conditions, adduct 4 underwent facile decomposition to afford Sc3 N@Ih -C80 , or isomerized into small amounts of 2 and 3. The relative stabilities of 2, 3, and 4 were rationalized through the results of theoretical calculations. In contrast, adducts 2, 3, and 4 were stable under the photolytic conditions employed for carbosilylation. The photochemical functionalization of Sc3 N@Ih -C80 represents a convenient synthetic method to obtain thermally labile fullerene-based products.

Experimental and Theoretical Examination of the Radical Cations Obtained from the Chemical and Electrochemical Oxidation of 5-Aminothiazoles.

Chemical or electrochemical one-electron oxidation of 5-N-arylaminothiazoles was found to afford stable radical cations. For chemical oxidation, 1 equivalent of [(4-BrC6H4)3N][SbCl6] (Magic Blue, MB) was added to CH2Cl2 solutions of the thiazoles, and the thus-obtained radicals showed light absorption in the near-infrared region. Electrochemical oxidation also led to bathochromic shifts in the absorption bands, and the obtained spectra were similar to those derived from the chemically oxidized species. These radicals afforded electron paramagnetic resonance (EPR) spectra that are consistent with the notion of stable nitrogen radicals (half-life≤385 h). The EPR spectrum of a thiazole containing 4-dimethylaminophenyl groups on the nitrogen atom at the 5-position changed significantly upon adding >3 equivalents of MB. Details of the electronic structures of the experimentally obtained radical cations were generated from theoretical calculations.

The synthesis and structure of a dianionic species with a bond between pentacoordinated tin atoms: bonding properties of the tin-tin bond.

The first dianionic compound bearing a bond between two pentacoordinated tin atoms, a distannate, was synthesized in a stable form by using two sets of an electron-withdrawing C,O-bidentate ligand on each tin atom. The structure of the distannate was determined by NMR spectroscopy and X-ray crystallographic analysis. The Sn-Sn bond of the distannate was shown to be a single bond featuring high s-character. The 1J(Sn-Sn) coupling constant was larger than that of Sn(sp3)-Sn(sp3) bonds found in most hexaorganodistannanes. This bond feature was also supported by computational studies. The Sn-Sn bond was cleaved by treatment with hydrochloric acid, which shows a different reactivity to the homonuclear bonds of pentacoordinated disilicates and digermanates.

Acid-Responsive Absorption and Emission of 5-N-Arylaminothiazoles: Emission of White Light from a Single Fluorescent Dye and a Lewis Acid.

The front cover artwork is provided by the group of Toshiaki Murai at Gifu University (Japan). The image shows structures of the key compounds and the change of the fluorescence. Although electron-donating and -accepting groups are not in the same plane, they show relatively strong fluorescence from blue to orange. For more details, read the full text of the Communication at 10.1002/open.201600059.

Acid-Responsive Absorption and Emission of 5-N-Arylaminothiazoles: Emission of White Light from a Single Fluorescent Dye and a Lewis Acid.

Solutions of 5-N-arylaminothiazoles containing pyridyl groups exhibited clear halochromism and halofluorism upon addition of Brønsted and Lewis acids. The addition of triflic acid to solutions of 5-N-arylaminothiazoles in Et2O induced bathochromic shifts of the absorption and emission bands. DFT calculations suggested that the spectral changes arise from the protonation of the pyridyl group of the thiazoles in Et2O. Single-crystal X-ray diffraction analysis of a thiazole and its protonated form revealed the change of the conformation around the thiazole ring. The emission of white light was accomplished from a single fluorescent dye by adjusting the ratio of dye and B(C6F5)3, whereby the International Commission on Illumination coordinates showed a linear change from blue to orange.

Dispersion Forces, Disproportionation, and Stable High-Valent Late Transition Metal Alkyls.

The transition metal tetra- and trinorbornyl bromide complexes, M(nor)4 (M=Fe, Co, Ni) and Ni(nor)3 Br (nor=1-bicyclo[2.2.1]hept-1-yl) and their homolytic fragmentations were studied computationally using hybrid density functional theory (DFT) at the B3PW91 and B3PW91-D3 dispersion-corrected levels. Experimental structures were well replicated; the dispersion correction resulted in shortened M-C bond lengths for the stable complexes, and it was found that Fe(nor)4 receives a remarkable 45.9 kcal mol-1 stabilization from the dispersion effects whereas the tetragonalized Co(nor)4 shows stabilization of 38.3 kcal mol-1 . Ni(nor)4 was calculated to be highly tetragonalized with long Ni-C bonds, providing a rationale for its current synthetic inaccessibility. Isodesmic exchange evaluation for Fe(nor)4 confirmed that dispersion force attraction between norbornyl substituents is fundamental to the stability of these species.

Synthesis of a 1-Aryl-2,2-chlorosilyl(phospha)silene Coordinated by an N-Heterocyclic Carbene.

Phosphasilenes, P=Si doubly bonded compounds, have received considerable attention due to their unique physical and chemical properties. We report on the synthesis and structure of a chlorophosphasilene coordinated by an N-heterocyclic carbene (NHC), which has the potential of functionalization at the Si-Cl moiety. Treatment of a silylphosphine, ArPH-SiCl2RSi (Ar = bulky aryl group, RSi = Si(SiMe3)3) with two equivalents of Im-Me4 (1,3,4,5-tetramethylimidazol-2-ylidene) afforded the corresponding NHC-coordinated phosphasilene, ArP=SiClRSi(Im-Me4) as a stable compound. Bonding properties of the P=Si bond coordinated to an NHC will be discussed on the basis of theoretical calculations.

(η(4)-Butadiene)Sn(0) Complexes: A New Approach for Zero-Valent p-Block Elements Utilizing a Butadiene as a 4π-Electron Donor.

Research on zero-valent p-block elements is a recent hot topic in synthetic and theoretical chemistry because of their novel electronic states having two lone pairs in both the s- and p-orbitals. It is considered that σ-donating ligands bearing large substituents are essential to stabilize these species. Herein, we propose a new approach using butadiene as a 4π-electron donor to stabilize zero-valent group 14 elements. During our study to explore the coordination chemistry of stannacyclopentadienyl ligands, unexpected products, in which the tin atom is coordinated by a butadiene in a η(4)-fashion, were obtained. Because butadiene is a neutral 4π-electron donating ligand, the formal oxidation number of the tin atoms of the products should be zero, which is supported by X-ray diffraction analysis and theoretical calculations. A mechanism for the formation of the products is also described.

Isolation and Ambident Reactivity of a Chlorogermylenoid.

Treatment of 2,5-di(3,5-tert-butylphenyl)-1-lithioferrocene with GeCl2 ⋅dioxane afforded the corresponding chlorogermylenoid that exhibited an ambident reactivity in different solvents; it displayed a behavior characteristic for a dichlorogermylene anion in THF, while it exhibited the typical reactivity of a chlorogermylene in toluene. X-Ray diffraction analysis of a single crystal of this chlorogermylenoid, obtained from recrystallization in THF, revealed a separated-ion-pair structure in the solid state.

Quasi-three-coordinate iron and cobalt terphenoxide complexes {Ar(iPr8)OM(µ-O)}2 (Ar(iPr8) = C6H-2,6-(C6H2-2,4,6-(i)Pr3)2-3,5-(i)Pr2; M = Fe or Co) with M(III)2(µ-O)2 core structures and the peroxide dimer of 2-oxepinoxy relevant to benzene oxidation.

The bis(µ-oxo) dimeric complexes {Ar(iPr8)OM(µ-O)}2 (Ar(iPr8) = C6H-2,6-(C6H2-2,4,6-(i)Pr3)2-3,5-(i)Pr2; M = Fe (1), Co (2)) were prepared by oxidation of the M(I) half-sandwich complexes {Ar(iPr8)M(η(6)-arene)} (arene = benzene or toluene). Iron species 1 was prepared by reacting {Ar(iPr8)Fe(η(6)-benzene)} with N2O or O2, and cobalt species 2 was prepared by reacting {Ar(iPr8)Co(η(6)-toluene)} with O2. Both 1 and 2 were characterized by X-ray crystallography, UV-vis spectroscopy, magnetic measurements, and, in the case of 1, Mössbauer spectroscopy. The solid-state structures of both compounds reveal unique M2(µ-O)2 (M = Fe (1), Co(2)) cores with formally three-coordinate metal ions. The Fe···Fe separation in 1 bears a resemblance to that in the Fe2(µ-O)2 diamond core proposed for the methane monooxygenase intermediate Q. The structural differences between 1 and 2 are reflected in rather differing magnetic behavior. Compound 2 is thermally unstable, and its decomposition at room temperature resulted in the oxidation of the Ar(iPr8) ligand via oxygen insertion and addition to the central aryl ring of the terphenyl ligand to produce the 5,5'-peroxy-bis[4,6-(i)Pr2-3,7-bis(2,4,6-(i)Pr3-phenyl)oxepin-2(5H)-one] (3). The structure of the oxidized terphenyl species is closely related to that of a key intermediate proposed for the oxidation of benzene.