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Ionic liquids (ILs) are promising electrolytes, although their often high viscosity remains a serious drawback. The latter can be addressed by the introduction of multiple ether functionalization. Based on the highly atom efficient synthesis of tris(2-ethoxyethyl) phosphine, several new phosphonium ionic liquids were prepared, which allows studying the influence of the ether side chains. Their most important physicochemical properties have been determined and will be interpreted using established approaches like ionicity, hole theory, and the Walden plot. There is striking evidence that the properties of phosphonium ionic liquids with the methanesulfonate anion are dominated by aggregation, whereas the two triple ether functionalized ILs with the highest fluidity show almost ideal behavior with other factors being dominant. It is furthermore found that the deviation from ideality is not significantly changed upon introduction of the ether side chains, although a very beneficial impact on the fluidity of ILs is observed. Multiple ether functionalization therefore proves as a powerful tool to overcome the disadvantages of phosphonium ionic liquids with large cations., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Syntheses and structures of anionic arylphosphate monoesters [ArOP(O) 2 (OH)] - (Ar = 2,6-CHPh 2 -4-R-C 6 H 2 ; R = Me/Et/ i Pr/ t Bu) with different counter cations are reported. The counter cations were varied systematically: imidazolium cation, 2-methyl imidazolium cation, N -methyl imidazolium cation, N , N '-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, 4,4'-bipyridinium dication, and magnesium(II) dication. The objective was to examine if the supramolecular structure of anionic arylphosphate monoesters could be modulated by varying the cation. It was found that an eight-membered P 2 O 4 H 2 -hydrogen-bonded dimeric motif involving intermolecular H-bonding between the [P(O)(OH)] unit of the anionic phosphate monoester along with the counter cation is formed with 2-methyl imidazolium cation, N -methyl imidazolium cation, N , N '-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, and magnesium(II) dication; both discrete and polymeric H-bonded structures are observed. In the case of imidazolium cations and 1,4-diazabicyclo[2.2.2]octan-1-ium cation, the formation of one-dimensional polymers (single lane/double lane) was observed. On the other hand, two types of phosphate motifs, intermolecular H-bonded dimer and an open-form, were observed in the case of 4,4'-bipyridinium dication., Competing Interests: The authors declare no competing financial interest.

Permethylated disila[2]metallocenophanes of silicon, germanium, tin, lead, 2 a-d, (tetrelocenophanes) and antimony, 3 a,b, (pnictogenocenophanes) are described. In the case of antimony, a chloro-substituted stibonocenophane, 3 a, as well as cationic stibonocenophanium tetrachloroaluminate and tetraphenylborate salts, 3 b[X] (X=[AlCl 4 ], [BPh 4 ]), were synthesized. These represent the first examples of metallocenophanes of any Group 15 element. All compounds were studied in solution and in the solid state. Without exception the ansa-bridge exerts a strong influence on the bending angle of the two Cp-ligands. For disila[2]plumbocenophane, 2 d, its reactivity towards Group 15 halides was probed. Treatment of disila[2]plumbocenophane, 2 d, with two equivalents of phosphorus(III) chloride or arsenic(III) chloride, results in a ring-opening reaction and gives the bis(dihalopnictogenyl)-substituted products, 4 a,b., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

1,2,3-Trisilacyclopentadienes are obtained from the reactions of cyclotrisilene c-Si3R4 (R = iPr3C6H2) with phenyl and diphenyl acetylene, respectively. With 1,4-diethynyl benzene the cross-conjugated bridging of two of the Si3C2 cycles by a para-phenylene linker is achieved. UV/vis spectroscopy indicates a small but significant effect of cross-conjugation, which is confirmed by TD-DFT calculations. The formation mechanism of the 1,2,3-trisilacyclopentadienes is elucidated by VT NMR.

Propellane-like structures are an important building block for group-14 compounds. Therefore, in the present work, we study theoretically various such structures of the form E[Formula: see text] starting with the pure molecules for which E = E ' = E ″ = C, Si, Ge, or Sn. Subsequently, we study the systems with E ≠ E ' = E ″ and finally, we consider some selected cases molecules for which E, E ' , and E ″ all are different. Special emphasis is put on identifying structural trends for the molecules with at least two different group-14 elements. The resulting scheme is, we believe, generally valid for group-14 based systems with more than one type of group-14 elements.

Unsaturated silicon clusters are key intermediates of silicon deposition processes from the gas phase, which is reflected in the recently introduced term "siliconoids" for silicon rich clusters with at least one hemispheroidally coordinated vertex. Unlike the case of metalloid clusters of heavier Group 14 elements, stable homonuclear derivatives containing silicon have become available just recently. This review summarizes the developments since the first report on a stable unsaturated silicon cluster in 2005. The synthesis, structure and reactivity under retention of the unsaturated vertices (i.e. the functionalization) of stable siliconoids is discussed within the broader context of soluble Zintl anions of silicon and metalloid clusters. A structural parameter is introduced as a quantitative measure to characterize the "hemispheroidality" of an unsubstituted vertex of a siliconoid.

Functionalized disilenes are valuable auxiliary tools in preparative chemistry. In the following review the various synthetic potential of disilenes as precursors is presented. Upon consumption of the Si=Si unit in disilenes, cyclic, polycyclic and cluster-like arrangements are obtainable., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Diphosphene Ter Mes P = PTer Mes ( 1 ; Ter Mes = 2,6-Mes 2 C 6 H 3 ; Mes = 2,4,6-Me 3 C 6 H 2 ) and NHC Me 4 2 (NHC Me 4 = 1,3,4,5-tetramethylimidazol-2-ylidene) exist in an equilibrium mixture with the NHC Me 4 -coordinated diphosphene 3 . While uncoordinated 1 is inert to hydrolysis, the NHC adduct 3 readily undergoes hydrolysis to afford a phosphino-substituted phosphine oxide with the liberation of NHC Me 4 . On this basis, conditions suitable for the catalytic use of NHC Me 4 were identified. Similarly, while the hydrogenation of free diphosphene 1 with H 3 N·BH 3 is very slow, 3 reacts instantaneously with H 3 N·BH 3 at room temperature to afford a dihydrodiphosphane.

We report the synthesis and isolation of molecular iron germanide motifs in the stoichiometry of Fe 3 Ge and Fe 2 Ge, which are stabilized by the coordination of N-heterocyclic carbene (NHC) and carbon monoxide (CO) ligands. NHCiPr2Me2 ⋅Ge[Fe(CO) 4 ][Fe 2 (CO) 8 ] (NHCiPr2Me2 =1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), a germanium(0) ligand with just one NHC as an auxiliary ligand, assumes a bridging coordination mode between a mononuclear [Fe(CO) 4 ] and a dinuclear [(CO) 4 FeFe(CO) 4 ] unit with rapid exchange between the two binding modes in solution. The electronic structure of this species is analysed by NBO and ELF calculations with DFT methods, as well as the mechanistic details of its fluxional coordination behaviour. Treatment of NHCiPr2Me2 ⋅Ge[Fe(CO) 4 ] [Fe 2 (CO) 8 ] with the sterically less demanding NHCMe4 (NHCMe4 =1,3,4,5-tetramethylimidazol-2-ylidene) leads to a dinuclear iron complex (NHCMe4 ) 2 Ge[Fe(CO) 4 ] 2 that contains a bridging germylone ligand with two stabilizing NHC equivalents., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The highly reactive silicon congeners of cyclopropene, cyclotrisilenes (c-Si 3 R 4 ), typically undergo either π-addition to the Si=Si double bond or σ-insertion into the Si-Si single bond. In contrast, treatment of c-Si 3 Tip 4 (Tip=2,4,6- i Pr 3 C 6 H 2 ) with styrene and benzil results in ring opening of the three-membered ring to formally yield the [1+2]- and [1+4] cycloaddition product of the isomeric disilenyl silylene to the C=C bond and the 1,2-diketone π system, respectively. At elevated temperature, styrene is released from the [1+2]-addition product leading to the thermodynamically favored housane species after [2+2] cycloaddition of styrene and c-Si 3 Tip 4 ., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A series of aryl disilenes Tip 2 Si[double bond, length as m-dash]Si(Tip)Ar (2a-c) and para-arylene bridged tetrasiladienes, Tip 2 Si[double bond, length as m-dash]Si(Tip)-LU-Si(Tip)[double bond, length as m-dash]SiTip 2 (3a-d) are synthesized by the transfer of the Tip 2 Si[double bond, length as m-dash]SiTip unit to aryl halides and dihalides by nucleophilic disilenides Tip 2 Si[double bond, length as m-dash]SiTipLi (Tip = 2,4,6-iPr 3 C 6 H 2 , Ar = aryl substituent, LU = para-arylene linking unit). The scope of the nucleophilic Si[double bond, length as m-dash]Si transfer reaction is demonstrated to also include substrates of considerable steric bulk such as mesityl or duryl halides Ar-X (Ar = Mes = 2,4,6-Me 3 C 6 H 2 ; Ar = Dur = 2,3,5,6-Me 4 C 6 H, X = Br or I). Bridged tetrasiladienes Tip 2 Si[double bond, length as m-dash]Si(Tip)-LU-Si(Tip)[double bond, length as m-dash]SiTip 2 with more extended linking units surprisingly exhibit fluorescence at room temperature, albeit weak. DFT calculations suggest that partial charge transfer character of the excited state is a possible explanation.

An isolable NHC-coordinated disilenyl germylene R 2 Si═SiR-GePh·NHC iPr 2 Me 2 (5b: R = 2,4,6-triisopropylphenyl, NHC iPr 2 Me 2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) is obtained from the reaction of the α-chloro silyl functionalized heavier vinylidene analogue R 2 (Cl)Si-RSi═Ge·NHC iPr 2 Me 2 with phenyllithium. The disilenyl germylene 5b isomerizes at 40 °C to the NHC-adduct of the corresponding heavier cyclopropylidene analogue 6b. Both 5b and 6b display near perfect regiodiscrimination in their reactions with phenylacetylene and xylyl isocyanide, affording in the case of each reagent two different regioisomers of NHC-coordinated cyclic germylenes. DFT calculations reveal that the Si═Si bond accounts for the high reactivity of 5b even at low temperature while in the case of cyclic 6b the low-valent germanium center requires a considerable thermal activation.

The reaction of MesLi (Mes=2,4,6-trimethylphenyl) with the electrophilic phosphasilene R2 (NMe2 )Si-RSi=PNMe2 (2, R=Tip=2,4,6-triisopropylphenyl) cleanly affords R2 (NMe2 )Si-RSi=PMes and thus provides the first example of a substitution reaction at an unperturbed Si=P bond. In toluene, the reaction of 2 with lithium disilenide, R2 Si=Si(R)Li (1), apparently proceeds via an initial nucleophilic substitution step as well (as suggested by DFT calculations), but affords a saturated bicyclo[1.1.0]butane analogue as the final product, which was further characterized as its Fe(CO)4 complex. In contrast, in 1,2-dimethoxyethane the reaction of 1 with 2 results in an unprecedented metal-amino exchange reaction., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Heavier group 14 multiple bonds have intrigued chemists since more than a century. The synthesis of stable compounds with double and triple bonds with silicon, germanium, tin and lead had considerable impact on modern ideas of chemical bonding. These developments were made possible by the use of bulky substituents that provide kinetic and thermodynamic protection. Since about a decade the compatibility of heavier multiple bonds with various functional groups has moved into focus. This review covers multiply bonded group 14 species with at least one additional reactive site. The vinylic functionalities of groups 1 and 17, resulting in nucleophilic and electrophilic disila vinyl groups, respectively, are the most prevalent and well-studied. They have been employed repeatedly for the transfer of heavier multiple bonds to yield low-valent group 14 compounds with novel structural motifs. Vinylic functionalities of groups 2 to 16 and a few σ-bonded transition metal complexes are experimentally known, but their reactivity has been studied to a lesser extent. Donor-coordinated multiple bonds are a relatively new field of research, but the large degree of unsaturation as isomers of alkynes (as well as residual functionality in some cases) offers considerable possibility for further manipulation, e.g. for the incorporation into more extended systems. Heavier allyl halides constitute the major part of heavier multiple bonds with a functional group in allylic position and some examples of successful transformations are given. At present, remote functionalities are basically limited to para-phenylene functionalised disilenes. The reported use of the latter for further derivatisation might encourage investigations in this direction. In summary, the study of peripherally functionalised multiple bonds with heavier group 14 elements is already well beyond its infancy and may be an instrumental factor in awakening the potential of group 14 chemistry for applications in polymers and other materials.

The characteristic features of bulk silicon surfaces are echoed in the related partially substituted-and thus unsaturated-neutral silicon clusters (siliconoids). The incorporation of siliconoids into more-extended frameworks is promising owing to their unique electronic features, but further developments in this regard are limited by the notable absence of functionalized siliconoid derivatives until now. Herein we report the isolation and full characterization of the lithium salt of an anionic R5 Si6 -siliconoid, thus providing the missing link between silicon-based Zintl anions and siliconoid clusters. Proof-of-principle for the high potential of this species for the efficient transfer of the intact unsaturated R5 Si6 moiety is demonstrated by clean reactions with representative electrophiles of Groups 13, 14, and 15., (© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Tetrasiladigermatricyclohexanes in two isomeric forms (chair and doubly-bridged tetrahedron) are obtained by the reaction of MeLi with an α-chlorosilyl functionalized NHC-stabilized silagermenylidene. (29)Si NMR at low temperature proves the initial formation of a monomeric NHC-adduct of a disilenyl germylene followed by cyclisation to the isomeric heavy cyclopropene. Addition of an excess of NHC stabilizes the both intermediates and demonstrates the reversibility of rate-determining initial equilibria involving NHC dissociation. Finally, a mixture of two isomeric tricyclic Si4Ge2 species is obtained: at elevated temperature the chair isomer converts to the doubly edge-bridged tetrahedron.

The complete reductive cleavage of the triple bond in carbon monoxide was achieved using a lithium disilenide at room temperature. The C-C-coupled product can be regarded as a silanone dimer with pending alkyne and silirene moieties and incorporates two equivalents of CO per disilenide unit. A formation mechanism via ketenyl intermediates is proposed on the basis of DFT calculations and elucidated experimentally by employing Group 6 metal carbonyls as both stabilizing entity and source of CO in the reaction with disilenide. The isolation of cyclic silylene complexes with weakly donating ketenyl donor groups further supports the mechanistic scenario., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reactivity of the 2-phosphaethynolate anion (PCO(-)) towards a cyclic trisilene (cSi3(Tip)4) is reported. The result is the net activation of the P-C and Si-Si multiple bonds of the precursors affording a heteroatomic bicyclo[1.1.1]pentan-2-one analogue ([P(CO)Si3(Tip)4](-); 1). This reaction can be interpreted as the formal addition of a phosphide and a carbonyl across the Si-Si double bond. Photolytic decarbonylation of 1 results in the incorporation of the phosphide vertex into the cyclotrisilene scaffold, yielding a congener of the cyclobutene anion with considerable allylic character., (© 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.)

Stable compounds with a GeGe bond are usually prepared under relatively harsh reaction conditions that are incompatible with many functional groups. In particular, unsaturated functionalities are not tolerated owing to their facile reaction with low-coordinate germanium compounds. We now report the synthesis of an imino-functionalized germanium(II) species, stabilized by coordination of an N-heterocyclic carbene (NHC), by reaction of an isonitrile with a heavier NHC-coordinated vinylidene. Removal of the NHC by a Lewis acid results in dimerization to the corresponding digermene with a GeGe bond. The reversibility of this process is demonstrated by addition of two equivalents of NHC to the isolated digermene., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Cyclopropylidene is a transient intermediate of the allene-propyne-cyclopropene isomerization. The incorporation of heavier Group 14 elements into the cyclopropylidene scaffold has to date been restricted to the formal replacement of the carbenic carbon atom by a base-coordinated silicon(II) center. Herein we report the synthesis and characterization of NHC-coordinated heavier cyclopropylidenes (Si2GeR3X, and Si3R3Br; X=Cl, Mes; R=Tip=2,4,6-iPr3C6H2) in which the three-membered ring is exclusively formed by silicon and germanium. In case of the chloro-substituted Si2Ge-cyclopropylidene, a stable heavier cycloprop-1-yl-2-ylidene cation is obtained by NHC-induced chloride dissociation., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In the presence of a Lewis acid or base, cyclotrisilene cSi3Tip4 (Tip = 2,4,6-triisopropylphenyl) reacts with CO to form stable adducts of a 1,3-disila-2-oxyallyl zwitterion. Coordination of an N-heterocyclic carbene (NHC) to one silicon center results in a localized Si=C bond, whereas B(C6F5)3 bonds to the carbonyl moiety leading to the delocalization of the positive charge over the oxyallyl moiety. Computational analysis of the electronic structure of the Lewis acid adduct reveals a considerable interaction between the two silicon atoms in 1,3-position and thus aromaticity akin to the homocyclopropenium cation., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A catalyst- and by-product-free protocol for the synthesis of σ-π conjugated organosilicon polymers is reported. The regiospecific [2+2] cycloaddition of C≡C triple bonds to Si=Si double bonds allowed the preparation of air-stable ethynyl-terminated extended monomers from 1,4-bis(ethynyl)benzene and the para-phenylene bridged tetrasiladiene, Tip2 Si=SiTip-pC6H4-SiTip=SiTip2 (Tip = 2,4,6-iPr3C6H2). The polymer obtained from the extended monomer and further tetrasiladiene exhibits pronounced σ-π conjugation, as was evident from the red-shift in the absorption spectrum compared to model systems. We show that the thermal stability of the employed bis(alkyne) co-monomer is translated into this polymer., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Vinylidenes are common in transition metal chemistry with catalytic applications in alkene and alkyne metathesis. We report here the isolation of a heavier analogue of vinylidene, an α-chlorosilyl functionalized silagermenylidene stabilized by an N-heterocyclic carbene (NHC). Silagermenylidene (Tip2Cl)Si-(Tip)Si=Ge·NHC(iPr2Me2) (4-E/Z; Tip = 2,4,6-(i)Pr3C6H2; NHC(iPr2Me2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) is available as an E/Z-equilibrium mixture from Tip2Si=Si(Tip)Li and NHC(iPr2Me2)·GeCl2. Reaction of 4-E/Z with Fe2(CO)9 affords a silagermenylidene Fe(CO)4 complex, which slowly isomerizes to its E-isomer at 25 °C. A rearranged Fe(CO)3 complex with an allylic SiGeSi ligand is obtained as a side product at 65 °C.

Aromatic species with heavier Group 14 elements show remarkable differences in terms of stability, structure, and reactivity. Herein we report our experimental and theoretical investigations regarding isomers of germanium- and tin-containing benzene analogues E2Si4R6 (E=Ge, Sn). The germanium-substituted dismutational isomer with a tricyclic six-membered scaffold is isolable, but unlike the homonuclear Si6 analogue slowly rearranges even at room temperature to give the propellane-type global minimum isomer. In case of E=Sn the dismutational isomer may be an intermediate on the pathway to the propellane-type species obtained, but cannot be detected even at low temperature. Unprecedentedly large chemical shift anisotropies in the (29)Si NMR spectra that increase from the Si6 species through Ge2Si4 to Sn2Si4 are rationalized by progressively larger paramagnetic-term contributions to the chemical shift tensor as a result of diminishing HOMO-LUMO gaps, which are also reflected in the absorption spectra, as well as by appearance and symmetry of these frontier orbitals., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The generation of heavier double-bond systems without by- or side-product formation is of considerable importance for their application in synthesis. Peripheral functional groups in such alkene homologues are promising in this regard owing to their inherent mobility. Depending on the steric demand of the N-alkyl substituent R, the reaction of disilenide Ar2Si=Si(Ar)Li (Ar = 2,4,6-iPr3C6H2) with ClP(NR2)2 either affords the phosphinodisilene Ar2 Si=Si(Ar)P(NR2)2 (for R = iPr) or P-amino functionalized phosphasilenes Ar2(R2N)Si=Si(Ar)=P(NR2) (for R = Et, Me) by 1,3-migration of one of the amino groups. In case of R = Me, upon addition of one equivalent of tert-butylisonitrile a second amino group shift occurs to yield the 1-aza-3-phosphaallene Ar2(R2N)Si=Si(NR2)(Ar)-P=C=NtBu with pronounced ylidic character. All new compounds were fully characterized by multinuclear NMR spectroscopy as well as single-crystal X-ray diffraction and DFT calculations in selected cases., (Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In organic chemistry, compounds with adjacent alkene and carbene functionalities (vinyl carbenes) are studied widely as fleeting intermediates and in the coordination sphere of transition metals. Stable derivatives of vinyl carbenes remain elusive, including the corresponding heavier group 14 homologues. Here we report the isolation and full characterization of a base-stabilized silicon version of a vinyl carbene that features a silicon-silicon double bond as well as a silylene functionality, coordinated by an N-heterocyclic carbene (NHC). In solution, the intensely green disilenyl silylene adduct exists in equilibrium with the corresponding silicon analogue of a cyclopropene and free NHC, which was quantified by nuclear magnetic resonance spectroscopy and ultraviolet-visible spectroscopy. The reversibility of this process raises exciting possibilities for the preparation of extended conjugated π systems of silicon.

The reactivity of two stable Si(6)R(6) clusters (4 and 5, R = 2,4,6-(i)Pr(3)C(6)H(2)) with unsymmetrical substitution patterns (including Si, SiR, and SiR(2) vertices) is reported. In order to account for the importance of such clusters as model systems for transient intermediates in the deposition of elemental silicon, we here propose the term "siliconoids" for silicon clusters with unsaturated valencies. With the hexasilaprismane 8a, a saturated-i.e., non-siliconoid-Si(6)R(6) isomer is accessible from a suitable Si(3) precursor. Thermal redistribution of the substituents converts 1,1,2-trichlorocyclotrisilane 6 into the corresponding 1,2,3-derivative 7 prior to the requisite reductive coupling step leading to 8a. On the other hand, a stable expanded Si(11)-siliconoid 9 was isolated as a minor side product of the thermal isomerization of 4 to 5, thus providing a first example of siliconoid cluster expansion in the condensed phase. In the solid-state structure, the two unsubstituted vertices of 9 strongly interact in a staggered propellane-like fashion. Oxidative cluster contraction of a siliconoid scaffold is observed upon treatment of siliconoid 5 with a large excess of iodine in refluxing toluene, thus providing access to a highly functionalized hexaiodocyclopentasilane 11 in high yield. Conversely, chlorination of the isomeric 4 with BiCl(3) as a mild source of Cl(2) results in a complex mixture of products from chlorination of the unsubstituted vertices as well as σ-bonds of the cluster framework of 4. The main product, 1,2-dichlorotricyclo[2.2.0.0(2,5)]hexasilane 12, undergoes thermal cluster contraction to give tricyclo[2.1.0.0(2,5)]pentasilane 14 with an exohedral chlorosilyl group.

The Cp*Si(+) cation acts as a stoichiometric source of silicon in the reaction with the disilenide Tip(2)Si=Si(Tip)Li (Tip = 2,4,6-(i)Pr(3)C(6)H(2)) affording known neutral unsaturated silicon clusters. It thereby provides a conceptually different approach to this novel class of compounds. The proposed mechanism involves a Cp*-substituted cyclotrisilene in which Cp*(-) acts as a leaving group upon single electron reduction or in a nucleophilic substitution step.

The first magnesium, copper and zinc disilenides were prepared via transmetallation reactions of a lithium disilenide and structurally characterised. The copper and zinc derivatives show red-shifted UV/vis absorptions due to admixture of metal d-orbitals to the highest occupied molecular orbital.

The reaction of 1 equiv of the disilenide Tip2Si═Si(Tip)Li (5; Tip = 2,4,6-(i)Pr3C6H2) with para-substituted phenyl iodides, 4-X-PhI, transfers the Tip2Si═Si(Tip) moiety with elimination of lithium iodide to yield the laterally functionalized disilenes Tip2Si═Si(Tip)(4-X-Ph) [X = H (6a), F (6b), Cl (6c), Br (6d), I (6e)]. The UV-vis absorptions of 6a-d suggest a linear correlation with electronic Hammett parameters. In addition, X-ray structural analyses of 6a-d verified the theoretically predicted linear dependence of the Si═Si bond length and trans-bent angles. The p-bromophenyl-substituted disilene 6d undergoes a metal-halogen exchange reaction to give 6f (X = Li), which was trapped with Me3SiCl to afford 6g (X = SiMe3). In the case of simple phenyl halides PhX without additional functionality, the reaction with 5 proceeded smoothly for X = Br, but phenyl chlorides and fluorides did not react at room temperature even after one week, hinting at an S(N)2-type aromatic substitution mechanism. Reactions of p- and m-diiodobenzene with 5 afford the corresponding phenylene-bridged tetrasiladienes p-7 and m-7. While red p-7 (λ(max) = 508 nm) exhibits efficient conjugation of the two Si═Si bonds with the phenylene linker, the conjugation in yellow m-7 (λ(max) = 449 nm) is much less effective. Electrochemical studies of m-7 and p-7 as well as density functional theory calculations and electron paramagnetic resonance studies of their respective radical anions provided further support for the notion of conjugation.

Legrand et al. (Reports, 16 July 2010, p. 299) reported on the photolytic reaction of an α-pyrone confined in a crystalline matrix. Their structural analysis invoked four products: activated precursor, isomeric Dewar β-lactone, and square and rectangular isomers of 1,3-dimethylcyclobutadiene. The reported x-ray data, however, suggest that all observed structures correspond to only one distinct species, the Dewar β-lactone.

The treatment of disilenide Tip2SiSi(Tip)Li (1, Tip = 2,4,6-iPr3C6H2) with P-chloro phosphines affords the phosphino disilenes (2a-d; a: R = Ph, b: R = iPr, c: R = Cy, d: R = tBu), which were characterised by multinuclear NMR spectroscopy for 2a-d and a single crystal X-ray diffraction study in case of 2c. As an alternate synthetic method, the diphenyl derivative 2a could also be prepared by reaction of LiPPh2 with the thermally unstable iododisilene, Tip2SiSi(Tip)I (3), which in turn was obtained by oxidation of 1 with stoichiometric amounts of iodine. Providing the first example for a SiSi bond with an iodo functionality, disilene 3 was fully characterised by multinuclear NMR and X-ray diffraction. The thermal rearrangement of phosphine disilene 2avia a C–H insertion reaction yields the diastereomeric mixture of a 1-phospha-2,3-disilaindane 4. The structure of the cis-diastereomer of 4 was determined by X-ray diffraction. Finally, the synthesis of first transition metal complex of 2a and 2b by their coordination to the [Pd(PCy3)] fragment is reported. The solid state structure of complex 5a reveals η2-coordination of the SiSi bond of the phosphino disilene 2a with an intermediate bonding mode between π-complex and metallacyclopropane rather than coordination of the phosphino group.

Benzene represents the showcase of Hückel aromaticity. The silicon analog, hexasilabenzene, has consequently been targeted for decades. We now report an intensely green isomer of Si6R6 (R being 2,4,6-triisopropylphenyl) with a tricyclic structure in the solid state featuring silicon atoms with two, one, and no substituents outside the ring framework. The highly dispersed 29Si nuclear magnetic resonance shifts in solution ranging from +125 to -90 parts per million indicate an inhomogeneous electron distribution due to the dismutation of formal oxidation numbers as compared with that of benzene. Theoretical analysis reveals nonetheless the cyclic delocalization of six mobile electrons of the pi-, sigma- and non-bonding type across the central four-membered ring. For this alternative form of aromaticity, in principle applicable to many Hückel aromatic species, we propose the term dismutational aromaticity.

E=Si transfer: Anionic compounds capable of transferring a silicon-containing double bond are reviewed (see figure), particularly reagents with Si=Si moieties (Tip=2,4,6-iPr(3)C(6)H(2), M=Li, Na, K) and their applications towards main-group and transition-metal electrophiles, as well as their reactivity towards organic compounds. A few recently reported derivatives with Si=C (Ad=1-adamantyl) and Si=P moieties are included for completeness.Anionic compounds capable of transferring a silicon double bond are summarized following an introduction to the differences between alkenes and their heavier homologues. The main focus is on reagents with Si=Si moieties and their applications towards main-group and transition-metal electrophiles, as well as their reactivity towards organic compounds, but a few recently reported derivatives with Si=C and Si=P bonds are also included.

The rearrangements of (chlorosilyl)disilenes R2(Cl)Si-(Tip)Si=SiTip2 (5a,b: Tip = 2,4,6-iPr3C6H2, a: R = Me, b: R = Ph) quantitatively yield the isomeric chlorocyclotrisilanes (6a,b). The disilene precursors 5a,b are, in turn, accessible from the reactions of the disilenide Tip2Si=Si(Tip)Li (1), that is, a disila analogue to vinyl anions, with dichlorosilanes R2SiCl2. This novel approach to cyclotrisilanes potentially allows for the facile variation of the substitution pattern and grants access to the first anionic derivatives; while the rearrangement of 5a,b to 6a,b is slow at room temperature and additionally requires the presence of THF or other n-donors, reduction of 5b with lithium instantly yields the corresponding cyclotrisilanide (7b) without detection of any open-chained isomer. Heating of a neat sample of 5b to 150 degrees C provides a completely characterized 1,2,3-trisilaindane derivative (13), strongly supporting the intermediacy of a disilanyl silylene species that inserts into an ortho-CH bond of the phenyl substituents. The X-ray diffraction studies on single crystals of 6a,b and 7b reveal that the Si-Si bond distance in cyclotrisilanes depends significantly on the electronegativity of the opposing silicon atom's substituents, which is rationalized by density functional theory (DFT) calculations on model systems.