Darstellung und Eigenschaften einiger Hochchlorierter Oligosilane

Preparation and Properties of Some Highly Chlorinated Oligosilanes Tetrakis(trichlorosilyl)silane (neo-Si₅Cl₁₂) is cleaved by HCl in SiCl₄ solution to tris(trichlorosilyl)silane, HSi(SiCl₃)₃, and SiCl₄. HSi(SiCl₃)₃ and bis(trichlorosilyl)silane, H₂Si(SiCl₃)₂, can also be prepared in good yield by reaction of the methoxy compounds HSi[Si(OCH₃)₃]₃ and H₂Si[Si(OCH₃)₃]₂ with BCl₃. The mass, ¹H-n.m.r., and vibrational spectra of HSi(SiCl₃)₃ and H₂Si(SiCl₃) as well as some ¹H-n.m.r. data of HClSi(SiCl₃)₂, HCl₂SiSiCl₃ and H₂ClSiSiCl₃ are reported and discussed. An improved synthesis for neo-Si₅Cl₁₂ is given.     

Enhancing the Variability of [Ge9] Cluster Chemistry through Phosphine Functionalization

The synthetic approach towards molecules that contain Ge atoms with oxidation state 0, and which are exclusively connected to other Ge atoms, is explored by using anionic clusters extracted from binary solids. Besides providing a novel variable method for the introduction of alkenyl moieties to [Ge₉] cluster compounds, this work expands the spectrum of mixed-functionalized [Ge₉] cluster anions, which are suitable for the straightforward synthesis of zwitterionic compounds upon coordination to metal cations. In detail, the synthesis of a series of mixed-functionalized [Ge₉] clusters is reported, including [Ge₉{Si(TMS)₃}₃PRR^I] (R=tBu, R^I=(CH₂)₃CH=CH₂; 2 ) and [Ge₉{Si(TMS)₃}₂PRR^I]⁻ (R and R^I: alkyl, alkenyl, aryl, aminoalkyl; 3 a to 11 a , TMS: (trimethyl)silyl). In 2 and 3 a , pentenyl functionalization of the [Ge₉] clusters was achieved by reaction of the novel chlorophosphine tBu{(CH₂)₃CH=CH₂}PCl ( 1 ) with silylated [Ge₉] clusters. Furthermore, the reactivity of the cluster anions 3 a to 11 a towards NHC^DippMCl (NHC^Dipp=1,3-di(2,6-diisopropylphenyl)imidazolylidine; M=Cu, Ag) showed a dependency on the steric demand of the phosphine either zwitterions ( 3 -MNHC^Dipp to 7 -MNHC^Dipp) featuring P–M interactions are formed, or Ge–M coordination ( 8 -MNHC^Dipp to 11 -MNHC^Dipp) occurs. For M=Ag, the formation of zwitterionic complexes was unequivocally proven by NMR investigations showing ¹J(³¹P-¹⁰⁷Ag/¹⁰⁹Ag) spin-spin coupling.     

Stabilization and Structure of the cis Tautomer of a Free‐Base Porphyrin

Single‐crystal X‐ray analysis of the β‐heptakis(trifluoromethyl)‐meso ‐tetrakis(p ‐fluorophenyl)porphyrin, H₂[(CF₃)₇TpFPP], has revealed the first example of a stable cis tautomer of a free‐base porphyrin, the long‐postulated intermediate of porphyrin tautomerism. The stability of the unique molecule appears to reflect a dual origin: a strongly saddled porphyrin skeleton, which alleviates electrostatic repulsion between the two NH protons, and two polarization‐enhanced, transannular N−H⋅⋅⋅O−H⋅⋅⋅N hydrogen bond chains, each involving a molecule of water. DFT calculations suggest that the observed tautomer has a lower energy than the alternative, doubly hydrated trans tautomer by some 8.3 kcal mol⁻¹. A fascinating prospect thus exists that H₂[(CF₃)₇TpFPP]⋅2 H₂O and cognate structures may act as supramolecular synthons, which, given their chirality, may even be amenable to resolution into optically pure enantiomers.     

Unprecedented Reaction Pathway of Sterically Crowded Calcium Complexes: Sequential C−N Bond Cleavage Reactions Induced by C−H Bond Activations

Five bis(quinolylmethyl)‐(1H ‐indolylmethyl)amine (BQIA) compounds, that is, {(quinol‐8‐yl‐CH₂)₂NCH₂(3‐Br‐1H ‐indol‐2‐yl)} ( L¹H ) and {[(8‐R³‐quinol‐2‐yl)CH₂]₂NCH(R²)[3‐R¹‐1H ‐indol‐2‐yl]} ( L^2–5H ) ( L²H : R¹=Br, R²=H, R³=H; L³H : R¹=Br, R²=H, R³=i Pr; L⁴H : R¹=H, R²=CH₃, R³=i Pr; L⁵H : R¹=H, R²=n Bu, R³=i Pr) were synthesized and used to prepare calcium complexes. The reactions of L^1–5H with silylamido calcium precursors (Ca[N(SiMe₂R)₂]₂(THF)₂, R=Me or H) at room temperature gave heteroleptic products ( L^{1, 2} )CaN(SiMe₃)₂ ( 1 , 2 ), ( L^{3, 4} )CaN(SiHMe₂)₂ ( 3 a , 4 a ) and homoleptic complexes ( L^{3, 5} )₂Ca ( D3 , D5 ). NMR and X‐ray analyses proved that these calcium complexes were stabilized through Ca⋅⋅⋅C−Si, Ca⋅⋅⋅H−Si or Ca⋅⋅⋅H−C agostic interactions. Unexpectedly, calcium complexes (( L^3–5 )CaN(SiMe₃)₂) bearing more sterically encumbered ligands of the same type were extremely unstable and underwent C−N bond cleavage processes as a consequence of intramolecular C−H bond activation, leading to the exclusive formation of (E )‐1,2‐bis(8‐isopropylquinol‐2‐yl)ethane.     

Seven-Membered Cyclic Potassium Diamidoalumanyls

The seven-membered cyclic potassium alumanyl species, [{SiN^Mes}AlK]₂ [{SiN^Mes}={CH₂SiMe₂N(Mes)}₂; Mes=2,4,6-Me₃C₆H₂], which adopts a dimeric structure supported by flanking K-aryl interactions, has been isolated either by direct reduction of the iodide precursor, [{SiN^Mes}AlI], or in a stepwise manner via the intermediate dialumane, [{SiN^Mes}Al]₂. Although the intermediate dialumane has not been observed by reduction of a Dipp-substituted analogue (Dipp=2,6-i-Pr₂C₆H₃), partial oxidation of the potassium alumanyl species, [{SiN^Dipp}AlK]₂, where {SiN^Dipp}={CH₂SiMe₂N(Dipp)}₂, provided the extremely encumbered dialumane [{SiN^Dipp}Al]₂. [{SiN^Dipp}AlK]₂ reacts with toluene by reductive activation of a methyl C(sp³)-H bond to provide the benzyl hydridoaluminate, [{SiN^Dipp}AlH(CH₂Ph)]K, and as a nucleophile with BPh₃ and RN=C=NR (R=i-Pr, Cy) to yield the respective Al-B- and Al-C-bonded potassium aluminaborate and alumina-amidinate products. The dimeric structure of [{SiN^Dipp}AlK]₂ can be disrupted by partial or complete sequestration of potassium. Equimolar reactions with 18-crown-6 result in the corresponding monomeric potassium alumanyl, [{SiN^Dipp}Al−K(18-cr-6)], which provides a rare example of a direct Al−K contact. In contrast, complete encapsulation of the potassium cation of [{SiN^Dipp}AlK]₂, either by an excess of 18-cr-6 or 2,2,2-cryptand, allows the respective isolation of bright orange charge-separated species comprising the ‘free’ [{SiN^Dipp}Al]⁻ alumanyl anion. Density functional theory (DFT) calculations performed on this moiety indicate HOMO-LUMO energy gaps in the of order 200–250 kJ mol⁻¹.     

A Systematic Study of Structure and E−H Bond Activation Chemistry by Sterically Encumbered Germylene Complexes

A series of new germylene compounds has been synthesized offering systematic variation in the σ‐ and π‐capabilities of the α‐substituent and differing levels of reactivity towards E?H bond activation (E=H, B, C, N, Si, Ge). Chloride metathesis utilizing [(terphenyl)GeCl] proves to be an effective synthetic route to complexes of the type [(terphenyl)Ge(ER_n)] ( 1 – 6 : ER_n=NHDipp, CH(SiMe₃)₂, P(SiMe₃)₂, Si(SiMe₃)₃ or B(NDippCH)₂; terphenyl=C₆H₃Mes₂‐2,6=Ar^Mes or C₆H₃Dipp₂‐2,6=Ar^Dipp; Dipp=C₆H₃iPr₂‐2,6, Mes=C₆H₂Me₃‐2,4,6), while the related complex [{(Me₃Si)₂N}Ge{B(NDippCH)₂}] ( 8 ) can be accessed by an amide/boryl exchange route. Metrical parameters have been probed by X‐ray crystallography, and are consistent with widening angles at the metal centre as more bulky and/or more electropositive substituents are employed. Thus, the widest germylene units (θ>110°) are found to be associated with strongly σ‐donating boryl or silyl ancillary donors. HOMO–LUMO gaps for the new germylene complexes have been appraised by DFT calculations. The aryl(boryl)‐germylene system [Ar^MesGe{B(NDippCH)₂}] ( 6 ‐Mes), which features a wide C‐Ge‐B angle (110.4(1)°) and (albeit relatively weak) ancillary π‐acceptor capabilities, has the smallest HOMO–LUMO gap (119 kJ mol^?1). These features result in 6 ‐Mes being remarkably reactive, undergoing facile intramolecular C?H activation involving one of the mesityl ortho‐methyl groups. The related aryl(silyl)‐germylene system, [Ar^MesGe{Si(SiMe₃)₃}] ( 5 ‐Mes) has a marginally wider HOMO–LUMO gap (134 kJ mol^?1), rendering it less labile towards decomposition, yet reactive enough to oxidatively cleave H₂ and NH₃ to give the corresponding dihydride and (amido)hydride. Mixed aryl/alkyl, aryl/amido and aryl/phosphido complexes are unreactive, but amido/boryl complex 8 is competent for the activation of E?H bonds (E=H, B, Si) to give hydrido, boryl and silyl products. The results of these reactivity studies imply that the use of the very strongly σ‐donating boryl or silyl substituents is an effective strategy for rendering metallylene complexes competent for E?H bond activation.     

Theoretical insights into the relative bonding of normal and abnormal N‐heterocyclic carbenes in [PdCl2(NHCR)2] and [PdCl2(NHCR)(aNHCR)] (R = H,Ph, Mes)

Quantum chemical insights into normal Pd‐C2(NHC^R) and abnormal Pd‐C5(aNHC^R) bonding, dominated by dispersion interactions in N‐hetereocyclic carbene complexes [PdCl₂(NHC^R)₂] ( I , R = H; II , R = Ph; III , R = Mes (2,4,6‐trimethyl)phenyl)) and [PdCl₂(NHC^R)(aNHC^R] ( IV , R = H; V , R = Ph; VI , R = Mes) have been investigated at DFT and DFT‐D3(BJ) level of theory with particular emphasis on the effects of the noncovalent interactions on the structures and the nature of Pd‐C2(NHC^R) and Pd‐C5(aNHC^R) bonds. The optimized geometries are good agreement with the experimental values. The Pd‐C bonds are essentially single bond. Hirshfeld charge distributions indicate that the abnormal aNHC^R carbene ligand is relatively better electron donor than the normal NHC^R carbene ligand. The C2 atom has larger %s contribution along Pd‐C2 bond than the C5 atom along Pd‐C5 bond. As a consequence the Pd‐C2(NHC^R) bonds are relative stronger than the Pd‐C5(aNHC^R) bonds. Thus, the results of natural hybrid orbital analysis support the key point of the present study. Calculations predict that for bulky substituent (R = Ph, Mes) at carbene, the Pd‐C2(NHC^R) bond is stronger than Pd‐C5(aNHC^R) bond due to large dispersion energy in [PdCl₂(NHC^R)₂] than in [PdCl₂(NHC^R)(aNHC^R)]. However, in case of non‐bulky substituent with small and almost equal contribution of dispersion energy, the Pd‐C2(NHC^R) bond is relative weaker than Pd‐C5(aNHC^R) bond. The bond dissociation energies are dependent on the R substituent, the DFT functional and the inclusion of dispersion interactions. Major point of this study is that the abnormal aNHCs are not always strongly bonded with metal center than the normal NHCs. Effects of dispersion interaction of substituent at nitrogen atoms of carbene ligand are found to play a crucial role on estimation of relative bonding strengths of the normal and abnormal aNHCs with metal center. © 2016 Wiley Periodicals, Inc.     

Reaction of silicon with alcohols in autoclave

A reaction of activated silicon with alcohols in an autoclave at 240—270 °C was studied. It was found that primary alcohols form tetraalkoxysilanes Si(OR)₄ with high selectivity (up to 97%), while the secondary PrⁱOH gave a mixture of compounds HSi(OPrⁱ)₃, Si(OPrⁱ)₄, HSi(OPrⁱ)₂OSi(OPrⁱ)₂H, HSi(OPrⁱ)₂OSi(OPrⁱ)₃, and Si(OPrⁱ)₃OSi(OPrⁱ)₃ with the predominance of trialkoxysilane (up to 67%). Carrying out the reaction under the indicated conditions has the advantage of experimental simplicity, reagent availability, high conversion of silicon, good isolated yields of products.     

Facile Access to an NHC‐Coordinated Silicon Ring Compound with a Si=N Group and a Two‐Coordinate Silicon Atom

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si₄{N(SiMe₃)Dipp}₄ 1 (Dipp=2,6‐diisopropylphenyl) with 5 equiv of the N‐heterocyclic carbene NHC^Me4 (1,3,4,5‐tetramethylimidazol‐2‐ylidene) affords a bifunctional carbene‐coordinated four‐membered‐ring compound with a Si=N group and a two‐coordinate silicon atom Si₄{N(SiMe₃)Dipp}₂(NHC^Me4)₂(NDipp) 2 . When 2 reacts with 0.25 equiv sulfur (S₈), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si₄ ring, which confirms the silylone character of the two‐coordinate silicon atom in 2 .     

Probing Charge-Transfer Processes in a Covalently Linked [Ge9]-Cluster Imine Dyad

C₆₀ donor dyads in which the carbon cage is covalently linked to an electron-donating unit have been discussed as one possibility for an electron-transfer system, and it has been shown that spherical [Ge₉] cluster anions show a close relation to fullerenes with respect to their electronic structure. However, the optical properties of these clusters and of functionalized cluster derivatives are almost unknown. We now report on the synthesis of the intensely red [Ge₉] cluster linked to an extended π-electron system. [Ge₉{Si(TMS)₃}₂{CH₃C=N}-DAB(II)^Dipp]⁻ ( 1⁻ ) is formed upon the reaction of [Ge₉{Si(TMS)₃}₂]²⁻ with bromo-diazaborole DAB(II)^Dipp-Br in CH₃CN (TMS=trimethylsilyl; DAB(II)=1,3,2-diazaborole with an unsaturated backbone; Dipp=2,6-di-iso-propylphenyl). Reversible protonation of the imine entity in 1⁻ yields the deep green, zwitterionic cluster [Ge₉{Si(TMS)₃}₂{CH₃C=N(H)}-DAB(II)^Dipp] ( 1-H ) and vice versa. Optical spectroscopy combined with time-dependent density functional theory suggests a charge-transfer excitation between the cluster and the antibonding π* orbital of the imine moiety as the cause of the intense coloration. An absorption maximum of 1-H in the red region of the electromagnetic spectrum and the corresponding lowest-energy excited state at λ=669 nm make the compound an interesting starting point for further investigations targeting the design of photo-active cluster compounds.     

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent‐Silyliumylidene Cation

The trapping of a silicon(I) radical with N‐heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAAC_Me] (cAAC_Me=:C(CMe₂)₂(CH₂)NAr, Ar=2,6‐i Pr₂C₆H₃) with H₂SiI₂ in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAAC_Me)₂Si:^.]⁺I⁻ ( 1 ), which features a cationic cAAC–silicon(I) radical, and [cAAC_Me−H]⁺I⁻. In addition, the reaction of the NHC–iodosilicon(I) dimer [I_Ar(I)Si:]₂ (I_Ar=:C{N(Ar)CH}₂) with 4 equiv of I_Me (:C{N(Me)CMe}₂), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(I_Me)₂SiH]⁺I⁻ ( 2 ) comprising the first NHC–parent‐silyliumylidene cation. Its further reaction with fluorobenzene afforded the C_Ar−H bond activation product [1‐F‐2‐I_Me‐C₆H₄]⁺I⁻ ( 3 ). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 . Compounds 1 – 3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X‐ray crystallography.     

The Nature of the UC Double Bond: Pushing the Stability of High‐Oxidation‐State Uranium Carbenes to the Limit

Treatment of [K(BIPM^MesH)] (BIPM^Mes={C(PPh₂NMes)₂}^2?; Mes=C₆H₂‐2,4,6‐Me₃) with [UCl₄(thf)₃] (1 equiv) afforded [U(BIPM^MesH)(Cl)₃(thf)] ( 1 ), which generated [U(BIPM^Mes)(Cl)₂(thf)₂] ( 2 ), following treatment with benzyl potassium. Attempts to oxidise 2 resulted in intractable mixtures, ligand scrambling to give [U(BIPM^Mes)₂] or the formation of [U(BIPM^MesH)(O)₂(Cl)(thf)] ( 3 ). The complex [U(BIPM^Dipp)(μ‐Cl)₄(Li)₂(OEt₂)(tmeda)] ( 4 ) (BIPM^Dipp={C(PPh₂NDipp)₂}^2?; Dipp=C₆H₃‐2,6‐iPr₂; tmeda=N,N,N′,N′‐tetramethylethylenediamine) was prepared from [Li₂(BIPM^Dipp)(tmeda)] and [UCl₄(thf)₃] and, following reflux in toluene, could be isolated as [U(BIPM^Dipp)(Cl)₂(thf)₂] ( 5 ). Treatment of 4 with iodine (0.5 equiv) afforded [U(BIPM^Dipp)(Cl)₂(μ‐Cl)₂(Li)(thf)₂] ( 6 ). Complex 6 resists oxidation, and treating 4 or 5 with N‐oxides gives [{U(BIPM^DippH)(O)₂‐ (μ‐Cl)₂Li(tmeda)] ( 7 ) and [{U(BIPM^DippH)(O)₂(μ‐Cl)}₂] ( 8 ). Treatment of 4 with tBuOLi (3 equiv) and I₂ (1 equiv) gives [U(BIPM^Dipp)(OtBu)₃(I)] ( 9 ), which represents an exceptionally rare example of a crystallographically authenticated uranium(VI)–carbon σ bond. Although 9 appears sterically saturated, it decomposes over time to give [U(BIPM^Dipp)(OtBu)₃]. Complex 4 reacts with PhCOtBu and Ph₂CO to form [U(BIPM^Dipp)(μ‐Cl)₄(Li)₂(tmeda)(OCPhtBu)] ( 10 ) and [U(BIPM^Dipp)(Cl)(μ‐Cl)₂(Li)(tmeda)(OCPh₂)] ( 11 ). In contrast, complex 5 does not react with PhCOtBu and Ph₂CO, which we attribute to steric blocking. However, complexes 5 and 6 react with PhCHO to afford (DippNPPh₂)₂C?C(H)Ph ( 12 ). Complex 9 does not react with PhCOtBu, Ph₂CO or PhCHO; this is attributed to steric blocking. Theoretical calculations have enabled a qualitative bracketing of the extent of covalency in early‐metal carbenes as a function of metal, oxidation state and the number of phosphanyl substituents, revealing modest covalent contributions to U?C double bonds.     

Hydrosilylation in Aryliminopyrrolide‐Substituted Silanes

A range of silanes was synthesized by the reaction of HSiCl₃ with iminopyrrole derivatives in the presence of NEt₃. In certain cases, intramolecular hydrosilylation converts the imine ligand into an amino substituent. This reaction is inhibited by factors such as electron‐donating substitution on Si and steric bulk. The monosubstituted (^DippIMP)SiHMeCl (^DippIMP=2‐[N‐(2,6‐diisopropylphenyl)iminomethyl]pyrrolide), is stable towards hydrosilylation, but slow hydrosilylation is observed for (^DippIMP)SiHCl₂. Reaction of two equivalents of ^DippIMPH with HSiCl₃ results in the hydrosilylation product (^DippAMP)(^DippIMP)SiCl (^DippAMP=2‐[N‐(2,6‐diisopropylphenyl)aminomethylene]pyrrolide), but the trisubsitituted (^DippIMP)₃SiH is stable. Monitoring the hydrosilylation reaction of (^DippIMP)SiHCl₂ reveals a reactive pathway involving ligand redistribution reactions to form the disubstituted (^DippAMP)(^DippIMP)SiCl as an intermediate. The reaction is strongly accelerated in the presence of chloride anions.     

Metal⋅⋅⋅F−C Bonding in Low-Coordinate Alkaline Earth Fluoroarylamides

A set of calcium and barium complexes containing the fluoroarylamide N(C₆F₅)₂⁻ is presented. These compounds illustrate the key role of stabilising M⋅⋅⋅F−C secondary interactions in the construction of low-coordinate alkaline earth complexes. The nature of Ca⋅⋅⋅F−C bonding in calcium complexes is examined in the light of structural data, bond valence sum (BVS) analysis and DFT computations. The molecular structures of [Ca{N(C₆F₅)₂}₂(Et₂O)₂] ( 4 ′), [Ca{μ-N(SiMe₃)₂}{N(C₆F₅)₂}]₂ ( 5₂ ), [Ba{μ-N(C₆F₅)₂}{N(C₆F₅)₂}⋅toluene]₂ ( 6₂ ), [{BDI^DiPP}CaN(C₆F₅)₂]₂ ( 7₂ ), [{N^N^DiPP}CaN(C₆F₅)₂]₂ ( 8₂ ), and [Ca{μ-OB(CH(SiMe₃)₂)₂}{N(C₆F₅)₂}]₂ ( 9₂ ), where {BDI^DiPP}⁻ and {N^N^DiPP}⁻ are the bidentate ligands CH[C(CH₃)NDipp]₂⁻ and DippNC₆H₄CNDipp⁻ (Dipp=2,6-iPr₂-C₆H₃), are detailed. Complex 6₂ displays strong Ba⋅⋅⋅F−C contacts at around 2.85 Å. The calcium complexes feature also very short intramolecular Ca−F interatomic distances at around 2.50 Å. In addition, the three-coordinate complexes 7₂ and 8₂ form dinuclear structures due to intermolecular Ca⋅⋅⋅F−C contacts. BVS analysis shows that Ca⋅⋅⋅F−C interactions contribute to 15–20 % of the bonding pattern around calcium. Computations demonstrate that Ca⋅⋅⋅F−C bonding is mostly electrostatic, but also contains a non-negligible covalent contribution. They also suggest that Ca⋅⋅⋅F−C are the strongest amongst the range of weak Ca⋅⋅⋅X (X=F, H, C_π) secondary interactions, due to the high positive charge of Ca²⁺ which favours electrostatic interactions.     

Hirshfeld surface analysis of two new phosphorothioic triamide structures

Hirshfeld surfaces and two‐dimensional fingerprint plots are used to analyse the intermolecular interactions in two new phosphorothioic triamide structures, namely N,N′,N′′‐tris(3,4‐dimethylphenyl)phosphorothioic triamide acetonitrile hemisolvate, P(S)[NHC₆H₃‐3,4‐(CH₃)₂]₃·0.5CH₃CN or C₂₄H₃₀N₃PS·0.5CH₃CN, (I), and N,N′,N′′‐tris(4‐methylphenyl)phosphorothioic triamide–3‐methylpiperidinium chloride (1/1), P(S)[NHC₆H₄(4‐CH₃)]₃·[3‐CH₃‐C₅H₉NH₂]⁺·Cl⁻ or C₂₁H₂₄N₃PS·C₆H₁₄N⁺·Cl⁻, (II). The asymmetric unit of (I) consists of two independent phosphorothioic triamide molecules and one acetonitrile solvent molecule, whereas for (II), the asymmetric unit is composed of three components (molecule, cation and anion). In the structure of (I), the different components are organized into a six‐molecule aggregate through N—H...S and N—H...N hydrogen bonds. The components of (II) are aggregated into a two‐dimensional array through N—H...S and N—H...Cl hydrogen bonds. Moreover, interesting features of packing arise in this structure due to the presence of a double hydrogen‐bond acceptor (the S atom of the phosphorothioic triamide molecule) and of a double hydrogen‐bond donor (the N—H unit of the cation). For both (I) and (II), the full fingerprint plot of each component is asymmetric as a consequence of the presence of three fragments. These analyses reveal that H...H interactions [67.7 and 64.3% for the two symmetry‐independent phosphorothioic triamide molecules of (I), 30.7% for the acetonitrile solvent of (I), 63.8% in the phosphorothioic triamide molecule of (II) and 62.9% in the 3‐methylpiperidinium cation of (II)] outnumber the other contacts for all the components in both structures, except for the chloride anion of (II), which only receives the Cl...H contact. The phosphorothioic triamide molecules of both structures include unsaturated C atoms, thus presenting C...H/H...C interactions: 17.6 and 21% for the two symmetry‐independent phosphorothioic triamide molecules in (I), and 22.7% for the phosphorothioic triamide molecule of (II). Furthermore, the N—H...S hydrogen bonds in both (I) and (II), and the N—H...Cl hydrogen bonds in (II), are the most prominent interactions, appearing as large red spots on the Hirshfeld surface maps. The N...H/H...N contacts in structure (I) are considerable, whereas for (II), they give a negligible contribution to the total interactions in the system.     

Auf dem Wege zu einem Disilin –Si≡Si–: Bildung von RHSi=SiHR und Hinweise auf die intermediäre Bildung von RSi≡SiR (R = SiH(SitBu3)2)

Compounds of Silicon. 146. Unsaturated Silicon Compounds. 59. On the Way to a Disilyne –Si≡Si–: Formation of RHSi=SiHR and Indication of the Intermediate Formation of RSi≡SiR (R = SiH(Si t Bu₃)₂) Reaction of R*₂HSi–SiHBr–SiHBr–SiHR*₂ with an equimolar amount of NaR* × 2 THF in THF at –78 °C (R* = supersilyl = SitBu₃) or reaction of R*₂ClSi–SiHBr–SiHBr–SiClR*₂ with a two‐molar amount of NaR* × 2 THF in C₆D₆ at room temperature leads in the first case to the yellow disilene (R*₂HSi)HSi=SiH(SiHR*₂) ( 1 ) and to the red cyclotetrasilene [–R*HSi–R*Si=SiR*–SiHR*–] ( 3 ), respectively, in the latter case to the colorless bicyclotetrasilane R*₄H₂Si₄ ( 4 ). The disilene 1 (for > Si= in ²⁹Si‐NMR: d of d at 141.32 ppm with ¹J_SiH = 149.9 Hz, ²J_SiH = 0.9 Hz) slowly isomerizes at room temperature (τ_1/2 ca. 3 h) under formation of the colorless trisilacyclopentane [–CH₂–SiH(SiH₂SiHR*₂)–SiHR*–SitBu₂–CMe₂–] ( 2 ) and adds methanole under formation of the colorless compound R*₂HSi–SiH₂–SiH(OMe)–SiHR*₂. As initial stage of 3 the disilyne R*₂HSi–Si≡Si–SiHR*₂ is in consideration, which by way of R*HSi=SiR*–SiR*=SiHR* may transform into 3 . The characterization of 2 and 3 results among others from X‐ray structure analyses, whereby for 3 an unexpectable long double bond (2.36 Å) has been found. The identification of 4 results from comparison with an authentic sample (formed from R*HBrSi–SiBr₂R* and NaR* × 2 THF).     

Low-Temperature Isolation of a Labile Silylated Hydrazinium-yl Radical Cation, [(Me3Si)2N−N(H)SiMe3].+

The oxidation of silylated hydrazine, (Me₃Si)₂N−N(H)SiMe₃, with silver salts led to the formation of a highly labile hydrazinium-yl radical cation, [(Me₃Si)₂N−N(H)SiMe₃]^.+, at very low temperatures (decomposition > −40 °C). EPR, NMR, DFT and Raman studies revealed the formation of a nitrogen-centered radical cation along the N−N unit of the hydrazine. In the presence of the weakly coordinating anion [Al{OCH(CF₃)₂}₄]⁻, crystallization and structural characterization in the solid state were achieved. The hydrazinium-yl radical cation has a significantly shortened N−N bond and a nearly planar N₂Si₃ framework, in contrast to the starting material. According to DFT calculations, the shortened N−N bond has a total bond order of 1.5 with a π-bond order of 0.5. The π bond can be regarded as a three-π-electron, two-center bond.     

Adduct Formation,B−H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1 ) with unsaturated and saturated NHCs as well as CAAC^Me. Mono‐NHC adducts of the type HBcat?NHC (NHC=nPr₂Im, iPr₂Im, iPr₂Im^Me, and Dipp₂Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC^Me with HBcat yielded the B?H activated product CAAC^Me(H)Bcat via insertion of the carbine‐carbon atom into the B?H bond. The saturated NHC Dipp₂SIm reacted in a 2:2 ratio yielding an NHC ring‐expanded product at room temperature forming a six‐membered ?B?C=N?C=C?N? ring via C?N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene‐carbon atom.     

Protonated silanoic acid HSi(OH)2+ and its neutral counterpart: a tandem mass spectrometric and CBS-QB3 computational study

Protonated silanoic acid, HSi(OH)₂⁺, 1a ⁺, is cleanly generated by the dissociative electron ionization of triethoxysilane, HSi(OC₂H₅)₃, and tetraethoxysilane, Si(OC₂H₅)₄. This follows from tandem mass spectrometric experiments and CBS-QB3 model chemistry calculations. The calculations predict that 1a ⁺ (ΔH_f(298 K) = 205 kJ mol⁻¹) is separated by high barriers from its isomers HOSiOH₂⁺, 1b ⁺ and HSi(O)OH₂⁺, 1c ⁺. Low-energy (metastable) ions 1a ⁺ dissociate by loss of H₂O via the pathway 1a ⁺ → 1b ⁺ → SiOH⁺ + H₂O. Analysis of the metastable peak for this process confirms that the isomerization step 1a ⁺ → 1b ⁺ is rate determining. The calculations further predict that the incipient ions 1b ⁺ communicate via a low barrier with the proton-bound dimer SiO···H···OH₂⁺, 1d ⁺. This dimer ion is much lower in energy than its counterpart OSi···H···OH₂⁺, 1e ⁺, which is calculated to be only marginally stable. A comparison of the potential energy diagram for the silicon-containing ions 1a ⁺– 1e ⁺ with that of their carbon analogues reveals that the dissociation chemistries of HSi(OH)₂⁺ and HC(OH)₂⁺ are only superficially similar. Neutralization–reionization experiments confirm the theoretical prediction that the HSi(OH)₂^• radical (ΔH_f(298 K) = −455 kJ mol⁻¹) is a stable species in the rarefied gas phase. However, owing to a mismatch of Franck–Condon factors a large fraction of the neutralized ions dissociates by loss of H^• yielding Si(OH)₂. Copyright © 2004 John Wiley & Sons, Ltd.     

NHC→SiCl4: An Ambivalent Carbene‐Transfer Reagent

The addition of BCl₃ to the carbene‐transfer reagent NHC→SiCl₄ (NHC=1,3‐dimethylimidazolidin‐2‐ylidene) gave the tetra‐ and pentacoordinate trichlorosilicon(IV) cations [(NHC)SiCl₃]⁺ and [(NHC)₂SiCl₃]⁺ with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC→SiCl₄ served as a transfer reagent for the NHC ligand. The addition of BF₃ ? OEt₂, on the other hand, gave NHC→BF₃ as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C₂F₅)₂SiCl₂ was treated with NHC→SiCl₄. In acetonitrile, the cationic silicon(IV) complexes [(NHC)SiCl₃]⁺ and [(NHC)₂SiCl₃]⁺ were detected with [(C₂F₅)SiCl₃]^? as counterion. A similar result was already reported for the reaction of NHC→SiCl₄ with (C₂F₅)₂SiH₂, which gave [(NHC)₂SiCl₂H][(C₂F₅)SiCl₃]. If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC→Si(C₂F₅)₂Cl₂ and NHC→Si(C₂F₅)₂ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron‐poor silylenes. Furthermore, the reactivity of NHC→SiCl₄ towards phosphines is discussed. The carbene complex NHC→PCl₃ shows similar reactivity to NHC→SiCl₄, and may even serve as a carbene‐transfer reagent as well.