A Stable Dimer of SiS2 Arranged between Two Carbene Molecules

The Me‐cAAC:‐stabilized dimer of silicon disulfide (SiS₂) has been isolated in the molecular form as (Me‐cAAC:)₂Si₂S₄ ( 2 ) at room temperature [Me‐cAAC:=cyclic alkyl(amino) carbene]. Compound 2 has been synthesized from the reaction of (Me‐cAAC:)₂Si₂ with elemental sulfur in a 1:4 molar ratio under oxidative addition. This is the smallest molecular unit of silicon disulfide characterized by X‐ray crystallography, electron ionization mass spectrometry, and NMR spectroscopy. Structures with three sulfur atoms arranged around a silicon atom are known; however, 2 is the first structurally characterized silicon–sulfur compound containing one terminal and two bridging sulfur atoms at each silicon atom. Compound 2 shows no decomposition after storing for three months in an inert atmosphere at ambient temperature. The bonding of 2 has been further studied by theoretical calculations.     

Rational Development of a Metal-Free Bifunctional System for the C−H Activation of Methane: A Density Functional Theory Investigation

The activation or heterolytic splitting of methane, a challenging substrate usually restricted to transition metals, has so far proven elusive in experimental frustrated Lewis pair (FLP) chemistry. In this article, we demonstrate, using density functional theory (DFT), that 1-aza-9-boratriptycene is a conceptually simple intramolecular FLP for the activation of methane. Systematic comparison with other FLP systems allows to gain insight into their reactivity with methane. The thermodynamics and kinetics of methane activation are interpreted by referring to the analysis of the natural charges and by employing the distortion-interaction/activation strain (DIAS) model. These showed that the nature of the Lewis base influences the selectivity over the reaction pathway, with N Lewis bases favoring the deprotonation mechanism and P bases the hydride abstraction one. The lower barrier of activation for 1-aza-9-boratriptycene and the higher products stability are due to a better interaction energy than its counterparts, itself due to electrostatic interactions with the methane moiety, favorable orbital overlaps allowed by the side-attack, and space proximity between the B and N atoms.     

The solution structure of (Me3Si)3CSiBr--an ab initio/NMR study

The first halosilylene stable in solution was investigated by ab initio/NMR calculations (IGLO SOS-DFPT PW91/B2//B3LYP/6-31+G(d)). The delta (29)Si(calc) of (Me(3)Si)(3)CSiBr (446 ppm) does not agree with the measured NMR signal at 106 ppm assigned to the free halosilylene. From the possible silylene complexes in the reaction solution, two structures agree with the observed NMR signal: the (Me(3)Si)(3)CSiBr(2) anion (delta (29)Si(calc)=124 ppm) and the unsolvated and solvated complex of the anion with two Li(+) (delta (29)Si(calc)=117 and estimated 134 ppm). Additionally the delta (29)Si(calc) of alkylsilylenes, R-Si-X, ranging from 200 to 900 ppm are presented to guide NMR identification in future silylene synthesis.     

A Triatomic Silicon(0) Cluster Stabilized by a Cyclic Alkyl(amino) Carbene

Reduction of the neutral carbene tetrachlorosilane adduct (cAAC)SiCl₄ (cAAC=cyclic alkyl(amino) carbene :C(CMe₂)₂(CH₂)N(2,6‐iPr₂C₆H₃) with potassium graphite produces stable (cAAC)₃Si₃, a carbene‐stabilized triatomic silicon(0) molecule. The Si?Si bond lengths in (cAAC)₃Si₃ are 2.399(8), 2.369(8) and 2.398(8) Å, which are in the range of Si?Si single bonds. Each trigonal pyramidal silicon atom of the triangular molecule (cAAC)₃Si₃ possesses a lone pair of electrons. Its bonding, stability, and electron density distributions were studied by quantum chemical calculations.     

Activation of SO2 and CO2 by Trivalent Uranium Leading to Sulfite/Dithionite and Carbonate/Oxalate Complexes

The first sulfite [{((^nP,MeArO)₃tacn)U^IV}₂(μ‐κ¹:κ²‐SO₃)] (tacn=triazacyclononane) and dithionite [{((^nP,MeArO)₃tacn)U^IV}₂(μ‐κ²:κ²‐S₂O₄)] complexes of uranium from reaction with gaseous SO₂ have been prepared. Additionally, the reductive activation of CO₂ was investigated with respect to the rare oxalate [{((^nP,MeArO)₃tacn)U^IV}₂(μ‐κ²:κ²‐C₂O₄)] formation. This ultimately provides the unique S₂O₄^2?/C₂O₄^2? and SO₃^2?/CO₃^2? complex pairs. All new complexes were characterized by a combination of single‐crystal X‐ray diffraction, elemental analysis, UV/Vis/NIR electronic absorption, IR vibrational, and ¹H NMR spectroscopy, as well as magnetization (VT SQUID) studies. Moreover, density functional theory (DFT) calculations were carried out to gain further insight into the reaction mechanisms. All observations, together with DFT, support the assumption that SO₂ and CO₂ show similar (dithionite/oxalate) to analogous (sulfite/carbonate) activation behavior with uranium complexes.     

Stable N‐Heterocyclic Carbene Adducts of Arylchlorosilylenes and Their Germanium Homologues

The first N‐heterocyclic carbene adducts of arylchlorosilylenes are reported and compared with the homologous germanium compounds. The arylsilicon(II) chlorides SiArCl(Im‐Me₄) [Ar=C₆H₃‐2,6‐Mes₂ (Mes=C₆H₂‐2,4,6‐Me₃), C₆H₃‐2,6‐Trip₂ (Trip=C₆H₂‐2,4,6‐iPr₃)] were obtained selectively on dehydrochlorination of the arylchlorosilanes SiArHCl₂ with 1,3,4,5‐tetramethylimidazol‐2‐ylidene (Im‐Me₄). The analogous arylgermanium(II) chlorides GeArCl(Im‐Me₄) were prepared by metathetical exchange of GeCl₂(Im‐Me₄) with LiC₆H₃‐2,6‐Mes₂ or addition of Im‐Me₄ to GeCl(C₆H₃‐2,6‐Trip₂). All compounds were fully characterized. Density functional calculations on ECl(C₆H₃‐2,6‐Trip₂)(Im‐Me₄), where E=Si, Ge, at different levels of theory show very good agreement between calculated and experimental bonding parameters, and NBO analyses reveal similar electronic structures of the two aryltetrel(II) chlorides. The low gas‐phase Gibbs free energy of bond dissociation of SiCl(C₆H₃‐2,6‐Trip₂)(Im‐Me₄) (Δ${G{{{\circ}\hfill \atop {\rm calcd}\hfill}}}$ =28.1 kJ mol^?1) suggests that the carbene adducts SiArCl(Im‐Me₄) may be valuable transfer reagents of the arylsilicon(II) chlorides SiArCl.     

Interactions of Molecules with cis and trans Double Bonds: A Theoretical Study of cis‐ and trans‐2‐Butene

Noncovalent interactions of cis‐ and trans‐2‐butene, as the smallest model systems of molecules with cis and trans double bonds, were studied to find potential differences in interactions of these molecules. The study was performed using quantum chemical methods including very accurate CCSD(T)/CBS method. We studied parallel and displaced parallel interactions in 2‐butene dimers, in butane dimers, and between 2‐butene and saturated butane. The results show the trend that interactions of 2‐butene with butane are the strongest, followed by interactions in butane dimers, whereas the interaction in 2‐butene dimers are the weakest. The strongest calculated interaction energy is between trans‐2‐butene and butane, with a CCSD(T)/CBS energy of ?2.80 kcal mol^?1. Interactions in cis‐2‐butene dimers are stronger than interactions in trans‐2‐butene dimers. Interestingly, some of the interactions involving 2‐butene are as strong as interactions in a benzene dimer. These insights into interactions of cis‐ and trans‐2‐butene can improve understanding of the properties and processes that involve molecules with cis and trans double bonds, such as fatty acids and polymers.     

IPr3Si3Cl5+: A Highly Reactive Cation with Silanide Character

The reaction of a metastable SiCl₂ solution with the sterically less‐demanding carbene N,N‐diisopropylimidazo‐2‐ylidene (IPr) yields the salt [(IPr₃Si₃Cl₅)⁺]Cl^? ( 1 ‐Cl), containing a silyl cation with a Si₃ backbone. Salt 1 is highly reactive, but it can be used as a reagent in deuterated dichloromethane, whereby dehalogenation with Me₃SiOTf (OTf=O₃SCF₃) gives the dicationic silyl halide [(IPr₃Si₃Cl₄)]²⁺ 2 . Quantum chemical calculations show that the HOMO is localized at the negatively charged central silicon atom of 1 and 2 , and thus although both compounds are cations they are better described as silanides, which was also corroborated by NMR investigations.     

Functionalization of a cyclo‐P5 Ligand by Main‐Group Element Nucleophiles

Unprecedented functionalized products with an η⁴‐P₅ ring are obtained by the reaction of [Cp*Fe(η⁵‐P₅)] ( 1 ; Cp*=η⁵‐C₅Me₅) with different nucleophiles. With LiCH₂SiMe₃ and LiNMe₂, the monoanionic products [Cp*Fe(η⁴‐P₅CH₂SiMe₃)]^? and [Cp*Fe(η⁴‐P₅NMe₂)]^?, respectively, are formed. The reaction of 1 with NaNH₂ leads to the formation of the trianionic compound [{Cp*Fe(η⁴‐P₅)}₂N]^3?, whereas the reaction with LiPH₂ yields [Cp*Fe(η⁴‐P₅PH₂)]^? as the main product, with {[Cp*Fe(η⁴‐P₅)]₂PH}^2? as a byproduct. The calculated energy profile of the reactions provides a rationale for the formation of the different products.     

Fixation and Release of Intact E4 Tetrahedra (E=P,As)

By the reaction of [NacnacCuCH₃CN] with white phosphorus (P₄) and yellow arsenic (As₄), the stabilization and enclosure of the intact E₄ tetrahedra are realized and the disubstituted complexes [(NacnacCu)₂(μ,η^2:2‐E₄)] ( 1 a : E=P, 1 b : E=As) are formed. The mono‐substituted complex [NacnacCu(η²‐P₄)] ( 2 ), was detected by the exchange reaction of 1 a with P₄ and was only isolated using low‐temperature work‐up. All products were comprehensively spectroscopically and crystallographically characterized. The bonding situation in the products as intact E₄ units (E=P, As) was confirmed by theory and was experimentally proven by the pyridine promoted release of the bridging E₄ tetrahedra in 1 .     

Oligonuclear Molecular Models of Intermetallic Phases: A Case Study on [Pd2Zn6Ga2(Cp*)5(CH3)3]

The synthesis, characterization, and theoretical investigation by means of quantum‐chemical calculations of an oligonuclear metal‐rich compound are presented. The reaction of homoleptic dinuclear palladium compound [Pd₂(μ‐GaCp*)₃(GaCp*)₂] with ZnMe₂ resulted in the formation of unprecedented ternary Pd/Ga/Zn compound [Pd₂Zn₆Ga₂(Cp*)₅(CH₃)₃] ( 1 ), which was analyzed by ¹H and ¹³C NMR spectroscopy, MS, elemental analysis, and single‐crystal X‐ray diffraction. Compound 1 consisted of two C_s‐symmetric molecular isomers, as revealed by NMR spectroscopy, at which distinct site‐preferences related to the Ga and Zn positions were observed by quantum‐chemical calculations. Structural characterization of compound 1 showed significantly different coordination environments for both palladium centers. Whilst one Pd atom sat in the central of a bi‐capped trigonal prism, thereby resulting in a formal 18‐valence electron fragment, {Pd(ZnMe)₂(ZnCp*)₄(GaMe)}, the other Pd atom occupied one capping unit, thereby resulting in a highly unsaturated 12‐valence electron fragment, {Pd(GaCp*)}. The bonding situation, as determined by atoms‐in‐molecules analysis (AIM), NBO partial charges, and molecular orbital (MO) analysis, pointed out that significant Pd? Pd interactions had a large stake in the stabilization of this unusual molecule. The characterization and quantum‐chemical calculations of compound 1 revealed distinct similarities to related M/Zn/Ga Hume–Rothery intermetallic solid‐state compounds, such as Ga/Zn‐exchange reactions, the site‐preferences of the Zn/Ga positions, and direct M? M bonding, which contributes to the overall stability of the metal‐rich compound.