Siloxane Coordination Revisited: Si−O Bond Character, Reactivity and Magnificent Molecular Shapes

Siloxanes have evolved into a multi-million dollar business due to their manifold of commercial and industrial applications. As siloxanes have high hydrophobicity, low basicity, high flexibility and also high chemical inertness in common, their chemistry differs significantly from that of organic ethers. The discovery of organic crown ethers, for instance, is commonly accepted as the birth of synthetic host-guest chemistry. Regarding the chemical properties of siloxanes, cyclic siloxanes which formally resemble silicon analogues of crown ethers, have received considerably less interest in terms of their host-guest chemistry. Hence, only little is known about siloxane coordination chemistry in the chemical community and the number of published works in this field has been very low till lately. In the last few years, the field has significantly advanced and elegant methods were established to enable the Si−O−Si unit for coordination. This review therefore summarizes the recent developments in the field, recapitulates the historical aspects of siloxane coordination chemistry and describes the specific Si−O bond character with regard to different siloxane linkages. Implications on Si−O bond activation are included and the limits of siloxane coordination are redefined

Swift C–C bond insertion by a 12-electron palladium(0) surrogate

The selective activation of C–C bonds holds vast promise for catalysis. So far, research has been primarily directed at rhodium and nickel under harsh reaction conditions. Herein, we report C–C insertion reactions of a 12-electron palladium(0) surrogate stabilized by a cyclic(alkyl)(amino) carbene (CAAC) ligand. Benzonitrile (1), biphenylene (2), benzocyclobutenone (3), and naphtho[b]cyclopropene (4) were studied. These substrates allow elucidation of the effect of ring strain as well as hybridization encompassing sp3 , sp2 and sp hybridized carbon atoms. All reactions proceed quantitatively at or below room temperature. This work therefore outlines perspectives for mild C–C bond functionalization catalysis

On the Reactivity of Phosphaalumenes towards C−C Multiple Bonds

Heterocycles containing group 13 and 15 elements such as borazines are an integral part of organic, biomedical and materials chemistry. Surprisingly, heterocycles containing P and Al are rare. We have now utilized phosphaalumenes in reactions with alkynes, alkenes and conjugated double bond systems. With sterically demanding alkynes 1,2-phosphaalumetes were afforded, whereas the reaction with HCCH or HCCSiMe3 gave 1,4-phosphaaluminabarrelenes. Using styrene saturated 1,2-phosphaalumates were formed, which reacted further with additional styrene to give different regio-isomers of 1,4-aluminaphosphorinanes. Using ethylene, a 1,4-aluminaphosphorinane is obtained, while with 1,3-butadiene a bicyclic system containing an aluminacyclopentane and a phosphirane unit was synthesized. The experimental work is supported by theoretical studies to shed light on the mechanism governing the formation of these heterocycles

Magnetic proximity in a van der Waals heterostructure of magnetic insulator and graphene

Engineering 2D material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr2Ge2Te6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr2Ge2Te6 results in significant modification of the spin transport and precession in graphene, which can be ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic interactions and spin filters in 2D material heterostructures and can form the basic building blocks for future spintronic and topological quantum devices

Contributions to siloxane coordination chemistry and silicon based crown-ether analogues via s-block metal templated Si-O bond activation

Silicone and Siloxane derivatives (e.g. (Me2SiO)n) are known to be chemically inert and hydrophobic materials. To bind cations thus requires sophisticated techniques to activate Si-O bonds for coordination. Central part of this thesis is therefor establishing Lewis acidic systems which allow effective Si-O bond activation. In addition, establishing suitable templates allow obtaining cation specific compounds in ring-opening oligomerization reactions. These reactions enable the synthesis of crown-ether analogues which formally resemble “inorganic” architectures of crown-ethers. So far, these ligands could not be synthesized by other means. Inorganic crown-ethers might show a reduced capability of binding cations but are remarkably stable in solution as long as a suitable metal ion is present which can act as a template. It could be shown that nine- and twelve membered cycles coordinate the cations Li+, Na+, Ca2+, Sr2+ and even H+ selectively. Hence, inorganic crown-ether analogues show an ion-selectivity towards chemically hard cations which has not been observed for their organic counterparts. In addition, templated synthesis has not yet been observed for conventional crown-ether analogues which is why a novel reactivity pattern is also presented here. The thesis does not only provide the synthesis of coordination compounds of inorganic crown-ether analogues but also that of related systems such as coordination compounds of hybrid disila-crown-ethers and cyclosiloxanes

Siloxane Coordination Revisited: Si‐O Bond Character, Reactivity and Magnificent Molecular Shapes

Siloxanes have evolved into a multi-million dollar business due to their manifold of commercial and industrial applications. As siloxanes have high hydrophobicity, low basicity, high flexibility and also high chemical inertness in common, their chemistry differs significantly from that of organic ethers. The discovery of organic crown ethers, for instance, is commonly accepted as the birth of synthetic host-guest chemistry. Regarding the chemical properties of siloxanes, cyclic siloxanes which formally resemble silicon analogues of crown ethers, have received considerably less interest in terms of their host-guest chemistry. Hence, only little is known about siloxane coordination chemistry in the chemical community and the number of published works in this field has been very low till lately. In the last few years, the field has significantly advanced and elegant methods were established to enable the Si−O−Si unit for coordination. This review therefore summarizes the recent developments in the field, recapitulates the historical aspects of siloxane coordination chemistry and describes the specific Si−O bond character with regard to different siloxane linkages. Implications on Si−O bond activation are included and the limits of siloxane coordination are redefined

On the ambiphilic character of phosphanylidenephosphoranes and manipulation of phosphinidenoid reactivity with Lewis acids

Phosphanylidenephosphoranes of the type R−P(PR’3), also known as phospha-Wittig reagents, can be utilized in a variety of bond activation reactions pursuing their phosphinidenoid reactivity. In here, we thoroughly show that a facile PMe3 for H2O exchange gives access to various primary phosphine oxides of the general formula RP(H)2O (R = Mes*, MesTer, DipTer). The molecular structure of DipTerP(H)2O was determined and provided the first picture of such species in the solid state. However, phosphanylidenephosphoranes are described to be highly nucleophilic as well. We show that the attachment of main group Lewis acids such as GaCl3 and GaI3 to2 R−P(PMe3) yielded highly sensitive, yet stable coordination compounds [RP(GaX3)PMe3] (R = Mes*, DipTer) or [(RPPMe3)2GaCl2]GaCl4 (R = MesTer). In contrast to the free phosphanylidenephosphoranes, these species reacted differently with H2O which was demonstrated for [(Mes*PPMe3)GaI3]. Here the formation of the phosphinophosphonium cation [Mes*P(H)PMe3]+ and different anions was observed with combined NMR spectroscopic and SC-XRD (SC-XRD = single crystal diffraction analysis) studies. This work demonstrates that the ambiphilic character of phosphanylidenephosphoranes can be utilized to manipulate the reactivity of R−P(PMe3) towards water, giving primary phosphine oxides, whereas the Lewis acid adducts [(RPPMe3)GaX3] gave phosphino-phosphonium species

Structural Study of Mismatched Disila-Crown Ether Complexes

Mismatched complexes of the alkali metals cations Li+ and Na+ were synthesized from 1,2-disila[18]crown-6 (1 and 2) and of K+ from 1,2,4,5-tetrasila[18]crown-6 (4). In these alkali metal complexes, not all crown ether O atoms participate in the coordination, which depicts the coordination ability of the C-, Si/C-, and Si-bonded O atoms. Furthermore, the inverse case—the coordination of the large Ba2+ ion by the relatively small ligand 1,2-disila[15]crown-5—was investigated, yielding the dinuclear complex 5. This structure represents a first outlook on sandwich complexes based on hybrid crown ethers

Hybrid Disila-Crown Ethers as Hosts for Ammonium Cations: The O–Si–Si–O Linkage as an Acceptor for Hydrogen Bonding

Host-guest chemistry was performed with disilane-bearing crown ethers and the ammonium cation. Equimolar reactions of 1,2-disila[18]crown-6 (1) or 1,2-disila-benzo[18]crown-6 (2) and NH4PF6 in dichloromethane yielded the respective compounds [NH4(1,2-disila[18]crown-6)]PF6 (3) and [NH4(1,2-disila-benzo[18]crown-6)]PF6 (4). According to X-ray crystallographic, NMR, and IR experiments, the uncommon hydrogen bonding motif O(Si)∙∙∙H could be observed and the use of cooperative effects of ethylene and disilane bridges as an effective way to incorporate guest molecules was illustrated

Metal-free NH-oxidative addition at phospha-Wittig reagents

N-containing molecules are mostly derived from ammonia (NH3). Ammonia activation has been demonstrated for single transition metal centers as well as for low-valent main group species. Phosphinidenes, mono-valent phosphorus species, can be stabilized by phosphines, giving so-called phosphanylidenephosphoranes of the type RP(PR’3). We demonstrate the facile, metal-free NH3 activation using ArP(PMe3), affording for the first time isolable secondary aminophosphines ArP(H)NH2 through oxidative addition at a phosphinidenoid P atom. DFT studies reveal that two molecules of NH3 act in concert to facilitate an NH3 for PMe3 exchange. Furthermore, H2NR and HNR2 activation is demonstrated