Annie Castel

Stable N‐Heterocyclic Carbene Complexes of Hypermetallyl Germanium(II) and Tin(II) Compounds

The chemistry of the heavier Group 14 element carbene analogues has received wide interest because of their special properties and reactivity. Recently, an unexpected application of cyclopentadienyl, amido, or alkoxy germylenes and cyclic diazastannylene as precursors of nanomaterials has been described, thus opening a wide and promising field for investigation. Germanium nanowires have also been obtained by decomposition of hexakis(trimethylsilyl)digermane, highlighting the labile character of trimethylsilyl groups. Thus, the hypermetallyl germylenes or stannylenes, which contain both a low-coordinate Group 14 atom and a good leaving substituent, might be suitable candidates for nanomaterial alloys preparations. However, to the best of our knowledge, germylenes or stannylenes having electropositive silyl or germyl substituents have only been postulated as transient species; for example, in the reaction of Cl2Ge·dioxane with (Me3Si)3ELi (E = Si, Ge), chloro(silyl)germylenes rapidly oligomerize with the formation of cyclotetragermanes [(Me3Si)3EGeCl]4 or rearrange leading to cyclotrimetallanes [(Me3Si)2E{Ge(SiMe3)2}E(SiMe3)2]. To date, only one bis(hypersilyl)stannylene has been reported, but in its dimeric form in equilibrium with the monomeric form in solution and as a dimer in the solid state. Recently, a thermally unstable bis(hypergermyl)stannylene was synthesized (requiring preparation and handling below 30 8C), but like its hypersilyl substituted analogue, the X-ray structural analysis revealed the presence of dimers in the solid state. These results show the limitations of the steric shielding of the hypersilyl or hypergermyl ligands for the stabilization of low-coordinate species. Among the stabilization strategies of germylenes or stannylenes, the intermolecular coordination has aroused a great interest in the last decades, particularly with the use of N-heterocyclic carbenes (NHC) as stabilizing co-ligand. The first examples of carbene–germanium(II) adducts were described by Arduengo et al. ((NHC)GeI2) [8] and then by Lappert et al. (NHC–heterocyclic germylene). Later, NHCs were successfully employed for the stabilization of transient germanium(II) species. In contrast, there are few examples of carbene–stannylene adducts. In all of these cases, the carbene coordination is one of the key factors to obtain divalent species in their monomeric state. Herein we describe the synthesis of hypermetallyl germylenes and stannylenes that are stabilized by complexation with carbene units. We investigated the reactivity of the carbene–germylene adduct 1 and of the carbene–stannylene adduct 3, which were obtained as previously reported from the known carbene [DC{N(iPr)C(Me)}2] [13] and Cl2Ge·dioxane or Cl2Sn, respectively, towards various sources of hypermetallyl units. All attempts to displace the chloride from germylenes with hypersilyl salts (Me3Si)3SiLi [14] or (Me3Si)3SiK [15] were unsuccessful and led to complex mixtures in which only some amounts of disilagermirane [(Me3Si)2Si{Ge(SiMe3)2}Si(SiMe3)2] could be identified. [4a] The latter has been previously obtained by mixing (Me3Si)3SiLi and Cl2Ge·dioxane. By contrast, treatment of 1 with one half equivalent of [(Me3Si)3Si]2Mg [16] in THF solution at room temperature gave the complex 2 a as the only product in a good yield (71%) (Scheme 1). Compound 2a is the first example of a donor-stabilized hypersilyl(chloro)germylene that could be isolated by the combination of both steric hindrance of the hypersilyl ligand and strong Lewis base coordination. Similarly, the addition of a stoichiometric amount of digermylmagnesium [(Me3Si)3Ge]2Mg [17] to 1 produces the hypergermyl-

Cycloaddition of germylenes to 3,5-di-t-butyl orthoquinone

Abstract The germylenes X 2 Ge, RGeX and R 2 Ge (X = halogen, OR; R = alkyl or aryl) react at room temperature with 3,5-di-t-butyl orthoquinone by regioselective cycloaddition. The corresponding substituted 2-germa-1,3-dioxolans are formed similarly in good yields, but their stabilities depend on the substituents on the metal. Some of them have also been synthesized by nucleophilic substitution from the corresponding chlorogermanes and 3,5-di-t-butyl catechol. 2-Halo(6,8-di-t-butyl)-4,5-benzo-2-germa-1,3-dioxolans undergo redistribution reactions, while the corresponding dialkyl or diaryl derivatives are very stable.

Polygermanes precurseurs d'especes du germanium a coordinance non usuelle

Several thermal or photochemical α-elimination reactions of functional polygermanes lead to germylenes, R2Ge. Photolysis of polygermanes, cyclopolygermanes and polygermylmercury compounds and also hydrogen abstraction from various organohydropolygermanes using t-BuO., lead to the formation of polymetallated chains containing one or two germanium-centered radicals. These polygermyl radicals give germylenes, R2Ge, germanium centered radicals , α-digermyl diradicals (or digermenes) [ or ] and β- or γ-polygermyl diradicals via a homolytic monoelectronic α-elimination process. In some cases the formation of α-digermyl diradicals or digermenes can also be seen as occurring through dimerization of germylenes but with lower yields. All these intermediates have been characterized by several trapping reactions with dimethyl disulfide, 2,3-dimethylbutadiene or biacetyl.

Bis(amidinato)germylenerhodium complexes: synthesis, structure, and density functional theory calculations.

The first monogermylenerhodium complexes stabilized by bulky amidinato ligands on the divalent germanium center have been synthesized and characterized by NMR spectroscopy and X-ray crystallography. Their stability strongly depends on the steric hindrance of the amidinato ligand. With trimethysilyl groups on the nitrogen atoms of the amidinato ligand, only the germylene oxide rhodium complex could be obtained; by contrast, with t-Bu groups, the germylenerhodium complex was isolated. In both cases, the formation of amidinatorhodium complexes was observed. The donating ability of the germylene ligand has been assessed from the CO stretching frequency of the corresponding dicarbonylrhodium complex and was confirmed by density functional theory calculations.

Synthese de composes a liaison MIVBmercure. Precurseurs d'especes monovalentes (germynes), et bivalentes (germylenes), de radicaux centrometalles et d'intermediaires

Abstract Polynuclear germylmercury compounds are obtained from the reaction of organohydrogermanes Ph n GeH 4− n or digermanes Ph 2 n Ge 2 H 4− n ( n = 1, 2) with dialkylmercury R 2 Hg. Di- or tri-mercurated geminal polygermates thus synthesized generally present a low stability and undergo thermal- or photo-decompositions leading to the corresponding monovalent (germynes), divalent (germylenes) or, trivalent (germanium centered radicals) species and also to intermediate biradicals which could be considered as limit forms of germanium doubly-bonded compounds . Such intermediates have been chemically and spectroscopically characterized. Extension of these reactions to the silicon analogs met with difficulties, and thus previously observed differences between silicon and germanium chemistry were confirmed.

Synthese et etude spectrale uv de chaines polymetallees (Ge, Si); analyse theorique des effets de substitution

Abstract An UV study of polymetallic chains of Group IVB elements (Ge, Si) was developed. The studied polygermanes and polygermasilanes were mainly synthetized either by reaction of organolithium with organochlorogermanes or by a Wurtz type reaction. An enhanced λ max value for UV absorption of the chromophore Ge-Ge in these polymetallic compounds is observed in alkyl substitution at the metal by phenyl groups or halogen and also when the polymetallic chain is lengthened. A theoretical approach of these phenomena is proposed based on excitonic coupling between phenyl and metallic chain transition moments.

Silica nanoparticles grown and stabilized in organic nonalcoholic media.

This work features an alternative approach to the well-documented preparation of silica nanoparticles in protic media. We present here the one-pot synthesis of silica nanoparticles of adjustable size (between 18 and 174 nm), prepared and stabilized in organic nonalcoholic solvents. This novel route is based on hydrolysis and condensation of tetraethoxysilane, using water as reactant and different primary amines (butylamine, octylamine, dodecylamine, hexadecylamine) as catalysts in tetrahydrofuran or dimethoxyethane. The growth rate can be finely adjusted, and the first stages of the formation are observed by transmission electronic microscopy, revealing a silicated network in which the silica particles are formed and then released in solution. The amine plays not only a catalyst role but is also implied, as well as the solvent, in the stabilization process and the size control of the particles. A detailed NMR study demonstrates a core-shell structure in which the silica core is surrounded by a layer of alkylammonium ions together with solvent.

Nanocomposites from Natural Templates based on Fatty Compound-Functionalised Siloxanes

This work describes the synthesis of new, environmentally friendly, robust and biocompatible organic–inorganic hybrid materials based on fatty acid methyl ester-functionalised silica. The hydrosilylation reaction was used to covalently anchor 10-undecenoic methyl ester to a cyclic and acyclic backbone based on methylsiloxane repeating units. These as-synthesised amphiphilic precursors exhibit a self-assembling ability as shown by the formation of nanoobjects evidenced by fluorescence experiments, transmission electronic microscopy (TEM) and dynamic light scattering (DLS) analyses. Spherical nanocomposites featuring unprecedented flexibility, hydrophobicity and improved hydrolytic and thermal properties were built using sol–gel condensation of tetraethoxysilane (TEOS) controlled by these new self-assembling nanobuilding blocks. The textural characteristics and the morphology of these composite materials were dependent on the type of catalyst (acidic or basic) and the nature of the solvent (polar or apolar) used during the sol–gel polymerisation. This strategy opens new opportunities for advanced applications in various fields of nanochemistry and biomaterials.

Reactions of germylenes with nitrosobenzene and phenyl-t-butyl oxaziridine and its isomeric nitrone

Abstract The reactions of germylenes with nitrosobenzene lead to nitrene and intermediates via the zwitterionic form of the appropriate germaoxa-aziridines. Interactions between the germylenes and the nitrene generated in the reactions give new -intermediates. The formation of germaoxa-azetidine is observed in the insertion reaction of germylene into the oxaziridine ring, in the 1,3-cycloaddition of germylene to the nitrone which is an isomer of the oxaziridine and also in the dehydrochlorination reaction of C-germylated hydroxylamines . The β-elimination process from germaoxa-azetidines leads to imine and intermediates

Assistance nucleophile dans des reactions d'organo-halogeno- et -halogeno-hydrogermanes: germylanions, germylenes, derives fonctionnels du germanium

Abstract Substitution reactions of halogermanes under nucleophilic assistance of tertiary amines or diazo derivatives leading readily to various functional derivatives of germanium have been carried out. Dehydrohalogenation of acidic halohydrogermanes under nucleophilic assistance is a useful way to divalent species, and the germylenes formed are stabilized by complexation with the nucleophilic agent. The reactions proceed via intermediate halogermylanions which lead to the formation of more stable germylenes. The transient halogermylanions have been characterized by means of their nucleophilic additions to carbonyl derivatives. Nucleophilic substitutions of halogermanes using these germylanions lead to the formation of polygermanes