8 research outputs found

    A New Area in Main-Group Chemistry: Zerovalent Monoatomic Silicon Compounds and Their Analogues

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    ConspectusMonoatomic zerovalent main-group element complexes emerged very recently and attracted increasing attention of both theoretical and experimental chemists. In particular, zerovalent silicon complexes and their congeners (metallylones) stabilized by neutral Lewis donors are of significant importance not only because of their intriguing electronic structure but also because they can serve as useful building blocks for novel chemical species. Featuring four valence electrons as two lone pairs at the central atoms, such complexes may form donor–acceptor adducts with Lewis acids. More interestingly, with the central atoms in the oxidation state of zero, they could pave a way to new classes of compounds and functional groups that are otherwise difficult to realize.In this Account, we mainly describe our contributions in the chemistry of monatomic zerovalent silicon (silylone) and germanium (germylone) supported by a chelate bis-<i>N</i>-heterocyclic carbene (bis-NHC) ligand in the context of related species developed by other groups in the meantime. Utilizing the bis-NHC stabilized chlorosilyliumylidene [:SiCl]<sup>+</sup> and chlorogermyliumylidene [:GeCl]<sup>+</sup> as suitable starting materials, we successfully isolated silylone (bis-NHC)Si and germylone (bis-NHC)­Ge, respectively. The electronic structures of the latter complexes established by theoretical calculations and spectroscopic data revealed that they are genuine metallylone species with electron-rich silicon(0) and germanium(0) centers. Accordingly, they can react with 1 molar equiv of GaCl<sub>3</sub> to form Lewis adducts (bis-NHC)­E­(GaCl<sub>3</sub>) (E = Si, Ge) and with 2 molar equiv of ZnCl<sub>2</sub> to furnish (bis-NHC)­Si­(ZnCl<sub>2</sub>)<sub>2</sub>. Conversion of the metallylones with elemental chalcogens affords isolable monomeric silicon­(II) and germanium­(II) monochalcogenides (bis-NHC)­EX­(GaCl<sub>3</sub>) (X = Se, Te), representing molecular heavier congeners of CO. Moreover, their reaction with elemental chalcogens can also yield monomeric silicon­(IV) and germanium­(IV) dichalcogenides (bis-NHC)­EX<sub>2</sub> (X = S, Se, Te) as the first isolable complexes of the molecular congeners of CO<sub>2</sub>. Moreover, (bis-NHC)­Si could even activate CO<sub>2</sub> to afford the monomolecular silicon dicarbonate complex (bis-NHC)­Si­(CO<sub>3</sub>)<sub>2</sub> via the formation of SiO and SiO<sub>2</sub> complexes as intermediates. Furthermore, starting with a chelate bis-<i>N</i>-heterocyclic silylene supported [:GeCl]<sup>+</sup>, we developed two bis-<i>N</i>-heterocyclic silylene stabilized germylone→Fe­(CO)<sub>4</sub> complexes. Our achievements in the chemistry of metallylones demonstrate that the characteristic of monatomic zerovalent silicon and its analogues can provide novel reaction patterns for access to unprecedented species and even extends the series of functional groups of these elements. With this, we can envision that more interesting zerovalent complexes of the main-group elements with unprecedented reactivity will follow in the near future

    Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-heterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins

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    The first chelating bis­(N-heterocyclic silylene)­xanthene ligand [Si<sup>II</sup>(Xant)­Si<sup>II</sup>] as well as its Ni complexes [Si<sup>II</sup>(Xant)­Si<sup>II</sup>]­Ni­(η<sup>2</sup>-1,3-cod) and [Si<sup>II</sup>(Xant)­Si<sup>II</sup>]­Ni­(PMe<sub>3</sub>)<sub>2</sub> were synthesized and fully characterized. Exposing [Si<sup>II</sup>(Xant)­Si<sup>II</sup>]­Ni­(η<sup>2</sup>-1,3-cod) to 1 bar H<sub>2</sub> at room temperature quantitatively generated an unexpected dinuclear hydrido Ni complex with a four-membered planar Ni<sub>2</sub>Si<sub>2</sub> core. Exchange of the 1,3-COD ligand by PMe<sub>3</sub> led to [Si<sup>II</sup>(Xant)­Si<sup>II</sup>]­Ni­(PMe<sub>3</sub>)<sub>2</sub>, which could activate H<sub>2</sub> reversibly to afford the first Si<sup>II</sup>-stabilized mononuclear dihydrido Ni complex characterized by multinuclear NMR and single-crystal X-ray diffraction analysis. [Si<sup>II</sup>(Xant)­Si<sup>II</sup>]­Ni­(η<sup>2</sup>-1,3-cod) is a strikingly efficient precatalyst for homogeneous hydrogenation of olefins with a wide substrate scope under 1 bar H<sub>2</sub> pressure at room temperature. DFT calculations reveal a novel mode of H<sub>2</sub> activation, in which the Si<sup>II</sup> atoms of the [Si<sup>II</sup>(Xant)­Si<sup>II</sup>] ligand are involved in the key step of H<sub>2</sub> cleavage and hydrogen transfer to the olefin

    Synthesis and Reactivity of an Anti-van’t Hoff/Le Bel Compound with a Planar Tetracoordinate Silicon(II) Atom

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    For a long time, planar tetracoordinate carbon (ptC) represented an exotic coordination mode in organic and organometallic chemistry, but it is now a useful synthetic building block. In contrast, realization of planar tetracoordinate silicon (ptSi), a heavier analogue of ptC, is still challenging. Herein we report the successful synthesis and unusual reactivity of the first ptSi species of divalent silicon present in 3, supported by the chelating bis(N-heterocyclic silylene)bipyridine ligand, 2,2′-{[(4-tBuPh)­C(NtBu)]2­SiNMe}2­(C5N)2, 1]. The compound resulted from direct reaction of 1 with Idipp-SiI2 [Idipp = 1,3-bis(2,6-diisopropyl­phenyl)­imidazol-2-ylidene]. Alternatively, it can also be synthesized by a two-electron reduction of the corresponding Si(IV) precursor 2 with 2 molar equiv of KC10H8. Density functional theory calculations show that the lone pair at the ptSi(II) resides almost completely in its 3pz orbital, very different from known four-coordinate silylenes. Oxidative addition of MeI to the ptSi(II) atom affords the corresponding pentacoordinate Si(IV) compound 4, with the methyl group located in an apical position. Remarkably, the reaction of 2 with [CuOtBu] leads to the regeneration of the bis(silylene) arms via Si–Si bond scission and induces the Si(II) → Si(IV) oxidation of the central Si(II) atom and concomitant two-electron reduction of the bipyridine moiety to form the neutral bis(silylene)silyl Cu(I) complex 5

    A Cyclic Germadicarbene (“Germylone”) from Germyliumylidene

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    By employing the chelate dicarbene <b>1</b>, the new chloro­germylium­ylidene complex <b>2</b> could be synthesized and isolated in 95% yield. Dechlorination of <b>2</b> with sodium naphthalenide furnishes the unique cyclic germadicarbene <b>3</b> which could be isolated in 45% yield. Compound <b>3</b> is the first isolable Ge(0) complex with a single germanium atom stabilized by a dicarbene. Its molecular structure is in accordance with DFT calculations which underline the peculiar electronic structure of <b>3</b> with two lone pairs of electrons at the Ge atom

    Synthesis and Unexpected Reactivity of Germyliumylidene Hydride [:GeH]<sup>+</sup> Stabilized by a Bis(<i>N</i>‑heterocyclic carbene)borate Ligand

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    Employing the potassium salt of the monoanionic bis­(<i>NHC</i>)­borate <b>1</b> (<i>NHC</i> = <i>N</i>-<i>H</i>eterocyclic <i>C</i>arbene) enables the synthesis and isolation of the bis­(<i>NHC</i>)­borate-stabilized chlorogermyliumylidene precursor <b>2</b> in 61% yield. A Cl/H exchange reaction of <b>2</b> using potassium tri<i>sec</i>.-butylborhydride as a hydride source leads to the isolation of the first germyliumylidene hydride [HGe:<sup>+</sup>] complex <b>3</b> in 91% yield. The Ge­(II)–H bond in the latter compound has an unexpected reactivity as shown by the reaction with the potential hydride scavenger [Ph<sub>3</sub>C]<sup>+</sup>[B­(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>−</sup>, furnishing the corresponding HGe: → CPh<sub>3</sub> cation in the ion pair <b>4</b> as initial product. Compound <b>4</b> liberates HCPh<sub>3</sub> in the presence of <b>3</b> to give the unusual dinuclear HGe: → Ge: cation in <b>5</b>. The latter represents the first three-coordinate dicationic Ge­(II) species stabilized by an anionic bis­(<i>NHC</i>) chelate ligand and a Ge­(II) donor. All novel compounds were fully characterized, including X-ray diffraction analyses

    A Cyclic Germadicarbene (“Germylone”) from Germyliumylidene

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    By employing the chelate dicarbene <b>1</b>, the new chloro­germylium­ylidene complex <b>2</b> could be synthesized and isolated in 95% yield. Dechlorination of <b>2</b> with sodium naphthalenide furnishes the unique cyclic germadicarbene <b>3</b> which could be isolated in 45% yield. Compound <b>3</b> is the first isolable Ge(0) complex with a single germanium atom stabilized by a dicarbene. Its molecular structure is in accordance with DFT calculations which underline the peculiar electronic structure of <b>3</b> with two lone pairs of electrons at the Ge atom

    Synthesis, Reactivity, and Electronic Structure of a Bioinspired Heterobimetallic [Ni(μ‑S<sub>2</sub>)Fe] Complex with Disulfur Monoradical character

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    The first synthesis of a monoradical Ni­(μ-S<sub>2</sub>)­Fe core in the [(Nacnac)­Ni­(μ-S<sub>2</sub>)­Fe­(dmpe)<sub>2</sub>] complex <b>3</b> could be accomplished in good yields by PMe<sub>3</sub> elimination from the zerovalent iron complex [(dmpe)<sub>2</sub>(PMe<sub>3</sub>)­Fe] (<b>2</b>; dmpe =1,2-bis­(dimethylphosphine)­ethane) upon reaction with the supersulfido nickel­(II) complex [(Nacnac)­Ni­(S<sub>2</sub>)] (<b>1</b>; Nacnac = CH­{(CMe)­(2,6-<sup><i>i</i></sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>N)}<sub>2</sub>). Complex <b>3</b> bears Ni­(II) and Fe­(II) centers, both of which are in a low-spin state. A single electron is located in the HOMO and is somewhat delocalized over the Ni­(μ-S<sub>2</sub>)Fe core, so that the bridging disulfur subunit exhibits some “subsulfide” S<sub>2</sub><sup>3–</sup> character. Compound <b>3</b> represents a bioinspired example of a monoradical with a Ni­(μ-S<sub>2</sub>)Fe structural motif, reminiscent of the Ni­(μ-S<sub>2</sub>)Fe core structure of the active site in [NiFe] hydrogenases. Its oxidation with [Fe­(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>]­[B­(C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)<sub>4</sub>] affords the product [(Nacnac)­Ni­(μ-S)<sub>2</sub>Fe­(dmpe)<sub>2</sub>]­[B­(C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)<sub>4</sub>] (<b>4</b>), and complex <b>3</b> can alternatively be prepared via a reductive route upon reaction of [Co­(η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>]­[(Nacnac)­NiS<sub>2</sub>] (<b>6</b>) with the Fe(0) precursor <b>2</b>. All synthesized complexes were fully characterized, including in some cases single-crystal X-ray diffraction analysis, magnetometry, EPR, NMR, and <sup>57</sup>Fe Mössbauer spectroscopy. DFT calculations were used to compute the spectroscopic parameters and to establish the electronic structure of <b>3</b> and its oxidized and reduced forms and related complexes

    Dual Reactivity of a Stable Zwitterionic N-Heterocyclic Silylene and Its Carbene Complex Probed with Muonium

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    The reactivity of the multifunctional cyclic silylene <b>4</b> and its carbene complex <b>5</b> have been investigated by a combination of muon spin spectroscopy and computation. The free radicals formed by muonium (Mu) addition to <b>4</b> were identified, showing that there are two dominant sites of free radical attack: on the Si atom and on the exocyclic methylene carbon. Reaction of muonium with <b>5</b> also produced two radicals, but with markedly different hyperfine constants. For both compounds avoided level-crossing resonance spectra and calculation of hyperfine constants show that one of the radicals results from Mu addition to the methylene group, yielding radicals <b>4a</b> and <b>5a</b>. Each contains a muoniated methyl group, −CH<sub>2</sub>Mu, which undergoes restricted rotation with respect to the plane of the ring. For <b>4</b> the second product is readily assigned as the muoniated silyl radical <b>4b</b>, on the grounds of its high muon hyperfine constant (716 MHz). The second product from <b>5</b> shows instead a very small coupling constant, 19 MHz, assignable to the muoniated complex <b>5b</b>, in which the spin density has been transferred from the silicon to the carbenic carbon
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