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