Silylene-Stabilized Neutral Dibora-Aromatics with a B═B Bond.

The unprecedented silylene-supported dibenzodiboraoxepin 2 and 9,10-diboraphenanthrene complexes 6 and 8 were synthesized. The (NHSi)2B2(xanthene) [NHSi = PhC(NtBu)2(Me2N)Si:] 2 results from debromination of the bis(NHSi)-stabilized bis(dibromoboryl)xanthene 1 with potassium graphite (KC8); 2 is capable of activating white phosphorus and ammonia to form the B2P4 cage compound 3 and H2N-B-B-H diborane species 4, respectively. The thermal rearrangement of 2 affords the 9,10-dihydro-9,10-diboraphenanthrene 5 through a bis(NHSi)-assisted intramolecular reductive C-O-C deoxygenation process. Notably, the 9,10-diboraphenanthrene derivatives 6 and 8 could be generated by deoxygenation of 2 with KC8 and 1,3,4,5-tetramethylimidazol-2-ylidene, respectively. The aromaticity of 6 and 8 was confirmed by computational studies. Strikingly, the NHSi ligand in 8 engenders the monodeoxygenation of carbon dioxide in toluene at room temperature to form the CO-stabilized 9,10-diboraphenanthrene derivative 9 via the silaoxadiborinanone intermediate 10.

Stabilizing Monoatomic Two-Coordinate Bismuth(I) and Bismuth(II) Using a Redox Noninnocent Bis(germylene) Ligand.

The formation of isolable monatomic BiI complexes and BiII radical species is challenging due to the pronounced reducing nature of metallic bismuth. Here, we report a convenient strategy to tame BiI and BiII atoms by taking advantage of the redox noninnocent character of a new chelating bis(germylene) ligand. The remarkably stable novel BiI cation complex 4, supported by the new bis(iminophosphonamido-germylene)xanthene ligand [(P)GeII(Xant)GeII(P)] 1, [(P)GeII(Xant)GeII(P) = Ph2P(NtBu)2GeII(Xant)GeII(NtBu)2PPh2, Xant = 9,9-dimethyl-xanthene-4,5-diyl], was synthesized by a two-electron reduction of the cationic BiIIII2 precursor complex 3 with cobaltocene (Cp2Co) in a molar ratio of 1:2. Notably, owing to the redox noninnocent character of the germylene moieties, the positive charge of BiI cation 4 migrates to one of the Ge atoms in the bis(germylene) ligand, giving rise to a germylium(germylene) BiI complex as suggested by DFT calculations and X-ray photoelectron spectroscopy (XPS). Likewise, migration of the positive charge of the BiIIII2 cation of 3 results in a bis(germylium)BiIIII2 complex. The delocalization of the positive charge in the ligand engenders a much higher stability of the BiI cation 4 in comparison to an isoelectronic two-coordinate Pb0 analogue (plumbylone; decomposition below -30 °C). Interestingly, 4[BArF] undergoes a reversible single-electron transfer (SET) reaction (oxidation) to afford the isolable BiII radical complex 5 in 5[BArF]2. According to electron paramagnetic resonance (EPR) spectroscopy, the unpaired electron predominantly resides at the BiII atom. Extending the redox reactivity of 4[OTf] employing AgOTf and MeOTf affords BiIII(OTf)2 complex 7 and BiIIIMe complex 8, respectively, demonstrating the high nucleophilic character of BiI cation 4.

Chelating Bis-silylenes As Powerful Ligands To Enable Unusual Low-Valent Main-Group Element Functions.

ConspectusSilylenes are divalent silicon species with an unoccupied 3p orbital and one lone pair of electrons at the SiII center. Owing to the excellent σ-donating ability of amidinato-based silylenes, which stems from the intramolecular imino-N donor interaction with the vacant 3p orbital of the silicon atom, N-heterocyclic amidinato bis(silylenes) [bis(NHSi)s] can serve as versatile strong donating ligands for cooperative stabilization of central atoms in unusually low oxidation states. Herein, we present our recent achievement on the application of bis(NHSi) ligands with electronically and spatially different spacers to main-group chemistry, which has allowed the isolation of a variety of low-valent compounds consisting of monatomic zero-valent group 14 E0 complexes (named "metallylones", E = Si, Ge, Sn, Pb); monovalent group 15 EI complexes (E = N, P, isoelectronic with metallylones); and diatomic low-valent E2 complexes (E = Si, Ge, P) with intriguing electronic structures and chemical reactivities.The role of the SiII···SiII distance was revealed to be crucial in this chemistry. Utilizing the pyridine-based bis(NHSi) (Si···Si distance: 7.8 Å) ligand, germanium(0) complexes with additional Fe(CO)4 protection at the Ge0 site have been isolated. Featuring a shorter Si···Si distance of 4.3 Å, the xanthene-based bis(NHSi) has allowed the realization of the full series of heavy zero-valent group 14 element E0 complexes (E = Si, Ge, Sn, Pb), while the o-carborane-based bis(NHSi) (Si···Si distance: 3.3 Å) has enabled the isolation of Si0 and Ge0 complexes. Remarkably, reduction of the o-carborane-based bis(NHSi)-supported Si0 and Ge0 complexes induces the movement of two electrons into the o-carborane core and provides access to SiI-SiI and GeI-GeI species as oxidation products. Additionally, the o-carborane-based bis(NHSi) reacts with adamantyl azide, leading to a series of nitrogen(I) complexes as isoelectronic species of a carbone (C0 complex). Moreover, cooperative activation of white phosphorus gives bis(NHSi)-supported phosphorus complexes with varying and unexpected electronic structures when employing the xanthene-, o-carborane-, and aniline-based bis(NHSi)s. With the better kinetic protection provided by the xanthene-based bis(NHSi), small-molecule activation and functionalization of the bis(NHSi)-supported central E or E2 atoms (E = Si, Ge, P) are possible and furnish several novel functionalized silicon, germanium, and phosphorus compounds.With knowledge of the ability of chelating bis(NHSi)s in coordinating and functionalizing low-valent group 14 and 15 elements, the application of these ligand systems to other main-group elements such as group 2 and 13 is quite promising. To fully understand the role of the NHSi in a bis(NHSi) ligand, introducing a mixed ligand, i.e., the combination of an NHSi with other functional groups, such as Lewis acidic borane or Lewis basic borylene, in one chelating ligand could lead to new types of low-valent main-group species. Furthermore, the development of a genuine acyclic silylene, without an imino-N interaction with the vacant 3p orbital at the silicon(II) atom, as part of a chelating bis(acyclic silylene) has the potential to form very electronically different main-group element complexes that could achieve even more challenging bond activations such as N2 or unactivated C-H bonds.

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.

Spectroscopic Properties of a Biologically Relevant [Fe2 (µ-O)2 ] Diamond Core Motif with a Short Iron-Iron Distance.

Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe-Fe distances of 2.5-3.4 Šwere attributed to "open" or "closed" cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a FeIII 2 (µ-O)2 complex (2) with tetrahedral (4C) centres and short Fe-Fe distance (2.52 Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the TD FeIII centres due to the short Fe-µ-O bonds. A ≈2.5 ŠFe-Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe2 (µ-O)2 core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe-Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O2 activation in biology.

Unexpected White Phosphorus (P4 ) Activation Modes with Silylene-Substituted o-Carboranes and Access to an Isolable 1,3-Diphospha-2,4-disilabutadiene.

New types of metal-free white phosphorus (P4 ) activation are reported. While the phosphine-silylene-substituted dicarborane 1, CB-SiP (CB=ortho-C,C'-C2 B10 H10 , Si=PhC(tBuN)2 Si, P=P[N(tBu)CH2 ]2 ), activates white phosphorus in a 2 : 1 molar ratio to yield the P5 -chain containing species 2, the analogous bis(silylene)-substituted compound 3, CB-Si2 , reacts with P4 in the molar ratio of 2 : 1 to furnish the first isolable 1,3-diphospha-2,4-disilabutadiene (Si=P-Si=P-containing) compound 4. For the latter reaction, two intermediates having a CB-Si2 P4 and CB-Si2 P2 core could be observed by multinuclear NMR spectroscopy. The compounds 2 and 4 were characterized including single-crystal X-ray diffraction analyses. Their electronic structures and mechanisms were investigated by density functional theory calculations.

The Heaviest Bottleable Metallylone: Synthesis of a Monatomic, Zero-Valent Lead Complex ("Plumbylone").

The elusive plumbylone {[SiII (Xant)SiII ]Pb0 } 3 stabilized by the bis(silylene)xanthene chelating ligand 1, [SiII (Xant)SiII =PhC(NtBu)2 Si(Xant)Si(NtBu)2 CPh], and its isolable carbonyl iron complex {[SiII (Xant)SiII ]Pb0 Fe(CO)4 } 4 are reported. The compounds 3 and 4 were obtained stepwise via reduction of the lead(II) dibromide complex {[SiII (Xant)SiII ]PbBr2 } 2, prepared from the bis(silylene)xanthene 1 and PbBr2 , employing potassium naphthalenide and K2 Fe(CO)4 , respectively. While the genuine plumbylone 3 is rather labile even at -60 °C, its Pb0 →Fe(CO)4 complex 4 turned out to be relatively stable and bottleable. However, solutions of 4 decompose readily to elemental Pb and {[SiII (Xant)SiII ]Fe(CO)3 } 5 at 80 °C. Reaction of 4 with [Rh(CO)2 Cl]2 leads to the formation of the unusual dimeric [(OC)2 RhPb(Cl)Fe(CO)4 ] complex 6 with trimetallic Rh-Pb-Fe bonds. The molecular and electronic structures of 3 and 4 were established by Density Functional Theory (DFT) calculations.

A Striking Mode of Activation of Carbon Disulfide with a Cooperative Bis(silylene).

The reactivity of the 1,4-substituted bis(silylenyl)terphenylene 1, 1,4-[ortho-(LSi)C6 H4 ]2 C6 H4 , (L=RC(NtBu)2 , R=Ph, Mes) towards CS2 is reported. It results in a dearomatization of the phenylene ring, affording the 1,3-substituted cyclohexadiene derivative 2. According to DFT calculations, a transient silene containing a Si=C bond capable of π(C=C) addition at the aromatic phenylene ring is a key intermediate. In contrast, addition of CS2 to the biphenyl-substituted mono-silylene ortho-(LSi)C6 H4 -C6 H5 3 leaves the aromatic π-system intact and forms, in a [1+2] cycloaddition reaction, the corresponding thiasilirane 4 with a three-membered SiSC ring. Further experimental studies led to the isolation of the novel mesoionic five-membered Si2 S2 C heterocycle 6, which reacts with CS2 under C-C bond formation. All isolated new compounds were fully characterized and their molecular structures determined by single-crystal X-ray diffraction analyses.

A Genuine Stannylone with a Monoatomic Two-Coordinate Tin(0) Atom Supported by a Bis(silylene) Ligand.

The monoatomic zero-valent tin complex (stannylone) {[SiII (Xant)SiII ]Sn0 } 5 stabilized by a bis(silylene)xanthene ligand, [SiII (Xant)SiII =PhC(NtBu)2 Si(Xant)Si(NtBu)2 CPh], and its bis-tetracarbonyliron complex {[SiII (Xant)SiII ]Sn0 [Fe(CO)4 ]2 } 4 are reported. The stannylone 5 bearing a two-coordinate zero-valent tin atom is synthesized by reduction of the precursor 4 with potassium graphite. Compound 4 results from the SnII halide precursor {[SiII (Xant)SiII ]SnII Cl}Cl 2 or {[SiII (Xant)SiII ]SnBr2 } 3 through reductive salt-metathesis reaction with K2 Fe(CO)4 . According to density functional theory (DFT) calculations, the highest occupied molecular orbital (HOMO) and HOMO-1 of 5 correspond to a π-type lone pair with delocalization into both adjacent vacant orbitals of the SiII atoms and a σ-type lone pair at the Sn0 center, respectively, indicating genuine stannylone character.

An Isolable 2,5-Disila-3,4-Diphosphapyrrole and a Conjugated Si=P-Si=P-Si=N Chain Through Degradation of White Phosphorus with a N,N-Bis(Silylenyl)Aniline.

White phosphorus (P4 ) undergoes degradation to P2 moieties if exposed to the new N,N-bis(silylenyl)aniline PhNSi2 1 (Si=Si[N(tBu)]2 CPh), furnishing the first isolable 2,5-disila-3,4-diphosphapyrrole 2 and the two novel functionalized Si=P doubly bonded compounds 3 and 4. The pathways for the transformation of the non-aromatic 2,5-disila-3,4-diphosphapyrrole PhNSi2 P2 2 into 3 and 4 could be uncovered. It became evident that 2 reacts readily with both reactants P4 and 1 to afford either the polycyclic Si=P-containing product [PhNSi2 P2 ]2 P2 3 or the unprecedented conjugated Si=P-Si=P-Si=NPh chain-containing compound 4, depending on the employed molar ratio of 1 and P4 as well as the reaction conditions. Compounds 3 and 4 can be converted into each other by reactions with 1 and P4 , respectively. All new compounds 1-4 were unequivocally characterized including by single-crystal X-ray diffraction analysis. In addition, the electronic structures of 2-4 were established by Density Functional Theory (DFT) calculations.

New Types of Ge2 and Ge4 Assemblies Stabilized by a Carbanionic Dicarborandiyl-Silylene Ligand.

The first Ge(0)-Ge(II) germylone-germylene-paired Ge2 complex (LSi)2Ge2 (4) and the molecular Ge4 cluster (LSi)2Ge4 (5) supported by the chelating carbanionic ortho-C,C'-dicarborandiyl-silylene ligand LSi [L = C,C'-C2B10H10, Si = PhC(tBuN)2Si] have been synthesized and isolated via reduction of the corresponding precursors chlorogermyl-germyliumylidene chloride (2), [(LSi)2Ge(Cl)Ge]+Cl-, and (LSi)2Ge4Cl4 (3) with C8K, respectively. The latter precursors were obtained from the unexpected outcome of the reaction of the ortho-C,C'-dicarborandiyl phosphine-silylene ligand PLSi (1) {P = P[N(tBu)CH2]2} and GeCl2·dioxane. Compound 2 is formed in higher yields (65% yields) by the salt metathesis reaction of the C-lithium dicarborandiyl-C'-silylene salt LiLSi (6) [Li = Li(OEt2)2] with GeCl2·dioxane. The molecular structures of all these species (1-6) have been established and confirmed spectroscopically and crystallographically. The electronic structures of 4 and 5 were elucidated by density functional theory calculations. While 4 possesses a localized dative Ge(0)→Ge(II) bond, the Ge-Ge σ bonds in 5 are delocalized in the Ge4 cluster core. Featuring a donor-acceptor interaction between two chelating silylenes and the Ge4 core, compound 5 represents a unique molecular model for a Ge4 cluster.

Where silylene-silicon centres matter in the activation of small molecules.

Small molecules such as H2, N2, CO, NH3, O2 are ubiquitous stable species and their activation and role in the formation of value-added products are of fundamental importance in nature and industry. The last few decades have witnessed significant advances in the chemistry of heavy low-coordinate main-group elements, with a plethora of newly synthesised functional compounds, behaving like transition-metal complexes with respect to facile activation of such small molecules. Among them, silylenes have received particular attention in this vivid area of research showing even metal-free bond activation and catalysis. Recent striking discoveries in the chemistry of silylenes take advantage of narrow HOMO-LUMO energy gap and Lewis acid-base bifunctionality of divalent Si centres. The review is devoted to recent advances of using isolable silylenes and corresponding silylene-metal complexes for the activation of fundamental but inert molecules such as H2, COx, N2O, O2, H2O, NH3, C2H4 and E4 (E = P, As).

Changing the Reactivity of Zero- and Mono-Valent Germanium with a Redox Non-Innocent Bis(silylenyl)carborane Ligand.

Using the chelating C,C'-bis(silylenyl)-ortho-dicarborane ligand, 1,2-(RSi)2 -1,2-C2 B10 H10 [R=PhC(NtBu)2 ], leads to the monoatomic zero-valent Ge complex ("germylone") 3. The redox non-innocent character of the carborane scaffold has a drastic influence on the reactivity of 3 towards reductants and oxidants. Reduction of 3 with one molar equivalent of potassium naphthalenide (KC10 H8 ) causes facile oxidation of Ge0 to GeI along with a two-electron reduction of the C2 B10 cluster core and subsequent GeI -GeI coupling to form the dianionic bis(silylene)-supported Ge2 complex 4. In contrast, oxidation of 3 with one molar equivalent of [Cp2 Fe][B{C6 H3 (CF3 )2 }4 ] as a one-electron oxidant furnishes the dicationic bis(silylene)-supported Ge2 complex 5. The Ge0 atom in 3 acts as donor towards GeCl2 to form the trinuclear mixed-valent Ge0 →GeII ←Ge0 complex 6, from which dechlorination with KC10 H8 affords the neutral Ge2 complex 7 as a diradical species.

Facile Access to an Active γ-NiOOH Electrocatalyst for Durable Water Oxidation Derived From an Intermetallic Nickel Germanide Precursor.

Identifying novel classes of precatalysts for the oxygen evolution reaction (OER by water oxidation) with enhanced catalytic activity and stability is a key strategy to enable chemical energy conversion. The vast chemical space of intermetallic phases offers plenty of opportunities to discover OER electrocatalysts with improved performance. Herein we report intermetallic nickel germanide (NiGe) acting as a superior activity and durable Ni-based electro(pre)catalyst for OER. It is produced from a molecular bis(germylene)-Ni precursor. The ultra-small NiGe nanocrystals deposited on both nickel foam and fluorinated tin oxide (FTO) electrodes showed lower overpotentials and a durability of over three weeks (505 h) in comparison to the state-of-the-art Ni-, Co-, Fe-, and benchmark NiFe-based electrocatalysts under identical alkaline OER conditions. In contrast to other Ni-based intermetallic precatalysts under alkaline OER conditions, an unexpected electroconversion of NiGe into γ-NiIII OOH with intercalated OH- /CO3 2- transpired that served as a highly active structure as shown by various ex situ methods and quasi in situ Raman spectroscopy.

Isolable Dibenzo[a,e]disilapentalene with a Dichotomic Reactivity toward CO2.

The first dibenzo[a,e]disilapentalene with two Si═C moieties in the heteropentalene core has been prepared. Its solid-state structure and density functional theory (DFT) calculations revealed that the Si═C bonds are involved in an expanded π-conjugated system. The Si═C bonds show a distinguished reactivity toward CO2, depending on the reaction conditions. While one product results from fixation of two CO2 molecules across one Si═C bond, two different products could be isolated from the reaction of three CO2 molecules with both Si═C bonds. The mechanism has been uncovered by DFT calculations.

Redox Noninnocent Monoatomic Silicon(0) Complex ("Silylone"): Its One-Electron-Reduction Induces an Intramolecular One-Electron-Oxidation of Silicon(0) to Silicon(I).

A monatomic zerovalent silicon(0) complex ("silylone") stabilized by the chelating bis(silylenyl)-ortho-carborane ligand, 1,2-(LSi)2-1,2-C2B10H10 [L = PhC(NtBu)2], has been synthesized from the redox reaction of the dipotassium bis(silylenyl)-nido-carboranate salt, 1,2-(LSi)2-1,2-C2B10H10K2, and NHC-SiCl2 (NHC = {[HCN(2,6-iPr2C6H3)]2C:}). Markedly different from previous examples, this silylone undergoes reduction due to the closo-C2B10 cluster backbone, which is prone to accept up to two electrons to form the cage-opened dianionic nido-C2B10 cluster core. Surprisingly, the closo-C2B10 core of the silylone consumes only one molar equiv of potassium naphthalenide, in addition, one electron is intramolecularly transferred from the Si0 atom to the C2B10 core to form an elusive bis(silylene)-stabilized SiI radical cation which undergoes homocoupling to the corresponding isolable dicationic SiI-SiI complex.

An Isolable Bis(Silanone-Borane) Adduct.

The reaction of bis(silylenyl)-substituted ferrocene 1 with two molar equivalents of BPh3 yields the corresponding bis(silylene-borane) Lewis adduct 2. The latter is capable to activate CO2 to furnish the borane-stabilized bis(silanone) 3 through mono-oxygenation of the dative SiII →B silicon centers under release of CO. Removal of BPh3 from 3 with PMe3 affords the corresponding 1,3,2,4-cyclodisiloxane and the Me3 P-BPh3 adduct. All isolated new compounds were characterized and their molecular structures were determined by single-crystal X-ray diffraction analyses.

Bis(silylene)-Stabilized Monovalent Nitrogen Complexes.

The first series of bis(silylene)-stabilized nitrogen(I) compounds is described. Starting from the 1,2-bis(N-heterocyclic silylenyl) 1,2-dicarba-closo-dedocaborane(12) scaffold 1, [1,2-(LSi)2 C2 B10 H10 ; L=PhC(Nt Bu)2 ], reaction with adamantyl azide (AdN3 ) affords the terminal N-µ2 -bridged zwitterionic carborane-1,2-bis(silylium) AdN3 adduct 2 with an open-cage dianionic nido-C2 B10 cluster core. Remarkably, upon one-electron reduction of 2 with C8 K and liberation of N2 and adamantane, the two silylene subunits are regenerated to furnish the isolable bis(silylene)-stabilized NI complex as an anion of 3 with the nido-C2 B10 cluster cage. On the other hand, one-electron oxidation of 2 with silver(I) yields the monocationic bis(silylene) NI complex 4 with the closo-C2 B10 cluster core. Moreover, the corresponding neutral NI radical complex 5 results from single-electron transfer from 3 to 4.

Synthesis of an Isolable Bis(silylene)-Stabilized Silylone and Its Reactivity Toward Small Gaseous Molecules.

The first bis(N-heterocyclic silylene)-stabilized zero-valent silicon compound, [SiII(Xant)SiII]Si0 (4, Xant = 9,9-dimethyl-xanthene-4,5-diyl), has been synthesized via the reduction of the corresponding chlorosilyliumylidene chloride precursor {[SiII(Xant)SiII]SiCl}+Cl- (2). The electronic structure of silylone 4, whose molecular structure is confirmed spectroscopically and crystallographically, is investigated by DFT calculations and Natural Bond Orbital analysis, showing two perpendicular lone-pairs of electrons on the central Si0 atom, i.e., an sp0.41-type lone-pair and a delocalized p lone-pair. With the electron-rich and oxophilic Si0 center, silylone 4 exhibits a striking reactivity toward small gaseous molecules. Remarkably, the oxidation of silylone 4 by N2O can be controlled to generate distinct products by regulating the amount of added N2O. Exposing 4 to an excess or two molar equivalents of N2O yields the unexpected oxidation product 5, bearing a central six-membered Si4O2 ring. When 4 is mixed with one molar equivalent of N2O, the unique compound 6 is obtained, resulting from a rare 1,4-addition of two central silicon atoms to a phenyl ring of an amidinate ligand coordinated to the SiII atom. In addition, cleavage of the strong N-H bond in ammonia is also readily accomplished by silylone 4, representing the first example of NH3 activation in silylone chemistry. In the presence of the Lewis acid BPh3, silylone 4 achieves heterolytic dihydrogen cleavage and ethylene addition to form the corresponding hydridosilyliumylidene hydroborate salt 8 and the zwitterionic compound 9, respectively, which represent a new type of frustrated Lewis pair based on an electron-rich Si0 donor and a borane acceptor.

An Isolable Bis(silylene)-Stabilized Germylone and Its Reactivity.

The first zerovalent germanium complex ("germylone") 3, [SiII(Xant)SiII]Ge0, stabilized by a chelating bis(N-heterocyclic silylene)xanthene donor ligand 1 was successfully synthesized via the dechlorination of the corresponding {[SiII(Xant)SiII]GeCl}+Cl- complex 2 with KC8; it was structurally and spectroscopically characterized, and also studied by density functional theory (DFT) calculations. Natural bond orbital (NBO) analysis of 3 unambiguously exhibits two lone pairs of electrons (one σ-type lone-pair and one 3p(Ge) lone-pair) on the zerovalent Ge atom. This is why the Ge atom can form the corresponding mono- and bis-AlBr3 Ge → Al Lewis adducts [SiII(Xant)SiII]Ge(AlBr3) 4 and [SiII(Xant)SiII]Ge(AlBr3)2 5, respectively. Due to the electron-rich character of the Ge0 atom, the germylone 3 displayed quite unusual reactivities. Thus, the reaction of 3 with 9-borabicyclo[3.3.1]nonane (9-BBN) as a potential Lewis acid furnished the first boryl(silyl)germylene complex 6, possessing a heteroallylic B···Ge···Si π-conjugation. When 3 was allowed to react with Ni(cod)2 (cod = 1,5-cyclooctadiene), the unique {[SiII(Xant)SiII]GeI}2NiII complex with a three-membered ring Ge2Ni-metallacycle was obtained via reductive coupling of two Ge0 atoms on the Ni center. Moreover, 3 was suitable to form a frustrated Lewis pair (FLP) with BPh3, which was capable of heterolytic H2 cleavage at 1 atm and room temperature, representing, for the first time, that a metallylone could be applied in FLP chemistry.