Bisgermylene-Stabilized Stannylone: Catalytic Reduction of Nitrous Oxide and Nitro Compounds via Element-Ligand Cooperativity.

This study describes the synthesis, structural characterization, and catalytic application of a bis(germylene)-stabilized stannylone (2). The reduction of digermylated stannylene (1) with 2.2 equiv of potassium graphite (KC8) leads to the formation of stannylone 2 as a green solid in 78% yield. Computational studies showed that stannylone 2 possesses a formal Sn(0) center and a delocalized 3-c-2-e π-bond in the Ge2Sn core, which arises from back-donation of the p-type lone pair electrons on the Sn atom to the vacant orbitals of the Ge atoms. Stannylone 2 can serve as an efficient precatalyst for the selective reduction of nitrous oxide (N2O) and nitroarenes (ArNO2) with the formation of dinitrogen (N2) and hydrazines (ArNH-NHAr), respectively. Exposure of 2 with N2O (1 atm) resulted in the insertion of two oxygen atoms into the Ge-Ge and Ge-Sn bonds, yielding the germyl(oxyl)stannylene (3). Moreover, the stoichiometric reaction of 2 with 1-chloro-4-nitrobenzene afforded an amido(oxyl)stannylene (4) through the complete scission of the N-O bonds of the nitroarene. Stannylenes 3 and 4 serve as catalytically active species for the catalytic reduction of nitrous oxide and nitroarenes, respectively. Mechanistic studies reveal that the cooperation of the low-valent Ge and Sn centers allows for multiple electron transfers to cleave the N-O bonds of N2O and ArNO2. This approach presents a new strategy for catalyzing the deoxygenation of N2O and ArNO2 using a zerovalent tin compound.

Double 1,2-Carbon Migration at Mixed Heavier Sn=Ge Vinylidenes.

1,2-migration is one recurring isomerization reaction in organic chemistry. In contrast, double 1,2-migration remains rare and limited to transition-metal complexes. Herein, we describe the synthesis, characterization and reactivity of mixed heavier Sn=Ge vinylidenes. Double 1,2-carbon migration enables the isomerization of the stannagermenylidene (3) to the germastannenylidene (4). X-ray diffraction analysis and DFT calculations revealed that 3 and 4 feature a Sn=Ge double bond. The reaction of 3 with IMe4 (1,3,4,5-tetramethylimidazoline-2-ylidene) results in the electron redistribution in the Sn=Ge core to give the germylone-stannylene adduct (5). Moreover, treatment of 3 with 0.25 equiv. of (AlCp*)4 produces the heteronuclear aluminyl stannagermyne (6).

A silylene-stabilized ditin(0) complex and its conversion to methylditin cation and distannavinylidene.

Due to their intrinsic high reactivity, isolation of tin(0) complexes remains challenging. Herein, we report the synthesis of a silylene-stabilized ditin(0) complex (2) by reduction of a silylene-supported dibromostannylene (1) with 1 equivalent of magnesium (I) dimer in toluene. The structure of 2 was established by single crystal X-ray diffraction analysis. Density Functional Theory calculations revealed that complex 2 bears a Sn=Sn double bond and one lone pair of electrons on each of the Sn(0) atoms. Remarkably, complex 2 is readily methylated to give a mixed-valent methylditin cation (4), which undergoes topomerization in solution though a reversible 1,2-Me migration along a Sn=Sn bond. Computational studies showed that the three-coordinate Sn atom in 4 is the dominant electrophilic center, and allows for facile reaction with KHBBus3 furnishing an unprecedented N-heterocyclic silylenes-stabilized distannavinylidene (5). The synthesis of 2, 4 and 5 demonstrates the exceptional ability of N-heterocyclic silylenes to stabilize low valent tin complexes.

Isolation and Reactivity of Homoleptic Diphosphene Lead Complexes.

Due to their limited capacity for π-backdonation, isolation of π-complexes of main-group elements remains a great challenge. We report herein the synthesis of a homoleptic diphosphene lead complex (2) from the degradation of P4 with a bis(germylene)-stabilized Pb(0) complex. Structural and computational studies showed that 2 possesses significant π bonding interactions between Pb atom and diphosphene ligands, which is reminiscent of transition-metal diphosphene complexes. Consistent with its unique electronic structure, complex 2 can deliver Pb(0) atoms to perform redox reaction with an iminoquinone to produce a cyclic plumbylene (4) and perform 2,5-dimethyl-3,4-dimethylimidazol-1-ylidene (IMe2 Me2 ) induced phosphorus cation abstraction to give an anionic PbP3 complex (6).

Isolation and characterization of bis(silylene)-stabilized antimony(I) and bismuth(I) cations.

Monovalent group 15 cations L2Pn + (L = σ-donor ligands, Pn = N, P, As, Sb, Bi) have attracted significant experimental and theoretical interest because of their unusual electronic structures and growing synthetic potential. Herein, we describe the synthesis of a family of antimony(I) and bismuth(I) cations supported by a bis(silylene) ligand [(TBDSi2)Pn][BArF4] (TBD = 1, 8, 10, 9-triazaboradecalin; ArF = 3,5-CF3-C6H3; Pn = Sb, (2); Bi, (3)). The structures of 2 and 3 have been unambiguously characterized spectroscopically and by X-ray diffraction analysis and DFT calculations. They feature bis-coordinated Sb and Bi atoms which exhibit two lone pairs of electrons. The reactions of 2 and 3 with methyl trifluoromethane sulfonate provide a approach for the preparation of dicationic antimony(III) and bismuth(III) methyl complexes. Compounds 2 and 3 serve as 2e donors to group 6 metals (Cr, Mo), giving rise to ionic antimony and bismuth metal carbonyl complexes 6-9.

Cooperative Bond Activation and Catalytic CO2 Functionalization with a Geometrically Constrained Bis(silylene)-Stabilized Borylene.

Metal-ligand cooperativity has emerged as an important strategy to tune the reactivity of transition-metal complexes for the catalysis and activation of small molecules. Studies of main-group compounds, however, are scarce. Here, we report the synthesis, structural characterization, and reactivity of a geometrically constrained bis(silylene)-stabilized borylene. The one-pot reaction of [(SiNSi)Li(OEt2)] (SiNSi = 4,5-bis(silylene)-2,7,9,9-tetramethyl-9H-acridin-10-ide) with 1 equiv of [BBr3(SMe2)] in toluene at room temperature followed by reduction with 2 equiv of potassium graphite (KC8) leads to borylene [(SiNSi)B] (1), isolated as blue crystals in 45% yield. X-ray crystallography shows that borylene (1) has a tricoordinate boron center with a distorted T-shaped geometry. Computational studies reveal that the HOMO of 1 represents the lone pair orbital on the boron center and is delocalized over the Si-B-Si unit, while the geometric perturbation significantly increases its energy. Borylene (1) shows single electron transfer reactivity toward tris(pentafluorophenyl)borane (B(C6F5)3), forming a frustrated radical pair [(SiNSi)B]•+[B(C6F5)3]•-, which can be trapped by its reaction with PhSSPh, affording an ion pair [(SiNSi)BSPh][PhSB(C6F5)3] (3). Remarkably, the cooperation between borylene and silylene allows the facile cleavage of the N-H bond of aniline, the P-P bond in white phosphorus, and the C═O bond in ketones and carbon dioxide, thus representing a new type of main-group element-ligand cooperativity for the activation of small molecules. In addition, 1 is a strikingly effective catalyst for carbon dioxide reduction. Computational studies reveal that the cooperation between borylene and silylene plays a key role in the catalytic chemical bond activation process.

An Isolable Bis(Germylene)-Stabilized Plumbylone.

We report herein the synthesis of a stable plumbylone (3) by reduction of a bromodigermylplumbylene (2) with 2.2 equiv of potassium graphite (KC8 ). The molecular structure of 3 was established by a single-crystal X-ray diffraction study and features a two-coordinated Pb center with an acute Ge-Pb-Ge bond angle. Computational studies showed that this complex (3) possesses a singlet electronic ground state with a Pb0 center. Its high thermal stability can be most likely ascribed to the delocalization of π electrons over the Ge-Pb-Ge moiety. A preliminary reactivity study demonstrates that complex 3 can deliver Pb0 atoms to an organic azide producing a tetrameric imido complex [(PbNDipp)4 ] (Dipp=2,6-i Pr-C6 H3 , 4) and perform a metathesis reaction with GeCl2 ⋅dioxane to produce a bis(germylene)-stabilized germylone (5), highlighting the synthetic utility of 3.

A Silylene-Stabilized Germanium Analogue of Alkynylaluminum.

In contrast to the well-developed metal acetylides, the heavier analogues of the type REEM (E=Si, Ge, Sn, Pb; M=metals) have not been reported to date. Herein, we describe the synthesis of a silylene-stabilized germanium analogue of alkynylaluminum (4) by the reaction of a silylene-stabilized digermavinylidene (3) with 0.25 equiv of (AlCp*)4 in THF at 80 °C. The structure of 4 has been unambiguously characterized by spectroscopic analysis, X-ray diffraction analyses and DFT calculations, and features a linear AlGeGe skeleton with a Ge=Ge double bond and a polarized Ge-Al bond. Complex 4 shows excellent reactivity towards small molecules such as H2 , CS2 and TolNCO, and this allows for the construction of the structurally intriguing germylenyl aluminum complex (5) and the germacycles (6, 7).

Dinitrogen Cleavage and Functionalization with Carbon Dioxide in a Dititanium Dihydride Framework.

The activation and functionalization of dinitrogen (N2) with carbon dioxide (CO2) are of great interest and importance but highly challenging. We report here for the first time the reaction of N2 with CO2 in a dititanium dihydride framework, which leads to N-C bond formation and N-N and C-O bond cleavage. Exposure of a dinitrogen dititanium hydride complex {[(acriPNP)Ti]2(µ2-η1:η2-N2)(µ2-H)2} (1) (acriPNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide) to a CO2 atmosphere at room temperature rapidly yielded a nitrido/N,N-dicarboxylamido complex {[(acriPNP)Ti]2(µ2-N)[µ2-N(CO2)2]} (2, 28%) and a diisocyanato/dioxo complex {[(acriPNP)Ti]2(NCO)2(µ2-O)2} (3, 52%) with release of H2. When the reaction of 1 with CO2 (1 atm) was carried out at -50 °C, complex 2 was selectively formed in 82% yield within 5 min. Heating 2 at 80 °C under 1 atm CO2 for 30 min afforded 3 in 67% yield. When 1 was allowed to react with 1.5 equiv of CO2 at room temperature, an isocyanato/nitrido/oxo complex {[(acriPNP)Ti]2(NCO)(µ2-N)(µ2-O)} (4) was exclusively formed in 89% yield within 5 min. The reaction of 4 with CO2 at room temperature almost quantitatively yielded the dioxo/diisocyanato complex 3 within 5 min. The mechanistic details were clarified by the 15N- and 13C-labeled experiments and density functional theory (DFT) calculations, providing unprecedented insights into the reaction of N2 with CO2. A titanium-mediated cycle for the synthesis of trimethylsilyl isocyanate Me3SiNCO from N2, CO2, and Me3SiCl using H2 as a reducing agent was also established.

Synthesis and Reactivity of N-Heterocyclic Silylene Stabilized Disilicon(0) Complexes.

Synthesis and reactivity of disilicon(0) complexes are of fundamental and application importance. Herein, we report the development of an N-heterocyclic imino-substituted silylene (1), which has strong σ-donating ability and is significantly sterically hindered. The one-pot reaction of this silylene with [IPr→SiCl2 ] (IPr=1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene) and KC8 (2 equiv) in THF at -30 °C leads to a silylene-ligated disilicon(0) complex (2), isolated as red crystals in 60 % yield. Characterization data and DFT calculations show that the trans-bent Si4 skeleton in 2 features a Si0 =Si0 double bond with significant π-π bonding and one lone pair of electrons on each of these two Si0 atoms. Complex 2 reacts readily with phenylacetylene, producing a structurally intriguing silatricyclic complex 6,8-diaza-1,2,5-trisilatricyclo-[3.2.1.02,7 ]-oct-3-ene (3), and revealing new aspects of low-valent silicon chemistry.

Deoxygenation of Nitrous Oxide and Nitro Compounds Using Bis(N-Heterocyclic Silylene)Amido Iron Complexes as Catalysts.

Herein, we report the efficient degradation of N2 O with a well-defined bis(silylene)amido iron complex as catalyst. The deoxygenation of N2 O using the iron silanone complex 4 as a catalyst and pinacolborane (HBpin) as a sacrificial reagent proceeds smoothly at 50 °C to form N2 , H2 , and (pinB)2 O. Mechanistic studies suggest that the iron-silicon cooperativity is the key to this catalytic transformation, which involves N2 O activation, H atom transfer, H2 release and oxygenation of the boron sites. This approach has been further developed to enable catalytic reductions of nitro compounds, producing amino-boranes with good functional-group tolerance and excellent chemoselectivity.

Regio- and Diastereoselective [3+2] Annulation of Aliphatic Aldimines with Alkenes by Scandium-Catalyzed ß-C(sp3 )-H Activation.

Here we report for the first time the regio- and diastereoselective [3+2] annulation of a wide range of aliphatic aldimines with alkenes via the activation of an unactivated ß-C(sp3 )-H bond by half-sandwich scandium catalysts. This protocol offers a straightforward and atom-efficient route for the synthesis of a new family of multi-substituted aminocyclopentane derivatives from easily accessible aliphatic aldimines and alkenes. The annulation of aldimines with styrenes exclusively afforded the 5-aryl-trans-substituted 1-aminocyclopentane derivatives with excellent diastereoselectivity through the 2,1-insertion of a styrene unit. The annulation of aldimines with aliphatic alkenes selectively gave the 4-alkyl-trans-substituted 1-aminocyclopentane products in a 1,2-insertion fashion. A catalytic amount of an appropriate amine such as adamantylamine (AdNH2 ) or dibenzylamine (Bn2 NH) showed significant effects on the catalyst activity and stereoselectivity.

Theoretical mechanistic insights into dinitrogen cleavage by a dititanium hydride complex bearing PNP-pincer ligands.

The mechanism of dinitrogen cleavage by a PNP-coordinated dititanium polyhydride complex has been computationally investigated. A "multi-state reactivity" scenario has been disclosed for the whole process of N2 coordination and activation. Remarkably, the H2 elimination prior to the N-N cleavage is accomplished by the coupling of two terminal hydrides, and planar PNP-pincer ligand could stabilize the corresponding transition state. Besides, the tetrahydrofuran (THF) solvent could also promote the H2 elimination due to the similar polarity of the corresponding intermediates or transition states to THF molecule.

Diastereodivergent [3 + 2] Annulation of Aromatic Aldimines with Alkenes via C-H Activation by Half-Sandwich Rare-Earth Catalysts.

Stereodivergent catalysis is of great importance, as it can allow efficient access to all possible stereoisomers of a given product with multiple stereocenters from the same set of starting materials. We report herein the first diastereodivergent [3 + 2] annulation of aromatic aldimines with alkenes via C-H activation by half-sandwich rare-earth catalysts. This protocol provides an efficient and general route for the selective synthesis of both trans and cis diastereoisomers of multisubstituted 1-aminoindanes from the same set of aldimines and alkenes, featuring 100% atom efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The diastereodivergence is achieved by fine-tuning the sterics or ligand/metal combination of the half-sandwich rare-earth metal complexes.

Synthesis and Diverse Transformations of a Dinitrogen Dititanium Hydride Complex Bearing Rigid Acridane-Based PNP-Pincer Ligands.

Studies on N2 activation and transformation by transition metal hydride complexes are of particular interest and importance. The synthesis and diverse transformations of a dinitrogen dititanium hydride complex bearing the rigid acridane-based acri PNP-pincer ligands {[(acri PNP)Ti]2 (µ2 -η1 :η2 -N2 )(µ2 -H)2 } are presented. This complex enabled N2 cleavage and hydrogenation even without additional H2 or other reducing agents. Furthermore, diverse transformations of the N2 unit with a variety of organometallic compounds such as ZnMe2 , MgMe2 , AlMe3 , B(C6 F5 )3 , PinBH, and PhSiH3 have been well established at the rigid acri PNP-ligated dititanium framework, such as reversible bonding-mode change between the end-on and side-on/end-on fashions, diborylative N=N bond cleavage, the formal insertion of two dimethylaluminum species into the N=N bond, and the formal insertion of two silylene units into the N=N bond. This work has revealed many unprecedented aspects of dinitrogen reaction chemistry.

Construction of All-Carbon Quaternary Stereocenters by Scandium-Catalyzed Intramolecular C-H Alkylation of Imidazoles with 1,1-Disubstituted Alkenes.

The exo-selective C-H cycloaddition of imidazoles to 1,1-disubstituted alkenes has been achieved for the first time by using half-sandwich scandium catalysts. A wide range of imidazole compounds bearing various 1,1-disubstituted aliphatic alkenes, styrenes, dienes, and enynes have been selectively converted in high yields to the corresponding bicyclic imidazole derivatives bearing ß-all-carbon-substituted quaternary stereocenters. By using a chiral half-sandwich scandium catalyst, the asymmetric exo-selective cyclization has also been achieved with a high level of enantioselectivity.

Chelate Silylene-Silyl Ligand Can Boost Rhodium-Catalyzed C-H Bond Functionalization Reactions.

The first N-heterocyclic silylene (NHSi)-silane scaffold LSi-R-Si(H)Mes2 (1) (L=PhC(NtBu)2 ; R=1,12-xanthendiyl spacer; Mes=2,4,6-Me3 C6 H2 ) was synthesized and used to form the unique rhodium(III) complex (LSi-R-SiMes2 )Rh(H)Cl 2 through its reaction with 0.5 molar equivalents of [Rh(coe)2 Cl]2 (coe=cyclooctene). An X-ray diffraction analysis revealed that 2 has a (SiII SiIV )Rh(H)Cl core with three short Rh⋅⋅⋅H-C contacts with Me groups of the ligand 1, which cause a distorted pentagonal bipyramidal coordination of the Rh center. Unexpectedly, the reaction of 2 with tBuONa gives the new bis(silyl)hydridorhodium(III) complex 4. Due to the strong donor ability of the chelate SiII -SiIV ligand, 2 and 4 can act as highly efficient pre-catalysts in the Rh-mediated selective C-H functionalization of 2-phenylpyridines with C-C unsaturated organic substrates under mild reaction conditions.

Reversible C-H Activation, Facile C-B/B-H Metathesis and Apparent Hydroboration Catalysis by a Dimethylxanthene-Based Frustrated Lewis Pair.

A dimethylxanthene-based phosphine/borane frustrated Lewis pair (FLP) is shown to effect reversible C-H activation, cleaving phenylacetylene, PhCCH, to give an equilibrium mixture of the free FLP and phosphonium acetylide in CD2 Cl2 solution at room temperature. This system also reacts with B-H bonds although in a different fashion: reactions with HBpin and HBcat proceed via C-B/B-H metathesis, leading to replacement of the -B(C6 F5 )2 Lewis acid component by -Bpin/-Bcat, and transfer of HB(C6 F5 )2 to the phosphine Lewis base. This transformation underpins the ability of the FLP to catalyze the hydroboration of alkynes by HBpin: the active species is derived from the HB(C6 F5 )2 fragment generated in this exchange process.

Stereoselective Transfer Semi-Hydrogenation of Alkynes to E-Olefins with N-Heterocyclic Silylene-Manganese Catalysts.

The synthesis and structures of the first SiII -donor supported manganese(II) complexes [L1]MnCl2 , [L2]MnCl2 , and [L3]2 MnCl2 are reported, bearing a pincer-type bis(NHSi)-pyridine ligand L1, bidentate bis(NHSi)-ferrocene ligand L2, and two monodentate NHSi ligands L3 (NHSi = N-heterocyclic silylene), respectively. They act as unprecedented very active and stereoselective Mn-based precatalysts (1 mol % loading) in transfer semi-hydrogenations of alkynes to give the corresponding E-olefins using ammonia-borane as a convenient hydrogen source under mild reaction conditions. Complex [L1]MnCl2 shows the best catalytic performance with quantitative conversion rates and excellent E-stereoselectivities (up to 98 %) for different alkyne substrates. Different types of functional groups can be tolerated, except CN, NH2 , NO2 , and OH groups at the phenyl group of 1-phenyl substituted alkynes.

An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules, Including Metal-Free Dehydrogenation of Water.

The first single-component N-heterocyclic silylene borane 1 (LSi-R-BMes2 ; L=PhC(Nt Bu)2 ; R=1,12-xanthendiyl spacer; Mes=2,4,6-Me3 C6 H2 ), acting as a frustrated Lewis pair (FLP) in small-molecule activation, can be synthesized in 65 % yields. Its HOMO is largely localized at the silicon(II) atom and the LUMO has mainly boron 2p character. In small-molecule activation 1 allows access to the intramolecular silanone-borane 3 featuring a Si=O→B interaction through reaction with O2 , N2 O, or CO2 , and formation of silanethione borane 4 from reaction with S8 . The SiII center in 1 undergoes immediate hydrogenation if exposed to H2 at 1 atm pressure in benzene, affording the silane borane 5-H2 , L(H2 )Si-R-BMes2 . Remarkably, no H2 activation occurs if the single silylene LSiPh and Mes3 B intermolecularly separated are exposed to dihydrogen. Unexpectedly, the pre-organized Si-B separation in 1 enables a metal-free dehydrogenation of H2 O to give the silanone-borane 3 as reactive intermediate.