Zero-valent species of group 13–15 elements

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.

Reversible Coupling of Germylone with Isocyanates

The germylone dimNHCGe (5, dimNHC=diimino N-heterocyclic carbene) undergoes a [2+2] cycloaddition with isocyanates RNCO (R=4-tolyl or 3,5-xylyl) to furnish novel alkyl carboxamido germylenes 7 (R=4-tolyl) and 8 (R=3,5-xylyl), featuring a C-C bond between the former carbene carbon and the isocyanate moiety. Heating a mixture of 8 with 4-tolyl isocyanate to 100 °C results in isocyanate metathesis, demonstrating reversible C-C bond formation on the reduced germanium compound. DFT calculations suggest that this process occurs via the reductive dissociation of isocyanate from 8 that regenerates the parent Ge(0) compound 5.

L→S Coordination Complexes Containing Benzothiazol-2-ylidene Ligand: Quantum Chemical Analysis and Synthesis

(NHC)→E coordination interactions were known where NHC is an N-heterocyclic carbene, and E is a main group element (B, C, N, Si, P). Recently, it was suggested that compounds with (NHC)→S coordination chemistry are also possible. This work reports quantum chemical analysis and synthesis of (NHC)→S-R(+) complexes in which benzothiazol-2-ylidene acts as a ligand. A Density functional study established that (NHC)→S interaction can best be described as a coordination interaction. Synthetic efforts were made, initially, to generate divalent sulfur compounds containing benzothiazole substituents. N-alkylation of the heterocyclic ring in these sulfides using methyl triflate led to the generation of the desired products with (NHC)→S coordination chemistry, which involves the in situ generation of NHC ring ligands. The observed changes in the 13C NMR spectra, before and after methylation, confirmed the change in the electronic character of the C-S bond from a covalent character to a coordination character.

NHC-Supported 2-Sila and 2-Germavinylidenes: Synthesis, Dynamics, First Reactivity and Theoretical Studies

2-tetrelavinylidenes (C=EH2 ; E=Si, Ge) are according to quantum chemical studies the least stable isomers on the [E,C,2H] potential energy hypersurface isomerizing easily via the trans-bent tetrelaacetylenes HE≡CH to the thermodynamically most stable 1-tetrelavinylidenes (E=CH2 ). Consequently, experimental studies on 2-tetrelavinylidenes (C=ER2 ) and their derivatives are lacking. Herein we report experimental and theoretical studies of the first N-heterocyclic carbene (NHC) supported 2-silavinylidene (NHC)C=SiBr(Tbb) (1-Si: NHC=C[N(Dipp)CH]2 , Dipp=2,6-diisopropylphenyl, Tbb=2,6-bis[bis(trimethylsilyl)methyl]-4-tert-butylphenyl) and the isovalent 2-germavinylidenes (NHC)C=GeBr(R) (1-Ge, 1-GeMind: R=Tbb, Mind (1,1,3,3,5,5,7,7-octamethyl-s-hydrindacene-4-yl)). The NHC-supported 2-tetrelavinylidenes were obtained selectively from the 1,2-dibromoditetrelenes (E)-(R)BrE=EBr(R) using the diazoolefin (NHC)CN2 as vinylidene transfer reagent. 1-E (E=Si, Ge) have a planar vinylidene core, a bent-dicoordinated vinylidene carbon atom (CVNL ), a very short E=CVNL bond and an almost orthogonal orientation of the NHC five-membered ring to the vinylidene core. Quantum chemical analysis of the electronic structures of 1-E suggest a significantly bent 1-tetrelaallene and tetrelyne character. NMR studies shed light into the dynamics of 1-E involving NHC-rotation around the CVNL -CNHC bond with a low activation barrier. Furthermore, the synthetic potential of 1-E is demonstrated by the synthesis and full characterization of the unprecedented NHC-supported bromogermynes BrGe=C(EBr2 Tbb)(NHC) (2-SiGe: E=Si; 2-GeGe: E=Ge).

Isolation of an Anionic Dicarbene Embedded Sn2 P2 Cluster and Reversible CO2 Uptake

Decarbonylation of a cyclic bis-phosphaethynolatostannylene [(ADC)Sn(PCO)]2 based on an anionic dicarbene framework (ADC = PhC{N(Dipp)C}2 ; Dipp = 2,6-iPr2 C6 H3 ) under UV light results in the formation of a Sn2 P2 cluster compound [(ADC)SnP]2 as a green crystalline solid. The electronic structure of [(ADC)SnP]2 is analyzed by quantum-chemical calculations. At room temperature, [(ADC)SnP]2 reversibly binds with CO2 and forms [(ADC)2 {SnOC(O)P}SnP]. [(ADC)SnP]2 enables catalytic hydroboration of CO2 and reacts with elemental selenium and Fe2 (CO)9 to afford [(ADC)2 {Sn(Se)P2 }SnSe] and [(ADC)Sn{Fe(CO)4 }P]2 , respectively. All compounds are characterized by multinuclear NMR spectroscopy and their solid-state molecular structures are determined by single-crystal X-ray diffraction.

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).

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.

A Guanidine-Supported π-Complex of Germanium Amenable to Intramolecular C-C Cleavage in Arene and Ge Atom Transfer

The germylone dimNHCGe (dimNHC=diimino N-heterocyclic carbene) reacts with azides N3 R (R=SiMe3 or p-tolyl) to furnish the first examples of germanium π-complexes, i. e. guanidine-ligated compounds (dimNHI-SiMe3 )Ge (NHI=N-heterocyclic imine, R=SiMe3 ) and (dimNHI-Tol)Ge (R=p-tolyl). DFT calculations suggest that these species are formed by a Staudinger type replacement of dinitrogen in the azide by a nucleophilic germylone, leading to a transient carbene adduct of iminogermylidene. Heating a solution of compound (dimNHI-SiMe3 )Ge to 70 °C results in extrusion of the iminogermylidene that further aggregates to produce the known [Me3 SiNGe]4 tetramer, whereas the imidazolylidene fragment transforms into an unusual heptatriene species that can be considered as a product of carbene insertion into the C-C bond of a pendant Ar substituent at the imidazolylidene nitrogen of the dimNHC. Reaction of (dimNHI-SiMe3 )Ge with tetrachloro-o-benzoquinone results in the net transfer of a germanium atom and formation of the free diimino-guanidine ligand. This ligand also forms when (dimNHI-SiMe3 )Ge is treated with azide N3 (p-Tol), with the germanium product being [(p-Tol)NGe]n.

Germanium Analogue of the Parent Phosphine-Borane FLP Compound

Diimino-carbene-supported germylone dimNHCGe does not react with BPh3 and does not activate dihydrogen in the FLP mode in the combination with this borane. However, it reacts with B(C6 F5 )3 to give the zwitterionic borate dimNHCGe-(C6 F4 )BF(C6 F5 )2 . This compound can be converted into the hydroborate dimNHCGe-(C6 F4 )BH(C6 F5 )2 (8) and further into [dimNHCGe-(C6 F4 )B(C6 F5 )2 ]+ (4). Compound 4 is a Ge/B analogue of Stephan's FLP parent P/B compound (C6 H2 Me3 )2 P-C6 F4 -B(C6 F5 )2 but unlike the latter cannot split dihydrogen. Moreover, attempts to prepare a Ge/B analogue of the zwitterion (C6 H2 Me3 )2 HP-C6 F4 -BH(C6 F5 )2 by protonation of borate 8 resulted in immediate elimination of H2.

N-Heterocyclic imine-based bis-gallium(I) carbene analogs featuring a four-membered Ga2N2 ring

A combination of Ga(I) centers as important building blocks and scaffolds containing N-heterocyclic imines gives new insights into low-valent Ga chemistry. In this study, a mixture of LDipNLi (LDip = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene), tBuOK, and Cp*Ga (Cp* = pentamethylcyclopentadienyl) in toluene afforded [LDipN-Ga]2 (1) via salt metathesis. X-ray structure analysis of 1 revealed a four-membered Ga2N2 ring, and DFT studies indicated the presence of a lone pair at each Ga center. In addition, compound 1 demonstrated diverse reactivities towards methyl trifluoromethanesulfonate, diphenyl disulfide, 9,10-phenanthrenequinone, and ECl2 (E = Ge or Sn).

Diazoalkenes: From an Elusive Intermediate to a Stable Substance Class in Organic Chemistry

Over decades diazoalkenes (R2 C=C=N2 ) were postulated as reactive intermediates in organic chemistry even though their direct spectroscopic detection proved very challenging. In the 1970/80ies several groups probed their existence mainly indirectly by trapping experiments or directly by matrix-isolation studies. In 2021, our group and the Severin group reported independently the synthesis and characterization of the first room-temperature stable diazoalkenes, which initiated a rapidly expanding research field. Up to now four different classes of N-heterocyclic substituted room-temperature stable diazoalkenes have been reported. Their properties and unique reactivity, such as N2 /CO exchange or utilization as vinylidene precursors in organic and transition metal chemistry are presented. This review summarizes the early discoveries of diazoalkenes from their initial postulation as transient, elusive species up to the recent findings of the room-temperature stable derivatives.

Synthesis and Reactivity of an Anionic Diazoolefin

Bent (hetero)allenes such as carbodicarbenes and carbodiphosphoranes can act as neutral C-donor ligands, and diverse applications in coordination chemistry have been reported. N-Heterocyclic diazoolefins are heterocumulenes, which can function in a similar fashion as L-type ligands. Herein, we describe the synthesis and the reactivity of an anionic diazoolefin. This compound displays distinct reactivity compared to neutral diazoolefins, as evidenced by the preparation of diazo compounds via protonation, alkylation, or silylation. The anionic diazoolefin can be employed as an ambidentate, X-type ligand in salt metathesis reactions with metal halide complexes. Extrusion of dinitrogen was observed in a reaction with PCl(NiPr2 )2 , resulting in a stable phosphinocarbene.

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][BAr^F₄] (TBD = 1, 8, 10, 9-triazaboradecalin; Ar^F = 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.

Counterintuitive Chemistry: Carbene Stabilization of Zero-Oxidation State Main Group Species

Carbenes have evolved from transient laboratory curiosities to a robust, diverse, and surprisingly impactful ligand class. A variety of different carbenes have significantly contributed to the development of low-oxidation state main group chemistry. This Perspective focuses upon advances in the chemistry of carbene complexes containing main group element cores in the formal oxidation state of zero, including their diverse synthetic strategies, unusual bonding and structural motifs, and utility in transition metal coordination chemistry and activation of small molecules.

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 Si^II 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 E^I 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 Si^II···Si^II 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 Si^I-Si^I and Ge^I-Ge^I 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.

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.

Theoretical Photoelectron Spectroscopy of Low-Valent Carbon Species: A ∼6 eV Range of Ionization Potentials among Carbenes, Ylides, and Carbodiphosphoranes

High-quality density functional theory calculations underscore a nearly 6 eV range for the ionization potentials (IPs) of neutral, low-valent carbon compounds, including carbenes, ylides, and zero-valent carbon compounds (carbones) such as carbodiphosphoranes (CDPs) and carbodicarbenes. Thus, adiabatic IPs as low as 5.5 ± 0.1 eV are predicted for CDPs, which are about 0.7-1.2 eV lower than those of simple phosphorus and sulfur ylides. In contrast, the corresponding values for N-heterocyclic carbenes are about 8.0 eV while those for simple singlet carbenes such as dichlorocarbene and difluorocarbene range from about 9.0 eV to well over 11.0 eV.

Isolation of an Arsenic Diradicaloid with a Cyclic C2 As2 -Core

Herein, we report on the synthesis, characterization, and reactivity studies of the first cyclic C2 As2 -diradicaloid {(IPr)CAs}2 (6) (IPr = C{N(Dipp)CH}2 ; Dipp = 2,6-iPr2 C6 H3 ). Treatment of (IPr)CH2 (1) with AsCl3 affords the Lewis adduct {(IPr)CH2 }AsCl3 (2). Compound 2 undergoes stepwise dehydrochlorination to yield {(IPr)CH}AsCl2 (3) and {(IPr)CAsCl}2 (5 a) or [{(IPr)CAs}2 Cl]OTf (5 b). Reduction of 5 a (or 5 b) with magnesium turnings gives 6 as a red crystalline solid in 90% yield. Compound 6 featuring a planar C2 As2 ring is diamagnetic and exhibits well resolved NMR signals. DFT calculations reveal a singlet ground state for 6 with a small singlet-triplet energy gap of 8.7 kcal mol-1 . The diradical character of 6 amounts to 20% (CASSCF, complete active space self consistent field) and 28% (DFT). Treatments of 6 with (PhSe)2 and Fe2 (CO)9 give rise to {(IPr)CAs(SePh)}2 (7) and {(IPr)CAs}2 Fe(CO)4 (8), respectively.