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.

Concatenating Structural Constraint Effects at Tin for the Sequential Generation, Stabilization, and Transfer of Acyclic Aminocarbenes

Structural constraint approaches have been employed toward different ends in recent years, from augmenting the nucleophilicity in pyramidalized low-valent p-block compounds to enhancing the Lewis acidities at planarized tetravalent p-block elements. While previous studies exploited these effects separately, this work introduces a strategy to concatenate structural constraint approaches at individual stages of a reaction sequence in a row to unlock a synthetic path unattainable by conventional methodologies. The boosted nucleophilicity resulting from the constrained tetracoordinated calix[4]pyrrolato stannate(II) dianion enables the reductive formation of sterically unprotected acyclic aminocarbenes. These amino carbenes are stabilized at the concomitantly formed square-planar stannane(IV) as air-stable adducts. Transfer of the carbenes onto copper(I) by cooperativity of the calix[4]pyrrole ligand finalizes this protocol to hitherto unreported yet prototypical carbene complexes. Detailed spectroscopic and quantum theoretical analyses establish the synergy of structural constraints and element-ligand cooperation as the linchpin to this reaction path and its selectivity.

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.

Lewis Acid Stabilized Diatomic Molecules of Group 14: A Computational Study on [(CO)4Fe]2E2 (E = C, Si, Ge, Sn, Pb)

A Lewis base and acid combination has been effectively employed to stabilize and isolate the low-valent group 14 compounds. We report DFT studies on stabilizing low-valent group 14 diatomics as adducts of Lewis acids employing transition metal carbonyl fragment iron tetracarbonyl [Fe(CO)4] as Lewis acid. Computational studies on [(CO)4Fe]2E2, E = C, Si, Ge, Sn, and Pb, predict five plausible isomers on its potential energy surface: linear (E2_L), bent (E2_B), three-membered (E2_T), dibridged (E2_D), and four-membered (E2_F). For the carbon analogue, the lowest energy configuration is linear and has a typical cumulenic structure, while silicon and germanium analogues favor three-membered cyclic isomers. Four-membered cyclic isomers are the most stable for tin and lead analogues.

Stabilization of Cyclic C4 by Four Donor Ligands: A Theoretical Study of (L)4C4 (L = Carbene)

Quantum chemical studies using density functional theory were carried out for the (L)4C4 complexes with L = cAAC, DAC, NHC, SNHC, MIC1, and MIC2. The results show that the title complexes are highly stable with respect to dissociation, (L)4C4 → C4 + 4L. However, their stability with respect to (L)4C4 → 2(L)2C2 is crucial for the assessment of their experimental viability. The (L)4C4 complexes with L = cAAC and DAC dissociate exergonically at room temperature into two (L)2C2 units. In contrast, the other (L)4C4 complexes with L = NHC, SNHC, MIC1, and MIC2 are thermochemically stable with respect to dissociation, (L)4C4 → 2(L)2C2. The computed adiabatic ionization potentials of (L)4C4 complexes with L = NHC, MIC1, and MIC2 are lower than those for the cesium atom. Particularly, (MIC1)4C4 and (MIC2)4C4 will very easily lose electrons to form cationic complexes. The SNHC ligand is the best for the experimental realization of (L)4C4 complexes, followed by NHC. The bonding analysis using charge and energy decomposition methods suggests that the (L)3C4-CL bond can be best described as a typical electron-sharing double bond with a strong σ-bond and a weaker π-bond. Therefore, the core bonding pictures in the title complexes resemble a [4]radialene. Larger substituents at the carbene ligands enhance the stability of the complexes (L)4C4 against dissociation.

Interaction of germanium analogue of organic isonitrile with Cu(I) imide in side-on mode

[NHC → GeN(Ar)Cu₂NAr]₂, the formal adduct of germanium analogue of organic isonitrile [GeNAr] with Cu(I) imide [(Cu₂NAr)₂] (Ar = 2,6-iPr₂C₆H₃) was prepared from the N-heterocyclic carbene (NHC) stabilized acyclic germylene Ge[N(H)Ar]₂ by reacting with two equivalents of nBuLi and CuCl(PPh₃)₃. As elucidated by X-ray crystal structural characterization, the two separated [GeNAr] moieties interacted with [(Cu₂NAr)₂] core in the side-on mode.

Monosubstituted Doublet Sn(I) Radical Featuring Substantial Unquenched Orbital Angular Momentum

Due to their intrinsic high reactivity, isolation of heavier analogues of carbynes remains a great challenge. Here, we report the synthesis and characterization of a neutral monosubstituted Sn(I) radical (2) supported by a sterically hindered hydrindacene ligand, which represents the first tin analogue of a free carbyne. Different from all Sn(I/III) species reported thus far, the presence of a sole Sn-C σ bond in 2 renders the remaining two Sn 5p orbitals energetically almost degenerate, of which one is singly occupied and the other is empty. Consequently, its S = 1/2 ground state possesses two-fold orbital pseudo-degeneracy and substantial unquenched orbital angular momentum, as evidenced by one component of its g matrix (1.957, 1.896, and 1.578) being considerably less than 2. Consistent with this unique electronic structure, 2 can bind to an N-heterocyclic carbene to afford a neutral two-coordinate Sn(I) radical and initiate a one-electron transfer to benzophenone to furnish a Sn(II)-ketyl radical anion adduct. As a manifestation of its Sn-centered radical nature, 2 reacts with diphenyl diselenide and p-benzoquinone to form Sn-S and Sn-O bonds, respectively.

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.

Organogermanium Analogues of Alkenes, Alkynes, 1,3-Dienes, Allenes, and Vinylidenes

In this review, the latest achievements in the field of multiply bonded organogermanium derivatives, mostly reported within the last two decades, are presented. The isolable Ge-containing analogues of alkenes, alkynes, 1,3-dienes, allenes, and vinylidenes are discussed, and for each class of unsaturated organogermanium compounds, the most representative examples are given. The synthetic approaches toward homonuclear multiply bonded combinations solely consisting of germanium atoms, and their heteronuclear variants containing germanium and other group 14 elements, both acyclic and cyclic, are discussed. The peculiar structural features and nonclassical bonding nature of the abovementioned compounds are discussed based on their spectroscopic and structural characteristics, in particular their crystallographic parameters (double bond length, trans-bending at the doubly bonded centers, and twisting about the double bond). The prospects for the practical use of the title compounds in synthetic and catalytic fields are also briefly discussed.

Ligand-stabilized heteronuclear diatomics of group 13 and 15

A theoretical investigation of ligand-stabilized MX diatomics (M = group 13, X = group 15 element) with N-heterocyclic carbene (NHC) ligands has been carried out to assess bonding and electronic structure. Binding of two ligands in the form L-MX-L is generally preferred over binding of a single ligand as L-MX or MX-L. Binding of carbene donor ligands is predicted to be thermodynamically favorable for all the systems, and is very favorable for the lighter group 15 systems (nitrogen and phosphorus). Detailed analysis of the bonding in these complexes has been carried out with energy decomposition analysis (EDA). In all cases, the carbene to boron and carbene to nitrogen bonding is described as an electron-sharing double bond with both σ and π bonding interactions. For the heavier elements, bonding to C (except for PC interactions) is best described as a donor-acceptor σ single bond.

N-heterocyclic carbene and cyclic (alkyl)(amino)carbene adducts of plumbanes and plumbylenes

Lewis-acid/base adducts of N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene cAAC^Me (1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) with selected lead(II) and lead(IV) compounds are presented. The reaction of the NHCs Me₂Im^Me (1,3,4,5-tetramethyl-imidazolin-2-ylidene), iPr₂Im^Me (1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene), Dipp₂Im (1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) and cAAC^Me (1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene) with PbI₂ yielded the NHC-containing plumbylenes NHC·PbI₂ (NHC = Me₂Im^Me (1), iPr₂Im^Me (2), Dipp₂Im (3) and cAAC^Me·PbI₂ (4)). Using the Pb(IV) compound PbCl₂Ph₂, the plumbane adducts NHC·PbCl₂Ph₂ (NHC = Me₂Im^Me (5), iPr₂Im^Me (6), Dipp₂Im (7)) and cAAC^Me·PbCl₂Ph₂ (8)) were isolated in high yields. Reduction of the lead(IV) adducts 5 and 6 with excess KC₈ afforded the diaryl substituted plumbylenes Me₂Im^Me·PbPh₂ (9) and iPr₂Im^Me·PbPh₂ (10), which are stable in the solid state but decompose in solution.

How to capture C2O2: structures and bonding of neutral and charged complexes [(NHC)-C2O2-(NHC)]q (NHC = N-heterocyclic carbene; q = 0, 1+, 2+)

We present the results of DFT calculations and a thorough bonding analysis of the neutral and charged complexes of the elusive C₂O₂ species stabilized by two NHC ligands. It is shown that the thermodynamic stability of the neutral complex [(NHC)-C₂O₂-(NHC)] is due to the low-lying triplet state of [NHC-CO] (T), which is only 3.2 kcal mol^-1 higher in energy than the singlet state [NHC-CO] (S), while the triplet state of CO is 131.9 kcal mol^-1 above the singlet. The much lower S/T gap of [NHC-CO] than in CO comes from the charge donation of NHC into the degenerate π* LUMO of CO and the concomitant mixing of the LUMO of NHC with the degenerate π* LUMO of CO, which strongly lowers the energy difference between HOMO and LUMO in the complex. The energy gain resulting from the formation of the CC double bond compensates the singlet-triplet gap and the thermodynamic instability of the fragments [NHC-CO] (S). The dissociation of neutral [(NHC)-C₂O₂-(NHC)] to 2NHC and 2CO molecules is calculated to be endothermic by D_o = 78.2 kcal mol^-1. The bonding analysis indicates that the neutral and the charged molecules [(NHC)-C₂O₂-(NHC)]^q have a central unit with C-C single bonds, where a combination of electron sharing and s dative interactions leads to very strong carbon-carbon bonds complemented by minor π-donation, which make all systems stable with respect to dissociation reactions. The central C₂O₂ fragment carries a large negative partial charge in the neutral and singly charged compounds [(NHC)-C₂O₂-(NHC)]^0,1+, while it is neutral in the dication [(NHC)-C₂O₂-(NHC)]²⁺.

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-[3.2.1.0^2,7 ]-oct-3-ene (3), and revealing new aspects of low-valent silicon chemistry.

Bonding analysis of the C2 precursor Me3E–C2–I(Ph)FBF3 (E = C, Si, Ge)

A series of possible precursors for generating C2 with the general formula Me3E–C2–I(Ph)FBF3 [E = C (1), Si (2), and Ge (3)] has been theoretically investigated using quantum chemical calculations. The equilibrium geometries of all species show a linear E–C2–I+ backbone. The inspection of the electronic structure of the Me3E–C2 bond by energy decomposition analysis coupled with the natural orbital for chemical valence (EDA-NOCV) method suggests a combination of electron sharing C–C σ-bond and v weak π-dative bond between Me3C and C2 fragments in the doublet state for species 1 (E = C). For species 2 (Si) and 3 (Ge), the analysis reveals σ-dative Me3E–C2 bonds (E = Si, Ge; Me3E←C2) resulting from the interaction of singly charged (Me3E)+ and (C2–IPh(BF4))− fragments in their singlet states. The C2–I bond is diagnosed as an electron sharing σ-bond in all three species, 1, 2 and 3.

Base-stabilized formally zero-valent mono and diatomic molecular main-group compounds

Various compounds are known for transition metals in their formal zero-oxidation state, while similar compounds of main-group elements are recently realized and limited to only a few examples. Lewis-base-stabilized mono and diatomic molecular species (B₂, C, C₂, Si, Si₂, Ge, Ge₂, Sn, P₂, As₂, Sb₂) represent groundbreaking examples of main-group compounds with formally zero-oxidation state. In recent years, the isolation of low-valent main-group compounds has attracted increasing attention of both experimental and theoretical chemists. This is not only due to their fascinating electronic structures and exceptional reactivities, but also their use as valuable precursors for the synthesis of exotic yet important chemical species. This has led to a better understanding of the intricate balance of the donor-acceptor properties of the ligand(s) used to stabilize elements in a formally zero-oxidation state. Owing to the unusual oxidation state of the central element, many compounds containing formally zero-valent elements can efficiently activate otherwise inert small molecules. This review describes the synthesis, characterization, and reactivity of reported mono and diatomic formal zero-oxidation state main-group compounds. This review also emphasizes the comparative description of systems where different ligands are used to stabilize an element in its formal zero-oxidation state.

Taming the Antiferromagnetic Beast: Computational Design of Ultrashort Mn-Mn Bonds Stabilized by N-Heterocyclic Carbenes

The development of complexes featuring low-valent, multiply bonded metal centers is an exciting field with several potential applications. In this work, we describe the design principles and extensive computational investigation of new organometallic platforms featuring the elusive manganese-manganese bond stabilized by experimentally realized N-heterocyclic carbenes (NHCs). By using DFT computations benchmarked against multireference calculations, as well as MO- and VB-based bonding analyses, we could disentangle the various electronic and structural effects contributing to the thermodynamic and kinetic stability, as well as the experimental feasibility, of the systems. In particular, we explored the nature of the metal-carbene interaction and the role of the ancillary η6 coordination to the generation of Mn2 systems featuring ultrashort metal-metal bonds, closed-shell singlet multiplicities, and positive adiabatic singlet-triplet gaps. Our analysis identifies two distinct classes of viable synthetic targets, whose electrostructural properties are thoroughly investigated.

Antimony(III) Halide-Assisted Stereospecific Coordination of Thione

The antimony halide-aided stereospecific coordination of a cyclic thiourea-type of ligand is observed for the first time. The antimony(III) imidazole thione complexes syn-[(L¹ )SbCl₃ ] (syn-1) and anti-[(L¹ )SbBr₃ ] (anti-2) have been synthesized in very good yield by the reaction between the spatially defined steric impact ligand [(IPaul)S] (L¹ ) ([(IPaul)S]=1,3-bis(2,4-methyl-6-diphenyl phenyl)imidazole thione) and corresponding antimony halide. The stereoselective formation of complexes syn-1 and anti-2 has been confirmed by both NMR and single-crystal X-ray diffraction studies. Interestingly the stereospecific nature of syn-1 and anti-2 remains intact in solution. Furthermore, the thermal stability of antimony(III) imidazole thione complexes were examined by TGA analysis.

Strongly reducing magnesium(0) complexes

A complex of a metal in its zero oxidation state can be considered a stabilized, but highly reactive, form of a single metal atom. Such complexes are common for the more noble transition metals. Although rare examples are known for electronegative late-main-group p-block metals or semimetals^1-6, it is a challenge to isolate early-main-group s-block metals in their zero oxidation state^7-11. This is directly related to their very low electronegativity and strong tendency to oxidize. Here we present examples of zero-oxidation-state magnesium (that is, magnesium(0)) complexes that are stabilized by superbulky, monoanionic, β-diketiminate ligands. Whereas the reactivity of an organomagnesium compound is typically defined by the nucleophilicity of its organic groups and the electrophilicity of Mg²⁺ cations, the Mg⁰ complexes reported here feature electron-rich Mg centres that are nucleophilic and strongly reducing. The latter property is exemplified by the ability to reduce Na⁺ to Na⁰. We also present a complex with a linear Mg₃ core that formally could be described as a Mg^I-Mg⁰-Mg^I unit. Such multinuclear mixed-valence Mg_n clusters are discussed as fleeting intermediates during the early stages of Grignard reagent formation. Their remarkably strong reducing power implies a rich reactivity and application as specialized reducing agents.

An Open-Shell Singlet SnI Diradical and H2 Splitting

The first Sn^I diradical [(ADC^Ph)Sn]₂ (4) based on an anionic dicarbene (ADC^Ph={CN(Dipp)}₂CPh; Dipp=2,6‐iPr₂C₆H₃) scaffold has been isolated as a green crystalline solid by KC₈ reduction of the corresponding bis‐chlorostannylene [(ADC^Ph)SnCl]₂ (3). The six‐membered C₄Sn₂‐ring of 4 containing six π‐electrons shows a diatropic ring current, thus 4 may also be regarded as the first 1,4‐distannabenzene derivative. DFT calculations suggest an open‐shell singlet (OS) ground state of 4 with a remarkably small singlet–triplet energy gap (ΔE_OS–T=4.4 kcal mol⁻¹), which is consistent with CASSCF (ΔE_S–T=6.6 kcal mol⁻¹ and diradical character y=37 %) calculations. The diradical 4 splits H₂ at room temperature to yield the bis‐hydridostannylene [(ADC^Ph)SnH]₂ (5). Further reactivity of 4 has been studied with PhSeSePh and MeOTf.

Bonding and stability of donor ligand-supported heavier analogues of cyanogen halides (L')PSi(X)(L)

Fluoro- and chloro-phosphasilynes [X-Si[triple bond, length as m-dash]P (X = F, Cl)] belong to a class of illusive chemical species which are expected to have Si[triple bond, length as m-dash]P multiple bonds. Theoretical investigations of the bonding and stability of the corresponding Lewis base-stabilized species (L')PSi(X)(L) [L' = cAAC^Me (cyclic alkyl(amino) carbene); L = cAAC^Me, NHC^Me (N-heterocyclic carbene), PMe₃, aAAC (acyclic alkyl(amino) carbene); X = Cl, F] have been studied using the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) method. The variation of the ligands (L) on the Si-atom leads to different bonding scenarios depending on their σ-donation and π-back acceptance properties. The ligands with higher lying HOMOs prefer profoundly different bonding scenarios than the ligands with lower lying HOMOs. The type of halogen (Cl or F) on the Si-atom was also found to have a significant influence on the overall bonding scenario. The reasonably higher value and endergonic nature of the dissociation energies along with the appreciable HOMO-LUMO energy gap may corroborate to the synthetic viability of the homo and heteroleptic ligand-stabilized elusive PSi(Cl/F) species in the laboratory.

Rare antimony(III) imidazole selone complexes: steric controlled structural and bonding aspects

Novel antimony(iii) imidazole selone complexes in a super crowded environment are reported for the first time. The super bulky selone antimony complexes, [{IPr*Se}(SbCl3)2] (1) and [{IPr*Se}(SbBr3)2] (2), were isolated from the reactions between IPr*Se (IPr*Se = [1,3-bis(2,6-diphenylmethylphenyl)imidazole selone]) and suitable antimony(iii) halides. 1 and 2 are dinuclear complexes with a Sb : Se ratio of 1 : 0.5 with an unusual coordination mode of selone. The molecules 1 and 2 consist of both Menshutkin-type Sbπaryl interactions and a Sb-Se coordination bond. However, the reaction between antimony(iii) halides and [(IPaul)Se] ([(IPaul)Se] = [1,3-bis(2,4-methyl-6-diphenyl phenyl)imidazole selone]) with a spatially defined steric impact gave the dinuclear complex [{(IPaul)Se}(SbCl3)]2 (3) and the mononuclear complex [{(IPaul)Se}(SbBr3)] (4) without Menshutkin-type interactions. The Sb : Se ratio in 3 and 4 is 1 : 1. Interestingly, the Menshutkin-type interaction was absent in 3 and 4 due to the efficient coordinating ability of the ligand [(IPaul)Se] with the Sb(iii) center compared to that of the super bulky ligand IPr*Se. The thermal property of these antimony selone complexes was also investigated. Density functional theory (DFT) calculations were carried out on the model systems [L(SbCl3)2] (1A), [L(SbCl3)] (1B), [L'(SbCl3)2] (1C), and [L'(SbCl3)] (1D), where L = [1,3-bis(2,6-diisopropyl-4-methyl phenyl)imidazole selone] and L' = [1,3-bis(phenyl)imidazole selone], to understand the nature of orbitals and bonding situations. The computed metrical parameters of 1A are in good agreement with the experimental values. Natural population analysis of the model system reveals that the natural charge and total population of antimony(iii) are comparable. The unequal interaction between selenium and antimony obtained using Wiberg bond indices (WBIs) is fully consistent with the findings of the single-crystal X-ray studies.

Isolable dicarbon stabilized by a single phosphine ligand

In contrast to naturally occurring F₂, O₂ and N₂, diatomic C₂ is an intriguing species that has only been observed indirectly in the gas phase, and because of its high reactivity has eluded isolation in the condensed phase. It has previously been stabilized in L→C₂←L compounds but the bonding situation of the central C₂ in this motif differs remarkably from that of free C₂. Here we have prepared and structurally characterized diatomic C₂ as a monoligated complex L→C₂ using a bulky phosphine ligand bearing two imidazolidin-2-iminato groups (L is (NHC^R=N)₂(CH₃)P, where NHC^R is an N-heterocyclic carbene). The compound is stable in solution at ambient temperature and has also been isolated in the solid state. Reactivity studies, in combination with quantum chemical analysis, suggest that the two carbon atoms of the L→C₂ complex both have carbene character. The complex underwent intermolecular C-H bond activation upon thermolysis and exhibited hydroalkoxylation-like reactivity with methanol.

New pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole, by DFT

We have investigated the pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole (1_a, 2_a, 3_a, and 4_a, respectively), at the M06/6-311++G** level of theory. Optimization and vibrational frequency calculations of ground states (GS) and transition states (TS) are performed to identify Gibbs free energies and nature of stationary points, respectively. Two possible pathways of stability for 1_a-4_a are compared and contrasted which entail dimerization through hydrogen bonding (HB) and covalent bonding (CB). The CB pathway comprises head to head (HH) and head to tail (HT) dimerizations. Plausible reaction profiles are illustrated for 1_a-4_a along with the mechanism of each dimerization. Structures 1_a-3_a show one possibility for HB while 4_a represents two possibilities. Structures 1_a and 4_a display two HH dimers while 2_a and 3_a show one. Structures 1_a-4_a undergo HT dimerizations to yield three possible dimers which include trans, cis, and [2+3] isomers. Interestingly, for all 1_a-4_a, HB dimerization turns out as the most favorable stability pathway for showing no barrier of reaction. Structures 4_b and 4_c indicate the highest stability with respect to their initial 4_a compared to remaining HB dimers 1_b-3_b. In addition, the 1,2-H shift appears as a possible rearrangement for 1_a-4_a to yield their corresponding tautomers (1_i, 2_h, 3_h, and 4_k, respectively). The reaction profile of this rearrangement indicates that 1_a-4_a favor HB dimerization pathway more than 1,2-H shift, in terms of kinetic and thermodynamic. Graphical Abstract.

Distannabarrelenes with Three Coordinated SnII Atoms

Crystalline 1,4-distannabarrelene compounds [(ADCAr )3 Sn2 ]SnCl3 (3-Ar) (ADCAr ={ArC(NDipp)2 CC}; Dipp=2,6-iPr2 C6 H3 , Ar=Ph or DMP; DMP=4-Me2 NC6 H4 ) derived from anionic dicarbenes Li(ADCAr ) (2-Ar) (Ar=Ph or DMP) have been reported. The cationic moiety of 3-Ar features a barrelene framework with three coordinated SnII atoms at the 1,4-positions, whereas the anionic unit SnCl3 is formally derived from SnCl2 and chloride ion. The all carbon substituted bis-stannylenes 3-Ar have been characterized by NMR spectroscopy and X-ray diffraction. DFT calculations reveal that the HOMO of 3-Ph (ϵ=-6.40 eV) is mainly the lone-pair orbital at the SnII atoms of the barrelene unit. 3-Ar readily react with sulfur and selenium to afford the mixed-valence SnII /SnIV compounds [(ADCAr )3 SnSn(E)](SnCl6 )0.5 (E=S 4-Ar, Ar=Ph or DMP; E=Se 5-Ph).

How to Bend a Cumulene

Allenes (carbodicarbenes) and [3]cumulenes are linear carbon chains that can be bent when the terminal group has a strong carbene nature. This bending can be quite pronounced in allenes but not in [3]cumulenes. In this study, how N-heterocyclic or cyclic (alkyl)(amino) carbene (NHC and CAAC, respectively) terminal groups can modify the linear structure of [n]cumulenes has been analyzed. A low π acidity of the terminal carbene affects the linearity of [2n]cumulenes. Indeed, it has been found that the NHC [4]cumulene is extremely bent, contrary to classical [4]cumulenes. The predicted NHC [4]cumulene or tricarbodicarbene has two lone pairs and the π electrons are delocalized over the whole molecule. More significantly, DFT calculations have shown that this bent [4]cumulene is very stable, considerably more so than the corresponding [3]cumulene, which has been elusive to synthesize. Remarkably, calculations have shown that all the NHC [2n]cumulenes are more than 25 kcal mol^-1 more stable than the [2n-1]cumulenes.

Access to the most sterically crowded anilines via non-catalysed C-C coupling reactions

Variously substituted 2,6-bis(1,1-diarylethyl)anilines and 2,6-bis(trityl)anilines were prepared by a three-step high-yield process. Dimethyl-2-aminoisophtalate was modified by reaction with arylmagnesium bromides, and the hydroxy-derivatives obtained were etherified. Under the non-catalysed C-C coupling protocol, the formed bis[methyl(methoxy)diaryl]anilines react with various Grignard reagents to give highly substituted products. The buried volumes around the central nitrogen atom of the prepared compounds exceed the parameters for the known most sterically hindered anilines by about 20%.

N-Heterocyclic Carbene-Supported Aryl- and Alk- oxides of Beryllium and Magnesium

Recently, we have witnessed significant progress with regard to the synthesis of molecular alkaline earth metal reagents and catalysts. To provide new precursors for light alkaline earth metal chemistry, molecular aryloxide and alkoxide complexes of beryllium and magnesium are reported. The reaction of beryllium chloride dietherate with two equivalents of 1,3-diisopropyl-4,5-dimethylimidizol-2-ylidine (sIPr) results in the formation of a bis(N-heterocyclic carbene) (NHC) beryllium dichloride complex, (sIPr)2BeCl2 (1). Compound 1 reacts with lithium diisopropylphenoxide (LiODipp) or sodium ethoxide (NaOEt) to form the terminal aryloxide (sIPr)Be(ODipp)2 (2) and alkoxide dimer [(sIPr)Be(OEt)Cl]2 (3), respectively. Compounds 2 and 3 represent the first beryllium alkoxide and aryloxide species supported by NHCs. Structurally related dimers of magnesium, [(sIPr)Mg(OEt)Brl]2 (4) and [(sIPr)Mg(OEt)Me]2 (5), were also prepared. Compounds 1-5 were characterized by single crystal X-ray diffraction studies, 1H, 13C, and 9Be NMR spectroscopy where applicable.

Isolation of Carbene-Stabilized Arsenic Monophosphide [AsP] and its Radical Cation [AsP]+. and Dication [AsP]2

Arsenic monophosphide (AsP) species supported by two different N‐heterocyclic carbenes were prepared by reaction of (IDipp)PSiMe₃ (1) (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) with (IMes)AsCl₃ (2) (IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene) to afford the dichloride [(IMes)As(Cl)P(IDipp)]Cl (3), which upon reduction with KC₈ furnished heteroleptic [(IMes)AsP(IDipp)] (4). The corresponding mono‐ and dications [(IMes)AsP(IDipp)][PF₆], [5]PF₆, and [(IMes)AsP(IDipp)][GaCl₄]_2, [6][GaCl₄]₂, respectively, were prepared by one‐electron oxidation of 4 with ferrocenium hexafluorophosphate, [Fc]PF_6, or by chloride abstraction from 3 with two equivalents of GaCl₃, respectively. Compounds 4–6 represent rare examples of heterodiatiomic interpnictogen compounds, and X‐ray crystal structure determinations together with density functional theory (DFT) calculations reveal a consecutive shortening of the As−P bond lengths and increasing bond order, in agreement with the presence of an arsenic–phosphorus single bond in 4 and a double bond in 6²⁺. The EPR signal of the cationic radical [5]^+. indicates a symmetric spin distribution on the AsP moiety through strong hyperfine coupling with the ⁷⁵As and ³¹P nuclei.

Stepwise Reduction at Magnesium and Beryllium: Cooperative Effects of Carbenes with Redox Non-Innocent α-Diimines

In the past two decades, the organometallic chemistry of the alkaline earth elements has experienced a renaissance due in part to developments in ligand stabilization strategies. In order to expand the scope of redox chemistry known for magnesium and beryllium, we have synthesized a set of reduced magnesium and beryllium complexes and compared their resulting structural and electronic properties. The carbene-coordinated alkaline earth-halides, (^Et2CAAC)MgBr₂ (1), (SIPr)MgBr₂ (2), (^Et2CAAC)BeCl₂ (3), and (SIPr)BeCl₂ (4) [^Et2CAAC = diethyl cyclic(alkyl)(amino) carbene; SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene] were combined with an α-diimine [2,2-bipyridine (bpy) or bis(2,6-diisopropylphenyl)-1,4-diazabutadiene (^DippDAB)] and the appropriate stoichiometric amount of potassium graphite to form singly- and doubly-reduced compounds (^Et2CAAC)MgBr(^DippDAB) (5), (^Et2CAAC)MgBr(bpy) (6), (^Et2CAAC)Mg(^DippDAB) (7), (^Et2CAAC)Be(bpy) (8), and (SIPr)Be(bpy) (9). The doubly-reduced compounds 7-9 exhibit substantial π-bonding interactions across the diimine core, metal center, and π-acidic carbene. Each complex was fully characterized by UV-vis, FT-IR, X-ray crystallography, ¹H, ¹³C, and ⁹Be NMR, or EPR where applicable. We use these compounds to highlight the differences in the organometallic chemistry of the lightest alkaline earth metals, magnesium and beryllium, in an otherwise identical chemical environment.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Donor Stabilized Diatomic Gr.14 E2 (E = C-Pb) Molecule D-E2-D (D = NHC, aNHC, NNHC, NHSi, NHGe, cAAC, cAASi, cAAGe): A Theoretical Insight

Quantum chemical calculations have been carried out to explore the detailed electronic structure and bonding scenario in various bis-donor stabilized E₂ compounds (E = C-Pb). Our computational findings reveal that the thermodynamic stabilities of the E₂ core gradually decrease as we move down the group. A linear D-E-E'-D framework is observed for C₂ systems, while the heavier group 14 analogues possess trans-bent geometries. Consideration of few compounds as viable targets for synthesis is suggested by their corresponding calculated formation energies. In addition, the thermodynamic stabilities of C₂ systems notably increase with the saturation of the donor ring framework and are even more pronounced for boron-substituted saturated NHD ligand. QTAIM calculations affirmed that the covalent nature of E-E' bonds shifts toward the donor-acceptor region as one traverses from top to bottom along group 14. The E-D and E'-D bonds in the C₂ systems have covalent nature, whereas those in Si₂-Pb₂ systems are characterized by donor-acceptor bonds. In addition, we have computed proton affinities and vertical ionization potentials (VIPs) of these compounds. An excellent correlation was obtained between calculated VIPs and orbital energies of HOMOs. Furthermore, in the present study, we also explored the effect of bis-donors in the stabilization of heterodiatomic SiC compounds. Our calculations indicate that a typical bonding description of the SiC(D)₂ compounds should be represented by a combination of a classical double bond between C-D with significant donor-acceptor interaction in Si-D, i.e., D → Si═C═D. The SiC(D)₂ systems are found to be less stable than the corresponding dicarbon compounds C₂(D)₂, but they show significant stabilization compared to the corresponding disilicon systems Si₂(D)₂.

Dicationic ditelluride salts stabilized by N-heterocyclic carbene

Two types of dicationic ditelluride salts stabilized by N-heterocyclic carbene have been synthesized and characterized by their spectroscopic data, X-ray diffraction, and DFT calculations.