pi-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements

Dehydrocoupling of Phosphine-Borane Adducts Under Ambient Conditions Using Aminoboranes as Hydrogen Acceptors

We report on the reactivity of aminoboranes (R2N=BH2; R = iPr, Et, Me) with phosphine-borane adducts (PhR'PH ⋅ BH3; R' = H, Ph): Sufficiently sterically unencumbered aminoboranes can accept hydrogen from phosphine-borane adducts. The hydrogen transfer results in the formation of amine-borane adducts (R2NH ⋅ BH3) and transient phosphinoboranes (PhR'P-BH2) in situ. These phosphinoboranes undergo subsequent reactivity to yield either polyphosphinoborane, [PhPH-BH2]n, or the linear dimer, Ph2PH ⋅ BH2-Ph2P ⋅ BH3. Unlike metal catalyzed phosphine-borane dehydrocoupling, which occurs at elevated temperatures (2 M, toluene, ≥100 °C, ≥24 h), these dehydrocoupling reactions occur under ambient conditions (2 M, Et2O or C6D6, 20 °C, ≤24 h). We performed a computational mechanistic study in which we identified that this transformation potentially occurs via a P-to-N and B-to-B hydrogen transfer through a 6-membered transition state.

Structural Snapshots on Stepwise Anionic Oxoborane Formation: Access to an Acyclic BO Ketone Analogue and Its Metathesis Chemistry with CO2 and CS2

In this work, we disclose the synthesis and characterization of non-acid/base-stabilized anionic oxoboranes [MesTer2BO][K(L)] (MesTer = -C6H3-2,6-(2,4,6-Me3-C6H2)2, L = [2.2.2]-cryptand or 18-crown-6), which are isoelectronic and isostructural with aryl-substituted ketones. The stepwise synthetic formation of these ion-separated oxoboranes is demonstrated on the one hand by the treatment of the parent borinic acid MesTer2BOH with N-heterocyclic carbenes (NHCs) to give [MesTer2BO][HNHC] derivatives, and on the other hand by a deprotonation-sequestration sequence. Bearing polarized boron-oxygen moieties, their inherent reactivity toward both carbon disulfide and carbon dioxide reveals a unique π-bond metathesis reactivity to yield [(MesTer)2B-μ-E2C=E][K(L)] (E = O, S) derivatives.

A trimetallic bismuth(I)-based allyl cation

The chemistry of low-valent bismuth compounds has recently unlocked new concepts in catalysis and unique electronic structure fundamentals. In this work, we describe the synthesis and characterization of a highly reduced bismuth salt featuring a cationic core based on three contiguous Bi(I) centres. The triatomic bismuth-based core exhibits an electronic configuration that mimics the canonical description of the archetypical carbon-based π-allyl cation. Structural, spectroscopic and theoretical analyses validate the unique π-delocalization between the bismuth's highly diffused 6p orbitals, resulting in a bonding situation in which the three bismuth atoms are interconnected by two bonds, formally possessing a 1.5 bond order each. This electronic situation defines this complex as the heaviest and stable π-allyl cation of the periodic table. Furthermore, we demonstrate that the newly synthesized complex is able to act as a synthon for the transfer of a Bi(I) cation to forge other low-valent organobismuth complexes.

Characterization of Iminobismuthanes and Catalytic Reduction of Organic Azides via Bi(I)/Bi(III) Redox Cycling

We report the stoichiometric and catalytic reactivity of organobismuth(I) complexes with organic azides. Treatment of N,C,N-pincer bismuthinidenes with organic azides (acyl, sulfonyl, and bulky aryl) results in monomeric iminobismuthanes which can be structurally characterized -including the formal Bi=N double bond- by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. Building upon the stoichiometric reactivity of the monomeric iminobismuthanes, catalytic reduction of a broad range of organic azides is developed. DFT calculations of the catalytic reaction pathway support the redox nature of the overall process.

Synthesis of a Transient Cationic Phosphaborene and its Trapping by Facile Aliphatic C-H Bond Activation

The synthesis of a transient cationic phosphaborene [(Mes*)P=B(CAAC)]+ (Mes*=2,4,6,-tri-tert-butylphenyl, CAAC=cyclic alkylamino carbene) by halide abstraction from the B-brominated analogue is reported. This species was found to undergo rapid and selective intramolecular aliphatic C-H bond activation to yield a phosphinoborenium cation, which undergoes facile deprotonation to give a cyclic base-stabilized phosphaborene. Computational investigation of the mechanism of C-H activation indicates a boron-centred activation route with an exceptionally low barrier of 8 kJ mol-1, followed by a nearly barrierless hydride migration from boron to phosphorus. This underscores the enhanced reactivity of phosphaborenes compared to their lighter iminoborane analogues and indicates that steric stabilization alone is likely insufficient to isolate persistent cationic phosphaborenes, necessitating the use of alternative design strategies.

Stabilization of Ethane-Like Dianionic Diborane(6) in Monometallic Titanium Complexes and its Subsequent B(sp3)-B(sp3) Bond Cleavage

Treatment of [Cp*TiCl3] with [LiBH4 ⋅ THF] followed by thermolysis with [Ph2E2] (E=S or Se) resulted in the formation of classical diborane(6) complexes, [(Cp*Ti)(η4-B2H4LL')] (L=C6H4E; L'=C6H5E; 1 a: E=S, 1 b: E=Se), stabilized at titanium template. To the best of our knowledge, they are the first examples of mono-metallic classical diborane(6) complexes. The bonding analysis and theoretical studies suggest that the stabilization of these diborane(6) species is due to the presence of four bridging ligands in ĸ4-fashion, where two of them are phenyl thiolates/selenolates that provide more electrons to the electron-deficient titanium center. Reactions of these diborane(6) species with [M(CO)5 ⋅ THF] (M=Mo, W) led to the cleavage of the electron-precise B(sp3)-B(sp3) bond that yielded ĸ3-hydridoborato complexes [(Cp*Ti)(ĸ3-BH3R)(μ-EPh)2{M(CO)4}] (2 a-c: R=H, 3 a-c: R=Ph). In an attempt to isolate the Te-analogue of 1 a-b, a similar reaction was performed; however, the complex was too unstable to be isolated. Interestingly, the treatment of this unstable intermediate with [W(CO)5 ⋅ THF] yielded [(Cp*Ti)(ĸ3-BH3R)(μ-TePh)2{W(CO)4}] (2 d: R=H, 3 d: R=Ph) that are analogues of 2 a-c and 3 a-c, respectively. Formation of these species provide indirect evidence for the existence of unstable [(Cp*Ti)(η4-B2H4LL')] (L=C6H4Te; L'=C6H5Te; 1 c).

A Crystalline Unsupported Phosphagallene and Phosphaindene

The synthesis and isolation of TerP═GaTer and TerP═InTer (Ter = 2,6-Dipp2-C6H3; Dipp = 2,6-diisopropylphenyl) is reported. These compounds feature unsupported P═Ga and P═In double bonds and two-coordinate triel element centers. Key to the stabilization of such compounds is the steric bulk of the terphenyl substituents, which serve to shield the highly reactive P═E bonds (E = Ga, In) and prevent further aggregation. When smaller aromatic substituents are employed on the phosphorus-containing precursor, the cyclic compounds Mes*P(ETer)2 (Mes* = 2,4,6-tBu3-C6H2) are isolated instead. These species contain weakly aromatic three-membered rings. The presence of an external base (PMe3) is required in order to stabilize a phosphagallene when the smaller Mes* substituent is used, allowing for the isolation of Mes*P═GaTer(PMe3).

Near-Infinite-Chain Polymers with Ge=Ge Double Bonds

Despite considerable interest in heteroatom-containing conjugated polymers, there are only few examples with heavier p-block elements in the conjugation path. The recently reported heavier acyclic diene metathesis (HADMET) allowed for the synthesis of a polymer containing Ge=Ge double bonds-albeit insoluble and with limited degree of polymerization. By incorporation of long alkyl chains, we now obtained soluble representatives, which exhibit degrees of polymerization near infinity according to diffusion-ordered NMR spectroscopy (DOSY) and dynamic light scattering (DLS). UV/Vis and NMR data confirm the presence of σ,π-conjugation across the silylene-phenylene linkers between the Ge=Ge double bonds. Favorable intermolecular dispersion interactions lead to ladder-like cylindrical assemblies as confirmed by X-ray diffraction (XRD), small angle X-ray scattering (SAXS) and DLS. AFM and TEM images of deposited thin films reveal lamellar ordering of extended polymer bundles.

Cationic Group 13 and 14 Element Clusters

Anionic and neutral clusters dominate the cluster chemistry of group 13 and 14 elements, many of which have become classic textbook examples of main group element clusters. However, facilitated by the development of unreactive, weakly coordinating anions, the number of known group 13 and 14 cationic cluster compounds has risen rapidly in recent years. Hence, this review aims to give an overview over this research field, which arouses increasing interest owing to the often unusual structures of the cationic clusters, as well as their application in bond activation chemistry. Challenges of the cluster formation are discussed and suitable starting materials are presented, as well as syntheses, structures and the rich follow-up chemistry of (also mixed) group 13 and 14 cluster cations.

A Lead(II) Substituted Triplet Carbene

Reaction of the pincer-type ligand L3 supported complex [L3PbBr][BArF24] (1) with Li[(C(═N2)TMS)] furnishes [L3Pb(C(═N2)TMS)][BArF24] (2). Diazo-compound 2 eliminates dinitrogen upon irradiation affording formal plumba-alkyne 3, which persists in cold fluoroarene solutions. Variable temperature UV/Vis and NMR spectroscopies in combination with quantum-chemical calculations identify 3 as a metal-substituted triplet carbene. In-crystallo irradiation of [L3Pb(C(═N2)TMS)(tol)][BArF24] (2·tol) provides a snapshot of intermolecular C-H bond insertion with toluene (4).

Mechanistic Insights into the Reactivity and Activation Barrier Origins for CO2 Capture by Heavy Group-14 Imine Analogues

Using M06-2X-D3/def2-TZVP, the [2 + 2] cycloaddition reactions of carbon dioxide with the heavy imine analogues G14=N-Rea (G14 = Group 14 element) were investigated. The theoretical evidence reveals that the nature of the doubly bonded G14=N moiety in heavy imine analogues, G14=N-Rea (L1L2G14=N-L3), is characterized by the electron-sharing interaction between triplet L1L2G14 and triplet N-L3 fragments. Based on our theoretical studies, except for the carbon-based imine, all four heavy imine analogues with Si=N, Ge=N, Sn=N, and Pb=N groups can easily engage in [2 + 2] cycloaddition reactions with CO2. Energy decomposition analysis-natural orbitals for chemical valence analyses and the FMO theory strongly suggest that in the CO2 capture reaction by heavy imine analogues G14=N-Rea, the primary bonding interaction is the occupied p-π orbital (G14=N-Rea) → vacant p-π* orbital (CO2) interaction, instead of the empty p-π* orbital (G14=N-Rea) ← filled p-π orbital (CO2) interaction. The activation barrier of the CO2 capture reactions by G14=N-Rea molecules is primarily determined by the deformation energy of CO2. Shaik's valence bond state correlation diagram model, used to rationalize the computed results, indicates that the singlet-triplet energy splitting of G14=N-Rea is a key factor in determining the reaction barrier for the current CO2 capture reactions.

An Unexpected Access to Phospholene Fused β-Phosphinolactams by the Reaction of α-C2-Bridged Biphospholes and Nitrones

An efficient and practical method for the preparation of the phospholene fused β-phosphinolactam skeleton from α-C2-bridged biphospholes and nitrones is described. The dissociation of biphospholes generates transient 1-phosphafulvenes, followed by oxidation with nitrones to give the 1-phosphafulvene oxides. The oxidation of 1-phosphafulvene boosts its reactivity toward imine, leading to the isolated products.

Heavier group 14-transition metal π-complex congeners

Since the dawn of organometallic chemistry, transition metal complexes of unsaturated organic molecules, namely π-complexes, have remained a central focus: our thorough understanding of the electronic nature of such species, and their importance in countless reactive processes continues to drive research in their synthesis and utilisation. Since the late 1900s, research regarding the related chemistry for the heavier group 14 elements has become increasingly more fervent. Today, heavier congeners of a vast array of classical π-complexes have been realised, from alkene to arene systems, involving Si, Ge, Sn, and Pb. This has given deeper insights into the bonding observed for these heavier elements, which typically involves a lessened degree of π-bonding and an increased polarisation. This review aims to summarise this field, identifying these disparities, and highlighting areas which we believe may be exciting for future exploration.

Theoretical Investigation on One-Electron ϕ···ϕ Bonding in Diuranium Inverse Sandwich U2B6 Complex Enabled by a B6 Ring

Traditional σ, π, and δ types of covalent chemical bonding have been extensively studied for nearly a century. In contrast, ϕ-type bonding involving nf (n = 4, 5) orbitals has received less attention due to their high contraction and minimal orbital overlap. Herein, we theoretically predict a singly occupied ϕ···ϕ bonding between two 5f orbitals, facilitated by B6 group orbitals in the hexa-boron diuranium inverse sandwich structure of U2B6. From ab initio quantum chemical calculations, the global minimum structure has a septuplet state with D6h symmetry. Chemical bonding analyses reveal that the 5f and 6d atomic orbitals of the two uranium atoms interact with the ligand orbitals of the central B6 ring, exhibiting favorable energy matching and symmetry compatibility to form delocalized σ-, π-, δ-, and ϕ-type bonding orbitals. Notably, even though the ϕ···ϕ bonding orbital is singly occupied, it still has a significant role in stability and cannot be overlooked. Furthermore, the U2B6 cluster model can be viewed as a building block of UB2 solid materials from both geometric and electronic perspectives. This work predicts the first example of ϕ···ϕ bonding, highlighting the complexity and diversity of chemical bonds formed in actinide boride clusters.

Macrocyclic bis-diphosphenes demonstrating bimetallic exo- and endo-cyclic binding modes

Macrocyclic bis-diphosphenes, formally heavier derivatives of macrocyclic azobenzenes, are accessed for the first time. These are synthesised in a reproducible fashion, through the nickel-mediated homocoupling of xanthene-derived NHC-stabilised bis-phosphinidene units. This gives direct access to target macrocyclic bis-diphosphenes 2, featuring exo-cyclic coordinated Ni0 fragments. The endo-cyclic binding mode in 3 is realised by NHC-abstraction using CuCl, so demonstrating two homometallic binding modes for this system. Additionally, reaction with CuCl in acetonitrile yields small amounts of a tetra-metallic NiII/CuI complex, which establishes simultaneous exo- and endo-cyclic metal binding. Fluctional solution state behavior in these systems is explored through variable temperature NMR spectroscopy, in addition to computational bonding analyses, giving the first insights into this novel class of compounds.

Diverse Functionality of Molecular Germanium: Emerging Opportunities as Catalysts

Fundamental research on germanium as the central element in compounds for bond activation chemistry and catalysis has achieved significant feats over the last two decades. Designing strategies for small molecule activations and the ultimate catalysts established capitalize on the orbital modalities of germanium, apparently imitating the transition-metal frontier orbitals. There is a growing body of examples in contemporary research implicating the tunability of the frontier orbitals through avant-garde approaches such as geometric constrained empowered reactivity, bimetallic orbital complementarity, cooperative reactivity, etc. The goal of this Perspective is to provide readers with an overview of the emerging opportunities in the field of germanium-based catalysis by perceiving the underlying key principles. This will help to convert the discrete set of findings into a more systematic vision for catalyst designs. Critical exposition on the germanium's frontier orbitals participations evokes the key challenges involved in innovative catalyst designs, wherein viewpoints are provided. We close by addressing the forward-looking directions for germanium-based catalytic manifold development. We hope that this Perspective will be motivational for applied research on germanium as a constituent of pragmatic catalysts.

Large Number of Direct or Pseudo-Direct Band Gap Semiconductors among A3TrPn2 Compounds with A = Li, Na, K, Rb, Cs; Tr = Al, Ga, In; Pn = P, As

Due to the high impact of semiconductors with respect to many applications for electronics and energy transformation, the search for new compounds and a deep understanding of the structure-property relationship in such materials has a high priority. Electron-precise Zintl compounds of the composition A3TrPn2 (A = Li - Cs, Tr = Al - In, Pn = P, As) have been reported for 22 possible element combinations and show a large variety of different crystal structures comprising zero-, one-, two- and three-dimensional polyanionic substructures. From Li to Cs, the compounds systematically lower the complexity of the anionic structure. For an insight into possible crystal-structure band-structure relations for all compounds (experimentally known or predicted), their band structures, density of states and crystal orbital Hamilton populations were calculated on a basis of DFT/PBE0 and SVP/TZVP basis sets. All but three (Na3AlP2, Na3GaP2 and Na3AlAs2) compounds show direct or pseudo-direct band gaps. Indirect band gaps seem to be linked to one specific structure type, but only for Al and Ga compounds. Arsenides show smaller band gaps than phosphides due to weaker Tr-As bonds. The bonding situation was confirmed by a Mullikan analysis, and most states close to the Fermi level were assigned to non-bonding orbitals.

Reactivity of a Linear 2-Germapropadiene with Acids, Ketones, and Amines

The reactivity of an isolable 2-germapropadiene with acids, ketones, and amines was investigated. The reactions of 2-germapropagiene 1 with hydrogen chloride and acetic acid afforded the corresponding dichlorogermane (2) and diacetoxygermane (3), respectively, indicating that the central germanium atom of 1 is electrophilic. The reaction of 1 with benzaldehyde proceeds via a formal [2+2] cycloaddition to afford the corresponding spiro compound (4). Moreover, 1 reacts smoothly with acetone to furnish germane 5, which contains a six-membered ring involving two acetone molecules. Furthermore, 1 undergoes N-H bond insertion with methylamine or aniline to afford diamino germanes 7 and 8, respectively. The reaction of 1 with urea selectively afforded the corresponding N-H-insertion product (8).

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Intramolecularly O,N,O-Coordinated Tin(II) Salts: Syntheses, Structures, Cyclization, and Transition Metal Complexation

We report the syntheses of tin(II) salts of the types [L1SnX]SnX3 [L1=2,6-{(i-PrO)2(O)P}2C5H3N: 1, X=Cl; 2, X=Br], [L2SnCl]SnCl3 [L2=2-{(i-PrO)Ph(O)P}-6-{(i-PrO)2(O)P}C5H3N: 3], [L3SnX]SnX3 [L3=2,6-{MeO(O)C}2C5H3N: 4, X=Cl; 5, X=Br], [L4SnX]SnX3 [L4=2,6-{Et2N(O)C}2C5H3N: 6, X=Cl; 7, X=Br]. These compounds were obtained by addition of SnX2 to the corresponding ligand inducing autoionization of the respective tin(II) halide. The thermal stability of 1, 3, and 4 was elucidated, giving, under ester cleavage and cyclisation, the tin(II) derivatives 8-12. The reaction of [L1SnCl]SnCl3 (1) with W(CO)4(thf)2 afforded the tungsten tetracarbonyl complex [{L1SnCl}{SnCl3}W(CO)4] (13), representing the first example in which a tin(II) stannate anion and a tin(II) stannylium cation simultaneously coordinate to a transition metal centre. The compounds were characterized by single crystal X-ray diffraction analyses and in part by elemental analyses, IR and NMR spectroscopy, electrospray ionization mass spectrometry. DFT calculations accompany the experimental work.

The borylamino-diborata-allyl anion

Reactions of β-diketiminato alkaline earth alkyldiboranate derivatives [(BDI)Ae{pinBB(R)pin}] (BDI = HC{(Me)CNDipp}2; Dipp = 2,6-i-Pr2C6H3; Ae = Mg, R = n-Bu or Ae = Ca, R = n-hexyl) with t-BuNC provide access to the respective group 2 derivatives of unprecedented diborata-allyl, {(pinB)2CNBpin(t-Bu)}-, anions. Although the necessary mode of B-C bond cleavage implicated in these transformations could not be elucidated, further studies of the reactivity of magnesium triboranates toward isonitriles delivered a more general and rational synthetic access to analogous anionic moieties. Extending this latter reactivity to a less symmetric triboranate variant also provided an isomeric Mg-C-bonded dibora-alkyl species and sufficient experimental insight to prompt theoretical evaluation of this reactivity. DFT calculations, thus, support a reaction pathway predicated on initial RNC attack at a peripheral boron centre and the intermediacy of such dibora-alkyl intermediates.

Carbene-Stabilized Phosphagermylenylidene: A Heavier Analog of Isonitrile

Phosphagermylenylidenes (R-P═Ge), as heavier analogs of isonitriles, whether in their free state or as complexes with a Lewis base, have not been previously identified as isolable entities. In this study, we report the synthesis of a stable monomeric phosphagermylenylidene within the coordination sphere of a Lewis base under ambient conditions. This species was synthesized by Lewis base-induced dedimerization of a cyclic phosphagermylenylidene dimer or via Me3SiCl elimination from a phosphinochlorogermylene framework. The deliberate integration of a bulky, electropositive N-heterocyclic boryl group at the phosphorus site, combined with coordination stabilization by a cyclic (alkyl)(amino)carbene at the low-valent germanium site, effectively mitigated its natural tendency toward oligomerization. Structural analyses and theoretical calculations have demonstrated that this unprecedented species features a P═Ge double bond, characterized by conventional electron-sharing π and σ bonds, complemented by lone pairs at both the phosphorus and germanium atoms. Preliminary reactivity studies show that this base-stabilized phosphagermylenylidene demonstrates facile release of ligands at the Ge atom, coordination to silver through the lone pair on P, and versatile reactivity including both (cyclo)addition and cleavage of the P═Ge double bond.

Theoretical Investigation on Dialumenes toward Dihydrogen Activation: Mechanism and Ligand Effect

Herein, we report a computation study based on the density functional theory calculations to understand the mechanism and ligand effect of the base-stabilized dialumenes toward dihydrogen activation. Among all of the examined modes of dihydrogen activation using the base-stabilized dialumene, we found that the concerted 1,2-hydrogenation of the Al═Al double bond is kinetically more preferable. The concerted 1,2-hydrogenation of the Al═Al double bond adopts an electron-transfer model with certain asynchrony. That is, the initial electron donation from the H-H σ bonding orbital to the empty 3p orbital of the Al1 center is followed by the backdonation from the lone pair electron of the Al2 center to the H-H σ antibonding orbital. Combined with the energy decomposition analysis on the transition states of the concerted 1,2-hydrogenation of the Al═Al double bond and the topographic steric mapping analysis on the free dialumenes, we ascribe the higher reactivity of the aryl-substituted dialumene over the silyl-substituted analogue in dihydrogen activation to the stronger electron-withdrawing effect of the aryl group, which not only increases the flexibility of the Al═Al double bond but also enhances the Lewis acidity of the Al═Al core. Consequently, the aryl-substituted dialumene fragment suffers less geometric deformation, and the orbital interactions between the dialumene and dihydrogen moieties are more attractive during the 1,2-hydrogenation process. Moreover, our calculations also predict that the Al═Al double bond has a good tolerance with the stronger electron-withdrawing group (-CF3) and the weaker σ-donating N-heterocyclic carbene (NHC) analogue (e.g., triazol carbene and NHSi). The reactivity of the dialumene in dihydrogen activation can be further improved by introducing these groups as the supporting ligand and the stabilizing base on the Al═Al core, respectively.

At the limits of bisphosphonio-substituted stannylenes

Donor stabilization of Sn(II) and Pb(II) halides with 1,1'-ferrocenylene bridged bisphosphanes has been explored for Fe(C5H4P(C6H5)2)2 (dppf), and Fe(C5H4PH(C4H9))2. These bisphosphanes are reacted with SnBr2 and PbCl2 with and without additional Lewis acid (AlCl3) forming acyclic and cyclic donor adducts from which the latter represent bisphosphoniotetrylenes. Since dynamic exchange in solution is observed, characterization includes solution and solid-state NMR in addition to SC-XRD, amended by DFT calculations.

Electronic Structure Analysis of the A10Tt2P6 System (A=Li-Cs; Tt=Si, Ge, Sn) and Synthesis of the Direct Band Gap Semiconductor K10Sn2P6

Investigating the relationship between atomic and electronic structures is a powerful tool to screen the wide variety of Zintl phases for interesting (opto-)electronic properties. To get an insight in such relations, the A10Tt2P6 system (A=Li-Cs; Tt=Si-Sn) was picked as model system to analyse the influence of structural motives, combination of elements and their properties on type and width of the band gaps. Those compounds comprise two interesting structural motives of their anions, which are either monomeric trigonal planar TtP3 5- units which are isostructural to CO3 2- or [Tt2P6]10- dimers which correspond to two edge-sharing TtP4 tetrahedra. The A10Tt2P6 compounds were structurally optimized for both polymorphs and subsequent frequency analysis, band structure as well as density of states calculations were performed. The Gibbs free energies were compared to determine temperature dependent stability, where Na10Si2P6, Na10Ge2P6 and K10Sn2P6 were found to be candidates for a high temperature phase transition between the two polymorphs. Additionally, the unknown, but predicted compound K10Sn2P6 was synthesized and characterized by single crystal and powder x-ray diffraction. It crystalizes in the monoclinic space group P 21/n and incorporates [Sn2P6]10- edge sharing double tetrahedra. It was determined to be a direct band gap semiconductor with a band gap of 2.57 eV.

Metal-metal cooperativity boosts Lewis basicity and reduction properties of the bis(gallanediyl) CyL2Ga2

The interplay of two proximate gallium centres equips the bimetallic complex CyL2Ga2 (1, CyL2 = 1,2-trans-Cy[NC(Me)C(H)C(Me)N(Dip)]2, Dip = 2,6-i-Pr2C6H3) with increased Lewis basicity and higher reducing power compared to the monometallic gallanediyl LGa (2, L = HC[MeCN(Dip)]2) as evidenced by cross-over experiments. Quantum chemical calculations were employed to support the experimental findings.

From Diaminosilylenes to Silapyramidanes: Making Sense of the Stability of Divalent Silicon Compounds

Since the discovery of decamethylsilicocene over three decades ago, chemists have successfully isolated a variety of divalent silicon compounds by orchestrating steric and electronic effects to their advantage. Two broad strategies of electronic stabilization appear to have been widely deployed, namely, π-conjugation as in diaminosilylenes and π-complexation as in decamethylsilicocene and silapyramidanes. Herein, we attempted to identify quantitative metrics for the electronic stabilization of silylenes. Singlet-triplet gaps and electron affinities, both physical observables, proved useful in this regard. Thus, the most stable silylenes exhibit unusually large singlet-triplet gaps and very low or negative gas-phase electron affinities. Both metrics signify low electrophilicity, i.e., a low susceptibility to nucleophilic attack. The chemical significance of the ionization potential associated with the Si-based lone pair, on the other hand, remains unclear.

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.

Selective monooxygenation of diphosphenes with molecular oxygen

The O2 splitting mediated by the bulky Rind-based diphosphenes resulted in the clean formation of the mixed-valent diphosphorus compounds, diphosphene oxides, with P2O moieties. Their structural features and electronic properties have been clearly characterized by experimental and theoretical methods.

Structures and Energetics of E2H3+ (E = As, Sb, and Bi) Cations

E2H2 (E = As, Sb, Bi) structures involving multiple bonds have attracted much attention recently. The E2H3+ cations (protonated E2H2) are predicted to be viable with substantial proton affinities (>180 kcal/mol). Herein, the bonding characters and energetics of a number of E2H3+ isomers are explored through CCSD(T) and DFT methods. For the As2H3+ system, the CCSD(T)/cc-pVQZ-PP method predicts that the vinylidene-like structure lies lowest in energy, with the trans and cis isomers higher by 6.7 and 9.3 kcal/mol, respectively. However, for Sb2H3+ and Bi2H3+ systems, the trans isomer is the global minimum, while the energies of the cis and vinylidene-like structures are higher, respectively, by 2.0 and 2.4 kcal/mol for Sb2H3+ and 1.6 and 15.0 kcal/mol for Bi2H3+. Thus, the vinyledene-like structure is the lowest energy for the arsenic system but only a transition state of the bismuth system. With permanent dipole moments, all minima may be observable in microwave experiments. Besides, we have also obtained transition states and planar-cis structures with higher energies. The current results should provide new insights into the various isomers and provide a number of predictions for future experiments.

Probing Radical Addition to 1-Phosphabutadienes by Employing Muonium as a "Light Isotope" of Hydrogen

Understanding free radical addition to multiple bonds is important to elucidating the mechanistic details of addition polymerization reactions, albeit the fleeting radical intermediates are very difficult to detect by conventional methodologies. Muon spin spectroscopy (μSR) is a highly sensitive method that can detect radical species at 106 spins (cf. EPR: 1012 spins, NMR: 1018 spins). Herein, we employ μSR to detect the radical-addition products from three 1-phosphabutadiene monomers, P-analogues of isoprene. We show that muonium (Mu), a "light" H-atom surrogate, adds predominantly at the C4 position of the P1 =C2 -C3 =C4 moiety to give unprecedented 1-phosphaallyl radicals as the major products. Our structural assignments are supported by assignment of muon, phosphorus and proton hyperfine coupling constants using DFT-calculations. A minor radical product is also detected that is tentatively assigned to an PC3 -heterocyclic free radical. On the basis of DFT-predictions, we speculate that its formation may involve initial addition of Mu+ at the C3 position followed by electron capture. These studies provide rare insights into the prospective radical (or cationic) polymerization of 1-phosphabutadienes, which have previously been polymerized using anionic initiation.

Synthesis and Reactivity of N-Heterocyclic Carbene Coordinated Formal Germanimidoyl-Phosphinidenes

Treatment of N-heterocyclic carbene (NHC) ligated germylidenylphosphinidene MsFluidtBu-GeP(NHCiPr) (where MsFluidtBu is a bulky hydrindacene substituent, and NHCiPr is 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene) with mesityl azide and 4-tertbutylphenyl azide afforded NHC coordinated formal germanimidoyl-phosphinidenes, which represent the first compounds bearing both Ge═N double bond and phosphinidene functionalities. Studies of the chemical properties revealed that the reactions preferred to occur at the Ge═N double bond, which underwent [2 + 2] cycloadditions with CO2 and ethyl isocyanate, and coordinated with coinage metals through the nitrogen atom.

Bis(tetrelocenes) - fusing tetrelocenes into close proximity

We report the synthesis and structure of two bis(germanocenes) and a bis(stannocene), obtained by the reaction of unsymmetric ansa bis(cyclopentadienyl) ligands with germanium and tin dichloride. DFT calculations show that the formation of these bis(tetrelocenes) is energetically favoured over the formation of the corresponding [1]tetrelocenophanes. In the crystal structure authenticated structural motif, the two tetrel(II) centers are forced into close proximity to each other, resulting in weak donor-acceptor interactions, according to Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) analyses.

A planar per-borylated digermene

A tetraboryl digermene synthesized by the reaction between a dianionic digermanide nucleophile and a boron halide electrophile is dimeric both in the solid state and in hydrocarbon solution. It features both a planar 'alkene-like' geometry for the Ge2B4 core, and an exceptionally short GeGe double bond. These structural features are consistent with the known electronic properties of the boryl group, and with lowest energy (in silico) fragmentation into two triplet bis(boryl)germylene fragments.

Structures, Bonding Analyses and Reactivity of a Dicationic Digallene and Diindene Mimicking trans-bent Ditetrylenes

The reaction of bisdicyclohexylphosphinoethane (dcpe) and the subvalent MI sources [MI (PhF)2 ][pf] (M=Ga+ , In+ ; [pf]- =[Al(ORF )4 ]- ; RF =C(CF3 )3 ) yielded the salts [{M(dcpe)}2 ][pf]2 , containing the first dicationic, trans-bent digallene and diindene structures reported so far. The non-classical MI ⇆MI double bonds are surprisingly short and display a ditetrylene-like structure. The bonding situation was extensively analyzed by quantum chemical calculations, QTAIM (Quantum Theory of Atoms in Molecules) and EDA-NOCV (Energy Decomposition Analysis with the combination of Natural Orbitals for Chemical Valence) analyses and is compared to that in the isoelectronic and isostructural, but neutral digermenes and distannenes. The dissolved [{Ga(dcpe)}2 ]2+ ([pf]- )2 readily reacts with 1-hexene, cyclooctyne, diphenyldisulfide, diphenylphosphine and under mild conditions at room temperature. This reactivity is analyzed and rationalized.

Base-Stabilized Phosphinidene Oxide, Imide and Sulfide

Oxidation of a base-stabilized phosphinidene (κ2 -NNP)P (12, NNP=phosphinoamidinate) with N2 O afforded a labile phosphinidene oxide (κ2 -NNP)P=O (16) which was characterized by NMR spectroscopy. Further oxidation of 16 by N2 O or reaction of 12 with two equivalents of pyridine oxide afforded the isolable dioxide (κ2 -NNP)PO2 which was characterized by NMR and SC XRD. Trapping of 16 with tolyl isocyanate resulted in P=O/N=C metathesis, eventually affording a urea-ligated phosphine (κ1 -NNP)P(NTol)2 C=O (17) The mechanism of this reaction was elucidated by DFT calculations. Reactions of phosphinidene 12 with azides generated transient imines (NNP)P=NR, which in the case of R=Tol underwent cycloaddition with tolyl Isocyanate to afford the urea product 17, and in the case of R=SiMe3 reacts with N3 SiMe3 via the addition of N-Si across the P=N bond affording, after the extrusion of dinitrogen, a P,N-heterocyclic compound. Both products of the reactions with azides have been fully characterized, both in solution and the solid-state. Finally, reaction of phosphinidene 12 with one equivalent of sulfur resulted in the isolation of the base-stabilized phosphinidene sulfide (κ2 -NNP)P=S that has also been fully characterized.

Stretched Central Double Bonds in Dialumene and Disilene by Amino Substituents: A Case of Lone Pair Repulsion

There have been remarkable advances in the syntheses and applications of groups 13 and 14 homonuclear ethene analogues. However, successes are largely limited to aryl- and/or silyl-substituted species. Analogues bearing two or more heteroatoms are still scarce. In this work, the block-localized wavefunction (BLW) method at the density functional theory (DFT) level was employed to study dialumene and disilene bearing two amino substituents whose optimal geometries exhibit significantly stretched central M=M (M=Al or Si) double bonds compared with aryl- and/or silyl-substituted species. Computational analyses showed that the repulsion between the lone electron pairs of amino substituents and M=M π bond plays a critical role in the elongation of the M=M bonds. Evidently, replacing the substituent groups -NH2 with -BH2 can enhance the planarity and shorten the central double bonds due to the absence of lone pair electrons in BH2 .

Inorganometallic allenes [(Mn(η5-C5H5)(CO)2)2(μ-E)] (E = Si-Pb): bis-allylic anionic delocalisation similar to organometallic allene but differential σ-donation and π-backdonation

The chemistry of heavy group-14 tetrel atoms is known to diverge from that of the lighter congener carbon. Here, we report the structure and bonding in inorganometallic allenes [(MnCp(CO)2)2(μ-E)] (2E, E = Si-Pb; Cp = η5-C5H5). These inorganometallic allenes are structurally similar to the lighter organometallic analog [(MnCp(CO)2)2(μ-C)] (2C). The bonding analysis of these compounds at the M06/def2-TZVPP//BP86/def2-SVP level of theory identifies a linear Mn-E-Mn spine with delocalised, mutually orthogonal π-systems across this back-bone. This results in a bis-allylic anionic bonding scenario. However, the strength of the Mn-E bonding is found to be weaker in these inorganometallic allenes. The energy decomposition analysis at the BP86/TZ2P//BP86/def2-SVP level of theory further reveals that the bonding in these compounds cannot be represented by one unique heuristic bonding model, but multiple bonding models. For all 2E (E = C-Pb), the Dewar-Chatt-Duncanson bonding model is one of the best bonding representations, where the central tetrel atom acts as a 4e- σ-donor and 4e- π-acceptor. The bonding analysis indicates that the carbon atom in the organometallic allene acts as a better π-acceptor than σ-donor, while the heavier tetrel atoms in the inorganometallic allenes are better σ-donors than π-acceptors. The npz-orbital is found to be a better σ-donor than the valence ns-orbital. However, when the bonding representation is changed to a traditional electron-sharing model, the contribution from the ns-orbital was found to be the largest in comparison to the interaction from the remaining three valence np-orbitals. It can be suggested that the ns-orbitals contribute more towards chemical bonding when participating via an electron-sharing interaction than a donor-acceptor interaction.

Room temperature stable E,Z-diphosphenes: their isomerization, coordination, and cycloaddition chemistry

E,Z-isomers display distinct physical properties and chemical reactivities. However, investigations on heavy main group elements remain limited. In this work, we present the isolation and X-ray crystallographic characterization of N-heterocyclic vinyl (NHV) substituted diphosphenes as both E- and Z-isomers (L[double bond, length as m-dash]CH-P[double bond, length as m-dash]P-CH[double bond, length as m-dash]L, E,Z-2b; L = N-heterocyclic carbene). E-2b is thermodynamically more stable and undergoes reversible photo-stimulated isomerization to Z-2b. The less stable Z-isomer Z-2b can be thermally reverted to E-2b. Theoretical studies support the view that this E ↔ Z isomerization proceeds via P[double bond, length as m-dash]P bond rotation, reminiscent of the isomerization observed in alkenes. Furthermore, both E,Z-2b coordinate to an AuCl fragment affording the complex [AuCl(η2-Z-2b)] with the diphosphene ligand in Z-conformation, exclusively. In contrast, E,Z-2b undergo [2 + 4] and [2 + 1] cycloadditions with dienes or diazo compounds, respectively, yielding identical cycloaddition products in which the phosphorus bound NHV groups are in trans-position to each other. DFT calculations provide insight into the E/Z-isomerisation and stereoselective formation of Au(i) complexes and cycloaddition products.

π-Aromaticity Dominating in a Saturated Ring: Neutral Aromatic Silicon Analogues of Cyclobutane-1,3-diyls

The synthesis, structures, and reactivity of the first neutral 2π-aromatic Si4 rings [LSiSiAr(X)]2 (3: X = Br; 4: X = Cl; L = PhC(NtBu)2, Ar = 2,4,6-Me3C6H2) were described. Compounds 3 and 4 were obtained by 1,3-halogenation of tetrasilacyclobutadiene (LSiSiAr)2 (2), which was prepared by the reductive cross-coupling of trisilane (ArSiCl2)2SiHAr with two equiv of chlorosilylene LSiCl. The reaction of 3 with two equiv of PhLi yielded the corresponding substitution Si4 ring [LSiSiAr(Ph)]2 (5). Single-crystal X-ray diffraction analysis of 3 disclosed that it adopts both puckered (3a) and planar (3b) structures in the solid state, whereas 4 and 5 exhibit only a puckered structure. DFT calculations suggested that the puckered 3a features almost the same electronic structure with fully delocalized 2π planar 3b. The dominant 2π-aromaticity of 3 in a σ-frame has been demonstrated by DFT calculations, providing the first example of aromatics featuring both planar and puckered structures.

2H-Phosphindole-Enabled Dearomatization and [4+2] Cycloaddition of (Hetero)Arenes

The heavier main group multiple bonds offer an effective tool for small molecule activation. Transient 2H-phosphinidole working as a reactive phosphadiene system undergoes phospha-Diels-Alder reaction with a wide range of non-activated aromatic carbocycles and heterocycles, including naphthalene, anthracene, phenanthrene, furan, thiophene, pyrrole, pyridine, and benzo-fused heterocycles, affording concise access to a range of polycyclic fused rings feature with phosphorus at the bridgehead. These results demonstrate that non-activated (hetero)arenes are capable of acting as 2π systems in [4+2] cycloaddition with highly reactive 2H-phosphindole complex.

A General Pathway towards NHC ⋅ GaH2 (OTf) Adducts - The Key for the Synthesis of NHC-Stabilized Cationic 13/15 Chain Compounds of Gallium

A general pathway towards NHC (NHC=N-heterocyclic carbene)-stabilized galliummonotriflates NHC ⋅ GaH2 (OTf) (NHC=IDipp, 1 a; IPr2 Me2 , 1 b; IMes, 1 c; IDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene, IPr2 Me2 =1,3-bis-(diisopropyl)-4,5-dimethyl-imidazolin-2-ylidene, IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene) is reported. Quantum chemical calculations give detailed insight into the underlying reaction pathway. The obtained NHC ⋅ GaH2 (OTf) compounds were employed in reactions with donor-stabilized pnictogenylboranes to synthesize the elusive cationic parent 13/15/13 chain compounds [IDipp ⋅ GaH2 ER2 E'H2  ⋅ D][OTf] (3 a: D=IDipp, E=P, E'=B, R=H; 3 b: D=NMe3 , E=P, E'=B, R=H, 3 c: D=NMe3 , E=P, E'=B, R=Ph, 3 d: D=IDipp, E=P, E'=Ga, R=H). Supporting computational studies highlight the electronic features of the products.

The First Planar, Not Twisted, Distannene - A Structural Alkene Analog. Synthesis, Isolation and X-ray Crystallography Characterization

The tetrasilyl-substituted distannene, (tBu2 HSi)2 Sn=Sn(SiHtBu2 )2 6, was synthesized by mild thermolysis (70 °C in hexane) of tris(di-tert-butyl-hydridosilyl)stannane 4. The X-ray crystallography structure of 6 reveals the following unusual structural properties: a planar geometry around both Sn atoms (Σ∡Sn=359.87°), a non-twisted Sn=Sn double bond, and the shortest Sn=Sn double bond of 2.599 Å among all acyclic distannenes. Thus, compound 6 is the first reported distannene having a structure closely analogous to a classic alkene. Reactions of 6 with CCl4 or with 2,3-dimethylbuta-1,3-diene to produce 1,2-dichlorodistannane 9 and the [2+4] cycloadduct 10, respectively, are characteristic for a Sn=Sn double bond.

Synthesis and Characterization of Substituted Phosphasilenes and its Rare Homologue Stibasilene >Si=Sb

Herein we report the reduction of R-EX2 (E=P, Sb) with two equivalents of KC8 in the presence of silylene (LSiR; L=PhC(NtBu)2 ) to give Trip-P=SiL(C6 H4 PPh2 ) (1), Ter Ph-P=(tBu)SiL (2) and Ter Ph-Sb=(tBu)SiL (3). The last (3) belongs to a new class of heavier analogues of Schiff bases (>C=N-), containing a formal >Si=Sb- double bond. The theoretical calculations suggest that lone pairs on the dicoordinated group-15 centers are stabilized by hyperconjugative interactions resulting in pseudo-Si-P/Si-Sb multiple bonds which are highly reactive as indicated by the high first and second proton affinities.

Coinage metal complexes of BN analogues of m-terphenyl ligands

We report the synthesis of a series of group 11 metal complexes with sterically demanding anionic nitrogen ligands based on the 1,2-azaborinine motif. The ligands, which share structural similarities with m-terphenyls, have been used to stabilize two-coordinate phosphine complexes and dimeric complexes with close contacts between the metal centers. Spectroscopic, crystallographic, and theoretical investigations reveal close parallels to the related m-terphenyl complexes, including metallophilic interactions in the dimers.

London Dispersion Effects on the Stability of Heavy Tetrel Molecules

London dispersion (LD) interactions, which stem from long-range electron correlations arising from instantaneously induced dipoles can occur between neighboring atoms or molecules, for example, between H atoms within ligand C-H groups. These interactions are currently of interest as a new method of stabilizing long bonds and species with unusual oxidation states. They can also limit reactivity by installing LD enhanced groups into organic frameworks or ligand substituents. Here, we address the most recent advances in the design of LD enhanced ligands, the sterically counterintuitive structures that can be generated and the consequences that these interactions can have on the structures and reactivity of sterically crowded heavy group 14 species.

Photochemical formation and reversible base-induced cleavage of a phosphagallene

The reactivity of Cp*Ga (Cp* = C₅Me₅) towards phosphanylidenephosphoranes of the type ^ArTerP(PMe₃) (^ArTer = ^DipTer 2,6-(2,6-iPr₂C₆H₃)₂C₆H₃), ^TipTer 2,6-(2,4,6-iPr₃C₆H₂)₂C₆H₃ was investigated. While no thermal reaction was observed (in line with DFT results), irradiation at 405 nm at low temperatures resulted in the formation of phosphagallenes ^DipTerP = GaCp* (1a) and ^TipTerP = GaCp* (1b) accompanied by release of PMe₃. When warming the reaction mixture to ambient temperatures without irradiation, the clean re-formation of ^ArTerP(PMe₃) and Cp*Ga in a second-order reaction was observed. Upon removal of PMe₃, 1a and 1b were isolated and fully characterized. Both derivatives were found to be labile and decomposed to the phosphafluorenes 2a and 2b, indicating generation of the transient phosphinidene ^ArTerP along with Cp*Ga. First reactivity studies show that CO₂ and H₂O cleanly reacted with 1a, affording ^DipTerPCO (3) and ^DipTerPH₂ (4), 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.

Excited state energy landscape of phosphorescent group 14 complexes

Great progress has been achieved on phosphorescent or photoactive complexes of the Earth-abundant transition metals, while examples for phosphorescent heavy main group element complexes are rare, in particular for group 14 complexes in the oxidation state +II. The known compounds often show only weak phosphorescence with fast non-radiative deactivation. The underlying photophysical processes and the nature of the phosphorescent electronic states have remained essentially unexplored. The present combined photophysical and theoretical study on tin(ii) and lead(ii) complexes E(bpep) with the dianionic tridentate ligand bpep^2- (E = Sn, Pb; H₂bpep = 2-[1,1-bis(1H-pyrrol-2-yl)ethyl]pyridine) provides unprecedented insight in the excited state energy landscape of tetrel(ii) complexes. The tin complex shows green intraligand charge transfer (ILCT) phosphorescence both in solution and in the solid state. In spite of its larger heavy-atom effect, the lead complex only shows very weak red phosphorescence from a strongly distorted ligand-to-metal charge transfer (LMCT) state at low temperatures in the solid state. Detailed (TD-)DFT calculations explain these observations and delineate the major path of non-radiative deactivation via distorted LMCT states. These novel insights provide rational design principles for tetrel(ii) complexes with long-lived phosphorescence.