Oxidative Addition and Reductive Elimination at Main-Group Element Centers

Oxidative addition of diaryldichalcogenides to the diferrocenylphosphenium ion: synthesis, structure and organocatalytic activity

The reaction of the phosphenium ion [Fc2P]+ with dichalcogenides gace rise to the respective phosphonium ions [Fc2P(ChR)2]+ (Ch = S, Se, Te; R = Ph, Fc, biphen), which were employed in chalcogen bond mediated Michael additions.

Assembly of Functionalized Organic Fragments via Reductive Activation and (Cross)-Coupling of C2H4, CO, CO2 and/or H2 Using a Magnesium-Dinitrogen Complex

Reactions of 1,2-dimagnesioethane compound [{K(TCHPNON)Mg}2(μ-C2H4)] (TCHPNON = 4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene), formed by the two-electron reduction of ethene with a dimagnesium/dipotassium complex of reduced N2, viz. [{K(TCHPNON)Mg}2(μ-N2)], with CO and CO2 have been explored. In the case of the reaction with CO, cross-coupling of the reduced ethene fragment with two molecules of CO gave a heterobimetallic complex of the parent cyclobutenediolate dianion, [{K(TCHPNON)Mg}2(μ-O2C4H4)], which when exposed to THF gave adduct [{K(TCHPNON)Mg}2(μ-O2C4H4)(THF)]. Treating [{K(TCHPNON)Mg}2(μ-C2H4)] with CO2 led to the insertion of CO2 into both Mg─C bonds and all Mg─N bonds, yielding a magnesium succinate complex, [{K(TCHPNON-C2O4)Mg}2(μ-O4C4H4)], in which the diamide ligands have been converted to xanthene bridged dicarbamates. The reaction of [{K(TCHPNON)Mg}2(μ-N2)] with CO2, proceeded via reductive coupling of the heterocumulene to give the oxalate dianion, in addition to the insertion of CO2 into all Mg─N bonds of the magnesium-dinitrogen complex, forming dimeric [{K(TCHPNON-C2O4)Mg}2(μ-O4C2)]2. When treated with THF this yields monomeric [{K(THF)(TCHPNON-C2O4)Mg(THF)}2(μ-O4C2)]. Related chemistry results from the reaction of a dianionic magnesium(I) compound with CO2. In contrast, C─C bond formation was not observed in the reaction of [{K(TCHPNON)Mg}2(μ-N2)] with a CO2 analog, i.e., the carbodiimide CyNCNCy (Cy = cyclohexyl). Instead, H abstraction by a proposed radical intermediate gave polymeric formamidinate complex [K(TCHPNON)Mg{(CyN)2CH}]∞. Reaction of CO2 with the magnesium hydride complex [{K(TCHPNON)Mg(μ-H)}2] gave the unusual trimeric magnesium formate complex [{K(TCHPNON-CO2)Mg}(μ-O2CH)]3 in which CO2 has inserted into only one Mg─N bond of each TCHPNON ligand. This study highlights the capacity of [{K(TCHPNON)Mg}2(μ-N2)] to act as a masked dimagnesium(I) diradical in reductive coupling or cross-coupling of the simple gaseous reagents, C2H4, CO, CO2 and H2, to give value-added organic fragments.

Singlet Carbenes Are Stereoinductive Main Group Ambiphiles

Stereogenic units are a critical source of molecular complexity, but their stereoselective formation via main group ambiphiles─which are suitable for derivatizing a wide scope of functionalities─is largely unexplored. Herein, using chiral cyclic (alkyl)(amino)carbenes (ChiCAACs), we study stereoinduction during the oxidative addition of E-H σ-bonds (E = C, N, O, Si, P). Through computational modeling, the relationship between stereochemical outcome and mechanism is elucidated, providing insight into when and why ChiCAACs exhibit excellent stereoselectivities. Altogether, these results demonstrate the potential for chiral main group ambiphiles to generate stereogenic units in a highly controlled manner opening avenues for applying "metal-like" reactivity in metal-free asymmetric syntheses.

Organotin(IV) compounds catalyzed cyanide-free synthesis of α-iminonitriles

Two polydentate pro-ligands (H2L1 and H3L2) have been reacted with different organotin(IV) halides such as Ph2SnCl2, (t-Bu)2SnCl2, and (n-Bu)2SnCl2 to synthesize six organotin(IV) compounds, [R2Sn(L1)] (R = Ph (1), t-Bu (2), n-Bu (3)) and [R2Sn(HL2)] (R = Ph (4), t-Bu (5), n-Bu (6)), respectively. All organotin(IV) compounds were characterized by FT-IR spectroscopy, 1H, 13C{1H}, and 119Sn{1H} NMR spectroscopy, HR-MS spectrometry, and single-crystal X-ray diffraction analysis. The single-crystal X-ray diffraction analyses reveal that all compounds contain a penta-coordinated tin atom except 1. Compound 1 is hexacoordinated. All organotin compounds show catalytic efficiency towards the synthesis of α-iminonitriles, with a maximum yield of up to 88%. The α-iminonitriles are synthesized from trans-β-nitrostyrene derivatives and 2-aminopyridine derivatives without using any cyanating agent.

Advancing metallomimetic catalysis through structural constraints of cationic PIII species

In recent years, the concept of structural constraints on the main-group (MG) centers has emerged as a powerful strategy to enhance their reactivity. Among these, structurally constrained (SC) phosphorus centers have garnered significant attention due to their ability to cycle between two stable oxidation states, P(III) and P(V), making them highly promising for small molecule activation and catalysis. Structural constraints grant phosphorus centers transition metal (TM)-like reactivity, enabling the activation of small molecules by these SC P(III) centers, a reactivity previously inaccessible with conventional phosphines or other phosphorus derivatives. This feature article reviews recent advances in the chemistry of cationic, structurally constrained P(III) (CSCP) compounds, emphasizing their ability to mimic TM behavior in small-molecule activation and catalysis, particularly through the key elementary steps of TM-based catalysis, such as oxidative addition (OA), migratory insertion (MI), ligand metathesis (LM), reductive elimination (RE), etc. The development of these SC cationic P(III) species highlights the interplay between structural constraints and cationic charge, facilitating analogous metallomimetic reactivity in other main-group elements.

GeCl2-Mediated Ring Contraction toward Endofullerenes

Germanium(II) dichloride dioxane complex (GeCl2·dioxane) is often utilized as a source of molecular germylene, i.e., GeCl2, which is known to undergo oxidative 1,4-addition to conjugated substrates. Inspired by this nature resembled to carbenes, we demonstrated a C═C bond formation from two carbonyl moieties engaged in an aromatic macrocycle. This germylene-mediated reaction enables us to realize efficient synthesis of endo[60]fullerenes through the consecutive ring contraction of open-[60]fullerenes.

Divergent reduction chemistry of NHC-aluminium(III) hydrides

Understanding and controlling facile reduction chemistry is a key challenge in molecular main group chemistry. Herein, we report the divergent reduction chemistry of aluminium(III) hydrides supported by N-heterocyclic carbenes (NHCs). Choice of reducing agent and NHC ligand are key, with Al(II) dialanes, Al(II) cations, asymmetric Al(II) dialanes and ligand exchange reactions all identified via NMR and single crystal X-ray diffraction.

The Lewis Acidity of Bare and Crown Ether-Complexed Germanium(II) and Tin(II) Cations

The Gutmann-Beckett and Fluoride Ion Affinity methods were used to assess the Lewis acidity of a variety of dicationic germanium(II) and tin(II) crown ether complexes and the corresponding neutral halides. The coordination of two or more equivalents of triethylphosphine oxide (TEPO) was observed which was accompanied by full or partial replacement of the crown ether or chloride ligands from the metal centre illustrating the importance of unambiguously identifying the species in solution to enable a meaningful discussion of relative Lewis acidities. From the coordination complexes observed, the germanium(II) centre was found, in general, to be more Lewis acidic than the tin(II) centre. The crown ether ligands, when retained, had little influence on the Lewis acidity of the complex and may, for ease of synthesis, be used as convenient precursors to "bare" Ge(II) and Sn(II) dicationic catalysts.

The Outstanding Ambiphilicity of Trialkylstibines among Trialkylpnictines: Power for Stepwise Deoxygenation and N-N Coupling of Nitroarenes

The ongoing discovery of highly reactive ambiphilic main-group species has significantly advanced the development of main-group chemistry, particularly in the realms of small molecule activation and catalysis. Theoretically, compounds featuring smaller HOMO-LUMO gaps gain stronger ambiphilicity and higher reactivity. In this work, we fundamentally demonstrate that Me3Sb holds the smallest HOMO-LUMO gap among trimethylpnictines, indicating its outstanding ambiphilicity. Correspondingly, the superior reactivity of Me3Sb toward deoxygenation of electron-deficient nitroarenes has been unambiguously revealed through control experiments. Furthermore, unprecedented SbIII/SbVO cycling between trialkylstibines and their oxides has been established for the catalytic transformation of nitroarenes into azoxyarenes/azoarenes. This study opens a new chapter for organoantimony derivatives in the fields of ambiphilic reactivity and redox catalysis.

Carbene-activated stannylenes to access selective C(sp3)-H bond scission at the steric limit

The ubiquity of N-heterocyclic carbenes (NHCs) in diverse areas of chemical research typically arises from their potent stabilising capabilities and role as innocent spectators to stabilise otherwise non-bottleable compounds and complexes. This has, until now, been particularly true for NHC-stabilised stannylenes, with no exceptions reported thus far. Herein, we demonstrate that the combination of heteroleptic terphenyl-/amido-based stannylenes and tetra-alkyl substituted NHCs renders the corresponding NHC-ligated stannylenes highly reactive, yet isolable. In solution, this induces sterically controlled inter- and intramolecular C(sp3)-H bond scissions, resulting in the selective formation of stannylene metallocycles that depend on both the NHC source and the meta-terphenyl ligand coordinated to tin.

Synthesis and Reactivity of Stable Open-Shell Gallylene

Treatment of an excess of gallium metal with iodine and 1,2-bis[(2,6-dibenzhydryl-4-methylphenyl)imino]acenaphthene (ArBIG-bian) in toluene at reflux affords the deep green radical [ArBIG-bianGaI2] (1). Product 1 can also be synthesized by reacting ArBIG-bian with "GaI" in toluene. The chloride and bromide derivatives, [ArBIG-bianGaCl2] (2) and [ArBIG-bianGaBr2] (3), were prepared by the reactions of corresponding gallium(III) halides with ArBIG-bian in the presence of an excess of gallium metal in toluene. The reduction of radical 1 by potassium metal in toluene produces stable gallylene with a paramagnetic ligand [ArBIG-bianGa] (4). The reaction of 4 with n-propyl bromide (1:1) affords gallium(III) derivatives, 3 and [ArBIG-bianGanPr] (5). In the course of the reaction of 4 with tetramethylthiuram disulfide (2:1), the gallium(I) center transfers two electrons: one to metallacycle and another one to the S-S bond of the substrate. The resulting product is the gallium(III) derivative [ArBIG-bianGaS2CNMe2] (6). Complexes 1-6 have been characterized by elemental analysis and IR spectroscopy, and their molecular structures have been determined by single-crystal X-ray analysis. The paramagnetic complexes 1-4 have been characterized by ESR spectroscopy and the diamagnetic compounds 5 and 6 by NMR spectroscopy. Based on DFT calculations, the electronic structures of molecules 2-5 and the thermodynamics of reactions accompanying the interaction of 4 with n-propyl bromide were studied. The computational results confirm the localization of the radical center in molecules 2-4 on the bian fragment and visualize the gallium lone pair directed along the NGaN bisector in 4. Thermodynamically favorable reactions leading to the 3 and 5 products have been identified.

Isolation and Reactivity of Carbene-Stabilized Carbon Disulfide Radical Anions

The reaction of CAAC-CS2 betaine (1; CAAC = cyclic(alkyl)(amino)carbene) and alkali metal reductants under ambient conditions yields carbene-stabilized carbon disulfide radical anions as crystalline alkali metal salts. The radicals 3-5 form multinuclear clusters featuring diverse metal sulfide and disulfide interactions, which promote unusual reductive coupling and cyclization of adjacent CS2 units to C2S3 heterocycles (6). The addition of crown ethers to 3-5 sequesters the alkali cations and facilitates disulfide cleavage to yield stable [CAAC-CS2]·- monomers (7 and 8). Calculated natural atomic spin populations suggest that the spin densities in the clustered and monomeric species are comparable and evenly distributed between the CAAC and CS2 subunits. Subsequent reductions afford [CAAC-CS2]2- dianions (9-12), which can be reoxidized to radicals by comproportionation reactions with 1. The radicals are, in turn, oxidized to betaine 1 through salt elimination reactions with transition metals. Cyclic voltammograms of 1 feature reversible 1/1·-/12- couples with a small separation between the events (ΔΔG = 11.1 kcal mol-1). All isolated compounds were characterized by a combination of electron paramagnetic resonance spectroscopy, heteronuclear NMR spectroscopy, infrared spectroscopy, and single-crystal X-ray diffraction. Insights into their electronic structure are supported by density functional theory calculations.

Hydride Rebound: A Frustrated Lewis Pair (FLP)-Type Cooperative Mechanism for H2 Activation by a Potassium Aluminyl Compound

Combining experiment and theory, the mechanisms of H2 activation by the potassium-bridged aluminyl dimer K2[Al(NON)]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tertbutyl-9,9-dimethylxanthene) and its monomeric K+-sequestered counterpart have been investigated. These systems show diverging reactivity towards the activation of dihydrogen, with the dimeric species undergoing formal oxidative addition of H2 at each Al centre under ambient conditions, and the monomer proving to be inert to dihydrogen addition. Noting that this K+ dependence is inconsistent with classical models of single-centre reactivity for carbene-like Al(I) species, we rationalize these observations instead by a cooperative frustrated Lewis pair (FLP)-type mechanism (for the dimer) in which the aluminium centre acts as the Lewis base and the K+ centres as Lewis acids. In contrast to previous theoretical work on this precise system by Schaefer and co-workers, the potassium ions are shown to play explicit roles in stabilizing a nascent 2-bridging hydride, formed by heterolytic H-H bond cleavage (with accompanying protonation of the aluminium-centred lone pair). K-to-Al hydride "rebound" into the vacant aluminium-centred p-orbital then completes the net addition of H2 via sequential H+/H- transfer. The experimentally determined kinetic isotope effect (kH/kD=2.6) reflects a high degree of bond activation in the transition state (as predicted quantum chemically).

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.

Facile, Reversible Hydrogen Activation by Low-Coordinate Magnesium Oxide Complexes

New approaches to achieve facile and reversible dihydrogen activation are of importance for synthesis, catalysis, and hydrogen storage. Here we show that low-coordinate magnesium oxide complexes [{(RDipnacnac)Mg}2(μ-O)] 1, with RDipnacnac = HC(RCNDip)2, Dip = 2,6-iPr2C6H3, R = Me (1a), Et (1b), iPr (1c), readily react with dihydrogen under mild conditions to afford mixed hydride-hydroxide complexes [{(RDipnacnac)Mg}2(μ-H)(μ-OH)] 4. Dehydrogenation of complexes 4 is strongly dependent on remote ligand substitution and can be achieved by simple vacuum-degassing of 4c (R = iPr) to regain 1c. Donor addition to complexes 4 also releases hydrogen and affords donor adducts of magnesium oxide complexes. Computational studies suggest that the hydrogen activation mechanism involves nucleophilic attack of an oxide lone pair at a weakly bound H2···Mg complex in an SN2-like manner that induces a heterolytic dihydrogen cleavage to yield an MgOH and an MgH unit. Alternative synthetic routes into complex 4b from a magnesium hydride complex have been investigated and the ability of complexes 1 or 4 to act as catalysts for the hydrogenation of 1,1-diphenylethene (DPE) has been tested.

Terphenyl-Ge as μ2-Ge-bis(hexahapto-Trip) Bridging Ligand to Form a New Transition Metal-Only Chelating Lewis Base

Terphenylgermanium Ar*Ge [Ar*=C6H3(2,6-Trip)2, Trip=2,4,6-C6H2iPr3] was found to act as a novel μ2-Ge-bis(hexahapto-Trip) bridging ligand. Deprotonated terphenyl germanium trihydride [Li(thf)3][Ar*GeH2] (1) undergoes reductive elimination and transfer of hydrogen in reaction with dimeric [(COD)RhCl]2 to yield the dinuclear complex [Ar*GeRh(COE)RhCl(COD)] (2). Subsequent chloride abstraction from compound 2 using Na[BArF 4] or Li[Al(OtBuF)4] results in the cationic complexes [Ar*GeRh(COE)Rh(COD)][WCA] (3) {WCA: [BArF 4]- (ArF=C6H3-3,5-(CF3)2), [Al(OtBuF)4]-}. Ligand exchange of olefin for CO yields the carbonyl complex [Ar*Ge(Rh(CO))2][BArF 4] (4). In an alternative approach to the synthesis of carbonyl complex 4, [Li(thf)3][Ar*GeH2] (1) was treated with [Rh(CO)2Cl]2 leading to the isolation of a hexanuclear rhodium cluster [(μ3-Ar*Ge)2{Rh(CO)2}6(μ3-H)2] (5) in reasonable yield. In reactions with [Rh(CO)2Cl]2 or [Ph3PAuCl] complex 4 abstracts the chloride ligand and forms tetranuclear complexes featuring a GeRh3- or GeRh2Au-rectangle, [Ar*GeCl(Rh3(CO)4)][BArF 4] (6), or [Ar*GeCl{Rh2(CO)2}(AuPPh3)][BArF 4] (7).

Carbon-carbon bond formation and cleavage at redox active bis(pyridylimino)isoindole (BPI) germylene compounds

Redox-active ligands provide alternative reaction pathways by facilitating redox events. Among these, tridentate bis(piridylimino)isoindole (BPI) fragments offer great potential, though their redox-active behaviour remains largely underdeveloped. We describe herein a family of BPI germanium(II) complexes and the study of their redox properties. Amido complexes (RBPI)Ge[N(SiMe3)2] 1-R (R = H, Me, Et) showing a κ2-Npy,Niso coordination to the germanium(II) centre were prepared. In contrast, chloride derivatives (RBPI)GeCl, 2-R, display dynamic κ3-Npy,Niso,Npy coordination to the metal centre. The addition of silver bis(trifluoromethane)sulfonimide to compound 1-H generates a dinuclear complex, 3, where the silver atoms are bound to the germanium and one of the imine nitrogen atoms of another BPI fragment. The reduction of 2-R with KC8 generates dinuclear complexes (4-R) characterized by the formation of new C-C bonds between the isoindoline five-membered rings of two different BPI ligands via a radical mechanism, a transformation that does not take place in the absence of germanium. Interestingly, computational and spectroscopic studies support that the reduction takes place exclusively over the RBPI ligand. Strikingly, the newly formed C-C bond is also readily cleaved. Thus, subsequent reduction of 4-R (R = H, Me) using additional KC8 affords dinuclear species 5-R, with polymeric structures between potassium atoms and the corresponding dinuclear Ge2(RBPI2) fragments.

Sb-to-P Metathesis: A Direct Route to Structurally Constrained, Cationic PIII Compound

Structurally constrained, cationic PIII compound [LP][SbCl4] with an OCO pincer-type ligand (L) having a central carbene donor was directly synthesized via an Sb-to-P metathesis reaction between PCl3 and LSb-Cl. [LP][SbCl4] was isolated and its reactivity with small molecules (ROH and RNH2) was studied, showing that [SbCl4]- is not an innocent counter anion, but an active participant in these reactions. When the [SbCl4]- was replaced with the [CB11H12]- ([Cb]-) anion, the reactions were redirected to [LP]+ cation only. The reactions with alcohols and amines led to the equilibrium between the products of the formal E-H (E=O, N) bond oxidative addition to the P-center and products of the P-center/ligand-assisted bond activation. Remarkably, [LP]+ activated the PhO-H and PhN(H)-H bonds in a reversible, thermoneutral fashion.

Understanding the Reactivity of "Naked" Acyclic Carbene-like Species Featuring a Group 13 Element in the [4+1] Cycloaddition with Benzene

The chemical reactivity between benzene and the "naked" acyclic carbene-like (G13X2)- species, having two bulky N-heterocyclic boryloxy ligands at the Group 13 center, was theoretically assessed using density functional theory computations. Our theoretical studies show that (BX2)- preferentially undergoes C-H bond insertion with benzene, both kinetically and thermodynamically, whereas the (AlX2)- analogue favors a reversible [4 + 1] cycloaddition. Conversely, the heavier carbene analogues ((GaX2)-, (InX2)-, and (TlX2)-) are not expected to engage in a reaction with benzene. The activation strain model analysis suggests that the geometric deformation energy of benzene, driven by the relativistic effects of the central G13 element in the (G13X2)- molecule, is crucial in determining the chemical reactivity of the [4 + 1] cycloaddition with benzene. According to our theoretical analyses, the stronger forward bonding is specifically the sp2-σ-orbital (G13) → vacant protruding p-π* orbitals (bent benzene). In contrast, the weaker backward bonding is the empty p-π orbital (G13) ←-filled protruding p-π orbital (bent benzene). Moreover, our theoretical findings indicate that the singlet-triplet splitting of (G13X2)- can be used as a diagnostic measure to predict the barrier height and reaction energy for their [4 + 1] cycloaddition with benzene.

Redox Cycling with Tellurium. Si-H Bond Activation by a Lewis Superacidic Tellurenyl Cation

The C,N-chelated aryltellurenyl triflate [2-(tBuNCH)C6H4Te][OTf] (1) activates the Si-H bonds in the tertiary silanes R3SiH via umpolung of H- to H+ to give rise to the iminium salts (tBuN(H)CH)C6H4TeSiR3][OTf] (2R, R=Et, Ph (elusive) and R=Si(CH3)3 isolated; OTf=O3SCF3) comprising Te-Si bonds, which are capable of generating silyl triflates, R3SiOTf, under attack of a second equivalent of 1. The unprecedented Si-H activation was utilized in main group redox catalysis using p-quinones, which were converted into (silylated) hydroquinones.

Borenium Ion Equivalents Stabilized by BICAAC and Their Implementation as Catalysts in Hydrosilylation of Carbonyls

Hydride abstraction from the borane adduct, (BICAAC) ⋅ BH3 afforded the hydride bridged dinuclear borenium ion equivalent complexes 1 and 2 that have been characterized by various spectroscopic and spectrometric techniques followed by the assessment of Lewis acidity using the Gutmann-Beckett method. The single crystal X-ray structure of complex 2 revealed the presence of discrete ions in the solid state. The complex (BICAAC) ⋅ BH2(OTf) (3), obtained from the reaction of (BICAAC) ⋅ BH3 with MeOTf, formed the corresponding boronium cations [(BICAAC) ⋅ BH2(L)]+(OTf)- on reaction with Lewis bases (L= pyridine (4) and DMAP (5)). Complexes 1 and 2 demonstrated notable catalytic activity in the hydrosilylation of a diverse array of carbonyls using 1.0 mol % catalyst loading (achieving the highest turnover frequency (TOFmax) of up to 1200 h-1 with benzaldehyde. A broad substrate scope has been presented for aldehydes and ketones decorated with various electron-donating and withdrawing substituents along with this the hydrosilylation of a few para-quinone methides has also been presented.

Terminal Alkyne Activation by an Al(I)-Centered Anion: Impact on the Mechanism of Alkali Metal Identity

The group 1 alumanyls, [{SiNDipp}AlM]2 (M = K, Rb, Cs; SiNDipp = {CH2SiMe2NDipp}2), display a variable kinetic facility (K < Rb < Cs) toward oxidative addition of the acidic C-H bond of terminal alkynes to provide the corresponding alkali metal hydrido(alkynyl)aluminate derivatives. Theoretical analysis of the formation of these compounds through density functional theory (DFT) calculations implies that the experimentally observed changes in reaction rate are a consequence of the variable stability of the [{SiNDipp}AlM]2 dimers, the integrity of which reflects the ability of M+ to maintain the polyhapto group 1-arene interactions necessary for dimer propagation. These observations highlight that such "on-dimer" reactivity takes place sequentially and also that the ability of each constituent Al(I) center to effect the activation of the organic substrate is kinetically differentiated.

Isolation of a NHC-stabilized heavier nitrile and its conversion into an isonitrile analogue

Nitriles (R-C≡N) have been investigated since the late eighteenth century and are ubiquitous encounters in organic and inorganic syntheses. In contrast, heavier nitriles, which contain the heavier analogues of carbon and nitrogen, are sparsely investigated species. Here we report the synthesis and isolation of a phosphino-silylene featuring an N-heterocyclic carbene-phosphinidene and a highly sterically demanding silyl group as substituents. Due to its unique structural motif, it can be regarded as a Lewis base-stabilized heavier nitrile. The Si-P bond displays multiple bond character and a bent R-Si-P geometry, the latter indicating fundamental differences between heavier and classical nitriles. In solution, a quantitative unusual rearrangement to a phosphasilenylidene occurs. This rearrangement is consistent with theoretical predictions of rearrangements from heavier nitriles to heavier isonitriles. Our preliminary reactivity studies revealed that both isomers exhibit highly nucleophilic silicon centres capable of oxidative addition and coordination to iron tetracarbonyl.

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.

Synthesis and redox catalysis of Carbodiphosphorane ligated stannylene

Heavier group 14 carbene analogues, exhibiting transition-metal-like behavior, display remarkable capability for small molecule activation and coordination chemistry. However, their application in redox catalysis remains elusive. In this paper, we report the synthesis and isolation of a stannylene with carbodiphosphorane ligand. The nucleophilic reactivity at the divalent tin center is elucidated by computational and reactivity studies. Moreover, this stannylene exhibits catalytic activity in the hydrodefluorination reaction of fluoroarenes. Mechanistic investigations into the elementary steps confirm a SnII/SnIV redox cycle involving C-F oxidative addition, F/H ligand metathesis, and C-H reductive elimination. This low-valent SnII catalytic system resembles the classical transition metal catalysis. Notably, this represents metallomimetic redox catalysis utilizing carbene analogue with heavier group 14 element as a catalyst.

A low-coordinate platinum(0)-germylene for E-H bond activation and catalytic hydrodehalogenation

Pairing transition metals and heavier tetrylenes (Si, Ge, Sn, Pb) holds great potential for cooperative bond activation and catalysis. In this work, we investigate the reactivity of a low-coordinate Pt(0)/Ge(II) system that emerges from the reaction between the monoligated platinum(0) precursor [(PMe2ArDipp2)Pt(olefin)] with germylene dimer [ArDipp2GeCl]2 (where ArDipp2 = C6H3-2,6-(C6H3-2,6-iPr2)2). The resulting complex reveals ability for cooperative bond activation. Stoichiometric reactions with dihydrogen, water, methanol, ammonia and alkynes unveil the formation of Pt(II)-germyl compounds, characterized by distinct isomeric forms, whose flexibility derives from the particularly low-coordination. We explore its catalytic potential in the hydrodehalogenation of aliphatic, aromatic and main-group halides under dihydrogen atmosphere using both thermal and photochemical conditions, demonstrating promising conversions even for more challenging alkyl chlorides.

Multielectron Reduction of Nitrosoarene via Aluminylene-Silylene Cooperation

The cooperative effects of main-group elements pave the way for novel chemical transformations. However, the potential of bimetallic complexes featuring the most abundant aluminum and silicon elements remains largely unexplored. In this study, we present the synthesis and characterization of bis(silylene)-stabilized aluminylene 2. The cooperation between aluminylene and silylene allows for the facile cleavage of the N-O bond in nitrosoarenes, producing an aluminum imide complex 4 and tetracyclic oxazasilaalanes 5 and 6, and also promotes the dearomatization of 2-methylquinoline, yielding a silylalane 7. In addition, 2 is an effective precatalyst for the reductive coupling of nitrosoarenes to azoxyarenes. These results outline an approach for orchestrating aluminum and silicon cooperation to facilitate chemical bond activation.

Reactions of an Anionic Gallylene with Azobenzene or Azide Compounds Through C(sp2)-H and C(sp3)-H Activation

The activation of inert C-H bonds remains a challenge in current chemistry. Here, we report the excellent reactivity of the anionic gallylene species [LGa:][Na(THF)3] (L = [(2,6-iPr2C6H3)NC(CH3)]22-, 1) that allows the selective activation one ortho sp2 C-H bond of several azobenzene and azide derivatives at ambient temperature, with the transfer of the hydrogen atom to one of the nitrogen atoms. The process leads to the formation of the aryl amido products [LGa-κ2N,C-PhNN(H)(p-R-C6H3)][Na(solvent)3] (2, R = H solvent = DME (1,2-Dimethoxyethane); 3, R = -OMe, solvent = DME; 4, R = -NMe2 solvent = THF), [LGa-κ2N,C-(m-CH3-C6H4)NN(H)(m-CH3-C6H3)][Na(15-C-5)2] (5) with new Ga-C and Ga-N bonds. Moreover, 1 is also effective for the C-H activation of two azides RN3 (R = 2,4,6-Me3C6H2 or 2,6-iPr2C6H3), resulting in the formation of gallium amides [LGa(NH-2-(CH2)-4,6-Me2C6H2)][Na(15-C-5)2] (6) and [LGa(NH-2,6-iPr2C6H3)2][Na(THF)5] (7) through intra- or intermolecular sp3 C-H amination. Significantly, these reactions occur for the highly challenging activation of inert C(sp2)-H and C(sp3)-H bonds, thus demonstrating the excellent reactivity of the Ga(I) species 1. The products 2-7 were characterized by X-ray crystallography, 1H and 13C NMR, UV-vis spectroscopy, and density functional theory (DFT) calculations.

Synthesis and Reactivity of an Alumanyl-Tin Species to Form an Al,N-Heteroallene Derivative

The molecular chemistry of metal-metal bonds is crucial for understanding the bonding and reactivity of metal-containing molecules as well as solid metals and their alloys, especially with respect to their application as catalysts for organic transformations and industrial processes. Despite the high efficiency of the recently developed nucleophilic alumanyl anions in the synthesis of diverse Al-metal bonds, the bonding between main group metals in a low oxidation state and the Al atom remains largely unexplored. This paper describes the reaction of an alumanyl anion with a chlorostannylene precursor to afford a compound with an unprecedented covalent Al-Sn bond. The lability of the covalent Al-Sn bond renders it reactive toward the insertion of carbodiimide, N2O, and phenylacetylene. Remarkably, the reaction with benzonitrile furnished an unprecedented Al,N-heteroallene species that contains a linear Al═N═C linkage.

A Masked Boryl-Substituted Oxo-Bridged Bis-Silylene: Synthesis and Reductive-Elimination and Synergistic Oxidative-Addition Reactivity

Controlled oxidation of NHB-stabilized disilyne (NHB)Si ≡ Si(NHB) (1, NHB = [ArN(CMe)2NAr]B, Ar = 2,6-iPr2C6H3) with one equivalent of trimethylamine N-oxide (Me3N+─O-) in dry n-hexane gave oxo-bridged bis-silepin 2 in high yields. DFT calculations disclosed that silepin 2 is only more stable by 13.4 kcal/mol than the corresponding oxo-bridged bis-silylene intermediate 2' (NHB)Si(μ-O)Si(NHB), and 2 was very likely to be formed by the insertion of the two divalent Si atoms into the pendant aryl rings in bis-silylene intermediate 2'. The two silicon atoms in bis-silepin 2 could undergo formal reductive-elimination of the aryl rings and sequential oxidative-insertion reactions with small molecules and organic substrates. Treatment of 2 with H2O, S8, and P4 at 60 °C yielded compounds 3-5 via reductive-elimination of the aryl rings, followed by the sequential oxidative-addition of these molecules at the two Si(II) centers. Similarly, reactions of 2 with PhSiH3, a diphenylalkyne, pyridines, 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe4), Ph2CO, and thiophene yielded the corresponding polycyclic bis-silanes 6-12 via reductive-elimination and oxidative-addition of C-H, Si-H, C≡C, and aromatic C═C, C-S, and C═N bonds at the two Si atoms. These novel reactions indicated the pronounced bis-silylene reactivity of bis-silepin 2, consistent with the low-energy barrier for the interconversion between 2 and 2', as disclosed by DFT calculations.

Uncovering diverse reactivity of NHCs with diazoalkane: C-H activation, C[double bond, length as m-dash]C bond formation, and access to N-heterocyclic methylenehydrazine

N-heterocyclic carbenes (NHCs) have attracted significant attention due to their strong σ-donating capabilities, as well as their transition-metal-like reactivity towards small molecules. However, their interaction with diazoalkanes remains understudied. In this manuscript, we explore the reactivity of a series of stable carbenes, encompassing a wide range of electronic properties, with Me3SiCHN2. 5-SIPr activates the C-H bond of Me3SiCHN2, resulting in the formation of a novel diazo derivative (1), while carbenes such as 5-IPr, 6-SIPr, and diamido carbene yield N-heterocyclic methylenehydrazine derivatives (3, 4, and 8). The reaction of Me3SiCHN2 with 5-I t Bu unexpectedly leads to the formation of a triazole ring linked with the imidazole moiety via a C[double bond, length as m-dash]C double bond (6) alongside the azine product (7). Substituting the diazoalkane with diazoester consistently yields azine derivatives (9-12 and 14). Only in the case of 5-I t Bu, an imidazolium salt with tetrazenide anion (13) was obtained as a side product. The reaction of 4 with HCl resulted in the desilylprotonation to form a salt, 5a, which undergoes deprotonation upon using bases such as Et3N and KHMDS to form N-heterocyclic methylene hydrazine, 5. Theoretical calculations have been conducted to elucidate the diverse mechanisms underlying product formation.

Non-covalent interactions and charge transfer in the CO 2 activation by low-valent group 14 complexes

CONTEXT

The CO2 activation by low-valent group 14 catalysts encompasses the rupture of varied covalent bonds in a single transition state through a concerted pathway. The bond between the central main group atom and the hydride in the complex is elongated to trigger the formation of the C-H bond with CO2 accompanied by the concomitant formation of the E-O bond between the complex and CO2 to lead the corresponding formate product. Prior studies have established that besides the apolar nature of CO2 , its initial interaction with the complex is primarily governed by electrostatic interactions. Notably, other stabilizing interactions and the transfer of charge between catalysts and CO2 during the initial phases of the reaction have been ignored. In this study, we have quantified the non-covalent interactions and charge transfer that facilitate the activation of CO2 by group 14 main group complex. Our findings indicate that electrostatic interactions predominantly stabilize the complex and CO2 in the reactant region. However, induction energy becomes the main stabilizing force as the reaction progresses towards the transition state, surpassing electrostatics. Induction contributes about 50% to the stabilization at the transition state, followed by electrostatics (40%) and dispersion interactions (10%). Atomic charges calculated with the minimal basis iterative stockholder (MBIS) method reveal larger charge transfer for the back-side reaction path in which CO2 approaches the catalysts in contrast to the front-side approach. Notably, it was discovered that a minor initial bending of CO2 to approximately 176 ∘ initiates the charge transfer process for all systems. Furthermore, our investigation of group 14 elements demonstrates a systematic reduction in both activation energies and charge transfer to CO2 while descending in group 14.

METHODS

All studied reactions were characterized along the reaction coordinate obtained with the intrinsic reaction coordinate (IRC) methodology at the M06-2X/6-31 g(d,p) level of theory. Gibbs free energy in toluene was computed using electronic energies at the DLPNO-CCSD(T)/cc-pVTZ-SSD(E) level of theory. Vibrational and translational entropy corrections were applied to provide a more accurate description of the obtained Gibbs free energies. To better characterize the changes in the reaction coordinate for all reactions, the reaction force analysis (RFA) has been employed. It enables the partition of the reaction coordinate into the reactant, transition state, and product regions where different stages of the mechanism occur. A detailed characterization of the main non-covalent driving forces in the initial stages of the activation of CO2 by low-valent group 14 complexes was performed using symmetry-adapted perturbation theory (SAPT). The SAPT0-CT/def2-SVP method was employed for these computations. Charge transfer descriptors based on atomic population using the MBIS scheme were also obtained to complement the SAPT analyses.

Carbodiphosphorane-Activated Distibene and Dibismuthene Dications

Low-valent antimony and bismuth have emerged as novel platforms for achieving reversible small-molecule activation at main-group metals. Although various examples of oxidative addition reactions at monomeric Sb(I) and Bi(I) have been reported, the chemistry of the heavy group 15 Sb(I)═Sb(I)/Bi(I)═Bi(I) double bonds toward small molecules remains largely unexplored. In this study, we present a straightforward synthesis of distibene and dibismuthene dications coordinated with a neutral carbodiphosphorane (CDP) ligand. The nonbonding interactions between the occupied p-orbital at the CDP ligand and the π-bonding orbital of the Sb═Sb/Bi═Bi bonds yield compounds with exceptionally small HOMO-LUMO gaps. In addition, the reduction of steric hindrance compared to known neutral derivatives stabilized with bulky aryl groups allows for better accessibility of the double bonds. This high reactivity is demonstrated in the oxidative addition of distibene to diphenyldisulfide as well as in [2+2] cycloadditions to alkynes. Additionally, the Sb═Sb bond reversibly adds to 2,3-dimethylbutadiene in a [4+2] cycloaddition reaction.

Main-group compounds selectively activate natural gas alkanes under room temperature and atmospheric pressure

Most C-H bond activations of natural gas alkanes rely on transition metal complexes. Activations by using main-group systems have been reported but required heating or photo-irradiation under high atmospheric pressure with rather low regioselectivity. Here we report that Lewis acid-carbene adducts facilely undergo oxidative additions to C-H bonds of ethane, propane and n-butane with high selectivity under room temperature and atmospheric pressure. The Lewis acids can be moved by the addition of a base and the carbene-derived products can be easily converted into aldehydes. This work offers a route for main-group element compounds to selectively functionalise C-H bonds of natural gas alkanes and other small molecules.

Transition Metal Parent Alumylene Complexes: Synthesis, Structures, and XPS Characterization of Aluminum Oxidation State

The first isolation and characterization of transition metal complexes with the parent Al(I)-H unit were achieved in base-stabilized forms. W and Fe complexes, Cp*(CO)n(H)M←:AlH(NHC)2 (NHC = N-heterocyclic carbene, n = 1 or 2), were synthesized in 43-63% yields by the one-step reaction of Cp*M(CO)n(py)Me with H3Al·NHC. The characterization included 1H and 27Al nuclear magnetic resonance (NMR), and infrared (IR) spectroscopic analysis, as well as DFT calculations, which revealed the extremely strong σ-donating ability of the :AlH(NHC)2 ligand, and the highly polarized M(δ-)←:Al(δ+) coordination bonds. The monovalent oxidation state of the Al center of these complexes was confirmed by X-ray photoelectron spectroscopy (XPS). The hydroalumination of carbodiimide and the reduction of CO2 to CO were also demonstrated.

Alumanyl Reduction, Reductive Coupling and C-H Isomerization of Organic Nitriles

The behavior of the potassium alumanyl, [{SiNDipp}AlK]2 ({SiNDipp} = {CH2SiMe2N(Dipp)}2; Dipp = 2,6-i-Pr2C6H3), toward organic nitriles has been investigated. In common with earlier studies of the reactivity of charge neutral Al(I) species with multiply bonded small molecules, it is suggested that the initial step in all the reactions involves [2 + 1] cycloaddition and the generation of an [η2-C=N-Al] alumina azacyclopropane unit. In the cases of o- and m-tolyl-substituted aryl nitriles, this species is too kinetically labile to allow its isolation and undergoes C-C coupling via immediate Al-C/C≡N insertion to yield the alumina diazabutadiene derivatives. In contrast, the increased steric profile of alkyl nitriles imposes a marked influence on the nature of the products formed. Consistent with the proposed sequential pathway, reaction of [{SiNDipp}AlK]2 with t-BuCN provides an isolable alumina cyclopropane species that is kinetically resistant to onward reaction with a further nitrile equivalent. While reduction in the alkyl nitrile steric demands by use of i-PrCN again facilitates C-C bond formation, the crowding of the Al center by the resultant alumina-diazabutadienediide moiety appears to be beyond the limit of kinetic viability, resulting in an unusual 2-fold C-H to N-H isomerization from one of the C-iso-propyl substituents and the isolation of a 1-alumina-2,5-diazabutadiene structure.

Regulating the photoluminescence of aluminium complexes from non-luminescence to room-temperature phosphorescence by tuning the metal substituents

Although luminescent aluminum compounds have been utilized for emitting and electron transporting layers in organic light-emitting diodes, most of them often exhibit not phosphorescence but fluorescence with lower photoluminescent quantum yields in the aggregated state than those in the amorphous state due to concentration quenching. Here we show the synthesis and optical properties of β-diketiminate aluminum complexes, such as crystallization-induced emission (CIE) and room-temperature phosphorescence (RTP), and the substituent effects of the central element. The dihaloaluminum complexes were found to exhibit the CIE property, especially RTP from the diiodo complex, while the dialkyl ones showed almost no emission in both solution and solid states. Theoretical calculations suggested that undesired structural relaxation in the singlet excited state of dialkyl complexes should be suppressed by introducing electronegative halogens instead of alkyl groups. Our findings could provide a molecular design not only for obtaining luminescent complexes but also for achieving triplet-harvesting materials.

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.

Dialumene as a Dimeric or Monomeric Al Synthon for C-F Activation in Monofluorobenzene

The activation of C-F bonds has long been regarded as the subject of research in organometallic chemistry, given their synthetic relevance and the fact that fluorine is the most abundant halogen in the Earth's crust. However, C-F bond activation remains a largely unsolved challenge due to the high bond dissociation energies, which was historically dominated by transition metal complexes. Main group elements that can cleave unactivated monofluorobenzene are still quite rare and restricted to s-block complexes with a biphilic nature. Herein, we demonstrate an Al-mediated activation of monofluorobenzene using a neutral dialumene, allowing for the synthesis of the formal oxidative addition products at either double or single aluminum centers. This neutral dialumene system introduces a novel methodology for C-F bond activation based on formal oxidative addition and reductive elimination processes around the two aluminum centers, as demonstrated by combined experimental and computational studies. A "masked" alumylene was unprecedentedly synthesized to prove the proposed reductive elimination pathway. Furthermore, the synthetic utility is highlighted by the functionalization of the resulting aryl-aluminum compounds.

Mechanochemical Synthesis of a Sodium Anion Complex [Na+(2,2,2-cryptand)Na-] and Studies of Its Reactivity: Two-Electron and One-Electron Reductions

Group 1 metal molecular chemistry is dominated by a +1 oxidation state, while a 0 oxidation state is widespread in the metals. A more exotic, yet still available, oxidation state of group 1 metal is -1, i.e., alkalide. Reported as early as the 1970s, the alkalides appear in every modern inorganic chemistry textbook as an iconic chemical curiosity, yet their reactivity remains unexplored. This is due to their synthetic hurdles. In this work, we report the first facile synthesis of the archetypical alkalide complex, [Na+(2,2,2-cryptand)Na-], which allows us to unveil a versatile reactivity profile of this once exotic species.

Carbene-Decorated Geometrically Constrained Borylenes for Bond Activations

While metal-ligand cooperativity is well-known, studies on element-ligand cooperativity involving main group species are comparatively much less explored. In this study, we computationally designed a few geometrically constrained borylenes supported by different carbenes. Our density functional theory studies indicate that they possess enhanced nucleophilicity as well as electrophilicity, thus rendering them promising candidates for exhibiting borylene-ligand cooperativity. The cooperation between the boron and adjacent carbene centers facilitates different bond activation processes, including the cycloaddition of acetylene across the boron-carbene bond as well as B-H/Si-H bond activation reactions, which have been analyzed in detail. To the best of our knowledge, the borylenes proposed in this study represent the first examples of theoretically proposed geometrically constrained bis(carbene)-stabilized borylenes capable of cooperative activation of enthalpically strong bonds.

Synthesis, Isolation, and Reactivity Studies of 'Naked' Acyclic Gallyl and Indyl Anions

By exploiting the electronic capabilities of the N-heterocyclic boryloxy (NHBO) ligand, we have synthesized "naked" acyclic gallyl [Ga{OB(NDippCH)2}2]- and indyl [In{OB(NDippCH)2}2]- anions (as their [K(2.2.2-crypt)]+ salts) through K+ abstraction from [KGa{OB(NDippCH)2}2] and [KIn{OB(NDippCH)2}2] using 2.2.2-crypt. These systems represent the first O-ligated gallyl/indyl systems, are ultimately accessed from cyclopentadienyl GaI/InI precursors by substitution chemistry, and display nucleophilic reactivity which is strongly influenced by the presence (or otherwise) of the K+ counterion.

Synthesis and Characterization of Bulky 1,3-Diamidopropane Complexes of Group 2 Metals (Be-Sr)

Reaction of lithium 1,3-diamidopropane Li2(TripNCN) (TripNCN=[{(Trip)NCH2}2CH2]2-, Trip=2,4,6-triisopropylphenyl) with BeBr2(OEt2)2 gave the diamido beryllium complex, [(TripNCN)Be(OEt2)]. Deprotonation reactions between the bulkier 1,3-diaminopropane (TCHPNCN)H2 (TCHPNCN=[{(TCHP)NCH2}2CH2]2-, TCHP=2,4,6-tricyclohexylphenyl) and magnesium alkyls afforded the adduct complexes [(TCHPNCN)Mg(OEt2)] and [(TCHPNCN)Mg(THF)2], depending on the reaction conditions employed. Treating [(TCHPNCN)Mg(THF)2] with the N-heterocyclic carbene :C{(MeNCMe)2} (TMC) gave [(TCHPNCN)Mg(TMC)2] via substitution of the THF ligands. Reactions of (ArNCN)H2 (Ar=Trip or TCHP) with Mg{CH2(SiMe3)}2, in the absence of Lewis bases, yielded the N-bridged dimers [{(ArNCN)Mg}2]. Salt metathesis reactions between alkali metal salts M2(TCHPNCN) (M=Li or K) and CaI2 or SrI2 led to the THF adduct compounds [(TCHPNCN)Ca(THF)3] and [(TCHPNCN)Sr(THF)4], the differing number of THF ligands in which is a result of the different sizes of the metals involved. The described complexes hold potential as precursors to kinetically protected, low oxidation state group 2 metal species.

Hydrogen splitting at a single phosphorus centre and its use for hydrogenation

Catalytic processes are largely dominated by transition-metal complexes. Main-group compounds that can mimic the behaviour of the transition-metal complexes are of great interest due to their potential to substitute or complement transition metals in catalysis. While a few main-group molecular centres were shown to activate dihydrogen via the oxidative addition process, catalytic hydrogenation using these species has remained challenging. Here we report the synthesis, isolation and full characterization of the geometrically constrained phosphenium cation with the 2,6-bis(o-carborano)pyridine pincer-type ligand. Notably, this cation can activate the H-H bond by oxidative addition to a single PIII cationic centre, producing a dihydrophosphonium cation. This phosphenium cation is also capable of catalysing hydrogenation reactions of C=C double bonds and fused aromatic systems, making it a main-group compound that can both activate H2 at a single molecular main-group centre and be used for catalytic hydrogenation. This finding shows the potential of main-group compounds, in particular phosphorus-based compounds, to serve as metallomimetic hydrogenation catalysts.

Redox-Active Carboranyl Diphosphine as an Electron and Proton Transfer Agent

In this work, we report the first example of the PCET reactivity for a boron cluster compound, the zwitterionic nido-carboranyl diphosphonium derivative 7-P(H)tBu2-10-P(H)iPr2-nido-C2B10H10. This main-group reagent efficiently transfers two electrons and two protons to quinones to yield hydroquinones and regenerate a neutral closo-carboranyl diphosphine, 1-PtBu2-2-PiPr2-closo-C2B10H10. As we have previously reported the conversion of this closo-carboranyl diphosphine into the zwitterionic nido- derivative upon reaction with main group hydrides, the transformation reported herein represents a complete synthetic cycle for the metal-free reduction of quinones, with the redox-active carboranyl diphosphine scaffold acting as a mediator. The proposed mechanism of this reduction, based on pKa determination, electrochemical studies, and kinetic isotope effect determination, involves the electron transfer from the nido- cluster to the quinone coupled with the delivery of protons.

Antimony Redox Catalysis: Hydroboration of Disulfides through Unique Sb(I)/Sb(III) Redox Cycling

The stibinidene ArSbI (Ar = [2,6-(tBuN═CH)2-C6H3], 1) reacts with S2Tol2 (Tol = p-tolyl) to form ArSbIII(STol)2 (2), which upon treatment with pinacolborane, regenerates 1. These processes unveil an unprecedented antimony redox catalysis involving Sb(I)/Sb(III) cycling for the hydroboration of organic disulfides. Elementary reaction studies and density functional theory calculations support that the catalysis mimics transition metal processes, proceeding through oxidative addition, ligand metathesis, and reductive elimination. The thiophenols and sulfidoborates generated from the hydroboration of disulfides react in situ with α,β-unsaturated carbonyl compounds with the assistance of 1 as a base catalyst. These tandem reactions establish a one-pot synthetic method for β-sulfido carbonyl compounds, in which a stibinidene functions as a redox catalyst and a base catalyst successively, illustrating the versatility and efficiency of antimony catalysis in organic synthesis.

ZnCl2-catalysed transfer hydrogenation of carbonyls and chemoselective reduction of the CC bond in α,β-unsaturated ketones

This manuscript describes chemoselective reduction of CC in α,β-unsaturated ketones and the transfer hydrogenation of aldehydes and ketones catalysed by ZnCl2-phosphinamino-triazolyl-pyridine (0.5 mol%) using KOH/iPrOH as a H2 source. A detailed mechanistic study using DFT calculations (B3LYP-D3/def2-TZVP) revealed the key role of metal-ligand cooperation (MLC) in the catalytic reaction demonstrating the non-innocent behaviour of the phosphine ligand.

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.

Understanding the Impact of Group 14 Elements on the Reactivity of [1 + 2] Cycloaddition Reaction between a Cyclic (Alkyl)(amino)carbene Analogue with a Group 14 Element and a Heavy Acetylene Molecule

Computational exploration using the density functional theory framework (M06-2X-D3/def2-TZVP) was undertaken to investigate the [1 + 2] cycloaddition reaction between a five-membered-ring heterocyclic carbene analogue (G14-Rea; G14 = group 14 element) and a heavy acetylene molecule (G14G14-Rea). It was theoretically observed that exclusively Si-Rea, Ge-Rea, and Sn-Rea demonstrate the capacity to participate in the [1 + 2] cycloaddition reaction with the triply bonded SiSi-Rea. In addition, only three heavy acetylenes (SiSi-Rea, GeGe-Rea, and SnSn-Rea) can catalyze the [1 + 2] cycloaddition reaction with Si-Rea. Our theoretical findings elucidated that the reactivity trend observed in these [1 + 2] cycloaddition reactions primarily arise from the deformation energies of the distorted G14G14-Rea. Also, our study reveals that the bonding characteristics of their respective transition states are controlled by the singlet-singlet interaction (donor-acceptor interaction), rather than the triplet-triplet interaction (electron-sharing interaction). Additionally, our work demonstrates that the bonding behavior between G14-Rea and G14G14-Rea is predominantly determined by the filled p-π orbital of G14G14-Rea (HOMO) → the empty perpendicular p-π orbital of G14-Rea (LUMO), rather than the vacant p-π* orbital of G14G14-Rea (LUMO) ← the filled sp2 orbital of G14-Rea (HOMO).