Formation of an Imino-Stabilized Cyclic Tin(II) Cation from an Amino(imino)stannylene

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Tetryliumylidene ions in synthesis and catalysis

Tetryliumylidene ions ([R-E:]+), recognised for their intriguing electronic properties, have attracted considerable interest. These positively charged species, with two vacant p-orbitals and a lone pair at the E(ii) centre (E = Si, Ge, Sn, Pb), can be viewed as the combination of tetrylenes (R2E:) and tetrylium ions ([R3E]+), which makes them potent Lewis ambiphiles. Such electronic features highlight the potential of tetryliumylidenes for single-site small molecule activation and transition metal-free catalysis. The effective utilisation of the electrophilicity and nucleophilicity of tetryliumylidenes is expected to stem from appropriate ligand choice. For most of the isolated tetryliumylidenes, electron donor- and/or kinetic stabilisation is necessary. This minireview highlights the developments in tetryliumylidene syntheses and the progress of research towards their reactivity and applications in catalytic reactions.

Reactivity of NHI-Stabilized Heavier Tetrylenes towards CO2 and N2 O

A heteroleptic amino(imino)stannylene (TMS2 N)(It BuN)Sn: (TMS=trimethylsilyl, It Bu=C[(N-t Bu)CH]2 ) as well as two homoleptic NHI-stabilized tetrylenes, (It BuN)2 E: (NHI=N-heterocyclic imine, E=Ge, Sn) are presented. VT-NMR investigations of (It BuN)2 Sn: (2) reveal an equilibrium between the monomeric stannylene at room temperature and the dimeric form at -80 °C as well as in the solid state. Upon reaction of the homoleptic tetrylenes with CO2 , both compounds insert two equivalents of CO2 , however differing bonding modes can be observed. (It BuN)2 Sn: (2) inserts one equivalent of CO2 into each Sn-N bond, giving carbamato groups coordinated κ2 O,O' to the metal center. With (It BuN)2 Ge: (3), the Ge-N bonds stay intact upon activation, being bridged by one molecule of CO2 respectively, forming 4-membered rings. Furthermore, the reactivity of 2 towards N2 O was investigated, resulting in partial oxidation to form stannylene dimer [((It BuN)3 SnO)(It BuN)Sn:]2 (6).

From Sn(II) to Sn(IV): Enhancing Lewis Acidity Via Oxidation

We demonstrate the increased Lewis acidity on going from Sn(II) to Sn(IV) by oxidizing TpMe2SnOTf (OTf = SO3CF3) to TpMe2SnF(OTf)2. Replacement of the fluoride ion in TpMe2SnF(OTf)2 by a triflate, resulting in TpMe2Sn(OTf)3 further enhances the Lewis acidity at tin. 119Sn NMR spectroscopy, modified Gutmann-Beckett test, computational analysis, and catalytic phosphine oxide deoxygenation support the claims.

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

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

Reactions of main group compounds with azides forming organic nitrogen-containing species

Since the seminal discovery of phenyl azide by Grieß in 1864, a variety of organic azides (R-N3) have been developed and extensively studied. The amenability of azides to a number of reactions has expanded their utility as building blocks not only in organic synthesis but also in bioorthogonal chemistry and materials science. Over the decades, it has been demonstrated that the reactions of main group compounds with azides lead to diverse N-containing main group molecules. In view of the pronounced progress in this area, this review summarizes the reactions of main group compounds with azides, emphatically introducing their reaction patterns and mechanisms. The reactions of forming inorganic nitrogen species are not included in this review.

Utilization of a Tris(carbene)borate Ligand for Umpolung Reactivity of a Nucleophilic Tin(II) Cation Salt

We show that a tris(carbene)borate (TCB) ligand, namely [PhB(^tBuIm)₃]^- ([PhB(^tBuIm)₃]^- = phenyltris(3-tert-butylimidazol-2-ylidene)borato), is capable of stabilizing an unprecedented nucleophilic Sn(II) cation salt. Unlike known Sn(II) cations, the strong electron-donating ability of [PhB(^tBuIm)₃]^- makes the cationic tin atom electron-rich, σ-donating yet slightly π-accepting, which allows for the ensuing facile oxidation with o-chloranil and S₈ as well as coordination with coinage metals. The former oxidations give the Sn(IV) cation salts, while the latter reactions produce the metal complexes. The electronic structures of these species are thoroughly probed by quantum chemical computations. These results uncover an added role for TCB ligands in isolating unprecedented p-block species.

An Air-Stable "Masked" Bis(imino)carbene: A Carbon-Based Dual Ambiphile

Carbenes, once considered laboratory curiosities, now serve as powerful tools in the chemical and material sciences. To date, all stable singlet carbenes are single-site ambiphiles. Here we describe the synthesis of a carbene which is a carbon-based dual ambiphile (both single-site and dual-site). The key is to employ imino substituents derived from a cyclic (alkyl)(amino)carbene (CAAC), which imparts a 1,3-dipolar character to the carbene. Its dual ambiphilic nature is consistent with the ability to activate simple organic molecules in both 1,1- and 1,3-fashion. Furthermore, its 1,3-ambiphilicity facilitates an unprecedented reversible intramolecular dearomative [3 + 2] cycloaddition with a proximal arene substituent, giving the carbene the ability to "mask" itself as an air-stable cycloadduct. We perceive that the concept of dual ambiphilicity opens a new dimension for future carbene chemistry, expanding the repertoire of applications beyond that known for classical single-site ambiphilic carbenes.

Isolation and Reactivity of Stannylenoids Stabilized by Amido/Imino Ligands

The reaction of the lithium aryl(silyl)amide Dipp(ⁱ Pr₃ Si)NLi (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ) with one equivalent of SnCl₂ in THF gave a novel stannylenoid Dipp(ⁱ Pr₃ Si)NSnCl⋅LiCl(THF)₂ . Heating the solution of amidostannylenoid in toluene to 80 °C resulted in dimeric amido(chloro)stannylene [Dipp(ⁱ Pr₃ Si)NSnCl]₂ , which can be converted to bis(amido)stannylene Sn[N(Dipp)(ⁱ Pr₃ Si)]₂ and amido(imino)stannylene Sn[N(Dipp)(ⁱ Pr₃ Si)][IPrN] (IPrN=bis(2,6-diisopropylphenyl)imidazolin-2-imino). Treatment of bis(imino)stannylenoid [IPrN]₂ Sn(Cl)Li with N₂ O resulted in the dimeric complex [IPrNSn(Cl)OLi]₂ . All compounds were characterized by NMR, elementary analysis, and X-ray structural determination.

Isolation and Reactivity of Tetrylene-Tetrylone-Iron Complexes Supported by Bis(N-Heterocyclic Imine) Ligands

The germanium iron carbonyl complex 3 was prepared by the reaction of dimeric chloro(imino)germylene [IPrNGeCl]2 (IPrN=bis(2,6-diisopropylphenyl)imidazolin-2-iminato) with one equivalent of Collman's reagent (Na2 Fe(CO)4 ) at room temperature. Similarly, the reaction of chloro(imino)stannylene [IPrNSnCl]2 with Na2 Fe(CO)4 (1 equiv) resulted in the Fe(CO)4 -bridged bis(stannylene) complex 4. We observed reversible formation of bis(tetrylene) and tetrylene-tetrylone character in complexes 3 vs. 5 and 4 vs. 6, which was supported by DFT calculations. Moreover, the Li/Sn/Fe trimetallic complex 12 has been isolated from the reaction of [IPrNSnCl]2 with cyclopentadienyl iron dicarbonyl anion. The computational analysis further rationalizes the reduction pathway from these chlorotetrylenes to the corresponding complexes.

Tuning the metal-ligand bond in the σ-complexes of stannylenes and azabenzenes

The metal-ligand bond in a set of 60 σ-complexes has been investigated by electronic structure computations. These σ-complexes originate from the unique combination of 12 stannylenes (SnX₂ ) with five azabenzene ligands (pyridine, pyrazine, pyrimidine, pyridazine, and s-triazine), where the nitrogen center of the ligand acts as σ-donor and the tin(II) center as σ-acceptor in a 1:1 fashion. The Sn ← N bond and the total interaction between the stannylene and azabenzene moieties of the σ-complexes are characterized in depth to relate the Sn ← N strength to the substitution pattern at SnX₂ and to the number and the positioning of N atoms in the azabenzenes. Such X substituents as (iso)cyano and trifluoromethyl groups enhance the interaction strength, while the presence of alkyl, phenyl, and silyl substituents in SnX₂ diminishes the stability of σ-complexes. A gradual weakening of the total interaction is associated with the growing number of N atoms in the azabenzenes, while the N-atom positioning in pyridazine is particularly effective in strengthening the interaction with stannylenes. Variations in the Sn ← N bond strength usually follow those in the total interaction between the moieties but the interacting quantum atoms picture of Sn ← N reveals certain intriguing exceptions.

Synthesis of Cationic Silaamidinate Germylenes and Stannylenes and the Catalytic Application for Hydroboration of Pyridines

The N-heterocyclic germylenes and stannylenes LSi(NAr)₂EX (L = PhC(NtBu)₂, Ar = 2,6-iPr₂C₆H₃; E = Ge, Sn; X = Cl, CF₃SO₃, BPh₄) supported by the bulky silaamidinate ligand [LSi(NAr)₂]^- have been synthesized and fully characterized. The germylene triflate LSi(NAr)₂GeOTf (3b) and dimeric borate [LSi(NAr)₂Ge]₂ClBPh₄ (3a) enabled highly regio- and chemoselective catalytic hydroboration of pyridines and may represent the most active catalytic system for the transformation. DFT calculations disclosed that the cationic germylene [LSi(NAr)₂Ge]⁺ with a low-lying LUMO energy initiated the catalytic process. In contrast, the analogous amidinate germylene triflates are almost inactive, indicating the silaamidinate ligand is essential for the stabilization of cationic species.

Me CAAC=N- : A Cyclic (Alkyl)(Amino)Carbene Imino Ligand

A cyclic (alkyl)(amino)carbene (CAAC) has been shown to react with a covalent azide similar to the Staudinger reaction. The reaction of Me CAAC with trimethylsilyl azide afforded the N-silylated 2-iminopyrrolidine (Me CAAC=NSiMe3 ), which was fully characterized. This compound undergoes hydrolysis to afford the 2-iminopyrrolidine and trimethylsiloxane which co-crystallize as a hydrogen-bonded adduct. The N-silylated 2-iminopyrrolidine was used to transfer the novel pyrrolidine-2-iminato ligand onto both main-group and transition-metal centers. The reaction of the tetrabromodiborane bis(dimethyl sulfide) adduct with two equivalents of Me CAAC=NSiMe3 afforded the disubstituted diborane. The reaction of Me CAAC=NSiMe3 with TiCl4 and CpTiCl3 afforded Me CAAC=NTiCl3 and Me CAAC=NTiCl2 Cp, respectively.

An N-Heterocyclic Boryloxy Ligand Isoelectronic with N-Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners

Introduced here is a new type of strongly donating N-heterocyclic boryloxy (NHBO) ligand, [(HCDippN)₂ BO]^- (Dipp=2,6-diisopropylphenyl), which is isoelectronic with the well-known N-heterocyclic iminato (NHI) donor class. This 1,3,2-diazaborole functionalized oxy ligand has been used to stabilize the first acyclic two-coordinate dioxysilylene and its Ge, Sn, and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X-ray crystallography and density-functional theory, enabling analysis of periodic trends: the potential for the [(HCDippN)₂ BO]^- ligand to subtly vary its electronic-donor capabilities is revealed by snapshots showing the gradual evolution of arene π coordination on going from Si to Pb.

Pseudo-One-Coordinate Tetrylenium Salts Bearing a Bulky Carbazolyl Substituent

Syntheses of a bulky carbazole-based substituent, the parent 1,8-bis(3,5-di-tert-butylphenyl)-3,6-di-tert-butyl-carbazole (R-H) and a series of chlorotetrylenes, RECl (E=Ge, Sn, Pb), are described. Detailed analysis of the properties of the carbazole-based substituent revealed that it features flexible high bulkiness and electronic non-innocence, which may make it suitable for many applications in both main-group and transition-metal chemistry. To further showcase the employability of the novel substituent, the chlorotetrylenes were subjected to halide abstraction reactions, affording the corresponding tetrylenium salts [RE][Al(OC₄ F₉ )₄ ] (E=Ge, Sn, Pb) as strongly coloured compounds.

Tetrazastannoles versus distannadiazanes - a question of the tin(ii) source

Various tin(ii) compounds such as Mes*2Sn (Mes* = 2,4,6-tri-tert-butylphenyl), Sn[N(SiMe3)2]2 and TerSnCl (Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl) could be readily oxidised by organic azides to release N2, forming nitrogen-tin compounds. Depending on the used Sn(ii) compound, the reactions with two equivalents of azide led to the formation of tetrazastannoles (R2N4SnR'2) or cyclo-distannadiazanes [R2Sn(μ-NR')]2. The reactivity was independent of the electronic situation of the organic azide. Additionally, Mes*2Sn formed an amido-azido compound of the type R(R')Sn(N(SiMe3)2)N3 in the presence of Me3SiN3. Presumably, the corresponding tetrazastannole was formed in the first step followed by a ring opening reaction. The same holds true for the reaction of Sn[N(SiMe3)2]2 with Me3SiN3 while TerSnCl showed no reaction in the presence of Me3SiN3, even after prolonged heating.

Labile Low-Valent Tin Azides: Syntheses, Structural Characterization, and Thermal Properties

The first two examples of the class of tetracoordinate low-valent, mixed-ligand tin azido complexes, Sn(N₃)₂(L)₂, are shown to form upon reaction of SnCl₂ with NaN₃ and SnF₂ with Me₃SiN₃ in either pyridine or 4-picoline (2, L = py; 3, L = pic). These adducts of Sn(N₃)₂ are shock- and friction-insensitive and stable at r.t. under an atmosphere of pyridine or picoline, respectively. A new, fast, and efficient method for the preparation of Sn(N₃)₂ (1) directly from SnF₂, and by the stepwise de-coordination of py from 2 at r.t., is reported that yields 1 in microcrystalline form, permitting powder X-ray diffraction studies. Reaction of 1 with a nonbulky cationic H-bond donor forms the salt-like compound {C(NH₂)₃}Sn(N₃)₃ (4) which is comparably stable despite its high nitrogen content (55%) and the absence of bulky weakly coordinating cations that are conventionally deemed essential in related systems of homoleptic azido metallates. The spectroscopic and crystallographic characterization of the polyazides 1-4 provides insight into azide-based H-bonded networks and unravels the previously unknown structure of 1 as an important lighter binary azide homologue of Pb(N₃)₂. The atomic coordinates for 1 and 2-4 were derived from powder and single crystal XRD data, respectively; those for 1 are consistent with predictions made by DFT-D calculations under periodic boundary conditions.

Insertion of a Transient Tin Nitride into Carbon-Carbon and Boron-Carbon Bonds

A simple exchange reaction between [Ar^iPr4Sn(μ-Cl)]₂ (1) and sodium azide afforded the doubly bridged Sn(II) azide, [Ar^iPr4Sn(μ-N₃)]₂ (2) (Ar^iPr4 = C₆H₃-2,6(C₆H₃-2,6-ⁱPr₂)₂) in 85% yield. Photolysis of a diethyl ether solution of 2 for ca. 16 h yielded an azepinyl-substituted insertion product, [C₆H₃-2-(C₆H₃-2,6-ⁱPr₂)-6-(C₆H₃N-3,7-ⁱPr₂)Sn]₂ (3). The reaction of the Lewis acid, B(C₆F₅)₃ (BCF), or the Lewis base, pyridine, with 2 dissociates the dimer to afford the corresponding complexed monomeric Sn(II) azide, Ar^iPr4SnN₃BCF (4) in which BCF coordinates the α-nitrogen, or Ar^iPr4Sn(pyridine)N₃ (6) in which pyridine coordinates to the tin atom. Photolysis of 4 in diethyl ether for 12 h results in the insertion of the α-nitrogen of the azide group into one of the B-C bonds of the BCF acceptor to yield the tin(II) amide, Ar^iPr4SnN(C₆F₅)B(C₆F₅)₂ (5). In contrast, photolysis of 6 for over 36 h afforded no apparent reaction. A highly reactive Sn nitride intermediate, Ar^iPr4Sn≡N, is proposed as part of the mechanistic pathway for the formation of 3 and 5 as a result of trapping the tin-centered radical isomers. This was effected by immediate freezing the samples of 2 or 4 after ca. 30 min of UV photolysis and recording their electron paramagnetic resonance spectra. These exhibited a rhombic g tensor of [g₁, g₂, g₃] = [2.029, 1.978, 1.933]. This radical intermediate could be related to the valence isomers of the nitride [-Sn^IV≡N] intermediate, in isomeric equilibrium with the nitrene [-Sn^II-N] and nitridyl [-Sn^III═N·] forms, but with the spin density on the nitrogen being quenched, possibly by the H atom abstraction to form an S = 1/2 species of formula -Sn·═N(H).

Reactivity of heavy carbene analogues towards oxidants: a redox active ligand-enabled isolation of a paramagnetic stannylene

We report the first isolated paramagnetic stannylene enabled by a redox active ligand.

Synthesis of a cyclopentadienyl(imino)stannylene and its direct conversion into halo(imino)stannylenes

We report the synthesis and structure of a dimeric Cp-substituted iminostannylene as well as its unusual reactivity towards haloalkanes, resulting in the formation of halogen-substituted iminostannylenes.

Facile Access to Stable Silylium Ions Stabilized by N-Heterocyclic Imines

Novel silylium ions with N-heterocyclic imines were successfully synthesized. The reaction of trimethylsilyl imidazolin-2-imine Me₃SiNIPr (NIPr = bis(2,6-diisopropylphenyl)-imidazolin-2-imino) with B(C₆F₅)₃ leads to dimeric imino-substituted silylium ions through a methyl group abstraction on the silicon atom. Meanwhile, the intermolecular imino-coordinated silylium ion is formed by using the less sterically crowded imine Me₃SiNItBu (NItBu = bis(tert-butyl)-imidazolin-2-imino). Furthermore, the treatment of dimethylchlorosilane Me₂(Cl)SiNIPr with AgOTf affords the contact ion pair Me₂(OTf)SiNIPr by substitution of the chloride. A novel complex with the formula [Me₂(DMAP)SiNIPr][OTf] was prepared by coordination with 4-dimethylamino-pyridine (DMAP). In the solid state, the DMAP adduct [Me₂(DMAP)SiNIPr][OTf] contains a distinct [Me₂(DMAP)SiNIPr]⁺ moiety.

A Ferrocenyl-Backboned Unsymmetric O,C-Coordinating Ligand and Its Tin Derivatives

The syntheses of the phosphonyl-substituted ferrocenyl stannane Fe[{η (5)-C5H3-1-SnPh3-2-P(O)(O-iPr)2}{η (5)-C5H4P(O)(O-iPr)2}] (1) and its iodine derivative Fe[{η (5)-C5H3-1-SnPh2I-2-P(O)(O-iPr)2}{η (5)-C5H4P(O)(O-iPr)2}] (2) are reported. The syntheses of the corresponding salts Fe[{η (5)-C5H3-1-SnPh2-2-P(O)(O-iPr)2}{η (5)-C5H4P(O)(O-iPr)2}]X (3, X=Al{OC(CF3)3}4, 4, X=ClO4, 5, X=HgI3), respectively, are also described. The compounds are characterized by elemental analyses, (1)H, (13)C, (31)P, (119)Sn NMR and IR spectroscopy, electrospray ionization mass spectrometry, and, except for 4 and 5, single-crystal X-ray diffraction analyses.

Applications of N-heterocyclic imines in main group chemistry

A survey of the most recent progress in the applications of N-heterocyclic imines in main group compounds is given.

Isolation of a germanium(ii) cation and a germylene iron carbonyl complex utilizing an imidazolin-2-iminato ligand

A novel amino(imino)germylene was synthesised. Its conversion with B(C₆F₅)₃ afforded a cyclic germanium(ii) cation in a methyl abstraction and ring closing reaction. The conversion with Fe₂(CO)₉ furnished a germylene iron carbonyl complex.