B–H Bond Activation by an Amidinate-Stabilized Amidosilylene: Non-Innocent Amidinate Ligand

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.

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.

Unusual nucleophilic reactivity of a dithiolene-based N-heterocyclic silane

While the dithiolene-based N-heterocyclic silane (4) reacts with two equivalents of BX3 (X = Br, I) to give zwitterionic Lewis adducts 5 and 8, respectively, the parallel reaction of 4 with BCl3 results in 10, a dithiolene-substituted N-heterocyclic silane, via the Si-S bond cleavage. Unlike 5, the labile 8 may be readily converted to 9via BI3-mediated cleavage of the Si-N bond. The formation of 5 and 8 confirms that 4 uniquely possesses dual nucleophilic sites: (a) the terminal sulphur atom of the dithiolene moiety; and (b) the backbone carbon of the N-heterocyclic silane unit.

From Carbene-Dithiolene Zwitterion Mediated B-H Bond Activation to BH3·SMe2-Assisted Boron-Boron Bond Formation

The 1:1 reaction of the carbene-stabilized dithiolene zwitterion 1 with BH3·SMe2 gave the dithiolene-based hydroborane 2 and the doubly hydrogen-capped CAAC species 3 via hydride-coupled reverse electron transfer processes. The mechanism of this transformation was probed computationally using density functional theory. The subsequent 2:1 reaction of 2 with 1 resulted in 4 and 3, suggesting that 1 can mediate the B-H bond activation not only for BH3 but also for monohydroboranes. In the presence of BH3·SMe2, 2 was unexpectedly converted to the corresponding diborane(4) complex 5 through a dehydrocoupling reaction at an elevated temperature.

Stereoselective formation of boron-stereogenic organoboron derivatives

Four-coordinate organoboron derivatives present interesting chemical, physical, biological, electronical, and optical properties. Given the increasing demand for the synthesis of smart functional materials based on chiral organoboron compounds, the exploration of stereoselective synthesis of boron-stereogenic organo-derivatives is highly desirable. However, the stereoselective construction of organoboron compounds stereogenic at boron has been far less studied than other elements of the main group due to configurational stability concerns. Nowadays, these species are no longer elusive and configurationally stable compounds have been highlighted. The idea is to show the potential of the stereoselective building of the four-coordinate boron centre and encourage future endeavors and developments in the field.

A Hydride-Substituted Homoleptic Silylborate: How Similar is it to its Diborane(6)-Dianion Isostere?

The B-nucleophilic 9H-9-borafluorene dianion reacts with 9-chloro-9-silafluorene to afford air- and moisture-stable silylborate salts M[Ar₂ (H)B-Si(H)Ar₂ ] (M[HBSiH], M=Li, Na). Li[HBSiH] and Me₃ SiCl give the B-pyridine adduct Ar₂ (py)B-Si(H)Ar₂ ((py)BSiH) or the chlorosilane Li[Ar₂ (H)B-Si(Cl)Ar₂ ] (Li[HBSiCl]) in C₆ H₆ -pyridine or THF. In both cases, the first step is H^- abstraction at the B center. The resulting free borane subsequently binds a py or thf ligand. While the py adduct is stable at room temperature, the thf adduct undergoes a 1,2-H shift via the cyclic B(μ-H)Si intermediate BHSi, which is afterwards attacked at the Si atom by a Cl^- ion to give Li[HBSiCl]. DFT calculations were employed to support the proposed reaction mechanism and to characterize the electronic structure of BHSi. Treatment of Li[HBSiCl] with the N-heterocyclic carbene IMe introduces the neutral donor at the Si atom and leads to Ar₂ (H)B-Si(IMe)Ar₂ (HBSi(IMe)), a donor-acceptor-stabilized silylene.

Amidinatoamidosilylene-Dibromodiborene

The amidinatoamidosilylene [LSiNMe₂] [1; L = PhC(NtBu)₂] was reacted with B₂Br₄(SMe₂)₂ in toluene at room temperature to form the bis(silylene)tetrabromodiborane [L{Me₂N}Si]₂B₂Br₄ (2). It was then reacted with excess KC₈ in tetrahydrofuran at room temperature to afford the bis(silylene)dibromodiborene [L{Me₂N}Si]₂B₂Br₂ (3).

Reactivity of a Phosphinoamidinate-Stabilized Disilylene toward H-X Bonds

An efficient method for the preparation of a phosphinoamidinate-supported disilylene was developed, and its reactivity toward H-E bonds (E = elements from Groups 13-15) was studied. With HBpin, transfer of the ligand from silicon to boron was observed to afford (NP)Bpin. Reaction with a silane (H₃SiPh) took place only at elevated temperatures, at which point oxidative addition of the N-P bond of the NP-ligand to one of the silicon atoms of the disilylene occurred prior to Si-H addition involving the remaining silylene center. In contrast, reaction of the disilylene with phosphine, HPPh₂ furnished the phosphidosilylene (NP)SiPPh₂ (16) together with a highly transient species that, on the basis of a trapping experiment with H₃SiPh, is proposed to be the hydridosilylene (NP)SiH, 17. Interestingly, 16 reacts with HPPh₂ to give the diphosphine (PPh₂)₂, most likely via a direct σ-bond metathesis process. The aforementioned products have been characterized by multinuclear NMR and single-crystal X-ray diffraction studies.

Selective Route to Stable Silicon-Boron Radicals and Their Corresponding Cations

Herein, we report on a facile and selective one-pot synthetic route to silicon-boron radicals. Reduction of Br₂BTip (Tip = 2,4,6-ⁱPrC₆H₂) with KC₈ in the presence of LSi-R affords LSi(^tBu)-B(Br)Tip (1) and LSi(N(TMS)₂)-B(Br)Tip (2) [L = PhC(N^tBu)₂]. These first examples of silicon-boron isolated radical species feature spin density on the silicon and boron atoms. 1 and 2 exhibit extraordinary stability to high temperatures under inert conditions in solution and air stability in the solid state. Both radicals have been isolated and fully characterized by electron paramagnetic resonance spectroscopy, SQUID magnetometry, mass spectrometry, cyclic voltammetry, single-crystal X-ray structure analysis, and density functional theory calculations. Moreover, compound 1 exhibits one-electron transfer when treated with 1 equiv of AgSO₃CF₃ and [Ph₃C]⁺[B(C₆F₅)₄]^-, respectively, resulting in the corresponding cations [LSi(^tBu)-B(Br)Tip]⁺[CF₃SO₃]^- (3) and [LSi(^tBu)-B(Br)Tip]⁺[B(C₆F₅)₄]^- (4). Compounds 3 and 4 have been characterized with multinuclear NMR and mass spectrometry.

A Stable N-Heterocyclic Silylene with a 1,1'-Ferrocenediyl Backbone

The N-heterocyclic silylene [{Fe(η5 -C5 H4 -NDipp)2 }Si] (1DippSi, Dipp=2,6-diisopropylphenyl) shows an excellent combination of pronounced thermal stability and high reactivity towards small molecules. It reacts readily with CO2 and N2 O, respectively affording (1DippSiO2 )2 C and (1DippSiO)2 as follow-up products of the silanone 1DippSiO. Its reactions with H2 O, NH3 , and FcPH2 (Fc=ferrocenyl) furnish the respective oxidative addition products 1DippSi(H)X (X=OH, NH2 , PHFc). Its reaction with H3 BNH3 unexpectedly results in B-H, instead of N-H, bond activation, affording 1DippSi(H)(BH2 NH3 ). DFT results suggest that dramatically different mechanisms are operative for these H-X insertions.

A cyclopentadienyl functionalized silylene - a flexible ligand for Si- and C-coordination

The synthesis of a 1,2,3,4-tetramethylcyclopentadienyl (Cp⁴) substituted four-membered N-heterocyclic silylene [{PhC(NtBu)₂}Si(C₅Me₄H)] is reported first. Then, selected reactions with transition metal and a calcium precursor are shown. The proton of the Cp⁴-unit is labile. This results in two different reaction pathways: (1) deprotonation and (2) rearrangement reactions. Deprotonation was achieved by the reaction of [{PhC(NtBu)₂}Si(C₅Me₄H)] with suitable zinc precursors. Rearrangement to [{PhC(NtBu)₂}(C₅Me₄)SiH], featuring a formally tetravalent silicon R₂C[double bond, length as m-dash]Si(R')-H unit, was observed when the proton of the Cp⁴ ring was shifted from the Cp⁴-ring to the silylene in the presence of a Lewis acid. This allows for the coordination of the Cp⁴-ring to a calcium compound. Furthermore, upon reaction with transition metal dimers [MCl(cod)]₂ (M = Rh, Ir; cod = 1,5-cyclooctadiene) the proton stays at the Cp⁴-ring and the silylene reacts as a sigma donor, which breaks the dimeric structure of the precursors.

Amidinate based indium(III) monohalides and β-diketiminate stabilized In(II)-In(II) bond: synthesis, crystal structure, and computational study

Synthesis and bonding aspects of mononuclear bis-amidinate indium(iii) monohalides L2InX (1-3), where L = PhC(NtBu)2; X is F (1), Br (2) or I (3) and β-diketiminate (NacNac) stabilized In(ii) dimer (MesNacNac)2In2Br2 (4) with In-In bond are reported along with the single-crystal X-ray structures of 2-4.

Silylene induced cooperative B–H bond activation and unprecedented aldehyde C–H bond splitting with amidinate ring expansion

Silylene mediated B–H and aldehyde C–H bond splitting were realized under ambient conditions.

(PhC(NtBu)2Al)2(SiH2)4 six-membered heterocycle: comparable in structure to cyclohexane

A (PhC(NtBu)₂Al)₂(SiH₂)₄ centrosymmetric six-membered heterocycle (LAl)₂(SiH₂)₄ (L = PhC(NtBu)₂) with chair conformation is prepared by reacting disilylene LSi–SiL with alane AlH₃·NEtMe₂. It is comparable in structure to cyclohexane containing four Si and two Al atoms as a core.