SiP Double Bonds: Experimental and Theoretical Study of an NHC-Stabilized Phosphasilenylidene

(NHC)Si═C═N-R: A Two-Coordinated Si0-Isocyanide Compound as Si(NHC) Transfer Reagent

Experimental and theoretical studies are reported of the first two-coordinated Si⁰-isocyanide compound (SIDipp)Si═C═N-Ar^Mes (1: SIDipp (NHC) = C[N(Dipp)CH₂]₂, Ar^Mes = 2,6-dimesitylphenyl), supported by an N-heterocyclic carbene (NHC). A Si atom economic two-step synthesis of 1 involves a 2e reduction of the isocyanide-stabilized silyliumylidene salt [SiBr(CNAr^Mes)(SIDipp)][B(Ar^F)₄] (2[B(Ar^F)₄], Ar^F = B(C₆H₃-3,5-(CF₃)₂)₄) with KC₈. 2[B(Ar^F)₄] was obtained from SiBr₂(SIDipp) after bromide abstraction with an equimolar mixture of Na[B(Ar^F)₄] and Ar^MesNC. Exact adherence to the stoichiometry is crucial in the latter reaction, since 2[B(Ar^F)₄] reacts with SiBr₂(SIDipp) via isocyanide exchange to afford the disilicon(II) salt [Si₂Br₃(SIDipp)₂)][B(Ar^F)₄] (3[B(Ar^F)₄]), the reaction leading to an equilibrium that favors 3[B(Ar^F)₄] (K_eq(298 K) = 10.6, ΔH° = -10.6 kJ mol^-1; ΔS^° = -16.0 J mol^-1 K^-1). 3[B(Ar^F)₄] was obtained selectively from the 2:1 reaction of SiBr₂(SIDipp) with Na[B(Ar^F)₄] and fully characterized. Detailed studies of 1 reveal an intriguing structure featuring a planar C_NHC-Si-C-N skeleton with a V-shaped geometry at the dicoordinated Si⁰ center, a slightly bent Si═C═N core, a C_NHC-Si-C_CNR 3c-2e out of plane π-bond (HOMO), and an anticlinal conformation of the SIDipp and Ar^Mes substituents leading to axial chirality and the presence of two enantiomers, (R_a)-1 and (S_a)-1. Compound 1 displays structural dynamics in solution, rapidly interconverting the enantiomers. The silacumulene 1 is a potent Si(SIDipp) transfer agent as demonstrated by the synthesis and full characterization of the NHC-supported germasilyne (Z)-(SIDipp)(Cl)Si═GeAr^Mes (4) from 1 and Ge(Ar^Mes)Cl.

31P NMR Chemical Shift Tensors: Windows into Ruthenium Phosphinidene Complex Electronic Structures

A series of octamethylcalix[4]pyrrole/ruthenium phosphinidene complexes (Na₂[1=PR]) can be accessed by phosphinidene transfer from the corresponding RPA (A = C₁₄H₁₀, anthracene) compounds (R = ^tBu, ⁱPr, OEt, NH₂, NMe₂, NEt₂, NⁱPr₂, NA, dimethylpiperidino). Isolation of the tert-butyl and dimethylamino derivatives allowed comparative studies of their ³¹P nuclear shielding tensors by magic-angle-spinning solid-state nuclear magnetic resonance spectroscopy. Density functional theory and natural chemical shielding analyses reveal the relationship between the ³¹P chemical shift tensor and the local ruthenium/phosphorus electronic structure. The general trend observed in the ³¹P isotropic chemical shifts for the ruthenium phosphinidene complexes was controlled by the degree of deshielding in the δ₁₁ principal tensor component, which can be linked to the σ_RuP/π_RuP* energy gap. A "δ₂₂-δ₃₃ crossover" effect for R = ^tBu was also observed, which was caused by different degrees of deshielding associated with polarizations of the σ_PR and σ_PR* natural bond orbitals.

Bonding Relationship between Silicon and Germanium with Group 13 and Heavier Elements of Groups 14-16

The topic of heavier main group compounds possessing multiple bonds is the subject of momentous interest in modern organometallic chemistry. Importantly, there is an excitement involving the discovery of unprecedented compounds with unique bonding modes. The research in this area is still expanding, particularly the reactivity aspects of these compounds. This article aims to describe the overall developments reported on the stable derivatives of silicon and germanium involved in multiple bond formation with other group 13, and heavier groups 14, 15, and 16 elements. The synthetic strategies, structural features, and their reactivity towards different nucleophiles, unsaturated organic substrates, and in small molecule activation are discussed. Further, their physical and chemical properties are described based on their spectroscopic and theoretical studies.

B-B Cleavage and Ring-Expansion of a 1,4,2,3-Diazadiborinine with N-Heterocyclic Carbenes

A 1,4,2,3-diazadiborinine derivative was found to form Lewis adducts with strong two-electron donors such as N-heterocyclic and cyclic (alkyl)(amino)carbenes. Depending on the donor, some of these Lewis pairs are thermally unstable, converting to sole B,N-embedded products upon gentle heating. The products of these reactions, which have been fully characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction, were identified as B,N-heterocycles with fused 1,5,2,4-diazadiborepine and 1,4,2-diazaborinine rings. Computational modelling of the reaction mechanism provides insight into the formation of these unique structures, suggesting that a series of B-H, C-N, and B-B bond activation steps are responsible for these "intercalation" reactions between the 1,4,2,3-diazadiborinine and NHCs.

Treatment of Silylene-Phosphinidene with Chalcogens Resulted Exclusively in the Formation of Silicon-Bonded Chalcogens

Chalcogen-bonded silicon phosphinidenes LSi(E)-P-Me cAAC (E=S (1); Se (2); Te (3); L=PhC(NtBu)2 ; Me cAAC=C(CH2 )(CMe2 )2 N-2,6-iPr2 C6 H3 )) were synthesized from the reactions of silylene-phosphinidene LSi-P-Me cAAC (A) with elemental chalcogens. All the compounds reported herein have been characterized by multinuclear NMR, elemental analyses, LIFDI-MS, and single-crystal X-ray diffraction techniques. Furthermore, the regeneration of silylene-phosphinidene (A) was achieved from the reactions of 2-3 with L'Al (L'=HC{(CMe)(2,6-iPr2 C6 H3 N)}2 ). Theoretical studies on chalcogen-bonded silicon phosphinidenes indicate that the Si-E (E=S, Se, Te) bond can be best represented as charge-separated electron-sharing σ-bonding interaction between [LSi-P-Me cAAC]+ and E- . The partial double-bond character of Si-E is attributed to significant hyperconjugative donation from the lone pair on E- to the Si-N and Si-P σ*-molecular orbitals.

Synthesis of Cyclic Alkyl(amino) Carbene Stabilized Silylenes with Small N-Donating Substituents

Lewis base cAACs stabilized monomeric silylenes with halogen or methyl substituents at the silicon center have not been reported due to the strong σ-donor and π-acceptor character of cAAC. To prepare these monomeric silylenes, we used the silicon(IV) precursors 5 and 6 with a nitrogen donor group L (L=o-C₆ H₄ NMe₂ ). The cAAC-stabilized (cAAC=C(CH₂ )(CMe₂ )₂ N-Ar, Ar=2,6-iPr₂ C₆ H₃ ) silylenes LSiCl(cAAC) (7) and LSiMe(cAAC) (8) were synthesized by reduction of LSiCl₃ and LSiMeCl₂ with two equivalents of KC₈ in the presence of one equivalent of cAAC, respectively. Compounds 7 and 8 were characterized by single-crystal X-ray crystallography, NMR spectroscopy, and elemental analysis. Compounds 7 and 8 are stable in the solid state as well as in solution at room temperature for at least four months under inert conditions.

A Free Silanide from Nucleophilic Substitution at Silicon(II)

A computationally guided synthetic route to a free silanide derived from tris(3-methylindol-2-yl)methane ([(tmim)Si]- ) through nucleophilic substitution on the SiII precursor (Idipp)SiCl2 is reported (Idipp=2,3-dihydro-1,3-bis(2,6-diisopropylphenyl)-1H-imidazol-2-ylidene). This approach circumvents the need for strained tetrahedral silanes as synthetic intermediates. Computational investigations show that the electron-donating properties of [(tmim)Si]- are close to those of PMe3. Experimentally, the [(tmim)Si]- anion is shown to undergo clean complexation to the base metal salts CuCl and FeCl2 , demonstrating the potential utility as a supporting ligand.