Computational Analysis of n→π* Back-Bonding in Metallylene–Isocyanide Complexes R2MCNR′ (M = Si, Ge, Sn; R = tBu, Ph; R′ = Me, tBu, Ph)

A crystalline stannyne

The synthesis of heteronuclear alkyne analogues incorporating heavier group 14 elements (R1-C≡E-R2, E = Si, Ge, Sn, Pb) has posed a long-standing challenge. Neutral silynes (R1-C≡Si(L)-R2) and germynes (R1-C≡Ge(L)-R2) stabilized by a Lewis base have achieved sufficient stability for structural characterization at low temperatures. Here we show the isolation of a base-free stannyne (R1-C≡Sn-R2) at room temperature, achieved through the strategic use of a bulky cyclic phosphino ligand in combination with a bulky terphenyl substituent. Despite an allenic structure with strong delocalization of π-electrons, this compound exhibits adjacent ambiphilic carbon and tin centres, forming a carbon-tin multiple bond with ionic character. The stannyne demonstrates reactivity similar to carbenes or stannylenes, reacting with 1-adamantyl isocyanide and 2,3-dimethyl-1,3-butadiene. Additionally, its carbon-tin bond can be saturated by Et3N·HCl or cleaved by isopropyl isocyanate.

The Scandium(II) Carbonyl Complex (C5H2tBu3)2Sc(CO) and Its Isocyanide Analog (C5H2tBu3)2Sc(CNC6H3Me2-2,6)

Treatment of the scandium(II) metallocene Cpttt2Sc (Cpttt = C5H2tBu3) with CO or the isocyanide CNXyl (Xyl = C6H3Me2-2,6) yields the carbonyl complex Cpttt2Sc(CO), 1, or the isocyanide complex Cpttt2Sc(CNXyl), 2, which were identified by X-ray crystallography. Isotopic labeling with 13CO shows the CO stretch of 1 at 1875 cm-1 shifts to 1838 cm-1 in 1-13CO. The CN stretch in 2 is shifted to 1939 cm-1 compared to 2118 cm-1 for the free isocyanide. The 80.1 MHz (28.7 G) 45Sc hyperfine coupling in 1 and 74.7 MHz (26.8 G) in 2 are similar to the 82.6 MHz (29.6 G) coupling constant in Cpttt2Sc and indicate that 1 and 2 are Sc(II) complexes. A comprehensive analysis of the electronic structures of 1 and 2 using DFT calculations is reported.

Homocoupling of Isocyanide at the Si(II) Center of Borylaminoamidinatosilylene

Two borylaminoamidinatosilylenes (L)[(1,5-C₈H₁₄)B(Ar)N]Si (L = PhC(NtBu)₂, Ar = 2,6-iPr₂C₆H₃ (1)) and (L)[(1,5-C₈H₁₄)B(Ar')N]Si (Ar' = 2,4,6-Me₃C₆H₂ (2)) have been prepared and utilized to investigate the reaction toward isocyanide. Reactions of 1 with the respective CN-2,6-Me₂C₆H₃ and CNCy (Cy = cyclo-C₆H₁₁) produced compounds (L)Si(NAr)C(N-2,6-Me₂C₆H₃)B(1,5-C₈H₁₄)(CN-2,6-Me₂C₆H₃) (3) and (L)Si(NAr)C(NCy)C(NCy)B(1,5-C₈H₁₄)(CNCy) (4). Reactions of 2 with the respective CNCy and CN-2,6-Me₂C₆H₃ yielded compounds cyclo-(L)SiN(Ar')C(NCy)B(1,5-C₈H₁₄)C(NCy) (5) and cyclo-(L)[(1,5-C₈H₁₄)B(Ar')N]SiC(CN-2,6-Me₂C₆H₃)N(2,6-Me₂C₆H₃)C(N-2,6-Me₂C₆H₃) (6). Compounds 3-6 have different compositions and structures from each other. Density functional theory (DFT) calculations suggest initial formation of (L)[(1,5-C₈H₁₄)B(←:CN-2,6-Me₂C₆H₃)(Ar)N]Si (A), (L)[(1,5-C₈H₁₄)B(←:CNCy)(Ar)N]Si (A'), (L)[(1,5-C₈H₁₄)B(←:CNCy)-(Ar')N]Si (A″), and (L)[(1,5-C₈H₁₄)B(←:CN-2,6-Me₂C₆H₃)(Ar')N]Si (A‴) as the respective intermediates. The as-followed transition states TS, TS1', TS1″, and TS‴ all feature probable Si:→C(═N):→B bonding with different Gibbs energies of 7.24, 2.46, 3.86, and 6.59 kcal/mol, respectively, due to variation among the Ar, Ar', 2,6-Me₂C₆H₃, and Cy groups in these species, and reacted in different ways.

(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.

A theoretical investigation into novel germylenes: effects of nitrogen substitution on stability and multiplicity

The effects of substituting nitrogen atoms on the stability of novel singlet (s) and triplet (t) forms of germylenes (1-20) are compared and contrasted, at B3LYP/AUG-cc-pVTZ level of theory. Every one of the 40 new divalents scrutinized appears as a minimum on its energy surface, for showing no negative force constant. Also, every singlet (1_s-20_s) appears more stable than its corresponding triplet (1_t-20_t). The highest stability (ΔE_s-t) is achieved by germylene (11) where all the three nitrogens are bonded to the central boron atom. The E_HOMO slightly decreases when the number of electronegative, σ-acceptor nitrogen atoms increases, and also causes it to be less electron-rich. Germylene 16_s with low stability (ΔE_s-t = 17.19 kcal/mol), bond gap (ΔE_HOMO-LUMO = 57.46 kcal/mol^-1), and atomic charge on -G̈e- (+ 0.9012), has high electrophilicity (ω = 3.78 eV) and nucleophilicity (N = 3.87 eV). Germylenes 8_s, 14_s, and 19_s with coordinate covalent bond between nitrogen (N(Y)) and germylene center have low ω and high ΔE_HOMO-LUMO. The purpose of the present work was, therefore, to assess the influence of nitrogen substituents on the stability (ΔΕ_s-t), band gaps (ΔΕ_HOMO-LUMO), N, ω, and heat of hydrogenation (ΔE_H). This investigation is aimed to introduce novel germylenes that can be applied as cumulated multi-dentate NHG̈e ligands.

Silylated silicon-carbonyl complexes as mimics of ubiquitous transition-metal carbonyls

Transition-metal-carbonyl complexes are common organometallic reagents that feature metal-CO bonds. These complexes have proven to be powerful catalysts for various applications. By contrast, silicon-carbonyl complexes, organosilicon reagents poised to be eco-friendly alternatives for transition-metal carbonyls, have remained largely elusive. They have mostly been explored theoretically and/or through low-temperature matrix isolation studies, but their instability had typically precluded isolation under ambient conditions. Here we present the synthesis, isolation and full characterization of stable silyl-substituted silicon-carbonyl complexes, along with bonding analysis. Initial reactivity investigations showed examples of CO liberation, which could be induced either thermally or photochemically, as well as substitution and functionalization of the CO moiety. Importantly, the complexes exhibit strong Si-CO bonding, with CO→Si σ-donation and Si→CO π-backbonding, which is reminiscent of transition-metal carbonyls. This similarity between the abundant semi-metal silicon and rare transition metals may provide new opportunities for the development of silicon-based catalysis.

Formation of Germa-ketenimine on the Ge(100) Surface by Adsorption of tert-Butyl Isocyanide

Reactions of the (100) surfaces of Ge and Si with organic molecules have been generally understood within the concept of "dimers" formed by the 2 × 1 surface reconstruction. In this work, the adsorption of tert-butyl isocyanide on the Ge(100)-2 × 1 surface at large exposures is investigated under ultrahigh vacuum conditions. A combination of infrared spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption experiments along with dispersion-corrected density functional theory calculations is used to determine the surface products. Upon adsorption of a dense monolayer of tert-butyl isocyanide, a product whose structure resembles a germa-ketenimine (N=C=Ge) with σ donation toward and π back-donation from the Ge(100) surface appears. Formation of this structure involves divalent-type surface Ge atoms that arise from cleavage of the Ge(100)-2 × 1 surface dimers. Our results reveal an unprecedented class of reactions of organic molecules at the Ge(100) surface.

Addition of Isocyanides to Tetramesityldigermene: A Comparison of the Reactivity between Surface and Molecular Digermenes

The reaction of benzyl isocyanide, tert-butyl isocyanide, and 2,6-dimethylphenyl isocyanide with tetramesityldigermene (Mes₂ Ge=GeMes₂ ) was examined. Whereas the addition of benzyl isocyanide gave the C-NC activation product, Mes₂ Ge(CH₂ Ph)Ge(CN)Mes₂ , tert-butyl isocyanide, and 2,6-dimethylphenyl isocyanide did not give stable adducts, rather the rate of conversion of the digermene to the corresponding cyclotrigermane was accelerated. A comparison between the reactivity of the isocyanides with Mes₂ Ge=GeMes₂ and the Ge(100)-2×1 surface was made and some insights into the surface chemistry are offered.

Metal-interacted histidine dimer: an ETS-NOCV and XANES study

We have analyzed the metal coordination in a histidine dimer, hydrated with a water molecule, based on the extended transition state scheme with the theory of natural orbitals for chemical valence (ETS-NOCV).