Synthesis of an Isolable Bis(silylene)-Stabilized Silylone and Its Reactivity Toward Small Gaseous Molecules.

The first bis(N-heterocyclic silylene)-stabilized zero-valent silicon compound, [Si(Xant)Si]Si (, Xant = 9,9-dimethyl-xanthene-4,5-diyl), has been synthesized via the reduction of the corresponding chlorosilyliumylidene chloride precursor {[Si(Xant)Si]SiCl}Cl (). The electronic structure of silylone , whose molecular structure is confirmed spectroscopically and crystallographically, is investigated by DFT calculations and Natural Bond Orbital analysis, showing two perpendicular lone-pairs of electrons on the central Si atom, i.e.

An Isolable Bis(silylene)-Stabilized Germylone and Its Reactivity.

The first zerovalent germanium complex ("germylone") 3, [Si(Xant)Si]Ge, stabilized by a chelating bis(N-heterocyclic silylene)xanthene donor ligand 1 was successfully synthesized via the dechlorination of the corresponding {[Si(Xant)Si]GeCl}Cl complex 2 with KC; it was structurally and spectroscopically characterized, and also studied by density functional theory (DFT) calculations. Natural bond orbital (NBO) analysis of 3 unambiguously exhibits two lone pairs of electrons (one σ-type lone-pair and one 3p(Ge) lone-pair) on the zerovalent Ge atom. This is why the Ge atom can form the corresponding mono- and bis-AlBr Ge → Al Lewis adducts [Si(Xant)Si]Ge(AlBr) 4 and [Si(Xant)Si]Ge(AlBr) 5, respectively.

A Bis(silylenyl)pyridine Zero-Valent Germanium Complex and Its Remarkable Reactivity.

The synthesis, reactivity, and electronic structure of the unique germylone iron carbonyl complex [SiNSi]Ge →Fe(CO) is reported. The compound was obtained in 49 % yield from the reaction of the bis(N-heterocyclic silylenyl)pyridine pincer ligand SiNSi (1,6-C NH -[EtNSi(N Bu) CPh] ) with GeCl ⋅(dioxane) to give the corresponding chlorogermyliumylidene chloride precursor [SiNSi]Ge Cl  Cl , which was further reduced with K Fe(CO) . Single-crystal X-ray diffraction analysis of [SiNSi]Ge→Fe(CO) revealed that the Ge center adopts a trigonal-pyramidal geometry with a Si-Ge-Si angle of 95.

Heavier congeners of CO and CO as ligands: from zero-valent germanium ('germylone') to isolable monomeric GeX and GeX complexes (X = S, Se, Te).

In contrast to molecular CO and CO, their heavier mono- and dichalcogenide homologues, EX and EX (E = Si, Ge, Sn, Pb; X = O, S, Se, Te), are important support materials (, SiO) and/or semiconductors (, SiS) and exist typically as insoluble crystalline or amorphous polymers under normal conditions. Herein, we report the first successful synthesis and characterisation of an extraordinary series of isolable monomeric GeX and GeX complexes (X = S, Se, Te), representing novel classes of compounds and heavier congeners of CO and CO. This could be achieved by solvent-dependent oxidation reactions of the new zero-valent germanium ('germylone')-GaCl precursor adduct (bis-NHC)Ge→GaCl (bis-NHC = HC[{NC(H)[double bond, length as m-dash]C(H)N(Dipp)}C:], Dipp = 2,6-PrCH) with elemental chalcogens, affording the donor-acceptor stabilised monomeric germanium(iv) dichalcogenide (bis-NHC)Ge([double bond, length as m-dash]X)[double bond, length as m-dash]X→GaCl (X = S, ; X = Se, ) and germanium(ii) monochalcogenide complexes (bis-NHC)Ge[double bond, length as m-dash]X→GaCl (X = Se, ; X = Te, ), respectively.

Isolation of cationic and neutral (allenylidene)(carbene) and bis(allenylidene)gold complexes.

The one-electron reduction of a cationic (allenylidene)[cyclic(alkyl) (amino)carbene]gold(i) complex leads to the corresponding neutral, paramagnetic, formally gold(0) complex. DFT calculations reveal that the spin density of this highly robust coinage metal complex is mainly located on the allenylidene fragment, with only 1.8 and 3.

Preparation of a silanone through oxygen atom transfer to a stable cyclic silylene.

We report the evaporation of a stable cyclic silylene and its oxidation (with ozone or N2 O) through oxygen atom transfer to form the corresponding silanone under matrix isolation conditions. As uncomplexed silanones are rare owing to their very high reactivity, this method provides an alternative route to these sought-after molecules. The silanone, as well as a novel bicyclic silane with a bridgehead silicon atom derived from an intramolecular silylene CH bond insertion, were characterized by comparison of high-resolution infrared spectra with density functional theory (DFT) computations at the M06-2X/cc-pVDZ level of theory.

Observation of a thermally accessible triplet state resulting from rotation around a main-group π bond.

We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical that is formed in a thermally induced rotation around a main-group π bond, that is, the SiSi double bond of tetrakis(di-tert-butylmethylsilyl)disilene (1). The highly twisted ground-state geometry of singlet 1 allows access to the perpendicular triplet diradical 2 at moderate temperatures of 350-410 K. DFT-calculated zero-field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half-field transition.

Formation of three new bonds and two stereocenters in acyclic systems by zinc-mediated enantioselective alkynylation of acylsilanes, Brook rearrangement, and ene-allene carbocyclization reactions.

Diastereoisomerically pure (dr > 99:1) and enantiomerically enriched (er up to 98:2) substituted propargyl diols possessing a tertiary hydroxyl group were synthesized in a single-pot operation from simple acylsilanes through a combined catalytic enantioselective alkynylation of acylsilanes, followed by an allenyl-Zn-Brook rearrangement and Zn-ene-allene (or Zn-yne-allene) cyclization reaction. Two remarkable features of these reactions are the near complete transfer of chirality in the allenyl-Zn-Brook rearrangement and the highly organized six-membered transition state of the Zn-ene-allene carbocyclization found by DFT calculations. In this process, three new bonds and two new stereogenic centers are created in a single-pot operation in excellent diastereo- and enantiomeric ratios.

One-pot zinc-promoted asymmetric alkynylation/brook-type rearrangement/ene-allene cyclization: highly selective formation of three new bonds and two stereocenters in acyclic systems.

It's as easy as 1, 2, 3: In a one-pot sequence, two stereocenters and three new bonds were created with high selectivity through an asymmetric alkynylation of acyl silanes, a tandem Brook-type rearrangement and Zn-ene-allene cyclization, the addition of an electrophile, and finally oxidation. The straightforward nature of the synthetic procedure contrasts strongly with the complexity of the densely functionalized products obtained.

Were reactions of triplet silylenes observed?

The observation that (iPr3Si)(tBu3Si)Si: (1) yields an intramolecular C-H bond insertion product at room temperature, together with earlier computational predictions that triplet 1 is slightly more stable (or isoenergetic) than singlet 1 and additional considerations, led previous investigators to conclude that 1 is the first silylene to exhibit triplet electronic state reactivity. In this paper we test, using DFT and ab initio methods, whether the occurrence of intramolecular C-H bond insertion indeed indicates the presence of a triplet-state silylene. DFT calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) level show that singlet (iPr3Si)(tBuMe2Si)Si: (9), a close model of singlet 1, inserts intramolecularly into a C-H bond of the tBu group with a barrier of 22.

Si=X multiple bonding with four-coordinate silicon? Insights into the nature of the Si=O and Si=S double bonds in stable silanoic esters and related thioesters: a combined NMR spectroscopic and computational study.

The electronic structures and nature of silicon-chalcogen double bonds Si=X (X = O, S) with four-coordinate silicon in the unique silanoic silylester 2 and silanoic thioester 3 have been investigated for the first time, by (29)Si solid state NMR measurements and detailed DFT and ab initio calculations. (29)Si solid state NMR spectroscopy of the precursor silylene 1 was also carried out. The experimental and computational study of 2 and 3, which was also supported by a detailed computational study of smaller model systems with Si=O and Si=S bonds, provides a deeper understanding of the isotropic and tensor components of their NMR chemical shifts.

Silabutadienes. Internal rotations and pi-conjugation. A density functional theory study.

The potential energy surfaces (PESs) for internal rotation around the central single bond of nine silabutadienes, which include all possible mono-, di-, tri-, and tetrasilabutadienes, are investigated computationally by using DFT with the B3LYP functional and the 6-311+G(d,p) basis set. For 1-silabutadiene (3), 2-silabutadiene (4), 1,4-disilabutadiene (5), 2,3-disilabutadiene (6), and 1,3-disilabutadiene (7), the s-trans rotamer is the most stable. For 1,2-disilabutadiene (8), 1,2,3-trisilabutadiene (9), and 1,2,4-trisilabutadiene (10), all having a trans-bent SiSi double bond, the most stable conformers are those having an antiperiplanar (ap) structure.

Role of negative hyperconjugation and anomeric effects in the stabilization of the intermediate in SNV reactions.

The role of negative hyperconjugation and anomeric and polar effects in stabilizing the XZHCbetaCalphaYY'- intermediates in SNV reactions was studied computationally by DFT methods. Destabilizing steric effects are also discussed. The following ions were studied: X = CH3O, CH3S, CF3CH2O and Y = Y' = Z = H (7b-7d), Y = Y' = H, Z = CH3O, CH3S, CF3CH2O (7e-7i), YY' = Meldrum's acid-like moiety (Mu), Z = H, (8b-8d), and YY' = Mu, Z = CH3O, CH3S, CF3CH2O (8e-8i).

Preparative separation of isomeric and stereoisomeric dicarboxylic acids by pH-zone-refining counter-current chromatography.

This work involves the preparative separation of some isomeric dicarboxylic acids using pH-zone-refining counter-current chromatography (CCC), a relatively new preparative technique for the separation of ionizable compounds. The paper concentrates especially on the separation of a synthetic mixture of closely related cis and trans pairs of 1-methyl- and 1,3-dimethyl-1,3-cyclohexanedicarboxylic acids. The elution sequence of the isomers is discussed in terms of their relative acidities (pK(a) values) in solution and gas phase, hydrophobicities, and steric configuration.

Solid-state (29)Si NMR study of RSiSiR: a tool for analyzing the nature of the Si-Si bond.

The first solid state 29Si NMR of a disilyne, that is, RSiSiR, R = Si(CH(SiMe3)2)2(i-Pr) (1) was measured: delta11 = 364 +/- 20; delta22 = 221 +/- 16 and delta33 = -350 +/- 13; CSA = -643 ppm. These measured values as well as calculations for model disilynes strongly support the description of the Si-Si bond in bent disilynes as a triple bond, although with weakened pi-bonds and a reduced bond order of 2.6.

Activation of pentafluorophenylsilanes by weak Lewis bases in reaction with iminium cations.

The Lewis base mediated carbon-carbon bond forming reactions between pentafluorophenylsilanes and iminium cations were studied theoretically and experimentally. The complexation of silanes with anionic Lewis bases was analyzed computationally using DFT methods at the B3LYP/6-31+G(d) level. The pentafluorophenyl group was found to exhibit a significant stabilizing effect on the formation of pentacoordinate silicon species, with (C6F5)3SiF and C6F5SiF3 being the strongest Lewis acids.

Continuous symmetry analysis of NMR chemical shielding anisotropy.

Molecular symmetry is a key parameter which dictates the NMR chemical shielding anisotropy (CSA). Whereas correlations between specific geometrical features of molecules and the CSA are known, the quantitative correlation with symmetry--a global structural feature--has been unknown. Here we demonstrate a CSA/symmetry quantitative relation for the first time: We study how continuous deviation from exact symmetry around a nucleus affects its shielding.

(tBu2MeSi)2Sn=Sn(SiMetBu2)2: a distannene with a >Sn=Sn< double bond that is stable both in the solid state and in solution.

((t)Bu(2)MeSi)(2)Sn=Sn(SiMe(t)Bu(2))(2) 1, prepared by the reaction of (t)Bu(2)MeSiNa with SnCl(2)-diox in THF and isolated as dark-green crystals, represents the first example of acyclic distannene with a Sn=Sn double bond that is stable both in the crystalline form and in solution. This was proved by the crystal and NMR spectral data of 1. Distannene 1 has these peculiar structural features: a shortest among all acyclic distannenes Sn=Sn double bond of 2.

Trisilaallene and the Relative Stability of Si3H4 Isomers.

A theoretical quantum-mechanical study of trisilaallene, H2Si [Formula: see text] Si [Formula: see text] SiH2, and of 15 other Si3H4 isomers was carried out using ab initio and DFT methods with a variety of basis sets. Values given below are at B3LYP/6-31G(d,p). Unlike H2C [Formula: see text] C [Formula: see text] CH2 which is linear, H2Si [Formula: see text] Si [Formula: see text] SiH2 is highly bent at the central silicon atom, with a SiSiSi bending angle of 69.

Synthesis, molecular structure, and reactivity of the isolable silylenoid with a tricoordinate silicon.

The first tricoordinate fluorosilylenoid, (t-Bu2MeSi)2SiFLi.3THF (1), was synthesized, and its X-ray molecular structure was determined. 1 was synthesized in 40% yield by a bromine-lithium exchange reaction in THF of the corresponding fluorobromosilane with t-Bu2MeSiLi.

How to design linear allenic-type trisilaallenes and trigermaallenes.

A theoretical prescription as to how to design allenic-type linear trisilaallenes and trigermaallenes is provided. Until now, all of the heavier group - 14 analogues of allene were found to be bent, unlike allene itself. DFT calculations show that linear trisilaallenes and trigermaallenes can be designed by using boryl substituents, e.

Silynes (RC≡SiR') and Disilynes (RSi≡SiR'): Why Are Less Bonds Worth Energetically More?

Bond stabilization through bending! Valence bond analysis shows that the σ frames of 1-3 (1: E = Si, E' = C; 2: E = E' = Si; 3: E = E' = C) are stabilized by trans bending (B), while π bonding is weakened. In acetylene (3) π bonding overrides the σ frame and establishes a linear molecule (3 L). In contrast, the σ frames dominate in silyne (1) and disilyne (2) and lead to trans-bent structures (1 B and 2 B).

HCSiF and HCSiCl: The First Detection of Molecules with Formal C≡Si Triple Bonds.

Neutralization of [C,H,Si,X]  .+ radical cations (X=F, Cl) in conjunction with electronic structure calculations provides the first experimental evidence for the formation of the neutral silynes HC≡SiF and HC≡SiCl, which have nonlinear structures (see picture).