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, [SiII(Xant)SiII]Si0 (4, Xant = 9,9-dimethyl-xanthene-4,5-diyl), has been synthesized via the reduction of the corresponding chlorosilyliumylidene chloride precursor {[SiII(Xant)SiII]SiCl}+Cl- (2). The electronic structure of silylone 4, 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 Si0 atom, i.e., an sp0.41-type lone-pair and a delocalized p lone-pair. With the electron-rich and oxophilic Si0 center, silylone 4 exhibits a striking reactivity toward small gaseous molecules. Remarkably, the oxidation of silylone 4 by N2O can be controlled to generate distinct products by regulating the amount of added N2O. Exposing 4 to an excess or two molar equivalents of N2O yields the unexpected oxidation product 5, bearing a central six-membered Si4O2 ring. When 4 is mixed with one molar equivalent of N2O, the unique compound 6 is obtained, resulting from a rare 1,4-addition of two central silicon atoms to a phenyl ring of an amidinate ligand coordinated to the SiII atom. In addition, cleavage of the strong N-H bond in ammonia is also readily accomplished by silylone 4, representing the first example of NH3 activation in silylone chemistry. In the presence of the Lewis acid BPh3, silylone 4 achieves heterolytic dihydrogen cleavage and ethylene addition to form the corresponding hydridosilyliumylidene hydroborate salt 8 and the zwitterionic compound 9, respectively, which represent a new type of frustrated Lewis pair based on an electron-rich Si0 donor and a borane acceptor.

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

The first zerovalent germanium complex ("germylone") 3, [SiII(Xant)SiII]Ge0, stabilized by a chelating bis(N-heterocyclic silylene)xanthene donor ligand 1 was successfully synthesized via the dechlorination of the corresponding {[SiII(Xant)SiII]GeCl}+Cl- complex 2 with KC8; 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-AlBr3 Ge → Al Lewis adducts [SiII(Xant)SiII]Ge(AlBr3) 4 and [SiII(Xant)SiII]Ge(AlBr3)2 5, respectively. Due to the electron-rich character of the Ge0 atom, the germylone 3 displayed quite unusual reactivities. Thus, the reaction of 3 with 9-borabicyclo[3.3.1]nonane (9-BBN) as a potential Lewis acid furnished the first boryl(silyl)germylene complex 6, possessing a heteroallylic B···Ge···Si π-conjugation. When 3 was allowed to react with Ni(cod)2 (cod = 1,5-cyclooctadiene), the unique {[SiII(Xant)SiII]GeI}2NiII complex with a three-membered ring Ge2Ni-metallacycle was obtained via reductive coupling of two Ge0 atoms on the Ni center. Moreover, 3 was suitable to form a frustrated Lewis pair (FLP) with BPh3, which was capable of heterolytic H2 cleavage at 1 atm and room temperature, representing, for the first time, that a metallylone could be applied in FLP chemistry.

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]Ge0 →Fe(CO)4 is reported. The compound was obtained in 49 % yield from the reaction of the bis(N-heterocyclic silylenyl)pyridine pincer ligand SiNSi (1,6-C5 NH3 -[EtNSi(Nt Bu)2 CPh]2 ) with GeCl2 ⋅(dioxane) to give the corresponding chlorogermyliumylidene chloride precursor [SiNSi]GeII Cl+ Cl- , which was further reduced with K2 Fe(CO)4 . Single-crystal X-ray diffraction analysis of [SiNSi]Ge→Fe(CO)4 revealed that the Ge0 center adopts a trigonal-pyramidal geometry with a Si-Ge-Si angle of 95.66(2)°. Remarkably, one of the SiII donor atoms in the complex is five-coordinated because of additional (pyridine)N→Si coordination. Unexpectedly, the reaction of [SiNSi]Ge→Fe(CO)4 with GeCl2 ⋅(dioxane) (one molar equivalent) yielded the first push-pull germylone-germylene donor-acceptor complex, [SiNSi]Ge→GeCl2 →Fe(CO)4 through the insertion of GeCl2 into the dative Ge0 →Fe bond. The electronic features of the new compounds were investigated by DFT calculations.

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. Experiment and theory suggest a thermal equilibrium between 1 and 2 with a very low singlet-triplet energy gap of only 7.3 kcal mol(-1) .

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.

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. DFT calculations show that the allenyl-Zn-Brook rearrangement occurs in preference to the classic [1,2]-Zn-Brook rearrangement owing to its significantly lower activation barrier.

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 22.7 kcal/mol (22.2 kcal/mol at SCS-MP2/cc-PVTZ). However, for triplet9 the barrier of this insertion reaction is significantly higher, 34.6 kcal/mol (41.9 kcal/mol at SCS-MP2/cc-PVTZ). This implies that at room temperature the intramolecular insertion reaction of the singlet is 10(9)-10(12) faster than that of the triplet. We conclude, in contrast to previous conclusions, that the observed intramolecular C-H bond insertion reactions of 1 as well as of (tBu3Si)2Si: (2) occur from the singlet state. Furthermore, the occurrence of an intramolecular C-H bond insertion cannot serve as evidence for the presence of a triplet silylene, either in cases where the singlet and triplet states are nearly isoenergetic (e.g., 1 and 9) or even for silylenes where the triplet state is the ground state (e.g., 2), because the corresponding singlet silylenes insert intramolecularly much faster. The search for a genuine reaction of a triplet silylene has to continue.

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.

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. The general agreement between the experimental NMR spectra and the calculations strongly support our previous NMR assignment deduced from experiment. The calculations revealed that in 2 delta((29)Si(=O))(iso) is shifted upfield relative to H(2)Si=O by as much as 175 ppm; the substituents are responsible for ca. 100 ppm of this shift, while the remaining upfield shift is caused by change in the coordination number from three to four at the Si=O moiety. The change in coordination number leads to a nearly cylindrical symmetry in the plane which is perpendicular to the Si=O molecular axis (delta(11) approximately delta(22)), in contrast to the significant anisotropy found in this plane in typical doubly bonded compounds. The change in r(Si=O) or in the degree of pyramidality at the Si=O center which accompanies the change in coordination number has practically no effect on the chemical shift. delta((29)Si(=S))(iso) in 3 is shifted downfield significantly relative to that in 2, and a similar trend is found in smaller models with Si=S vs those with Si=O subunits. This downfield shift can be explained by the smaller sigma-pi* energy difference in the Si=S bond, relative to that of the Si=O bond. The NMR measurements of 2 and 3 having a four-coordinate silicon-chalcogen moiety, and the calculations of their tensor components, their bond polarities, and their Wiberg bond indices revealed that the Si=X moieties in both 2 and 3 have a significant pi(Si=X) character; yet, in both molecules there is a substantial contribution from a zwitterionic Si(+)-X(-) resonance structure, which is more pronounced in 2.

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. For tetrasilabutadiene (11), the global minimum is the gauche rotamer. The internal rotation barriers (RB) (relative to the global minimum) follow the order (kcal/mol) 5 (10.0) > 3 (7.4) > 1,3-butadiene (12, (6.6)) > 10 (4.9) > or = 7 (4.4) > or = 4 (4.0) approximately = 8 (3.9) > 9 (2.7) approximately = 6 (2.6) > 11 (2.4). The barriers are slightly smaller at CCSD(T)/cc-PVTZ, but the trend remains the same. The size of the rotation barrier is mainly dictated by the length of the central single bond; that is, it is the largest for dienes with the shorter C-C central bond (5, 3, and 12), and it is smaller for dienes with the longer Si-C and Si-Si central bonds. The strength of pi-conjugation in the s-trans conformers of silabutadienes was estimated by resonance stabilization energies (RE) calculated by using the Natural Bond Orbital (NBO) and Block Localized Wave function (BLW) methods and bond separation equations. A linear correlation is found between the barrier heights for internal rotation and pi-conjugation energies. The calculated RBs are significantly smaller than the corresponding REs, indicating that pi-resonance energies are not the only factor that dictate the RB, and therefore, RBs, although suitable for estimating trends in pi-conjugation in a series of compounds, cannot be used for estimating absolute resonance energies.

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. Two possible explanations are suggested for the mechanism of separation. They both involve the amount of retainer acid used, as it affects the separation and plays a role in the chemohydrodynamic equilibrium of the dicarboxylic acids in the column.

Heavier congeners of CO and CO2 as ligands: from zero-valent germanium ('germylone') to isolable monomeric GeX and GeX2 complexes (X = S, Se, Te).

In contrast to molecular CO and CO2, their heavier mono- and dichalcogenide homologues, EX and EX2 (E = Si, Ge, Sn, Pb; X = O, S, Se, Te), are important support materials (e.g., SiO2) and/or semiconductors (e.g., SiS2) 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 GeX2 complexes (X = S, Se, Te), representing novel classes of compounds and heavier congeners of CO and CO2. This could be achieved by solvent-dependent oxidation reactions of the new zero-valent germanium ('germylone')-GaCl3 precursor adduct (bis-NHC)Ge0→GaCl3 1 (bis-NHC = H2C[{NC(H)[double bond, length as m-dash]C(H)N(Dipp)}C:]2, Dipp = 2,6-iPr2C6H3) with elemental chalcogens, affording the donor-acceptor stabilised monomeric germanium(iv) dichalcogenide (bis-NHC)GeIV([double bond, length as m-dash]X)[double bond, length as m-dash]X→GaCl3 (X = S, 2; X = Se, 3) and germanium(ii) monochalcogenide complexes (bis-NHC)GeII[double bond, length as m-dash]X→GaCl3 (X = Se, 4; X = Te, 5), respectively. Moreover, the reactivity of 4 and 5 towards elemental sulphur, selenium, and tellurium has been investigated. In THF, the germanium(ii) monoselenide complex 4 reacts with activated elemental selenium to afford the desired germanium(iv) diselenide complex 3. Unexpectedly, both reactions of 4 and 5 with elemental sulphur, however, lead to the formation of germanium(iv) disulfide complex 2 under liberation of elemental Se and Te as a result of further oxidation of the germanium centre and replacement of the Se and Te atoms by sulphur atoms. All novel compounds 1-5 have been fully characterised, including single-crystal X-ray diffraction analyses, and studied by DFT calculations.

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 3.1% on the gold center and the CAAC ligand, respectively. In addition, the first homoleptic bis(allenylidene)gold(i) complex has been prepared and fully characterized.

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).

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).

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). The electron-withdrawing Mu substituent at Calpha stabilizes considerably the intermediates and allows their accumulation. The hyperconjugation ability (HCA) (i.e., the stabilization due to 2p(Calpha) --> sigma*(Cbeta-X) interaction) in 8b-8d follows the order (for X, kcal/mol) CH3S (8.5) > CF3CH2O (7.6) approximately CH3O (7.5). The HCA in 8b-8d is significantly smaller than that in 7b-7d due to charge delocalization in Mu in the former. The calculated solvent (1:1 DMSO/H2O) effect is small. The stability of disubstituted ions (7e-7i and 8e-8i) is larger than that of monosubstituted ions due to additional stabilization by negative hyperconjugation and an anomeric effect. However, steric repulsion between the geminal Cbeta substituents destabilizes these ions. The steric effects are larger when one or both substituents are CH3S. The anomeric stabilization (the energy difference between the anti,anti and gauche,gauche conformers) in the disubstituted anions contributes only a small fraction to their total stabilization. Its order (for the following X/Z pairs, kcal/mol) is CF3CH2O/CH3S (8i, 4.9) > CF3CH2O/CH3O (8h, 3.9) > CH3O/CH3S (8g, 3.3) > CH3S/CH3S (8f, 2.9) > CH3O/CH3O (8e, 2.4). Significantly larger anomeric effects of ca. 8-9 kcal/mol are calculated for the corresponding conjugate 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. To achieve this we employed the continuous symmetry measures methodology, which allows one to quantify the degree of content of a given symmetry. The model case we use for this purpose is a population of distorted SiH(4) structures, for which we follow the (29)Si CSA as a function of the degree of tetrahedral symmetry and of square-planar symmetry. Quantitative correlations between the degree of these symmetries and the NMR shielding parameters emerge.

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 69.4°. The Si [Formula: see text] Si bond length is 2.269 Å, longer than a regular Si [Formula: see text] Si double bond (2.179 Å) but shorter than a Si-Si single bond (2.351 Å). The distance between the terminal silicon atoms is 2.583 Å, significantly longer than a Si-Si single bond. The geometry and electronic properties of H2Si [Formula: see text] Si [Formula: see text] SiH2 are similar to those of the corresponding trisilacyclopropylidene, which is only 2.7 kcal/mol higher in energy. A barrier of only 0.1 kcal/mol separates trisilacyclopropylidene and trisilaallene which can be described as bond-stretch isomers. Sixteen minima were located on the Si3H4 PES, most of them within a narrow energy range of ca. 10 kcal/mol. Six of the Si3H4 isomers are analogous to the classic C3H4 minima structures; however, the other Si3H4 isomers do not have carbon analogues, and they are characterized by hydrogen-bridged structures.