Vladimir Ya. Lee

Lithiosilanes and their application to the synthesis of polysilane dendrimers

Abstract An account is given of the preparation and structural investigations of silyllithium compounds, including X-ray and NMR data. This review covers the chemistry of lithiosilanes with alkyl, aryl, functional substituents and lithiooligosilanes with SiSi bonds. Application of lithiosilanes for the synthesis of polysilane dendritic molecules is also described.

Stable aromatic compounds containing heavier Group 14 elements

Abstract This account is devoted to the problem of aromaticity in the chemistry of heavier Group 14 elements: namely cyclic compounds including Si and Ge atoms in the ring. Three-membered, five-membered and six-membered ring systems, possessing aromatic character are described.

Si3C2-rings: from a nonconjugated trisilacyclopentadiene to an aromatic trisilacyclopentadienide and cyclic disilenide.

1,2,3-Trisilacyclopenta-1,4-diene 2, featuring three skeletal Si atoms in the five-membered ring, was synthesized by the thermolysis of the 1,2,3-trisilabicyclo[1.1.0]butane derivative 1 at 130 degrees C in the presence of hex-3-yne. Possessing the properties of nonconjugated cyclopentadiene, 2 readily underwent reduction with KC(8), which was followed by treatment with LiBr to form the lithium salt of 1,2,3-trisilacyclopentadienide 3(-)*[Li(+)(thf)], from which the ketone-coordinated derivative 3(-)*[Li(+)(O=C(t)Bu(2))] was prepared. Both 3(-)*[Li(+)(L)] (L = thf, O=C(t)Bu(2)) are classified as novel 6pi-electron aromatic systems based on their characteristic X-ray crystal and NMR spectral data. Addition of 12-crown-4 to 3(-)*[Li(+)(thf)] resulted in the unexpected formation of 4(-)*[Li(+)(12-crown-4)(2)], featuring the unprecedented cyclic disilenide ion 4(-).

Base-free molybdenum and tungsten bicyclic silylene complexes stabilized by a homoaromatic contribution.

Unprecedented base-free bicyclic silylene complexes of molybdenum and tungsten 3a,b (a, M = Mo; b, M = W) were prepared by the reaction of the calcium salt of 1,2,3,4-tetrakis(di-tert-butylmethylsilyl)tetrasilabicyclo[1.1.0]butane-2,4-diide 2(2-) x Ca(2+) with Cp(2)MCl(2) (M = Mo, W). The unusual structural features of 3b (long SiW double bond, short skeletal (sp(2)-Si)-(sp(3)-Si) bonds and extremely long Si-Si bridge) are indicative of a contribution from the zwitterionic resonance form W(delta-)-Si(delta+), in which the electrophilic silicon center is stabilized by the Si(3)-homoaromatic conjugation. This was supported by the theoretical calculations (NBO, NPA) revealing the presence of pi(WSi) and sigma(Si-Si bridge) interactions.

From tetragermacyclobutene to tetragermacyclobutadiene dianion to tetragermacyclobutadiene transition metal complexes.

The reaction of 3,4-dichlorotetragermetene derivative 2 with Na(2)[Fe(CO)(4)] in THF produced a (tetragermacyclobutadiene)tricarbonyliron complex, [{η(4)-((t)Bu(2)MeSi)(4)Ge(4)}]Fe(CO)(3)4, which has a slightly folded Ge(4) ring perhaptocoordinated to the Fe center. Structural and spectral characteristics of 4 show a remarkable π-donating ability of the tetragermacyclobutadiene ligand toward the transition metal, surpassing that of tetrasilacyclobutadiene and cyclobutadiene ligands. Reduction of 2 with KC(8) resulted in exclusive formation of the dipotassium salt of the tetragermacyclobutadiene dianion derivative 3(2-)·[K(+)(thf)(2)](2), representing a rare example of a 6π-electron compound that, on the basis of its structural and magnetic properties, was recognized as a nonaromatic species. Reaction of 3(2-)·[K(+)(thf)(2)](2) with CpCoI(2)(PPh(3)) produced a (cyclopentadienyl)(tetragermacyclobutadiene)cobalt complex, [{η(4)-((t)Bu(2)MeSi)(4)Ge(4)}]CoCp 7, as the first example of a sandwich compound featuring an all-germanium-containing cyclic polyene ligand.

Isomeric Metamorphosis: Si3E (E = S, Se, and Te) Bicyclo[1.1.0]butane and Cyclobutene

Group 14 and 16 hybrid heavy bicyclo[1.1.0]butanes (tBu2MeSi)4Si3E (E = S, Se, and Te) 2a-c have been prepared by the [1 + 2] cycloaddition reaction of trisilirene 1 and the corresponding chalcogen. Bicyclo[1.1.0]butanes 2 have exceedingly short bridging Si-Si bonds (2.2616(19) A for 2b and 2.2771(13) A for 2c), a phenomenon explained by the important contribution of the trisilirene-chalcogen pi-complex character to the overall bonding of 2. Photolysis of 2a and 2b produced their valence isomers, the heavy cyclobutenes 3a and 3b, featuring flat four-membered Si3E rings and a planar geometry of the Si=Si double bond. The mechanism of such isomerization was studied using deuterium-labeled 2a-d6 to ascertain the preference of the pathway, involving the direct concerted symmetry-allowed transformation of bicyclo[1.1.0]butane 2 to cyclobutene 3.

η5-1,2,3-Trisilacyclopentadienyl - A Ligand for Transition Metal Complexes: Rhodium Half-Sandwich and Ruthenium Sandwich

The lithium salt of 1,2,3-trisilacyclopentadienide [eta(5)-((t)Bu(2)MeSi)(3)Si(3)C(2)Et(2)](-) 1(-)*[Li(+)(thf)] reacted with an equivalent amount of Rh(CO)(2)(acac) in toluene to form the half-sandwich (trisilacyclopentadienyl)dicarbonylrhodium complex [eta(5)-((t)Bu(2)MeSi)(3)Si(3)C(2)Et(2)]Rh(CO)(2) 2. Pi-electron delocalization in the trisilacyclopentadienyl ring in 2 was manifested in the shielding of the ring atoms upon complexation, as well as by the observation of (1)J(Rh-C) and (1)J(Rh-Si) coupling constants. In the solid state, cyclic delocalization in 2 was seen in the diagnostic eta(5)-coordination of the heavy cyclopentadienyl ring to the Rh center and the remarkable flattening of the Si(3)C(2) five-membered ring. Reaction of 1(-)*[Li(+)(thf)] with 0.25 equiv of [Cp*RuCl](4) (Cp* = eta(5)-C(5)Me(5)) resulted in the formation of the sandwich complex Cp*Ru[eta(5)-((t)Bu(2)MeSi)(3)Si(3)C(2)Et(2)] 3 as the heavy analogue of ruthenocene.

Toward a Silicon Version of Metathesis: From Schrock-Type Titanium Silylidenes to Silatitanacyclobutenes

Olefin metathesis is one of the most important industrial processes for the production of alkenes. In contrast, silicon versions of metathesis are unknown given the lack of available silylene transition-metal complexes suitable for [2 + 2] cycloaddition with unsaturated substrates. Here, we report the synthesis of 18-electron titanium silylene complexes featuring different Lewis base ligands and classified on the basis of structural, computational, and reactivity studies as Schrock-type silylene complexes. Because of the presence of loosely bound Lewis base ligands, such silylene complexes readily undergo reaction with simple unsaturated hydrocarbons, such as alkynes, forming the corresponding [2 + 2] cycloaddition products.

Novel Organometallic Reagents: Geminal Dianionic Derivatives of the Heavy Group 14 Elements

This Forum review describes the most recent achievements in the novel prospective field of highly reactive main-group organometallics, namely, geminal dianionic derivatives of the heavy group 14 elements (Si, Ge, Sn). A brief historical introduction to the topic is followed by discussion of the current state of affairs in the field of stable derivatives and prospects for future efforts, highlighting our own synthetic approach and recent results. The most important experimental contributions, including synthesis of 1,1-dilithiosilane, -germane, and -stannane derivatives; dilithio(halo)silanes (lithiosilylenoids); metallole 1,1-dianions; and heavy analogues of the cyclobutadiene dianion derivatives, are presented, along with a discussion of the synthetic applications of the above-mentioned organometallic compounds.

Reaction of disilagermirenes with phenylacetylene: from a germasilene GeSi to a metalladiene of the type SiGeCC

Abstract The reaction of tetrakis[di-tert-butyl(methyl)silyl]-2-disilagermirene (1b) with phenylacetylene at room temperature produced highly air- and moisture-sensitive bright orange crystals of 1,1,2,3-tetrakis[di-tert-butyl(methyl)silyl]-4-phenyl-1,2-disila-3-germacyclopenta-2,4-diene (2b), which represents a previously unknown metalladiene with one SiGe and one CC double bond. The crystal structure of 2b was determined by X-ray crystallography, which showed a trans-bent configuration around the SiGe double bond with a bond length of 2.250(1) A. The reaction mechanism to form 2b, the question of conjugation of the two double bonds in a cyclopentadiene ring of 2b, as well as its reactivity are discussed. The reaction of tetrakis[di-tert-butyl(methyl)silyl]-1-disilagermirene (1a), which is an isomer of 1b, with phenylacetylene is also examined.