Silicon Chalcogenide Cage Compounds: New Structures Derived from the Bicyclic Silicon(I) Ring Compound Si4 {N(SiMe3 )Mes}4.

The bicyclic silicon(I) ring compound Si4 {N(SiMe3 )Mes}4 (2) was used as starting material in reactions with chalcogens and chalcogen transfer agents at low temperatures. This resulted in the selective formation of new cage compounds. With Me3 NO, a silicon oxide with adamantane-type cage 3 was isolated that represents the first isolated T4 silsesquioxane. Reactions with propylenesulfide and red selenium gave direct access to defect heterocubane-type cages 4 and 5 with three Si-Si bonds wherein the silicon atoms adopt different low oxidation states of +I and +III. A reaction with elemental tellurium even occurs below room temperature to provide ditelluro-tetrasila-tricyclohexane 6.

Manipulating the Substituent Sphere of a Bicyclic Silicon(I) Ring Compound Results in Anionic Cubane-Type Clusters and a Tetrahedral Silanide.

The reaction of the bicyclic silicon(I) ring compound Si4{N(SiMe3)Mes}4 1 with strong zwitterionic character and moderate sterical demand of the amido substituents with two equivalents of KC8 was investigated. This resulted in the unexpected abstraction of two amido substituents from 1 and additionally in dimerization to a dianionic Si8 cluster compound 2 with four unsubstituted silicon atoms and two [K([18]crown-6)]+ counter cations. Performing this reaction in the absence of [18]crown-6 results in release of only one amido substituent from 1 and dimerization to a dianionic Si8 cluster compound 3 with only two unsubstituted silicon atoms. This reaction with KC8 was repeated and trapping agents such as SiMe3Cl and tBuCl were added in-situ whereupon the second isolated homocyclic silylene 4 and a monoanionic hydride and tBu substituted Si8 cluster 5 with one unsubstituted silicon atom were isolated. Furthermore, 1 was reacted with KOtBu which resulted in the selective abstraction of one SiMe3 group and formation of the tetrahedral silanide 6 with one imido substituent bridging an edge of the tetrahedron.

An Anionic Amido-Substituted Seven-Vertex Siliconoid Cluster.

The silanide [Si4 {N(SiMe3 )Dipp}3 ]- (1) transforms into the anionic siliconoid cluster [Si7 {N(SiMe3 )Dipp}3 ]- (2) with four unsubstituted silicon atoms as a contact ion pair with [K([18]crown-6)] in C6 D6 at room temperature within five weeks. Anion 2 was investigated by natural population analysis and visualization of intrinsic atomic orbitals. Magnetically induced current-density calculations of 2 revealed two distinct strong diatropic vortices that sum up in one direction and create a strongly shielded apical silicon atom in 2.

Synthesis and Reactivity of a Neutral Homocyclic Silylene.

Isolation of the neutral homocyclic silylene 2 is possible via amine ligand abstraction with potassium graphite (KC8 ) and subsequent reaction with SiMe3 Cl from a bicyclic silicon(I) amide J. This reaction proceeds via an anionic homoaromatic silicon ring compound 1 as an intermediate. The twofold-coordinated silicon atom in the homocyclic silylene 2 is stabilized by an allyl-type π-electron delocalization. 2 reacts in an oxidative addition with two equivalents of MeOH and in cycloadditions with ethene, phenylacetylene, diphenylacetylene and with 2,3-dimethyl-1,3-butadiene to afford novel functionalized ring compounds.

Reactivity of the Bicyclic Amido-Substituted Silicon(I) Ring Compound Si4 {N(SiMe3 )Mes}4 with FLP-Type Character.

The bicyclic amido-substituted silicon(I) ring compound Si4 {N(SiMe3 )Mes}4 2 (Mes=Mesityl=2,4,6-Me3 C6 H2 ) features enhanced zwitterionic character and different reactivity from the analogous compound Si4 {N(SiMe3 )Dipp}4 1 (Dipp=2,6-i Pr2 C6 H3 ) due to the smaller mesityl substituents. In a reaction with the N-heterocyclic carbene NHC Me 4 (1,3,4,5-tetramethyl-imidazol-2-ylidene), we observe adduct formation to give Si4 {N(SiMe3 )Mes}4 ⋅ NHC Me 4 (3). This adduct reacts further with the Lewis acid BH3 to yield the Lewis acid-base complex Si4 {N(SiMe3 )Mes}4 ⋅ NHC Me 4 ⋅ BH3 (4). Coordination of AlBr3 to 2 leads to the adduct 5. Calculated proton affinities and fluoride ion affinities reveal highly Lewis basic and very weak Lewis acidic character of the low-valent silicon atoms in 1 and 2. This is confirmed by protonation of 1 and 2 with Brookharts acid yielding 6 and 7. Reaction with diphenylacetylene only occurs at 111 °C with 2 in toluene and is accompanied by fragmentation of 2 to afford the silacyclopropene 8 and the trisilanorbornadiene species 9.

Cycloadditions with a Stable Charge-Separated Cyclobutadiene-Type Amido-Substituted Silicon Ring Compound.

Reductive debromination of {N(SiMe3 )2 }SiBr3 with Rieke magnesium results in the formation of the five-vertex silicon cluster with one bromine substituent Si5 {N(SiMe3 )2 }5 Br, 1, and the cyclobutadiene analogue 2 in a 1:1 ratio. The latter features a planar four-membered silicon ring with a charge-separated electronic situation. Two silicon atoms in 2 are trigonal planar and the other two trigonal pyramidal. In cycloadditions with ethylene, diethylacetylene, 1,5-cyclooctadiene, and 2,3-dimethyl-1,3-butadiene cyclic unsaturated ring compounds (3-6) were formed at room temperature in quantitative reactions. Two of the products (3 and 6) show photochemical isomerization with LED light (λ=405 nm) to afford saturated ring compounds 4 e and 6'.

Reactions of a Silicon-Based Cyclic Silylone with Chalcogens.

Tetrasilasilylone N is composed of a silicon(0) atom, two NHC-coordinated silylenes, and a silaimine (Si═N) group. It reacts rapidly and selectively with the chalcogens selenium and tellurium to afford corresponding dichalcogenides 1 and 2 in high yields of 69-79%. In these compounds, one chalcogen atom is connected to the silicon(0) atom with a short bond in the range of Si═E double bonds. The second chalcogen atom adopts the bridging position between the Si(0) atom and the Si═N group. Using only 1 equiv of tellurium affords the selective formation of monotelluride 3 in 84% yield. In this monotelluride, the tellurium atom adopts a bridging position between the silylone functionality and the Si═N group.

Facile Access to an NHC-Coordinated Silicon Ring Compound with a Si=N Group and a Two-Coordinate Silicon Atom.

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si4 {N(SiMe3 )Dipp}4 1 (Dipp=2,6-diisopropylphenyl) with 5 equiv of the N-heterocyclic carbene NHCMe4 (1,3,4,5-tetramethylimidazol-2-ylidene) affords a bifunctional carbene-coordinated four-membered-ring compound with a Si=N group and a two-coordinate silicon atom Si4 {N(SiMe3 )Dipp}2 (NHCMe4 )2 (NDipp) 2. When 2 reacts with 0.25 equiv sulfur (S8 ), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si4 ring, which confirms the silylone character of the two-coordinate silicon atom in 2.

Efficient Synthesis of a NHC-Coordinated Trisilacyclopropylidene and Its Coordination Behavior.

With a co-reduction procedure using 2 equivalents Mes2 SiCl2 and 1 equivalent NHCiPr2Me2 →SiCl4 an NHC-adduct of a trisilacyclopropylidene (SiMes2 )2 SiNHCiPr2Me2 1 was synthesized. Addition of NHCMe4 to 1 results in quantitative carbene exchange under release of NHCiPr2Me2 to yield (SiMes2 )2 SiNHCMe4 2. Both NHC-coordinated trisilacyclopropylidenes coordinate to BH3 . In reaction of 1 with SiMe3 N3 a NHC-coordinated trisilane with an exocyclic Si=N double bond is formed. With diphenylacetylene 1 undergoes a ring expansion at room temperature to afford a five-membered ring compound with an NHC-coordinated silylene functionality.

Diradicaloid or Zwitterionic Character: The Non-Tetrahedral Unsaturated Compound [Si4 {N(SiMe3 )Dipp}4 ] with a Butterfly-type Si4 Substructure.

The reduction of the tribromoamidosilane {N(SiMe3 )Dipp}SiBr3 (Dipp=2,6-iPr2 C6 H3 ) with potassium graphite or magnesium resulted in the formation of [Si4 {N(SiMe3 )Dipp}4 ] (1), a bicyclo[1.1.0]tetrasilatetraamide. The Si4 motif in 1 does not adopt a tetrahedral substructure and exhibits two three-coordinate and two four-coordinate silicon atoms. The electronic situation on the three-coordinate silicon atoms is rationalized with positive and negative polarization based on EPR analysis, magnetization measurements, and DFT calculations as well as 29 Si CP MAS NMR and multinuclear NMR spectroscopy in solution. Reactivity studies with 1 and radical scavengers confirmed the partial charge separation. Compound 1 reacts with sulfur to give a novel type of silicon sulfur cage compound substituted with an amido ligand, [Si4 S3 {N(SiMe3 )Dipp}4 ] (2).

Reversible complexation of ethylene by a silylene under ambient conditions.

Treatment of toluene solutions of the silylenes Si(SAr(Me6))2 (Ar(Me6) = C6H3-2,6(C6H2-2,4,6-Me3)2, 1) or Si(SAr(Pr(i)4))2 (Ar(Pr(i)4) = C6H3-2,6(C6H3-2,6-Pr(i)2)2, 2) with excess ethylene gas affords the siliranes (Ar(Me6)S)2SiCH2CH2 (3) or (Ar(Pr(i)4)S)2SiCH2CH2 (4). Silirane 4 evolves ethylene spontaneously at room temperature in toluene solution. A Van't Hoff analysis by variable-temperature (1)H NMR spectroscopy showed that ΔG(assn) = -24.9(2.5) kJ mol(-1) for 4. A computational study of the reaction mechanism using a model silylene Si(SPh)2 (Ph = C6H5) was in harmony with the Van't Hoff analysis, yielding ΔG(assn) = -24 kJ mol(-1) and an activation energy ΔG(‡) = 54 kJ mol(-1).

Dispersion forces and counterintuitive steric effects in main group molecules: heavier group 14 (Si-Pb) dichalcogenolate carbene analogues with sub-90° interligand bond angles.

The synthesis and spectroscopic and structural characterization of an extensive series of acyclic, monomeric tetrylene dichalcogenolates of formula M(ChAr)2 (M = Si, Ge, Sn, Pb; Ch = O, S, or Se; Ar = bulky m-terphenyl ligand, including two new acyclic silylenes) are described. They were found to possess several unusual features-the most notable of which is their strong tendency to display acute interligand, Ch-M-Ch, bond angles that are often well below 90°. Furthermore, and contrary to normal steric expectations, the interligand angles were found to become narrower as the size of the ligand was increased. Experimental and structural data in conjunction with high-level DFT calculations, including corrections for dispersion effects, led to the conclusion that dispersion forces play an important role in stabilizing their acute interligand angles.

Synthesis and functionalization of the six-vertex anionic amido-substituted silicon cluster [Si6{N(SiMe3)Ph}5].

The reaction of the six-vertex amido-substituted silicon cluster Si6{N(SiMe3)Ph}6 1 with two equiv. of KC8 results in the abstraction of K{N(SiMe3)Ph} and leads to the contact ion pair 2 including an anionic silicon cluster with two unsubstituted pyramidal vertices. Facile functionalization of 2 was achieved with MeI, SiCl4 and SiBr4 and results in neutral two-fold functionalized silicon clusters.

Doped semimetal clusters: ternary, intermetalloid anions [Ln@Sn7Bi7]4- and [Ln@Sn4Bi9]4- (Ln = La, Ce) with adjustable magnetic properties.

Two K([2.2.2]crypt) salts of lanthanide-doped semimetal clusters were prepared, both of which contain at the same time two types of ternary intermetalloid anions, [Ln@Sn(7)Bi(7)](4-) and [Ln@Sn(4)Bi(9)](4-), in 0.70:0.30 (Ln = La) or 0.39:0.61 (Ln = Ce) ratios. The cluster shells represent nondeltahedral, fullerane-type arrangements of 14 or 13 main group metal atoms that embed the Ln(3+) cations. The assignment of formal +III oxidation states for the Ln sites was confirmed by means of magnetic measurements that reveal a diamagnetic La(III) compound and a paramagnetic Ce(III) analogue. Whereas the cluster anions with a 14-atomic main-group metal cage represent the second examples in addition to a related Eu(II) cluster published just recently, the 13-atomic cages exhibit a yet unprecedented enneahedral topology. In contrast to the larger cages, which accord to the Zintl-Klemm-Busmann electron number-structure correlation, the smaller clusters require a more profound interpretation of the bonding situation. Quantum chemical investigations served to shed light on these unusual complexes and showed significant narrowing of the HOMO-LUMO gap upon incorporation of Ce(3+) within the semimetal cages.

A strongly twisted SiSi bond with resemblance to a buckled dimer in an unexpected isomer of hexasilabenzene.

The reductive debromination of {N(SiMe3)Ph}SiBr31 with Rieke magnesium yields the six-vertex amido-substituted silicon cluster 2 with zwitterionic character that represents an unprecedented isomer of hexasilabenzene. The topology of Si1 and Si2 in 2 has bonding features of a highly twisted disilene and resembles that of a buckled dimer of Si(100)2 × 1 reconstructed surfaces. Cluster 2 forms the adducts 3 and 4 with NHCMe4 and DMAP, respectively. The NHC adduct 4 additionally coordinates to BH3 which affords the saturated cluster BH3NHCMe4Si6{N(SiMe3)Ph}6 (5). Furthermore, 2 undergoes addition with MeI and iodine to form the halogenated silicon clusters 6 and 7, respectively.

Unsaturated amido-substituted six-vertex mixed silicon germanium clusters.

The synthesis of mixed silicon and germanium clusters SixGe6-x{N(SiMe3)Dipp}41-3 (x = 3.7, 3.1, 2.1) with amido-substituents and two unsubstituted germanium atoms was achieved in co-reductions using the tribromosilane {N(SiMe3)Dipp}SiBr3 and the tribromogermane {N(SiMe3)Dipp}GeBr3 in three different ratios. The ratio of Si and Ge used for the synthesis of 1-3 approximately corresponds to that in the final products. All compounds were characterized by single crystal X-ray diffraction and EI mass spectrometry. In addition, 1 and 2 were characterized by solid-state 29Si{1H} CP/MAS NMR spectroscopy and multinuclear NMR spectroscopy in solution.

Tetracyclic silaheterocycle formed through a pericyclic reaction cascade including a two-fold intramolecular C-C bond activation.

Reductive debromination of the tribromoamidosilane 2 gave the tetracyclic silaheterocycle 3 through a unique reaction cascade involving unprecedented two-fold intramolecular cycloaddition by transient silylenes. Experimental and computational analyses of the reaction mechanism allowed the identification of the key intermediates that lead to the silaheterocycle 3 or, alternatively, to the cyclotrisilene 19.

[Pd3Sn8Bi6]4-: a 14-vertex Sn/Bi cluster embedding a Pd3 triangle.

The endohedral cluster anion [Pd(3)Sn(8)Bi(6)](4-) crystallizes as its K([2.2.2]crypt)(+) salt 1 upon reaction of [K([2.2.2]crypt)](2)[Sn(2)Bi(2)]·en and Pd(dppe)(2) in 1,2-diaminoethane (en)/toluene and incorporates a complete Pd(3) triangular cluster within a medium-size 14-vertex cage of Sn and Bi atoms. 1 was characterized by a combination of single crystal diffraction, ESI mass spectrometry, elemental analysis, and magnetic measurements. According to quantum chemical investigations, the Pd(3) triangle interacts only weakly with the Sn/Bi cluster shell despite the relatively small cavity inside the cage.

An unsaturated amido-substituted six-vertex germanium cluster and its reactions with alkenes and alkynes.

The unsaturated metalloid germanium cluster Ge6{N(SiMe3)Dipp}42 with two ligand-free germanium atoms and only four amine substituents was obtained starting from the base-coordinated germylene {N(SiMe3)Dipp}GeCl·DMAP 1 in 50% yield (DMAP = 4-(dimethylamino)-pyridine). This cluster reacts as a masked digermyne in cycloadditions with ethylene, diphenylacetylene and 2,3-dimethyl-1,3-butadiene in toluene at 100 °C to yield 3-5.

An intermolecular FLP System derived from an NHC-coordinated trisilacyclopropylidene.

The NHC-coordinated trisilacyclopropylidene (A) is shown to behave as the basic component of an FLP used in combination with the Lewis acid B(C6F4H)3 (i.e. B(2,3,5,6-C6F4H)3). This FLP cleaves dihydrogen highly selectively at room temperature giving rise to the ionic compound [(NHC)SiH(Mes2SiSiMes2)][HB(C6F4H)3] 1 in 90% isolated yield. Further reaction of the FLP with Ph2NH and acetone yielded compounds [(NHC)SiH(Mes2SiSiMes2)][Ph2NB(C6F4H)3] 2 and [(NHC)SiH(Mes2SiSiMes2)][MeC(CH2)OB(C6F4H)3] 3 in 75% and 80% yield, respectively. Reaction of the FLP with N2O results in the oxidation of the silylene center affording [((NHC)SiOB(C6F4H)3)(Mes2SiSiMes2)] 4 in 53% yield. These products are spectroscopically characterized and an X-ray structure of 4 is reported.