Manfred Weidenbruch

Some recent advances in the chemistry of silicon and its homologues in low coordination states

Abstract This review summarises some recent results on synthesis, structures, and reactivity of the heavier analogues of carbenes, alkenes, and dienes. With the isolation of the first diplumbenes, the series R 2 EER 2 , E=CPb, has been completed. These double bond systems show enormous variations in the EE bond lengths, E=SiPb, and the trans -bent angles. Furthermore, new families of multiple-bonded species have been discovered, such as a spiropentasiladiene, molecules with conjugated SiSi, GeGe, and GeC double bonds, and a diplumbylene RPbPbR. However, no stable compounds featuring homonuclear triple bonding between heavier Group 14 elements are known. Theoretical insights on the nature of such triple bonds as well as first attempts to achieve this target are discussed.

Some Silicon, Germanium, Tin, and Lead Analogues of Carbenes, Alkenes, and Dienes

The isolation and characterization of thermally stable diaminocarbenes and diaminosilylenes has reawakened interest in the low-coordinated compounds of group-14 elements. The existence of these species is presumably due to interactions between the free electron pairs on the nitrogen atoms and the carbene carbon atom or the silicon atom. The present review is mainly concerned with the heavier analogues R2Ge:, R2Sn:, and R2Pb:, systems without intramolecular donor stabilization that owe their existence principally to steric shielding by the voluminous alkyl or aryl groups R. Dimerizations of these electron-sextet molecules give rise to the double-bond systems of the digermenes, distannenes, and diplumbenes; the latter species have for a long time been considered as being incapable of existence. The properties of these molecules are compared with the results of quantum chemical calculations. A separate section is devoted to the isomers of Si4R6, which include novel tetrasilacyclobutenes and the diene analogues, the tetrasilabuta-1,3-dienes.

Tetrakis(2,4,6-triisopropylphenyl)diplumbene: A Molecule with a Lead–Lead Double Bond

There is good agreement between the length of the lead–lead double bond calculated for H2Pb=PbH2 and that present in the tetraaryldiplumbene 1 (R=2,4,6-iPr3C6H2), which has now been synthesized and structurally characterized. Furthermore, the calculated and experimentally observed trans bent angles Θ are similar. Thus, compounds with homonuclear double bonds between all elements of Group 14 are now known.

Siliciumverbindungen mit starken intramolekularen sterischen wechselwirkungen: XIX. Simultane bildung und reaktionen von di-t-butylsilandiyl und tetra-t-butyldisilen☆☆☆

Abstract The photochemically induced or palladium salt-catalyzed cleavage of hexa-t-butylcyclotrisilane leads to di-t-butylsilanediyl ( 2 ) and tetra-t-butyldisilene ( 3 ), which can be trapped by a variety of multiple-bonded compounds. Reaction of the cyclotrisilane with phenylacetylene gives the 1-sila-2,4-cyclopentadiene ( 4 ), the disilacyclobutene ( 5 ), and a 1/2 addition product between 3 and the acetylene. Reaction of the cyclotrisilane with ketones RCOR′ (R = R′ = CH 3 , CF 3 , C 6 H 5 R = CH 3 , R′=C 6 H 5 ) is thought to proceed by addition of 2 and 3 to give the silaoxiranes and disilaoxetanes, which can rearrange to the corresponding silyl and disilyl enol ethers, respectively. An X-ray structure analysis of diphenyl-1,1,2,2-tetra-t-butyl-1,2-disilaoxetane reveals an almost planar ring with slightly elongated SiSi, SiO, CO bond distances, but an extremely long SiC ring bond.

A carbene–stannylene adduct with a long tin–carbon double bond?

Bis(2,4,6-triisopropylphenyl)stannylene, formed by thermolysis of the cyclotristannane 4, reacts with 4,5-dimethyl-1,3-diisopropylimidazol-2-ylidene to furnish the adduct 8; an X-ray structural analysis of this compound reveals the presence of a long tin–carbon ‘double bond’ with a length of 237. 9(5) pm and a distinctly pyramidal tin centre.

Hexa-t-butyldigerman und Hexa-t-butylcyclotrigerman: Moleküle mit den derzeit längsten GeGe- und GeC-Bindungen☆

Reductive halogen elimination from di-t-butyldichlorogermane gives hexa-t-butylcyclotrigermane (2) in low yield. The X-ray structure analysis of 2 reveals an equilateral triangle of D3 symmetry with very long GeGe (256.3(1) pm) and GeC (205.6(3) pm) bonds. Hexa-t-butyldigermane (6) is formed analogously by treatment of tri-t-butylchlorogermane with lithium naphthalenide. The molecular structure of 6 has the longest GeGe (d|− 271.0 pm) and GeC (d|− 207.6 pm) bonds found in any molecule up to now. Despite the extreme bond lengths, 6 is thermally and solvolytically very stable.

Pentafluorphenylverbindungen des Zinks and Cadmiums: Bildung und Strukturen von (C6F5)2Zn(thf)2 und von tetramerem C6F5CdOH

Abstract Although single crystals of bis(pentafluorphenyl)zinc ( 1 ) and -cadmium ( 2 ) suitable for X-ray structure analyses could not be isolated, crystallization of 1 in the presence of tetrahydrofuran (THF) gives the adduct (C 6 H 5 ) 2 Zn(thf) 2 , which has tetrahedrally coordinated molecules in the solid state. Toluene, containing small amounts of water, reacts with 2 to give tetrameric pentafluorophenyl cadmium hydroxide, which has a cubane-type structure with the cadmium and the oxygen atoms occupying the corner positions.

Silicium-verbindungen mit starken intramolekularen sterischen wechselwirkungen : VI. Darstellung und eigenschaften des tri-t-butylsilans und der tri-t-butylhalogensilane

Abstract Tri-t-butylsilane has been prepared by treatment of t-butyllithium with HSiCl 3 or HSiF 3 . Reactions of this compound with the halogens Cl 2 , Br 2 , I 2 , and the pentafluorides PF 5 or SbF 5 gave the expected tri-t-butylhalosilanes. With SO 3 bis(tri-t-butylsilyl) sulfate is obtained via tri-t-butylsilanole. Reaction with dihalocarbenes obtained from phenyl(trihalomethyl)mercury compounds or the system haloform/base leads to the corresponding tri-t-butyl(dihalomethyl)silanes. The reactions with nucleophiles proceed much less readily. Until now only the reaction of tri-t-butylhalosilanes with LiAlD 4 and with KOH giving tri-t-butylsilane- d 1 and tri-t-butylsilanole could be carried out. The attempted preparation of long-living tri-butylsilyl-radicals as well as the direct observation of tri-t-butylsilicenium ions was unsuccessful. In the first case intermolecular hydrogen abstraction by the radical leads to the hydrosilane, and in the second case the experimental results are better explained by Lewis-acid—base interactions than by silicenium ion formation.

A Heteroleptic Diplumbene and a Magnesium Dibromide Stabilized Plumbylene Dimer

Treatment of the diarylplumbylene R2Pb: (R = 2,4,6-iPr3C6H2) with the disilylplumbylene R′2Pb: [R′ = Si(SiMe3)3] furnishes the heteroleptic plumbylene RR′Pb: which exists as the diplumbene RR′Pb=PbRR′ (7) in the solid state. The X-ray structure analysis of 7 reveals a trans-bent angle of 42.7° and a Pb–Pb bond length of 298.99(5) pm, the shortest observed so far for diplumbenes with a lead–lead double bond. Reaction of mesitylmagnesium bromide with PbCl2 yielded black crystals of an MgBr2 stabilized dimesitylplumbylene dimer with a large trans-bent angle of 71°, a lead–lead separation of 335.49(6) pm, and long Pb···Br contacts of 315.71(8) pm.

Aminosilylgermylenes: (E)‐Digermene versus Germylene Crystallization [1]

The diaminosilylene tBuNCH2CH2NtBuSi: reacted with the diaminogermylenes RNCH2CH2NRGe: R = 2,6-Me2C6H3, iPr, by silylene insertion into one of the Ge–N bonds to furnish the aminosilylgermylenes 8, R = 2,6-Me2C6H3, and 9, R = iPr. The X-ray structure analyses of these compounds revealed that 8 remains monomeric in the crystal with weak Ge … Ge interactions to the germanium atom of a neighbouring germylene molecule, whereas 9 dimerizes to give the strongly twisted (E)-1,2-diamino-1,2-disilyldigermene (E)-10 with a long Ge–Ge double bond of 246 pm and a large trans-bent angle of 47.3°. Aminosilylgermylene: Kristallisation als Germylen oder als (E)-Digermen [1] Die Reaktionen des Diaminosilylens tBuNCH2CH2NtBuSi: mit den Diaminogermylenen RNCH2CH2NRGe: R = 2,6-Me2C6H3, iPr, lieferten unter Insertion des Silylens in eine der Ge–N-Bindungen die Aminosilylgermylene 8, R = 2,6-Me2C6H3, und 9, R = iPr. Die Rontgenstrukturanalysen dieser Verbindungen ergaben, das 8 im Kristall monomer mit schwachen Ge … Ge Wechselwirkungen zum Germaniumatom eines benachbarten Germylenmolekuls vorliegt, wahrend 9 zum stark verdrillten (E)-1,2-Diamino-1,2-disilyldigermen (E)-10 mit einer langen Ge–Ge-Doppelbindung von 246 pm und grosem trans-Abknickwinkel von 47.3° dimerisiert.