Heavy Analogs of Alkenes, 1,3‐Dienes, Allenes and Alkynes: Multiply Bonded Derivatives of Si, Ge, Sn and Pb

Abstract

Unsaturated hydrocarbons, alkenes, alkynes and dienes are among the most fundamental classes of organic compounds. The great variety and richness of organic chemistry are to a large extent caused by the disposition of carbon to form highly reactive carbon-carbon and carbon-heteroatom multiple bonds. Standard textbooks on organic chemistry describe the C=C double bond as one made of a σ -bond, formed by the linear end-on overlap of sp 2 -hybrid orbitals of each carbon partner (sp 2 -sp 2 interaction), and a π -bond, formed by the side-on overlap of unhybridized 2p-orbitals (2p π -2p π interaction). Analogously, the carbon-carbon triple bond consists of a σ -bond, formed by the linear end-on overlap of the two sp-hybrids (sp-sp interaction), and two mutually orthogonal π -bonds, each formed by the side-on overlap of unhybridized 2p-orbitals (2p π -2p π interaction). Because the s-character of the hybrid orbitals used for the formation of carbon-carbon bonds progressively increases from the single (sp 3 -sp 3 ) to the double (sp 2 -sp 2 ) and triple (sp-sp) bonds, C=C double bonds are shorter (and stronger) than C-C single bonds, and C≡C triple bonds are shorter (and stronger) than C=C double bonds. In accord with their hybridization types, the geometry of the carbon-carbon double bond in alkenes R 2 C=CR 2 is planar with an idealized R-C=C bond angle of 120 • , and the geometry of the carbon-carbon triple bond in alkynes R-C≡C-R is linear with an idealized R-C≡C bond angle of 180 • .The structures and bonding of the alkene analogs of heavy group 14 elements, both in the crystalline form and in solution, are sharply distinctive from those of their organic counterparts (alkenes); therefore, before proceeding to the discussion on each class of heavy alkenes we will briefly comment on the specific structural features of the title compounds. Structure and bondingIn contrast to organic alkenes, which are typically planar and feature diagnostically short double bonds (av. 1.34Å for >C=C< vs av. 1.54Å for >C-C<), their analogs of the heavy group 14 elements >E=E< (E = Si-Pb) revealed diverse types of structural modes sharply distinctive from those of their organic counterparts. Thus, apart from the stretching of the E=E bond descending group 14, there are two fundamental structural deformations characteristic of heavy alkenes >E=E< and leading to their departure from planarity: trans-bending at E centers and twisting about the E=E bond. The peculiar trans-bending geometry, resulting in the overall pyramidalization at the heavy group 14 elements, was rationalized in the framework of two different MO models providing alternative explanations for this phenomenon. 5 The first qualitative model considered formation of the double bond as a result of the interaction of two monomeric carbene units, taking into account the different groundstate multiplicities of carbenes and their heavy analogs. Thus, interaction of the two monomeric carbenes, which typically have triplet ground states (or alternatively, singlet ground states with ...

Hybrid Bicyclo[1.1.0]butanes [(R4Si3)CH2] and [(R4Si2Ge)CH2] (R = SiMetBu2): Synthesis and Thermal Isomerization to Alkyl-Substituted Cyclopropenes [R3Si3]CH2R and [R3Si2Ge]CH2R

“…These Si–Si bonds are greatly shorter than those in all other currently known neutral tetrasilabicyclo[1.1.0]­butane derivatives (2.367(1)–2.871(1) Å) . And even more so, the lengths of these exceptionally short Si–Si bridging bonds in 1a,b are on the top border of the SiSi double bond length range in the cyclic disilenes [2.132(2)–2.2906(6) Å]. , Thus, in terms of the bicyclo[1.1.0]­butane SB/LB-isomers concept (Chart ), , both 1a and 1b should be recognized as the extreme cases of the SB-isomers. Calculated values for the bridging Si–Si bond in 1a [2.276(2.252) Å at the DFT1(DFT2) (DFT = density functional theory) levels of theory; DFT1: TPSSh/Def2TZVP, DFT2: M06/Def2TZVP] well agree with the crystallographic data (2.2664(9)/2.2727(9) Å).…”

“…In 2a , the doubly bonded silicon atoms are substituted by the electropositive silyl-substituents R, whereas in the intermediate 7 one of the substituents at the sp 2 -Si atom is the electronegative alkyl RCH 2 -group. It is well-established that the presence of the electronegative substituents in disilenes greatly destabilize the latter, whereas the electropositive groups have the opposite effect on the stability of the SiSi bond, and this could be the main reason that drives isomerization of 7 to 2a to its completion. , …”