Germacyclobutenes: Generation by 1,1‐Carbalumination or 1,1‐Carbagallation and Their Photophysical Properties

Abstract

Aluminium- and gallium-functionalised alkenylalkynylgermanes, R(1) 2 Ge(C≡C-R(2) )[C{E(CMe3 )2 }=C(H)-R(2) ] (E=Al, Ga), exhibit a close contact between the coordinatively unsaturated Al or Ga atoms and the α-C atoms of the intact ethynyl groups. These interactions activate the Ge-C(alkynyl) bonds and favour the thermally induced insertion of these C atoms into the E-C(vinyl) bonds by means of 1,1-carbalumination or 1,1-carbagallation reactions. For the first time the latter method was shown to be a powerful alternative to known metallation processes. Germacyclobutenes with an unsaturated GeC3 heterocycle and endo- and exocyclic C=C bonds resulted from concomitant Ge-C bond formation to the β-C atoms of the alkynyl groups. These heterocyclic compounds show an interesting photoluminescence behaviour with Stokes shifts of >110 nm. The fascinating properties are based on extended π-delocalisation including σ*-orbitals localised at Ge and Al. High-level quantum chemical DFT and TD-DFT calculations for an Al compound were applied to elucidate their absorption and emission properties. They revealed a biradical excited state with the transfer of a π-electron into the empty p-orbital at Al and a pyramidalisation of the metal atom.

Marriage of heavy main group elements with π-conjugated materials for optoelectronic applications

“…8). 110 Upon excitation of 103 at ca. 265 nm, deep blue fluorescence in the solid state and solution was noted.…”

σ‐Bond Activation in Aluminium‐Functionalized Alkynylchlorogermanes: Facile Insertion of Isocyanate and Azide into Al–C and Ge–Cl Bonds

“…The weakening of the E–X bonds was in selected cases confirmed by quantum‐chemical calculations indicating a decrease of the Wiberg bond indices . Interactions of Si‐ or Ge‐bound alkynyl groups with Al or Ga atoms resulted in an activation of Si–C and Ge–C bonds,, which favoured rearrangement to sila‐ and germacyclobutenes (1,1‐carbametallation) , . Intramolecular coordination of NR 2 groups or Cl atoms to the Lewis‐acidic metal atoms facilitated the unique elimination of an imine at room temperature from a (diethylamino)germane or a Cl/ t Bu exchange between Al and Si/Ge atoms (dyotropic rearrangement) , .…”

Hydroalumination of Oligoalkynylgermanes and ‐digermanes – Reactions with Heterocumulenes by Al–C or Ge–C Bond Activation and Formation of a Hexazenedialuminum Complex

“…Treatment of dialkynyl derivatives with one equivalent of an aluminum or gallium hydride yielded mixed alkenyl‐alkynyl compounds ( A , Scheme ), in which the Lewis acidic metal atoms have a close contact to the C α atoms of the remaining alkynyl groups . This interaction resulted in an activation of the E –C(≡C– R ) bond ( E = Si, Ge) and favored thermal rearrangement (1,1‐carbometallation) to yield sila‐ or germacyclobutenes, which showed interesting fluorescence properties , . Halogen atoms attached to Si or Ge resulted in strong intramolecular Al– X or Ga– X interactions ( B ) and a significant lengthening and activation of E – X bonds ( X = Cl, Br), which resulted in rearrangement and exchange of halogen atoms and Al bound tert ‐butyl groups , .…”