2,3‐Dialkynyl‐1,4‐diazabuta‐1,3‐dienes as Novel π‐Systems: Synthesis, Structure, and Electronic Properties

The introduction of alkynyl groups into the 2,3‐positions of the 1,4‐diazabuta‐1,3‐diene (DAD) backbone succeeds along two complementary synthetic routes either by condensing triisopropylsilyl‐terminated dialkynyl‐1,2‐diones with primary aromatic amines or by a palladium‐mediated alkynylation of bis(imidoyl chlorides). X‐ray crystallographic analyses of two dialkynyl DAD derivatives reveals their planar ( E‐s‐trans‐E ) conformations in the solid state. However, the central CC bond of both DAD backbones investigated has a length of 1.491(2) Å, and is therefore too long to indicate efficient delocalization across the DAD core. The UV/Vis spectra of dialkynyl DADs demonstrate that their absorptions in comparison to those of non‐alkynyl DADs are bathochromically shifted by more than 40 nm, thereby demonstrating the suitability of the title compounds for developing NIR chromophores. The electron absorption properties of differentially N,N′ ‐disubstituted dialkynyl DADs gave no indication for a push‐pull effect across the DAD skeleton and suggest the ynimine moiety as the active chromophore of dialkynyl DADs. The electrochemical properties of the title compounds were determined by cyclo‐ and steady state voltammetry and show that introducing alkynyl groups leads to more easily reducible DAD systems. Again, the perception of dialkynyl DADs as covalently linked, but electronically decoupled ynimine units is corroborated by the redox potential of a differentially N,N′ ‐disubstituted dialkynyl DAD. Similar conclusions were drawn from semiempirical MO (PM3) calculations of dialkynyl DADs.