Modular Approach to Kekulé Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes

A modular approach for the synthesis
of Kekulé diradicaloids
is reported. The key step is the insertion of a carbene, namely, a
cyclic (alkyl)­(amino)­carbene (CAAC), into the C–H bonds of
two terminal alkynes linked by a spacer. Subsequent hydride abstraction,
followed by two-electron reduction of the corresponding bis­(iminium)
salts, affords the desired diradicaloids. This synthetic route readily
allows for the installation of communicating spacers, featuring different
degrees of aromaticity and lengths, and gives the possibility of generating
unsymmetrical compounds with two different CAACs. Electron paramagnetic
resonance (EPR), NMR, UV–vis, and X-ray studies in combination
with quantum-chemical calculations give insight into the electronic
nature of the deeply colored Kekulé diradicaloids. They feature
a singlet ground state with varying degrees of diradical character
in combination with small singlet/triplet gaps. Upon lengthening of
the spacer, the properties of the compounds approach those of monoradicals
in which steric protection of the propargyl radical moiety is necessary
to inhibit decomposition pathways. Most of these diradicaloids are
stable at room temperature, both in solution and in the solid state,
but are highly oxygen-sensitive. They represent the first diradicaloids
derived from iminium salts.