Dimerization of a Reactive Azaacene Diradical: Synthesis of a Covalent Azaacene Cage.

Two series of regioisomeric dicyanomethylene substituted dithienodiazatetracenes with formal para- or ortho- quinodimethane subunitswere synthesized and characterized. Whereas the para-isomers (p-n, diradical index y0=0.01) are stable and isolable, the ortho-isomer (are quinoidal and closed-shell species, the elusive ortho-isomer (o-1, y0=0.98) dimerizes into a covalent azaacene cage. Four elongated σ-CC bonds are formed and the former triisopropylsilyl(TIPS)-ethynylene groups transformed into cumulene-units. The azaacene cage dimer (o-1)2 was characterized by X-ray single crystal structure analysis and temperature-dependent infrared (IR), electron paramagnetic resonance (EPR, solid state), nuclear magnetic resonance (NMR) and ultraviolet-visible (UV-Vis) spectroscopies (solution) indicating reformation of o-1.