BNB-Doped Phenalenyls: Modular Synthesis, Optoelectronic Properties, and One-Electron Reduction

A highly modular
synthesis of BNB- and BOB-doped phenalenyls is
presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid
anhydride 1 with LiAlH4/Me3SiCl
afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent
transformations. Upon addition of MesMgBr/Me3SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through
the reaction of 2 with (Me3Si)2NR′ or 5 with H2NR′ or H2O (R′ = H, Me, p-Tol). A helically
twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed
by a Grignard-mediated double ring-closure reaction. All compounds
devoid of B–H bonds show favorable optoelectronic properties,
such as luminescence and reversible reduction behavior. In the case
of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion
salt K­[7•] was generated through chemical
reduction with K metal and characterized by EPR spectroscopy. K­[7•] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation
of the diamagnetic BNB-doped 1H-phenalene K­[7H].