Three-Coordinate Organoboron Compounds BAr₂R (Ar = Mesityl, R = 7-Azaindolyl- or 2,2′-Dipyridylamino-Functionalized Aryl or Thienyl) for Electroluminescent Devices and Supramolecular Assembly

Eight novel three-coordinate boron compounds with the general formula BAr₂L, in which Ar is mesityl and L is a 7-azaindolyl- or a 2,2′-dipyridylamino-functionalized aryl or thienyl ligand, have been synthesized by Suzuki coupling, Ullmann condensation methods, or simple substitution reactions (L = p-(2,2′-dipyridylamino)-phenyl, 1; p-(2,2′-dipyridylamino)biphenyl, 2; p-(7-azaindolyl)phenyl, 3; p-(7-azaindolyl) biphenyl, 4; 3,5-bis(2,2′-dipyridylamino) phenyl, 5; 3,5-bis(7-azaindolyl) phenyl, 6; p-[3,5-bis(2,2′-dipyridylamino) phenyl]phenyl, 7; 5-[p-(2, 2′-dipyridylamino)phenyl]-2-thienyl, 8). The structures of 1, 3, and 5-7 have been determined by X-ray diffraction analyses. These new boron compounds are bright blue emitters. Electroluminescent devices using compound 2 or 8 as the emitter and the electron-transport layer have been successfully fabricated. Molecular orbital calculations (Gaussian 98) have established that the blue emission of compounds 1-8 originates from charge transfer between the πorbital of the ligand L and the pπ orbital of the boron center. The ability of these boron compounds to bind to metal centers to form supramolecular assemblies was demonstrated by treatment of compound 2 with Zn(O₂CCF₃)₂, which generated a 1:1 chelate complex {2·Zn(O₂CCF₃)₂} (10), and also by treatment of compound 4 with AgNO₃, yielding a 2:1 coordination compound {(4)2·Ag(NO₃} (11). In the solid state, compounds 10 and 11 form interesting head-to-head and tail-to-tail extended structures that host solvent molecules such as benzene.