Syntheses, structures, luminescence and electrochemistry of benzene- and biphenyl-centered bis- and tris-1,3,2-diazaboroles and -1,3,2-diazaborolidines

Abstract

Reaction of 1,4-bis(dibromoboryl)benzene (1a) with 2 equiv. of the diazabutadiene ^tBuNCH–CHN^tBu and subsequent reduction of the obtained bis(1,3,2-diazaborolium)salt 2a with sodium amalgam afforded the 1,4-bis(1,3,2-diazaborolyl)benzene 3a. Similary, 1,3-bis(dibromoboryl)benzene (1b), 1,3,5-tris(dibromoboryl)benzene (1c) and 4,4′-bis(dibromoboryl)biphenyl (1d) were converted into compounds 3b, 3c and 3d which contain two or three diazaborolyl substituents at the arene core. Treatment of precursors 1a,b,d with two equiv. or 1c with three equiv. of N,N′-di-tert-butylethane-1,2-diamine in the presence of an excess of NEt₃ gave rise to the diazaborolidine derivatives 4a–4d. Reaction of 1,3-bis(diiodoboryl)benzene 1e with two equivalents of N,N′-dimethylethane-1,2-diamine in the presence of NEt₃ furnished the corresponding 1,3-bis(diazaborolidinyl)benzene 4e. The novel compounds were characterized by elemental analyses and spectroscopy (¹H, ¹³C, ¹¹B NMR, MS). The molecular structures of 3c, 4a and 4e were eludicated by X-ray-diffraction analyses. In addition to this, the oxidative cyclovoltammograms and blue emission spectra of these novel compounds were discussed. Here, the electronic communication between boron heterocycles on the different spacer-units and the luminescence of the oligo-diazaborolylarenes were of interest.