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 tBuN=CH-CH=NtBu 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. Similarly, 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 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 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.

Recent developments in the chemistry of 1,3,2-diazaborolines-(2,3-dihydro-1H-1,3,2-diazaboroles)

“…1). The BN bond lengths vary from 1.41 to 1.45Å and show no significant differences to the BN bonds in the fully saturated borolidines [17]. In the planar aminoborane (CF 3 ) 2 B N(iPr) 2 the BN distance was determined to 1.37(1)Å [45] which compares well with the BN bond length of 1.38Å calculated for H 2 NBH 2 with an efficient p()-p() interaction [46].…”

“…Their reduction with an excess of sodium amalgam in hexane solution led the formation of 15a-e, as colorless crystals (69-84%) yield (Scheme 9) [17,18].…”

“…Generally this shift is less pronounced in hexane. Such a solvatochromism was rationalized by the stabilization of a polar excited state by the polar solvent [17].…”

“…In contrast to this, two irreversible waves were observed for 15d (245 and 1066 mV). The strong luminescence and the electrochemical behavior of this species was taken as evidence for interaction and -conjugation between the heterocycles [17].…”

Diazaborolyl-boryl push–pull systems with ethynylene–arylene bridges as ‘turn-on’ fluoride sensors

“…Bond lengths and angles within the benzodiazaborole part of 15 are comparable to 3 and other benzodiazaboroles. 19,[21][22][23][25][26][27][28] In summary, molecule 15 is closer to planarity than the BMes 2 -analogue 3. …”