A Chemiluminescent Tetraaryl Diborane(4) Tetraanion

Abstract

Two subvalent, redox‐active diborane(4) anions, [3]4− and [3]2−, carrying exceptionally high negative charge densities are reported: Reduction of 9‐methoxy‐9‐borafluorene with Li granules without stirring leads to the crystallization of the B(sp3)−B(sp2) diborane(5) anion salt Li[5]. [5]− contains a 2,2′‐biphenyldiyl‐bridged B−B core, a chelating 2,2′‐biphenyldiyl moiety, and a MeO substituent. Reduction of Li[5] with Na metal gives the Na+ salt of the tetraanion [3]4− in which two doubly reduced 9‐borafluorenyl fragments are linked via a B−B single bond. Comproportionation of Li[5] and Na4[3] quantitatively furnishes the diborane(4) dianion salt Na2[3], the doubly boron‐doped congener of 9,9′‐bis(fluorenylidene). Under acid catalysis, Na2[3] undergoes a formal Stone–Wales rearrangement to yield a dibenzo[g,p]chrysene derivative with B=B core. Na2[3] shows boron‐centered nucleophilicity toward n‐butyl chloride. Na4[3] produces bright blue chemiluminescence when exposed to air.

Cationic Symmetrically and Unsymmetrically Substituted Diboranes and Bis(diboranes) with Direct Boron‐Boron Bond: Synthesis by Substitution, Stability and Properties

“…Such borates, generally formed in situ, for example by addition of an alkoxide, OR − , to B 2 (cat) 2 (see structure on the left of Figure 1a), are useful reagents for (catalyst‐free) C−B bond formations in synthetic chemistry [14] . In addition, Figure 1a includes Lewis structures of a diboranate formed from B 2 (pin) 2 and a β‐diketiminato magnesium n ‐butyl derivative, [15–17] and of a tetraaryl diborane(4) tetraanion, [18] both being nucleophilic.…”