A Chemiluminescent Tetraaryl Diborane(4) Tetraanion

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(sp(3))−B(sp(2)) 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 Na(4)[3] quantitatively furnishes the diborane(4) dianion salt Na(2)[3], the doubly boron‐doped congener of 9,9′‐bis(fluorenylidene). Under acid catalysis, Na(2)[3] undergoes a formal Stone–Wales rearrangement to yield a dibenzo[g,p]chrysene derivative with B=B core. Na(2)[3] shows boron‐centered nucleophilicity toward n‐butyl chloride. Na(4)[3] produces bright blue chemiluminescence when exposed to air.