A Chemiluminescent Tetraaryl Diborane(4) Tetraanion.

Two subvalent, redox-active diborane(4) anions, [3] and [3] , 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 )-B(sp ) 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] in which two doubly reduced 9-borafluorenyl fragments are linked via a B-B single bond. Comproportionation of Li[5] and Na [3] quantitatively furnishes the diborane(4) dianion salt Na [3], the doubly boron-doped congener of 9,9'-bis(fluorenylidene). Under acid catalysis, Na [3] undergoes a formal Stone-Wales rearrangement to yield a dibenzo[g,p]chrysene derivative with B=B core. Na [3] shows boron-centered nucleophilicity toward n-butyl chloride. Na [3] produces bright blue chemiluminescence when exposed to air.