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(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.