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.