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 B(sp3)?B(sp2) diborane(5) anion salt Li[5]. [5]? contains a 2,2?-biphenyldiyl-bridged B?B core, chelating 2,2?-biphenyldiyl moiety, MeO substituent. Li[5] Na metal gives Na+ tetraanion in which two doubly reduced 9-borafluorenyl fragments linked via single bond. Comproportionation Na4[3] quantitatively furnishes dianion Na2[3], boron-doped congener 9,9?-bis(fluorenylidene). Under acid catalysis, Na2[3] undergoes formal Stone–Wales rearrangement yield dibenzo[g,p]chrysene derivative B=B core. shows boron-centered nucleophilicity toward n-butyl chloride. produces bright blue chemiluminescence when exposed air.