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 Li5. 5(-) contains a 2,2'-biphenyldiyl-bridged B-B core, a chelating 2,2'-biphenyldiyl moiety, and a MeO substituent. Reduction of Li5 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 Li5 and Na-43 quantitatively furnishes the diborane(4) dianion salt Na-23, the doubly boron-doped congener of 9,9'-bis(fluorenylidene). Under acid catalysis, Na-23 undergoes a formal Stone-Wales rearrangement to yield a dibenzog,pchrysene derivative with B=B core. Na-23 shows boron-centered nucleophilicity toward n-butyl chloride. Na-43 produces bright blue chemiluminescence when exposed to air.
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