Reactions of B2 (o-tolyl)4 with Boranes: Assembly of the Pentaborane(9), HB[B(o-tolyl)(μ-H)]4.

Reactions of the diborane(4) B2 (o-tolyl)4 and monohydridoboranes are shown to give B(o-tolyl)3 and (o-tolyl)BR2 (R2 =(C8 H14 ) 3, cat 4, pin 5, (C6 F5 )2 6) as the major products. The corresponding reaction with BH3 -sources gives complex mixtures, resulting from hydride/aryl exchange, dimerization and borane elimination. This led to the isolation of the first tetra-substituted pentaborane(9) HB[B(o-tolyl)(μ-H)]4 8. The reaction pathways are probed experimentally and by computations.