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=(C8H14) 3, cat 4, pin 5, (C6F5)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. The diborane(4) B2(o‐tolyl)4 reacts with species with B−H bonds giving a complex mixture arising from hydride/aryl exchange, dimerization and borane elimination. Use of BH3 sources led to the isolation of the first tetra‐substituted pentaborane(9) HB[B(o‐tolyl)(μ‐H)]4.