Umpolung at Boron: Ancillary-Ligand-Induced Formation of Boron-Based Donor Ligands from Phosphine-Boranes.

Easy-to-prepare η2 -coordinated phosphine-borane ligands are demonstrated to liberate hydrogen upon treatment with different σ-donor/π-acceptor ligands (CO, tBuNC, CN- ). Depending on the utilized ancillary ligand, different reaction pathways are observed, ranging from simple hydride protonation to iron-boron bond formation and subsequent rearrangement to pincer-type ligands based on a tricoordinate boron centre. The last-named reactivity is in line with a formal umpolung at the boron centre from a Lewis acidic borane to a Lewis basic boron-based donor ligand.