Monomeric Triphosphinoboranes: Intramolecular Lewis Acid-Base Interactions between Boron and Phosphorus Atoms

Herein, we present the synthesis of the first fully characterized monomeric triphosphinoboranes. The simple reaction of boron tribromide with 3 equiv of bulky lithium phosphide tBu₂PLi yielded triphosphinoborane (tBu₂P)₃B. Triphosphinoboranes with diversified phosphanyl substituents were obtained via a two-step reaction, in which isolable bromodiphosphinoborane (tBu₂P)₂BBr is first formed and then reacts with 1 equiv of less bulky phosphide R₂PLi (R₂P = Cy₂P, iPr₂P, tBuPhP, or Ph₂P). By utilizing this method, we obtained a series of triphosphinoboranes with the general formula (tBu₂P)₂BPR₂. On the basis of structural and theoretical studies, two main types of triphosphinoborane structures can be distinguished. In the first type, all three electron lone pairs interact with the formally empty p orbital of the central boron atom, resulting in delocalized π bonding, whereas in the second type, one localized P═B bond and two P-B bonds are observed. The Lewis acidic-basic properties of triphosphinoboranes during the reaction of (tBu₂P)₂BPiPr₂ with H₃B·SMe₂ were analyzed. The P-B bond-containing compound mentioned above not only formed an adduct with BH₃ but also activated the B-H bond of the borane molecule, resulting in the incorporation of the BH₂ unit into two phosphorus atoms and migration of a hydride to the boron atom of the parent triphosphinoborane. The structures of the triphosphinoboranes were confirmed by single-crystal X-ray analysis, multinuclear nuclear magnetic resonance spectroscopy, and elemental analysis.