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 t Bu 2 PLi yielded triphosphinoborane ( t Bu 2 P) 3 B. Triphosphinoboranes with diversified phosphanyl substituents were obtained via a two-step reaction, in which isolable bromodiphosphinoborane ( t Bu 2 P) 2 BBr is first formed and then reacts with 1 equiv of less bulky phosphide R 2 PLi (R 2 P = Cy 2 P, i Pr 2 P, t BuPhP, or Ph 2 P). By utilizing this method, we obtained a series of triphosphinoboranes with the general formula ( t Bu 2 P) 2 BPR 2 . 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 ( t Bu 2 P) 2 BP i Pr 2 with H 3 B·SMe 2 were analyzed. The P-B bond-containing compound mentioned above not only formed an adduct with BH 3 but also activated the B-H bond of the borane molecule, resulting in the incorporation of the BH 2 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.