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 tBu(2)PLi yielded triphosphinoborane (tBu(2)P)(3)B. Triphosphinoboranes with diversified phosphanyl substituents were obtained via a two-step reaction, in which isolable bromodiphosphinoborane (tBu(2)P)(2)BBr is first formed and then reacts with 1 equiv of less bulky phosphide R2PLi (R2P = Cy2P, iPr(2)P, tBuPhP, or Ph2P). By utilizing this method, we obtained a series of triphosphinoboranes with the general formula (tBu(2)P)(2)BPR2. 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 pi 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 (tBu(2)P)(2)BPiPr(2) with H3B center dot 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.