Diphosphinoboranes as Intramolecular Frustrated Lewis Pairs: P–B–P Bond Systems for the Activation of Dihydrogen, Carbon Dioxide, and Phenyl Isocyanate

Abstract

Herein, we present the first example of the activation of small molecules by P–B–P bond systems. The reactivity study involves reactions of two selected diphosphinoboranes, ( t -Bu 2 P) 2 BPh ( 1′ ) and (Cy 2 P) 2 BN i Pr 2 ( 2 ), that differ in terms of their structural and electronic properties for the activation of dihydrogen, carbon dioxide, and phenyl isocyanate. Diphosphinoborane 1′ activates H 2 under very mild conditions in the absence of a catalyst with the formation of the dimer ( t -Bu 2 PB(Ph)H) 2 and t -Bu 2 PH. Conversely, diphosphinoborane 2 did not react with H 2 under the same conditions. The reaction of 1′ with CO 2 led to the formation of a compound with an unusual structure, where two phosphinoformate units were coordinated to the PhBOBPh moiety. In addition, 2 reacted with CO 2 to insert two CO 2 molecules into the P–B bonds of the parent diphosphinoborane. Both diphosphinoboranes activated PhNCO, yielding products resulting from the addition of two and/or three PhNCO molecules and the formation of new P–C, B–O, B–N, and C–N bonds. The products of the activation of small molecules by diphosphinoboranes were characterized with nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, single-crystal X-ray diffraction, and elemental analysis. Additionally, the reaction mechanisms of the activation of small molecules by diphosphinoboranes were elucidated by theoretical methods.

Phosphinoborinium cation: a synthon for cationic B–P bond systems

“…26 Despite the presence of one to three B-P bonds, these systems retain Lewis acidic and Lewis basic character of the reactive boron and phosphorus centres and behave as intramolecular FLPs to activate H 2 , CO 2 , other heterocumulenes, SO 2 , N 2 O, and BH 3 . 24,[26][27][28] As a continuation of our previous studies, we decided to introduce this research concept to cationic boron compounds and synthesize low-coordinate boronic cations possessing B-P functionality.…”

“…Interestingly, in the case of ambiphilic species containing tri-coordinate B atoms, such as phosphinoboranes and diphosphinoboranes, the small molecules are inserted into the B-P bond (except H 2 ). 24,27,28,36 Similar to the product of the reaction of the phosphinoborinium cation with benzophenone, the B-P bond is retained in adducts of neutral phosphaborene (containing dicoordinate B atom) with ketones, which were further utilized in phospha-bora-Wittig reaction. 37 In conclusion, we have provided a simple method for the generation of phosphinoborinium cation intermediates utilizing synthetically accessible bromophosphinoboranes.…”

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

“…The chemistry of diphosphinoboranes has been explored to a lesser extent than that of other P–B systems (Chart B). Most synthesis attempts have been made by Nöth et al − Recently, we vastly expanded the chemistry of these compounds, not only by the synthesis and isolation of several new diphosphinoboranes but also by revealing their potential in the activation of small molecules . Our preliminary research on the reactivity of selected diphosphinoboranes revealed that these species react with isocyanates, CO 2 , and H 2 .…”

“…Our preliminary research on the reactivity of selected diphosphinoboranes revealed that these species react with isocyanates, CO 2 , and H 2 . We have also reported the very first P–B system that activates both H 2 and CO 2 …”

“…Most synthesis attempts have been made by Nöth et al 19 − 21 Recently, we vastly expanded the chemistry of these compounds, not only by the synthesis and isolation of several new diphosphinoboranes 22 but also by revealing their potential in the activation of small molecules. 23 Our preliminary research on the reactivity of selected diphosphinoboranes revealed that these species react with isocyanates, CO 2 , and H 2 . We have also reported the very first P–B system that activates both H 2 and CO 2 .…”