Synthesis and Reactivity of the Phosphinoboranes R2PB(C6F5)2

The phosphinoboranes [R2PB(C6F5)2]2 (R = Et 1, Ph 2) and R2PB(C6F5)2 (R = tBu 3, Cy 4, Mes 5) were synthesized from the reaction of (C6F5)2BCl and the corresponding lithium phosphide. The relationships between B−P distance, P pyramidality, and the extent of BP multiple bonding were further explored computationally. Natural Bond Order (NBO) analyses of 3 and 4 showed that the π-bonding highest occupied molecular orbitals (HOMOs) were highly polarized. In addition the Lewis acid−base adducts, R2(H)P·B(H)(C6F5)2 (R = Et 6; Ph 7; tBu 8; Cy 9; Mes 10) were prepared via the reaction of the phosphines R2PH with the borane HB(C6F5)2. Compounds 1 and 2 showed no signs of reaction with H2; however, reaction of compounds 3 and 4 with H2 was observed to give 8 and 9. In a related set of reactions compounds 3 and 4 were reacted with H3NBH3 or Me2(H)NBH3 also led to the generation of 8 and 9, respectively. The reaction profile of the reaction of (CF3)2BPR2 with H2 was examined computationally and shown to be exothermic. Efforts to effect the reverse reaction, that is, dehydrogenation of adducts 6−10 were unsuccessful. Compound 4 was also shown to react with 4-tert-butylpyridine to give Cy2PB(C6F5)2(4-tBuC5H4N) 11 while reactions of 3 and 4 with the Lewis acid BCl3 gave the dimers (R2PBCl2)2 (R = tBu 12, Cy 13) and the byproduct ClB(C6F5)2.