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)(2)BCl 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 pi-bonding highest occupied molecular orbitals (HOMOs) were highly polarized. In addition the Lewis acid-base adducts, R(H)P center dot 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 H-2; 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 Me-2(H)NBH3 also led to the generation of 8 and 9, respectively. The reaction profile of the reaction of (CF3)(2)BPR2 with H-2 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-tBuC(5)H(4)N) 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).