Excited state dynamics of P-H···H-B and P-H···H-Si hydride-hydride interactions and potential dehydrogenation reactions in frustrated lewis pair

Abstract

Dihydrogen interactions plays a crucial role in frustrated Lewis pairs (FLPs) strategy and promotes dehydrogenation reactions due to its low directionality and saturability. This work focuses on the excited-state dynamics of the P-H···H-B and P-H···H-Si intramolecular hydride-hydride interactions in 4-bis(pentafluorophenyl)-borane-5-dimesitylphosphino-9,9-dimethylxanthine (FPB-MP-MX) and 5-dimethylsilyl-9,9-dimethylxanthene-4-yl-diphenylphosphonium cation (MSi-PP-MX), respectively. The interaction between B-H, C-H and π electron of benzene ring are also considered. Upon photoexcitation to the first singlet (S1) state, the P-H···H-B of FPB-MP-MX and the P-H···H-Si of MSi-PP-MX are both strengthened, while B-H···π and C−H···π interaction are all weakened. These changes are induced by the charge transfer from the 9,9-dimethyl-xanthene moiety to the electron-deficient phosphorus cation and mesitylene group, which are demonstrated by the molecule orbital, electron–hole, and charge analysis. In addition, the potential energy curve of FPB-MP-MX scanned the P-H···H-B shows low energy barriers of 20.68 kcal mol−1 in the S0 state, implying the potential dehydrogenation reaction with lower energy of hydrogen product. In contrast to FPB-MP-MX, the potential energy curve of MSi-PP-MX scanned the P-H···H-Si has large energy barrier of 42.66 kcal mol−1 in the S0 state and 32.01 kcal mol−1 in the S1 state, which demonstrates the difficulty of dehydrogenation reaction. This works presents reasonable interpretation on the dynamics changes of dihydrogen bonding and dehydrogenation reaction of frustrated Lewis pairs upon photoexcitation.