Probing substituent effects on the activation of H2 by phosphorus and boron frustrated Lewis pairs

Abstract

The impact of substituent changes on phosphorus and boron-containing frustrated Lewis pairs (FLPs) has been examined. The phosphites (RO)₃P R = Me, Ph form classical Lewis acid–base adducts of the formula (RO)₃PB(C₆F₅)₃ R = Me 1; R = Ph 2, whereas P(O-2,4-^tBu₂C₆H₃)₃ and P(O-2,6-Me₂C₆H₃)₃ generate FLPs. Nonetheless, these latter combinations do not react with H₂. The more basic phosphinite tBu₂POR, R = tBu 3 reacts with B(C₆F₅)₃ to give (tBu₂(H)PO)B(C₆F₅)₃6. The related species tBu₂POR, R = Ph 4; 2,6-Me₂C₆H₃5 showed no reaction with B(C₆F₅)₃ but the FLPs react under H₂ (4 atm) to give [tBu₂P(OR)H][HB(C₆F₅)₃] R = Ph 7 and 2,6-Me₂C₆H₃8. Similarly, tBu₂PCl in combination with B(C₆F₅)₃, generates an FLP that upon addition of H₂, gives [tBu₂PH₂][ClB(C₆F₅)₃] 9 albeit in low yield. The diborane 1,4-(C₆F₅)₂B(C₆F₄)B(C₆F₅)₂ in combination with either tBu₃P or (C₆H₂Me₃)₃P generates FLPs that react with H₂ to give [R₃PH]₂[1,4-(C₆F₅)₂HB(C₆F₄)BH(C₆F₅)₂] (R = tBu 11, C₆H₂Me₃12). Similarly PhB(C₆F₅)₂ and tBu₃P react with H₂ giving [tBu₃PH][HBPh(C₆F₅)₂] 13. The combination of B(OC₆F₅)₃ and PtBu₃ also generate an FLP which reacts with H₂ to give [HPtBu₃][B(OC₆F₅)₄] 14, the product of substituent redistribution. The boronic esters, (C₆H₄O₂)BC₆F₅15, (C₆H₃FO₂)BC₆F₅16 and (C₆F₄O₂)BC₆F₅17, and the borate esters B(OC₆H₃(CF₃)₂)₃18, B(OC₆H₂F₃)₃19 and B(OC₆H₄CF₃)₃20 were prepared and shown to generate FLPs with tBu₃P or (C₆H₂Me₃)₃P. Nonetheless, no reaction with H₂ was observed for 15-17. Collectively these data suggest that there is a threshold of combined Lewis acidity and basicity that is required to effect the splitting of H₂.