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

The impact of substituent changes on phosphorus and boron-containing frustrated Lewis pairs (FLPs) has been examined. The phosphites (RO)(3)P R = Me, Ph form classical Lewis acid-base adducts of the formula (RO)(3)PB(C6F5)(3) R = Me 1; R = Ph 2, whereas P(O-2,4-(Bu2C6H3)-Bu-t)(3) and P(O-2,6-Me2C6H3)(3) generate FLPs. Nonetheless, these latter combinations do not react with H-2. The more basic phosphinite tBu(2)POR, R = tBu 3 reacts with B(C6F5)(3) to give (tBu(2)(H)PO)B(C6F5)(3) 6. The related species tBu(2)POR, R = Ph 4; 2,6-Me2C6H3 5 showed no reaction with B(C6F5)(3) but the FLPs react under H-2 (4 atm) to give [tBu(2)P(OR) H][HB(C6F5)(3)] R = Ph 7 and 2,6-Me2C6H3 8. Similarly, tBu(2)PCl in combination with B(C6F5)(3), generates an FLP that upon addition of H-2, gives [tBu(2)PH(2)][ClB(C6F5)(3)] 9 albeit in low yield. The diborane 1,4-(C6F5)(2)B(C6F4) B(C6F5)(2) in combination with either tBu(3)P or (C6H2Me3)(3)P generates FLPs that react with H-2 to give [R3PH](2)[1,4-(C6F5)(2)HB(C6F4) BH(C6F5)(2)] (R = tBu 11, C6H2Me3 12). Similarly PhB(C6F5)(2) and tBu(3)P react with H-2 giving [tBu(3)PH][HBPh(C6F5)(2)] 13. The combination of B(OC6F5)(3) and PtBu3 also generate an FLP which reacts with H-2 to give [HPtBu3][B(OC6F5)(4)] 14, the product of substituent redistribution. The boronic esters, (C6H4O2) BC6F5 15, (C6H3FO2) BC6F5 16 and (C6F4O2) BC6F5 17, and the borate esters B(OC6H3(CF3)(2))(3) 18, B(OC6H2F3)(3) 19 and B(OC6H4CF3)(3) 20 were prepared and shown to generate FLPs with tBu(3)P or (C6H2Me3)(3)P. Nonetheless, no reaction with H-2 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 H-2.