Water Reduction and Dihydrogen Addition in Aqueous Conditions With ansa-Phosphinoborane.

Ortho-phenylene-bridged phosphinoborane (2,6-Cl2 Ph)2 B-C6 H4 -PCy2 1 was synthesized in three steps from commercially available starting materials. 1 reacts with H2 or H2 O under mild conditions to form corresponding zwitterionic phosphonium borates 1-H2 or 1-H2 O. NMR studies revealed both reactions to be remarkably reversible. Thus, when exposed to H2 , 1-H2 O partially converts to 1-H2 even in the presence of multiple equivalents of water in the solution. The addition of parahydrogen to 1 leads to nuclear spin hyperpolarization both in dry and hydrous solvents, confirming the dissociation of 1-H2 O to free 1. These observations were supported by computational studies indicating that the formation of 1-H2 and 1-H2 O from 1 are thermodynamically favored. Unexpectedly, 1-H2 O can release molecular hydrogen to form phosphine oxide 1-O. Kinetic, mechanistic, and computational (DFT) studies were used to elucidate the unique "umpolung" water reduction mechanism.