Water Reduction and Dihydrogen Addition in Aqueous Conditions With ansa‐Phosphinoborane

Ortho‐phenylene‐bridged phosphinoborane (2,6‐Cl(2)Ph)(2)B‐C(6)H(4)‐PCy(2) 1 was synthesized in three steps from commercially available starting materials. 1 reacts with H(2) or H(2)O under mild conditions to form corresponding zwitterionic phosphonium borates 1‐H(2) or 1‐H(2)O. NMR studies revealed both reactions to be remarkably reversible. Thus, when exposed to H(2), 1‐H(2)O partially converts to 1‐H(2) 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‐H(2)O to free 1. These observations were supported by computational studies indicating that the formation of 1‐H(2) and 1‐H(2)O from 1 are thermodynamically favored. Unexpectedly, 1‐H(2)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.