Abstract

Ortho-phenylene-bridged phosphinoborane (2,6-Cl₂Ph)₂B-C₆H₄-PCy₂ 1 was synthesized in three steps from commercially available starting materials. 1 reacts with H2 or H2O under mild conditions to form corresponding zwitterionic phosphonium borates 1-H₂ or 1-H₂O. NMR studies revealed both reactions to be remarkably reversible. Thus, when exposed to H₂, 1-H₂O 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-H₂O to free 1. These observations were supported by computational studies indicating that the formation of 1-H₂ and 1-H₂O from 1 are thermodynamically favored. Unexpectedly, 1-H₂O 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.