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

Abstract

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.

Keywords: DFT; frustrated Lewis pairs; hydrogen activation; mechanism; umpolung of proton; water reduction.

Figure 1

Efforts in designing water tolerant ansa‐FLPs.

Scheme 1

Three‐step synthesis of ansa‐phosphinoborane 1.

Figure 2

Crystal structure of ansa‐phosphinoborane 1 reveals P^…B separation 3.170(4) Å (displacement parameters are drawn at 50 % probability level).

Scheme 2

Reactivity of the ansa‐phosphinoborane 1 towards H₂O and H₂.

Figure 3

Crystal structure of phosphinoborane 1‐H₂O reveals exo‐configuration (displacement parameters are drawn at 50 % probability level). Selected distances (Å): B1‐O1 1.463(7), P1⋅⋅⋅O1 2.311(4), P1⋅⋅⋅B1 3.195(6).

Figure 4

DFT studies of water and hydrogen addition to 1. Solution phase Gibbs free energies computed at ωB97XD/6‐311++G(3df,3pd) level of theory are given in kcal/mol with respect to reactants in dichloromethane.

Figure 5

Crystal structure of phosphinoborane oxide 1‐O (displacement parameters are drawn at 50 % probability level). Selected distances (Å): B1‐O1 1.569(0), P1‐O1 1.550(1), P1⋅B1 2.6074(16).

Scheme 3

In situ generation of H₂ via the stoichiometric reduction of H₂O with 1 and its utilization as a reductant using a two‐chamber reactor.

Scheme 4

Transformations of 1‐H₂O upon heating with and without H₂ pressure monitored by NMR spectroscopy (¹H, ¹¹B) in 1 : 1 CD₂Cl₂:CD₃CN.

Figure 6

DFT studies of the water reduction mechanism with a model des‐chlorophosphinoborane 4. Solution phase Gibbs free energies computed at the ωB97XD/6‐311++G(3df,3pd) level of theory (DFT) are given in kcal/mol with respect to 4‐H₂O‐exo in dichloromethane (blue), acetonitrile (black), and water (red).

Figure 7

¹H NMR spectra of 1‐H₂ (a) and 1‐H₂O (b). (c) ¹H NMR spectrum of 1 and 6 equivalents H₂O after 1 h of heating at 80 °C under 10 bars of H₂. ¹H (d) and ¹¹B (e) NMR spectra of the same sample recorded after additional 2 h of heating at 80 °C. All spectra were recorded in the 1 : 1 CD₂Cl₂:CD₃CN mixture at 27 °C.

Figure 8

¹H and ³¹P NMR spectra detected after exposing a 0.03 M solution of 1 in (a) dry and (b) moist (3 equivalents H₂O) 1 : 1 CD₂Cl₂:CD₃CN solvent at 25 and 80 °C, respectively. The signals revealing the hyperpolarization effects are colored in red. The intensity scale is adjusted for better visibility of the hyperpolarization.