The chemistry of water is intimately tied to energy production, notably in the steam reforming of methane . 1 However, the quest for sustainable energy sources has focused attention on the efficient photo-or electrochemical splitting of water into hydrogen and oxygen. 2 Owing to their intrinsic redox activity and rich hydride and oxygen-derived ligand chemistry, transition metal species of both molecular and heterogeneous types have been a central focus of research in this area. 3 In the realm of molecular organometallic chemistry, O-H bond oxidative addition to give M-H and M-OH fragments has been considered as a potential strategy for activating water towards redox transformations, 4 with most examples featuring heavier late metals. 5 Approaches to small molecule activation that forgo transition metals, 6 such as frustrated Lewis-pairs (FLPs), 7 maingroup multiple bonds, 8 and low-valent p-block elements, 9 have Fig. 1 Examples of E-H bond activation processes at 1,4 diboron heterocycles.

garnered considerable attention as these systems are capable of activating a range of small molecules, including via oxidative addition. 10 Boron-containing heterocycles have demonstrated promise in this regard, as they have been shown to activate a wide range of single bonds as well as binding unsaturated molecules via [4+2] or [4+1] cycloaddition chemistry. 11 In 2010, Wagner demonstrated the C-H activation of alkynes at a reduced 9,10-diboraanthracene (DBA) 12 (Figure 1, top) and has since extended this reactivity to H-H cleavage. 13 Kinjo has developed a range of diazadiborinines capable of activating C-O, P-H, Si-H, and B-H bonds 14 as well as H2 and NH3 (Figure 1, middle). 15 Reports of HO-H cleavage with boron heterocycles are limited, however, and have largely involved irreversible B-C or B-H hydrolysis of the heterocycle or its substituents. 16 Recently we developed a DBA based disphosphine ligand (B2P2) and reported its Ni, 17 Cu, Ag 18 and Au complexes. 19 The auride complex of B2P2, [Au(B2P2)] -(Figure 1, bottom), exhibits diverse two-electron reductive chemistry with H + , CO2, 20 and organic carbonyls. 21 Herein we report that this species also can react directly with two equivalents of water to yield H2 via a pathway involving the oxidative addition of an O-H bond of water across the two boron atoms. The [Au(B2P2)] scaffold is generally stable in excess H2O and the hydroxide byproducts can be liberated from the boron centers with acid, formally closing a synthetic cycle for water reduction to H2 mediated by the DBA core.

Scheme 1 Water stability and water reduction from the Au(B2P2) platform.

Steric protection of borane centers (e.g. with mesityl substituents) is an established method of stabilizing DBA molecules against borane hydrolysis, 22 and we wondered if the rigid phenylene substituents presented by [Au(B2P2)] might offer similar protection. The water stability of [Au(B2P2)]Cl (1) was explored by allowing a 0.02 M solution in CD3CN:D2O (2:1) to stand at 22 °C for two weeks. (Compound 1 is insoluble in pure water.) Although we cannot rule out reversible chloride ionization under these conditions, the 1 H and 31 P NMR spectra are indistinguishable from the same in pure CD3CN (Figs. S1-S2), and pure 1 is recovered upon solvent removal. Analogous results were obtained in CDCl3:D2O suspensions, suggesting a negligible role for solvent donor ability on stability. Addition of triethylamine (2 equiv.) to a suspension of 1 in toluene:H2O (10:1) formed the hydroxide substituted compound, Au(B2P2)OH (2), in 89% yield as a pale-yellow solid (Scheme 1). The 31 P NMR spectrum of 2 in toluene-d8 at 22 °C contained a singlet at 48.5 ppm. However, single-crystal X-ray diffraction studies of 2 (Fig. 3a) revealed distinct boron atoms, with hydroxide bound to one (ΣCBC∠ = 336.9 °) with a distance of 1.529(2) Å. A Au-B contact of 2.615(1) Å occupies the essentially planar B atom on the opposite face of the DBA ring and is slightly longer than the analogous distance in Au(B2P2)Cl (dAu-B = 2.575(2) Å).

The discrepancy between the solid-state and apparent solution symmetries of 2 led us to consider a rapid hydroxide exchange mechanism that would symmetrize the molecule on the NMR timescale. A solution of 2 in toluene-d8 was cooled to -45 °C causing 31 P NMR singlet at 48.6 ppm to broaden and resolve to a set of doublets at 47.5 and 50.3 ppm (JPP = 242 Hz) (Fig. 2). The 1 H NMR at -45 °C of 2 was consistent with Cs symmetry, and the 11 B{ 1 H} NMR contained a broad peak at 36.36 ppm and a sharp signal at -5.57 ppm corresponding to distinct, three-and four-coordinate B atoms, respectively. Eyring analysis of these data gave activation parameters of DH = 12(1) kcalmol -1 and DS = 4.3(2) calmol -1 K -1 (Fig. S31). The small, positive entropy of activation is inconsistent with a bimolecular mechanism, supporting instead an intramolecular process for hydroxide shuttling between boron sites. 23 Given the solvent and temperature employed, an ionic dissociation/reassociation pathway is unlikely. To better understand the reactivity of these complexes with water, we measured the relative Lewis acidity of 1 and 2 by the Gutmann-Becket method, 24 along with [Au(B2P2)]Cl and the complex salt, [Au(B2P2)][BAr F 4] (Fig. S10). Acceptor numbers (ANs) of 0, 0, and 69 were determined in THF (AN = 45.25) for the series 1, 2, and [Au(B2P2)][BAr F 4] when exposed to 1.05 eq. of triethylphosphine oxide, respectively. These results are consistent with the observed H2O stability of 1 and 2. However, in contrast with 1, when allowing a 0.02 M benzene solution of 2 to stand in the presence water (10 equiv.), colorless crystals formed over the course of 3 days. Single-crystal XRD revealed a water addition product in which the previously threecoordinate B atom in 2 binds an equivalent of H2O, affording Au(B2P2)(OH)(H2O) (2-H2O, Fig. 3b). Each B atom is puckered from the DBA ring to adopt a pseudo-tetrahedral geometry (ΣCBC∠ = 338.4 and 339.7°) with B-O bond lengths of 1.596(2) and 1.587(2) Å. Disordered H atoms were located in the electron difference map between the two O atoms, suggesting some degree of H-bonding between each B-OH unit in the solid

Please do not adjust margins state. NMR spectra collected in toluene-d8 after exposure of solid 2-H2O to 10 -2 mbar vacuum for 15 minutes were consistent with pure 2, and a crystal grown from this material was identified by preliminary XRD to be 2, confirming that the formation of 2-H2O is reversible.

We next turned to the reaction of the reduced species [Au(B2P2)][K (18-c-6)] (3) with H2O (Scheme 1). Addition of excess H2O (3 equiv.) to 3 resulted in immediate loss of color and effervescence to yield the dihydroxide complex [Au(B2P2)(OH)2][K (18-c-6)] 4 in essentially quantitative yield.foot_0 H NMR spectroscopy confirmed the evolution of H2 and singlecrystal XRD studies on 4 (Fig. 3c) revealed B-OH bonds of 1.530(3) and 1.509(3) Å, significantly shorter than those in 2-H2O. When 1 equiv. of H2O is added slowly to a solution of 3 at 0° C, 31 P NMR spectroscopy revealed a new set of coupled doublets at 46.0 and 50.6 ppm (JPP = 276.9 Hz) along with a singlet at 45.5 ppm corresponding to the dihydroxide 4, in a 4:1 ratio, respectively. We formulate this new product as [Au(B2P2)(OH)(H)][K (18-c-6)] (5) on the basis of the following data. 1 H NMR analysis of the major product was consistent with Cs symmetry due to different substituents at the two B atoms, with a distinct four-line signal arising from one-bond B-H coupling at 4.19 ppm (JB-H = 72.0 Hz). The 1 H-coupled 11 B NMR displayed a corresponding doublet at -9.73 ppm (JB-H = 75.7 Hz) and a broader singlet at -0.85 ppm (Fig. 4), strongly suggestive of a B-H unit. Although crystals could be obtained from this mixture, they invariably consisted of cocrystallization of 5 with roughly equimolar amounts of 4, resulting in significant disorder (Fig. S37). Despite this, a satisfactory crystallographic model could be constructed consistent with an approximately equimolar cocrystallization, which can be thought of as a 50% OH occupancy in the hydride site of 5 (Fig. 3d). There are no other compelling hypotheses for the lack of electron density in this position given the clear pyramidalization of the B atom. Although these crystallographic data are consistent with this structure yet not definitive, the solution spectroscopic characterization of 5, including the incontrovertible signature of a B-H moiety, provides strong evidence for this formulation. As implied by the difficulty of isolating 5 in pure form, it is extraordinarily water sensitive, rapidly converting to 4 upon addition of H2O or by scavenging adventitious water from the glovebox atmosphere. To close a synthetic cycle for H2 evolution from H2O, we investigated reaction conditions to regenerate 1 from 5. HCl•Et2O (3 equiv.) was identified to cleanly induce this reaction, with no other products observed by 1 H and 31 P NMR. This reaction highlights the unique stability of the [Au(B2P2)] system to both water and acid and provides an outline for the potential catalysis of H2 evolution from H2O with this and related systems.

In conclusion, we have demonstrated the reduction of H2O to H2 mediated by a redox-active borane. As boron-containing materials such as boron-doped graphene, 25 boron nanoparticles 26 and other boron-doped materials 27 continue to attract interest as metal-free alternatives to H2 production from H2O, molecular platforms like the ones discussed here can play a key role in informing underlying mechanistic discussion and aiding in rational design. Modifications to the [Au(B2P2)] system directed at performing electrocatalytic H2O reduction are currently being explored.

This work was supported by the National Science Foundation (CHE-1752876 and CHE-162673). Dr. Charlene Tsay is acknowledged for X-ray crystallographic assistance.

There are no conflicts to declare.

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