Takumi
Oishi
a,
Leonardo I.
Lugo-Fuentes
b,
Yichuan
Jing
a,
J. Oscar C.
Jimenez-Halla
b,
Joaquín
Barroso-Flores
cd,
Masaaki
Nakamoto
a,
Yohsuke
Yamamoto
a,
Nao
Tsunoji
e and
Rong
Shang
*a
aDepartment of Chemistry, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526, Japan. E-mail: rshang@hiroshima-u.ac.jp
bDepartment of Chemistry, Division of Natural and Exact Sciences, University of Guanajuato, Campus Gto, Noria Alta s/n, 36050 Guanajuato, Mexico
cCentro Conjunto de Investigación en Química Sustentable UAEM-UNAM, Unidad San Cayetano, 50200 Toluca de lerdo, México
dInstituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior S/N, Ciudad Universitaria, alcaldía de Coyoacán, CP 04510, Ciudad de México, México
eDepartment of Chemistry, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, 739-8527, Japan
First published on 15th November 2021
Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H–OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH−) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations.
Chart 1 Metal- (A) and p-block element (B and C)-based mechanisms for water activation and reduction. |
This general heterolytic mechanism of water cleavage (Chart 1A) has proven difficult to replicate by main-group elements. Although water HO–H bond cleavage, to generate element–hydride (E–H) bonds, has been achieved by oxidative addition at low valent elements (e.g. Al/Ga(I)19–23 and Si/Ge(II)24 species) and 1,2-addition across main-group multiple bonds (e.g. disilenes),25–34 a p-block E–H bond is usually less reactive than an s-/d-block M–H bond. Their reduced reactivity and large steric bulk (usually employed for their isolation) tend to favor the activation product instead of liberation of dihydrogen. A remarkable p-block example of water reduction was reported by Nielsen and Skrydstrup, which took advantage of the reducing power of the B–B bond in an sp3–sp3 diboron(4) system,35 resembling a binuclear metal system.
Another main-group strategy for facile H2O activation involves a bulky Lewis acid and base pair (frustrated Lewis pair, FLP, Chart 1B). In this case, group 15 functionalities, such as phosphines with electron-donating substituents, are usually employed as a Lewis base to deprotonate the Lewis acid-coordinated water molecule. However, the conversion of a proton, in a water molecule, into hydride cannot be achieved and thus the reaction stops at the HO–H bond cleavage step.36–44 By using both low-valent p-block elements and FLP chemistry, Driess and coworkers reported a metal-free water reduction example, from an intramolecular silylene/borane system (Chart 1B).45 However, since silylene is in a highly reduced form, this water reduction is not entirely unexpected. The necessity of borane in the water reduction was not discussed explicitly in the report, though its involvement in H2 generation was proposed based on the calculated reaction mechanism.
Herein we report a new FLP design for metal-free water reduction based on phosphine and borane functionalities. While triphenyl phosphine does not react with water under ambient conditions, we found that a bisborane-functionalized triaryl phosphine can instantaneously and quantitatively reduce water to generate dihydrogen and phosphine oxide.46–49 This reaction features an electron relay mechanism among the borane-phosphine-borane centers, which allowed an umpolung of a proton in water to a hydride on borane driven at a phosphonium center. This then led to formation of phosphine oxide and H2, leaving both Lewis acidic boranes intact after the reaction.
Compound 1b is a highly reactive crystalline solid. Its 31P{1H} NMR spectrum showed a sharp singlet at −7.0 ppm. The 11B{1H} NMR spectrum showed a broad singlet at 79.2 ppm, similar to that of Bourissou's phosphorus/borane ambiphilic ligand bearing the cyclohexylborane moiety.54 The down-field 11B NMR signal suggests a highly Lewis acidic borane center with no strong intra- or intermolecular coordination from the phosphine moiety. Attempts to obtain single crystals of 1b were not successful in our hands. However, its isocyanide-Lewis adduct 2 could be obtained as colorless crystals in 85% yield (Scheme 1). X-ray analysis of a single crystal of 2 showed the molecular structure of 1b with each borane coordinated by an isocyanide moiety (Fig. 1b). The solution NMR spectrum of 2 was fully consistent with its solid-state structure, showing no dissociation of the Lewis base in solution at room temperature.
Fig. 1 Solid-state structures of 1a (a), 2 (b) and 3 (c). Thermal ellipsoids are drawn at 30% probability. Peripheral ellipsoids are omitted for clarity. CCDC: 2096462 (1a), 2096464 (2), 2096465 (3). See the ESI for 1-int, CCDC 2096463 and 3·H2O, CCDC 2109052.† |
Compound 1b did not react with H2 under ambient conditions. However, it reacted with H2O at room temperature with vigorous evolution of gas (Fig. 2). The 1H NMR spectrum of the reaction mixture showed a singlet at 4.47 ppm, assignable to H2. An analogous reaction with D2O in toluene led to clean generation of D2, detected by 2H NMR. These results confirmed that both hydrogen atoms in the generated dihydrogen are from the water molecule (Fig. 2). In addition, phosphine oxide 3 was isolated in 97% yield. The solid-state structure of 3 showed a POBCC 5-membered ring,55 with a short P–O distance of 1.5322(13) Å and a remarkably long B1–O distance of 1.633(2) Å. The tetrahedral geometry at B1 (ΣB1CCC = 339.9°) suggests a definite B–O interaction. In solution however, only one boron signal at 48.2 ppm could be observed at room temperature, which broadened into the baseline below 0 °C (see spectra in the ESI†). The 1H NMR spectrum also suggested a symmetrical structure in solution at room temperature. These revealed a fluxional coordination of the oxide to both borane centres. The 31P{1H} NMR signal of compound 3 showed a sharp signal at 60.4 ppm. Interestingly, compound 3 does not react with additional H2O under an inert atmosphere but decomposes readily in air at room temperature.
Fig. 2 Reaction schemes and NMR spectra of 1b with H2O and D2O at room temperature (a) and with frozen water at −80 °C (b). |
Although FLPs are well known for heterolytic bond cleavages in water36–44 and dihydrogen,56–61 to the best of our knowledge, there is only one exmple of water reduction by metal-free FLP systems (Chart 1C). A control reaction of free PPh3 and BCy3 in a 1:2 ratio at 50 °C in benzene did not yield any H2 overnight (see spectra in the ESI†), which means the architecture of the rigid phenylene backbone and the proximity of two boranes are essential for this reaction. In addition, numerous examples of phenylene linked phosphine/borane FLPs for small molecule activation studies have been reported previously. However, water reduction has not been described.62–76
In the search for possible HO–H bond splitting intermediates like those observed in FLP chemistry,36–44 a toluene-d8 solution of 1b was frozen while water was added by syringe. The 31P{1H} NMR measurement of the reaction mixture at −80 °C revealed a small singlet at 18.3 ppm which splits into a doublet (1JPH = 474 Hz) in a subsequent 31P NMR measurement, confirming the generation of a phosphonium ([Ar3PH]+) intermediate (Fig. 2b). This signal could not be observed at temperatures higher than −30 °C, where the reaction proceeds too quickly. Attempts to isolate a similar O–H bond cleavage product from the reaction of 1b and methanol were not successful. A large excess of methanol (ca. 50 equiv.) was needed for an observable change in the NMR reactions with 1b, where the 31P{1H} NMR spectrum of the reaction mixture revealed a singlet which splits into a doublet in a proton-coupled 31P NMR experiment (see spectra in the ESI†). Removal of solvent led to a complete recovery of 1b. This suggested that the cleavage of the MeO–H bond by 1b is slightly uphill energetically, and 1b and the activation product (4, see the ESI†) are in equilibrium in solution.
To obtain more insights into the mechanistic details on the water reduction by 1b, we performed theoretical calculations at the SMD(benzene):ω-B97XD/6-311G(d) level (see the ESI† for computational details). The calculated reaction mechanism (Fig. 3) suggests that the water activation starts with its coordination to one borane to give adduct H1 (ΔGR1 = +4.7 kcal mol−1). Then the phosphine serves as a base to deprotonate the coordinated HO–H bond (via transition state TSH1, ΔG‡1 = +7.7 kcal mol−1) which generates intermediate H2 (observed by NMR at −80 °C as 3-I, ΔGR2 = −1.2 kcal mol−1). The resulting phosphonium cation now behaves as an electrophile77 to receive the pair of electrons of the hydroxyl group while transferring the first hydrogen as a hydride to the second boron (TSH2, ΔG‡2 = +14.5 kcal mol−1) in a concerted fashion to form H3 (ΔGR3 = +8.6 kcal mol−1). This is followed by the decoordination of hydroxyl from the first boron (TSH3, ΔG‡3 = +1.0 kcal mol−1) generating rotamer H4 (ΔGR4 = −2.5 kcal mol−1) which keeps closer the hydrogens to finally lead to formation of H2 (viaTSH4, ΔG‡4 = +1.2 kcal mol−1) and the phosphine oxide 3 (ΔGR5 = −31.7 kcal mol−1). Overall, this reaction is exergonic (ΔG0R = −22.1 kcal mol−1). The calculated total energy barrier of +18.0 kcal mol−1 is in line with our reported reaction conditions. H2 was also observed experimentally at −80 °C by NMR experiments.
The natural bond orbital (NBO) charges of the phosphorus atom and the transferred hydrogen atom between H2 and H3 reflected the charge transfer corresponding to oxidation and reduction respectively, showing a simultaneous increase in the charge of P and decrease in that of H (Fig. 3b). Also, our theoretical calculations suggest that there is a fluxional coordination of the oxygen to each boron atom in 3 (see Fig. S5†) with a low barrier energy of 8.4 kcal mol−1, indicating a weak O–B coordination which is consistent with the experimental observations.
Following the same mechanistic route as described above, the reaction of 1b with MeOH (see Fig. S7†) was also investigated computationally. The coordination of methanol to the first boron gives adduct Me1 (ΔGR1 = −0.4 kcal mol−1) followed by the proton transfer leading to intermediate 4 (Me2 in Fig. S7,† ΔGR2 = 4.1 kcal mol−1) through TSMe1 (ΔG‡1 = +6.5 kcal mol−1) which are accessible at room temperature. However, the following reaction steps are very high in energy and the reaction stops at this point. In general, the reduction of MeOH by 1b to produce CH4 and 3 must overcome a total energy barrier of 58.4 kcal mol−1 and, therefore, is practically unreachable. The reaction to afford the MeO–H bond cleavage intermediate (4) is exergonic, and because of the excess of methanol, this intermediate could be detected by NMR spectroscopy but could not be isolated.
Although triphenyl phosphine and trialkyl/aryl boranes are not particularly reactive in comparison to other derivatives often employed in FLP chemistry, the preorganization of the borane-phosphine-borane framework in 1b is very important to coordinate the phosphine/phosphonium bifunctionality, taking advantage of phosphorus' ability of hypervalent bonding and its non-polar P–H bond for the umpolung of proton to hydride. This simple design of a bisborane-functionalized phosphine FLP has demonstrated a new working paradigm for metal-free water reduction, showing a new strategy for generation of hydride at a main-group element from water that was previously only possible at metal centres.
Footnote |
† Electronic supplementary information (ESI) available: Synthetic and computational details, structural and spectroscopic data of 1a–b, 1-int, 2 and 3. Computational details. CCDC 2096462–2096465 and 2109052. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc05135k |
This journal is © The Royal Society of Chemistry 2021 |