Germanium hydrides as an efficient hydrogen-storage material operated by an iron catalyst

Abstract

The use of metal hydrides such as NaBH₄ as hydrogen-storage materials has recently received substantial research attention on account of the worldwide demand for the development of efficient hydrogen-production, -storage, and -transportation systems. Here, we report the quantitative production of H₂ gas from a germanium hydride, Ph₂GeH₂, mediated by an iron catalyst at room temperature via dehydrogenative coupling, concomitant with the formation of (GePh₂)₅. Of particular importance is that Ph₂GeH₂ can be facilely recovered from (GePh₂)₅ by contact with 1 atm of H₂ or PhICl₂/LiAlH₄ at 0 °C or 40 °C, respectively. A detailed reaction mechanism for the iron-catalyzed dehydrogenative coupling of Ph₂GeH₂ is proposed based on the isolation of four intermediate iron species.

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Introduction

Economic hydrogen production/storage is the key issue preventing the application of hydrogen as an energy carrier in the context of a global low-carbon strategy to address the ever-increasing energy challenges that face humanity. Recently, a number of hydrogen storage materials, such as metal hydrides, liquid organic hydrogen carriers, ammonia and ammonia borane, were applied for the development of effective hydrogen production/storage systems.¹ Among them, metal hydrides have attracted great attention recently as potential efficient hydrogen-storage materials. Sodium borohydride (NaBH₄) is one of the most studied hydrides for this purpose, as it is able to deliver H₂ gas under mild conditions, e.g., at room temperature, via hydrolysis.² However, one major issue with this approach is that the regeneration of NaBH₄ from the hydrolytic products formed after H₂ production generally carries a high energy penalty and thus requires, e.g., high temperatures (400–500 °C). Thus, the development of hydrogen-production/-storage systems in which the generation of H₂ can proceed under mild conditions and hydrogen-storage materials that can be regenerated easily is highly desirable.

In this paper, we focus on the use of group-14 hydrides as potentially reusable hydrogen-storage materials. It is well known that group-14 hydrides such as hydrosilanes, hydrogermanes, and hydrostannanes undergo dehydrogenative coupling reactions in the presence of appropriate transition-metal catalysts to form E–E (E = Si, Ge, Sn) bond(s), concomitant with the generation of H₂.³ A pioneering study in this area has been published by Harrod and co-workers, who reported that hydrosilanes such as PhSiH₃ effectively undergo multiple Si–Si-bond-forming reactions in a dehydrogenative manner to afford polysilanes and/or a mixture of oligosilanes.⁴ This study revealed that hydrosilanes can act as reagents for hydrogen-production and E–E-bond-formation reactions; however, the regeneration of the starting hydrosilanes is considered to be challenging because high-molecular-weight polymeric compounds are obtained as the major products after the production of H₂. Although polymeric products have mainly been obtained in catalyst systems based on early transition metals,⁵ it has been reported that late-transition-metal catalysts often tend to afford relatively short-chain oligomers in the dehydrogenative coupling of group-14 hydrides.⁶ For instance, Rosenberg et al. have indicated that the dehydrogenative coupling of Ph₂SiH₂ catalyzed by Rh(PPh₃)₃Cl affords the disilane Ph₂(H)Si–Si(H)Ph₂ selectively under the optimal reaction conditions;^5b however, no attempts to regenerate Ph₂SiH₂ from Ph₂(H)Si–Si(H)Ph₂ have been reported in the same study. Although some examples of the reverse reaction of the dehydrogenative coupling of hydrosilanes, i.e., hydrogenolysis of disilanes to afford hydrosilanes, have been reported for Ni or Pt catalysts,⁷ reversible and reusable hydrogen-production/-storage systems with group-14 hydrides have not been explored so far.

In this work, we developed a catalytic system based on iron for the reversible production/storage of H₂ using germanium hydrides as the potential hydrogen-storage material because iron is the least expensive and least toxic late transition metal (Scheme 1). We found that the combination of the iron precursor [Fe(mesityl)₂]₂ (mesityl = 2,4,6-Me₃-C₆H₂) with N-heterocyclic carbene (NHC) ligands promotes the effective production of H₂ in the dehydrogenative coupling of R₂GeH₂ (R = Ph or Et) (hydrogen content: ca. 0.87 wt% for Ph₂GeH₂ and ca. 1.51 wt% for Et₂GeH₂). The quantitative generation of H₂ at room temperature using Ph₂GeH₂ as a hydrogen-storage material was confirmed, along with the formation of (GePh₂)₅ as the dehydrogenative coupling product. It is noteworthy that the starting Ph₂GeH₂ can be readily regenerated via the hydrogenation of (GePh₂)₅ under 1 atm of H₂ at 0 °C catalyzed by the same iron catalyst system. Alternatively, Ph₂GeH₂ can also be recovered from (GePh₂)₅ by treatment with PhICl₂ followed by LiAlH₄ at 40 °C. These results show the promising potential of germanium hydrides as prospective reusable hydrogen-production/-storage materials that can evolve H₂ at room temperature using an iron catalyst. Moreover, four possible intermediary iron species formed in this catalysis were structurally characterized, based on which, a detailed reaction mechanism is proposed. The present catalyst system is also applicable to the dehydrogenative coupling of other group-14 hydrides, such as hydrosilanes and hydrostannanes R₂EH₂ (E = Si, Sn).

Scheme 1 Overall scheme of this work; Ph₂GeH₂ as a reusable hydrogen-storage material in the presence of an iron catalyst.

Results and discussion

Hydrogen production from secondary germanes

First, the diphenylgermane Ph₂GeH₂ was used as the substrate, and the dinuclear iron(II) complex [Fe(mesityl)₂]₂ (mesityl = 2,4,6-trimethylphenyl) was used as the catalyst precursor, because it has been reported to effectively activate group-14-element–hydrogen bonds.⁸ The dehydrogenative coupling of Ph₂GeH₂ proceeded effectively at room temperature in THF under an N₂ atmosphere when a mixture of 2.5 mol% of [Fe(mesityl)₂]₂ (5 mol% based on Fe) and 10 mol% of the N-heterocyclic carbene ⁱPrIM^Me (ⁱPrIM^Me = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) was used as the catalyst (Scheme 2). This reaction furnished cyclic pentagermane (GePh₂)₅ as the sole product in 62% isolated yield after 40 h. The formation of (GePh₂)₅ was confirmed based on a comparison of the ¹H and ¹³C NMR spectra of the product with those of an authentic sample.⁹ It is noteworthy that the formation of (GePh₂)₅ was accompanied by the generation of H₂ gas, which was confirmed by ¹H NMR spectroscopy. The quantity of H₂ gas produced was determined using a gas burette; when the dehydrogenative coupling of 1 mmol of Ph₂GeH₂ was performed at room temperature under the catalytic conditions described above, ca. 21 mL of gaseous product(s) were obtained in the gas burette. These results indicate that the dehydrogenation of Ph₂GeH₂ to afford (GePh₂)₅ proceeded effectively at room temperature to give a quantitative amount of H₂ gas.

Scheme 2 Catalytic dehydrogenative coupling of Ph₂GeH₂ or Et₂GeH₂ catalyzed by the [Fe(mesityl)₂]₂/NHC catalyst system.

It should be emphasized here that this catalysis is highly sensitive to the electronic and/or steric environment of the ligands used. Thus, ⁱPrIM^Me was found to be the only ligand that selectively furnished (GePh₂)₅ for a quantitative conversion of Ph₂GeH₂, and that other NHC ligands used in this study with different substituents on the nitrogen atoms or the ligand backbone barely generated any (GePh₂)₅. Although the use of certain NHC ligands led to a high conversion of Ph₂GeH₂, the formation of a mixture of oligogermanes other than (GePh₂)₅ was suggested by the ¹H NMR spectra of the corresponding crude products (for details, see Table S2 in the ESI†). Subsequently, we examined the effect of the solvent on the reaction of Ph₂GeH₂ catalyzed by [Fe(mesityl)₂]₂/ⁱPrIM^Me, which revealed that the use of ethereal solvents such as Et₂O and THF afforded (GePh₂)₅ in medium to good yield.

Subsequently, the dehydrogenative coupling of Et₂GeH₂ was performed using [Fe(mesityl)₂]₂/ⁱPrIM^Me. Although the conversion of Et₂GeH₂ reached >99% in THF after 40 h, the ¹H NMR spectrum of the crude product suggested the formation of a complex mixture of oligogermanes, and almost no formation of cyclopentagermane (GeEt₂)₅ was observed. This result stands in stark contrast to the fact that the selective production of (GeEt₂)₅ with >99% conversion (52% isolated yield) of Et₂GeH₂ was confirmed when EtIM^Me was used instead of ⁱPrIM^Me (Scheme 2). These results indicate that sophisticated control of the steric environment around the iron center on the catalytically active species can be expected to be crucial to achieving the dehydrogenative coupling of secondary germanes in a selective manner to produce the corresponding cyclopentagermanes (GeR₂)₅ (R = Ph, Et) as a single product. It should also be noted here that although cyclooligogermanes are usually generated via the Wurtz-type reductive coupling of R₂GeX₂ (R = alkyl, aryl; X = halide) using a stoichiometric amount of alkali or alkaline earth metals,¹⁰ the development of alternative synthetic methods would be highly desirable due to the harsh reaction conditions required. The dehydrogenative coupling of secondary germanes could be a potential alternative to such Wurtz-type reductive coupling reactions in terms of the formation of Ge–Ge bonds, although previously reported examples of the catalytic dehydrogenative coupling of secondary germanes only furnished dimers. To the best of our knowledge, the results shown here thus represent the first example of the formation of cyclooligogermanes via the sequential catalytic dehydrogenative coupling of secondary germanes under the formation of multiple Ge–Ge bonds.

Regeneration of Ph₂GeH₂ from (GePh₂)₅

We discovered that the starting material Ph₂GeH₂ can be readily regenerated from (GePh₂)₅ by treatment with 1 atom of H₂ in the presence of [Fe(mesityl)₂]₂/ⁱPrIM^Me in THF at 0 °C for 24 h, i.e., Ph₂GeH₂ was formed in 51% yield. It was also confirmed that no reaction occurred when (GePh₂)₅ was exposed to 1 atm of H₂ in the absence of an iron catalyst. The regeneration of Ph₂GeH₂ was accompanied by the formation of germoxane (Ph₂GeO)₃ in 49% yield; this is presumably due to the rapid hydrolysis of the formed Ph₂GeH₂ induced by ⁱPrIM^Me and unremovable traces of H₂O. It should also be noted here that hydrosilanes such as Ph₂SiH₂ undergo fast hydrolysis in the presence of a catalytic amount of N-heterocyclic carbenes.¹¹ Although the quantitative formation of Ph₂GeH₂ from (GePh₂)₅ and H₂ gas was hampered despite many attempts to remove the aforementioned trace amounts of H₂O, we found that Ph₂GeH₂ could be readily recovered from (GePh₂)₅via an alternative method; (GePh₂)₅ can be converted quantitatively into Ph₂GeCl₂ (64% isolated yield) by treatment with PhICl₂ in THF at 40 °C, before Ph₂GeCl₂ can be quantitatively transformed into Ph₂GeH₂ by reaction with LiAlH₄ in THF at room temperature (82% isolated yield) (Scheme 3). Thus, we concluded that the germanium hydride Ph₂GeH₂ was easily recovered and reused via two independent reaction protocols. These results corroborate the notion that Ph₂GeH₂ is an effective and reusable hydrogen-storage material that can quantitatively produce H₂ gas at mild operating temperatures and that can be readily regenerated from (GePh₂)₅ through hydrogenation under 1 atm of H₂ gas or sequential treatment with PhICl₂ and LiAlH₄.

Scheme 3 Regeneration of Ph₂GeH₂ from (GePh₂)₅.

Isolation of four possible intermediary iron species

To obtain insight into the reaction mechanism of the [Fe(mesityl)₂]₂/ⁱPrIM^Me-catalyzed dehydrogenative coupling of Ph₂GeH₂ to produce (GePh₂)₅, several investigations were carried out to isolate any potential intermediary iron species. Initially, the reaction of [Fe(mesityl)₂]₂ and 4 equivalents of ⁱPrIM^Me was carried out in Et₂O at room temperature for 16 h, which provided mononuclear trans-Fe(mesityl)₂(ⁱPrIM^Me)₂ (1) in 52% isolated yield (Scheme 4A). A single-crystal X-ray diffraction (XRD) analysis revealed that the iron center in 1 adopts a square-planar geometry with two ⁱPrIM^Me ligands located at the trans-positions, which is similar to that observed in a previously reported analogous Fe(mesityl)₂(ⁱPmIM^H)₂ complex.^8b Subsequently, isolated complex 1 was treated with 2 equivalents of Ph₂GeH₂, which produced a trace amount of orange crystals of trans-Fe(GePh₂H)₂(ⁱPrIM^Me)₂ (2), together with a white powder of (GePh₂)₅. Although the isolation of pure 2 from this reaction was hampered by the concomitant formation of (GePh₂)₅, 2 was isolated using an alternative reaction protocol. Namely, the reaction of FeCl₂ with in situ-generated LiGe(H)Ph₂ in the presence of 2 equivalents of ⁱPrIM^Me in THF led to the isolation of 2 in 43% yield (Scheme 4B). The molecular structure of 2 was determined by single-crystal XRD analysis (Fig. 1A). Similar to that in complex 1, the iron center in 2 adopts a square-planar coordination geometry wherein two ⁱPrIM^Me ligands occupy trans-positions. To the best of our knowledge, complex 2 is the first example of an Fe(II)-germyl complex with a four-coordinate square-planar coordination geometry. The Fe–Ge bond distances of 2.4488(8) Å are longer than those in previously reported iron(II) germyl complexes with octahedral structures (Cp*Fe(CO)₂(GeMe₂SPh): 2.3633(4) Å;¹² CpFe(CO)(pyridine)(GeEt₃): 2.4055(11) Å;¹³ Cp(CO)₂[Ge(C₂F₅)₃]: for 2.3232(3) Å;¹⁴). This is presumably due to the strong trans-influence of the germyl moieties. In the IR spectrum of 2, an absorption band observed at 1848 cm⁻¹ implied the presence of Ge–H bonds in 2.

Scheme 4 Synthesis of iron complexes 1–4. (A) Synthesis and isolation of 1. (B) Two reaction schemes for the synthesis of 2. (C) Synthesis and isolation of 3. (D) Synthesis of ca. a 1 : 1 mixture of iron complexes 3 and 4.

Fig. 1 Molecular structures of 2, 3 and 4 with thermal ellipsoids at 50% probability; hydrogen atoms, except for the hydrides in 2, have been omitted for clarity. (A) Molecular structure of 2. (B) Molecular structure of 3. (C) Molecular structure of 4.

Based on the molecular structure of 2, the first H–Ge bond activation mediated by 1 can be assumed to take place through the Fe-mesityl moiety. A similar reaction, i.e., the formation of square-planar trans-(NHC)₂Fe(SnⁿBu₃)₂via the reaction of trans-(NHC)Fe(mesityl)₂ with ⁿBu₃SnH, has recently been reported by Radius et al.^8b Further treatment of 2 with 1 equivalent of Ph₂GeH₂ and 1 equivalent of Et₂GeH₂ led to the formation of a Ge–Ge bond on the iron center to afford complex 3, which contains a metallatrigermacycle skeleton, in 25% isolated yield (Scheme 4C). Although the actual role of Et₂GeH₂ in this reaction remains unclear so far, Et₂GeH₂ may contribute to retarding the rate of the reaction between 2 and Ph₂GeH₂, thus leading to the production of complex 3 as the main product. The molecular structure of 3 (Fig. 1B) revealed that the iron center adopts a tetrahedral coordination geometry, which is different from those found in 1 and 2. The Fe–Ge bond distances in 3 (2.5712(6)/2.5557(5) Å) are elongated compared to those in 2 and other previously reported iron(II) germyl complexes,^12–14 although they are comparable to that of our previously reported iron(II) digermyl complex with a tetrahedral coordination geometry, (THF)₂Fe[Ge(SiMe₃)₃]₂ (2.5589(8) Å).¹⁵ The iron center and three germanium atoms form a nearly planar metallacyclic skeleton, wherein the deviation of all atoms from the least-squares plane ranges from 0.116 Å to 0.124 Å. It is worth noting here that the Ge–Ge-bond-forming reaction apparently occurs during the reaction of 2 with Ph₂GeH₂ to produce 3. In other words, the bond formation involving two Fe–Ge bonds and an incoming molecule of Ph₂GeH₂ to construct a cyclic trigermyl moiety is effectively promoted on the iron center supported by ⁱPrIM^Me ligands. The ¹H NMR spectrum of isolated 3 in C₆D₆ at room temperature features broad resonances at δ = 27.2, 21.3, 19.8, 15.8, 14.3, 7.5, and −1.2 ppm, indicating that 3 is paramagnetic.

We found that a new iron species 4 is formed together with 3 and (GePh₂)₅ in the reaction between 2 and 2 equivalents of Ph₂GeH₂ (Scheme 4D). Although several attempts to isolate 4 failed, a subsequent recrystallization led to the generation of reddish crystals, whose ¹H NMR spectrum in C₆D₆ indicated that they consist of a ca. 1 : 1 mixture of 3 and 4. A careful single-crystal XRD analysis revealed the molecular structure of 4, which consists of a five-membered metallatetragermacycle skeleton composed of four ‘GePh₂’ moieties (Fig. 1C). The iron center of 4 adopts a slightly distorted tetrahedral coordination geometry with Fe–Ge bond lengths of 2.5714(10) and 2.5840(9) Å. These structural features are similar to those in 3; however, there are apparent, significant differences in the metallacycle framework. Namely, the four-membered-ring structure in 3 is almost planar, whereas that of 4 exhibits a ^Ge(2)T_Ge(3)-type twisted structure.¹⁶ The average Ge–Ge bond length (∼2.459 Å) in 4 is also comparable to that in 3 (∼2.477 Å). Based on the molecular structure of 4, one might consider that the incorporation of one additional ‘GePh₂’ unit into 3 led to the generation of 4via Ge–Ge bond propagation.

With the four possible intermediary iron species 1–4 in hand, we would like to propose a reaction mechanism for the dehydrogenative coupling of Ph₂GeH₂ catalyzed by the [Fe(mesityl)₂]₂/ⁱPrIM^Me catalyst system as shown in Scheme 5. First, [Fe(mesityl)₂]₂ reacts with ⁱPrIM^Me to generate catalyst precursor 1. Then, Ge–H bond activation of two moles of Ph₂GeH₂ occurs via the Fe-mesityl bonds in 1 to generate the catalytically active species 2. A sequence of Ge–Ge-bond-forming and Ge–Ge-bond-propagation reactions occurs on the iron center supported by the two ⁱPrIM^Me ligands to generate 3 followed by 4, both of which feature metallacyclooligogermane frameworks. Finally, the incorporation of an additional ‘GePh₂’ unit proceeds when 4 comes into contact with Ph₂GeH₂ to produce (GePh₂)₅ with concomitant regeneration of 2. In summary, ‘GePh₂’ units sequentially assemble on the iron center to realize the selective production of (GePh₂)₅via dehydrogenative coupling. It should be noted here that the catalytic activity of the isolated complexes 1, 2 and 3 was examined independently and that both showed good catalytic activity toward the dehydrogenative coupling of Ph₂GeH₂ to afford (GePh₂)₅ in quantitative yield in THF at room temperature after 40 h (5 mol% of 1, 2 or 3). This result strongly supports the notion that the iron species 2 and 3 play a crucial role in the catalytic dehydrogenative coupling of Ph₂GeH₂.

Scheme 5 Plausible reaction mechanism for the sequential production of H₂ and Ge–Ge bond propagation to afford cyclopentagermane (GePh₂)₅via the dehydrogenative coupling of Ph₂GeH₂ catalyzed by [Fe(mesityl)₂]₂/ⁱPrIM^Me.

Dehydrogenative coupling of other group-14 hydrides

As described above, the [Fe(mesityl)₂]₂/NHC catalyst system is effective for H₂ production via the dehydrogenative coupling of secondary germanes. We found that the dehydrogenative coupling of tertiary and primary germanes can also be achieved by this catalyst system. For instance, the dehydrogenative coupling of Ph₃GeH proceeded effectively in THF at room temperature mediated by 2.5 mol% of [Fe(mesityl)₂]₂ in the presence of 10 mol% of MeIM^Et, which contains sterically less demanding methyl groups on the nitrogen atoms of the NHC ligand, to selectively afford the dimeric product (GePh₃)₂. Similarly, the reaction with ^tBuGeH₃ was briefly examined, and the formation of oligogermanes via the dehydrogenative coupling mediated by a catalytic amount of [Fe(mesityl)₂]₂/ⁱPrIM^Me was observed using ¹H NMR and FAB-MS.

Tin hydrides such as Ph₂SnH₂ and ⁿBu₂SnH₂ were also found to be suitable for this catalytic system (hydrogen content: ca. 0.73 wt% for Ph₂SnH₂ and ca. 0.85 wt% for ⁿBu₂SnH₂). Namely, 1 mmol of Ph₂SnH₂ was dehydrogenatively coupled in the presence of 1 mol% of [Fe(mesityl)₂]₂ and 4 mol% of ⁱPrIM^Me to exclusively produce cyclohexastannane (SnPh₂)₆, which was isolated in 70% yield. The quantity of H₂ gas produced in the course of this reaction was determined using a gas burette, from which ca. 15.4 mL of gaseous product(s) were obtained after 2.5 h at room temperature, and then production of ca. 16.7 mL of gaseous product(s) was detected after 19 h. In this reaction, the production of H₂ immediately took place once Ph₂SnH₂ came into contact with [Fe(mesityl)₂]₂/ⁱPrIM^Me, and this may cause decrease of the amount of detectable gaseous H₂ compared with the theoretical value (ca. 22 mL). In contrast to the reaction with germanium congener Ph₂GeH₂, the reaction with Ph₂SnH₂ exclusively afforded cyclohexastannane. ⁿBu₂SnH₂ was also found to be suitable for this catalyst system (1 mol% of [Fe(mesityl)₂]₂ and 4 mol% of ⁱPrIM^Me) to give a ca. 3 : 1 mixture of cyclopentastannane (SnⁿBu₂)₅ and cyclohexastannane (SnⁿBu₂)₆ with complete conversion of ⁿBu₂SnH₂ at room temperature (Scheme 6). In this context, it should also be noted that the formation of a mixture of (SnⁿBu₂)₅ and (SnⁿBu₂)₆ has already been reported by Jousseaume et al. in the (PPh₃)₂PdCl₂-catalyzed dehydrogenative coupling of ⁿBu₂SnH₂.¹⁷

Scheme 6 Catalytic dehydrogenative coupling of Ph₂SnH₂ catalyzed by the [Fe(mesityl)₂]₂/NHC catalyst system.

The iron-based catalyst system described here was found to be effective under mild reaction conditions for the dehydrogenative coupling of primary, secondary, and tertiary germanes as well as secondary stannanes. Although a relatively high reaction temperature was required, the dehydrogenative coupling of a secondary silane was also achieved by an [Fe(mesityl)₂]₂/ⁱPrIM^Me catalyst system. Neat Ph₂SiH₂ (hydrogen content: ca. 1.01 wt%) was treated with 2.5 mol% of [Fe(mesityl)₂]₂ and 10 mol% of ⁱPrIM^Me at 80 °C. The ¹H NMR spectrum of the crude product indicated 82% conversion of Ph₂SiH₂ and the generation of the disilane Ph₂(H)Si–Si(H)Ph₂ and trisilane Ph₂(H)Si–SiPh₂–Si(H)Ph₂ in 54% and 8% yield, respectively. In contrast to the reaction systems with secondary germanes and stannanes, no further propagation of the ‘SiPh₂’ units took place in this reaction; instead, the generation of Ph₃SiH and PhSiH₃ was detected due to concomitant redistribution. It should be mentioned here that Rosenberg et al. have already reported the selective formation of Ph₂(H)Si–Si(H)Ph₂via the dehydrogenative coupling of Ph₂SiH₂ catalyzed by Rh(PPh₃)₃Cl. In their report, the authors mentioned that the chemoselectivity of this reaction strongly depends on the reaction conditions, including the size of the reaction vial relative to the volume of the substrate, as well as the rate and efficacy of stirring, and that trisilanes and other redistribution products are often generated under non-optimized reaction conditions. This precedent might suggest that the chemoselectivity of our reaction with Ph₂SiH₂ could be improved by the optimization of the reaction conditions. However, unfortunately, neither the conversion of Ph₂SiH₂ nor the chemoselectivity were improved under either different reaction conditions or the use of catalyst systems that contained different auxiliary ligands.

Conclusions

Germanium hydride Ph₂GeH₂ can act as an efficient and reusable hydrogen-production/-storage material with the aid of iron catalysts. Hydrogen evolution from Ph₂GeH₂ was effectively realized at room temperature, concomitant with the quantitative formation of (GePh₂)₅. The regeneration of Ph₂GeH₂ is very facile, i.e., by simple treatment of (GePh₂)₅ with 1 atm of H₂ at 0 °C, or by treatment with PhICl₂ and LiAlH₄ at 40 °C. This hydrogen-production/-storage system has some advantages compared to systems based on conventional hydrogen-storage materials such as NaBH₄, in which both the hydrogen-production and -storage processes can be carried out under relatively mild operating conditions, and that they do not require precious-metal catalysts. Furthermore, germanium hydrides are generally sufficiently stable under aerobic conditions, easy-to-handle, and show less toxicity toward living organisms. We expect that the results presented in this paper will help in developing the next generation of chemical hydrogen-storage/-production systems and support efforts to use group-14 hydrides such as germanium hydrides as reusable hydrogen carriers in practical applications, which are currently in progress in our laboratory.

Data availability

All experimental data are provided in the ESI.†

Author contributions

Y. Kobayashi conducted all experiments. All authors analysed the data. Y. Sunada supervised this study and wrote the manuscript. All authors discussed the results and contributed to the preparation of the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by a project of the Kanagawa Institute of Industrial Science and Technology (KISTEC) as well as by a Grant in Aid for Scientific Research (B) (20H02751) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The synchrotron radiation experiments were performed at the BL02B1 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2021A1139).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2204746, 2204747, 2204748 and 2204749. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2sc06011f

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