Mo^IV₃-Polyoxomolybdates with frustrated Lewis pairs for high-performance hydrogenation catalysis

Abstract

Bifunctional catalysis of frustrated Lewis pairs (FLPs) is one of the most active frontier areas in catalytic hydrogenation. The first Mo^IV-γ-Keggin-like cluster H₂[(py₃MO^IV₃)₂MO^VI₇O₃₂(OH)₄] (γ-1, py = pyridine), which contains unprecedented Mo^IV–O–Mo^VI=O FLPs (POM-FLPs), was prepared in a high yield. The synergistic effect of the POM-FLPs accounts for the high efficiency in catalytic hydrogenation as exemplified by the excellent performance (yield > 99%) of γ-1 as the heterogeneous catalyst in the hydrogenation reduction of nitroarenes to anilines with hydrazine hydrate under mild conditions, revealing the promising potential of such unique MO^IV₃–POMo^VI FLP systems in catalytic hydrogenation.

---

Catalytic hydrogenation is utilized industrially on enormous scales to produce commodity and fine chemicals and pharmaceuticals.^1,2 One active frontier area is the bifunctional catalysis of frustrated Lewis pairs (FLPs) built of main-group and/or transition-metal elements.^3–6 On the other hand, polyoxometalates (POMs) have been extensively employed in epoxidation, dehydrogenation, photoreductants, water oxidation, electron-transfer and storage mediators in Li-ion batteries among many others,^7–10 but their catalytic hydrogenation applications by either high-temperature (>300 °C)¹¹ or electrochemical reduction¹² were extremely rare^11,12 as a consequence of the absence of Lewis acidic low-valence metal centers to build FLPs.^10,13 Recently, we successfully prepared a series of MO^IV₃–POMo^VI hybrids.¹⁴ As illustrated in Fig. 1(b), they contain a triangular Mo–Mo-bonded MO^IV₃ unit wherein each Mo^IV requires a pair of electrons to achieve its stable 16-electron configuration and hence can serve as a Lewis acid (LA).^15–18 The surrounding O–Mo^VI=O groups have been well established as a Lewis base (LB) induced by the high negative charge and large number of bridging¹⁸ and terminal¹⁹ oxygen atoms as electron-pair donors. Such unique POM-FLP systems have several advantages: (1) they can simultaneously activate several hydrogen donors and hence provide more protons/hybrids in single catalytic cycles; (2) close proximity between Mo^IV LAs allows for their cooperative action in binding and activating substrates; (3) the system can be markedly strengthened by the extensive Mo^IV → Mo^VI electron transfer that increases Mo^IV electron-pair accepting and O–Mo^VI=O electron-donating abilities; (4) the peripheral ligands can suffer from functional modification for further applications such as surface decoration²⁰ and engineering.⁷ In this article, the novel MO^IV₆-γ-Keggin-like hybrid [(py₃MO^IV₃)₂MO^VI₇O₃₂(OH)₄]·2(CH₃)₂NH₂·(CH₃)₂NH·2H₂O (γ-1) was prepared in a repeatable and high yield, and hence employed to examine the catalytic hydrogenation activity of such MO^IV₃–POMo^VI FLP systems for the first time. Expectedly and also excitingly, γ-1 has been found to be an excellent heterogeneous catalyst in the catalytic transfer hydrogenation of nitroarenes to anilines, which are vital building blocks and central intermediates for the manufacture of dyes, pigments, agrochemicals, and pharmaceuticals,²¹ under mild conditions as will be described in this work.

Fig. 1 (a, left) Structure of the γ-Keggin-like dianion (γ-1a); (b, right) Mo^IV–O–Mo^VI=O FLP systems in MO^IV₃–POMo^VI hybrids.

The structure of the hybrid [(py₃MO^IV₃)₂MO^VI₇O₃₂(OH)₄]²⁻ (γ-1a) (Fig. 1(a), S1 and S2†) is of crystallographic C_2v symmetry with two mirror planes defined by Mo1–Mo1′–Mo5 and Mo3–Mo3′–Mo5, respectively. It can be described as resulting from the 60° rotation of one py₃MO^IV₃O₄ of the asymmetric isomer cluster [(py₃MO^IV₃)₂MO^VI₇O₃₂(OH)₄]²⁻ (β-2), which was prepared from the uncontrollable oxidation of the [Mo₃O₄(H₂O)₉]⁴⁺ precursor in an unrepeatable and low yield.^14a It features the two (μ₂-OH)₂-bridged [py₃Mo^IV₃O₄] incomplete cuboidal units stabilized by the strong triangular Mo–Mo bonds (Mo1–Mo2, 2.5680(1); Mo2–Mo2′, 2.5215(2) Å) and confirmed by XPS (Fig. S3†) as well as the low bond valence sum (BVS) values (Table S3,† Mo1, 4.33; Mo2, 4.35). Each Mo^IV has an octahedral coordination geometry (without counting Mo–Mo bonds) with an apical pyridine trans to the capping oxygen (O2) to provide the required electron pair. Each pyridine is loosely coordinated to Mo^IV (2.1408(1)–2.3664(1) Å) and hence easily replaced to activate Mo^IV LAs. The two [MO^IV₃O₄] LAs are supported by the MO^VI₇O₂₆ unit featuring the multiple Mo^VI=O and Mo^VI=O₂ LBs. The electrophilic addition of the strongly Lewis-basic Mo^VIO₂ (Mo5) occurs at the four μ₂-O trans to [Mo^IV(μ₂-OH)]₂. The low BVS values of the edge-bridged O1 (1.34) and capping O7 (1.33) reveal that they appear as OH groups. The resulting γ-Keggin-like dianion (γ-1a) is charge-balanced by two protonated Me₂NH from the decomposition of DMF. Of note is that reported γ-Keggin anions, one of the five Baker–Figgis–Keggin isomers,²² are all stabilized by interstitial MO₄ groups.²³ γ-1 represents the first example of empty γ-Keggin-like clusters. The facile formation of γ-1 in a high yield can be partly rationalized on the fact that the electrostatic repulsion in Mo^VI-γ-Keggin²⁴ is highly alleviated in Mo^IV-γ-1.

The DFT calculations of γ-1a, β-2a and α-[H₄MO^VI₁₂O₄₀]⁴⁻ clearly reveal the Mo^IV–Mo^IV-bonded HOMO of the former two (Fig. S4c and S5†), which is higher in energy than that of the latter by about 0.97 eV. Also notable is the decreased HOMO–LUMO gap by 0.3 eV (Fig. S4b†). The markedly increased HOMO energy levels of the 12e-reduced γ-1a and β-2a make them more catalytically active as a consequence of the easier electron transfer from frontier orbitals. The DFT calculations (Table S4†) reveal remarkable Mo^IV → Mo^VI electron transfer in γ-1, especially from the tetravalent Mo1(+0.164)/Mo2(+0.125) to hexavalent Mo4(−0.166)/Mo5(−0.246), which appreciably enhances their Lewis acidity (Mo^IV) and basicity (Mo^VI=O) and hence contributes their synergistic action to be described below.

Unlike previously reported β-2, the insoluble γ-1 was obtained in a repeatable and high yield and selected as the heterogeneous catalyst with the advantages of easy separation and recovery and hence recycling. The catalytic hydrogenation activity to be examined is the reduction of nitroarenes to anilines. Hydrazine hydrate (NH₂NH₂·H₂O), a stable hydrogen transfer reagent, is employed since in principle after hydrogen transfer, the only by-products are water and molecular nitrogen.²⁵ Compared to its unstable anhydrous form, hydrazine hydrate is relatively safe and easy to handle. As shown in Fig. 2 and Table S1,† it is a highly selective and very effective catalyst in the hydrogenation reduction of nitrobenzene to aniline under mild conditions. γ-1 exhibits time-independent perfect selectivity in the catalytic transfer hydrogenation reduction reaction. More than 80% nitroarenes were converted to their corresponding anilines in about thirty minutes. Within two hours, the complete conversion of nitroarenes into the desired anilines in ethanol at 80 °C was achieved. γ-1 can be reused by simple centrifugation with more than 99% conversion and 98% yield in the hydrogenation reduction of nitrobenzene to aniline after the first use as the heterogeneous catalyst shown in Fig. S11 and Table S1.† The examination of molybdenum in reaction solution after centrifugation shows that the concentration of Mo is about 14 mg L⁻¹ corresponding to 0.7% weight of γ-1, clearly indicative of the heterogeneous catalysis. To figure out if the catalytic reactions started from the substitution of the coordinated pyridine to activate Mo^IV LAs, the reaction solution after the first centrifugation is detected to show the presence of about one-third the amount of pyridine of γ-1 and only traces of pyridine are found after the second centrifugation. This observation reveals that almost all surface pyridines of γ-1 were substituted after the first cycle, indicative of the fact that the catalytic reactions begin with the substitution of the surface pyridine to activate Mo^IV LAs. This observation was also supported by the IR spectra of 2, 3 and 4 (Fig. S12†) in the second, third and fourth cycle, respectively, wherein the pyridine IR peaks are not well recognized as in the case of the IR spectra of fresh and 1 (1607 cm⁻¹, 1449 cm⁻¹, and 644 cm⁻¹) which may account for the weakened structural integrity and the lowered conversion in the second, third and fourth cycle, as shown in Fig. S11.† Remarkably, as displayed in Fig. 2(left), the larger amount of hydrazine and prolonged catalytic reaction time can afford yields of more than 98% in the second, third, fourth and fifth cycle.

Fig. 2 Reusable profiles (left) and catalyst-dependent performance (right) of γ-1 for the reduction of nitrobenzene to aniline (reaction conditions see the ESI† for more details).

The scope of this catalytic system has also been examined by testing a variety of nitroarenes with different substituents. As shown in Table S1,† the nitroarenes with both electron-donating (H, CH₃) and electron-withdrawing (Br, CO₂CH₃) groups are smoothly reduced to the desired anilines with a quantitative yield. Meanwhile, bromide-substituted aniline is obtained in excellent yield without the formation of any dehalogenation products. γ-1 is also able to reduce the nitro group with excellent selectivity in a substrate bearing a reducible ester moiety to afford a more demanding functionalized aniline.

Fig. S13† shows the solvent-dependent catalytic performance of γ-1 under the same reaction conditions. These Lewis basic solvents are of decreasing Mo^IV-coordinating capacity: pyridine > CH₃CN > H₂O > C₂H₅OH ~ isopropanol > THF > ethyl acetate, of which water, ethanol and isopropanol are the protic ones and only H₂O can't dissolve nitroarene. The appropriate and reversible solvent→Mo^IV dative coordination by replacing the surface pyridine^13–15 plays an essential role in both activating and in situ stabilizing Mo^IV LAs. As revealed in Fig. S13,† both water incapable of dissolving nitroarenes and THF/ethyl acetate with very poor coordination ability exhibit the lowest activity. The selectivity and, to a much less extent, conversion can be significantly improved by using pyridine and acetonitrile of enhanced datively coordinating capacity. The moderate yields for both pyridine and acetonitrile should be assigned to the fact that they are not protic solvents, which is supported by the observation that the protic isopropanol with the required coordination capacity markedly enhances the yield (90%). However, its performance is still not perfect because of its steric bulkiness. This has been confirmed by the perfect performance (yield, 99.8%) of the smaller ethanol molecules (Fig. S13†). These observations reveal the essential roles of protic, dissolving and datively coordinating capacities in a solvent, which acts cooperatively to deliver both hydrogen transfer reagent (hydrazine) and nitroarenes to the active Mo^IV LAs by forming hydrogen bonds and coordination bonds (PhNO₂⋯HOEt→Mo^IV₃ and H₂N⋯HOEt→Mo^IV₃) for subsequent hydrazine heterolysis and H⁺/H⁻ transfer. Another important role of the protic solvent is to facilitate the dissociation of the coordinated pyridine driven by the C₅H₅N⋯HOEt hydrogen bond for subsequent solvent substitution.

A comparison of the catalytic performance of γ-1 and the triangular Mo^IV oxoclusters [MO^IV₃O₂(O₂CCH₃)₆L₃]ZnCl₄ (L = H₂O, 3a; py, 3b), [Pro₄N]₂[MO^IV₃O₄(C₂O₄)₃L₃] (L = H₂O, 4a; py, 4b; Pro = CH₃CH₂CH₂) offers further insights into the hydrogenation mechanism. As shown in Fig. 2(right), the right two Mo^IV-free blank systems show negligible reactivity. Surprisingly, the discrete [MO^IV₃O₄]-type clusters of 4 have poor selectivity (10%), somewhat similar to the cases of THF and ethyl acetate of poor coordination capacity. This probably results from the presence of strong electron-pair donors C₂O₄²⁻ in 4 that deactivate Mo^IV LAs. The replacement of C₂O₄²⁻ in 4 with acetate groups in 3 remarkably enhances the selectivity (>90%). The low conversions in both 3a and 3b are assignable to the absence of both enhanced Mo^IV LAs and more efficient edge-bridging and terminal oxygen LBs that are believed to account for the perfect performance of γ-1 (yield, 99.8%). As revealed by the DFT calculations (Table S4†), the Mo^IV Lewis acidity is significantly boosted by Mo^IV → Mo^VI electron transfer, which, together with the numerous bridging/terminal oxygen atoms with the correspondingly enhanced Lewis basicity, leads to the excellent efficiency of γ-1 in the hydrogenation reduction of nitroarenes to anilines.

The comparative investigation into the catalytic performance described above offers us invaluable information to understand the hydrogenation reduction mechanism. It is commonly recognized that bifunctional hydrogenation usually starts with the LP-promoted heterolysis of hydrogen donors, which produces a hydride and a proton stabilized by Lewis acidic and basic sites, respectively. The related examples include [CpMo(CO)(κ³-P₂N₂)]⁺ with the non-frustrated N→Mo Lewis pair, which can efficiently catalyze H₂ heterolysis to form MoH⁻ and NH⁺.^5b The density functional theory calculations suggest the stepwise hydrogen transfer via the prior cleavage of the N–H bond rather than the N–N bond,^26a while [MO^IV₃S₄] clusters have a strong tendency to accept a hydride for transfer hydrogenation.^26b,c The heterolytic H → N → H cleavage mechanism of hydrazine by γ-1 bearing the multiple py→Mo^IV Lewis pairs with removable pyridines is thus proposed. The Mo^IV₃ Lewis acidic active sites, which are further promoted by the Mo^IV → Mo^VI electron transfer, actuate the N → H heterolysis to form Mo^IVH⁻. Meanwhile, the Mo^VI=O and even stronger O=Mo^VI=O Lewis basic sites have a pronounced capacity to drive the heterolytic H → N cleavage and stabilize the protons, which is in turn promoted by the Mo^IV → Mo^VI electron transfer. With the above considerations in mind, we propose the following catalytic cycle exemplified by the [pyMo^IV(μ₂-OH)]₂ unit of γ-1 involved in the hydrogenation reduction of nitrobenzene to aniline, as depicted in Scheme 1. Initially, the two pyridine ligands are replaced by hydrazine (N₂H₄) to form [Mo^IV(μ₂-OH)]₂(μ-N₂H₄) (Scheme 1b). The subsequent dual N → H heterolysis of the bridging N₂H₄ through transition state c promoted by the synergistic action of the two Mo^IV LAs leads to the N → H electron transfer to form the Mo^IVH⁻ hydride intermediates. Meanwhile, the surrounding O–Mo^VI=O and hydrazine LBs facilitate the H → N heterolytic breakage to form Mo^VIOH⁺ and/or H⁺₂N₂H₄ that are boosted and stabilized by O–Mo^VI=O (Scheme 1d, Fig. S6 and S7†). This step is accelerated by the formation of a N₂H₂N₂ six-membered ring and N₂ release. The last multi-step (d → b, Scheme S1†) closes the cycle via the hydride and proton transfer to produce aniline which starts the next one. The very low azobenzene → aniline conversion (11.6%) by γ-1 and the undetected former reveals the direct route (Scheme S1†). The other two couples of the Mo^IV₂ LAs (Mo2–Mo2′ and its equivalent) are proposed to suffer from another two similar catalytic cycles, as shown in Scheme 1, to provide a total of four hydrides and four protons. As a consequence, every catalytic cycle produces six hydrides and six protons to reduce two PhNO₂ to PhNH₂ concomitant with the formation of four H₂O, as proposed in Scheme 1d. The countercations [H₂N(CH₃)₂]⁺ may also be involved in the proton transfer process.

Scheme 1 Proposed catalytic cycle for γ-1 involved in the reduction of nitrobenzene. (a) Structure of γ-1a (b) hydrazine coordination to Mo(IV) LAs (c) Mo(IV)/Mo(VI) promoted H → N → H heterolysis of hydrazine (d) HMo(IV) hydrides and protonated Mo(VI)OH intermediates.

Conclusions

The Mo^IV₆-γ-Keggin-like hybrid (γ-1) with the unprecedented Mo^IV₃–O–Mo^VI=O FLPs was prepared in a high yield. The high efficiency of such unique FLP systems in catalytic hydrogenation has been established by the excellent performance of γ-1 as the heterogeneous catalyst in the hydrogenation reduction of nitroarenes to anilines under mild conditions, revealing the promising potential of the unique POM-FLP systems in catalytic hydrogenation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21873102 and 21073192) and the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB 20000000.

Notes and references

1. H.-J. Arpe, Industrial Organic Chemistry, Wiley-VCH, Weinheim, 5th edn, 2010 (a) Modern Reduction Methods, ed. P. G. Andersson and I. J. Munslow, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008; (b) Catalytic Hydrogenation, ed. L. Cerveny, Elsevier, Amsterdam, 1986.
2. (a) A. E. Allen and D. W. C. MacMillan, Chem. Sci., 2012, 3, 633–658; (b) M. K. Karunananda and N. P. Mankad, ACS Catal., 2017, 7, 6110–6119; (c) D. H. Paull, C. J. Abraham, M. T. Scerba, E. Alden-Danforth and T. Lectka, Acc. Chem. Res., 2008, 41, 655–663; (d) W. P. Deng, P. Wang, B. J. Wang, Y. L. Wang, L. F. Yan, Y. Y. Li, Q. H. Zhang, Z. X. Cao and Y. Wang, Green Chem., 2018, 20, 735–744.
3. G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science, 2006, 314, 1124–1126.
4. (a) A. Stute, G. Kehr, C. G. Daniliuc, R. Fröhlich and G. Erker, Dalton Trans., 2013, 42, 4487–4499; (b) L. J. Hounjet, C. Bannwarth, C. N. Garon, C. B. Caputo, S. Grimme and D. W. Stephan, Angew. Chem., Int. Ed., 2013, 52, 7492–7495.
5. (a) S. R. Flynn and D. W. Wass, ACS Catal., 2013, 3, 2574–2581; (b) S. G. Zhang, A. M. Appel and R. M. Bullock, J. Am. Chem. Soc., 2017, 139, 7376–7387.
6. D. W. Stephan, Science, 2016, 354, 7229.
7. (a) M. T. Pope and A. Müller, Angew. Chem., Int. Ed. Engl., 1991, 30, 34–48; (b) C. L. Hill, Chem. Rev., 1998, 98, 1–2.
8. (a) D. L. Long, R. Tsunashima and L. Cronin, Angew. Chem., Int. Ed., 2010, 49, 1736–1758; (b) H. N. Miras, J. Yan, D. L. Long and L. Cronin, Chem. Soc. Rev., 2012, 41, 7403–7430.
9. (a) S. S. Wang and G. Y. Yang, Chem. Rev., 2015, 115, 4893–4962; (b) Y. F. Song and R. Tsunashima, Chem. Soc. Rev., 2012, 41, 7384–7402; (c) R. F. Renneke, M. Pasquali and C. L. Hill, J. Am. Chem. Soc., 1990, 112, 6585–6594; (d) E. Haviv, L. J. W. Shimon and R. Neumann, Chem. – Eur. J., 2017, 23, 92–95; (e) C. Zhang, X. Lin, Z. Zhang, L. S. Long and W. Lin, Chem. Commun., 2014, 50, 11591–11594.
10. N. I. Gumerova and A. Rompel, Nat. Rev. Chem., 2018, 2, 0112.
11. (a) H. Benaissa, P. N. Davey, E. F. Kozhevnikova and I. V. Kozhevnikov, Appl. Catal., A, 2008, 351, 88–92; (b) H. Benaissa, P. N. Davey, Y. Z. Khimyak and I. V. Kozhevnikov, J. Catal., 2008, 253, 244–252; (c) F. Lin and Y.-H. Chin, J. Catal., 2016, 341, 136–148.
12. L. MacDonald, B. Rausch, M. D. Symes and L. Cronin, Chem. Commun., 2018, 54, 1093–1096.
13. (a) K. Piepgrass and M. T. Pope, J. Am. Chem. Soc., 1987, 109, 1586–1587; (b) M. H. Dickman, T. Ozeki, H. T. Evans-Jr., C. Rong, G. B. Jameson and M. T. Pope, J. Chem. Soc., Dalton Trans., 2000, 149–154.
14. (a) W. P. Chen, R. L. Sang, Y. Wang and L. Xu, Chem. Commun., 2013, 49, 5883–5885; (b) X. Xu, B. L. Luo, L. L. Wang and L. Xu, Dalton Trans., 2018, 47, 3218–3222; (c) J. Zhao and L. Xu, Eur. J. Inorg. Chem., 2011, 26, 4096–4102.
15. (a) D. T. Richens, The Chemistry of Aqua Ions, John Wiley & Sons, Chichester, 1997; (b) D. T. Richens, Chem. Rev., 2005, 105, 1961–2002.
16. (a) A. Bino, F. A. Cotton and Z. Dori, J. Am. Chem. Soc., 1978, 100, 5252–5253; (b) R. K. Murmam and M. E. Shelton, J. Am. Chem. Soc., 1980, 102, 3984–3985; (c) F. A. Cotton, D. O. Marler and W. Schwotzer, Inorg. Chem., 1984, 23, 3671–3673; (d) M. Brorson, A. Hazell, C. J. H. Jaconson, I. Schmidt and J. Villadsen, Inorg. Chem., 2000, 39, 1346–1350; (e) K. Nakata, A. Nagasawa, N. Soyama, Y. Sasaki and T. Ito, Inorg. Chem., 1991, 30, 1575–1579; (f) B.-L. Ooi and A. G. Sykes, Inorg. Chem., 1988, 27, 310–315; (g) P. Kathirgamanathan, A. B. Soares, D. T. Richens and A. G. Sykes, Inorg. Chem., 1985, 24, 2950–2954.
17. S. F. Gheller, T. W. Hambey, R. T. Brownlee, M. J. Oconnor, M. R. Show and A. G. Wedd, J. Am. Chem. Soc., 1983, 105, 1527–1532.
18. (a) L. Xu, D. Yan, H. Liu, J. S. Huang and Q. E. Zhang, J. Chem. Soc., Chem. Commun., 1993, 1507–1510; (b) L. Xu, H. Liu, J. S. Huang and Q. E. Zhang, Chem. – Eur. J., 1997, 3, 226–231; (c) L. Xu, Z. F. Li and J. S. Huang, Inorg. Chem., 1996, 35, 5097–5100; (d) L. Xu, H. Liu, D. C. Yan, J. S. Huang and Q. E. Zhang, J. Chem. Soc., Dalton Trans., 1994, 2099–2107.
19. (a) K. Sugahara, T. Kimura, K. Kamata, K. Yamaguchi and N. Mizuno, Chem. Commun., 2012, 48, 8422–8424; (b) K. Sugahara, N. Satake, K. Kamata, T. Nakajima and N. Mizuno, Angew. Chem., Int. Ed., 2014, 53, 13248–13252.
20. X. F. Yi, N. V. Izarova, M. Stuckart, D. Guerin, L. Thomas, S. Lenfant, D. Vuillaume, J. V. Leusen, T. Duchoň, S. Nemsăk, S. D. M. Bourone, S. Schmitz and P. Kögerler, J. Am. Chem. Soc., 2017, 139, 14501–14510.
21. (a) R. S. Downing, P. J. Kunkeler and H. V. Bekkum, Catal. Today, 1997, 37, 121–136; (b) N. Ono, The Nitro Group in Organic Synthesis, Wiley-VCH, New York, 2001; (c) A. Stephen, Amines: Synthesis, Properties and Applications, Cambridge University Press, Cambridge, UK, 2006, pp. 205–215.
22. L. C. W. Baker and J. S. Figgis, J. Am. Chem. Soc., 1970, 92, 3794–3797.
23. (a) B. Botar, Y. V. Geletii, P. Kögerler, D. G. Musaev, K. Morokuma, I. A. Weinstock and C. L. Hill, J. Am. Chem. Soc., 2006, 128, 11268–11277; (b) Y. Kikukawa, K. Suzuki, K. Yamaguchi and N. Mizuno, Inorg. Chem., 2013, 52, 8644–8652; (c) A. Tézé, E. Cadot, V. Béreau and G. Hervé, Inorg. Chem., 2001, 40, 2000–2004.
24. M. T. Pope, Inorg. Chem., 1976, 15, 2008–2010.
25. A. Furst, R. C. Berlo and S. Hooton, Chem. Rev., 1965, 65, 51–68.
26. (a) C. F. Zhang, J. M. Lu, M. R. Li, Y. H. Wang, Z. Zhang, H. J. Chen and F. Wang, Green Chem., 2016, 18, 2435–2442; (b) A. G. Algarra, M. G. Basallote, M. J. Fernández-Trujillo, M. Feliz, E. Guillamón, R. Llusar, I. Sorribes and C. Vicent, Inorg. Chem., 2010, 49, 5935–5942; (c) I. Sorribes, G. Wienhóer, C. Vicent, K. Junge, R. Llusar and M. Beller, Angew. Chem., Int. Ed., 2012, 51, 7794–7798.

---

Footnotes

† Electronic supplementary information (ESI) available: Experimental details, chemicals, instrumentation, molecular and crystal structure figures, XPS, DFT calculations, catalysis (PDF) and crystal data (CIF). CCDC 1575811. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cy01771a

‡ Dedicated to Prof. Jin-Shun Huang on the occasion of his 80th birthday.

---