Comproportionation of a dialuminyne with alane or dialane dihalides as a clean route to dialuminenes

Abstract

Dialuminenes RAlAlR (R = m-terphenyl or bulky aryl) react with the aromatic solvents (e.g. benzene or toluene) in which they dissolve. We synthesized –SiMe₃ substituted derivatives of known terphenyl ligands to increase their solubility in alkanes which have lower reactivity than arenes. The new dialuminene was synthesized via the comproportionation reaction of Na₂(AlAr^iPr4-4-SiMe₃)₂ (3) (Ar^iPr4-4-SiMe₃ = 2,6-(2,6-ⁱPr₂C₆H₃)₂-4-SiMe₃C₆H₂) with either the diiodide Al(Et₂O)I₂Ar^iPr4-4-SiMe₃ (1) or the 1,2-diiododialane 4-SiMe₃Ar^iPr4(I)Al–Al(I)Ar^iPr4-4-SiMe₃ (2). This cleanly generates the dialuminene 4-SiMe₃Ar^iPr4AlAlAr^iPr4-4-SiMe₃ which was trapped as its cycloaddition product (4) with benzene. Even in non-aromatic, essentially inert, solvents red 4 decomposes to colorless solutions. This indicates that the instability of the free dialuminene is an inherent property rather than arising from of the method of synthesis, solvent employed, or the presence of impurities.

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The recognition in the 1970s and 80s that the heavier p-block elements can form multiple bonds to each other^1–3 led to the synthesis of series of alkene and alkyne analogues derived from the heavier members of the group 13, 14, and 15 elements.⁴ The compounds with multiple bonds between aluminium atoms are of special interest due to their very high reactivity^5–9 and their calculated diradical character.¹⁰ However this apparent diradical character makes them difficult to isolate as neutral species. Thus, many structurally characterized Al–Al multiply bonded molecules (Scheme 1) are anionic^11–13 and derived from the reduction of a bonding π-type LUMO in a neutral species. Alternatively, complexation of the aluminium atoms by Lewis bases confers stability on the multiple bonded compounds.^14–16 Another important compound class related to the multiple bonded species are the cycloaddition derivatives of Al=Al bonded moieties and solvent arenes.^17–21 This is a reversible process in some cases.^18–20

Scheme 1 Representative Al–Al multiple bonded species. Ar^iPr4 = 2,6-(2,6-ⁱPr₂C₆H₃)₂C₆H₃, Ar^Me6 = 2,6-Mes₂C₆H₃, Bbt = 2,6-{CH(SiMe₃)₂}₂C₆H₃, Tbt = 2,6-{CH(SiMe₃)₂}₂-4-^tBuC₆H₃.

The first isolated Al–Al π-bonded compounds resulted from one electron reduction of the tetraorganodialanes R₂Al–AlR₂ (R = CH(SiMe₃)₂, C₆H₃-2,4,6-ⁱPr₃).^12,22 Their EPR spectra showed the unpaired electron to be located predominantly in the π-type SOMO between the two Al atoms thereby generating multiple bond character and shortened Al–Al bonds.^11,12

While the sterically demanding terphenyl ligand Ar^iPr4 (Ar^iPr4 = 2,6-(2,6-ⁱPr₂C₆H₃)₂C₆H₃) allowed the isolation of the neutral dimetallenes Ar^iPr4EEAr^iPr4 (E = Ga–Tl)^23–25 which dissociate to :EAr^iPr4 monomers in solution, the Al congener Ar^iPr4AlAlAr^iPr4 could not be isolated from hexanes or ether, due to its poor solubility.¹⁷ Rather, it could only be trapped and characterized as its cycloaddition products with toluene or Me₃SiCCSiMe₃ following the reduction of the halide derivative AlI₂Ar^iPr4 in Et₂O.^17,26 However, the dianionic dialuminyne Na₂(AlAr^iPr4)₂ and the trimeric Na₂(AlAr^Me6)₃ (Ar^Me6 = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃) were isolable by reduction of AlI₂Ar^iPr4 and AlI₂Ar^Me6 with excess Na metal.¹³ The anions contained in these salts are analogues of the corresponding Ga species Na₂(GaAr^iPr6)₂²⁷ (Ar^iPr6 = 2,6-(2,4,6-ⁱPr₃C₆H₂)₂C₆H₃) and Na₂(GaAr^Me6)₃²⁸ isolated earlier by Robinson and coworkers. By increasing the steric demand of the terphenyl ligand on the group 13 elements, the crystalline metallanediyls :EAr^iPr6 (E = In, Tl),^29,30 :GaAr^iPr8 (Ar^iPr8 = 2,6-(2,4,6-ⁱPr₃C₆H₂)₂-3,5-ⁱPr₂C₆H) and :Ga(2,6-(2,6-ⁱPr₂C₆H₃)₂-3,5-ⁱPr₂C₆H)³¹ were isolated. Following these results, we recently reported the synthesis of a stable one-coordinate aluminium species :AlAr^iPr8.³² A favorable dimerization energy of :AlAr^iPr8 to yield Ar^iPr8AlAlAr^iPr8 (ΔG = −20 kJ mol⁻¹) was calculated although this dimer was not isolated and only the monomer :AlAr^iPr8 was obtained from solution. Nonetheless, calculations suggested Ar^iPr8AlAlAr^iPr8 as the reactive species with H₂ (cf. monomeric :GaAr^iPr8 does not react with H₂ although the dimer Ar^iPr4GaGaAr^iPr4 is highly reactive).³³

Tokitoh and coworkers isolated dialuminene–benzene cycloaddition products by reduction of the 1,2-dibromodialanes Bbt(Br)Al–Al(Br)Bbt (Bbt = 2,6-{CH(SiMe₃)₂}₂C₆H₃) and Tbt(Br)Al–Al(Br)Tbt (Tbt = 2,6-{CH(SiMe₃)₂}₂-4-^tBuC₆H₂) with KC₈ in benzene.^18,34 They further showed that these complexes act as synthetic equivalents to dialuminenes by the reversible dissociation of the benzene to yield ArAlAlAr in solution. The complexed benzene could also be irreversibly exchanged with naphthalene, anthracene, or Me₃SiCCSiMe₃ while the reaction with H₂ to give {AlH(μ-H)Ar}₂ (Ar = Bbt, Tbt) was also observed under ambient conditions.¹⁹

As mentioned above, stable compounds containing Al=Al double bonds were only recently isolated through coordination of Lewis bases to the reactive Al centers and are so far limited to a few examples. Thus Inoue and coworkers reported the carbene complexed dialuminenes (NHC)RAl=AlR(NHC) (R = Si^tBu₂Me, 2,4,6-ⁱPr₃C₆H₂; NHC = 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene)^14,15 and Cowley and coworkers reported the synthesis of an amidophosphine supported dialuminene which reversibly dissociates to monomers in solution.¹⁶ However the coordination of Lewis bases to the aluminium atoms does not guarantee stability of the Al=Al bonded moiety. The amidinate supported dialuminene (Am^Dipp)Al=Al(Am^Dipp) (Am^Dipp = C{N(2,6-ⁱPr₂C₆H₃)}₂(4-MeC₆H₄)) of Bakewell and coworkers, generated by hydrogen abstraction from AlH₂Am^Dipp by Al(BDI^Dipp) (BDI^Dipp = C{C(Me)N(2,6-ⁱPr₂C₆H₃)}₂) at 80 °C, went on to react with the benzene solvent to give the respective cycloaddition product.²¹ Braunschweig and coworkers reported that the reduction of the carbene supported terphenyl aluminium diiodide Al(NHC′)I₂Ar^Me6 (NHC′ = 1,3,4,5-tetramethylimidazol-2-ylidene) apparently gave Ar^Me6(NHC′)AlAl(NHC′)Ar^Me6 which added across a C=C bond of a flanking aryl ring, and the monomer Al(NHC′)Ar^Me6 which ring opened the benzene or toluene solvent to give dihydropentalene type structures.²⁰

Despite these advances, compounds of the type RAlAlR free of complexing ligands remain unisolated. Recent reports on the synthesis of the one-coordinate Al species^32,35,36 have illustrated the importance of reducing agent selection to access low oxidation state Al compounds—the products are obtained only when Na metal dispersed on NaCl powder is used. As the chemistry of dialuminenes remains underdeveloped, we revisited their synthesis with the objective of developing isolable ArAlAlAr species that are soluble in solvents with which they do not react. The direct stoichiometric reduction of the aluminium iodide precursors with alkali metals did not yield the targeted dialuminene, instead decomposition occured with elimination of the terphenyl arene. This led us to investigate alternative methods of their preparation free of alkali metal reductants. We found that the comproportionation reaction between a dialuminyne salt and aluminium iodides proceeded rapidly in benzene to cleanly afford a dialuminene-benzene complex. However, the comproportionation reaction in nonreactive ether or alkane solvents led to decomposition of the mixture, indicating an inherent instability of the dialuminene. Despite this, the reaction offers a new synthetic route to reactive Al–Al multiple bonded species.

Using the -SiMe₃ modified ligand Ar^iPr4-4-SiMe₃ (Ar^iPr4-4-SiMe₃ = 2,6-(2,6-ⁱPr₂C₆H₃)₂-4-SiMe₃C₆H₂), we anticipated that the extra aliphatic groups would increase the solubility of ArAlAlAr sufficiently to permit its handling in non-aromatic solvents with which it does not react (e.g. hexane, ether). The Hammett constant σ_para = −0.07 for –SiMe₃³⁷ also suggested it would have minimal electronic effects on the Al=Al moiety in comparison to the Ar^iPr4 derivative (σ_para for H = 0). Addition of LiAr^iPr4-4-SiMe₃ to AlH₃NMe₃ in Et₂O afforded the aluminate salt Li(Et₂O)AlH₃Ar^iPr4-4-SiMe₃ as a white solid. Subsequent treatment with CH₃I (5 eq.) gave the iodide Al(Et₂O)I₂Ar^iPr4-4-SiMe₃ (1). The Al-I distances in 1 (Fig. 1) are 2.5288(7) Å and 2.5578(7) Å, an Al–C bond length of 1.9936(19) Å and Al–O bond being 1.8703(15) Å. Treating 1 with KC₈ (1.3 eq.) gave the yellow 1,2-diiododialane 4-SiMe₃Ar^iPr4(I)Al–Al(I)Ar^iPr4-4-SiMe₃ (2) (Scheme 2). The Al–Al distance of 2.604(2) Å in 2 is nearly identical to that of Ar^iPr4(I)Al–Al(I)Ar^iPr4 (2.609(2) Å). The I atoms are disordered over two sites with Al-I distances of 2.5083(11) and 2.5354(13) Å but in each case the coordination around the Al atom is trigonal planar. The UV-Visible spectrum of 2 showed a single absorbance at 386 nm.

Fig. 1 Thermal ellipsoid plots (30%) of 1 (left), 2 (middle), and 3 (right). Disordered atoms, solvent molecules, and hydrogen atoms are not shown. Selected bond distances (Å) and angles (°): [1]: Al(1)–I(1): 2.5287(7), Al(1)–I(2): 2.5578(8), Al(1)–C(1): 1.993(2), Al(1)–O(1): 1.8704(16), C(1)–Al(1)–I(1): 124.12(6), C(1)–Al(1)–I(2): 108.44(6), I(1)–Al(1)–I(2): 106.87(2), I(1)–Al(1)–O(1): 104.87(5) I(2)–Al(1)–O(1): 96.68(5). [2]: Al(1)–Al(1A): 2.604(3), Al(1)–I(1): 2.5087(17), Al(1)–C(1): 1.955(4), C(1)–Al(1)–Al(1A): 130.09(12), C(1)–Al(1)–I(1): 113.73(11), I(1)–Al(1)–Al(1A): 115.95(10), C(1)–Al(1)–Al(1A)–C(1A): 180. [3]: Al(1)–Al(1A): 2.4255(9), Al(1)–C(1): 2.0410(15), Al(1)–Na(1): 3.1403(8), Al(1)–Na(1A): 3.1138(9), Na–C(avg):2.968, C(1)–Al(1)–Al(1A)–C(1A): 180.

Scheme 2 Synthetic routes to compounds 2, 3 and 4.

The reduction of 1 with excess Na in the form of either Na mirror or 5% w/w Na/NaCl in Et₂O gave a dark green-brown solution. Removal of the volatile components and extraction of the residue with hexanes and then toluene gave two fractions: the hexanes extract, despite having a dark red color, contained 4-SiMe₃-Ar^iPr4H as the only identifiable product (isolated yield 62% based on 1), indicating a significant amount of decomposition under reducing conditions. From the dark green toluene extract the anionic dialuminyne complex Na₂(4-SiMe₃-Ar^iPr4AlAlAr^iPr4-4-SiMe₃) 3 was isolated as dark green crystals in ca. 30% yield. The structure of 3 shows two crystallographically independent molecules with Al–Al distances of ca. 2.43 Å which is near to the 2.428(1) Å in Na₂(Ar^iPr4AlAlAr^iPr4).¹³ The Na–Al distances are in the range 3.1016(8) to 3.1403(8) Å, and the average Na-C distance is 2.968 Å. The UV-Visible spectrum of 3 displays absorbances at 344 nm, 470 nm, and at 612 nm with a shoulder at 660 nm (cf. 354, 456, and 600 nm in Na₂(Ar^iPr4Al-AlAr^iPr4). Overall, the new compounds showed little deviation from their Ar^iPr4 substituted counterparts while having the desired increased solubility properties. However, attempts to reduce 1 or 2 with alkali metals to the dialuminene 4-SiMe₃-Ar^iPr4AlAlAr^iPr4-4-SiMe₃ under a variety of conditions were unsuccessful.

As a result of the difficulty with direct reduction to the dialuminene (see above), we tested the comproportionation reaction of the dialuminyne 3 with the aluminium iodides 1 or 2. Treating 3 with excess 1 in C₆D₆ resulted in the formation of 2 with a trace amount of a new product that was later determined to be the dialuminene-benzene cycloaddition species 4. Mixtures of 1 or 2 with a slight excess of 3 in C₆D₆ gave dark brown solutions that indicated consumption of the iodide materials along with formation of 4 in the ¹H NMR spectra. On a preparative scale an equimolar mixture of 1 and 3 was dissolved in benzene, resulting in rapid formation of a red solution. Filtration and concentration of the solution to ca. 1 mL followed by storage at ca. 8 °C afforded red crystals of the dialuminene-benzene cycloaddition complex 4 (Fig. 2). ¹H NMR monitoring of the reaction indicated the formation of no other Ar^iPr4-4-SiMe₃ substituted species except 4.

Fig. 2 Thermal ellipsoid plot (30%) of 4. Selected bond lengths (Å) and angles (°): Al(1)–Al(2): 2.5585(6), Al(1)–C(1): 1.9681(14), Al(2)–C(34): 1.9743(13), Al(1)–C(67): 2.0000(15), Al(2)–C(70): 1.9976(14), C(67)–C(68): 1.500(3), C(68)–C(69): 1.337(2), C(69)–C(70): 1.502(2), C(70)–C(71): 1.498(2), C(71)–C(72): 1.338(2), C(72)–C(67): 1.500(2), C(1)–Al(1)–Al(2): 136.83(4), C(1)–Al(1)–C(67): 128.87(6), C(67)–Al(1)–Al(2): 93.87(4), C(34)–Al(2)–Al(1): 136.76(4), C(34)–Al(2)–C(70): 127.02(6), C(70)–Al(2)–Al(1): 94.79(4), C(1)–Al(1)–Al(2)–C(34): 34.18(9), C(67)–Al(1)–Al(2)–C(70): 12.95(7).

The Al–Al bond length in 4 is 2.5585(6) Å with Al–C_ipso distances of 1.9681(14) and 1.9743(13) Å while the Al–C distances to the bridging cyclohexadiene moiety are 2.0000(15) and 1.9976(14) Å. The Al atoms are almost planar coordinated (∑°Al = 358.57(9)°, 359.57(9)°) and the C(1)–Al(1)–Al(2)–C(34) torsion angle is 34.18(9)°. UV-Visible spectroscopy in hexanes showed an absorbance at 323 nm with a weaker shoulder spanning the range ca. 430 to 530 nm. A broad absorbance in the same region was found earlier for the BbtAlAlBbt benzene complex mentioned above.¹⁸

The isolation of 4 is consistent with the growing number of cycloaddition products that have been isolated from the reaction of arenes and Al=Al bonded species.^17,18,20,21 Its formation indicates that the dialuminene ArAlAlAr is obtained cleanly from the reaction between 3 and 1 or 2. The direct reduction of 1 in benzene with ≥2 eq. of alkali metal reducing agents did not afford the dialuminene but instead gave red solutions for which ¹H NMR spectroscopy showed the generation of the free arene 4-SiMe₃-Ar^iPr4H as the major product with no signals assignable to 4.

Complex 4 was observed to be soluble in benzene and cyclohexane as well as being sparingly soluble in hexanes or ether. While 4 showed no signs of decomposition in benzene after several days, solutions of 4 in hexane or cyclohexane gradually lost their color overnight. This is probably due to reversible dissociation of 4 and a shift of the equilibrium toward benzene and the free dialuminene, which subsequently decomposes or rearranges. Attempts to comproportionate 1 or 2 with 3 in Et₂O or hexanes afforded a dark purple solution that also rapidly faded to colorless. We conclude that the instability of the dialuminene 4-SiMe₃-Ar^iPr4AlAlAr^iPr4-4-SiMe₃ is an inherent property of the compound rather than an artifact of the synthetic route employed or impurities in the system. It is possible that in the future suitable electronic modification of the ligand may yield a less reactive, isolable dialuminene species.

In summary, we have shown that the dialuminene 4-SiMe₃-Ar^iPr4AlAlAr^iPr4-4-SiMe₃ is cleanly generated stoichiometrically in solution via the reaction of 1 or 2 with 3. Its decomposition in hexanes or ether indicates that it is inherently unstable, probably as a result of its singlet diradical character.^10,38 However, it can be trapped as 4 by its reaction with benzene. Further investigations into the use of 3 as a reducing agent to generate Al–Al multiply bonded species are underway.

This work is dedicated to the memory of Robert West. We thank the U.S. Department of Energy (DE-FG02-07ER46475) for supporting this work.

Note added after first publication: This article replaces the version published on 22nd November 2022, which contained errors in the display of chemical formulae throughout.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. D. E. Goldberg, D. H. Harris, M. F. Lappert and K. M. Thomas, J. Chem. Soc., Chem. Commun., 1976, 261–262.
2. R. West, M. J. Fink and J. Michl, Science, 1981, 214, 1343–1344.
3. M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu and T. Higuchi, J. Am. Chem. Soc., 1981, 103, 4587–4589.
4. R. C. Fischer and P. P. Power, Chem. Rev., 2010, 110, 3877–3923.
5. A. J. Downs, Coord. Chem. Rev., 1999, 189, 59–100.
6. G. H. Robinson, Adv. Organomet. Chem., 2001, 47, 283–294.
7. Y. Wang and G. H. Robinson, Organometallics, 2007, 26, 2–11.
8. Y. Wang and G. H. Robinson, Chem. Commun., 2009, 5201–5213.
9. P. Bag, C. Weetman and S. Inoue, Angew. Chem., Int. Ed., 2018, 57, 14394–14413.
10. J. Moilanen, P. P. Power and H. M. Tuononen, Inorg. Chem., 2010, 49, 10992–11000.
11. W. Uhl, A. Vester, W. Kaim and J. Poppe, J. Organomet. Chem., 1993, 454, 9–13.
12. R. J. Wehmschulte, K. Ruhlandt-Senge, M. M. Olmstead, H. Hope, B. E. Sturgeon and P. P. Power, Inorg. Chem., 1993, 32, 2983–2984.
13. R. J. Wright, M. Brynda and P. P. Power, Angew. Chem., Int. Ed., 2006, 45, 5953–5956.
14. P. Bag, A. Porzelt, P. J. Altmann and S. Inoue, J. Am. Chem. Soc., 2017, 139, 14384–14387.
15. C. Weetman, A. Porzelt, P. Bag, F. Hanusch and S. Inoue, Chem. Sci., 2020, 11, 4817–4827.
16. R. L. Falconer, K. M. Byrne, G. S. Nichol, T. Krämer and M. J. Cowley, Angew. Chem., Int. Ed., 2021, 60, 24702–24708.
17. R. J. Wright, A. D. Phillips and P. P. Power, J. Am. Chem. Soc., 2003, 125, 10784–10785.
18. T. Agou, K. Nagata and N. Tokitoh, Angew. Chem., Int. Ed., 2013, 52, 10818–10821.
19. K. Nagata, T. Murosaki, T. Agou, T. Sasamori, T. Matsuo and N. Tokitoh, Angew. Chem., Int. Ed., 2016, 55, 12877–12880.
20. D. Dhara, A. Jayaraman, M. Härterich, R. D. Dewhurst and H. Braunschweig, Chem. Sci., 2022, 13, 5631–5638.
21. C. Bakewell, K. Hobson and C. J. Carmalt, Angew. Chem., Int. Ed., 2022, 61, e202205901.
22. W. Uhl, Z. Naturforsch., B: J. Chem. Sci., 1988, 43, 1113–1118.
23. N. J. Hardman, R. J. Wright, A. D. Phillips and P. P. Power, Angew. Chem., Int. Ed., 2002, 41, 2842–2844.
24. R. J. Wright, A. D. Phillips, N. J. Hardman and P. P. Power, J. Am. Chem. Soc., 2002, 124, 8538–8539.
25. R. J. Wright, A. D. Phillips, S. Hino and P. P. Power, J. Am. Chem. Soc., 2005, 127, 4794–4799.
26. C. Cui, X. Li, C. Wang, J. Zhang, J. Cheng and X. Zhu, Angew. Chem., Int. Ed., 2006, 45, 2245–2247.
27. J. Su, X.-W. Li, R. C. Crittendon and G. H. Robinson, J. Am. Chem. Soc., 1997, 119, 5471–5472.
28. X.-W. Li, W. T. Pennington and G. H. Robinson, J. Am. Chem. Soc., 1995, 117, 7578–7579.
29. S. T. Haubrich and P. P. Power, J. Am. Chem. Soc., 1998, 120, 2202–2203.
30. M. Niemeyer and P. P. Power, Angew. Chem., Int. Ed., 1998, 37, 1277–1279.
31. Z. Zhu, R. C. Fischer, B. D. Ellis, E. Rivard, W. A. Merrill, M. M. Olmstead, P. P. Power, J. D. Guo, S. Nagase and L. Pu, Chem. – Eur. J., 2009, 15, 5263–5272.
32. J. Queen, A. Lehmann, J. Fettinger, H. Tuononen and P. Power, J. Am. Chem. Soc., 2020, 142, 20554–20559.
33. C. A. Caputo, J. Koivistoinen, J. Moilanen, J. N. Boynton, H. M. Tuononen and P. P. Power, J. Am. Chem. Soc., 2013, 135, 1952–1960.
34. K. Nagata, T. Agou, T. Sasamori and N. Tokitoh, Chem. Lett., 2015, 44, 1610–1612.
35. X. Zhang and L. L. Liu, Angew. Chem., Int. Ed., 2021, 60, 27062–27069.
36. A. Hinz and M. P. Müller, Chem. Commun., 2021, 57, 12532–12535.
37. C. Hansch, A. Leo and R. W. Taft, Chem. Rev., 1991, 91, 165–195.
38. Y. Jung, M. Brynda, P. P. Power and M. Head-Gordon, J. Am. Chem. Soc., 2006, 128, 7185–7192.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2212327–2212330. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2cc05646a

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