Superacidic aluminum azido species: precursors to triazolyl dimers, trimers and tetramers

Abstract

The aluminum-azido species (Me₃SiN₃)Al(C₆F₅)₃ thermally converts to [(C₆F₅)₂Al(μ-N₃)]₂ 2. Further reactions with Me₃SiN₃ afford [(C₆F₅)₂Al(μ-N₃)₂]₃Al which contains unique four Al centers bound to six bridging azides. Compound 2 is shown to be a Lewis superacid both experimentally and theoretically, forming dimeric products in reactions with diphenylphosphine oxide or tetrahydrofuran and a monomeric adduct with an NHC donor. Compound 2 also undergoes “click” reactions with alkynes to give dimeric, trimeric and tetrameric products depending on the nature of the substitution on the alkyne, which demonstrates the ability of the reagent to control the nature of aggregation.

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Introduction

Since the first synthesis of azido benzene by Griess in 1864,^1,2 organic azides have been extensively studied for applications in organic synthesis, bioorthogonal chemistry and materials science.^3–13 Polyazido compounds are high-energy materials thanks to the high nitrogen content and the energetically favorable release of N₂.¹⁴ As a result they often exhibit poor thermal stability and are sensitive to mechanical stimuli.^15–20 Thus, a major theme in the study of azido compounds has been to probe their reactivity and stability.^{11,19,21–23} The incorporation of strongly Lewis acidic or steric bulky moieties has been shown to significantly increase the stability of azido compounds, leading to the formation of isolable molecular species.^24–26 While monoazido boranes such as R_(3−n)B(N₃)_n^19,27 1-azido-2,3,4,5-tetraphenylborole²⁸ and carbene-stabilized azidoborane,^29–31 have been reported, more electrophilic boranes generally form more diverse molecular structures. For example, the trimers (Cl₂B(μ-N₃))₃ and (Br₂B(μ-N₃))₃^32,33 were reported in the 1960s, while Klapoke reported the dimer ((C₆F₅)₂B(μ-N₃))₂^24,25 in 2000. Subsequently Bettinger et al. described the trimeric 9-azido-9-borafluorene (Scheme 1)^34,35 while Braunschweig et al. synthesized the diborane(4) diazides (Scheme 1).^36,37 The reactivity of borane–azides has drawn limited attention. We previously reported the use of ((C₆F₅)₂B(μ-N₃))₂ in uncatalyzed “click” reactions with various alkynes.^38–40 Ye et al. reported the reactivity of the super Lewis acid carborane-derived 9,10-diazido-9,10-dibora-anthracene (Scheme 1)²¹ with B–H, Si–H and Ge–H bonds. Most recently, Guan, Lin, Shi and co-workers published the use of bis-azidoboranes fused by biphenylene/xanthrene (Scheme 1)⁴¹ to generate transient nitrene species for further reactions.

Scheme 1 Selected examples of group 13 azides.

Heavier analogues of group 13 polyazides are generally less studied. For gallium, indium and thallium, the polyazido species M(N₃)₃, (CH₃CN)M(N₃)₃ (M = Ga, In, Tl), [Ga(N₃)₅]²⁻, and [M(N₃)₆]³⁻ (M = In, Tl) (Scheme 1) have been fully characterized by Haiges, Christe and co-workers (Scheme 1).⁴² In the case of aluminum, while the first Al-azide (Me₂Al(μ-N₃))₃ (Scheme 1) was reported by Dehnicke in 1968, only a few other Al-azides have been described since. The Al-monoazides LAlN₃ (L = [(Me₃SiNCH₂CH₂)₂NR]²⁻) and Ar^iPr8Al(N₃)[N(SiMe₃)₂] have been described by Bertrand et al.⁴³ and Power et al.,⁴⁴ respectively, while Uhl et al. and Fischer and coworkers reported the trimeric [(CH₃Si)₂CH₂Al(μ-N₃)]₃ ⁴⁵ and the monomeric triazide py₃Al(N₃)₃ ⁴⁶ (Scheme 1), respectively. Apart from the use of azido-alane Et₂AlN₃ ⁴⁷ as an N₃ source in the preparation of N-rich compounds and materials,^48–52 little is known about the reactivity of Al-polyazides.

We reasoned that the incorporation of strong electron withdrawing groups on aluminum would enhance the stability and Lewis acidity of polyazido species and thus favor expanded coordination geometries for aggregation. Herein, we report the preparation of aluminum-azido derivatives incorporating C₆F₅ substituents and demonstrated that such species provide access to polytriazolyl derivatives via “click” reactions resulting in dimeric, trimeric and tetrameric aluminum species, where the reagent determines the nature of the aggregation.

Results and discussion

We initiated our investigation with the reaction of trimethylsilylazide (Me₃SiN₃) and Al(C₆F₅)₃ in a ratio of 1 : 1 at room temperature. A white solid 1 was obtained and isolated in 90% yield (Scheme 2). This species exhibited ¹⁹F NMR signals at −123.0, −150.0 and −160.1 ppm, inferring a tetracoordinated Al center based on the gap between the signal for the meta- and para-fluorines.^53–55 The IR absorption peaks of 1 at 2170 and 1358 cm⁻¹ confirm the presence of the azido moiety.⁵⁶ Single crystal X-ray diffraction (XRD) analysis of the colorless crystals of 1 revealed the presence of the azide–alane adduct (Me₃SiN₃)Al(C₆F₅)₃ (Fig. 1a) with a N–Al bond of 1.9925(14) Å. The two N–N bond lengths in the azido-fragment were 1.258(2) and 1.113(2) Å, which are similar to those found in the known boron analogue [1.259(1) and 1.117(2) Å].^57,58

Scheme 2 Synthesis of 1–6.

Fig. 1 POV-ray depiction of (a) 1, (b) 2, and (c) 3; C: black, Al: brown, F: pink, N: blue, and Si: salmon. Hydrogen atoms have been omitted for clarity.

Heating 1 in a toluene solution at 60 °C for 12 h led to the observation of two new sets of peaks at −124.3, −146.7 and −157.7 ppm and −128.0, −152.4 and −161.7 ppm in the ¹⁹F NMR spectrum. The latter signals affirm the generation of Me₃Si–C₆F₅.⁵⁹ The remaining signals were attributed to 2 which was isolated as colorless crystals upon recrystallization from a toluene solution (Scheme 2). The persistence of IR absorptions at 2174 and 1365 cm⁻¹ suggests the retention of the azido-fragment. Upon redissolution, a ¹⁹F–¹⁹F DOSY NMR spectrum confirmed a single species in solution. In addition, compound 2 was found to be unstable in dichloromethane and chloroform giving a complex mixture of products. X-ray crystallography revealed 2 as the dimer [(C₆F₅)₂Al(μ-N₃)]₂ (Fig. 1b) structurally similar to the boron analog,²⁵ in which two azido groups bridge the two Al atoms. The four Al–N bond lengths are found to be 1.914(2), 1.918(2), 1.918(2) and 1.925(2) Å.

The formation of 2 from 1 is thought to occur via a formal β-elimination of Me₃SiC₆F₅. This notion prompted us to probe the corresponding reaction of [HAl(C₆F₅)₂]₃ with Me₃SiN₃. Indeed upon β-elimination of the silane, HSiMe₃ was evidenced by the ¹H NMR resonances at 4.17 (Si–H) and 0.01 (SiCH₃) ppm and the product 2 could be isolated in 90% yield.

Heating compound 2 in the presence of additional Me₃SiN₃ at 80 °C for 3 days resulted in the formation of a new species 3 as evidenced by a new set of ¹⁹F NMR signals and the resonances attributable to Me₃SiC₆F₅. Species 3 was isolated in 73% yield (Scheme 2) and showed IR absorptions at 2169 and 1360 cm⁻¹. An X-ray crystallographic study confirmed the formulation of 3 as [(C₆F₅)₂Al(μ-N₃)₂]₃Al (Fig. 1c) featuring a pseudo-octahedral Al center surrounded by six azido fragments, each of which bridges three other Al centers. The twelve Al–N bond lengths range from 1.893(2) to 1.996(2) Å. Each of the three Al₂N₂ rings are nearly planar with the N–Al–N angles ranging from 76.23(8)° to 80.8(1)°. Compound 3 represents an unusual polyazido-alane, incorporating an unprecedented hexaazido-aluminum center.

The Lewis acidity of 2 was assessed by Gutmann–Beckett Lewis acidity test revealing an acceptor number (AN) of 82.4 for 2. This is significantly higher than that observed for B(C₆F₅)₃ (AN = 76.0).^55,60 Density functional theory (DFT) calculations performed at the BP86-D3/def2-SVP level of theory revealed that the gas-phase/solvent-corrected (toluene) isodesmic fluoride ion affinity values (FIA) of 545.1 kJ mol⁻¹ and 387.6 kJ mol⁻¹ exceed those of SbF₅ (FIA: 483.2 kJ mol⁻¹; FIA_toluene: 383.5 kJ mol⁻¹). Similarly, the hydride ion affinities (HIA) of 487.7 kJ mol⁻¹ and 324.5 kJ mol⁻¹ for 2 are similar to those of B(C₆F₅)₃ (HIA: 488.1 kJ mol⁻¹; HIA_toluene: 286.4 kJ mol⁻¹). Collectively these data suggest that 2 is a Lewis superacid,^61–63 with the lowest unoccupied molecular orbital (LUMO) of 2 partly associated with the empty p-orbital on Al (Fig. 2).

Fig. 2 Selected molecular orbitals for 2 (dimer) at the M062X/def2-SVP level. Isovalue: 0.05.

Treatment of 2 with Ph₂P(O)H afforded colorless crystals of 4 (Scheme 2) which exhibited a ³¹P NMR doublet at −34.5 ppm, with a P–H coupling constant of 539.2 Hz. The molecular structure of 4 (Scheme 2 and Fig. 3a) confirmed the coordination of a phosphine-oxide to each of the Al centers of 2, with ((C₆F₅)₂Al(OP(H)Ph₂)(μ-N₃))₂ generating five coordinate Al centers. The analogous tetrahydrofuran adduct ((C₆F₅)₂Al(thf)(μ-N₃))₂, 5 was also isolated (Scheme 2, see the ESI†). In contrast, the reaction of 2 with the N-heterocyclic carbene [1,3-bis-(2,6-diisoproylphenyl)imidazol-2-ylidene] (IPr) gave the product 6 (Scheme 2) which was shown crystallographically to be the monomeric species (IPr)Al(N₃)(C₆F₅)₂ (Fig. 3b). This contrasting reactivity is thought to result from both the strong donor ability and the steric demands of the IPr ligand.

Fig. 3 POV-ray depiction of (a) 4 and (b) 6; C: black, Al: brown, F: pink, N: blue, O: red, and P: orange. Hydrogen atoms have been omitted for clarity.

We speculated that alkynes react with 2 in a ‘click reaction’ and that the superacidity of 2 and the nature of the alkyne substitution would have an impact on the degree of aggregation. To probe this, 2 was reacted with ethyl phenylpropiolate in a toluene solution. The in situ ¹⁹F NMR spectrum showed new resonances at −123.1, −153.6 and −161.5 ppm (Scheme 3). An X-ray crystallographic study of the product 7 showed it to be the dimeric species [(C₆F₅)₂Al(μ-EtO₂CC₂PhN₃)]₃ (Fig. 4a) in which two penta-coordinate Al centers are bridged by two triazole rings in a 1,3-fashion with Al–N bond lengths of 1.9547(14) and 2.0337(15) Å.

Scheme 3 Synthesis of 7–9.

Fig. 4 POV-ray depiction of (a) 7, (b) 8, and (c) 9; C: black, Al: brown, F: pink, N: blue, and O: red. Hydrogen atoms have been omitted for clarity.

The analogue reaction of 2 with dimethyl acetylenedicarboxylate in toluene solution led to the formation of a new species 8 which exhibited ¹⁹F NMR signals at −139.1, −154.0 and −162.3 ppm. Colourless crystals of 8 (Scheme 3) suitable for single-crystal XRD analysis were obtained by storing a C₆H₆/hexane solution of 8 at −30 °C. The molecular structure showed 8 to be the trimeric species ((C₆F₅)₂Al((μ-MeO₂C)₂C₂N₃))₃ (Fig. 4b). In this case, three triazole rings bridged three Al(C₆F₅)₂ moieties with Al–N bond lengths ranging from 2.030(3) to 2.118(2) Å. The Al centers are also coordinated to C=O fragments of the ester with Al–O distances of 1.9804(19) to 2.155(2) Å generating a pseudo-octahedral Al coordination sphere.

Compound 2 also reacted with an equivalent of ethyl propiolate in a benzene solution, affording colorless crystals of 9 in a yield of 76%. This species exhibited ¹⁹F NMR signals at −123.2, 125.3, −151.7, 156.2, −160.1, and 162.9, inferring a tetracoordinated Al center with the inequivalent C₆F₅ groups. A single-crystal X-ray diffraction study of 9 (Scheme 3 and Fig. 4c) revealed a tetrameric formulation ((C₆F₅)₂Al(μ-EtO₂CC₂HN₃))₄ in which the four triazole rings bridged four Al atoms in a 1, 3-fashion with Al–N bond lengths of 1.936(3) and 2.002(3) Å. Coordination of the ester's carbonyl fragment to Al affords penta-coordinate Al centers.

In contrast to boron azides which undergo “click reactions”, affording four coordinate boron centers,^38–40 the present reactions of superacidic aluminum centers incorporated in 7–9 provide interactions with the carbonyl substituents to give penta- or hexa-coordinated Al centers. As a consequence, the nature of the substituents on the alkyne determines the degree of aggregation, permitting the formation of dimers, trimers and tetramers, 7–9. Thus, the super-Lewis acidity of the Al centers in 2 and the nature of the substitutions on the alkynes can be tuned to control the nature of aggregates formed.

Conclusions

In summary, we have reported the synthesis of the aluminum azido derivative 1 and its thermal conversion to 2. In addition, 2 was shown to react with excess Me₃SiN₃ to give 3, a species containing an unusual hexaazido-Al center. The superacidity of 2 was confirmed experimentally and computationally while reactions with donors revealed that steric demands have an impact on the nature of the resulting adducts. Exploiting compound 2 for click reactions with substituted alkynyl esters afforded oligomeric aluminum triazole derivatives 7–9. These findings demonstrate the ability of highly electron deficient aluminum-azides to access a range of structurally diverse aggregates, the nature of which is determined by the steric demands of the substitution on the alkyne. This provides a strategy to control the degree of aggregation. Further exploration of the utility of such Lewis superacidic aluminum azides in organic and materials chemistry is the subject of ongoing work.

Author contributions

S. J., J. T. and T. C. performed the experiments; S. J. performed DFT calculations; T. C., W. Y and N. X. assisted S. J. and J. T. with data collection; D. W. S. and Y. W. prepared the manuscript and managed the project.

Data availability

The data supporting this article have been included as part of the ESI.† Deposition numbers 2164107 (2), 2329987 (3), 2329988 (1), 2329989 (8), 2329990 (5), 2329991 (6), 2329992 (4), 2356544 (7), and 2359713 (9) contain the supplementary crystallographic data for this paper.†

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 22401163, 42388101 and 92256203), Ningbo Natural Science Foundation (No. 2022J108) and the Technology and Engineering Center for Space Utilization, the Chinese Academy of Sciences (No. KJZ-YY-NSM0406). We acknowledge the Analysis Center of Institute of Drug Discovery Technology for collecting spectral data.

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Footnotes

† Electronic supplementary information (ESI) available: synthesis and characterization of all compounds, NMR spectra, crystallographic data, and details of the DFT calculations. CCDC 2164107, 2329987–2329992, 2356544 and 2359713. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi00038f

‡ These authors contributed equally to this work.

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