Stepwise azido transfer to cerous tris(tert-butoxy)siloxide via trans-silylation with Me₃SiN₃

Abstract

Trimethylsilylhalogenides and -pseudohalogenides are frequently used reactants for (pseudo)halogen transfer, including derivatizations of cerium compounds. The present study describes the facile ligand exchange of cerium(III) complex [Ce{OSi(OtBu)₃}₃]₂ with Me₃SiN₃ generating the mixed azido/siloxy Ce^III complexes Ce₂[OSi(OtBu)₃]₅(N₃) and [Ce{OSi(OtBu)₃}₂(N₃)]₄. Although cerium siloxide complexes are a topical research field, thermal or photolytic ligand degradation as described in Ce^IV alkoxide chemistry has not been described. The activation of the azide complexes by irradiation with UV-visible light is easily achieved for such mixed azido/siloxy cerous complexes, as revealed by the identification of the homoleptic ceric complex Ce[OSi(OtBu)₃]₄.

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Organoazide-promoted formation of organoimide and nitride complexes has emerged as a productive/popular synthesis protocol in uranium 5f chemisitry.^1,2 Such redox transformations build on the release of N₂ according to photolytic and thermal pathways as well as the facile accessibility of uranium in high oxidation states (U^IV–VI).^1,2 Reinforcing similarities between U^III/IV and Ce^III/IV ions,^3,4 it was recently shown that the 4f-element cerium can engage in a similar redox transformation. For example, anionic cerium(IV) terminal imide was obtained by two-electron oxidation of a “Ce^II” complex supported by a tripodal tris(amido)arene ligand using organoazide N₃Ar^CF₃ (Ar^CF₃ = C₆H₃(CF₃)₂-3,5).⁵ In contrast, the only molecular cerium nitride [(TriNO_x)Ce(Li₂μ-N)Ce(TriNO_x)][BAr^F₄] did not involve an azide derivative but a reaction sequence comprising the cationization of the chloride precursor in liquid ammonia and subsequent exhaustive deprotonation of the formed ammonia complex with Li[N(SiMe₃)₂].⁶ The few reported molecular Ce^III and Ce^IV azide complexes were accessed via salt-metathesis protocols with an inorganic azide, AgN₃ or NaN₃ (ref. 7–10), but also by applying organoazide-promoted trans-silylation (Scheme 1).^11–14 The latter include the terminal Ce^IV species Ce[N(SiMe₃)₂]₃(N₃)¹² and the terminal and bridging Ce^III complexes (Tp^Me2)₂Ce(N₃) (Tp^Me2 = hydrotris(3,5-dimethylpyrazolyl)borate)¹³ and [LCe{N(SiMe₃)₂}(μ-N₃)]₂ (L = alkoxy-N-heterocyclic carbene Me₃Si{C(NDippCH₂CH₂N)}CH₂CMe₂O).¹⁴

Scheme 1 Organoazide-promoted formation of cerium azides and organoimides.

Given that the monoanionic tris(tert-butoxy)siloxy ligand [OSi(OtBu)₃] was successfully employed by Mazzanti et al. for the syntheses of uranium imido-, imino- and nitride complexes^1e and because it provides a stabilizing platform for both ceric and cerous chemistry,^15,16 we set out to examine [OSi(OtBu)₃]/N₃SiMe₃ exchange reactions driven by silicon oxophilicity. In this communication, a stepwise azide transfer via trans-silylation of cerous tris(tert-butoxy)siloxide is presented, affording well-defined mixed azido/siloxy cerium complexes of distinct aggregation. Preliminary experiments revealed that the heteroleptic Ce^III complexes are potential precursors for photolytic and thermal oxidation generating Ce^IV compounds.

Treatment of a slurry of [Ce{OSi(OtBu)₃}₃]₂ (ref. 15) in n-hexane with one equiv. of Me₃SiN₃ gave a clear solution (Scheme 2). The ambient temperature ¹H NMR spectrum in C₆D₆ of the obtained colourless reaction product revealed distinct paramagnetic resonances in the range from 10.31 to −8.77 ppm (Fig. S1†). These were accompanied by diamagnetic signals, attributable to the side-product Me₃SiOSi(OtBu)₃ (1.39, 0.28 ppm). Recrystallization at −40 °C from a toluene solution allowed for the effective removal of the silyl ether and isolation of complex Ce₂[OSi(OtBu)₃]₅(N₃) (1), with the composition as suggested by elemental analysis. In non-coordinating solvents, slow ligand rearrangement was observed, as confirmed by formation of the complex [Ce{OSi(OtBu)₃}₂(μ-N₃)] (2, vide infra). Complex 2 also appeared in the supernatants of crystallization attempts of 1 (see also Fig. S1†). The ambient temperature ¹H NMR spectrum of crystalline 1 showed paramagnetically broadened resonances at 10.31/9.75 and −6.02 ppm (ratio 6 : 9) for the tBu protons (Fig. S2†) in accordance with distinct siloxy ligands. FT IR spectroscopy showed a vibration band at ν = 2076 cm⁻¹ (Fig. S17†), characteristic of cerium-coordinated azido ligands. The X-ray structural analysis of complex 1 revealed a binuclear structural motif slightly different from that of the dimeric precursor [Ce{OSi(OtBu)₃}₃]₂.¹⁵ In the solid state, bimetallic complex 1 displays two 6-coordinate cerium atoms asymmetrically bridged by three siloxy ligands, each involving a κ²-bonding mode (Fig. 1 and Scheme 2). Whereas Ce(2) completes its coordination sphere with one terminal siloxy ligand, Ce(1) bears a terminal siloxy and a terminal azido ligand. For comparison, the cerium centres in the dimeric homoleptic precursor are 5-coordinate with two terminal and two bridging κ²-bonded siloxy ligands.

Scheme 2 Stepwise functionalization of [Ce{OSi(OtBu)₃}₃]₂ with azido ligands via a trans-silylation reaction with Me₃SiN₃.

Fig. 1 ORTEP representations of the molecular structure of [Ce{OSi(OtBu)₃}{μ-OSi(OtBu)₃}₃Ce{OSi(OtBu)₃}N₃] (1, left) and of complex [{Ce₂{μ₂-OSi(OtBu)₃}₂{OSi(OtBu)₃}}₂(μ_2(1,3)-N₃)₃(μ_{4(1,1′,3,3′)}-N₃)] (2, right). The asymmetric unit of molecule 1 is shown in the ESI (Fig. S10).† Thermal ellipsoids are set at the 30% probability level; hydrogen atoms are omitted for clarity. Selected interatomic distances [Å] and angles [°]: 1: Ce1–O1 2.223(4), Ce1–O5 2.572(4), Ce1–O8 2.634(4), Ce1–O9 2.391(4), Ce1–O13 2.436(4), Ce2–O8 2.306(4), Ce2–O9 2.407(4), Ce2–O12 2.597(4), Ce2–O13 2.560(4), Ce2–O16 2.547(4), Ce2–O17 2.215(4), Ce1–N1 2.375(6), N1–N2 1.194(8), N2–N3 1.137(8), O1–Si1 1.598(4), O8–Si2 1.606(4), O9–Si3 1.605(4), O13–Si4 1.594(4), O17–Si5 1.597(4), Ce1–N1–N2 166.8(6), N1–N2–N3 179.4(9), O1–Ce1–O8 156.03(14), O1–Ce1–O9 126.53(14), O1–Ce1–O13 100.45(14), O9–Ce1–N1 83.08(18), O8–Ce2–O17 105.47(15), O9–Ce2–O17 123.63(14), O13–Ce2–O17 167.45(14). 2: Ce1–O74 2.6340(14), Ce1–O83 2.4019(14), Ce1–O84 2.2007(13), Ce1–O85 2.5665(13), Ce2–O3 2.4934(14), Ce2–O10 2.1931(13), Ce2–O83 2.5054(13), Ce2–O85 2.3128(14), Ce1–N1 2.5578(18), Ce1–N2 2.5569(17), Ce1–N4 2.7528(18), Ce2–N4 2.6198(18), Ce2–N6 2.5475(18), N1′–N3 1.177(2), N1–N3′ 1.176(2), N2–N3 1.173(2), N4–N5 1.1693(17), N6–N7 1.1717(18), O84–Si1 1.5998(15), O85–Si2 1.6098(15), O83–Si3 1.6061(15), O10–Si4 1.6037(15), Ce1–N1–N3′ 132.82(15), Ce1–N2–N3 126.21(14), Ce2–N6–N7 143.83(17), N2–N3–N1′ 178.8(2), N4–N5–N4′ 179.2(3), N6–N7–N6′ 178.5(3), Ce1–O84–Si1 157.12(9), Ce2–O10–Si4 154.36(9), O10–Ce1–O83 125.03(5), O10–Ce1–O85 107.65(5), O83–Ce1–O84 107.11(5), O84–Ce1–O85 120.59(5). ′ = −x, y, −z + 3/2.

The terminal azido ligand in 1 features a Ce(1)–N bond length of 2.375(6) Å, quite similar to the 2.340(6) Å reported for trivalent [(Tp^Me2)₂Ce(N₃)].¹³ Further metrics of the Ce–N₃ moiety include the interatomic angles Ce1–N1–N2 (166.8(6)°) and N1–N2–N3 (179.4(9)°), while the distances N1–N2 (1.194(8) Å) and N2–N3 (1.137(8) Å) are in accordance with an ionic azido bonding.^17,18 The terminal Ce–O(siloxy) distances of 2.223(4) and 2.215(4) Å are slightly longer than those reported for [Ce{OSi(OtBu)₃}₃]₂ reflecting the higher coordination number of complex 1.¹⁵ The bridging Ce–O(siloxy) distances range from 2.306(4) to 2.634(4) Å. For comparison, the bridging distances in [Ce{OSi(OtBu)₃}₃]₂ were detected as 2.532(2) and 2.396(2) Å. The Ce–OtBu donor distances in 1 amount to 2.547(4), 2.572(4) and 2.597(4) Å, and match those in [Ce{OSi(OtBu)₃}₃]₂.¹⁵

Treatment of [Ce{OSi(OtBu)₃}₃]₂ with two equiv. of Me₃SiN₃ in n-hexane again led to the complete dissolution of the cerous siloxide upon stirring overnight (Scheme 2). The ambient temperature ¹H NMR spectrum of the obtained colourless product in C₆D₆ indicated distinct paramagnetic resonances, as well as diamagnetic signals of the side-product Me₃SiOSi(OtBu)₃. Once more, the silyl ether could be effectively separated via recrystallization from a toluene solution at −40 °C. The ambient temperature ¹H NMR spectrum of pure [Ce{OSi(OtBu)₃}₂(μ-N₃)]₄ (2) in C₆D₆ (Fig. S3†) showed broad paramagnetic resonances at 8.24 and −8.77 ppm (ratio 54 : 54), indicative of two chemically inequivalent tris(tert-butoxy)siloxy ligand environments.

The results obtained from elemental analysis displayed CHN values as expected for a compound of composition [Ce{OSi(OtBu)₃}₂(N₃)] (2). In contrast to compound 1, the IR spectrum of 2 revealed distinct azido vibration bands at ν = 2144 and 2076 cm⁻¹ (Fig. S8†), indicative of different azido bondings. The X-ray structural analysis of complex 2 revealed a tetrametallic complex of composition [Ce₄{OSi(OtBu)₃}₄{μ₂-OSi(OtBu)₃}₄(μ₂-N₃)₃(μ₄-N₃)] (2) (Fig. 1). The asymmetric unit consists of a bimetallic entity in which each cerium ion is coordinated by one terminal, as well as two bridging siloxy ligands adopting the κ²-bonding mode (Fig. 1). The asymmetric units are connected by four azido moieties (Scheme 2 and Fig. 1), which display two distinct coordination modes. While three of the azido ligands each bridge two of the four cerium atoms asymmetrically in a μ_2(1,3)-fashion, the fourth one connects to all four cerium centres in a μ_{4(1,1′,3,3′)} fashion.¹⁹ As a consequence, both 6- and 7-coordinate cerium atoms are present. In complex 2, the terminal Ce–O siloxy bonds are 2.1931(13) and 2.2007(13) Å, while the bridging ones range from 2.3128(14) to 2.5665(13) Å. The donor Ce–OtBu moieties show interatomic Ce−O distances of 2.4934(14) and 2.6340(14) Å. As expected, the Ce–N distances of the bridging azido moieties are significantly elongated (2.5475(18)–2.7528(18) Å) in agreement with those reported for the 5-coordiante Ce^III complex [LCe{N(SiMe₃)₂}(μ-N₃)]₂ (vide supra, 2.538(2) and 2.585(2) Å).¹⁴ The N–N distances are slightly enlarged (1.169(2)–1.177(2) Å) as a result of the end-on coordination to cerium.

Exposing the colourless complex 1 to ambient light resulted in a colour change within hours. The ambient temperature ¹H NMR spectrum of the obtained yellow substance in C₆D₆ displayed resonances of unknown diamagnetic species (1.64–1.60 ppm), with Ce[OSi(OtBu)₃]₄ emerging (Fig. S4†), and presumably HOSi(OtBu)₃ (1.40 ppm). Interestingly, excitation of a C₆D₆ solution containing complex 1 with UV light (254 nm) for 17 h allowed the generation of the aforementioned diamagnetic intermediate product (1.61 ppm) more selectively (see ESI Fig. S5†). The photoreaction also produced isobutene. After prolonged treatment for 3.5 days, complete decomposition was observed, which was revealed by undefined paramagnetically shifted resonances in the ambient temperature ¹H NMR spectrum (Fig. S6†). Short exposure times of 10 min to UV irradiation (260–330 nm) revealed almost complete consumption of complex 1 with isobutene as the main decomposition product detectable (Fig. S7†). The degradation reaction was revealed by the DRIFT spectrum, which showed almost complete disappearance of the azido bands (Fig. S19†). Applying this short UV irradiation to complex 2 produced significant amounts of both isobutene and homoleptic complex Ce[OSi(OtBu)₃]₄ (Fig. S8 and S21†). Unfortunately, any other cerium- or nitrogen-containing product could neither be identified spectroscopically nor isolated. Overall, our observations are similar to those of the thermal decomposition study of the uranium(III) siloxide congener [U{OSi(OtBu)₃}₃]₂ reported by Mazzanti et al. which led to the isolation of the mixed siloxy/silanediolato uranium(IV) complex [U{OSi(OtBu)₃}₂{μ-O₂Si(OtBu)₂}₂]₂.^20,21 The formation of the latter uranium(IV) complex takes place at ambient temperature both in the solid state and in solution, and is accompanied by the elimination of isobutene.

Thermal activation of an azido moiety was also suggested by the treatment of complex 1 dissolved in C₆D₆ at 50 °C. Over a period of 5 days increasing resonances of diamagnetic Ce[OSi(OtBu)₃]₄ (Fig. S9†) and HOSi(OtBu)₃ (1.40 ppm) were detected. These were accompanied by low intensity paramagnetically shifted resonances of complex 2 occurring from concomitant ligand rearrangement. Conducting the thermal treatment of complexes 1 and 2 at 80 °C in toluene for 38 h showed only negligible decomposition, and formation of significant amounts of Ce[OSi(OtBu)₃]₄ was not detected by NMR/DRIFT spectroscopy (Fig. S10, S11 and S21†). Furthermore, performing such prolonged heating on homoleptic cerium tris(tert-butoxy)siloxide complexes, trivalent [Ce{OSi(OtBu)₃}₃]₂ as well as tetravalent Ce[OSi(OtBu)₃]₄, did not result in any significant decomposition (Fig. S13 and S14†). In contrast, isobutene formation features a prominent thermal decomposition path of both cerous alkoxide Ce(OCtBu₃)₃ (ref. 22) and dimeric ceric complex [Ce(OtBu)₃(μ-OtBu)]₂.²³

In conclusion, mixed ligand siloxy/azido complexes of cerium(III) are readily accessible via a trans-silylation protocol. Complexes [Ce₂{OSi(OtBu)₃}₅N₃] and [Ce{OSi(OtBu)₃}₂N₃]₄ can be isolated in good yields but possess limited stability under light. It is revealed that both moderate thermal and photolytic treatment can lead to the oxidation of Ce^III ions, as corroborated by the identification of ceric complex Ce[OSi(OtBu)₃]₄. Intermediate products generated from the photolytic path tend to slow ligand degradation, while the thermally activated product is prone to rearrangement reactions. Given the stabilizing effect of siloxy ligands on rare-earth-metal complexes, such transazidation reactions might provide a new avenue for cerium redox transformations.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for the compounds have been deposited with the Cambridge Crystallographic Data Centre (deposition numbers 2448269 and 2448270†).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, NMR, and FTIR data. CCDC 2448269 and 2448270. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt01051a

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