The first structurally characterized cationic lanthanide–alkyl complexes

Abstract

Reaction of rare earth metal–alkyl complexes [Ln(CH₂SiMe₃)₃(THF)₂] (Ln = Y, Lu) with B(C₆X₅)₃ (X = H, F) in the presence of crown ethers gives crystallographically characterized ion pairs [Ln(CH₂SiMe₃)₂(CE)(THF)_n]⁺[B(CH₂SiMe₃)(C₆X₅)₃]^– (CE = [12]-crown-4, n = 1; CE = [15]-crown-5 and [18]-crown-6, n = 0).

---

In contrast to the intensely investigated chemistry of the group 4 metal–alkyl cations in the context of homogeneous olefin polymerization catalysis,¹ related cationic alkyl complexes of the group 3 metals and lanthanides have not attracted much attention.^2–5 Recently, yttrium–alkyl cations [Y(L)(CH₂SiMe₃)]⁺[B(C₆F₅)₄]⁻ (L = N,N′-R₂-tacn-N″-(CH₂)₂N^tBu; tacn = 1,4,7-triazacyclononane, R = ⁱPr, Me) were reported as single-site polymerization catalysts for ethylene.² However, these and other alkyl cations of the rare earths reported in the literature, such as [Y(L)(CH₂SiMe₃)₂]⁺ (L = deprotonated aza-[18]-crown-6) were generated in situ and only characterized by NMR spectroscopy.^2,4 The thermal sensitivity coupled with the extreme electrophilicity of the metal center due to its low valence electron count were cited as reasons for the difficulty in isolating such species. We present here, surprisingly robust rare earth metal–dialkyl cations supported by crown ethers, which allow full crystallographic characterization. Crown ethers have long been known to coordinate the cations of lanthanide chlorides and nitrates,⁶ but their use in organolanthanide chemistry has only been described recently.⁷

When a THF solution of the lutetium–trialkyl complex [Lu(CH₂SiMe₃)₃(THF)₂]⁸ was treated at 25 °C with one equivalent of the borane B(C₆X₅)₃ (X = H, F), followed by one equivalent of crown ether CE, analytically pure, colorless crystals of the ion pair [Lu(CH₂SiMe₃)₂(CE)(THF)_n]⁺[B(CH₂SiMe₃)(C₆X₅)₃]⁻ were isolated in practically quantitative yield (Scheme 1). In contrast to cationic d⁰ metal–benzyl complexes,^1,5 the anions were shown to be non-coordinating. Irrespective of the anion, the LuCH₂ resonances shift to higher field in the ¹H and ¹³C NMR spectra with increasing size of the crown ether; this indicates enhanced shielding in this series. The ion pairs are soluble in THF and CH₂Cl₂, but insoluble in aliphatic or aromatic hydrocarbons. Surprisingly, THF solutions of the ionic complexes are stable for more than 12 h. As alkyl abstraction with triphenylborane resulted in complete conversion at ambient temperature, there was no obvious advantage in using the more electrophilic perfluorinated triphenylborane. The ¹¹B{¹H} NMR spectra of all cations with [B(CH₂SiMe₃)Ph₃]⁻ as anion, at 25 °C in THF-d₈, contains a signal at δ −10.4, whilst in the ¹H NMR spectra the BCH₂ resonance appears as a quartet at δ 0.17 with ²J_BH = 4.7 Hz, indicating the presence of the identical anion.

Scheme 1 Conditions: i. THF, 25 °C, 20 min; ii. crown ether (CE), 25 °C, 5 min. CE = [12]-crown-4, n = 1; CE = [15]-crown-5, n = 0; CE = [18]-crown-6, n = 0.

¹H and ¹³C NMR spectroscopy have shown that the cationic lutetium complex derived from [12]-crown-4 contains one molecule of THF that is labile on the NMR timescale.⁹ In the yttrium analogue [Y(CH₂SiMe₃)₂([12]-crown-4)(THF)]⁺, the CH₂ protons of the two equivalent alkyl groups at the yttrium center in CD₂Cl₂ give rise to a doublet at δ −0.86 with ²J_YH = 3.2 Hz, whilst the carbon atoms were recorded as a doublet at δ 39.8 with ¹J_YC = 41.0 Hz. The CH₂ protons of the facially coordinated [12]-crown-4 appear in the ¹H NMR spectrum as two separate broad multiplets at δ 3.27 and 3.65, suggesting rigid coordination at the yttrium center. The lutetium complex shows similar NMR spectra. As shown in Fig. 1, the lutetium derivative exhibits a seven-coordinate metal center best described as a capped trigonal prism. Two parallel trigonal planes are formed by the atoms O(1), O(2), C(9) and O(3), O(4), C(13). The capping atom O(5) is located over the square plane O(2), O(3), C(9), C(13). The angles at the alkyl carbon atoms, 127.3(1) and 135.4(1)°, suggest the absence of any strong agostic interaction.

Fig. 1 ORTEP diagram of [Lu(CH₂SiMe₃)₂([12]-crown-4)(THF)]⁺[B(CH₂SiMe₃)Ph₃]^–. Anion and hydrogen atoms omitted for clarity, thermal ellipsoids drawn at 30% probability level. Selected bond lengths (Å): Lu–O(1) 2.438(1), Lu–O(2) 2.451(1), Lu–O(3) 2.503(1), Lu–O(4) 2.406(1), Lu–O(5) 2.307(1), Lu–C(9) 2.340(2), Lu–C(13) 2.354(2).

[15]-Crown-5 stabilizes the formally four-electron dialkyl cation fragment without an additional molecule of THF.⁹ The CH₂ resonance of the alkyl groups at the lutetium center of [Lu(CH₂SiMe₃)₂([15]-crown-5)]⁺ in CD₂Cl₂ appear in the ¹H NMR spectrum as a sharp singlet at δ −1.17. The diastereotopic methylene protons of the [15]-crown-5 give rise to two broad multiplets at δ 4.46 and 3.75. Again, the coordination polyhedron around the seven-coordinate metal center can be regarded as a capped trigonal prism (Fig. 2). Two parallel trigonal planes are formed by the atoms O(2), O(3), C(15) and O(4), O(5), C(11), with the capping atom O(1) located over the square plane O(2), O(5), C(11), C(15).

Fig. 2 ORTEP diagram of [Lu(CH₂SiMe₃)₂([15]-crown-5)]⁺[B(CH₂SiMe₃)Ph₃]⁻·0.5(CH₂Cl)₂. Anion, non-coordinating solvent molecule and hydrogen atoms omitted for clarity, thermal ellipsoids drawn at 30% probability level. Selected bond lengths (Å): Lu–O(1) 2.359(5), Lu–O(2) 2.419(5), Lu–O(3) 2.376(5), Lu–O(4) 2.406(5), Lu–O(5) 2.421(5), Lu–C(11) 2.364(7), Lu–C(15) 2.345(7).

Use of [18]-crown-6 makes further expansion of the coordination sphere of the dialkyl–lanthanide complex possible.⁹ The NMR spectroscopic features are similar to those of the related cations with smaller crown ethers. As Fig. 3 shows, the lutetium center adopts a coordination polyhedron of a doubly capped trigonal prism, where all six oxygen atoms of [18]-crown-6 are coordinated to the eight-coordinate lutetium ion. The two parallel trigonal planes are formed by the atoms O(1), O(6), C(13) and O(3), O(4), C(17). The oxygen atoms O(2) and O(5) cap the square planes formed by O(1), O(3), C(13), C(17), and by O(4), O(6), C(13), C(17), respectively.

Fig. 3 ORTEP diagram of [Lu(CH₂SiMe₃)₂([18]-crown-6)]⁺[B(CH₂SiMe₃)Ph₃]⁻·0.5(CH₂Cl)₂. Anion, non-coordinating solvent molecule and hydrogen atoms omitted for clarity, thermal ellipsoids drawn at 30% probability level. Selected bond lengths (Å): Lu–O(1) 2.532(5), Lu–O(2) 2.422(5), Lu–O(3) 2.431(5), Lu–O(4) 2.433(5), Lu–O(5) 2.399(5), Lu–O(6) 2.524(5), Lu–C(13) 2.366(8), Lu–C(17) 2.371(8).

In the absence of crown ethers, the reaction of [Lu(CH₂SiMe₃)₃(THF)₂] with B(C₆F₅)₃ in THF-d₈ gives [Lu(CH₂SiMe₃)₂(THF)_n]⁺[B(CH₂SiMe₃)(C₆F₅)₃]⁻, whose ¹⁹F NMR spectrum exhibits meta/para chemical shift differences Δδ of 2.2 ppm, consistent with solvent-separated ion pairs.¹⁰ However, reaction between [Lu(CH₂SiMe₃)₃(THF)₂] and BPh₃ in THF at ambient temperature resulted in colorless crystals of [Lu(CH₂SiMe₃)₂(THF)₃]⁺[B(CH₂SiMe₃)Ph₃]⁻. The ¹H NMR spectrum in CD₂Cl₂ shows that the cation of this complex contains three THF molecules. Comparison of its ¹H and ¹³C NMR data with that of [Lu(CH₂SiMe₃)₂(THF)_n]⁺[B(CH₂SiMe₃)(C₆F₅)₃]⁻ shows that the LuCH₂ resonances appear at significantly higher field (δ −1.03 vs. −0.92 and δ 39.8 vs. 40.4, respectively). This finding strongly suggests that the [B(CH₂SiMe₃)Ph₃]⁻ anion is loosely coordinating in the absence of crown ethers.¹¹

In conclusion, we have shown that in the presence of oxygen donors, thermally robust lutetium and yttrium alkyl cations [Ln(CH₂SiMe₃)₂(CE)(THF)_n]⁺ become easily available. Preliminary experiments with these extremely oxygen- and moisture-sensitive cationic lanthanide alkyl complexes indicate that they not only react with Broensted acids such as HOC₆H₂^tBu₂-2,6-Me-4, but exhibit significant ethylene polymerization activity.

This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.

Notes and references

1. C. Pellecchia, A. Grassi and A. Immirzi, J. Am. Chem. Soc., 1993, 115, 1160; R. F. Jordan, Adv. Organomet. Chem., 1991, 32, 325; W. Kaminsky and M. Arndt, Adv. Polym. Sci., 1997, 127, 144.
2. S. Bambirra, D. van Leusen, A. Meetsma, B. Hessen and J. H. Teuben, Chem. Commun., 2001, 637.
3. C. J. Schaverien, Organometallics, 1992, 11, 3476.
4. L. Lee, D. J. Berg, F. W. Einstein and R. J. Batchelor, Organometallics, 1997, 16, 1819.
5. L. W. M. Lee, W. E. Piers, M. R. J. Elsegood, W. Clegg and M. Parvez, Organometallics, 1999, 18, 2947.
6. R. D. Rogers and C. B. Bauer, in Comprehensive Supramolecular Chemistry, ed. G. W. Gokel, Pergamon, Oxford, vol. 1, 1996; J.-C. G. Bünzli, B. Klein and D. Wessner, Inorg. Chim. Acta, 1980, 44, 147; J. D. J. Backer-Dirks, J. E. Cooke, A. M. R. Galas, J. S. Ghotra, C. J. Gray, F. A. Hart and M. B. Hursthouse, J. Chem. Soc., Dalton Trans., 2191, 1980; J.-C. G. Bünzli, B. Klein, D. Wessner and N. W. Alcock, Inorg. Chim. Acta, 1982, 59, 269; J.-C. G. Bünzli, B. Klein, G. Chapuis and K. J. Schenk, Inorg. Chem., 1982, 21, 808.
7. Y. K. Gun′ko, P. B. Hitchcock and M. F. Lappert, Chem. Commun., 1998, 1843.
8. M. F. Lappert and R. Pearce, Chem. Commun., 1973, 126. For the crystal structure of the tris(THF) adduct, see: W. J. Evans, J. C. Brady and J. W. Ziller, J. Am. Chem. Soc., 2001, 123, 7711.
9. See electronic supplementary information (ESI) for experimental and spectroscopic details. [Ln(CH₂SiMe₃)₂([12]-crown-4)(THF)]⁺[B(CH₂SiMe₃)Ph₃]^– (Ln = Lu, Y) and [Lu(CH₂SiMe₃)₂([18]-crown-6)(THF)]⁺[B(CH₂SiMe₃)Ph₃]⁻ were prepared according to the method used for the synthesis of [Lu(CH₂SiMe₃)₂([15]-crown-5)]⁺[B(CH₂SiMe₃)Ph₃]^–: A solution of Lu(CH₂SiMe₃)₃(THF)₂ (200 mg, 344 μmol) and BPh₃ (83 mg, 344 μmol) in THF (1 mL) was stirred for 20 min at 25 °C, and treated with [15]-crown-5 (75 μL, 344 μmol). After evaporation of the solvent, pentane (3 mL) was added to give a colorless powder which was washed with pentane (3 × 2 mL). Yield: 284 mg, 92%. ¹H NMR (CD₂Cl₂, 25 °C), δ −1.17 (s, 2 × 2 H, LuCH₂), –0.44 (s, 9 H, BCH₂SiCH₃), –0.07 (s, 2 × 9 H, LuCH₂SiCH₃), 0.18 (br, 2 H, BCH₂), 3,45, 3.75 (br m, 10 H, OCH), 6.87 (t, ³J_HH 7.0 Hz, 3 H, 4-Ph), 7.03 (t, ³J_HH 7.1 Hz, 3 × 2 H, 3-Ph), 7.45 (br 3 × 2 H, 2-Ph). ¹³C{¹H} NMR (CD₂Cl₂, 25 °C), δ 2.4 (BCH₂SiCH₃), 4.2 (LuCH₂SiCH₃), 36.7 (LuCH₂), 69.2 (OCH), 122.0 (4-Ph), 126.0 (3-Ph), 135.3 (2-Ph), 167.5 (q, ¹J_BC 48.3 Hz, 1-Ph). The signal of the BCH₂ group was not detected. ¹¹B{¹H} NMR (THF-d₈, 25 °C), δ −10.4. Anal. Calc. for C₄₀H₆₈BLuO₅Si₃: C, 53.44; H, 7.56. Found: C, 52.56; H, 8.32. Crystallographic data: CCDC reference number 179961, [Lu(CH₂SiMe₃)₂([12]-crown-4)(THF)]⁺[B(CH₂SiMe₃)Ph₃]⁻; 179962, [Lu(CH₂SiMe₃)₂([15]-crown-5)]⁺[B(CH₂SiMe₃)Ph₃]⁻·0.5(CH₂Cl)₂; 179963, [Lu(CH₂SiMe₃)₂([18]-crown-6)]⁺[B(CH₂SiMe₃)Ph₃]⁻·0.5(CH₂Cl)₂. See http://www.rsc.org/suppdata/cc/b2/b201613n/ for crystallographic data in CIF or other electronic format.
10. ¹⁹F chemical shift differences between meta and para F atoms of less than 3 ppm are reported to be characteristic of a non-coordinating anion: A. D. Horton, J. de With, A. J. van der Linden and H. van de Weg, Organometallics, 1996, 15, 2672.
11. A strong ηⁿ coordination of the aryl groups is unlikely because of the high fluxionality. Furthermore, no reduction of the anion’s C_3v symmetry is observed on the NMR timescale..

---

Footnote

† Electronic supplementary information (ESI) available: experimental and spectroscopic details. See http://www.rsc.org/suppdata/cc/b2/b201613n/

---