A valence isomer of a dialane

Abstract

The compound (η⁵-C₅Me₅)Al→Al(C₆F ₅)₃, which is the first valence isomer of a dialane, has been prepared by treatment of [Al(η⁵-C₅Me₅)]₄ with Al(C₆F₅)₃ and characterized by X-ray crystallography and NMR spectroscopy.

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Compounds with aluminium–aluminium bonds are attracting considerable recent attention. The simplest such compounds are the dialanes, R₂AlAlR₂, and a number of these have now been structurally authenticated.¹ It occurred to us that valence isomers of dialanes, viz. RAl→AlR₃, might be capable of existence if the appropriate substituents were employed. DFT calculations² on the prototypical dialane, H₂AlAlH₂, revealed that the valence isomer HAl→AlH₃, is less stable than H₂AlAlH₂ by 9.17 kcal mol⁻¹. However, replacement of one of the dialane hydride substituents by cyclopentadienide inverted this order and (η⁵-C₅H₅)Al→AlH₃1 is more stable than the dialane (η²-C₅H₅)(H)Al→AlH₂ by 10.79 kcal mol⁻¹. In view of the foregoing, [Al(η⁵-C₅Me₅)]₄ [65 mg, 0.40 mmol of Al(η⁵-C₅Me₅) units]³ was treated with Al(C₆F₅)₃·PhCH₃ ⁴ (250 mg, 0.40 mmol) in 30 mL of toluene at 25 °C. After being stirred for 4 h at 25 °C, the yellow reaction mixture was heated to 50 °C for 30 min. Upon cooling to 25 °C, the reaction mixture was filtered and the solvent and volatiles were removed from the filtrate to afford a dark amber oil from which yellow crystalline (η⁵-C₅Me₅)Al→Al(C₆F ₅)₃2 (220 mg, 80% yield, mp 131–133 °C) deposited over a period of 24 h. The mass spectral data† for 2 are consistent with the proposed dialane isomer formulation. The presence of (η⁵-C₅Me₅)Al and Al(C₆F₅)₃ moieties in 2 is evident from the ¹H, ¹³C, and ¹⁹F NMR spectroscopic data,† noting however that the equivalence of the C₅Me₅ ring carbon and Me resonances could be due to the well known fluxional behaviour of cyclopentadienyl–aluminium systems.⁵ The ²⁷Al NMR spectrum of 2 comprises singlet resonances at δ −115.7 and 106.9. Given that the ²⁷Al chemical shifts for the model compound 1, as computed by the GAIO method,^2b,6 are δ −107.9 and 109.0 for the (η⁵-C₅Me₅)Al and AlH₃ centres, respectively, analogous assignments have been made for 2.† Further support for the proposed assignments stems from the experimentally observed ²⁷Al chemical shifts for monomeric (η⁵-C₅Me₅)Al (δ −150)⁷ and Al(C₆F₅)₃·arene [δ 52 (benzene); δ 61 (toluene)].⁴ The overall trend of ²⁷Al chemical shifts is consistent with the transfer of electron density from the alanediyl to the Al(C₆F₅)₃ fragment upon formation of the Al→Al donor acceptor bond of 2.

Fig. 1 Thermal ellipsoid plot (30% probability level) for (η⁵-C₅Me₅)Al→Al(C₆F ₅)₃2. Selected bond lengths (Å) and bond angles (°): Al(2)–Al(1) 2.591(2), Al(1)–X(1A) 2.591(8), Al(1)–C(11) 2.172(7), Al(1)–C(12) 2.162(6), Al(1)–C(13) 2.165(7), Al(1)–C(14) 2.200(7), Al(1)–C(15) 2.189(6), Al(2)–C(21) 1.982(7), Al(2)–C(31) 1.999(7); Al(2)–C(41) 1.997(7); Al(2)–Al(1)–X(1A) 170.1(3), C(21)–Al(2)–C(41) 111.0(3), C(21)–Al(2)–C(31) 108.5(3), C(41)–Al(2)–C(31) 113.5(3), C(21)–Al(2)–Al(1) 104.1(2), C(41)–Al(2)–Al(1) 111.2(2), C(31)–Al(2)–Al(1) 108.0(2).

The foregoing spectroscopic conclusions were confirmed by X-ray crystallography.‡ Compound 2 crystallizes in the C2/c space group with Z = 8; the solid state consists of individual molecules of the dialane isomer and there are no unusually short intermolecular contacts. The pentamethylcyclopentadienyl substituent is attached in an η⁵ fashion and the ring centroid–Al–Al moiety deviates only modestly from linearity [170.1(3)°]. The Al–Al bond length in 2 [2.591(3) Å] is shorter than those in the dialanes {(Me₃Si)₂CH}₄Al₂ [2.660(1) Å],^1a {2,4,6-Prⁱ₃- C₆H₂}₄Al₂ [2.647(3) Å],^1b and {Bu^t₃Si}₄Al₂ [2.751(2) Å]^1c but identical to that in [RIAl–AlClR] {R = [(Me₃Si)₂C(Ph)C(Me₃- Si)N]) [2.593(2) Å]}^1d within experimental error. The average Al(1)–C bond length of 2.178(7) Å [Al–centroid 1.810(8) Å] is considerably shorter than those reported for Al(η⁵-C₅Me₅) [2.388(7) Å]⁸ and [Al(η⁵-C₅Me₅)]₄ (2.344 Å, av. Al–centroid 2.011 Å).⁷ Such a shortening is anticipated as the partially antibonding aluminium ‘lone pair’ orbital of Al(η⁵-C₅Me₅) is transformed into the donor–acceptor bond with the concomitant development of positive and negative charges on the aluminium centres.⁹ The same trend is evident for other group 13 (η⁵-C₅Me₅)M→acceptor complexes¹⁰ and is true for both main-group and transition element acceptors.

In conclusion, we have prepared (η⁵-C₅Me₅)Al→Al(C₆F ₅)₃, a valence isomer of a dialane. This compound also features the first example of an Al→Al donor acceptor bond.

Acknowledgements

We are grateful to the National Science Foundation, Robert A. Welch Foundation, and the National Academy of Sciences, through Sigma Xi, The Scientific Research Society for financial support.

Notes and references

1. (a) For a review, see: W. Uhl, Angew Chem., Int. Ed. Engl., 1993, 32, 1386 see also:; (b) R. J. Wehmschulte, K. Ruhlandt-Senge, M. M. Olmstead, H. Hope, B. E. Sturgeon and P. P. Power, Inorg. Chem., 1993, 32, 2983; (c) N. Wiberg, K. Amelunxen, T. Blank, H. Nöth and J. Knizek, Organometallics, 1998, 17, 5431; (d) K. S. Klimek, C. Cui, H. W. Roesky, M. Noltemeyer and H.-G. Schmidt, Organometallics, 2000, 19, 3085.
2. B3LYP: (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; (b) A. D. Becke, Phys. Rev. A, 1988, 38, 3098; (c) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785; (d) S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 1980, 58, 1200 All DFT calculations were performed using the Gaussian 94 (revision B2) suite of programs. All-electron basis sets were used for C, H [6-31G(d)] and the group 13 elements [6-31 + G(d])..
3. Prepared according to the method of S. Schulz, H. W. Roesky, H. J. Koch, G. M. Sheldrick, D. Stalke and A. Kuhn, Angew. Chem., Int. Ed. Engl., 1993, 32, 1729.
4. G. S. Hair, A. H. Cowley, R. A. Jones, B. G. McBurnett and A. Voigt, J. Am. Chem. Soc., 1999, 121, 4922.
5. P. J. Shapiro, Coord. Chem. Rev., 1999, 189, 1.
6. R. Ditchfield, Mol. Phys., 1974, 27, 789; K. Wolinski, J. F. Hinton and P. Pulay, J. Am. Chem. Soc., 1990, 122, 8251 BP86: J. P. Perdew, Phys. Rev. B, 1986, 33, 8822.
7. C. Dohmeier, D. Loos and H. Schnöckel, Angew. Chem., Int. Ed. Engl., 1996, 35, 129.
8. A. Haaland, K.-G. Martinsen, S. A. Shlykov, H. V. Volden, C. Dohmeier and H. Schnöckel, Organometallics, 1995, 14, 3116.
9. C. L. B. Macdonald and A. H. Cowley, J. Am. Chem. Soc., 1999, 121, 12113.
10. J. D. Gorden, A. Voigt, C. L. B. Macdonald, J. S. Silverman and A. H. Cowley, J. Am. Chem. Soc., 2000, 122, 950.

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Footnotes

† 2: HRMS (CI, CH₄) calc. for C₂₈H₁₅Al₂F₁₅m/z 690.0565; found 690.0572. ¹H NMR (499.35 MHz, 295 K, C₆D₆) δ 1.49 (s, 15H, C₅Me₅). ¹³C{¹H} NMR (125.69 MHz, 295 K, C₆D₆) δ 149.99 (d, o-C₆F₅, ¹J_CF 224 Hz), 141.83 (d, p-C₆F₅, ¹J_CF 239 Hz), 137.34 (d, m-C₆F₅, ¹J_CF 226 Hz), 129.28 (s, ipso-C₆F₅), 115.94 [s, C₅(CH₃)₅], 8.44 [s, C₅(CH₃)₅]. ¹⁹F NMR (469.81 MHz, 295 K, C₆D₆) δ −122.03 (s, m-C₆F₅), −153.19 (s, p-C₆F₅), 161.77 (s, o-C₆F₅). ²⁷Al NMR (130.25 MHz, 295 K, C₆D₆) δ 106.9 [br, (C₆F₅)₃AlAlC₅Me₅ , w_1/2 6122 Hz], −115.7 [s, (C₆F₅)₃AlAlC₅Me₅ ].

‡ Crystal data for 2: C₂₈H₁₅Al₂F₁₅, monoclinic, space group C2/c, a = 30.635(6), b = 9.814(2), c = 20.236(4) Å, β = 111.10(3), V = 5676(2) ų, Z = 8, D_c = 1.616 g cm⁻³, μ(Mo-Kα) = 0.220 mm⁻¹. A suitable single crystal of 2 was covered with mineral oil and mounted on a Nonius-Kappa CCD diffractometer at 123 K. A total of 8481 independent reflections were collected in the range 5.96 < 2θ < 50.20° using Mo-Kα radiation (λ = 0.71073 Å). Of these, 3815 were considered observed [I > 2.0σ(I)] and were used to solve (direct methods) and refine (full matrix, least squares on F²) the structure of 2; R = 0.0767, wR2 = 0.1944.

CCDC 182/1856. See http://www.rsc.org/suppdata/cc/b0/b007341p/ for crystallographic files in .cif format

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