A valence isomer of a dialane
John D. Gorden, Charles L. B. Macdonald and Alan H. Cowley*
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, USA.
E-mail: cowley@mail.utexas.edu
Received (in Columbia, MO, USA) 14th August 2000, Accepted 13th November 2000
First published as an Advance Article on the web
The compound (h5-C5Me5)Al?Al(C6F5)3, which is the first
valence isomer of a dialane, has been prepared by treatment
of [Al(h5-C5Me5)]4 with Al(C6F5)3 and characterized by X-
ray crystallography and NMR spectroscopy.
respectively, analogous assignments have been made for 2.†
Further support for the proposed assignments stems from the
experimentally observed 27Al chemical shifts for monomeric
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(h -C5Me5)Al (d 2150)7 and Al(C6F5)3·arene [d 52 (benzene);
d 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.
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 mol21. However, replacement of one of
the dialane hydride substituents by cyclopentadienide inverted
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 sub-
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stituent is attached in an h 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 {But3Si}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
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this order and (h -C5H5)Al?AlH3 1 is more stable than the
2
dialane (h -C5H5)(H)Al?AlH2 by 10.79 kcal mol21. In view of
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the foregoing, [Al(h -C5Me5)]4 [65 mg, 0.40 mmol of Al(h -
C5Me5) units]3 was treated with Al(C6F5)3·PhCH3 (250 mg,
4
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
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considerably shorter than those reported for Al(h -C5Me5)
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[2.388(7) Å]8 and [Al(h -C5Me5)]4 (2.344 Å, av. Al–centroid
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afford a dark amber oil from which yellow crystalline (h -
2.011 Å).7 Such a shortening is anticipated as the partially
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C5Me5)Al?Al(C6F5)3 2 (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.
antibonding aluminium ‘lone pair’ orbital of Al(h -C5Me5) is
transformed into the donor–acceptor bond with the concomitant
development of positive and negative charges on the aluminium
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The presence of (h -C5Me5)Al and Al(C6F5)3 moieties in 2 is
centres.9 The same trend is evident for other group 13 (h -
1
evident from the H, 13C, and 19F NMR spectroscopic data,†
C5Me5)M?acceptor complexes10 and is true for both main-
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 d 2115.7
and 106.9. Given that the 27Al chemical shifts for the model
compound 1, as computed by the GAIO method,2b,6 are d
group and transition element acceptors.
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In conclusion, we have prepared (h -C5Me5)Al?Al(C6F5)3,
a valence isomer of a dialane. This compound also features the
first example of an Al?Al donor acceptor bond.
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.
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2107.9 and 109.0 for the (h -C5Me5)Al and AlH3 centres,
Notes and references
† 2: HRMS (CI, CH4) calc. for C28H15Al2F15 m/z 690.0565; found
1
690.0572. H NMR (499.35 MHz, 295 K, C6D6) d 1.49 (s, 15H, C5Me5).
13C{1H} NMR (125.69 MHz, 295 K, C6D6) d 149.99 (d, o-C6F5, 1JCF 224
1
1
Hz), 141.83 (d, p-C6F5, JCF 239 Hz), 137.34 (d, m-C6F5, JCF 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) d 2122.03 (s, m-C6F5), 2153.19 (s, p-C6F5),
161.77 (s, o-C6F5). 27Al NMR (130.25 MHz, 295 K, C6D6) d 106.9 [br,
(C6F5)3AlAlC5Me5, w1/2 6122 Hz], 2115.7 [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, b = 111.10(3), V = 5676(2) Å3,
Z = 8, Dc = 1.616 g cm23, m(Mo-Ka) = 0.220 mm21. 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 < 2q < 50.20° using Mo-Ka radiation (l =
0.71073 Å). Of these, 3815 were considered observed [I > 2.0s(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.
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Fig. 1 Thermal ellipsoid plot (30% probability level) for (h -C5Me5)-
Al?Al(C6F5)3 2. 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).
crystallographic files in .cif format
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.
DOI: 10.1039/b007341p
Chem. Commun., 2001, 75–76
This journal is © The Royal Society of Chemistry 2001
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