Bis(η5-pentamethylcyclopentadienyl)aluminium tetrabromidoaluminate

The title compound, [Al(C10H15)2][AlBr4], was formed during the reduction of a mixture of Cp*AlBr2 and AlBr3. The AlIII atoms of the two crystallographically independent cations each lie on an inversion center, and the [AlBr4]− anions are on general positions. At 123 K, the structure exhibits disorder in two of the Br atoms of the [AlBr4]− ion, with a ratio occupancy of 0.733 (6): 0.267 (3). In the crystal, there is possible weak hydrogen bonding between some methyl groups and Br atoms. The interactions link the moieties in a three-dimensional array.


Experimental
Crystal data [Al(C 10 Table 1 Hydrogen-bond geometry (Å , ). We thank the Office of Naval Research for financial support. RJB wishes to acknowledge the NSF-MRI program (grant CHE-0619278) for funds to purchase the diffractometer.

Synthesis and crystallization
The starting material Cp*AlBr 2 was prepared as previously reported (Schormann et al., 2001), and all compounds were handled in an Ar filled dry box or Schlenk glassware. Aluminium tribromide (0.4367 g, 1.63 mmol) was combined with Cp*AlBr 2 (0.1607 g, 0.50 mmol) in about 10 mL of dry toluene, and NaK 2 eutectic (0.2002 g, 1.97 mmol) was added.
The Kontes valve on the tube was closed and the tube sonicated for 1 day in an ordinary sonic cleaner. Inside the dry box, the mixture was filtered to afford a yellow solution, which was pumped to dryness, affording 0.0619 g of raw product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The bromine atoms Br3 and Br4 are disordered over two positions with occupancies of 0.733 (6) and 0.267 (6), respectively. These Br's along with Br1 and Br2 were restrained to be tetrahedral, i.e. Br1/Br2/Br3a/Br4a as one tetrahedral set and Br1/Br2/Br3b/Br4b as another tetrahedral set. The H atoms were placed in idealized positions, with CH 3 groups as rigid groups free to rotate and C-H bond lengths constrained to 0.98 Å.

Results and discussion
The cluster compound (AlCp*) 4 is easily made from alkali metal reduction of Cp*AlX 2 (X=Cl, Br, I) compounds (Schormann et al., 2001;Minasian & Arnold, 2008), but larger Cp*Al clusters have only been reported from syntheses that start with Al(I) halide clusters, and that starting material requires a difficult and expensive apparatus to synthesize (Vollet et al., 2004(Vollet et al., , 2005. We attempted to prepare larger clusters by alkali reduction of mixtures of Cp*AlBr 2 and AlBr 3 in toluene. The soluble portion of the reaction mixture was recrystallized to afford the colorless title compound (Fig. 1).
Other unidentified species with a broad 27 Al-NMR peak at -20 ppm and a sharp peak at 97 ppm were also present, in addition to Cp* 2 Al + AlBr 4 -.
Decamethylaluminocenium tetrabromoaluminate has 1/2 of two Cp* 2 Al + cations in its asymmetric unit, as the two cations are crystallographically independent. As is normal for this cation, the Al atom is linear with the centroids of both rings, with the methyl groups staggered, and the linearity is in this case by symmetry. The two distinct decamethylaluminocenium cations are tilted with respect to each other as the dihedral angle between the ring planes of the two cations is 47.51°. This is in contrast with the AlCl 4analog (Schurko et al., 2002;Macdonald et al., 2008), which has two whole molecules in its unit cell, dihedral angles of 43.45-43.98° between the various Cp* ring planes, and near, but not exact, linearity of the centroids of the Cp* rings and their sandwiched Al atoms. The Cp* 2 Al + cation has also been reported in other compounds (Üffing et al., 1999;Kruczyński et al., 2012;Vollet et al., 2006;Burns et al., 1999). The supplementary materials AlBr 4anion shows disorder in two of the bromine atoms, and there appears to be weak hydrogen bonding interactions between some of the methyl protons and Br1 and Br3 (Fig. 2). While the angle between the undisordered Br atoms and Al3 (Br1-Al3-Br2) of 109.71 (4)° is close to the ideal tetrahedral angle of 109.5°, not much can really be said about the angles to the disordered Br atoms except that the geometry is approximately tetrahedral.
Based on other reports of Cp* 2 Al + formation (Dohmeier et al., 1993;Üffing et al., 1998), we presume that interactions between (AlCp*) 4 and AlBr 3 is probably responsible for the formation of the title compound. A repeat of the same reduction reaction at 0 °C resulted in (AlCp*) 4 as the only soluble Al-containing product, based on its 27 Al-NMR peak at -80.7 ppm.   Packing diagram showing the crystal structure and the hydrogen bonding scheme (dashed lines).