The crystal structure of the decaaluminum alkoxide cluster Al10O4(OH)8 L 14 (L = 1,1,1,3,3,3-hexafluoropropan-2-olate)

The title centrosymmetric decaaluminum cluster, Al10O4(OH)8(C3HF6O)14, contains two OAl4 units and a central Al2(O)2 bridge.

Apart from the simpler homoleptic aluminum alkoxides containing two, three, and four aluminum atoms, in the larger aggregates the important building block appears to be a central O atom surrounded by four Al atoms in a distorted tetrahedral arrangement, i.e. OAl 4 [five Al atoms in the case of Al 5 O(Oi-Bu) 13 (Abrahams et al., 2002) but this is an exception and also not an aggregate]. In each case in this OAl 4 building block, five of the six edges are occupied by a (O)-alkoxide bridge while the sixth edge is vacant to allow for dimerization. In larger aggregates, in the case of Al 8 O 2 (OH) 2 (OiBu) 18 (Abrahams et al., 2002), these building blocks are linked by two -OH units. For Al 9 O 3 (OEt) 21 (Nachtigall et al. 2018), these building blocks are linked by two moieties. The first is a 3 (O) group linking the two halves as well as the ninth Al atom. The second link is provided by a central Al(OEt) 4 group, which links the two building blocks through two (OEt) on each side of the ninth Al atom. In the case of Al 10 O 4 (OEt) 22 (Yanovsky et al., 1987), these units are again linked by two moieties somewhat analogous to the situation for Al 9 O 3 (OEt) 21 . Both contain a 3 (O) group linking the two halves as well as an additional Al(OEt) 4 group, which links the two building blocks through two (OEt) on each side of the group. However, in this instance this both linking moieties are located about a center of inversion The situation for Al 11 O 6 (OnPr) 10 (OiPr) 10 (Oi/nPr)(HOi/nPr) 2 (Starikova et al., 2004) is slightly more complex: in this case the two building blocks are linked by group containing three Al atoms of which the central Al is located on a twofold crystallographic axis. This central Al is linked to both the O 4 Al building blocks and the other Al in the linking moiety by both two 2 (O) and 3 (O) linkages and also contains a terminal OEt ligand. From this survey of aluminum alkoxide aggregates containing more than five Al centers, it can be seen that the present structure is unique in both its building block and the method of aggregation. In this instance, the edges of the OAl 4 block are made up by three (O)-1,1,1,3,3,3-hexafluoropropan-2-olate (L) and two -OH bridges with the sixth edge vacant to allow for dimerization. Aggregation is achieved by a 3 (O) group as in the other cases as well as a Al(OH) 2 (O)(L) moiety containing both (OH) and (O) groups where the latter are used to achieve dimerization. The molecular structure of the decaaluminium cluster in 1 showing labeling for Al and O only for clarity (major component only; unlabeled atoms are generated by Àx, 1 À y, 1 À z). Atomic displacement parameters are shown at the 30% probability level. Intramolecular O-HÁ Á ÁO, O-HÁ Á ÁF and C-HÁ Á ÁF interactions are shown by dashed lines.
Typically the Al centers in these aluminum alkoxide aggregates have varying coordination numbers from four to six with angles that vary widely from regular geometry and this is true in 1 (Table 1 and Fig. 1) where Al5 is four-coordinate [ 4 0 = 0.886 (Okuniewski et al., 2015) indicating slightly distorted tetrahedral], while Al1, Al3, and Al4 are all fivecoordinate [ 5 values are 0.098, 1.028, and 0.338, respectively (Addison et al., 1984)] while Al2 is distorted six-coordinate with O-Al-O bond angles ranging from 74.22 (9) to 171.59 (12) . A 5 value of 1.028 is outside the normal range from 0 to 1 (Addison et al., 1984) so some comment should be made. A recent paper (Blackman et al., 2020) gave examples of this situation in which the geometries were all distorted trigonal pyramidal with the metal out of the trigonal plane, as is the case for Al3 (Fig. 2). The geometry about the central O atom in the OAl 4 block is significantly distorted tetrahedral [ 4 0 = 0.630 (Okuniewski et al., 2015)] with Al-O-Al angles ranging from 95.50 (9) to 147.74 (13) .

Figure 3
Packing diagram of the decaaluminium cluster in 1 viewed along the caxis direction. Inter-cluster FÁ Á ÁF interactions and both intra-cluster and inter-cluster O-HÁ Á ÁO, O-HÁ Á ÁF and C-HÁ Á ÁF interactions are shown with dashed lines.

Synthesis and crystallization
A solution of Al(BH 4 ) 3 (Olson and Sanderson, 1958) in toluene was prepared by a reaction of AlCl 3 with 3 eq. of LiBH 4 in toluene, followed by distillation. In a bulb, 21.18 mmol of hexafluoroisopropanol were condensed into 1.76 mmol of Al(BH 4 ) 3 solution in several portions, and allowed to react to completion. Two phases formed, and then the second phase redissolved. The yellow liquid product was stored in a vial in a dry box, and on a day where the room temperature was very cold (<15 C), colorless crystals formed.
The crystals quickly melt at normal room temperature, and had to be placed into the cold stream immediately upon isolation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Several of the hexafluoropropyl groups are disordered and each was refined with two equivalent conformations with occupancies of 0.770 (3) (3). The H atoms attached to C were refined in idealized positions using a riding model with C-H = 1.00 Å and U iso (H) = 1.2U eq (C), while those attached to O were refined isotropically.

hydroxido-di-µ 4 -oxido-di-µ 3 -oxido-decaaluminium
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.