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

Abstract

Chemical context  

The inter­est in metal alkoxides (Turova et al., 2002 ▸) is due to their potential use as precursors of oxide materials in sol–gel technology (Brinker & Scherer, 1990 ▸) with applications in many fields including biomaterials (Avnir et al., 2006 ▸), and in the synthesis of single-phase materials, which provide unique possibilities to tailor the mechanical, electrical, and optical properties (Schottner, 2001 ▸). Within this class of compounds, the alkoxides of aluminum are of great inter­est and the first aluminum compounds with monodentate alkoxide ligands have been known since 1881. However, in spite of this inter­est, there are few examples of simple monodentate aluminum alkoxides that have been structurally characterized by single crystal X-ray analysis. In order of complexity, the dinuclear structure, Al₂(OtBu)₆ [tBu = tert-but­yl], was published in 1991 (Cayton et al., 1991 ▸) followed by trinuclear Al₃(OCHex)₉ [CHex = cyclo­hex­yl] in 2000 (Pauls & Neumüller, 2000 ▸). The crystal structure of the tetra­nuclear compound Al₄(OiPr)₁₂ [iPr = isoprop­yl] was first reported in 1979 (Turova et al., 1979 ▸) and re-determined in 1991 (Folting et al., 1991 ▸). An additional structure with four Al atoms and containing a μ₄-O atom bridging all four Al atoms, [Al₄(OCH₂CF₃)₁₁]⁻ (one H atom could not be located) has been reported (Sangokoya et al., 1993 ▸). A penta­nuclear, Al₅O(Oi-Bu)₁₃, and octa­nuclear structure, Al₈O₂(OH)₂(OiBu)₁₈ (iBu = iso-but­yl), was determined in 2002 (Abrahams et al., 2002 ▸). In 2018, the structure of a nona­nuclear structure, Al₉O₃(OEt)₂₁, was reported (Nachtigall et al. 2018 ▸). In 1987, the deca­nuclear compound, Al₁₀O₄(OEt)₂₂, was reported (Yanovsky et al., 1987 ▸). The polynuclear aluminum oxoalkoxide structure containing the largest number of Al atoms solely from simple alcohols reported to date was Al₁₁O₆(OnPr)₁₀(OiPr)₁₀(Oi/nPr)(HOi/nPr)₂ (nPr = n-prop­yl) in 2004 (Starikova et al., 2004 ▸). In a continuation of these studies, the structure of the complex derived from perfluorinated 2-propanol and aluminum ions, Al₁₀O₄(OH)₈ L ₁₄ [L = 1,1,1,3,3,3-hexa­fluoro­propan-2-olate], 1, is now reported.

Structural commentary  

The structure of the title compound (C₄₂H₂₂Al₁₀F₈₄O₂₆) is best described in terms of its building blocks. First there is a μ₄-OAl₄ moiety (O1, Al1–Al4), which has six edges of which three contain μ(O)-1,1,1,3,3,3-hexa­fluoro­propan-2-olate (L) ligands and two contain μ-OH groups, each bridging two Al atoms along an edge (Al1–Al2, Al2–Al4, and Al3–Al4 for L and Al1–Al3 and Al2–Al3 for the μ-OH groups). The sixth edge (Al1–Al4) is occupied by a group containing a fifth Al atom [bis-μ(OH)-, μ₃(O)-AlL] where one μ(OH) bridges Al4–Al5 and the μ₃(O) group bridges A11–Al5, while the second μ(OH) bridges Al2–Al5. This last μ₃(O) group allows this overall moiety to form a centrosymmetric Al₂O₂ deca-aluminum dimer, thus each μ₃(O) group is linked to Al1 and Al5 in the asymmetric unit as well as a second Al1 atom through a center of inversion (symmetry operation −x, 1 − y, 1 − z).

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. OAl₄ [five Al atoms in the case of Al₅O(Oi-Bu)₁₃ (Abrahams et al., 2002 ▸) but this is an exception and also not an aggregate]. In each case in this OAl₄ building block, five of the six edges are occupied by a μ(O)-alkoxide bridge while the sixth edge is vacant to allow for dimerization. In larger aggregates, in the case of Al₈O₂(OH)₂(OiBu)₁₈ (Abrahams et al., 2002 ▸), these building blocks are linked by two μ-OH units. For Al₉O₃(OEt)₂₁ (Nachtigall et al. 2018 ▸), these building blocks are linked by two moieties. The first is a μ₃(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 μ(OEt) on each side of the ninth Al atom. In the case of Al₁₀O₄(OEt)₂₂ (Yanovsky et al., 1987 ▸), these units are again linked by two moieties somewhat analogous to the situation for Al₉O₃(OEt)₂₁. Both contain a μ₃(O) group linking the two halves as well as an additional Al(OEt)₄ group, which links the two building blocks through two μ(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 Al₁₁O₆(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 O₄Al building blocks and the other Al in the linking moiety by both two μ₂(O) and μ₃(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 OAl₄ block are made up by three μ(O)-1,1,1,3,3,3-hexa­fluoro­propan-2-olate (L) and two μ-OH bridges with the sixth edge vacant to allow for dimerization. Aggregation is achieved by a μ₃(O) group as in the other cases as well as a Al(OH)₂(O)(L) moiety containing both μ(OH) and μ(O) groups where the latter are used to achieve dimerization.

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 [τ₄′ = 0.886 (Okuniewski et al., 2015 ▸) indicating slightly distorted tetra­hedral], while Al1, Al3, and Al4 are all five-coordinate [τ₅ 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 τ₅ 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 OAl₄ block is significantly distorted tetra­hedral [τ₄′ = 0.630 (Okuniewski et al., 2015 ▸)] with Al—O—Al angles ranging from 95.50 (9) to 147.74 (13)°.

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Figure 1

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 −x, 1 − y, 1 − z). Atomic displacement parameters are shown at the 30% probability level. Intra­molecular O—H⋯O, O—H⋯F and C—H⋯F inter­actions are shown by dashed lines.

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Figure 2

Diagram showing the five-coordinate environment about Al3 in which the metal ion is displaced out of the trigonal plane leading to a τ₅ value of 1.028 (> 1).

Supra­molecular features  

In the extended structure of 1, the deca-aluminum clusters make numerous inter­molecular F⋯F contacts, which are less than the sum of their van der Waals (Alvarez, 2013 ▸) radii, ranging in length from 2.641 (4) [F143⋯F211(1 − x, 2 − y, 1 − z) to 2.921 (4) Å [F31⋯F202(x, −1 + y, z) (see Fig. 3 ▸). In addition there are strong O—H⋯O and O—H⋯F and weak C—H⋯O and C—H⋯F hydrogen bonds, which help to consolidate the aluminum aggregates (Table 2 ▸).

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Figure 3

Packing diagram of the deca­aluminium cluster in 1 viewed along the c-axis direction. Inter-cluster F⋯F inter­actions and both intra-cluster and inter-cluster O—H⋯O, O—H⋯F and C—H⋯F inter­actions are shown with dashed lines.

Database survey  

A search of the Cambridge Structural Database [CSD version 5.41 (November 2019); Groom et al., 2016 ▸] for fragments based on the structure of 1 gave five hits [ERUBEY (Starikova et al., 2004 ▸); QESHOO (Nachtigall et al. 2018 ▸); UDOTAI and UDOTEM (Abrahams et al., 2002 ▸) and ZZZGIE11 (Yanovsky et al., 1987 ▸)]. A survey of the literature also revealed other structures not found from this search (Cayton et al., 1991 ▸; Pauls & Neumüller, 2000 ▸; Folting et al., 1991 ▸; Sangokoya et al., 1993 ▸).

Synthesis and crystallization  

A solution of Al(BH₄)₃ (Olson and Sanderson, 1958 ▸) in toluene was prepared by a reaction of AlCl₃ with 3 eq. of LiBH₄ in toluene, followed by distillation. In a bulb, 21.18 mmol of hexa­fluoro­iso­propanol were condensed into 1.76 mmol of Al(BH₄)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)/0.230 (3), 0.772 (3)/0.228 (3) and 0.775 (3)/0.225 (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.

Table 3

Experimental details

Crystal data
Chemical formula	[Al10(C3HF6O)14(OH)8O4]
M r	2808.39
Crystal system, space group	Triclinic, P
Temperature (K)	100
a, b, c (Å)	11.8721 (8), 12.4448 (8), 16.3091 (11)
α, β, γ (°)	108.754 (3), 102.232 (3), 98.650 (3)
V (Å3)	2166.8 (3)
Z	1
Radiation type	Mo Kα
μ (mm−1)	0.37
Crystal size (mm)	0.20 × 0.20 × 0.20
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2016 ▸)
T min, T max	0.634, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	13173, 13173, 8076
R int	0.075
(sin θ/λ)max (Å−1)	0.714
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.059, 0.171, 1.02
No. of reflections	13173
No. of parameters	935
No. of restraints	307
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.79, −0.87

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Computer programs: APEX2 and SAINT (Bruker, 2016 ▸), SHELXT (Sheldrick 2015a ▸), SHELXL2018/3 (Sheldrick, 2015b ▸) and SHELXTL (Sheldrick 2008 ▸).

Supplementary Material

supplementary crystallographic information

Funding Statement

This work was funded by Office of Naval Research grant .

References