μ-Methyl­ene-bis­[di­bromido(diethyl ether-κO)aluminium(III)]: crystal structure and chemical exchange in solution

Butcher, R.J.; Purdy, A.P.

doi:10.1107/S2056989021005302

research communications

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ISSN: 2056-9890

Volume 77| Part 6| June 2021| Pages 647-652

https://doi.org/10.1107/S2056989021005302

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μ-Methylene-bis[dibromido(diethyl ether-κO)aluminium(III)]: crystal structure and chemical exchange in solution

Ray J. Butcher ^a ^* and Andrew P. Purdy ^b

^aDepartment of Chemistry, Howard University, 525 College Street NW, Washington DC 20059, USA, and ^bChemistry Division, Code 6123, Naval Research Laboratory, 4555 Overlook Av, SW, Washington DC 20375-5342, USA
^*Correspondence e-mail: rbutcher99@yahoo.com

Edited by V. Jancik, Universidad Nacional Autónoma de México, México (Received 11 December 2020; accepted 19 May 2021; online 21 May 2021)

In the title compound, [Al₂Br₄(CH₂)(C₄H₁₀O)₂], the molecule lies on a crystallographic twofold axis passing through the bridging C atom. Each Al^III atom is four-coordinate, being bonded to two bromide ions, bridging the CH₂ group as well as the oxygen atom of a diethyl ether ligand in a slightly distorted tetrahedral arrangement with angles ranging from 101.52 (8) to 116.44 (5)°. The Al—CH₂—Al angle, 118.4 (2)°, is the smallest observed for a structure where this moiety is not part of a ring. In the crystal, weak C—H⋯Br interactions, characterized as R²₂(12) rings, link the molecules into ribbons in the [101] direction. The title compound is monomeric and coordinatively saturated in the solid state, as each aluminum is four-coordinate, but in solution the ether molecules from either or both Al atoms can dissociate, and would be expected to rapidly exchange, and this is supported by NMR data.

Keywords: crystal structure; methylene bridged aluminum; aluminum diethyl ether complex; solution NMR studies.

CCDC reference: 2084539

1. Chemical context

There is great current interest in the chemistry of reduced aluminum (Klemp et al., 2001 , Bonyhady et al., 2018 ) and aluminum carbon (carbaalanes) clusters (Stasch et al., 2002 ; Uhl & Roesky, 2002 ; Kumar et al., 2004 ) as well as aluminum–carbon nanoparticles (Diaz-Droguett et al., 2020 ) because of their interesting structural chemistry and many theoretical studies have been carried out on potential derivatives and as analogs of the better known boron examples (Attia et al., 2017 ). This has lead to a renaissance in the chemistry of aluminum (Roesky, 2004 ). In view of this chemistry, there is a need for easily prepared precursors for the synthesis of these reduced aluminum and carbaalane clusters, and this is the motivation behind preparing organometallics with two or more Al atoms on a carbon atom. The synthesis of methylene bis(aluminum halides) has been described before (Ort & Mottus, 1973 ; Lehmkuhl & Schäfer, 1966 ).

2. Structural commentary

In the structure of the title compound, [Al₂(C₉H₂₂Br₄O₂)] (1), the molecule lies on a crystallographic twofold axis passing through C1 (see Fig. 1). Each Al atom is four-coordinate, being bonded to two bromide ions and the bridging CH₂ group as well as the oxygen of a diethyl ether ligand in a slightly distorted tetrahedral arrangement (τ₄ = 0.907; Okuniewski et al., 2015 ) with angles ranging from 101.52 (8) to 116.44 (5)° (see Table 1). In the literature there are eight structures containing an AlBr₂ fragment coordinated to a diethyl ether ligand (LOCMEY, Yanagisawa et al., 2018 ; NOJYIW, Lips et al., 2014 ; QQQGXV, QQQGYA, Semenenko et al., 1973 ; RABCOM, Wehmschulte et al., 1996 ; TEXNIV, Agou et al., 2012 ; YANKON, Petrie et al., 1993 ; YERLUD, Quillian et al., 2006 ). In each of these structures, there is both a longer and shorter Al—Br bond distance [average Al—Br distances of 2.315 (18) and 2.30 (2) Å] with an average Al—O distance of 1.874 (14) Å. The comparable distances in 1 are 2.3046 (10), 2.3029 (9) and 1.881 (2) Å.

Table 1
Selected geometric parameters (Å, °)

Al—O1	1.881 (2)	Al—Br2	2.3029 (9)
Al—C1	1.927 (2)	Al—Br1	2.3046 (10)

O1—Al—C1	110.42 (13)	O1—Al—Br1	101.52 (8)
O1—Al—Br2	101.60 (7)	C1—Al—Br1	116.44 (5)
C1—Al—Br2	114.90 (9)	Br2—Al—Br1	110.07 (4)

Figure 1
Molecular diagram showing the atom labeling (symmetry operation to generate the complete molecule, −x, y, $[{1\over 2}]$ − z). Atomic displacement parameters are at the 30% level.

As indicated below, there are many instances of structures containing the Al–CH₂–Al fragment but only one which combines this fragment along with aluminum–halogen bonding (Uhl & Layh, 1991 ). In this structure {[(Me₃Si)₂CHAlCl]₂CH₂}₂, this moiety is not isolated but part of a ring in an adamantanoid cage, which would influence both its bond lengths and angles. However, there are ten instances (BELLAH, BELLEL, BELLIP, BELLOW, BELLUP (Uhl et al., 2012a ); JEZFID (Layh & Uhl, 1990 ); JUWMOD (Uhl et al., 1993 ); PENSEI (Uhl et al., 2012b ); WOZJUQ, WOZKAX (Knabel et al., 2002 ) where the metrical parameters of the Al–CH₂–Al fragment are not influenced by being part of a ring. In these structures, apart from JEZFID (Layh & Uhl, 1990) and WOZJUQ (Knabel et al., 2002), there are two independent Al—C bond lengths, which average 2.003 and 1.922 Å, with an overall average Al—C—Al bond angle of 132.5°. As a result of the unconstrained nature of this angle, it varies over a wide range from 126.3 to 144.4° and the value depends on the steric bulk of the Al substituents. In the smallest value in the list [BELLOV, 126.29 (13)°; Uhl et al., 2012a], the substituents attached to Al are (trimethylsilyl)methyl moieties, while the largest [JUWMOD, 144.4 (2)°; Uhl et al., 1993] has a neopentyl as well as two [bis(trimethylsilyl)methyl] groups attached. There are two structures, QQQGXV and QQQGYA (Semenenko et al., 1973), which only have Br₃ and Br₂H as substituents on the Al, but the angles cannot be calculated since the coordinates are not available. In 1, which lacks this steric bulk and where atom C1 lies on a crystallographic twofold axis, these values are 1.927 (2) Å and 118.4 (2)°, respectively. This latter value reflects this lack of steric bulk in the groups attached to the Al atoms.

3. Supramolecular features

As shown in Fig. 2, there are weak C—H⋯Br interactions, which link the molecules into ribbons in the [101] direction (see Table 2). In graph-set notation (Etter et al., 1990 ), these interactions can be characterized as R₂²(12) rings and this is shown in Fig. 3. These interactions can be highlighted in a Hirshfeld fingerprint plot as shown in Fig. 4 (Spackman & Jayatilaka, 2009 ), which shows these features. If this is expanded to take interactions beyond the van der Waals radii sum cutoff (Desiraju & Steiner, 1999 ; Desiraju, 2011a ,b ), this plot indicates that these weak C—H⋯Br interactions dominate the packing and make up 52.6% of all intermolecular interactions.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C2—H2A⋯Br1	0.99	2.98	3.481 (3)	113
C5—H5B⋯Br1ⁱ	0.98	3.10	3.871 (4)	136
Symmetry code: (i) $[-x+{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1]$ .

Figure 2
Packing diagram of 1 viewed from the [010] direction.

Figure 3
Diagram showing the C—H⋯Br interactions (as dashed lines) that link the molecules into ribbons via the formation of R₂²(12) rings (symmetry operation, $[{1\over 2}]$ − x, $[{3\over 2}]$ − y, 1 − z).

Figure 4
Hirshfeld surface plot highlighting the C—H⋯Br interactions, which make up 52.6% of all interactions.

4. 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 revealed there are eight structures in the literature containing an AlBr₂ fragment coordinated to a diethyl ether ligand (LOCMEY, Yanagisawa et al., 2018; NOJYIW, Lips et al., 2014; QQQGXV, QQQGYA, Semenenko et al., 1973; RABCOM, Wehmschulte et al., 1996; TEXNIV, Agou et al., 2012; YANKON, Petrie et al., 1993; YERLUD, Quillian et al., 2006). There were 99 examples containing the Al–CH₂–Al fragment, of which there are ten instances (BELLAH, BELLEL, BELLIP, BELLOW, BELLUP (Uhl et al., 2012a); JEZFID (Layh & Uhl, 1990); JUWMOD (Uhl et al., 1993); PENSEI (Uhl et al., 2012b); WOZJUQ, WOZKAX (Knabel et al., 2002) where the metrical parameters of the Al–CH₂–Al fragment are not influenced by being part of a ring.

5. Synthesis and crystallization

Aluminum wire, cut into small pieces (3.19 g), was added slowly over several days to a stirred, dry CH₂Br₂ (50 mL) under N₂ by inserting the wire through a hole in a rubber septum. After the aluminum had reacted, the mixture was filtered inside an N₂ flow dry box and the solids were collected and pumped dry. A total of 20.22 g (96% based on Al) was isolated. A portion of this white solid was dissolved in Et₂O and allowed to slowly evaporate inside the dry box to produce crystals of the title compound. IR (neat smeared on KBr plates, cm⁻¹): [3002.90, 2982.63, 2871.84, 2964.61, 2935.43, 2920.37, 2850.50] (m, C—H str), 2213.10 (w), 1635.50 (w), 1463.47 (m), 1442.72 (m), 1390.14 (s), 1326.09 (m), 1281.45 (w), 1260.61 (m), 1189.28 (m), 1146.67 (m), 1088.57 (m), 999.64 (s), 985.03 (s), 904.06 (w), 879.11 (s), 827.67 (m), 796.06 (w), 763.57 (s), 723.29 (m), 606.38 (s), 545.18 (s), 530.16 (s), 463.05 (w). The NMR solvents were dried from sodium–potassium alloy. NMR spectra were recorded in C₆D₆ solution in flame-sealed tubes and were found to be concentration dependent. Proton spectra were recorded at 400 MHz on three different concentrations, and two samples of the intermediate concentration with added ether, and are displayed in Fig. 5. ¹³C spectrum (C₆D₆, 100 MHz): δ 1.34 (CH₂, sharp), −1.46 ppm (CH₂, broad, HHLW ≃ 150 Hz). ²⁷Al spectrum (C₆D₆, 104 MHz): δ 93 (sharp), 132 ppm (broad, HHLW ≃ 4000 Hz).

Figure 5
¹H NMR spectra of the title compound in C₆D₆ at three different concentrations (bottom three spectra), and at an intermediate concentration with added ether (top two spectra). The CH₂ group attached to Al has peaks A, B, C, and D, which are concentration dependent, and an expanded view from δ 1 to −1 ppm is shown in the lower part of the figure. The concentration of the small peak at 2.5 ppm (probably OH) is invariant in all samples and is undoubtedly due to hydrolysis caused by the release of a small amount of water during flame sealing of the NMR tubes.

Safety Note:

This reaction should be carried out with caution as when finely divided Al flakes were used instead of Al wire, an explosion occurred.

6. Chemical exchange in solution

The title compound (1) is monomeric and coordinatively saturated in the solid state, as each aluminum is four-coordinate, but in solution, the ether molecules from either or both Al atoms can dissociate and would be expected to exchange rapidly. Once an ether molecule dissociates, the aluminum atom can regain four-coordination by association to a bromine atom from the other half of the same or another molecule. In the C₆D₆ solution, there are four main proton NMR peaks visible for the CH₂ moieties on aluminum, as shown in Fig. 5, and those peaks are labeled A, B, C, and D. Both the relative amounts and chemical shifts of peaks A–D are concentration dependent. Additionally, the NMR peaks for the ether moieties are dependent on concentration as well, as is most obvious at the lowest concentration (where the ether CH₂ peak splits), and adding additional ether to the solution does affect the spectra, as also shown in Fig. 5. The unsolvated parent compound, CH₂(AlBr₂)₂, has extremely low solubility in non-coordinating solvents such as C₆D₆, as one would expect if it is polymeric. While the structures of unsolvated compounds of this type are unknown, association through Al₂Br₂ rings is common, and this compound can easily form such rings on each end linking into an extended structure.

One can determine some information as to the identity of the peaks from the concentration dependence of the spectra. If there is an exchange process between different degrees of association (for example between monomer and trimer), the ratio of oligomerization between the species can be determined by the slope of a ln–ln plot of the molar concentrations represented by each NMR peak (Purdy et al., 1987 ). Fig. 6 shows a natural ln–ln plot for the integral fraction of the CH₂ NMR peaks multiplied by the absolute concentration of the title compound in solution, for all six binary combinations of peaks A–D, with the linear equation between the points displayed on the chart. All combinations involving only peaks A, C, and D have R² factors near 1, showing a high linear correlation. The species with the NMR peak C clearly has three times the degree of association of A, and D has 2.5 times the degree of association as A. However, all combinations involving peak B with A, C, or D do not have as good a linear correlation, but do show that and A and B have approximately the same degree of association. Fig. 7 displays a ln–ln plot for the concentrations of peaks A–D against the ether concentration for three solutions of approximately the same concentration of the title compound with varying amounts of ether. Clearly, peak B correlates positively to the ether concentration, and the relative amounts of peaks A, C, and D have a slightly negative correlation to the concentration of ether. Therefore, we conclude that B is for a solution species that is more coordinatively saturated by ether than A, C, or D, and is probably the title compound. A is probably formed by the dissociation of a single ether molecule, C is its trimer, and D is a partially associated trimer. Fig. 8 illustrates some possible structures of monomeric and trimeric species, with varying degrees of ether solvation, although the drawings of trimers do not exhaust the possible structures that may exist. The NMR data do not allow definite structural conclusions to be drawn for the trimers. While substantial precedent exists for compounds associated through Al₂Br₂ rings, and an example exists for a four-membered Al–CH₂–Al–Br ring (PENSOS; Uhl et al. 2012a), six- and eight-membered aluminum–halogen (AlX)_n rings are mostly known for X=F, although a structurally constrained Cl example does exist (GOTNEI; Tschinkl et al. 1999 ). An example exists for a linearly associated –CH₂–AlBr₃–AlBr₃ moiety (KIXBEA; Ménard et al. 2013 ), which opens the possibility that a partially associated trimer could have a single dative bond in place of Al₂Br₂ or Al₃Br₃ rings.

Figure 6
ln–ln plots of the concentrations of the molecules represented by the CH₂—Al peaks in the proton spectra. The concentration is calculated from the integral fraction of those CH₂ resonances multiplied by the total concentration of CH₂(AlBr₂OEt₂)₂ dissolved. The slope of the line is the ratio of the degree of association of the species in solution.

Figure 7
ln–ln plots of the total ether concentration on the x-axis and the concentration of the species represented by the CH₂—Al peaks on the y-axis for the three samples with approximately equal total concentration of CH₂(AlBr₂OEt₂)₂. The relative amount of the species with peak B increases with ether concentration, while the other peaks decrease.

Figure 8
Drawings of some possible structures of monomeric and trimeric aggregates of 1, with varying degrees of ether coordination. The possibilities exhibited here are not exhaustive.

7. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. For the CH₂ bridging group, the H-atom position was refined isotropically while the other H atoms were refined in idealized positions using a riding model with atomic displacement parameters of U_iso(H) = 1.2U_eq(C) [1.5U_eq(C) for CH₃], with C—H distances ranging from 0.98 to 0.99 Å.

Table 3
Experimental details

Crystal data
Chemical formula	[Al₂Br₄(CH₂)(C₄H₁₀O)₂]
M_r	535.86
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	8.3872 (6), 12.1039 (6), 18.1504 (12)
β (°)	95.646 (3)
V (Å³)	1833.7 (2)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	8.87
Crystal size (mm)	0.31 × 0.25 × 0.08

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996 )
T_min, T_max	0.291, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	10600, 2027, 1736
R_int	0.073
(sin θ/λ)_max (Å⁻¹)	0.641

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.031, 0.076, 1.07
No. of reflections	2027
No. of parameters	83
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.74, −0.73
Computer programs: APEX2 (Bruker, 2005 ), SAINT (Bruker, 2002 ), SHELXT (Sheldrick 2015a ), SHELXL2018/3 (Sheldrick, 2015b ), and SHELXTL (Sheldrick 2008 ).

Supporting information

CCDC reference: 2084539

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989021005302/jq2003sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989021005302/jq2003Isup2.hkl

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXT (Sheldrick 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick 2008); software used to prepare material for publication: SHELXTL (Sheldrick 2008).

µ-Methylene-bis[dibromido(diethyl ether-κO)aluminium(III)] top

Crystal data top

[Al₂Br₄(CH₂)(C₄H₁₀O)₂]	F(000) = 1032
M_r = 535.86	D_x = 1.941 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 8.3872 (6) Å	Cell parameters from 4301 reflections
b = 12.1039 (6) Å	θ = 3.1–32.5°
c = 18.1504 (12) Å	µ = 8.87 mm⁻¹
β = 95.646 (3)°	T = 100 K
V = 1833.7 (2) Å³	Plate, colorless
Z = 4	0.31 × 0.25 × 0.08 mm

Data collection top

Bruker APEXII CCD diffractometer	1736 reflections with I > 2σ(I)
φ and ω scans	R_int = 0.073
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	θ_max = 27.1°, θ_min = 3.0°
T_min = 0.291, T_max = 0.747	h = −9→10
10600 measured reflections	k = −15→15
2027 independent reflections	l = −23→23

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.031	Hydrogen site location: mixed
wR(F²) = 0.076	H atoms treated by a mixture of independent and constrained refinement
S = 1.07	w = 1/[σ²(F_o²) + (0.0337P)² + 0.0465P] where P = (F_o² + 2F_c²)/3
2027 reflections	(Δ/σ)_max = 0.001
83 parameters	Δρ_max = 0.74 e Å⁻³
0 restraints	Δρ_min = −0.73 e Å⁻³

Special details top

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Al	0.11619 (11)	0.71423 (7)	0.32918 (6)	0.0103 (2)
Br1	−0.02876 (4)	0.77240 (3)	0.42346 (2)	0.01923 (12)
Br2	0.27035 (4)	0.85713 (3)	0.29158 (2)	0.02154 (12)
O1	0.2720 (3)	0.62389 (16)	0.38074 (13)	0.0112 (4)
C1	0.000000	0.6327 (3)	0.250000	0.0131 (9)
H1	−0.066 (4)	0.589 (3)	0.272 (2)	0.016*
C2	0.2436 (4)	0.5550 (2)	0.44484 (19)	0.0150 (7)
H2A	0.204255	0.601947	0.483872	0.018*
H2B	0.345754	0.520776	0.465274	0.018*
C3	0.1224 (4)	0.4655 (3)	0.4234 (2)	0.0202 (8)
H3A	0.111673	0.417489	0.466116	0.030*
H3B	0.158243	0.421669	0.382748	0.030*
H3C	0.018501	0.499297	0.407455	0.030*
C4	0.4055 (4)	0.5842 (3)	0.34074 (19)	0.0151 (7)
H4A	0.388925	0.608372	0.288425	0.018*
H4B	0.408161	0.502482	0.341600	0.018*
C5	0.5618 (4)	0.6288 (3)	0.3758 (2)	0.0201 (7)
H5A	0.650079	0.596834	0.351275	0.030*
H5B	0.574490	0.609271	0.428443	0.030*
H5C	0.563012	0.709361	0.370553	0.030*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Al	0.0122 (5)	0.0062 (4)	0.0126 (5)	−0.0002 (3)	0.0013 (4)	0.0007 (3)
Br1	0.0223 (2)	0.01703 (18)	0.0192 (2)	0.00542 (12)	0.00634 (15)	−0.00335 (13)
Br2	0.0235 (2)	0.01272 (17)	0.0279 (2)	−0.00778 (12)	−0.00023 (15)	0.00757 (13)
O1	0.0110 (11)	0.0102 (10)	0.0127 (12)	0.0012 (8)	0.0025 (9)	0.0028 (8)
C1	0.016 (2)	0.0088 (19)	0.014 (2)	0.000	0.0011 (19)	0.000
C2	0.0167 (17)	0.0151 (15)	0.0128 (17)	0.0009 (12)	−0.0001 (13)	0.0045 (13)
C3	0.0199 (18)	0.0159 (16)	0.025 (2)	−0.0023 (13)	0.0051 (15)	0.0042 (14)
C4	0.0163 (17)	0.0143 (14)	0.0157 (17)	0.0016 (12)	0.0064 (14)	−0.0030 (13)
C5	0.0147 (18)	0.0257 (17)	0.0206 (19)	0.0015 (13)	0.0059 (14)	−0.0021 (14)

Geometric parameters (Å, º) top

Al—O1	1.881 (2)	C2—H2B	0.9900
Al—C1	1.927 (2)	C3—H3A	0.9800
Al—Br2	2.3029 (9)	C3—H3B	0.9800
Al—Br1	2.3046 (10)	C3—H3C	0.9800
O1—C2	1.470 (4)	C4—C5	1.500 (5)
O1—C4	1.473 (4)	C4—H4A	0.9900
C1—H1	0.89 (4)	C4—H4B	0.9900
C1—H1ⁱ	0.89 (4)	C5—H5A	0.9800
C2—C3	1.510 (4)	C5—H5B	0.9800
C2—H2A	0.9900	C5—H5C	0.9800

O1—Al—C1	110.42 (13)	H2A—C2—H2B	108.0
O1—Al—Br2	101.60 (7)	C2—C3—H3A	109.5
C1—Al—Br2	114.90 (9)	C2—C3—H3B	109.5
O1—Al—Br1	101.52 (8)	H3A—C3—H3B	109.5
C1—Al—Br1	116.44 (5)	C2—C3—H3C	109.5
Br2—Al—Br1	110.07 (4)	H3A—C3—H3C	109.5
C2—O1—C4	113.3 (2)	H3B—C3—H3C	109.5
C2—O1—Al	124.38 (19)	O1—C4—C5	110.4 (3)
C4—O1—Al	117.9 (2)	O1—C4—H4A	109.6
Al—C1—Alⁱ	118.4 (2)	C5—C4—H4A	109.6
Al—C1—H1	105 (2)	O1—C4—H4B	109.6
Alⁱ—C1—H1	111 (2)	C5—C4—H4B	109.6
Al—C1—H1ⁱ	111 (2)	H4A—C4—H4B	108.1
Alⁱ—C1—H1ⁱ	105 (2)	C4—C5—H5A	109.5
H1—C1—H1ⁱ	107 (5)	C4—C5—H5B	109.5
O1—C2—C3	111.1 (3)	H5A—C5—H5B	109.5
O1—C2—H2A	109.4	C4—C5—H5C	109.5
C3—C2—H2A	109.4	H5A—C5—H5C	109.5
O1—C2—H2B	109.4	H5B—C5—H5C	109.5
C3—C2—H2B	109.4

C1—Al—O1—C2	−89.5 (2)	Br1—Al—O1—C4	−170.52 (18)
Br2—Al—O1—C2	148.1 (2)	C4—O1—C2—C3	−91.5 (3)
Br1—Al—O1—C2	34.6 (2)	Al—O1—C2—C3	64.4 (3)
C1—Al—O1—C4	65.4 (2)	C2—O1—C4—C5	−85.6 (3)
Br2—Al—O1—C4	−57.0 (2)	Al—O1—C4—C5	116.9 (3)

Symmetry code: (i) −x, y, −z+1/2.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2A···Br1	0.99	2.98	3.481 (3)	113
C5—H5B···Br1ⁱⁱ	0.98	3.10	3.871 (4)	136

Symmetry code: (ii) −x+1/2, −y+3/2, −z+1.

Acknowledgements

RJB wishes to acknowledge the ONR Summer Faculty Research Program for funding in 2019 and 2020.

Funding information

Funding for this research was provided by: The Office of Naval Research.

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