Crystal structure of bis(triphenylphosphonium) hexabromodigallate(II) in the correct space group: Conformational complexity in a heteroethane

Figures & data

Figure 1. Disordered [Ga₂Br₆]²⁻ anion in the refinement model for FUPSIS.

Figure 2. Displacement ellipsoids (50% probability) plot of the asymmetric unit depicting the molecular structures of the two independent [Ga₂Br₂]²⁻ anions and the three independent [Ph₃PH]⁺ cations (Ga1 with site symmetry $\overline{3}$; Ga2,3 with site symmetry 3). False colours indicate symmetry-related atoms (sym codes on diagram). A symmetry-equivalent P1 cation additional to the asymmetric unit is shown in light grey (‘‘). Phenyl ring T-interactions from C–H to C are shown with dashed blue lines once for each symmetry-replicated interaction. H atoms on carbon are omitted, as are atom labels for symmetry related atoms.

Table 1. Selected interatomic distances, angles and torsions in the crystal structure of the title compound

Atoms	Distance (Å)	Atoms	Angle (°)	Atoms	Dihedral (°)
Ga1–Ga1^i	2.4124 (7)	Br1–Ga1–Ga1^i	115.115 (9)	Br1–Ga1–Ga^i–Br1^vii	60 (exact)
Br1–Ga1	2.3732 (3)	Br1–Ga1–Br1^iv	103.284 (11)	Br2–Ga2–Ga3–Br3 ^viii	36.91 (1)
Ga2–Ga3	2.3978 (5)	Br2–Ga2–Ga3	113.700 (9)	C1–P1⋯P^ix–C1^ix	60 (exact)
Br2–Ga2	2.3731 (3)	Br3–Ga3–Ga2	114.136 (9)	C7–P2⋯P3–C13^x	55.60 (1)
Br3–Ga3	2.3770 (3)	Br2–Ga2–Br2^v	104.931 (11)
P1–C1	1.7794 (19)	Br3^vi–Ga3–Br3	104.431 (11)
P1–H1	1.321 (18)	C1–P1–C1^vii	110.04 (6)
P2–C7	1.785 (2)	C1–P1–H1	108.90 (7)
P2–H2A	1.314 (18)	C7–P2–C7^v	111.09 (6)
P3–C13	1.780 (2)	C7–P2–H2A	107.80 (7)
P3–H3A	1.304 (18)	C13^vi–P3–C13	109.57 (7)
Ga1⋯P1	4.518 (1)	C13–P3–H3A	109.37 (7)
P1⋯∙P1^ii	6.156 (1)
Ga2⋯P2	4.604 (1)
Ga3⋯P3^iii	4.460 (2)
P2⋯P3	6.145 (1)

ⁱ −x, −y, −z.

ⁱⁱ −x, −y, 1 − z.

ⁱⁱⁱ x, y, 1 + z.

^iv −y, x − y, z.

^v −y + 1, x − y + 1, z.

^vi −x + y, 1 − x, z.

^vii y, −x + y, z. −x.

^viii −x + y, 1 − x, z.

^ix −x, −y, 1 − z.

^x −x + y, 1 − x, z.

Figure 3. Packing diagram of the structure of the title compound in P$\overline{3}$. H atoms on the phenyl rings are omitted for clarity, as are all atoms along the front (1 1 0) unit cell edge. The phosphonium ions are rendered as rods, whereas the hexabromodigallate ions are rendered as “space filling” spheres.

Figure 4. Overlaid unit cells of the title structure in P$\overline{3}$ with the alternate refinement in R$\overline{3}$ with apparent disorder as in (Khan et al., 1986), drawn as 30% displacement ellipsoids. The $\overline{3}$ body diagonal of the rhombohedral cell is aligned with the 3 axis of the trigonal cell along (⅔ ⅓ 0).

Figure 5. Representation of part of the crystal structure of Li₂[Ga₂Br₆]; there is a mirror plane through Br11, Ga1, Ga2, Br22. Li⁺ ions coordinated to Br22 and all remaining bromine atoms completing the coordination spheres of Li1,2 have been omitted for clarity. The data is taken from Hönle and Simon (1986).

Figure 6. Conformational energy profile for the title anion computed at the B3LYP/6–311+G(fd,) level of theory for gas-phase [Ga₂Cl₆]^2– and [Ga₂Br₆]^2– ions. The calculations were undertaken in 10° steps in the X–Ga–Ga–X torsion angle starting from 63.3°.

Table 2. Experimental or calculated barriers to rotation in [X₃EEX₃] species

Molecule	Exp/calc	kJ/mol	References
H3CCH3	c	12.8	Mo and Gao (2007)
H3SiSiH3	c	4.1	Mo and Gao (2007)
H3GeGeH3	c	3.1	Mo and Gao (2007)
H3CSiH3	c	6.1	Mo and Gao (2007)
Cl3CCCl3	e	73.2	Morino and Hirota (1958)
Br3CCBr3	e	83.7	Morino and Hirota (1958)
Cl3SiSiCl3	e	4.2	Morino and Hirota (1958)
[Ga2Cl6]^2–	c	4.34	This work
[Ga2Br6]^2–	c	5.09	This work

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Mo, Y., & Gao, J. (2007). Theoretical analysis of the rotational barrier of ethane. Accounts of Chemical Research, 40, 113–119. doi:10.1021/ar068073w

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