Investigation of mesitylene-solvated group 13 mixed-metal halides: syntheses and crystal structures of bis(1,3,5-trimethylbenzene)gallium(I) tetrachlorido- and tetrabromidoaluminate(III) and (1,3,5-trimethylbenzene)gallium(I) tetraiodidoaluminate(III). Variation of the gallium-π-arene bond strength

2.1 Syntheses and crystal structures of the bis(arene)gallium(I) complexes [(1,3,5-(CH₃)₃C₆H₃)₂Ga][AlCl₄] (1) and [(1,3,5-(CH₃)₃C₆H₃)₂Ga][AlBr₄] (2)

The mixed metal ternary halides Ga[AlX₄] (X=Cl, Br) [19] easily dissolve in mesitylene at elevated temperature. After cooling to room temperature, not the halides but bis(mesitylene) complexes of the general formula [(1,3,5-(CH₃)₃C₆H₃)₂Ga][AlX₄] (1 (X=Cl); 2 (X=Br)) crystallize from these solutions. The colorless compounds are highly sensitive against moisture and strictly anhydrous conditions are needed for successful syntheses. Unfortunately, in vacuo1 and 2 lose mesitylene to give compounds of still unknown composition that contain less mesitylene. In the case of 1 Ga[AlCl₄] as a minor component is observed, too. As documented by X-ray powder diffraction measurements (see Figs. 1 and 2), partial loss of arene during the drying or storing process is unavoidable and acquisition of convincing C,H analyses is ambitious. Raman lines at 120 cm⁻¹ (ν₂), 181 cm⁻¹ (ν₄) and 350 cm⁻¹ (ν₁) resp. 115 cm⁻¹ (ν₄) and 210 cm⁻¹ (ν₁) can be assigned to the tetrachlorido- and the tetrabromidoaluminate tetrahedra [20].

Fig. 1:

X-ray powder pattern of 1 after drying under reduced pressure (blue, λ=1.54056 Å) and the simulated pattern (orange).

Fig. 2:

X-ray powder pattern of 2 (blue, λ=1.54056 Å) and the simulated pattern (red).

Crystallographic data of 1 and 2 is summarized in Table 1. The chloridoaluminate 1 crystallizes in the noncentrosymmetric monoclinic space group Cc, isotypic to [(1,3,5-(CH₃)₃C₆H₃)₂Ga][GaCl₄] [5], the bromidoaluminate 2 in the centrosymmetric monoclinic space group P2₁/n, isotypic to [(1,3,5-(CH₃)₃C₆H₃)₂In][InBr₄] [12]. The asymmetric units of the crystal structures of both 1 and 2 contain one Ga⁺ cation and coordinated to it one tetrahalogenidoaluminate anion and two mesitylene molecules, the latter bonded to the cation in the η⁶ coordination mode (Fig. 3).

Fig. 3:

Asymmetric units of the crystal structures of 1 (top) and 2 (bottom). The hydrogen atoms are omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level. Further connections in the solid are indicated by sharpened sticks.

Reflecting its role as bridging moiety between (arene)₂Ga⁺ fragments, in both 1 and 2 the tetrahalogenidoaluminate tetrahedra are significantly distorted with X–Al–X angles between 108.17(16) and 113.05(19)° (1; X=Cl) and 106.50(7) and 110.74(8)° (2; X=Br). The mesitylene ligands of 1 and 2 are coordinated in bent sandwich arrangements with a centroid–Ga–centroid angle of 136.24 (1) and 133.48° (2), respectively. In the case of 1, bonding of equal strength is observed between the arene molecules and the gallium cation with gallium best-plane distances of 2.667(3) and 2.666(3) Å and calculated gallium centroid distances of 2.668 and 2.668 Å. In 2, the corresponding distances are greater and significantly more different with 2.683(3) and 2.753(2) Å (gallium best-plane distances) and 2.684 and 2.757 Å (gallium centroid distances). The related ring slippages of 0.098 and 0.095 Å for 1 resp. 0.077 and 0.148 Å for 2 are small and do not indicate substantial distortion of the η⁶ coordination. Taking into account the additional contacts to symmetry related neighboring anions, the coordination spheres of the Ga⁺ central ions are completed by two chlorido (1) and three bromido ligands (2). To build up strands in an “zigzag-design” along the crystallographic b or c axis, respectively (Fig. 4). The arene molecules of different strands in 2 are ordered parallel. All in all, the strands in both compounds do not feature any π-π interactions but form contacts through van-der-Waals interactions. A comparison of the tetrachloridoaluminate 1 to the isotypic tetrachloridogallate [4] shows no significant differences of Ga^I–Cl and Ga^I–centroid distances. The only significant difference is a slightly more acute ­centroid–Ga–centroid angle in 1 as compared to the gallate (140.3°) [4], which can be explained by the smaller radius of the tetrachloridoaluminate anion.

Fig. 4:

Coordination-polymeric chains in the crystals of 1 (top; view direction [001]) and 2 (bottom; view direction approximately [100]); note the different single-chain zigzag patterns displaying ‘in line’ and ‘out of line’ arrangements of the Ga⁺ ions.

As mentioned above, 2 crystallizes isotypic to bis(1,3,5-trimethylbenzene)indium(I) tetrabromidoindate(III) [12]. Due to the difference of the ionic radii of Ga⁺ and In⁺, for a comparative evaluation of the metal-π-arene and the metal-halogen bond strength in the isotypic solids, it is not possible to compare the M^I–arene and M^I–Br distances directly and empirical bond orders s(M^I–arene) had to be calculated using the bond valence method [21], [22] according to s=exp[(r₀ – r)/B] in an indirect manner defining s(M^I–arene)=1 – Σs(M^I–X) [23]. Unfortunately, there are no bond valence parameters for Ga^I–X (X=Cl, Br, I) and In^I–X (X=Cl, Br, I). However, r₀=2.411 and B=0.37 that give Σs(Ga^I–Cl)=1 for α-Ga₂Cl₄ should be acceptable for a qualitative exploration of the bond strength in the coordination environment of the Ga⁺ ion in 1 and the isotypic gallate. Furthermore, r₀=2.570 A, B=0.35 and r₀=2.720, B=0.37 that give Σs(Ga^I–Br)=1 for α-Ga₂Br₄ and Σs(In^I–Br)=1 for In₂Br₄, respectively, should be acceptable for a qualitative exploration of the bond strength in the coordination environment of the M⁺ ions in 2 and the isotypic bromidoindate. Alternatively, r₀ values can be taken from data of gas phase spectroscopic measurements of the monohalides and the B value set to 0.35 [21], [24]. Results corresponding to this approach demonstrate that the indium-π-arene interaction is significantly weaker (Σs(In^I–arene)=0.800) than the gallium-π-arene interaction in 2 (Σs(Ga^I–arene)=0.874), which can be explained by the substantially lower Lewis acidity of InBr₃ as compared to GaCl₃. For 1 Σs(Ga^I–arene)=0.907 suggests a stronger bonding in the chloridoaluminate as compared to the bromidoaluminate in line with expectations. However, as exemplified in the case of Ga₃Cl₇ [25], this method to some extent underestimates the M^I–X and overestimates the M^I–arene bond strength. All in all, the π bonding interaction in 1 and 2 is medium strong on the overall scale including all types of arene π bonding, but strong on the scale of non-covalent main-group metal-arene bonding.

2.2 Synthesis and crystal structure of the mono(arene) complex [(1,3,5-(CH₃)₃C₆H₃)Ga][AlI₄] (3)

Following the same experimental procedures as given for 1 and 2, the mono(arene) complex [(1,3,5-(CH₃)₃C₆H₃)Ga][AlI₄] (3) – the first mono(mesitylene) complex of Ga^I – has been obtained. The Raman lines at 83 cm⁻¹ (ν₄), 149 cm⁻¹ (ν₁) and 341 cm⁻¹ (ν₃) can be assigned to the tetraiodidoaluminate tetrahedra [20]. The crystallographic data of the colourless, moisture sensitive compound is given in Table 1, the powder diffractogram is shown in Fig. 5. The asymmetric unit of the crystal structure contains one gallium(I) cation, one tetraiodidoaluminate anion and one mesitylene molecule that coordinates the cation in η⁶ mode (Fig. 6). The AlI₄⁻ ­tetrahedron is slightly distorted with I–Al–I angles between 108.43(6) and 110.89(6)°. The gallium arene distances are 2.693 (gallium centroid distance) and 2.692(2) Å (gallium best-plane distance); the ring slippage is 0.052 Å.

Fig. 5:

X-ray powder pattern of 3 (blue, λ=1.54056 Å) and the simulated pattern (red).

Fig. 6:

Asymmetric unit of the crystal structure of 3. The hydrogen atoms are omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level. Further connections in the solid are indicated by sharpened sticks.

In total, the coordination sphere of the Ga⁺ central ion is established by the π-coordinating mesitylene molecule and five iodine atoms of three AlI₄⁻ anions with Ga–I distances ranging from 3.4302(8) to 3.6277(8) Å (Fig. 7). Considering the arene π-ligand as occupying one coordination site only, the coordination number is six. The coordination polyhedra are connected via secondary M^I–X interactions to give strands along the crystallographic a axis (Fig. 7). As in 1 and 2, between the strands connectivity is limited to van-der-Waals interactions.

Fig. 7:

(Top) Coordination-polymeric chain in the crystals of 3 (view direction approximately [001]); note the enhanced double-chain character of the arrangement; (bottom) van-der-Waals packing of the chains (view direction [100]).

By looking at related compounds, it is to mention that [[(1,3,5-(CH₃)₃C₆H₃)₆Tl₄][GaBr₄]]₄ has structural similarities to 3. By omitting the two terminal mesitylene molecules of the bis(arene) coordinated Tl⁺ cations, the entire structural motif is very similar to a section of the strand in 3. Furthermore, these centrosymmetric ‘dimeric’ sections of 3, are closely related to the inorganic structural moiety of the cationic bismuth-π-arene complex [[(CH₃)₆C₆)₆BiCl₂][AlCl₄]]₂ [26].

In general, compared to the other members of the class of mesitylene M^I complexes of the third main group it is unusual that 3 is found to be a mono(mesitylene) complex. This could be explained by the greater radius of iodido in comparison to bromido and chlorido ligands. As shown in Fig. 8, one half of the Ga⁺ cation is entirely surrounded by five iodine atoms. Consequently, this covering could prevent the approach of another mesitylene molecule.

Fig. 8:

Space filling model in the environment of the gallium cation in 3.

4.1 Materials and methods

All experiments have been performed under argon atmosphere (Argon 99.999%) by using Schlenk line techniques or in a MBraun Labstar glove box (preparation of samples for X-ray powder diffraction, C,H analyses and Raman spectroscopy). Mesitylene was treated with AlCl₃, washed with water, dried with CaCl₂, distilled and stored over 4 Å molecular sieve. GaAlCl₄, GaAlBr₄ and GaAlI₄ were synthesized following the procedures described by McMullan and Corbett [19]. For the drying process of 1, 2 and 3 the solvent was removed with a syringe and the solid dried under reduced pressure (1×10⁻¹ mbar).

Samples for Raman spectroscopy were sealed in glass tubes and measured with a Bruker MultiRAM spectrometer (resolution 2 cm⁻¹, ND:YAG laser, λ=1064 nm). C,H analyses were performed with an Elementar Vario MICRO cube elemental analyzer. Samples for PXRD investigations were sealed in 0.4 mm diameter thin walled glass capillaries and measured on a STOE Stadi-P diffractometer [Ge(111) monochromator, CuKα₁, λ=1.54056 Å]. Simulated patterns were fitted with the program Jana2006 [27].

4.2 Bis(mesitylene)gallium(I) tetrachloridoaluminate(III), [(1,3,5-(CH₃)₃C₆H₃)₂Ga][AlCl₄] (1)

0.117 g (0.491 mmol) GaAlCl₄ was dissolved in 7 mL mesitylene at 80°C. After cooling to room temperature, within some days colorless crystals formed at the wall of the glass flask. Finally, the solution was removed with a syringe and the colorless solid dried carefully under reduced pressure as described above. Compound 1 is very sensitive against hydrolysis. Yield 23% (the yield was calculated by considering the minority phase). – Raman (cm⁻¹): ν˜=88 (vs), 120 (s), 181 (m), 248 (m), 279 (m), 350 (s), 511 (m), 520 (w), 578 (vs), 997 (s), 1030 (vw), 1168 (vw), 1303 (m), 1386 (m), 1445 (vw), 1589 (vw), 1603 (w), 2738 (vw), 2869 (w), 2921 (s), 2963 (w), 3034 (m). – Anal. calcd. for (C₃H₉C₆H₃)₂GaAlCl₄ (478.9 g·mol⁻¹): C 45.14, H 5.05; found C 45.40, H 4.64.

4.3 Bis(mesitylene)gallium(I) tetrabromidoaluminate(III), [(1,3,5-(CH₃)₃C₆H₃)₂Ga][AlBr₄] (2)

0.251 g (0.603 mmol) GaAlBr₄ was dissolved in 3 mL mesitylene at 80°C. After cooling to room temperature, within some days colorless crystals formed. The mother liquor was removed and the colourless solid was dried carefully under reduced pressure as described before. Compound 2 is very sensitive against oxidation and hydrolysis. Yield 26% (the yield was calculated by considering the minority phase). – Raman (cm⁻¹): ν˜=78 (s), 86 (s), 115 (m), 210 (vs), 243 (w), 278 (w), 410 (w), 509 (w), 518 (w), 579 (m), 996 (m), 1029 (vw), 1166 (vw), 1303 (w), 1386 (w), 1601 (vw), 2864 (vw), 2920 (m), 3024 (w). – Anal. calcd. for (C₃H₉C₆H₃)₂GaAlBr₄ (656.7 g·mol⁻¹): C 32.92, H 3.68; found C 32.40, H 3.49.

4.4 (Mesitylene)gallium(I) tetraiodidoaluminate(III), [(1,3,5-(CH₃)₃C₆H₃)Ga][AlI₄] (3)

0.090 g (0.124 mmol) GaAlI₄ was dissolved in a mixture of 3 mL mesitylene and 2.6 mL toluene at 80°C. After cooling to room temperature growth of colorless crystals started immediately. Some days later isolation and removal of adhering arene were performed as described above. Compound 3 is very sensitive against oxidation and hydrolysis. Yield 55%. – Raman (cm⁻¹): ν˜=67 (s), 83 (s), 101 (m), 149 (vs), 241 (w), 279 (w), 341 (w), 512 (w), 579 (m), 996 (w), 1309 (w), 1379 (vw), 2916 (w), 3024 (vw). – Anal. calcd. for C₉H₁₂AlGaI₄ (724.5 g·mol⁻¹): C 14.92, H 1.67; found C 14.73, H 1.53.

4.5 Crystal structure determination and refinement

Crystals for single-crystal X-ray structure determination were selected under a constant flow of nitrogen, fixed on top of a broken glass capillary and investigated with a STOE IPDS diffractometer using graphite-monochromated MoKα radiation (λ=0.71073 Å) and applying a cold nitrogen gas stream (T=−60°C). Absorption corrections were applied based on multi-scans. For structure solution and refinement Shelxt and Shelxl were used, respectively [28], [29]. All hydrogen atoms were identified via difference Fourier synthesis and treated as riding on their parent atoms in idealized positions (with U_iso(H)=1.5U_eq(C_methyl)). Crystal data and some details of data collection and structure refinement are given in Table 1. Further crystallographic data is given as Supporting information available online (see below). Diamond [30] was used for preparing Figs. 3, 4, 6 and 8.

CCDC 1949383 (1), 1949384 (2) and 1949385 (3) contain the deposited supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.