Thio-, seleno- and telluro-ether complexes of aluminium(III) halides: synthesis, structures and properties

Abstract

The reaction of AlCl₃ with Me₂E (E = S, Se or Te) or ⁿBu₂E (E = Se or Te) in CH₂Cl₂ under rigorously anhydrous conditions gave the pseudo-tetrahedral complexes [AlCl₃(R₂E)]. The [AlX₃(Me₂E)] (X = Br or I, E = S; X = Br, E = Te) were made from toluene solution since attempted syntheses in CH₂Cl₂ resulted in substantial chloride incorporation. The synthesis of [(AlCl₃)₂{o-C₆H₄(CH₂SEt)₂}], in which the ligand bridges two tetrahedral aluminium centres, and of the six-coordinate trans-[AlX₂{MeE(CH₂)₂EMe}₂][AlX₄] (X = Cl or Br, E = S, and X = Cl, E = Se) and cis-[AlI₂{MeS(CH₂)₂SMe}₂][AlI₄] are reported. The tripodal thioether forms [AlCl₃{MeC(CH₂SMe)₃}], which is a chain polymer with κ²-coordinated ligand and a tbp arrangement at Al(III). Chalcogenoether macrocycle complexes [AlCl₃([9]aneS₃)], [AlCl₂([14]aneS₄)][AlCl₄] and [AlCl₂([16]aneSe₄)] [AlCl₄] are also described. All complexes were characterised by microanalysis, IR and multinuclear NMR (¹H, ²⁷Al, ⁷⁷Se or ¹²⁵Te) spectroscopy as appropriate. In CH₂Cl₂ solution [AlCl₃(Me₂S)] with added Me₂S forms [AlCl₃(Me₂S)₂], and the [AlX₂{MeS(CH₂)₂SMe}₂][AlX₄] exist as mixtures of cis and trans isomers which undergo rapid exchange at ambient temperatures. X-Ray crystal structures are reported for [AlCl₃(Me₂Se], [AlX₃(Me₂Te)] (X = Cl or Br), trans-[AlCl₂{MeE(CH₂)₂EMe}₂][AlCl₄] (E = S or Se), cis-[AlI₂{MeS(CH₂)₂SMe}₂][AlI₄], [AlCl₃{MeC(CH₂SMe)₃}], and for the sulfonium salt [Me₂SH][AlCl₄]. The aluminium halide chalcogenoether chemistry is compared with the corresponding gallium and indium systems, and the relative Lewis acidities of the metals discussed. Attempts to use [AlCl₃(ⁿBu₂E)] (E = Se or Te) as LPCVD reagents to form aluminium chalcogenide films were unsuccessful.

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Introduction

Aluminium chloride is widely used both in the laboratory and industry as a Friedel–Crafts catalyst for alkylation and acylation reactions, and also catalyses condensation, isomerisation and polymerisation reactions.¹ The reactions depend upon the strong Lewis acidity which produces incipient carbocations in combination with [AlCl₄]⁻ anions. Aluminium bromide and iodide promote similar chemistry and have some niche applications.¹ The strong Lewis acidity of these three halides has resulted in the characterisation of very many adducts, mostly with N- or O-centres in solvents or donor ligands.² In contrast, AlF₃ is an inert polymer which forms few complexes.³ Surprisingly little is known about AlX₃ adducts of chalcogenoethers, which are limited to early studies of adducts with Me₂S or Et₂S that reported phase diagrams, IR and ¹H NMR spectra,^4–6 and several studies of solution enthalpies.⁷ The only crystallographically characterised example of a neutral thioether coordinated to AlX₃ is the recently reported⁸ [AlCl₃(thianthrene)], although there are a few AlMe₃ complexes with thiamacrocycles.⁹ There are no reports of studies with telluroethers, and the single complex of a selenium ligand is [AlCl₃(selenoxan)], characterised only by microanalysis.¹⁰

Recent studies of gallium(III) halides GaX₃ (X = Cl, Br or I) with chalcogenoethers have established that most contain pseudo-tetrahedral gallium centres [GaX₃L] (L = Me₂S, Me₂Se, Me₂Te, etc.) or dinuclear [X₃Ga(μ-L–L)GaX₃] (L–L = RS(CH₂)₂SR, MeSe(CH₂)₂SeMe, MeTe(CH₂)₃TeMe etc.).^11–14 Higher coordination numbers are rare in the gallium systems, but found with the thia-macrocycle [14]aneS₄,‡ which binds exodentate via two sulfur centres, affording the chain polymer [GaCl₃([14]aneS₄)] with a trigonal bipyramidal geometry.¹⁵ The larger rings [16]aneS₄ and [16]aneSe₄ form octahedral cations trans-[GaCl₂(macrocycle)][GaCl₄].¹⁵ The larger indium centre forms 4-, 5- or 6-coordinate complexes of types [InX₃(R₂E)] (E = S, Se or Te), [InX₃(R₂S)₂], [In₂X₆{RE(CH₂)₂ER₂}₂], trans-[InX₂{RE(CH₂)₂ER₂}₂][InX₄],^11–16cis-[InCl₂{[14]aneS₄}][InCl₄] and trans-[InCl₂{[16]aneSe₄}][InCl₄].^15,16

We have recently reported that [GaCl₃(ⁿBu₂E)] (E = Se or Te) or [GaCl₃{μ-ⁿBuE(CH₂)_nEⁿBu)GaCl₃] are effective single source precursors for low pressure chemical vapour deposition (LPCVD) of Ga₂E₃ thin films, and that preferential deposition occurs onto TiN in photolithographically patterned SiO₂/TiN substrates.¹⁷ Here we report systematic studies of the reactions of AlX₃ (X = Cl, Br or I) with a range of thio-, seleno- and telluro-ethers, detailed spectroscopic and structural data, and comparisons with their gallium and indium analogues. We also explored whether selected complexes would function as CVD reagents for deposition of aluminium chalcogenide films.

Results and discussion

Aluminium trihalides (AlX₃ (X = Cl, Br or I), are strong hard Lewis acids with a very high affinity for water.² Successful synthesis of their complexes with soft donor ligands such as thio-, seleno- or telluro-ethers requires anhydrous AlX₃, rigorously anhydrous solvents and ligands and exclusion of water at all stages of the manipulations. Trace water displaces the neutral ligands and also generates [AlX₄]⁻ which are readily identified in the solids by their characteristic IR spectra ([AlCl₄]⁻t₂ = 498, [AlBr₄]⁻ 394, [AlI₄]⁻ 336 cm⁻¹)¹⁸ and in solution by ²⁷Al NMR spectroscopy (Table 1), where they have sharp characteristic resonances. The moisture sensitivity of the halides and the complexes is much greater than observed in the corresponding gallium(III) systems.^12,13 The higher reactivity of the aluminium halides also affects the choice of solvent for the synthesis. Whilst complexes of AlCl₃ are readily made in anhydrous CH₂Cl₂, use of this solvent for the AlBr₃ or AlI₃ reactions results in incorporation of substantial amounts of chloride into the products, and the heavier halides are best made from anhydrous toluene solution. Similar observations were made by Burford et al.¹⁹ in AlX₃/R₃PO/CH₂Cl₂ systems. However, the pre-isolated pure [AlX₃L] (X = Br or I) react only slowly with CH₂Cl₂ (or CD₂Cl₂), which remains the NMR solvent of choice, as non-coordinating and useable down to 180 K. The reactivity of AlCl₃ in CH₂Cl₂ has been ascribed to the formation of the intermediate carbenium ion [CH₂Cl][AlCl₄],^8,20 and in previous studies of GaCl₃ complexes of thioethers we observed the formation of [o-C₆H₄(SMeCH₂Cl)₂][GaCl₄]₂ when [(GaCl₃)₂{μ-o-C₆H₄(SMe)₂}] is allowed to stand in CH₂Cl₂ solution for several days.¹³ Cleavage of C–Se or C–Te bonds is also observed, e.g. the formation of the selenonium cation in [o-C₆H₄(CH₂)₂SeMe][GaCl₄] from o-C₆H₄(CH₂SeMe)₂.¹² In the aluminium systems many of the complexes degrade on standing in solution at ambient temperatures (below), hence rapid isolation of the complexes from solution is advisable. Decomposition in solution is much slower at low temperatures, permitting growth of X-ray quality crystals overnight at −18 °C.

Table 1 ²⁷Al NMR data^a

Compound^b	δ (^27Al), 298 K	δ (^27Al), 190 K	Comments
a Chemical shifts relative to [Al(H2O)6]^3+ in H2O at pH = 1. b In CH2Cl2–CD2Cl2 solution at temperature specified.
[AlCl4]^−	103.5	103.6	W 1/2 ∼ 5 Hz
[AlBr4]^−	80.6	81.4	W 1/2 ∼ 15 Hz
[AlI4]^−	−23.0		W 1/2 ∼ 25 Hz
[AlCl3(Me2S)]	111.6 W1/2 ∼ 310 Hz	111.2	73.8 with excess Me2S (298 K)
[AlBr3(Me2S)]	81.2 W1/2 ∼ 250 Hz	81.6	No change with added Me2S
[AIl3(Me2S)]	48.7 W1/2 ∼ 360 Hz	—	No change with added Me2S
[AlCl3(Me2Se)]	110.5 W1/2 ∼ 300 Hz	110.1	No change with added Me2Se
[AlBr3(Me2Se)]	99.7 W1/2 ∼ 170 Hz	—
[AlCl3(Me2Te)]	105.9 W1/2 ∼ 310 Hz	103.4
[AlBr3(Me2Te)]	92.6 W1/2 ∼ 320 Hz	—	Resonance lost on cooling
[AlCl3(^nBu2Se)]	109.9 W1/2 ∼ 490 Hz	—
[AlCl3(^nBu2Te)]	92.6 W1/2 ∼ 555 Hz	—
[AlCl2{MeS(CH2)2SMe}2][AlCl4]	105.5, 37.4
[AlBr2{MeS(CH2)2SMe}2][AlBr4]	80.6	81.1	Only anion resonance seen
[AlI2{MeS(CH2)2SMe}2][AlI4]	−23.0	−23.0	Only anion resonance seen
[AlCl2{MeSe(CH2)2SeMe}2][AlCl4]	105.4	—
[(AlCl3)2{C6H4(CH2SEt)2}]	111.3 W1/2 ∼ 900 Hz	—

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[AlX₃(R₂E)] complexes

The reaction of AlCl₃ in CH₂Cl₂ or AlX₃ (X = Br or I) in toluene with Me₂S affords [AlX₃(Me₂S)] in high yield. The chloro-complex is an oil at room temperature, the others crystalline solids. The complexes can also be made by condensing excess Me₂S onto the appropriate AlX₃ at −196 °C, allowing the mixture to thaw, when the halide dissolves to give a clear solution, and then removing excess Me₂S in vacuo. The phase diagram of the AlCl₃/Me₂S system⁴ shows both [AlCl₃(Me₂S)] and [AlCl₃(Me₂S)₂], but on removing the volatiles in vacuo from a mixture of AlCl₃ and 3Me₂S, only the 1 : 1 complex was isolated.

The IR spectrum of [AlCl₃(Me₂S)] shows features at 541 and 410 cm⁻¹ assigned to the E and A₁ modes of the pyramidal AlCl₃ unit; both bands are quite broad and the E mode shows some evidence of further splitting. Similar observations were made in the spectra of many of the aluminium complexes in this work, and are probably due to solid state effects, such as lower site symmetry or cation–anion interactions. The ¹H NMR spectrum of [AlCl₃(Me₂S)] in CD₂Cl₂ (295 K) shows a singlet at δ = 2.53 which does not change significantly on cooling the solution to 190 K. Addition of aliquots of Me₂S to the solution produces progressive shifts in the single resonance to low frequency, and on cooling to 185 K, two broad resonances are resolved at δ = 2.24 and 2.20, suggesting ligand exchange is slowing, but that the low temperature limit has not been reached. The latter shift is similar to that of free Me₂S (δ = 2.15), whilst the former is assigned to a new aluminium complex. The ²⁷Al NMR spectra are more informative (Table 1). At ambient temperatures a broad singlet at δ = 111.6 is present in the spectrum of [AlCl₃(Me₂S)], which is little changed on cooling the solution to 190 K. However, addition of 1 mol. equivalent of Me₂S to the solution generates a new resonance at δ = 73.5 (W_1/2 = 650 Hz), and this resonance is unchanged upon addition of more Me₂S and shows only a small low frequency drift on cooling the solution to 190 K. The new ²⁷Al chemical shift is in the range expected for five-coordinate Al species.²¹ The combination of the ¹H and ²⁷Al NMR results show that in the presence of excess Me₂S in CH₂Cl₂ solution the 2 : 1 complex [AlCl₃(Me₂S)₂] forms; further addition of Me₂S does not produce any evidence for a 3 : 1 complex. As noted above, work-up of the solution results in decomposition to reform [AlCl₃(Me₂S)]. Five-coordination is established in the solid state with aluminium–phosphine complexes, e.g. [AlI₃(PEt₃)₂], which has a tbp geometry with axial phosphines.²²

The complexes [AlX₃(Me₂S)] (X = Br or I) are generally similar to the chloride complex, and exhibit progressively lower frequency shifts in the ²⁷Al NMR spectra as the halogen becomes heavier (Table 1). However, although the ¹H NMR spectra show fast exchange with added Me₂S in CH₂Cl₂ solution, no new resonances were evident in the ²⁷Al NMR spectra in the presence of a large excess of Me₂S, indicating that in these cases 2 : 1 complexes do not form.

The [AlX₃(Me₂E)] (X = Cl, Br, E = Se or Te) were obtained in high yields and their ¹H NMR and IR spectroscopic properties are similar to those of the thioether analogues. The ²⁷Al NMR spectra (Table 1) show only small low frequency shifts along the series E = S > Se > Te, and no new complexes are formed by adding excess Me₂E to CH₂Cl₂ solutions of the appropriate [AlX₃(Me₂E)]. [AlCl₃(Me₂Se)] shows a ⁷⁷Se NMR chemical shift of δ = −11.3, which corresponds to a small low frequency coordination shift (Δ = −11.3); this can be compared with small high frequency coordination shifts observed in [GaX₃(Me₂Se)].¹² Although high frequency coordination shifts are seen in most transition metal selenoether (and telluroether) complexes, in p-block complexes both high and low frequency shifts are seen in different systems, and the causes are not understood.²³ We were unable to observe a ⁷⁷Se NMR resonance from [AlBr₃(Me₂Se)] or ¹²⁵Te resonances from [AlX₃(Me₂Te)] over the temperature range 295–190 K, presumably due to fast exchange. The solutions of the selenoether and telluroether complexes develop new resonances on standing, some of which may be due to Me₂E₂, Me₃E⁺ or Me₂EX₂ from their chemical shifts, but given the sensitivity of ⁷⁷Se and ¹²⁵Te chemical shifts to concentration, solvent etc.^21,24 their identification was not pursued. They do, however, provide evidence of the fragility in solution of the AlX₃ complexes with the heavier chalcogenoethers.

In view of the scarcity of aluminium complexes of the heavier chalcogenoethers, structures of three examples, [AlCl₃(Me₂Se)], [AlCl₃(Me₂Te)] and [AlBr₃(Me₂Te)] were determined. The structures (Fig. 1–3) are isomorphous (orthorhombic, Pbcm) and show the expected pseudo-tetrahedral geometry. The bond lengths are mostly unexceptional, although the Al–Te distances in [AlX₃(Me₂Te)] (X = Cl or Br) are the same within experimental error.

Fig. 1 The structure of [AlCl₃(Me₂Se)] showing the numbering scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: a = x, y, 3/2 − z. Selected bond lengths (Å) and angles (°) Se1–Al1 = 2.486(2), Cl1–Al1 = 2.121(2), Cl2–Al1 = 2.1130(14), Cl2–Al1–Cl2a = 115.30(9), Cl2–Al1–Cl1 = 111.32(5), Cl2–Al1–Se1 = 106.47(5), Cl1–Al1–Se1 = 105.20(6).

Fig. 2 The structure of [AlCl₃(Me₂Te)] showing the atom numbering scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: a = x, y, 1/2 − z. Selected bond lengths (Å) and angles (°): Al1–Cl2 = 2.1207(6), Al1–Cl1 = 2.1295(10), Al1–Te1 = 2.6871(9), Cl2–Al1–Cl2a = 115.20(4), Cl2–Al1–Cl1 = 111.14(3), Cl2–Al1–Te1 = 106.25(2), Cl1–Al1–Te1 = 106.21(3).

Fig. 3 The structure of [AlBr₃(Me₂Te)] showing the atom numbering scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: a = x, y, 3/2 − z. Selected bond lengths (Å) and angles (°): Al1–Br1 = 2.286(3), Al1–Br2 = 2.287(2), Al1–Te1 = 2.692(4), Br1–Al1–Br2 = 111.19(9), Br2–Al1–Br2a = 114.01(15), Br1–Al1–Te1 = 106.34(12), Br2–Al1–Te1 = 106.80(9).

Storing a solution of [AlCl₃(Me₂S)] in CH₂Cl₂ in the refrigerator, produced a few small crystals which were identified as the sulfonium salt [Me₂SH][AlCl₄] (Fig. 4), by the X-ray structure solution, and probably formed by adventitious hydrolysis. Solid sulfonium salts are rare, but we have previously obtained examples from serendipitous hydrolysis of some niobium(V) fluoride–thioether complexes,²⁵ the formation being promoted by “anhydrous” conditions and a large weakly coordinating anion. The S–H proton was identified in the difference map of residual electron density. A bulk sample of [Me₂SH][AlCl₄] was obtained by saturating a dry CH₂Cl₂ solution of [AlCl₃(Me₂S)] with HCl gas.

Fig. 4 The structure of [Me₂SH][AlCl₄] showing the S2 centred cation and the H-bond (dashed bond) to an adjacent Cl atom. Ellipsoids are drawn at the 50% probability level and H atoms on C are not shown. The other crystallographically independent cation is similar. Selected bond lengths (Å): Al1–Cl3 = 2.1110(11), Al1–Cl4 = 2.1349(11), Al1–Cl2 = 2.1361(10), Al1–Cl1 = 2.1412(10), S2–H6 = 1.26(3), H6⋯Cl4 = 2.51(3).

The n-butyl derivatives [AlCl₃(ⁿBu₂E)] (E = Se, Te) were also synthesised as potential single source precursors for LPCVD as discussed below. Their characterisation data matched well with that of the equivalent methyl substituted complexes. The telluroether complexes are not stable when stored at room temperature over a period of days to weeks, even in an N₂ filled glove box. They gradually become darker and a quantity of black solid (elemental Te) forms. Samples can be stored at −18 °C for a few weeks with minimal decomposition. A sample of [AlCl₃(ⁿBu₂Te)] that had been stored in the glove box for several weeks and had visibly decomposed was analysed by ESI⁺ mass spectrometry, which showed the presence of [ⁿBu₃Te]⁺, and this ion was also present in the ¹²⁵Te NMR spectrum (δ = 491).¹³

Complexes with bidentate ligands

By analogy with the corresponding GaX₃ adducts,¹² and considering the preference for four-coordination in [AlX₃(R₂E)] adducts described above, it was expected that flexible dithioethers or diselenoethers (L–L) would produce complexes of the type [X₃Al(μ-L–L)AlX₃]. In fact this was only true for the bulky o-xylyl-backboned dithioether, o-C₆H₄(CH₂SEt)₂, which gave yellow crystals of [(AlCl₃)₂{μ-o-C₆H₄(CH₂SEt)₂}]. The crystals were of modest quality and had the characteristics of a modulated structure (see Experimental), and thus comparison of the detailed bond lengths and angles is not warranted. However, they are isomorphous with the gallium(III) analogue,¹³ and serve as an example of this structure type (Fig. 5).

Fig. 5 The structure of [(AlCl₃)₂{o-C₆H₄(CH₂SEt)₂}] showing the atom labelling scheme. The crystals had the properties of a modulated structure and the solution shown was derived from a smaller subcell. For this reason bond lengths and angles are less reliable than usual and not presented here.

The spectroscopic data are consistent with four-coordinate aluminium in solution. The corresponding diselenoether o-C₆H₄(CH₂SeMe)₂ was completely converted to the selenonium cation [o-C₆H₄(CH₂)₂SeMe][AlCl₄] upon reaction with AlCl₃ in CH₂Cl₂ (see ESI†). The same selenonium cation is formed upon reaction of this ligand with GaCl₃ or InCl₃.^12,15

Unexpectedly, reaction of AlX₃ with MeE(CH₂)₂EMe (E = S or Se) failed to give [X₃Al{μ-MeE(CH₂)₂EMe}AlX₃], and the products had an AlX₃ : MeE(CH₂)₂EMe ratio of 1 : 1 irrespective of the ratio of reactants used. Crystals of three examples were grown and showed the presence of pseudo-octahedral cations and tetrahedral anions, [AlX₂{MeE(CH₂)₂EMe}₂][AlX₄] (X = Cl, E = S or Se; X = I, E = S). The structures of [AlCl₂{MeE(CH₂)₂EMe}₂][AlCl₄] (Fig. 6 and 7) reveal centrosymmetric cations (trans isomer) with identical d(Al–Cl), which are as expected longer than in the four-coordinate complexes. The d(Al–S) and d(Al–Se) differ by ∼0.14 Å, which approximates to the difference in covalent radii of the chalcogens.²⁶ The bond angles around the aluminium show only small deviations from 90°.

Fig. 6 The structure of a cation in trans-[AlCl₂{MeS(CH₂)₂SMe}₂][AlCl₄] showing the numbering scheme for the centrosymmetric Al2 centred cation. Ellipsoids are drawn at the 50% probability level and H atoms are not shown for clarity. The other cation is similar. Symmetry operation: a = 2 − x, −y, 1 − z. Selected bond lengths (Å) and angles (°): Al2–Cl5 = 2.2131(10), Al2–S1 = 2.4595(11), Al2–S2 = 2.4809(10), S1–Al2–S2 = 87.92(4), S1–Al2–S2a = 92.08(4), Cl5–Al–S1a = 94.49(2), Cl5–Al2–S1 = 85.51(2), Cl5–Al2–S2a = 86.50(3), Cl5–Al2–S2 = 93.50(3).

Fig. 7 The structure of a cation in trans-[AlCl₂{MeSe(CH₂)₂SeMe}₂][AlCl₄] showing the numbering scheme for the centrosymmetric Al2 centred cation. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. The other crystallographically independent cation is similar. Symmetry operation: a = 1 − x, 1 − y, 1 − z. Selected bond lengths (Å) and angles (°): Al2–Cl5 = 2.211(2), Al2–Se1 = 2.5950(13), Al2–Se2 = 2.6232(13), Se1–Al2–Se2 = 88.11(4), Se1–Al2–Se2a = 91.89(4), Cl5–Al2–Se1a = 94.50(6), Cl5–Al2–Se1 = 85.50(6), Cl5–Al2–Se2a = 86.51(6), Cl5–Al2–Se2 = 93.49(6).

Although both of the structures contain the chalcogenoether in the DL conformation, the two crystals are not isomorphous and in the selenoether complex there are short contacts Se2⋯Se2′ (3.436 Å) and Se3⋯Se3′′ (3.542 Å). These link Al2-centred cations into chains through Se2 (along the a direction) and similarly, the Al3-centred cations are linked into chains through Se3 (again along the a direction) (Fig. 8).

Fig. 8 View along the a axis of the packing diagrams of [AlCl₂{MeS(CH₂)₂SMe}₂][AlCl₄] (left) and [AlCl₂{MeSe(CH₂)₂SeMe}₂][AlCl₄] (right) showing the different molecular packing.

The structure of the [AlI₂{MeS(CH₂)₂SMe}₂][AlI₄] (Fig. 9) also contains a pseudo-octahedral cation, but this has a cis-geometry and with the dithioethers in the meso form. In cis-[AlI₂{MeS(CH₂)₂SMe}₂]⁺ the Al–S_transI are longer by ∼0.1 Å than Al–S_transS, but the Al–S_transS distances are not significantly different to those in trans-[AlCl₂(MeSCH₂CH₂SMe)₂][AlCl₄].

Fig. 9 The structure of the cation in cis-[AlI₂{MeS(CH₂)₂SMe}₂][AlI₄] showing the atom labelling scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Al1–S1 = 2.431(4), Al1–S4 = 2.448(4), Al1–S3 = 2.533(5), Al1–S2 = 2.546(5), Al1–I1 = 2.612(4), Al1–I2 = 2.635(4), I1 Al1–I2 = 97.30(13), S1–Al1–S3 = 84.84(15), S4–Al1–S3 = 83.28(14), S1–Al1–S2 = 83.46(15), S4–Al1–S2 = 79.24(14), S3–Al1–S2 = 76.13(15), S1–Al1–I1 = 99.17(14), S4–Al1–I1 = 89.96(14), S2–Al1–I1 = 93.69(14), S1–Al1–I2 = 94.43(13), S4–Al1–I2 = 101.02(15), S3–Al1–I2 = 92.95(14).

The Nujol mull IR spectra for these salts all confirm the presence of [AlX₄]⁻, but the Al–X stretches of the cations could not be identified with certainty. The solution speciation is less clear, and all the complexes are extremely moisture sensitive in solution. All four complexes show ²⁷Al resonances assignable to the [AlX₄]⁻ (Table 1), but only [AlCl₂{MeS(CH₂)₂SMe}₂][AlCl₄] shows a second resonance (δ²⁷Al = 37.4) which is assigned to the six-coordinate cation. For the other complexes it is likely that the increasing electric field gradients promote fast quadrupolar relaxation, resulting in the loss of the cation resonance.

At room temperature a CD₂Cl₂ solution of [AlCl₂{MeS(CH₂)₂SMe}₂][AlCl₄] shows singlet CH₃ (δ = 2.26) and CH₂ (δ = 3.08) resonances. On cooling to 223 K the spectrum shows three CH₃ resonances (δ = 2.26, 2.50, 2.66) and overlapping CH₂ resonances (δ = 3.08–3.31), which we tentatively assign to a mixture of cis and trans isomers of [AlCl₂{MeS(CH₂)₂SMe}₂]⁺, the changes reversing on warming the solution. The bromo- and iodo-complexes behave similarly. Notably, none of the complexes show resonances due to free dithioether, which would seem to rule out significant amounts of [AlCl₂{MeS(CH₂)₂SMe}]⁺, being present. At 185 K further splitting of the resonances is evident, which is probably due to slowing of the pyramidal inversion at S, leading to separate resonances for the individual invertomers. The solutions decompose slowly on standing.

The trans-[AlCl₂{MeSe(CH₂)₂SeMe}₂][AlCl₄] exhibits a singlet ⁷⁷Se NMR resonance at room temperature (δ = 95.5) which is a low frequency coordination shift (Δ = −25.5) and singlet CH₃ (δ = 2.36) and CH₂ (δ = 3.21) resonances in the ¹H NMR spectrum. Cooling the solution to 190 K produces little change in the ¹H NMR spectrum, although the ⁷⁷Se resonance is lost below ∼240 K. In the selenoether complex only the trans isomer appears to be present in significant amounts. On standing, new resonances grow in due to decomposition. Attempts to record spectra in CD₃CN solution resulted in displacement of the thio- or seleno-ethers by the nitrile.

Reaction of AlCl₃ with the ditelluroether ^tBuTe(CH₂)₃Te^tBu produced a mixture of species resulting from ligand fragmentation. The ¹H NMR spectrum of the product showed multiple resonances for the t-butyl groups and the CH₂ units. Multiple signals were also observed in the ¹²⁵Te NMR spectrum, whilst the ²⁷Al NMR spectrum indicated the presence of [AlCl₄]⁻. Very air sensitive, yellow crystals were isolated of one of the decomposition products, which proved to be [^tBuTe(CH₂)₃Te(^tBu)Te(CH₂)₃Te^tBu][AlCl₄], derived from fragmentation of the ditelluroether (see ESI†).

Polydentates and macrocycles

The reaction of the tripodal trithioether MeC(CH₂SMe)₃ with AlCl₃ in a 1 : 1 molar ratio in anhydrous CH₂Cl₂ gave colourless crystals whose structure (Fig. 10) showed a chain polymer with the ligand binding as a bridging bidentate with one uncoordinated –CH₂SMe arm. The structure forms a chain in the a direction The geometry at aluminium is a distorted trigonal bipyramid with equatorial chlorines and there are two slightly different aluminium environments in the unit cell.

Fig. 10 The structure of the asymmetric unit in [AlCl₃{MeC(CH₂SMe)₃}] showing the atom labelling scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Al1–Cl1 = 2.151(3), Al1–Cl2 = 2.152(3), Al1–Cl3 = 2.154(3), Al1–S5 = 2.473(3), Al1–S2 = 2.489(3), Al2–Cl5 = 2.158(3), Al2–Cl6 = 2.160(3), Al2–Cl4 = 2.169(3), Al2–S3 = 2.469(3), Al2–S6 = 2.488(3), Cl1–Al1–Cl2 = 117.05(12), Cl1–Al1–Cl3 = 118.80(13), Cl2–Al1–Cl3 = 124.15(13), Cl1–Al1–S5 = 94.98(11), Cl2–Al1–S5 = 88.33(11), Cl3–Al1–S5 = 86.31(10), Cl1–Al1–S2 = 95.32(11), Cl2–Al1–S2 = 89.27(11), Cl3–Al1–S2 = 86.49(10), S5–Al1–S2 = 169.38(11), Cl5–Al2–Cl6 = 117.44(13), Cl5–Al2–Cl4 = 119.86(12), Cl6–Al2–Cl4 = 122.70(12), Cl5–Al2–S3 = 94.48(10), Cl6–Al2–S3 = 89.38(11), Cl4–Al2–S3 = 86.84(11), Cl5–Al2–S6 = 90.27(10), Cl6–Al2–S6 = 92.63(10), Cl4–Al2–S6 = 86.71(11), S3–Al2–S6 = 173.30(12).

In solution the ¹H NMR spectrum shows single, sharp –CH₂, –SMe and –CMe resonances consistent with fast exchange. Attempts to isolate complexes with higher AlCl₃ : tripod stoichiometries were unsuccessful. The [AlCl₃{MeC(CH₂SMe)₃}] stoichiometry contrasts with the gallium complex which was [(GaCl₃)₃{MeC(CH₂SMe)₃}], assumed to contain tetrahedral gallium coordination.¹²

The reaction of the small ring thia-macrocycle [9]aneS₃ with an equimolar amount of AlCl₃ in toluene gave a white powder, [AlCl₃([9]aneS₃)]. The complex is insoluble in CH₂Cl₂ and the macrocycle is displaced by coordinating solvents like MeCN, precluding solution measurements. Both gallium and indium form similarly intractable 1 : 1 complexes with this ligand,¹¹ and it seems likely that all three contain fac-octahedral structures. The larger macrocycles [14]aneS₄ and [16]aneSe₄ reacted with two molar equivalents of AlCl₃ in anhydrous CH₂Cl₂ to form insoluble white complexes. The analytical data supported a formulation [AlCl₂(macrocycle)][AlCl₄] (macrocycle = [14]aneS₄ and [16]aneSe₄) and the far IR spectra confirmed the presence of [AlCl₄]⁻ anions. Poor quality crystals of [AlCl₂([14]aneS₄)][AlCl₄] were obtained from the filtrate from the reaction solution and the structure shows the cation with endodentate macrocycle and a cis AlCl₂ unit which serve to confirm the constitution (see ESI†). Although not isomorphous, the structure is similar to that of cis-[InCl₂([14]aneS₄}][InCl₄].¹⁵ In contrast, gallium chloride forms an exodentate chain polymer [GaCl₃([14]aneS₄)] with trigonal bipyramidal coordination.¹³ The larger ring [16]aneSe₄ forms trans-[MCl₂([16]aneSe₄)][MCl₄] (M = Ga or In)¹³ and the aluminium complex probably has an analogous structure. The 16-membered ring macrocycle complexes were insoluble in CH₂Cl₂ and the macrocycle was displaced by stronger donor solvents.

LPCVD investigation

Following previous success in using neutral chalcogenoether adducts of GaCl₃ to deposit thin films of crystalline Ga₂Se₃ and Ga₂Te₃,¹⁷ several coordination complexes of AlCl₃ were synthesised as potential single source precursors to Al₂E₃ films. Ligands with ⁿBu substituents were selected as these had previously been shown to be more effective than ligands with Me substituents, probably because they have the β-hydride elimination route available.¹⁷ [AlCl₃(ⁿBu₂E)] (E = Se, Te) were synthesised as yellow oils as described above. LPCVD using both complexes was attempted at temperatures between 723 and 873 K, using the CVD equipment described previously.¹³ In all cases the precursor evaporated cleanly, having changed colour to dark brown during the evaporation. There was no deposition on the substrates and some elemental selenium or tellurium was deposited on the tube at the exit of the furnace. We conclude that these reagents are unsuitable for LPCVD under these conditions.

Comparisons of Lewis acid behaviour of Al, Ga and In towards chalcogenoethers

There has been much recent interest in exploring Group 13 Lewis acid–Lewis base adducts both by modelling and experiment. The modelling work has mostly focussed on boron and aluminium complexes with O- or N-donor ligands, with trends sometimes extrapolated to gallium. The conclusions of these studies, although sometimes disputed in detail, were that Lewis acidity falls AlCl₃ > AlBr₃ > AlI₃, and AlCl₃ > AlBr₃ > GaCl₃ > GaBr₃ (ref. 27–31 and references therein). One should note in passing that the order with halogen is reversed for boron. The modelling work is based upon gas phase molecules and does not take into account solid state effects (lattice energies, intermolecular interactions and packing effects), or the effects of lattice solvent, which may complicate the interpretation of experimental data, and in some cases lead to apparently anomalous results.³⁰ The various contributions listed above, means that interpreting changes in metal–ligand bond lengths simply in terms of Lewis acidity must be done with care, and one might expect the occasional anomaly, but as a result of recent studies, there are sufficient data available to attempt some comparisons for Al/Ga/In–Group 16 donor complexes. Table 2 shows some illustrative data.

Table 2 Structural data on Group 16 ligand adducts of Al, Ga and In halides

Compound	M–X/Å	M–E/Å	Coordination number of M	Reference
a Terminal M–X.
[AlCl3(Me2Se)]	2.121(2), 2.113(1)	2.486(2)	4	This work
[AlCl3(Me2Te)]	2.1207(6), 2.1295(10)	2.6871(9)	4	This work
[AlBr3(Me2Te)]	2.286(3), 2.287(2)	2.692(4)	4	This work
trans-[AlCl2{MeS(CH2)2SMe}2]^+	2.2131(10)	2.4595(11), 2.4809(10)	6	This work
cis-[AlI2{MeS(CH2)2SMe}2]^+	2.612(4), 2.635(4)	2.431(4)–2.546(5)	6	This work
trans-[AlCl2{MeSe(CH2)2SeMe}2]^+	2.211(2)	2.5950(13), 2.6232(13)	6	This work
[(GaI3)2{MeS(CH2)2SMe}2]	2.4997(7)–2.5316(7)	2.4048(12)	4	12
[(GaCl3)2{o-C6H4(CH2SMe)2}]	2.1478(14)–2.1650(15)	2.3573(15)	4	12
[GaCl3(Me2Se)]	2.1606(8), 2.1700(10)	2.4637(7)	4	12
[GaI3(Me2Se)]	2.5209(2)–2.5277(2)	2.479(2)	4	14
[(GaCl3)2{^nBuSe(CH2)2Se^nBu}]	2.149(2)–2.166(2)	2.468(1)	4	12
[(GaCl3)2{^tBuTe(CH2)3Te^tBu}]	2.158(3)–2.181(3)	2.6378(14), 2.6356(13)	4	17
trans-[GaCl2([16]aneS4]^+	2.276(3)–2.296(3)	2.482(4)–2.518(4)	6	13
trans-[GaCl2([16]aneSe4]^+	2.3058(11), 2.3220(12)	2.5906(7)–2.6106(8)	6	13
[InBr3(Me2Se)]	2.4901(12), 2.4981(7)	2.6455(11)	4	15
[(InCl3)2(Me2Se)4]	2.4243(1), 2.4457(1)^a	2.7722(6), 2.7802(6)	6	14
trans-[InCl2({^iPrS(CH2)2S^iPr}2]^+	2.4237(18), 2.393(18)	2.645(2)–2.696(2)	6	15
trans-[InBr2({^iPrS(CH2)2S^iPr}2]^+	2.5775(5), 2.5918(5)	2.6518(10)–2.6901(11)	6	15
trans-[InBr2{MeSe(CH2)2SeMe}2]^+	2.6076(15), 2.6113(15)	2.7370(16)–2.7762(15)	6	15
trans-[InCl2([16]aneSe4]^+	2.475(2)	2.7189(10), 2.7394(11)	6	13

---

The data show firstly, that if one compares complexes of the same element with the same coordination number, the M–X distances seem unaffected by the specific chalcogen donor type present, which is consistent with the metal–halogen being the dominant interaction. A similar comparison of the M–E bond lengths shows that these increase (sometimes only marginally) with halide, Cl < Br < I, consistent with the trends deduced for lighter donor atoms. The M–X, and M–E bond lengths in comparable complexes of Al and Ga are also nearly identical, consistent with their almost identical covalent radii, resulting from the “3d block contraction”, i.e. the increased nuclear charge resulting from the 3d metals only partially screened by the d electron shell.²⁶ As expected, In–X and In–E bonds are typically ∼0.2 Å longer. A very recent dft study³¹ suggested that whilst Ga and In halide complexes of Me₂Se had a high degree of covalency in the M–Se bonds, those of aluminium had a markedly higher electrostatic component to the bonding. Our experimental data reported in the present paper, show no evidence for a significant change in the bonding type present along the series of group 16 donor complexes, the differences noted being due to the higher Lewis acidity of Al(III). We note that the dft calculations predict³¹ an Al–Se bond length (for the gas phase molecule) of 2.53 Å compared to the X-ray crystallographic result (for the solid) of 2.486(2) Å.

Our data also show the ready formation of six-coordinate cations with aluminium, [AlX₂(L–L)₂]⁺, contrasting with the reluctance of gallium to exceed four-coordination, except in macrocyclic compounds, cannot be due to steric effects, but must be a further consequence of the stronger Lewis acidity of aluminium. These differences must originate in the donor/acceptor orbital energy match (or mis-match) rather than in charge/radius effects. The larger indium centre has a less clear preference, easily accommodating four-, five- or six-coordination depending upon the ligand and reaction conditions.

Conclusions

Chalcogenoether complexes of aluminium(III) halides with four-, six- and (rarely) five-coordinate metal centres have been prepared, and their structures and properties compared with those of the heavier analogues GaX₃ and InX₃. The aluminium complexes are extremely moisture sensitive, and complexes with selenium or tellurium ligands are prone to slow E–C bond cleavage in solution. Nonetheless, the formation and structural characterisation of telluroether complexes of the hard AlX₃ acceptors is notable. In contrast to the gallium complexes, it does not appear that the aluminium systems are suitable for LPCVD applications. The detailed study of Al(III) complexes with soft, modest donor chalcogenoethers has confirmed the trends in Lewis acidity observed with hard O or N donor ligands and are broadly in line with expectations based upon the dft calculations. The work further demonstrates that a significant range of chalcogenoether complexes with hard p-block Lewis acids are obtainable despite the hard/soft- acceptor/donor mismatch.

Experimental

Infrared spectra were recorded as Nujol mulls between CsI plates using a Perkin-Elmer Spectrum 100 spectrometer over the range 4000–200 cm⁻¹. ¹H NMR spectra were recorded using a Bruker AV300 spectrometer. ⁷⁷Se{¹H}, ¹²⁵Te{¹H} and ²⁷Al NMR spectra were recorded using a Bruker DPX400 spectrometer and are referenced to external neat SeMe₂, TeMe₂ and aqueous [Al(H₂O)₆]³⁺ at pH = 1, respectively. Microanalyses were undertaken by Medac Ltd or London Metropolitan University. Solvents were dried prior to use: toluene by distillation from sodium benzophenone ketyl, CH₂Cl₂ from CaH₂, and all preparations were undertaken using standard Schlenk techniques under a N₂ atmosphere. Anhydrous aluminium(III) halides (Aldrich or Strem) were used as received. SMe₂ and SeMe₂ (Aldrich) were stored over molecular sieves. Other thio- seleno- and telluroethers were made by literature methods^32–36 and stored under dinitrogen over molecular sieves.

[AlCl₃(Me₂S)]: Me₂S (0.09 g, 1.5 mmol) was added dropwise to a suspension of AlCl₃ (0.20 g, 1.5 mmol) in anhydrous CH₂Cl₂ (15 mL) with stirring to give a colourless solution. After 30 minutes, all solvent was removed in vacuo to give a colourless oil. Yield 0.26 g, 89%. Anal. Calcd for C₂H₆AlCl₃S: C, 12.3; H, 3.1. Found: C, 12.4; H, 3.1%. ¹H NMR (CDCl₃, 295 K): 2.53 (s). ²⁷Al NMR (CD₂Cl₂, 295 K): 111.6. IR (cm⁻¹): 541 (s), 410 (m). Raman (cm⁻¹): 404 (m) Al–Cl.

[AlBr₃(Me₂S)]: AlBr₃ (0.10 g, 0.37 mmol) was dissolved in toluene (10 mL) to give a pale yellow solution and Me₂S (0.023 g, 0.37 mmol) was added dropwise when the solution became colourless. After stirring for 15 minutes, all solvent was removed in vacuo to give a white solid. Yield 0.10 g, 79%. Anal. Calcd for C₂H₆AlBr₃S: C, 7.3; H, 1.8. Found: C, 6.4; H, 2.0%. ¹H NMR (CD₂Cl₂, 295 K): 2.51 (s). ²⁷Al NMR (CD₂Cl₂, 295 K): 81.2; (185 K): 81.6. IR (cm⁻¹, Nujol): 392 (br) Al–Br.

[AlI₃(Me₂S)]: AlI₃ (0.20 g, 0.5 mmol) was dissolved in toluene (10 mL) to give a pale yellow solution and Me₂S (0.03 g, 0.5 mmol) was added dropwise. After stirring for 15 minutes, all solvent was removed in vacuo to give a pale yellow solid. Yield 0.17 g, 75%. Anal. Calcd for C₂H₆AlI₃S: C, 5.1; H, 1.3. Found: C, 5.0; H, 1.3%. ¹H NMR (CD₂Cl₂, 295 K): 2.44 (s). ²⁷Al NMR (CD₂Cl₂, 295 K): 48.7. IR (cm⁻¹, Nujol): 336 (br) Al–I.

[Me₂SH][AlCl₄]: Me₂S (0.09 g, 1.5 mmol) was added dropwise to a suspension of AlCl₃ (0.20 g, 1.5 mmol) in anhydrous CH₂Cl₂ (25 mL) with stirring to give a colourless solution. HCl was slowly bubbled through the solution for ∼30 seconds. After stirring for 30 minutes, some white precipitate formed. The white solid was collected by filtration and dried in vacuo. The volume of the pale yellow filtrate was reduced to ∼10 mL. Colourless crystals suitable for single crystal X-ray diffraction were obtained after storage of the filtrate at −18 °C for 24 hours. Yield 82%. Anal. Calcd for C₂H₇AlCl₄S: C, 10.4; H, 3.0. Found: C, 10.5; H, 2.9%. IR (cm⁻¹, Nujol): 2482 (br, w) SH, 488 (br), 467 (br), 395 (s) [AlCl₄]⁻; the strong S–H⋯Cl–Al interactions causes extensive splitting of the t₂ Al–Cl stretching mode in the anion.

[AlCl₃(Me₂Se)]: Me₂Se (0.16 g, 1.5 mmol) was added dropwise to a suspension of AlCl₃ (0.2 g, 1.5 mmol) in anhydrous CH₂Cl₂ (15 mL) with stirring to give a colourless solution. After 30 minutes, the volume of solvent was reduced in vacuo to ∼5 mL. Storage at −18 °C for 24 hours produced colourless crystals. Yield 0.30 g, 82%. Anal. Calcd for C₂H₆AlCl₃Se: C, 9.9; H, 2.5. Found: C, 10.0; H, 2.6%. ¹H NMR (CDCl₃, 295 K): 2.92 (s); (283 K): 2.83 (s). ²⁷Al NMR (CDCl₃, 295 K): 110.5. ⁷⁷Se NMR (CDCl₃, 273 K): −11.3; (183 K): −14.0. IR (cm⁻¹, Nujol): 531 (s), 399 (m). Raman (cm⁻¹): 521 (w), 399 (s) Al–Cl.

[AlBr₃(Me₂Se)]: AlBr₃ (0.10 g, 0.37 mmol) was dissolved in toluene (10 mL) to give a pale yellow solution and Me₂Se (0.04 g, 0.37 mmol) was added dropwise when the solution became colourless. After stirring for 15 minutes, all solvent was removed in vacuo to give a white solid. Yield 0.11 g, 80%. Anal. Calcd for C₂H₆AlBr₃Se: C, 6.4; H, 1.6. Found: C, 6.1; H, 1.9%. ¹H NMR (CD₂Cl₂, 295 K): 2.47 (s). ²⁷Al NMR (CD₂Cl₂, 295 K): 99.7. IR (cm⁻¹, Nujol): 446 (s), 397 (s) Al–Br.

[AlCl₃(ⁿBu₂Se)]: A solution of ⁿBu₂Se (0.29 g, 1.5 mmol) in anhydrous CH₂Cl₂ (7 mL) was added dropwise to a suspension of AlCl₃ (0.20 g, 1.5 mmol) in anhydrous CH₂Cl₂ (8 mL) with stirring to give a colourless solution. After 30 minutes, all solvent was removed in vacuo to yield a pale yellow oil. Yield 0.36 g, 73%. Anal. Calcd for C₈H₁₈AlCl₃Se: C, 29.4; H, 5.6. Found: C, 29.7; H, 5.8%. ¹H NMR (CD₂Cl₂, 295 K): 3.03 (t, [4H]), 1.82 (m, [4H]), 1.46 (m, [4H]), 0.97 (t, [6H]). ²⁷Al NMR (CD₂Cl₂, 295 K): 109.9. ⁷⁷Se NMR (CDCl₃, 295 K): 102.9. IR (cm⁻¹): 539 (s), 399 (m) Al–Cl.

[AlCl₃(Me₂Te)]: Me₂Te (0.12 g, 0.75 mmol) was added dropwise to a suspension of AlCl₃ (0.10 g, 0.75 mmol) in anhydrous CH₂Cl₂ (10 mL) with stirring to give a colourless solution. After 30 minutes, the volume of solvent was reduced in vacuo to ∼5 mL. Storage at −18 °C for 24 hours produced yellow crystals. Yield: 0.135 g, 62%. Anal. Calcd for C₂H₆AlCl₃Te: C, 8.3; H, 2.1. Found: C, 8.6; H, 2.1%. ¹H NMR (CD₂Cl₂, 295 K): 2.19 (s). ²⁷Al NMR (CD₂Cl₂, 295 K): 105.9. IR (cm⁻¹, Nujol): 491 (s), 395 (m) Al–Cl.

[AlBr₃(Me₂Te)]: AlBr₃ (0.10 g, 0.37 mmol) was dissolved in toluene (10 mL) to give a pale yellow solution. Me₂Te (0.06 g, 0.37 mmol) was added dropwise to give a paler yellow solution. After stirring for 15 minutes, all solvent was removed in vacuo to give a yellow solid. Yield: 0.11 g, 68%. Anal. Calcd for C₂H₆AlBr₃Te: C, 5.7; H, 1.4. Found: C, 5.7; H, 1.8%. ¹H NMR (CD₂Cl₂, 295 K): 2.66 (s). ²⁷Al NMR (CD₂Cl₂, 295 K): 92.6. IR (cm⁻¹, Nujol): 404 (m), 391 (m) Al–Br.

[AlCl₃(ⁿBu₂Te)]: A solution of ⁿBu₂Te (0.18 g, 0.75 mmol) in anhydrous CH₂Cl₂ (7 mL) was added dropwise to a suspension of AlCl₃ (0.10 g, 0.75 mmol) in anhydrous CH₂Cl₂ (8 mL) with stirring to give a yellow solution. After 15 minutes, all solvent was removed in vacuo to yield a yellow oil. Yield 0.22 g, 78%. Anal. Calcd for C₈H₁₈AlCl₃Te: C, 25.6; H, 4.8. Found: C, 26.1; H, 5.1%. ¹H NMR (CD₂Cl₂, 295 K): 2.95 (t, [4H]), 1.83 (m, [4H]), 1.43 (m, [4H]), 0.96 (t, [6H]). ²⁷Al NMR (CD₂Cl₂, 295 K): 106.8. IR (cm⁻¹): 495 (s), 395 (m) Al–Cl.

[AlCl₂{MeS(CH₂)₂SMe}₂][AlCl₄]: MeS(CH₂)₂SMe (0.18 g, 1.5 mmol) was added dropwise to a suspension of AlCl₃ (0.20 g, 1.5 mmol) in anhydrous CH₂Cl₂ (15 mL) with stirring to give a colourless solution. After 30 minutes the volume of solvent was reduced in vacuo to ∼5 mL. A large quantity of white solid precipitated after storage at −18 °C for 24 hours. The solid was collected by filtration and dried in vacuo. Yield 0.26 g, 68%. Anal. Calcd for C₈H₂₀Al₂Cl₆S₄: C, 18.8; H, 3.9. Found: C, 18.3; H, 3.8%. ¹H NMR (CD₂Cl₂, 295 K): 3.07 (s, [4H]), 2.25 (s, [6H]). ²⁷Al NMR (CDCl₃, 295 K): 105.5, 37.4. IR (cm⁻¹, Nujol): 489 (s), 463 (s), 346 (w). Raman (cm⁻¹): 490 (w), 350 (s), 295 (s).

[AlBr₂{MeS(CH₂)₂SMe}₂][AlBr₄]: MeS(CH₂)₂SMe (0.09 g, 0.75 mmol) was added dropwise to a pale yellow solution of AlBr₃ (0.20 g, 0.75 mmol) in toluene (15 mL) with stirring to give a colourless solution and a white precipitate. After 30 minutes the reaction mixture was filtered and the white solid dried in vacuo. Yield 0.21 g, 72%. Anal. Calcd for C₈H₂₀Al₂Br₆S₄: C, 12.3; H, 2.6. Found: C, 11.7; H, 2.9%. ¹H NMR (CD₂Cl₂, 295 K): 3.10 (s, [4H]), 2.26 (s, [6H]). ²⁷Al NMR (CD₂Cl₂, 295 K): 80.6. IR (cm⁻¹, Nujol): 405 (s, br), 355 (s, br).

[AlI₂{MeS(CH₂)₂SMe}₂][AlI₄]: MeS(CH₂)₂SMe (0.06 g, 0.5 mmol) was added dropwise to a pale yellow solution of AlI₃ (0.20 g, 0.5 mmol) in toluene (15 mL) with stirring to give an orange solution and some pale yellow precipitate. After 30 minutes the reaction mixture was filtered and the pale yellow solid dried in vacuo. Crystals suitable for single crystal X-ray diffraction were obtained after storage of the filtrate at −18 °C for 48 hours. Yield 0.19 g, 74%. Anal. Calcd for C₈H₂₀Al₂I₆S₄: C, 9.1; H, 1.9. Found: C, 8.9; H, 3.3%. ¹H NMR (CD₂Cl₂, 295 K): 2.77 (s, [4H]), 2.17 (s, [6H]). ²⁷Al NMR (CD₂Cl₂, 295 K): −23.0. IR (cm⁻¹, Nujol): 338 (s), 304 (m).

[AlCl₂{MeSe(CH₂)₂SeMe}₂][AlCl₄]: MeSe(CH₂)₂SeMe (0.16 g, 0.75 mmol) was added dropwise to a suspension of AlCl₃ (0.10 g, 0.75 mmol) in anhydrous CH₂Cl₂ (10 mL) with stirring to give a yellow solution. After 30 minutes the volume of solvent was reduced in vacuo to ∼5 mL. Storage of the solution at −18 °C for 24 hours produced colourless crystals. Yield 0.18 g, 69%. Anal. Calcd for C₈H₂₀Al₂Cl₆Se₄: C, 13.8; H, 2.9. Found: C, 12.8; H, 2.7%. ¹H NMR (CDCl₃, 295 K): 3.21 (s, [4H]), 2.36 (s, [6H]). ²⁷Al NMR (CDCl₃, 295 K): 105.4. ⁷⁷Se NMR (CDCl₃, 295 K): 95.4. IR (cm⁻¹, Nujol): 488 (s), 442 (m).

[AlCl₃{MeC(CH₂SMe)₃}]: MeC(CH₂SMe)₃ (0.16 g, 0.75 mmol) was added dropwise to a suspension of AlCl₃ (0.10 g, 0.75 mmol) in anhydrous CH₂Cl₂ (10 mL) with stirring to give a colourless solution. After 30 minutes the volume of solvent was reduced in vacuo to ∼5 mL. Colourless crystals suitable for single crystal X-ray diffraction were obtained after storage at −18 °C for 48 hours. Yield 0.18 g, 70%. Anal. Calcd for C₈H₁₈AlCl₃S₃·1/2CH₂Cl₂: C, 26.4; H, 5.0. Found: C, 26.9; H, 4.9%. ¹H NMR (CDCl₃, 295 K): 2.76 (s, [6H]), 2.25 (s, [9H]), 1.16 (s, [3H]). IR (cm⁻¹, Nujol): 534 (s), 491 (s).

[(AlCl₃)₂{C₆H₄(CH₂SEt)₂}]: o-C₆H₄(CH₂SEt)₂ (0.17 g, 0.75 mmol) was added dropwise to a suspension of AlCl₃ (0.20 g, 1.5 mmol) in anhydrous CH₂Cl₂ (15 mL) with stirring to give a yellow solution. After 30 minutes the volume of solvent was reduced in vacuo to ∼5 mL. Storage of the solution at −18 °C for 24 hours produced yellow crystals. Yield 0.22 g, 60%. Anal. Calcd for C₁₂H₁₈Al₂Cl₆S₂: C, 29.2; H, 3.7. Found: C, 29.3; H, 3.7%. ¹H NMR (CDCl₃, 295 K): 7.47 (m, [4H]), 4.37 (s, [4H]), 3.08 (q, [4H]), 1.49 (t, [6H]). ²⁷Al NMR (CDCl₃, 295 K): 111.3. IR (cm⁻¹, Nujol): 565–500 (vbr, s), 394 (s). Raman (cm⁻¹): 518 (m), 402 (m).

[AlCl₃([9]aneS₃)]: A solution of [9]aneS₃ (0.14 g, 0.75 mmol) in anhydrous toluene (10 mL) was added to a solution of AlCl₃ (0.10 g, 0.75 mmol) in anhydrous toluene (10 mL) with stirring to give a colourless solution and a large quantity of white precipitate. After 60 minutes, the precipitate was collected by filtration and dried in vacuo. Yield 0.16 g, 68%. Anal. Calcd for C₆H₁₂AlCl₃S₃: C, 23.0; H, 3.9. Found: C, 23.1; H, 3.9%. IR (cm⁻¹, Nujol): 408 (s), 375 (m) Al–Cl.

[AlCl₂([14]aneS₄)][AlCl₄]: A solution of [14]aneS₄ (0.10 g, 0.37 mmol) in anhydrous CH₂Cl₂ (10 mL) was added to a suspension of AlCl₃ (0.10 g, 0.75 mmol) in anhydrous CH₂Cl₂ (10 mL) with stirring to give a colourless solution. After approximately 5 minutes a large quantity of white precipitate formed. After 60 minutes, the precipitate was collected by filtration and dried in vacuo. Yield 0.14 g, 70%. Anal. Calcd for C₁₀H₂₀Al₂Cl₆S₄: C, 22.4; H, 3.8. Found: C, 22.6; H, 3.7%. IR (cm⁻¹, Nujol): 481 (br, s), 419 (m).

[AlCl₂([16]aneSe₄)][AlCl₄]: was made similarly from a solution of [16]aneSe₄ (0.18 g, 0.37 mmol) in anhydrous CH₂Cl₂ (10 mL) and AlCl₃ (0.1 g, 0.75 mmol) in anhydrous CH₂Cl₂ (10 mL). The precipitate was collected by filtration and dried in vacuo. Yield 0.20 g, 70%. Anal. Calcd for C₁₂H₂₄Al₂Cl₆Se₄: C, 19.2; H, 3.2. Found: C, 19.0; H, 3.1%. IR (cm⁻¹): 483 (s), 434 (br).

X-Ray experimental

Details of the crystallographic data collection and refinement parameters are given in Table 3. Crystals suitable for single crystal X-ray analysis were obtained as described above. Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073 Å) rotating anode generator with VHF Varimax optics (100 μm focus) with the crystal held at 100 K (N₂ cryostream). Structure solution and refinement were straightforward,^37,38 except as detailed below, with H atoms bonded to C being placed in calculated positions using the default C–H distance. For [AlCl₃{MeC(CH₂SMe)₃}] the data were collected using the Rigaku automated routines which normally gives close to 100% of the data out to 2θ of 55°. For reasons that are not clear this did not happen in this case and it proved difficult to obtain more suitable crystals for a re-collection. Judged by the high R_int value, the data are of modest quality although the intensities seems satisfactory (80% exceed the Shelxl test, I > 2σ(I)). The structure that emerges from the analysis appears sound, with no unusual adp values or other causes for concern. For [(AlCl₃)₂{C₆H₄(CH₂SEt)₂}] the diffraction pattern exhibits many additional reflections in the 100 projection. These are likely due to a modulation. In fact, the Ga analogue¹³ exhibits the same behaviour and in that case it was possible to index the modulated cell. As with the Ga analogue, the data were indexed on a strong sub-cell making refinement of the basic structure possible; however, the ignored reflections result in unrealistic thermal parameters for many of the atoms. For both of these structures therefore, detailed comparisons of the geometric parameters require caution.

Table 3 Crystal data and structure refinement details^a

Compound	[AlCl3(Me2Se)]	[AlCl3(Me2Te)]	[AlBr3(Me2Te)]	[AlCl2{MeS(CH2)2SMe}2] [AlCl4]	[AlCl2{MeSe(CH2)2SeMe}2] [AlCl4]
Formula	C2H6AlCl3Se	C2H6AlCl3Te	C2H6AlBr3Te	C8H20Al2Cl6S4	C8H20Al2Cl6Se4
M	242.36	291.00	424.38	511.14	698.74
Crystal system	Orthorhombic	Orthorhombic	Orthorhombic	Triclinic	Monoclinic
Space group	Pbcm (no. 57)	Pbcm (no. 57)	Pbcm (no. 57)	P1̄ (no. 2)	P21/c (no. 14)
a [Å]	6.205(4)	6.248(1)	6.562(3)	6.708(5)	7.122(4)
b [Å]	12.603(8)	13.114(3)	13.400(5)	10.583(5)	17.744(8)
c [Å]	10.479(6)	10.514(2)	10.724(4)	15.383(5)	17.582(8)
α [°]	90	90	90	90.070(5)	90
β [°]	90	90	90	102.422(5)	93.169(8)
γ [°]	90	90	90	96.512(5)	90
U [Å^3]	819.6(8)	861.4(3)	943.0(6)	1059.2(10)	2218.5(18)
Z	4	4	4	2	4
μ(Mo Kα) [mm^−1]	5.564	4.388	15.870	1.276	7.405
Total no. reflns	4909	3839	8126	11 612	10 277
Unique reflns	1315	1312	1140	6072	5039
R int	0.103	0.018	0.297	0.026	0.114
No. of params, restraints	38, 0	38, 0	38, 0	188, 0	188, 0
R 1 [Io > 2σ(Io)]	0.055	0.018	0.065	0.046	0.077
R 1 [all data]	0.065	0.022	0.068	0.059	0.091
wR2^b [Io > 2σ(Io)]	0.137	0.034	0.163	0.072	0.196
wR2 [all data]	0.148	0.035	0.168	0.077	0.207

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Compound	[AlI2{MeS(CH2)2SMe}2] [AlI4]	[{o-C6H4(CH2SEt)2}(AlCl3)2]	[AlCl3(MeC(CH2SMe)3)]	[Me2SH][AlCl4]
a Common items: temperature = 100 K; wavelength (Mo-Kα) = 0.71073 Å; θ(max) = 27.5°. b R 1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo^2 − Fc^2)^2/∑wFo^4]^1/2.
Formula	C8H20Al2I6S4	C12H18Al2Cl6S2	C16H36Al2Cl6S6	C2H7AlCl4S
M	1059.84	493.04	687.47	231.92
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c (no. 14)	C2/c (no. 15)	P21/c (no. 14)	P21/c (no. 14)
a [Å]	14.825(3)	18.745(2)	16.131(7)	14.3783(7)
b [Å]	12.181(2)	10.810(10)	8.310(3)	10.9075(7)
c [Å]	15.150(4)	13.608(12)	24.976(11)	12.3742(5)
α [°]	90	90	90	90
β [°]	108.449(8)	125.779(1)	107.156(8)	90.260(3)
γ [°]	90	90	90	90
U [Å^3]	2595.3(10)	2237(3)	3199(2)	1940.64(17)
Z	4	4	4	8
μ(Mo Kα) [mm^−1]	7.563	1.026	0.991	1.443
Total no. reflns	24 085	8120	10 176	11 008
Unique reflns	5928	2199	4613	4432
R int	0.152	0.041	0.133	0.035
No. of params, restraints	185, 0	100, 3	279, 0	153, 0
R 1 [Io > 2σ(Io)]	0.057	0.108	0.085	0.045
R 1 [all data]	0.135	0.121	0.103	0.055
wR2^b [Io > 2σ(Io)]	0.106	0.258	0.193	0.156
wR2 [all data]	0.115	0.277	0.207	0.189

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Acknowledgements

We thank the CMSD at STFC and the EPSRC for support, and Dr M. Webster for assistance with the X-ray data analyses. We also thank Professor J. M. Dyke for helpful discussions.

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Footnotes

† Electronic supplementary information (ESI) available: Structural data on [o-C₆H₄CH₂Se(Me)CH₂][AlCl₄], [AlCl₂([14]aneS₄)][AlCl₄] and [^tBuTe(CH₂)₃Te(^tBu)Te(CH₂)₃Te^tBu][AlCl₄]. CCDC 966342–966353. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52991f

‡ [14]aneS₄ = 1,4,8,11-tetrathiacyclotetradecane, [16]aneSe₄ = 1,5,9,13-tetraselanacyclohexadecane, [16]aneS₄ = 1,5,9,13-tetrathiacyclohexadecane, [9]aneS₃ = 1,4,7-trithiacyclononane.

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