Phosphine complexes of aluminium(III) halides – preparation and structural and spectroscopic systematics

Abstract

Six-coordinate pseudo-octahedral complexes trans-[AlX₂(L–L)₂][AlX₄] (X = Cl, Br or I; L–L = o-C₆H₄(PMe₂)₂, Me₂P(CH₂)₂PMe₂) are produced from reaction of AlX₃ with the diphosphine in CH₂Cl₂ (X = Cl) or toluene (X = Br or I) solution. Four-coordinate dimers [Cl₃Al(μ-L′–L′)AlCl₃] (L′–L′ = Me₂P(CH₂)₂PMe₂, Cy₂P(CH₂)₂PCy₂), and the tetrahedral cation [AlCl₂{o-C₆H₄(PPh₂)₂}][AlCl₄] were also obtained. Both four- and five-coordinate complexes [AlX₃(PMe₃)] and [AlX₃(PMe₃)₂] could be isolated with PMe₃ depending upon the ratio of reagents used. These extremely moisture sensitive complexes have been characterised by microanalysis, IR and multinuclear NMR (¹H, ³¹P{¹H} and ²⁷Al) spectroscopy. X-ray crystal structures are reported for [AlCl₂{o-C₆H₄(PMe₂)₂}₂][AlCl₄], [AlCl₂{Me₂P(CH₂)₂PMe₂}₂][AlCl₄], [Cl₃Al{μ-Me₂P(CH₂)₂PMe₂}AlCl₃], [Cl₃Al{μ-Cy₂P(CH₂)₂PCy₂}AlCl₃], [AlCl₃(PMe₃)], [AlCl₃(PMe₃)₂], and for the six-coordinate cation complex [AlCl₂{o-C₆H₄(PPh₂)₂}₂][AlCl₄], although a bulk sample of the last could not be isolated. Tertiary arsines (AsPh₃ or AsEt₃) form only 1 : 1 complexes even with excess arsine present. The unstable [AlCl₂{o-C₆H₄(AsMe₂)₂}][AlCl₄] is also described, and shown to decompose rapidly in CH₂Cl₂ solution to form the diquaternised diarsine cation [o-C₆H₄(AsMe₂)₂(CH₂)][AlCl₄]₂, which was fully characterised. Comparisons are drawn with the corresponding gallium(III) systems (Cheng et al., Inorg. Chem., 2007, 46, 7215–7223) and with AlX₃ complexes of Group 16 ligands (George et al., Dalton Trans., 2014, 43, 3637–3648), and it is concluded that the differences between the Al and Ga systems reflect the higher Lewis acidity of aluminium(III) towards soft donor ligands.

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Introduction

Aluminium chloride is an archetypal Lewis acid, widely used in industry and in the laboratory as a catalyst for condensation, polymerisation and isomerisation reactions. It is perhaps best known as a Friedel–Crafts catalyst for alkylations or acylations where the strong affinity for chloride generates incipient carbocations in combination with [AlCl₄]⁻ anions.^1,2 Typically, AlBr₃ and AlI₃ are weaker Lewis acids, but chemically similar. Solid AlCl₃ contains six-coordinate aluminium, but in the melt or when dissolved in inert solvents, dimeric tetrahedral molecules [Cl₂Al(μ-Cl)₂AlCl₂] are present. Solid AlBr₃ is also dimeric, [Br₂Al(μ-Br)₂AlBr₂], whereas AlI₃ is a chain polymer [{I₂Al(μ-I)}_n].³ The extensive coordination chemistry of these three halides, mostly with hard N- or O-donor ligands, continues to attract much effort.³ In contrast, AlF₃ is an inert polymer containing six-coordinate aluminium; few complexes are known.⁴ Complexes of aluminium halides with neutral soft-donor ligands have received much less attention and current knowledge is not systematic.³ We recently reported⁵ complexes with thio-, seleno- and telluro-ether ligands, including examples of four-coordinate [X₃Al(ER₂)] (X = Cl, Br or I; E = S, Se or Te; R = alkyl) and [(AlCl₃)₂{o-C₆H₄(CH₂SEt)₂}], as well as six-coordinate [AlX₂{MeE(CH₂)₂EMe}₂][AlX₄] (E = S or Se). The isolation of the latter was unexpected since gallium(III) halides gave only four-coordinate complexes with the bidentate chalcogenoethers.⁶ Both Al and Ga can achieve six-coordination with thia- and selena-macrocycles.^6,7 There are a substantial number of phosphine and arsine complexes of trialkylaluminiums, and even some examples with stibine and bismuthine ligands; all contain four-coordinate aluminium with a single Group 15 donor per Al centre, even when polydentate ligands are used.⁸ In contrast, complexes with AlX₃ are few, and almost all are of the type [X₃Al(E′R₃)] (E′R₃ = PPh₃,⁹ AsPh₃,¹⁰ AsEt₃,¹⁰ AsMe₃,¹⁰ P(mesityl)₃ ^11a or P(SiMe₃)₃ ^11b) containing tetrahedral aluminium centres.⁸ A rare example of a higher coordination number (five) is found in the trigonal bipyramidal [AlI₃(PEt₃)₂].¹² Surprisingly, no examples with bidentate phosphine or arsine ligands have been prepared. In a previous study with gallium(III) halides,¹³ four-coordination was dominant, either as neutral dimers [X₃Ga{μ-Et₂P(CH₂)₂PEt₂}GaX₃] or [X₃Ga{μ-Ph₂As(CH₂)₂AsPh₂}GaX₃], or tetrahedral cations [GaX₂(L–L)]⁺ (L–L = o-C₆H₄(PPh₂)₂ or o-C₆H₄(AsMe₂)₂). Octahedral cations, [GaX₂{o-C₆H₄(PMe₂)₂}₂]⁺, are only known with o-C₆H₄(PMe₂)₂, which is a very strong σ-donor ligand, has small steric demands and is pre-organised for chelation.¹³ The larger In(III) centre forms both four-coordinate [InX₂(L–L)]⁺ and six-coordinate [InX₂(L–L)₂]⁺ cations, as well as neutral six-coordinate dimers, [In₂X₆(L–L)₂], and very rare six-coordinate iodoanions [InI₄(L–L)]⁻ (L–L = o-C₆H₄(PMe₂)₂, o-C₆H₄(AsMe₂)₂).^8,14

There has also been much recent interest in exploring the relative Lewis acidity among the Group 13 halides. DFT calculations have predicted that Lewis acidity generally falls AlCl₃ > AlBr₃ ≫ AlI₃ and AlCl₃ > AlBr₃ > GaCl₃ > GaBr₃ (gas phase) (ref. 8,9,15 and references therein) and this is supported by experimental data, although in some cases intermolecular interactions or solvate molecules can mask these trends in the solid state.⁹ Here we report systematic studies of AlX₃ (X = Cl, Br or I) complexes with some diphosphine and diarsine ligands and detailed comparisons with the gallium(III) analogues.

Experimental section

All preparations were carried out under rigorously anhydrous conditions via a dry dinitrogen atmosphere and standard Schlenk and glove-box techniques. Anhydrous grade aluminium trihalides were obtained commercially (Aldrich) and used as received. The ligands were obtained commercially (Strem or Aldrich), apart from o-C₆H₄(PMe₂)₂ and o-C₆H₄(AsMe₂)₂ which were made by the literature methods.¹⁶ Solvents were dried by distillation from CaH₂ (CH₂Cl₂, MeCN) or sodium benzophenone ketyl (hexane, toluene). IR spectra were recorded as Nujol mulls between CsI plates using a Perkin Elmer Spectrum 100 spectrometer over the range 4000–200 cm⁻¹. ¹H and ³¹P{¹H} NMR spectra were recorded using a Bruker AV300 or DPX400 spectrometer and referenced to the residual solvent resonance and external 85% H₃PO₄ respectively. ²⁷Al NMR spectra were recorded with a Bruker DPX400 spectrometer and referenced to external [Al(H₂O)₆]³⁺. Microanalytical measurements were performed by Medac Ltd or London Metropolitan University.

trans-[AlCl₂{o-C₆H₄(PMe₂)₂}₂][AlCl₄]

To a suspension of AlCl₃ (0.067 g, 0.51 mmol) in CH₂Cl₂ (5 mL) was added o-C₆H₄(PMe₂)₂ (0.100 g, 0.51 mmol) in CH₂Cl₂ (5 mL) to form a colourless solution, which was allowed to stir for 1.5 h. The solvent volume was reduced to about 3 mL in vacuo. The resulting white precipitate was isolated by filtration and dried in vacuo to give a white powder. Yield: 0.071 g (43%). Small colourless blocks suitable for single crystal X-ray diffraction study were grown from a CH₂Cl₂ solution containing AlCl₃ and o-C₆H₄(PMe₂)₂ in a 2 : 1 mol ratio kept at −18 °C. Anal. Calc. for C₂₀H₃₂Al₂Cl₆P₄: C, 36.2; H, 4.9. Found: C, 36.0; H, 4.8%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.74 (s, [24H], CH₃), 7.67–7.77 (m, [8H], C₆H₄). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −42.8 (sextet, ¹J_PAl = 155 Hz); (253 K): −42.6 (sextet). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 103.3 (s, AlCl₄^–), 0.7 (quintet, ¹J_AlP = 155 Hz); (253 K): 103.4 (s), 0.4 (quintet). IR (Nujol): ν = 488 (vs, AlCl₄^–), 410 (s, AlCl) cm⁻¹.

trans-[AlBr₂{o-C₆H₄(PMe₂)₂}₂][AlBr₄]

AlBr₃ (0.135 g, 0.51 mmol) was dissolved in toluene (5 mL) to form a yellow solution. To this was added o-C₆H₄(PMe₂)₂ (0.100 g, 0.51 mmol) in toluene (5 mL) which immediately led to the solution turning colourless, with evidence of a large amount of white precipitate. The reaction was stirred for 2 h, then the white powder was isolated by filtration and dried in vacuo. Yield: 0.213 g (91%). Anal. Calc. for C₂₀H₃₂Al₂Br₆P₄: C, 25.8; H, 3.5. Found: C, 26.0; H, 3.3%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.80 (s, [24H], CH₃), 7.71–7.78 (m, [8H], C₆H₄). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −42.6 (br s); (223 K): −42.6 (br s). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 80.5 (s, AlBr₄^–), −12.0 (br s); (253 K): 80.5 (s), −11.4 (br s). IR (Nujol): ν = 398 (vs, AlBr₄^–), 326 (m, AlBr) cm⁻¹.

trans-[AlI₂{o-C₆H₄(PMe₂)₂}₂][AlI₄]

This was made similarly from AlI₃ (0.166 g, 0.41 mmol) and o-C₆H₄(PMe₂)₂ (0.080 g, 0.41 mmol) in toluene (10 mL). Yield: 0.231 g (94%). Anal. Calc. for C₂₀H₃₂Al₂I₆P₄: C, 19.8; H, 2.7. Found: C, 20.0; H, 2.8%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.90 (s, [24H], CH₃), 7.79–7.89 (m, [8H], C₆H₄). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −42.3 (br s); (223 K): −42.2 (br s). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −26.5 (s, AlI₄^–); (223 K): −23.9 (s). IR (Nujol): ν = 334 (vs, AlI₄^–) 281 (m, AlI) cm⁻¹.

trans-[AlCl₂{Me₂P(CH₂)₂Me₂}₂][AlCl₄]

To a suspension of AlCl₃ (0.090 g, 0.67 mmol) in CH₂Cl₂ (5 mL) was added Me₂P(CH₂)₂PMe₂ (0.100 g, 0.67 mmol) in CH₂Cl₂ (5 mL). The resulting solution was stirred for 2 h, then the solvent volume was reduced to about 2 mL in vacuo and layered with hexane (2.5 mL), whereupon small colourless blocks suitable for single crystal X-ray diffraction study grew. The crystalline material was then isolated by filtration and dried in vacuo to give a white solid. Yield: 0.145 g (77%). Anal. Calc. for C₁₂H₃₂Al₂Cl₆P₄: C, 25.4; H, 5.7. Found: C, 25.2; H, 5.9%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.44 (br s, [24H], CH₃), 2.00 (br s, [8H], CH₂). ³¹P{¹H} NMR (CD₂Cl₂, 295 K): δ = −40.6 (sextet, ¹J_PAl = 164 Hz). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 103.3 (s, AlCl₄^–), 1.2 (quintet, ¹J_AlP = 164 Hz). IR (Nujol): ν = 478 (vs, br, AlCl₄^–), 377 (s, AlCl) cm⁻¹.

trans-[AlBr₂{Me₂P(CH₂)₂Me₂}₂][AlBr₄]

AlBr₃ (0.133 g, 0.50 mmol) was dissolved in toluene (5 mL) to form a yellow solution. To this was added Me₂P(CH₂)₂PMe₂ (0.075 g, 0.50 mmol) in toluene (5 mL) which caused the immediate precipitation of a white solid. The reaction was stirred for 1 h, then the white powder was isolated by filtration and dried in vacuo. Yield: 0.176 g (85%). Anal. Calc. for C₁₂H₃₂Al₂Br₆P₄: C, 17.3; H, 3.9. Found: C, 17.2; H, 3.9%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.52 (br s, [24H], CH₃), 2.04 (br s, [8H], CH₂). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −38.0 (br s). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 80.4 (s, AlBr₄^–), −12.3 (br s). IR (Nujol): ν = 394 (vs, AlBr₄^–), 326 (m, AlBr) cm⁻¹.

trans-[AlI₂{Me₂P(CH₂)₂Me₂}₂][AlI₄]

AlI₃ (0.204 g, 0.50 mmol) was dissolved in toluene (5 mL) to form a yellow solution. To this was added Me₂P(CH₂)₂PMe₂ (0.078 g, 0.51 mmol) in toluene (5 mL) which caused the immediate precipitation of a white solid. The reaction was stirred for 2 h, then the white powder was isolated by filtration and dried in vacuo. Yield: 0.270 g (97%). Anal. Calc. for C₁₂H₃₂Al₂I₆P₄: C, 12.9; H, 2.9. Found: C, 13.0; H, 3.0%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.63 (br s, [24H], CH₃), 2.09 (br s, [8H], CH₂). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −39.1 (br s); (203 K): −37.7 (br s). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −27.1 (s, AlI₄^–), −33.9 (br s); (203 K): −26.1 (s). IR (Nujol): ν = 337 (vs, AlI₄^–), 285 (s, AlI) cm⁻¹.

[(AlCl₃)₂{μ-Me₂P(CH₂)₂PMe₂}]

To a suspension of AlCl₃ (0.178 g, 1.33 mmol) in CH₂Cl₂ (5 mL) was added Me₂P(CH₂)₂PMe₂ (0.099 g, 0.66 mmol) in CH₂Cl₂ (5 mL). The resulting solution was stirred for 1 h, then the solvent volume was reduced to about 4 mL in vacuo whereupon a white solid precipitated out. The solid was isolated by filtration and dried in vacuo. Yield: 0.055 g. The solid is a mixture of trans-[AlCl₂{Me₂P(CH₂)₂Me₂}₂][AlCl₄] and [(AlCl₃)₂{μ-Me₂P(CH₂)₂PMe₂}]. Small colourless blocks suitable for single crystal X-ray diffraction study were grown from a CH₂Cl₂ solution containing AlCl₃ and Me₂P(CH₂)₂PMe₂ in a 1.5 : 1 ratio kept at −18 °C. Spectroscopic data for this complex obtained from the mixture, the resonances of the trans-[AlCl₂{Me₂P(CH₂)₂Me₂}₂][AlCl₄] present were identical to those listed above: ¹H NMR (CD₂Cl₂, 295 K): δ = 1.90 (m, [12H], CH₃), 2.17 (d, [4H], CH₂). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −31.6 (br s). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 298 K): δ = 109.3 (br s); (253 K): 108.9 (s).

[AlCl₂{o-C₆H₄(PPh₂)₂}][AlCl₄]

To a solution of o-C₆H₄(PPh₂)₂ (0.075 g, 0.17 mmol) in CH₂Cl₂ (10 mL) was added AlCl₃ (0.045 g, 0.34 mmol). The resulting solution was stirred for 1.5 h, then the solvent volume was reduced to about 3 mL in vacuo and hexane (3 mL) was added. The resulting white precipitate was isolated by filtration and dried in vacuo. Yield = 0.035 g (29%). Anal. Calc. for C₃₀H₂₄Al₂Cl₆P₂: C, 50.5; H, 3.4. Found: C, 50.5; H, 3.7%. ¹H NMR (CD₂Cl₂, 295 K): δ = 7.22–7.53 (aromatics). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −8.3 (v br); (180 K): −12.4 (v br). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 103.3 (s, AlCl₄^–). IR (Nujol): ν = 483 (br, s, AlCl₄^–) cm⁻¹. Some small crystals of [AlCl₂{o-C₆H₄(PPh₂)₂}₂][AlCl₄] were obtained from a similar preparation by careful hexane layering.

[(AlCl₃)₂{μ-Cy₂P(CH₂)₂PCy₂}]

To a suspension of AlCl₃ (0.063 g, 0.47 mmol) in CH₂Cl₂ (5 mL) was added a solution of Cy₂P(CH₂)₂PCy₂ (0.101 g, 0.24 mmol) in CH₂Cl₂ (5 mL). The resulting solution was stirred for 1 h, then the solvent volume was reduced to about 3 mL in vacuo. Small colourless blocks suitable for single crystal X-ray diffraction study grew upon cooling the solution to −18 °C. Anal. Calc. for C₂₆H₄₈Al₂Cl₆P₂: C, 45.3; H, 7.0. Found: C, 45.1; H, 7.0%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.32–1.89 (m [44H]), 2.53 (m, [4H], ²J_PH = 13 Hz). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −0.5 (s); (213 K): −1.4 (br). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 110.6 (s); (213 K): 111.3 (s). IR (Nujol): ν = 478 (vs, br), 375 (w, br, AlCl) cm⁻¹.

[AlCl₃(PMe₃)]

To a suspension of AlCl₃ (0.177 g, 1.33 mmol) in CH₂Cl₂ (5 mL) was added PMe₃ (0.100 g, 1.31 mmol) in CH₂Cl₂ (5 mL). The resulting colourless solution was stored at −18 °C and over a few days large, colourless, temperature-sensitive crystals formed. These were separated and dried. The solvent was then removed in vacuo and the resulting white solid was washed with hexane (5 mL), and dried in vacuo. Yield: 0.174 g (63%). Anal. Calc. for C₃H₉AlCl₃P: C, 17.2; H, 4.3. Found: C, 17.2; H, 4.4%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.50 (d, ²J_HP = 10 Hz, CH₃). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −42.6 (br s); (248 K): −42.3 (sextet); (223 K): −42.3 (sextet, ¹J_PAl = 275 Hz). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 108.8 (s); (223 K): 111.4 (d, ¹J_AlP = 275 Hz). IR (Nujol): ν = 482 (vs, br, AlCl) cm⁻¹.

trans-[AlCl₃(PMe₃)₂]

To a suspension of AlCl₃ (0.133 g, 1.00 mmol) in CH₂Cl₂ (5 mL) was added PMe₃ (0.154 g, 2.02 mmol) in CH₂Cl₂ (5 mL). The resulting solution was stirred for 1.5 h, then the solvent was removed in vacuo and the resulting white solid was washed with hexane (6 mL). The hexane was decanted off and the white powder was dried in vacuo. Yield: 0.191 g (67%). Small colourless crystals suitable for single crystal X-ray diffraction study were grown from a concentrated CH₂Cl₂ reaction solution kept at −18 °C. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.28 (d, ²J_HP = 8.0 Hz, CH₃). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −37.9 (s); (253 K): −38.6 (br s); (183 K): −39.7 (broad, ill-defined coupling). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 66.0 (s); (253 K): 61.3 (s). IR (Nujol): ν = 488 (vs, br, AlCl) cm⁻¹.

[AlBr₃(PMe₃)]

To a yellow solution of AlBr₃ (0.175 g, 0.66 mmol) in toluene (3 mL) was added PMe₃ (0.050 g, 0.66 mmol) in toluene (3 mL). The resulting colourless solution was stirred for 1.5 h, then the solvent was removed in vacuo to yield a white powder, which was dried in vacuo. Yield: 0.186 g (83%). Anal. Calc. for C₃H₉AlBr₃P: C, 10.5; H, 2.6. Found: C, 10.6; H, 2.6%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.39 (d, ²J_HP = 9.6 Hz, CH₃). ³¹P{¹H} NMR (toluene-d⁸ toluene, 295 K): δ = −40.9 (sextet, ¹J_AlP = 242 Hz); (CH₂Cl₂–CD₂Cl₂, 193 K): δ = −37.1 (sextet, ¹J_PAl = 259 Hz). ²⁷Al NMR (toluene-d⁸ toluene, 295 K): δ = 101.5 (d, ¹J_AlP = 242 Hz); (CH₂Cl₂–CD₂Cl₂, 193 K): δ = 102.7 (d, ¹J_AlP = 258 Hz). IR (Nujol): ν = 414 (m), 393 (s, AlBr) cm⁻¹.

[AlI₃(PMe₃)]

This was made similarly from AlI₃ (0.282 g, 0.69 mmol) and PMe₃ (0.053 g, 0.69 mmol) in toluene (8 mL). Yield: 0.264 g (78%). Anal. Calc. for C₃H₉AlI₃P: C, 7.4; H, 1.9. Found: C, 7.5; H, 2.0%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.45 (d, ²J_HP = 10.0 Hz, CH₃). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −42.8 (br s); (193 K): −40.8 (sextet, ¹J_PAl = 205 Hz). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 50.2 (br s); (193 K): 52.6 (d, ¹J_AlP = 209 Hz). IR (Nujol): ν = 367 (s), 340 (s, AlI) cm⁻¹.

[AlBr₃(PMe₃)₂]

To a yellow solution of AlBr₃ (0.173 g, 0.65 mmol) in toluene (2 mL) was added PMe₃ (0.100 g, 1.31 mmol) in toluene (3 mL) which immediately led to the solution turning colourless, with formation of a large amount of white precipitate. The reaction was stirred for 45 min, then the solvent volume was reduced to about 1 mL in vacuo. A white powder was isolated by filtration and dried in vacuo for 15 min. Yield: 0.182 g (66%). Anal. Calc. for C₆H₁₈AlBr₃P₂: C, 17.2; H, 4.3. Found: C, 17.1; H, 4.4%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.25 (d, ²J_HP = 7.0 Hz, CH₃). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −37.4 (br s); (223 K): −32.6 (br s); (193 K): −30.3 (s). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 44.5 (br s); (193 K): no resonance. IR (Nujol): ν = 373 (s, AlBr) cm⁻¹.

[AlI₃(PMe₃)₂]

This was made similarly from AlI₃ (0.135 g, 0.33 mmol) and PMe₃ (0.050 g, 0.66 mmol) in toluene (10 mL). Yield: 0.090 g (49%). Anal. Calc. for C₆H₁₈AlI₃P₂: C, 12.9; H, 3.2. Found: C, 12.8; H, 3.3%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.33 (d, ²J_HP = 9.2 Hz, CH₃). ³¹P{¹H} NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = −31.6 (br s); (223 K): −28.1 (br s); (193 K): −27.2 (s). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 4.4 (br s); (193 K): no resonance. IR (Nujol): ν = 324 (s, AlI) cm⁻¹.

The two [AlI₃(AsR₃)] (R = Et or Ph)¹⁰ were made by reaction of a 1 : 1 molar ratio of AlI₃ and AsR₃ in toluene. [AlI₃(AsPh₃)]: ²⁷Al NMR (toluene–toluene-d⁸, 295 K): δ = 33.1 (br s); (203 K): no resonance; [AlI₃(AsEt₃)]: ²⁷Al NMR (toluene–toluene-d⁸, 295 K): δ = 31.9 (s); (243 K): 32.4 (s); (203 K): 31.4 (br s).

[AlCl₂{o-C₆H₄(AsMe₂)₂}][AlCl₄]

To a suspension of AlCl₃ (0.093 g, 0.70 mmol) in toluene (5 mL) was added o-C₆H₄(AsMe₂)₂ (0.101 g, 0.35 mmol) in toluene (5 mL), resulting in a colourless solution with a white solid suspended in it. After the reaction was stirred for 1.5 h the solid was isolated by filtration, washed with hexane and dried in vacuo to give a sticky white solid. Anal. Calc. for C₁₀H₁₆Al₂As₂Cl₆: C, 21.7; H, 2.9. Found: C, 21.6; H, 3.1%. ¹H NMR (CD₂Cl₂, 295 K): δ = 1.76 (s, [12H], CH₃), 7.62–7.70 (m, [4H], C₆H₄). ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K): δ = 103.2 (s, AlCl₄^–); (203 K): 103.2 (s). IR (Nujol): ν = 490 (s, br, AlCl) cm⁻¹.

[o-C₆H₄(AsMe₂)₂(CH₂)][AlCl₄]₂

To a suspension of AlCl₃ (0.094 g, 0.70 mmol) in CH₂Cl₂ (5 mL) was added o-C₆H₄(AsMe₂)₂ (0.100 g, 0.35 mmol) in CH₂Cl₂ (5 mL). Initially a colourless solution formed, with a white solid precipitating out after 15 min stirring. The reaction was stirred for 2.5 h and then allowed to sit overnight, whereupon small colourless crystals suitable for single crystal X-ray diffraction study grew. Over the period of a week the white precipitate slowly redissolved and more colourless crystals grew, which were isolated and dried in vacuo to give a white powder. Yield: 0.052 g (23%). Anal. Calc. for C₁₁H₁₈Al₂As₂Cl₈: C, 20.7; H, 2.8. Found: C, 20.6; H, 2.7%. ESI+ (CH₃CN): m/z 191.2 [M + CH₃CN]²⁺. ¹H NMR (CD₃CN, 295 K): δ = 2.43 (s, [12H], CH₃), 3.43 (s, [2H], CH₂), 8.02–8.15 (m, [4H], C₆H₄). ²⁷Al NMR (CH₃CN–CD₃CN, 295 K): δ = 103.5 (s, AlCl₄^–). IR (Nujol): ν = 478 (vs, br, AlCl₄^–) cm⁻¹.

X-Ray experimental

Details of the crystallographic data collection and refinement parameters are given in Table 1. 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 (70 μm focus) with the crystal held at 100 K (N₂ cryostream). Structure solution and refinements were performed with either SHELX(S/L)97 or SHELX(S/L)2013¹⁷ and were straightforward, except where detailed below. H atoms bonded to C were placed in calculated positions using the default C–H distance and refined using a riding model. Distance restraints were used in [AlCl₃(PMe₃)] to stop hydrogen atoms disordering across the mirror plane. [o-C₆H₄(AsMe₂)₂(CH₂)] [AlCl₄]₂ was treated as a racemic twin with the batch scale of 0.241.

Table 1 X-Ray crystallographic data^a

Compound	[AlCl3(PMe3)]	[AlCl3(PMe3)2]	[AlCl2{o-C6H4(PMe2)2}2][AlCl4]	[AlCl2{o-C6H4(PPh2)2}2][AlCl4]
Formula	C3H9AlCl3P	C6H18AlCl3P2	C20H32Al2Cl6P4	C60H48Al2Cl6P4
M	209.40	285.47	663.00	1159.52
Crystal system	Monoclinic	Orthorhombic	Monoclinic	Triclinic
Space group	P21/m (no. 11)	Pnma (no. 62)	C2/c (no. 15)	P1̄ (no. 2)
a/Å	6.414(3)	9.970(3)	13.278(4)	10.024(3)
b/Å	10.292(4)	10.599(4)	14.545(4)	11.752(4)
c/Å	7.649(4)	13.562(4)	16.554(5)	24.438(8)
α/°	90	90	90	81.926(14)
β/°	113.698(12)	90	104.126(6)	85.52(2)
γ/°	90	90	90	82.25(2)
U/Å^3	462.4(4)	1433.1(8)	3100.2(16)	2818.9(16)
Z	2	4	4	2
μ(Mo-Kα/mm^−1	1.174	0.883	0.828	0.489
F(000)	212	592	1360	1192
Total no. reflns	4220	14 433	14 366	24 430
Unique reflns	1113	1486	3544	12 194
R int	0.056	0.140	0.032	0.076
No. of params, restraints	44, 9	66, 0	151, 0	652, 0
R 1, wR2 [I > 2σ(I)]^b	0.052, 0.124	0.065, 0.087	0.030, 0.065	0.079, 0159
R 1, wR2 (all data)	0.055, 0.125	0.088, 0.091	0.035, 0.067	0.135, 0.193

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Compound	[(AlCl3)2{μ-Me2P(CH2)2PMe2}]	[AlCl2{Me2P(CH2)2PMe2}2][AlCl4]	[(AlCl3)2{μ-Cy2P(CH2)2PCy2}]	[o-C6H4(AsMe2)2(CH2)][AlCl4]2
a Common items: T = 100 K; wavelength (Mo-Kα) = 0.71073 Å; θ(max) = 27.5°. b R 1 = ∑||Fσ| − |Fc||/∑|Fo|; wR2 = [∑w(Fo^2 − Fc^2)^2/∑wFo^2]^1/2.
Formula	C6H16Al2Cl6P2	C12H32Al2Cl6P4	C26H48Al2Cl6P2	C11H18Al2As2Cl8
M	416.82	566.92	689.24	637.65
Crystal system	Monoclinic	Orthorhombic	Triclinic	Orthorhombic
Space group	P21/c (no. 14)	Pbca (no. 61)	P1̄ (no. 2)	Pna21 (no. 33)
a/Å	10.738(6)	19.631(5)	8.251(2)	14.7537(10)
b/Å	7.948(4)	16.157(4)	9.830(2)	14.9361(10)
c/Å	11.772(6)	34.831(8)	11.921(3)	10.9633(8)
α/°	90	90	113.905(8)	90
β/°	113.938(9)	90	91.329(6)	90
γ/°	90	90	103.390(7)	90
U/Å^3	918.2(8)	11 048(5)	852.4(4)	2415.9(3)
Z	2	16	1	4
μ(Mo-Kα/mm^−1	1.182	0.916	0.666	3.719
F(000)	420	4672	362	1248
Total no. reflns	10 169	92 232	8428	19 941
Unique reflns	2093	10 858	3884	4813
R int	0.062	0.1169	0.114	0.049
No. of params, restraints	73, 0	449, 0	163, 0	213, 1
R 1, wR2 [I > 2σ(I)]^b	0.051, 0.076	0.0503, 0.0985	0.0859, 0.1751	0.0409, 0.0954
R 1, wR2 (all data)	0.065, 0.081	0.0871, 0.1148	0.1743, 0.2276	0.0499, 0.1002

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Results and discussion

All syntheses, manipulations and spectroscopic measurements on the aluminium pnictogen complexes require rigorous exclusion of moisture, which otherwise causes displacement of the phosphorus donor from the aluminium by water, and generation of phosphonium salts, [R₂P(H) ∩ PR₂][AlX₄] or [R₂P(H) ∩ PR₂(H)][AlX₄]₂. The latter are readily identified by ¹H, ³¹P{¹H} and ³¹P NMR spectroscopy and during the course of this work X-ray structures were obtained on several examples (see ESI†). The diarsine is very easily displaced from the aluminium by moisture, but protonates less readily. We also found that while CH₂Cl₂ can be used to prepare the complexes with AlCl₃, attempts to use CH₂Cl₂ for the synthesis of AlBr₃ or AlI₃ complexes result in fast Cl/X exchange and incorporation of substantial amounts of chloride. This arises from the ability of the aluminium halides to promote reaction with CH₂Cl₂, as seen in other systems.^5,18 In these cases toluene was used for the syntheses. Once isolated, the pure complexes with AlBr₃ and AlI₃ react only slowly with CH₂Cl₂, and it remains the NMR solvent of choice since it is very weakly coordinating and freezes at 176 K, with data collected immediately from freshly prepared solutions.

Diphosphine complexes

The reaction of o-C₆H₄(PMe₂)₂ with AlCl₃ in CH₂Cl₂, irrespective of the mol ratio of reactants used (1 : 1 or 1 : 2), formed white crystals with a 1 : 1 o-C₆H₄(PMe₂)₂–AlCl₃ stoichiometry (see Scheme 1). These were shown by the X-ray structure determination to be trans-[AlCl₂{o-C₆H₄(PMe₂)₂}₂][AlCl₄], containing a pseudo-octahedral cation (Fig. 1) and the familiar tetrahedral anion. Even with a four-fold excess of AlCl₃, the ³¹P{¹H} and ²⁷Al NMR spectra showed no evidence for the presence of a pseudo-tetrahedral cation, [AlCl₂{o-C₆H₄(PMe₂)₂}]⁺. The crystals are isomorphous with trans-[GaCl₂{o-C₆H₄(PMe₂)₂}₂][GaCl₄],¹³ and the cell dimensions, d(M–P) (M = Al or Ga), <Cl–M–P and <P–M–P are very similar, while d(M–Cl) is ∼0.05 Å longer for M = Ga. We return to these comparisons later. The corresponding reactions with AlBr₃ and AlI₃ also formed only [AlX₂{o-C₆H₄(PMe₂)₂}₂][AlX₄]. They precipitate as fine powders from toluene and slow reaction with CH₂Cl₂ (see above) precludes using this solvent for crystallization for the bromo and iodo complexes, although it was used satisfactorily in the less reactive gallium systems.¹³ The complexes are also decomposed by MeCN with partial displacement of the phosphine. The spectroscopy of the three complexes show that all contain six-coordinate trans isomers; the first examples of six-coordination in aluminium phosphine complexes.⁸

Scheme 1 The types of Al(III) phosphine complexes formed.

Fig. 1 The structure of the centrosymmetric cation trans-[AlCl₂{o-C₆H₄(PMe₂)₂}₂]⁺ 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 − z. Selected bond lengths (Å) and angles (°): Al1–Cl1 = 2.2692(7), Al1–P1 = 2.4745(8), Al1–P2 = 2.4832(7), Cl1–Al1–P1 = 87.703(14), Cl1–Al1–P2 = 87.46(3), P1–Al1–P2 = 81.415(13).

The far-IR spectra contain strong peaks due to the t₂ mode of [AlX₄]⁻ at 488 (X = Cl), 398 (X = Br) or 334 (X = I) cm⁻¹ (ref. 19) and weaker features at lower energy assigned to the a_2u modes in the cations (with local D_4h symmetry) at 410 (X = Cl), 326 (X = Br) or 281 (X = I) cm⁻¹.

Their ¹H NMR spectra in CD₂Cl₂ at 295 K each show single broad δ(Me) resonances without resolved ²J_PH couplings, confirming that only the trans forms are present. Similarly, the ³¹P{¹H} NMR spectra of all three complexes show resonances with modest high frequency coordination shifts which differ very little with X (Table 2). For X = Cl, the ³¹P{¹H} resonance shows a well-defined six line pattern due to coupling to the quadrupolar ²⁷Al nucleus (²⁷Al, I = 5/2, 100%), but for X = Br the coupling is not clearly resolved, and for X = I only a broad singlet (w_1/2 ∼ 500 Hz) is present. The ²⁷Al NMR spectrum of trans-[AlCl₂{o-C₆H₄(PMe₂)₂}₂][AlCl₄] shows a very sharp singlet at δ = 103.4, due to [AlCl₄]⁻ (ref. 20) and a binomial quintet at δ = +0.7 from coupling to four equivalent P atoms in the cation (Fig. 2).

Fig. 2 (a) ²⁷Al NMR of [AlCl₂{o-C₆H₄(PMe₂)₂}₂]⁺ and (b) ³¹P{¹H} NMR of [AlCl₂{o-C₆H₄(PMe₂)₂}₂]⁺ in CH₂Cl₂ at 295 K.

Table 2 Selected NMR spectroscopic data

Complex	δ(^31P{^1H}) 295 K^a (^1JAl–P/Hz)	Δ = δ(^31Pcomplex − δ(^31Pligand)^a (295 K)	δ (^27Al)^a^,^b (295 K)
a CH2Cl2–CD2Cl2 except c., s = singlet, br s = broad singlet, v br = very broad. b ^27Al resonances of [AlX4]^− anions not listed; δ([AlCl4]^−) = 103, δ([AlBr4]^−) = 80.5, δ([AlI4]^−) = −26.5. c Toluene/d^8 toluene.
[AlCl2{o-C6H4(PMe2)2}2][AlCl4]	−42.8 (sextet, 155)	+12.2	0.7 (quintet)
[AlBr2{o-C6H4(PMe2)2}2][AlBr4]	−42.6 (s)	+12.4	−12.0 (s)
[AlI2{o-C6H4(PMe2)2}2][AlI4]	−42.3(s)	+12.7	–
[AlCl2{Me2P(CH2)2Me2}2] [AlCl4]	−40.6 (sextet, 164)	+6.4	1.2 (quintet)
[AlBr2{Me2P(CH2)2Me2}2] [AlBr4]	−38.0 (s)	+9	−12.3 (s)
[AlI2{Me2P(CH2)2Me2}2] [AlI4]	−39.1 (br s)	+8	−33.9 (br s)
[(AlCl3)2{μ-Me2P(CH2)2PMe2}]	−31.6 (br s).	+15.4	109.3 (s)
[AlCl2{o-C6H4(PPh2)2}][AlCl4]	−8.3 (v br)	∼+5	—
[(AlCl3)2{μ-Cy2P(CH2)2PCy2}]	−0.5 (br, s)	−2.7	110.6 (s)
[AlCl3(PMe3)]	−42.6 (br s)	+19.4	108.8 (s)
[AlCl3(PMe3)2]	−37.9 (s)	+24	66.0 (s)
[AlBr3(PMe3)]	−40.9 (sextet, 240)^c	+21	101.5 (d)^c
[AlBr3(PMe3)2]	−37.4 (s)	+24.5	44.5 (s)
[AlI3(PMe3)]	−42.8 (br s)	+19	50.2 (s)
[AlI3(PMe3)2]	−31.6 (br s);	+30.5	4.4 (s)
[AlI3(AsEt3)]	—	—	31.9 (s)^c
[AlI3(AsPh3)]	—	—	33.1 (s)^c

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The NMR spectra are essentially invariant over the temperature range 295–233 K, in contrast to those of the corresponding thioether or selenoether complexes, which show fast dissociative ligand exchange is present at ambient temperatures.⁵trans-[AlBr₂{o-C₆H₄(PMe₂)₂}₂][AlBr₄] similarly shows ²⁷Al resonances at δ = 80.5 (s) assigned to [AlBr₄]⁻ and a broad singlet at δ −12.0 (s) from the cation, whereas for trans-[AlI₂{o-C₆H₄(PMe₂)₂}₂][AlI₄] only the ²⁷Al resonance of the anion (δ = −26.5 (s)) is observed. The loss of coupling and then the loss of the cation resonance along the series X = Cl → Br → I can be ascribed to increasing quadrupolar relaxation rates as the electric field gradients around the Al nucleus increase with the changing donor sets.

The reaction of AlCl₃ with the sterically bulkier o-C₆H₄(PPh₂)₂ in either toluene or CH₂Cl₂ using a variety of conditions and molar ratios, invariably produced white powders with an AlCl₃–o-C₆H₄(PPh₂)₂ stoichiometry of 2 : 1. These showed a strong band in the IR spectrum at 483 cm⁻¹ indicative of [AlCl₄]⁻, leading to the proposed composition, [AlCl₂{o-C₆H₄(PPh₂)₂}][AlCl₄], containing a pseudo-tetrahedral cation. The ²⁷Al NMR spectrum shows only the sharp resonance for the anion over the range 295 to 180 K, whilst the ³¹P{¹H} NMR spectrum (CH₂Cl₂ solution) at ambient temperatures is a very broad feature at δ ∼ −8. On cooling the solution, the latter drifts to low frequency, reaching δ ∼ −12 at 180 K, but remaining broad. Addition of o-C₆H₄(PPh₂)₂ to the solution shows a second resonance at δ = −13.3 (295 K) which is the value for the free diphosphine, showing intermolecular exchange is slow on the NMR timescale at room temperature. Gallium(III) halides form only the [GaX₂{o-C₆H₄(PPh₂)₂}][GaX₄] complexes with this ligand, irrespective of the reaction conditions or ratio of reagents used.^13,21 The reason for the broad ³¹P{¹H} NMR resonance in the aluminium complex is not entirely clear; the X-ray crystal structures of [SnCl₂{o-C₆H₄(PPh₂)₂}]²² and [GeCl₂{o-C₆H₄(PPh₂)₂}]²³ show the diphosphine coordinated in a very asymmetric manner, probably best described as κ¹, and these complexes also exhibit broad ³¹P{¹H} NMR resonances at all temperatures 295–190 K, which were attributed to intramolecular exchange between the ‘free’ and coordinated phosphorus donor groups in the diphosphine. The ³¹P{¹H}NMR spectrum of the aluminium complex also rules out other than occasional traces of the phosphonium species [o-C₆H₄(PPh₂)(PPh₂H)]⁺ or [o-C₆H₄(PPh₂H)₂]²⁺.²⁴ The preparation of [AlCl₂{o-C₆H₄(PPh₂)₂}][AlCl₄] produced a small number of colourless crystals, which were found to be trans-[AlCl₂{o-C₆H₄(PPh₂)₂}₂][AlCl₄], containing a pseudo-octahedral cation, although the bulk product was reproducibly [AlCl₂{o-C₆H₄(PPh₂)₂}][AlCl₄], based upon microanalysis and spectroscopic data. We assume that the trans-[AlCl₂{o-C₆H₄(PPh₂)₂}₂][AlCl₄] crystallised preferentially from a solution in which it was a minor constituent. The structure of the cation is shown in Fig. 3. The d(Al–P) are ∼0.1 Å longer than those in trans-[AlCl₂{o-C₆H₄(PMe₂)₂}₂]⁺, reflecting the relatively weaker donor power of the diphenylphosphino groups, and possible steric hindrance. All attempts to isolate the six-coordinate complex as a bulk product have failed, but its existence was very surprising given the presence of two very bulky ligands chelating to the small aluminium centre, and the absence of similar gallium complexes.

Fig. 3 The structure of the centrosymmetric Al1 centred cation in trans-[AlCl₂{o-C₆H₄(PPh₂)₂}₂][AlCl₄] showing the atom labelling scheme. The C₆ rings are numbered cyclically with only the ipso-carbon atoms labelled and H atoms are excluded for clarity. Ellipsoids are drawn at the 50% probability level. The other cation is similar. Symmetry operation: a = 2 − x, 1 − y, 1 − z. Selected bond lengths (Å) and angles (°): Al1–Cl1 = 2.2222(13), Al1–P1 = 2.5574(15), Al1–P2 = 2.5843(15), P1–Al1–P2 = 78.69(4), Cl1–Al1–P1 = 87.80(5), Cl1a–Al1–P1 = 92.20(5), Cl1–Al1–P2 = 87.13(5), Cl1a–Al1–P2 = 92.87(5).

The reaction of AlX₃ (X = Cl, Br or I) with Me₂P(CH₂)₂PMe₂ in a 1 : 1 mol ratio produces trans-[AlX₂{Me₂P(CH₂)₂PMe₂}₂][AlX₄], which contrasts with the corresponding GaX₃ complexes of diphosphinoethanes which produce only the dimers [X₃Ga{μ-R₂P(CH₂)₂PR₂}GaX₃].¹³ The presence of six-coordinate cations and [AlX₄]⁻ was confirmed by the X-ray structure of trans-[AlCl₂{Me₂P(CH₂)₂PMe₂}₂][AlCl₄] (Fig. 4).

Fig. 4 The structure of the cation in trans-[AlCl₂{Me₂P(CH₂)₂PMe₂}₂][AlCl₄] showing the atom labelling scheme and with H atoms excluded for clarity. Ellipsoids are drawn at the 50% probability level. The other cation is similar. Selected bond lengths (Å) and angles (°): Cl1–Al1 = 2.2911(16), Cl2–Al1 = 2.2959(16), P1–Al1 = 2.4929(16), P2–Al1 = 2.4773(16), P3–Al1 = 2.4863(16), P4–Al1 = 2.4899(16), P3–Al1–P4 = 83.56(5), P2–Al1–P1 = 83.51(5), Cl1–Al1–Cl2 = 179.83(8), Cl1–Al1–P2 = 91.96(6), Cl2–Al1–P2 = 87.88(5), Cl1–Al1–P1 = 89.50(5), Cl2–Al1–P1 = 90.44(5), Cl1–Al1–P3 = 89.03(5), Cl1–Al1–P4 = 91.35(5), Cl2–Al1–P4 = 88.81(5), Cl2–Al1–P3 = 91.02(5).

The multinuclear NMR data obtained for the three trans-[AlX₂{Me₂P(CH₂)₂PMe₂}₂][AlX₄] (Table 2 and Experimental section) show these to be the only forms present in CH₂Cl₂ solution, with the chemical shifts and coupling patterns similar to those discussed above for the complexes of o-C₆H₄(PMe₂)₂. Notably, for trans-[AlI₂{Me₂P(CH₂)₂PMe₂}₂][AlI₄] the ²⁷Al NMR resonance of the cation, δ = −33.9, was observed as a broad singlet, illustrating how the presence or absence of a resonance due to the quadrupolar nucleus varies with very small changes in the coordination environment, which clearly affect the electric field gradient and hence the relaxation rate. The solution NMR data showed no evidence for cis-isomers, whereas [AlX₂{MeS(CH₂)₂SMe}₂]⁺ show mixtures of cis and trans forms in solution at low temperatures, both of which were crystallographically characterised in trans-[AlCl₂{MeS(CH₂)₂SMe}₂][AlCl₄] and cis-[AlI₂{MeS(CH₂)₂SMe}₂][AlI₄].⁵

Reaction of Me₂P(CH₂)₂PMe₂ with AlCl₃ in a 1 : ≤2 mol ratio in CH₂Cl₂ or toluene resulted in a mixture containing varying amounts of trans-[AlCl₂{Me₂P(CH₂)₂PMe₂}₂][AlCl₄] and a second complex identified by a combination of ¹H, ³¹P{¹H} and ²⁷Al NMR spectroscopy as [Cl₃Al{μ-Me₂P(CH₂)₂PMe₂}AlCl₃]. Despite many attempts, we have been unable to isolate a pure bulk sample of the diphosphine-bridged complex, and similar studies with AlBr₃ or AlI₃ showed that only the [AlX₂{Me₂P(CH₂)₂PMe₂}₂][AlX₄] type formed, even with excess AlX₃, although the solids containing excess aluminium halide were extremely moisture sensitive, and prone to forming [Me₂P(CH₂)₂PMe₂H][AlX₄]. A few colourless crystals formed from one reaction mixture containing AlCl₃–Me₂P(CH₂)₂PMe₂ in a 1.5 : 1 mol ratio were found to be of the bridged complex, thus confirming its identity (Fig. 5).

Fig. 5 The structure of centrosymmetric [(AlCl₃)₂{μ-Me₂P(CH₂)₂PMe₂}] showing the atom labelling scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: a = −x, 1 − y, −z. Selected bond lengths (Å) and angles (°): Al1–Cl2 = 2.1166(14), Al1–Cl3 = 2.1203(15), Al1–Cl1 = 2.1284(15), Al1–P1 = 2.4059(14), Cl2–Al1–Cl3 112.15(6), Cl2–Al1–Cl1 = 112.54(6), Cl3–Al1–Cl1 = 110.78(5), Cl2–Al1–P1 = 106.34(5), Cl3–Al1–P1 = 109.97(6), Cl1–Al1–P1 = 104.68(5).

The molecule is centrosymmetric with rather shorter d(Al–P) and d(Al–Cl) than in the corresponding six-coordinate complexes described above, due to the decreased coordination number. Although not isomorphous, it is also generally similar to the structures reported¹³ for [(GaX₃)₂{Et₂P(CH₂)₂PEt₂}] (X = Br or I). The formation of the ligand-bridged dimer in the chloride system alone shows that steric factors from the increased radius of the halide co-ligands are not the driving force, but increasing the cone angle at phosphorus would be expected to favour a lower coordination number at Al. To explore this we examined the AlCl₃ complexes of Et₂P(CH₂)₂PEt₂ and Cy₂P(CH₂)₂PCy₂. The former gave a similar mixture of six- and four-coordinate species to that described and these complexes were not studied further, but the more bulky cyclohexyl-substituted ligand gave only the dimer, [(AlCl₃)₂{μ-Cy₂P(CH₂)₂PCy₂}].

The [(AlCl₃)₂{μ-Cy₂P(CH₂)₂PCy₂}] is extremely sensitive and very readily decomposed by trace moisture, and quite quickly by standing in CH₂Cl₂ solution at room temperature, generating [AlCl₄]⁻ and phosphonium cations. The X-ray structure (Fig. 6) was obtained from a very small crystal, and confirms the centrosymmetric dimer. The d(Al–P), d(Al–Cl), <Cl–Al–Cl and <Cl–Al–P are similar to those in the dimer shown in Fig. 5, showing the bulky cyclohexyl groups do not exert any great steric effects at the four-coordinate aluminium centre.

Fig. 6 The structure of centrosymmetric [(AlCl₃)₂{μ-Cy₂P(CH₂)₂PCy₂}] showing the atom labelling scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: a = 2 − x, 1 − y, 1 − z. Selected bond lengths (Å) and angles (°): Al1–Cl3 = 2.114(3), Al1–Cl1 = 2.123(3), Al1–Cl2 = 2.129(3), Al1–P1 = 2.407(3), Cl3–Al1–Cl1 = 112.98(12), Cl3–Al1–Cl2 = 113.23(12), Cl1–Al1–Cl2 112.00(12), Cl3–Al1–P1 = 106.20(11), Cl–Al1–P1 = 108.76(11), Cl2–Al1–P1 = 102.90(10).

The ²⁷Al NMR (CH₂Cl₂–CD₂Cl₂, 295 K) of this complex shows a singlet at δ = 110.6, consistent with a tetrahedral aluminium centre, whilst the ³¹P{¹H} NMR is a singlet at δ = −0.5. The coordination shift (Δ) for this complex is very small and negative (−0.3) which contrasts with the small positive values of Δ seen for the other diphosphine complexes. This observation parallels those observed in trimethylaluminium–phosphine complexes, where small phosphines exhibit small positive coordination shifts, but as the steric bulk of the phosphine increases the Δ values become negative, attributed to steric effects.²⁵

Monophosphine complexes

The reaction of AlCl₃ with PMe₃ in a 1 : 1 molar ratio in anhydrous CH₂Cl₂ forms four-coordinate [AlCl₃(PMe₃)], whilst using a 1 : ≥2 molar ratio produced five-coordinate trans-[AlCl₃(PMe₃)₂]. The X-ray structures of both were determined (Fig. 7 and 8) and reveal the expected C_3v and D_3h geometries respectively. The d(Al–Cl) and d(Al–P) distances are ∼0.07 Å longer in the five-coordinate complex, attributed to the increased coordination number. The d(Al–P) in trans-[AlCl₃(PMe₃)₂] is also shorter (by 0.06 Å) than that in trans-[AlI₃(PEt₃)₂],¹² consistent with the rather weaker Lewis acidity expected for aluminium iodide.

Fig. 7 The structure of [AlCl₃(PMe₃)] showing the atom labelling scheme. The molecule has a mirror plane. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: a = x, 1/2 − y, z. Selected bond lengths (Å) and angles (°): Al1–Cl1 = 2.1291(15), Al1–Cl2 = 2.130(2), Al1–P1 = 2.392(2), Cl1–Al1–Cl1a = 111.15(10), Cl1–Al1–Cl2 = 111.06(6), Cl1–Al1–P1 = 107.22(6), Cl2–Al1–P1 = 108.94(9).

Fig. 8 The structure of trans-[AlCl₃(PMe₃)₂] showing the atom numbering scheme. The molecule lies on a mirror plane going through C4, P2, Al1, Cl2, P1, C1. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Symmetry operation: a = x, 1/2 − y, z. Selected bond lengths (Å) and angles (°): Al1–Cl2 = 2.203(2), Al1–Cl1 = 2.2029(14), Al1–P1 = 2.452(2), Al1–P2 = 2.459(2), Cl2–Al1–Cl1 = 121.22(5), Cl1–Al1–Cl1 = 117.56(9), P1–Al1–P2 = 177.31(9), Cl2–Al1–P1 88.79(8), Cl1–Al1–P1 = 90.44(6), Cl2–Al1–P2 88.52(8), Cl1–Al1–P2 = 90.96(6).

The ³¹P{¹H} NMR spectrum of [AlCl₃(PMe₃)] at 295 K in CH₂Cl₂–CD₂Cl₂ exhibits a broad resonance at δ = −42.6, which on cooling the solution, resolves into a six-line coupling pattern with ¹J_PAl = 275 Hz. The corresponding ²⁷Al NMR spectrum at 295 K is a singlet at δ = 108.8, which splits into a doublet with the same ¹J_PAl on cooling. In contrast, the ³¹P{¹H} NMR spectrum of [AlCl₃(PMe₃)₂] shows ill-defined coupling even at low temperatures, whilst the ²⁷Al NMR spectrum is a singlet at δ = 66.0 (295 K). Examination of the ³¹P{¹H} spectra of either complex in CH₂Cl₂ solution containing added PMe₃, shows singlets at room temperature due to fast exchange between the ‘free’ and coordinated phosphine, but on cooling the exchange slows and separate resonances for the complex and PMe₃ are observed. Even with a large excess of PMe₃ there was no evidence for the formation of a six-coordinate species such as [AlCl₃(PMe₃)₃] or [AlCl₂(PMe₃)₄]⁺.

The corresponding reactions of PMe₃ with AlX₃ (X = Br or I) were carried out in toluene, since in CH₂Cl₂ rapid formation of aluminium chloro-species was observed, as discussed. Depending on the PMe₃ : AlX₃ ratio used, either [AlX₃(PMe₃)] or [AlX₃(PMe₃)₂] were isolated as very moisture sensitive white powders. The pure complexes dissolve in CD₂Cl₂ and NMR spectra can be obtained from the freshly prepared solutions without significant Cl/X exchange. However, if some AlX₃ is present (or added to the solution) rapid halide scrambling occurs. The spectroscopic properties of the bromide and iodide complexes (Experimental section) are much as expected and the NMR spectra (Table 2) show systematic chemical shift changes with the aluminium coordination number and the halogen present.

Diarsine complexes

A series of arsine complexes [AlX₃(AsR₃)] (X = Cl, Br or I; R = Me, Et or Ph) have been described, along with the X-ray structures of [AlX₃(AsPh₃)] (X = Cl or I).¹⁰ These show the presence of the expected pseudo-tetrahedral molecules. In passing we note that ²⁷Al NMR spectra reported (obtained from in situ reaction mixtures in CH₂Cl₂) for X = Br or I, show resonances in the regions typical of chloro-species (δ(²⁷Al) = 100–112), indicating Cl/X exchange with the solvent had most likely occurred. To confirm this we obtained ²⁷Al NMR spectra for [AlI₃(AsR₃)] prepared and recorded in toluene solution, which showed δ = 33.1 (R = Ph) or δ = 31.9 (s) (R = Et), similar to the values reported for the phosphine complexes (above). We also found that addition of AsEt₃ to a toluene solution of [AlI₃(AsEt₃)] failed to generate any new resonance, indicating that a five-coordinate complex did not form, which is in contrast to the phosphine systems.

The reaction of AlCl₃ with the diarsine, o-C₆H₄(AsMe₂)₂, in toluene solution gave a slightly sticky white solid with an AlCl₃ : o-C₆H₄(AsMe₂)₂ composition of 2 : 1, which we formulate as [AlCl₂{o-C₆H₄(AsMe₂)₂}][AlCl₄] based upon IR and ²⁷Al NMR spectroscopic evidence for the presence of the anion. Storing the CH₂Cl₂ solution produced from reacting AlCl₃ with o-C₆H₄(AsMe₂)₂ in a 2 : 1 molar ratio for a few days formed a modest yield (23%) of colourless crystals. These were identified as [o-C₆H₄(AsMe₂)₂(CH₂)][AlCl₄]₂, in which the diarsine has been diquaternized by the solvent, yielding a dication with a CH₂ bridge between the arsenic centres. Spectroscopic data for the cation are reported in the Experimental section and the structure is shown in Fig. 9. The dimensions are unexceptional, but the formation of this species again shows that the reactivity of the aluminium halide systems is not limited to ligand coordination. The diphosphine analogue, [o-C₆H₄(PMe₂)₂(CH₂)]²⁺ was obtained in (failed) attempts to make phosphine complexes of SnF₂, and may be synthesised from the diphosphine and CH₂I₂ in toluene.²² A related diarsine cation with an oxido bridge, [o-C₆H₄(AsMe₂Cl)(μ-O)AsMe₂]⁺ is formed by hydrolysis of [VCl₄{o-C₆H₄(AsMe₂)₂}_x] (x = 1 or 2) or [VOCl₂{o-C₆H₄(AsMe₂)₂}] in thf solution.²⁶

Fig. 9 The structure of the cation in [o-C₆H₄(AsMe₂)₂(CH₂)][AlCl₄]₂ showing the atom numbering scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): As1–C2 = 1.907(6), As1–C5 = 1.910(5), As1–C1 = 1.910(7), As1–C11 = 1.968(7), As2–C3 = 1.906(6), As2–C10 = 1.907(5), As2–C4 = 1. 910(7), As2–C11 = 1.969(7), As1–C11–As2 = 104.2(3).

Comparisons with gallium systems

Gallium(III) halides form four-coordinate [GaX₃(PR₃)], [X₃Ga(μ-L–L)GaX₃] or [GaX₂(L–L)]⁺ with most phosphines and diphosphines (L–L), with six-coordination observed only with o-C₆H₄(PMe₂)₂, in the cations [GaX₂{o-C₆H₄(PMe₂)₂}₂]⁺.^8,13 In contrast, as described above, aluminium(III) halides can produce four-, five- or six-coordinate complexes depending upon the phosphine present. In p-block chemistry coordination numbers and geometries are controlled by a combination of steric and electronic effects, and the structural and composition data reported above, combined with literature data,^8–13,21 should allow some elucidation of the factors involved. Considering steric effects first, the single bond covalent radii of Al(III) and Ga(III) are quoted in standard texts as very similar (∼1.25 Å), although the ionic radius of Ga³⁺ is ∼0.07 Å larger than that of Al³⁺.²⁷ In a study of complexes with Group 16 donor ligands, we found that for comparable complexes, d(M–X) and d(M–E) (E = S, Se, Te) were very similar for M = Al or Ga and typically ∼0.2 Å longer for M = In.⁵ DFT calculations predict that the bond lengths involving neutral ligands for a fixed metal (Al, Ga or In) generally increase (for gas phase molecules) Cl < Br < I, reflecting decreasing Lewis acidity down the series.^5,7–9 However, in some cases solid state effects (packing effects, intermolecular interactions, hydrogen bonding or interaction with solvent molecules) can mask this trend.⁹ In order to explore the issue of size between Al and Ga centres, we compared d(M–X) (X = Cl, Br or I) for the four-coordinate [MX₄]⁻ ions, with data taken from the Cambridge Structural Database.²⁸ The results are shown in Table 3 and as histograms in the ESI† and show that the d(M–X) are essentially the same for gallium and aluminium. The fact that aluminium(III) has the same covalent radius to gallium(III) allows us to rule out steric effects as an explanation for the ability of aluminium to achieve a higher coordination number than gallium in many of the phosphine systems described in the present work. The similar size of Al and Ga is explained as being due to the “3d block contraction”, i.e. the increased nuclear charge resulting from insertion of the 3d transition metals, not completely offset by the screening effect of the 3d electrons.

Table 3 Summary of the mean M–X bond lengths (d(M–X) in Å), with associated errors, in [MX₄]⁻

[MX4]^−	X = Cl	X = Br	X = I
M = Al	2.126 ± 0.018	2.289 ± 0.015	2.527 ± 0.012
M = Ga	2.166 ± 0.018	2.321 ± 0.016	2.544 ± 0.018
M = In	2.337 ± 0.022	2.484 ± 0.018	2.702 ± 0.014

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The trend to higher coordination numbers found for aluminium(III) is thus an electronic effect, resulting from higher Lewis acidity towards phosphine ligands, as predicted by the DFT studies.^7,15

Conclusions

The synthesis, structures and spectroscopic characteristics of a range of four-, five, and six-coordinate aluminium complexes with phosphine donor ligands have been described. The contrasts with gallium(III) phosphine chemistry have been highlighted and shown to be due to greater Lewis acidity of the aluminium centre in these systems, consistent with computational predictions.^9,15 In contrast to many d-block metals where analogous phosphine and arsine ligands produce very similar chemistries, the affinity of AlX₃ for arsines is markedly less, and only four-coordinate complexes were identified. This discrimination is also seen in complexes of other light p-block elements; SiX₄ (X = Cl, Br or I) form phosphine complexes but do react with AsMe₃ or o-C₆H₄(AsMe₂)₂,²⁹ whilst GeCl₄ produces the structurally authenticated trans-[GeCl₄(AsR₃)₂] (R = Me or Et), but does not form a complex with o-C₆H₄(AsMe₂)₂.³⁰ The high reactivity of AlX₃ also contributes to the synthetic challenges in this area, with their ability to activate chlorocarbon solvents toward reaction at the P- or As-centres.

Acknowledgements

We thank EPSRC for support (EP/I033394/1) and Dr M. Webster for helpful discussions. The SCFED Project (http://www.scfed.net) is a multidisciplinary collaboration of British universities investigating the fundamental and applied aspects of supercritical fluids.

References

1. (a) S. Dagorne and S. Bellemin-Laponnaz, in The group 13 metals aluminium, gallium, indium and thallium, ed. S. Aldridge and A. J. Downs, Wiley, NY, 2011, p. 654; (b) J. A. Miller, in The chemistry of aluminium, gallium, indium and thallium, ed. A. J. Downs, Blackie, London, 1993, p. 372.
2. (a) Comprehensive organic synthesis, ed. B. M. Tost, Pergamon, Oxford, 1991, ch. 3, vol. 2, p. 702; (b) T. Ooi and K. Maruoka, in Lewis acids in organic synthesis, ed. H. Yamamoto, Wiley VCH, Weinheim, Germany, 2000, p. 191.
3. (a) G. H. Robinson, in Comprehensive Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier, Oxford, 2004, vol. 3, p. 347; (b) S. Aldridge, in The group 13 metals aluminium, gallium, indium and thallium, ed. S. Aldridge and A. J. Downs, Wiley, NY, 2011, p. 75.
4. S. L. Benjamin, W. Levason and G. Reid, Chem. Soc. Rev., 2013, 42, 1460.
5. K. George, M. Jura, W. Levason, M. E. Light and G. Reid, Dalton Trans., 2014, 43, 3637.
6. C. Gurnani, M. Jura, W. Levason, R. Ratnani, G. Reid and M. Webster, Dalton Trans., 2008, 6274.
7. K. George, M. Jura, W. Levason, M. E. Light, L. P. Ollivere and G. Reid, Inorg. Chem., 2012, 51, 2231.
8. J. Burt, W. Levason and G. Reid, Coord. Chem. Rev., 2014, 260, 65.
9. A. Y. Timoshkin, M. Bodensteiner, T. N. Sevastianova, A. S. Lisovenko, E. I. Davydova, M. Scheer, C. Grassi and A. V. Butlak, Inorg. Chem., 2012, 51, 11602.
10. E. Conrad, J. Pickup, N. Burford, R. McDonald and M. J. Ferguson, Can. J. Chem., 2010, 88, 797.
11. (a) G. Menard and D. W. Stephan Stephan, J. Am. Chem. Soc., 2010, 132, 1796; (b) R. L. Wells, A. T. McPhail, J. A. Laske and P. S. White, Polyhedron, 1994, 13, 2737.
12. A. Ecker and H. Schnoeckel, Z. Anorg. Allg. Chem., 1998, 624, 813.
13. F. Cheng, A. L. Hector, W. Levason, G. Reid, M. Webster and W. Zhang, Inorg. Chem., 2007, 46, 7215.
14. F. Cheng, S. I. Friend, A. L. Hector, W. Levason, G. Reid, M. Webster and W. Zhang, Inorg. Chem., 2008, 47, 9691.
15. F. Bessac and G. Frenking, Inorg. Chem., 2006, 45, 6956.
16. (a) R. D. Feltham, A. Kasenally and R. S. Nyholm, J. Organomet. Chem., 1967, 7, 285; (b) E. P. Kyba, S. T. Liu and R. L. Harris, Organometallics, 1983, 2, 1877.
17. G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2008, 64, 112.
18. (a) R. T. Tjahjanto, M. F. Peintinger, T. Bredow and J. Beck, Eur. J. Inorg. Chem., 2112, 3625; (b) N. Burford, B. W. Royan, R. E. vH. Spence and T. S. Cameron, J. Chem. Soc., Dalton Trans., 1990, 1521.
19. K. Nakamoto, Infrared and Raman spectra of inorganic and coordination compounds, Wiley, NY, 4th edn, 1986.
20. J. Mason, Multinuclear NMR, Plenum, NY, 1987.
21. M. Sigl, A. Schier and H. Schmidbaur, Eur. J. Inorg. Chem., 1998, 203.
22. C. Gurnani, A. L. Hector, E. Jager, W. Levason, D. Pugh and G. Reid, Dalton Trans., 2013, 42, 8364.
23. F. Cheng, A. L. Hector, W. Levason, G. Reid, M. Webster and W. Zhang, Inorg. Chem., 2010, 49, 752.
24. (a) M. Sigl, A. Schier and H. Schmidbaur, Z. Naturforsch., B: Chem. Sci., 1998, 53, 1313; (b) M. Jura, W. Levason, E. Petts, G. Reid, M. Webster and W. Zhang, Dalton Trans., 2010, 10264.
25. A. R. Barron, J. Chem. Soc., Dalton Trans., 1988, 3047.
26. B. M. Gray, A. L. Hector, W. Levason, G. Reid, M. Webster, W. Zhang and M. Jura, Polyhedron, 2010, 29, 1630.
27. (a) M. J. Taylor and P. J. Brothers, in The chemistry of aluminium, gallium, indium and thallium, ed. A. J. Downs, Blackie, London, 1993, p. 111; (b) B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves, J. Echeverria, E. Cremades, F. Barragan and S. Alvarez, Dalton Trans., 2008, 2832.
28. Cambridge Structural Database (CSD version 5.34; latest update = May 2013), accessed on the 21^st January 2014, http://www.ccdc.cam.ac.uk.
29. W. Levason, D. Pugh and G. Reid, Inorg. Chem., 2013, 52, 5185.
30. (a) M. F. Davis, W. Levason, G. Reid, M. Webster and W. Zhang, Dalton Trans., 2008, 353; (b) S. M. Godfrey, I. Mushtaq and R. Pritchard, Dalton Trans., 1999, 1319.

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Footnote

† Electronic supplementary information (ESI) available: The X-ray structural data for [o-C₆H₄(PMe₂)(PHMe₂)]₂[AlX₄][X] (X = Cl or Br) and [Cy₂P(H)(CH₂)₂PCy₂(H)][AlCl₄]₂ and an analysis of M–X bond lengths in [MX₄]⁻ (M = Al, Ga, I) based upon data taken from the Cambridge Structural Database, with derived histograms, are also given. CCDC 1005860–1005870. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02051k

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