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 ((1)H, (31)P{(1)H} and (27)Al) 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.
Highlights
Aluminium chloride is an archetypal Lewis acid, widely used in industry and in the laboratory as a catalyst for condensation, polymerisation and isomerisation reactions
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]
We 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
Summary
Aluminium chloride is an archetypal Lewis acid, widely used in industry and in the laboratory as a catalyst for condensation, polymerisation and isomerisation reactions. 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.
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