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

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.