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.