Solution complexes of the aluminum halides in acetonitrile and acetonitrile-water studied by high-field 27AI NMR

Abstract

The complexes coexisting in acetonitrile upon dissolution of AIX 3 (X = CI, Br, I) are shown to consist essentially of the pure anionic complex AIX 4 − and mixed hexacoordinated species of type [A1X n S 6− n ] 3− n (S = acetonitrile), all exhibiting distinctly different 27 Al shieldings. The presence of both geometric isomers was unambiguously established for [AICl 2S 4] + and AICl 3S 3, which could be identified on the basis of their characteristic linewidths, dictated by the distinctly different electric field gradient tensor invariants acting on the Al nucleus. The relative populations of the various mixed species are found to be highly dependent upon solute concentration, and the more chloride-rich species become appreciably populated only at high dilution. By contrast only three different hexacoordinated species were identified for AIBr 3 whereas ALL 3 provides only AI(CH 3CN) 6 3+ and All 4 −. Addition of small amounts of water consecutively displaces chloride or bromide ions and acetonitrile from the ligand sphere affording mixed solvates of stoichiometry [AlS n H 2O) 6− n ] 3+ and, with AICl 3, probably also chloride-containing species. All of the complexes described are found to be kinetically inert on the 27Al NMR time scale. Nine out of ten theoretically possible mixed solvates including the cis-trans pairs for [AIS 3(H 2O) 3] 3+ and [AIS 4(H 2O) 2] 3+ were identified.