Bis(tetramethylpiperidino)aluminum Halide Adducts tmp2AlX · Do and Tetrahaloaluminates of Tricoordinated Aluminum Cations [tmp2Al(Do)]AlX4

Abstract

Upon treatment with Lewis bases Do (Do = pyridine bases or THF), the Lewis acids tmp2AlX (X = Cl, Br, I) are converted exclusively to the monoadducts tmp2AlX · Do (2–4). The Al–X bonds of these addition compounds are considerably elongated, indicating a tendency towards the formation of ionic species [tmp2Al(Do)]X. Due to the steric requirements of the bulky tmp ligands, addition of an excess of the Lewis base does not force these compounds to form tetracoordinated aluminum cations [tmp2Al(Do)2]+ or pentacoordinated adducts tmp2AlX · Do2. Attempts to prepare ionic representatives by reaction of tmp2AlX · Do with “ate” complexes of comparatively low nucleophilicity [MY = NaBPh4, AgBPh4, LiB(C6F5)4, AgBF4, AgOtos] result in phenylation products [e.g. tmp2AlPh (5a) and BPh3 · py (5b)] or tetracoordinated addition compounds tmp2AlY · Do (Y = anion). However, addition of one equivalent of AlX3 (X = Br, I) initiates halide abstraction with formation of the ionic [tmp2Al(Do)]AlX4 species 6a–f, as indicated by 27Al-NMR data and conductivity measurements. Solid [tmp2Al(py)]AlI4 (6b) decomposes readily into tmpAII2 and tmpAlI2 · py (7c). Addition of non-polar aliphatic solvents to solutions of [tmp2Al(Do)]AlX4 (6) leads to slow decomposition into tmp2AlX and AlX3 · Do (7a–b, d). This also occurs in polar donor solvents, where compounds AlX3 · Do are favoured due to the formation of penta- or hexacoordinated species AlX3 · Do · Solvn (n = 1, 2). Semiempirical AM1 calculations reveal the gas-phase stability of the tricoordinated bis(tmp)aluminum cation in the salt [tmp2AlPy]AlCl4 as the only representative in a series of calculated aluminum cations [(R2N)2AlPy]AlCl4 (R2N = Me2N, Et2N, iPr2N, tmp). According to these calculations, the stability of a given cation increases when tetrachloroaluminate is replaced by tetraiodoaluminate. Ab initio calculations have been performed on two cations [(H2N)2Al(Do)]+ (Do = NH3, py) and indicate very short Al–N bond lengths owing to ionic bonding contributions.