Publication
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. Crystal and mol. structure of these monoadducts were detd. The Al-X bonds of these addn. compds. 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, addn. of an excess of the Lewis base does not force these compds. to form tetracoordinated Al cations [tmp2Al(Do)2]+ or pentacoordinated adducts tmp2AlX.Do2. Attempts to prep. ionic representatives by reaction of tmp2AlX.Do with "ate" complexes of comparatively low nucleophilicity [MY = NaBP4, AgBPh4, LiB(C6F5)4, AgBF4, AgOtos] result in phenylation products (e.g. tmp2AlPh and BPh3.py) or tetracoordinated addn. compds. tmp2AlY.Do (Y = anion). However, addn. of 1 equiv. of AlX3 (X = Br, I) initiates halide abstraction with formation of the ionic [tmp2Al(Do)]AlX4 species, as indicated by 27Al-NMR data and cond. measurements. Solid [tmp2Al(py)]AlI4 decomps. readily into tmpAlI2 and tmpAlI2.py. Addn. of non-polar aliph. solvents to solns. of [tmp2Al(Do)]AlX4 leads to slow decompn. into tmp2AlX and AlX3.Do. This also occurs in polar donor solvents, where compds. AlX3.Do are favored due to the formation of penta- or hexacoordinated species AlX3.Do.Solvn (n = 1, 2). Semiempirical AM1 calcns. 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 calcd. Al cations [(R2N)2A1Py]AlCl4 (R2N = Me2N, Et2N, iPr2N, tmp). According to these calcns., the stability of a given cation increases when tetrachloroaluminate is replaced by tetraiodoaluminate. Ab initio calcns. were performed on 2 cations [(H2N)2Al(Do)]+ (Do = NH3, py) and indicate very short Al-N bond lengths owing to ionic bonding contributions. [on SciFinder (R)]
Arianna Marchioro, Marie Bischoff