Organoaluminium complexes with sterically demanding oximato ligands: does a bulky and rigid ligand backbone change the aggregation motif?

Abstract

The four organoaluminium ketoximates [(2,4-dimethylpentane-3-one oximato)AlMe₂]₂ (1), (meso)-[(norcamphor oximato)AlMe₂]₂ (2), {[(R,R)-camphor oximato]AlMe₂}₃ (3) and {[(R,S)-fenchone oximato]AlMe₂}₂ (4) have been prepared by the reactions of the corresponding oximes with trimethylaluminium. All compounds have been fully characterized by means of IR, mass and multi-nuclear NMR spectroscopy (¹H, ¹³C, ²⁷Al) and by elemental analyses. The crystal structures of three of these compounds (2, 3 and 4) were determined, revealing the aggregation motif of organometallic group 13 oximates to vary from the hitherto predominant six-membered dimeric M₂N₂O₂ array to others (six-membered M₃O₃ core ring in the case of the camphor derivative; five-membered M₂NO₂ core ring in the case of the fenchone derivative) so far only found e.g. in hydroxylamino complexes of group 13 metals. Furthermore, complexes 3 and 4 exhibit an unusual behaviour in solution, as indicated by upfield-shifted additional signals in their ²⁷Al NMR spectra, pointing at aluminium atoms in penta- or hexacoordination, respectively.