Aluminium compounds containing bidentate ligands: ligand base strength andremote geometric control over degree of association

Dialkylaluminium compounds with bi-functional ligands, [E(CH3)(x)NR2](-) (x = 2, 3; E = S, NR') have been prepared, and compared to those that contain [O(CH2)(x)NR2](-), in order to investigate the effect of the anionic termi ni on the structure of the aluminium compounds, [R2Al{E(CH2)(x)NR'(2)}](n). The reaction of R2NCH2CH2SH . HCl with Li[Al-(Bu-t)(3)Me], formed in situ from Al(tBu)(3) with MeLi, yields (Bu-t)(2)Al(SCH2CH2NR2), R = Me 1 and Et 2. Reaction of Al(Bu-t)(3) with HN(Me)CH2CH2NMe2 and HN(Me)CH2CH2CH2NMe2 ul timately yields (Bu-t)(2)Al[N(Me)CH2CH2-NMe2] 3 and (Bu-t)(2)Al[N(Me)CH2CH2 CH2NMe2] 4, respectively. Reaction of HAl(Bu-1)(2) with HN(Me)CH2CH2-NMe2 y ields [(Bu-i)(2)Al{mu-N(Me)CH2CH2NMe2}](2) 5, while [H2Al{mu-N(R)CH2CH2NM2} ](2), R = Me 6 and Et 7, are formed from the reaction of AlH3(NMe3) with HN (R)CH2CH2NMe2. The molecular structures of compounds 1, 2, 6 and 7 have bee n determined by X-ray crystallography. Compounds 1-4 are monomeric with fiv e (1-3) and six (4) membered chelate heterocyclic rings. Compound 5 exists as a monomer/dimer equilibrium in solution, in contrast, compounds 6 and 7 exist as bridged dimers. The formation of monomeric chelate structures for compounds 1 and 2, rather than the bridged dimers found for the alkoxide an alogs is due to the relative ligand base strength. However, the formation o f monomers in the case of compounds 3 and 4 is found to be due to a combina tion of the steric bulk of the aluminium alkyl and the geometry at the pote ntially bridging amide group. These results demonstrate a remote geometric control over the degree of association.