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

Dialkylaluminium compounds with bi-functional ligands, [E(CH2)xNR2]– (x = 2, 3; E = S, NR′) have been prepared, and compared to those that contain [O(CH2)xNR2]–, in order to investigate the effect of the anionic termini on the structure of the aluminium compounds, [R2Al{E(CH2)xNR′2}]n. The reaction of R2NCH2CH2SH·HCl with Li[Al(tBu)3Me], formed in situ from Al(tBu)3 with MeLi, yields (tBu)2Al(SCH2CH2NR2), R = Me 1 and Et 2. Reaction of Al(tBu)3 with HN(Me)CH2CH2NMe2 and HN(Me)CH2CH2CH2NMe2 ultimately yields (tBu)2Al[N(Me)CH2CH2NMe2] 3 and (tBu)2Al[N(Me)CH2CH2CH2NMe2] 4, respectively. Reaction of HAl(iBu)2 with HN(Me)CH2CH2NMe2 yields [(iBu)2Al{µ-N(Me)CH2CH2NMe2}]2 5, while [H2Al{µ-N(R)CH2CH2NMe2}]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 been determined by X-ray crystallography. Compounds 1–4 are monomeric with five (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 analogs is due to the relative ligand base strength. However, the formation of monomers in the case of compounds 3 and 4 is found to be due to a combination of the steric bulk of the aluminium alkyl and the geometry at the potentially bridging amide group. These results demonstrate a remote geometric control over the degree of association.