Steric effects in aluminum compounds containing monoanionic potentially bidentate ligands: Toward a quantitative measure of steric bulk

To develop an understanding of the factors that control the strength of the Lewis acid base interactions in five-coordinate compounds of aluminum, dim eric dialkylaluminum compounds [R2Al{mu-O(CH2),ERx'}](2) (n = 2, 3; ERx' = OR', SR', NR2') have been prepared from AlR3 and the appropriate substitute d alcohol, thiol, or amine: [R2Al(mu-OCH2CH2OMe)](2), R = Bu-t (1), Bu-i (2 ), and Et (3); [R2Al(mu-(OCH2CH2OBu)-Bu-n)](2), R = Bu-t (4) and Me (5); [R 2Al-(mu-OCH2CH2CH2OMe)](2), R = Bu-t (6) and Me (7); (Bu-t)(2)Al(mu-(OBu)-B u-n)(mu-OCH2CH2OMe)Al(Bu-t)(2) (11); [R2Al(mu-OCH2CH2SMe)](2), R = Bu-t (12 ), Bu-i (13), Et (14), and Me (15); [R2Al(mu-OCH2CH2CH2SMe)](2), R = Bu-t ( 16) and Me (17); [R2Al(mu-OCH2CH2NMe2)](2), R = Bu-t (18), Bu-i (19). Et (2 0), and H (21); [(Bu-t)(2)Al(mu-OCH2CH2NH2)](2) (22); [(Bu-t)(2)Al(mu-OCH2C H2CH2NH2)](2) (23). The molecular structures of compounds 1, 2, 6, 7, 12, 1 3, 16, 18-22, and [Me2Al(mu-OCH2CH2NMe2)](2) have been determined by XLray crystallography. The solution structures have been probed by C-13 NMR spect roscopy using the alkoxide derivatives, [R2Al(mu-(OBu)-Bu-n)](2), R = Bu-t (8), Bu-i (9), and Et (10), as a comparison, which show that compounds 1-7 and 11-23 exist as an equilibrium mixture between the four-coordinate isome r and its five-coordinate isomer, where K-eq = [4-coord]/[5-coord]. Factors that control the extent of this equilibrium and hence the coordination abo ut aluminum include the steric bulk of the substituents at aluminum (R) and the Lewis base donor (R'), the basicity of the neutral donor group (ERx'), and the chelate ring size (as determined by n). The intramolecular bond di ssociation energies (BDEs) of the Lewis base donor (ERx'), as determined by variable-temperature NMR spectroscopy, for compounds 1, 12, 15, 16, [Me2Al (mu-OCH2CH2OMe)](2), and [R2Al(mu-OCH2CH2NMe2)](2) (2.3-13.2 kJ.mol(-1)) ar e significantly lower than observed for their four-coordinate analogues, R3 Al(ERx+1') (63.8-125.5 kJ.mol(-1)). In addition, the relative donor ability (thioether > ether > amine) is the opposite of that expected. The similari ty in BDE between compound 1 and 11 suggests that no cooperative effects ar e present. Ab initio calculations on the four- and five-coordinate isomers of the model compound [H2Al(mu-OCH2CH2OH)](2) indicate that the surprisingl y weak fifth coordination ligation in [R2Al{mu-O(CH2)(n)-ERx'}](2) is a con sequence of significant steric hindrance between the aluminum alkyl groups (R) and the Lewis base donor group. A quantitative measure of the thermodynamic destabilization that occurs upo n substitution of H with Me or Bu-t is proposed on the basis of the derivat ion of the Lennard-Jones (12,6) potential from solution thermodynamic and X -ray structural data.