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, dimeric dialkylaluminum compounds [R2Al{mu-O(CH2),ERx'}](2) (n = 2, 3; ERx' = OR', SR', NR2') have been prepared from AlR3 and the appropriate substituted 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); [R2Al-(mu-OCH2CH2CH2OMe)](2), R = Bu-t (6) and Me (7); (Bu-t)(2)Al(mu-(OBu)-Bu-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 (20), and H (21); [(Bu-t)(2)Al(mu-OCH2CH2NH2)](2) (22); [(Bu-t)(2)Al(mu-OCH2CH2CH2NH2)](2) (23). The molecular structures of compounds 1, 2, 6, 7, 12, 13, 16, 18-22, and [Me2Al(mu-OCH2CH2NMe2)](2) have been determined by XLray crystallography. The solution structures have been probed by C-13 NMR spectroscopy 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 isomer and its five-coordinate isomer, where K-eq = [4-coord]/[5-coord]. Factors that control the extent of this equilibrium and hence the coordination about 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 dissociation 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)) are significantly lower than observed for their four-coordinate analogues, R3Al(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 similarity in BDE between compound 1 and 11 suggests that no cooperative effects are present. Ab initio calculations on the four- and five-coordinate isomers of the model compound [H2Al(mu-OCH2CH2OH)](2) indicate that the surprisingly weak fifth coordination ligation in [R2Al{mu-O(CH2)(n)-ERx'}](2) is a consequence 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 upon substitution of H with Me or Bu-t is proposed on the basis of the derivation of the Lennard-Jones (12,6) potential from solution thermodynamic and X-ray structural data.