Phenolate Substituent Effects on Ring-Opening Polymerization of ε-Caprolactone by Aluminum Complexes Bearing 2-(Phenyl-2-olate)-6-(1-amidoalkyl)pyridine Pincers

Interaction of the 2-(phenyl-2-ol)-6-ketiminopyridines 2-(4′-R 1 -C 6 H 3 -2′-OH)-6-{CMe═N(2″,6″- i -Pr 2 C 6 H 3 )}C 5 H 3 N (R 1 = H ( L1 a -H), Bu t ( L1 b -H), Cl ( L1 c -H), F ( L1 d -H)) with AlMe 3 at elevated temperature and subsequent crystallization from acetonitrile affords the five-coordinate 2-(phenyl-2-olate)-6-(2-amidoprop-2-yl)pyridine aluminum–methyl complexes [2-(4′-R 1 C 6 H 3 -2′-O)-6-{CMe 2 N(2″,6″- i -Pr 2 C 6 H 3 )}C 5 H 3 N]AlMe(NCMe) (R 1 = H ( 1a ), Bu t ( 1b ), Cl ( 1c ), F ( 1d )), as their acetonitrile adducts, in good yield. In each case, complexation results in concomitant C–C bond formation via methyl migration from aluminum to the corresponding imino unit in L1 -H. On the other hand, reactions of the aldimine-containing compounds 2-(4′-R 1 -C 6 H 3 -2′–OH)-6-{CH═N(2″,6″- i -Pr 2 C 6 H 3 )}C 5 H 3 N (R 1 = H ( L1 e -H), Bu t ( L1 f -H), Cl ( L1 g -H)) afford as the major crystallized products [2-(4′-R 1 C 6 H 3 -2′-O)-6-{CH(Me))N(2″,6″- i -Pr 2 C 6 H 3 )}C 5 H 3 N]AlMe(NCMe) (R 1 = H ( 2a ), Bu t ( 2b ), Cl ( 2c )), in which the migrated methyl group and aluminum–methyl are disposed mutually cis; evidence for the minor trans isomers 2a ′– c ′ is presented. The ring-opening polymerization of ε-caprolactone employing 1 and 2 in the presence of PhCH 2 OH proceeded efficiently, producing polymers of narrow molecular weight distribution with the catalytic activities highly dependent on the nature of the phenolate-containing 4-R 1 substituent with the F and Bu t initiators showing the highest activities ( 1b ≈ 1d > 1a ≈ 1c and 2b > 2a > 2c ); in general the CMe 2 -containing series 1 were more active than CH(Me)-containing 2 at 30 °C. The bimetallic complex [{2-(4′-Bu t -C 6 H 3 -2′-O)-6-{CHMeN(2″,6″- i -Pr 2 C 6 H 3 )}C 5 H 3 N}AlMe(μ-OMe)AlMe 2 ] ( 3 ), the result of adventitious oxygenation, is also reported. The single-crystal X-ray structures are reported for L1 d -H, L1 f -H, 1a – d, 2a, 2b / 2b ′, 2c, and 3 .