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-2-01)-6-ketiminopyridines 2-(4'-R-1-C6H3-2'-OH)-6-{CMe=N(2 '',6 ''-i-Pr2C6H3)}C5H3N (R-1 = H (L1(a)-H), Bu-t (L1(b)-H), Cl (L1(c)-H), F (L1(d)-H)) with AlMe3 at elevated temperature and subsequent crystallization from acetonitrile affords the five-coordinate 2-(phenyl-2-olate)-6-(2-amidoprop-2-yppyridine aluminum-methyl complexes [2-(4'-(RC6H3)-C-1-2'-O)-6-{CMe2N(2 '',6 ''-i-Pr2C6H3)}C5H3N]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-C6H3-2'-OH)-6-{CH=N(2 '',6 ''-i-Pr2C6H3)}C3H3N (R-1 = H (L1(e)-H), Bu-t (L1(f)-H), Cl (L1(g)-H)) afford as the major crystallized products [2-(4'-(RC6H3)-C-1-2'-O)-6-{CH(Me))N(2 '',6 ''-i-Pr2C6H3)}C5H3N]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 epsilon-caprolactone employing 1 and 2 in the presence of PhCH2OH 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 But initiators showing the highest activities (1b approximate to 1d > 1a approximate to 1c and 2b > 2a> 2c); in general the CMe2-containing series I were more active than CH(Me)-containing 2 at 30 degrees C. The bimetallic complex {{2-4'-Bu-t-C6H3-2'- O)-6-{CHMeN(2 '',6 ''-i-Pr2C6H3)}C3H3N}AlMe(mu-OMe)AlMe2] (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.