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′-R1-C6H3-2′-OH)-6-{CMeN­(2″,6″-i-Pr2C6H3)}­C5H3N (R1 = H (L1a-H),
But (L1b-H), Cl (L1c-H), F (L1d-H)) with AlMe3 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′-R1C6H3-2′-O)-6-{CMe2N­(2″,6″-i-Pr2C6H3)}­C5H3N]­AlMe­(NCMe) (R1 = H (1a), But (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′-R1-C6H3-2′–OH)-6-{CHN­(2″,6″-i-Pr2C6H3)}­C5H3N (R1 = H (L1e-H),
But (L1f-H), Cl (L1g-H)) afford as the major crystallized products [2-(4′-R1C6H3-2′-O)-6-{CH­(Me))­N­(2″,6″-i-Pr2C6H3)}­C5H3N]­AlMe­(NCMe) (R1 = H (2a), But (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 PhCH2OH proceeded efficiently, producing
polymers of narrow molecular weight distribution with the catalytic
activities highly dependent on the nature of the phenolate-containing
4-R1 substituent with the F and But initiators
showing the highest activities (1b ≈ 1d > 1a ≈ 1c and 2b > 2a > 2c); in general the CMe2-containing series 1 were more active than CH­(Me)-containing 2 at 30 °C. The bimetallic complex [{2-(4′-But-C6H3-2′-O)-6-{CHMeN­(2″,6″-i-Pr2C6H3)}­C5H3N}­AlMe­(μ-OMe)­AlMe2] (3), the result of adventitious oxygenation, is also reported. The
single-crystal X-ray structures are reported for L1d-H, L1f-H, 1a–d, 2a, 2b/2b′, 2c, and 3.