Pronounced effect of cycloalkyl substituents in Bis(imino)pyridine cobalt catalysts for high 1-hexene selectivity in propylene oligomerization

Abstract

Bis(imino)pyridine cobalt catalysts offer a tunable platform for selective propylene oligomerization to linear α-olefins, yet the interplay between steric and electronic effects remains incompletely understood. This work systematically modulates the steric environment via cycloalkyl substituents (cyclopropyl, cyclopentyl, cyclohexyl) at the ortho -positions of N -aryl groups and the electronic properties via remote functionalization at the 4-pyridyl and para-N -aryl positions. The ring size of the cycloalkyl group dictates catalytic performance: the cyclopentyl-substituted complex Co-2 delivers the optimal balance of activity (5.47 × 10⁵ g·mol⁻¹·h⁻¹) and 1-hexene selectivity (57.5%), outperforming the cyclohexyl analogue (excessive rigidity favors branched products) and the cyclopropyl analogue (conformational inertia limits turnover). Electronic fine-tuning reveals position-dependent behavior: electron-withdrawing groups at the 4-pyridyl position (Cl, Br) markedly enhance activity but erode regioselectivity, whereas excessive withdrawal (CN) suppresses activity entirely; at the para-N -aryl position, electron-donating groups prove deleterious to both parameters, while moderate withdrawal (F) preserves selectivity with acceptable activity loss. Reaction optimization identifies 20 μmol catalyst loading and an Al/Co molar ratio of 100 as optimal, achieving 62.2% 1-hexene selectivity—the highest reported for this catalyst class. These results establish clear structure–performance relationships, demonstrating that precise steric control through ring-size selection combined with judicious, position-specific electronic tuning enables selective conversion of propylene to linear α-olefins, providing practical design principles for next-generation bis(imino)pyridine cobalt catalysts.