Subtle dihedral angle effect of cycloalkyl-bridged PCCP diphosphine ligands on activity in chromium-catalyzed ethylene tri-/tetramerization

Abstract

Chromium complexes supported by alkyl-bridged PCCP ligands (Ph₂PCH(R¹)-C(R²)PPh₂) demonstrate exceptional catalytic activity and selectivity in ethylene tri-/tetramerization reactions. To elucidate the influence of PCCP ligand geometry on catalytic performance, a series of bisphosphine ligands featuring five- to eight-membered cycloalkane bridges were systematically synthesized. Notably, the catalytic behavior is profoundly dependent on the ring size of the cycloalkane bridge. As the bridge cycloalkane increases in ring size, the chromium complexes exhibit a 3.4 fold enhancement in activity (from 813 kg g⁻¹ Cr per h for the five-membered ring to 2891 kg g⁻¹ Cr per h for the eight-membered variant) alongside a progressive improvement in α-olefin selectivity (total selectivity of 1-C₆ and 1-C₈ from 76.5% to 90.3%). Concurrently, polyethylene formation is dramatically suppressed (from 38.6% to 0.14%). Under optimal conditions, complex bearing ligand 4 achieves a peak activity of 3120 kg g⁻¹ Cr per h with 48.9% 1-C₈ selectivity, 89.9% α-olefin selectivity, and near-complete suppression of polymer. Structural analysis reveals a critical correlation between the ligand backbone dihedral angle and catalytic performance: smaller dihedral angles correlate with higher activity, underscoring the pivotal role of ligand structure in tuning reactivity.