Chromium(III) catalysts based on tridentate silicon-bridged tris(diphenylphosphine) ligands for selective ethylene tri-/tetramerization

The chromium(III) catalysts based on tridentate silicon-bridged tris(diphenylphosphine) ligands of the form RSi(CH₂PPh₂)₃ (L¹: R = Me; L²: R = Cy; L³: R = Ph) were investigated for selective ethylene tri-/tetramerization. The steric and electronic properties of the substituents attached to the silicon moiety have been observed to have a great impact on the catalytic performance of these catalysts. Single-crystal analysis, high-resolution mass spectrometry (HR-MS), and elemental analysis revealed that these complexes may adopt mononuclear tridentate coordination mode (k³-P, P, P) with Cr center. However, one of the phosphorus atoms in these complexes may act as a hemilabile donor and presumably loses its coordination with chromium in the presence of a competent donor which consequently transformed the tridentate complexes to bidentate (k²-P, P) coordination mode. Backbone modification of L¹ of the precatalyst 1 via successive abstraction of methylene spacers may offer MeSi(CH₂)_n(PPh₂)₃ (L⁴: n = 2; L⁵: n = 1; L⁶: n = 0) type chromium(III) complexes. Ethylene oligomerization of these systems suggested that the methylene spacers may effectively tune the complex structure and catalytic performance. Precatalyst 1 and 4 respectively based on L¹ and L⁴ deliver 70% C₈ selectivity in the liquid oligomeric fraction and considerable activity under experimental conditions. DFT investigations based on catalyst 4 revealed that the catalyst may simultaneously facilitate the single and double coordination pathways for C₈ formation, however, the single coordination pathway is thermodynamically more favorable for this system.