Cobalt(II)-based ethylene dimerization catalysts with silicon-bridged diphosphine ligands

A series of ligands and cobalt complexes were synthesized and characterized using NMR spectroscopy, elemental analysis and single-crystal X-ray diffraction. Complex C1 [CoCl₂{N-(2,6-diisopropylphenyl)-N-[(diphenylphosphino)dimethylsilyl]-1,1-diphenylphosphine}] features a monomeric structure with κ² coordination of the silicon-bridged diphosphine ligand to the cobalt center. Using dried methylaluminoxane (DMAO) and AlEt₃ as co-catalysts and methylcyclohexane as solvent, the Co(II) complex proved to be moderately active and highly selective for ethylene dimerization. The effects of ligand structure, catalyst loading, reaction temperature, Al/Co molar ratio, DMAO/AlEt₃ molar ratio and reaction time were investigated with respect to the catalytic activity and product selectivity of ethylene dimerization. Cobalt complex C1 exhibited moderate catalytic activity of 2.3 × 10⁵ g (mol_Co)⁻¹ h⁻¹, with 100% selectivity towards C₄ (87% 1-C₄ in the C₄ fraction).