C-Substituted Bis(diphenylphosphino)methane-Type Ligands for Chromium-Catalyzed Selective Ethylene Oligomerization Reactions

Ligand backbone alkylation of the complex [Cr(CO)(4)dppnt)] (dppm = bis(disphenylphosphino)methane) with alkyl iodides yields the C-substituted dppm ligand complexes [Cr(CO)(4){Ph2PCH(R)PPh2}] (R = methyl, n-hexyl, benzyl). Activation of these complexes via one-electron oxidation with Ag[(Al(OC4F9)(4)] and CO removal with triethylaluminium, or (in the cas of R = methyl) by in situ treatment of the free ligand with a chromium salt and modified methyl alumoxane (MMAO), leads to catalysts showing some selectivity for ethylene trimerization and tetramerization. NMR spectroscopic studies of the parent dppm or [Cr(CO)(4)(dppm)] compounds suggest that ligand deprotonation and decomplexation may be the cause of the surprisingly poor catalytic performance of these specific derivatives.