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

Ligand backbone alkylation of the complex [Cr(CO) 4 (dppm)] (dppm = bis(diphenylphosphino)methane) with alkyl iodides yields the C-substituted dppm ligand complexes [Cr(CO) 4 {Ph 2 PCH(R)PPh 2 }] (R = methyl, n -hexyl, benzyl). Activation of these complexes via one-electron oxidation with Ag[(Al(OC 4 F 9 ) 4 ] and CO removal with triethylaluminium, or (in the case 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.