Aminophosphine-Based Chromium Catalysts for Selective Ethylene Tetramerization

Chromium complexes of three new ligands, Ph2PN­(Me)­(CH2)2-X [X = NMe2 (PNN); PPh2 (PNP); Py (PNPy)], have been examined vis-à-vis their ability to promote ethylene tetramerization, (PNN)­CrCl3(L) [L = THF (1); CH3CN (2)], (PNPy)­CrCl3(L) [L = THF (3); CH3CN (4)], and (PNP)­CrCl3(THF) (5). In the case of 2 and 4, it was possible to grow crystals suitable for X-ray diffraction. The reaction of 3 with Et3Al afforded the dinuclear [(HN­(Me)­(CH2)2Py)­CrCl2Et]2 (6) containing a trivalent chromium connected to an Et group. During the alkylation though, the ligand has been fragmented with removal of the side arm and protonation of the N atom of the remaining NP residue. All the complexes have been tested for ethylene oligomerization activity. Complex 1 displayed the highest selectivity for 1-octene, upon activation with DMAO in MeCy. Contrary to expectations, complex 6 is not a self-activating catalyst.