We previously explored about Philips trimerization system and elucidated the zwitterion-type η5-pyrrole Cr(II) complexes. Herein, we report a series of
zwitterion-type Cr(II) complexes to investigate ethylene oligomerization activity. Lithium bis(phosphino)aluminate (4, [Et2Al(PPh2)2]Li(THF); 5, [Et2Al(PCy2)2]
Li(THF)2), lithium bis(amino)aluminate (6, [Et2Al(NC4H8)2]Li(THF); 7, [Et2Al(NiPr2)2]Li; 8, [iBu2Al(NiPr2)2]Li), and potassium bis(phosphino)borate ligands (11, [(C6F5)2B(PPh2)2]K; 12, [(C6F5)2B(PCy2)2]K) were prepared. Reaction of 4-6 with CrCl3(THF)3 did not provide any discrete chromium complexes, but the reaction of 5-6 with CrCl2(THF) yielded zwitterion-type Cr(II) complexes (13,
[Et2Al(PCy2)2]2Cr(II); 14, [Et2Al(NC4H8)2]2Cr(II)). Reaction of THF-free 7-8 with CrCl3(THF)3 resulted in the formation of Cl-bridged Cr(II) dimer complexes {[Et(Cl)Al(NiPr2)2]Cr(II)- 2-Cl}2 (15) and {[iBu(Cl)Al(NiPr2)2]Cr(II)- 2-Cl}2 (16).
A similar Cl-bridged Cr(II) dimer complex {[(C6F5)2B(PCy2)2]Cr(II)-μ2-Cl}2 (17) was also obtained by reacting the borate ligand 12 with CrCl2(THF). The structures of 13-17 were elucidated by X-ray crystallography. None of the prepared zwitterion-type Cr(II) complexes 13-17 exhibited any activity after activation with MAO, supporting catalytic cycle involving Cr(I/III) oxidation states in ethylene oligomerization.
