Synthesis, Structure, and Olefin Polymerization Behavior of Nickel Complexes with Carborane [S,C] or [S,S] Ligands

The o-carborane [S,C] ligand 1 (1-(2′-(S)PPh2)-o-carborane) was prepared by the reaction of a monophosphino o-carborane with elemental sulfur in the presence of Et3N. Ligand 1 was lithiated with n-BuLi and then reacted with (Ph3P)Ni(Ph)Cl2, Ni(PPh3)2Cl2, and (DME)NiBr2, respectively, to give the same mononuclear Ni complex [1-(2′-(S)PPh2)-o-carborane]2Ni (2). Additionally, the lithium salt of ligand 1 was treated with elemental sulfur and then reacted with (Ph3P)Ni(Ph)Cl2, affording the mononuclear Ni complex [1-S-(2′-(S)PPh2)-o-carborane]2Ni (3). Ni complexes 2 and 3 were characterized by IR, 1H NMR, and 31P NMR spectroscopy and elemental analysis. In addition, an X-ray structure analysis was performed on complex 2, where the o-carborane [S,C] ligand 1 was found to coordinate in a bidentate mode. EXAFS spectroscopy was performed on complex 3 to confirm that the coordination geometry was similar to that for complex 2. Two nickel complexes with carborane [S,C] or [S,S] ligands show good catalytic activities for the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as cocatalyst. Catalytic activities, molecular weights, and molecular weight distributions of polynorbornene (PNB) have been investigated under various reaction conditions.