Boroxide Complexes of the Group 4 Metals:  A “Noninnocent” Ligand in Olefin Polymerization

Abstract

The synthesis of a series of group 4 metal complexes supported by the boroxide anion, [OB(mes) 2 ] -, is described and their application in α-olefin polymerization discussed. The compounds M{OB(mes) 2 } 2 Cl 2 (THF) 2 [ 1, M = Ti; 2, M = Zr; 3, M = Hf] have been synthesized from MCl 4 (THF) n [M = Ti, n = 0; M = Zr/Hf, n = 2] and 2 equiv of lithiated dimesitylborinic acid. Accidental hydrolysis during the preparation of the zirconium analogue afforded the dimeric hydroxyl species [Zr{OB(mes) 2 } 3 ( μ -OH)] 2 ( 4 ), which was characterized crystallographically. The four-coordinate titanium bis(amide) complex, Ti{OB(mes) 2 } 2 (NEt 2 ) 2 ( 5 ), was prepared by protonolysis of the titanium-tetra(amide) using the parent borinic acid. X-ray structural analyses of 1 and 3 showed distorted, octahedral geometry at the metal with cis -boroxide and trans -chloride ligands. A series of mixed-ligand cyclopentadienyl/boroxide chlorides of the group 4 metals are also described. TiCp‘{OB(mes) 2 }Cl 2 (Cp‘ = η -C 5 Me 4 H, 6 ) and MCp 2 {OB(mes) 2 }Cl [ 7, M = Ti; 8, M = Zr] are formed upon replacement of a chloride with a boroxide ligand using the appropriate TiCp‘Cl 3 and MCp 2 Cl 2 starting materials. X-ray diffraction studies showed compounds 6 and 7 exist as monomers in the solid state, with distorted tetrahedral geometry around the metal. Selected complexes were tested for ethylene polymerization activity, using MAO as activator. Polymerization behavior was observed similar to that of ZrCp 2 Cl 2 tested under the same conditions, which, supplemented by complementary reactivity studies and polymer analysis, suggests possible transfer of the boroxide ligand to aluminum during polymerization. To circumvent the problems associated with activation using MAO, a family of group 4 alkyl compounds have been synthesized. The titanium and hafnium compounds M{OB(mes) 2 } n (CH 2 Ph) 4 - n [ 9, M = Ti, n = 2; 10, M = Ti, n = 3; 11, M = Hf, n = 3; 12, M = Hf, n = 4] were synthesized by protonolysis of the tetra(benzyl) compounds. NMR studies of the reaction between 9 and B(C 6 F 5 ) 3 demonstrated that a well-defined cationic titanium compound was generated in solution; however, strong interaction with the anion prevented the system from being active for polymerization.