Boroxide Complexes of the Group 4 metals: A 'Non-Innocent' Ligand in Olefin Polymerization

Coles, Martyn P, Cole, Sarah C and Hitchcock, Peter B (2005) Boroxide Complexes of the Group 4 metals: A 'Non-Innocent' Ligand in Olefin Polymerization. Organometallics, 24 (13). 3279 -3 289. ISSN 0276-7333

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The synthesis of a series of group 4 metal complexes supported by the boroxide anion, [OB(mes)2]-, is described and their application in -olefin polymerization discussed. The compounds M{OB(mes)2}2Cl2(THF)2 [1, M = Ti; 2, M = Zr; 3, M = Hf] have been synthesized from MCl4(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(-OH)]2 (4), which was characterized crystallographically. The four-coordinate titanium bis(amide) complex, Ti{OB(mes)2}2(NEt2)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}Cl2 (Cp' = -C5Me4H, 6) and MCp2{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'Cl3 and MCp2Cl2 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 ZrCp2Cl2 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(CH2Ph)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(C6F5)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.

Item Type: Article
Schools and Departments: School of Life Sciences > Chemistry
Depositing User: Martyn Coles
Date Deposited: 06 Feb 2012 19:53
Last Modified: 26 Mar 2012 09:12
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