Univalent Gallium Complexes of Simple and ansa-Arene Ligands: Effects on the Polymerization of Isobutylene

Using [Ga(C₆ H₅ F)₂ ]⁺ [Al(OR^F )₄ ]^- (1) (R^F =C(CF₃ )₃ ) as starting material, we isolated bis- and tris-η⁶ -coordinated gallium(I) arene complex salts of p-xylene (1,4-Me₂ C₆ H₄ ), hexamethylbenzene (C₆ Me₆ ), diphenylethane (PhC₂ H₄ Ph), and m-terphenyl (1,3-Ph₂ C₆ H₄ ): [Ga(1,4-Me₂ C₆ H₄ )_2.5 ]⁺ (2⁺ ), [Ga(C₆ Me₆ )₂ ]⁺ (3⁺ ), [Ga(PhC₂ H₄ Ph)]⁺ (4⁺ ) and [(C₆ H₅ F)Ga(μ-1,3-Ph₂ C₆ H₄ )₂ Ga(C₆ H₅ F)]²⁺ (5²⁺ ). 4⁺ is the first structurally characterized ansa-like bent sandwich chelate of univalent gallium and 5²⁺ the first binuclear gallium(I) complex without a GaGa bond. Beyond confirming the structural findings by multinuclear NMR spectroscopic investigations and density functional calculations (RI-BP86/SV(P) level), [Ga(PhC₂ H₄ Ph)]⁺ [Al(OR^F )₄ ]^- (4) and [(C₆ H₅ F)Ga(μ-1,3-Ph₂ C₆ H₄ )₂ Ga(C₆ H₅ F)]²⁺ {[Al(OR^F )₄ ] ^- }₂ (5), featuring ansa-arene ligands, were tested as catalysts for the synthesis of highly reactive polyisobutylene (HR-PIB). In comparison to the recently published 1 and the [Ga(1,3,5-Me₃ C₆ H₃ )₂ ]⁺ [Al(OR^F )₄ ]^- salt (6) (1,3,5-Me₃ C₆ H₃ =mesitylene), 4 and 5 gave slightly reduced reactivities. This allowed for favorably increased polymerization temperatures of up to +15 °C, while yielding HR-PIB with high contents of terminal olefinic double bonds (α-contents=84-93 %), low molecular weights (M_n =1000-3000 g mol^-1 ) and good monomer conversions (up to 83 % in two hours). While the chelate complexes delivered more favorable results than 1 and 6, the reaction kinetics resembled and thus concurred with the recently proposed coordinative polymerization mechanism.