Israel Journal of Chemistry Vol. 42 2002 pp. 383–392 *Author to whom correspondence should be addressed. E-mail: okuda@mail.uni-mainz The Kinetic Stability of Cationic Benzyl Titanium Complexes that Contain a Linked Amido-Cyclopentadienyl Ligand: The Influence of the Amido- Substituent on the Ethylene Polymerization Activity of “Constrained Geometry Catalysts” JUN OKUDA,* KITTICHOTE MUSIKABHUMMA, AND PIET-JAN SINNEMA Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität Mainz, Duesbergweg 10–14, Mainz D-55099, Germany (Received 30 September 2002) Abstract. Cationic benzyl titanium complexes [Ti(η 5 : η 1 -C 5 Me 4 SiMe 2 NR′)- (CH 2 Ph)] + were cleanly formed by the reaction of the dibenzyl titanium complexes [Ti(η 5 : η 1 -C 5 Me 4 SiMe 2 NR′)(CH 2 Ph) 2 ] with B(C 6 F 5 ) 3 and [Ph 3 C][B(C 6 F 5 ) 4 ] in bromobenzene. NMR spectroscopic studies suggest that the benzyl titanium cations contain a fluxional η 2 -coordinated benzyl ligand. Kinetic analysis showed that the benzyl titanium cations decompose according to first-order kinetics and that the amido substituents R′ (R′ = Me, i Pr, t Bu) in the linked amido-cyclopentadienyl ligand influence the lability of these benzyl titanium cations. The order of the kinetic stability of the benzyl titanium cations was found for both anions to follow the order R′ = Me > i Pr > t Bu. The benzyl titanium cations generated with [Ph 3 C][B(C 6 F 5 ) 4 ] were found to undergo faster decomposition than those generated with B(C 6 F 5 ) 3 . The ethylene polymerization activity order for both systems was found to be the reverse: R′ = t Bu > i Pr > Me. The decomposition of the benzyl titanium cations was suggested to occur via C–H activation with concomitant toluene elimination. INTRODUCTION Based on (C 5 Me 4 SiMe 2 N t Bu) 2– , the prototypical linked amido-cyclopentadienyl ligand system first developed by Bercaw and coworkers to develop single-component α-olefin polymerization catalysts of scandium, 1 a novel class of Group IV metal single-site catalyst precursors for the α-olefin polymerization has been introduced. 2 The sterically more open and electronically more unsat- urated metal center in these catalysts was particularly made responsible for efficient copolymerization of ethyl- ene with α-olefin, styrene, norbornene, and even isobuty- lene. 3 It is conspicuous that the ligand most broadly and evidently most successfully used for α-olefin polymeriza- tion is Bercaw’s “original” system (C 5 Me 4 SiMe 2 N t Bu) 2– . However, it has been long recognized 4 that a broad variation is easily possible in the ligand (C 5 R 4 ZNR′), 2–5 where, independently, the ring C 5 R 4 , the linking group Z, and the amido substituent NR′ can be varied. The active species in the polymerization of such “con- strained geometry” catalysts is assumed to be the 12- electron alkyl cation [Ti(η 5 : η 1 -C 5 Me 4 SiMe 2 NR′)R′′] + . 6 Previously, we showed that by introducing an additional donor function in the amido substituent, benzyl titanium cations of the type [Ti(η 5 : η 1 -C 5 Me 4 SiMe 2 NCH 2 CH 2 X)- (CH 2 Ph)] + can be conveniently observed by NMR spec- troscopy. 7 By comparing the stability of the benzyl cat- ions containing different amido substituents R′ [Ti(η 5 : η 1 -C 5 Me 4 SiMe 2 NR′)(CH 2 Ph)] + , we have now found that simple alkyl groups such as the amido substituent R′ (R′ = Me, i Pr, t Bu) show a notable influence on the lability and ethylene polymerization activity of the benzyl cations [Ti(η 5 : η 1 -C 5 Me 4 SiMe 2 NR′)(CH 2 Ph)] + .