Metallacyclopentane Formation:  A Deactivation Pathway for a Tungsten(VI) Alkylidene Complex in Olefin Metathesis Reactions

W(NPh)[ o -(Me 3 SiN) 2 C 6 H 4 ](CH 2 CMe 3 ) 2, 2h, is quantitatively converted to a new metallacyclopentane complex, W(CH 2 CH 2 CH 2 CH 2 )(NPh)[ o -(Me 3 SiN) 2 C 6 H 4 ], 4, upon the addition of ethylene at T > 70 °C. The complex W(CH 2 CH 2 CH 2 CH 2 )(NPh)[ o -(Me 3 SiN) 2 C 6 H 4 ], 4, was also formed by the transmetalation reaction of W(NPh)[ o -(Me 3 SiN) 2 C 6 H 4 ](Cl) 2 with the corresponding Grignard reagent, BrMg(CH 2 ) 4 MgBr, and also when W(NPh)[ o -(Me 3 SiN) 2 C 6 H 4 ](PMe 3 )( C(H) t -Bu), 3, reacts with ethylene. Mechanistic studies suggest that 2h and 3 react with ethylene through a common base-free alkylidene intermediate which undergoes metathesis with ethylene and then generates a W(IV) intermediate which is converted to 4 by coupling two molecules of ethylene. This reaction is a deactivation pathway when 3 is used as an olefin metathesis catalyst for terminal olefins because ethylene is generated during the reaction. The tungsten(IV) complex W(NPh)[ o -(Me 3 SiN) 2 C 6 H 4 ](PMe 3 ) 2 (η 2 -C 2 H 4 ), 5, was observed during the reaction between 3 and ethylene. Complex 5 can also be synthesized from the diethyl complex W(NPh)[ o -(Me 3 SiN) 2 C 6 H 4 ](CH 2 CH 3 ) 2, 2b, in the presence of PMe 3 . Addition of excess ethylene to 5 gives 4 . The X-ray structure of 4 was determined.