Structural Factors that Influence the Course of Overall [2 + 2] Cycloaddition Reactions between Imidozirconocene Complexes and Heterocumulenes

A study has been carried out to examine the factors that influence the overall [2 + 2] cycloaddition reactions between imidozirconocene complexes Cp 2 (THF)Zr NR (R = 2,6- i -Pr 2 C 6 H 3, 2,6-Me 2 C 6 H 3, t -Bu) and small, unsaturated organic molecules. Steric factors in the cycloaddition reactions investigated were found to be significant in both the imido fragment and the heterocumulene reactant partner. When Cp 2 (THF)Zr NR (R = 2,6-Me 2 C 6 H 3 ) was treated with symmetrical carbodiimides RN C NR (R = t -Bu, i -Pr, SiMe 3, cyclohexyl, p -tolyl), the corresponding diazametallacycles formed in high yields. However, with the more sterically encumbered zirconocene imido complex Cp 2 (THF)Zr NR (R = 2,6- i -Pr 2 C 6 H 3 ), diazametallacycle formation was observed only with the heterocumulenes 1,3-diisopropylcarbodiimide, 1,3-dicyclohexylcarbodiimide, and 1,3-di- p -tolylcarbodiimide. The 16-electron diazametallacyle Cp 2 Zr(N(2,6- i -Pr 2 C 6 H 3 )C N(Tol)N(Tol)) ( 9c ) underwent rearrangement of the Zr−N bound moiety upon heating, forming a new metallacycle with the bulkier N-aryl substrate located on the exocyclic position of the four-membered zirconacycle and the less sterically demanding NR group (R = p -tolyl) bound to zirconium. Imido group metathesis reactions between isolated diazametallacycle complexes and carbodiimides were pursued as a method for generating new zirconium-containing metallacycles and unsymmetrical carbodiimides. When diazametallacycle Cp 2 Zr(N( t -Bu)C N(SiMe 3 )N(SiMe 3 )) ( 7d ) was treated with an excess of bis-1,3-trimethylsilylcarbodiimide, Cp 2 Zr(N(SiMe 3 ) 2 (N C NSiMe 3 ) ( 13 ) was obtained. The symmetrical diazametallacycle Cp 2 Zr(N( i -Pr)C N( i -Pr)N( i -Pr)) ( 14 ) was generated with the concurrent formation of the unsymmetrical carbodiimide i -PrN C N- t -Bu when diazametallacycle Cp 2 Zr(N( t -Bu)C N( i -Pr)N( i -Pr)) ( 7a ) was treated with 1 equiv of 1,3-diisopropylcarbodiimide. A variable-temperature NMR experiment (−20 °C to 80 °C) revealed that complex 14 exhibits fluxional behavior with a coalescence temperature of 46 °C and a Δ G ⧧ of 15.3 k cal/mol. This fluxionality is proposed to be due to syn/anti isomerism about the exocyclic C N( i -Pr) group.