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 Cp2(THF)ZrNR (R = 2,6-i-Pr2C6H3, 2,6-Me2C6H3, 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 Cp2(THF)ZrNR (R = 2,6-Me2C6H3) was treated with symmetrical carbodiimides RNCNR (R = t-Bu, i-Pr, SiMe3, cyclohexyl, p-tolyl), the corresponding diazametallacycles formed in high yields. However, with the more sterically encumbered zirconocene imido complex Cp2(THF)ZrNR (R = 2,6-i-Pr2C6H3), diazametallacycle formation was observed only with the heterocumulenes 1,3-diisopropylcarbodiimide, 1,3-dicyclohexylcarbodiimide, and 1,3-di-p-tolylcarbodiimide. The 16-electron diazametallacyle Cp2Zr(N(2,6-i-Pr2C6H3)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 Cp2Zr(N(t-Bu)CN(SiMe3)N(SiMe3)) (7d) was treated with an excess of bis-1,3-trimethylsilylcarbodiimide, Cp2Zr(N(SiMe3)2(NCNSiMe3) (13) was obtained. The symmetrical diazametallacycle Cp2Zr(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 Cp2Zr(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 ΔG ⧧ of 15.3 kcal/mol. This fluxionality is proposed to be due to syn/anti isomerism about the exocyclic CN(i-Pr) group.