Formation and Structural and Dynamic Features of Atropisomeric η2-Iminoacyl Zirconium Complexes

The Cp2ZrCl(μ-1,4-diphenylbutenyne)PX2 complexes 7a [−P(C6H5)2] and 10 [−P(C6F5)2] insert tert-butylisonitrile into the Zr−C(sp2) σ bond to yield the N-inside η2-iminoacyl zirconocene complexes 13a and 13b. X-ray crystal structure analysis of complexes 13a and 13b revealed the presence of a chiral atropisomeric structure with a torsion angle of 74.8(2)° (13a) and 72.9(6)° (13b), respectively, around the central iminoacyl/alkenyl C(sp2)−C(sp2) σ bond. In solution an analogous chiral structure is observed. The barrier of interconversion of the enantiomeric atropisomers of 13a and 13b was determined at ΔG⧧ (327K) = 14.9 ± 0.3 kcal mol-1 (13a) and ΔG⧧ (325K) = 14.8 ± 0.3 kcal mol-1 (13b) by temperature-dependent dynamic NMR spectroscopy. Reaction of 7a and 10 with methyllithium followed by treatment with B(C6F5)3 gave the corresponding cationic zirconocene complexes 12a and 12b. These complexes took up 2 mol equiv of tert-butylisonitrile to yield the cationic N-inside η2-iminoacyl zirconocene systems 14a and 14b as isonitrile adducts. The cationic complexes 14a and 14b are also axially chiral. The barriers of enantiomerization (ΔG⧧ (288 K) = 13.1 ± 0.3 kcal mol-1 (14a), ΔG⧧ (293 K) = 13.4 ± 0.3 kcal mol-1 (14b)) were also determined by dynamic NMR spectroscopy.