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

The Cp 2 ZrCl(μ-1,4-diphenylbutenyne)PX 2 complexes 7a [−P(C 6 H 5 ) 2 ] and 10 [−P(C 6 F 5 ) 2 ] insert tert -butylisonitrile into the Zr−C(sp 2 ) σ bond to yield the N-inside η 2 -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(sp 2 )−C(sp 2 ) 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 Δ G ⧧ (327K) = 14.9 ± 0.3 kcal mol -1 ( 13a ) and Δ G ⧧ (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(C 6 F 5 ) 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 η 2 -iminoacyl zirconocene systems 14a and 14b as isonitrile adducts. The cationic complexes 14a and 14b are also axially chiral. The barriers of enantiomerization (Δ G ⧧ (288 K) = 13.1 ± 0.3 kcal mol -1 ( 14a ), Δ G ⧧ (293 K) = 13.4 ± 0.3 kcal mol -1 ( 14b )) were also determined by dynamic NMR spectroscopy.