Probing the Dynamic Features of Bis(aminocyclopentadienyl) and Bis(aminoindenyl) Zirconium Complexes

Treatment of 3-phenylcyclopent-2-enone ( 1 ) with either morpholine or pyrrolidine in the presence of 3 Å molecular sieves yielded the corresponding cyclopentadienamines 2a and 2b . Subsequent deprotonation with n -butyllithium followed by the reaction with zirconium tetrachloride led to the formation of the amino-substituted bent metallocenes 4a and 4b . Crystallization furnished the isomerically pure complex meso- bis(phenyl-3-pyrrolidinylcyclopentadienyl)zirconium dichloride ( meso- 4b ), which was characterized by X-ray diffraction. Complex meso- 4b exhibits a C(Cp)−N bond length of 1.349(3) Å. A rotational barrier of Δ G ⧧ (CN - rot) (248 K) = 12.1 ± 0.2 kcal mol - 1 was measured for meso- 4b in solution by dynamic 600 MHz 1 H NMR spectroscopy. Similar treatment of 2-indanone with morpholine, followed by deprotonation and reaction with ZrCl 4, gave bis(2-morpholinoindenyl)ZrCl 2 ( 7a ), which was also characterized by an X-ray crystal structure analysis (the indenyl-C2/12−N bond lengths are 1.349(5)/1.381(5) Å). Two rotational processes were subsequently frozen out on the 600 MHz 1 H NMR time scale upon decreasing the temperature from ambient to give a C−N rotational barrier of Δ G ⧧ (CN - rot) (268 K) = 12.9 ± 0.2 kcal mol - 1 and an activation energy of the rotation about the zirconium−Cp(indenyl) vector of Δ G ⧧ (Ind - rot) (178 K) = 7.6 ± 0.5 kcal mol - 1 . In solution and in the solid state only a single C 2 -symmetric metallocene rotamer of 7a was detected. The same is found for bis(2-pyrrolidinoindenyl)ZrCl 2 ( 7b ). By dynamic 1 H NMR spectroscopy the two rotational activation barriers of complex 7b were determined as Δ G ⧧ (CN - rot) (338 K) = 16.4 ± 0.2 kcal mol - 1 and Δ G ⧧ (Ind - rot) (198 K) = 9.1 ± 0.5 kcal mol - 1 .