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 Angstrom 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) Angstrom. A rotational barrier of Delta G((CN-rot))(not equal)(248 K) = 12.1+/- 0.2 kcal mol(-1) was measured for meso-4b in solution by dynamic 600 MHz H-1 NMR spectroscopy. Similar treatment of 2-indanone with morpholine, followed by deprotonation and reaction with ZrCl4, gave bis(2-morpholinoindenyl)ZrCl2 (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) Angstrom). Two rotational processes were subsequently frozen out on the 600 MHz H-1 NMR time scale upon decreasing the temperature from ambient to give a C-N rotational barrier of Delta G((CN-rot))(not equal)(268 K) = 12.9 +/- 0.2 kcal mol(-1) and an activation energy of the rotation about the zirconium-Cp(indenyl) vector of Delta G(not equal)((Ind-rot))(178 K) = 7.6 +/- 0.5 kcal mol(-1). In solution and in the solid state only a single Ct-symmetric metallocene rotamer of 7a was detected. The same is found for bis(2-pyrrolidinoindenyl)ZrCl2 (7b). By dynamic H-1 NMR spectroscopy the two rotational activation barriers of complex 7b were determined as Delta G((CN-rot))(not equal)(338 K) = 16.4 +/- 0.2 kcal mol(-1) and Delta G((Ind-rot))(not equal)(198 K) = 9.1 +/- 0.5 kcal mol(-1).