Sterically encumbered pyrrolyl ligands and their incorporation into the cycloheptatrienyl zirconium coordination sphere

摘要

An improved synthesis of 2,5-di(tert-butyl)-3,4-dimethylpyrrole (2-H, HPy~rtBu_2Me_2) and a subsequent reaction with KH yield K(PyrtBu2Me2) (2-K) in multi-gram quantities. Four different pyrrolyl (Pyr) and imidazolyl (Im) ligands were used in salt metathesis reactions with [(η7-C7H 7)ZrCl(tmeda)] (7) to afford a series of azatrozircenes: [(η~7-C_7H_7)Zr(η~5-Pyr t~Bu_2)] (1-Zr), [(η~7-C_7H_7)Zr(η~5-Pyr~tBu_2Me_2)] (2-Zr), [(η~7-C_7H_7)Zr(η~5- Pyr~tBu_3)] (3-Zr) and [(η~7-C_7H_7)Zr(η~5-Im~tBu_3)] (4-Zr), which were characterized by NMR spectroscopy and elemental analysis. In addition, the molecular structures of 2-H, 2-K·18-crown-6, 1-Zr, 2-Zr and 4-Zr were determined by X-ray diffraction analysis, revealing η~5- rather than κ~1-N-coordination of the N-heterocyclic ligands. Cone angle measurements on the sandwich complexes 1-Zr-4-Zr showed that their nitrogen-containing ligands belong to the class of very sterically encumbered π-ligands, but DFT calculations suggest lower stabilities compared to their all-carbon analogues.