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

An improved synthesis of 2,5-di(tert-butyl)-3,4-dimethylpyrrole (2-H, HPyr(tBu(2)Me(2))) and a subsequent reaction with KH yield K(Pyr(tBu(2)Me(2))) (2-K) in multi-gram quantities. Four different pyrrolyl (Pyr) and imidazolyl (Im) ligands were used in salt metathesis reactions with [(η(7)-C(7)H(7))ZrCl(tmeda)] (7) to afford a series of azatrozircenes: [(η(7)-C(7)H(7))Zr(η(5)-Pyr(tBu(2)))] (1-Zr), [(η(7)-C(7)H(7))Zr(η(5)-Pyr(tBu(2)Me(2)))] (2-Zr), [(η(7)-C(7)H(7))Zr(η(5)-Pyr(tBu(3)))] (3-Zr) and [(η(7)-C(7)H(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 nitrogencontaining ligands belong to the class of very sterically encumbered π-ligands, but DFT calculations suggest lower stabilities compared to their all-carbon analogues.