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

Abstract

An improved synthesis of 2,5-di(tert-butyl)-3,4-dimethylpyrrole (2-H, HPyr^tBu₂Me₂) and a subsequent reaction with KH yield K(Pyr^tBu₂Me₂) (2-K) in multi-gram quantities. Four different pyrrolyl (Pyr) and imidazolyl (Im) ligands were used in salt metathesis reactions with [(η⁷-C₇H₇)ZrCl(tmeda)] (7) to afford a series of azatrozircenes: [(η⁷-C₇H₇)Zr(η⁵-Pyr^tBu₂)] (1-Zr), [(η⁷-C₇H₇)Zr(η⁵-Pyr^tBu₂Me₂)] (2-Zr), [(η⁷-C₇H₇)Zr(η⁵-Pyr^tBu₃)] (3-Zr) and [(η⁷-C₇H₇)Zr(η⁵-Im^tBu₃)] (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 η⁵- rather than κ¹-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.