Synthesis and Characterization of N<i>-</i>Donor-Functionalized Enantiomerically Pure Pentadienyl Ligands Derived from (1<i>R</i>)‑(−)-Myrtenal

A series of enantiomerically pure −SiMe<sub>2</sub>NR<sub>2</sub> (R = Me, Et) substituted pentadienyl ligands were prepared starting from the natural product (1<i>R</i>)-(−)-myrtenal. Deprotonation with a Schlosser superbase yields the corresponding potassium salts, which were characterized by various spectroscopic techniques. In solution these neutral N-donor-substituted pentadienyl systems predominantly adopt a <i>U</i> conformation, but in two cases the rare <i>S</i> conformation was also observed as a minor component in solution. Addition of 18-crown-6 allowed the molecular structures of two of these potassium pentadienyls to be determined by X-ray diffraction. Interestingly, η<sup>5</sup> and κ<i>N</i> coordination of the pentadienyl system to the [K­(18-crown-6)]<sup>+</sup> cation was observed. Furthermore, these ligand systems also coordinate to transition metals and form an open titanocene, open vanadocenes, open chromocenes, and half-open trozircenes with [TiCl<sub>3</sub>(thf)<sub>3</sub>], [VCl<sub>3</sub>(thf)<sub>3</sub>], CrCl<sub>2</sub>, and [(η<sup>7</sup>-C<sub>7</sub>H<sub>7</sub>)­ZrCl­(tmeda)], respectively. These complexes were characterized by elemental analyses and various spectroscopic techniques. However, no coordination of the pendant −SiMe<sub>2</sub>NR<sub>2</sub> group to the metal centers was observed. In addition, significant steric crowding in these open metallocenes prevents the formation of isolable CO or PMe<sub>3</sub> adducts. This was further corroborated by EPR studies on an open vandadocene, which showed that no adduct formation occurs at ambient temperature in solution, but a weak PMe<sub>3</sub> adduct was detected at 26 K.