Mixed pyrazolylborate/cyclopentadienyl derivatives of divalent lanthanides: synthesis and structure of (TptBu,Me)Yb(C5H4R) (R=H, SiMe3)

Reaction of [(TptBu,Me)YbH]2 with cyclopentadiene (C5H6) and trimethylsilyl cyclopentadiene (C5H5SiMe3) resulted in the formation of the corresponding mixed-ligand complexes, (TptBu,Me)Yb(C5H4R) (R=H, 1a; SiMe3, 1b), in an essentially quantitative yield. The complexes were characterized by multinuclear NMR spectroscopy and, 1b, by single-crystal X-ray diffraction. The solid-state structure of 1b consists of well-separated monomeric units with a η5-C5H4SiMe3 ring, but the TptBu,Me ligand exhibits an unusual distortion; one of the pyrazolyl rings is rotated in such a way as to bring both pyrazolyl nitrogens in bonding contact with ytterbium. The simple 1H- and 13C-NMR spectra of one set of TptBu,Me resonances indicate fluxional solution behavior. The barrier to pyrazolyl ring exchange is very small since no line broadening was observed down to −100°C.