posted on 2022-01-06, 19:05authored byJingjing Zhai, Fen You, Suting Xu, Ao Zhu, Xiaohui Kang, Yat-Ming So, Xiaochao Shi
A PNP-pincer ligand provides a versatile
ligation framework, which
is highly useful in organometallic chemistry and catalytic chemistry.
In this work, by a de novo strategy, a simple and
efficient synthetic pathway, has been developed to prepare the new
iminodibenzyl-based PNP pincer proligand imin‑RPNP(Li
or H) (R = isopropyl, phenyl). By employing salt metathesis or direct
alkyl elimination, we successfully synthesized a series of iminodibenzyl-PNP
rare-earth-metal (Ln = Sc, Y, Dy, Ho, Er, Tm, Lu) complexes and characterized
them by NMR and X-ray diffraction analyses. Upon addition of a borate
and triisobutylaluminum (TIBA), the rare-earth-metal complexes 2-Y, 2-Dy, 2-Ho, 2-Er, and 2-Tm bearing the imin‑PhPNP
ligand exhibited unexpectedly high 3,4-selectivity (up to 95%) for
the polymerization of 1,3-dienes (isoprene and myrcene); in particular,
the chosen yttrium complex 2-Y promoted the 1,3-diene
polymerization in a living manner. A computational study suggested
that the sterically congested configuration around the metal center
imposed by the imin‑RPNP ligand might be the main
reason for this unusual selectivity.