Multifunctional Behavior by a Bis-(phosphinimino)methanide Ligand: η~2- vs η~3-coordination vs Bronsted Basicity

摘要

Recent years have witnessed an explosion of results involving monoanionic bidentate ligands where the two donor atoms each carry very bulky substituents. When the donors are conjugated and separated by three intervening atoms, the general class includes the extraordinarily popular β-diketiminates1, I (D = NR, G = R'C). A less studied class includes the bis-(phosphinimino)methanide, I (D = NR, G = R'2P), in which pentavalent phosphorus introduces two electron-withdrawing ring members and also substituents which project out of the ring. A surprising feature of the interaction of this ligand with electrophilic centers is that the resulting six-membered ring may not be planar and η~2 on a metal, and that the metal and the γ-carbon fold into a sort of boat conformation to establish a transannular M/C interaction. This interaction is sometimes quite long and can be annihilated by seemingly modest modification of peripheral groups (II) or by no modification at all (III). While the transannular interaction is often long and weak, deprotonation of this C_γ has been shown, in one instance, to yield a molecule, IV, with a most unusual angular geometry (