Structures, Electronics, and Reactivity of Strained Phosphazane Cages: A Combined Experimental and Computational Study

A series of formamidine-bridged P2N2 cages
have been prepared. Upon deprotonation, these compounds serve as valuable
precursors to hybrid N-heterocyclic carbene ligands,
whereas direct metalation gives rearranged dimetallic complexes as
a result of cleavage of the formamidine bridge. The latter metal complexes
contain an intact cyclophosphazane moiety that coordinates two distinct
metal centers in a monodentate and a chelating fashion. A computational
study has been carried out to elucidate the bonding within the P2N2 framework as well as the reactivity patterns.
Natural bond orbital analysis indicates that the cage motif is poorly
described by localized Lewis structures and that negative hyperconjugation
effects govern the stability of the bicyclic framework. The donor
capacity of the cyclophosphazane unit was assessed by inspection of
the frontier molecular orbitals, highlighting the fact that π-back-donation
from the metal fragments is crucial for effective metal–ligand
binding.