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.