Aza-Diphosphazenanes: Inorganic Cyclohexane-like Connectors with Stereochemically Tunable Exit Vectors for Molecular and Macromolecular Chemistry.

Molecules that place functional groups at well-defined angular and radial separations are valued as connectors in synthetic chemistry. Examples include rigid, polyvalent arenes and bicyclic compounds featuring strong C─C or C─H bonds. Aliphatic rings are rare in this context due to the challenge of stereochemical control over C─H functionalization. Here we investigate inorganic cyclohexane-like P2N4 rings (aza-diphosphinanes) and develop a model based on negative hyperconjugation and diaxial strain to explain their cis/trans isomerization. Informed by this model, we rationally make 1,4-disubstituted aza-diphosphinanes, and stereoselectively oxidize them to aza-diphosphazenanes, which retain their cis (77°) or trans (180°) covalent exit vectors. These compounds show high stability but nevertheless react as 1,4-dinucleophiles, supporting their classification as new cyclohexane-like connectors. Appropriate connector choice in polycondensations enables the divergent synthesis of low-molecular weight macrocyclic oligomers (from the cis variant) or the first example of high molecular weight cyclo-linear polyphosphazenes (from the trans variant). These results provide fundamental insight into the stereochemistry of inorganic heterocycles and molecular dimensionality, debut connectors that fill a niche left by organic ones, and offer access to new classes of inorganic polymers for future investigation.