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

Robust organic 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. Non-aromatic rings like cyclohexane 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 patterns. Informed by this model, we rationally make cis-1,4 or trans-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 smoothly as 1,4-dinucleophiles, supporting their classification as new cyclohexane-like connectors. Use of the appropriate connector in polycondensations enables the preferential 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 the consequences of molecular dimensionality, debut connectors that fill a niche left by organic ones, and offer access to new classes of inorganic polymers for future investigation.