1,3-Dipolar cyclisation reactions of nitriles with sterically encumbered cyclic triphosphanes: synthesis and electronic structure of phosphorus-rich heterocycles with tunable colour.

We describe the synthesis, solid state and electronic structures of a series of tunable five-membered cationic and charge-neutral inorganic heterocycles featuring a P3CN core. 1-Aza-2,3,4-triphospholenium cations [(PR)3N(H)CR']+, [1R]+ (R' = Me, Ph, 4-MeOC6H4, 4-CF3C6H4) were formed as triflate salts by the formal [3 + 2]-cyclisation reactions of strained cyclic triphosphanes (PR)3 (R = t Bu, 2,4,6-Me3C6H2 (Mes), 2,6- i Pr2C6H3 (Dipp), 2,4,6- i Pr3C6H2 (Tipp)) with nitriles R'CN in the presence of triflic acid. The corresponding neutral free bases (PR)3NCR' (2R) were readily obtained by subsequent deprotonation with NEt3. The P3CN cores in 2R show an envelope conformation typical for cyclopentenes and present as yellow to orange compounds in the solid state as well as in solution depending on both substituents R and R' in (PR)3NCR'. The P3CN cores in [1R]+ show a significant deviation from planarity with increasing steric bulk of the R groups at phosphorus, which results in a decrease in the HOMO-LUMO gap and distinct low-energy UV-Visible absorption bands. This allows access to colours spanning red, blue, indigo, and magenta. TD-DFT calculations provide valuable insight into this phenomenon and indicate an intramolecular charge-transfer from the HOMO located on the P3 framework to the N[double bond, length as m-dash]C-R'-based LUMO in the cationic species. The cations [1R]+ represent rare examples of phosphorus-rich heterocycles with tunable colour, which can be incorporated into polymers by post-polymerization modification to afford coloured polymers, which demonstrate utility as both proton and ammonia sensors.