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']⁺, [1_R]⁺ (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' (2_R) were readily obtained by subsequent deprotonation with NEt₃. The P₃CN cores in 2_R 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 [1_R]⁺ 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 [1_R]⁺ 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.