Puzzle of Phosphorus(V) Phthalocyanine Distortion: Crystallographic vs Computational Viewpoints

Phosphorus(V) porphyrinoids typically exhibit significant macrocycle distortion, which is almost unquestionably attributed to the small ionic radius of the central atom. However, the influence of counterions that balance the positive charge of P(V) complexes has never been considered a factor affecting macrocycle distortion. To investigate this, we performed single-crystal XRD studies on a series of P(V) complexes featuring unsubstituted phthalocyanine ligand, PcP(OMe)₂⁺X^-, paired with anions of various sizes (X^- = iodide, trifluoroacetate, formate, triflate, tosylate, and tetraphenylborate). The results demonstrated anion-dependent distortion, yielding geometries ranging from highly distorted for X^- = BPh₄^- to nearly perfectly flat with X^- = I^-. The latter planar structure had not been observed previously in phosphorus(V) porphyrinoids; therefore, further quantum-chemical modeling was used to solve the puzzle of the flat P(V) complex. Calculations of the gas-phase geometry of the isolated PcP(OMe)₂⁺ cation revealed a previously unknown effect of the orientation of the axial MeO groups on Pc ring distortion, with flat geometries stabilized by the antiperiplanar H₃C-O-P-O-CH₃ fragment conformation. However, this trend is not strictly followed in the crystal state, suggesting that the PcP(OMe)₂⁺ cation adjusts its distortion according to its crystal packing advantages.