Synthesis, structures, and aromaticity of phosphole-containing porphyrins and their metal complexes*

Our recent studies on the synthesis, structures, and aromaticity of phosphole-containing porphyrins and their metal complexes are summarized. Core-modified P,X,N2-porphyrins (X = S, N) are accessible from a σ4-phosphatripyrrane and the corresponding 2,5-difunctionalized heteroles in a few steps. X-ray structural analysis of the σ-P,N3 porphyrin revealed that it possesses a slightly distorted 18π plane. The phosphorus atom incorporated at the core plays an important role in producing unprecedented reactivity and coordinating ability for the porphyrin ring. The P,N3 free base reacts with [RhCl(CO)2]2 in dichloromethane, ultimately yielding an 18π P,N3–rhodium(III) complex, whereas the P,S,N2 free base undergoes redox-coupled complexation with zero valent group 10 metals to afford the corresponding P,S,N2-isophlorin–metal(II) complexes. The aromaticity of the free-base porphyrins and the metal complexes was uncovered based on both experimental and theoretical results. It is of particular interest that the P,S,N2-isophlorin –metal complexes exhibit only a weak antiaromaticity in terms of the magnetic criterion.