Synthesis, structure, and properties of antiaromatic PVO complexes of triphyrins(2.1.1)

Abstract

A series of antiaromatic stable P^VO complexes of meso-tetra(p-tolyl) triphyrin(2.1.1) and its β-substituted derivatives were synthesized in 38–48% yields by treating an appropriate aromatic free base triphyrin(2.1.1) with PCl₃ in toluene/Et₃N at reflux for 4–6 h. During the reaction, 14π aromatic meso-tetra(p-tolyl) triphyrin(2.1.1) is reduced to a 16π macrocycle and complexes with P^VO to form antiaromatic P^VO complexes of meso-tetra(p-tolyl) triphyrins(2.1.1). The X-ray structure obtained for one of the P^VO triphyrin(2.1.1) complexes revealed that unlike the almost planar structure of free base meso-tetraaryl triphyrin(2.1.1), P^VO triphyrin(2.1.1) was highly distorted and adopted a domed structure with P(V) placed at a distance of 0.927 Å above the mean plane defined by the four meso-carbon atoms. NMR studies indicated the paratropic ring current effect in the P^VO triphyrin(2.1.1) complexes and the outer β-pyrrole protons which appear in the downfield region of 7.50–8.20 ppm in free base triphyrin(2.1.1) experienced significant upfield shifts and appeared in the region of 3.50–4.50 ppm in P^VO triphyrin(2.1.1) complexes, supporting the switching of aromatic 14π free base triphyrin(2.1.1) to antiaromatic 16π P^VO triphyrin(2.1.1) complexes. The P^VO triphyrin(2.1.1) complexes show one intense absorption band in the 300–350 nm region and an antiaromatic characteristic broad band in the lower energy region of 450–1000 nm. The electrochemical studies revealed that the P^VO triphyrin(2.1.1) complexes undergo very easy oxidations and difficult reductions compared to their corresponding free base triphyrins(2.1.1). DFT/TD-DFT calculations were carried out to support the experimental observations.