Synthesis, structure, and properties of antiaromatic P ^V O complexes of triphyrins(2.1.1)

A series of antiaromatic stable P^VO complexes of <i>meso</i>-tetra(<i>p</i>-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 <i>meso</i>-tetra(<i>p</i>-tolyl) triphyrin(2.1.1) is reduced to a 16π macrocycle and complexes with P^VO to form antiaromatic P^VO complexes of <i>meso</i>-tetra(<i>p</i>-tolyl) triphyrins(2.1.1). The X-ray structure obtained for one of the P^VO triphyrin(2.1.1) complexes revealed that unlike the almost planar structure of free base <i>meso</i>-tetraaryl triphyrin(2.1.1), P^VO 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 <i>meso</i>-carbon atoms. NMR studies indicated the paratropic ring current effect in the P^VO 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^VO triphyrin(2.1.1) complexes, supporting the switching of aromatic 14π free base triphyrin(2.1.1) to antiaromatic 16π P^VO triphyrin(2.1.1) complexes. The P^VO 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^VO 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.