Pd(II) complexes of meso-tetraaryl triphyrin(2.1.1)s: synthesis and studies

Three examples of Pd(II) meso-tetraaryl triphyrin(2.1.1) complexes were synthesized by reacting the appropriate meso-tetraaryl triphyrin(2.1.1) with PdCl₂ in CHCl₃/CH₃OH under reflux conditions for 6 h, followed by basic alumina column chromatographic purification. The X-ray structure obtained for one of the complexes revealed that the Pd(II) complex exhibits a rigid cis square-planar geometry. In the Pd(II) complex, the Pd(II) ion was coordinated by two pyrrolic nitrogen atoms of the dipyrroethene moiety of the triphyrin macrocycle and two chloride ligands and adopts a characteristic saddle-shaped conformation, in which the two coordinated pyrrole rings were tilted upwards from the mean plane, while the non-coordinated pyrrole ring was tilted downwards. The spectral and redox studies of Pd(II) triphyrin complexes compared to free base triphyrins reveal the following: (1) in the ¹H NMR spectrum, the triphyrin core protons and the inner amine NH proton were upfield/downfield shifted; (2) absorption bands were red-shifted; (3) reductions were easier and oxidations were difficult; and (4) the HOMO-LUMO energy gap was reduced. DFT/TD-DFT studies of Pd(II) triphyrin complexes were in support of experimental observations.