Insights on the UV/Vis, Fluorescence, and Cyclic Voltammetry Properties and the Molecular Structures of ZnII Tetraphenylporphyrin Complexes with Pseudohalide Axial Azido, Cyanato‐N, Thiocyanato‐N, and Cyanido Ligands

Abstract

AbstractThe syntheses, spectroscopic and photophysical properties, and molecular structures of azido‐ (1), cyanato‐N‐ (2), thiocyanato‐N‐ (3), and cyanido‐ (4) (meso‐tetraphenylporphyrinato)zinc(II) complexes are reported. These species were prepared by using cryptand‐222 to solubilize the pseudohalide salts in organic solvents. The UV/Vis spectra of these zinc metalloporphyrins are solvent‐dependent and exhibit large redshifted Soret bands compared with those of the [Zn(Porph)L] derivatives in which Porph is a meso‐porphyrinato ligand and L is a monodentate neutral axial ligand. The room‐temperature fluorescence spectra of the zinc complexes 1–4 indicate that the Q bands are not very affected by the nature of the axial ligands, and their positions are very close to those of previously reported (meso‐porphyrinato)zinc complexes. The quantum yields of the S1 → So fluorescence of 1–4 range between 2.8 and 5.5 %, and their fluorescence lifetimes are the same (1.7 ns). Cyclic voltammetry investigations on 1–4 show that the characteristic potentials for the reduction and the two first oxidations of the porphyrin ring are not very affected by the nature of the axial ligand. A third irreversible oxidation of the porphyrin ring is observed. Additional anodic irreversible waves are observed for the thiocyanato‐N (3) and cyanido (4) species. The solid‐state molecular structures of 1–4 are the first examples of zinc porphyrin complexes with anionic ligands. The average equatorial zinc–pyrrole N atom (Zn–Np) distances for 1–4 range between 2.083(1) and 2.117(2) Å and are much longer than those of the related pentacoordinate zinc porphyrin complexes with monodentate neutral ligands. As a consequence, the displacement of the Zn2+ cation from the mean 24‐atom plan of the porphyrin core is significant (ca. 0.5 Å), and the porphyrin core is very distorted. The crystal structures of 1–4 are stabilized by weak intermolecular π interactions, C–H···Cg (Cg are the centroids of some six‐membered phenyl rings and five‐membered pyrrole rings). The molecular structure of 1 is further stabilized by weak intermolecular C–H···N hydrogen bonds between one carbon atom of cryptand‐222 and the terminal nitrogen atom of the azido ligand.