Exploring the Effects of Axial Pseudohalide Ligands on the Photophysical and Cyclic Voltammetry Properties and Molecular Structures of MgII Tetraphenyl/porphyrin Complexes

The (meso-tetraphenylporphyrinato) magnesium(II) complexes with azido (1), cyanato-N (2), and thiocyanato-N (3) ligands were prepared by using 2.2.2-cryptand to solubilize the azide, cyanato, and thiocyanato salts in dichloromethane solvent. These species were characterized by UV/Vis and IR spectroscopy, mass spectrometry, and electrochemistry. The first reduction potential and the two first oxidation potentials of the porphyrin rings of these species are not affected by the nature of the axial ligand, and an unusual third irreversible oxidation of the porphyrin ring is observed. The anodic behavior of the magnesium azide derivative is complicated by the appearance of additional signals for ligand-centered electron transfers that originate from the release of the azido ligand of 1. The room-temperature fluorescence spectra of the magnesium complexes 1-3 indicate that the Soret and Q bands are not particularly affected by the nature of the axial ligands. The quantum yields of the S-1 -> S-0 fluorescence are between 0.10 and 0.19, and the fluorescence lifetimes range between 3.7 and 6.1 ns at room temperature. Complexes 1-3 crystallize in the monoclinic crystal system in the same space group, P2(1)/n. The molecular structure of 1 is the first example of a magnesium azide complex. The average equatorial magnesium-N-pyrrole bond lengths (Mg-N-p) are higher than those of the related pentacoordinate magnesium porphyrin species and decreases from 1 [2.1187(16) angstrom] to 2 [2.1108(15) angstrom] to 3 [2.0962(13) angstrom]; the distance between the magnesium center and the 24-atom mean plane of the porphyrin ring (Mg-P-C) also decreases from 1 to 2 to 3 with values of 0.6629(7), 0.6598(7), and 0.5797(6) angstrom, respectively. Complex 1 shows major doming and saddle distortions, whereas 2-3 exhibit relatively high ruffling and moderate doming deformations. The molecular structure of 1 is stabilized by weak intermolecular C-H center dot center dot center dot N hydrogen bonds between one carbon atom of the phenyl ring and the terminal nitrogen atom of the azido ligand, and the lattice of 2 exhibits weak intermolecular C-H center dot center dot center dot OH bonds between one carbon atom of the phenyl ring and the terminal oxygen atom of the NCO-ligand. The crystal structure of 3 is mainly sustained by weak intermolecular C-H center dot center dot center dot C-g pi interactions between a carbon atom of 2.2.2-cryptand and the centroid of one pyrrole ring.