Département de chimie moléculaire

The (meso-tetraphenylporphyrinato)magnesium(II) complexes with azido (1), cyanato-N (2), and thiocyanato-N (3) ligands were prepd. by using 2.2.2-cryptand to solubilize the azide, cyanato, and thiocyanato salts in dichloromethane solvent. These species were characterized by UV/visible and IR spectroscopy, mass spectrometry, and electrochem. The first redn. potential and the two first oxidn. potentials of the porphyrin rings of these species are not affected by the nature of the axial ligand, and an unusual third irreversible oxidn. of the porphyrin ring is obsd. The anodic behavior of the magnesium azide deriv. is complicated by the appearance of addnl. signals for ligand-centered electron transfers that originate from the release of the azido ligand of 1. The room-temp. 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 S1→S0 fluorescence are 0.10-0.19, and the fluorescence lifetimes range between 3.7 and 6.1 ns at room temp. Complexes 1-3 crystallize in the monoclinic crystal system in the same space group, P21/n. The mol. structure of 1 is the first example of a magnesium azide complex. The av. equatorial magnesium-Npyrrole bond lengths (Mg-Np) are higher than those of the related pentacoordinate magnesium porphyrin species and decreases from 1 [2.1187(16) {\AA}] to 2 [2.1108(15) {\AA}] to 3 [2.0962(13) {\AA}]; the distance between the magnesium center and the 24-atom mean plane of the porphyrin ring (Mg-PC) also decreases from 1 to 2 to 3 with values of 0.6629(7), 0.6598(7), and 0.5797(6) {\AA}, resp. Complex 1 shows major doming and saddle distortions, whereas 2-3 exhibit relatively high ruffling and moderate doming deformations. The mol. structure of 1 is stabilized by weak intermol. C-H···N hydrogen bonds between one carbon atom of the Ph ring and the terminal nitrogen atom of the azido ligand, and the lattice of 2 exhibits weak intermol. C-H···O H bonds between one carbon atom of the Ph ring and the terminal oxygen atom of the NCO- ligand. The crystal structure of 3 is mainly sustained by weak intermol. C-H···Cg $π$ interactions between a carbon atom of 2.2.2-cryptand and the centroid of one pyrrole ring. [on SciFinder(R)]