Synthesis, Aromaticity, and Optical and Redox Properties of Silicon(IV) and Tin(IV) Complexes of meso -Substituted 5-Monoazaporphyrin Derivatives

Abstract

Herein, we report the first examples of group-14 metal complexes of 5-monoazaporphyrins. Six-coordinate silicon(IV) and tin(IV) complexes of 10,15,20-triaryl-5-monoazaporphyrins (Ar3MAPMX2; M = Si or Sn, X = F, Cl, OH, or OCOPh) were prepared by metalation of the freebases of Ar3MAP (Ar3MAPH2) with the corresponding metal salts, followed by sequential axial-ligand exchange reactions. Furthermore, N-phenylation and N-ethynylation reactions of Ar3MAPSiF2 with the corresponding iodonium salts afforded 5,10,15,20-tetraaryl-MAP salts and 5-ethynyl-10,15,20-triaryl-MAP salts, respectively. We also established an environmentally benign method for synthesizing Ar3MAPH2 without a column-separation process. The structures, aromaticity, and optical and electrochemical properties of the SiIV and SnIV complexes of the MAP derivatives were investigated using X-ray crystallography, spectroscopic techniques, cyclic voltammetry, and DFT calculations. The SiIV and SnIV centers stabilized the HOMO and LUMO energy levels and weakened the diatropic ring-current effect of their MAP ligands. The positive charge generated by introducing substituents on the meso nitrogen considerably stabilized the frontier orbitals, resulting in a decrease in aromaticity. Time-resolved fluorescence spectroscopy of fluorescent SiIV complexes also provided valuable information on the effects of the meso-nitrogen atom and its substituents on the excited-state dynamics of MAP chromophores.