Distorted five-coordinate square pyramidal geometry of a cadmium(II) complex containing a 2-methylimidazole ligand: Crystal structure and axial ligand effect on spectroscopic properties

Abstract In this work, we describe a novel cadmium(II) complex based on the insertion of the Cd(II) metal ion into the meso-tetrakis(4-tert-butylphenyl)porphyrin (TBPP) ring, followed by the coordination of the 2-methylimidazole (2-MeHIm) N-donor axial ligand. Single crystal X-ray diffraction of [Cd(TBPP)(2-MeHIm)] (I) revealed that the Cd(II) metal ion induces a significantly distorted five-coordinate square pyramidal geometry of the metalloporphyrin (MP). This particular geometry provides a high displacement of the Cd(II) metal atom out of the porphyrinato mean plane by 0.860 A, which is slightly higher than in related [Cd(II)(porphyrin)(ligand)] complexes. This high value induces a significant porphyrinato core doming and a moderate ruffling. An UV-visible spectroscopy study highlighted the red-shift effect of the absorption bands after the insertion of the Cd(II) metal ion into the TBPP ring and this effect is greater after coordination of the 2-MeHIm axial ligand. The calculated Gap energy Eg from the UV-visible spectra indicate interesting organic semiconducting properties. A fluorescence emission spectroscopy study showed a remarkable blue-shift effect in the Q bands accompanied by a decrease of the fluorescence emission, quantum yield (ɸf) and lifetime (τf) after the insertion of Cd(II) metal ion into TBPP, while the coordination of 2-MeHIm axial ligand had a minor effect.