Synthesis and characterization of chromium(III) octaphenylporphyrin complexes with various axial ligands: An insight into porphyrin distortion

Influence of axial ligands, MeCN, H2O, py, and piperidine (pip), on distortion of (2,3,5,10,12,13,15,20-octaphenylporphinato)chromium(III), [Cr(OPP)]+, was investigated by X-ray crystallography and UV–vis and ESR spectroscopies. In crystal structures of [Cr(OPP)(MeCN)(H2O)]ClO4 · MeCN and [Cr(OPP)(H2O)2]ClO4 · 3THF, the OPP2− ligand had a planar structure. On the other hand, crystal structures of [Cr(OPP)(pip)2]ClO4 · 2CH2Cl2 and [Cr(OPP)(py)2]ClO4 exhibited a waved structure and a saddle-shaped structure with ruffling, respectively. In the UV–vis spectrum of [Cr(OPP)(py)2]ClO4 in CH2Cl2, the large red shift of the Soret band was observed. Furthermore, the small D value of 0.10 cm−1 was obtained for [Cr(OPP)(py)2]ClO4 from the ESR spectrum in frozen 1,2-dichloroethane solution at 5 K. These results suggest that the OPP2− ligand is distorted both in solid and in solution, and that the axial ligand would exert some effects on the porphyrin distortion.

Graphical abstractThe origins of distortion of the porphyrin ligand in Cr(III)–OPP (OPP = octaphenylporphinato) complexes with various axial ligands have been investigated by X-ray crystallography and spectroscopic methods. In the crystals, [Cr(OPP)(pip)2]ClO4 (pip = piperidine) shows a waved structure and a saddle-shaped structure with ruffling for [Cr(OPP)(py)2]ClO4. The large red shift of Soret band in a UV–vis spectrum of [Cr(OPP)(py)2]ClO4 and the small D value obtained from its ESR spectrum indicates that the OPP2− ligand is highly distorted also in solution.Figure optionsDownload full-size imageDownload as PowerPoint slide