X-ray absorption spectroscopy of archetypal chromium porphyrin and corrole derivatives

Abstract

A combination of Cr K-edge XAS and DFT calculations have been performed on archetypal chromium porphyrinoid complexes Cr^IV[TMP]O (2) and Cr^V[TPC]O (3), and the results have been compared to the reference compound Cr^III[TPP]Cl (1), where TPP²⁻, TMP²⁻, and TPC³⁻ are the anions of meso-tetraphenylporphyrin, meso-tetramesitylporphyrin, and meso-triphenylcorrole, respectively. The intensity-weighted average energy position in 1 (5990.9 eV), 2 (5992.0 eV) and 3 (5992.6 eV) are consistent with increasing the metal oxidation state along the series. EXAFS and theoretical analysis of 2 and 3 reveal that the Cr–O bond is longer and weaker in 3 relative to 2, despite the structural similarity and increase in oxidation state in 3. This is also reflected in a comparison of the Cr K-pre-edge transitions. The roughly twenty-fold increase in intensity in the two oxo complexes is unsurprising and well-precedented for other first-row transition metals. However, although 3 had greater overall intensity, the intensity of transitions along the Cr–O bond is greater. EXAFS, DFT and TDDFT analyses show that the more contracted N₄ core of the corrole results in a greater out-of-plane displacement of the Cr in 3 relative to 2, which in turn accentuates the lower local symmetry of the Cr in 3 relative to 2. This difference helps us to appreciate the TDDFT result that whereas two 1s→ 3d_z² transitions make up the majority of the intensity in the pre-edge of 2, the pre-edge of 3 also includes fairly intense transitions to molecular orbitals with 3d_xz, 3d_yz and 3d_z² character, thus accounting for the somewhat higher overall intensity for 3.