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

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 TPP(2−), TMP(2−), and TPC(3−) 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 N(4) 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(z(2)) 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(z(2)) character, thus accounting for the somewhat higher overall intensity for 3.