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 (<b>2</b>) and Cr^V[TPC]O (<b>3</b>), and the results have been compared to the reference compound Cr^III[TPP]Cl (<b>1</b>), where TPP²⁻, TMP²⁻, and TPC³⁻ are the anions of <i>meso</i>-tetraphenylporphyrin, <i>meso</i>-tetramesitylporphyrin, and <i>meso</i>-triphenylcorrole, respectively. The intensity-weighted average energy position in <b>1</b> (5990.9 eV), <b>2</b> (5992.0 eV) and <b>3</b> (5992.6 eV) are consistent with increasing the metal oxidation state along the series. EXAFS and theoretical analysis of <b>2</b> and <b>3</b> reveal that the Cr–O bond is longer and weaker in <b>3</b> relative to <b>2</b>, despite the structural similarity and increase in oxidation state in <b>3</b>. 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 <b>3</b> 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 <b>3</b> relative to <b>2</b>, which in turn accentuates the lower local symmetry of the Cr in <b>3</b> relative to <b>2</b>. 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 <b>2</b>, the pre-edge of <b>3</b> 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 <b>3</b>.