X-ray Absorption Spectroscopy as a Probe of Ligand Noninnocence in Metallocorroles: The Case of Copper Corroles
The question of ligand noninnocence in Cu corroles has long been a topic of discussion. This work introduces a Cu K-edge X-ray absorption spectroscopy (XAS) study, which allows a direct probe of the metal oxidation state, of three Cu corroles, Cu[TPC], Cu[Br 8 TPC], and Cu[(CF 3 ) 8 TPC] (TPC = meso -triphenylcorrole), and the analogous Cu(II) porphyrins, Cu[TPP], Cu[Br 8 TPP], and Cu[(CF 3 ) 8 TPP] (TPP = meso -tetraphenylporphyrin). The Cu K rising-edges of the Cu corroles were found to be about 0–1 eV upshifted relative to the analogous porphyrins, which is substantially lower than the 1–2 eV shifts typically exhibited by authentic Cu(II)/Cu(III) model complex pairs. In an unusual twist, the Cu K pre-edge regions of both the Cu corroles and the Cu porphyrins exhibit two peaks split by 0.8–1.3 eV. Based on time-dependent density functional theory calculations, the lower- and higher-energy peaks were assigned to a Cu 1s → 3d$_{x^{2}–y^{2}}$ transition and a Cu 1s → corrole/porphyrin π* transition, respectively. From the Cu(II) porphyrins to the corresponding Cu corroles, the energy of the Cu 1s → 3d$_{x^{2}–y^{2}}$ transition peak was found to upshift by 0.6–0.8 eV. This shift is approximately half that observed between Cu(II) to Cu(III) states for well-defined complexes. The Cu K-edge XAS spectra thus show that although the metal sites in the Cu corroles are more oxidized relative to those in their Cu(II) porphyrin analogues, they are not oxidized to the Cu(III) level, consistent with the notion of a noninnocent corrole. The relative importance of σ-donation versus corrole π-radical character is discussed.