Electronic Absorption, Resonance Raman, and Electrochemical Studies of Planar and Saddled Copper(III) meso-Triarylcorroles. Highly Substituent-Sensitive Soret Bands as a Distinctive Feature of High-Valent Transition Metal Corroles

We present here a first systematic study of substituent effects in metallocorroles, based on electronic absorption, resonance Raman (RR), and infrared (IR) spectroscopic studies and electrochemical measurements on 10 copper(III) meso-triarylcorroles, Cu^III[β-Y₈TArC], where the β-substituent Y = H or Br and the meso-aryl group Ar = C₆F₅ or p-X-C₆H₄ and X = CF₃, H, CH₃, and OCH₃. The results afford a number of significant inisights. (1) The RR (and IR) results show that at least two and possibly more high-frequency bands in the 1400−1550 cm^-1 region exhibit significant frequency downshifts on β-octabromination and, thus, qualify as structure-sensitive marker bands. DFT geometry optimizations indicate that the saddled conformation should be clearly preferred for the β-octabromo-meso-triarylcorrole derivatives studied and that β-octabromination results in expansion of a number of skeletal bond distances of the corrole macrocycle, consistent with observed frequency downshifts. (2) Electrochemical measurements on planar Cu^III[TArC] derivatives have shown that the para substituents on the meso-aryl groups exert a strong influence on the half-wave potentials for oxidation (ρ_ox = ΔE_1/2ox/Δ(3σ) = 95 mV), suggesting that oxidation involves removal of an electron from the corrole "b₁" HOMO, which has significant amplitudes at the meso postions and crudely resembles a porphyrin a_2u HOMO in shape. In contrast, the Hammett ρ_ox is much lower for the nonplanar Cu^III[Br₈TArC] derivatives and we suggest that this ultimately results from a b₁-to-a₂ HOMO reversal which in turn stems from a metal (d_x²-y²)−corrole ("b₁") orbital interaction that becomes symmetry-allowed under a saddle distortion of the corrole macrocycle. In contrast to what has been observed for metallotetraphenylporphyrins, β-octabromination dramatically raises the half-wave potential for one-electron oxidation of the triarylcorrole derivatives studied. This appears to be due to the fact that both the “a₂” and “b₁” HOMOs of a corrole (in C_2v notation) have significantly higher amplitudes at the β positions, compared to a porphyrin a_2u HOMO. Thus, although many metallocorroles are significantly more easily oxidizable than analogous metalloporphyrins, certain β-octahalogeno-meso-triarylcorrole derivatives can indeed be extremely electron deficient and oxidation resistant and may, therefore, find use as rugged catalysts or reagents under highly oxidizing conditions. (3) Finally, the Soret absorption maxima of high-valent metallotriarylcorroles exhibit a uniquely sensitive dependence on the substituents on the meso-aryl groups. Thus, on going from Cu^III[T(p-CF₃-P)C] (T(p-CF₃-P)C = meso-tris((p-trifluoromethyl)phenyl)corrolato) to Cu^III[T(p-OM-P)C] (T(p-OM-P)C = meso-tris(p-methoxyphenyl)corrolato), the Soret maximum red shifts by 26 nm, from 407 to 433 nm. Similarly, on going from Cu^III[Br₈T(p-CF₃-P)C] (Br₈T(p-CF₃-P)C = β-octabromo-meso-tris((p-trifluoromethyl)phenyl)corrolato) to Cu^III[Br₈T(p-OM-P)C] (Br₈T(p-CF₃-P)C = β-octabromo-meso-tris(p-methoxyphenyl)corrolato), the Soret maximum red shifts by 34 nm, from 434 to 468 nm. Time-dependent DFT calculations suggest that this substituent dependence reflects significant ligand-to-metal charge-transfer character of certain transitions in the Soret region. The optical spectra of free-base and non-high-valent transition metal tetrapyrroles, in general, do not exhibit a similar substituent dependence.