Experimental and Theoretical Evidence for an Unusual Almost Triply Degenerate Electronic Ground State of Ferrous Tetraphenylporphyrin.

Iron porphyrins exhibit unrivalled catalytic activity for electrochemical CO-to-CO conversion. Despite intensive experimental and computational studies in the last 4 decades, the exact nature of the prototypical square-planar [Fe(TPP)] complex (; TPP = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin ( = 1) ground state was contradictorily assigned to either a nondegenerate A state with a (d)(d)(d) configuration or a degenerate E state with a (d)(d)(d)/(d)(d)(d) configuration. To address this question, we present herein a comprehensive, spectroscopy-based theoretical and experimental electronic-structure investigation on complex . Highly correlated wave-function-based computations predicted that A and E are well-isolated from other triplet states by ca. 4000 cm, whereas their splitting Δ is on par with the effective spin-orbit coupling (SOC) constant of iron(II) (≈400 cm). Therfore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the A and E states. This approach enabled us to successfully simulate all spectroscopic data of obtained by variable-temperature and variable-field magnetization, applied-field Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, Δ, an angle θ that describes the mixing of (d)(d)(d) and (d)(d)(d) configurations, and the ⟨⟩ expectation value of the iron d orbitals that is necessary to estimate the Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of is genuinely multiconfigurational with substantial parentages of both A (<88%) and E (>12%), owing to their accidental near-triple degeneracy with Δ = +950 cm. As a consequence of this peculiar electronic structure, exhibits a huge effective magnetic moment (4.2 μB at 300 K), large temperature-independent paramagnetism, a large and positive axial zero-field splitting, strong easy-plane magnetization ( ≈ 3 and ≈ 1.7) and a large and positive internal field at the Fe nucleus aligned in the plane. Further in-depth analyses suggested that ≫ is a general spectroscopic signature of near-triple orbital degeneracy with more than half-filled pseudodegenerate orbital sets. Implications of the unusual electronic structure of for CO reduction are discussed.