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 2 -to-CO conversion. Despite intensive experimental and computational studies in the last 4 decades, the exact nature of the prototypical square-planar [Fe II (TPP)] complex ( 1; TPP 2– = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin ( S = 1) ground state was contradictorily assigned to either a nondegenerate 3 A 2g state with a (d xy ) 2 (d z 2 ) 2 (d xz, yz ) 2 configuration or a degenerate 3 E g θ state with a (d xy ) 2 (d xz, yz ) 3 (d z 2 ) 1 /(d z 2 ) 2 (d xy ) 1 (d xz, yz ) 3 configuration. To address this question, we present herein a comprehensive, spectroscopy-based theoretical and experimental electronic-structure investigation on complex 1 . Highly correlated wave-function-based computations predicted that 3 A 2g and 3 E g θ are well-isolated from other triplet states by ca. 4000 cm –1, whereas their splitting Δ A–E is on par with the effective spin–orbit coupling (SOC) constant of iron(II) (≈400 cm –1 ). Therfore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the 3 A 2g and 3 E g θ states. This approach enabled us to successfully simulate all spectroscopic data of 1 obtained by variable-temperature and variable-field magnetization, applied-field 57 Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, Δ A–E, an angle θ that describes the mixing of (d xy ) 2 (d xz, yz ) 3 (d z 2 ) 1 and (d z 2 ) 2 (d xy ) 1 (d xz, yz ) 3 configurations, and the ⟨ r d –3 ⟩ expectation value of the iron d orbitals that is necessary to estimate the 57 Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of 1 is genuinely multiconfigurational with substantial parentages of both 3 A 2g (<88%) and 3 E g (>12%), owing to their accidental near-triple degeneracy with Δ A–E = +950 cm –1 . As a consequence of this peculiar electronic structure, 1 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 ( g ⊥ ≈ 3 and g ∥ ≈ 1.7) and a large and positive internal field at the 57 Fe nucleus aligned in the xy plane. Further in-depth analyses suggested that g ⊥ ≫ g ∥ 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 1 for CO 2 reduction are discussed.