Experimental And Theoretical Evidence For An Unusual Almost Triply Degenerate Electronic Ground State Of Ferrous Tetraphenylporphyrin

Iron porphyrins exhibit unrivalled catalytic activity for electrochemical CO2-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; TPP2- = tetraphe-nylporphyrinate 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(z2))(2)(d(xz,yz))(2) configuration or a degenerate E-3(g)theta state with a (d(xy))(2)(d(xz,yz)(3)())(dz(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 wavefunction-based computations predicted that 3A2g and 3Eg. are well-isolated from other triplet states by ca. 4000 cm(-1), whereas their splitting.A- E is on par with the effective spin-orbit coupling (SOC) constant of iron(II) (similar to 400 cm(-1)). Therfore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the 3A2g and 3Eg. states. This approach enabled us to successfully simulate all spectroscopic data of 1 obtained by variable-temperature and variable-field magnetization, applied-field Fe-57 Mossbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC,.A-E, an angle. that describes the mixing of (dxy)2(dxz,yz)3(dz2)1 and (d(z)(2))(2)(d(xy))(1)(d(xz,yz))(3) configurations, and the.rd-3. expectation value of the iron d orbitals that is necessary to estimate the 57Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of 1 is genuinely multiconfigurational with substantial parentages of both 3A2g (<88%) and 3Eg (>12%), owing to their accidental near-triple degeneracy with.A- E = +950 cm(-1). As a consequence of this peculiar electronic structure, 1 exhibits a huge effective magnetic moment (4.2 mu B at 300 K), large temperature-independent paramagnetism, a large and positive axial zero-field splitting, strong easy-plane magnetization (g perpendicular to approximate to 3 and g perpendicular to approximate to 1.7) and a large and positive internal field at the 57Fe 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 CO2 reduction are discussed.