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 [FeII(TPP)] complex (1; TPP2- = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin (S = 1) ground state was contradictorily assigned to either a nondegenerate 3A2g state with a (dxy)2(dz2)2(dxz,yz)2 configuration or a degenerate 3Egθ state with a (dxy)2(dxz,yz)3(dz2)1/(dz2)2(dxy)1(dxz,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 3A2g and 3Egθ are well-isolated from other triplet states by ca. 4000 cm-1, whereas their splitting ΔA-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 3A2g and 3Egθ states. This approach enabled us to successfully simulate all spectroscopic data of 1 obtained by variable-temperature and variable-field magnetization, applied-field 57Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, ΔA-E, an angle θ that describes the mixing of (dxy)2(dxz,yz)3(dz2)1 and (dz2)2(dxy)1(dxz,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 ΔA-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 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.

Modulating Effect of Ligand Charge on the Electronic Properties of 2Ni-2S Structures and Implications for Biological 2M-2S Sites.

Sulfur-bridged bimetallic 2M-2S type structures are essential cofactors that participate in biological long-range electron transport and metabolism. Metal-sulfur bond covalency is a decisive property for inner sphere (through-bond) type electron transfer that dominates in buried or hydrophobic protein environments. This work reports on a combined experimental and computational study of the effect of ligand charge on the electronic structure of a 2Ni-2S model site that adopts the biologically relevant S = 1/2 redox state. Starting out from an isostructural dinickel(1.5+)-dithiophenolate platform with sulfur-bridged tetrahedral Ni sites, η2:η2-µ-coordination of the S = 1/2 [2Ni-2S]+ core to either a neutral π-system or strongly σ-donating cyclohexadienido renders its electronic structure substantially different. Density functional theory analysis corroborates pulse and continuous wave electron paramagnetic resonance data that associate co-ligand charge with the significant change in the mechanism and size of electron-31P nuclear spin hyperfine coupling to a phosphine reporter ligand at each nickel center. An increasing level of charge donation attenuates direct and through-bridge electronic coupling of the metal sites, resulting in a stronger electronic coupling of the 2Ni-2S core to its terminal phosphine donors. Drawing a connection to biological 2M-2S sites, our 2Ni-2S system indicates that a fine balance of intracore and core-protein electronic coupling is key to biological function for which the degree of charge donation by peripheral donors appears to be a significant parameter.

Coordination Modes, Oxidation, and Protonation Levels of 2,6-Pyridinediimine and 2,2':6',2'́-Terpyridine Ligands in New Complexes of Cobalt, Zirconium, and Ruthenium. An Experimental and Density Functional Theory Computational Study.

The syntheses and molecular and electronic structures of the following complexes have been established by single crystal X-ray crystallography and UV-vis-NIR spectroscopy, and verified by density functional theory calculations (DFT B3LYP): [(η5-Cp)2ZrIV(tpy2-)]0 ( S = 0) 1, [(η5-Cp)2ZrIV(OMepdi2-)]0 ( S = 0) 2, [CoII(OMepdi•)(η2-BH4)]0 ( S = 0) 4, [RuII(OMepdi-H)Cl(PPh3)2]0 ( S = 0) 5, cis-[RuII(OMepdi0)Cl2(PPh3)]0 ( S = 0) 6, and [RuII(η2-OMepdi0)(η2-OMepdi-H)2]0 ( S = 0) 7, with (tpy0) being neutral 2,2':6',2'́-terpyridine, (tpy•)1- its π radical anion, (tpy2-)2- its dianion; (OMepdi0) neutral 2,6-bis(4-methoxyphenylmethylimine)pyridine, (OMepdi•)1- its radical anion and (OMepdi2-)2- its dianion; (OMepdi-H)1- represents the deprotonated form of the (OMepdi0) ligand where deprotonation takes place at the meta-position of the pyridine ring. Density functional theory calculations using the B3LYP functional were performed, establishing geometry optimized molecular and electronic structures. The structural parameter Δ = [(average distance Cpy-Cimine) - (av. distance Cpy-Npy + av. distance Cimine-Nimime)] is introduced for the characterization of the oxidation level of pdi (and analogously of tpy) ligands of M(pdi) (or M(tpy)) motifs for first row transition metals. The M(L0) unit in second and third row low-valent transition metal ion complexes may exhibit significant π-backdonation M → L0 structural effects.

Electronic Structure and Spin Multiplicity of Iron Tetraphenylporphyrins in Their Reduced States as Determined by a Combination of Resonance Raman Spectroscopy and Quantum Chemistry.

Iron tetraphenylporphyrins are prime candidates as catalysts for CO2 reduction. Yet, even after 40 years of research, fundamental questions about the electronic structure of their reduced states remain, in particular as to whether the reducing equivalents are stored at the iron center or at the porphyrin ligand. In this contribution, we address this question by a combination of resonance Raman spectroscopy and quantum chemistry. Analysis of the data allows for an unequivocal identification of the porphyrin as the redox active moiety. Additionally, determination of the spin state of iron is possible by comparing the characteristic shifts of spin and oxidation-state-sensitive marker bands in the Raman spectrum with calculations of planar porphyrin model structures.

Electronic Structure of a Formal Iron(0) Porphyrin Complex Relevant to CO2 Reduction.

Iron porphyrins can act as potent electrocatalysts for CO2 functionalization. The catalytically active species has been proposed to be a formal Fe(0) porphyrin complex, [Fe(TPP)]2- (TPP = tetraphenylporphyrin), generated by two-electron reduction of [FeII(TPP)]. Our combined spectroscopic and computational investigations reveal that the reduction is ligand-centered and that [Fe(TPP)]2- is best formulated as an intermediate-spin Fe(II) center that is antiferromagnetically coupled to a porphyrin diradical anion, yielding an overall singlet ground state. As such, [Fe(TPP)]2- contains two readily accessible electrons, setting the stage for CO2 reduction.

A Novel Cathode Material for Cathodic Dehalogenation of 1,1-Dibromo Cyclopropane Derivatives.

Leaded bronze turned out to be an excellent cathode material for the dehalogenation reaction of cyclopropanes without affecting the strained molecular entity. With this particular alloy, beneficial properties of lead cathodes are conserved, whereas the corrosion of cathode is efficiently suppressed. The solvent in the electrolyte determines whether a complete debromination reaction is achieved or if the process can be selectively stopped at the monobromo cyclopropane intermediate. The electroorganic conversion tolerates a variety of functional groups and can be conducted at rather complex substrates like cyclosporine A. This approach allows the sustainable preparation of cyclopropane derivatives.