Measuring Spin-Allowed and Spin-Forbidden d–d Excitations in Vanadium Complexes with 2p3d Resonant Inelastic X‑ray Scattering

Spectroscopic probes of the electronic structure of transition metal-containing materials are invaluable to the design of new molecular catalysts and magnetic systems. Herein, we show that 2p3d resonant inelastic X-ray scattering (RIXS) can be used to observe both spin-allowed and (in the V^III case) spin-forbidden d–d excitation energies in molecular vanadium complexes. The spin-allowed d–d excitation energies determined by 2p3d RIXS are in good agreement with available optical data. In V­(acac)₃, a previously undetected spin-forbidden singlet state has been observed. The presence of this feature provides a ligand-field independent signature of V^III. It is also shown that d–d excitations may be obtained for porphyrin complexes. This is generally prohibitive using optical approaches due to intense porphyrin π-to-π* transitions. In addition, the intensities of charge-transfer features in 2p3d RIXS spectroscopy are shown to be a clear indication of metal–ligand covalency. The utility of 2p3d RIXS for future studies of complex inorganic systems is highlighted

Measuring Spin-Allowed and Spin-Forbidden d–d Excitations in Vanadium Complexes with 2p3d Resonant Inelastic X‑ray Scattering

Spectroscopic probes of the electronic structure of transition metal-containing materials are invaluable to the design of new molecular catalysts and magnetic systems. Herein, we show that 2p3d resonant inelastic X-ray scattering (RIXS) can be used to observe both spin-allowed and (in the V^III case) spin-forbidden d–d excitation energies in molecular vanadium complexes. The spin-allowed d–d excitation energies determined by 2p3d RIXS are in good agreement with available optical data. In V­(acac)₃, a previously undetected spin-forbidden singlet state has been observed. The presence of this feature provides a ligand-field independent signature of V^III. It is also shown that d–d excitations may be obtained for porphyrin complexes. This is generally prohibitive using optical approaches due to intense porphyrin π-to-π* transitions. In addition, the intensities of charge-transfer features in 2p3d RIXS spectroscopy are shown to be a clear indication of metal–ligand covalency. The utility of 2p3d RIXS for future studies of complex inorganic systems is highlighted

X‑ray Absorption and Emission Spectroscopic Studies of [L₂Fe₂S₂]^<i>n</i> Model Complexes: Implications for the Experimental Evaluation of Redox States in Iron–Sulfur Clusters

Herein, a systematic study of [L₂Fe₂S₂]^<i>n</i> model complexes (where L = bis­(benzimidazolato) and <i>n</i> = 2-, 3-, 4-) has been carried out using iron and sulfur K-edge X-ray absorption (XAS) and iron Kβ and valence-to-core X-ray emission spectroscopies (XES). These data are used as a test set to evaluate the relative strengths and weaknesses of X-ray core level spectroscopies in assessing redox changes in iron–sulfur clusters. The results are correlated to density functional theory (DFT) calculations of the spectra in order to further support the quantitative information that can be extracted from the experimental data. It is demonstrated that due to canceling effects of covalency and spin state, the information that can be extracted from Fe Kβ XES mainlines is limited. However, a careful analysis of the Fe K-edge XAS data shows that localized valence vs delocalized valence species may be differentiated on the basis of the pre-edge and K-edge energies. These findings are then applied to existing literature Fe K-edge XAS data on the iron protein, P-cluster, and FeMoco sites of nitrogenase. The ability to assess the extent of delocalization in the iron protein vs the P-cluster is highlighted. In addition, possible charge states for FeMoco on the basis of Fe K-edge XAS data are discussed. This study provides an important reference for future X-ray spectroscopic studies of iron–sulfur clusters

Measurement of the Ligand Field Spectra of Ferrous and Ferric Iron Chlorides Using 2p3d RIXS

Ligand field spectra provide direct information about the electronic structure of transition metal complexes. However, these spectra are difficult to measure by conventional optical techniques due to small cross sections for d-to-d transitions and instrumental limitations below 4000 cm^–1. 2p3d resonant inelastic X-ray scattering (RIXS) is a second order process that utilizes dipole allowed 2p to 3d transitions to access d–d excited states. The measurement of ligand field excitation spectra by RIXS is demonstrated for a series of tetrahedral and octahedral Fe­(II) and Fe­(III) chlorides, which are denoted Fe­(III)-<i>T</i>_<i>d</i>, Fe­(II)-<i>T</i>_<i>d</i>, Fe­(III)-<i>O</i>_<i>h</i>, and Fe­(II)-<i>O</i>_<i>h</i>. The strong 2p spin–orbit coupling allows the measurement of spin forbidden transitions in RIXS spectroscopy. The Fe­(III) spectra are dominated by transitions from the sextet ground state to quartet excited states, and the Fe­(II) spectra contain transitions to triplet states in addition to the spin allowed ⁵Γ → ⁵Γ transition. Each experimental spectrum is simulated using a ligand field multiplet model to extract the ligand field splitting parameter 10Dq and the Racah parameters <i>B</i> and <i>C</i>. The 10Dq values for Fe­(III)-<i>T</i>_<i>d</i>, Fe­(II)-<i>T</i>_<i>d</i>, and Fe­(III)-<i>O</i>_<i>h</i> are found to be −0.7, −0.32, and 1.47 eV, respectively. In the case of Fe­(II)-<i>O</i>_<i>h</i>, a single 10Dq parameter cannot be assigned because Fe­(II)-<i>O</i>_<i>h</i> is a coordination polymer exhibiting axially compressed Fe­(II)Cl ₆ units. The ⁵T → ⁵E transition is split by the axial compression resulting in features at 0.51 and 0.88 eV. The present study forms the foundation for future applications of 2p3d RIXS to molecular iron sites in more complex systems, including iron-based catalysts and enzymes