Probing the Valence Electronic Structure of Low-Spin Ferrous and Ferric Complexes Using 2p3d Resonant Inelastic X-ray Scattering (RIXS)

Abstract

Understanding the detailed electronic structure of transition metal ions is essential in numerous areas of inorganic chemistry. In particular, the ability to map out the many particle d-d spectrum of a transition metal catalyst is key to understanding and predicting reactivity. However, from a practical perspective, there are often experimental limitations on the ability to determine the energetic ordering, and multiplicity of all the excited states. These limitations derive in part from parity and spin-selection rules, as well as from the limited energy range of many standard laboratory instruments. Herein, we demonstrate the ability of 2p3d resonant inelastic X-ray scattering (RIXS) to obtain detailed insights into the many particle spectrum of simple inorganic molecular iron complexes. The present study focuses on low-spin ferrous and ferric iron complexes, including [Fe(tacn)] and [Fe(CN)]. This series thus allows us to assess the contribution of d-count and ligand donor type, by comparing the purely σ-donating tacn ligand to the π-accepting cyanide. In order to highlight the conceptual difference between RIXS and traditional optical spectroscopy, we compare first RIXS results with UV-vis and magnetic circular dichroism spectroscopy. We then highlight the ability of 2p3d RIXS to (1) separate d-d transitions from charge transfer transitions and (2) to determine the many particle d-d spectrum over a much wider energy range than is possible by optical spectroscopy. Our experimental results are correlated with semiempirical multiplet simulations and ab initio complete active space self-consistent field calculations in order to obtain detailed assignments of the excited states. These results show that Δ S = 1, and possibly Δ S = 2, transitions may be observed in 2p3d RIXS spectra. Hence, this methodology has great promise for future applications in all areas of transition metal inorganic chemistry.

Probing Physical Oxidation State by Resonant X‐ray Emission Spectroscopy: Applications to Iron Model Complexes and Nitrogenase

“…It is here that we draw inspiration from the words of Linus Pauling in his 1948 Liversidge lecture where he stated with regard to our understanding of valency, "…and we may hope that powerful methods of investigation that are not yet known will be discovered". [43] In our view, resonant X-ray emission spectroscopy (RXES also known as resonant inelastic X-ray scattering (RIXS)) is atool with the potential to fulfill this role.R XES has historically been at ool for physicists, [44][45][46][47][48] and, while various types of RXES experiments have recently been utilized by the chemistry community, [49][50][51][52][53][54][55][56][57] it is clear that we are still evolving our understanding of the rich chemical information content of these spectra. In particular,s ignificant progress has been made utilizing 2p3d and 1s2p RXES to extract d-d transition energies and L-edgelike information, respectively.…”

“…In particular,s ignificant progress has been made utilizing 2p3d and 1s2p RXES to extract d-d transition energies and L-edgelike information, respectively. [49,52,53,58,59] Additionally,1 s3p RXES has been utilized to obtain spin and oxidation state selective XAS,o ften referred to as Kb-detected XAS. [60][61][62][63][64] However,t oo ur knowledge the detailed chemical and electronic structural information contained in the 1s3p RXES spectra themselves has yet to be fully explored.…”

Theoretical X‐ray spectroscopy of transition metal compounds

“…Since with increasing energy, core excitations are characterized by an increasing quintet–triplet mixing, the elastic peak decreases and spin‐forbidden features grow in intensity. Different spin states of iron complexes have been also studied in Reference .…”

Simulations of valence excited states in coordination complexes reached through hard X-ray scattering

“…The rich spectral information has been used to extract detailed electronic structure information and to follow ultrafast excited-state dynamics. [9][10][11][12] A challenge with soft X-ray RIXS is the significant background absorption associated with the generation of photoelectrons from lighter elements, which also can lead to sample damage even at low X-ray doses. 13,14 For first-row transition metals excitations in the metal K edge, which involves the metal 1s orbital, instead requires photon energies of thousands of eV.…”

“…Metal hexacyanides have been widely used as model systems for new experimental and theoretical X-ray techniques. 11,12,23,41,[48][49][50][51][52][53][54] Valence K-edge RIXS spectra have been collected for three systems with different d-electron counts: [Fe II (CN) 6 ] 4À (ferrocyanide), [Fe III (CN) 6 ] 3À (ferricyanide), and [Mn III (CN) 6 ] 3À (manganicyanide). 23,27 The spectrum for ferrocyanide is shown in Fig.…”