Experimentally Quantifying Small-Molecule Bond Activation Using Valence-to-Core X-ray Emission Spectroscopy

Abstract

This work establishes the ability of valence-to-core x-ray emission spectroscopy (XES) to serve as a direct probe of N2 bond activation. A systematic series of iron-N2 complexes has been experimentally investigated and the energy of a valence-to-core XES peak correlated with N-N bond length and stretching frequency. Computations demonstrate that, in a simple one-electron picture, this peak arises from the N2 2s2s σ* orbital, which becomes less antibonding as the N-N bond is weakened and broken. Changes as small as 0.02 Å in the N-N bond length may be distinguished using this approach. The results thus establish valence-to-core XES as an effective probe of small molecule activation, which should have broad applicability in transition metal mediated catalysis.

X-ray Absorption and Emission Study of Dioxygen Activation by a Small-Molecule Manganese Complex

“…The peroxo ligand is expected to have VtC transitions arising from both 2s–2s bonding and 2s–2s antibonding combinations. 40 Though not resolved in this spectral region, the significant increase in the fwhm is consistent with discrete transitions at different energies in place of a single transition expected in 3 . Furthermore, the decreased intensity can be attributed to the longer (by 0.072 Å) Mn–O bonds in 2 versus 3 .…”

“…As the optimized Mn–O–O bond angle is increased to 100° for the peroxo 2 and finally to 112° for the alkylperoxo, the ratio of bonding and antibonding intensity is found to change dramatically, as the 2s–2s bonding combination moves further from the Mn and the antibonding combination is increasingly oriented toward the Mn 3p σ . It is noted that investigation of a hypothetical end-on coordination mode was attempted in light of previous work on dinitrogen with a small-molecule iron complex ( vide infra ); 40 however, the sp hybridization of the oxygens required to accommodate this geometry is somewhat unrealistic and produced unsatisfactory results. The coordination mode of the peroxo unit in 2 thus has suboptimal orientation to provide appreciable K β ″ intensity, and is likely the reason for the poor experimental resolution of this feature.…”

A Practical Guide to High‐resolution X‐ray Spectroscopic Measurements and their Applications in Bioinorganic Chemistry

“…Hence C, N, and O, may be distinguished by VtC XES, as can individual protonation events . Moreover, insight into metal‐ligand bond lengths, and the activation of diatomic ligands (CO, N 2 , O 2 , or NO) may also be extracted from analysis of VtC spectra ,…”

Two‐Color Valence‐to‐Core X‐ray Emission Spectroscopy Tracks Cofactor Protonation State in a Class I Ribonucleotide Reductase

“…VtC transitions), [10] the resulting spectra are sensitive to the electronic structure of the ligands. The identity [11] and number of ligands, [12] the degree of activation of diatomic ligands, [13] and, in some cases, bond angles [14] can all be probed by VtC XES. Indeed, the sensitivity of VtC XES to ligand electronic structure is sufficient to identify ligand protonation events in small molecule systems, both theoretically [15] and experimentally.…”