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

This work establishes the ability of valence-to-core X-ray emission spectroscopy (XES) to serve as a direct probe of N-2 bond activation. A systematic series of iron-N-2 complexes has been experimentally investigated and the energy of a valence-to-core XES peak was correlated with N-N bond length and stretching frequency. Computations demonstrate that, in a simple one-electron picture, this peak arises from the N-2 2s2s sigma* orbital, which becomes less antibonding as the N-N bond is weakened and broken. Changes as small as 0.02 angstrom 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.