Using Soft X-ray Absorption Spectroscopy to Evaluate the Bond Lengths of sp2-hybridized Carbons in Organic Solvents: Tracking the C–Si Bond Length during the Hiyama Cross-Coupling Reaction

The reactivity of an organic compound is often discussed in the context of the bond length at the site of the reaction. Yet, evaluating the length of a reactive bond in organic solvents to understand its reactivity, where the majority of an organic reaction occurs, is a demanding task. Here, we showcase the analysis of the electronic structure of organic molecules that contain sp2-hybridized carbons in organic solvents using carbon K-edge X-ray absorption spectroscopy (XAS) with a transmission-type liquid cell, a technique which we have named ‘Soft-XAS-OS’. The energies of various organic molecules containing sp2-hybridized carbons and the energy thresholds of their absorbances in various organic solvents were measured using Soft-XAS-OS, thus revealing the electronic states of the sp2-hybridized carbons free of influence from the organic solvents. Through comparison with inner-shell calculations, Soft-XAS-OS experimentally evaluates bond lengths. Soft-XAS-OS was subsequently applied to investigate the reactivity of an arylsilane and various silicates using the Hiyama cross-coupling reaction as a model reaction. The obtained results revealed that the reactivities of an arylsilicate that bears ethylene glycol and an aryltrimethoxysilane are correlated to their C‒Si bond lengths in tetrahydrofuran.