Soft X-ray absorption spectroscopy of organic molecules in organic solvents for applying organic reactions

・　Soft X-ray absorption spectroscopy (XAS) was adopted to measure organic molecules that contain sp²-hybridized carbons in organic solvents (Soft-XAS-OS) for applying the Hiyama cross-coupling reaction, a carbon-carbon bond-forming reaction.

・　The C=C π* peaks of phenyl groups of an arylsilane and silicates in organic solvents were observed using the C K-edge XAS.

・　Combining the XAS spectra with the inner-shell quantum chemical calculations, the energy shifts of the C=C π* peaks were related to the C‒Si bond lengths of an arylsilane and silicates, which are key factors for the reactivities in the Hiyama cross-coupling reaction.

The electronic structures of organic molecules including sp²-hybridized carbons in organic solvents were studied using the C K-edge X-ray absorption spectroscopy (XAS), termed "Soft-XAS-OS". The C‒Si bond lengths of an arylsilane and silicates, which exhibit different reactivity in the Hiyama cross-coupling reaction, were studied using the C=C π* peaks of phenyl groups in the C K-edge XAS spectra with the comparison of the inner-shell quantum chemical calculations combined with the molecular dynamics (MD) simulations.

The length of a chemical bond at a reaction site is known to influence reactivity. Single-crystal X-ray diffraction usually determines the bond lengths in solid samples and cannot be applied to solutions where most organic reactions take place. Solution-phase techniques such as NMR, UV-visible, and infrared spectroscopy provide valuable insights into chemical structures, but are generally unsuitable for evaluating the bond lengths. XAS is an element-specific method to study the electronic structures of organic molecules. The previous work measured the C and N K-edge XAS spectra of organic molecules in aqueous solutions (Soft-XAS-H₂O). There is no established way to apply the XAS measurements to organic molecules in organic solvents, where most organic reactions are conducted.

The C K-edge XAS spectra of organic molecules in organic solvents (Soft-XAS-OS) were measured using the transmission-type liquid cell, where the thickness of the liquid layer is precisely controlled from 20 nm to 40 μm. In the C K-edge XAS spectrum of 100 mM trimethoxyphenylsilane (Ph‒Si(OMe)₃) in tetrahydrofuran (THF), as shown in Fig. 1, the C=C π* peak of phenyl groups of Ph‒Si(OMe)₃ appears below the energy thresholds of solvent THF. The measurements of the energy thresholds of representative organic solutions and the C=C and C=N π* peak of organic molecules confirmed that the electronic states of sp²-hybridized carbons can be distinguished from absorbances of organic solvents.

For studying the mechanism of Hiyama cross-coupling reaction, a carbon-carbon bond-forming reaction, Ph‒Si(OMe)₃, fluorotrimethoxyphenylsilicate (Ph‒Si(OMe)₃F), and ethylene-glycol-derived spirosilicate Ph‒Si(OCH₂CH₂O)₂ were prepared, and these reactivities were analyzed. The structures of different Ph‒Si(OMe)₃F were determined using the ¹⁹F NMR. The C=C π* peak of Ph‒Si(OMe)₃ in the C K-edge XAS can be divided in the ipso, ortho + para, and meta + para carbons of the phenyl group. The ipso C=C π* peak of Ph‒Si(OMe)₃ appears at 284.56 eV, and those of Ph‒Si(OCH₂CH₂O)₂ (284.54 eV) and Ph‒Si(OMe)₃F (284.50 eV) show lower energy shifts. Because the changes of the C‒Si bond lengths affect the energy shits of the XAS peaks, the C K-edge inner-shell quantum chemical calculations of the arylsilane and silicates in organic solvents were performed from the snapshots of the MD simulations. As a result, the energy shifts of the ipso C=C π* peaks can be interpreted in terms of the C(sp²)‒Si bond lengths. In the optimized molecular structures, the C‒Si bond lengths in organic solvents follow the order: Ph‒Si(OMe)₃ < Ph‒Si(OCH₂CH₂O)₂ < Ph‒Si(OMe)₃F. The C‒Si bond length is one of the key factors influencing their reactivity.

The present study established a new soft X-ray absorption spectroscopy technique, Soft-XAS-OS, that enables the analysis of sp²-hybridized carbons in organic solvents. This development is significant because most organic reactions are conducted in organic solvents. The Soft-XAS-OS, combined with inner-shell calculations, provides a powerful approach for probing the bond lengths of organic molecules in a variety of bond-forming organic reactions involving sp²-hybridized carbons, including various cross-coupling reactions.