Observation of proton-transfer-coupled spin transition by single-crystal neutron-diffraction measurement

The application of single-crystal neutron diffraction (SCND) to observe proton-transfer phenomena in crystalline compounds exhibiting unusual protonation states or proton dynamics has garnered significant research interest in recent years. However, proton tautomerism, which results in different protonation states before and after proton transfer, has never been observed using the SCND technique. Thus, to observe the proton tautomerism phenomenon by SCND measurements, we developed an iron(ii) complex that forms a large crystal and exhibits a proton-transfer-coupled spin transition (PCST). The presence of the two types of proton tautomers was determined by conventional analysis of the proton position by X-ray crystallography, infrared spectroscopy, and density functional theory calculations. Finally, our results confirmed that proton tautomerism was successfully observed for the first time using variable-temperature SCND measurements.

Automated summary

The application of single-crystal neutron diffraction to observe proton-transfer phenomena in crystalline compounds has gained research interest. However, proton tautomerism, which involves different protonation states before and after proton transfer, has never been observed using this technique. In this study, we successfully observed proton tautomerism by developing an iron(ii) complex with a proton-transfer-coupled spin transition and using variable-temperature single-crystal neutron diffraction measurements.