The ligand field in low-crystallinity metal-organic frameworks investigated by soft X-ray core-level absorption spectroscopy

Kohei Yamagami; Haruka Yoshino; Hirona Yamagishi; Hiroyuki Setoyama; Arata Tanaka; Ryo Ohtani; Masaaki Ohba; Hiroki Wadati

doi:10.1039/d2cp01415g

The ligand field in low-crystallinity metal-organic frameworks investigated by soft X-ray core-level absorption spectroscopy

Phys Chem Chem Phys. 2022 Jul 13;24(27):16680-16686. doi: 10.1039/d2cp01415g.

Authors

Kohei Yamagami¹, Haruka Yoshino^{2

3

4}, Hirona Yamagishi⁵, Hiroyuki Setoyama⁶, Arata Tanaka⁷, Ryo Ohtani², Masaaki Ohba², Hiroki Wadati^{1

8}

Affiliations

¹ Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwanoha, Chiba 277-8581, Japan.
² Department of Chemistry, Graduate School of Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
³ Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
⁴ Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan.
⁵ Synchrotron Radiation Center, Ritsumeikan University, Kusatsu, Shiga 525-0058, Japan.
⁶ Kyushu Synchrotron Light Research Center, 8-7 Yayoigaoka, Tosu, Saga, 841-0005, Japan.
⁷ Department of Quantum Matter, ADSM, Hiroshima University, Higashihiroshima, Hiroshima 739-8530, Japan.
⁸ Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan.

PMID: 35766583
DOI: 10.1039/d2cp01415g

Abstract

The ligand field (LF) of transition metal ions is a crucial factor in realizing the mechanism of novel physical and chemical properties. However, the low-crystallinity state, including the amorphous state, precludes the clarification of the electronic structural relationship of transition metal ions using crystallographic techniques, ultraviolet and infrared optical methods, and magnetometry. Here, we demonstrate that soft X-ray 2p → 3d core-level absorption spectroscopy (L_2,3-edge XAS) systematically revealed the local 3d electronic states, including in the LF, of nitrogen-coordinated transition-metal ions for low-crystallinity cyanide-bridged metal-organic frameworks (MOFs) M[Ni(CN)₄] (MNi; M = Mn, Fe, Co, Ni) and Ni[Pd(CN)₄] (NiPd). In NiNi and NiPd, N-coordinated Ni ions with square-planar symmetry exhibit strong orbital hybridization and ligand-to-metal charge transfer effects. In MnNi, FeNi, and CoNi, the correlation between the crystalline electric field splitting in the LF and the transition metal-nitrogen bonding length is revealed using the multiplet LF theory. Regardless of the different local symmetries, our results indicate that L_2,3-edge XAS is a powerful tool for gaining element-specific knowledge about the transition-metal ion characterizing the functionality of low-crystallinity MOFs and will be the foundation for an attractive platform, such as adsorption/desorption materials.