Structural Insight into Order-Disorder Transition and Charge-Transfer Phase Transition in an Iron Mixed-Valence Complex (n-C3H7)4N[FeIIFeIII(dto)3] with a Two-Dimensional Honeycomb Network

Miho Itoi; Atsushi Okazawa; Jun-Ichi Yamaura; Sachiko Maki; Tokutaro Komatsu; Isabelle Maurin; Epiphane Codjovi; Kamel Boukheddaden; Norimichi Kojima

doi:10.1021/acs.inorgchem.8b02211

Structural Insight into Order-Disorder Transition and Charge-Transfer Phase Transition in an Iron Mixed-Valence Complex (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] with a Two-Dimensional Honeycomb Network

Inorg Chem. 2018 Nov 5;57(21):13728-13738. doi: 10.1021/acs.inorgchem.8b02211. Epub 2018 Oct 19.

Authors

Miho Itoi¹, Atsushi Okazawa², Jun-Ichi Yamaura³, Sachiko Maki³, Tokutaro Komatsu⁴, Isabelle Maurin⁵, Epiphane Codjovi⁶, Kamel Boukheddaden⁶, Norimichi Kojima⁷

Affiliations

¹ Division of Physics, Institute of Liberal Education , Nihon University School of Medicine , Itabashi-Ku , Tokyo 173-8610 , Japan.
² Department of Basic Science, Graduate School of Arts and Sciences , The University of Tokyo , Meguro-Ku , Tokyo 153-8902 , Japan.
³ Materials Research Center for Element Strategy , Tokyo Institute of Technology , Yokohama , Kanagawa 226-8503 , Japan.
⁴ Division of Chemistry, Institute of Liberal Education , Nihon University School of Medicine , Itabashi-Ku , Tokyo 173-8610 , Japan.
⁵ Physique de la Matière Condensée, Ecole Polytechnique, CNRS , Université Paris-Saclay , Route de Saclay , 91120 Palaiseau , France.
⁶ Groupe d'Etudes de la Matière Condensée, UMR 7643 , CNRS-Université de Versailles Saint-Quentin-en-Yvelines , 45 Avenue des Etats Unis , FR-78035 Versailles , France.
⁷ Toyota Physical and Chemical Research Institute , Nagakute , Aichi 480-1192 , Japan.

PMID: 30339001
DOI: 10.1021/acs.inorgchem.8b02211

Abstract

The structural properties of the iron mixed-valence complex ( n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto = dithiooxalato, C₂O₂S₂) have been investigated by single-crystal X-ray diffraction (SCXRD) at low temperatures. ( n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] has two-dimensional (2D) honeycomb layers consisting of alternating Fe^II and Fe^III arrays bonded by bis-bidentate dithiooxalato ligands. Upon cooling, a superlattice structure with q = (1/3, 1/3, 0) was observed below 260 K, which corresponds to an order-disorder transition of the ( n-C₃H₇)₄N⁺ ions between the honeycomb layers. The charge-transfer phase transition (CTPT) occurs at T_C↑1/2 ∼ 120 K and T_C↓1/2 ∼ 90 K upon heating and cooling, respectively, with an electron transfer between the Fe^II and Fe^III ions, accompanied by a spin-state change, Fe^II ( S = 2; HS)-O₂C₂S₂-Fe^III ( S = 1/2; LS) ↔ Fe^III ( S = 5/2; HS)-O₂C₂S₂-Fe^II ( S = 0; LS). During the CTPT, the intersheet [Fe^IIFe^III(dto)₃] distance decreased monotonously upon cooling, and an abrupt structural contraction was observed in the hexagonal 2D network. The volume contraction during the CTPT was quite small (∼0.7%), and differences in the structural distortions at the FeS₆ and FeO₆ sites were not found in the vicinity of the CTPT. We also calculated the orbital energies and the occupied spin states for the [Fe(O₂C₂S₂)₃] and [Fe(S₂C₂O₂)₃] octahedra in the vicinity of the CTPT by density functional theory (DFT). Because the local symmetry around the two coordinating iron ions is already lowered to trigonal symmetry, the CTPT did not cause any further deformation. This symmetry invariance resulted in an absence of orbital contributions to the total entropy change (Δ S) in the CTPT, which is in agreement with the previous heat capacity measurements. [Nakamoto, T; Miyazaki, Y; Itoi, M; Ono, Y; Kojima, N; Sorai, M. Heat Capacity of the Mixed-Valence Complex {[( n-C₃H₇)₄N][Fe^IIFe^III(dto)₃]}_∞, Phase Transition because of Electron Transfer, and a Change in Spin-State of the Whole System. Angew. Chem., Int. Ed. 2001, 40, 4716-4719.].