Structural Insight into Order-Disorder Transition and Charge-Transfer Phase Transition in an Iron Mixed-Valence Complex (n-C 3 H 7 ) 4 N[Fe II Fe III (dto) 3 ] with a Two-Dimensional Honeycomb Network

The structural properties of the iron mixed-valence complex ( n-C H ) N[Fe Fe (dto) ] (dto = dithiooxalato, C O S ) have been investigated by single-crystal X-ray diffraction (SCXRD) at low temperatures. ( n-C H ) N[Fe Fe (dto) ] has two-dimensional (2D) honeycomb layers consisting of alternating Fe and Fe 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-C H ) N ions between the honeycomb layers. The charge-transfer phase transition (CTPT) occurs at T ∼ 120 K and T ∼ 90 K upon heating and cooling, respectively, with an electron transfer between the Fe and Fe ions, accompanied by a spin-state change, Fe ( S = 2; HS)-O C S -Fe ( S = 1/2; LS) ↔ Fe ( S = 5/2; HS)-O C S -Fe ( S = 0; LS). During the CTPT, the intersheet [Fe Fe (dto) ] 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 FeS and FeO sites were not found in the vicinity of the CTPT. We also calculated the orbital energies and the occupied spin states for the [Fe(O C S ) ] and [Fe(S C O ) ] 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-C H ) N][Fe Fe (dto) ]} , Phase Transition because of Electron Transfer, and a Change in Spin-State of the Whole System. Angew. Chem., Int. Ed. 2001, 40, 4716-4719.].