Structural Study by Wide-Angle X-ray Scattering of the Spin Transition Molecular Materials [Fe(Htrz)₂(trz)](BF₄) and [Fe(NH₂trz)₃](NO₃)₂(Htrz = 1,2,4-4H-Triazole, trz = 1,2,4-Triazolato)

A precise structural study by wide-angle X-ray scattering (WAXS) has been carried out on two spin transition materials: [Fe(Htrz) 2 (trz)](BF 4 ) ( 1 ) and [Fe(NH 2 trz) 3 ](NO 3 ) 2 ( 2 ), with Htrz = 1,2,4-4H-triazole, trz = 1,2,4-triazolato, and NH 2 trz = 4-amino-1,2,4-triazole, both at room temperature in the low-spin state (LS) and at higher temperature in the high-spin state (HS). These compounds exhibit cooperative spin transitions between LS and HS state above room temperature with a large thermal hysteresis and a pronounced thermochromic effect. Both compounds have a polymeric chain structure where each Fe 2+ metal ion is triply bridged to their neighbors through nitrogen atoms of the triazole or triazolato groups. In the LS state the polymeric chain is linear, whereas at higher temperature, in the HS state, the magnetic transition causes an elongation and a deformation of the chain. The chain loses its linear character. We explain this behavior by a rhombic-type distortion of the iron octahedral environment associated with the spin transition. Compound 1 presents an important interchain order in the LS state, probably due to a hydrogen-bond network connecting together linear chains through N−H···BF 4 ···H−N linkages, where the nitrogen atoms occupy the 4-position of the triazole ligands. When the spin transition occurs, the interchain long-range order vanishes. For compound 2 each polymeric chain seems to be independent from its neighbors.