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Two‐Step Spin Transition in an Iron(II) Coordination Network Based on Flexible Bitopic Ligand 1‐(Tetrazol‐1‐yl)‐3‐(1,2,3‐triazol‐1‐yl)propane
Author(s) -
Białońska Agata,
Bronisz Robert,
Kusz Joachim,
Zubko Maciej
Publication year - 2013
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201200960
Subject(s) - chemistry , spin crossover , crystallography , tetrazole , ion , molecule , ligand (biochemistry) , spin transition , spin states , methylene , stereochemistry , inorganic chemistry , medicinal chemistry , biochemistry , receptor , organic chemistry
Flexible bitopic ligand 1‐(tetrazol‐1‐yl)‐3‐(1,2,3‐triazol‐1‐yl)propane (ptrtz) reacts with Fe(ClO 4 ) 2 · 6H 2 O to form two‐dimensional (2D) coordination network [Fe(ptrtz) 3 ](ClO 4 ) 2 · MeCN ( 1 ) in which two chemically different spin crossover (SCO) Fe II centers are observed. Although two 1,2,3‐triazole and four tetrazole rings are in the first coordination sphere of the Fe1 ions, Fe2 ions are coordinated to four 1,2,3‐triazole and two tetrazole rings. Symmetry‐related Fe II ions are bridged by two disordered ptrtz molecules resulting in the formation of chains that are extended in the same direction. Alternately arranged chains containing Fe1 or Fe2 ions are cross‐linked by single ptrtz molecules to form 2D layers. At room temperature, the complex remains in a high‐spin (HS) form ( χ M T = 3.46 cm 3 K mol –1 ). Spin crossover in 1 occurs in two steps separated by an inflection point at 140 K ( χ M T = 2.68 cm 3 K mol –1 , γ HS ≈ 0.76). The first step is gradual and between 220 and 140 K about a quarter of all the iron(II) ions change their spin state ( T 1 ↓ = T 1 ↑ = 165 K). Single‐crystal X‐ray diffraction studies revealed that upon lowering the temperature from 220 to 140 K, spin crossover occurs for only half of the iron(II) ions occupying Fe2 sites. It is accompanied by an alteration of the occupancy factor of ptrtz molecules bridging the Fe2 ions. Iron(II) ions occupying Fe1 sites remain in the HS form, which inhibits compression of the polymeric layer in the Fe1 ··· Fe1 and Fe2 ··· Fe2 linking direction. Upon further lowering the temperature, iron(II) ions occupying Fe1 sites and the rest of the iron(II) ions occupying Fe2 sites change their spin state. The second step of the spin transition is very abrupt and is accompanied by a hysteresis loop ( T 2 ↓ = 90 K, T 2 ↑ = 96 K). The change in the spin state by iron(II) ions occupying Fe1 and Fe2 sites involves, contrary to first step, a significant shortening of the Fe1 ··· Fe1 and Fe2 ··· Fe2 separations.