Cyanocarbanion-Based Spin-Crossover Materials: Photocrystallographic and Photomagnetic Studies of a New Iron(II) Neutral Chain

Abstract

A new iron(II) chain of formula [Fe(abpt)(2)(tcpd)] [1; (tcpd)(2-) = [C(10)N(6)](2-) = (C[C(CN)(2)](3))(2-) = 2-dicyanomethylene-1,1,3,3-tetracyanopropanediide anion, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] has been synthesized and characterized by IR spectroscopy, detailed variable-temperature single-crystal X-ray diffraction, magnetic and photomagnetic measurements. The crystal structure determination of 1 reveals a one-dimensional structural architecture in which the (tcpd)(2-) cyanocarbanion acts as a μ(2)-bridging ligand and the two abpt molecules act as chelating ligands. Detailed X-ray diffraction studies as a function of the temperature (293-10 K) showed a strong modification of the iron coordination sphere, whose characteristics are in agreement with the presence of a spin-crossover transition from high spin (HS) to low spin (LS) in 1. The average Fe-N distances at room temperature, at 10 K following a flash cooling, and at 10 K after subsequent HS-to-LS relaxation are in the range expected for 100%, 50%, and 25% fractions of HS Fe(II), respectively. These observations are consistent with the presence of ca. 25% residual HS species at low temperatures, as derived from the magnetic data. The signature of a photoinduced metastable HS state in 1 has been detected by performing coupled photomagnetic and photocrystallographic analyses. The limiting T(LIESST) value associated with the light-induced excited-spin-state trapping effect was derived as 35 K, in good agreement with the thermal dependence of the unit cell volume upon irradiation. Kinetic studies governing the photoinduced HS/LS process have been recorded at different temperatures; a reverse-LIESST effect has been evidenced at 10 K as a reduction of the residual HS fraction by irradiating the sample at 830 nm.

Spin state switching in iron coordination compounds

“…Noticeably, numerous examples of 1D chains made of flexible single bridge spacers systematically reveal a gradual spin conversion as noted for 1,2-bis(4-pyridyl)etha(e)ne [308], 4,4’-bpy [309–310], substituted triazine/pyrimidine [307,311–313], pyrazine [314], dca [315] and tcpd anions [316]. Some other very interesting 1D chains were recently prepared by Weber et al using tetradentate Schiff bases allowing them to set up a N 4 O 2 core and to observe a variety of magnetic behavior including gradual, hysteretic, complete, incomplete and even double-step spin conversions [317–324].…”

Polypyridyl-based Cu(II) coordination polymers: Synthesis, structural and magnetic characterizations

“…Taking into account the crucial role of these anionic ligands, we are interested in using them in combination with other chelating or bridging neutral co-ligands such as the aromatic N-containing ligands to explore the role of their structural and electronic characteristics in the field of molecular materials including switchable materials exhibiting magnetic bistability [7][8][9][10][11][12][13], as in the intriguing derivative [Cu(bpym)(tcnoet) 2 ].H 2 O (bpym = 2,2'-bipyrimidine; (tcnoet) -= 1,1,3,3-tetracyano-2-ethoxypropenide) that displays a zigzag infinite chain with an unusual magnetic transition and reversible thermochromism [13]. Experimental and theoretical studies revealed that both magnetic and thermochromic transitions are induced by several structural and electronic effects such as the flexibility of the cyanocarbanion ligand, the rigidity and the bis-chelating coordination mode of the bpym ligand and the dynamic Jahn-Teller effect of the Cu(II) metal ion.…”

A Spin Crossover Transition in a Mn(II) Chain Compound

“…This kind of incomplete and smooth SCO transition is typical of chain compounds, since when the metallic centers are connected, the transition of one of them exerts a chemical pressure on the bridging ligands that hampers the transition of the neighboring metal centers [45][46][47].…”