pubs.acs.org/IC Published on Web 09/17/2010 r 2010 American Chemical Society 9358 Inorg. Chem. 2010, 49, 9358–9368 DOI: 10.1021/ic101038z Cyanocarbanion-Based Spin-Crossover Materials: Photocrystallographic and Photomagnetic Studies of a New Iron(II) Neutral Chain Gaelle Dupouy, † Smail Triki,* ,† Mathieu Marchivie, †,‡ Nathalie Cosquer, † Carlos J. G  omez-Garcı ´a, § S  ebastien Pillet, ^ El-Eulmi Bendeif, ^ Claude Lecomte, ^ Saket Asthana, z and Jean-Franc -ois L  etard z † Universit e Europ eenne de Bretagne (ueb), Universit e de Brest, UMR CNRS 6521, 6 Avenue V. Le Gorgeu, C.S. 93837, 29238 Brest, France, § Instituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Valencia, Spain, ^ CRM2, UMR CNRS 7036, Institut J. Barriol, Nancy-Universit e, BP239, 54506 Vandoeuvre les Nancy, France, and z CNRS, Universit e Bordeaux, ICMCB, 87 Av. Doc. A. Schweitzer, F-33608 Pessac, France. ‡ Present address: EA 4138, Universit e de Bordeaux, 146 rue L eo Saignat, 33076 Bordeaux, France. Received May 22, 2010 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. Introduction Spin-crossover (SCO) complexes have been widely inves- tigated over the last 30 years 1,2 and are still attracting intense research interest given the many possible applications of such complexes in the development of novel electronic devices, especially as molecular switches. 3,4 The electronic activity of the SCO complexes is inherent to the potential of some pseudo-octahedral transition-metal complexes to display a magnetic transition between the high-spin (HS) and low- spin (LS) states through external stimuli such as temperature, pressure, magnetic field, or light irradiation. 4-7 Such a SCO phenomenon occurs in the d 4 -d 7 transition-metal complexes, 8-15 but the most studied examples to date are those based on Fe II (the d 6 configuration), for which a *To whom correspondence should be addressed. E-mail: smail.triki@ univ-brest.fr. (1) G€ utlich, P., Goodwin, H. A., Eds. Spin Crossover in Transition Metal Compounds I-III. 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