Spin crossover (SCO) iron(II) coordination polymer chain: Synthesis, structural and magnetic characterizations of [Fe(abpt)2(μ-M(CN)4)] (M = PtII and NiII)
Graphical abstract
Two iron(II) coordination neutral chain of formula [Fe(abpt)2(μ-M(CN)4)] (M = PtII and NiII) have been synthesized and structurally characterized. Both compounds exhibit SCO transition transitions above room temperature.
Introduction
Metal transition compounds exhibiting spin crossover (SCO) behavior have been widely investigated over the last decade and are still attracting intense research interest given their many potential applications in the development of novel electronic devices [1]. The applied perspectives of the SCO complexes are inherent to the potential of some pseudo-octahedral transition metal complexes to display a magnetic transition between the high spin (HS) and the low spin (LS) states through external stimuli such as temperature, pressure, magnetic field or light irradiation [2], [3], [4], [5]. The SCO phenomenon occurs in the d4–d7 transition metal complexes, but the most studied examples to date are those based on Fe(II) (d6 configuration), for which a reversible paramagnetic–diamagnetic transition from the HS (S = 2, 5T2g) to the LS (S = 0, 1A1g) state is observed.
Up to now, numerous mononuclear compounds (mainly Fe(II) complexes) have been extensively studied [2], [3], [4], [5]. In such discrete systems, the intermolecular interactions (π-stacking, hydrogen bonding and van der Waals interactions), which generate the supramolecular architecture in the solid state, play a crucial role in the transmission of the magneto-elastic cooperative interactions at the origin of the hysteretic behavior. Unfortunately, their non covalent character precludes any control on the way they extend in the solid structure, and consequently on the SCO characteristics. In order to better explore the cooperative effect between the active metal sites, an alternative approach, based on the use of suitable bridging ligands to covalently connect the metal centers, has been introduced [6]. This strategy has already resulted in the syntheses of several Fe(II) SCO coordination polymers exhibiting rich and fascinating structural features coupled to their magnetic behaviors. However, in contrast to the mononuclear compounds, these studies remain scarcely reported because of the limited number of potentially appropriate bridging ligands. In this context, we have extended this polymeric approach to the highly conjugated cyanocarbanion ligands involving several potentially donating nitrogen atoms juxtaposed in such a way that they cannot all coordinate to the same metal ion [7]. We have recently reported a detailed photocrystallographic and photomagnetic study [4] on the first SCO iron(II) molecular neutral chain [Fe(abpt)2(tcpd)] involving an anion as bridging ligand ((tcpd)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). Continuing our investigations on such SCO polymeric materials, we have extended this study to more rigid anionic bridging ligands.
In this context, we have prepared two new SCO 1D-polymeric coordination compounds [Fe(abpt)2(μ-M(CN)4)]·xH2O, with M = PtII (1) and NiII (2), where the tetracyanometallate planar anions [M(CN)4]2− act as μ2-bridging ligands. The magnetic properties of a hemihydrated form of the Ni derivative (compound 2) have been reported very recently by Trávníček et al. [8], although the crystal structure was not reported. Therefore, here we will focus on the crystal structure characterizations of both compounds (1 and 2) and the magnetic properties of the Pt derivative (1).
Section snippets
General remarks
All reactions were carried out under aerobic conditions. The starting materials and solvents were purchased from commercial sources (analytical reagent grade) and used without further purification.
Syntheses of [Fe(abpt)2(μ-M(CN)4)] (M = PtII (1), NiII (2))
An ethanolic solution (10 mL) of abpt (95.3 mg, 0.4 mmol) was added progressively, under continuous stirring, to 10 mL of a 1:1 water/methanol solution containing Fe(BF4)2·6H2O (67.5 mg, 0.2 mmol) and the corresponding salt of K2[M(CN)4] (M = PtII (1): 75.5 mg, 0.2 mmol; M = NiII (2): 48.2 mg, 0.2 mmol). For both
Results and discussions
The IR spectrum of 1 shows sharp peaks attributed to the distinctive bond vibrations modes of the abpt ligand, i.e. (i) the ν(NH) absorption bands of the amino group in the 3250-3140 cm−1 region, (ii) the stretching modes of the aromatic (CH) bonds detected in the 3080–3015 cm−1 region and (iii) the stretching modes of the aromatic (CC) and (CN) bonds localized in the 1635–1500 and 1500–1400 cm−1 regions, respectively [14]. The modifications and the shifts of those bands, with respect to the IR
Conclusions
Using single and powder X-ray diffraction, we have structurally characterized the first two examples of one dimensional neutral chain involving tetracyanometallate bridging ligands, [Fe(abpt)2(μ-M(CN)4)] (M = PtII (1), NiII (2)). As observed previously for the NiII analogue (2) [8], compound 1 displays a SCO transition above room temperature, making difficult to reach any structural and electronic information on the HS state. Thus, in order to better control the transition temperature in such SCO
Acknowledgments
The authors acknowledge the CNRS (Centre National de la Recherche Scientifique), the Brest University, the french “Ministère de la Recherche and Ministère des Affaires Etrangères et Européennes” (PHC Maghreb project N° 30255ZJ), Agence Nationale de la Recherche (ANR Project BISTA-MAT: ANR-12-BS07-0030-01), the Spanish Ministerio de Economía y Competitividad (Project CTQ2011-26507), the Generalitat Valenciana (Project Prometeo 2009/095) and the “Université de Lorraine” for financial support.
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