Magnetic bistability in new molecular systems based on bridging anionic ligands – BISTA-MAT

Up to now, numerous SCO materials have been synthesized and described in the literature, on the basis of the coordination of a cationic metallic active site by neutral and/or anionic organic ligands. However it has never been investigated the potential impact of using an anionic network for connecting the active iron(II) metal centers on the switching properties (hysteresis width, abruptness of the transition,…). The use of anionic bridging ligands effectively reveals an attractive strategy with promising SCO characteristics, such as the design of neutral polymeric chain materials, absence of counter-ions in the lattice, possibly enhanced cooperativity and magnetic coupling.

BISTA-MAT project, deals with molecular bistability, in the spin crossover (SCO) complexes, which is, undoubtedly, one of the most searched properties in molecular materials nowadays and is considered as the relevant base for the next generation of information storage devices. The main objective is to design SCO materials with perfectly controlled magnetic and photo-magnetic behaviours in a rational chemical synthetic strategy. Up to day, we have prepared and studied several new series of switchable cooperative materials. The most significant results concerns the following systems: (i) a new cooperative and robust spin crossover triazole-based FeII 1D coordination polymer [Fe(bntrz)3][Pt(CN)4].H2O, exhibiting a sharp transition at 240 K, accompanied with an anisotropic deformation of the unit cell and a significant gliding of the chains from each other. These two features have been identified as the key parameters of the non-conventional mechanical resilience of this system (Chem. Mater., 2017, 29, 490–494); (ii) two-dimensional Hofmann-like spin-crossover material that experiences strong elastic frustration, leading to an incomplete spin transition. Under light, a hidden stable low-spin state is reached, revealing the existence of a hidden thermal hysteresis and multistability features. These characteristics pave the way for a multidirectional photoswitching and allow potential applications for electronic devices based on ternary digits (Inorg. Chem. 2016, 55, 11652, and Volume 55, Issue 22, pubs.acs.org/toc/inocaj/55/22, for invited cover picture). In parallel, this study has been extended to the design of new polyfunctional materials exhibiting two differents properties. In this new way, the most significant result an original discrete Fe(II) complex, based on triazole ligand, exhibiting synergetic SCO and luminescent behaviours.

This project, gathering four complementary French groups which are recognized in their respective fields, deals with molecular bistability which is, undoubtedly, one of the most searched properties in molecular materials for the next generation of information storage devices. In addition to the clearly achieved objectives (see Main results), this project opened a new way for innovative perspectives such as the design of polyfunctional materials involving SCO and photoluminescent behaviours, and European collaborations (J-R Galán Mascarós ICIQ, Tarragona, Spain ; E. Colacio ; University of Grenada, Spain; G. Morgan, Dublin, Irlande and U. Jonas, Siegen, Germany).

Up today, this project led us to 12 publications in international journals involving at least two partners (see below), 11 publications authored by one partner and two chapters. The publication N°7, authored by the four partners was the object of the invited cover picture (see Volume 55, Issue 22, pubs.acs.org/toc/inocaj/55/22, for invited cover picture).
1. F. Setifi, S. Benmansour, M. Marchivie, G. Dupouy, S. Triki, J. Sala-Pala, J.-Y. Salaün, C. J. Gómez-García, S. Pillet, C. Lecomte, E. Ruiz, Inorg. Chem. 2009, 48, 1269.
2. G. Dupouy, M. Marchivie, S. Triki, J. Sala-Pala, C. J. Gomez-Garcia, S. Pillet, C. Lecomte, J.-F. Létard, Chem. Commun., 2009, 3404.
3. G. Dupouy, S. Triki, M. Marchivie, N. Cosquer, C. J. Gómez-García, S. Pillet, E.-E. Bendeif, C. Lecomte, S. Asthana, J.-F. Létard, Inorg. Chem. 2010, 49, 9358.
4. C. Charles, F. Setifi, F. Thétiot, S. Triki, C. J. Gómez-García, Sébastien Pillet, Polyhedron, 2013, 61, 242.
5. Varret F., Boukheddaden K ., Chastanet, G ., Paradis, N. and Létard, J.-F. , Eur. J. Inorg. Chem., 2013, 763.
6. N. Paradis, G. Chastanet, T. Palamarciuc, P. Rosa, F. Varret, K. Boukheddaden, J.-F. Létard, J. Phys. Chem. C. 2015, 119, 20039
7. E. Milin, V. Patinec, S. Triki, E.-E. Bendeif, S. Pillet, M. Marchivie, G. Chastanet, K. Boukheddaden, Inorg. Chem. 2016, 55, 11652
8. E. Milin, S. Belaid, V. Patinec, S. Triki, G. Chastanet, M. Marchivie, Inorg. Chem. 2016, 55, 9038
9. E. Milin, B. Benaicha, F. El Hajj, V. Patinec, S.Triki, M. Marchivie, C. J. Gómez-García, S. Pillet, Eur. J. Inorg. Chem. 2016, 34, 5305.
10. N. Pittala, F. Thétiot, S. Triki, K. Boukheddaden, G. Chastanet, M. Marchivie, Chem. Mater. 2017, 29, 490.
11. N. Pittala, F. Thétiot, C. Charles, S. Triki, K. Boukheddaden, G. Chastanet, M. Marchivie, Chem. Commun., 2017, 53, 8356.
12. M. Nguenar Ndiaye, E.E. Bendeiff, S. Pillet, M. Marchivie, G. Chastanet, K. Boukheddaden (submitted).

This project, gathering four complementary French groups which are recognized in their respective fields (Brest: Partner 1, Nancy: Partner 2, Bordeaux: Partner 3 and Versailles: Partner 4), deals with molecular bistability which is, undoubtedly, one of the most searched properties in molecular materials nowadays and is considered as the relevant base for the next generation of information storage devices. The molecular bistability resulting from a magnetic transition accompanied by a hysteretic behavior still constitutes a challenging aim in the scientific community. Spin crossover (SCO) complexes are one of the most representative examples of molecular switchable materials. The goal nowadays is to design SCO materials with perfectly controlled magnetic and photo-magnetic behaviors in a rational chemical synthetic strategy. However, fascinating challenges have to be solved before considering them for potential applications:
1) optimization of the cooperativity responsible for the existence of thermal hysteresis
2) stabilization of the photo-induced meta-stable high spin (HS) state
3) synergetic combination in the same molecular material of the SCO behavior and a second property such as magnetic ordering.
Up to now, numerous SCO materials have been synthesized and described in the literature, but it has never been investigated the potential impact of using an anionic network for connecting the active iron(II) metal centers on the switching properties (hysteresis width, abruptness of the transition,…). The use of anionic bridging ligands effectively reveals an attractive strategy with promising SCO characteristics, such as the design of neutral polymeric chain materials, absence of counter-ions in the lattice, possibly enhanced cooperativity and magnetic coupling. Based on the complementary expertises of the four Partners, the main objective of this project concerns the design and synthesis of a new generation of SCO molecular materials; the proposed detailed physical characterization should provide important hints to reach the three important points (1-3) mentioned above. In this context, the work plan of this project will be organized into four phases which will be articulated on the principal challenges enumerated above (1-3): synthesis and characterization of appropriate anionic ligands, neutral co-ligands and polynitrile radical ligands (phase 1); design and syntheses of new polymeric SCO materials, using the ligands previously elaborated in phase 1 (phase 2); accurate structural, spectroscopic (including Mössbauer investigations) and magnetic properties of the series of synthesized materials in a systematic manner (phase 3); and modelling of the observed physical and chemical properties of synthesized materials, such as the impact of the rigidity of the ligands on the thermodynamical parameters (phase 4).