Xin Bao

The Effect of an Active Guest on the Spin Crossover Phenomenon

A straightforward method for the reversible modification ofsolid-state properties is a goal being constantly pursued in thedevelopment of molecular switches, and molecular electronicand photonic devices. As one of the most attractive molecule-based switchable materials, spin crossover (SCO) complexespresent different magnetic, optical, electrical and structuralproperties in response to external stimuli (such as temper-ature, pressure, light, or magnetic fields), driven by conver-sionoftheelectronconfigurationbetweenhigh spin(HS)andlow spin (LS) states.

Remarkably high-temperature spin transition exhibited by new 2D metal–organic frameworks

Highly stable two-dimensional metal–organic frameworks, 2∞[FeII(L)2] (HL = 3-(2-pyridyl)-5-(3-pyridyl)-1,2,4-triazole (1) and 3-(3-methyl-2-pyridyl)-5-(3-pyridyl)-1,2,4-triazole (2), display well defined two-step spin crossover properties at remarkably high temperatures, namely, Tc1 = 329 K, Tc2 = 501 K for 1 and Tc1 = 351 K, Tc2 = 520 K for 2, which are the highest critical temperatures reported so far.

Tuning the Spin States of Two Apical Iron(II) Ions in the Trigonal‐Bipyramidal [{FeII(μ‐bpt)3}2FeII3(μ3‐O)]2+ Cations Through the Choice of Anions

When located in octahedral environment, the iron(II) ion with a d electronic configuration may adopt two different stable electronic states, namely, a diamagnetic low-spin (LS) state and a paramagnetic high-spin (HS) state, which both give rise to different magnetic, optical and electronic properties. So tuning the spin state of the iron(II) ion is significant and contributes to the development of amazing materials that can be used as molecular switches, sensors and display devices. As is well established, the spin state depends on relative strength of spin paring energy (P) and splitting energy (D0). If the former is much stronger than the latter, the HS state will be stabilised, and if D0 is stronger then the LS state will be the ground state. If P and D0 are comparable a spin crossover (SCO) may occur between the HS and LS state by external perturbations such as temperature, pressure or light irradiation. So the main task is to make a judicious choice of ligand, which can impose a proper ligand-field strength. However, in reality, the spin state of the iron(II) ion is quite sensitive to even more subtle changes such as solvent molecules, polymorphism and counterions. As one of the best representatives of switchable molecules, SCO materials have attracted considerable interest in the chemistry and materials fields. Current work mainly focuses on the enhancement of cooperativity, which results in an abrupt transition and a wider thermal hysteresis. Another promising research area, but with few examples, is the combination of magnetic-exchange and spin-transition (ST) phenomena in the same molecule or polymeric network, which could eventually afford new switching materials with considerable amplification of the response signal. Much more work should be done to expand our knowledge of spin electronics. To induce a ST, the most common method is by varying the temperature. However, light-and pressure-induced SCOs play an increasingly important role owing to their potential applications as optical and pressure switches, for example. Moreover, the latter two methods are not restricted to thermal SCO compounds: LIESST (light-induced excited spinstate trapping) may also be observed in LS compounds, whereas HS compounds may experience STs with the application of external hydrostatic pressure. Although there are many examples that exhibit thermal STs, rare spin-crossover clusters of iron(II) have been found to exhibit a mixed-spin structure and synergy between ST and magnetic interaction. Fortunately, by introducing counterions, we have successfully tuned the spin states of two apical iron(II) ions in the pentanuclear [{Fe ACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3-O)]2+ (Hbpt=3,5-bis(pyridin-2-yl)-1,2,4-triazole) cations through anions. Both apical ions are of LS states in [{Fe ACHTUNGTRENNUNG(mbpt)3}2Fe II 3ACHTUNGTRENNUNG(m3-O)] ACHTUNGTRENNUNG(NCS)2·10H2O (1), [{Fe ACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3O)] ACHTUNGTRENNUNG(ClO4)2·3H2O (2) and [{FeIIACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3-O)]I2· 4MeCN (3), and are of HS states in [{Fe ACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3O)] ACHTUNGTRENNUNG[FeIII2ACHTUNGTRENNUNG(m-O)Cl6]·1/2H2O (4). In this new {Fe5} family, two new developments have been achieved: 1) The ligand 4amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole (abpt) has been, for the first time, used to produce the oxo-centred polynuACHTUNGTRENNUNGcle ACHTUNGTRENNUNGar iron(II) complexes; 2) Two types of spin topology have been trapped in the [{Fe ACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3-O)]2+ cluster that are controlled by counterions. Note that only one [a] X. Bao, J.-D. Leng, Z.-S. Meng, Dr. Z.-J. Lin, Prof. Dr. M.-L. Tong Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education State Key Laboratory of Optoelectronic Materials and Technologies School of Chemistry & Chemical Engineering Sun Yat-Sen University, Guangzhou 510275 (P.R. China) Fax: (+86) 20-8411-2245 E-mail : tongml@mail.sysu.edu.cn [b] Dr. M. Nihei, Prof. Dr. H. Oshio Graduate School of Pure and Applied Sciences University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571 (Japan) Fax: (+81) 29-853-4238 E-mail : oshio@chem.tsukuba.ac.jp Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000526.

Spin Crossover versus Low‐Spin Behaviour Exhibited in 2D and 3D Supramolecular Isomers of [FeII(2,4‐bpt)2]⋅Guest

The bistable properties (magnetic, structure and colour) of spin crossover (SCO) materials have resulted in a bright future for their applications as molecular memory, molecular switches, molecular sensors, data storage and display devices. Spin transitions (STs) between high(HS) and lowspin (LS) states can be achieved by external stimuli, such as temperature, pressure, light, magnetic or electrical switching. From practical aspects, the design and the synthesis of SCO compounds with high transition temperatures, wide thermal hysteresis and thermochroism are of crucial importance. Two strategies have been universally acknowledged to improve the cooperatives: 1) to construct covalently bridged coordination polymers; 2) to enrich intermolecular interactions, such as hydrogen bonding and p–p stacking. However, in reality, it is hard to predict theoretically the SCO property of a material until it has been characterised. The sensitivity of the SCO property to subtle changes does cause problems, but from another perspective, it may also be viewed as an opportunity to deepen our knowledge of the nature of SCO. To this end, the rich structural diversity of supramolecular isomers provided us a useful and unique perspective to understand structure–property relationships. As a branch of isomerism, polymorphism-dependent SCO compounds are not uncommon, however, most of which are mononuclear. To the best of our knowledge, only two examples focus on structural isomerism and another shows catenane isomerism-dependent SCO properties. Herein, we report the crystal structures and physical properties of four supramolecular isomers based on [FeACHTUNGTRENNUNG(2,4bpt)2]·guest (2,4-Hbpt= 3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole): a 2D SCO polymer [FeACHTUNGTRENNUNG(2,4-bpt)2] (1·Fe), two LS twofold interpenetrated 3D coordination polymers with NbO topology, [Fe ACHTUNGTRENNUNG(2,4-bpt)2]·2.17 H2O (2 a) and [Fe ACHTUNGTRENNUNG(2,4bpt)2]·2.5 H2O (2 b) and a LS non-interpenetrated 3D coordination polymer with NbO topology, [FeACHTUNGTRENNUNG(2,4bpt)2]·4 dioxane·4 H2O (3). It should be mentioned that these complexes include all kinds of isomers except for optical isomers, that is, structural, conformational and catenane supramolecular isomerism. Moreover, a cobalt analogue and a Fe Co solid solution species: [Co ACHTUNGTRENNUNG(2,4-bpt)2]·0.5 H2O (1·Co) and [FexCo1 x ACHTUNGTRENNUNG(2,4-bpt)2] (x=0.93) (1·Fe–Co) have also been synthesised and characterised. The latter compound allows us to investigate SCO properties in a doped system. Our work provides a good example to study magneto–structural relationships. We choose the 2,4-Hbpt ligand mainly for the following reasons: 1) as a variant of the well-studied ligand 2,2-Rbpt (2,2-Rbpt= 4-substituted 3,5-di(2-pyridyl)-1,2,4-triazole), it should also provide an appropriate ligand field favouring the occurrence of SCO; 2) it is a good candidate to construct a coordination polymer by using the 4-position N atom, thus enhancing cooperation between SCO sites; 3) the ligand can take on abundant conformations due to its rotatable feature, thus resulting in a variety of isomers and fits our need to study structure-dependent SCO properties. All products are synthesised in solvothermal conditions at 160 8C for 3 days. However, different solvent media have an important influence on the final products. By hydrothermal treatment, five products are captured, namely, two mononuclear polymorphs based on [FeACHTUNGTRENNUNG(2,4-bpt)2ACHTUNGTRENNUNG(H2O)2],[10] 1·Fe, 2 a and 2 b. However, there are problems with low yields for all products and also poor reproducibility. So it was laborious work to collect pure 1·Fe to study its magnetic behaviour. [a] X. Bao, J.-L. Liu, J.-D. Leng, Dr. Z. Lin, Prof. Dr. M.-L. Tong Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education State Key Laboratory of Optoelectronic Materials and Technologies School of Chemistry & Chemical Engineering Sun Yat-Sen University Guangzhou 510275 (P.R.China) Fax: (+86) 20-8411-2245 E-mail : tongml@mail.sysu.edu.cn [b] Dr. M. Nihei, Prof. Dr. H. Oshio Graduate School of Pure and Applied Sciences University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571 (Japan) [**] 2,4-Hbpt=3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001179.

Polymorphism-Dependent Spin-Crossover: Hysteretic Two-Step Spin Transition with an Ordered [HS–HS–LS] Intermediate Phase

A mononuclear iron(II) complex has been isolated in two polymorphs. Polymorph 1b remains high-spin over all temperatures, whereas polymorph 1a undergoes a cooperative two-step spin crossover accompanied by symmetry breaking, showing an ordered 2:1 high-spin-low-spin intermediate phase.

High-Temperature Spin Crossover in Two Solvent-Free Coordination Polymers with Unusual High Thermal Stability

Two solvent-free two-dimensional (2D) coordination polymers, (2)∞[Fe(ptim)2] (1) and (2)∞[Fe(ptpy)2] (2) (Hptim = 2-(5-(4-(1H-imidazol-1-yl)phenyl)-4H-1,2,4-triazol-3-yl)pyridine; Hptpy = 2-(5-(4-(pyridin-3-yl)phenyl)-4H-1,2,4-triazol-3-yl)pyridine), have been successfully prepared by solvothermal reactions. Their iron atoms are bridged by the corresponding multidentate anionic ligands into dense neutral structures. The magnetic data reveal that complexes 1 and 2 are rare examples exhibiting reversible one-step high-temperature spin crossover behaviors with spin transition temperatures of 419 and 416 K, respectively. Moreover, these structures also display remarkable thermal stability up to 714 K (for 1) and 690 K (for 2), which are confirmed by thermogravimetric and variable-temperature powder X-ray diffraction analyses.

Structurally Perfect Ni3(μ1,3-N3)3 Triangles for a Magnetic Model

Two ideal triangular Ni3 compounds [Ni3(μ1,3-N3)3(2,2′-bpy)3]X3·3H2O (X = ClO4- (1) and BF4- (2); 2,2′-bpy = 2,2′-bipyridine) have, for the first time, been structurally and magnetically characterized, and represent a good model for us to understand the magnetic behaviours in extended azide-bridged nickel(II) complexes with trinuclear subunits.

CCDC 804234: Experimental Crystal Structure Determination

Related Article: Jun-Liang Liu, Xin Bao, Ji-Dong Leng, Zhuo-Jia Lin, Ming-Liang Tong|2011|Cryst.Growth Des.|11|2398|doi:10.1021/cg200490v