External Pressure Effect on a Two-fold Interpenetrated 3D PtS-type

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External Pressure Effect on a Two-fold Interpenetrated 3D PtS-type Spin-crossover Coordination Polymer Jia Li, Sheng Chen, Lianyan Jiang, Yanshuo Li, and Bao Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01701 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 24, 2018

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Crystal Growth & Design

External Pressure Effect on a Two-fold Interpenetrated 3D PtS-type Spin-crossover Coordination Polymer. Jia Li,a Sheng Chen,a Lianyan Jiang,a Yanshuo Li, a,* Bao Li,b,* a

School of Materials Science and Chemical Engineering, Ningbo University,Ningbo, 315211, China ;

b

Key laboratory of Material Chemistry for Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China; Supporting Information Placeholder ABSTRACT: A two-fold interpenetrated three-dimensional iron coordination polymer with PtS-type topology, Fe (NCS)2(TINM)•1/2TINM (1, TINM= tetrakis(isonicotinoxymethyl)methane), constructed by a semirigid tetradentate ligand, displays an incomplete spin-crossover behavior with T1/2 at 118 K. Furthermore, the correlation between the external-pressure and spin transition is also discussed following theoretical approaches.

The phenomenon of spin-crossover (SCO) between high-spin (HS) and low-spin (LS) states of 3d4-3d7 transition metal ions have been sparked great substantive discussions in the field of coordination chemistry during the past decades.1-3 The electronically active SCO complexes (commonly octahedral Fe(II) and Fe(III) complexes)4,5 could behave as bistable materials, 6,7 whose structure, color, optical or magnetic properties can be dramatically changed by external stimuli such as thermal, pressure and magnetic-field switching and light irradiation.8-10 Hence, SCO complexes could be of desirable function materials for their potential applications as nano-switch, micro-memory devices and medicinal diagnostics. Coordination Polymers (CPs), as one class of prominent functional materials, are ever-increasing in their degree of structural diversity and tunability as well as their wide range of physical and chemical properties.11,12 As the intersection of two important research field between SCO iron complexes and high-dimensional CPs, SCO CPs have been attracting more and more attentions due to their fascinating magnetic properties response to versatile external stimuli.13,14,15 The isolated SCO active centers could be directly interconnected by proper linking ligands to form the extended systems, which is very important to improve the cooperativity or synergistic effect compared to oligonuclear ones.16 Further, thus supramolecular or polymeric networks not only include various short- and long-range interactions,17 but exhibit intriguing properties such as modularity, selectivity and reversibly absorbability or desorbability.18,19 However, the assemble of thus high-dimensional coordination networks usually depends on many factors, and directly obtaining the anticipated structure of topologies still poses a great challenge. In this regard, proper design and selection of organic linkers with modifiable backbone and high connectivity would be of facile route to modulate novel network topologies with anticipated properties. The incorporation of multi-dentate ligands have been

verified to promote the formation of 3-dimensional geometry, and several 3-D coordination frameworks with fascinating SCO properties have been constructed by utilizing bidentate and tridentate N-ligands.20,21 Hence, we are committed to search flexible multidentate bridging ligands to enrich the scanty toolbox of SCO CPs, which are inclined to be the optimal functional materials that combine the advantages of structural diversity of CPs and electronic switching capability of SCO complexes together.

Figure 1. ORTEP drawing of the structure of 1 with atom labels. Symmetry operations: (A) 1/2-x, 3/2-y, z; (B) 2-y, 1/2+x, -1/2+z; (C)-1/2+y, 1-x, -1/2+z;(D) 3/2-x, 3/2-y, z; (E) 1/2-x, 1/2-y, z; (F) y, 1/2-x, 1/2-z; (G) 1/2-y, x, 1/2-z. The semi-rigid dendritic tetradentate ligand TINM,22 inherently, adopts a highly distorted tetrahedral disposition of the four pyridyl groups, each of which could connect a distinct metal atom, which could be seen as an ideal candidate for the selfassembly of the higher-dimensional SCO CPs. In addition, topological analysis has been identified as a useful tool for understanding the intricate structure of CPs, and lots of topological configurations such as NbO23, NiAs24 and PtS25 have been reported. The PtS network, which is the simplest structure type known for the assembly of tetrahedral and square-planar shapes, has been

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discussed thoroughly before, but seldom with SCO identities. Herein, we present the synthesis, fascinating architecture and magnetic characterization of the 3-D iron complex formulated as Fe (NCS)2(TINM)•1/2TINM (1) with two-fold interpenetrated PtS topology.

Figure 2. The square (a) and tetrahedral (b) building blocks constructed the three-dimensional PtS-type structure of 1(c); The augmented PtS net (d). Interpenetrated topology of 1 along with discrete tetra-dentate ligand (e). The glittering red crystal aggregates formed on the wall of test tube by solvent diffusion method after two or three days. Singlecrystal X-ray diffraction data of 1 were collected at 106 and 250 K. An ORTEP drawing (250 K) of the basic unit is shown in Figure 1. Crystal 1 adapts the tetragonal P4(2)/n space group and its asymmetric unit contains one distinct FeII atom, two SCN- groups (N3, N3D) and a half of the bridging TINM ligand (N1, N2), additionally, a quarter of TINM is discrete. The FeII ion is located in octahedral coordination geometry formed with four N atoms from four TINM ligands in equatorial direction and the other two are occupied by SCN- group of axial position. The equatorial bond length [Fe-N1B = Fe-N1C = 2.238 Å(106 K), 2.258 Å (250K), Fe-N2 = Fe-N2A = 2.221 Å(106 K), 2.252 Å (250K)] are longer than those of the axial positions [Fe-N3 = Fe-N3A = 2.081 Å (106K), 2.071Å (250K)]. The average change of the equatorial and axial bond distances at different temperature is 0.02 Å and 0.01 Å respectively, which is indistinctive compared with the alteration of 0.15-0.2 Å in typical FeII SCO compounds. In consistent with the spin transition behavior of iron complex, incomplete SCO behavior could be observed in the whole temperature region. At ambient pressure, about 35% high spin centers have transferred to low spin state. Therefore, only little difference of bond lengths between high- and low-temperature region could be detected due to the mean bond length of all iron centers. If the four-connected FeII centers with the four equatorial nitrogen atoms and the tetra-topic ligands are regarded as square units and tetrahedral units (formed by the four methylenes connected to the quaternary carbon center) (Figure 2a, 2b), respectively, the whole structure of 1 can be simply viewed as a decorated PtS topology, shown in Figure 2c. However, due to the twisted con-

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figuration of the semi-rigid pyridyl arms, the whole framework is not a perfect PtS-type framework, whose tetrahedral units directly distort compared to prototype (Figure 2d). Further, a two-fold interpenetrated framework is resulted from the arrangement of the two same PtS-type networks, which possesses channels along the crystallographic a and b axes in the structure (Figure 2e). However, these cavities are occupied by the uncoordinated TINM ligands. The trapping of the uncoordinated ligands into the two-fold interpenetrated framework was attributed to the weak interactions between the components of the geometry and their preferably accommodation to the framework. For example, there are strong interactions as S···π and hydrogen bonding interaction between the discrete ligand and the sulfur atom of SCN- group, and CC···π and C-H···π interactions between the pyridyl rings of the trapped and coordinated ligands. As shown in Figure S4 and Table S3, Hydrogen bond between uncoordinated oxygen atoms and hydrogen atoms of the methylene group/ pyridyl rings were found with the distance of [O1···H23B =2.378 Å(106 K), 2.399 Å (250K), O1···H20A = 2.792Å(106 K), 2.723Å (250K), O6···H8A = 2.820Å(106 K), 2.979 Å (250K)]. Calculations of the whole structure showed the percentage of the void to be about 9%, indicating the possibility that some solvent molecules are captured in the solid state, which could be further confirmed by thermogravimetric analysis (shown in the supporting information). About 6.10% weight loss may be attributed to the release of two MeOH molecules up to ∼70 °C (with ca.5.98%).

Figure 3. χMT versus T plots of 1 measured at 1 bar , 2.53, 4.41, 6.48, 8.07 and 9.68 kbar under an applied field of 5 kOe in the temperature range of 2–300 K. Inset: T1/2 versus P plots for 1.

Table 1. T1/2 at Different External Pressures. P (kbar) 0.001 2.53

T1/2(K) 118 121

P (kbar) 6.48 8.07

T1/2(K) 126 136.5

4.41

123

9.68

143

The magnetic properties of 1 at different pressures with an applied field of 5 kOe over 2-300K range were presented in Figure 3, which also confirmed the SCO characterization of 1. Under normal pressure, the χMT value is 4.22 cm3 K mol-1 at room temperature, corresponding to the values of HS state for iron (II), slightly larger than the common values of HS state for iron (II), which could be ascribed to the unstable solvent molecules and spinorbital coupling of iron centers. Upon cooling, χMT value remains almost constant down to 170 K, and then decreases gradually till a new plateau from 60 K to 17.5 K. The χMT value is 2.26 cm3 K mol-1 at 17.5 K, indicating the incomplete SCO behavior of 1. Calculated from the χMT value, almost half of the iron active

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Crystal Growth & Design

centers had been triggered to occur the spin transition from HS state to LS one. The sharp decrease of χMT values below 17.5 K should be ascribed to zero-field splitting of the remaining HS-FeII centers. No obvious hysteresis phenomenon could be observed during cooling and warming processes. Plenty of investigations have revealed that the spin transition is sensitive to external pressure with the changed molecule size and volume during this process.26 Variable temperature magnetic behaviors of 1 under gradually increasing external pressure have been also monitored. Variable temperature magnetic behaviors of 1 under gradually increasing external pressure have been also monitored by utilizing the crystal sample immersed in silicon oil as pressure transmitting medium. As expected, χMT value gradually decreases with the increasing external pressure, while Tc moves upwards to higher temperature region. The parameters and plot of transition temperature T1/2 under different external pressures in the cooling mode are listed in Table 1 and shown in Figure 3 inset. By utilizing mean field theory,27 two parts of good linear dependence of T1/2 and P were observed when external pressure below 7 Kbar and up 7 Kbar, respectively. The presentation of two different linear curves would to some extend reflect the mechanism and effect of applied external pressure to SCO behavior in 3D framework. At the beginning, applying the external pressure mainly causes the volume contraction because of the fluffy packing structure of 1. When the applied pressure exceeds 7 Kbar, the pressure effect tends to not only cause the successive volume contraction, but also change the strength of the crystal field around the FeII center, which causes higher slope compared to the low pressure region. In addition, the incomplete SCO behaviors of 1 under variable external pressure could be also ascribed to the “internal pressure effect”28 originated from the huge and slightly clumsy body of quadridentate ligand and host-guest repulsion. Larger guest molecules, TINM ligands, would support greater suppression in the LS state compared to HS state, and possess less extended configuration in the smaller pores of LS state, which totally tend to stabilize HS state over LS state. Therefore, even high external pressure up to 10 kbar had been applied onto the samples, only 75% HS irons transfer to LS state. Moreover, the ∆H and ∆S under different external pressure have been simulated according to the Van't Hoff equation (Figure S7,Table S2).29 As shown in Figure S7, the nonlinear tendency of ∆H and ∆S versus external pressure could be observed. These values became larger along with the external increasing, especially at high pressure region. It was well known that the values of ∆H and ∆S would become larger along with the increasing pressure.30 However, the tendency is tender, and different to the phenomenon for 1, which was caused by the incomplete transition behavior. The primary values of ∆H and ∆S were far smaller than the typical values of low pressure due to the low ratio of spin transition centers. These values became closer to the normal values observed for the complete spin transitions, due to the high ratio of spin transition centers. Whatever, the simulated values of ∆H and ∆S could also validate the obvious effect of external pressure to spin transition, which tends to favor low spin state and trigger the spin transition at high temperature region. In conclusion, we describe a two-fold interpenetrated 3D CP with PtS-type network, which exhibits incomplete SCO behavior confirmed by variable-temperature crystal structures, and magnetic susceptibilities under variable external pressure. Thus architecture traps a large bulk of uncoordinated guest that occupies the space and stabilizes the whole framework. Further work on searching new 3-D SCO compounds which exhibit complete spin transition behavior and obvious hysteresis phenomenon is urgently needed and going on by us.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:.... Detailed experimental section including synthesis, tga and applied external pressure; selected bond and structural parameters under two different temperature (PDF) Accession code

CCDC 1571293 and 1571294 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author * author email: [email protected]; [email protected];

ORCID

Jia Li: 0000-0002-8392-1125 Yanshuo Li: 0000-0002-7722-7962 Bao Li: 0000-0003-4627-072X Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We gratefully acknowledge the National Natural Science Foundation of China (21701091, 21622607, 21761132009) and the K. C. Wong Magna Fund in Ningbo University.

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For Table of Contents Use Only External Pressure Effect on a Two-fold Interpenetrated 3D PtS-type Spin-crossover Coordination Polymer. Jia Li,a Sheng Chen,a Lianyan Jiang,a Yanshuo Li, a,* Bao Li,b,*

SYNOPSIS：

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Crystal Growth & Design

A three-dimensional iron coordination polymer displays an incomplete spin-crossover behavior with T1/2 at 118 K. The correlation between the external-pressure and spin transition is also discussed following theoretical approaches.

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