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. 2019 Dec 30;23(1):100812. doi: 10.1016/j.isci.2019.100812

Synthetic Route to a Triphenylenehexaselenol-Based Metal Organic Framework with Semi-conductive and Glassy Magnetic Properties

Yutao Cui 1,2, Jie Yan 1,2, Zhijun Chen 1,2, Weilong Xing 1,2, Chunhui Ye 1,2, Xiang Li 3, Ye Zou 1,2, Yimeng Sun 1, Caiming Liu 1,2,∗, Wei Xu 1,2,4,∗∗, Daoben Zhu 1,2
PMCID: PMC6965734  PMID: 31945732

Summary

In the latest decade, two-dimensional (2D) π-d conjugated metal organic frameworks (MOFs) constructed from metal ions with square-planar coordination geometry and benzene- or triphenylene-derived ligands with ortho-disubstituted N, O, or S donor atoms have received great research interests because of their exceptional physical properties and promising applications. New MOFs of this class are constantly being reported, but 2D metal bis(diselenolene) MOFs based on organic ligands with ortho-disubstituted Se donor atoms have not been synthesized. Herein, a Lewis-acid-induced dealkylation protocol is introduced to the synthesis of arenepolyselenols and related coordination polymers. A triphenylene-derived diselenaborole compound is synthesized and employed as precursor reagent for the synthesis of 2,3,6,7,10,11-triphenylenehexaselenol (H6TPHS) and the first conductive metal organic framework namely Co-TPHS based on triphenylenehexaselenolate (TPHS6−). Co-TPHS exhibits porous honeycomb 2D structure and electrically conductive and glassy magnetic properties.

Subject Areas: Chemical Synthesis, Magnetic Property, Polymers

Graphical Abstract

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Highlights

  • •

    A BBr3-induced dealkylation protocol is used to synthesize arenepolyselenols ligands

  • •

    Triphenylenehexaselenol (TPHS) and a Co-TPHS MOF were synthesized for the first time

  • •

    The MOF exhibits electrically conductive and glassy magnetic properties

Subject Areas: Chemical Synthesis; Magnetic Property; Polymers

Introduction

Two dimensional (2D) π-d conjugated metal organic frameworks (MOFs) perform exceptional electrically conductive properties and potential applications in electronics (Sun et al., 2016, Stassen et al., 2017, Ko et al., 2018, Zhao et al., 2018). The structurally related 2D MOFs are made from π-conjugated planer ligands and single metal ion with square-planar coordinative geometry (Sun et al., 2016, Ko et al., 2018). The in-planer π-d conjunction, in favors of charge carriers' delocalization, leads to exceptional electrical conductivity different from traditional MOFs. Due to their intrinsic electrical conductivity and porosity, conductive MOFs in this class exhibit promising applications in electrocatalysis, transparent electrodes, energy storage, and chemoreceptive sensors (Campbell et al., 2015, Dong et al., 2015, Feng et al., 2018, Huang et al., 2017, Huang et al., 2018, Jin et al., 2017, Li et al., 2017, Miner et al., 2016, Miner et al., 2017, Miner et al., 2018, Park et al., 2018b, Sheberla et al., 2017, Smith et al., 2016, Wu et al., 2017). As bottom-up fabricated solid materials, MOFs with tunable structures and different properties are mainly defined by the two building blocks: metal centers and organic ligands. Up to now, a series of benzene-, triphenylene-, coronene-, or phthalocyanine-derived π-conjugated ligands with different donor atoms (O, S, and N) have been employed for the construction of π-d conjugated MOFs with metal bis(dithiolene), metal catecholates, metal bis(diaminoene) and metal bis(aminothiolato) units (Kambe et al., 2013, Kambe et al., 2014, Huang et al., 2015, Pal et al., 2015, Clough et al., 2017, Dong et al., 2018a, Dong et al., 2018b, Sheberla et al., 2014, Dou et al., 2017, Lahiri et al., 2017, Hmadeh et al., 2012, Park et al., 2018a, Meng et al., 2019, Yang et al., 2019, Zhong et al., 2019). Theoretical studies indicate that replacing sulfur by selenium in a hybrid organic–inorganic semiconductor is predicted to decrease the electronic band gap (Stott et al., 2012). Although the synthesis of the protected benzenepolyselenols has been reported, metal bis(diselenolene) 2D MOFs have not been synthesized and reported up to now (Turner and Vaid, 2012, Stott et al., 2012). Synthesis of arenepolyselenols and MOFs based on these ligands is significant for the research of structure-property relationship on the conductive MOFs, as well as providing diversity of conductive 2D materials for further studies on physical properties and electronic applications.

Arene(polysthiols), as arene(polyselenols) analogous, are frequently prepared via reductive cleavage of arenepoly(alkylthioethers) under Birch conditions using alkali metal in liquid ammonia (Figure S1). The starting arenepoly(alkylthioethers) are commonly synthesized via the nucleophilic aromatic substitution of arene(polyhalides) with alkylthiolates. An analogous route was applied to the synthesis of two acetyl-protected benzene(polyselenols) (Turner and Vaid, 2012). As arene(polyselenols) are air-sensitive, it is difficult to obtain arene(polyselenols) in the form of selenol. An alternative route (Figure S2) to arene(polyselenols) is the Lewis-acid-promoted dealkylation of arenepoly(alkylselenoethers). A Lewis-acid (BBr3)-induced deprotection procedure has been applied in the production of 2-bromobenzo-1,3,2-dithiaboroles (Schlindwein et al., 2016). We introduce the Lewis-acid-induced deprotection protocol to the synthesis of arene(polyselenols). The resulting diselenaboroles are available as precursor for the synthesis of arene(polyselenols) and related MOFs. Recently, we have successfully synthesized the ligand benzenehexaselenol (H6BHS) via the method of boron tribromide as Lewis acid to promote the reproduction of the alkyl benzene-polyselenols (Cui et al., 2019). Here we report the first synthesis of selenium-substituted triphenylene ligand 2,3,6,7,10,11-triphenylenehexaselenol (H6TPHS) and the first member of 2D MOFs based on triphenylenehexaselenolate namely Co-TPHS.

Results and Discussion

Synthesis

As illustrated in Figure 1, the MOF Co-TPHS was synthesized by the coordination reaction of 2,3,6,7,10,11-triphenylenehexa(selenolate) ((TPHS)6−) and cobaltous ions in the presence of base. Under inert atmosphere, the as-prepared (BBr)3TPHS was treated with ethyl alcohol and potassium hydroxide and stirred for a while. During which, (TPHS)6− was formed. Then cobaltous salt was added and the reaction mixture was heated to reflux and stirred for 24h. The as-made MOF is air sensitive. The color of the MOF turned from green to black-green in about twenty seconds following the exposure to air (Figure S3), indicating the oxidation of Co-TPHS. The reaction mixture was stirred under air for 12h to ensure the complete oxidization of the as-made MOF. Then the material Co-TPHS (Co1.5(TPHS)) was yielded as bulk black-green powder. It is noteworthy that the addition of base and getting the reaction temperature reaching reflux temperature of ethyl alcohol are critical to obtain samples with high crystallinity.

Figure 1.

Figure 1

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Synthetic Route of H6TPHS and Co-TPHS

To understand the synthetic process, model reactions aiming at benzene-1,2,4,5-tetraselenol (H4BTS) and a reported coordination polymer Co-BTS were carried (Figure 2). (Turner and Vaid, 2012, Downes and Marinescu, 2017) The corresponding diselenaboroles (BBr)2BTS was synthesized via the BBr3-promoted cleavage of protected alkyl groups. A solution of 1,2,4,5-benzenetetra(selenolate) ((BTS)4−) in D2O was prepared by treating the diselenaborole compound with LiOD/D2O for the NMR characterization. The H1NMR and C13NMR spectrum (Figures S25 and S26) showed reasonable peaks corresponding to Li4C6H2Se4. As good solubility of (BBr)2BTS in hot PhF, single-crystals of (BBr)2BTS were obtained via slowly cooling a hot solution of (BBr)2BTS in PhF. And as shown in Figure S4 and Table S1, the structure was further confirmed via single-crystal X-ray diffraction. The expected five-member cyclic 2-bromo-1,3,2-diselenaboroles were observed. The organic ligand benzene-1,2,4,5-tetraselenol (H4BTS) was generated in good yield by methanolysis of (BBr)2BTS. It's the first time that the molecule H4BTS is obtained in the form of benzeneselenol. It is noteworthy that the ligand H4BTS is unstable and just can be exposed to air for a while and stored under argon atmosphere for about three days. The one-dimensional coordination polymer namely Co-BTS was synthesized by the coordination reaction of cobaltous ions and (BTS)4−, which was generated in situ via treating the diselenaborole compound with a solution of KOH in alcohol solvents. The structural characterization results are consistent with that of Co-BTS in reported literature previously (presented in Supplemental Information, Figures S5 and S6).

Figure 2.

Figure 2

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Synthesis of Benzene-1,2,4,5-tetraselenol (H4BTS) and 1D Coordination Polymer Co-BTS and Molecular Structure of (BBr)2BTS

Structural Resolution

Powder X-ray diffraction (PXRD) measurements reveal a layered structure of Co-TPHS with honeycomb lattice, which is similar to the MOFs constructed from triphenylene-derived ligands. As showed in Figure 3, the prominent diffraction peaks at 2θ = 4.3° and 8.8°, corresponding to the [100] and [200] reflections, indicate long-range order within the ab plane. The 100 spacing is ∼20.5Å. The weak and broad peak at 2θ = ∼27°, corresponding the [001] reflection, indicates poor coherence along the c direction or a small dimension along the c-axis in the Co-TPHS crystallites. The interspace of 001 crystal face is ∼3.3Å. To determine the staking arrangement of the 2D sheets, we simulated two possible eclipsed (AA) and slipped-parallel (AB) staking patterns. As the limitation of the few numbers of diffraction peaks in experimental PXRD patterns and the similar PXRD patterns simulated from the AA and AB models, it is difficult to determine the accurate stacking geometry. Both the positions and intensity profile of the PXRD calculated from the AA staking are more consistent with the experimental data than that of AB staking. The cell parameters calculated from the AA stacking (a = 23.718Å, b = 23.718Å, c = 3.400Å, α = 90°, β = 90°, γ = 120° in P 6/mmm space group) are consistent with the experimental result of Co-TPHS.

Figure 3.

Figure 3

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PXRD Analysis and Simulated Structure of Co-TPHS

(A) A comparison of the PXRD of the Co-TPHS and simulated patterns of structure models with AA and AB stacking patterns.

(B) Illustration of the 2D lattice of Co-TPHS.

A distinct flaky morphology of Co-TPHS was observed in the scanning electron microscopy (SEM) photos. As shown in Figure S7, Co-TPHS is composed of nanosheets with the size varying from several tens to more than three hundreds of nanometers in diameter and about ten nanometers in thickness. The arrangement of the nanosheets is irregular and incompact. Transmission electron microscopy (TEM) images were also obtained to further confirm the structure of Co-TPHS (Figure 4). Irregularly arranged nanosheets were observed and uniform honeycomb lattice with d100 = ∼20Å was shown in the nanosheet (along [001]) just in parallel with the observation plane, which is consistent with the 2D honeycomb model obtained from PXRD results (d100 = 20.5Å). The elemental mapping shows homogeneous distribution of Co, Se, and C (Figure S8).

Figure 4.

Figure 4

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TEM Images of the Co-TPHS

(A–D) TEM images of the Co-TPHS nanosheets at different magnifications. The insets of Figure B are larger version of the red highlighted region (top and right) and the fast Fourier transform of the image (middle and right).

As shown in Figure S9, the porous character of Co-TPHS was revealed in N2 sorption experiments (at 77 K). The Brunauer–Emmett–Teller (BET) surface area of Co-TPHS is 246 m2 g−1, and the surface area data are comparable to catecholate and dithiolene systems that also build upon the triphenylene core.

Component Analysis

The electron probe microanalysis (EPMA) results of Co-TPHS indicate the existence of cobalt, selenium, and carbon, and the calculated atom ratio of selenium to cobalt is ∼4.0 (or 6:1.51). The molecular formula of Co-TPHS is Co1.5(C18H6Se6) according to the EPMA result. The content of C was analyzed by elemental analysis (EA). The weight percentage of carbon was determined to be 27.22%, which are coincident with the calculated result (27.56%) of the formula of Co1.5(C18H6Se6), showing that the cobalt bis(diselenolene) unit was oxidized to be electroneutral. Most of the reported MOFs that exhibit staked honeycomb lattices and are made from triphenylene-derived ligands with ortho-disubstituted S, O, or N donor atoms and square-planar coordination late-transition-metals are expected to be oxidized to be electroneutral (Sun et al., 2016).

X-ray photoelectron spectroscopy (XPS) experiments were performed for further characterization of the MOFs. As showed in Figure S10, the full XPS spectrum reveals the presence of C, Se, and Co. In the Co 2p region (Figure S11A), two peaks at binding energies of 778.5 and 793.6 eV correspond to the 2p3/2 and 2p1/2. Two corresponding satellite peaks locate at the higher-binding energy side, which signifies a +2 oxidation state on Co. The XPS result of Co 2p suggests no electron transfer between metal ions and organic ligands. And the oxidation of Co-TPHS by air occurred on the organic ligands, which was oxidized to be TPHS(3-) (3⋅) to achieve charge balance with Co2+. In the Se 3d region (Figure S11B), two dominating peaks located at binding energies of 778.5 and 793.6 eV are observed for 3d5/2 and 3d3/2, in agreement with those of previously reported metal-diselenolene polymers (Downes and Marinescu, 2017, Cui et al., 2018).

Electrical Conductivity

The electrical conductivity (σ) of Co-TPHS was measured on the pressed pellet sample via a standard two-probe method. The σ of Co-TPHS reaches 10−6 S cm−1 at 300 K. As shown in Figure 5, temperature dependence of electrical conductivity reveals typically semi-conductive behavior of Co-TPHS. The electrical conductivity increases with the elevated temperature and reaches 10−5 S cm−1 at 400 K. The ligands with stable radicals provide unpaired electrons as charge carriers. The in-planer π-d conjunction, in favors of charge carriers' delocalization, provide through bonds pathway for charge transport. The activation energy (Ea) of electrical conductivity can be estimated by fitting the temperature-dependent electrical conductivities to the Arrhenius equation (Givaja et al., 2012). Figure S12A shows the linear plot of natural logarithm of conductivity (ln σ) versus the reciprocal of the absolute temperature (1/T). The slope of the curve can be seen as -Ea/k (k = Boltzmann constant). The Ea of Co-TPHS is estimated to be 294 meV. As shown in Figure S12B, the linear dependence of ln σ versus T−1/4 indicates that charge carrier transport in Co-TPHS sample is well fitted to the three-dimensional variable range hopping (3D-VRH) model. The optical band gap of Co-TPHS was confirmed to be 2.33 eV. Bulk Co-TPHS is a wide-band semiconductor.

Figure 5.

Figure 5

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Electrically Conductive Properties of Co-TPHS

(A) Temperature dependence of electrical conductivity of Co-TPHS.

(B) Tauc plots of (αhν)2 versus photon energy (hν) for Co-TPHS. A linear fit (red line) was used to estimate the band gap by extrapolating to zero absorption.

Band structure of monolayer of the 2D lattice and eclipsed AA stacking bulk crystal are calculated by using the plane-wave technique as implemented in CASTEP code (Figures S13 and S14). The valence and conducting band of monolayer structure show relatively narrow dispersing comparing with that of the previously reported 2D MOFs based on BHT(benzenehexathiol) and TPHT (triphenylenehexathiol), indicating the weaker electronic coupling between the d-orbital of Co ion and π-orbital of the ligand. The calculated band gap is much smaller compared with what is observed in the optical spectroscopy. This might attribute to the fact that the band gaps are usually underestimated in the DFT calculations. The influences of the existence of defects and uncertainty of the edge structure have not been addressed in the theoretical calculation yet. The absorption observed in optical spectroscopy may come from the defects and few layer nanostructures of this 2D MOF, which should be different from what expected for an ideal bulk crystal. And the metal organic coordination solid might be doped by defects or uncompleted oxidization sites (Co-TPHS was oxidized to be electroneutral by air), both of which will influence the band structure diverging from theoretical calculation. The bulk crystal shows a highly dispersed band structure with bandwidth over 2 eV in the valence band, which indicates the strong interlayer electronic couplings between this 2D lattices. With the Fermi energy crossing the highly dispersed valence band, large charge carrier density as well as high carrier mobility should be expected. But only poor electrical conductivity can be observed. Considering the weak and broad [001] peak at 2θ = ∼27°, which refers poor coherence along the c direction or a small dimension along the c-axis, indicating that the material obtained is not a real bulk crystal, especially in the stacking direction. It is more like a nanosheet containing only a few layers of the 2D lattice, in which the electrical transport behavior is obviously different from a real bulk material.

Magnetism

DC magnetic susceptibility is measured in a 1000 Oe field and a 2-300K temperature range. As shown in Figure 6A, the room temperature χT value of 2.764 cm3 K mol−1 is consist with the value of 2.625 cm3 K mol−1 expected for one spin-only Co2+ cations (S = 3/2, assuming g = 2.0) and 2/3 ligand of TPHS(3-) (3⋅). The χT value decreases slowly with decreasing temperature, indicating the existence of weak intramolecular antiferromagnetic exchange. The 1/χ data above 100 K is in line with the Curie-Weiss law well, with the θ value of −14.48 K and the C value of 5.33 cm3 K mol−1 (Figure S15). After reaching a minimum value of 1.99 cm3 K mol−1 at 44 K, the χT value suddenly increases sharply to the maximum value of 6.50 cm3 K mol−1 at 7 K, indicating the occurrence of magnetic phase transition. The χT product drops sharply when it is less than 7 K, which may be caused by Zero field splitting, Zeeman effect, and/or weak interlayer antiferromagnetic interactions.

Figure 6.

Figure 6

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Magnetic Properties of Co-TPHS

(A) Plot of χT versus T of Co-TPHS under 1k Oe field.

(B) Plots of FC and ZFC susceptibilities vs. temperature of Co-TPHS at an applied field of 30 Oe.

(C) Ac susceptibilities of Co-TPHS measured in an ac field of 2.5 Oe.

(D) Hysteresis loop at 2.0 (K).

The zero-field cooling (ZFC) and the field cooling (FC) magnetic susceptibility divergence appear below about 3.6 K (Figure 6B), indicating that magnetization irreversibility occurs. Alternating-current (ac) magnetic susceptibility investigation reveals that both the in-phase component (χ′) and the out-of-phase component (χ″) of Co-TPHS are frequency dependent (Figure 6C). These phenomena indicate that each magnetic moment is frozen at low temperatures, which are characteristic of spin-glass or single-molecule magnets (Liu et al., 2008). In order to determine whether it is spin-glass or single-molecule magnet, a parameter, Φ = (ΔTp/Tp)/Δ(logf) (Mydosh, 1993), in which Tp and f are the peak temperature and the frequency, respectively, must be introduced. The Φ value of Co-TPHS is calculated to be 0.081; this value is more consistent with the behavior of the spin-glass (≈0.01) but smaller than single-molecule magnet (≥0.1) (Mydosh, 1993). A small hysteresis can be clearly observed at 1.9 K of Co-TPHS (Figure 6D), with a coercive field (Hc) of 160 Oe. Furthermore, the field-dependent magnetization at 2 K for Co-TPHS (Figure S16) reveals that only a low magnetization value of 0.86 Nβ is observed even at the highest field measured (50 kOe), and saturation cannot occur, which is consistent with weak ferromagnetism owing to glassy magnet. Such glassiness might result from the not necessarily homogeneous topology of Co-TPHS (Jeon Ie et al., 2015). The oxidation of Co-TPHS might introduce defects to the as-made Cp-TPHS. What is more, the existence of two types of spin carrier (Co2+ and TPHS(3-) (3⋅)) in Co-TPHS leads to complicated magnetism.

Conclusion

In conclusion, the H6TPHS and 2D conductive MOFs Co-TPHS were synthesized for the first time. A novel Lewis-acid-induced dealkylation protocol was applied for the synthesis of a TPHS-derived diselenaborole, which was used as a precursor reagent for the preparation of H6TPHS. Co-TPHS was synthesized via simple homogeneous reaction. Co-TPHS is composed of nanosheets with honeycomb lattice and exhibit porous, electrically conductive, and magnetic properties. The synthesis of H6TPHS and Co-TPHS verify an easy route to arenepolyselenols ligand and metal bis(diselenolene) MOFs, simultaneously providing new materials for the study of further applications and structure-property relationship.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.

Acknowledgments

We gratefully acknowledge the financial support from the National Key R&D Program of China (Grant No. 2017YFA0204701), the Chinese Academy of Sciences (Strategic Priority Research Program No. XDB12000000, QYZDY-SSW-SLH024), and National Natural Science Foundation of China (21333011, 21661132006, 21790051, 51403215).

Author Contributions

D.Z. and W.X. initiated and supervised the research program. Y.C. Z. C. and C. Y. were responsible for the material preparation and characterizations. W-L. X. and Y. Z. collected and analyzed X-ray photoelectron spectroscopy data. Y. C. and Y. S. collected and analyzed electrical conductivity data. C. L. performed the measurement magnetic properties. J. Y. and X. L. carried out the TEM measurement. Y. C., C. L., and W. X. wrote the manuscript. All the authors participated in the discussion and interpretation of data, read the manuscript, and provided input.

Declaration of Interests

The authors declare no competing interests.

Published: January 24, 2020

Footnotes

Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2019.100812.

Contributor Information

Caiming Liu, Email: cmliu@iccas.ac.cn.

Wei Xu, Email: wxu@iccas.ac.cn.

Supplemental Information

Document S1. Transparent Methods and Figures S1–S28, and Table S1
mmc1.pdf (2.1MB, pdf)

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Associated Data

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Supplementary Materials

Document S1. Transparent Methods and Figures S1–S28, and Table S1
mmc1.pdf (2.1MB, pdf)

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