Surfactant 1-Hexadecyl-3-methylimidazolium Chloride Can Convert

Apr 24, 2017 - Synopsis. A facile surfactant-mediated crystallization strategy was applied to the preparation of hybrid bromoplumbates with tailored l...
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Surfactant 1‑Hexadecyl-3-methylimidazolium Chloride Can Convert One-Dimensional Viologen Bromoplumbate into Zero-Dimensional Guangfeng Liu,† Jie Liu,‡ Lina Nie,† Rui Ban,† Gerasimos S. Armatas,§ Xutang Tao,‡ and Qichun Zhang*,†,⊥ †

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China § Department of Materials Science and Technology, University of Crete, 71003 Heraklion, Greece ⊥ Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡

S Supporting Information *

(PEG), and sodium dodecyl sulfate (SDS) as additives to grow MV bromoplumbate crystals.7 Our results indicated that these organic surfactants can make the as-obtained structures diverse, and as a result, the first three-dimensional (3D) open framework of viologen bromoplumbate, [MV]2[Pb7Br18], was obtained by this method. Very recently, this 3D compound, [MV]2[Pb7Br18], turned out to be a bistable semiconductor with electron-transfer thermochromic functionality.8 Continuing on this research direction, here we report the synthesis of a new [BV]6[Pb9Br30] (BV2+ = N,N′-dibutyl-4,4′-dipyridinium) viologen bromoplumbate from the surfactant-mediated coupling between PbBr2 and N,N′-dialkyl-4,4′-bipyridinium. We show that the [BV]6[Pb9Br30] compound has a 0D structure that consists of unusual discrete [Pb9Br30]12− clusters and exhibits a lower band gap than the 1D plumbate architectures. Because the surfactant-assisted fabrication of MV halogenoplumbates is a simple one-pot method and an effective way of discovering new structural variants, in this work, we conducted the synthesis of novel viologen bromoplumbates by using this method. We choose different alcohols (i.e., ethanol, n-propanol, n-butanol, and n-pentanol) as alkylation reagents in the presence of 4,4′-dipyridinium, PbBr2, and HBr to prepare a new family of N,N′-dialkyl-4,4′-bipyridinium bromoplumbate compounds via a solvothermal process. The synthetic route and typical optical microscopy images of the as-obtained crystals are shown in Figure 1. At first, we implemented all reactions at different temperatures (i.e., 100, 120, 140, 160, and 180 °C) without surfactants. The as-obtained crystals always show sheet-like morphology and yellow color. The X-ray analysis indicated that as-prepared viologen bromoplumbates have a similar molecular formula of [V]1.5[Pb3Br9] ([EV]1.5[Pb3Br9], [PV]1.5[Pb3Br9], [BV]1.5[Pb3Br9], and [PeV]1.5[Pb3Br9], where PeV2+ = N,N′dipentyl-4,4′-dipyridinium) and exhibit the same 1D [Pb3Br9]n3n− chain structures (Table S1). For convenience, we abbreviated the compound names as C2, C3, C4-1, and C5, respectively. Next, we added various surfactants, such as PVP, PEG, and SDS, as additives into the precursors’ solution in an effort to make the structures diverse; however, the as-obtained crystals were still pristine [V]1.5[Pb3Br9] structures. This suggests

ABSTRACT: A zero-dimensional N,N′-dibutyl-4,4′-dipyridinium bromoplumbate, [BV]6[Pb9Br30], with unusual discrete [Pb9Br30]12− anionic clusters was prepared via a facile surfactant-mediated solvothermal process. This bromoplumbate exhibits a narrower optical band gap relative to the congeneric one-dimensional viologen bromoplumbates.

T

he alkylation of 4,4′-dipyridinium is of great technological importance because the resulting viologen products show excellent redox activity and chemical stability,1 which eventually can lead to many applications from catalysis and energy storage to electrochromic devices and biological sensors.2 In addition, viologens can act as effective templates to construct various organic−inorganic hybrids, charge-transfer complexes, and supramolecular systems.3 Among them, viologen halogenoplumbates have received a lot of attention because of their versatile molecular structures and potential applications in optoelectronic devices. An appealing method to construct halogenoplumbate compounds is to adjust the substituent groups of the viologens. By this approach, one-dimensional (1D) chains such as [PbI3]nn−, [Pb3I9]n3n−, and [Pb5I14]n4n− can be isolated as [MV][Pb2I6] (MV2+ = N,N′-dimethyl-4,4′-dipyridinium), [iPV]2[Pb4I12] (iPV2+ = N,N′-diisopropyl-4,4′-bipyridinium), [EV] 1.5 [Pb 3 I 9 ] (EV 2+ = N,N′-diethyl-4,4′-dipyridinium), [PV]1.5[Pb3I9] (PV2+ = N,N′-dipropyl-4,4′-dipyridinium), and [BzV]2[Pb5I14] (BzV2+ = N,N′-dibenzyl-4,4′-dipyridinium) ionic compounds.4 So far, only a few examples of discrete zerodimensional (0D) viologen iodoplumbate anions have been reported. These include [Pb13I38]12− in [Bz′V]6[Pb13I38] (Bz′V2+ = 1,2,1′-tribenzyl-4,4′-bipyridinium) and [Pb 6 I 22 ] 10− in [BV]5[DMF]2[Pb6I22] (BV2+ = N,N′-dibutyl-4,4′-dipyridinium).5 As for viologen bromoplumbates, less explored compounds have been reported, although these materials might exhibit fascinating architectural structures and unique physical properties. We recently reported a new surfactantmediated strategy to prepare inorganic crystalline materials such as chalcogenides and metal−organic frameworks.6 Following this method, we employed various types of amphiphilic molecules such as poly(vinylpyrrolidone) (PVP), poly(ethylene glycol) © 2017 American Chemical Society

Received: March 11, 2017 Published: April 24, 2017 5498

DOI: 10.1021/acs.inorgchem.7b00648 Inorg. Chem. 2017, 56, 5498−5501

Communication

Inorganic Chemistry

Figure 2. (a and b) Packing diagrams of compounds C2 and C3 along the a direction. (c−e) Packing diagrams of compounds C4-1, C5, and C4-2 along the c direction. Color scheme: PbII, yellow; Br, brown; C, gray; N, blue. H atoms are omitted for clarity.

Figure 1. Reaction routes, crystal morphologies, molecular formulas, and dimensionality of the as-obtained C2−C5 viologen bromoplumbates. The chemical formula of [HMIM]Cl is shown.

that conventional anionic and neutral surfactants have no effect on the structural transformation of [Pb3Br9]-based viologens. In contrast, when the [HMIM]Cl surfactant was added to the BV bromoplumbate solution, a new compound with a formula of [BV]6[Pb9Br30] (denoted as C4-2) was isolated with about 51% yield. Interestingly, the crystal structure of [BV]6[Pb9Br30] consists of unusual [Pb9Br30]12− discrete clusters and has a large unit cell volume (more than 15000 Å3). The C4-2 compound shows a deeper color and a narrower optical band gap than the 1D C2−C5 viologen bromoplumbates. Note that [HMIM]Cl did not produce the 0D analogues if we chose EV, PV, and PeV bromoplumbate systems to prepare bromoplumbates at 120 and 180 °C with the same conditions. The final products were still C2, C3, and C5 crystals. Single-crystal X-ray diffraction (XRD) analysis revealed that compounds C2, C3, C4-1, and C5 crystallize in monoclinic structures with P21/c lattice symmetry. The asymmetric unit of these compounds always consists of one [Pb3Br9]3− anion with three crystallographically independent Pb atoms and one and a half EV2+, PV2+, BV2+, and PeV2+ cations, respectively (Figure S1a−c,e). Parts a−d of Figure 2 show that every six organic V2+ species form honeycomb cavities to encompass the anionic [Pb6Br24]12− clusters. The polyhedral stacking pattern of [Pb6Br24]12− in Figure 3a−d shows that all PbBr6 octahedral units adopt an edge-sharing close-packed mode with different distortion orientations. These [Pb6Br24]12− species are linked to their neighbors on either side, constructing 1D [Pb3Br9]n3n− polymer chains along the a direction (C2 and C3) or the c direction (C4-1 and C5). These crystallographic data indicate that the C2, C3, C4-1, and C5 polymers are almost isomorphic with those of [EV]1.5[Pb3I9] and [PV]1.5[Pb3I9]. As for the hybrid C4-2, it is crystallized in the monoclinic C2/c structure with lattice parameters a = 11.8158(11) Å, b = 13.3127(13) Å, c = 23.777(2) Å, and β = 102.1900(10)° [V = 15987.9(4) Å3]. The asymmetric unit of C4-2 comprises three BV2+ cations and half of the [Pb9Br30]12− anion (Figure S1d). The packing diagram of C4-2 in Figure 2e shows that every six BV2+ cations form an eyelike lattice to perfectly encapsulate for each discrete [Pb9Br30]12− cluster. The [Pb9Br30]12− clusters

Figure 3. Views of the polyhedral stacking patterns of inorganic components in the compounds C2, C3, C4-1, C5, and C4-2. Color scheme: PbII, yellow; Br, brown.

adopt a C2v symmetry, where Pb1, Pb4, and Pb5 atoms lie along a 2-fold axis, and each cluster contains nine PbBr6 octahedral edgesharing units (Figure 3e). These [Pb9Br30]12− anions remain separate and distinct within the crystal structure; the distance from Br1 to Br1c is ca 20.1 Å. To the best of our knowledge, the [Pb9Br30]12− anion is the largest discrete fragment observed in bromoplumbates, while holding the highest negative charge (12−). It should be stressed that, without the [HMIM]Cl surfactant, only the C4-1 crystals were obtained at all reaction temperatures. In stark contrast, when the same experiment was performed in the presence of [HMIM]Cl ([HMIM]Cl/HBr = 1 g/mL), compound C4−2 can be isolated in relatively high yields (>50%; see details in the Supporting Information). The mixed C4-1 and C4-2 crystals can be separated by sieving with a 30-μm sieve (Figure S2). These results clearly suggest that the surfactant [HMIM]Cl acts as a template to induce heterogeneous nucleation and to tailor the dimensionality of the viologen bromoplumbate. 5499

DOI: 10.1021/acs.inorgchem.7b00648 Inorg. Chem. 2017, 56, 5498−5501

Communication

Inorganic Chemistry

bates. The optical absorption spectra and TGA measurements showed that C4-2 bromoplumbate has a narrower optical band gap and exhibits lower thermostability than the 1D variants. The results of this study show that the proposed surfactant-mediated synthesis of bromoplumbates can be applied to the preparation of other hybrid halogenoplumbates with tailored lattice architecture and dimensionality. The prepared crystalline plumbates may exhibit superior optoelectronic properties and improved device stability.

All of the prepared compounds are stable in air. Thus, before the optical and thermal properties of the as-prepared crystals were tested, the purity of all compounds was verified by powder XRD in the ambient environment. All XRD patterns are consistent with the corresponding calculated results from the single-crystal XRD data (Figure S3). The optical diffusereflectance spectra of crystalline samples C2−C5 were recorded at room temperature. The absorption (α/S) data calculated from the reflectance spectra using the Kubelka−Munk function show that the optical band gaps (Eg) are 2.45 eV for C2, 2.34 eV for C3, 2.18 eV for C4-1, and 2.14 eV for C5 (Figure 4). This gradual

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00648. Experimental details, Figures S1−S5, and Table S1 (PDF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 4. Optical absorption spectra of C2, C3, C4-1, C5, and C4-2 viologen bromoplumbates.

ORCID

Xutang Tao: 0000-0001-5957-2271 Qichun Zhang: 0000-0003-1854-8659

decrease in the energy gap observed for bromoplumbates (from C2 to C5) implies that the longer alkyl chain in viologen results in a significant reduction of the band gap. Despite this, the C4-2 crystals have a brown color and their optical band gap is found to be 1.97 eV, suggesting that 0D viologen bromoplumbate has a lower band gap than 1D analogous. The Fourier transform infrared spectra (Figure S4) of all compounds display strong and medium absorption peaks at around 3050 cm−1 and strong peaks in the range of 1450−1650 cm−1, confirming the existence of pyridyl groups. The medium peaks in the 2800−3000 cm−1 range observed in IR spectra can be assigned to the stretching vibration modes of alkyl groups, suggesting that the alkylation of 4,4′dipyridinium was successfully achieved in all compounds.9 In order to investigate the thermostability of C2−C5 compounds, thermogravimetric analysis (TGA) experiments were carried out from 50 to 400 °C under a nitrogen flow. As shown in Figure S5, the starting decomposition temperature of C4-1 and C5 is much higher than that of the C2 and C3 compounds (267−270 vs 208 °C), indicating that the long alkyl chain in viologen can improve the thermostability of these hybrid polymers. However, the TGA profile of C4-2 displays the starting weight loss at only 197 °C, which is much lower than that of 1D C4-1 bromoplumbate. These results suggest that the lower dimensionality of bromoplumbates deteriorates the thermostability of the structure. In summary, we have demonstrated an efficient approach to preparing a series of viologen bromoplumbates C2, C3, C4−1, and C5 by a solvothermal process, in which the viologen cations can be generated in situ from alcohols and 4,4′-dipyridinium precursors. XRD data indicated that these compounds have a similar [Pb3Br9]n3n−-based 1D chain structure. More importantly, a 0D viologen bromoplumbate C4-2 consisting of unusual discrete [Pb9Br30]12− anions was obtained upon the addition of [HMIM]Cl surfactant. To the best of our knowledge, [Pb9Br30]12− is the largest metal halide cluster of bromoplum-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.Z. acknowledges financial support from AcRF Tier 1 (RG8/ 16, RG133/14, and RG 13/15), Singapore. REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00648 Inorg. Chem. 2017, 56, 5498−5501