WU Guo-Xing WANG Zhong-Hui LI Xu-Peng

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Structure and Optical Property of a New 1D Iodoplumbate Hybrid Directed bySynthesized Asymmetrical N-benzyl-1,4-diazabicyclo[2.2.2]octane①

WU Guo-Xinga②WANG Zhong-HuiaLI Xu-Pengb

a(033001)b(041004)

Simple solvothermal reaction of PbI2, 1,4-diazabicyclo[2.2.2]octane and benzyl alcohol in the presence of HI acid creates a new organically templated iodoplumbate hybrid: [(N-Bz-DABCO)(PbI3)]n(1, N-Bz-DABCO+= N-benzyl-1,4-diazabicyclo[2.2.2]octane). Com- pound 1 crystallizes in orthorhombicspace group212121with= 20.044(9),= 8.002(3),= 12.345(5) ?,= 1980.1(14) ?3,= 4, C13H19N2PbI3,M= 791.19,D= 2.654 g/cm3,(000) = 1408,(Mo)= 13.189 mm–1, the final= 0.0303 and= 0.0849 for 3777 observed reflections (> 2()). Compound 1 contains one-dimensional anionic [PbI3]nn-chains that are enclosed into the cationic channel formed from the N-benzyl-1,4-diazabicyclo[2.2.2]octane. Noteworthy, compared with common symmetrical N,N?-dialkyl-DABCO2+cations,formed asymmetrical N-monoalkyl-DABCO+cation is first used as structure directing agents/templating agents in the haloplumbate system, finally leading to the occurrence of noncentrosymmetric feature in compound 1. In addition, UV-Vis absorption spectrum analysis reveals that compound 1 is a potential semiconductor material with energy gap of 2.80 eV.

iodoplunbate hybrid, template synthesis, asymmetry, semiconductor material;

1 INTRODUCTION

Structural and functional researches of haloplum- bate hybrids based on molecular and energy-band engineering represent a new trend in solid-state chemistry owing to their structural diversity and excellent optoelectronic properties[1-4], which have been used as visible-light sensitizers for photovoltaic cells[5], as semiconducting channels in thin-film field-effect transistors[6], as switchable nonlinear- optic devices[7], and in ferroelectric materials[8], chromism[9, 10]and so on[11, 12]. Usually, the organic moieties play two important roles in the structures and functions of haloplumbate hybrids: (a) acting as templating agents/structure directing agents (SDAs) to influence the structures and consequent optoelec- tronic properties of the inorganic framework[13, 14]; (b) producing novel properties by the synergetic interactions between two countercomponents[9, 15, 16].Obviously, subtle changes of the organic cationicparameters, such as size, shape, charge density, symmetry and electron accepting ability, is an effective route for realizing the structural and function modulation of haloplumbate hybrids.

1,4-Diazabicyclo[2.2.2]octane (DABCO) pos- sesses two potential N-alkylated sites, suitable rigidity and excellent space-filling ability, and its alkylated products as templating agents/structure directing agents have been proved to be very successful in the syntheses of iodoplumbate hybrids. For example, the obtained halometallates include the zero-dimensional (0-D) (Me2DABCO)2(PbI6)[17], [Bu2DABCO]3[Pb5I16]?·?4?DMF[18], the one-dimensional(1-D) (Et2DABCO)2(Pb3I10)[17], (Pr2DABCO)2(Pb3I10)[17],(Me2DABCO)3(H2DABCO)2(Pb7I24)[17], (Bz2DABCO)(Pb2I6)·0.5H2O[17], [Pr2DABCO][Pb2I6][19]as well as the coexistence of different anions (Pr2DABCO)21[Pb18I54(I2)9][Pb2I9]2I5·13H2O[12]. However, to the best of our knowledge, only symmetrical N,N?-dialkyl-DABCO2+cations with the same substituent at nitrogen have been widely studied, while the asymmetric N-monoalkyl- DABCO+cation and its corresponding iodoplumbate hybrid have not been explored. In order to investigate the impact of N-monoalkyl-DABCO+cation on the structure and of iodoplumbate hybrids, we herein report the successful preparation and structural characterization of a new 1D iodoplumbate hybrid directing byformed N-Bz-DABCO+cation: [(N-Bz-DABCO)(PbI3)]n(1), which exhibits interes- ting chiral and semiconductor feature.

2 EXPERIMENTAL

2. 1 Materials and instruments

All reagents were purchased commercially and used without further purification. Powder X-ray diffraction (PXRD) intensities was measured on a Bruker D8Xdiffractometer equipped with mono- chromatized Cu(= 1.541 ?) radiation at room temperature. Elemental analysis (C, H, N) was carried on a Perkin-Elmer 240 elemental analyzer. The FT-IR spectrum was recorded from KBr pellets in the range of 400～4000 cm?1on a Nicolet 5DX spectrometer. UV/Vis diffuse reflectance spectrum was recorded with a TU-1950 UV/Vis spectro- photometer at room temperature, and the BaSO4plate was used as the reference.

2. 2 Synthesis of compound 1

A mixture of PbI2(0.231g, 0.5mmol), con- centrated HI (0.1 mL, 45%)and DABCO (0.056g, 0.5mmol) in benzyl alcohol (4mL) was stirred for 20 min at room temperature, and then the resultant turbid liquid sealed in a 15 mL Teflon-lined stainless-steel autoclave and heated at 140 ℃ for 6 days. Pale-yellow stick crystals of1were obtained in 15.9% yield (based on Pb) after washing with benzyl alcohol. Anal. Calcd. (%) forC13H19N2PbI3:C, 19.73; H, 2.42; N, 3.54. Found(%): C, 19.70; H, 2.53; N, 3.62. IR data (KBr, cm?1): 3423s, 3048w, 2931w, 2854w, 1675s, 1610s, 1449w, 1397m, 1274w, 1190w, 1131w, 853m, 756m, 574m.

2. 3 Crystal structure determination

A stick pale-yellow singlecrystal of 1 with dimensions of 0.39mm ×0.12mm ×0.11mm was selectedand mounted on a glass fiber. Thesingle-crystal X-ray diffraction data were collected on a Bruker SMART APEX CCD diffractometer equipped with graphite-monochromated Moradiation (= 0.71073 ?)at 293(2) K. A total of 45692 reflections were collected by a-scan mode in the range of 5.24≤2≤52.00o, of which 3860 were unique withint= 0.0577 and 3777 were observed with> 2(). Data reductions and empiri- cal absorption correction were performed using the SAINT and SADABS program[20], respectively.The structure was solved by direct methods using SHELXS and refinedby full-matrix least-squares methods on2with the SHELXL software[21].All non-hydrogen atoms were refined anisotropically. All hydrogen atomswere placed geometrically and refined isotropically. The final= 0.0303 and= 0.0849 (= 1/[2(F2) + (0.0556)2+ 3.1188], where= (F2+ 2F2)/3) for 3777 observed reflections (> 2()). (Δ/)max= 0.001,= 1.103, (Δ)max= 1.58 and (Δ)min= – 2.05 e/?3. Selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for 1

Symmetry codes:A1–, –1/2+, 3/2–;B1–, 1/2+, 3/2–

3 RESULTS AND DISCUSSION

3. 1 Synthetic aspects

alkylation of organic amines as a green, effective, and economic way has been widely used for the synthesis of halometallate hybrids[16, 22-25], among which acid environment plays the key role in forming the resultant structures. In this work, benzyl alohol solvent first N-alkylates DABCO to form N-Bz-DABCO+iodide in acidic HI solution, and then N-Bz-DABCO+cation actsas structure directing agents/templating agents to direct the formation of polymeric iodoplumbate anion. It should be mentioned that although N,N?-dialkyl-DABCO2+cations were widely used in the haloplumbate system[11, 12, 17-19],formed N-monoalkyl- DABCO+cation was first discovered here. Mean- while, with increasing the amount of concentrated HI, DABCO molecule is inclined to exhibit dialkylation/protonation.

3. 2 Structure description

Single-crystal X-ray diffraction analysis revealed that compound 1 crystallizes in the chiral space group212121with a Flack parameter of 0.001(11), and its asymmetric unit consists of one Pb atom, three iodine atoms and one N-Bz-DABCO+cation (Fig. 1). The crystallographically independent Pb(1) atom adopts a distorted octahedral coordination geometrybonding to six iodine atoms. The Pb–I bond distances range from 3.117(9) to 3.377(2) ?, and the I–Pb–I bond angles fall in the range of 83.3(1)～179.4(6)°. As shown in Fig. 2, similar to reported {[PC][PbI3]}n[16], the adjacent PbI6octahedra share faces to form a 1D a-type [PbI3]nn-chain, in which the six I atoms in each PbI6octahedron is overlapped with their neighbouring ones along the Pb → Pb vector of the chain. The Pb···Pb distance in the chain is 4.001 ?, indicating the absence of metal-metal interactions.For 1, the N-Bz-DABCO+cations stacked along theaxis to form rhombic channels where the 1D a-type iodoplumbate chains are encapsulated via electro- static contribution and five C–H···I weak nontra- ditional hydrogen bonds, while thirteen C–H···I weak hydrogen bonds exist in compound (Bz2DABCO)(Pb2I6)·0.5H2O.

It is well known that the structures of iodo- plumbate anions closely depend on the organic ammonium templates[13, 15, 26, 27]. As an example, Wang etc. investigated the impact of different derivatives of 1,4-diazabicyclo[2.2.2]octane on the crystal structures of iodoplumbates by systematically increasing the size of the same organic amine[17], and found the anion tends to form an isolated (PbI6)4?octahedron for small (H2DABCO)2+or (Me2DABCO)2+template but a 1-D chain with larger templates such as (Et2DABCO)2+, (Pr2DABCO)2+and (Bz2DABCO)2+. It is noteworthy that in contrast to common symmetrical N,N?-dialkyl-DABCO2+cations, asymmetrical N-Bz-DABCO+cation as structure directing agents/templating agents is the first example in haloplumbate hybrid system where DABCO molecule only generates monoalkylation. Furthermore, with changing the alkylation number on DABCO molecule from two to one, the structures of iodoplumbates turn from ribbon-like [Pb2I6]n2n-chains to rod-like [PbI3]nn-chains, and their space group change from central to chiral, which can be assigned to the use of asymmetrical N-Bz-DABCO+cation and the reduction of hydrogen bondingnumber[8, 28]. Obviously, the introduction of asymmetrical N-monoalkyl-DABCO+cation into halometallate system can not only greatly enrich the types of structural directing agents, but also provide a potential strategy for the construction of novel acentric halometallate hybrid materials.

Fig. 1. Asymmetric unit diagram of compound 1 with 30% ellipsoid probability(Symmetry code: A:1–, –1/2+, 3/2–)

Fig. 2. View of the A-type (PbI3)nn?anionic chain and their packing diagram

3. 3 PXRDand optical property

As shown in Fig. 3, the experimental powder X-ray diffraction (PXRD) pattern for 1 shows good agreement with the simulated patterns, proving that the as-synthesized products are a single phase. The difference in reflection intensity is probably caused by the preferred orientation effect of the powder sample.

The optical absorption spectrum of 1 was calculated from the diffuse reflectance data (R) by using the Kubelka-Munk function (F(R) = (1 –)2/2). As shown in Fig. 4, the band gap of 1 was estimated to be 2.80 eV, suggesting a semiconductor nature (the energy band gap of Eg was obtained by using a straightforward extrapolation method[26]), which indicates a 0.50 eV blue-shift of the absorption edge compared with the bulk PbI2(2.30 eV) and slightly larger than that of (Bz2DABCO)(Pb2I6)·0.5H2O (2.62 eV)[17]. Accor- ding to the theoretical calculationsfor compound (Bz2DABCO)(Pb2I6)·0.5H2O (Bz = benzyl) by Wang, the absorption edge of compound 1 might be attributed to the charge transitions from the 5orbitals of the iodine atoms to p-* anti-bonding orbitals of the BzDABCO+cations[17]. That is to say, the benzylation of DABCO cation can make a certain contribution to the bottom of the conduction band and modulates the band gap of hybrid directly.

Fig. 3. Experimental (top) and simulated (bottom) PXRD patterns of compound 1

Fig. 4. Optical absorption spectra for 1

4 CONCLUSION

In summary, our research is first reported on using asymmetrical N-Bz-DABCO+cation as structure directing agents/templating agents to construct iodoplumbate hybrid, which exhibits interesting chiral and semiconductor nature. Meanwhile, struc- ture directing agents/templating agents with more substituents present more C–H···I weak hydrogen bonds, which tends to form more complicated inor- ganic moieties. This result may provide an effective avenue for the rational construction of iodoplumbate hybrid materials with various structures and properties via utilizing asymmetrical template. Research in this field is ongoing.

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27 July 2017;

19 October 2017 (CCDC 1562186)

①This work was financially supported by the Natural Scientific Foundation of Lvliang University(No.ZRXN201603) and the characteristic & preponderant discipline of “1331 engineering” of Shanxi Province,Materials Science and Engineering (2050205 higher education)

. Wu Guo-Xing, born in 1981, lecturer, majoring in coordination chemistry. E-mail: wuguoxing8@163.com

10.14102/j.cnki.0254-5861.2011-1795