Using alcohols as alkylation reagents for 4-cyanopyridinium and N,N′-dialkyl-4,4′-bipyridinium and their one-dimensional iodoplumbates

Abstract

Solvothermal reactions of PbI₂ with 4-cyanopyridine (4-cypy) or 4,4′-bipyridine (4,4′-bipy) and alcohols (ethanol, propanol, iso-propanol, benzyl alcohol, α,α′-dihydroxy-p-xylene) in the presence of PbI₂, I₂ and a trace amount of water in acetonitrile gave rise to a family of 1D iodoplumbate complexes of 4-cyanopyridinium and N,N′-dialkyl-4,4′-bipyridinium including {[EC]₄[(Pb₂I₆)₂]·2MeCN}_n (EC⁺ = N-ethyl-4-cyanopyridium) (1), {[PC][PbI₃]}_n (PC⁺ = N-propyl-4-cyanopyridium) (2), {[iPC]₃[(PbI₃)(Pb₂I₆)]}_n (iPC⁺ = N-isopropyl-4-cyanopyridium) (3), {[BzC][PbI₃]}_n (BzC⁺ = N-benzyl-4-cyanopyridium) (4), {[Cxy]₂[(Pb₂I₆)₂]}_n (Cxy²⁺ = 1,4-bis(4-cyanopyridium)-xylene) (5), {[EV]_1.5[Pb₃I₉]}_n (EV²⁺ = N,N′-diethyl-4,4′-bipyridinium) (6), {[PV]_1.5[Pb₃I₉]}_n (PV²⁺ = N,N′-dipropyl-4,4′-bipyridinium) (7), and {[iPV]₂[Pb₄I₁₂]}_n (iPV²⁺ = N,N′-diisopropyl-4,4′-bipyridinium) (8). The resulting 4-cyanopyridinium and viologen cations were generated in situ via the cleavage of C–O bond of alcohols followed by alkylation of 4-cypy or 4,4′-bipy. X-Ray analysis revealed that compounds 1–8 contain one-dimensional anionic [PbI₃]_nⁿ⁻ (1–5, 8) and [Pb₃I₉]_n³ⁿ⁻ (6 and 7) chains that are enclosed into the different cationic channels formed from the 4-cyanopyridinium and N,N′-dialkyl-4,4′-bipyridinium. In addition, the optoelectronic and dielectric properties of 1–8 were also investigated.

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Introduction

The alkylation of pyridyl groups is of importance because the resulting products, pyridinium cations, have many applications in advanced materials such as catalytic,¹agrochemical,²surfactant,³ and optoelectronic materials.⁴ The traditional approach to pyridinium cations is to react pyridine derivatives with halohydrocarbons. In recent years, some reports indicated that alcohols, instead of halohydrocarbons, could be used as the alkylation reagents under the presence of metal salts and strong acids.⁵ On the other hand, the pyridinium cations have been widely used as templates to tailor out different types of low-dimensional metal-halide-pyridinium complexes.⁶ Among them, the low-dimensional pyridinium iodoplumbate hybrids have received much attention due to their intriguing architectures and their potential applications in the optical and electronic fields.⁷ The normal approach to iodoplumbate-pyridinium complexes is to combine PbI₂ with the preformed pyridinium salts directly in certain solvents, and in many cases, NaI, KI or HI is added to increase the solubility of the products and to promote crystallization.^8,6a,7g Because the readily-made pyridinium salts are limited in number, the above method can work only on a small scale, especially for iodoplumbate-pyridinium complexes.

In this article, we have developed a new approach to one-dimensional (1D) iodoplumbate-pyridinium complexes viasolvothermal reactions of 4-cyanopyridine (4-cypy) or 4,4′-bipyridine (4,4′-bipy) with various alcohols in the presence of PbI₂, I₂ and a trace amount of water in acetonitrile. A set of 1D iodoplumbate complexes of 4-cyanopyridinium (C⁺/C²⁺) and viologen (V²⁺) including {[EC]₄[(Pb₂I₆)₂]·2MeCN}_n (EC⁺ = N-ethyl-4-cyanopyridium) (1), {[PC][PbI₃]}_n (PC⁺ = N-propyl-4-cyanopyridium) (2), {[iPC]₃[(PbI₃)(Pb₂I₆)]}_n (iPC⁺ = N-isopropyl-4-cyanopyridium) (3), {[BzC][PbI₃]}_n (BzC⁺ = N-benzyl-4-cyanopyridium) (4), {[Cxy]₂[(Pb₂I₆)₂]}_n (Cxy²⁺ = 1,4-bis(4-cyanopyridium)-xylene) (5), {[EV]_1.5[Pb₃I₉]}_n (EV²⁺ = N,N′-diethyl-4,4′-bipyridinium) (6), {[PV]_1.5[Pb₃I₉]}_n (PV²⁺ = N,N′-dipropyl-4,4′-bipyridinium) (7), and {[iPV]₂[Pb₄I₁₂]}_n (iPV²⁺ = N,N′-diisopropyl-4,4′-bipyridinium) (8) were isolated there from. Considering that the low dielectric constant materials (low-k materials) is of importance in modern semiconductor industry,⁹ and most of the low-k materials are based on inorganosilica¹⁰ and organosilica¹¹ complexes, aromatic polymers,¹² fluorinated polymers,¹³polyimides,¹⁴ low-dimensional nanomaterials,¹⁵copper and aluminium composites,¹⁶ we have also investigated dielectric properties of 1–8 in order to screen out suitable low-k complexes for potential applications. Herein, we report their syntheses, structures along with their optoelectronic and dielectric properties.

Experimental

Materials and physical measurements

All starting materials were purchased from commercial sources and used without further purification. Solvents were purchased as reagent grade and distilled prior to use. The elemental analyses for C, H, and N were performed on a Carlo–Erba CHNO–S microanalyzer. The IR spectra were recorded on a Varian 1000 FT–IR spectrometer as KBr disks (4000–400 cm⁻¹). Solid-state UV-vis-NIR spectra were measured with a Shimadzu UV-3150 spectrometer at room temperature in the range 200∼2000 nm. The permittivity measurements were performed on a Novocontrol Concept 80 broadband dielectric spectrometer.

Preparation of complexes 1–8

{[EC]₄[(Pb₂I₆)₂]·2MeCN}_n (1). To a Pyrex glass tube (15 cm in length, 7 mm in inner diameter) was added PbI₂ (23 mg, 0.05 mmol), I₂ (13 mg, 0.05 mmol), 4-cypy (10 mg, 0.1 mmol), water (3 μL), ethanol (0.5 mL) and acetonitrile (1.5 mL). The tube was sealed and heated in an oven at 150 °C for 35 h and then cooled to room temperature at a rate of 5 °C 100 min⁻¹ to form brown-yellow rod crystals of 1, which were collected by filtration, washed with ethyl acetate, and dried in air. Yield: 11 mg (30% based on PbI₂). Anal. calcd for C₃₆H₄₂N₁₀Pb₄I₁₂: C 14.58, H 1.43, N 4.72%. Found: C 14.81, H 1.45, N 4.98%. IR (KBr, cm⁻¹): 3107 w, 3045 m, 2973 w, 2932 w, 2244 w, 1732 w, 1638 m, 1561 m, 1506 w, 1452 s, 1384 w, 1331 w, 1298 m, 1231 m, 1158 m, 1085 w, 1053 w, 971 w, 838 s, 760 w, 722 w, 670 w, 559 s.

{[PC][PbI₃]}_n (2). The brown rod crystals of 2 were prepared by a similar method used in the synthesis of 1 except that ethanol was replaced by propanol (0.15 g, 2.50 mmol). Yield: 6 mg (17% based on PbI₂). Anal. calcd for C₉H₁₁I₃N₂Pb: C 14.70, H 1.51, N 3.81%. Found: C 14.76, H 1.51, N 3.75%. IR (KBr, cm⁻¹): 3104 m, 3083 m, 3043 s, 2964 m, 2931 m, 2873 w, 2244 w, 1637 s, 1561 m, 1507 w, 1453 s, 1384 w, 1333 w, 1295 m, 1214 m, 1156 m, 1085 w, 1052 w, 846 m, 829 m, 670 w, 560 m.

{[iPC]₃[(PbI₃)(Pb₂I₆)]}_n (3). The orange rod crystals of 3 were prepared by a similar method used in the synthesis of 2 except that propanol was replaced by isopropanol (0.20 g, 3.33 mmol). Yield: 18 mg (16% based on PbI₂). Anal. calcd for C₂₇H₃₃I₉N₆Pb₃: C 14.70, H 1.51, N 3.81%. Found: C 14.84, H 1.52, N 3.73%. IR (KBr, cm⁻¹): 3109 m, 3074 m, 3045 m, 2979 m, 2932 w, 2873 w, 2817 w, 2244 w, 1923 w, 1732 w, 1683 w, 1635 s, 1562 m, 1504 m, 1454 s, 1374 m, 1320 w, 1297 w, 1249 s, 1178 m, 1137 s, 1073 w, 1049 m, 952 w, 891 w, 842 s, 760 w, 726 w, 671 w, 566 s, 426 w.

{[BzC][PbI₃]}_n (4). The orange rod crystals of 4 were prepared by a similar method used in the synthesis of 2 except that propanol was replaced by benzyl alcohol (0.15 g, 1.40 mmol). Yield: 16 mg (42% based on PbI₂). Anal. calcd for C₁₃H₁₁I₃N₂Pb: C 19.94, H 1.42, N 3.58%. Found: C 20.11, H 1.45, N 3.69%. IR (KBr, cm⁻¹): 3098 m, 3075 w, 3038 m, 2983 w, 2245 w, 1634 m, 1558 m, 1495 m, 1450 s, 1335 w, 1286 m, 1204 w, 1139 m, 1080 w, 1051 w, 1028 w, 1002 w, 855 w, 817 m, 769 w, 726 m, 702 m, 598 m, 543 m, 492 w.

{[Cxy]₂[(Pb₂I₆)₂]}_n (5). The red rod crystals of 5 were prepared by a similar method used in the synthesis of 2 except that propanol was replaced by α,α′-dihydroxy-p-xylene (14 mg, 0.1 mmol). Yield: 4 mg (10% based on PbI₂). Anal. calcd for C₄₀H₃₂I₁₂N₈Pb₄: C 16.14, H 1.08, N 3.76%. Found: C 16.28, H 1.07, N 3.83%. IR (KBr, cm⁻¹): 3103 w, 3039 m, 2963 w, 2934 w, 2244 w, 1923 w, 1635 s, 1559 m, 1506 m, 1450 s, 1363 w, 1285 w, 1212 m, 1126 m, 1084 w, 1050 w, 1021 w, 965 w, 826 m, 734 w, 700 w, 609 w, 552 m.

{[EV]_1.5[Pb₃I₉]}_n (6). The dark red rod crystals of 6 were prepared by a similar method used in the synthesis of 1 except that 4-cypy was replaced by 4,4′-bipy (7.8 mg, 0.05 mmol). Yield: 16 mg (45% based on PbI₂). Anal. calcd for C₂₁H₂₇I₉N₃Pb₃: C 12.10, H 1.31, N 2.02%. Found: C 12.17, H 1.30, N 1.95%. IR (KBr, cm⁻¹): 3109 w, 3083 w, 3046 m, 2966 w, 2928 w, 1918 w, 1635 s, 1557 m, 1500 w, 1441 s, 1377 w, 1340 w, 1290 w, 1234 w, 1214 w, 1172 m, 1084 m, 969 w, 864 w, 822 m, 767 w, 713 w, 561 w, 507 w.

{[PV]_1.5[Pb₃I₉]}_n (7). The dark red block crystals of 7 were prepared by a similar method used in the synthesis of 6 except that ethanol was replaced by propanol (0.5 mL). Yield: 22 mg (63% based on PbI₂). Anal. calcd for C₂₄H₃₃I₉N₃Pb₃: C 13.55, H 1.56, N 1.98%. Found: C 13.78, H 1.51, N 2.14%. IR (KBr, cm⁻¹): 3085 w, 3046 m, 2958 m, 2928 w, 2871 w, 1922 w, 1635 s, 1556 m, 1501 m, 1440 s, 1384 w, 1338 m, 1283 w, 1214 m, 1172 m, 1099 w, 1049 w, 830 m, 805 m, 714 w, 573 w, 505 w.

{[iPV]₂[Pb₄I₁₂]}_n (8). The black-red block crystals of 8 were prepared by a similar method used in the synthesis of 6 except that ethanol was replaced by isopropanol (0.5 mL). Yield: 12 mg (35% based on PbI₂). Anal. calcd for C₃₂H₄₄I₁₂N₄Pb₄: C 13.55, H 1.56, N 1.98%. Found: C 13.41, H 1.54, N 1.87%. IR (KBr, cm⁻¹): 3112 w, 3080 w, 3050 m, 2976 w, 2927 w, 1632 s, 1557 m, 1496 w, 1439 s, 1376 m, 1249 m, 1160 m, 1079 m, 1045 w, 824 m, 737 w, 712 w, 534 w.

X-Ray data collection and structure determination

Single crystals of 1–8 were obtained directly from the above preparations. Diffraction intensities of 1–8 were collected on a Rigaku Mercury CCD X-ray diffractometer (Mo-Kα, λ = 0.71073 Å). Each single crystal was mounted at the top of a glass fiber at 293 K for 4 and 223 K for others in a stream of gaseous nitrogen. Cell parameters were refined on all observed reflections by using the program CrystalClear (Rigaku and MSc, Ver. 1.3, 2001). The collected data were reduced by the program CrystalClear, and an absorption correction (multi-scan) was applied. The reflection data were also corrected for Lorentz and polarization effects.

The crystal structures of 1–8 were solved by direct methods and refined on F² by full-matrix least-squares techniques with SHELXTL-97 program.¹⁷ For 7, two propyl groups on a N,N′-dipropyl-4,4′-bipyridinium cation were disordered over two positions with the occupancy factors of 0.45/0.55 for C17–C18/C17′–C18′ and C21/C21′, respectively. For 8, the Pb centers and some I atoms were disordered over two positions with the occupancy factors of 0.83/0.17 for Pb1/Pb1′, Pb2/Pb2′, Pb3/Pb3′, Pb4/Pb4′, I1/I1′, I2/I2′, I4/I4′, I8/I8′, I9/I9′ and I11/I11′. All non-hydrogen atoms were refined anisotropically while all hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms. Presence of heavy elements like Pb and I may unfavourably affect the crystallographic results, even if absorption correction is applied. Checkcif results showed several Hirshfeld-test failures, unexpected deviations from expected ADP's, unusual bond distances, etc. However, the geometric details of the organic part may not affect too much the significance of these structures. A summary of key crystallographic information for 1–8 is given in Table 1. Selected bond lengths and angles for 1–8 are listed in Table 2.

Table 1 Summary of crystallographic data for complexes 1–8

Compound	1	2	3
a R = Σ‖Fo| − |Fc‖/Σ|Fo|. b wR = {Σw(Fo^2 − Fc^2)^2/Σw(Fo^2)^2}^1/2. c GOF = {Σ[w((Fo^2 − Fc^2)^2)/(n − p)}^1/2, where n = number of reflections and p = total numbers of parameters refined.
Formula	C36H42N10Pb4I12	C9H11N2PbI3	C27H33N6Pb3I9
FW/g mol^−1	2966.44	735.10	2205.29
Crystal system	Monoclinic	Monoclinic	Trigonal
Space group	C2/c	P21/c	P-3
a/Å	16.743(3)	7.8610(11)	19.208(3)
b/Å	14.793(3)	12.7769(19)	19.208(3)
c/Å	28.814(6)	16.133(2)	7.8129(16)
β/°	94.97(3)	97.655(4)
γ/°			120.00
V/Å^3	7110(2)	1606.0(4)	2496.4(8)
Z	4	4	2
T/K	223(2)	223(2)	223(2)
D c/g cm^−3	2.810	3.040	2.934
F(000)	5256.0	1280	1920
μ (Mo-Kα/mm^−1)	14.686	16.249	15.680
Total no. of reflections	16 517	9048	12 015
No. of unique reflections	7997	3651	3804
No. of observed reflections	4774	2354	2813
R ^a (I > 2.00σ (I))	0.0725	0.0650	0.0403
wR ^b	0.1437	0.2498	0.0640
GOF^c	1.045	1.153	0.933
Δρmax/Δρmin/e Å^−3	1.692/−2.210	2.708/−2.791	2.803/−2.054

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Compound	4	5	6
Formula	C13H11N2PbI3	C40H32N8Pb4I12	C21H27N3Pb3I9
FW/g mol^−1	783.14	2976.34	2085.16
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	P21/c	P21/c
a/Å	8.0356(16)	7.7261(15)	12.026(2)
b/Å	14.126(3)	21.194(4)	14.039(3)
c/Å	16.065(3)	19.674(4)	24.770(5)
β/°	93.41(3)	94.01(3)	90.44(3)
V/Å^3	1820.3(6)	3213.7(11)	4181.9(14)
Z	4	2	4
T/K	293(2)	223(2)	223(2)
D c/g cm^−3	2.858	3.076	3.312
F(000)	1376	2584	3588
μ (Mo-Kα/mm^−1)	14.346	16.243	18.708
Total no. of reflections	17 293	12 948	19 117
No. of unique reflections	3323	7200	9316
No. of observed reflections	2709	4202	6458
R ^a (I > 2.00σ (I))	0.0528	0.0372	0.0785
wR^b	0.1100	0.0596	0.1435
GOF^c	1.023	0.815	1.097
Δρmax/Δρmin/e Å^−3	1.170/−0.967	1.339/−1.338	1.444/−1.771

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Compound	7	8
Formula	C24H33N3Pb3I9	C32H44N4Pb4I12
FW/g mol^−1	2127.23	2836.31
Crystal system	Monoclinic	Monoclinic
Space group	P21/c	P21/c
a/Å	12.328(3)	16.182(3)
b/Å	14.341(3)	21.323(4)
c/Å	24.982(5)	17.966(4)
β/°	90.24(3)	103.44(3)
V/Å^3	4416.7(17)	6029(2)
Z	4	4
T/K	223(2)	223(2)
D c/g cm^−3	3.199	3.125
F(000)	3684	4912
μ (Mo-Kα/mm^−1)	17.717	17.305
Total no. of reflections	20 775	19 681
No. of unique reflections	10 019	10 380
No. of observed reflections	7579	4568
R ^a (I > 2.00σ (I))	0.0722	0.0986
wR^b	0.1244	0.1842
GOF^c	1.109	1.062
Δρmax/Δρmin/e Å^−3	1.256/−2.152	1.554/−1.421

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Table 2 Selected bond lengths (Å) and angles (°) for complexes 1–8^a

a Symmetry codes for 1: A: −x + 1/2, −y + 1/2, −z; B: −x, y, −z − 1/2. Symmetry codes for 2: A: −x + 2, −y + 1, −z + 1; B: −x + 1, −y + 1, −z + 1. Symmetry codes for 3: A: −y + 1, x − y, z; C: −y + 1, x − y, z + 1; F: −x + 2, −y, −z + 2; M: −x + 2, −y, −z + 3. Symmetry codes for 4: A: −x + 1, −y + 1, −z; B: −x + 2, −y + 1, −z; C: x + 1, y, z. Symmetry codes for 5: A: x + 1, y, z; B: x − 1, y, z. Symmetry codes for 6: A: −x + 1, −y + 2, −z; B: x + 1, y, z; C: −x + 2, −y + 2, −z. Symmetry codes for 7: A: −x, −y + 2, −z; B: x − 1, y, z; C: −x − 1, −y + 2, −z. Symmetry codes for 8: A: x − 1, y, z.
Complex 1
av. Pb(1)–I	3.2335(13)	av. Pb(2)–I	3.2265(12)
av. Pb(3)–I	3.2263(12)
I(1)–Pb(1)–I(3)	172.99(3)	I(2)–Pb(1)–I(2B)	174.90(5)
I(6A)–Pb(2)–I(1B)	178.16(3)	I(5)–Pb(2)–I(2B)	176.36(3)
I(4)–Pb(2)–I(3)	174.19(3)	I(5)–Pb(3)–I(5A)	180.0
I(4)–Pb(3)–I(4A)	180.0	I(6A)–Pb(3)–I(6)	180.0
Complex 2
av. Pb(1)–I	3.2244(15)	av. Pb(2)–I	3.2258(15)
I(1)–Pb(1)–I(1B)	180.0	I(3)–Pb(1)–I(3B)	180.0
I(2)–Pb(1)–I(2B)	180.0	I(1)–Pb(2)–I(1A)	180.0
I(3)–Pb(2)–I(3A)	180.0	I(2)–Pb(2)–I(2A)	180.0
Complex 3
av. Pb(1)–I	3.2227(7)	av. Pb(2)–I	3.2265(8)
Pb(3)–I(3)	3.2235(7)	Pb(4)–I(3)	3.2390(7)
I(1A)–Pb(1)–I(2)	178.139(18)	I(2)–Pb(2)–I(1C)	179.051(17)
I(3)–Pb(3)–I(3F)	180.0	I(3M)–Pb(4)–I(3)	180.0
Complex 4
av. Pb(1)–I	3.2272(11)	av. Pb(2)–I	3.2482(12)
I(2)–Pb(1)–I(2A)	180.0	I(3)–Pb(1)–I(3A)	180.0
I(1A)–Pb(1)–I(1)	180.0	I(1B)–Pb(2)–I(1)	180.0
I(2B)–Pb(2)–I(2)	180.0	I(3C)–Pb(2)–I(3A)	180.0
Complex 5
av. Pb(1)–I	3.2335(10)	av. Pb(2)–I	3.2259(10)
I(5)–Pb(1)–I(2)	174.70(2)	I(6A)–Pb(1)–I(1)	175.284(19)
I(3)–Pb(1)–I(4)	175.96(2)	I(5B)–Pb(2)–I(2)	174.52(2)
I(6)–Pb(2)–I(1)	178.04(2)	I(3)–Pb(2)–I(4B)	175.51(2)
Complex 6
av. Pb(1)–I	3.2315(14)	av. Pb(2)–I	3.2740(17)
av. Pb(3)–I	3.2485(15)
I(4)–Pb(1)–I(5A)	177.75(3)	I(7)–Pb(1)–I(5)	168.81(4)
I(8)–Pb(1)–I(6)	174.63(4)	I(9)–Pb(2)–I(8)	179.48(4)
I(7C)–Pb(3)–I(4)	173.71(4)	I(9B)–Pb(3)–I(6)	166.18(4)
I(1)–Pb(3)–I(6C)	171.94(4)
Complex 7
av. Pb(1)–I	3.2370(13)	av. Pb(2)–I	3.1474(14)
av. Pb(3)–I	3.2489(13)
I(7)–Pb(1)–I(5A)	165.73(3)	I(4)–Pb(1)–I(5)	177.91(3)
I(8)–Pb(1)–I(6)	172.28(3)	I(8)–Pb(2)–I(9)	174.98(3)
I(4)–Pb(3)–I(7C)	173.67(3)	I(9B)–Pb(3)–I(6)	166.52(3)
I(1)–Pb(3)–I(6C)	168.91(4)
Complex 8
av. Pb(1)–I	3.230(4)	av. Pb(2)–I	3.283(4)
av. Pb(3)–I	3.227(4)	av. Pb(4)–I	3.283(4)
I(3)–Pb(1)–I(10A)	172.30(11)	I(11A)–Pb(1)–I(2)	177.64(11)
I(1)–Pb(1)–I(12A)	174.14(12)	I(3)–Pb(1)–I(12A)	101.63(9)
I(6)–Pb(2)–I(2)	108.31(9)	I(3)–Pb(2)–I(4)	114.32(9)
I(2)–Pb(2)–I(4)	158.30(10)	I(6)–Pb(2)–I(1)	155.12(9)
I(3)–Pb(2)–I(5)	154.10(8)	I(1)–Pb(2)–I(5)	120.52(9)
I(6)–Pb(3)–I(9)	174.03(10)	I(4)–Pb(3)–I(8)	174.05(12)
I(9)–Pb(3)–I(5)	100.36(10)	I(7)–Pb(3)–I(5)	175.62(8)
I(9)–Pb(4)–I(11)	110.81(13)	I(8)–Pb(4)–I(11)	160.38(12)
I(9)–Pb(4)–I(12)	155.76(11)	I(12)–Pb(4)–I(7)	119.75(10)
I(8)–Pb(4)–I(10)	107.49(10)	I(7)–Pb(4)–I(10)	154.86(10)

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Results and discussion

Synthetic and spectral aspects

Complexes 1–8 were generated in situ under solvothermal conditions, in which the C⁺/C²⁺ and V²⁺ cations were formed via cleavage of the C–O bond of alcohols followed by alkylation of 4-cypy or 4,4′-bipy (Scheme 1). In these reactions, elemental iodine played a key role in realizing the synthetic processes. The possible mechanisms had been suggested in ESI, Scheme S1.†Elemental iodine may react with water to produce HI, which reacts with alcohols to form iodohydrocarbons. The resulting iodohydrocarbons further react with 4-cypy or 4,4′-bipy to generate a series of C⁺/C²⁺ or V²⁺ cations (Fig. 1). In the case of α,α′-dihydroxy-p-xylene, an unprecedented Cxy²⁺ dication was produced (Fig. 1e). With these C⁺/C²⁺ or V²⁺ cations in solution, they worked as various templates to induce Pb²⁺ and I⁻ to self-assemble into 1D anionic [PbI₃]_nⁿ⁻ (1–5, 8) and [Pb₃I₉]_n³ⁿ⁻ (6 and 7) chains.

Scheme 1 The reactions for the synthesis of complexes 1–8

Fig. 1 View of the in situ generated C⁺/C²⁺ and V²⁺ cations in 1–8. Atom colour codes: C = grey; H = white; N = cyan (the same hereinafter).

Compounds 1–8 are air- and moisture-stable, and insoluble in common solvents but soluble in DMF and DMSO. The elemental analysis was consistent with their chemical formula. The IR spectra of 1–8 show medium peaks at about 3050 cm⁻¹ and strong or medium peaks in the range of 1650–1450 cm⁻¹ indicating the existence of pyridyl groups; the medium peaks at ca. 2244 cm⁻¹ in 1–5 are assigned to be the terminal cyanide stretching vibration; and the medium peaks in the range of 3000–2800 cm⁻¹ denotes the existence of alkyl groups in 1–8.¹⁸ The identities of the eight compounds were further confirmed by single-crystal diffraction analysis.

Crystal structure of 1

Complex 1 crystallizes in the monoclinic space groupC2/c, and its asymmetric unit consists of one [Pb₂I₆]²⁻ dianion, two discrete EC⁺ cations, and one and one-half MeCN molecules. 1 features its b-type [PbI₃]_nⁿ⁻ chain, which is constructed by PbI₆ octahedra being linked by sharing I–I–I faces (Fig. 2, top). Pb1 is sitting on one mirror plane while a two-fold axis goes through Pb3. Looking along the Pb → Pb vector of the chain, the six I atoms in each PbI₆ octahedron are not fully overlapped with their corresponding ones of the neighbouring octahedra (Fig. 2b) because each PbI₆ octahedron is slightly distorted (Table 2). The mean Pb⋯Pb contact in the chain is 4.009 Å, which excludes any metal-metal interactions. In the crystal of 1, the EC⁺ cations surround the 1D anionic chains through static forces and stacked along the [101] direction to form rhombic channels where the 1D iodoplumbate chains are encapsulated (Fig. 3).

Fig. 2 View of the three types of [PbI₃]_nⁿ⁻ chains observed in 1–5 and 8. Atom colour codes: Pb = blue; I = pink (the same hereinafter).

Crystal structure of 2–5

Complexes 2, 4 and 5 crystallize in the monoclinic space groupP2₁/c and the asymmetric units of 2 and 4 contain half a [Pb₂I₆]²⁻ dianion, and one PC⁺ or BzC⁺ cation, while that of 5 contains one [Pb₂I₆]²⁻ dianion and two one-halves of Cxy²⁺ dications. Complex 3 crystallizes in the trigonal space groupP3̄, and the asymmetric unit consists of one-third of a [Pb₂I₆]²⁻ dianion, and one-sixth of the other [Pb₂I₆]²⁻ dianion, and one discrete iPC⁺ cation. Complexes 2–5 possess the a-type [PbI₃]_nⁿ⁻ chain, which is constructed by interconnections of nearly perfect PbI₆ octahedra. Looking along the Pb → Pb vector of the chain, the six I atoms in each PbI₆ octahedron is overlapped with their corresponding ones of the neighbouring octahedral, forming a six-vanes windmill-like configuration (Fig. 2a). Each PbI₆ unit in 2–5 adopts an almost perfect octahedron and the mean Pb⋯Pb contact in the chain varies from 3.817 Å to 4.018 Å. The arrangement of the resulting pyridinium cations around the inorganic chains in 2–5 deserves comments. For 2 and 4, the PC⁺ or BzC⁺ cations stack along the a-axis to form 1D rhombic channels where the a-type [PbI₃]_nⁿ⁻ chains are enveloped (ESI, Fig. S1†). In the case of 3, the iPC⁺ cations stack along the c-axis, forming a Kagomé-shaped channels where the a-type [PbI₃]_nⁿ⁻ chains are included (Fig. 4). In addition, the similar chains are also observed in triangle channels constructed by three Kagomé-shaped channels. While in 5, each four Cxy²⁺ dications stack along the a-axis to form a honeycomb-like channels where the a-type [PbI₃]_nⁿ⁻ chains are located (ESI, Fig. S2†).

Fig. 3 View of the b-type [PbI₃]_nⁿ⁻ chains enclosed into rhombic channels assembled by N-ethyl-4-cyanopyridium cations in 1 (looking along the [101] direction). All H atoms are omitted for clarity.

Fig. 4 View of the a-type [PbI₃]_nⁿ⁻ chains enclosed into Kagomé-shaped channels assembled by iPC⁺ cations in 3 (looking along the c-axis). All H atoms are omitted for clarity.

Crystal structures of 6 and 7

Complexes 6 and 7 crystallize in the monoclinic space groupP2₁/c and the asymmetric unit consists of one [Pb₃I₉]³⁻ trianion, and one and one-half EV²⁺ or PV²⁺ dications. 6 or 7 possesses an uncommon 1D [Pb₃I₉]_n³ⁿ⁻ chain that is constructed by edge-sharing PbI₆ octahedra (Fig. 5a). Such a chain is similar to that observed in (Et₂NH₂)₃Pb₃I₉·0.5H₂O.^8h Intriguingly, a group of six EV²⁺ or PV²⁺ cations forms a chair-like cyclohexane cavity that encompasses the [Pb₆I₂₄]¹²⁻ species (Fig. 5b). Such a [Pb₆I₂₄]¹²⁻ species links its neighbouring ones to form a 1D [Pb₃I₉]_n³ⁿ⁻ chain extending along the a-axis (Fig. 5a).

Fig. 5 (a) View of the 1D step-shaped [Pb₃I₉]_n³ⁿ⁻ chain in 6 and 7 (top). Each octahedron represents the PbI₆ unit (bottom). (b) View of the [Pb₆I₂₄]¹²⁻ species enclosed by a chair-like cyclohexane cavity constructed through a group of six EV²⁺ in 6.

Crystal structure of 8

Complex 8 crystallizes in the monoclinic space groupP2₁/c. The asymmetric unit consists of one [Pb₄I₁₂]⁴⁻tetraanion, and one and two haves of iPV²⁺. The four Pb centers show different coordination geometries. Pb1 and Pb3 adopt a PbI₆ octahedral geometry (O) while Pb2 and Pb4 take a trigonal prism geometry (TP). The PbI₆ units interconnect alternately via sharing the I–I–I faces in the manner [O-TP-O-TP] to form a rare c-type [PbI₃]_nⁿ⁻ chain (Fig. 2c). Because Pb centers show different coordination geometries, all the I atoms in the octahedral PbI₆ can not overlap those of the trigonal prismatic PbI₆ unit. The iPV²⁺ dications (Fig. 1h) stack along the a axis, thereby forming a honeycomb-like channels where these c-type [PbI₃]_nⁿ⁻ chains are encircled (Fig. 6).

Fig. 6 View of the c-type [PbI₃]_nⁿ⁻ chains enclosed into honeycomb-like channels assembled by iPV²⁺ cations in 8 (looking along the a-axis). All H atoms are omitted for clarity.

Optoelectronic and dielectric properties of 1–8

The optical diffuse-reflection spectra of crystalline solids 1–8 were measured at room temperature. The absorption (α/S) data were calculated from the reflectance using the Kubelka–Munk function.¹⁹ The energy band gaps (E_onset) obtained by extrapolation of the linear portion of the absorption edges were estimated to be 2.22 eV (1), 1.99 eV (2), 2.12 eV (3), 1.79 eV (4), 1.85 eV (5), 1.85 eV (6), 1.82 eV (7), and 1.61 eV (8) (Fig. 7), indicating a semiconductor nature. At the same time, the values of energy band gaps of 1–8 are all red-shifted with respect to that of the layered compound PbI₂ (2.30 eV),²⁰ which suggests that the introduction of pyridinium cations into the framework of PbI₂ may increase the activities of electrons due to the charge transfer effect.²¹

Fig. 7 Solid-state optical diffuse-reflection spectra of 1–8 and PbI₂ derived from diffuse reflectance data at ambient temperature.

The dielectric behaviours of 1–8 had been measured based on continuous variation of frequency at 298 K. As shown in Fig. 8(a), the dielectric constants of 1–8 in the range of 100 Hz to 10⁶ Hz were nearly frequency-independent with the values being estimated to be 4.57 (1), 6.88 (2), 5.98 (3), 5.92 (4), 4.74 (5), 6.55 (6), 7.22 (7), and 9.64 (8), respectively, which were all smaller than the observed value of PbI₂ (10.60). It is noted that the values of 1 and 5 were close to that of SiO₂ (4.3).⁹ The permittivity (real part) of the iodoplumbate-V²⁺ complexes (6–8) were generally larger than those of iodoplumbate-C⁺/C²⁺ ones (1–5). Complex 8 exhibited the maximum permittivity value compared with 1–7. From 1 Hz to 10⁶ Hz, the dielectric loss of 1–8 and PbI₂ was gradually close to zero, and 5 showed the maximum value of 0.03 (average) compared with other complexes in the range of 100 Hz to 10⁶ Hz.

Fig. 8 (a) Frequency dependence of permittivity (ε_r) of 1–8 and PbI₂ at 298 K. (b) Frequency dependence of dielectric loss (ε_i/ε_r) of 1–8 and PbI₂ at 298 K. The ε_r and ε_i are the real and imaginary parts of permittivity, respectively.

Conclusions

In summary, we have demonstrated an efficient approach to a series of 1D 4-cyanopyridinium and viologen iodoplumbates 1–8 by solvothermal reactions of alcohols with 4-cypy/4,4′-bipy, PbI₂, I₂ and a small amount of water in acetonitrile, in which the 4-cyanopyridinium and viologen cations were generated in situ. The optoelectronic and dielectric properties of 1–8 were explored. It was found that the introduction of C⁺/C²⁺ and V⁺ cations into bulk PbI₂ could decrease the energy band gaps and dielectric constant, obviously. It is anticipated that the as-synthesized complexes may be have potential application as low-k materials in the microelectronic field. The aforementioned synthetic methodology may be applied to prepare other low-dimensional charge transfer pyridinium-iodometalates with new structures, better optoelectronic performances and low dielectric constant characteristics.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (20525101, 20871088 and 90922018), the Nature Science Key Basic Research of Jiangsu Province for Higher Education (09KJA150002), the Specialized Research Fund for the Doctoral Program of higher Education of Ministry of Education (20093201110017), the State Key Laboratory of Organometallic Chemistry of Shanghai Institute of Organic Chemistry (08–25), the Qin-Lan and the “333” Projects of Jiangsu Province, and the “Soochow Scholar” Program and the Program for Innovative Research Team of Suzhou University, and the Innovative Research Program for Postgraduates in Universities of Jiangsu Province (CX09B_020Z). The authors also greatly thanked the helpful comments from the editor and the reviewers

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Footnote

† Electronic supplementary information (ESI) available: the possible mechanisms on generating complexes 1–8, 3D packing diagrams of 2, 4, 5 and 7, and frequency dependence of permittivity (imaginary part) of 1–8. CCDC reference numbers 776895–776899 and 765929–765931. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0ce00309c

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