Charge-Transfer Salts via Cocrystallization of the Cationic Ferrocenyl

Feb 5, 2010 - ABSTRACT: Four new charge-transfer (CT) salts based on the cationic ferrocenylmethylpyridinium (fmp) donor (CpFeCp-. CH2-Py. +. ) ...
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DOI: 10.1021/cg900777g

Charge-Transfer Salts via Cocrystallization of the Cationic Ferrocenyl Donor with Polyoxometalate Acceptors

2010, Vol. 10 1096–1103

Haisheng Xu,†,# Zhanfeng Li,† Bin Liu,† Ganglin Xue,*,† Huaiming Hu,† Feng Fu,‡ and Jiwu Wang‡ †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of Education), Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi’an 710069, China, ‡Shaanxi Key Laboratory of Chemical Reaction Engineering, Yanan University, Yan’an, Shaanxi 716000, China, and #College of Chemistry and Chemical Engineering, Xian Shiyou University, Xi’an 710065, China Received July 7, 2009; Revised Manuscript Received January 24, 2010

ABSTRACT: Four new charge-transfer (CT) salts based on the cationic ferrocenylmethylpyridinium (fmp) donor (CpFeCpCH2-Pyþ) and polyoxometalate (POM) acceptors, Lindqvist-type [M6O19]2- (M=Mo or W), the decatungstate isopolyanion [W10O32]4- and Keggin-type [PMo12O40]3-, namely, [CpFeCp-CH2-Py]2[Mo6O19] (1), [CpFeCp-CH2-Py]2[W6O19] (2), [CpFeCp-CH2-Py]4[W10O32] (3), and (NBu4)[CpFeCp-CH2-Py]2[PMo12O40] (4), were synthesized by the traditional solution synthetic method. X-ray crystallographic studies of the dark-colored 1 and brownish red 2 reveal that they are isostructural and crystallize in the monoclinic space group P21/c. Salt 3 crystallizes in the orthorhombic space group Pbca, and salt 4 crystallizes in the orthorhombic space group Pccn. In salts 1-4, the CpFeCp-CH2-Pyþ and the polyoxoanions are cocrystallized by Coulombic forces, and complex C-H 3 3 3 π and π 3 3 3 π stacking interactions also exist between the adjacent ferrocenylmethylpyridinium (fmp) cations. The orbital overlap between the π-donor plane of the pyridyl ring and one oxygen facet of the acceptor octahedron in the solid state is observed in salts 1 and 2. The UV-vis diffuse reflectance spectra indicate the presence of a broad CT band between 500 and 850 nm for 1-4, and the CT character of 1 and 2 is also confirmed by the linear Mulliken correlation between the CT transition energies and the reduction potentials of the POM acceptors. The luminescent spectroscopy of both salts 1 and 2 shows an intense emission at about 394 nm, and they may be excellent candidates for potential solid-state photofunctional materials. The emission band of 3 exhibits weakened fluorescence signals compared to that of the corresponding POM and the cationic donor, while the fluorescence of 4 observed at 400 nm for (NBu4)3PMo12O40 was almost totally quenched owing to the occurrence of an efficient CT process from the ferrocenyl donor to POM acceptors.

Introduction Polyoxometalates (POMs) or metal oxide clusters are versatile in many aspects, and in recent years new efforts have been made to explore their applications in catalysis, medicine, and material sciences,1-3 which is based on the ability of POMs to act as electron reservoirs as well as the extreme variability of their molecular properties, including size, shape, charge, charge density, redox potential, acidity, solubility, etc. 4,5 In particular, the potentialities of molecular materials of these metal oxide clusters are exemplified by the POMs in their use as electron-accepting moieties in charge-transfer (CT) materials prepared by cocrystallization with electron-rich organic donors, such as tetrathiafulvalene,3-13 ferrocene, and their derivatives via an initial electron transfer.3,4,14-20 Most of them are stabilized by hydrogen bonding or Coulombic forces in ion pairs consisting of (partially or fully) reduced POMs and oxidized donors.4,14,21 The first CT salts containing the cationic ferrocenyl donor CpFeCpCH2Nþ(CH3)3 and POM acceptors of the Lindqvist structural type ([M6O19]2-), namely, [CpFeCpCH2N(CH3)3]2[M6O19] (M = Mo, W), were reported in 1995;19 the UV-vis diffuse reflectance spectrum and the laser-flash photolysis spectroscopy in the solid state confirmed the presence of a new CT band and the relevant CT interactions between the ferrocenyl and POM *To whom correspondence should be addressed: E-mail: xglin707@ 163.com. pubs.acs.org/crystal

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moieties in the electron donor-acceptor (EDA) complexes. Recently, our group reported the CT salts based on the cationic ferrocenyl donor CpFeCpCH2Nþ(CH3)3 and Keggin-type acceptors, [CpFeCpCH2N(CH3)3]4[PMo12O40] 3 CH3CN and [CpFeCpCH2N(CH3)3]4[GeMo12O40].20 Continuing the effort in this direction, and with the aim to obtain novel CT salts with interesting properties by modulating the size and shape of the ferrocenyl donors and POM acceptors, herein we present four new CT salts based on the cationic ferrocenylmethylpyridinium (fmp) donor (CpFeCp-CH2Pyþ) and different types of POM acceptors (Lindqvist-type [M6O19]2- (M = Mo or W), the decatungstate isopolyanion [W10O32]4- and Keggin-type [PMo12O40]3-), namely, [CpFeCp-CH2-Py]2[Mo6O19] (1), [CpFeCp-CH2-Py]2[W6O19] (2), [CpFeCp-CH2-Py]4[W10O32] (3), and (NBu4)[CpFeCpCH2Py]2[PMo12O40] (4); this is the first report on the CT salts of POMs with ferrocene derivatives containing a pyridyl ring. The structural comparability of the new intensively colored materials is explored by X-ray crystallography, and the CT properties are examined by solid-state diffuse reflectance spectra, electronic spectra in acetonitrile solution, and luminescent spectroscopy. Experimental Section General Considerations. The experimental procedure was carried out in open air. The (NBu4)2M6O19 (M = Mo or W), (NBu4)4W10O32, (NBu4)3PMo12O40, and CpFeCp-CH2-PyþI- were prepared according r 2010 American Chemical Society

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Table 1. Crystallographic Parameters and Refinement Details for 1-4 empirical formula M, g mol-1 T, K space group a, A˚ b, A˚ c, A˚ R (deg) β (deg) γ (deg) Volume, A˚3 Z dcalcd, g cm-3 μ, mm-1 total reflns indep reflns parameters GOF R1 (I > 2σ(I)) wR2 (I > 2σ(I)) R1 (all data) wR2 (all data) diff peak and hole, e A-3

1

2

3

4

C32H32O19Fe2 N2Mo6 1435.93 293(2) P21/c 8.2227(14) 9.9797(17) 24.352(4) 90 93.029(2) 90 1995.6(6) 2 2.390 2.608 9551 3550 254 0.997 0.0261 0.0638 0.0321 0.0651 0.643, -0.485

C32H32O19Fe2 N2W6 1963.40 293(2) P21/c 8.1887(12) 10.0651(14) 24.424(3) 90 92.623(2) 90 2010.9(5) 2 3.243 17.865 9087 3464 273 0.995 0.0401 0.0926 0.0453 0.0947 2.196,-3.875

C64H64O32Fe4N4 W10 3462.98 293(2) Pbca 18.328(2) 17.4640(18) 23.660(3) 90 90 90 7573.1(14) 4 3.037 15.944 36331 6735 443 0.907 0.0347 0.0718 0.0531 0.0754 1.217, -1.844

C48H68Fe2Mo12N3 O40P 2620.99 293(2) Pccn 20.639(5) 21.396(5) 16.918(4) 90 90 90 7471(3) 4 2.330 2.428 35213 6512 216 1.054 0.0928 0.2208 0.1378 0.2356 1.793, -0.917

to the literature procedures,22-26 and other starting materials were AR grade and used as purchased. IR spectra were obtained on an EQUINOX55 IR spectrometer with KBr pellets. Electronic spectra (λ = 300-800 nm) were recorded on a Shimazu UV-2550 spectrophotometer in acetonitrile solution. Solid-state diffuse reflectance spectra of the samples were obtained for the dry pressed disk samples using a Shimazu UV-2550 spectrophotometer, equipped with an integrating sphere coated with polytetrafluoroethylene (PTFE), between 350 and 850 nm. Absorption spectra were referenced to barium sulfate. Luminescent spectra were measured at room temperature on a Hitachi F4500 fluorescence spectrophotometer equipped with a 450 W xenon lamp as the excitation source. Synthesis. [CpFeCp-CH2-Py]2[Mo6O19] (1). (NBu4)2Mo6O19 (0.68 g, 0.5 mmol) was dissolved in 20 mL of acetonitrile. To the solution CpFeCp-CH2-PyþI- (0.41 g, 1 mmol) dissolved in 10 mL acetonitrile was added with stirring. The mixture was heated for 30 min at 60 °C and then filtered. The filtrate was slowly evaporated at ambient conditions. Within 2 days, brownish red block crystals of 1 were isolated in about 69% (based on (NBu4)2Mo6O19). Elemental analysis (%) calcd for C32H32O19Fe2N2Mo6: C, 26.7; H, 2.2; N, 1.9; Fe, 7.8; Mo, 40.0. Found: C, 27.1; H, 1.7; N, 2.0; Fe, 7.7; Mo, 39.8. IR (KBr): 3436.1(m), 3075.6(m), 1625.5(m), 1483.0(s), 1144.3(s), 940.7(vs), 906.2(vs), 844.1(s), 707.7(s), 504.6(m) cm-1. 1H NMR [d6DMSO, 400 MHz]: δ 9.12 (2 H, py), 8.58 (1 H, py), 8.14 (2 H, py), 5.62 (2 H, CH2), 4.57 (2 H, Cp H), 4.30 (2 H, Cp H), 4.28 (5 H, Cp H). [CpFeCp-CH2-Py]2[W6O19] (2). The synthetic procedure for 2 is similar to that of 1 except for using (NBu4)2W6O19 (0.95 g, 0.5 mmol) instead of (NBu4)2Mo6O19. The brownish red block crystals of 2 were isolated in about 65% (based on (NBu4)2W6O19). Elemental analysis (%) calcd for C32H32O19Fe2N2W6: C, 19.5; H, 1.6; N, 1.4; Fe, 5.7; W, 56.1. Found: C, 20.1; H, 1.7; N, 1.3; Fe, 5.6; W, 57.0. IR (KBr): 3743.6(s), 3085.4(s), 2362.3(vs), 1626.6(s), 1480.4(s), 1238.9(w), 1147.4(s), 1096.7(s), 1038.2(s), 976.0(vs), 803.0(vs), 579.8(s), 493.4(s), 443.2(vs) cm-1. [CpFeCp-CH2-Py]4[W10O32] (3). (NBu4)4W10O32 (0.83 g, 0.25 mmol) was dissolved in 20 mL of acetonitrile. To the solution CpFeCp-CH2-PyþI- (0.41 g, 1 mmol) dissolved in 10 mL of acetonitrile was added with stirring. The mixture was heated for 30 min at 60 °C and then filtered. The filtrate was slowly evaporated at ambient conditions. Within 2 days, brownish red block crystals of 3 were isolated in about 62% (based on (NBu4)4W10O32). Elemental analysis (%) calcd for C64H64 O32Fe4N4W10: C, 22.2; H, 1.8; N, 1.6; Fe, 6.5; W, 53.1. Found: C, 22.6; H, 1.7; N, 1.5; Fe, 6.8; W, 53.8. IR (KBr): 3447.4(m), 3055.3(m), 1627.8(m), 1483.8(w), 1403.5(vw), 1333.8 (w), 1240.5(w), 1214.5(w), 1142.9(w), 1102.4(w), 997.9(vw), 955.6(vs), 890.5(m), 793.6(vs), 671.4(m), 582.7(w), 501.6(w) cm-1.

(NBu4)[CpFeCp-CH2-Py]2[PMo12O40] (4). (NBu4)3PMo12O40 (0.85 g, 0.33 mmol) was dissolved in 20 mL of acetonitrile. To the solution CpFeCp-CH2-PyþI- (0.41 g, 1 mmol) dissolved in 10 mL of acetonitrile was added with stirring. The mixture was heated for 30 min at 60 °C and then filtered. The filtrate was slowly evaporated at ambient conditions. Within 2 days, dark-red block crystals of 4 were isolated in about 70% (based on (NBu4)3PMo12O40). Elemental analysis (%) calcd for C48H68Fe2Mo12N3O40P: C, 22.0; H, 2.6; N, 1.6; Fe, 4.3; Mo, 43.9. Found: C, 22.3; H, 2.5; N, 1.5; Fe, 4.4; Mo, 44.2. IR (KBr): 3451.3(m), 3081.4(w), 2961.6(w), 1621.8(m), 1475.5(m), 1238.8(w), 1140.7(m), 1062.8(vs), 958.8(vs), 880.6(s), 795.2(vs), 495.9(m) cm-1. X-ray Crystallography. A selected crystal of the salts 1-4 was respectively mounted on a glass fiber capillary which was put on a BRUKER SMART APEX II CCD diffractometer equipped with graphite monochromatic radiation and used for data collection. Data were collected at 293(2) K using MoKR radiation (λ = 0.71073 A˚). The structures were solved by direct methods (SHELXTL-97) and refined by the full-matrix-block least-squares method on F2. All non-hydrogen atoms except O1 in 2 were refined with anisotropic displacement parameters for salts 1-3. Heavy atoms (W and Fe) were refined with anisotropic displacement parameters and other atoms (O, C, N, and P) were refined isotropically for 4. Hydrogen atoms were included at calculated positions and refined with a riding model. A summary of the crystal data and refinement results for 1-4 are listed in Table 1. The CCDC reference numbers are CCDC 734252 for 1, 734253 for 2, 734254 for 3, and 734255 for 4.

Results and Discussion Synthesis. The mixture of dilute acetonitrile solutions of the iodide of CpFeCp-CH2-Pyþ and of (Bu4N)2M6O19 (M= Mo, W) at 60 °C resulted in brown-yellow solutions, indicating that the electronic interaction between the cationic ferrocenyl donor and the polyoxometalate acceptor has occurred.27,28 On slow evaporation (1 or 2 days) of the solvent, intensely colored crystals were formed according to 2:1 stoichiometry, 2CpFeCp-CH2 -Pyþ I- þ ðNBu4 Þ2 ½M6 O19  f ½CpFeCp-CH2 -Pyþ 2 ½M6 O19 2- þ 2NBu4 þ I-

ð1Þ

Similar treatment of acetonitrile solutions of the iodide of CpFeCp-CH2-Pyþ and of (NBu4)4W10O32 afforded strongly

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Figure 2. Projection of the crystal structure of 1 in the ac plane.

Figure 1. ORTEP diagram of the asymmetric unit of 1 with the atomic numbering scheme and 30% thermal ellipsoids. H atoms are omitted for clarity.

colored crystals of 4:1 salts, 4CpFeCp-CH2 -Pyþ I - þ ðNBu4 Þ4 ½W10 O32  f ½CpFeCp-CH2 -Pyþ 4 ½W10 O32 4 - þ 4NBu4 þ I - ð2Þ Interestingly, similar treatment of acetonitrile solutions of the iodide of CpFeCp-CH2-Pyþ and of (NBu4)3PMo12O40 afforded strongly colored crystals with a ratio of the donor/ acceptor of 2:1, and one additional tetrabutylammonium cation satisfies the electroneutrality of the cocrystals, 2CpFeCp-CH2 -Pyþ I - þ ðNBu4 Þ3 ½PMo12 O40  f ðNBu4 Þþ ½CpFeCp-CH2 -Pyþ 2 ½PMo12 O40 3 - þ2NBu4 þ I ð3Þ All of salts 1-4 are the intensely colored crystals, soluble in acetonitrile and methylene chloride; however, they are almost insoluble in n-hexane, and all of them are stable to air and moisture. Crystal Structures of 1 and 2. Salts 1 and 2 are isostructural and crystallize in the monoclinic space group P21/c; therefore, only the structure of 1 is discussed here. The asymmetric unit is constituted from half of a Lindqvist anion and one fmp cation (Figure 1). The central oxygens (Oc) of [Mo6O19]2- in the unit cell are located at the center of b- and c-axis, respectively. The average bond lengths of Mo-Oa, Mo-Ob, and Mo-Oc are 1.682, 1.928, and 2.320 A˚. The cyclopentadienyl (Cp) rings of ferrocene are almost parallel, having a dihedral angle of 1.78°. The rings are twisted by about 10° relative to each other about the axis connecting the two ring centroids. Adjacent ferrocene units are linked via pairs of C-H 3 3 3 π interactions with the nearest distance of 2.486 A˚ between the pyridyl hydrogen atom of one molecule and the bare Cp ring of the next in the a direction, and between the ortho Cp hydrogen atoms on each Cp ring of one molecule and the pyridyl rings of the next with the nearest distance of 2.793 A˚ in the c direction (Figure 2). These interactions are supplemented by a π 3 3 3 π stacking interaction between adjacent pyridyl rings along the c direction, which are nearly parallel with the dihedral angle of 0.87° and with the mean interplanar separation of 3.213 A˚. Close interactions exist between the surface of oxygen atoms of the hexamolybdate and the hydrogen atoms of the CpFeCp-CH2-Pyþ; for example, the distances of C2-H2 3 3 3 O6, C11-H11A 3 3 3 O4, C11-H11A 3 3 3 O8,

Figure 3. The π-donor plane of the pyridyl ring and one oxygen facet of the [M6O19]2- acceptor face each other in the crystals of 1.

C13-H13 3 3 3 O3, and C13-H13 3 3 3 O9 are between 2.485 and 2.610 A˚, and the C-H 3 3 3 O angles are between 124.5 and 155.7°. Very interestingly, the short methylene bridge and the flat shape of the pyridinium cause the latter to approach the oxygen face of POMs and the π-donor plane of the pyridyl ring, and one oxygen facet of the acceptor octahedron face each other at a dihedral angle of 9.25° and an interplanar distance of about 2.919 A˚ (Figure 3), which means that the orbital overlap take places between the π-donor plane of the pyridyl ring and one oxygen facet of the acceptor octahedron in the solid state.29 Crystal Structure of 3. Salt 3 crystallizes in the orthorhombic space group Pbca. The asymmetric unit is composed from half of the decatungstate isopolyanion and two fmp cations (Figure 4). The decatungstate isopolyanion [W10O32]4- consists of two defect Lindquist ([W5O14]2-) fragments linked by four corner-sharing oxygens with an unusually wide W-O-W angle of 174.2° and 175.9°. Adjacent ferrocene units are linked via pairs of the complex C-H 3 3 3 π interactions with the nearest distance of 2.882 A˚ between the ortho Cp hydrogen atoms of one molecule and the Cp ring of the next, and between the ortho Cp hydrogen atoms on the bare Cp ring of one molecule and the pyridyl rings of the next with the nearest distance of 2.892 A˚. And no face-to-face π 3 3 3 π interactions are observed. Close interactions exist between the surface of oxygen atoms of the decatungstate isopolyanion and the hydrogen atoms of the CpFeCp-CH2-Pyþ cation (Figure 5); the distances of C3-H3 3 3 3 O10, C11-H11B 3 3 3 O10, C16H16 3 3 3 O3, C17-H17 3 3 3 O8, C26-H26 3 3 3 O8, and C32H32 3 3 3 O10 are between 2.398 and 2.707 A˚ and the C-H 3 3 3 O angles are between 127.1 and 158.0°. The dihedral angle and an interplanar distance of the π plane of the pyridyl

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ring and one oxygen facet of the POMs are 31.24° and 3.144 A˚, respectively, much larger than 9.25° and 2.919 A˚ in 1. Crystal Structure of 4. Salt 4 crystallizes in the orthorhombic space group Pccn. The asymmetric unit is composed from half of a Keggin anion, one fmp cation, and half of a tetrabutylammonium cation (Figure 6). The polyoxoanion [PMo12O40]3- presents the same type of crystallographic disorder as found in many other crystal structures with the Keggin anions;11,17 the central P atom is surrounded by a cube of eight oxygen atoms, with each oxygen site halfoccupied, and the P and these oxygen atoms formed two groups tetrahedron, the central P-O distances of polyoxoanion vary from 1.447 to 1.660 A˚, and the range of bond angles of O-P-O varies from 103.9° to 115.5°, which are all far from the regular tetrahedral angle. Each polyanion is surrounded by eight adjacent CpFeCpCH2-Pyþ units and four adjacent tetrabutylammonium

Figure 4. ORTEP diagram of the asymmetric unit of 3 with the atomic numbering scheme and 30% thermal ellipsoids. H atoms are omitted for clarity.

Figure 5. Projection of the crystal structure of 3 in the bc plane.

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cations. The fmp and tetrabutylammonium cations alternatively arranged in the empty spaces between the polyanions (Figure 7). Close interactions exist between the surface of oxygen atoms of Keggin anion and the pyridyl units; the distances of C5-H5 3 3 3 O16, C11-H11A 3 3 3 O15, and C8-H8 3 3 3 O6 are 2.316, 2.333, and 2.611 A˚, and the C-H 3 3 3 O angles are 166.3, and 144.0 and 170.1°, respectively. The fmp cation can cocrystallize with different POM acceptors, Lindqvist-type [M6O19]2- (M = Mo or W), the decatungstate isopolyanion [W10O32]4- and Keggin-type [PMo12O40]3-, with charges from -2 to -4 to form the CT salt by Coulombic forces, C-H 3 3 3 π and π 3 3 3 π stacking interactions. As usual, the charge of polyanions in salts determines the donor/acceptor ratio (such as, 2:1 for the Lindqvist type and 4:1 for the decatungstate isopolyanion) to satisfy the electroneutrality of the cocrystals. However, salt 4 gives a ratio of 2:1 (CpFeCp-CH2-Pyþ/[PMo12O40]3-) and has one additional tetrabutylammonium cation in terms of the charge balance of salt. To date, the ratio of ferrocenyl/POM salts of 1:2, 2:1, 3:1, and 4:1 has been reported, such as 1:2 species, ([NBu4]6[Fe(C5H5)2][HXMo12O40][XMo12O40],17,18 X = P or As; 2:1 species, [NBu4]2H[Fe(C5H5)2]2[SiMo12O40] 3 2CH3CN,18 Na[Fe(C5H5)2]2[Cr(OH)6Mo6O18] 3 3H2O,15 and [C5H5FeC5H4CH2N(CH3)3]2M6O19 (M = Mo, W);19 3:1 species, ([Fe(C5H5)2]3[WVWVI5O19],16 [Fe(C5Me5)2]3Cr(OH)6Mo6O18 3 20H2O;15 and 4:1 species, ([Fe(C5Me5)2]4(POM) 3 n(solv) (POM = [SiMo12O40]4-, [SiW12O40]4-, [PMo12O40]4-, [HFeW12O40]4-, solv=H2O,DMF,CH3CN);14 [Fe(C5Me5)2]4[HPCu(H2O)W11O39] 3 6CH3CN,15 and [CpFeCpCH2N(CH3)3]4[XMo12O40] 3 nCH3CN (n=0 for X=P or n=1 for X= Ge).20 The experimental studies and comparative literature reviews suggest that there are numerous factors that influence the ratio of ferrocenyl/POM salts, such as the size, shape, charge, and reducibility of the POM acceptors, and factors of donors and solvents, and synthetic conditions. It is noticeable that the charge of the polyanion is one of the important factors of the cocrystallization of the cationic ferrocenyl donor with POM acceptors by self-assembly. The salts with a ferrocenyl/ POM ratio > 4 and with a negative charge of the polyanion 500 nm. In contrast, the crystalline 2:1 complex of CpFeCp-CH2-Pyþ with both POMs showed strong absorptions at λ > 500 nm (Figure 8a,b), and the broad additional absorption of the [Mo6O19]2- salt is stronger than that of [W6O19]2- salt from 550 to 850 nm. In fact, the [W6O19]2- salt showed a weak absorption tailing over 850 nm. The bathochromic shift with λo (W6O19)2- < λo (Mo6O19)2- (the wavelength λo at which the absorbance reached the baseline value) corresponds to the order of the reduction potentials with E°red(W6O192-) < E°red(Mo6O192-) (E°red (NBu4)2[W6O19]=-0.96 V and E°red (NBu4)2[Mo6O19]=-0.44 V).29 Similarly, the physical mixture of the iodide salt of CpFeCp-CH2-Pyþ and the Bu4Nþ salt of [W10O32]4- did not show any absorption at λ > 400 nm. In contrast, the

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Figure 8. Diffuse reflectance spectra of the charge-transfer salts of CpFeCp-CH2-Pyþ donor with the POM acceptors: (a) [Mo6O19]2- (s), (b) [W6O19]2- (s), (c) [W10O32]4- (s), and (d) [PMo12O40]3-. The dashed lines represent the diffuse reflectance spectra of the physical mixtures of the iodide salt of CpFeCp-CH2-Pyþ with the Bu4Nþ salt of (a) [Mo6O19]2-, (b) [W6O19]2-, and (d) [PMo12O40]3- in a 2:1 ratio and the Bu4Nþ salt of (c) [W10O32]4- in a 4:1 ratio.

Figure 9. (a, b) Electronic spectra of CTPs 1-2 (s) in 0.1 mmol/L acetonitrile solutions in comparison with electronic spectra of 0.1 mmol/L POMs (red dotted line) and 0.2 mmol/L CpFeCp-CH2-PyþI- (green dotted line) acetonitrile solutions. All spectra were obtained in a 1 cm cell at 20 °C.

4:1 complex of CpFeCp-CH2-Pyþ with [W10O32]4- showed new absorptions extending up to λ = 850 nm (Figure 8c). And the physical mixture of the iodide salt of CpFeCpCH2-Pyþ and the (Bu4Nþ) salt of [PMo12O40]3- showed very weak absorption at λ > 520 nm, while the dark-red crystals of 4 showed strong parallel absorptions from 350 to 850 nm (Figure 8d); the results are similar to those of reported CT salts.18-20 Thus, in accord with Mulliken theory,31-33 the new (visible) absorption bands should be ascribed to CT transitions between the cationic ferrocenyl donor CpFeCp-CH2-Pyþ and the POM acceptors, and the strongly colored crystals were identified as CT salts.29,34 The UV-Vis spectra in 0.1 and 0.01 mmol/L weak coordination acetonitrile solution of the title compounds and

the starting materials (POMs and CpFeCp-CH2-PyþI-) are shown in Figures 9 and 10, respectively. Compared with solid-state diffuse reflectance spectra, the absorption over 350 nm of the title compounds in solution is very weak, and the absorption gets weaker with reduced concentration, meaning that the interaction between the cationic ferrocenyl donor and the polyoxoanion must play a role in the observed CT effect if the distance of both is short enough (in concentrated solution or the solid state).30,35 Indeed, in a very dilute solution, the complexes are almost entirely disrupted and the absorption bands vanish. These results indicate that under the specified conditions, such as in the solid state and the concentrated solution, CT between the ferrocenyl electron donor and the POM acceptors would take place in the compounds.

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Figure 10. (a, b) Electronic spectra of CTPs 1-2 (s) in 0.01 mmol/L acetonitrile solutions in comparison with electronic spectra of 0.01 mmol/L POMs (red dotted line) and 0.02 mmol/L CpFeCp-CH2-PyþI- (green dotted line) acetonitrile solutions. All spectra were obtained in a 1 cm cell at 20 °C.

Figure 11. Emission spectra of the charge-transfer salts of CpFeCp-CH2-Pyþ donor with the POM acceptors: (a) [Mo6O19]2- (s), (b) [W6O19]2(s), (c) [W10O32]4- (s), and (d) [PMo12O40]3-(s) in the solid state at 20 °C. The dotted lines represent emission spectra of the corresponding POM acceptors: (a) [Mo6O19]2-, (b) [W6O19]2-, (c) [W10O32]4-, and (d) [PMo12O40]3- and the dashed lines represent emission spectra of the CpFeCpCH2-Pyþ donor.

Fluorescence Properties. The photoluminescent properties of CpFeCp-CH2-Pyþ, POMs, and salts 1-4 were investigated in the solid state at room temperature (Figure 11). The weak fluorescence emission band for the ferrocenyl donor CpFeCp-CH2-Pyþ is at ca. 398 nm in the solid state upon excitation at 260 nm, which corresponds to π 3 3 3 π* transition. Excitation at 260 nm leads to broad fluorescence signals with the emission peaks at about 394 nm for 1 and 2, which may mainly be attributed to O2p to Mo4d (or W5d) charge transfer. The emission bands of 1 and 2 are compared to that of the corresponding POMs and exhibit the enhanced fluorescence signals, which means that salts 1

and 2 may be excellent candidates for potential solid-state photofunctional material since both are stable to air and moisture. The emission band of 3 is compared to that of the corresponding POM and exhibit the weakened fluorescence signals while, the fluorescence of 4 observed at 400 nm for (NBu4)3PMo12O40 was almost totally quenched. When the ferrocenyl donors were appended to the different POM acceptors by Coulombic forces in ion pairs consisting of (partially or fully) oxidized donors and reduced POMs to yield charge-transfer salts, the fluorescences were obviously distinct (strengthened or weakened, and even quenched).

Article

Crystal Growth & Design, Vol. 10, No. 3, 2010

These findings suggested the occurrence of an efficient CT process from the ferrocenyl donor to POM acceptors.36-41 Conclusions In summary, four new charge-transfer salts based on the fmp donor and different sizes and shapes of the POM acceptors with the charges from -2 to -4, namely, Lindqvist-type [M6O19]2- (M = Mo or W), the decatungstate isopolyanion [W10O32]4-, and Keggin-type [PMo12O40]3-, were cocrystallized by Coulombic forces. C-H 3 3 3 π and π 3 3 3 π stacking interactions also exist between the ferrocenyl electron donors, and the orbital overlap between the π-donor plane of the pyridyl ring and one oxygen facet of the acceptor octahedron in salts 1-2. The CT transitions in the dark-colored (brownish red) crystalline materials composed of partially oxidized donors and reduced acceptor were confirmed by the progressive bathochromic shift of the new absorption bands. The UV-vis spectra in weak coordination acetonitrile solution showed that the absorption gets weaker with reduced concentration. Both salts 1 and 2 have an intense emission at about 394 nm and may be excellent candidates for potential solid-state photofunctional material. This work also confirms that fmp is a very good donor for the cocrystallization with some of polyoxometalate acceptors, and more transfer salts with more interesting properties based on ferrocenetype donors and POM acceptors could be expected by modulating the species of the ferrocenyl donors and POM acceptors.

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Acknowledgment. This work was supported by the National Natural Science Foundation of China (20973133), the Education Commission of Shaanxi Province (09JK783), and Funded Projects of Independent Innovation of Northwest University Postgraduates (08YZZ41).

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Supporting Information Available: X-ray crystallographic files in CIF format are available free of charge via the Internet at http:// pubs.acs.org.

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