Synthesis and properties of new (μ-oxo)bis[trichloroferrate(III)] dianion salts incorporated with dicationic moiety

Polyhedron 29 (2010) 2976–2984

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Synthesis and properties of new (l-oxo)bis[trichloroferrate(III)] dianion salts incorporated with dicationic moiety Jui-Cheng Chang a, Wen-Yueh Ho b,⇑, I-Wen Sun a,⇑⇑, Yu-Kai Chou b, Hsin-Hsiu Hsieh b, Tzi-Yi Wu a, Shih-Shin Liang a a b

Department of Chemistry, National Cheng Kung University, Tainan 70101, Taiwan, ROC Institute of Cosmetic Science, Chia Nan University of Pharmacy & Science, Tainan 71710, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 22 July 2010 Accepted 7 August 2010 Available online 18 August 2010 Keywords: Dication Ionic liquid TGA curves X-ray single crystal Iron(III) anion Diferrate anion Magnetic susceptibility Magnetic coupling constant

a b s t r a c t New (l-oxo)bis[trichloroferrate(III)] dianions-based ionic compounds that contain various counterdications were synthesized and characterized with regards to their crystal structures, thermal properties, and magnetic susceptibility. These salts are soluble in polar solvents such as methanol and water. The melting point of these compounds is affected by the dication following the order of triphenylphosphinium > pyridinium > imidazolium dications, and symmetrical dicationic salts > unsymmetrical ones. In these compounds, the trichloroferrate dianion exists in either a linear or a bent form, which is affected by the dications. Interestingly, the dicationic diferrate compounds show magnetic coupling constants fairly smaller than those reported in literature for diferrate salts in which monocations are the counterion. Furthermore, unlike the diferrate salts associated with separate monocations, the linear diferrate dicationic compounds show magnetic coupling constant lower than that of bent diferrate dicationic salts. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Ionic compounds composed of organic cations often exhibit much lower melting point than the inorganic analogous. To distinguish from the high temperature molten salts, salts that are liquids at temperature lower than 100 °C have been arbitrarily defined as ionic liquids (ILs). Most common ILs are obtained from quaternary ammonium monocations, such as imidazolium or pyridinium, with monoanions, such as Cl, Br, PF6  , BF4  , or NTf 2  (bis(trifluoromethanesulfonyl)amide). The ILs possess unique properties and have attracted intensive interests for various applications [1,2]. For certain applications such as the stationary phase of gas chromatography [3], and electrolytes for dye-sensitize solar cells (DSSC) [4] and high temperature batteries [5], thermal stability of the ILs is important. Recently, it was demonstrated that by linking two monocations into a dication significantly improved the thermal stability of the resulted ILs [6–9]. Although many of the dicationic salts have melting points higher than 100 °C, they are still termed as ILs for convenience [6,7]. The dicationic ILs are further classified as geminal (the two monocations that form the ⇑ Corresponding author. ⇑⇑ Corresponding author. E-mail addresses: [email protected] (W.-Y. Ho), [email protected] (I-Wen Sun). 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.08.010

dication are the same) or unsymmetrical (the monocations are different). While many dicationic ILs using monoanions as the counters for the dications have been investigated, no dicationic ILs (or salts) using counterdianions has been reported. This paper for the first time studies a series of dicationic compounds in which the (l-oxo)bis[trichloroferrate(III)] dianion, [Fe2OCl6]2, is used as the counter ion for the dications. The (l-oxo)bis[trichloroferrate(III)] dianion, [Fe2OCl6]2, was first introduced in 1978 in the study of synthesis, crystal structure, and magnetic properties of bispyridinium (l-oxo)bis[trichloroferrate(III)]-pyridine, [Hpy]2[Fe2Cl6O]py) [10], and has become an important example in the coordination chemistry of trivalent iron. Compounds involving the [Fe2OCl6]2 anion have been suggested as model for the active center of nitrogenase [11] and non-heme Fe oxygenase [12]. In addition to [Hpy]2[Fe2Cl6O]py, a large number of salts containing [Fe2OCl6]2 anion have been synthesized and crystallographically characterized [13–16]. In the compounds reported in the early literatures, each [Fe2OCl6]2 anion is almost unexceptionally associated with two separate monocations as counterions [13–16]. Crystal structure determinations indicated that the Fe–O–Fe conformation can be either linear (angle = 180°) or bent (angle ranges from 145° to 170°) depending on the respective countercation. Magnetochemical studies on these dinuclear ferric salts have unambiguously established that the magnitude of the antiferromagnetic spin exchange coupling constant J as

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el: TGA-50, Shimadzu) with a heating rate of 10 °C/min (the Td selected at the given temperature corresponded to 10% weight loss), melting points were measured using a capillary melting point apparatus (model: Buchi B-540) with a heating rate of 10 °C/min, and direct current magnetic susceptibility (called vMT value) was measured using a superconducting quantum interference device (SQUID, model: MPMS7, Quantum Design).

defined by the spin Hamiltonian H = 2JS1S2 (S1 = S2 = 5/2) is strongly related to the Fe–O–Fe bond angle, and the J value of linear Fe–O–Fe was found larger than that of bent one [15,16]. In view of the fact that no dinuclear ferric salts in which each [Fe2OCl6]2 anion is associated with a counterdication rather than two countermonocations, this paper offers the first series of examples of [Fe2OCl6]2 salts incorporated with dicationic moiety. The thermal properties (melting temperature and decomposition temperature), solubility in organic solvents, and magnetic susceptibility of the new dication-[Fe2OCl6]2 salts are discussed. The steric difference between the monocations and dications leaded to different yet interesting behaviors of the salts.

2.2. General procedure for the synthesis of 1a and 1b Pyridine (1.0 mmol) was mixed with 1,3-dibromopropane (1.5 mmol) and 1,4-dibromobutane (1.5 mmol), respectively, and stirred under neat conditions at room temperature for 4 days, and then washed using ethyl acetate to obtain the required products (all white solids for 1a and 1b). [1-(3-Bromopropyl)pyridinium] bromide (1a): Yield 99%. 1H NMR (200 MHz, D2O): d 2.58 (quin, J = 6.6 Hz, 2H), 3.48 (t, J = 6.6 Hz, 2H), 4.85 (t, J = 6.6 Hz, 2H), 8.08 (t, J = 6.8 Hz, 2H), 8.54 (t, J = 7.2 Hz, 1H), 8.89 (d, J = 6.3 Hz, 2H). [1-(4-Bromobutyl)pyridinium] bromide (1b): Yield 99%. 1H NMR (200 MHz, D2O): d 1.81 (quin, J = 6.4 Hz, 2H), 2.04 (quin, J = 6.4 Hz, 2H), 3.41 (t, J = 6.4 Hz, 2H), 4.55 (t, J = 6.4 Hz, 2H), 7.97 (t, J = 6.8 Hz, 2H), 8.44 (t, J = 7.2 Hz, 1H), 8.76 (d, J = 6.3 Hz, 2H).

2. Experimental 2.1. General 1,3-Dibromopropane (Alfa Aesar, purity: 99.9%), 1,4-dibromobutane (Alfa Aesar, purity: 99.9%), pyridine (Fluka, purity: 98%), 1-methylimidazole (Fluka, purity: 99.9%), triphenylphosphine (Fluka, purity: 99.9%), iron trichloride hexahydrate (Showa, purity: 99.9%), methanol (J.T. Baker, ACS grade, 60.01% H2O), and ethanol (J.T. Baker, ACS grade, 60.01% H2O) were purchased from a commercial supplier and used without any pre-treatment. The 1H spectra of the purified products (for 1a, 1b, 2a–2g) were recorded in D2O (Cambridge Isotope Laboratories Inc., 99.9% D) on a Bruker Avance 200 spectrometer at 200 MHz at a temperature of 25 °C. The crystal structures, which were dissolved in methanol and crystallized by ethyl ether, were characterized with an X-ray single crystal diffractometer (model: Siemens Smart CCD, Germany). Elemental analyses were conducted using an Elementar analyzer (model: Vario EL III), thermal decomposition temperatures were measured using thermogravimetry analysis (TGA, mod-

Br

n Br

N

N

2.3. General procedure for the synthesis of bromide anion salts containing unsymmetrical dications (2a and 2b) Compounds 1a (1.0 mmol) and 1b (1.0 mmol), respectively, were reacted with 1-methylimidazole (2.0 mmol), stirred in methanol under reflux for 24 h, and then precipitated from ethyl acetate to obtain the required products (all white solids for 2a and 2b). [1-(1-Pyridinium-yl-propyl)-3-methylimidazolium] dibromide (2a): Yield 95%. 1H NMR (200 MHz, D2O): d 2.70 (quin, J = 7.5 Hz,

N

n Br

neat condition

N N

MeOH/reflux

Br

Br

25o C

N

n N

N

Br

2a, n=1 (95%) 2b, n=2 (95%)

1a, n=1 (99%) 1b, n=2 (99%)

FeCl 3.6H 2O

n N

N

Cl3 Fe

O

2 FeCl3

EtOH 25o C 3a, n=1 (71%) 3b, n=2 (72%)

Nu Br

n

Br

FeCl 3.6H 2O Nu

MeOH/reflux Br

n Nu Br

2c, n=1, Nu=Im (95%) 2d, n=1, Nu=Py (95%) 2e, n=2, Nu=Im (95%) 2f , n=2, Nu=Py (95%) 2g, n=2, Nu=PPh3 (95%)

n Nu

Nu

EtOH 25o C

Cl3 Fe

O

2 FeCl3

(Nu=nucleophile) 3c, n=1, Nu=Im (74%) 3d, n=1, Nu=Py (70%) 3e, n=2, Nu=Im (67%) 3f , n=2, Nu=Py (69%) 3g, n=2, Nu=PPh3 (70%)

Scheme 1. Synthesis procedures of the magnetic diferrate anion ionic liquids.

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2H), 3.92 (s, 3H), 4.43 (t, J = 7.5 Hz, 2H), 4.77 (t, J = 7.5 Hz, 2H), 7.49 (s, 1H), 7.56 (s, 1H), 8.12 (t, J = 7.1 Hz, 2H), 8.61 (t, J = 7.3 Hz, 1H), 8.84 (s, 1H), 8.92 (d, J = 6.5 Hz, 2H). [1-(1-Pyridinium-yl-butyl)-3-methylimidazolium] dibromide (2b): Yield 95%. 1H NMR (200 MHz, D2O): d 1.86–2.17 (m, 2H), 3.87 (s, 3H), 4.21–4.30 (m, 2H), 4.61–4.71 (m, 2H), 7.43 (s, 1H), 7.46 (s, 1H), 8.07 (t, J = 7.0 Hz, 2H), 8.56 (t, J = 7.2 Hz, 1H), 8.85 (d, J = 6.6 Hz, 2H). 2.4. General procedure for the synthesis of bromide anion salts containing geminal dications and propyl linkage chains (2c and 2d) 1,3-Dibromopropane (1.0 mmol) was reacted with 1-methylimidazole and pyridine (both 2.0 mmol), respectively, stirred in methanol, refluxed for 24 h, and then precipitated from ethyl acetate to obtain the required products (all white solids for 2c and 2d). [1,10 -(Propane-1,3-diyl)-bis(3-methylimidazolium)] dibromide (2c): Yield 95%. 1H NMR (200 MHz, D2O): d 2.56 (quin, J = 7.3 Hz, 2H), 3.94 (s, 6H), 4.36 (t, J = 7.3 Hz, 4H), 7.51 (s, 2H), 7.55 (s, 2H), 8.65 (s, 2H). [1,10 -(Propane-1,3-diyl)-bis(pyridinium)] dibromide (2d): Yield 95%. 1H NMR (200 MHz, D2O): d 2.86 (quin, J = 7.8 Hz, 2H), 4.86 (t, J = 7.8 Hz, 4H), 8.14 (t, J = 7.3 Hz, 4H), 8.62 (t, J = 7.3 Hz, 2H), 8.96 (t, J = 6.5 Hz, 4H). 2.5. General procedure for the synthesis of bromide anion salts containing geminal dications and butyl linkage chains (2e, 2f, and 2g)

[1,10 -(Propane-1,3-diyl)-bis(pyridinium)] (l-oxo)bis[trichloroferrate(III)] (3d): Yield 70%. Elem. Anal. Calc. for C13H16Cl6Fe2N2O: C, 28.88; H, 2.98; N, 5.18. Found: C, 28.60; H, 2.98; N, 5.04%. [1,10 -(Butane-1,4-diyl)-bis(3-methylimidazolium)] (l-oxo)bis[trichloroferrate(III)] (3e): Yield 67%. Elem. Anal. Calc. for C12H20Cl6Fe2N4O: C, 25.70; H, 3.60; N, 9.99. Found: C, 25.67; H, 3.67; N, 9.93%. [1,10 -(Butane-1,4-diyl)-bis(dipyridinium)] (l-oxo)bis[trichloroferrate(III)] (3f): Yield 69%. Elem. Anal. Calc. for C14H18Cl6Fe2N2O: C, 30.31; H, 3.27; N, 5.05. Found: C, 29.94; H, 3.37; N, 5.03%. [1,10 -(Butane-1,4-diyl)-bis(triphenylphosphonium)] (l-oxo)bis[trichloroferrate(III)] (3g): Yield 70%. Elem. Anal. Calc. for C40H38Cl6Fe2P2O0.7H2O: C, 51.45; H, 4.25. Found: C, 51.62; H, 4.23%.

3. Results and discussion 3.1. Synthesis of compounds and structure characterization of Fe(III) anion system The synthesis procedure and the product yields are summarized in Scheme 1, and the chemical structures are shown in Table 1. Target dicationic salts, 3a–3g, were prepared using previously

Table 1 Summary of melting points and thermal decomposition temperatures of the magnetic diferrate anion ionic liquids. M.p. (°C)

Td (°C)a

Td (°C)b

104–105

310

335

146–147

318

341

138–139

354

383

2

179–180

340

373

2

143–144

317

338

2

175–176

300

334

2

260–261

360

378

Structure

1,4-Dibromopropane (1.0 mmol) was reacted with 1-methylimidazole, pyridine, and triphenylphosphine (all 2.0 mmol), respectively, stirred in methanol, refluxed for 24 h, and then precipitated from ethyl acetate to obtain the required products (all white solids for 2e, 2f, and 2g). [1,10 -(Butane-1,4-diyl)-bis(3-methylimidazolium)] dibromide (2e): Yield 95%. 1H NMR (200 MHz, D2O): d 1.94 (quin, J = 7.3 Hz, 4H), 3.91 (s, 6H), 4.28 (t, J = 7.3 Hz, 4H), 7.47 (s, 2H), 7.50 (s, 2H), 8.58 (s, 2H). [1,10 -(Butane-1,4-diyl)-bis(dipyridinium)] dibromide (2f): Yield 95%. 1H NMR (200 MHz, D2O): d 2.14 (quin, J = 7.3 Hz, 4H), 4.70 (t, J = 7.3 Hz, 4H), 8.09 (t, J = 7.1 Hz, 4H), 8.57 (t, J = 7.1 Hz, 2H), 8.87 (t, J = 7.1 Hz, 4H). [1,10 -(Butane-1,4-diyl)-bis(triphenylphosphonium)] dibromide (2g): Yield 95%. 1H NMR (200 MHz, D2O): d 1.74–1.78 (m, 4H), 3.24–3.30 (m, 4H), 7.64–7.86 (m, 30H).

N

3a

N N

2 O

Cl3 Fe

N

N

3b

FeCl3

2

N

O

Cl3 Fe

3c N

FeCl 3

N

N N

Cl3 Fe

2

O

FeCl 3

3d N

2.6. General procedure for the synthesis of [Fe2Cl6O]2 anion salts (3a, 3b, 3c, 3d, 3e, 3f, and 3g) Compounds 2a, 2b, 2c, 2d, 2e, 2f, and 2g (all 1.0 mmol), respectively, were reacted with iron trichloride hexahydrate (2.5 mmol), and stirred in ethanol at room temperature for 4 days. The required products were then precipitated from ethanol (all yellow solids for 3a–3g). The detailed crystal structure refinements are summarized in Supplementary material. The element analysis data are as follows: [1-(1-Pyridinium-yl-propyl)-3-methylimidazolium] (l-oxo)bis[trichloroferrate(III)] (3a): Yield 71%. Elem. Anal. Calc. for C12H17Cl6Fe2N3O: C, 26.51; H, 3.15; N, 7.73. Found: C, 26.76; H, 3.46; N, 7.81%. [1-(1-Pyridinium-yl-butyl)-3-methylimidazolium] (l-oxo)bis[trichloroferrate(III)] (3b): Yield 72%. Elem. Anal. Calc. for C13H19Cl6Fe2N3O2H2O: C, 26.30; H, 3.90; N, 7.08. Found: C, 26.23; H, 3.53; N, 7.14%. [1,10 -(Propane-1,3-diyl)-bis(3-methylimidazolium)] (l-oxo)bis[trichloroferrate(III)] (3c): Yield 74%. Elem. Anal. Calc. for C11H18Cl6Fe2N4O: C, 24.17; H, 3.32; N, 10.25. Found: C, 23.84; H, 3.37; N, 10.05%.

N

3e

Cl3 Fe

N

O

Cl3 Fe

3f

FeCl 3

N

N

N

O

FeCl 3

N N Cl3 Fe

O

FeCl 3

3g Ph3 P

PPh 3 Cl3 Fe

a b

Td onset. Td at 10% weight loss.

O

FeCl 3

J.-C. Chang et al. / Polyhedron 29 (2010) 2976–2984

reported methodology [6,7]. In this work, each dication salt is associated with one [Fe2Cl6O]2 dianion. For the synthesis route for compounds 3a and 3b, pyridine is used rather than a strong nucleophile (for example: 1-methylimidazole) to react with a dibromo reagent. Pyridine is used because it attacks only one side of the dibromo reagent whereas a strong nucleophile attacks both sides. A base or nucleophile is then used to conduct the SN2 substitution reaction and the resulted dibromide anions are anion exchanged with FeCl3.6H2O in ethanol in the final step. For the synthesis of geminal compounds 3c–3g, 1-methylimidazole, pyridine, and triphenylphosphine, respectively, are reacted with a dibromo reagent to form dibromide salts. Dibromide anions are then anion exchanged with the diferrate anion. The unsymmetrical dicationic compounds 3a and 3b have pyridinium on one side

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and 1-methylimidazolium on the other side, whereas geminal compounds have a dipyridinium group (3c and 3e), a di-1-methylimidazolium group (3d and 3f), and a di-triphenylphosphinium group (3g), respectively. The linkage chain system is a propyl chain –(CH2)3–) in compounds 3a, 3c, and 3d, and a butyl chain (–(CH2)4–) in compounds 3b, 3e, 3f, and 3g. The anion is [Fe2Cl6O]2 in all the synthesized products (3a–3g). The final structure of the synthesized products was determined using an X-ray single crystal diffractometer. Their ORTEP pictures are shown in Fig. 1a–g. The anion consists of a l-oxo diiron(III) (also called diferrate) unit in all synthesized dicationic ionic liquids. Table 2 shows the selected bond lengths of Fe–O and Fe–Cl, and the bond angles of Fe–O–Fe, Cl–Fe–O, and Cl–Fe–Cl. Table 3a and b lists some crystal data and structure refinements, and Table 4

Fig. 1. ORTEP diagram of the magnetic diferrate anion ionic liquids. (a) Compound 3a, (b) compound 3b, (c) compound 3c, (d) compound 3d, (e) compound 3e, (f) compound 3f, and (g) compound 3g.

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Fig. 1 (continued)

presents the p–p stacking distance and hydrogen bonding distance between the [Fe2Cl6O]2 anion and the dicationic moiety in crystal packing (other detailed crystallographic data and structure refinements are shown in Supplementary material, Fig. S1a–1g and Table S1–S42). The formation of the diiron(III)l-oxo anion in this system may be the result of the partial hydrolysis of either FeCl3 or [FeCl4] by the small amount of water in the ethanol [11]. Take the compound 3d as an example. Diffraction-quality crystals of compound 3d were obtained from methanol/ethyl ether at 25 °C. Compound 3d consists of a Cs symmetrical Fe2(l–O) rhomb, with the three chloride atoms completing the distorted tetrahedron coordination model of each iron center. The coordination environments of the two iron centers in compound 3d are not identical. Compound 3d has Fe–lO bond lengths of 1.755(3) and 1.754(3) Å,

with both the Fe–O bonds trans to the chloride atom. These bonds are nearly identical to those previously reported, such as those in [BzEt3N]2[Fe2Cl6O]2 (Bz = benzyl), which has Fe–lO bond length of 1.752(5) and 1.758(5) Å [16]. The Fe–Cl distances of compound 3d range from 2.215(14) to 2.237(11) Å, with an average of 2.226 Å; these distances are consistent with those previously reported. For example, distance of 2.206–2.231 Å with an average of 2.219 Å were reported for [BzEt3N]2[Fe2Cl6O]2 crystal [16]. The conformation of compounds 3a–3g has two forms. In the bent form, the bond angle of Fe–O–Fe ranges from 145.7° to 162.1° (compounds 3a–d). In the linear form, the bond angle of Fe–O–Fe is close to 180° (compounds 3b, 3e, and 3f). This difference may be due to the oxygen atom in diferrate anion forming hydrogen bonds with the NCH in the dicationic moiety in the bent

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Fig. 1 (continued)

Table 2 Selected bond lengths (Fe–Cl, Fe–O) and bond angles (Fe–O–Fe, Cl–Fe–O, Cl–Fe–Cl) in the [Fe2Cl6O]2 anion for compounds 3a–3g. Compound

Fe–O, Å

Fe–Cl, Å

a(Fe–O–Fe), °

a(Cl–Fe–O), °

a(Cl–Fe–Cl), °

3a

1.765(18), 1.766(18) 1.768(2), 1.761(2) 1.758(8), 1.768(16) 1.758 (8), 1.768 (16) 1.760(17), 1.771(17) 1.755(3), 1.754(3) 1.752(4), 1.752(4) 1.765(6), 1.765(6) 1.756(4), 1.755(4)

2.223–2.240 2.220–2.249 2.219–2.243

147.9(12) 148.5(14) 180.0(3) 148.2(3) 145.7 (11) 162.1(3) 180.0(5) 180.0 158.2(3)

105.80(7)–111.87(7)

105.60(3)–112.64(4)

106.92(16)–112.30(5)

107.04(6)–112.07(6)

107.61(6)–112.51(7) 110.25(7)–114.49(13) 109.53(3)–110.97(3) 109.81(4)–110.31(4) 106.65(18)–113.22(15)

108.81(3)–111.41(3) 104.96(3)–111.76(7) 106.82(3)–110.34(4) 108.25(5)–109.20(4) 106.41(8)–111.74(8)

3b 3c 3d 3e 3f 3g

2.218–2.241 2.215–2.237 2.230–2.252 2.227–2.252 2.192–2.247

form (see Table 4; the defined range of the hydrogen bonding distance is 2.0–3.1 Å) [6,17]. The Fe–O–Fe bond angle of compounds 3a–3g ranges from 145.7° to 180°, which is consistent with those previously reported [16,17]. The bond angles of Cl–Fe–O and Cl–Fe–Cl of compounds 3a–3g are remarkably different from those reported in previous research. For compounds 3a–3g, the Cl–Fe–O bond angles vary from 105.80° to 114.49° whereas the Cl–Fe–Cl bond angles vary from 104.96° to 112.64°. A previous study indicated that the Cl–Fe–O bond angles are close to 111.9° and that the Cl–Fe–Cl bond angles approach 110.0° [17]. This discrepancy may be due to the cationic functional groups, such as pyridine, 1-methylimidazole, and triphenylphos-

phine being close to the iron center in the diferrate structure, resulting in the distortion of the Cl–Fe–O and Cl–Fe–Cl bond angles. This behavior is similar to that the ligand approaches metal center would cause the distortion of the metal complex [18]. 3.2. Thermal stability characterizations As shown by the TGA curves in Fig. 2, the dicationic salts that contain diferrate anions exhibit good thermal stability in the range 330–385 °C. As indicated by the data in Table 1, the salt that contains the triphenylphosphine group (3g) is thermally more stable than those containing the imidazole (3c and 3e) and the pyridine

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Table 3 Summary of crystal data for (a) propyl linkage chains in compounds 3a, 3c, and 3d and (b) butyl linkage chains in compounds 3b, 3e, 3f, and 3g. Compound (a) Formula Formula weight (g) T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (Mg/m3) Absorption coefficient (mm1) F(0 0 0) Reflections collected Goodness-of-fit (GOF)

3a

3c

3d

C12H17Cl6Fe2N3O 543.68 100(2) Triclinic  P1

C11H18Cl6Fe2N4O 546.69 150(2) Monoclinic P2(1)/c 13.7207(3) 9.3432(3) 16.6574(4) 90 91.343 90 2134.81(10) 4 1.701 2.117 1096 13 914 1.067

C13H16Cl6Fe2N2O 540.69 296(2) Monoclinic P2(1)/m 6.5742(3) 15.1406(5) 10.9176(4) 90 91.524(2) 90 1086.32(7) 4 1.653 2.077 540 10 476 1.126

10.5430(9) 14.5740(12) 14.9656(13) 73.9050(10) 75.2170 84.6640 2135.6(3) 2 1.691 2.114 1088 25 065 1.089

Compound

3b

3e

3f

3g

(b) Formula Formula weight (g) T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (Mg/m3) Absorption coefficient (mm1) F(0 0 0) Reflections collected Goodness-of-fit (GOF)

C13H19Cl6Fe2N3O 557.70 100(2) Monoclinic C2/c 15.766(3) 14.925(2) 18.644(3) 90 94.947(4) 90 4370.8(12) 8 1.692 2.068 2232 7394 1.034

C12H20Cl6Fe2N4O 560.72 100(2) Monoclinic C2/c 20.877(2) 8.9501(10) 15.7442(18) 90 128.2480(10) 90 2310.3(5) 4 1.658 1.964 1160 8235 1.089

C14H18Cl6Fe2N2O 554.71 100(2) Triclinic  P1 7.6242(2) 8.0217(2) 9.3068(3) 83.777(2) 88.106(2) 73.983(2) 543.87(3) 2 1.694 2.076 278 2151 1.088

C40H38Cl6Fe2OP2 921.04 296(2) Orthorhombic Pna2(1) 30.2366(12) 9.7391(3) 14.5465(6) 90 90 90 4283.6(3) 4 1.428 1.156 1880 34 268 0.931

Table 4 Hydrogen bonding and intermolecular p–p stacking distances for compounds 3a–3g. Compound

p–p Stacking distance (Å)a

Hydrogen bonding distance (Å)a

3a 3b 3c 3d 3e 3f 3g

4.066b, 16.973 10.855 6.066e 8.307 20.877 7.624 8.149f, 15.120f

2.752 3.497c, 3.039d 2.687 5.467 3.160 5.336 2.984g

a

Study in crystal packing, distances were calculated using mercury. Two pyridinium rings are partially stacked. c Hydrogen bonding distance in linear form. d Hydrogen bonding distance in bent form. e Distance between two double bonds in 1-methylimidazolium ring but not between two 1-methylimidazolium rings. f Two phenyl rings partially stacked. g Hydrogen bonding distance between the oxygen in [Fe2Cl6O]2 anion and PCH in dicationic moiety. b

ring (3d and 3f). A possible factor is the degree of positive charge dispersion. The phosphine group has better positive charge dispersion than those of the 1-methylimidazole and pyridine groups, which give it a stronger attraction to anions [19]. Therefore, dicationic salts containing the phosphine group decompose at higher temperature. These results also show that the thermal stability of dicationic salts can be higher than that of a monocationic system [6,7,20].

3.3. Melting point characterizations All the diferrate dicationic compounds synthesized in this study were yellow solids at room temperature. The melting points for these diferrate dicationic salts were determined by using a capillary melting point apparatus. As indicated in Table 1, the dicationic salt containing the phosphine group (compound 3g) has the highest melting point among the diferrate dicationic salts. Geminal salts containing two pyridine groups have higher melting points than that containing two 1-methylimidazole groups (compound 3d > 3c; 3f > 3e). The unsymmetrical dicationic salts have lower melting points than the geminal salts (3d > 3c > 3a with propyl linkages, and 3f > 3e  3b with butyl linkages). The high melting point of compound 3g may be due to its high symmetry in the space group of orthorhombic Pan 2(1); it is known that more symmetrical compounds have higher melting points due to easier packing [6]. The higher melting points of 3d and 3f compared to those of 3c and 3e may also be attributed to the added p–p stacking [7]. Unsymmetrical dicationic compounds 3a and 3b have relatively poor p–p stacking, and resulting in lower melting points (the calculated p–p stacking values are presented in Table 4). 3.4. Magnetic property characterizations The temperature dependence of the DC magnetic susceptibilities for powder samples of compounds 3a–3g measured in the

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a

100

Weight Loss (%)

90 80

3d 3c

70

3a

Table 5 Comparison of the bond angle of Fe–O–Fe, magnetic susceptibility, and magnetic coupling constant between the compounds 3a–3g and [Fe2Cl6O]2 salts reported in the previous literature.a Fe–O–Fe structure

Fe–O–Fe angle

vMT

60

3a

1.75

62

50

3b

1.54

65

40

3c

1.40

73

3d

147.9(12) 148.5(14) 180.0(3) 148.2(3) 145.7 (11) 162.1(3)

1.26

80

3e

180.0(5)

1.61

64

3f

180.0

1.48

69

3g

158.2(3)

2.88

37

(BzPh3P)2[Fe2OCl6] [Fe(phen)3][Fe2OCl6]2CH3CN Bis(ferrocenium)[Fe2OCl6] [PhCH2N(CH3)3]2[Fe2OCl6] [PhCH2N(CH3)3]2[Fe2OCl6] (Ferrocenium)2[Fe2OCl6] [Hpy]2[Fe2OCl6]py [Fe(bpy)3][Fe2OCl6] [PhCH2N(Et)3]2[Fe2OCl6] (Ph4P)2[Fe2OCl6] (BzPh3P)2[Fe2OCl6] (Ph4As)2[Fe2OCl6] [{Ph3P}2CSe]2[Fe2OCl6] (Et4N)2[Fe2OCl6] (PhMe3N)2[Fe2OCl6] (BzMe3N)2[Fe2OCl6] (MePh3P)2[Fe2OCl6] (EtPh3P)2[Fe2OCl6] [Fe2O(OAsPh3)4Cl3]2[Fe2OCl6]

160.2(5) 141.6 162.1(5) 144.6 180.0 162.4 155.6 148.1(2) 155.3(3) 170.4(4) 160.2(5) 170.8(5) 180 155.5 170 170 170 170 170.3

nr 1.15 5.66 nr nr 1.34 0.47 nr nr nr nr nr nr nr nr nr nr nr 0.45

90 106 nr 119 130 nr 92 107 116 112 117 117 190 109 113 107 113 107 170

30

|J| reference (emu K mol1)b (cm1)c

20 10 0 0

100

200

300

400

Temperature

500 o

600

700

800

C

b

100 90

3g 3f 3e 3b

Weight Loss (%)

80 70 60 50 40 30 20 10 0

a

0

100

200

300

400

Temperature

500 o

600

700

800

900

c

C

Fig. 2. TGA curves for the magnetic diferrate anion ionic liquids at a heating rate of 10 °C min1. (a) Propyl chain (3a, 3c, 3d) and (b) butyl chain (3b, 3e, 3f, 3g).

3.0 2.8 2.6 2.4

χMT emu K mole

-1

2.2 2.0 1.8 1.6 1.4

3g 3f 3e 3d Ο 3c 3b 3a

Abbreviation: nr = not reported. At 300 K. Susceptibility data 75–300 K.

ues of vMT (magnetic susceptibility) that were measured at 300 K are collected in Table 5. The spin-only effective magnetic susceptibility value, leff, is defined by the g value (anisotropy) and the S value (the total quantum number of electron spin) with Eq. (1) [21].

g½SðS þ 1Þ1=2 ¼ leff ¼ 2:828ðvM TÞ1=2

ð1Þ

Because the leff values found for compounds 3a–3g are lower than the calculated spin-only value of 10.954 emu K mol1 for the [Fe2Cl6O]2 anion with noninteracting metal centers with g = 2.0 and S = 5, compounds 3a–3g are antiferromagnetism. It has been reported that the shorter Fe–Ooxo bond (i.e., the larger the overlap of half-filled 3d orbitals of the ferric ions and a filled p orbital of the oxo bridge (magnetic superexchange pathway)), the more efficient the unpaired spins of one Fe(III) ion coupled to those of a second Fe(III) ion in an antiparallel fashion [16,17,22]. The antiferromagnetism of the [Fe2Cl6O]2 anion is well established and described by Eq. (2) [16,17],

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

b

This work This work This work This work This work This work This work [14] [24] [15] [17] [17] [25] [10] [16] [16] [16] [16] [16] [16] [16] [16] [16] [16] [16] [26]

50

100

150

200

250

300

Temperature (K) Fig. 3. vMT versus T plot for the magnetic diferrate anion ionic liquids (compounds 3a–3g) at 1000 G.

temperature range of 2.0–300 K in a 1.0 kG magnetic field are displayed in Fig. 3. The vMT values decrease from 1.75 emu K mol1 for compound 3a and 1.54 emu K mol1 for compound 3b at 300 K to 0.03 and 0.05 emu K mol1 at 2 K, respectively. The other compounds, 3c–3g, exhibit similar temperature dependence. Val-

vM ¼

  Ng 2 b2 2e2x þ 10e6x þ 28e12x þ 60e20x þ 110e30x ð1  pÞ 2x 6x 12x 20x 30x kT 1 þ 3e þ 5e þ 7e þ 9e þ 11e þp

35Nb2 kðT  hÞ

ð2Þ

which has been developed under the usual Heisenberg–Dirac–Van Vleck HDVV spin Hamiltonian H = 2JS1S2 (S1 = S2 = 5/2). In Eq. (2), p represents the fraction of the paramagnetic impurity, x = J/ kT, J being the magnetic coupling constant. The g value was fixed to 2.00 in all cases and least square fitting parameters were only

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J.-C. Chang et al. / Polyhedron 29 (2010) 2976–2984

J, p and h. The values for the magnetic coupling constant, J, obtained from the fitting for diiron(III) in each of these compounds are collected in Table 5. Several interesting facts are revealed from this table. Firstly, the triphenylphosphine compound 3g has the lowest J value among these dicationic diferrate salts. Secondly, the unsymmetrical dicationic diferrate salts have lower J values than geminal diferrate ILs (3a < 3c < 3d; 3b  3e < 3f). Thirdly, the J values of the dicationic diferrate salts synthesized in this work are apparently lower than those of monocation diferrate salts reported previously. Last and most striking fact is that the J values of compounds containing bent [Cl3Fe–O–FeCl3]2 (in which the dication contains propyl linkage) are either close or higher than those of the corresponding compounds containing linear [Cl3Fe–O–FeCl3]2 (in which the dication contains butyl linkage), that is, 3a  3b; 3c > 3e; 3d > 3f. This trend is reverse to what were reported in previous studies which indicated that the linear [Cl3Fe–O–FeCl3]2 has larger antiferromagnetic interaction (lower S and vMT values but higher J value) than that of the bent [Cl3Fe–O–FeCl3]2 [16,17]. These facts indicate that the J value is not solely determined by the Fe–O–Fe bond angle. It is the result of combined effects of several factors such as conformational arrangement in space (or called steric effect), and weak hydrogen bonding between the bridged oxygen atom of the [Fe2Cl6O]2 anion and the dicationic moiety. It is worth mention that in the monocationic diferrate compound, it is possible for the two separate monocations to locate and refine their positions compared to the dications in the dicationic diferrate compounds. The accurate resolve the weights of each factor apparently require further more careful and thorough study.

3.5. Solubility description All synthesized diferrate dicationic salts exhibited good solubility in polar solvent such as water (H2O), methanol (MeOH), acetonitrile (CH3CN), and dimethylsulfoxide (DMSO). They cannot be dissolved in methanol, acetone, ethyl acetate, and ethyl ether. This may be attributed to the diferrate dicationic salts containing bridged oxygen atoms in anions that generate hydrogen bonds with methanol and water [23]. The solubility of salts may also be affected by the dielectric constant and polarity of the solvents, and p–p interaction. Therefore, more detailed studies (such as NMR and Raman spectroscopy) are needed to gain more insight into the solubility behavior of the salts in various solvents. Although these diferrate dicationic salts can only be dissolved in highly polar solvents, they have better solubility than that of Fe2O3 particles, which are not soluble in any polar or non-polar solvent. This advantage is important for blending with polymers, such as conducting polymer to develop magnetic composites which have higher thermal stability and good solubility in polar organic solvents.

4. Conclusions A new type of dicationic salts, which contain [Fe2Cl6O]2 anions, was synthesized. These diferrate salts have good thermal stability and are soluble in polar solvent. The dicationic structure affects their magnetic susceptibility and magnetic coupling constant. The magnetic diferrate dicationic salts which use the triphenylphosphine group had the highest magnetic susceptibility. It was found that the magnetic susceptibility in a dicationic system is

higher than that of monocationic salts whereas the magnetic coupling constant in dication is lower than that of monocationic ones. Acknowledgments The financial support from the National Science Council of the Republic of China is gratefully acknowledged (NSC-96-2113-M006-014-MY3). We would like to thank the instrument centers of National Taiwan University (Mr. Un-Cheong Sou for the magnetic susceptibility data), National Tsing Hua University (Mrs. Pei-Lin Chen for the X-ray single crystal data), and National Cheng Kung University (Mr. Shr-Tza Lin for the magnetic susceptibility data, Mrs. Chia-Chen Tsai for the element analysis data, and Mrs. RuRong Wu for NMR measurements). We would also like to thank professor Hua-Fen Hsu, Dr. Chen-I Yang, and Yi-Fang Tsai for magnetic discussions. Appendix A. Supplementary data The packing structure of dicationic diferrate ionic liquids in the unit cell, crystallographic system, and space group, and the element analysis data of these ionic liquids are shown in Fig. S1. The crystallographic data in CIF and Word format are shown in Tables S1–S42. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2010.08.010. References [1] S. Sowmiah, V. Srinivasadesikan, M.C. Tseng, Y.H. Chu, Molecules 14 (2009) 3780. [2] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, 2008. [3] Y.N. Hsieh, R.S. Horng, W.Y. Ho, P.C. Huang, C.Y. Hsu, T.J. Whang, C.H. Kuei, Chromatographia 67 (2008) 413. [4] P. Wang, S.M. Zakeeruddin, P. Comte, I. Exnar, M. Gratzel, J. Am. Chem. Soc. 125 (2003) 1166. [5] W. Ogihara, S. Washiro, H. Nakajima, H. Ohno, Electrochim. Acta 51 (2006) 2614. [6] J.L. Anderson, R. Ding, A. Ellern, D.W. Armstrong, J. Am. Chem. Soc. 127 (2005) 593. [7] T. Payagala, J. Huang, Z.S. Breitbach, P.S. Sharma, D.W. Armstrong, Chem. Mater. 19 (2007) 5848. [8] J.M. Jin, C. Ye, B.S. Phillips, J.S. Zabinski, X. Liu, W. Liu, J.M. Shreeve, J. Mater. Chem. 16 (2006) 1529. [9] Z. Zeng, B.S. Phillips, J. Xiao, J.M. Shreeve, Chem. Mater. 20 (2008) 2719. [10] M.G.B. Drew, V. McKee, S.M. Nelson, J. Chem. Soc. Dalton Trans. (1978) 80–84. [11] K.B. Girma, V. Lorenz, S. Blaurock, F.T. Edelmann, Inorg. Chim. Acta 361 (2008) 346. [12] F. Li, M. Wang, C. Ma, A. Gao, H. Chen, L. Sun, Dalton Trans. (2006) 2427–2434. [13] W.H. Armstrong, S.J. Lippard, Inorg. Chem. 24 (1985) 981. [14] D. Petridis, A. Terzis, Inorg. Chim. Acta 118 (1986) 129. [15] P. Carty, K.C. Clare, J.R. Creighton, E. Metcalfe, E.S. Raper, H.M. Dawes, Inorg. Chim. Acta 112 (1986) 113. [16] G. Haselhorst, K. Wieghardt, S. Keller, B. Schrader, Inorg. Chem. 32 (1993) 520. [17] A. Lledos, M. Moreno-Manas, M. Sodupe, A. Vallribera, I. Mata, B. Martinez, E. Molins, Eur. J. Inorg. Chem. (2003) 4187–4194. [18] R.H. Crabtree, The Organometallic Chemistry of the Transition Metals, fourth ed., Wiley-Interscience, 2005. [19] J.M. Crosthwaite, M.J. Muldoon, J.K. Dixon, J.L. Anderson, J.F. Brennecke, J. Chem. Thermodyn. 37 (2005) 559. [20] Y. Yoshida, G. Saito, J. Mater. Chem. 16 (2006) 1254. [21] R.S. Drago, Physical Methods for Chemists, second ed., Saunders College, 1992. [22] O. Kahn, Molecular Magnetism, Wiley-VCH, New York, 1993. [23] W.Y. Hsu, C.C. Tai, W.L. Su, C.H. Chang, S.P. Wang, Inorg. Chim. Acta 361 (2008) 1281. [24] B. Yan, Z.D. Chen, S.X. Wang, J. Chin. Chem. Soc. 51 (2004) 367. [25] G.J. Bullen, B.J. Howlin, J. Silver, B.W. Fitzsimmons, I. Sayer, L.F. Larkworthy, J. Chem. Soc., Dalton Trans. (1986) 1937–1940. [26] I. Ondrejkovicova, T. Lis, J. Mrozinski, V. Vancova, M. Melnik, Polyhedron 17 (1998) 3181.