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Calorimetric and vibrational spectroscopic investigations of phase transitions in crystalline [Cr(OS(CH3)2)6](BF4)3
Vibrational Spectroscopy 62 (2012) 222–228
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Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Calorimetric and vibrational spectroscopic investigations of phase transitions in crystalline [Cr(OS(CH3 )2 )6 ](BF4 )3 Natalia Górska a,∗ , Akira Inaba a , Anna Migdał-Mikuli b a b
Research Center for Structural Thermodynamics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Faculty of Chemistry, Jagiellonian University, Ingarden 3, 30-060 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 17 February 2012 Received in revised form 24 July 2012 Accepted 26 July 2012 Available online 3 August 2012 Keywords: Dimethyl sulfoxide complex Order–disorder phase transition Vibrational and reorientational dynamics Adiabatic calorimetry FT-MIR RS
a b s t r a c t Two phase transitions – one sharp at T1 = 247.4 K and the other very broad at T2 ∼ 135 K – were observed for [Cr(OS(CH3 )2 )6 ](BF4 )3 by adiabatic calorimetry. They are of the order–disorder type, and their transition entropies are 28.4 and 8.3 J K−1 mol−1 , respectively. IR absorption and Raman scattering spectroscopy measurements revealed distinct changes in the molecular dynamics of both CH3 groups and BF4 − anions at T1 and reduced symmetry of the [Cr(OS(CH3 )2 )6 ]3+ complex cations and OS(CH3 )2 ligands below T2 . The spectral width of an IR and Raman active band associated with the ıas (FBF)F2 mode yielded an activation energy (Ea ) ∼ 9 kJ mol−1 for the reorientation of BF4 − anions, which is comparable with that obtained for other coordination compounds with BF4 − anions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dimethyl sulfoxide (OS(CH3 )2 , DMSO) is a versatile compound used as a solvent in various organic syntheses and also acts as a ligand in coordination chemistry. In liquid, the molecules form stable dimers which are held together by weak intermolecular C H· · ·O interactions and long-range electrostatic attraction between oxygen and sulfur deriving from adjacent dipoles [1,2]. When attached to a metal ion in coordination compounds, DMSO can form bonds via oxygen or sulfur atom depending on a type of metal based on Lewis classification [3,4]. The oxygen-coordinated ligands are often disordered especially with symmetric anions such as BF4 − , which causes difficulties in determination of the crystal structure. In such cases vibrational spectroscopy is an important tool to identify the molecular structure and dynamics of particular species and their interactions across temperature. Thermal behavior of coordination compounds with DMSO ligands coordinated octahedrally to transition metals and with tetrahedral BF4 − uncoordinated anions has been investigated for three crystalline compounds – [Ni(DMSO)6 ](BF4 )2 , [Co(DMSO)6 ](BF4 )2 , and [Mn(DMSO)6 ](BF4 )2 – by differential scanning calorimetry (DSC) [5–7]. These compounds are characterized by solid
∗ Corresponding author. Tel.: +81 06 6850 5526; fax: +81 06 6850 5526. E-mail addresses: [email protected], [email protected] (N. Górska). 0924-2031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vibspec.2012.07.010
polymorphism above room temperature with several phase transitions occurring between both stable and metastable phases. [Mn(DMSO)6 ](BF4 )2 undergoes an additional phase transition at 215 K accompanied by a small entropy change S = 1.3 J K−1 mol−1 [7]. However, except for the spectroscopic analysis at room temperature, there is no information concerning the phase behavior of any analogous compounds with trivalent central metals. The compound that we study here – [Cr(DMSO)6 ](BF4 )3 (hereafter referred to as CrHB) – exhibits a completely different thermal behavior from others. As described below, we observed two transitions at T1 = 247.4 K and T2 ∼ 135 K, both accompanied by a large entropy change. Interestingly, similar phase transitions were observed for [Cr(NH3 )6 ](BF4 )3 [8] at nearly the same temperatures. This study precisely examines the phase behavior of CrHB between 5 and 300 K using adiabatic calorimetry and two complementary spectroscopic methods: Fourier-transform infrared absorption and Raman light scattering. We discuss the possible mechanism of the phase transitions. 2. Experimental 2.1. Materials First, 2 g of Cr(NO3 )3 ·9H2 O (Kanto Chem. Co. Inc.) was dissolved in about 15 mL of distilled water and then 5 mL of 40% HBF4 solution (Wako Pure Chem. Ind. Ltd.) was added. A polypropylene temperature resistant beaker was used. Next, the solution was slowly
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heated up to evaporate the excess solvent and then chilled to room temperature and fine green crystals of Cr(BF4 )3 ·xH2 O were precipitated and dried over phosphorus(V) oxide (P4 O10 ). The crystals obtained were dissolved during heating in anhydrous DMSO (99.5%, Wako Pure Chem. Ind. Ltd.). The solution was chilled to room temperature and a green powder of [Cr(OS(CH3 )2 )x ](BF4 )3 precipitated, where x = 2–6. To obtain the compound with the appropriate coordination, it was recrystallized four times from DMSO with final percent yield value of more than 70%. The final compound CrHB is hygroscopic; therefore, it was placed in a sealed vessel and stored in a desiccator over P2 O10 . Its composition was determined by elemental analysis on a EURO EA 3000 instrument based on the C and H content of the DMSO ligands. Theoretical contents: C, 18.45%; H, 4.64%. Found: C, 18.41%; H, 4.50%. 2.2. Methods Heat capacity measurements were carried out between 5 and 300 K using a laboratory-made adiabatic calorimeter. As a sample is hygroscopic it was placed in a gold-plated copper cell with an inner volume of 2.77 cm3 and sealed under a helium atmosphere and then weighed. The sample mass was 1.5111 g after buoyancy correction. The temperature was measured with a rhodium–iron alloy resistance thermometer. The heat capacity of the empty cell was measured in advance and subtracted from the total heat capacity. The dead space of the sample cell was filled with helium gas at ambient pressure to advance equilibration. More details concerning this experiment can be found in previous papers [9,10]. Additional calorimetric measurements were also performed by DSC between 300 and 400 K to confirm that no phase transition exists above 300 K. Fourier-transform far-infrared (FT-FIR) and Fourier-transform mid-infrared (FT-MIR) absorption measurements were performed using BIO-RAD FTS-40V and JASCO FT-IR-6100 spectrometers, respectively. The room temperature FT-FIR spectrum was recorded at 40–500 cm−1 with a resolution of 2 cm−1 . The sample was suspended in paraffin wax and placed between polyethylene windows. FT-MIR spectra were obtained at 500–4000 cm−1 with a resolution of 2 cm−1 using a liquid helium cryostat. Two samples – one suspended in Nujol and the other in Fluorolube – were prepared and placed between two KBr plates. The sample in Fluorolube was measured during cooling from 300 K down to 15 K and the sample in Nujol was measured during heating from 15 K up to 285 K, both under vacuum conditions. The temperature stabilization accuracy was ±1 K. Raman scattering (RS) measurements were performed with a JASCO NR-1800 spectrometer with a resolution of 2 cm−1 . The incident radiation ( = 1064 nm) came from a Nd:YAG laser (Spectra-Physics). All spectra were recorded in the Raman shift range of 50–4000 cm−1 while cooling the sample from 300 K to 30 K using a liquid helium cryostat and under vacuum conditions.
Fig. 1. Molar heat capacity Cp obtained for [Cr(DMSO)6 ](BF4 )3 . Inset: plotted in the form of Cp T−1 against T. Baseline for the phase transitions is shown.
heat capacity is plotted in the form of Cp T−1 against T, another phase transition was observed with a maximum at T2 ∼ 135 K. While this transition is very broad, the shape of the heat capacity anomaly is still symmetric. The enthalpy of transition was 1.03 kJ mol−1 , and the corresponding entropy was 8.3 J K−1 mol−1 . Fig. 2 illustrates the temperature dependence of the cumulative entropy due to the phase transitions. A two-step sigmoidal curve with a large entropy change is evident, suggesting that both transitions are of the order–disorder type. The total entropy amounted to 36.7 J K−1 mol−1 , demonstrating that the compound at 300 K is highly disordered. 3.2. Spectroscopic investigations (FT-IR and RS spectra) Table 1 presents a list of IR and Raman band positions, their relative intensities, and assignments denoted by comparison with the literature data for several [M(DMSO)6 ]X3 complexes and liquid DMSO [11–15]. There is no crystallographic data available for this particular compound, but the crystal structures of [Cr(DMSO)6 ]Cl3 , [Cr(DMSO)6 ](NO3 )3 , and [Cr(DMSO)6 ](ClO4 )3 obtained at room temperature show that the complex cation Cr(DMSO)6 3+ has an octahedral symmetry with DMSO ligands coordinated to the central metal Cr3+ through oxygen atoms [16–18]. Moreover, the IR
3. Results and discussion 3.1. Calorimetric investigations Fig. 1 illustrates the molar heat capacity of CrHB obtained by adiabatic calorimetry. The data are given in Supplementary information (Table S1). The compound exhibits a sharp phase transition at T1 = 247.4 K, where the shape of the anomalous heat capacity curve is rather symmetric, tailing to the both temperature sides. The enthalpy of transition was evaluated by estimating a “normal” heat capacity curve, as indicated in Fig. 1. The enthalpy was 6.96 kJ mol−1 , and the corresponding entropy of transition was 28.4 J K−1 mol−1 . As shown in the inset of Fig. 1, where the
Fig. 2. Cumulative entropy across the two phase transitions. The size of 3R ln 2 is shown.
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Fig. 3. Temperature change for the IR spectra in the temperature range of 300–15 K, in four spectral ranges: (a) 1470–1380 cm−1 , (b) 1350–1280 cm−1 , (c) 3055–2985 cm−1 , and (d) 2970–2915 cm−1 .
Table 1 List of band positions for Raman and IR spectra of [Cr(DMSO)6 ](BF4 )3 at room temperature (vw, very weak; w, weak; sh, shoulder; m, medium; st, strong; vst, very strong; br, broad; , stretching; ı, bending; , rocking). Vibrational frequencies (cm−1 ) RS
IR
3019(m) 3014(m) 2993(w)
3024(m) 3007(sh)
2927(vst) 1437(vw) 1420(m) 1328(vw)
1122(vw)
1007(w) 995(w) ∼955(sh) 942(m) 766(m) 727(st) 690(vst) 571(w) 518(vw) 498(m) 354(m) 342(st) 324(m) 298(vw) 217(w) 180(w) ∼66(vst)
2961(m) 2927(m) 1437(sh) 1423(m) 1412(sh) 1330(m) 1307(w) 1288(w) 1102(st,sh) 1059(vst) 1040(st,sh) 993(st) 957(st,sh) 935(vst) 876(vw) 766(vw) 724(m) 689(vw) 573(m) 518(st) 497(vw) 366(st) 357(st) 327(w,sh) 294(m) 216(w) 184(w) 90(m)
Tentative assignments
as (CH3 ) as (CH3 ) as (CH3 ) s (CH3 ) s (CH3 ) ıas (CH3 ) ıas (CH3 ) ıas (CH3 ) ıs (CH3 ) ıs (CH3 ) ıs (CH3 ) as (FBF)F2 as (FBF)F2 as (FBF)F2 or r (CH3 ) r (CH3 ) r (CH3 ) r (CH3 ) (SO) r (CH3 ) s (BF)A1 as (CS) s (CS) ıas (FBF)F2 (CrO) (CrO) ıs (CSO) ıas (CSO) ıs (FBF)E ı(CSC) ı(OCrO) ı (CrOS) L (lattice)
frequencies of (S O) and (CrO) modes observed for CrHB are 935 and 518 cm−1 , respectively. These values are sufficiently close to those observed for [Cr(DMSO)6 ](NO3 )3 at 931 and 527 cm−1 [16] and for [Cr(DMSO)6 ](ClO4 )3 at 928 and 529 cm−1 [19,20], respectively, which indicates the same O-type coordination. The highest possible symmetry of the complex cation is Oh , with possible symmetry reduction going through Oh → D3d → S6 → C3 → C1 . In the C3 symmetry, all vibrations are infrared and Raman active [13]. In our study, the complementary character of the IR and Raman spectra obtained at 300 K shows that the geometry of the Cr(DMSO)6 3+ can successfully be described by the S6 point group. The complex cation has 177 vibrational degrees of freedom, and according to the group theory, the vibrational representation under S6 symmetry is 29Ag + 29Eg + 30Au + 30Eu , with E vibrations representing two degrees of freedom for each. All symmetric modes (g) should be Raman active, whereas the asymmetric ones (u) should be IR active. A free BF4 − anion has tetrahedral symmetry (Td point group) with four normal vibrations of A1 , E, and 2F2 symmetry. The first two are infrared (IR) inactive, but all of them are Raman active. For the crystal CrHB, the weak bands at 766 cm−1 , which are connected with the s (BF)A1 mode, and those at 327 cm−1 , which are connected with the ıs (FBF)E mode, were observed in the IR spectrum at 300 K. Moreover, we could observe the splitting of the degenerate as (FBF)F2 mode. All these facts can support that the BF4 − anion is not perfectly tetrahedral with possible symmetry reduction. On the other hand, we did not observe the splitting of the ıas (FBF)F2 mode, which is also indicative for symmetry reduction.
3.2.1. Temperature variation of IR spectra Fig. 3 illustrates the IR spectra of CrHB (for the sample suspended in Fluorolube) recorded between 15 and 300 K with four spectral ranges. Different temperature ranges are marked with different colors. The spectra obtained reflect some dynamic changes
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Fig. 4. Temperature change for the IR band assigned to the r (CH3 ) mode (a) and the temperature dependence of the peak position (b).
with decreasing temperature. Bands at 3024, 1437, and 1330 cm−1 assigned to the as (CH3 ), ıas (CH3 ), and ıs (CH3 ) vibrations, respectively, split into two components below T1 and then further into three components below T2 . All these changes are a result of the loss of degeneracy of modes. The most remarkable changes are visible at T2 . This fact is a strong piece of evidence, demonstrating that the symmetry reduction of Cr(DMSO)6 3+ cations occurred during the phase transition at T2 . It is also interesting to note that the position of the IR band at 2961 cm−1 assigned to s (CH3 ) shifts significantly toward lower frequency, whereas the position of the IR band at 1330 cm−1 connected with the ıs (CH3 ) mode shifts toward higher frequency with decreasing temperature. This behavior indicates that the C H· · ·F interactions become stronger at low temperature. To analyze the spectra in detail, the peak position and the full width at half maximum (FWHM) values were calculated for all bands by fitting the combination of Lorentz and Gauss functions using Spectra Manager. Fig. 4 illustrates the temperature dependence of the IR band assigned to the rocking vibration of the CH3 groups. The peak position of this band shifts toward higher frequency with decreasing temperature. A significant change occurs at T1 , whereas the value is almost constant below T2 . The temperature dependence of the FWHM of the IR band assigned to the s (CH) mode is presented in Fig. 5. The FWHM value decreases gradually with decreasing temperature, showing a two-step sigmoidal curve, one step below 250 K and the other below 150 K. The FWHM values become nearly constant below 75 K and can be connected only with the vibrational relaxation process. It is evident that the distinct change in the rotational dynamics of methyl groups occurs in both phase transitions. It is also interesting to note that this curve in Fig. 5 is similar to the one obtained in the cumulative entropy showing a measure of disorder (see Fig. 2). The only difference is that the high-temperature step is vertically squashed. To determine the activation energy for the reorientational dynamics of BF4 − groups, we followed the FWHM analysis described by Carabatos–Nédelec and Becker, which is based on the theory used for damping associated with an order–disorder mechanism [21]. The orientational correlation time R is the mean time between instantaneous jumps from one potential well to another and is given by
R = ∞ exp
E a
RT
(1)
where ∞ is the relaxation time at infinite temperature, Ea is the height of the potential barrier for BF4 − groups, and R is the gas constant. When 2 R2 1 ( is the frequency of a particular mode), which is the case here, the temperature dependence of the band width is described by [22]
E a
FWHM = (a + bT ) + c exp −
RT
(2)
where a, b, c, and Ea are the fitting parameters. The linear part of Eq. (2) represents the influence of the vibrational relaxation, and the exponential term represents the thermal orientational mechanism of diffuse nature (reorientational relaxation). Fig. 6 illustrates the temperature change for the IR band at 573 cm−1 associated with the ıas (FBF)F2 mode. The temperature dependencies of the peak position and the FWHM are also plotted, where a large reduction at T1 is evident. The peak position and FWHM values decrease exponentially below T1 , and the values become nearly constant in the T2 region. The solid curve in Fig. 6 (b) is the one fitted with Eq. (2), and the parameters are listed in Table 2. The estimated activation energy for the rotations of BF4 − anions below 250 K is (8.3 ± 2.0) kJ mol−1 .
Fig. 5. Temperature dependence of the FWHM for the IR band assigned to the s (CH3 ) mode.
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Fig. 6. Temperature change for the IR band assigned to the ıas (FBF)F2 mode (a) and temperature dependencies of the peak position (triangles) and the FWHM (squares), where the solid curve represents the fitting result according to Eq. (2) (b).
Table 2 Fitted parameters a, b, c, and Ea for the temperature dependence of the FWHM of the IR and Raman bands connected with ıas (FBF)F2 . Parameters −1
a (cm ) b (cm−1 K−1 ) c (cm−1 ) Ea (kJ mol−1 )
Band at 573 cm−1 IR
Band at 571 cm−1 Raman
11.2 1.98 × 10−3 267.2 8.3
6.14 8.4 × 10−4 767.5 10.1
3.2.2. Temperature variation of Raman spectra The selected Raman spectra recorded from 300 K to 30 K are illustrated in Figs. 7 and 8. In general, they show a tendency similar to what we observed in the IR spectra. The bands at 342, 690, 942, and 1420 cm−1 assigned to the internal vibrations of DMSO ligands are mainly split below T2 . The shifts of the bands connected with the vibrations of complex BF4 − anion are distinct. Specifically, the peak position of the band at 766 cm−1 assigned to the s (BF) mode shifts toward higher frequency with decreasing temperature, whereas the band at 571 cm−1 assigned to the ıas (FBF)F2 mode shifts toward
Fig. 7. Temperature change for the Raman spectra in the temperature range 300–30 K in two spectral ranges: 590–460 cm−1 and 390–300 cm−1 .
lower frequency. These changes occur immediately below T1 . The most intense Raman band at 690 cm−1 associated with the s (CS) mode is split into two components below T1 . Fig. 9 illustrates the temperature dependencies of the peak position and the FWHM of the Raman band at 571 cm−1 associated with the ıas (FBF)F2 mode. The results are consistent with the ones obtained for the same band by IR spectroscopy (see in Fig. 6). The estimated activation energy for the BF4 − anions rotations is (10.1 ± 3.0) kJ mol−1 , which is in reasonable agreement with that obtained by IR (Table 2). The average activation energy (∼9 kJ mol−1 ) is of the same order as the values obtained for the other coordination compounds with BF4 − anions: [Zn(NH3 )4 ](BF4 )2 (by RS) [23], [Cd(NH3 )6 ](BF4 )2 , [Cd(H2 O)6 ](BF4 )2 , and [Zn(ptz)6 ](BF4 )2 (by NMR) [24–26]. 3.3. Mechanism of phase transitions It is evident from our spectroscopic investigations that the orientational ordering of BF4 − anions is fairly abrupt and strongly involved in the phase transition at T1 . If we suppose that the anions become ordered completely and independently from each other below T1 , then we simply get the entropy change of 3R ln 2 = 17.3 J K−1 mol−1 . Here we assume a disorder with two possible orientations for each anion above T1 as was observed for several complexes [17,27–29]. This entropy value amounts to 60% of that obtained experimentally for the transition at T1 . Interestingly, this contribution corresponds to the vertical portion of the cumulative entropy at T1 (see in Fig. 2). On the other hand, the ordering of the DMSO molecules is rather gradual and involved in both transitions. There are two possibilities for the ordering process for the DMSO molecules: either with two different kinds of molecules in the crystal or with all the molecules ordered at two different temperatures. In either case, the ordering of the DMSO molecules can be triggered at T1 by the BF4 − anions ordering through the hydrogen bonds connecting between them. However, to understand the mechanism in more detail we need crystallographic evidence. As described earlier, the structure of the [Cr(DMSO)6 ]X3 compounds with X = Cl− , NO3 − , and ClO4 − was investigated by single-crystal X-ray diffraction [16–18]. [Cr(DMSO)6 ]Cl3 (at 120 K) and [Cr(DMSO)6 ](NO3 )3 (at 298 K) crystallize in a trigonal space group R3¯ (Z = 3), where the DMSO molecules are completely
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Fig. 8. Temperature change for the Raman spectra in the temperature range of 300–30 K in three spectral ranges: (a) 780–670 cm−1 , (b) 1025–900 cm−1 , and (c) 1460–1380 cm−1 .
ordered. In contrast, [Cr(DMSO)6 ](ClO4 )3 (at 298 K and 153 K) crystallizes in a monoclinic space group P21 /c (Z = 4), where all DMSO ligands are disordered at 298 K with two alternative positions for oxygen and sulfur atoms. Moreover, four ClO4 − anions (out of twelve) are orientationally disordered. At 153 K, all DMSO ligands together with eight (out of twelve) anions become perfectly ordered. It was reported [17] that other trivalent complexes of the [M(DMSO)6 ](ClO4 )3 type with M = Al, Ga, and Sc also exhibit similar disorder for DMSO ligands as well as ClO4 − anions at room temperature, which diminishes at low temperatures. Although there is no structural study for the [M(DMSO)6 ](BF4 )3 complexes, it is likely that the behavior of BF4 − anions in the crystal is similar to that of ClO4 − anions because of the same symmetry and similar size.
4. Conclusion [Cr(DMSO)6 ](BF4 )3 exhibits two phase transitions at T1 = 247.4 K and at T2 ∼ 135 K, accompanied by entropy changes of 28.4 and 8.3 J K−1 mol−1 , respectively. The former is very sharp and the latter very broad, but both are the order–disorder type. A distinct change in the molecular dynamics of CH3 groups and BF4 − anions is revealed at the transition at T1 . Below T2 , a reduction in the symmetry of the [Cr(OS(CH3 )2 )6 ]3+ complex cation and the DMSO ligand is revealed. An activation energy Ea ∼9 kJ mol−1 for the reorientation of BF4 − anions is obtained from the spectral width of an IR and Raman active band associated with the ıas (FBF)F2 mode. Acknowledgment This work is Contribution No. 24 from the Research Center for Structural Thermodynamics. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.vibspec.2012.07.010. References [1] [2] [3] [4]
Fig. 9. Temperature dependencies of the peak position (triangles) and the FWHM (squares) for the Raman band assigned to ıas (FBF)F2 . The solid curve represents the fitting result according to Eq. (2).
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Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Calorimetric and vibrational spectroscopic investigations of phase transitions in crystalline [Cr(OS(CH3 )2 )6 ](BF4 )3 Natalia Górska a,∗ , Akira Inaba a , Anna Migdał-Mikuli b a b
Research Center for Structural Thermodynamics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Faculty of Chemistry, Jagiellonian University, Ingarden 3, 30-060 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 17 February 2012 Received in revised form 24 July 2012 Accepted 26 July 2012 Available online 3 August 2012 Keywords: Dimethyl sulfoxide complex Order–disorder phase transition Vibrational and reorientational dynamics Adiabatic calorimetry FT-MIR RS
a b s t r a c t Two phase transitions – one sharp at T1 = 247.4 K and the other very broad at T2 ∼ 135 K – were observed for [Cr(OS(CH3 )2 )6 ](BF4 )3 by adiabatic calorimetry. They are of the order–disorder type, and their transition entropies are 28.4 and 8.3 J K−1 mol−1 , respectively. IR absorption and Raman scattering spectroscopy measurements revealed distinct changes in the molecular dynamics of both CH3 groups and BF4 − anions at T1 and reduced symmetry of the [Cr(OS(CH3 )2 )6 ]3+ complex cations and OS(CH3 )2 ligands below T2 . The spectral width of an IR and Raman active band associated with the ıas (FBF)F2 mode yielded an activation energy (Ea ) ∼ 9 kJ mol−1 for the reorientation of BF4 − anions, which is comparable with that obtained for other coordination compounds with BF4 − anions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Dimethyl sulfoxide (OS(CH3 )2 , DMSO) is a versatile compound used as a solvent in various organic syntheses and also acts as a ligand in coordination chemistry. In liquid, the molecules form stable dimers which are held together by weak intermolecular C H· · ·O interactions and long-range electrostatic attraction between oxygen and sulfur deriving from adjacent dipoles [1,2]. When attached to a metal ion in coordination compounds, DMSO can form bonds via oxygen or sulfur atom depending on a type of metal based on Lewis classification [3,4]. The oxygen-coordinated ligands are often disordered especially with symmetric anions such as BF4 − , which causes difficulties in determination of the crystal structure. In such cases vibrational spectroscopy is an important tool to identify the molecular structure and dynamics of particular species and their interactions across temperature. Thermal behavior of coordination compounds with DMSO ligands coordinated octahedrally to transition metals and with tetrahedral BF4 − uncoordinated anions has been investigated for three crystalline compounds – [Ni(DMSO)6 ](BF4 )2 , [Co(DMSO)6 ](BF4 )2 , and [Mn(DMSO)6 ](BF4 )2 – by differential scanning calorimetry (DSC) [5–7]. These compounds are characterized by solid
∗ Corresponding author. Tel.: +81 06 6850 5526; fax: +81 06 6850 5526. E-mail addresses: [email protected], [email protected] (N. Górska). 0924-2031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vibspec.2012.07.010
polymorphism above room temperature with several phase transitions occurring between both stable and metastable phases. [Mn(DMSO)6 ](BF4 )2 undergoes an additional phase transition at 215 K accompanied by a small entropy change S = 1.3 J K−1 mol−1 [7]. However, except for the spectroscopic analysis at room temperature, there is no information concerning the phase behavior of any analogous compounds with trivalent central metals. The compound that we study here – [Cr(DMSO)6 ](BF4 )3 (hereafter referred to as CrHB) – exhibits a completely different thermal behavior from others. As described below, we observed two transitions at T1 = 247.4 K and T2 ∼ 135 K, both accompanied by a large entropy change. Interestingly, similar phase transitions were observed for [Cr(NH3 )6 ](BF4 )3 [8] at nearly the same temperatures. This study precisely examines the phase behavior of CrHB between 5 and 300 K using adiabatic calorimetry and two complementary spectroscopic methods: Fourier-transform infrared absorption and Raman light scattering. We discuss the possible mechanism of the phase transitions. 2. Experimental 2.1. Materials First, 2 g of Cr(NO3 )3 ·9H2 O (Kanto Chem. Co. Inc.) was dissolved in about 15 mL of distilled water and then 5 mL of 40% HBF4 solution (Wako Pure Chem. Ind. Ltd.) was added. A polypropylene temperature resistant beaker was used. Next, the solution was slowly
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heated up to evaporate the excess solvent and then chilled to room temperature and fine green crystals of Cr(BF4 )3 ·xH2 O were precipitated and dried over phosphorus(V) oxide (P4 O10 ). The crystals obtained were dissolved during heating in anhydrous DMSO (99.5%, Wako Pure Chem. Ind. Ltd.). The solution was chilled to room temperature and a green powder of [Cr(OS(CH3 )2 )x ](BF4 )3 precipitated, where x = 2–6. To obtain the compound with the appropriate coordination, it was recrystallized four times from DMSO with final percent yield value of more than 70%. The final compound CrHB is hygroscopic; therefore, it was placed in a sealed vessel and stored in a desiccator over P2 O10 . Its composition was determined by elemental analysis on a EURO EA 3000 instrument based on the C and H content of the DMSO ligands. Theoretical contents: C, 18.45%; H, 4.64%. Found: C, 18.41%; H, 4.50%. 2.2. Methods Heat capacity measurements were carried out between 5 and 300 K using a laboratory-made adiabatic calorimeter. As a sample is hygroscopic it was placed in a gold-plated copper cell with an inner volume of 2.77 cm3 and sealed under a helium atmosphere and then weighed. The sample mass was 1.5111 g after buoyancy correction. The temperature was measured with a rhodium–iron alloy resistance thermometer. The heat capacity of the empty cell was measured in advance and subtracted from the total heat capacity. The dead space of the sample cell was filled with helium gas at ambient pressure to advance equilibration. More details concerning this experiment can be found in previous papers [9,10]. Additional calorimetric measurements were also performed by DSC between 300 and 400 K to confirm that no phase transition exists above 300 K. Fourier-transform far-infrared (FT-FIR) and Fourier-transform mid-infrared (FT-MIR) absorption measurements were performed using BIO-RAD FTS-40V and JASCO FT-IR-6100 spectrometers, respectively. The room temperature FT-FIR spectrum was recorded at 40–500 cm−1 with a resolution of 2 cm−1 . The sample was suspended in paraffin wax and placed between polyethylene windows. FT-MIR spectra were obtained at 500–4000 cm−1 with a resolution of 2 cm−1 using a liquid helium cryostat. Two samples – one suspended in Nujol and the other in Fluorolube – were prepared and placed between two KBr plates. The sample in Fluorolube was measured during cooling from 300 K down to 15 K and the sample in Nujol was measured during heating from 15 K up to 285 K, both under vacuum conditions. The temperature stabilization accuracy was ±1 K. Raman scattering (RS) measurements were performed with a JASCO NR-1800 spectrometer with a resolution of 2 cm−1 . The incident radiation ( = 1064 nm) came from a Nd:YAG laser (Spectra-Physics). All spectra were recorded in the Raman shift range of 50–4000 cm−1 while cooling the sample from 300 K to 30 K using a liquid helium cryostat and under vacuum conditions.
Fig. 1. Molar heat capacity Cp obtained for [Cr(DMSO)6 ](BF4 )3 . Inset: plotted in the form of Cp T−1 against T. Baseline for the phase transitions is shown.
heat capacity is plotted in the form of Cp T−1 against T, another phase transition was observed with a maximum at T2 ∼ 135 K. While this transition is very broad, the shape of the heat capacity anomaly is still symmetric. The enthalpy of transition was 1.03 kJ mol−1 , and the corresponding entropy was 8.3 J K−1 mol−1 . Fig. 2 illustrates the temperature dependence of the cumulative entropy due to the phase transitions. A two-step sigmoidal curve with a large entropy change is evident, suggesting that both transitions are of the order–disorder type. The total entropy amounted to 36.7 J K−1 mol−1 , demonstrating that the compound at 300 K is highly disordered. 3.2. Spectroscopic investigations (FT-IR and RS spectra) Table 1 presents a list of IR and Raman band positions, their relative intensities, and assignments denoted by comparison with the literature data for several [M(DMSO)6 ]X3 complexes and liquid DMSO [11–15]. There is no crystallographic data available for this particular compound, but the crystal structures of [Cr(DMSO)6 ]Cl3 , [Cr(DMSO)6 ](NO3 )3 , and [Cr(DMSO)6 ](ClO4 )3 obtained at room temperature show that the complex cation Cr(DMSO)6 3+ has an octahedral symmetry with DMSO ligands coordinated to the central metal Cr3+ through oxygen atoms [16–18]. Moreover, the IR
3. Results and discussion 3.1. Calorimetric investigations Fig. 1 illustrates the molar heat capacity of CrHB obtained by adiabatic calorimetry. The data are given in Supplementary information (Table S1). The compound exhibits a sharp phase transition at T1 = 247.4 K, where the shape of the anomalous heat capacity curve is rather symmetric, tailing to the both temperature sides. The enthalpy of transition was evaluated by estimating a “normal” heat capacity curve, as indicated in Fig. 1. The enthalpy was 6.96 kJ mol−1 , and the corresponding entropy of transition was 28.4 J K−1 mol−1 . As shown in the inset of Fig. 1, where the
Fig. 2. Cumulative entropy across the two phase transitions. The size of 3R ln 2 is shown.
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Fig. 3. Temperature change for the IR spectra in the temperature range of 300–15 K, in four spectral ranges: (a) 1470–1380 cm−1 , (b) 1350–1280 cm−1 , (c) 3055–2985 cm−1 , and (d) 2970–2915 cm−1 .
Table 1 List of band positions for Raman and IR spectra of [Cr(DMSO)6 ](BF4 )3 at room temperature (vw, very weak; w, weak; sh, shoulder; m, medium; st, strong; vst, very strong; br, broad; , stretching; ı, bending; , rocking). Vibrational frequencies (cm−1 ) RS
IR
3019(m) 3014(m) 2993(w)
3024(m) 3007(sh)
2927(vst) 1437(vw) 1420(m) 1328(vw)
1122(vw)
1007(w) 995(w) ∼955(sh) 942(m) 766(m) 727(st) 690(vst) 571(w) 518(vw) 498(m) 354(m) 342(st) 324(m) 298(vw) 217(w) 180(w) ∼66(vst)
2961(m) 2927(m) 1437(sh) 1423(m) 1412(sh) 1330(m) 1307(w) 1288(w) 1102(st,sh) 1059(vst) 1040(st,sh) 993(st) 957(st,sh) 935(vst) 876(vw) 766(vw) 724(m) 689(vw) 573(m) 518(st) 497(vw) 366(st) 357(st) 327(w,sh) 294(m) 216(w) 184(w) 90(m)
Tentative assignments
as (CH3 ) as (CH3 ) as (CH3 ) s (CH3 ) s (CH3 ) ıas (CH3 ) ıas (CH3 ) ıas (CH3 ) ıs (CH3 ) ıs (CH3 ) ıs (CH3 ) as (FBF)F2 as (FBF)F2 as (FBF)F2 or r (CH3 ) r (CH3 ) r (CH3 ) r (CH3 ) (SO) r (CH3 ) s (BF)A1 as (CS) s (CS) ıas (FBF)F2 (CrO) (CrO) ıs (CSO) ıas (CSO) ıs (FBF)E ı(CSC) ı(OCrO) ı (CrOS) L (lattice)
frequencies of (S O) and (CrO) modes observed for CrHB are 935 and 518 cm−1 , respectively. These values are sufficiently close to those observed for [Cr(DMSO)6 ](NO3 )3 at 931 and 527 cm−1 [16] and for [Cr(DMSO)6 ](ClO4 )3 at 928 and 529 cm−1 [19,20], respectively, which indicates the same O-type coordination. The highest possible symmetry of the complex cation is Oh , with possible symmetry reduction going through Oh → D3d → S6 → C3 → C1 . In the C3 symmetry, all vibrations are infrared and Raman active [13]. In our study, the complementary character of the IR and Raman spectra obtained at 300 K shows that the geometry of the Cr(DMSO)6 3+ can successfully be described by the S6 point group. The complex cation has 177 vibrational degrees of freedom, and according to the group theory, the vibrational representation under S6 symmetry is 29Ag + 29Eg + 30Au + 30Eu , with E vibrations representing two degrees of freedom for each. All symmetric modes (g) should be Raman active, whereas the asymmetric ones (u) should be IR active. A free BF4 − anion has tetrahedral symmetry (Td point group) with four normal vibrations of A1 , E, and 2F2 symmetry. The first two are infrared (IR) inactive, but all of them are Raman active. For the crystal CrHB, the weak bands at 766 cm−1 , which are connected with the s (BF)A1 mode, and those at 327 cm−1 , which are connected with the ıs (FBF)E mode, were observed in the IR spectrum at 300 K. Moreover, we could observe the splitting of the degenerate as (FBF)F2 mode. All these facts can support that the BF4 − anion is not perfectly tetrahedral with possible symmetry reduction. On the other hand, we did not observe the splitting of the ıas (FBF)F2 mode, which is also indicative for symmetry reduction.
3.2.1. Temperature variation of IR spectra Fig. 3 illustrates the IR spectra of CrHB (for the sample suspended in Fluorolube) recorded between 15 and 300 K with four spectral ranges. Different temperature ranges are marked with different colors. The spectra obtained reflect some dynamic changes
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Fig. 4. Temperature change for the IR band assigned to the r (CH3 ) mode (a) and the temperature dependence of the peak position (b).
with decreasing temperature. Bands at 3024, 1437, and 1330 cm−1 assigned to the as (CH3 ), ıas (CH3 ), and ıs (CH3 ) vibrations, respectively, split into two components below T1 and then further into three components below T2 . All these changes are a result of the loss of degeneracy of modes. The most remarkable changes are visible at T2 . This fact is a strong piece of evidence, demonstrating that the symmetry reduction of Cr(DMSO)6 3+ cations occurred during the phase transition at T2 . It is also interesting to note that the position of the IR band at 2961 cm−1 assigned to s (CH3 ) shifts significantly toward lower frequency, whereas the position of the IR band at 1330 cm−1 connected with the ıs (CH3 ) mode shifts toward higher frequency with decreasing temperature. This behavior indicates that the C H· · ·F interactions become stronger at low temperature. To analyze the spectra in detail, the peak position and the full width at half maximum (FWHM) values were calculated for all bands by fitting the combination of Lorentz and Gauss functions using Spectra Manager. Fig. 4 illustrates the temperature dependence of the IR band assigned to the rocking vibration of the CH3 groups. The peak position of this band shifts toward higher frequency with decreasing temperature. A significant change occurs at T1 , whereas the value is almost constant below T2 . The temperature dependence of the FWHM of the IR band assigned to the s (CH) mode is presented in Fig. 5. The FWHM value decreases gradually with decreasing temperature, showing a two-step sigmoidal curve, one step below 250 K and the other below 150 K. The FWHM values become nearly constant below 75 K and can be connected only with the vibrational relaxation process. It is evident that the distinct change in the rotational dynamics of methyl groups occurs in both phase transitions. It is also interesting to note that this curve in Fig. 5 is similar to the one obtained in the cumulative entropy showing a measure of disorder (see Fig. 2). The only difference is that the high-temperature step is vertically squashed. To determine the activation energy for the reorientational dynamics of BF4 − groups, we followed the FWHM analysis described by Carabatos–Nédelec and Becker, which is based on the theory used for damping associated with an order–disorder mechanism [21]. The orientational correlation time R is the mean time between instantaneous jumps from one potential well to another and is given by
R = ∞ exp
E a
RT
(1)
where ∞ is the relaxation time at infinite temperature, Ea is the height of the potential barrier for BF4 − groups, and R is the gas constant. When 2 R2 1 ( is the frequency of a particular mode), which is the case here, the temperature dependence of the band width is described by [22]
E a
FWHM = (a + bT ) + c exp −
RT
(2)
where a, b, c, and Ea are the fitting parameters. The linear part of Eq. (2) represents the influence of the vibrational relaxation, and the exponential term represents the thermal orientational mechanism of diffuse nature (reorientational relaxation). Fig. 6 illustrates the temperature change for the IR band at 573 cm−1 associated with the ıas (FBF)F2 mode. The temperature dependencies of the peak position and the FWHM are also plotted, where a large reduction at T1 is evident. The peak position and FWHM values decrease exponentially below T1 , and the values become nearly constant in the T2 region. The solid curve in Fig. 6 (b) is the one fitted with Eq. (2), and the parameters are listed in Table 2. The estimated activation energy for the rotations of BF4 − anions below 250 K is (8.3 ± 2.0) kJ mol−1 .
Fig. 5. Temperature dependence of the FWHM for the IR band assigned to the s (CH3 ) mode.
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Fig. 6. Temperature change for the IR band assigned to the ıas (FBF)F2 mode (a) and temperature dependencies of the peak position (triangles) and the FWHM (squares), where the solid curve represents the fitting result according to Eq. (2) (b).
Table 2 Fitted parameters a, b, c, and Ea for the temperature dependence of the FWHM of the IR and Raman bands connected with ıas (FBF)F2 . Parameters −1
a (cm ) b (cm−1 K−1 ) c (cm−1 ) Ea (kJ mol−1 )
Band at 573 cm−1 IR
Band at 571 cm−1 Raman
11.2 1.98 × 10−3 267.2 8.3
6.14 8.4 × 10−4 767.5 10.1
3.2.2. Temperature variation of Raman spectra The selected Raman spectra recorded from 300 K to 30 K are illustrated in Figs. 7 and 8. In general, they show a tendency similar to what we observed in the IR spectra. The bands at 342, 690, 942, and 1420 cm−1 assigned to the internal vibrations of DMSO ligands are mainly split below T2 . The shifts of the bands connected with the vibrations of complex BF4 − anion are distinct. Specifically, the peak position of the band at 766 cm−1 assigned to the s (BF) mode shifts toward higher frequency with decreasing temperature, whereas the band at 571 cm−1 assigned to the ıas (FBF)F2 mode shifts toward
Fig. 7. Temperature change for the Raman spectra in the temperature range 300–30 K in two spectral ranges: 590–460 cm−1 and 390–300 cm−1 .
lower frequency. These changes occur immediately below T1 . The most intense Raman band at 690 cm−1 associated with the s (CS) mode is split into two components below T1 . Fig. 9 illustrates the temperature dependencies of the peak position and the FWHM of the Raman band at 571 cm−1 associated with the ıas (FBF)F2 mode. The results are consistent with the ones obtained for the same band by IR spectroscopy (see in Fig. 6). The estimated activation energy for the BF4 − anions rotations is (10.1 ± 3.0) kJ mol−1 , which is in reasonable agreement with that obtained by IR (Table 2). The average activation energy (∼9 kJ mol−1 ) is of the same order as the values obtained for the other coordination compounds with BF4 − anions: [Zn(NH3 )4 ](BF4 )2 (by RS) [23], [Cd(NH3 )6 ](BF4 )2 , [Cd(H2 O)6 ](BF4 )2 , and [Zn(ptz)6 ](BF4 )2 (by NMR) [24–26]. 3.3. Mechanism of phase transitions It is evident from our spectroscopic investigations that the orientational ordering of BF4 − anions is fairly abrupt and strongly involved in the phase transition at T1 . If we suppose that the anions become ordered completely and independently from each other below T1 , then we simply get the entropy change of 3R ln 2 = 17.3 J K−1 mol−1 . Here we assume a disorder with two possible orientations for each anion above T1 as was observed for several complexes [17,27–29]. This entropy value amounts to 60% of that obtained experimentally for the transition at T1 . Interestingly, this contribution corresponds to the vertical portion of the cumulative entropy at T1 (see in Fig. 2). On the other hand, the ordering of the DMSO molecules is rather gradual and involved in both transitions. There are two possibilities for the ordering process for the DMSO molecules: either with two different kinds of molecules in the crystal or with all the molecules ordered at two different temperatures. In either case, the ordering of the DMSO molecules can be triggered at T1 by the BF4 − anions ordering through the hydrogen bonds connecting between them. However, to understand the mechanism in more detail we need crystallographic evidence. As described earlier, the structure of the [Cr(DMSO)6 ]X3 compounds with X = Cl− , NO3 − , and ClO4 − was investigated by single-crystal X-ray diffraction [16–18]. [Cr(DMSO)6 ]Cl3 (at 120 K) and [Cr(DMSO)6 ](NO3 )3 (at 298 K) crystallize in a trigonal space group R3¯ (Z = 3), where the DMSO molecules are completely
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Fig. 8. Temperature change for the Raman spectra in the temperature range of 300–30 K in three spectral ranges: (a) 780–670 cm−1 , (b) 1025–900 cm−1 , and (c) 1460–1380 cm−1 .
ordered. In contrast, [Cr(DMSO)6 ](ClO4 )3 (at 298 K and 153 K) crystallizes in a monoclinic space group P21 /c (Z = 4), where all DMSO ligands are disordered at 298 K with two alternative positions for oxygen and sulfur atoms. Moreover, four ClO4 − anions (out of twelve) are orientationally disordered. At 153 K, all DMSO ligands together with eight (out of twelve) anions become perfectly ordered. It was reported [17] that other trivalent complexes of the [M(DMSO)6 ](ClO4 )3 type with M = Al, Ga, and Sc also exhibit similar disorder for DMSO ligands as well as ClO4 − anions at room temperature, which diminishes at low temperatures. Although there is no structural study for the [M(DMSO)6 ](BF4 )3 complexes, it is likely that the behavior of BF4 − anions in the crystal is similar to that of ClO4 − anions because of the same symmetry and similar size.
4. Conclusion [Cr(DMSO)6 ](BF4 )3 exhibits two phase transitions at T1 = 247.4 K and at T2 ∼ 135 K, accompanied by entropy changes of 28.4 and 8.3 J K−1 mol−1 , respectively. The former is very sharp and the latter very broad, but both are the order–disorder type. A distinct change in the molecular dynamics of CH3 groups and BF4 − anions is revealed at the transition at T1 . Below T2 , a reduction in the symmetry of the [Cr(OS(CH3 )2 )6 ]3+ complex cation and the DMSO ligand is revealed. An activation energy Ea ∼9 kJ mol−1 for the reorientation of BF4 − anions is obtained from the spectral width of an IR and Raman active band associated with the ıas (FBF)F2 mode. Acknowledgment This work is Contribution No. 24 from the Research Center for Structural Thermodynamics. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.vibspec.2012.07.010. References [1] [2] [3] [4]
Fig. 9. Temperature dependencies of the peak position (triangles) and the FWHM (squares) for the Raman band assigned to ıas (FBF)F2 . The solid curve represents the fitting result according to Eq. (2).
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