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Characterization of compounds derived from copper-oxamate and imidazolium by X-ray absorption and vibrational spectroscopies
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 303–310
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Characterization of compounds derived from copper-oxamate and imidazolium by X-ray absorption and vibrational spectroscopies Gustavo M. do Nascimento a,b,⇑, Walace D. do Pim a,c, Daniella O. Reis a, Tatiana R.G. Simões a, Noriberto A. Pradie d, Humberto O. Stumpf a a
Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Brazil Centro de Ciências Naturais e Humanas (CCNH), Universidade Federal do ABC, Brazil Centro Federal de Educação Tecnológica de Minas Gerais, CEFET-MG, Brazil d Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Brazil b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Salts of copper-oxamate anions and
imidazolium cations were studied. Raman and IR band assignments were
supported by DFT calculations. Electronic distribution around
ACuANA sites is changed by anion– cation interaction.
a r t i c l e
i n f o
Article history: Received 29 September 2014 Received in revised form 9 December 2014 Accepted 4 February 2015 Available online 12 February 2015 Keywords: Ionic liquids Imidazolium Raman XANES DFT calculations
a b s t r a c t In this work, compounds derived from copper-oxamate anions (ortho, meta, and para)-phenylenebis (oxamate) and imidazolium cations (1-butyl-3-methylimidazolium) were synthesized. The compounds were characterized by Raman and FTIR spectroscopies and the band assignments were supported by DFT calculations. Strong IR bands from 1610 to 1700 cm 1 dominated the spectra of the complex and can be assigned to mC@O vibrations of the [Cu(opba)]2 anions by the comparison with the DFT data. In opposition to the FTIR spectra, the main vibrational bands in the Raman spectra are observed in the 1350– 1600 cm 1 range. All bands in this region are associated to the modified benzene vibrations of the copper-phenylenebis(oxamate) anions. X-ray absorption near edge (XANES) at different energies (NK and Cu L2,3 edges) was also used to probe the interionic interactions. XANES data show that anion–cation interaction in the Cu-oxamate–imidazolium changes the electronic structure around the ACuANA sites in the oxamate anion. Ó 2015 Elsevier B.V. All rights reserved.
Introduction The emerging field of molecular magnetism has the potential to design unparallel new magnetic materials which can exhibit bulk ⇑ Corresponding author at: Centro de Ciências Naturais e Humanas (CCNH), Universidade Federal do ABC, Brazil. E-mail address: [email protected] (G.M. do Nascimento). http://dx.doi.org/10.1016/j.saa.2015.02.012 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
physical properties, such as long-range magnetic ordering. Our group has been developed a series of copper complexes with phenylenebis(oxamate) derivates as ligands [1–8]. In addition, the combination of different cations to these anionic copper-complexes can change the crystal packing and also the magnetic behavior. Nowadays the family of salts derived from imidazolium cations (known as ionic liquids, ILs) open up the possibility to modulate the
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structural characteristics of the cations bonded to the copperphenylene-bis(oxamate) anions in order to obtain molecule-based magnets with new behavior and in a liquid state. Ionic liquids (ILs) are a broad family of salts that are liquid at temperatures lower than 100 °C. Commonly the ionic liquids are composed by a large cation and a weakly coordinating anion [9]. There are a very large range of combinations between cations and anions those can form ionic liquids, about 1018 different combinations [9]. The ionic liquids derived from imidazolic ring are the most studied ones [10,11]. Parameters like the carbonic chain length bonded to the cation or the anion type can drastically change the properties of the ILs, thus being possible to achieve a desired function by designing the structure of a specific IL [12]. In most cases, ILs have high thermal stability, a broad electrochemical window, an organized structure at medium distances (in opposition to molecular liquids, that are organized at small distances) and very low vapor pressure, properties that make them important in many areas like green chemistry, organic and inorganic synthesis and many others [13]. The ILs are salts with low melting points and the most unusual characteristic of these systems is that, although they are liquids, they present features similar to solids, such as structural organization at intermediate distances and negligible vapor pressure [9]. Our group start the preparation of compounds derived from Cu(ortho, meta, and para)-phenylenebis (oxamate) anions combined with different imidazolium cations. The main objective is to prepare compounds derived from ionic liquids with different magnetic behavior. In addition, the presence of a carbonic chain opens the possibility to enhance the interactions between the metal-complex and carbon nanotubes [14,15]. The electronic interactions between the [Cu(opba)]2 anions (where opba = orthophenylenebis(oxamate)) and single wall carbon nanotubes (SWCNTs) were recently investigated by our group [14–16]. It is observed that the electronic interactions show a dependence on the SWCNT diameter, being the interaction stronger for metallic tubes. This interaction is also influenced by the amount of complex that is probably adsorbed on the carbon surface of the SWCNTs. Some charge transfer can be also occurring between the metallic complex and the SWCNTs. Hence, the characterization of the vibrational and electronic signatures of these new ionic liquids is crucial for their future use with single or double walled carbon tubes [17,18]. The Raman and FTIR are commonly used as fundamental techniques in the study of vibrational behavior of these kinds of compounds. The X-ray absorption spectroscopy near edge spectroscopy (XANES) involves the excitation of ‘‘core’’ electrons of the atoms [19]. Each absorption energy edge is related to a specific atom present in the material and, more specifically, to a quantum-mechanical transition that excites an electron in a particular atomic core-orbital to the free or unoccupied continuum levels (ionization of the core orbital above the Fermi energy). In addition, the wavelength associated to the photoelectron formed by the absorption process is higher than the atomic distances. Thus, the mean free path of the photoelectron is sufficient to cause multiple scatterings; this is the main characteristic of the XANES regime [20]. This phenomenon turns the XANES spectra sensible to the electronic density around the absorbing atom, implying that it can be used to monitor the oxidation state and also the density of unoccupied electronic states [21]. In fact, the XANES spectra, particularly for molecules with high electronic delocalization, are very complicated, because many pre-edge peaks can appear due to the resonance and/or conjugation effects [22–27]. XANES spectroscopy was used for investigation of the electronic structure around the Cu atoms in the copper-oxamate anions ((ortho, meta, and para)-phenylenebis(oxamate)) bonded to the imidazolium cations (1-butyl-3-methylimidazolium). For this trend, XANES spectra at Cu L-edge and N K-edge were used for
investigation the changes in the electronic densities around Cu and N atoms in these new copper complexes. Hence, in the present work, a vibrational (Raman and FTIR) and electronic (UV–Vis, XANES and XPS) spectroscopies were used for investigation of the ionic liquids derived from copper-oxamate anions ((ortho, meta, and para)-phenylenebis(oxamate)) and imidazolium cations (1-butyl-3-methylimidazolium). The experimental findings are supported by DFT calculations. Experimental methods Reagents The water used in the experiments was deionized from a MilliQ (Millipore System) system. The dichloromethane (Synth), acetonitrile (Synth) and CuCl22H2O (Sigma–Aldrich) were analytical grade and they were used as received without any further purification. Tetrahydrofuran (THF, Synth) was treated with CaCl2 and metallic sodium for removing the water, as described by Morita [28]. The [Bu4N]2Cu(opba) (where Bu4N = tetra-n-butyl-ammonium and opba = ortho-phenylenebis(oxamate)) material was prepared according to the Ref. [2]. The Et2H2mpba (where mpba = meta-phenylenebis(oxamate)) material was prepared according to the Ref. [29], the Et2H2ppba (where ppba = para-phenylenebis (oxamate)) material was prepared according to the Ref. [30], and the [C4MIm]ClO4 was prepared according to the Ref. [31]. In this work, imidazolic ILs will be abbreviated as [CnMIm]+ for 1-alkyl3-methylimidazolium, where n is the number of carbons in the alkyl chain. The description of the synthetic routes used to acquire the [C4MIm]2Cu(opba)3H2O, [C4MIm]4[Cu2(mpba)2], and [C4MIm]4[Cu2(ppba)2] compounds were described in the Appendix A. Instrumentation Raman spectra for the ILs samples at 647.1 nm (1.92 eV, Kr+ laser) were taken with a triple monochromator, XY Dilor Micro-Raman System, equipped with a CCD detector. For Raman spectra at 785.0 nm (1.58 eV, solid state laser) were taken with a Senterra Bruker Raman spectrometer, equipped with a CCD detector, and finally for Raman spectra at 632.8 nm (1.96 eV, He–Ne laser) were taken with a Renishaw in via Raman spectrometer, equipped with a CCD detector. All Raman spectra were collected at room temperature and using a backscattering geometry and the laser line was focused on the sample using a 50 objective and the power incident on the sample was kept lower than 2 mW to avoid heating effects. Different acquisition times between 5 and 30 s were used for each sample in an attempt to optimize the signal-to-noise ratio of the Raman spectra. Different laser lines were used in order to avoid some fluorescence and/or to maximize the signal-to-noise relation for each IL sample. The equilibrium geometry of the [Cu(opba)]2 , [Cu2(mpba)2]4 , [Cu2(ppba)2]4 , and [C4MIm]+ as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software [32] using B3LYP density functional theory. This method uses Becker’s three-parameter exchange functional (B3) [33,34], in combination with the (LYP) correlation functional [35]. The calculations were done using the 6-31+G(d) or 6311++G(d,p) basis set [36,37]. The scale factor values of 0.96 [38,39] or 0.9679 [40] were used in the correction of vibrational frequencies calculated by 6-31+G(d) or 6-311++G(d,p) basis set, respectively. The geometry of [Cu(opba)]2 was constrained to the C2v point group. The cutoffs on forces and step size that are used to determine optimization convergence were tightened, and the ultrafine grid option was also used to ensure the calculated values for the low frequency vibrational modes.
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magnetic field of 1.65 T and critical energy of 2.08 keV) at the Brazilian National Synchrotron Light Laboratory, LNLS (electron energy of the storage ring is 1.37 GeV). The SGM beam line (the spectral resolution E/DE is better than 3.000) has a focused beam of, roughly, 0.5 mm2 spot size, and the spectra were recorded in total electron yield (TEY) detection mode, with the sample compartment pressure at 10 8 mbar. Measurements were done with the sample surface normal to the beam. All energy values observed in the N K-edge and Cu L1,2-edge spectra were calibrated using the first resonant peak at 405.5 eV in N K-edge XANES spectra for potassium nitrate [41–43]. The XANES spectra for meta- and para-complexes were not possible to obtain due to the difficult to dry this samples at the point to acquire the spectra in ultra-high vacuum conditions. Results and discussion
A
Vibrational (FTIR and Raman) characterization
Raman IR 1 2
1000
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Fig. 1. Experimental and DFT calculated Raman and FTIR spectra for [C4MIm]2[Cu(opba)] sample. (A) Experimental Raman (laser line 647.1 nm or Elaser = 1.92 eV) and FTIR spectra (spectrum A1, KBr pellets) of [C4MIm]2[Cu(opba)] sample. For comparison purposes the FTIR spectrum of K2[(Cu(opba)] is also given (spectrum A2) and (B) DFT calculated Raman and FTIR spectra for [Cu(opba)]2 anion. The optimized structure of the anion, used in the calculations, is given in the top of the figure. The DFT spectra are shown without adjustment by the numeric factor correction of 0.96 [38,39].
Nitrogen K-edge and Cu L2,3-edge XANES spectra were obtained at the Spherical Grating Monochromator (SGM) beam line (dipole
Fig. 1 shows the experimental and DFT calculated Raman and FTIR spectra for synthesized [C4MIm]2[Cu(opba)] sample. For comparison purposes the FTIR spectrum of K2[(Cu(opba)] is also given in the figure. As can be seen, the experimental FTIR spectra of the K2[(Cu(opba)] complex is dominated by strong bands from 1610 to 1700 cm 1 (spectrum A2). Very similar spectrum is obtained for [C4MIm]2[Cu(opba)] sample. When the experimental data is compared with the calculated FTIR spectrum, the bands from 1610 to 1700 cm 1 can be assigned to mC@O vibrations of the [Cu(opba)]2 anions (see Table 1). Only the band near 1170 cm 1 can be associate to the [C4MIm]+ cation. For comparison purposes the FTIR spectra (experimental and calculated) of [C4MIm]Cl is also shown (see Supporting information Fig. 1) and it can be seen that the strongest band is near 1170 cm 1 can be related to imidazolium cation. In addition, this band is not observed in IR spectrum of K2[(Cu(opba)], it confirms the assignment. In opposition to the FTIR spectra, the main vibrational bands in the Raman spectra of the [C4MIm]2 [Cu(opba)] complex are observed from 1350 to 1600 cm 1. All bands
Table 1 Experimental values observed in the FTIR and in the Raman spectra (laser line 647.1 nm or Elaser = 1.92 eV) for the [C4MIm]2[Cu(opba)] compound. The equilibrium geometry of the [Cu(opba)]2 anion, as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-31+G(d) basis set. The band values (with (bold) and without (italic) adjustment by scale factor of 0.96 [38,39]) and the assignments of the vibrational bands are given. The correlation between calculated and experimental band values was done considering the wavenumber values and relative intensities. The qualitative normal mode description for the vibrational modes was done using the visualization mode of chemcraft v.13 quantum chemistry software in scaled vectors mode. The abbreviations s, m, w, sh, are related to the band intensities, and mean strong, medium, weak and shoulder. i.p. means in-plane modes. The abbreviations m, c and b are related to the kind of vibration modes and mean stretching, torsion and bending, respectively. Raman bands
IR bands
Calculated values B3LYP/631+G(d)
Experimental
Calculated values B3LYP/6-31+G(d)
1708 1693 1669 1596 1512 1479 1414 1341 1312 1207 –
1708 1693 1667 1600 1512 1479 1414 – 1312 – 1174 1187 1052
(A1) (B2) (A1) (A1) (A1) (B2) (A1) (A1) (A1) (A1)
1639 1626 1603 1533 1452 1420 1358 1288 1260 1158 –
1680 1660 1630 1571 1475 1453 1400 1326 1280 1191 –
1169 (A1) 1052 (A1)
1122 1010
1150 1030
Experimental
Qualitative assignments
1639 1626 1600 1536 1452 1420 1358 – 1260 –
1669 (1673)2 1637 (1620) 1620 (1605) 1580 (1575) 1473 (1469) 1458 (1455) 1333 (1357) (1333) (1300) 1292 (1287)
1127 1140 1010
11703 1122 (1151) 1034 (1034)
m(C@O) m(C@O) m(C@O) m/(CAC) + c/(CAH)1 m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) + m(CAN) m/(CAC) + c/(CAH) + m(CAC) m/(CAC) + c/(CAH) + m(CAO) m(CAN) + m(CAO) + b/(CAH) ci-ring + bi-ring(CAH) b/(CAH) b/(CAH)
The underlined values were assigned to the imidazolium ring (see Appendix B). 1 The symbol / means that the vibrations are related to the benzene-like ring in the molecular structure of the [Cu(opba)]2 anion. 2 For comparison purposes the IR bands of the Na2[Cu(opba)] complex are also given between the parentheses. 3 This band shows more signal intensity than any other DFT calculated band in this region. However, the experimental and calculated spectra of the imidazolium ring (see Appendix B) show a strong band in this region (1174 cm 1 band value obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6–311++G(d,p) basis set without adjustment by the scale factor of 0.9679 [40]). Hence, the band at 1170 cm 1 observed for [C4MIm]2[Cu(opba)] sample was associated to the vibration mode comes from the imidazolium ring.
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Fig. 2. Experimental and DFT calculated Raman and FTIR spectra for [C4MIm]4[Cu2(ppba)2] sample. (A) Experimental Raman (laser line 785.0 nm or Elaser = 1.58 eV) and FTIR spectra (KBr pellets) of [C4MIm]4[Cu2(ppba)2] sample and (B) DFT calculated Raman and FTIR spectra for [Cu2(ppba)2]4 anion. The optimized structure of the anion, used in the calculations, is given in the top of the figure. The DFT spectra are shown without adjustment by the scale factor of 0.96 [38,39].
in this region are associated to the modified benzene vibrations of the [Cu(opba)]2 anions (see Table 1). The experimental and calculated Raman bands display a high correspondence. By combining, the Raman and the FTIR data is it possible to see any modifications in the principal chemical groups of the anion molecule. Fig. 2 displays the experimental and DFT calculated Raman and FTIR spectra for [C4MIm]4[Cu2(ppba)2] sample. As can be seen, the experimental FTIR spectra of the [C4MIm]4[Cu2(ppba)2] complex is dominated by strong bands from 1550 to 1700 cm 1 (spectrum A). The presence of adsorbed water is clearly seen by the presence of broad band near to 1640 cm–1, assigned to the O–H bending. From the comparison to the DFT calculated FTIR spectrum of the [C4MIm]4[Cu2(ppba)2] is it possible to attribute the bands from 1550 to 1700 cm–1 to mC@O vibrations of the [Cu2(ppba)2]4 anions (see Table 2). The main Raman bands are observed in a more broad range here than that observed for [C4MIm]2[Cu(opba)] complex. In fact, the experimental data indicated that the [C4MIm]4[Cu2(ppba)2] complex has a centre of symmetry, in accordance to the theoretical model used in the ab initio calculations (see Fig. 2). Fig. 3 displays the experimental and DFT calculated Raman and FTIR spectra for [C4MIm]4[Cu2(mpba)2] sample. Similarly to observed for the ortho and para analogues, the FTIR spectra is dominated by strong bands from 1610 to 1700 cm 1, assigned to mC@O vibrations of the [Cu2(mpba)2]4 anions (see Table 3) and the main Raman bands can be observed in lower wavenumbers. Two different electronic configurations were considered in the computational simulation (singlet and triplet) and the vibrational spectra obtained for triplet configuration of the [Cu2(mpba)2]4 anions showed more similitude with the experimental data (see Fig. 3) and therefore only this data were plotted. In addition, the vibrational bands observed for [C4MIm]4[Cu2(mpba)2] sample are very broad, may be owed to a strong fluorescence. Similarly to that observed for ortho and para complexes, only in the IR spectrum is possible to see a band at near 1170 cm 1 attributed to the imidazolium cation.
Table 2 Experimental values observed in the FTIR and in the Raman spectra (laser line 785.0 nm or Elaser = 1.58 eV) for the [C4MIm]4[Cu2(ppba)2] compound. The equilibrium geometry of the [Cu2(ppba)2]4 anion, as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-31+G(d) basis set. The band values with (bold) and without (italic) adjustment by the scale factor of 0.96 [38,39] and the assignments of the vibrational bands are given. The correlation between calculated and experimental band values was done considering the wavenumber values and relative intensities. The qualitative normal mode description for the vibrational modes was done using the visualization mode of chemcraft v.13 quantum chemistry software in scaled vectors mode. The abbreviations s, m, w, sh, are related to the band intensities, and mean strong, medium, weak and shoulder. i.p. means in-plane modes. The abbreviations m, c and b are related to the kind of vibration modes and mean stretching, torsion and bending, respectively. Raman bands
IR bands
Calculated values B3LYP/ 6-31+G(d)
Experimental
Calculated values B3LYP/ 6-31+G(d)
Experimental
Qualitative assignments
–
–
–
–
1595 1572 1549 – – 1349 – – 1261 – 1170 –
– 1626 1591 – – 1406 – – 1304 – 1195 –
1660 1639 – 1602 1529 – 1395 1322 – 1302 –
1594 1573 – 1538 1468 – 1340 1269 – 1250 –
1640 1607 1578 – 1500 1437 – 1331 1280 – 1260 –
b(OAH)1 m(C@O) m(C@O) m(C@O) + m/(CAC) + c/(CAH) m(C@O) + m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) + m(CAN) m(CAC) + m(CAN) + m(CAO) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAN) + b/(CAH) m(CAN) + b/(CAH) m(CAN) + b/(CAH)
1127 -
1167 –
1127 – 1123
1169 – 1117
m(CAN) + b/(CAH)2 b/(CAH) b/(CAH)
1662 1638 1612 – – 1405 – – 1314 – 1219 –
(A) (A) (A)
(A)
(A) (A)
1173 (A) –
– (A) (A) (A) (A) (A) (A) (A)
1174 – 1170 (A)
The underlined values were assigned to the imidazolium ring (see Appendix B). 1 This band is assigned to the bending vibrational of the adsorbed water molecules; this is consequence of the very high hygroscopic characteristic of the sample. 2 This band shows more signal intensity than any other DFT calculated band in this region. However, the experimental and calculated spectra of the imidazolium ring (see Appendix B) show a strong band in this region (1174 cm 1 band value obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-311++G(d,p) basis set without adjustment by the scale factor of 0.9679 [40]). Hence, the band at 1169 cm 1 observed for [C4MIm]4[Cu2(ppba)2] sample was associated to the vibration mode comes from the imidazolium ring.
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Fig. 3. Experimental and DFT calculated Raman and FTIR spectra for [C4MIm]4[Cu2(mpba)2] sample. (A) Experimental Raman (laser line 632.8 nm or Elaser = 1.96 eV) and FTIR spectra (KBr pellets) of [C4MIm]4[Cu2(mpba)2] sample and (B) DFT calculated Raman and FTIR spectra for [Cu2(mpba)2]4 anion in triplet electronic configuration. The optimized structure of the anion, used in the calculations, is given in the top of the figure. The DFT spectra are shown without adjustment by the scale factor of 0.9679 [40].
Fig. 4 shows the XANES spectra at the Nitrogen K edge for some synthesized copper complexes and some compounds previously studied and used as standards in order to guarantee the reproducibility of the experiment (methyl orange and Cu(NO3)2). The methyl orange N K XANES spectrum (see spectrum A in Fig. 4) is dominated by a strong band at 398.7 eV, assigned to AN@NA nitrogen [44], and the Cu(NO3)2 (see spectrum B in Fig. 4) shows three strong bands associated to NO3 group, this behavior was previously observed by our group and it indicates that the experimental setup was configured in a condition to avoid the sample degradation by the radiation [44]. For comparison purposes the N K XANES spectrum of [C4MIm]Br sample (see spectrum C in Fig. 4) is displayed, the spectrum is dominated by a strong band at 401.9 eV. According to our previous studies this peak is assigned to the 1s ? 2pp⁄ of the nitrogen in the imidazolium ring [27], and the other bands at higher energies are related to 1s ? r⁄ transitions. It can be clearly seen in the spectra of the copper complex containing oxamate anions (spectra D, E and F) strong variations in the relative intensities and also in the position value of the bands. This behavior can be related to the electronic change around the nitrogen atoms bounded to the cation. Probably this behavior is associated to changes in the kind of cation Na+, K+ and [C4MIm]+ and also to the structure of the complex (ortho- and meta-). The oxamate anions bonded to small cations (Na+ and K+, see spectra D, E in Fig. 4) have the lowest energy peak at 399.7 eV. According to our previous studies about the N K XANES spectra for several organic molecules having nitrogen atoms with different oxidation states [25,44], the nitrogen atoms in these oxamate complex, having small cations, are with the oxidation state similar to the @N . The N K XANES spectrum of [C4MIm]2[Cu(opba)] sample (see spectrum F in Fig. 4) shows two main peaks that can be easily associated to 1s ? 2pp⁄ transitions of nitrogen atoms to the oxamate anion (399.3 eV) and also to the imidazolium cation (401.9 eV). Only the peak associated to the oxamate anion is shifted to lower ener-
Table 3 Experimental values observed in the FTIR and in the Raman spectra (laser line 632.8 nm or Elaser = 1.96 eV) for the [C4MIm]4[Cu2(mpba)2] compound. The equilibrium geometry of the [Cu2(mpba)2]4 anion in a triplet state, as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-311++G(d,p) basis set. The band values (with (bold) and without (italic) adjustment by the scale factor of 0.9679 [40]) and the assignments of the vibrational bands are given. The correlation between calculated and experimental band values was done considering the wavenumber values and relative intensities. The qualitative normal mode description for the vibrational modes was done using the visualization mode of chemcraft v.13 quantum chemistry software in scaled vectors mode. The abbreviations s, m, w, sh, are related to the band intensities, and mean strong, medium, weak and shoulder. i.p. means in-plane modes. The abbreviations m, c and b are related to the kind of vibration modes and mean stretching, torsion and bending, respectively. In this table just bands due to triplet configuration were considered. Raman bands
IR bands
Calculated values B3LYP/ 6-311++G(d,p)
Experimental
Calculated values B3LYP/6-311++G(d,p)
Experimental
Qualitative assignments
1682 1676 1609 1608 1590 1500 1441 1387 – 1377 1317 1278 1219 1145 1009
– – 1593 1568 1506 1440 1406 1386 – 1368 1334 1265 1185 1110 1022
1682 1676 1609 1608 – 1514 – – 1379 – 1317 1278
1640 1619 1598 1574 – 1478 1448 1419 1340/1328 – 1167 – – – –
m(C@O)1 m(C@O) m(C@O) + m/(CAC) + c/(CAH) m(C@O) + m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH)2 m(CAN) + b/(CAH) m(CAN) + b/(CAH)
(A) (A) (A) (A) (A)
1628 1622 1557 1556 1539 1452 1395 1342 – 1333 1275 1237 1180 1108 977
– –
1628 1622 1557 1556 – 1452 – – 1334 – 1275 1237 – – –
b/(CAH) Breathing/(CAH)
The underlined values were assigned to the imidazolium ring (see Appendix B). 1 This band is assigned to the bending vibrational of the adsorbed water molecules; this is consequence of the very high hygroscopic characteristic of the sample. 2 This band shows more signal intensity than any other DFT calculated band in this region. However, the experimental and calculated spectra of the imidazolium ring (see Appendix B) show a strong band in this region (1174 cm 1 band value obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-311++G(d,p) basis set without adjustment by the scale factor of 0.9679 [40]). Hence, the band at 1167 cm 1observed for [C4MIm]4[Cu2(mpba)2] sample was associated to the vibration mode comes from the imidazolium ring.
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It clearly observed that Cu atoms in oxamate ion (see spectra B and C in Fig. 5) show peaks in higher energies (931.7 eV and 951.7 eV) compared to the standard compound. These two peaks are shifted a little bit more for [C4MIm]2[Cu(opba)] sample (932.0 eV and 952.0 eV), it indicates that the presence of imidazolium cation changes the electronic structure around the Cu atoms in the complex oxamate anions. Hence, the N K and Cu L2,3 XANES data indicates that the imidazolium cation changes the electronic structure around the ACuANA sites in the oxamate anion. Probably this interaction can cause changes in the magnetic behavior of the copper-oxamate complexes. Contrarily to that observed in the vibrational characterization, where mainly C@O and benzene rings can be easily monitored, in the XANES spectra is it possible to monitor the change in the electronic structure around the ACuANA sites. In addition, it can be seen in the XANES of the [C4MIm]2[Cu(opba)] sample (spectrum F in Fig. 4) a clear presence of the strong transition peak 1s ? 2pp⁄ at 401.9 eV that is assigned to the 1s ? 2pp⁄ transition of the nitrogen in the imidazolium ring (for comparison purposes see the spectrum for the [C4MIm]Br sample in Fig. 4) [27,42,43].
F
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The vibrational data of all compounds clearly show mainly bands associated to the mC@O and benzene ring modes of the oxamate anions, but the presence of the imidazolic cation is not so clear. On contrary, the XANES spectra show clearly the presence of the imidazolic cation and also the influence of the cation over the electronic density of the ACuANA sites in the copper-oxamate anions. XANES data show that the anion–cation interaction is not weak, as previously observed by us for very small anions, like Br and Cl [27,42], probably due to the large structure of the anion copper-oxamate. Hence, this behavior suggests that the magnetic properties of these copper-oxamate-imidazolium samples can be changed by the kind of imidazolium cation bonded in the system. In fact, the magnetic properties of these compounds are under investigation in our lab. Acknowledgments
C B A 910
920
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Energy/ eV Fig. 5. Cu L2,3 XANES spectra of: (A) Cu(NO3)2; (B) Na4[Cu2(mpba)2]; (C) K2[Cu(opba)]; and (D) [C4MIm]2[Cu(opba)].
This work was supported by CNPq, FAPEMIG, CAPES and FAPESP (Brazilian agencies). G.M. do Nascimento acknowledges CAPES for his post-doctoral fellowship (PNPD program) and N.A. Pradie acknowledges CNPq for their doctoral fellowship. These authors are indebted to the LCCA (Laboratory of Advanced Scientific Computation of the University of São Paulo) for the use of its computational resources. The authors would like to thank the LNLS for the use of SGM beam line (Proposal No 10685 and 9281) and XAFS1 beam line (Proposal No 2706). Special thanks to Dr. Fabio Rodrigues and Dr. Douglas Galante for their helpful support during the XANES and some Raman experiments. Appendix A. Synthesis of the [C4MIm]2Cu(opba)3H2O compound
gies, it indicates the changes could be occurring around the CuAN bonding. Indeed, higher charger delocalization or changes around the Cu coordination geometry owed to the presence of imidazolium cation could be responsible for this behavior. Fig. 5 shows the Cu L2,3-edge XANES spectra. The Cu L2,3 XANES spectra are sensible to the valence state, ionic character, and ligand field and coordination state of Cu species [45,46]. The Cu L-edge XANES spectrum (see Fig. 5) for Cu(NO3)2 was used as reference, the spectrum is dominated by two sharp bands (white lines), the first one at 930.0 eV is assigned to 2p3/2 ? 3d transitions and the second one at 950.0 eV can be assigned to 2p3/2 ? 3d transitions.
2.39 g (3.00 mmol) of the [Bu4N]2Cu(opba) crystalline powder were dissolved in 50.0 mL of deionized water and afterwards 1.47 g (6.00 mmol) of the [C4MIm]ClO40.25H2O were added to the solution and immediately a white flocculent powder was precipitated. This mixture was left under stirring for 1 h at room temperature. Afterwards, the mixture was filtered and the precipitate was washed with deionized water, dried in a desiccator under vacuum. The preparation of the [Bu4N]ClO4 salt was confirmed by FTIR data. The bluish filtered solution was extracted with 5.0 mL of dichloromethane for elimination of the non-reacting
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Wavenumber/ cm-1 Additional Fig. 1. Experimental and DFT calculated FTIR spectra for [C4MIm]Cl sample. A. Experimental FTIR spectrum (KBr pellets) of [C4MIm]Cl sample and B. DFT calculated FTIR spectrum for [C4MIm]+ cation. The equilibrium geometry of the [C4MIm]+cation, as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software30 using B3LYP density functional theory and 6-311++G(d,p) basis set. The optimized structure of the cation, used in the calculations, is given in the top of the figure. The DFT spectra are shown without adjustment by the scale factor of 0.9679.38
reagents and byproducts that are very soluble in this solvent. Then, the volume of the resulting solution was reduced in a rotating evaporator until the formation a very viscous liquid. This material was dried under vacuum and a very deliquescent purple solid was formed. The reaction yield was about 71.0%. Appendix B. Synthesis of the [C4MIm]4[Cu2(mpba)2] compound A total of 0.308 g (1.00 mmol) of the Et2H2mpba solid was suspended in 25.0 mL of deionized water and it was added 2.60 mL (4.00 mmol) of a solution of Bu4NOH (40.0% in water, tetra-nbutyl-ammonium hydroxide), the resulting solution was left under stirring at 60.0 °C for 30 min. After cooling, it was slowly dropped 5.00 mL of the aqueous solution containing 0.169 g (1.00 mmol) of the CuCl22H2O. After complete mixture, it was added 0.486 g (2.00 mmol) of the [C4MIm]ClO40.25H2O to the solution above. A flocculent white solid was formed. The solution was left under stirring for one hour and after the precipitate was separated from the solution by filtration. The greenish filtered solution was extracted with 5.0 mL of dichloromethane for elimination of the non-reacting reagents and byproducts that are very soluble in this solvent. Then, the volume of the resulting solution was reduced in a rotating evaporator until the formation a very viscous liquid. This liquid was washed with tetrahydrofuran (THF) until the solvent was colorless. The material was dried under vacuum for 2 days and a very viscous liquid was formed. The reaction yield was about 62.0%. Appendix C. Synthesis of [C4MIm]4[Cu2(ppba)2] compound The preparation of [C4MIm]4[Cu2(ppba)2] was similar to the synthesis described above for [C4MIm]4[Cu2(mpba)2]. For the for-
mation of the final product was added 0.486 g (2.00 mmol) of the [C4MIm]ClO40.25H2O to the solution containing [Bu4N]+ and [Cu2(ppba)2]4–. A flocculent white solid was formed. The solution was left under stirring for 1 h and after the precipitated was separated from the solution by filtration. The greenish filtered solution was extracted with 5.0 mL of dichloromethane for elimination of the non-reacting reagents and byproducts that are very soluble in this solvent. Hence, the volume of the resulting solution was reduced in a rotating evaporator until the formation a very viscous liquid. This liquid was washed with acetonitrile. The material was dried under vacuum for 1 days and a green deliquescent solid was formed. The reaction yield was about 65.0%.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Characterization of compounds derived from copper-oxamate and imidazolium by X-ray absorption and vibrational spectroscopies Gustavo M. do Nascimento a,b,⇑, Walace D. do Pim a,c, Daniella O. Reis a, Tatiana R.G. Simões a, Noriberto A. Pradie d, Humberto O. Stumpf a a
Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Brazil Centro de Ciências Naturais e Humanas (CCNH), Universidade Federal do ABC, Brazil Centro Federal de Educação Tecnológica de Minas Gerais, CEFET-MG, Brazil d Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Brazil b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Salts of copper-oxamate anions and
imidazolium cations were studied. Raman and IR band assignments were
supported by DFT calculations. Electronic distribution around
ACuANA sites is changed by anion– cation interaction.
a r t i c l e
i n f o
Article history: Received 29 September 2014 Received in revised form 9 December 2014 Accepted 4 February 2015 Available online 12 February 2015 Keywords: Ionic liquids Imidazolium Raman XANES DFT calculations
a b s t r a c t In this work, compounds derived from copper-oxamate anions (ortho, meta, and para)-phenylenebis (oxamate) and imidazolium cations (1-butyl-3-methylimidazolium) were synthesized. The compounds were characterized by Raman and FTIR spectroscopies and the band assignments were supported by DFT calculations. Strong IR bands from 1610 to 1700 cm 1 dominated the spectra of the complex and can be assigned to mC@O vibrations of the [Cu(opba)]2 anions by the comparison with the DFT data. In opposition to the FTIR spectra, the main vibrational bands in the Raman spectra are observed in the 1350– 1600 cm 1 range. All bands in this region are associated to the modified benzene vibrations of the copper-phenylenebis(oxamate) anions. X-ray absorption near edge (XANES) at different energies (NK and Cu L2,3 edges) was also used to probe the interionic interactions. XANES data show that anion–cation interaction in the Cu-oxamate–imidazolium changes the electronic structure around the ACuANA sites in the oxamate anion. Ó 2015 Elsevier B.V. All rights reserved.
Introduction The emerging field of molecular magnetism has the potential to design unparallel new magnetic materials which can exhibit bulk ⇑ Corresponding author at: Centro de Ciências Naturais e Humanas (CCNH), Universidade Federal do ABC, Brazil. E-mail address: [email protected] (G.M. do Nascimento). http://dx.doi.org/10.1016/j.saa.2015.02.012 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
physical properties, such as long-range magnetic ordering. Our group has been developed a series of copper complexes with phenylenebis(oxamate) derivates as ligands [1–8]. In addition, the combination of different cations to these anionic copper-complexes can change the crystal packing and also the magnetic behavior. Nowadays the family of salts derived from imidazolium cations (known as ionic liquids, ILs) open up the possibility to modulate the
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structural characteristics of the cations bonded to the copperphenylene-bis(oxamate) anions in order to obtain molecule-based magnets with new behavior and in a liquid state. Ionic liquids (ILs) are a broad family of salts that are liquid at temperatures lower than 100 °C. Commonly the ionic liquids are composed by a large cation and a weakly coordinating anion [9]. There are a very large range of combinations between cations and anions those can form ionic liquids, about 1018 different combinations [9]. The ionic liquids derived from imidazolic ring are the most studied ones [10,11]. Parameters like the carbonic chain length bonded to the cation or the anion type can drastically change the properties of the ILs, thus being possible to achieve a desired function by designing the structure of a specific IL [12]. In most cases, ILs have high thermal stability, a broad electrochemical window, an organized structure at medium distances (in opposition to molecular liquids, that are organized at small distances) and very low vapor pressure, properties that make them important in many areas like green chemistry, organic and inorganic synthesis and many others [13]. The ILs are salts with low melting points and the most unusual characteristic of these systems is that, although they are liquids, they present features similar to solids, such as structural organization at intermediate distances and negligible vapor pressure [9]. Our group start the preparation of compounds derived from Cu(ortho, meta, and para)-phenylenebis (oxamate) anions combined with different imidazolium cations. The main objective is to prepare compounds derived from ionic liquids with different magnetic behavior. In addition, the presence of a carbonic chain opens the possibility to enhance the interactions between the metal-complex and carbon nanotubes [14,15]. The electronic interactions between the [Cu(opba)]2 anions (where opba = orthophenylenebis(oxamate)) and single wall carbon nanotubes (SWCNTs) were recently investigated by our group [14–16]. It is observed that the electronic interactions show a dependence on the SWCNT diameter, being the interaction stronger for metallic tubes. This interaction is also influenced by the amount of complex that is probably adsorbed on the carbon surface of the SWCNTs. Some charge transfer can be also occurring between the metallic complex and the SWCNTs. Hence, the characterization of the vibrational and electronic signatures of these new ionic liquids is crucial for their future use with single or double walled carbon tubes [17,18]. The Raman and FTIR are commonly used as fundamental techniques in the study of vibrational behavior of these kinds of compounds. The X-ray absorption spectroscopy near edge spectroscopy (XANES) involves the excitation of ‘‘core’’ electrons of the atoms [19]. Each absorption energy edge is related to a specific atom present in the material and, more specifically, to a quantum-mechanical transition that excites an electron in a particular atomic core-orbital to the free or unoccupied continuum levels (ionization of the core orbital above the Fermi energy). In addition, the wavelength associated to the photoelectron formed by the absorption process is higher than the atomic distances. Thus, the mean free path of the photoelectron is sufficient to cause multiple scatterings; this is the main characteristic of the XANES regime [20]. This phenomenon turns the XANES spectra sensible to the electronic density around the absorbing atom, implying that it can be used to monitor the oxidation state and also the density of unoccupied electronic states [21]. In fact, the XANES spectra, particularly for molecules with high electronic delocalization, are very complicated, because many pre-edge peaks can appear due to the resonance and/or conjugation effects [22–27]. XANES spectroscopy was used for investigation of the electronic structure around the Cu atoms in the copper-oxamate anions ((ortho, meta, and para)-phenylenebis(oxamate)) bonded to the imidazolium cations (1-butyl-3-methylimidazolium). For this trend, XANES spectra at Cu L-edge and N K-edge were used for
investigation the changes in the electronic densities around Cu and N atoms in these new copper complexes. Hence, in the present work, a vibrational (Raman and FTIR) and electronic (UV–Vis, XANES and XPS) spectroscopies were used for investigation of the ionic liquids derived from copper-oxamate anions ((ortho, meta, and para)-phenylenebis(oxamate)) and imidazolium cations (1-butyl-3-methylimidazolium). The experimental findings are supported by DFT calculations. Experimental methods Reagents The water used in the experiments was deionized from a MilliQ (Millipore System) system. The dichloromethane (Synth), acetonitrile (Synth) and CuCl22H2O (Sigma–Aldrich) were analytical grade and they were used as received without any further purification. Tetrahydrofuran (THF, Synth) was treated with CaCl2 and metallic sodium for removing the water, as described by Morita [28]. The [Bu4N]2Cu(opba) (where Bu4N = tetra-n-butyl-ammonium and opba = ortho-phenylenebis(oxamate)) material was prepared according to the Ref. [2]. The Et2H2mpba (where mpba = meta-phenylenebis(oxamate)) material was prepared according to the Ref. [29], the Et2H2ppba (where ppba = para-phenylenebis (oxamate)) material was prepared according to the Ref. [30], and the [C4MIm]ClO4 was prepared according to the Ref. [31]. In this work, imidazolic ILs will be abbreviated as [CnMIm]+ for 1-alkyl3-methylimidazolium, where n is the number of carbons in the alkyl chain. The description of the synthetic routes used to acquire the [C4MIm]2Cu(opba)3H2O, [C4MIm]4[Cu2(mpba)2], and [C4MIm]4[Cu2(ppba)2] compounds were described in the Appendix A. Instrumentation Raman spectra for the ILs samples at 647.1 nm (1.92 eV, Kr+ laser) were taken with a triple monochromator, XY Dilor Micro-Raman System, equipped with a CCD detector. For Raman spectra at 785.0 nm (1.58 eV, solid state laser) were taken with a Senterra Bruker Raman spectrometer, equipped with a CCD detector, and finally for Raman spectra at 632.8 nm (1.96 eV, He–Ne laser) were taken with a Renishaw in via Raman spectrometer, equipped with a CCD detector. All Raman spectra were collected at room temperature and using a backscattering geometry and the laser line was focused on the sample using a 50 objective and the power incident on the sample was kept lower than 2 mW to avoid heating effects. Different acquisition times between 5 and 30 s were used for each sample in an attempt to optimize the signal-to-noise ratio of the Raman spectra. Different laser lines were used in order to avoid some fluorescence and/or to maximize the signal-to-noise relation for each IL sample. The equilibrium geometry of the [Cu(opba)]2 , [Cu2(mpba)2]4 , [Cu2(ppba)2]4 , and [C4MIm]+ as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software [32] using B3LYP density functional theory. This method uses Becker’s three-parameter exchange functional (B3) [33,34], in combination with the (LYP) correlation functional [35]. The calculations were done using the 6-31+G(d) or 6311++G(d,p) basis set [36,37]. The scale factor values of 0.96 [38,39] or 0.9679 [40] were used in the correction of vibrational frequencies calculated by 6-31+G(d) or 6-311++G(d,p) basis set, respectively. The geometry of [Cu(opba)]2 was constrained to the C2v point group. The cutoffs on forces and step size that are used to determine optimization convergence were tightened, and the ultrafine grid option was also used to ensure the calculated values for the low frequency vibrational modes.
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magnetic field of 1.65 T and critical energy of 2.08 keV) at the Brazilian National Synchrotron Light Laboratory, LNLS (electron energy of the storage ring is 1.37 GeV). The SGM beam line (the spectral resolution E/DE is better than 3.000) has a focused beam of, roughly, 0.5 mm2 spot size, and the spectra were recorded in total electron yield (TEY) detection mode, with the sample compartment pressure at 10 8 mbar. Measurements were done with the sample surface normal to the beam. All energy values observed in the N K-edge and Cu L1,2-edge spectra were calibrated using the first resonant peak at 405.5 eV in N K-edge XANES spectra for potassium nitrate [41–43]. The XANES spectra for meta- and para-complexes were not possible to obtain due to the difficult to dry this samples at the point to acquire the spectra in ultra-high vacuum conditions. Results and discussion
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-1
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Fig. 1. Experimental and DFT calculated Raman and FTIR spectra for [C4MIm]2[Cu(opba)] sample. (A) Experimental Raman (laser line 647.1 nm or Elaser = 1.92 eV) and FTIR spectra (spectrum A1, KBr pellets) of [C4MIm]2[Cu(opba)] sample. For comparison purposes the FTIR spectrum of K2[(Cu(opba)] is also given (spectrum A2) and (B) DFT calculated Raman and FTIR spectra for [Cu(opba)]2 anion. The optimized structure of the anion, used in the calculations, is given in the top of the figure. The DFT spectra are shown without adjustment by the numeric factor correction of 0.96 [38,39].
Nitrogen K-edge and Cu L2,3-edge XANES spectra were obtained at the Spherical Grating Monochromator (SGM) beam line (dipole
Fig. 1 shows the experimental and DFT calculated Raman and FTIR spectra for synthesized [C4MIm]2[Cu(opba)] sample. For comparison purposes the FTIR spectrum of K2[(Cu(opba)] is also given in the figure. As can be seen, the experimental FTIR spectra of the K2[(Cu(opba)] complex is dominated by strong bands from 1610 to 1700 cm 1 (spectrum A2). Very similar spectrum is obtained for [C4MIm]2[Cu(opba)] sample. When the experimental data is compared with the calculated FTIR spectrum, the bands from 1610 to 1700 cm 1 can be assigned to mC@O vibrations of the [Cu(opba)]2 anions (see Table 1). Only the band near 1170 cm 1 can be associate to the [C4MIm]+ cation. For comparison purposes the FTIR spectra (experimental and calculated) of [C4MIm]Cl is also shown (see Supporting information Fig. 1) and it can be seen that the strongest band is near 1170 cm 1 can be related to imidazolium cation. In addition, this band is not observed in IR spectrum of K2[(Cu(opba)], it confirms the assignment. In opposition to the FTIR spectra, the main vibrational bands in the Raman spectra of the [C4MIm]2 [Cu(opba)] complex are observed from 1350 to 1600 cm 1. All bands
Table 1 Experimental values observed in the FTIR and in the Raman spectra (laser line 647.1 nm or Elaser = 1.92 eV) for the [C4MIm]2[Cu(opba)] compound. The equilibrium geometry of the [Cu(opba)]2 anion, as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-31+G(d) basis set. The band values (with (bold) and without (italic) adjustment by scale factor of 0.96 [38,39]) and the assignments of the vibrational bands are given. The correlation between calculated and experimental band values was done considering the wavenumber values and relative intensities. The qualitative normal mode description for the vibrational modes was done using the visualization mode of chemcraft v.13 quantum chemistry software in scaled vectors mode. The abbreviations s, m, w, sh, are related to the band intensities, and mean strong, medium, weak and shoulder. i.p. means in-plane modes. The abbreviations m, c and b are related to the kind of vibration modes and mean stretching, torsion and bending, respectively. Raman bands
IR bands
Calculated values B3LYP/631+G(d)
Experimental
Calculated values B3LYP/6-31+G(d)
1708 1693 1669 1596 1512 1479 1414 1341 1312 1207 –
1708 1693 1667 1600 1512 1479 1414 – 1312 – 1174 1187 1052
(A1) (B2) (A1) (A1) (A1) (B2) (A1) (A1) (A1) (A1)
1639 1626 1603 1533 1452 1420 1358 1288 1260 1158 –
1680 1660 1630 1571 1475 1453 1400 1326 1280 1191 –
1169 (A1) 1052 (A1)
1122 1010
1150 1030
Experimental
Qualitative assignments
1639 1626 1600 1536 1452 1420 1358 – 1260 –
1669 (1673)2 1637 (1620) 1620 (1605) 1580 (1575) 1473 (1469) 1458 (1455) 1333 (1357) (1333) (1300) 1292 (1287)
1127 1140 1010
11703 1122 (1151) 1034 (1034)
m(C@O) m(C@O) m(C@O) m/(CAC) + c/(CAH)1 m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) + m(CAN) m/(CAC) + c/(CAH) + m(CAC) m/(CAC) + c/(CAH) + m(CAO) m(CAN) + m(CAO) + b/(CAH) ci-ring + bi-ring(CAH) b/(CAH) b/(CAH)
The underlined values were assigned to the imidazolium ring (see Appendix B). 1 The symbol / means that the vibrations are related to the benzene-like ring in the molecular structure of the [Cu(opba)]2 anion. 2 For comparison purposes the IR bands of the Na2[Cu(opba)] complex are also given between the parentheses. 3 This band shows more signal intensity than any other DFT calculated band in this region. However, the experimental and calculated spectra of the imidazolium ring (see Appendix B) show a strong band in this region (1174 cm 1 band value obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6–311++G(d,p) basis set without adjustment by the scale factor of 0.9679 [40]). Hence, the band at 1170 cm 1 observed for [C4MIm]2[Cu(opba)] sample was associated to the vibration mode comes from the imidazolium ring.
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Absorbance Intensity/ a.u.
306
B Raman
IR
Absorbance Intensity/ a.u.
1000
A
1100
1200
1300
1400
1500
1600
1700
1800
1300
1400
1500
1600
1700
1800
Raman
IR
1000
1100
1200
Wavenumber/ cm-1
Fig. 2. Experimental and DFT calculated Raman and FTIR spectra for [C4MIm]4[Cu2(ppba)2] sample. (A) Experimental Raman (laser line 785.0 nm or Elaser = 1.58 eV) and FTIR spectra (KBr pellets) of [C4MIm]4[Cu2(ppba)2] sample and (B) DFT calculated Raman and FTIR spectra for [Cu2(ppba)2]4 anion. The optimized structure of the anion, used in the calculations, is given in the top of the figure. The DFT spectra are shown without adjustment by the scale factor of 0.96 [38,39].
in this region are associated to the modified benzene vibrations of the [Cu(opba)]2 anions (see Table 1). The experimental and calculated Raman bands display a high correspondence. By combining, the Raman and the FTIR data is it possible to see any modifications in the principal chemical groups of the anion molecule. Fig. 2 displays the experimental and DFT calculated Raman and FTIR spectra for [C4MIm]4[Cu2(ppba)2] sample. As can be seen, the experimental FTIR spectra of the [C4MIm]4[Cu2(ppba)2] complex is dominated by strong bands from 1550 to 1700 cm 1 (spectrum A). The presence of adsorbed water is clearly seen by the presence of broad band near to 1640 cm–1, assigned to the O–H bending. From the comparison to the DFT calculated FTIR spectrum of the [C4MIm]4[Cu2(ppba)2] is it possible to attribute the bands from 1550 to 1700 cm–1 to mC@O vibrations of the [Cu2(ppba)2]4 anions (see Table 2). The main Raman bands are observed in a more broad range here than that observed for [C4MIm]2[Cu(opba)] complex. In fact, the experimental data indicated that the [C4MIm]4[Cu2(ppba)2] complex has a centre of symmetry, in accordance to the theoretical model used in the ab initio calculations (see Fig. 2). Fig. 3 displays the experimental and DFT calculated Raman and FTIR spectra for [C4MIm]4[Cu2(mpba)2] sample. Similarly to observed for the ortho and para analogues, the FTIR spectra is dominated by strong bands from 1610 to 1700 cm 1, assigned to mC@O vibrations of the [Cu2(mpba)2]4 anions (see Table 3) and the main Raman bands can be observed in lower wavenumbers. Two different electronic configurations were considered in the computational simulation (singlet and triplet) and the vibrational spectra obtained for triplet configuration of the [Cu2(mpba)2]4 anions showed more similitude with the experimental data (see Fig. 3) and therefore only this data were plotted. In addition, the vibrational bands observed for [C4MIm]4[Cu2(mpba)2] sample are very broad, may be owed to a strong fluorescence. Similarly to that observed for ortho and para complexes, only in the IR spectrum is possible to see a band at near 1170 cm 1 attributed to the imidazolium cation.
Table 2 Experimental values observed in the FTIR and in the Raman spectra (laser line 785.0 nm or Elaser = 1.58 eV) for the [C4MIm]4[Cu2(ppba)2] compound. The equilibrium geometry of the [Cu2(ppba)2]4 anion, as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-31+G(d) basis set. The band values with (bold) and without (italic) adjustment by the scale factor of 0.96 [38,39] and the assignments of the vibrational bands are given. The correlation between calculated and experimental band values was done considering the wavenumber values and relative intensities. The qualitative normal mode description for the vibrational modes was done using the visualization mode of chemcraft v.13 quantum chemistry software in scaled vectors mode. The abbreviations s, m, w, sh, are related to the band intensities, and mean strong, medium, weak and shoulder. i.p. means in-plane modes. The abbreviations m, c and b are related to the kind of vibration modes and mean stretching, torsion and bending, respectively. Raman bands
IR bands
Calculated values B3LYP/ 6-31+G(d)
Experimental
Calculated values B3LYP/ 6-31+G(d)
Experimental
Qualitative assignments
–
–
–
–
1595 1572 1549 – – 1349 – – 1261 – 1170 –
– 1626 1591 – – 1406 – – 1304 – 1195 –
1660 1639 – 1602 1529 – 1395 1322 – 1302 –
1594 1573 – 1538 1468 – 1340 1269 – 1250 –
1640 1607 1578 – 1500 1437 – 1331 1280 – 1260 –
b(OAH)1 m(C@O) m(C@O) m(C@O) + m/(CAC) + c/(CAH) m(C@O) + m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) + m(CAN) m(CAC) + m(CAN) + m(CAO) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAN) + b/(CAH) m(CAN) + b/(CAH) m(CAN) + b/(CAH)
1127 -
1167 –
1127 – 1123
1169 – 1117
m(CAN) + b/(CAH)2 b/(CAH) b/(CAH)
1662 1638 1612 – – 1405 – – 1314 – 1219 –
(A) (A) (A)
(A)
(A) (A)
1173 (A) –
– (A) (A) (A) (A) (A) (A) (A)
1174 – 1170 (A)
The underlined values were assigned to the imidazolium ring (see Appendix B). 1 This band is assigned to the bending vibrational of the adsorbed water molecules; this is consequence of the very high hygroscopic characteristic of the sample. 2 This band shows more signal intensity than any other DFT calculated band in this region. However, the experimental and calculated spectra of the imidazolium ring (see Appendix B) show a strong band in this region (1174 cm 1 band value obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-311++G(d,p) basis set without adjustment by the scale factor of 0.9679 [40]). Hence, the band at 1169 cm 1 observed for [C4MIm]4[Cu2(ppba)2] sample was associated to the vibration mode comes from the imidazolium ring.
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307
Absorbance Intensity/ a.u.
X-ray absorption characterization
B
IR
1000
Absorbance Intensity/ a.u.
Raman
1100
1200
1300
1400
1500
1600
1700
1800
1200
1300
1400
1500
1600
1700
1800
A Raman
IR 1000
1100
Wavenumber/ cm-1
Fig. 3. Experimental and DFT calculated Raman and FTIR spectra for [C4MIm]4[Cu2(mpba)2] sample. (A) Experimental Raman (laser line 632.8 nm or Elaser = 1.96 eV) and FTIR spectra (KBr pellets) of [C4MIm]4[Cu2(mpba)2] sample and (B) DFT calculated Raman and FTIR spectra for [Cu2(mpba)2]4 anion in triplet electronic configuration. The optimized structure of the anion, used in the calculations, is given in the top of the figure. The DFT spectra are shown without adjustment by the scale factor of 0.9679 [40].
Fig. 4 shows the XANES spectra at the Nitrogen K edge for some synthesized copper complexes and some compounds previously studied and used as standards in order to guarantee the reproducibility of the experiment (methyl orange and Cu(NO3)2). The methyl orange N K XANES spectrum (see spectrum A in Fig. 4) is dominated by a strong band at 398.7 eV, assigned to AN@NA nitrogen [44], and the Cu(NO3)2 (see spectrum B in Fig. 4) shows three strong bands associated to NO3 group, this behavior was previously observed by our group and it indicates that the experimental setup was configured in a condition to avoid the sample degradation by the radiation [44]. For comparison purposes the N K XANES spectrum of [C4MIm]Br sample (see spectrum C in Fig. 4) is displayed, the spectrum is dominated by a strong band at 401.9 eV. According to our previous studies this peak is assigned to the 1s ? 2pp⁄ of the nitrogen in the imidazolium ring [27], and the other bands at higher energies are related to 1s ? r⁄ transitions. It can be clearly seen in the spectra of the copper complex containing oxamate anions (spectra D, E and F) strong variations in the relative intensities and also in the position value of the bands. This behavior can be related to the electronic change around the nitrogen atoms bounded to the cation. Probably this behavior is associated to changes in the kind of cation Na+, K+ and [C4MIm]+ and also to the structure of the complex (ortho- and meta-). The oxamate anions bonded to small cations (Na+ and K+, see spectra D, E in Fig. 4) have the lowest energy peak at 399.7 eV. According to our previous studies about the N K XANES spectra for several organic molecules having nitrogen atoms with different oxidation states [25,44], the nitrogen atoms in these oxamate complex, having small cations, are with the oxidation state similar to the @N . The N K XANES spectrum of [C4MIm]2[Cu(opba)] sample (see spectrum F in Fig. 4) shows two main peaks that can be easily associated to 1s ? 2pp⁄ transitions of nitrogen atoms to the oxamate anion (399.3 eV) and also to the imidazolium cation (401.9 eV). Only the peak associated to the oxamate anion is shifted to lower ener-
Table 3 Experimental values observed in the FTIR and in the Raman spectra (laser line 632.8 nm or Elaser = 1.96 eV) for the [C4MIm]4[Cu2(mpba)2] compound. The equilibrium geometry of the [Cu2(mpba)2]4 anion in a triplet state, as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-311++G(d,p) basis set. The band values (with (bold) and without (italic) adjustment by the scale factor of 0.9679 [40]) and the assignments of the vibrational bands are given. The correlation between calculated and experimental band values was done considering the wavenumber values and relative intensities. The qualitative normal mode description for the vibrational modes was done using the visualization mode of chemcraft v.13 quantum chemistry software in scaled vectors mode. The abbreviations s, m, w, sh, are related to the band intensities, and mean strong, medium, weak and shoulder. i.p. means in-plane modes. The abbreviations m, c and b are related to the kind of vibration modes and mean stretching, torsion and bending, respectively. In this table just bands due to triplet configuration were considered. Raman bands
IR bands
Calculated values B3LYP/ 6-311++G(d,p)
Experimental
Calculated values B3LYP/6-311++G(d,p)
Experimental
Qualitative assignments
1682 1676 1609 1608 1590 1500 1441 1387 – 1377 1317 1278 1219 1145 1009
– – 1593 1568 1506 1440 1406 1386 – 1368 1334 1265 1185 1110 1022
1682 1676 1609 1608 – 1514 – – 1379 – 1317 1278
1640 1619 1598 1574 – 1478 1448 1419 1340/1328 – 1167 – – – –
m(C@O)1 m(C@O) m(C@O) + m/(CAC) + c/(CAH) m(C@O) + m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) m/(CAC) + c/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH) m(CAC) + m(CAN) + m(CAO) + b/(CAH)2 m(CAN) + b/(CAH) m(CAN) + b/(CAH)
(A) (A) (A) (A) (A)
1628 1622 1557 1556 1539 1452 1395 1342 – 1333 1275 1237 1180 1108 977
– –
1628 1622 1557 1556 – 1452 – – 1334 – 1275 1237 – – –
b/(CAH) Breathing/(CAH)
The underlined values were assigned to the imidazolium ring (see Appendix B). 1 This band is assigned to the bending vibrational of the adsorbed water molecules; this is consequence of the very high hygroscopic characteristic of the sample. 2 This band shows more signal intensity than any other DFT calculated band in this region. However, the experimental and calculated spectra of the imidazolium ring (see Appendix B) show a strong band in this region (1174 cm 1 band value obtained with the Gaussian 03 software [32] using B3LYP density functional theory and 6-311++G(d,p) basis set without adjustment by the scale factor of 0.9679 [40]). Hence, the band at 1167 cm 1observed for [C4MIm]4[Cu2(mpba)2] sample was associated to the vibration mode comes from the imidazolium ring.
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It clearly observed that Cu atoms in oxamate ion (see spectra B and C in Fig. 5) show peaks in higher energies (931.7 eV and 951.7 eV) compared to the standard compound. These two peaks are shifted a little bit more for [C4MIm]2[Cu(opba)] sample (932.0 eV and 952.0 eV), it indicates that the presence of imidazolium cation changes the electronic structure around the Cu atoms in the complex oxamate anions. Hence, the N K and Cu L2,3 XANES data indicates that the imidazolium cation changes the electronic structure around the ACuANA sites in the oxamate anion. Probably this interaction can cause changes in the magnetic behavior of the copper-oxamate complexes. Contrarily to that observed in the vibrational characterization, where mainly C@O and benzene rings can be easily monitored, in the XANES spectra is it possible to monitor the change in the electronic structure around the ACuANA sites. In addition, it can be seen in the XANES of the [C4MIm]2[Cu(opba)] sample (spectrum F in Fig. 4) a clear presence of the strong transition peak 1s ? 2pp⁄ at 401.9 eV that is assigned to the 1s ? 2pp⁄ transition of the nitrogen in the imidazolium ring (for comparison purposes see the spectrum for the [C4MIm]Br sample in Fig. 4) [27,42,43].
F
Intensity/ arb. units
E D C
B
A
Conclusion 390
395
400
405
410
415
420
425
430
Energy/ eV Fig. 4. N K XANES spectra of: (A) methyl orange; (B) Cu(NO3)2; (C) [C4MIm][Br]; (D) Na4[Cu2(mpba)2]; (E) K2[Cu(opba)]; and (F) [C4MIm]2[Cu(opba)].
Intensity/ arb. units
D
The vibrational data of all compounds clearly show mainly bands associated to the mC@O and benzene ring modes of the oxamate anions, but the presence of the imidazolic cation is not so clear. On contrary, the XANES spectra show clearly the presence of the imidazolic cation and also the influence of the cation over the electronic density of the ACuANA sites in the copper-oxamate anions. XANES data show that the anion–cation interaction is not weak, as previously observed by us for very small anions, like Br and Cl [27,42], probably due to the large structure of the anion copper-oxamate. Hence, this behavior suggests that the magnetic properties of these copper-oxamate-imidazolium samples can be changed by the kind of imidazolium cation bonded in the system. In fact, the magnetic properties of these compounds are under investigation in our lab. Acknowledgments
C B A 910
920
930
940
950
960
970
Energy/ eV Fig. 5. Cu L2,3 XANES spectra of: (A) Cu(NO3)2; (B) Na4[Cu2(mpba)2]; (C) K2[Cu(opba)]; and (D) [C4MIm]2[Cu(opba)].
This work was supported by CNPq, FAPEMIG, CAPES and FAPESP (Brazilian agencies). G.M. do Nascimento acknowledges CAPES for his post-doctoral fellowship (PNPD program) and N.A. Pradie acknowledges CNPq for their doctoral fellowship. These authors are indebted to the LCCA (Laboratory of Advanced Scientific Computation of the University of São Paulo) for the use of its computational resources. The authors would like to thank the LNLS for the use of SGM beam line (Proposal No 10685 and 9281) and XAFS1 beam line (Proposal No 2706). Special thanks to Dr. Fabio Rodrigues and Dr. Douglas Galante for their helpful support during the XANES and some Raman experiments. Appendix A. Synthesis of the [C4MIm]2Cu(opba)3H2O compound
gies, it indicates the changes could be occurring around the CuAN bonding. Indeed, higher charger delocalization or changes around the Cu coordination geometry owed to the presence of imidazolium cation could be responsible for this behavior. Fig. 5 shows the Cu L2,3-edge XANES spectra. The Cu L2,3 XANES spectra are sensible to the valence state, ionic character, and ligand field and coordination state of Cu species [45,46]. The Cu L-edge XANES spectrum (see Fig. 5) for Cu(NO3)2 was used as reference, the spectrum is dominated by two sharp bands (white lines), the first one at 930.0 eV is assigned to 2p3/2 ? 3d transitions and the second one at 950.0 eV can be assigned to 2p3/2 ? 3d transitions.
2.39 g (3.00 mmol) of the [Bu4N]2Cu(opba) crystalline powder were dissolved in 50.0 mL of deionized water and afterwards 1.47 g (6.00 mmol) of the [C4MIm]ClO40.25H2O were added to the solution and immediately a white flocculent powder was precipitated. This mixture was left under stirring for 1 h at room temperature. Afterwards, the mixture was filtered and the precipitate was washed with deionized water, dried in a desiccator under vacuum. The preparation of the [Bu4N]ClO4 salt was confirmed by FTIR data. The bluish filtered solution was extracted with 5.0 mL of dichloromethane for elimination of the non-reacting
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IR
Absorbance
B
A
1000
1100
1200
1300
1400
1500
1600
1700
1800
Wavenumber/ cm-1 Additional Fig. 1. Experimental and DFT calculated FTIR spectra for [C4MIm]Cl sample. A. Experimental FTIR spectrum (KBr pellets) of [C4MIm]Cl sample and B. DFT calculated FTIR spectrum for [C4MIm]+ cation. The equilibrium geometry of the [C4MIm]+cation, as well its respective vibrational frequencies (Raman and IR bands) were obtained with the Gaussian 03 software30 using B3LYP density functional theory and 6-311++G(d,p) basis set. The optimized structure of the cation, used in the calculations, is given in the top of the figure. The DFT spectra are shown without adjustment by the scale factor of 0.9679.38
reagents and byproducts that are very soluble in this solvent. Then, the volume of the resulting solution was reduced in a rotating evaporator until the formation a very viscous liquid. This material was dried under vacuum and a very deliquescent purple solid was formed. The reaction yield was about 71.0%. Appendix B. Synthesis of the [C4MIm]4[Cu2(mpba)2] compound A total of 0.308 g (1.00 mmol) of the Et2H2mpba solid was suspended in 25.0 mL of deionized water and it was added 2.60 mL (4.00 mmol) of a solution of Bu4NOH (40.0% in water, tetra-nbutyl-ammonium hydroxide), the resulting solution was left under stirring at 60.0 °C for 30 min. After cooling, it was slowly dropped 5.00 mL of the aqueous solution containing 0.169 g (1.00 mmol) of the CuCl22H2O. After complete mixture, it was added 0.486 g (2.00 mmol) of the [C4MIm]ClO40.25H2O to the solution above. A flocculent white solid was formed. The solution was left under stirring for one hour and after the precipitate was separated from the solution by filtration. The greenish filtered solution was extracted with 5.0 mL of dichloromethane for elimination of the non-reacting reagents and byproducts that are very soluble in this solvent. Then, the volume of the resulting solution was reduced in a rotating evaporator until the formation a very viscous liquid. This liquid was washed with tetrahydrofuran (THF) until the solvent was colorless. The material was dried under vacuum for 2 days and a very viscous liquid was formed. The reaction yield was about 62.0%. Appendix C. Synthesis of [C4MIm]4[Cu2(ppba)2] compound The preparation of [C4MIm]4[Cu2(ppba)2] was similar to the synthesis described above for [C4MIm]4[Cu2(mpba)2]. For the for-
mation of the final product was added 0.486 g (2.00 mmol) of the [C4MIm]ClO40.25H2O to the solution containing [Bu4N]+ and [Cu2(ppba)2]4–. A flocculent white solid was formed. The solution was left under stirring for 1 h and after the precipitated was separated from the solution by filtration. The greenish filtered solution was extracted with 5.0 mL of dichloromethane for elimination of the non-reacting reagents and byproducts that are very soluble in this solvent. Hence, the volume of the resulting solution was reduced in a rotating evaporator until the formation a very viscous liquid. This liquid was washed with acetonitrile. The material was dried under vacuum for 1 days and a green deliquescent solid was formed. The reaction yield was about 65.0%.
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