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FT-Raman spectra of cis-bis(thiourea)tellurium(II) halides (Cl−, Br−, I−) and thiocyanate
Journal of Molecular Structure 508 (1999) 51–58
FT-Raman spectra of cis-bis(thiourea)tellurium(II) halides (Cl 2, Br 2, I 2) and thiocyanate J.M. Alı´a a,*, H.G.M. Edwards b, F.J. Garcı´a-Navarro a a
E.U.I.T.A., Departamento de Quı´mica-Fı´sica, Universidad de Castilla-La Mancha, Ronda de Calatrava 5, ES-13071Ciudad Real, Spain b Chemical and Forensic Sciences, University of Bradford, Richmond Road, BD7 1DPBradford, UK Received 5 October 1998; accepted 11 December 1998
Abstract The FT-Raman spectra of some cis-bis(thiourea)tellurium(II) coordination compounds [Te(tu)2Cl2, Te(tu)2Br2, Te(tu)2I2 and Te(tu)2(SCN)2] are reported. The observed spectral modifications affect all the Raman active modes of thiourea and can be interpreted as the result of a strong coordination between the Te(II) ion and the sulphur atom of thiourea that weakens the CyS bond and subsequently strengthens the C–N bonds. The Raman bands assigned to (TeS) stretching are located at 266 1 253 cm 21 (thiocyanate), 276 1 262 cm 21 (chloride), 258 1 250 cm 21 (bromide) and 232 cm 21 (iodide). The corresponding wave numbers for (TeX) stretching are: 162 cm 21 for (TeCl) and (TeSCN), 150 cm 21 for (TeBr) and 139 cm 21 for (TeI). q 1999 Elsevier Science B.V. All rights reserved. Keywords: FT-Raman spectroscopy; Thiourea coordination complexes; Tellurium halides; Tellurium thiocyanate
1. Introduction The existence of coordination complexes between thiourea (tu) and different salts of tellurium(II) has been known for forty years [1–4]. Crystal structures of bis(thiourea)tellurium(II) complexes in the form of chloride, bromide, iodide and thiocyanate are also reported [5,6]. Table 1 summarises several parameters of these structures. In all cases, the central element Te(II) is tetra-coordinated, showing a more or less distorted square-planar configuration, with the ligands in a cis position. The only previous vibrational spectroscopic study for these compounds is that of Hendra and Jovic [7], which deals with the low frequency Raman and IR spectra of cis-bis(thiourea)tellurium(II) chloride and bromide. * Corresponding author. E-mail address: [email protected] (J.M. Alı´a)
In contrast, the IR studies of thiourea coordination complexes are quite numerous [8–13], although there are some contradictory interpretations and assignments arising from the complex vibrational dynamics of the organic ligand [14–17] whose normal modes of vibration are considerably mixed. Despite the relative abundance of the IR studies, Raman studies are scarce. A comprehensive vibrational spectroscopic study of platinum(II) and palladium(II) complexes with thiourea and selenourea has been made [18] and recently, two Raman spectroscopic studies of oriented single crystals of Zn(tu)3SO4 [19] and Cd(tu)2Cl2 [20] have appeared. The interest of these compounds arises from their non-linear optical properties. The Raman spectrum at low temperature (10 K) of the complex Pb(tu)2Cl2 has also been reported by Faulques et al. [20]. The main objective of the present study is the description and interpretation of the modifications
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J.M. Alı´a et al. / Journal of Molecular Structure 508 (1999) 51–58
Table 1 Crystallographic data for the complexes studied (XyCl, Br, I or SCN) Compound
Te(tu)2Cl2
Te(tu)2Br2
Te(tu)2I2
Te(tu)2(SCN)2
System ˚) a (A ˚) b (A ˚) c (A b (8) Sp ace group Z ˚) S(tu)–Te bond length (A ˚) X–Tebond length (A Ref.
Monoclinic 9.878 7.610 14.104 100.84 I2/a (#15) 4 2.457 2.936 [5]
Monoclinic 10.020 7.727 14.633 100.46 I2/a (#15) 4 2.476 3.038 [5]
Monoclinic 10.209 7.974 15.500 99.89 I2/a (#15) 4 2.521 3.162 [5]
Orthorhombic 18.430 18.560 7.861 Fdd2 (#43) 8 2.458 3.039 [6]
observed in the Raman spectrum of thiourea as a result of its coordination with a soft acceptor such as Te(II) in several complexes, namely Te(tu)2Cl2, Te(tu)2Br2, Te(tu)2I2 and Te(tu)2(SCN)2, whose full FT-Raman spectra are published here for the first time. The low frequency region of the Raman spectra of these complexes is studied in detail, providing some new information about the S–Te stretching bands. Revision of the published vibrational frequencies of the halogen–Te(II) bonds is also proposed. 2. Experimental 2.1. Preparation of the thiourea complexes The complexes were obtained by the procedure of Foss and Hauge [2,3]. The absence in the final product of the unreacted thiourea and/or the formamidinium disulphide salts, by-products of the synthetic route, was confirmed by powder X-ray diffraction carried out using a Philips PW 1710 automatic diffractometer with CuKa radiation. The stoichiometry of the complexes was confirmed by an elemental CNS analyses with a NA1500 analyser (Carlo Erba Instrumentazione). 2.2. FT-Raman spectroscopy
Fig. 1. FT-Raman spectra of thiourea and its complexes with tellurium(II) in the region 3500–2900 cm 21.
FT-Raman spectra were excited at 1064 nm using a Nd : YAG laser and a Bruker IFS66 optical bench with an FRA 106 Raman accessory. The laser power was set at 95–100 mW, and 1000 scans were accumulated with a resolution of 2 cm 21. The mathematical
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Table 2 Wave numbers (n /cm 21) of Raman bands in the NH stretching region and their assignments for Te(II)-thiourea complexes (sh shoulder; b broad) Iodide
Bromide 3347 3267
3347 3313 3274
3178 3115 3123 (sh, b)
3172 3125 3120 (sh, b)
Chloride
Thiocyanate
3346
3342
3277 3206 (sh) 3173 3142 3115 3057 (sh, b)
3294 3218 (sh) 3193 3156 3134 (sh, b) 3032 (sh, b)
(curve fitting) treatment of the spectra was carried out using the commercial software GRAMS/32w (Galactic Industries). The smoothing procedures or the baseline correction routines were not applied in
Assignment
n as NH n as NH hydrogen-bonded 2 d NH2 in Fermi resonance n as NH hydrogen-bonded n s NH n NH n NH hydrogen-bonded n NH hydrogen-bonded
this work. Band were normalised using the corresponding integrated area. 3. Results and discussion The recorded FT-Raman spectra can be conveniently divided into three regions, viz. 3500– 3000 cm 21 (NH stretching region), 1700–400 cm 21 and 300–80 cm 21 (low frequency region). 3.1. N–H stretching region
Fig. 2. FT-Raman spectra of thiourea and its complexes with tellurium(II) in the region 1700–950 cm 21.
Fig. 1 shows the region of the Raman spectra that contains thiourea NH stretching bands. A complex envelope can be observed whose assignments [14,15,17,22] in thiourea spectrum are as follows: the band at 3375 cm 21 is the antisymmetric stretching; the peak at 3283 cm 21, whose symmetry is Ag [14], represents the first overtone of the NH2 antisymmetric bending component. The feature at 3233 cm 21 is the first overtone of the corresponding symmetric component, in Fermi resonance with the NH symmetric stretching band located at 3183 cm 21. Moreover, some unresolved shoulders and low intensity broad bands (e.g. at 3090 cm 21) are evident that could arise from relatively strong hydrogen-bonded NH oscillators, whose relevance has been recognised both in thiourea temperatureinduced phase transformations [23] as well in the normal coordinate treatment [24]. The bands n (NH) of the spectra of thiourea in the coordination complexes, whose enumeration and assignment are given in Table 2, are shifted towards lower wave numbers except for the band assigned to 2 d (NH2) in Fermi resonance with the symmetric
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Table 3 Wave numbers (n /cm 21) of Raman bands in the region 1700–350 cm 21 and their assignments for Te(II)-thiourea complexes Thiourea 1636 1613 1485 1471 1384 1370 1094
733 572 (vb) 500 (sh) 479 401
Assignment
Thiocyanate
Chloride
Bromide
Iodide
d s (NH2) d as (NH2) n as (NCN) B1g n as (NCN) B3g n 1 (SCN2H4) B2g n 1 (SCN2H4) Ag n 2 (SCN2H4)
1639
1633 1624 1537 1531 1404 1360 1102 1034
1624
1614
1527
1514
1402 1359 1098 1024
1406 1359 1095 1020
686 589
684 577
683 582
2n 2 (SCN 2)? 2n 2 (SCN 2)? n 3 (SCN 2) n 3 (SCN2H4) v (NH2) t (NH2) See text d SCN
1541 1398 1352 1109 1032 920 896 737 682 609 470 449 1 439
n (NH) mode. This arises as a consequence of the d (NH2) positive shift promoted by the hydrogen bond. Moreover, there is a general broadening of the vibrational bands. These characteristics are indicative of stronger hydrogen bonds in the coordination complexes than in thiourea. A crude, although useful, qualitative approach to the mean relative strength of the hydrogen bonds can be made by calculating the centroid of the n (NH) envelope. In the Raman spectrum of thiourea, this point is located at 3237 cm 21, reaching the values of 3225, 3210, 3175 and 3182 cm 21 for iodide, bromide, chloride and thiocyanate, respectively. In consequence, the hydrogen bonding mean strength would increase in the following order: thiourea , I 2 , Br 2 , Cl 2 < SCN 2, in good agreement with the relative acidity these such anions [25]. 3.2. 1700–350 cm 21 region To demonstrate more effectively the number and the shape of the Raman bands, this spectral region was arbitrarily divided into two parts: 1700– 850 cm 21 and 800–350 cm 21. Fig. 2 shows the first part of this spectral region with the band wave numbers and assignments given in Table 3. Bands at 1636 and 1613 cm 21 in the Raman spectrum of thiourea are assigned to the symmetric and antisymmetric-coupled bending mode d (NH2). This mode is
471 1 467 451 1 438
467 1 463 442 1 431
460 1 463 427 1 420
shifted towards higher wave numbers in the Raman spectra of the complexes in an inverse trend to that discussed for the n (NH) band envelope. The increase in the corresponding integrated intensity of the bands in the Raman spectra of the complexes with respect the the corresponding thiourea spectrum is noteworthy. The antisymmetric stretching n as(NCN) bands of thiourea subject to a crystal-field splitting are at 1485 and 1471 cm 21. The strong shifts towards higher wave numbers observed in the spectra of the complexes (63, 56, 49 and 36 cm 21 for the thiocyanate, chloride, bromide and iodide, respectively) are significantly greater than those reported in the Raman spectra of Zn(tu)3SO4 (37 cm 1) [19], Zn(tu)2Cl2 (24 cm 21) [20] or Pb(tu)2Cl2 (30 cm 1) [21]. This may be correlated with the C–N bond lengths observed in Te(II)/thiourea coordination complexes, ˚ [5,6], which are signifiin the range 1.302–1.316 A ˚) cantly shorter than that reported in thiourea (1.34 A [24,26,27]. The assignment of the doublet that appears at 1384 and 1370 cm 21 in the Raman spectrum of thiourea has been controversial. Stewart [28] reports a single, intense and broad band at 1374 cm 21 assigned to the n (CS) stretching. Schrader et al.[14] designate it as n 1(SCN2H4) and confirm that affects the complete thiourea molecule, calculating the potential energy distribution (PED) as 20% n (CN) 1 19% d (NCN) 1 26% d (CNH) 1 10% n (CS). The spectral data
J.M. Alı´a et al. / Journal of Molecular Structure 508 (1999) 51–58
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Fig. 3. FT-Raman spectra of thiourea and its complexes with tellurium(II) in the region 800–350 cm 21.
reported by these authors (components at 1369 and 1384 cm 21, with relative intensities of 8 and 10.5, respectively), obtained from the oriented single crystals, are fully comparable with those observed in the present work. Hadzi et al. [24] carried out a normal coordinate analysis of thiourea and assigned the band that appears at 1414 cm 21 in the IR spectrum to a vibrational mode with a PED of 40% n (CN), 39% r (NH2), 23% d (NCN) and 20% n (CS). Finally, Vijay and Sathyanarayana [17] equate the IR band at 1412 cm 21 with the Raman band at 1372 cm 1 and calculate for the corresponding normal mode a PED very close with those previously reported. The effect of the cation coordination on the assigned n (CS) vibrational mode is complex. The
stronger component, at 1384 cm in the Raman spectrum of thiourea, shifts towards higher wave numbers, while the band at 1370 cm 21 moves in the opposite direction. The different anions do not have any appreciable effect. Swaminathan and Irving [9] reported a similar conclusion from a comprehensive IR study of several complexes of thiourea with transition metals. The IR band at 1412 cm 21 splits into two components at higher (ca. 1440 cm 21) and lower (ca. 1404 cm 1) wave numbers than the original band. Bailey and Peterson [12] observe a similar splitting in the IR spectra of Fe(tu)2(SCN)2 and Fe(tu)4Cl2. However, none of these authors discusses this observation or offers an explanation. A possible explanation for the spectral behaviour of this doublet is afforded taking into consideration the different effects caused by the coordination between the cation and thiourea through the sulphur atom. Firstly, the n (CS) stretching mode must shift towards lower wave numbers because the coordination weakens the CyS double bond. However, the subsequent strengthening of the C–N bonds must shift the stretching n (CN) as well the bending d (NCN) modes in the opposite direction. In this fundamental, as there are components of both kinds, the effect of the coordination is rather complex. The feature observed at 1094 cm 21 in the Raman spectrum of thiourea arises from a mixed normal mode whose PED can be divided [17,24,29] into contributions of approximately equal amounts of the stretching n (CN) and rocking r (NH2) modes. The effect of the coordination is similar to that described earlier. The band is now split into two new components: one peak appears slightly displaced towards higher wave numbers, following the trend SCN 2 . Cl 2 . Br 2 . I 2, while the other appears at lower wave numbers with the opposite trend in anions. The band at higher wave numbers is assigned to the n (CN) mode, whereas the other component corresponds to the rocking r (NH2) mode. The shift towards lower wave numbers increases as the mean strength of the hydrogen bonds decreases, because two opposed effects appear in this vibrational mode: on the one hand, the N–H bond is weakened because the C–N bonds have some partially double character which results in a shift towards lower wave numbers of the rocking mode; on the other hand, the hydrogen atoms are more or less anchored by the corresponding anion
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Fig. 4. FT-Raman spectra of thiourea and its complexes with tellurium(II) in the low frequency region, 350–60 cm 21.
through the hydrogen bond, and this hinders the rocking motion. Fig. 3 shows the 800–350 cm 21 region in the Raman spectra. Band wave numbers and assignments are given in Table 3. The strongest band in the Raman spectrum of thiourea, which appears at 733 cm 21, arises from a normal mode whose PED can be interpreted in terms of nearly pure n (CS) [17,24,29] stretching, ruling out previous assignments of this band to symmetric NCN stretching [28]. However, this confusion has been maintained in recent publications [19,20]. The effect of the coordination of thiourea with tellurium(II) is to decrease by about 50 cm 21 the wave number position of this band, showing clearly the weakening of the CyS bond that, further, seems to be independent of the anion.
This important effect can be correlated with the elongation of the CyS bond observed in these Te(II)/ thiourea complexes, where the mean bond length is ˚ as opposed to 1.681 A ˚ which is the average 1.745 A CyS bond length for 96 different thiourea complexes [30]. The reported wave number shifts (Dn 249 cm 21, see Table 3) are significantly greater than those observed in the Raman spectra of Zn(tu)3SO4 (215 cm 21) [19], Zn(tu)2Cl2 (29 cm 21) [20] or Pb(tu)2Cl2 (219 cm 21) [21]. The decrease evident in the relative intensity of this band in the Raman spectra of the complexes can be interpreted as a direct consequence of the effective reduction of the CyS bond order [31]. The Raman spectra of thiourea and its complexes show a number of broad, weak bands in the range 600–500 cm 21 assigned to wagging and twisting motions of the NH2 groups (see Table 3). The peak observed at 479 cm 21 in the Raman spectrum of thiourea deserves special attention. Stewart [28] describes this band as d (NCN) exclusively; Bleckmann et al. [29] calculate its PED as 40% n (CS) and 38% d (NCN) while Hadzi et al. [24] give a composition of 62% d (NCN) and 19% n (CS); according to Vijay and Sathyanarayana [17] this band lacks of n (CS) contribution and is characterised by a PED of 79% d (NCN) and 12% r (NH2). The effect of the coordination with Te(II) is an observed shift towards lower wave numbers, which becomes greater as the anion size increases. This observation could be explained on the basis of opposing effects of the coordination on n (CS) and d (NCN). As has been discussed previously, the band at 733 cm 21 (n (CS) essentially) is significantly shifted towards lower wave numbers without any appreciable effect of the anion. However, the d (NCN) mode behaves differently because of the increase in the effective order of the C–N bond. Moreover, this effect seems to be anion-dependent, as thiocyanate is the most disturbing anion. Therefore, the effect of the coordination on the n (CS) mode, promoting shifts towards lower wave numbers, could be partially compensated by an opposing effect on the d (NCN) mode. There is no controversy about the assignment of the band at 401 cm 21 to d (SCN) mode; this is shifted towards higher wave numbers in the Raman spectra of the complexes. These shifts demonstrate the anchor
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Table 4 Wave numbers (n /cm 21) of Raman bands in the low frequency region and their assignments for Te(II)-thiourea complexes Thiourea
117 101 84 (sh)
Assignment
Thiocyanate
Chloride
Bromide
Iodide
n Te–S n Te–S d S–Te–S n Te–X d X–Te–X Lattice mode Lattice mode Lattice mode Lattice mode Lattice mode Lattice mode
266 253 178 162 143 124
276 262 192 162 144 124 117 109 98 91 84
258 250 184 150 137 127 118 111 101 93 79
232
111 103 92 83
effect that the coordination with Te(II) has over the sulphur atom. The significant increase of the integrated band intensity as a result of coordination seems to be a general conclusion in these bending modes. Similar effects have been reported for the bending C–CxN [32–34] mode in the coordination of mono- and bivalent cations with acrylonitrile in solution. 3.3. Low frequency region, 350–60 cm 21 Fig. 4 shows the Raman spectra of the complexes in the low frequency region. From Table 4, thiourea has a simple low frequency spectrum [7,14,18,21], which facilitates the study and the assignment of the spectra of the coordination complexes. The only assignment available for this region has been published by Hendra and Jovic [7] without knowledge of the crystal structures which were reported in 1987 [5,6], some 20 years later. This makes it necessary to revise the cited assignments. It is important to state that the square coordination around Te(II) is not regular because the Te–S bond distances are significantly shorter (ca. 0.5– ˚ ) than those corresponding with the Te–X 0.6 A (XxCl, Br, I, SCN) bonds, as can be seen from Table 1. Moreover, the molecular structure is not planar as the planes S–Te–S and X–Te–X have a dihedral angle of ca.10–118. Hence, the assignment of the Te(tu)2Cl2 Raman bands at 276 and 262 cm 21 (278 and 264 cm 21 in the study of Hendra and Jovic [7]) as n (TeCl) and n (TeS), respectively, must be reconsidered. In other thiourea complexes, such as
175 139 124 111 103 92 83
Zn(tu)2Cl2 and Cd(tu)2Cl2, where the central cation has a tetrahedral coordination, the sequence of the vibrational bands is that assigned by Hendra and Jovic [7] and, moreover, the wave number of both bands are quite close because the cation–Cl and cation–S bond lengths are very similar; these are, ˚ for Zn–S and 2.32 A ˚ for Zn–Cl for example, 2.35 A [27] in the complex Zn(tu)2Cl2. In the case of ˚ Cd(tu)2Cl2, the bond lengths are Cd–S 2.45 A ˚ [20]. The data in Table 1 for and Cd–Cl 2.50 A the complexes studied in the present work show a significantly different situation and the Te–S bond isobserved to be always rather shorter than the corresponding Te–X bond, as expected for the interaction between two soft entities like organic sulphur and Te(II). The assignment proposed here considers the first two peaks (only one in the Raman spectrum of the iodide), the strongest in the low frequency region, as the stretching S–Te modes. The observed wave numbers, which follow the order Cl . SCN . Br . I, can be correlated very well with the corresponding bond length. The presence of the two components whose splitting decreases and vanishes in the spectrum of the iodide, has been observed [13] for similar vibrations in the IR spectra of cis-MCl2L2 complexes and can be ascribed to the crystal field effects. The remaining assignments, although necessarily tentative, are based on several well-established criteria [31] such as the usually stronger intensity of the stretching modes as opposed to the bending modes. Then, the Te–X (X Cl, Br, I, SCN) stretching bands in the Raman spectrum must be
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stronger than the corresponding S–Te–S bending bands. In contrast, the Te–X stretching bands should appear at lower wave numbers because of the long Te–X bond lengths. Moreover, as both bonds are quite similar, the Te–Cl and Te–SCN stretching bands must be close. The assignment of the lattice modes was effected by assuming that they must be similar in the different complexes and to the external modes of thiourea itself. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
E.E. Aynsley, W.A. Campbell, J. Chem. Soc. (1958) 3290. O. Foss, S. Hauge, Acta Chem. Scand. 13 (1959) 1252. O. Foss, S. Hauge, Acta Chem. Scand. 15 (1961) 1615. J. Vrestal, Collect. Czech. Chem. Commun. 25 (1960) 443. O. Foss, K. Maartmann-Moe, Acta Chem. Scand. A41 (1987) 121. O. Foss, J. Henjum, K. Maartmann-Moe, K. Maroy, Acta Chem. Scand. A41 (1987) 77. P.J. Hendra, Z. Jovic, J. Chem. Soc. (A) (1967) 735. A. Yamaguchi, R.B. Penland, S.-I. Mizushima, T.J. Lane, C. Curran, J.V. Quagliano, J. Am. Chem. Soc. 80 (1958) 527. K. Swaminathan, H.M.N.H. Irving, J. Inorg. Nucl. Chem. 26 (1964) 1291. R.W. Olliff, J. Chem. Soc. (1965) 2036. C.D. Flint, M. Goodgame, J. Chem. Soc. (1966) 744. R.A. Bailey, T.R. Peterson, Can. J. Chem. 45 (1967) 1135. D.M. Adams, J.B. Cornell, J. Chem. Soc. (A) (1967) 884. B. Schrader, W. Meier, K. Gottlieb, H. Agatha, H. Barentzen, P. Bleckmann, Ber. Bunsen-Ges. Phys. Chem. 75 (1971) 1263. G.B. Aitken, J.L. Duncan, G.P. McQuillan, J. Chem. Soc. (A) (1971) 2695.
[16] J.F. Arenas, F. Marquez, A. Cardenete, Spectrochim. Acta, Part A 40 (1984) 1033. [17] A. Vijay, D.N. Sathyanarayana, Spectrochim. Acta, Part A 49 (1993) 1565. [18] P.J. Hendra, Z. Jovic, Spectrochim. Acta, Part A 24 (1968) 1713. [19] V. Venkataramanan, M.R. Srinivasan, H.L. Bhat, J. Raman Spectrosc. 25 (1994) 805. [20] V. Venkataramanan, H.L. Bhat, M.R. Srinivasan, P. Ayyub, M.S. Multani, J. Raman Spectrosc. 28 (1997) 779. [21] E. Faulques, J.D. Zubkowski, D.L. Perry, Spectrosc. Lett. 29 (1996) 1275. [22] R.M. Badger, R.D. Waldron, J. Chem. Phys. 26 (1957) 255. [23] I. Takahashi, A. Onodera, Y. Shiozaki, Acta Crystallogr., Sect. B: Struct. Sci. 46 (1990) 661. [24] D. Hadzi, J. Kidric, Z.V. Knezevic, B. Barlic, Spectrochim. Acta, Part A 32 (1976) 693. [25] Y. Marcus, Ion Properties, Wiley, Chichester, 1998. [26] R. Vega, A. Lo´pez-Castro, R. Ma´rquez, Acta Crystallogr., Sect. B: Struct. Sci. 34 (1978) 2297. [27] N.R. Kunchur, M.R. Truter, J. Chem. Soc. (1958) 3478. [28] J.E. Stewart, J. Chem. Phys. 26 (1957) 248. [29] P. Bleckmann, B. Schrader, W. Meier, H. Takahashi, Ber. Bunsen-Ges. Phys. Chem. 75 (1971) 1279. [30] CRC Handbook of Chemistry and Physics, 74th ed., CRC Press, Boca Raton, 1993. [31] B.J. Bulkin, in: J.G. Graselli, B.J. Bulkin (Eds.), Analytical Raman Spectroscopy, Wiley, New York, 1991, pp. 1. [32] J.M. Alı´a, H.G.M. Edwards, J. Moore, J. Raman Spectrosc. 26 (1995) 715. [33] J.M. Alı´a, H.G.M. Edwards, J. Moore, J. Chem. Soc. Faraday Trans. 92 (1996) 1187. [34] J.M. Alı´a, Y. Dı´az de Mera, H.G.M. Edwards, F.J. Garcı´a, E.E. Lawson, Z. Phys. Chem. (Munich) 196 (1996) 209.
FT-Raman spectra of cis-bis(thiourea)tellurium(II) halides (Cl 2, Br 2, I 2) and thiocyanate J.M. Alı´a a,*, H.G.M. Edwards b, F.J. Garcı´a-Navarro a a
E.U.I.T.A., Departamento de Quı´mica-Fı´sica, Universidad de Castilla-La Mancha, Ronda de Calatrava 5, ES-13071Ciudad Real, Spain b Chemical and Forensic Sciences, University of Bradford, Richmond Road, BD7 1DPBradford, UK Received 5 October 1998; accepted 11 December 1998
Abstract The FT-Raman spectra of some cis-bis(thiourea)tellurium(II) coordination compounds [Te(tu)2Cl2, Te(tu)2Br2, Te(tu)2I2 and Te(tu)2(SCN)2] are reported. The observed spectral modifications affect all the Raman active modes of thiourea and can be interpreted as the result of a strong coordination between the Te(II) ion and the sulphur atom of thiourea that weakens the CyS bond and subsequently strengthens the C–N bonds. The Raman bands assigned to (TeS) stretching are located at 266 1 253 cm 21 (thiocyanate), 276 1 262 cm 21 (chloride), 258 1 250 cm 21 (bromide) and 232 cm 21 (iodide). The corresponding wave numbers for (TeX) stretching are: 162 cm 21 for (TeCl) and (TeSCN), 150 cm 21 for (TeBr) and 139 cm 21 for (TeI). q 1999 Elsevier Science B.V. All rights reserved. Keywords: FT-Raman spectroscopy; Thiourea coordination complexes; Tellurium halides; Tellurium thiocyanate
1. Introduction The existence of coordination complexes between thiourea (tu) and different salts of tellurium(II) has been known for forty years [1–4]. Crystal structures of bis(thiourea)tellurium(II) complexes in the form of chloride, bromide, iodide and thiocyanate are also reported [5,6]. Table 1 summarises several parameters of these structures. In all cases, the central element Te(II) is tetra-coordinated, showing a more or less distorted square-planar configuration, with the ligands in a cis position. The only previous vibrational spectroscopic study for these compounds is that of Hendra and Jovic [7], which deals with the low frequency Raman and IR spectra of cis-bis(thiourea)tellurium(II) chloride and bromide. * Corresponding author. E-mail address: [email protected] (J.M. Alı´a)
In contrast, the IR studies of thiourea coordination complexes are quite numerous [8–13], although there are some contradictory interpretations and assignments arising from the complex vibrational dynamics of the organic ligand [14–17] whose normal modes of vibration are considerably mixed. Despite the relative abundance of the IR studies, Raman studies are scarce. A comprehensive vibrational spectroscopic study of platinum(II) and palladium(II) complexes with thiourea and selenourea has been made [18] and recently, two Raman spectroscopic studies of oriented single crystals of Zn(tu)3SO4 [19] and Cd(tu)2Cl2 [20] have appeared. The interest of these compounds arises from their non-linear optical properties. The Raman spectrum at low temperature (10 K) of the complex Pb(tu)2Cl2 has also been reported by Faulques et al. [20]. The main objective of the present study is the description and interpretation of the modifications
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00005-8
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J.M. Alı´a et al. / Journal of Molecular Structure 508 (1999) 51–58
Table 1 Crystallographic data for the complexes studied (XyCl, Br, I or SCN) Compound
Te(tu)2Cl2
Te(tu)2Br2
Te(tu)2I2
Te(tu)2(SCN)2
System ˚) a (A ˚) b (A ˚) c (A b (8) Sp ace group Z ˚) S(tu)–Te bond length (A ˚) X–Tebond length (A Ref.
Monoclinic 9.878 7.610 14.104 100.84 I2/a (#15) 4 2.457 2.936 [5]
Monoclinic 10.020 7.727 14.633 100.46 I2/a (#15) 4 2.476 3.038 [5]
Monoclinic 10.209 7.974 15.500 99.89 I2/a (#15) 4 2.521 3.162 [5]
Orthorhombic 18.430 18.560 7.861 Fdd2 (#43) 8 2.458 3.039 [6]
observed in the Raman spectrum of thiourea as a result of its coordination with a soft acceptor such as Te(II) in several complexes, namely Te(tu)2Cl2, Te(tu)2Br2, Te(tu)2I2 and Te(tu)2(SCN)2, whose full FT-Raman spectra are published here for the first time. The low frequency region of the Raman spectra of these complexes is studied in detail, providing some new information about the S–Te stretching bands. Revision of the published vibrational frequencies of the halogen–Te(II) bonds is also proposed. 2. Experimental 2.1. Preparation of the thiourea complexes The complexes were obtained by the procedure of Foss and Hauge [2,3]. The absence in the final product of the unreacted thiourea and/or the formamidinium disulphide salts, by-products of the synthetic route, was confirmed by powder X-ray diffraction carried out using a Philips PW 1710 automatic diffractometer with CuKa radiation. The stoichiometry of the complexes was confirmed by an elemental CNS analyses with a NA1500 analyser (Carlo Erba Instrumentazione). 2.2. FT-Raman spectroscopy
Fig. 1. FT-Raman spectra of thiourea and its complexes with tellurium(II) in the region 3500–2900 cm 21.
FT-Raman spectra were excited at 1064 nm using a Nd : YAG laser and a Bruker IFS66 optical bench with an FRA 106 Raman accessory. The laser power was set at 95–100 mW, and 1000 scans were accumulated with a resolution of 2 cm 21. The mathematical
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53
Table 2 Wave numbers (n /cm 21) of Raman bands in the NH stretching region and their assignments for Te(II)-thiourea complexes (sh shoulder; b broad) Iodide
Bromide 3347 3267
3347 3313 3274
3178 3115 3123 (sh, b)
3172 3125 3120 (sh, b)
Chloride
Thiocyanate
3346
3342
3277 3206 (sh) 3173 3142 3115 3057 (sh, b)
3294 3218 (sh) 3193 3156 3134 (sh, b) 3032 (sh, b)
(curve fitting) treatment of the spectra was carried out using the commercial software GRAMS/32w (Galactic Industries). The smoothing procedures or the baseline correction routines were not applied in
Assignment
n as NH n as NH hydrogen-bonded 2 d NH2 in Fermi resonance n as NH hydrogen-bonded n s NH n NH n NH hydrogen-bonded n NH hydrogen-bonded
this work. Band were normalised using the corresponding integrated area. 3. Results and discussion The recorded FT-Raman spectra can be conveniently divided into three regions, viz. 3500– 3000 cm 21 (NH stretching region), 1700–400 cm 21 and 300–80 cm 21 (low frequency region). 3.1. N–H stretching region
Fig. 2. FT-Raman spectra of thiourea and its complexes with tellurium(II) in the region 1700–950 cm 21.
Fig. 1 shows the region of the Raman spectra that contains thiourea NH stretching bands. A complex envelope can be observed whose assignments [14,15,17,22] in thiourea spectrum are as follows: the band at 3375 cm 21 is the antisymmetric stretching; the peak at 3283 cm 21, whose symmetry is Ag [14], represents the first overtone of the NH2 antisymmetric bending component. The feature at 3233 cm 21 is the first overtone of the corresponding symmetric component, in Fermi resonance with the NH symmetric stretching band located at 3183 cm 21. Moreover, some unresolved shoulders and low intensity broad bands (e.g. at 3090 cm 21) are evident that could arise from relatively strong hydrogen-bonded NH oscillators, whose relevance has been recognised both in thiourea temperatureinduced phase transformations [23] as well in the normal coordinate treatment [24]. The bands n (NH) of the spectra of thiourea in the coordination complexes, whose enumeration and assignment are given in Table 2, are shifted towards lower wave numbers except for the band assigned to 2 d (NH2) in Fermi resonance with the symmetric
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54
Table 3 Wave numbers (n /cm 21) of Raman bands in the region 1700–350 cm 21 and their assignments for Te(II)-thiourea complexes Thiourea 1636 1613 1485 1471 1384 1370 1094
733 572 (vb) 500 (sh) 479 401
Assignment
Thiocyanate
Chloride
Bromide
Iodide
d s (NH2) d as (NH2) n as (NCN) B1g n as (NCN) B3g n 1 (SCN2H4) B2g n 1 (SCN2H4) Ag n 2 (SCN2H4)
1639
1633 1624 1537 1531 1404 1360 1102 1034
1624
1614
1527
1514
1402 1359 1098 1024
1406 1359 1095 1020
686 589
684 577
683 582
2n 2 (SCN 2)? 2n 2 (SCN 2)? n 3 (SCN 2) n 3 (SCN2H4) v (NH2) t (NH2) See text d SCN
1541 1398 1352 1109 1032 920 896 737 682 609 470 449 1 439
n (NH) mode. This arises as a consequence of the d (NH2) positive shift promoted by the hydrogen bond. Moreover, there is a general broadening of the vibrational bands. These characteristics are indicative of stronger hydrogen bonds in the coordination complexes than in thiourea. A crude, although useful, qualitative approach to the mean relative strength of the hydrogen bonds can be made by calculating the centroid of the n (NH) envelope. In the Raman spectrum of thiourea, this point is located at 3237 cm 21, reaching the values of 3225, 3210, 3175 and 3182 cm 21 for iodide, bromide, chloride and thiocyanate, respectively. In consequence, the hydrogen bonding mean strength would increase in the following order: thiourea , I 2 , Br 2 , Cl 2 < SCN 2, in good agreement with the relative acidity these such anions [25]. 3.2. 1700–350 cm 21 region To demonstrate more effectively the number and the shape of the Raman bands, this spectral region was arbitrarily divided into two parts: 1700– 850 cm 21 and 800–350 cm 21. Fig. 2 shows the first part of this spectral region with the band wave numbers and assignments given in Table 3. Bands at 1636 and 1613 cm 21 in the Raman spectrum of thiourea are assigned to the symmetric and antisymmetric-coupled bending mode d (NH2). This mode is
471 1 467 451 1 438
467 1 463 442 1 431
460 1 463 427 1 420
shifted towards higher wave numbers in the Raman spectra of the complexes in an inverse trend to that discussed for the n (NH) band envelope. The increase in the corresponding integrated intensity of the bands in the Raman spectra of the complexes with respect the the corresponding thiourea spectrum is noteworthy. The antisymmetric stretching n as(NCN) bands of thiourea subject to a crystal-field splitting are at 1485 and 1471 cm 21. The strong shifts towards higher wave numbers observed in the spectra of the complexes (63, 56, 49 and 36 cm 21 for the thiocyanate, chloride, bromide and iodide, respectively) are significantly greater than those reported in the Raman spectra of Zn(tu)3SO4 (37 cm 1) [19], Zn(tu)2Cl2 (24 cm 21) [20] or Pb(tu)2Cl2 (30 cm 1) [21]. This may be correlated with the C–N bond lengths observed in Te(II)/thiourea coordination complexes, ˚ [5,6], which are signifiin the range 1.302–1.316 A ˚) cantly shorter than that reported in thiourea (1.34 A [24,26,27]. The assignment of the doublet that appears at 1384 and 1370 cm 21 in the Raman spectrum of thiourea has been controversial. Stewart [28] reports a single, intense and broad band at 1374 cm 21 assigned to the n (CS) stretching. Schrader et al.[14] designate it as n 1(SCN2H4) and confirm that affects the complete thiourea molecule, calculating the potential energy distribution (PED) as 20% n (CN) 1 19% d (NCN) 1 26% d (CNH) 1 10% n (CS). The spectral data
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55 21
Fig. 3. FT-Raman spectra of thiourea and its complexes with tellurium(II) in the region 800–350 cm 21.
reported by these authors (components at 1369 and 1384 cm 21, with relative intensities of 8 and 10.5, respectively), obtained from the oriented single crystals, are fully comparable with those observed in the present work. Hadzi et al. [24] carried out a normal coordinate analysis of thiourea and assigned the band that appears at 1414 cm 21 in the IR spectrum to a vibrational mode with a PED of 40% n (CN), 39% r (NH2), 23% d (NCN) and 20% n (CS). Finally, Vijay and Sathyanarayana [17] equate the IR band at 1412 cm 21 with the Raman band at 1372 cm 1 and calculate for the corresponding normal mode a PED very close with those previously reported. The effect of the cation coordination on the assigned n (CS) vibrational mode is complex. The
stronger component, at 1384 cm in the Raman spectrum of thiourea, shifts towards higher wave numbers, while the band at 1370 cm 21 moves in the opposite direction. The different anions do not have any appreciable effect. Swaminathan and Irving [9] reported a similar conclusion from a comprehensive IR study of several complexes of thiourea with transition metals. The IR band at 1412 cm 21 splits into two components at higher (ca. 1440 cm 21) and lower (ca. 1404 cm 1) wave numbers than the original band. Bailey and Peterson [12] observe a similar splitting in the IR spectra of Fe(tu)2(SCN)2 and Fe(tu)4Cl2. However, none of these authors discusses this observation or offers an explanation. A possible explanation for the spectral behaviour of this doublet is afforded taking into consideration the different effects caused by the coordination between the cation and thiourea through the sulphur atom. Firstly, the n (CS) stretching mode must shift towards lower wave numbers because the coordination weakens the CyS double bond. However, the subsequent strengthening of the C–N bonds must shift the stretching n (CN) as well the bending d (NCN) modes in the opposite direction. In this fundamental, as there are components of both kinds, the effect of the coordination is rather complex. The feature observed at 1094 cm 21 in the Raman spectrum of thiourea arises from a mixed normal mode whose PED can be divided [17,24,29] into contributions of approximately equal amounts of the stretching n (CN) and rocking r (NH2) modes. The effect of the coordination is similar to that described earlier. The band is now split into two new components: one peak appears slightly displaced towards higher wave numbers, following the trend SCN 2 . Cl 2 . Br 2 . I 2, while the other appears at lower wave numbers with the opposite trend in anions. The band at higher wave numbers is assigned to the n (CN) mode, whereas the other component corresponds to the rocking r (NH2) mode. The shift towards lower wave numbers increases as the mean strength of the hydrogen bonds decreases, because two opposed effects appear in this vibrational mode: on the one hand, the N–H bond is weakened because the C–N bonds have some partially double character which results in a shift towards lower wave numbers of the rocking mode; on the other hand, the hydrogen atoms are more or less anchored by the corresponding anion
56
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Fig. 4. FT-Raman spectra of thiourea and its complexes with tellurium(II) in the low frequency region, 350–60 cm 21.
through the hydrogen bond, and this hinders the rocking motion. Fig. 3 shows the 800–350 cm 21 region in the Raman spectra. Band wave numbers and assignments are given in Table 3. The strongest band in the Raman spectrum of thiourea, which appears at 733 cm 21, arises from a normal mode whose PED can be interpreted in terms of nearly pure n (CS) [17,24,29] stretching, ruling out previous assignments of this band to symmetric NCN stretching [28]. However, this confusion has been maintained in recent publications [19,20]. The effect of the coordination of thiourea with tellurium(II) is to decrease by about 50 cm 21 the wave number position of this band, showing clearly the weakening of the CyS bond that, further, seems to be independent of the anion.
This important effect can be correlated with the elongation of the CyS bond observed in these Te(II)/ thiourea complexes, where the mean bond length is ˚ as opposed to 1.681 A ˚ which is the average 1.745 A CyS bond length for 96 different thiourea complexes [30]. The reported wave number shifts (Dn 249 cm 21, see Table 3) are significantly greater than those observed in the Raman spectra of Zn(tu)3SO4 (215 cm 21) [19], Zn(tu)2Cl2 (29 cm 21) [20] or Pb(tu)2Cl2 (219 cm 21) [21]. The decrease evident in the relative intensity of this band in the Raman spectra of the complexes can be interpreted as a direct consequence of the effective reduction of the CyS bond order [31]. The Raman spectra of thiourea and its complexes show a number of broad, weak bands in the range 600–500 cm 21 assigned to wagging and twisting motions of the NH2 groups (see Table 3). The peak observed at 479 cm 21 in the Raman spectrum of thiourea deserves special attention. Stewart [28] describes this band as d (NCN) exclusively; Bleckmann et al. [29] calculate its PED as 40% n (CS) and 38% d (NCN) while Hadzi et al. [24] give a composition of 62% d (NCN) and 19% n (CS); according to Vijay and Sathyanarayana [17] this band lacks of n (CS) contribution and is characterised by a PED of 79% d (NCN) and 12% r (NH2). The effect of the coordination with Te(II) is an observed shift towards lower wave numbers, which becomes greater as the anion size increases. This observation could be explained on the basis of opposing effects of the coordination on n (CS) and d (NCN). As has been discussed previously, the band at 733 cm 21 (n (CS) essentially) is significantly shifted towards lower wave numbers without any appreciable effect of the anion. However, the d (NCN) mode behaves differently because of the increase in the effective order of the C–N bond. Moreover, this effect seems to be anion-dependent, as thiocyanate is the most disturbing anion. Therefore, the effect of the coordination on the n (CS) mode, promoting shifts towards lower wave numbers, could be partially compensated by an opposing effect on the d (NCN) mode. There is no controversy about the assignment of the band at 401 cm 21 to d (SCN) mode; this is shifted towards higher wave numbers in the Raman spectra of the complexes. These shifts demonstrate the anchor
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Table 4 Wave numbers (n /cm 21) of Raman bands in the low frequency region and their assignments for Te(II)-thiourea complexes Thiourea
117 101 84 (sh)
Assignment
Thiocyanate
Chloride
Bromide
Iodide
n Te–S n Te–S d S–Te–S n Te–X d X–Te–X Lattice mode Lattice mode Lattice mode Lattice mode Lattice mode Lattice mode
266 253 178 162 143 124
276 262 192 162 144 124 117 109 98 91 84
258 250 184 150 137 127 118 111 101 93 79
232
111 103 92 83
effect that the coordination with Te(II) has over the sulphur atom. The significant increase of the integrated band intensity as a result of coordination seems to be a general conclusion in these bending modes. Similar effects have been reported for the bending C–CxN [32–34] mode in the coordination of mono- and bivalent cations with acrylonitrile in solution. 3.3. Low frequency region, 350–60 cm 21 Fig. 4 shows the Raman spectra of the complexes in the low frequency region. From Table 4, thiourea has a simple low frequency spectrum [7,14,18,21], which facilitates the study and the assignment of the spectra of the coordination complexes. The only assignment available for this region has been published by Hendra and Jovic [7] without knowledge of the crystal structures which were reported in 1987 [5,6], some 20 years later. This makes it necessary to revise the cited assignments. It is important to state that the square coordination around Te(II) is not regular because the Te–S bond distances are significantly shorter (ca. 0.5– ˚ ) than those corresponding with the Te–X 0.6 A (XxCl, Br, I, SCN) bonds, as can be seen from Table 1. Moreover, the molecular structure is not planar as the planes S–Te–S and X–Te–X have a dihedral angle of ca.10–118. Hence, the assignment of the Te(tu)2Cl2 Raman bands at 276 and 262 cm 21 (278 and 264 cm 21 in the study of Hendra and Jovic [7]) as n (TeCl) and n (TeS), respectively, must be reconsidered. In other thiourea complexes, such as
175 139 124 111 103 92 83
Zn(tu)2Cl2 and Cd(tu)2Cl2, where the central cation has a tetrahedral coordination, the sequence of the vibrational bands is that assigned by Hendra and Jovic [7] and, moreover, the wave number of both bands are quite close because the cation–Cl and cation–S bond lengths are very similar; these are, ˚ for Zn–S and 2.32 A ˚ for Zn–Cl for example, 2.35 A [27] in the complex Zn(tu)2Cl2. In the case of ˚ Cd(tu)2Cl2, the bond lengths are Cd–S 2.45 A ˚ [20]. The data in Table 1 for and Cd–Cl 2.50 A the complexes studied in the present work show a significantly different situation and the Te–S bond isobserved to be always rather shorter than the corresponding Te–X bond, as expected for the interaction between two soft entities like organic sulphur and Te(II). The assignment proposed here considers the first two peaks (only one in the Raman spectrum of the iodide), the strongest in the low frequency region, as the stretching S–Te modes. The observed wave numbers, which follow the order Cl . SCN . Br . I, can be correlated very well with the corresponding bond length. The presence of the two components whose splitting decreases and vanishes in the spectrum of the iodide, has been observed [13] for similar vibrations in the IR spectra of cis-MCl2L2 complexes and can be ascribed to the crystal field effects. The remaining assignments, although necessarily tentative, are based on several well-established criteria [31] such as the usually stronger intensity of the stretching modes as opposed to the bending modes. Then, the Te–X (X Cl, Br, I, SCN) stretching bands in the Raman spectrum must be
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stronger than the corresponding S–Te–S bending bands. In contrast, the Te–X stretching bands should appear at lower wave numbers because of the long Te–X bond lengths. Moreover, as both bonds are quite similar, the Te–Cl and Te–SCN stretching bands must be close. The assignment of the lattice modes was effected by assuming that they must be similar in the different complexes and to the external modes of thiourea itself. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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