Crystal structure and Raman spectra of rubidium hydrogen squarate

Journal of Molecular Structure 741 (2005) 61–66 www.elsevier.com/locate/molstruc

Crystal structure and Raman spectra of rubidium hydrogen squarate Ste´fanos L. Georgopoulosa, Renata Diniza, Bernardo L. Rodriguesb, Luiz F.C. de Oliveiraa,* a

Nu´cleo de Espectroscopia e Estrutura Molecular, Departamento de Quı´mica, Universidade Federal de Juiz de Fora, Campus Universita´rio-Bairro Martelos-Juiz de Fora, MG 36036-900, Brazil b Instituto de Fı´sica, Universidade de Sa˜o Paulo, Sa˜o Carlos, SP, Brazil Received 16 December 2004; revised 21 January 2005; accepted 24 January 2005 Available online 8 March 2005

Abstract Rubidium hydrogen squarate (RbHC4O4, RbHSQ) crystallized in monoclinic space group P21/c. This compound form a short asymmetric ˚ . The hydrogen squarate anions are forming head-to-tail infinite chain intermolecular hydrogen bond whose O–O distance is 2.482(4) A ˚ hydrogen-bonding motifs. A long interplanar separation (4.15 A) indicates that a weak p interaction occurs between hydrogen squarate anions in RbHSQ. The hydrogen bond and cation–anion interactions are the predominant driving forces in the crystal packing. The Raman spectrum of RbHSQ shows an average behaviour between squaric acid and squarate dianion, however, the vibrational modes at ca. 1800 cmK1 (CO stretching mode) and in the region 1500–1700 cmK1 (COCCC stretching modes) are the most affected by the presence of strong hydrogen bonding interactions. q 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen squarate; Crystal structure; Raman spectra

1. Introduction Investigation of supramolecular organization systems in general analyses intermolecular interaction that present a strong correlation to crystal packing. Compounds that present groups which can be involved in hydrogen bonds are very interesting in this kind of study. Hydrogen bonds are probably the most widely used interaction to generate supramolecular organized systems [1]. In cyclic compounds that present electronic delocalization it can be observed a p–p interaction (p-stacking) that play an important rule in the crystal packing [2]. Oxocarbon ions are cyclic compounds of general formula (CnOn)2K, where n varies from 3 (deltate), 4 (squarate), 5 (croconate) to 6 (rhodizonate), presenting very unusual electronic and vibrational properties, as can be seen in several investigations in literature [3–9]. In the last years there has been a great interest in the crystallographic properties of supramolecular structures involving oxocarbon ions, specifically squarate and croconate, since, they * Corresponding author. Tel.: C55 32 3229 3310; fax: C55 32 3229 3314. E-mail address: [email protected] (L.F.C. de Oliveira). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.01.048

can present a very rich crystal engineering to be researched, and according to Braga and coworkers [10], they can be used for evaluating some fundamental aspects of hydrogen bonding interactions between ions. Hydrogen squarate anion is a very interesting system to identify supramolecular organization. This ion present donor and acceptors sites of hydrogen bonds and p delocalization system. Braga and co-workers [2] studied these interactions in lithium, sodium, potassium and cesium hydrogen squarate, observing the formation of short hydrogen bonds between the anions and also p-stacking interaction. Many of the interesting characteristics of oxocarbon ions can be also discussed in terms of the vibrational spectra, more specifically the Raman spectra, since, they can provide useful information about the chemical environment they are experiencing. In the last years, several compounds involving squarate and croconate ions have been investigated by our group, aiming to understand the relationship between structure and vibrational properties of such a species [3–9]. In particular, the Raman spectrum of potassium hydrogen squarate has been described in the literature [11] and more recently some of the interesting discussions about hydrogen bonds have been made in the description of the vibrational

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properties of squarate ion in restrained geometry, i.e. in the situation of squarate ion caged inside zeolite structure [3]. In this work crystal structure and vibrational (Raman) spectra of rubidium hydrogen squarate are reported. The main purpose of such investigation is to correlate the unambiguous crystallographic data with the other compounds from the series, and also to understand the Raman spectrum of such a species, trying to evaluate how p-stacking and hydrogen bonds are active in the structure and their reflections in the vibrational spectrum. 2. Experimental 2.1. Synthesis In an aqueous solution of lithium squarate (1.53 mmol), rubidium chloride aqueous solution (1.48 mmol) was slowly added under stirring. Single crystal of RBHSQ (0.48 mmol) were obtained by slow evaporation of the standing final solution at room temperature. Analysis of RbHC4O4 (198.52 g molK1): calc. C 24.19 and H 0.50; found C 23.93 and H 0.65. 2.2. X-ray diffraction Single crystal X-ray data were collected in a Nonius Kappa ˚ ) at room CCD diffratometer with Mo Ka (lZ0.71073 A temperature. Data collection and reduction, and cell refinement were performed by DENZO and SCALEPACK programs [12]. Analytical absorption effects were corrected with maximum and minimum transmission of 0.594 and 0.199, respectively. The structure was solved and refined using SHELXL-97 [13]. An empirical isotropic extinction parameter x was refined, according to the method describe by Larson [14]. The structure was drawn by ORTEP-3 for windows [15]. Hydrogen atom was located from Fourier difference maps and its atomic coordinates was fixed. Anisotropic displacement parameters were assigned to all non-hydrogen atoms. CCDC-258327 contains the supplementary crystallographic data for this investigation. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK [fax: (internat.) C1 44 1223/336 033; e-mail: [email protected]]. 2.3. Raman spectra Fourier-transform Raman spectroscopy was carried out using a Bruker RFS 100 instrument and a Nd3C/YAG laser operating at 1064 nm in the near infrared and CCD detector cooled with liquid N2. To improve the signal-to-noise ratios, 2000 scans were accumulated over a period of about 30 min, using 4 cmK1 as spectral resolution. All spectra were obtained more than two times to show reproducibility, and no changes in band positions and intensities were observed.

Table 1 Crystal data of rubidium hydrogen squarate RbHC4O4 198.52 Monoclinic P21/c 4.1472(2) 15.8467(10) 8.0608(4) 96.250(3) 526.60(5) 4 0.15!0.07!0.07 2.504 9.325 0.1988/0.5940 1218 1080 84 0.0351 0.0807 1.109 0.11

Formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) V (A Z Crystal size (mm) dcalc. (g cmK3) m(Mo Ka) (cmK1) Min/max transmission factors No of unique reflexions No. of observed reflexions, F2oO2s(F2o) No. of parameters refined R(Fo) wR(F2o) S ˚ K3) RMS peak (eK A

It was also obtained the Raman spectrum of RbHSQ using 632.8 nm laser excitation (He–Ne laser from Spectra Physics model 127) in a Renishaw Raman microscope fitted with a thermoelectrically cooled CCD detector (Wright, 600!400 pixels) and with an Olympus metallurgical microscope (80!lens), with ca. 7 mW of laser output.

3. Results and discussion Crystal data of RbHSQ are listed in Table 1; some selected bond distances and bond angles are shown in Table 2, whereas the crystal structure is shown in Fig. 1. Table 2 Selected bond distances and bond angles of RbHSQ. ˚) Bond distance (A Rb–O1 Rb–O2 Rb–O2 Rb–O3 Rb–O3 Rb–O4 Rb–O4 Rb–O4 Bond angle (8) C1–C2–C3 C3–C4–C1 C1–C2–O2 C2–C3–O3 C3–C4–O4 C4–C1–O1 Torsion atom (8) O1–C1–C2–O2 O2–C2–C3–O3 Hydrogen bond DHA O1–H1–O2

2.928(3) 3.223(3) 3.275(3) 2.876(3) 3.059(3) 2.944(3) 3.054(3) 3.073(3)

C1–O1 C2–O2 C3–O3 C4–O4 C1–C2 C1–C4 C2–C3 C3–C4

1.299(5) 1.268(5) 1.231(5) 1.230(6) 1.417(5) 1.454(7) 1.470(7) 1.504(6)

90.5(3) 87.7(3) 133.5(4) 137.4(4) 135.9(4) 135.9(4)

C2–C3–C4 C4–C1–C2 C1–C4–O4 C2–C1–O1 C3–C2–O2 C4–C3–O3

88.7(3) 92.8(3) 136.4(4) 131.2(4) 136.0(3) 133.9(4)

3.1(7) K4.4(8)

O1–C1–C4–O4 O3–C3–C4–O4

K5.1(8) 6.2(9)

˚) D–H/H–A (A 0.82/1.68

˚) D/A (A 2.482(4)

D–H/A (8) 164(3)

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Fig. 1. (a) Ortep [15]view of RbHSQ and (b) infinite chain of short hydrogen bond view along of a-axis. Symmetry codes: i (1Cx, 1⁄2 Ky, 1⁄2 Cz); ii (1Kx, 1⁄2 Cy, 1 1⁄2 Kz); iii (Kx, 1⁄2 Cy, 1 1⁄2 Kz); iv (x, 1⁄2 Ky,K 1⁄2 Cz) and n (1Cx, y, z).

RbHSQ is an anhydrous compound and the rubidium atom is bonded to eight oxygen atoms in a very complex coordination geometry. As can be seen in Fig. 1a, three kinds of coordination were observed in this compound:

mododentate (O2), quelate (O1, O2 and O3, O4), and bridge (O3 and O4) forming an infinite chain structure. The range of ˚ . The carbon atoms Rb–O distance is 2.876–3.275 A ˚ of mean deviation. of HSQK form a plane with 0.022 A

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The average of C–C and C–O bonds is, respectively, 1.461(7) ˚ . The biggest difference in C–C and C–O and 1.257(5) A ˚ , respectively. This last result bonds is 0.087 and 0.069 A indicates some electronic delocalization in the anion units. Topological analysis [10] of HSQK crystal packing in RbHSQ shows that the planes containing anion units are ˚ ) but the distance between the centroids of close (3.27 A K ˚ ) is bigger than the observed in HSQ units (4.15 A compounds presenting an effective p-stacking interaction ˚ and the ˚ ) [2]. HSQK rings are shift of 1.96 A (3.15–3.34 A angle between the normals to the ring planes is 0.018. This analysis shows that in despite of the planes which contain the HSQK anions are closely, the molecular units are not so closed. The anions are connected by a short asymmetric intermolecular hydrogen bond forming an extended structure in one-dimension (1D), and this interaction forms a head-totail infinite chain hydrogen bonds motifs, as shown in Fig. 1b [1]. The O–O distance and O–H–O angle of this interaction ˚ and 164(3)8. Analyzing interare, respectively, 2.482(4) A molecular interactions, it can be pointed out that hydrogen bond and Rb–O are the main force driving responsible for crystal packing of RbHSQ, and these interactions are more effective in crystal organization than p-stacking interaction. Braga and co-workers examined the crystal structure of hydrogen squarate of lithium, sodium, potassium and cesium [2]. In these compounds the HC4OK 4 anions are connected via short hydrogen bond interaction like observed in RbHSQ. All hydrogen bond interactions are short, where the shortest O–O distance was observed in LiHC4O4 ˚ ) and the longest in NaHC4O4 (2.503 A ˚ ). The (2.417 A ˚ O–O distance of RbHSQ (2.484 A) is very similar to that ˚ ), noticing that both comobserved in CsHSQ (2.482 A pounds do not present water molecules in their structures, while the others present one water molecule in the MHC4O4 unit. Considering p-stacking interaction, the interplanar distance of MHC4O4 is much related to the type of alkali cation. This distance increases slightly on increasing the cation size, and the same is observed for M–O distance. In despite of interplanar distance of HC4OK 4 observed in RbHSQ has been in agreement to the observed in other alkali hydrogen squarates, the p-stacking is not the most important interaction in the solid state, since, the anion units are not so close, what can be observed analyzing the ˚ ). distance between the centroids of HSQK units (4.15 A These interactions are listed in Table 3. In Fig. 2 are shown the Raman spectra of RbHSQ obtained with 1064 and 632.8 nm excitations, and Table 4 Table 3 ˚ ) of MHSQ (MZLi, Na, K, Rb and Cs). Some interionic parameters (in A Compound

O–O

M–O

Interplanar distance

LiHSQ [2] NaHSQ [2] KHSQ [2] RbHSQ CsHSQ [2]

2.417 2.503 2.472 2.484 2.482

2.063 2.345 2.850 2.876 3.123

3.130 3.155 3.205 3.270 3.315

Fig. 2. FT-Raman spectra of RbHSQ and H2SQ (excitation at 1064 nm).

displays the vibrational assignment of the main bands of the oxocarbon species. It must be noticed that the assignments are tentative, based on vibrational investigations of similar chemical systems. In an attempt to check the dependence of the Raman intensities, even out of resonance conditions, we obtained Raman spectra with different laser excitations: 1064 and 632.8 nm; inspection of Fig. 2 shows that both spectra are very similar in intensities. This fact suggests the hydrogen squarate is not showing the same behaviour of enhancement as squarate ion itself [7], since, in the case of dianion even far from resonance conditions (as in the case showed here) it can be observed an enhancement of the not totally symmetric modes, evidencing the Jahn-Teller effect. On the other hand, no sign of OH stretching vibrations can also be observed with 632.8 nm excitation (Fig. 3) since, in the obtained FT-Raman spectrum (1064 nm) (Fig. 2) this region is not accessible due to equipment restrictions. The early investigations concerning Raman spectra of hydrogen squarate ion were done involving potassium as counter ion [3,11]; as we can see in this investigation, the presence of such a different counter ion as rubidium ion do not provides discrepancies in the vibrational spectra. Table 4 Vibrational wavenumbers and tentative assignment of some most prominent bands of RbHSQ. Raman (cmK1)

Assignment

207 m 261 w 300 s 628 w 643 s 715 s 797 w 1058 m 1172 s 1569 m 1650 s 1802 w

g (O–H) g (C]O) d (C]O) g (C]O) Ring bending Ring breathing d (O–H) n (C–C) n (C–C) n (C]C)Cn (C]O) n (C]C)Cn (C]O) n (C]O)

Mode symmetry

Ag Ag Ag Ag B1g Ag

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The hydrogen squarate ions in both compounds are involved in similar short hydrogen bond, forming an infinite linear chain with head-to-tail hydrogen bonds motifs, and both of them crystallize in the same monoclinic space group (P21/c). Raman spectrum of RbHSQ is quite similar to that obtained for squaric acid (H2SQ) in region from 500 to 1200 cmK1. This region is relative to ring modes, and the results indicate that the rings of RbHSQ and H2SQ are similar. Neutron diffraction crystal structure of H2SQ [16] shows that the difference between the biggest C–C bond ˚ and the average of these distance and the shortest is 0.086 A ˚ bond distances is 1.46 A. The comparison with the same geometric parameters obtained by X-ray crystal structure of ˚ , respectively) shows that the RbHSQ (0.088 and 1.46 A rings of these compounds are very similar. On the other hand, in region from 1200 to 2000 cmK1 the spectra are quite distinct. In both spectra three bands are observed in this region. The band around 1800 cmK1 is assigned to carbonyl stretching mode. In RbHSQ this mode is observed at 1802 cmK1 and at 1821 cmK1 in H2SQ. The relative intensity of this band is also different. In RbHSQ spectrum it is more intense and defined than H2SQ spectrum. The shift of this band could be related to different hydrogen bond interactions that are observed in both compounds. In H2SQ, each ring is involved in four hydrogen bonds forming a polymeric interaction with pseudotetragonal body centered ˚ and this fashion. The average of O–O distance is 2.554 A interaction is little longer than interaction defined as short ˚ ). As can be observed in Table 2, hydrogen bond (2.4–2.5 A a short hydrogen bond interaction is observed in RbHSQ, ˚ . This interaction forms a which O–O distance is 2.482(4) A head-to-tail infinite chain (Fig. 1b). The bands observed from 1500 to 1700 cmK1 are quite different in the Raman spectra. In H2SQ, two weak and broad bands are observed centered at 1617 and 1513 cmK1, otherwise a very intense band are observed at 1650 cmK1 and a medium intense band at 1569 cmK1 in RbHSQ. These

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band are tentative assigned, respectively, to C]C stretching mode coupled to C]O stretching mode. The shifting of these bands are 33 and 56 cmK1, respectively. Diffraction data show very similar C]O bond distances for both ˚ ), but only in H2SQ the compounds (average of 1.230 A ˚ ). Two distinct carbonyl bond distances are similar (1.288 A C–O bonds are observed in RbHSQ, respectively, 1.268 and ˚ . The difference between the biggest C–O bond and 1.299 A ˚ in the shortest is very similar in both compounds: 0.062 A ˚ H2SQ and 0.069 A in RbHSQ. The geometric parameters of the carbonyl bonds for these compounds are similar and do not explain the spectral differences observed in the Raman spectra. This could be interpreted by the association of hydrogen bond interaction and p interaction between the oxocarbon rings in the solid structure. In both compounds, the p interaction is not as effective as observed in alkali metal croconate salts [10], but in RbHSQ this interaction is a little more effective than H2SQ. Braga and coworkers [10] have used mainly three parameters to characterize the p interaction: distance between ring centroids, distance between ring centroid and the underlying plane, and the shift between ring centroids. The distance of the first ˚ and the shortest distance in parameter in RbHSQ is 4.15 A ˚ H2SQ is 4.99 A, although the second parameter is smaller in ˚ ) than in Rb salt (3.27 A ˚ ). The third parameter H2SQ (2.64 A discussed by the authors can explain these results, since, the oxocarbon rings are translated and present average shift of ˚ in H2SQ and 1.97 A ˚ in RbHSQ. The ring centroids of 3.97 A rings in RbHSQ are closer than in H2SQ and could be present an p interaction little more effective than in H2SQ. However, in both salts the hydrogen bond interactions are more important in crystal packing. Previous studies of vibrational spectra of H2SQ [17] had assigned the broad and weak band around 1300 cmK1 to O–H stretching. Similar band is not observed in RbHSQ, although this compound also presents hydrogen bond interaction. The hydrogen bond crystal packing is very distinct in these compounds and it could be responsible to observed spectral difference. The stretching modes of short hydrogen bonds are expected around 850 and 300 cmK1 that are related to asymmetric and symmetric mode, respectively, [18]. These bands are very weak in Raman spectra and could not be identified in RbHSQ.

4. Conclusions

Fig. 3. Raman spectra of RbHSQ and H2SQ excited at 632.8 nm.

Supramolecular investigation of RbHSQ shows that the short and asymmetric intermolecular hydrogen bond, and also Rb–O interactions are responsible for crystal packing. The hydrogen bond interaction forms a linear extended structure of one dimensionality. Although the interplanar ˚ ) is short, this distance observed in RbHSQ (3.27 A compound does not present a strong p-stacking interaction. ˚ ) of the rings The distance between the centroids (4.15 A ˚. shows that these units are horizontally translated by 1.96 A

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Comparing interionic structural parameters of MHSQ (MZ Li, Na, K, Rb and Cs) can be observed that M–O and interplanar distances are related to cation size. These parameters increase slightly on increasing of cation size. On the other hand the hydrogen bond interactions (O–O distance) are not associated to cation type. Comparison of Raman spectra of RbHSQ and H2SQ indicates that carbonyl modes are more affected by crystal packing than the carbon–carbon from the oxocarbon ring modes, due to the wavenumber shifting. The region from 1500 to 1800 cmK1 show very broad and weak features for H2SQ that becomes more intense and best defined in RbHSQ. X-ray crystal data show that M–O and hydrogen bond interaction are more important to crystal packing than p-stacking interaction. This same result is also reached when the Raman spectrum is analyzed, since, the carbonyl modes are more affected in wavenumber shifting and intensity changes than oxocarbon ring modes.

Acknowledgements The authors thank to CNPq, CAPES and FAPEMIG for financial support, and also to Laborato´rio de Espectroscopia Molecular (USP-SP) for the Raman facilities. BLR is grateful to FAPESP for a Postdoctoral fellowship.

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