The imino–amino tautomeric equilibrium in cyanoguanidine dissolved in several aprotic solvents; an FT-Raman spectroscopic study

Journal of Molecular Structure 597 (2001) 49±55

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The imino±amino tautomeric equilibrium in cyanoguanidine dissolved in several aprotic solvents; an FT-Raman spectroscopic study J.M. AlõÂa a,*, H.G.M. Edwards b, F.J. GarcõÂa Navarro a a

E.U.I.T.A. Ð Departamento de QuõÂmica FõÂsica, Universidad de Castilla-La Mancha, Ronda de Calatrava 5, ES-13071 Ciudad Real, Spain b Chemical and Forensic Sciences, University of Bradford, Richmond Road, Bradford BD7 1DP, UK Received 31 August 2000; revised 8 March 2001; accepted 8 March 2001

Abstract The FT-Raman spectra of cyanoguanidine solutions in N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO) and hexamethylphosphoric triamide (HMPA), in a range of concentrations between R ˆ 3 and R ˆ 10 (R ˆ solvent/solute mole ratio), are studied. The intense doublet that appears in the spectral region of the nitrile stretching fundamental is attributed to the presence of the imino (2147 cm 21) and amino (2186 cm 21) tautomeric forms. Furthermore, a shoulder located in the higher frequency wing that could arise from cyanoguanidine molecules associated through hydrogen bonds affecting the nitrogen atom of the nitrile group. When the solute concentration is increased, the relative intensity of that band arising from the imino tautomer increases and a further increase of the associated species can also be observed. These observations are common for the three solvents studied. The concentration of the imino form strongly depends on the solvent, increasing in the order DMF , DMSO , HMPA, which qualitatively corresponds with the donor number ranking. The presence of associated species, however, seems to be controlled by the dielectric constant of the solvent, being scarce in DMSO, moderate in DMF and more important in HMPA. q 2001 Elsevier Science B.V. All rights reserved. Keywords: FT-Raman spectroscopy; Cyanoguanidine; Imino±amino tautomerism; Hydrogen bond

1. Introduction Cyanoguanidine (dicyandiamide) [cgne] is an interesting molecule from both biological and chemical points of view, being widely employed as a ligand of the later transition metals in the synthesis of coordination compounds [1±3]. Its solid state structure has attracted the attention of chemists since the begin* Corresponding author. Tel.: 134-926-295-352; fax: 134-926295351. E-mail address: jmalia@qi®-cr.uclm.es (J.M. AlõÂa).

ning of this century [4,5] and raised several recent theoretical as well as experimental studies [6±10]. It has been suggested that the structural peculiarities observed in the lengths of the different C±N bonds (excluding the nitrile group), which are very similar and whose bond distances are between those corresponding to the single and the double bond, could be explained assuming the coexistence of both the imino (I) and amino (II) tautomeric forms of the compound [11]. Although in the solid state, the predominant form should correspond to the imino tautomer [6,7], which is theoretically more stable, the dissolution

0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00579-8

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DMSO

DMF

HMPA

Molar ratio a

Density (kg dm 23)

[CGN] (mol dm 23)

Molar ratio a

Density (kg dm 23)

[CGN] (mol 23)

Molar ratio a

Density (kg dm 23)

[CGN] (mol dm 23)

10 8 6 5 4 3

1.1238 1.1279 1.1350 1.1407 1.1490 1.1617

1.30 1.59 2.05 2.40 2.90 3.65

10 8 6 5 4 3

0.9766 0.9839 0.9997 1.0033 1.0192 1.0422

1.20 1.47 1.91 2.23 2.69 3.44

10.6 8 6.2 5 4 3

1.0417 1.0457 1.0505 1.0558 1.0629 1.0754

0.53 0.69 0.88 1.08 1.33 1.73

a

Solvent/solute.

J.M. AlõÂa et al. / Journal of Molecular Structure 597 (2001) 49±55

Table 1 General characteristics of the solutions studied

J.M. AlõÂa et al. / Journal of Molecular Structure 597 (2001) 49±55

of cyanoguanidine in appropriate solvents reveals the presence of both tautomers in equilibrium.

In this work, we present vibrational evidence of the simultaneous existence of both tautomeric forms in moderate-to-concentrate solutions of cgne in three aprotic solvents, namely N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO) and hexametylphosphoric triamide (HMPA). Furthermore, the presence of hydrogen-bonded associated forms can be inferred from the CxN stretching bandshape. 2. Experimental Cyanoguanidine [dicyandiamide] (Aldrich, 99%), N,N-dimethylformamide (Aldrich, anhydrous, 99.8%, water ,0.005%), dimethylsulfoxide (Aldrich, anhydrous, 99.8%, water ,0.005%) and hexamethylphosphoramide (Aldrich, 99%), were used as purchased. Solutions were prepared by weight, and

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their densities measured at 208 C with a conventional pycnometer. Table 1 gives some relevant data on solution composition. FT-Raman spectra were always recorded from freshly prepared solutions ®ltered through 0.65 mm Millipore ®lters. FT-Raman spectra were excited at 1064 nm using an Nd:YAG laser and a Bruker IFS66 optical bench with an FRA 106 Raman accesory. Laser power was set at 100±110 mW, and 2000 scans were accumulated with a resolution of 2 cm 21. The temperature was 20 ^ 18 C. The integral intensities of the following Raman bands were used as internal standards in order to normalise the different spectra: C±N±C symmetric stretching at 866 cm 21 in DMF; CH3 deformation at 1420 cm 21 in DMSO; and CH3 symmetric and antisymmetric deformations at 1450 and 1486 cm 21 in HMPA. Original FT-Raman spectral data in an OPUSe (Bruker) format were transferred to JCAMP format and processed with the commercial software GRAMS/32e (Galactic Industries). 3. Results and discussion The effect of the cnge dissolution on the Raman bands attributable to the n (CxN) mode can be observed in Fig. 1, which shows the spectra of both

Fig. 1. FT-Raman spectra of cyaniguanidine in solid state (solid line) and dissolved in DMF, 1:3 molar ratio (dotted line).n (CxN) region.

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J.M. AlõÂa et al. / Journal of Molecular Structure 597 (2001) 49±55

the solid phase and the most concentrated solution in DMF. The doublet observed in the spectrum of the solid sample, with maxima at 2204 and 2158 cm 21, was justi®ed by Jones and Orville-Thomas [12] as an effect of the crystal ®eld. Other authors, such as Sukhorukov and Finkelshtein ([11]) have attributed the doublet, likewise observable in the IR spectrum although showing a marked difference in the corresponding intensity ratio, to a Fermi resonance process. Both possibilities (Davydov effect and Fermi resonance) can be ruled out in Fig. 1, which shows the presence of a new doublet at 2186 and 2148 cm 21 in the Raman spectrum of the cgne dissolution. Remarkably, the relative intensities of both components are inverse to those observed in the spectrum of the solid phase. Sheludyakova et al. [11] have assigned the band at lower wavenumbers, which is stronger in the solid phase spectrum, to the imino form (I), because the conjugation between both carbon± nitrogen multiple bonds should partially reduce the effective order of the CxN triple bond. On the contrary, the amino form (II), although energetically less stable by about 42±50 kJ mol 21 [8±10], clearly predominates in the spectrum of the DMF-dissolved cgne. In order to gain information about the possible tautomeric equilibrium in dissolution, the Raman spectra showed in Fig. 2 were obtained. These spectroscopic results deserve several comments. It is clear that, in all the aprotic solvents studied here, the concentration of the amino form (II) predominates over the whole range of concentrations investigated, contrary to that observed in the solid state where the predominant species should be the imino form (I). Furthermore, the relative content of the imino form increases with the molar concentration of the solutions. It is also evident that the proportion between both tautomers is solvent-dependent, as can be observed in Fig. 3 which shows the n (CxN) doublet in three isoconcentrated (1.5 mol dm 23) dissolutions. The relative concentration of the imino form increases with the trend DMF , DMSO , HMPA. It seems, therefore, reasonable to conclude that a true tautomeric equilibrium must be present in the cgne solutions studied here. The differences observed between the three aprotic solvents, which are quantitatively given in Fig. 4, can be justi®ed taking into account the relative acidity of

both isomers and the basicity of the different solvents, expressed through their corresponding donor number (DN) [13]. Both the macroscopic measurements [14] as well as the theoretical calculations [10] indicate that the amino form (II) must be more acidic than the imino form (I), which would be a relatively stronger base. Considering the DN of the solvents investigated (DMF: 26.6; DMSO: 29.8; HMPA: 38.8), it is evident that the corresponding basicities increase exactly in the same order as that observed in the relative concentration of the imino form. It seems clear that raising the basic character of the solvent results in a shift of the tautomeric equilibrium towards the formation of the most basic species, the imino form (I), as expected. This experimental result con®rms, once more, that dissolved cgne undergoes a tautomeric equilibrium which gives rise to the presence of both amino and imino isomers. Even though the theoretical calculations demonstrate the high value of the activation energy for such equilibrium (170±210 kJ mol 21) [9,10], it has been suggested that the solvation enthalpy could sustantially reduce this barrier [11]. These results could be also discussed in terms of the corresponding dipolar moments of both isomers. The experimental value for cyanoguanidine is 8.16 D [14], although theoretical studies [9] demonstrate a higher polarity in the imino (I) (7.417 D) than in the amino (II) form (3.135 D). Thus, taking into account the dipolar moment of the solvents (DMSO: 3.90 D; DMF: 3.86 D; HMPA: 5.54 D) [13], it is easy to explain the previously discussed results. However, as the dipolar moments of both tautomers are calculated in gaseous phase, it seems preferable in our opinion, to discuss the results in terms of parameters more appropriate for condensed phases, such as relative acidity or solvent's donor number, as Arbuznikov et al. [9] suggest. The spectra shown in Figs. 2 and 3 reveal a complex band structure in both components, visible as a clear shoulder attributable to the imino form (I) and as a broadening towards higher wavenumbers in the stronger component, assignable to the amino form (II). Furthermore, both features are concentrationdependent, as can be observed in Fig. 2: they became more evident as the concentration increases. A reasonable interpretation of these results is the presence of cnge molecules associated through hydrogen bonds,

J.M. AlõÂa et al. / Journal of Molecular Structure 597 (2001) 49±55 Fig. 2. FT-Raman spectra of several solutions of cyanoguanidine dissolved in DMSO, DMF and HMPA. Arrows indicate increasing concentrations (see Table 1).n (CxN) region.

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J.M. AlõÂa et al. / Journal of Molecular Structure 597 (2001) 49±55

Fig. 3. FT-Raman spectra of isoconcentrated (1.5 mol dm 23) solutions of cyaniguanidine in DMSO, DMF and HMPA. n (CxN) region.

Fig. 4. Fraction of imino form (I) calculated as the fraction of area corresponding to the component at lower wavenumbers in the n (CxN) doublet.

J.M. AlõÂa et al. / Journal of Molecular Structure 597 (2001) 49±55

in which the cyano group acts as an acceptor. As has been abundantly demonstrated [15±20], the presence of hydrogen bond directed towards the lone pair of the cyanic nitrogen implies the appearance of new components, usually shoulders, in the n (CxN) band, located in the high frequency wing. The cgne tendency towards the formation of strong hydrogen bonds in the solid state is well documented, both in its own crystal structure [6,7] and in co-ordination [21]. On the other hand, diphenylguanidine, a rather similar system, is strongly associated by hydrogen bonds in dissolution in aprotic solvents [22]. Qualitatively, it seems clear from the spectra of Figs. 2 and 3 that the imino form (I) should be the most strongly affected by the hydrogen bonding, which agrees with its basic character. Furthermore, the shoulder evident in the component assignable to such tautomer became sharper with the trend DMSO , DMF , HMPA. This strongly suggests that the cgne self-association is mainly promoted by the dielectric constant of the solvent, being greater in the solvent medium of lower constant. References [1] A.S. Batsanov, P. Hubberstey, C.E. Russell, P.H. Walton, J. Chem. Soc., Dalton Trans. (1997) 2667. [2] J. Pickardt, B. KuÈhn, Z. Naturforsch., B: Chem. Sci. 51 (1996) 1701. [3] L.M.D.R.S. Martins, J.J.R. FrauÂsto da Silva, A.J. L. Pombeiro,

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