Solid state NMR study of 1 , 3-imidazolidine-2-thione , 1 , 3-imidazolidine-2-selenone and some of their N-substituted derivatives

Solid-state NMR spectra were recorded for 1,3-imidazolidine-2-thione, 1,3-imidazolidine-2-selenone and some of their N-substituted derivatives. Spinning side-bands of thione and selenone carbons were analysed to yield chemical shift anisotropies for these carbons. The NMR spectrum of imidazolidine-2-thione (Imt) showed some evidence for the presence of thiol tautomer. Molecular computations were carried out for Imt and its N-methyl derivative to yield relative energies of various tautomers.


Introduction
There has been considerable interest in 1,3-imidazolidine-2-thiones (Imt), 1,3-imidazolidine-2selenones (ImSe) and their derivatives, because of their ambidentate nature [1].That is, they may coordinate to a metal through S (or Se), nitrogen or by various chelating modes.These penta-atomic ligands exist in a thiol thione equilibrium [2].However, it has been established that the thione form dominates in the solid state of these complexes [3,4].We have studied extensively the interaction of metal ions with Imt and its derivatives [5].Most of the complexations with these ligands were carried out in solution and investigated by NMR spectroscopy.The X-ray structures of some of the complexes only show a specific unit for the ligand, i.e. thione form in the case of ligand and thiolate form in the case of complexation.This was explained as due to the weakening and elongation of the thione when bonded to metal, as demonstrated by IR [5].
In this study, we have carried out a solid state NMR studies of these penta-atomic ligands.Examination of solid state NMR of powder samples provides principal components of the chemical shift tensors, which potentially contain a wealth of structural information.Before embarking on a solid-state NMR study of Imt-metal and ImSe-metal complexes, it is important to understand the shielding tensors for the uncoordinated ligand.Towards this end we present the results of solid state 13 C NMR for Imt and various related ligands and some of their selenium counterparts.

Preparation of Imt and ImSe ligands
The thione ligands were synthesized according to the procedure described in the literature (by the addition of CS 2 to diamines in ether and then heating the resulting adduct at 100 • C for 2-3 hrs, followed by its crystallization in methanol) [6,7].
The selenone ligands were synthesized according to the procedure described in the literature [8,9].

Solid-state NMR studies
Natural abundance 13 C solid state NMR spectra were obtained on a JEOL LAMBDA 500 spectrometer operating at 125.65 MHz (11.74 T), at ambient temperature of 25 • C. Samples were packed into mm zirconium oxide rotors.Cross polarization and high power decoupling were employed.Pulse delay of 7.0 s and a contact time of 5.0 ms were used in the CPMAS experiments.The magic angle spinning rates were from 2000 to 4000 Hz.Carbon chemical shifts were referenced to TMS by setting the high frequency isotropic peak of solid adamantine to 38.56 ppm.The CPMAS spectra of thione and selenone  carbons were analysed using a program based on Maricq and Waugh [10] and developed by Durham University, UK, and run on a unix platform.The calculated chemical shift tensors are given in Table 1.

Molecular computations
Using the Gaussian 98 program system [11], calculations at the density functional theory (DFT) [12] levels using Becke's nonlocal three-parameter exchange and correlated functional with Lee-Yang-Parr correlation (B3LYP) [13] functional methods were performed.The optimized molecular geometries and energies were obtained using 6-311+G * * basis sets.Imidazolidine-2-thione (and its N-methyl derivative) has three expected tautomers, Scheme 1.These tautomers could be classified as the thione form (T1) and two thiol tautomers, where the S-H bond maybe eclipse the C=N bond (T2) or being anti to it (T3).The B3LYP/6-311+G** optimized total energies, relative energies, dipole moments and rotational constants are listed in Table 2.The calculated energies suggest the higher stability of the T1 relative to the other tautomeric forms.In Imt, the cis (T2) and trans (T3) thiol tautomers are found to be of equal energy that is about 72.5 kJ/mol higher than that of the thione form.However, in N-methylimidazolidine-2-thione, the stability order based on the calculated relative energies of the three tautomers is T1 < T2 < T3.
The optimized structural parameters, namely, intermolecular bond lengths and angles for the three tautomers of both the molecules are listed in Tables 3 and 4 and are compared to the X-ray data for N,N-dimethyl-2-imidazolidinethione [14].The geometries calculated by this method are expected [15] to have rms errors of ca.0.006 Å for the bond lengths and ca.±4 kJ/mol for the energies.It is clear that the X-ray data agrees well with the calculated results for the thione tautomer (T1).In Imt, the C3-N1 182.8 X-ray data for N,N-dimethyl-2-imidazolidinethione taken from [14].and S-C bond lengths in T1 were about 1.369 (Å) and 1.666 (Å), respectively, these values changed to 1.275 (Å) and 1.775 (Å) in T2 and T3 due to the formation and/or disappearance of double bonds.The calculated dihedral angles showed that the molecule is non-planar.
The dihedral angle CCNC shows nonplanarity of about 24.4 • out of the plane.Furthermore, in the cis (T2) and trans (T3) tautomers, the HSCN dihedral angle shows a deviation from nonplanarity by about 5.5 • in T2 and about 3 • in the T3.

Results and discussion
The solid-state CPMAS 13 C spectrum Imt (1a) is shown in Fig. 1.Based on the gas phase calculation, we expect only two isotropic peaks for this compound in its thione form.However, there is an extra set of peaks which can only be explained in terms of a tautomeric isomer.This phenomenon was not observed in any of the other compounds studied.Imt was prepared three times independently, and spectra recorded showed similar pattern indicating the reproducibility of the result.The splitting observed in the high field region cannot be attributed to the second order quadrupolar effects due to the nitrogen nuclei as this was not observed in any of the other related molecules.Moreover, at the high fields (11.74 T) used in our experiment, this effect is expected to be very low.The second order effect due to two equivalent nitrogens should result in a 4 : 4 : 1 triplet for the thione carbon.However, we do not observe any splitting.So we can safely assume that the second order effects are negligible in these case.
A similar tautomerism has been suggested in imidazoline in its coordination compounds [16].The tautomers present in five-membered rings that contain nitrogen (azoles) have been reported [17].In spite  of the higher energy of the thiol tautomer in the gas phase, the stability of the thiol tautomer in the solid state may be explained due to the hydrogen bonding possible in the solid state (Scheme 2).
The Imt (1a) peaks are broader than the peaks in the other compounds and this can be explained due to the interproton transfer [17,18] possible in the catenation form shown above.This also explains the fact that N-mono substituted Imt compounds do not show tautomerism, due to the absence of the stabilizing H bonding possible in these compounds.This would imply we have got mixed crystals rather than having the two tautomers in the same unit cell.A similar situation where two crystal forms of a tetrazole containing compound was observed [19].
In the IR spectrum the S-H stretching is generally expected to be weak around 2550-2600 cm −1 .The IR spectrum of solid Imt (1a) does show a weak peak at 2575 cm −1 , confirming the presence of thiol form in the solid state.The solution C-13 NMR spectrum of 1a in DMSO, on the other hand, showed only two peaks at 183.4 ppm and 44.1 ppm corresponding to the thione structure.This indicates that the hydrogen bonded structure of the thiol form breaks down in solution giving rise to near 100% thione form.
The chemical shift tensors, anisotropy and asymmetry factors for the compounds studies are given in Table 1.The shielding tensor component 3 is defined as the one farthest from the isotropic value and component 2 is the closest to the isotropic.When the component 2 is more shielded than the isotropic value the anisotropy will be negative, as observed in the compounds 1b-1g.At the same time components 1 and 3 appear to swap.There seem to be only a minor effect in the tensors by the substitution on the nitrogen atoms in the Imt moiety.The thione carbon peaks (isotropic as well as side-bands) of compound 1d are 1 : 1 doublets, indicating two non-equivalent molecules in the unit cell.

Table 1 13
C chemical shift tensors a for the C=X (X = S or Se) carbons