Structural, Spectroscopic, and Thermal Behaviour of Bis -(thiosaccharinate)- aqua-cadmium(II)

Summary. The crystal structure of the title complex, [Cd( tsac ) 2 (H 2 O)], has been determined by single crystal X-ray diffraction methods. It crystallizes in the monoclinic space group C2/c ( a (cid:136) 12.236(3), b (cid:136) 8.919(3), c (cid:136) 16.655(3) A˚, (cid:12) (cid:136) 96.18(2) (cid:14) , Z (cid:136) 4). The molecular structure was solved from 1705 independent reﬂections with I > 2 (cid:27) (cid:133) I (cid:134) and reﬁned to R 1 (cid:136) 0 : 0489. Infrared and Raman spectra of the complex were recorded and are brieﬂy discussed. Its thermal behaviour was investigated by thermogravimetry and differential thermal analysis.


Introduction
The coordination chemistry of metal thioamides has received increasing attention during the last decades taking into account the participation of sulfur-containing molecules and ions in a variety of biological systems and processes [1,2].
Only the crystal structures of thiosaccharin itself [16] and of its sodium [17] and potassium [18] salts have been reported so far. Additionally, some of its transition metal complexes have recently been characterized by IR spectroscopy; the results suggested coordination through the nitrogen atom [19].
Considering the interesting coordination possibilities of the tautomeric form b (Fig. 1), we simultaneously started theoretical studies of the electronic properties of the thioamide [20] and experimental ones about its interaction with some heavy metal cations which should be favoured by Pearson's rule [1,21]. We could isolate insoluble compounds with Ag(I), Cd(II), Hg(II), Pb(II), and Tl(I). However, only in the case of Cd(II) we were able to obtain single crystals adequate for structural analysis.
In this paper we report the synthesis of this new complex, the results of the single crystal X-ray investigation, and its vibrational spectroscopic as well as thermal behaviour.

Structural analysis
Crystal parameters and details of the applied re®nement procedures are given in Table 1, whereas Fig. 2 shows a PLUTON2 [22,23] plot of the structure of [Cd(tsac) 2 (H 2 O)] including the atom labelling of the non-hydrogen atoms. Selected interatomic distances and angles are compiled in Table 2.
The Cd(II) ion is located on a crystallographic two fold-axis in a ®ve-fold environment. It is coordinated to four symmetry related thiosaccharinate anions through the thiocarbonyl sulfur atoms (d(Cd-S) 2.467(1) A Ê ) and one of the sulfonyl O-atoms (d(Cd-O(2)) 2.512(3) A Ê ). These ligands reside at the corners of a distorted tetrahedron, squashed along the two-fold axis (S-Cd-S and O-Cd-O bond angles of 134.52(6) and 139.2(2) , respectively). The ®fth ligand is a water molecule located on the axis (d(Cd-O(3)) 2.219(6) A Ê ). This distance is lower than the sum of the Cd(II) ionic radius and the van der Waals radius of oxygen [24].
The Cd-S bonds are in the order of the shortest known metal-sulfur distances of covalently bonded thiolates or thioamidates [25±27], suggesting the existence of a negative charge at the sulfur atom. The Cd-O bond distances, on the other hand, are of the order of the sum of the Cd(II) ionic radius and the van der Waals radius of oxygen, indicating only a weak interaction between these atoms.
The compound is arranged in the crystal as a layered structure parallel to the ab plane. In each layer, the Cd(II) ions form a centered rectangular lattice, with nearest   The thiosaccharinate moieties are almost planar. The principal features of the thioamidic functional group are that the C(1)-S(1) distance is larger and the C(1)-N distance is shorter than in the thiosaccharin molecule (C(1)-S(1) 1.622 A Ê , C(1)-N 1.384 A Ê [16]). This observation is in agreement with the results of our theoretical calculations about the electronic structure of the thiosaccharin molecule and its anion [20], indicating that after deprotonation the negative charge at the nitrogen atom is essentially transferred to the C(1)-N bond which acquires double bond character. Simultaneously, thiocarbonyl %-bonding electrons are transferred to the sulfur atom, and the C(1)-S(1) bonds adopt single bond character, thus increasing the negative charge at the heteroatom and facilitating the Cd(II)-sulfur interaction expected from Pearson's rule [1,21].
It should be noted that in most heavy metal saccharinates, like mercury derivatives (cf. Ref. [28] and references cited therein) or [Cd(sac) 2 [13] or bridging structures as in [Cd(sac) 2 (im) 2 ] [31]. It is also interesting that the CdS 2 O 3 environment found for the Cd(II) cation of the title compound seems to be rather uncommon. Apparently, it has so far only been observed in a complex with an arenephosphinothiol ligand [32].
In a simpli®ed analysis the 42 internal vibrational modes of the thiosaccharinate anion can be divided into aromatic ring motions, sulfothiocarboximide (C(S)NSO 2 ) group motions, and isoindole group motions. Most of the aromatic ring vibrations, particularly CH stretchings and bendings, CC stretchings, and CCC deformations, have been identi®ed on the basis of our previous studies of saccharinate compounds [7,8,10,12] complemented by the theoretical study of Binev et al. [40] and data from related substances like benzothiophene [41] and phthalimide [42].
The internal motions of the water molecules could also be clearly identi®ed. The O-H stretching vibrations are of medium intensity, relatively broad, and centered around 3610 cm À1 in the IR spectrum. The HOH bending mode produces a IR band at 1636 cm À1 , near to a stretching vibration of the thiosaccharinate ligand. The spectroscopic behaviour of the ®ve-membered ring, including the chalcogen atom and the SO 2 moiety, seems to be of special interest because this part of the molecule must re¯ect the properties of the metal-ligand interactions. In the same way as for the saccharinate anion, the stretching vibrations of the SO 2 group are related to some of the most intense IR bands. These vibrations are expected to appear at lower wavenumbers in the Cd(II) complex compared to thiosaccharin due to the increased S-O bond lengths (d(S-O) 1.431/1.425 A Ê in thiosaccharin [16], 1.441/1.432 A Ê in the complex (cf. Table 2)) as a consequence of the interaction of this group with the metallic center and the electronic charge reorganization after deprotonation [20]. As can be seen from Table 3, this expectation is clearly ful®lled for # as (SO 2 ).
The assignment of the respective symmetric stretching mode is not so straightforward, because this mode is coupled with other vibrations of the isoindole moiety. In free thiosaccharin, this band has been found at 1155 cm À1 [16,19]. However, due to the interaction of the Cd(II) cation with the O-atoms of the sulfonyl groups, for this vibrational mode a frequency lowering may be expected, too. On this basis we assume that the strong IR band at 1118 cm À1 is mostly related to this mode. Therefore, the very strong IR band at 1156 cm À1 and the medium intensity Raman line at 1150 cm À1 , found at almost the same position as in thiosaccharin, must be essentially related to breathing modes of the ®ve-membered ring.
A comparison of the C(1)-S(1) and C(1)-N bond lengths in the thiosaccharinate complex and in the free ligand (1.622 and 1.384 A Ê , respectively [16]), shows a lengthening of the ®rst one with a concomitant important reduction of the latter one, in good agreement with the results of our theoretical study of the electronic structures of both species [20]. Thus, one can assume that the band related essentially to the C(1)-N bond stretching (1247 cm À1 in the IR spectrum of thiosaccharin) may be strongly displaced to higher wavenumbers in the cadmium complex, and we have assigned the 1418 cm À1 entity to this vibration. On the other hand, and although the C=S stretching mode is not a good group vibration because it is coupled with other motions of the sulfothioamide skeleton [16], the band mainly related to the C(1)-S(1) stretching motion (1039 cm À1 in thiosaccharin) should be displaced to lower energies in the complex, in which it is actually found at 1005 cm À1 (cf. Table 3).
Other bands in the spectral range below 1000 cm À1 are more dif®cult to assign. They are usually of complex origin and involve strong couplings of different motions. Therefore, the assignment proposed in Table 3 for some of these bands is only tentative. It was also dif®cult to identify with certainty a band related to the Cd-S stretching mode. The band assigned to this vibration in Table 3 lies in the spectral range usually found for this mode in other complexes containing M-S bonds [43].

Thermal behaviour
The analysis of the TG and DTA curves shows a weight loss of 3.6% between 75 and 128 C as the ®rst TG step, accompanied by a weak and relatively broad endothermic DTA-signal centered at about 115 C. This corresponds to the loss of the bonded water molecule (theoretical loss: 3.42%). Subsequently, three further TG-steps are observed between 128±310, 310±450, and 450±750 C. The ®rst one is related to a well-de®ned and medium intensity exothermic DTA peak at 295 C, whereas the last two are associated to a unique, broad, and¯at exothermic signal extending between 450 and 650 C. These three successive processes are related to the gradual decomposition of the thiosaccharinate ligands, involving a total weight loss of 69.1% (theoretical value: 69.2%). The ®nal pyrolysis residue was identi®ed as CdS by IR spectroscopy.

Synthesis
Thiosaccharin was prepared by reaction of saccharin (Mallinckrodt) with Lawesson's reagent (Fluka) in toluene according to the procedure of Schibye et al. [44]. The cadmium complex was obtained by slow addition of 0.4 mmol of cadmium nitrate dissolved in 10 cm 3 of water to 0.8 mmol of thiosaccharin dissolved in 40 cm 3 of water under continuous stirring. The precipitated solid complex was ®ltered off, washed several times with small portions of cold water, and ®nally dried over CaCl 2 . Amber-like single crystals adequate for X-ray studies were grown by slow evaporation from saturated aqueous solutions.
All hydrogen atoms were found among the ®rst nine peaks of a difference Fourier map. However, the thiosaccharinate H-atoms were positioned stereochemically and re®ned with the riding model. During the re®nement, the independent water hydrogen atom was kept ®xed at its located position.
Tables containing complete information on atomic coordinates and equivalent isotropic parameters, bond distances, angles, anisotropic thermal parameters, and hydrogen atomic positions are available from the authors upon request and have been deposited at the Cambridge Crystallographic Data Centre (reference number CCDC 157696).

Spectroscopic measurements
Infrared spectra were recorded up to 400 cm À1 on a Perkin Elmer GXFT-IR instrument using the KBr pellet technique. Spectra obtained from suspensions of the powdered samples in Nujol gave identical results. Raman spectra of the powdered sample in a capillary tube were scanned on a Spex Ramalog double-monochromator spectrometer using the 514.5 nm line of a Spectra Physics Ar laser for excitation.

Thermal analysis
Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out on a Rigaku Denki Thermo¯ex instrument under a nitrogen¯ow of 30 cm 3 Á min À1 up to a ®nal temperature of 750 C.
The heating rate was 5 C Á min À1 , and employed sample masses were about 10 mg. Al 2 O 3 was used as a DTA reference standard.