A comparative study of the coordination of saccharinate (sac), thiosaccharinate (tsac) and benzisothiazolinate (bit) ligands to trans-[PdCl2(H2NBz)2]: molecular structure of cis-[Pd(bit)2(H2NBz)2]

A comparative study of reactions of saccharinate (sac), thiosaccharinate (tsac) and benzisothiozolinate (bit) with trans-[PdCl2(H2NBz)2] is reported. While in all cases substitution of both chlorides occurs, product types differ for the three closely related ligands. With sodium saccharinate, trans-[Pd(N-sac)2(H2NBz)2] results in which the sac ligands are N-bound. A similar N-bound coordination is observed with sodium benzisothiazolinate, but a crystal structure shows that they adopt a mutual cis arrangement in cis-[Pd(N-bit)2(H2NBz)2]. In contrast, with sodium thiosaccharinate it is proposed that the new ligands adopt an S-bound coordination mode in trans-[Pd(S-tsac)2(H2NBz)2].


Experimental
General methods 1 H NMR spectra were recorded on a Varian Unity spectrometer in CDCl 3 or d 6 -dmso. IR spectra were recorded on a Shimadzu FT-IR 8400 spectrophotometer in the 400-4000 cm -1 range using KBr discs and in the 200-600 cm -1 using CsI discs. Elemental analysis was carried out at Al Al-Bayt University, Jordan, using a Euro-vector EURO EA 300 elemental analyzer. Melting points were measured on a Gallenkamp melting point apparatus and are uncorrected. Conductivity measurements were carried out on 10 -3 M solutions using a digital conductivity meter. Na 2 PdCl 4 , benzisothiazolinone (Hbit), benzylamine and sodium saccharinate were purchased and used as received. Thiosaccharin [27] and trans-[PdCl 2 (H 2 NBz) 2 ] (1) [28] were prepared by literature methods.

Synthesis of 2
A solution of Nasac (0.285 g, 1.35 mmol) in MeOH (5 cm 3 ) was added to a solution of 1 (0.244 g, 0.62 mmol) in MeOH (10 cm 3 ). The mixture was stirred at room temperature for 3 h. The resulting yellow solid was collected by filtration, washed with MeOH and dried in vacuum. It was recrystallised from CHCl 3 /MeOH to afford 2 as a yellow crystalline solid. Yield 0.341 g, 73%. Anal.
Calc. for C 28

Synthesis of 3
A solution of tsac (0.051 g, 0.26 mmol) in MeOH (5 cm 3 ) was added to a solution of 1 (0.051 g, 0.13 mmol) in MeOH (10 cm 3 ). The mixture was stirred at 30°C for 2 h. The yellow-orange solid formed was collected by filtration and dried under vacuum. Yield 0.068 g, 75%. Anal. Calc. for C 28

Synthesis of 4
A solution of Nabit (0.048 g, 0.28 mmol) in MeOH (5 cm 3 ) was added to a solution of 1 (0.055 g, 0.14 mmol) in MeOH (10 cm 3 ) and stirred for 3 h at room temperature to give a yellow-brown solution. The solution was filtered and left to evaporate to afford yellow crystals. These were collected by filtration, washed with water and dried in a vacuum oven. Yield 0.075 g, 87%. Anal. Calc. for C 28

X-ray crystallography
Crystals of cis-[Pd(bit) 2 (H 2 NBz) 2 ] (4) suitable for X-ray crystallography were produced by slow evaporation of a methanol solution. A yellow crystal with approximate dimensions 0.10 9 0.10 9 0.10 mm 3 was mounted on a glass fibre, and all geometric and intensity data were taken from this sample using a STOE-IPDS diffractometer with Mo-Ka radiation (k = 0.7103 Å , graphite monochromator). Absorption corrections were made using the IPDS software package [29]. All structures were solved by direct methods and refined using full-matrix least-square routines against F 2 with SHELXL-97 [30]. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were included in the models by calculating the positions (riding model) and refined with calculated isotropic displacement parameters. Illustrations were generated using DIAMOND 3.0 [31].

Results and discussion
Addition of two equivalents of sodium saccharinate to a methanol solution of trans-[PdCl 2 (H 2 NBz) 2 ] (1) resulted in the slow formation of trans-[Pd(N-sac) 2 (H 2 NBz) 2 ] (2) isolated in 73% yield as a yellow solid (Scheme 1). Elemental analysis supports the substitution of both halides in 1, as does the symmetrical nature of the 1 H NMR spectrum. This simple substitution and formation of the trans-saccharinate complexes mirrors behaviour previously noted by us [17,18] and others [13]. Reaction of 1 with thiosaccharin in methanol at 30°C resulted in formation of trans-[Pd(S-tsac) 2 (H 2 NBz) 2 ] (3) as a yellow-orange solid in 75% yield (Scheme). Elemental analysis was indicative of the substitution of both chlorides, and this is consistent with Fig. 1 Saccharin (sacH), benzisothiazoline (bitH) and thiosaccharin (tsacH) the 1 H NMR spectrum. On the basis of the observation of an IR band at 1004 cm -1 , which is attributed to the C-S vibration and is shifted some 35 cm -1 from the corresponding vibration in thiosaccharin, we propose that binding of the tsac ligands occurs through sulphur. This is not unexpected and is in accord with the established chalcogenophilic nature of Pd(II) and also with previous work from our laboratory [18]. While we have been unable to crystallographically characterise 2 and 3, we strongly believe that the trans arrangement confirmed in 1 is maintained upon chloride substitution. The basis of this is the relatively simple nature of their IR spectra and the aromatic region of the 1 H NMR spectra, both being consistent with retention of the (approximate) D 2h symmetry. This assignment is also made on the basis of the chemical shifts of the amine protons at d 4.34 and 4.58, respectively (see below).
Reaction of two equivalents of sodium benzisothiozolinate with 1 in methanol gave a yellow-brown solution and, unlike previous reactions with sodium saccharinate and thiosaccharin, no solids initially precipitated from the solution. However, after filtration and upon standing for a few days, slow evaporation of the methanol led to the growth of yellow crystals identified as cis-[Pd(N-bit) 2 (H 2 NBz) 2 ] (4) in 87% yield. The 1 H NMR spectrum was significantly different to those of 1-3, being more complicated with overlapping signals in the aromatic region (indicative of a lowering of the D 2h symmetry), while the amine protons appeared at d 5.56. We have recently reported [Pd(H 2 NBz) 3 Cl][Cl] and note that its 1 H NMR spectrum shows two amine resonances in an approximate 2:1 ratio at d 4.70 (4H) and 5.26 (2H) [34] assigned to the mutual trans amines and that lying trans to the chloride, respectively. This suggested to us that the amines in 4 adopted a relative cis orientation. A single-crystal analysis was carried out in order to determine the coordination mode of the bit ligands and relative arrangement of amines. The results of this are shown in Fig. 2 and its caption ( Table 1).
The structure confirms that the two bit ligands bind in a monodentate fashion through nitrogen, but the main surprise was their relative cis arrangement. All four palladium-nitrogen bond lengths are similar, although those to the benzisothiazolinate ligands  [25], suggesting that they may be sensitive to a trans-influence.
Complex 4 is the third example of a palladiumbis(benzisothiozolinate) complex, and like the diphosphine and diamine derivatives, it also contains a cis arrangement of benzisothiazolinate ligands. Thus, it may be that these ligands inherently prefer to adopt a relative cis orientation, although in 4 this is the first example where the arrangement is not imposed by a chelating co-ligand. A possible explanation for the cis geometry in 4 comes from inspection of the intermolecular packing of the individual molecules. Thus, as shown in Fig. 3, pairs of molecules are strongly associated by hydrogen bonds between the amine protons and the oxygen atoms of the benzisothiazolinate  Fig. 3 Packing of two molecules of 4 with intermolecular bond lengths (Å ). Symmetry operator: i: -x ? 2, -y, -z. We located the protons on nitrogen from Fourier difference maps and acknowledge that this leads to abnormally short N-H distances but we favour this approach over that of using computationally generated positions.