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]
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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].
KeywordsMeOH Palladium Coordination Chemistry Coordination Mode PdCl2
1H NMR spectra were recorded on a Varian Unity spectrometer in CDCl3 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. Na2PdCl4, benzisothiazolinone (Hbit), benzylamine and sodium saccharinate were purchased and used as received. Thiosaccharin  and trans-[PdCl2(H2NBz)2] (1)  were prepared by literature methods.
Synthesis of 2
A solution of Nasac (0.285 g, 1.35 mmol) in MeOH (5 cm3) was added to a solution of 1 (0.244 g, 0.62 mmol) in MeOH (10 cm3). 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 CHCl3/MeOH to afford 2 as a yellow crystalline solid. Yield 0.341 g, 73%. Anal. Calc. for C28H26N4O6PdS2: C, 49.1, H, 3.8, N, 8.2. Found: C, 49.2, H, 3.7, N, 8.2. Molar conductivity (DMSO): 0.40 (Ω−1 mol−1 cm−1). IR (KBr): 3265w, 3130w, 3029w, 1672s, 1593w, 1451w, 1290s, 1155m, 563m cm−1. 1H NMR (CDCl3): δ 7.94–7.92 (m, 2H, sac), 7.87–7.85 (m, 2H, sac), 7.75–7.72 (m, 4H, sac), 7.30–7.22 (m, 10H, Ph), 4.34 (bs, 4H, 2NH2), 3.97–3.93 (m, 4H, 2CH2) ppm. Mp: 224–226 °C.
Synthesis of 3
A solution of tsac (0.051 g, 0.26 mmol) in MeOH (5 cm3) was added to a solution of 1 (0.051 g, 0.13 mmol) in MeOH (10 cm3). 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 C28H26N4O4PdS4: C, 46.9, H, 3.7, N, 7.8. Found: C, 46.9, H, 3.8, N, 8.0. Molar conductivity (DMSO): 0.40 (Ω−1 mol−1 cm−1). IR (KBr): 3425sb, 3051w, 2922w, 1541m, 1463m, 1384s, 1163s, 1004m, 806m, 370s cm−1. 1H NMR (DMSO-d6): δ 7.89 (dd, J 8.0, J 3.2, 4H, tsac), 7.71 (t, J 8.0, 2H, tsac), 7.58 (t, J 8.0, 2H, tsac), 7.29 (s, 10H, Ph), 4.58 (bs, 4H, 2NH2), 3.69 (s, 4H, 2CH2) ppm.
Synthesis of 4
A solution of Nabit (0.048 g, 0.28 mmol) in MeOH (5 cm3) was added to a solution of 1 (0.055 g, 0.14 mmol) in MeOH (10 cm3) 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 C28H26N4O2PdS2: C, 53.3, H, 4.1, N, 9.2. Found: C, 53.4, H, 4.4, N, 9.5. Molar conductivity (DMSO): 0.80 (Ω−1 mol−1 cm−1). IR (KBr): 3195w, 3112w, 2927w, 1650s, 1539s, 1450m, 1290m, 1155m, 459w, 342m cm−1. 1H NMR (DMSO-d6): δ 7.76 ppm (d, J 7.7, 2H, bit), 7.66 (d, J 7.7, 2H, bit), 7.57–7.23 (m, 14H, Ph + bit), 5.56 (s, 4H, 2NH2), 3.56 (s, 4H, 2CH2) ppm. Mp: 208–210 °C.
Crystals of cis-[Pd(bit)2(H2NBz)2] (4) suitable for X-ray crystallography were produced by slow evaporation of a methanol solution. A yellow crystal with approximate dimensions 0.10 × 0.10 × 0.10 mm3 was mounted on a glass fibre, and all geometric and intensity data were taken from this sample using a STOE-IPDS diffractometer with Mo-Kα radiation (λ = 0.7103 Å, graphite monochromator). Absorption corrections were made using the IPDS software package . All structures were solved by direct methods and refined using full-matrix least-square routines against F 2 with SHELXL-97 . 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 .
Results and discussion
Crystallographic data for cis-[Pd(bit)2(H2NBz)2] (4)
Crystal system, space group
Unit cell dimensions
a = 9.8581(4) Å, α = 90°
b = 23.7295(8) Å, β= 102.856(3)°
c = 11.6318(5) Å, γ = 90°
Z, Calculated density
4, 1.555 mg/m3
0.10 × 0.10 × 0.10 mm
Theta range for data collection
−13 ≤ h ≤ 13, −32≤ k ≤ 32, −15 ≤ l ≤ 14
19,921/7115 [R(int) = 0.0597]
Completeness to θ = 25.00
Max. and min. transmission
0.9162 and 0.9162
Full-matrix least-squares on F 2
Goodness-of-fit on F 2
Final R indices [I > 2sigma(I)]
R 1 = 0.0377, wR 2 = 0.0732
R indices (all data)
R 1 = 0.0696, wR 2 = 0.0851
Largest diff. peak and hole
0.605 and −0.628 e A−3
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 [Pd–N(1) 2.022(2), Pd–N(2) 2.015(3) Å] are slightly shorter than to the benzylamine groups [Pd–N(3) 2.045(2), Pd–N(4) 2.056(3) Å]. The latter compare well with the related bonds in trans-[PdCl2(H2NBz)2] [Pd–N 2.050(4) and 2.046(2) Å] [32, 33] and [PdCl(H2NBz)3]Cl. H2O [Pd(1)–N(1) 2.061(2), Pd(1)–N(2) 2.053(2), Pd(1)–N(3) 2.063(2) Å] . Both Pd–N(bit) bond lengths in 4 are significantly shorter than those in [Pd(N-bit)2(κ 2-Ph2PCH2CH2PPh2)] [Pd–N 2.070(3) & 2.100(3) Å] , being closer to [Pd(N-bit)2(κ 2-H2NCH2CH2NH2)] [Pd–N 2.029(2) & 2.031(2) Å] , suggesting that they may be sensitive to a trans-influence.
In this contribution, we have shown that simple exchange of both chlorides in trans-[PdCl2(H2NBz)2] (1) for the related mono-anionic (X) N-heterocyclic saccharinate, thiosaccharinate and benzisothiozolinate ligands in all cases affords the expected palladium(II) complexes [PdX2(H2NBz)2]. The molecular structure of the product is, however, sensitive to the nature of the incoming ligand with products trans-[Pd(N-sac)2(H2NBz)2] (2), trans-[Pd(S-tsac)2(H2NBz)2] (3) and cis-[Pd(N-bit)2(H2NBz)2] (4) resulting, respectively. Formation of N-coordinated saccharinate and S-bound thiosaccharinate ligands to the same metal fragments has been previously noted  and likely results from a preference of Pd(II) to bind to a soft sulphur centre when available. Palladium(II) bis(benzisothiozolinate) complexes are far less common [25, 26] but the three crystallographically characterised examples all contain a cis arrangement of benzisothiozolinate ligands. In [Pd(N-bit)2(H2NBz)2] (4), this is the first time that this cis arrangement has not been imposed by the presence of a chelating co-ligand and the preferential precipitation of cis-4 over its trans isomer (which may be initially formed) may result from the ability of the cis complex to form strong intermolecular hydrogen bonds with a neighbour, thus favouring crystallisation of this isomer.
CCDC 1503153 contains the supplementary crystallographic data for 4. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data-request/cif.
We would like to thank the University of Tikrit for partial support of this work.
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