Mixed Ligand, Palladium(II) and Platinum(II) Complexes of Tertiary Diphosphines with S-1H Benzo[D] Imidazole-2-Yl Benzothioate

Introduction

Benzimidazole is the heterocyclic aromatic organic compound¹. This bicyclic ring system consist of benzene ring joined with 4- and 5- situation of imidazole ring, imidazole ring having non adjacent two nitrogen atoms². 2-Mercaptobenzimidazole derived from benzimidazole with thiol group in the 2-position. It holds extra chemical names such as, o-phenylen thiourea, benzimidazol-2-thion with formula of C₇H₆N₂S^3,4. In 2-mercaptobenzimidazole have two N atoms and one S atom and they can independently contribute to its binding to metal⁵. 2-mercaptobenzimidazole derivatives, one of the most important derivatives of benzimidazole exhibited a wide range of importance biological activities such as antimicrobial⁶, antihistamine⁷, neutropic⁸ and analgesic⁹, antiprotozoal, antiviral, and anticancer activities¹⁰. Metal derivatives of heterocyclic-2-thiones have shown a variety of biochemical application: as antidandruff, antifungal, antibacterial, antibiotic and in shampoos, hair creams, plant diseases and hepatitis¹¹.

Thioesters are important class of organosulfur compounds that play a major role in the construction of pharmaceutical, biological, industrial and natural products. In this respect, thioesters synthesis is one of the most important tasks in organosulfur chemistry. The esterification of thiols is the best and most common strategy for the synthesis of Thioesters¹². Diphosphines are a class of chelating ligands that contain two phosphine groups connected by a bridge (also referred to as a backbone). The bridge, for instance, might consist of one or more methylene groups or multiple aromatic rings with heteroatoms attached¹³. The structure of the backbone and the substituents attached to the phosphorus atoms influence the chemical reactivity of the diphosphine ligand in metal complexes through steric and electronic effects¹³. Phosphorus ligands typically bind to the metal centers via the lone pair of electrons on the phosphorus¹⁴.

Experimental

Materials and Instrumentation

The compounds PtCl₂, PdCl₂, dppm, dppp, dppf and S-1H benzo[d] imidazole-2-yl benzothioate (HSBIBT) were commercially available and obtained from Yacoo chem. China. The complexes [Pd(k²-dppf)(HSBIBT)Cl]Cl, [Pd(k²-dppp) (HSBIBT)Cl]Cl, [Pt(k²-dppm) (HSBIBT)Cl]Cl were prepared according to the literature.

Elemental CHNS analysis were carried out on a EuroEA 3000 instrument. The FT-IR spectra in range (4000-200 cm^-1) were recorded as CsI disc on Shimadzu IRAffinity-1S FTIR Spectrophotometer, and FT-IR spectra in range (4000-400 cm^-1) were recorded as KBr disc on Shimadzu, FT-IR spectroscopy Mod IR Affinity-1CE spectrophotometer. UV-Visible spectra were measured using AE-UV1609 (UK) CO., LTD Shimadzu, in the range (200-800) nm. The magnetic susceptibility values of the prepared complexes were carried out at room temperature using Bruker Magnet BM6. Conductivity measurements were made on a conductivity meter type Senz µSiemen tester 4200 (093 cell constant) (UK.). Melting points were measured on Melting Point-MPD-100 Pixel Technology CO., Limited apparatus.

Synthesis of [Pd(k²-dppf)Cl2].CH2Cl2

The solution of dppf (1.6632g, 3mmol) in 15ml CH₂Cl₂ was added to a solution of PdCl₂ (0.5320g, 3mmol) that dissolved in 2 cm³ ethanol and 2 cm³ concentrate HCl. The mixture was reflux for 3 hrs. And then the solvent was evaporate at room temperature to give red precipitate. A small portion of this complex was dissolve in CH₂Cl₂ and set to slow evaporation at room temperature after a few days, the red colored needle crystals were formed suitable for single-crystal diffraction analysis¹⁵. The melting point is 253°C, yield is 2.086gm (95%), formula: [Pd(k²-dppf)Cl₂] and  M.wt.is 816.58 gm/mole, as shown in Figure 1.

Synthesis of [Pd(k²-dppf)(HSBIBT)Cl]Cl

A solution of ligand (0.0509 g, 0.2 mmol) in CH₂Cl₂ (15 cm³) was added to a solution of [Pd(k²-dppf)Cl₂] (0.1463 g, 0.2 mmol) in CH₂Cl₂ (15cm³), and the mixture was heated under reflux for 4 hrs and then the Solvent was evaporated at room temperature to give a red solid. Yield= 91 %, m.p= 139-141°C. as shown in Figure 1.

Synthesis of [Pd(k²-dppp)Cl₂]

The solution of dppp (1.2373g, 3mmol) in 15ml CH₂Cl₂ was added to a solution of PdCl₂ (0.5320g, 3mmol) that dissolved in 2 cm³ ethanol and 2cm³ concentrate HCl. The mixture was reflux for 3 hrs. And then the solvent was evaporate at room temperature to give pale yellow precipitate. A small portion of this complex was dissolve in CH₂Cl₂ and set to slow evaporation at room temperature after a one week, the yellow colored needle crystals were formed suitable for single-crystal diffraction analysis¹⁵. The Melting point is 277 °C, yield is 1.664gm (94%), formula: [Pd(k²-dppp)Cl₂] and M.wt.is 589.72 gm/mole. as shown in Figure 1.

Synthesis of [Pd(k²-dppp)(HSBIBT)Cl]Cl

A solution of ligand (0.0509 g, 0.2 mmol) in CH₂Cl₂ (15 cm³) was added to a solution of [Pd(k²-dppp)Cl₂] (0.1179 g, 0.2 mmol) in CH₂Cl₂ (15cm³), and the mixture was heated under reflux for 4 hrs and then the Solvent was evaporated at room temperature to give a yellow solid. Yield= 93 %, m.p= 142-144°C. as shown in Figure 1.

Synthesis of [Pt(k²-dppmCl)Cl2]

The solution of dppm (1.1532g, 3mmol) in 15ml CH₂Cl₂ was added to a solution of PtCl₂ (0.7979g, 3mmol) that dissolved in 2 cm³ ethanol and 2cm³ concentrate HCl. The mixture was reflux for 3 hrs. And then the solvent was evaporate at room temperature to give yellow precipitate. A small portion of this complex was dissolve in CH₂Cl₂ and set to slow evaporation at room temperature after a one week, the yellow colored needle crystals were formed suitable for single-crystal diffraction analysis¹⁵. The melting point is  ˃ 300°C, yield is 1.756gm (90%), formula: [Pt(k²-dppmCl)Cl₂] and M.wt.is 667.43 gm/mole. as shown in Figure 1.

Synthesis of [Pt(k²-dppmCl)(HSBIBT)Cl]Cl

A solution of ligand (0.0509 g, 0.2 mmol) in CH₂Cl₂ (15 cm³) was added to a solution of [Pt(k²-dppmCl)Cl₂] (0.1300 g, 0.2 mmol) in CH₂Cl₂ (15cm³), and the mixture was heated under reflux for 4 hrs and then the Solvent was evaporated at room temperature to give a yellow solid. Yield= 90 %, m.p= 117-119°C. as shown in Figure 1.

Figure 1: synthesis route of complexes Click here to View figure

 

X-ray Crystallography

Single crystal X-ray crystallography studies were performed at 200 k using a Bruker Kappa Apex II diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 ^°A). For data collection, Apex-II was used while for cell refinement and data reduction, SAINT was used¹⁶. The structures were solved by direct methods applying SHELXL-2013¹⁷, with SHELXLE¹⁸ as a graphical interface.

Red crystals of 1, yellow crystals of 3 and yellow crystals of 5 suitable for X-ray crystallographic measurements were obtained by slow evaporation of dichloromethane solutions of the respective complex. Table 4 gives the crystallographic data and collection parameters. All 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.

Results and Discussion

The mixed ligand complexes were synthesized by two steps. The first step include the preparation of metal complexes with tertiary diphosphines ligands [MCl₂(k²-PPh₂(CH₂)nPPh₂)] where n= 1,3 with [MCl₂(k²-dppf)], and M= Pd and Pt. The second step include the synthesis of mixed ligand complexes by addition of S-1H-benzo[d]imidazole-2-yl benzothioate (HSBIBT) ligand to tertiary diphosphines metal complexes. The physical data of complexes are given in Table (1), the suggested molecular formula are supported by IR spectra, UV-Visible sepectra, elemental analysis, magnetic moment and conductivity measurement.

Table 1: physical characteristics and analytical data for metal complexes

Complex	Color	Yield %	M.P °C	Elemental analysis, calc. (found) %
				C	H	N	S
[Pd(k2-dppf)(HSBIBT)Cl]Cl	Red	91	142-144	58.47(58.47)	3.84(3.75)	2.75(2.84)	3.15(3.24)
[Pd(k2-dppp)(HSBIBT)Cl]Cl	Yellow	93	139-141	58.45(58.35)	4.18(4.15)	3.26(3.32)	3.46(3.79)
[Pt(k2-dppmCl)(HSBIBT)Cl]Cl	Yellow	90	117-119	51.46(51.78)	3.60(3.43)	2.93(3.09)	3.44(3.54)

 

IR Spectral Studies

The infrared spectrum of free (HSBIBT) ligand is characterized by intense absorption band at 1712 cm^-1. This band is assigned to ν(C=O) group^{19, 4}. The stretching frequency of ν(C=N) band which appeared at 1618 cm^-1 forfree (HSBIBT) ligand. This band was shifted to lower frequency during complexation due to the coordination of nitrogen atom to the metals²⁰ (Table 2).

The infrared spectra of complexes showed medium intensity bands in the range 513 to 570 cm^-1, these bands are attributed to ν(M-N)²¹. Also showed a bands in the range 370 to 391cm^-1, these bands are related to ν(M-Cl)²². Further support for the coordinated phosphines ligands has been provided by the appearance of new bands in the range 285 to 291cm^-1, these bands are assigned to ν(M-P)²³.

Magnetic Moments

The magnetic moments of complexes [Pd(k²-dppf)(HSBIBT)Cl]Cl (2), [Pd(k²-dppp)(HSBIBT)Cl]Cl  (4) and [Pt(k²-dppmCl)(HSBIBT)Cl]Cl  (6)  are 0.54, 0.6 and 0.44 B.M. respectively (Table 3), suggesting a square planer  arrangement around the metal and complexes are diamagnetic^24,25.

Electronic Spectral Studies

The UV-Vis spectrum in 10^-3 M chloroform (Table 3) shows three absorption bands of Pd(II) complex (2), at 20661 cm^-1, 28571 cm^-1, and 33783 cm^-1 which are assigned to the ¹A₁g→ ¹A₂g , ¹A₁g→ ¹Eg  and charge transfer transition respectively, these bands are reasonable to square planner geometry²⁶. The UV-visible spectrum of Pd(II) complex(4), gave three spins allowed transitions at 24038 cm^-1, 29411 cm^-1, and 32679  cm^-1 which are assigned to the ¹A₁g→ ¹A₂g , ¹A₁g→ ¹Eg and charge transfer transition respectively. The UV-visible spectrum of Pt(II) complex(6), shows three absorption bands at 23923 cm^-1, 27777 cm^-1, and 32573 cm^-1 which are  assigned to the ¹A₁g→ ¹A₂g , ¹A₁g→ ¹Eg and charge transfer transition respectively, These transitions value are indicated to square planner geometry²⁷.

Conductivity Measurement

The molar conductivity of complexes [Pd(k²-dppf)(HSBIBT)Cl]Cl (2), [Pd(k²-dppp)(HSBIBT)Cl]Cl  (4) and [Pt(k²-dppmCl)(HSBIBT)Cl]Cl  (6) in dimethyl sulfoxide (DMSO) are 34.2, 41.4 and 36 (cm².ohm^-1. mol^-1) (Table 3) suggesting that they are electrolytes complexes^28,29.

Table 2: IR frequencies (cm^-1) of the compounds 

complex	ν(N-H)	ν(C=O)	ν(C=N)	ν(P-Ph)	ν(P-C)	ν(M-N)	ν(M-P)	ν(M-Cl)
HSBIBT	3151	1712	1618	–	–	–	–	–
2	3165	1712	1608	1435	–	513	285	370
4	3159	1712	1612	1435	513	540	288	383
6	3153	1712	1608	1436	509	559	291	391

 

Table 3: Electronic spectra, magnetic moments and molar conductivity of complexes

complex	λmax(nm)/ Chloroform	assignments	µeff (BM)a	ɅM(Ω-1cm2 mol-1)/ DMSO
2	484350296	1A1g→ 1A2g1A1g→ 1EgC.T	0.54	34.2
4	416340306	1A1g→ 1B1g1A1g→ 1EgC.T	0.6	41.4
6	418360307	1A1g→ 1A2g1A1g→ 1EgC.T	0.44	36

 

X-ray Crystal Structures

The [Pd(k²-dppf)Cl₂].CH₂Cl₂ complex was crystallized in a dichorometahne (CH₂Cl₂) at room temperature. After one week, the red colored needle type crystals had grown from the CH₂Cl₂ solvent and these crystals were subjected to single-crystal X-ray diffraction study. The molecular structure of [Pd(k²-dppf)Cl₂].CH₂Cl₂ complex is shown in Figure 1. The [Pd(k²-dppf)Cl₂].CH₂Cl₂ complex crystallizes in a monoclinic space group Monoclinic, P21/c. a= 9.7960(3) A, b= 18.1527(7) A, c=19.0713(7) A, α= γ =90°, β=102.536(1). Structural analysis reveals that [Pd(k²-dppf)Cl₂].CH₂Cl₂ complex has a square planer geometry around the metal center, two chlorines and two phosphorus atoms are coordinated with the palladium ion. The dppf behave as bidentate chelating ligand. The crystal determination data and selected bond distances and bond angles for [Pd(k²-dppf)Cl₂].CH₂Cl₂ complex are summarized in Tables 4 and5. The palladium center was found to a distorted square planar geometry, with Cl1-Pd1-P1, Cl2-Pd1-P2, Cl1-Pd1-P2 and Cl2-Pd1-P1 angles of 87.79(2), 83.96(2), 171.50(3), and 177.75(2) respectively [30, 31].

The [Pd(k²-dppp)Cl₂] complex was crystallized from a CH₂Cl₂ solution of the complex at room temperature, after a one week, as yellow colored needle type crystals are formed, these crystals were subjected to single-crystal X-ray diffractometer study. The molecular structure of [Pd(k²-dppp)Cl₂] complex is shown in Fig. 2. The structural analysis reveals that [Pd(k²-dppp)Cl₂] complex has a distorted square planer geometry around the metal center and that the two phosphorus and two chlorine atoms are coordinated to the palladium ion. The dppm behave as behave as bidentate chelating ligand. Whereas the Pd-P bond distance of 2.2424(4) and 2.2456(4) Ǻ for P(1) and P(2) atoms of the [Pd(k²-dppp)Cl₂] complex^30,31. The crystal determination data and selected bond distance and bond angles for [Pd(k2- dppp)Cl₂] complex is summarized in Tables 4 and  6.

Figure 2: Molecular Structure of [Pd(k2-dppf)Cl2].CH2Cl2 Complex Click here to View figure

 

The [Pt(k²-dppmCl)Cl₂] complex was crystallized from a CH₂Cl₂ solution of the complex at room temperature, after a five days, a yellow colored needle type crystals are formed, these crystals were subjected to single-crystal X-ray diffractometer study. The molecular structure of [Pt(k²-dppmCl)Cl₂] complex is shown in Fig. 3. The structural analysis reveals that [Pt(k²-dppmCl)Cl₂] complex has a square planer geometry around the metal center and that the two phosphorus and two chlorine atoms are coordinated to the platinum ion. The dppm behave as behave as bidentate chelating ligand. Whereas the Pt-P bond length of 2.217(2) and 2.217(2) Ǻ for P(1) and P(2) atoms of the [Pt(k²-dppmCl)Cl₂] complex^30,31. The crystal determination data and selected bond distances and bond angles for [Pt(k²-dppmCl)Cl₂] complex is summarized in Tables 4 and 7.

Figure 3: Molecular Structure of [Pd(k2-dppp)Cl2] Complex Click here to View figure

 

Table 4: Crystallographic Data and structure refinement for [Pd(k²-dppf)Cl₂].CH₂Cl₂, [Pd(k²-dppp)Cl₂] and [Pt(k²-dppmCl)Cl₂]complexes.

	[Pd(k2-dppf)Cl2].CH2Cl2	[Pd(k2-dppp)Cl2]	[Pt(k2-dppmCl)Cl2]
Empirical formula	C34H28Cl2FeP2Pd.CH2Cl2	C27H26Cl2P2Pd	C24.27H21.46Cl2P2Pt(Cl)0.729 (H) 0.54
Formula weight	816.58	589.72	667.43
R	0.03	0.02	0.03
Temperature	200 K	200 K	200 k
Radiation	0.71073 A	0.71073 A	0.71073 A
Crstal system	Monoclinic	triclinic	Monoclinic
space group	P21/c	P-1	C2/c
a(°A)	9.7960(3)	8.4865(3)	16.2034(5)
b(°A)	18.1527(7)	10.5484(4)	7.8274(2)
c(°A)	19.0713(7)	14.4020(6)	19.2496(6)
Α(deg)	90	88.388(2)	90
β(deg)	102.536(1)	80.064(2)	98.918(1)
γ(deg)	90	73.475(2)	90
volume	3310.5(2)A^3	1217.16(8) A^3	2411.92(12)A^3
F(000)	1640	596	4, 1.838 Mg/m^3
Z, Calculated density	4, 1.638 Mg/m^3	2, 1.609 g/m^3	6.263 mm^-1
Absorption coefficient	1.426 mm^-1	1.128 mm^-1	1288
Crystal size	0.41 x 0.46 x 0.47 mm	0.19 x 0.31 x 0.51 mm	0.04 x 0.20 x 0.21 mm
Dataset	-13: 13 ; -20: 24 ; -25: 25	-11: 11 ; -13: 14 ; -18: 19	-13:21; -10:10; -25:25
Reflections collected / unique	60083,8203,[R(int)= 0.018]	32265, 6016,[R(int)= 0.016]	16294,3007,[R(int)= 0.030]
Observed Data [I > 2.0 sigma(I)]	7414	5668	2930
R, wR2, S	R= 0.0284, wR2= 0.0693,S= 1.04	R= 0.0177, wR2= 0.0451, S= 1.03	R= 0.0478 wR2= 0.1178 S= 1.30
Min. and Max. Resd. Dens	[-2.08, 1.73] e/Ang^3	[-0.44,0.44] e/Ang^3	[-1.71, 4.83] e/Ang^3
Max. and Av	0.00 and 0.00	0.00 and 0.00	0.00 and 0.00

 

Table 5: Selected bond distances (in A°) and bond angles (in degree) for [Pd(k²-dppf) Cl₂].CH₂Cl₂

Bond distance		Bond angle
Pd(1)-Cl(1)	2.3417(7)	Cl(1)-Pd(1)-P(1)	87.79(2)
Pd(1)-Cl(2)	2.3577(7)	Cl(1)-Pd(1)-P(2)	171.50(3)
Pd(1)-P(1)	2.2774(7)	Cl(2)-Pd(1)-P(1)	177.75(2)
Pd(1)-P(2)	2.2883(7)	Cl(2)-Pd(1)-P(2)	83.96(2)
Fe(1)-C1(1)	1.996(2)	P(1)-Pd(1)-P(2)	97.94(2)
Fe(1)-C1(2)	2.028(2)	C1(3)-Fe-C1(4)	40.16(10)
P(1)-C1(1)	1.802(2)	Pd(1)-P(1)-C(31)	108.46(7)

 

Table 6: Selected bond distances (in A°) and bond angles (in degree) for [Pd(k²-dppp)Cl₂]

Bond distance		Bond angle
Pd(1)-Cl(1)	2.3575(4)	Cl(1)-Pd(1)-Cl(2)	90.54(1)
Pd(1)-Cl(2)	2.3557(4)	Cl(1)-Pd(1)-P(1)	87.77(1)
Pd(1)-P(1)	2.2424(4)	Cl(1)-Pd(1)-P(2)	177.80(2)
Pd(1)-P(2)	2.2456(4)	Cl(2)-Pd(1)-P(1)	172.11(2)
P(1)-C(1)	1.8243(16)	Cl(2)-Pd(1)-P(2)	91.39(1)
P(2)-C(3)	1.8355(16)	P(1)-Pd(1)-P(2)	90.47(1)
C(3)-H(3)A	0.9900	Pd(1)-P(1)-C(1)	116.42(5)

 

Table 7: Selected bond distances (in A°) and bond angles (in degree) for [Pt(k²-dppmCl)Cl₂]

Bond distance		Bond angle
Pt(1)-Cl(1)	2.3661(19)	Cl(1)-Pt(1)-P(2)	98.10(7)
Pt(1)-P(2)	2.217(2)	Cl(1)-Pt(1)-P(1)	98.10(7)
Pt(1)-P(1)	2.217(2)	Cl(1)-Pt(1)-Cl(1)_a	90.02(6)
Pt(1)-Cl(1)_a	2.3661(19)	Cl(1)-Pt(1)-P(2)_a	171.58(7)
Pt(1)-P(2)_a	2.217(2)	Cl(1)_a-Pt(1)-P(2)	171.58(7)
P(1)-C(21)	1.816(7)	P(2)-Pt(1)-P(2)_a	73.87(8)
P(2)-C(11)	1.808(8)	Pt(1)-P(1)-C(21)	117.2(3)

 

Scheme 1   Click here to View scheme

 

Figure 4: Molecular Structure of [Pt(k2-dppmCl)Cl2] Complex   Click here to View figure

 

Figure 5: The Proposed Geometrical Structure of [Pd(k2-dppp)(HSBIBT)Cl]Cl Complex Click here to View figure

 

Figure 6: The Proposed Geometrical Structure of [Pt(k2-dppmCl)(HSBIBT)Cl]Cl Complex Click here to View figure

 

Conclusions

In this research we have describe the synthesis and characterization of some Palladium and platinum complexes obtained from the reaction of the Thioester ligand with tertiary diphosphines metal complexes as shown in Figure 1. The manner of bonding and overall structure of complexes were determined through elemental analysis, magnetic susceptibility, molar conductance, IR spectral data and UV-Visible. All synthesized complexes are square planar and diamagnetic.

Acknowledgements

The authors are thankful to the chemistry Department. College of Education-Salahaddin University for their accomplishing our present work and Prof T.I.A. Gerber and Eric. C. Hosten at Nelson Mandela Metropolitan University for their help in obtaining the X-ray crystallographic data.

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