Synthesis and Characterization of Palladium(II) Complexes with 2-(2'-hydroxylphenyl)benzoxazole, 2-(2'-hydroxyl phenyl)benzothiazole or the Mixed Ligands

Introduction                                   

Currently there is a considerable interests in the coordination chemistry of heterocyclic ligands contain nitrogen, oxygen and sulfur donors[1-10]. Partially because of the biological significance of such ligands. Thus benzothiazole derivatives are key components in the bioactive compounds of both natural and synthetic origin. However benzoxazoles are important biological substances, covering an extensive spectrum of essential and selective antimicrobial activities [11-16]. In addition to their biological importance they are strongly coordinating agents and form stable complexes with various transition metal ions [17]. The biological activity of these ligands increase on complex formation with metal ions [18,19]. It is believed that they react selectively toward certain biological system[20].

The use of biological ligands with hard base characters for chelation of heavy metals like uranium, thorium or lanthanides has been a subject of interest of some research groups [21-25] as those kinds of ligands generally produce very stable chelate complexes. This makes them highly appropriates for therapeutic development of clinical chelators in safety procedures related to contamination through natural and depleted thorium and uranium and in similar chemoprevention processes against uranium, thorium heavy metals intoxication.

In view of the above mentioned importance of these ligands and their complexes and as a continuous of our interests in synthesis of new complexes with heterocyclic ligands containing nitrogen, oxygen or sulfur donors [26-30] we report here the syntheses and characterization of palladium(II) complexes with 2-(2^¢-hydroxylphenyl)benzoxazole (Hpbo), 2-(2^¢-hydroxylphenyl)benzothiazole (Hpbt) and  mixed ligands (Hpbo) and (Hpbt).

Experimental

General

The experimental techniques were the same as those used in our recent paper from this laboratory [31].

Starting materials

The compounds 2-(2^¢-hydroxylphenyl)benzoxazole, 2-(2^¢-hydroxylphenyl) benzothiazole and Na₂PdCl₄ were commercial products and were used as supplied.

Synthesis of the complexes

trans-[PdCl₂(Hpbo)₂] (1)

A suspended mixture of Na₂PdCl₄ (0.074g, 0.25mmol) and Hpbo (0.10g, 0.5mmol) in methanol (5ml) was stirred at room temperature for 3h .The produce yellow solid was filtered off washed with cold methanol and dried under vacuum (0.12g , 82% yield).

Complex 3 was prepared and isolated in a similar manner.

trans-[Pd(pbo)₂] (2)

NEt₃ (0.02g, 0.2mmol) was added to a suspension of trans-[PdCl₂(Hpbo)₂] (0.10g, 0.1mmol) in methanol (5ml).The mixture was stirred for 3h during this period the colour of the mixture changed from yellow to orange. The produce orange solid was filtered off washed with cold methanol, dried in a vacuum oven and recrystallized from CHCl₃ to give the product as orange crystals (0.08g, 90% yield).

The following complexes were prepared and isolated in a similar manner (4) and (6).

trans-[PdCl₂(Hpbo)(Hpbt)] (5)

Methanol (10ml) was added to a solid mixture of Hpbt (0.051g, 0.2mmol), Hpbo (0.046g, 0.2mmol) and Na₂PdCl₄ (0.059g, 0.2mmol). The resulting mixture was stirred at room temperature for 3h to give a yellow solid, which was filtered off washed with cold methanol and dried in a vacuum oven (0.11g, 89%yield)

Results and Discussion

Synthesis of the complexes

Treatment of Na₂PdCl₄ with two mole equivalent of 2-(2^¢-hydroxylphenyl) benzoxazole (Hpbo) or 2-(2^¢-hydroxylphenyl)benzothiazole (Hpbt) in methanol gave complexes of the type trans-[PdCl₂(Hpbo)₂] (1) or trans-[PdCl₂(Hpbt)₂] (3) respectively Figure (1). The ligands Hpbo or Hpbt were coordinated as monodentate ligands to palladium through the heterocyclic N-atom. Mixed ligands complex of the type trans-[PdCl₂(Hpbo)(Hpbt)] was also prepared by treatment of Na₂PdCl₄ with one mole equivalent of (Hpbo) followed by (Hpbt).

The prepared complexes are stable at room temperature, soluble in DMSO solvent but are insoluble in other common organic solvent such as CH₂Cl₂,CHCl_3, MeOH or EtOH.

 

Figure 1: structures of the ligands and the prepared complexes Click here to View figure

 

Complex (1) has been reported previously [32]. It has been made starting with a rather different starting palladium compound, trans-bis(benzonitrile)dichloro palladium(II), and a long reaction time (20 days).

Attempts to displace the chloride ions of the above complexes with saccharinate ion by treatment with sodium saccharinate [33] were unsuccessful and always the starting complexes were obtained. It might be the stirric effect caused by the (hydroxyl groups prevent such reaction to occur.

Treatment of the prepared palladium complexes (1) and (5) with triethylamine in methanol gave the chelated palladium complexes of the type trans-[Pd(pbo)₂], trans-[Pd(pbt)₂] or trans-[Pd(pbo)(pbt)] respectively Figure(2). The deprotonated anion ligands pbo or pbt behave as chelate ligands coordinated to palladium through the N-atom of the heterocyclic ring at the O-atom of the deprotonated OH group.

The prepared chelated complexes are soluble in CHCl₃ or CH₂Cl₂ but insoluble inMeOH or EtOH solvents.

Attempts to open the chelate ring in the above chelate palladium complexes (2,4,6) with diphosphine such as dppm or dppe in a similar way to that reported for other chelate complexes [26], were unsuccessful and always the starting complexes were obtained.This probably reflect the high stability of chelate ring to be open by diphosphine.

Characterization of complexes

The prepared complexes were identified by Elemental analysis, i.r. spectra, conductivity measurements and ¹H nmr. spectra and their data are listed in Tables (1-3). The molar conductivity of complexes in DMSO or CHCl₃ is low enough to suggest that they are non-electrolytes [34].                 

Figure 2: structures of the prepared chelate complexes (4)-(6) Click here to View figure

 

Nuclear magnetic resonance 

The ¹H-nmr data of the prepared complexes are given in Table 3. The ¹H-nmr spectra of complexes trans-[PdCl₂(Hpbo)₂] (1), trans-[PdCl₂(Hpbt)₂] (3) showed a broad singlet each at dH=11.20 and 11.54 ppm a singed to the hydroxyl proton in complexes (1) and (2) respectively, while trans-[PdCl₂(Hpbo)(Hpbt)] (5) showed two broad singlets at  dH=11.18 and 11.58 ppm assigned to two different hydroxyl protons of the two ligands (Hpbo) and (Hpbt) respectively. The phenyl protons appeared within the dH=7.0-8.026 ppm ring. Integration showed that the ratio of the OH: aromatic protons are 1:8.

The hydroxyl signals disappeared in the spectra of the chelated complexes (2), (4) and (6) confirming the deprotontion of the hydroxyl protons. The aromatic protons appeared within the chemical shift dH=6.70-8.0 ppm.

On the basis of the above n.m.r. data and other identification data given in Tables 1 and 2, the structures shown in Figures 1 and 2 have been suggested.

Infrared spectra

The i.r. data of the prepared complexes are given in Table 2. The i.r. spectra of complexes 1, 3 and 5 have a broad medium intensity bond centered at 3299, 3390 and 3284 cm^-1 respectively due to the vibration of the hydroxyl group u(OH).

The spectra also showed a strong bond at 338, 342 and 335 cm^-1 assigned to u(Pd-Cl) in a trans arrangement around palladium [35,36]. The u(OH) and u(Pd-Cl) bands disappeared in the spectra of complexes (2), (4) and (6) and a new medium intensity band at around 420-433 cm^-1 appeared. This band assigned to u(Pd-O) [37]. The spectra of the prepared complexes (1)-(6) showed a medium to strong band at 1596-1606 cm^-1 shifted to a lower frequency from that in the corregonding free ligands. This band assigned to u(C=N) of the heterocyclic coordinated to palladium through the N-atom [38]. Further evidence came from the appearance of a new medium intarsity band at 474-485 cm^-1 which was absent in the spectra of the ligands this band assigned to u(Pd-N).

Conclusions

In summary reaction of Na₂PdCl₄ with two mole equivalents of (Hpbo), (Hpbt) or the mixed ligands (Hpbo) and (Hpbt) in methanol gave complexes of the type trans-[PdCl₂(Hpbo)₂], trans-[PdCl₂(Hpbt)₂] or trans-[PdCl₂(Hpbo)(Hpbt)]. These complexes do not seem to react with sodium saccharinate.

Treatment of the prepared complexes with triethylamine gave the chelate complexes trans-[Pd(pbo)₂], trans-[Pd(pbt)₂] and trans-[Pd(pbo)(pbt)]. These complexes failed to react with diphosphine such as dppm or dppe.             

Table 1:  Color, M.P., Yield, Elemental analysis and conductivity of complexes (1)-(6)

 

 

 

 

 

 

 

 

 

 

Table 2:  I.r. spectra date (cm^-1) of the ligands and complexes (1)-(6)

Compound	υ(OH)	υ(Ar-H)	υ(C=N)	υ(C=C)	υ(C-O)	υas(COC)	υ(CSC)	δ(CH)	υ(Pd-Cl)	υ(Pd-N)	υ(Pd-O)
Hpbo	3190b	3058m	1631s	1595s 1485s	1253s	1155m	—	750s	—	—	—
[PdCl2(Hpbo)2]	3299b	3064m	1606s	1550s 1487s	1261s	1197m	—	752s	338s	485m	—
[Pd(pbo)2]	—	3056m	1604s	1531m	1253s	1193w	—	746s	—	480m	420m
Hpbt	3163b	3000m	1620s	1605s 1487s	1300s	—	1106m	756s	—	—	—
[PdCl2(Hpbt)2]	3390b	3006m	1600s	1508m	1307m	—	1105m	756s	342s	480m	—
[Pd(pbt)2]	—	3058m	1596s	1542s 1487s	1290w	—	1141w	748s	—	515m	440m
[PdCl2(Hpbo)(Hpbt)]	3284b	3070w	1600m	1546m 1500m	1259w 1299w	1209w	1108w	752s	335s	474m	—
[Pd(pbo)(pbt)]	—	3062m	1604s	1537s 1463s	1251s	1225w	1160w	746s	—	480m	433m

Table 3:  ¹H-nmr data for complexes (1)-(6)

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