Synthesis and characterization of labile ylide of 1-(p-tolyl)-2-(tri-p- tolylphosphoranylidene)ethanone and its related complexes with mercury(II) halides

Article history: Received June 28, 2013 Received in Revised form December 10, 2013 Accepted 20 December 2013 Available online 21 December 2013 The novel α­phosphorus labile ylide of 1­(p­tolyl)­2­(tri­p­tolylphosphoranylidene)ethanone (PTTPPY) is prepared by the reaction of 2­bromo­1­(p­tolyl)ethanone with tri­p­tolylphosphine in chloroform as solvent. The complexes of the type [HgX2(p­tolyl)3PCHCOC6H4CH3)] (X = Cl, Br and I) are prepared by the reaction of title ylide with mercury(II) halides in good yields, using methanol as solvent. The final products were characterized by IR, H, C, P NMR spectroscopic methods and microanalysis.


Introduction
Phosphorus ylides are important reagents in organic chemistry, especially in the synthesis of naturally occurring products with biological and pharmacological activities. 1 The organometallic chemistry of phosphorus ylides R 3 P=C(R ′ )COR ′′ (R, R ′ , R ′′ = alkyl or aryl groups) has undergone great growth over the last few years, mainly due to their interesting application as reactants in organometallic and metalmediated organic synthesis. 23Resonance stabilized ylides, particularly the keto ylides are also successfully used as ligands in organometallic and coordination chemistry owing to their accessibility and stability towards air and moisture. 4Due to delocalization of electrons in labile ylide, we have an ambidentate ligand and thus bond to a metal center through either the carbanion or the enolate oxygen (Scheme 1).Coordination through carbon is more predominant and observed with soft metal ions, e.g., Pd(II), Pt(II), Hg(II), Au(I), and Au(III), 57 whereas, O coordination dominates when the metals involved are hard, e.g., Ti(IV), Zr(IV), and Hf(IV). 8

Scheme 1.
The reaction of BPPY ylide with mercury(II) chloride has already been reported by Nesmeyanov et al. 9 We are currently interested in synthesis and reactivity of metal derivatives of such ylides as α keto stabilized ylide of (ptolyl) 3 PCHCOC 6 H 4 CH 3 and its related mercury(II) halide complexes.In this work, the complexes obtained from the reaction of the new prepared αcarbonylstabilized ylide of 1(ptolyl)2(triptolylphosphoranylidene)ethanone with HgX 2 (X = Cl, Br and I), have been reported.All of the products were characterized by IR, 1 H, 13 C, 31 P NMR spectroscopic methods and microanalysis.

Results and Discussion
The ν (CO) which is sensitive to complexation occurs at 1599 cm 1 in the parent ylides, as in the case of other resonance stabilized ylides. 6Coordination of ylide through carbon causes an increase in ν (CO) while for Ocoordination a lowering of ν (CO) is expected.The infrared spectra of complexes in the solid state show ν (CO) in the range of 1638, 1626 and 1621 cm 1 , indicate coordination of the ylide through carbon at higher wave numbers with respect to the free labile ylide (PTTPPY ν (CO) 1599 cm 1 ) (Table 1).The ν (P + ─C ) which is also diagnostic for the coordination, occurs at 881 cm 1 in the parent ylide.These assignments confirmed by comparing the IR spectra of the corresponding 13 C substituted ylides. 8 the present study, the ν (P + ─C ) stretching frequencies for all four complexes were shifted to lower frequencies and observed at 852, 848 and 837 cm 1 for 1, 2 and 3, respectively, suggesting partial removal of electron density of the P−C bond.The 1 H and 31 P NMR data of the mercury(II) ylide complexes along with those of the parent ylide are listed in Table 2.The CH signals of the three complexes are shifted downfield compared to that of the free ylide, as a consequence of Ccoordination character.The expected downfield shifts of 31 P signals for PCH group upon complexation were observed in their corresponding spectra.The appearance of single signals for PCH group in the 31 P NMR indicates the presence of only one molecule for all three complexes.The resonances of 31 P NMR complexes 1, 2 and 3 were observed at a lower field with respect to the free ylide (Table 2), thus suggesting a direct bond to methyl carbon with mercury (Fig. 1).It must be noted that Ocoordination of the ylide generally leads to the formation of cis and trans isomers giving rise to two different signals in 31 P and 1 H NMR. 6 The 13 C NMR data of the complexes and the title ylide are listed in Table 3 along with possible assignments.The 13 C NMR shifts of the CO group in the complexes 1 and 2 are higher than 191.34 ppm noted for the same carbon in the parent ylide, indicating much lower shielding of CO group carbon in the complexes but this is not in agreement with complex 3 and the reason is not clear yet.Such upfield shift in complexes 2 and 3 also observed in [PdCl(η 3 2XC 3 H 4 )(C 6 H 5 ) 3 PCHCOR] (X = H, CH 3 ; R = CH 3 , C 6 H 5 ) and was attributed to change in hybridization of the ylidic carbon. 11Similar upfield shifts of 12 ppm with reference to the parent ylide were also observed in the case of [HgI 2 (C 6 H 5 ) 3 PC 5 H 4 ] 2 and in our synthesized mercury complexes. 13No coupling constant for mercury was observed at room temperature in 1 H and 13 C NMR spectra of complexes is due to the broad coupling of proton with mercury and phosphorus atoms.The suggested dimeric type structure of complexes are shown in Fig. 2. 14,15

Conclusion
This work describes the synthesis and characterization of a keto ylides (PTTPPY) and its mercury(II) complexes of the general formula [HgX 2 (ptolyl) 3 PCHCOC 6 H 4 CH 3 )] (X = Cl, Br and I).The physical, chemical and spectroscopic data propose that PTTPPY herein exhibits as monodentate Ccoordination to the metal center.Theoretical studies on the gas phase structures of the latter complexes, confirm the translike dimeric structure for all complex compounds.The observation of similar analytical and spectroscopic data after further support the presence of a centrosymmeteric dimeric structure containing the labile ylide and HgX 2 .

Experimental
Methanol was distilled over magnesium powder and diethyl ether (Et 2 O) over CaH 2 just before use.All other solvents were reagent grade and used without further purification. 1H, 31 P, 13 C NMR NMR measurements were recorded on a Bruker 300 spectrometer in DMSOd 6 and CDCl 3 using TMS as the internal reference.Solid state IR spectra in the region 2004000 cm 1 using KBr pellets were obtained on a FTIR Perkin Elmer spectrophotometer.Elemental analyses were performed using a Heraeus CHNORapid analyzer.

Synthesis of ylide [(p-tolyl) 3 PCHCOC 6 H 4 CH 3 ]
2bromo1(ptolyl)ethanone (0.21 g, 1 mmol) was dissolved in chloroform (20 mL), then a solution of triparatolylphosphine (0.30 g, 1 mmol) in the same solvent (5 mL) was added to the above solution dropwise, and the pale yellow solution was stirred for 4 h.The solution was concentrated under reduced pressure to 10 mL, and diethyl ether (20 mL) was added to it.The white formed solid was filtered off, washed with petroleum benzene (2 × 10 mL), and dried under reduced pressure.In order to get final product, whole of the crude solid (0.51 g, 97%), was treated with alkaline solution of 5% NaOH, and stirred at 40ºC for about 24 h, the pale yellow precipitate of 1(ptolyl)2(trip tolylphosphorane ylidene)ethanone was obtained.

Table 1 .
ν (CO) of selected phosphoranes and their metal complexes 10