Reaction of N-ethoxycarbonylpyrrole-2-thiocarboxamide and N-phenylcarbamoylpyrrole-2 thiocarboxamide with [M(PEt3)4] (M=Ni, Pd, and Pt)

Introduction

A large number of papers have appeared on the study of complexes with the ligands having  Sulphur and Nitrogen donor atoms.^1,2 Although the purpose of these investigations were mainly to get a clear understanding of the nature of bond formed between Sulphur atom of the ligand and the central metal ion, but a coherent picture of the donor acceptor relationship between Sulphur and metal ions is still not clear. Further, the metal Complexes of b-diketones and their derivatives exhibit interesting Properties,³ there exist however, a Possibility to modify these Properties if one replaces –CH group between to Corbonyl  or thiocarbonyl groups by a more electronegative N atom in b-diketones. It will, therefore, be interesting to study metal Complexes of b-diketones type ligands having sulphur as donor atom’s, so  that a Correlation could be sought between different Physico-chemical Properties of the Various metal Complexes. This paper reports the synthesis and characterization of several complexes of N- Ethoxycarbonylphyrrole-2 thiocarboxamide(A) and N- Phenylcarbamoylphyrrole-2 thiocarboxamide(B) with various metal ions.

Experimental

N-Ethoxycarbonylpyrrole-2-thiocarboxamide and N-Phenyl Carbamoylpyrrole-2-thicarboxamide according to the Procedure described in [M(PEt₃)₄] (M=Ni, Pd and Pt) were prepared according to the literature methods⁴.

Preparation of metal complexes.

[Ni(PEt₃)₂ (LH)] (LH=ETH on PTH)

To a solution of [Ni(PEt₃)₄ (0.42g~ 0.8mmol) in 20ml CH₂Cl₂ was added one equivalent of ligand LH(0.16g or  0.20g respectively, 0.8mmol). The resultant   solution   was stirred   for  3h   and   filtered.  The  filtrate   is   removed    to

smaller   volume  20ml  Et₂– Hexane ( 1!1VïV) was added to this solution where by a coloured Precipitate of the complex was formed. It was feltered and washed with Et₂0-Hexane(2x2ml) and dried in vacuo.

Yield. (Ni(PEt₃)₂ (ETH)], 0.31g, 80%,

(Ni(PEt₃)₂ (PTH)], 0.28g, 65%

 [Pd(PEt₃)₂ (LH)] (LH=ETH or PTH)

A suspension of  [Pd(PEt₃)₄] (0.29g~0.5mmol) in benzene (15ml) was added one equivalent (0.5mmol) of LH(ETH,0.1g, PTH, 0.12g) with constant stirring. The resultant solution was refluxed for  3h then filtered and reduced to smaller volume. It was kept in referigerator for 30 minutes where by crystalline precipitate of the complex formed. It was filtered and washed with Et₂0(2x2ml), Pet ether (60-80) (2x2ml) and dried in vacuo.

Yield: [Pd(PEt₃)₂ (ETH)], 0.19g, 70%

[Pd(PEt₃)₂ (PTH)], 0.20g, 70%

 [Pt(PEt3)₂(LH)] (LH= ETH or PTH)

A Suspension Of [Pt(PEt₃)₄] (0.20g, 0.3mmol) and one Equivalent of LH (0.3mmol) (ETH, 0.05g; PTH, 0.07g) in acetone- TMF (20ml, 1:1 V çV) was heated under reflex for 10 min, during which the colour of the soluation changes. It was feltered and concentrated to 5ml. On adding 20ml petroleum ether (60-80⁰) Precipitate of the complex formed. It was feltered, washed with petroleum ether (60-80⁰) (2×2ml) and dried in vacuo.

Analyses

A weighed amount of complex was taken in a Pyrexbeaker. It was decomposed by digesting it with aquaregia for some time until the residue gaves transparent solution with dil. mineral  acids. This soluation was used to estimate metal ions and sulphur.

Estimation of Nickel, Palladium and Platinum

Ni and Pd were estimated as their dimethyl  glyoximate and weighing as M(C₄H₇O₂N₂)₂ (M=Ni or Pd) and Pt as K₂PtCl₆.

Estimation of Sulphur

Sulphur was estimated as BaSo₄. Carbon, Hydrogen and Nitrogen analyses were Performed by the microanalytical section of the Indian Institute of Technology, Kanpur(U.P.) India. The analytical results are given in Table II-1.

Infrared Spectra

Infrared spectra of the ligand and metal complexes were recorded with a Perkin- Elmer 580 Diffraction grating Infrared Spectrophotometer in the range 4000-250cm^-1 range. Samples were Prepared as Potassium bromide Pellets. The mejor bands of the infrared spectra of the ligand and complexes are given in Table II-2.

Magnetic Susceptibility Measurements

Magnetic Properties of the Complexes were checked with the help of Gowy balance at room temperature.

Visible Spectra

The absorption Spectra of the ligand and Complexes were measured in CH₂Cl₂ on Cystromic 108 spectrometer from 200-900nm. The results are given in Table II-3.

Results and Discussion

Treatment of [M(PEt₃)₄] (M= Ni, Pd, or Pt) with one equivalent of LH(ETH or PTH) in a solvent (CH₂Cl₂ or Benzene or Acetone- DMF) undergo replacement of two molecules of PEt₃ by one molecule of LH.

The Compound are air stable, soluble in CH₂Cl₂ or benzene, Partialy soluble in DMF or Benzene and insoluble in CHCl₃, DMF or DMSO. On Comparing the Stoichemetry of these. Compound with known Preferences, the analytical data suggest that ligands in all the complexes is functioning as bidentate. As a bidentate, it can link, through any two of the five donor sites viz., Nitrogen, Oxygen and Sulphur. The IR Spectral studies of ligand and complexes are taken to know the possible modes of linkages in the complexes. The Positions of mejor I.R bands  present in the spectra of the ligand and complexes are give in Table II-1`. Since attempt to deuterate the ligand, failed, our tentative assignments are based on the comparative IR spectral studies  of a number of related compounds.⁴

Characteristic IR bands of pyrrole moiety (1550, 1470, 1400, 1140, 1040, 995, and 750 cm^-1) were present in the spectrum of lignad⁵. The position of these bands did not shift in the spectra of complexes (maximum shift ± 5 cm^-1). This indicates that NH group of pyrrole moiety is not participating in the bond formation. It is highly unlikely that Oxygen of – OEt group will participate in bond formation with the metal. NH group of – NHPH group in PTH is less  basic owing to the deleralization of  lone pair of electron in phenyl ring. It is, therefore, assumed that – CSNH C(O) moiety of the ligand is participating the bond formation with the metal and the mejor shift in the positions of the bands are expected to be in thioamide and amide bands. The mejor changes in the band positions in the spectra of the complexes compared to those of ligand along with the discussions are as follows.

M(PEt₃)₂ (ETH)] (M= Ni, Pd or Pt)

The ligand bond at 3360 cm^-1 due to(NH) shifted towards lower wave numbers and  appeared around 3310 cm^-1 suggesting increased electro negativity of   nitrogen  atom of the-CSNHC(O)- moiety and absence of deprotonation in the complexes.

The thioamide^6-8 band I and II arising due to coupled vibration of δ(NH) and (CN) at 1540 and 1340 cm^-1 shifted towards higher ware number.

The broad band at 1050 and 870 cm^-1 of the free ligand   shifted towards   lower  wave numbers (~20 cm^-1) in the spectra of the complexes.  The  band  at 1070 cm^-1   have  been assigned to  thioamide band III having mejor contribution from (C-N) and (C-S) in  the normal coordinate. The band at 870 cm^-1 in assigned toC ^………S (thioamide band IV). The shift in the positions of these bands towards lower wave numbers in the complexes further confirmed the bonding of the metal through thiocanbonyl sulphur.

The position of ligand band at 495 cm^-1 assigned to δ (C=S), shifted towards lower ware numbers (~20 cm^-1).

The characteristic bands of triethyl  phasphine at 2962, 2872, 2853, 1467, 700 and 500cm^-1 were present in all the complexes¹.

The new bands around 470-350, and 450-300 in  the spectra of  all  the complexes  are assigned to mixed  ( M-N) and  (M-S) vibrations.¹⁰

From the forgoing discussions of IR spectra, the following bonding scheme is assumed.

The NH group(1) will not Participate in bonding due to following two reason.

 NH(1) is less basic than NH(2) to the deleralization of nitrogen lone pair electron density in the ring.

 There is a steric hindrance of phenyl group when nitrogen lone pair undergoes bond formation with a lewis acid (metal atom).

Thus the bonding pattern of PTH with metal atom is almost same as ETH. The shift of characteristic PTH bands are as follows.

All the characteristic bands of pyrrole moiety are Present in complexes.

The ligand band 3410 and 3260 cm^-1 becomes broad and that at 3160 cm^-1 due to(NH) of –CSNHCO- moiety remains stationary (±5cm^-1) in all the complexes.

 The thioamide band  I and II (drising due to coupled vibrations of δ(NH) and(CN) at 1525 and 1350 cm^-1) shifted towards the higher wave number (10-15cm^-1)

 The broad band at 1010 and 860 cm^-1 (arising due to thioamide band III and IV respectively) shifted to lower wave number (10cm^-1) in all the complexes.

Position of the ligand band 490 cm^-1 assigned to δ(C=S) shifted to lower wave number (20cm^-1)

The characteristic bands of triethylphosphire at 2962, 2872, 2853, 1467, 700 and 500 cm^-1 were Present in all the complexes.

The new bands around 470-350 and 400-300 cm^-1 in the spectra of all the complexes are assigned to (M-N) and  (M-S) vibrations.

 

The similar bonding scheme of ETH and PTH are proposed on the basis of forgoing I.R spectral studies of ligand and complexes.

The ligand band 3410 and 3260 cm^-1 becomes broad and that at 3160 cm^-1 due to(NH) of –CSNHCO- moiety remains stationary (±5cm^-1) in all the complexes.

The thioamide band  I and II (drising due to coupled vibrations of δ(NH) and(CN) at 1525 and 1350 cm^-1) shifted towards the higher wave number (10-15cm^-1)

The broad band at 1010 and 860 cm^-1 (arising due to thioamide band III and IV respectively) shifted to lower wave number (10cm^-1) in all the complexes.

Position of the ligand band 490 cm^-1 assigned to δ(C=S) shifted to lower wave number (20cm^-1)

The characteristic bands of triethylphosphire at 2962, 2872, 2853, 1467, 700 and 500 cm^-1 were Present in all the complexes.

The new bands around 470-350 and 400-300 cm^-1 in the spectra of all the complexes are assigned to (M-N) and  (M-S) vibrations.

The similar bonding scheme of ETH and PTH are proposed on the basis of forgoing I.R spectral studies of ligand and complexes.

Magnetic Properties

All the complexes were found to be diamagnetic, since the number of ligands two PEt₃ (monodentate) and One LH (bidentate). It is assumed that these metal atom are four coordinated tetrahedral.

Electronic Spectra

The electronic spectrum of ETH showed two bands at 440 and 365 nm which are assigned to n ® π^* and π ® π^*intraligand transitions respectively. The electronic spectrum of  PTH showed three bands at 415, 390 and 360 nm. Which are assigned to n®π^*, π ®π^* and CT bands respectively. Usually n ® π^* transitions involving N and

Sulphur

occur at lower energy and are less  intense  than π® π^* transition.^11-13 The spectra of almost all the complexes show the Presence of these bands with hypsochromic shift. The one  intense  band at  400-430  nm in all the  complexes is  assigned to   dπ(M)®  dπ(P) of PEt₃. The high intensity of these bands suggest that they should be charge transfer an intraligand band. NO d-d transition bands are expected in these metal system (d¹⁰). The hypsochromic shift of the bands at 440 and 415 nm of ligand ETH and PTH respectively, in their complexes may be due to stabilization of the energy of lone pair of electrons on complex formation. Similarly the hypstochromic shift of other bands of ligands in the spectra of complexes may be explained as that metal atom are bonded through thioccrbonyl sulphur and Nitrogen¹⁴. The bond formation lower the energy of non bonding Orbitals as well as π levels thus now more energy will be required to promote the electron from the n or π level to π^* orbital hence the observed hypsochromic shift. The assignments of all the bands are given in Table II 1.

 Table II.1: Analytical Data Colour and melting Points of Complexes.

Compound	Colour	M.Poc	Found (Calcd) %
			C	H	N	S	P	Metal
[Ni(PEt3)2(ETH)]	Brown	171	28.78 (29.22)	4.12 (4.05)	5.42 (5.68)	6.34 (6.50)	12.12 (12.58)	11.6 (11.9)
[Ni(PEt3)2(PTH)]	Brownish Black	233	35.22 (35.57)	4.05 (3.89)	7.12 (7.28)	5.62 (5.92)	10.92 (11.48)	10.68 (10.87)
[Pd(PEt3)2(ETH)]	Yellow	184	26.44 (26.6)	3.88 (3.70)	5.06 (5.18)	5.64 (5.92)	11.24 (11.47)	19.14 (19.68)
[Pd(PEt3)2(PTH)]	Yellow	255	23.10 (23.6)	3.88 (3.57)	6.90 (7.15)	5.18 (5.44)	10.42 (10.55)	18.00 (18.11)
[Pt(PEt3)2(ETH)]	Orange	198	22.68 (22.89)	3.36 (3.17)	4.28 (4.45)	5.16 (5.08)	9.48 (9.85)	30.6 (31.0)
[Pt(PEt3)2(PTH)]	Orange	261	28.12 (28.40)	3.26 (3.10)	5.96 (6.21)	4.62 (4.73)	8.90 (9.17)	28.2 (28.81)

 Table II.2: Comparison of the mejor IR Bonds of  ligands and Complexes.

Compund	(NH) (CO)   (CS)	Thioamide Band    I            II          III        IV	Coordination
ETH	3360m  1775s    1120s 3140m	1540s   1340s    1070s   870m
[Ni(PEt3)2(ETH)]	3310s    1785      1100	1550s   1350s    1050s   850m
[Pd(PEt3)2(ETH)]	3310s     1790     1100	1550s   1350s    1045s   855m
[Pt(PEt3)2(ETH)]	3310s    1785      1100	1555s   1350s    1045s   850m
PTH	3410m    1720s   1130s 3260m 3160m	1525s   1350s    1010s   860m
[Ni(PEt3)2 (PTH)2]	3405br     1730    1110    3240	1540s   1375s    1000s   840m
[Pd(PEt3)2 (PTH)2]	3400br    1735     1110 3240	1540s   1360s    990s     840m
[Pt(PEt3)2(PTH)]	3400br    1735     1110 3240	1545s    1370s    995s     840m

Table II.3: Electronic Spectra Of ligand and Complexes.

Compounds	Band Position λmax (nm)	Assignments
ETH Ni(PEt3)2 (ETH) Pd(PEt3)2 (ETH) Pt(PEt3)2(ETH) PTH [Ni(PEt3)2 (PTH)] [Pd(PEt3)2(PTH)] [Pt(PEt3)2(PTH)]	440 365 400 300 260 410 310 260 425 300 270 415 390 360 410 300 260 425 310 270 430 310 275	n              π* π             π* CT CT IL CT CT IL CT CT IL n             π* π             π* CT CT CT IL CT CT IL CT CT IL

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