Reaction of N-ethoxycarbonylphyrrole-2-thiocarboxamide and N-phenylcarbamoylphyrrole-2-thiocarboxamide with [Rh NOX2 (PPh3)2] (X=Cl-, Br-, I- and NO2-)

Introduction

Nitric  Oxide is a versatile ligand for Coordination and Organometallic compounds. It can bind transition metals as a linear or bent molecule (NOˉ). The over all structure and reactivity features of  metal nitrosyls  has been classically rationalized in terms of their [MNO]ⁿ description. Where n stands for total number of metal d electrons and nitrosyl π^* electrons. This description was given by Enemrak and  Fetham¹. Thousands of metal nitrosyls are known out of which 2200 compounds having linear M-NO (NO⁺) have been fully characterised. It has been Observed that lability of M-NO bond depends on Coligands donor atoms, electron delocalization inside the ligands, solvent and steric  effects.²

Experimental

N-Ethoxycarbonylphyrrole-2-thiocarboxamid and N-Phenylcarbamoylpyrrole-2-thiocarboxamide were prepared according +to the procedure described in [RhX₂NO(PPh₃)₂] (X=Clˉ, Brˉ, Iˉ and NO₂ˉ) were prepared according to the method described else where.^3,4

 Preparation of Metal Complexes

 [Rh NO(Cl₂) (ETH)₂]

To a suspension of  [RhCl₂NO(PPh₃)₂] (0.18g, 0.25mmol) in THF (10ml) was added 2 equivalent of ETH (0.1g, 0.50mmol), and the reaction mixture was stirred at room temperature for 5h. The solvent was removed and the residue was extracted with hexane- Et₂0(1:1V1V ). Concentration and cooling at -10⁰c afforded deep red crystals.

[Rh NO(Br₂) (ETH)₂]

A mixture of  [Rh Br₂ NO(PPh₃)₂] (0.20g, 0.25mmol) and two equivalent of ETH (0.1g, 0.50mmol) in THF (10ml) was heated at reflux for 1h. The solvent was pumped off and the residue was washed with hexane. Recrystalization from Et₂0-hexane (1:1V1V) afforded dark brown crystals.

[Rh NO(I₂) (ETH)₂]

An excess of lithium iodide (0.15g) was added to a solution of dichoronitrosyl rohodium (I) (0.05g) in a atone (5ml) and the mixture was shaken for Ca. 1min. 0.28g of ETH was added to this solution and the mixture was heated under reflux for 1h. The violet brown crystals which separated were washed with water (3x5ml), acetone (2x2ml) and pentane (2x5ml) and recrystalized with bezenelight Petroleum (1:1V1V).

[Rh NO(NO₂)₂ (ETH)₂]

To a continuously stirred suspension of  [Rh NO(NO₂)₂ (PPh₃)₂] (0.19g, 0.25mmol) in THF (10ml) was added  2 equivalent (0.1g, 0.50mmol) of ETH and resultant mixture was furthered stirred to then filtered. On adding petroleum ether (60-80⁰c) to the filtrate on yellow precipitate of the complex was formed. It was filtered and washed with petroleum ether (2x2ml) and ether (2x2ml) and dried in vacuo.

[Rh NO (Cl₂) (PTH)₂]

A suspension of  [Rh NO Cl₂ (PPh₃)₂] (0.18g, 0.25mml) and 2 equivalent of PTH (0.12g, 0.50mmol) in 20ml THF was heated of reflux for 2-3h, during which the colour changed  from light brown to orange brown. The solvent was pumped off and residue was washed with Et₂0 and then extracted with CH₂Cl₂ Recrystallization from CH₂Cl₂-Et₂0-hexane afforded orange brown crystals.

[Rh NO(Br₂) (PTH)₂]         

A suspension of [Rh NO(Br₂) (PPh₃)₂] (0.20g, 0.25mmol) and 2 equivalent of PTH (0.12g, 0.50mmol) in THF- acetone (20ml, 1:1VïV) was heated under reflux for 5min during which the colour of soluation change from orange brown to red brown. It was filtered and concentrated to 5ml and Rept in refrigerator for 1h during which the red brown crystals of complex separated. It was filtered, washed with ethanol (1x1ml), acetone (1x1ml), ether (1x1ml) and dried in vacuo.

[Rh NO(I₂) (PTH)₂]

An excess of lithium iodide (0.15g) was added to the soluation of dichoronitroysl rhodium (1) (0.05g) in acetone (5ml) and the mixture was shaken for 1h. The excess of PTH (0.15g) in 10ml ethanol- dichloromethane (1:1VïV) was slowly added and the reaction mixture was stirred for 1h then heated to reflux for 5min. The violet brown crystals which separated were washed with water (3x5ml), acetone (2x2ml), and pentane (2x5ml) and recrystallized from benzenelight petroleum.

[Rh NO (NO₂)₂ (PTH)₂]

A suspension  of  [Rh NO (NO₂)₂ (PPh₃)₂] (0.19g, 0.25mmol) and 2 equivalent of PTH (0.12g, 0.50mmol) in THF (25ml) was stirred for 5h and heated to reflux for 30 min then  filtered. The soluation was concentrated to 5ml under reduced pressure and Kept in referigerator for 1h where by an pale yellow crystalline complex was separated. It was filtered and washed with (1x1ml) THF (1x1ml)  CH₂cl₂, (1x1ml) Et₂0 and dried in vacuo.

Analyses 

Rhodium and Sulpher were estimated by started procuder  Corbon, hydrogen and nitrogen analyses were performed by the microanalytical section of B.H.U. Varanasi. The analytical   results are given in Table. I.R spectra were recorded on PerRin- Elemer Model 580 in KBr Disc. The IR spectral data are given Table. U.V, Visible spectra of the ligand and complexes were recorded on cystronic Model 108 and the data are given in Table magnetic properties of the complexes were determined on Gouy balaue at room temperature.

Results and Discussion

Treatment  of  [Rh NOX₂ (PPh₃)₂] with LH, ETH or PTH in THF aforded [Rh N0X₂ (LH)₂] air stable crystalline compounds.

[Rh NOX₂ (PPh₃)₂] + LH     THF       [Rh NOX₂ (LH)₂]

reflux

X= CIˉ, Brˉ, Iˉ ,  NOˉ₂

The IR NO stretching frequency 1635-1660 Cm^-1 for the compounds (1-8) are within the renge  of expected for bent nitrosyl compound.¹ Although they are higher than that for [Rh NOX₂ (PPh₃)₂]  1560 Cm^-1. This suggest that PPh₃ ligand in stronger donor than sulpher counterpart. Further, the stretching frequencies of NO in a given series of compounds [Rh NOX₂ (ETH)₂]  are found to reduce in the order NOˉ₂ > Iˉ > Brˉ > Clˉ. The larger π- donor ability of the ligand X causes a  smaller (NO).

Attempt have been made to activate the nitrosyl ligand of the Rh nitrosyl. Compound by treatment with nucleophiles and electrophiles. NO reactions were found when [Rh(NO)X₂(LH)₂] were reacted with MeLi or LiBEt₃H. The position of nitrosyl band(NO) the IR spectra of [Rh(NO)X₂ (LH₂] is not affected (< 20Cm^-1) by change of the medium from KBr to CH₂Cl₂ soluation and we therefore, conclude that essentially the same molecular structure prevails for these complexes in the solid state and solution. However, splitting of the nitrosyl band observed in some of the solid state spectra, and tentatively affributed to solid state effects, are absent from the corresponding solution spectra.

The title ligand LH is capable of existing ( I, II and III) in three tautomeric form and clearly posses a variety of coordinating possibilities.

R=2-Phyrrolyl,R’=OEt;N-EthoxycarbonylPyrrole-2-thiocarboxamide(ETH).

R=2-Phyrrolyl,R’=NHPh;N-Phenylcarbamoylpyrrol-2-thiocarboxamide(PTH).

The eventaual out come of a particular interaction will be decided by the character of acceptor cation, the reaction conditions as well as other factors which are known to effect the behavior of the ambidentate ligands⁵, Free ligand extist as I but II and III forms do become operative when the ligand gets coordinated to the metal ions^6,7.

The nitrogen atom of Pyrrole ring is not capable to coordinate to metal ion under the reaction condition. The protan of the NH group can be ionized to get uninegative ligand in presence of highly basic medium but under this condition ligand is converted to pyrrole- 2- thiocorboxamide. N and O/N or O of RCSNHCOR’ moiety will Prefer to coordinate more effectively  with hard lewis acids (Metal ion) In series of complexes [RhNOX₂(PPh₃)₂] (X= Cl^–, Bi^–, I^–, NO₂^–), Rh is present in +3 state, Rh⁺³ is a border line lewis acid, which may prefer to bind the ligand through N,O or S side Thus, Inorder to Know the made of linkage of ligand, the IR spectrum of ligand and complexes were compared. Followings are the result of IR spectral studies.

[RhNOX₂(ETH)₂] (X= Cl^–, Br^–, I^–, and NO^–₂)

Characteristic bands of pyrrole ring⁹ (3340, 1550, 1470, 1400, 1140, 1040, 995, 890 and 750cm^-1) were not shifted in the spectra of complexes (maximum shift ±5cm^-1), suggesting that NH group of pyrrole ring is not participating in the bond formation.

The band due to (NH) and~(NH) (3360, 3145, 630 and 600 cm^-1), The amide band I at 1775cm^-1 and (C-OEt) band at 1190 cm^-1 did not shift in the spectra of complexes, indicating no bond formation with NH and CO group.

The thioamide bands^10-12 I (δ(C-H) + δ(N-H) + (C=N);) II ((C=S) + (C=N) + δ(N-H); III ((C-N) + (C-S)., and IV ((C-S) modes of vibrations of ligand appearing at 1540, 1340, 1070 and 870 cm^-1 respectively, undergo blue shift of the band I(10-15)cm^-1 and red shift (20-25cm^-1) of the band II-IV, indicating the coordination of the ligand through Sulphur atom.

The characteristic¹³ bands of NO₂ at 1346cm^-1 as (NO₂) and 1330 s (NO₂) were present in corresponding nitro complexes.

The weak intersits bands at 325-270cm^-1 appeared in the IR spectra of [RhNOX₂(ETH)₂] are assigned to (Rh – x). The Stretching frequency of these bands increases in the order I^– < Br^– < Cl^– < NO^–₂.

The IR Spectra of [RhNOX₂(ETH)₂] show broad bands at 1635-1660 cm^-1. The position of these bands depend upon nature ligand x. These bands are assigned to (NO)¹⁴.

The medium intensity band appeared at 350-480cm^-1 in spectra of all the complexes are assigned to (Rh – S).

The Systematic assignments of all the bands are given in Table.

[RhNOX₂(PTH)₂] (X = Cl^–, Br^–, I^–, NO^–₂)

On Comparing the stoichiometrixes of these complexes with Known preferences of Rh(III) for coordination number five, it appears that ligand PTH is acting as unidentate. As a consequence of having five potential donor sites in the ligand molecules, these arises a number of possibilities of bond formation with metal ion. The study of IR spectrum of ligand and Nu complexes provided a possible mode of linkage.

The IR spectra of the ligand and complexes were quite complicated owing to strefehing and bonding vibration of CN, CH, CC, NH and CS groups. Further, the (NO) also appears in the region of these groups. However, the systematic shift of few bands can be used to decide the mode of linkage of ligand to the metal ion. There in a gross similarity in the spectra of all the complexes except a little charge in the position of (NO), suggesting that the ligand has same mode of bonding. The systematic shifts of the mejor bands and their assignments are discussed below.

The characteristic bands of pyrrole moiety were not shifted in the spectra of complexes (maximum shift ±cm^-1), suggesting that NH of the pyrrole ring is not participating in the bond formation.

The bands due to (NH) present at 3410, 3260 and 3160 cm^-1 in the spectrum of ligand were present in those of complexes in stightly shifted position (±5cm^-1).

The amide bands¹⁵ I at 1720 cm^-1 remained practically stationary in the spectra of all the complexes (maximum shift ± 5cm^-1).

The weak band due to (NH) at 630 and 680cm^-1 did not shift.

The  band due to (C = S)¹⁶ at 860cm^-1 shifted to the lower wave number (25cm^-1) indicating the bonding of the metal ion with thiocarbonyl sulpher.

These in no systematic shift; in the position of the bands 1590-1200cm^-1. Since these bands arise due to modes of vibrations  having  contributions from (C = N), δ(NH),(C = C),(C = O), the  mixing  pattern  in these

bands will be different from those present in the spectrum of ligand and hence one should expect a non systematic shift in the positions of these bands.

The new bands appeared in the spectra of complexes around 350 cm^-1 is assigned to (Rh-S).

The characteristic bands of NO₂ (as (NO₂) and s(NO₂) were present at 1346 and 1330 cm^-1 respectively.

All the complex [Rh NOX₂(PTH)] exhibit bands from 270-330 cm^-1 which is assigned to (Rh – X) modes of vibration. The energy of these bands increases in the order I^– < BI^– <Cl^– < NO^–₂. The systematic assignmente of all these bands are given is Table.

Electronic Spectra

The electronic spectrum of the chloroform solution of ETH showed two absorption bands at 365 nm (S) and 440 nm(W). These bands are assigned to π ® π^* and n ® π^* transition respectively. The reasons for assigning 440nm bands to n ® π^* transition are (a) usually n ®  π^* transitions involving sulphur and nitrogen atom occur at lower energy are less interse than π ® π^* transitions^{17, 18}. The nature and positions of these band is solvent dependent. The band is absent from the spectra of Zinc and silver complex. This may be owing to the stabilization of the energy of the lone pairs of the electron on complex formation. The electronic spectrum of the chloroform soluation of the ligand PTH exhibited three bands at 415, 390 and 360 nm respectively. These bands are assigned to n  ®  π^*, π ® π^* and  CT band respectively.

The electronic spectra of complexes were taken in CH₂Cl₂ from 200 to 900nm. These absorption bands were compared with the ligands absorption bands. The ligand bands at 440 and 415nm in ETH and PTH respectively disappeared owing to the stabilization of lone pair of electrons on complex formation one intense band around 400nm present in all the complexes is assigned to dπ (Rh) ® π^x (NO) transition, indicative¹⁹ of {Rh NO}⁷. Rh (III) being d⁶ (t₂g)⁶ system^20-22 can show fourd-d transition bands corresponding to ¹A₁g ® ³T₁g, ³T₂g, ¹T₂g and ¹T_1­g. The two bands ¹A₁g® ³T₁g and ³T₂g are spin forbidden so they generally do not appear in electronic absorption spectrum of Rh (III) complexes. But the two  spin allowed  transions, corresponding  to ¹A₁g® ¹T₂g  and ¹T₁ g are expected to appear. But, due to presence of intense charge transfer bands the one corresponding to ¹A₁g ® ¹T₂g is masked and other band (¹A₁g  ® ¹T₁g) of weak  intensity at 450-470nm appear in all the complexes. The systematic assignments of these absorption bands are given in Table.

All the compounds are diamagnetic. Thus, on the basis of analytical, spectral (IR, UV and visible) and magnetic studies, the following structure ²³ of complexes is propsed.

Table IV.I: Analytical Data, Colour and Metting Point of the Complexes.

Compound	Colour	M.P.C	Found (Calcd) %
			C	H	N	S	Cl	Br	I	Rh
Rh(NO)Cl2(ETH)2	Deep Red	180	32.25 (32.00)	3.56 (3.32)	11.42 (11.6)	10.88 (10.66)	11.76 (11.83)	—	—	17.20 (17.11)
Rh(NO)Br2(ETH)2	Dark Brown	210	28.02 (27.86)	3.05 (2.90)	10.26 (10.15)	9.44 (9.28)	—	23.34 (23.22)	—	15.12 (14.04)
Rh (NO)I2(ETH)2	Violet Brown	270	24.62 (24.52)	2.68 (2.55)	8.60 (8.93)	8.02 (8.17)	—	—	32.22 (32.43)	13.05 (13.15)
Rh(NO)(NO2)2(ETH)2	Yellow	235	30.48 (30.91)	3.34 (3.22)	15.58 (15.78)	10.34 (10.30)	—	—	—	16.64 (16.58)
Rh(NO)Cl2(PTH)2	Orange Brown	215	41.76(41.49)	3.30 (3.17)	14.02 (14.12)	9.38 (9.22)	10.18 (10.23)	—	—	14.28   (14.84)
Rh(NO)Br2(PTH)2	Red Brown	218	36.46 (36.78)	3.10 (2.80)	12.28 (12.51)	8.35 (8.17)	—	20.58 (20.43)	—	13.26 (13.15)
Rh(NO)I2(PTH)2	Violet Brown	280	32.6 (32.8)	2.70 (2.50)	11.02 (11.17)	7.56 (7.29)	—	—	28.70 (28.96)	11.54 (11.74)
Rh(NO)(NO2)2(PTH)2	Pale Yellow	245	40.02 (40.27)	3.18 (3.07)	17.40 (17.62)	8.68 (8.95)	—	—	—	14.20 (14.40)

 

 

 

 

 

 

 

 

 

 

 

 

Table IV.2: Characteristic IR Bands of ligands and Complexes.

 

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