International Journal of Scientific & Engineering Research, Volume 3, Issue 11, November-2012 1

ISSN 2229-5518

On the molecular adducts of tris(pentafluorophenyl)antimony(V)

diisothiocyanate with neutral monodentate


Dharmendra K. Srivastava, Ram Nath Prasad Yadav, Prem Raj, Isha Rizvi, Prof. Amarika Singh

Abstract- Hexa-coordinated neutral adducts (C6F5)3Sb(NCS)2.L [L = DPF, α-Pic., β-Pic., γ-Pic., DMF, Ph3AsO, Ph3PO, DMSO, TU, Py] have been synthesized. Molecular adducts are monomeric in benzene and non electrolyte in acetonitrile. IR spectra and conductance measurement suggest the presence of coordination of O, N and S donor ligands in adduct. Spectroscopic data conform to the requirement of octahedral configuration for these neutral adducts.

KeywordsMolecular adducts, monomeric, metathetical, nonelectrolyte, neutral ligand, octahedral, tris(pentafluorophenyl)antimony(V)



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The Lewis acidity of pentavalent organoantimony com- pounds, RnSbX5-n has extensively been investigated in the last two decades by various groups of workers. The studied are not confined to R2SbCl3 and RSbCl4 having more chlorine con- tent but has been extended to R3SbCl2 derivatives as well. The letter classes of compounds based on hydrocarbon ligand are not good acceptor but the introduction of CF3 and C6F5 groups on to the metal atom (Sb) considerably enhances the Lewis acidity as evident by the formation of hexacoordinated com- plexes of the type (CF3)3SbCl2.L and (C6F5)3SbCl2.L . The syn- thesis and stereochemistry of tris(pentafluorophenyl)antimony(V) dichloride with a number of ligands viz. dimethyl formamide(DMF), diphenyl forma- mide (DPF), picoline (α-Pic., β-Pic., γ-Pic.), triphenylarsine oxide (Ph3AsO), triphenyl phosphine oxide (Ph3PO), dimethyl sulphoxide (DMSO), thiourea ( TU), pyridine (Py) has been reported [1]. An octahedral environment around antimony has tentatively been proposed for such complexes. On the basis of analytical and spectroscopic data, it may be noted that except to a single reference on the formation and charecterisation of (C6F5)3SbCl2.L no other study related to the synthesis of mo- lecular adducts has been reported to date [1]. In view of our interest in the chemistry and various aspect of fluorocarbon based organoantimony compounds including their antimicro- bial and antitumor activity, coupled with the paucity of pub- lished data in the field, the author considered it worth to syn- thesize a series of molecular
-adducts of tris (pentafluorophenyl) antimony (V)


Author name is Dharmendra K. Srivastava; he is about to submittion of Ph. D. thesis in Organometallic Chemistry at Chemistry Department, Lucknow University, India,

Co-Author name is Isha Rizvi, currently pursuing Ph. D. fromInstitute of Engineering & Technology, Constituent College of G. B. T. U., Sitapur Road Lucknow, India.

-diisothiocyanate, (C6F5)3Sb(NCS)2 with oxygen, nitrogen and sulphur donor Lewis bases. A few complexes of (C6F5)3SbCl2 have also been synthesized for the sake of comparison.


Tris(pentafluorophenyl)antimony(V) diisothiocyanate ob-
tained by the metathetical reaction of
tris(pentafluorophenyl)antimony(III) dichloride, (C6F5)3SbCl2
and potassium thiocyanate, KNCS, recrystallized and dried
before use, was treated with the desired ligand in equimolar ratio in anhydrous methanol. The reactions were carried out
under anhydrous oxygen free conditions.
All the reactions were found to proceed smoothly under mild
conditions. The completion of reactions takes place within 3h.
In most of the cases products were obtained as solid after
evaporating the solvent which were crystallized with petrole- um ether (40-60°C) or the mixture of diethyl ether and petro-
leum ether (60-80°C). The complexes are soluble in common organic solvents such as chloroform, acetonitrile etc. They show monomeric constitution in freezing benzene. The com- plexes are stable, non susceptible to oxygen and can be stored for several weeks without decomposition. The constancy in melting point after repeated crystallization as well as TLC run in polar solvents with a single spot excluded the presence of mixture of reactant.
Elemental analysis, conductance and molecular weight data are given in table 1 and 2 are correspond well to the proposed formulation of the complexes. The observed values of molecu- lar weight indicate their monomeric constitution while the values of molar conductance of 10-3M solution in acetonitrile ranges between 20-30 ohm-1 cm2 mol-1 at the room temperature (30°C) which shows the absence of ionic species in solution.

3 IR Spectroscopy

The entire complexes (listed in Table 1) were characterized in the solid state by their IR spectra in the region 4000-200 cm-1. Important IR frequencies for the complexes together with their assignment are listed in Table-3. These assignments have been

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International Journal of Scientific & Engineering Research, Volume 3, Issue 11, November-2012 2

ISSN 2229-5518

made by comparing the spectra in 4000-200 cm-1 region in the solid state of complexes with those of free ligands. The IR ab- sorption due to C6F5 group attached to antimony atom are almost identical and do not differ significantly from those ob- served for pentafluorophenyl (V) compounds earlier [2], [3], [4].

3.1 IR spectra of the adducts with oxygen donors

The ν(C=O) modes in various amides bases appearing at
1650±15 cm-1 undergo negative shift and are identified at
1608±10 cm-1 in the spectra of adducts suggesting weakening of C=O bond and coordination through the oxygen atom of
the base. On the basis of -Δ ν (CO), the DPF was found better
donors as compared to DMF [5].
An absorption of strong intensity for ν(S=O), ν(As=O) and
ν(P=O) lying at 1045, 880 and 1195 cm-1 respectively, in the
spectra of free ligands undergoes a distinct negative shift on complexation. The corresponding absorption in the spectra of
adduct appear at 940, 835 and 1162 cm-1 suggesting coordina- tion from oxygen atom of the base.
The relative donor ability of the ligand as apparent from the values of-Δ ν (C=O), follow the sequence DMSO > Ph3AsO > Ph3PO. On the basis of present and some previous studies a medium strong band in the region 380-410 cm-1 is assigned to ν(Sb-O) stretching frequency [5].

3.2 IR spectra of the adducts with nitrogen donors

The ν(CN) frequency in (C6F5)3Sb(NCS)2.Py and
(C6F5)3Sb(NCS)2.3-Pic is seen to decrease significantly to
1610±5 cm-1 . In addition to this a band at 3310±10 cm-1 assign-
able to ν(NH) mode in free ligand is shifted to slight lower
frequency 3310±20 cm-1 [5]. In the free IR spectra of ligand the
assignment of the Sb-N bond is tentatively assigned at about
385±5 cm-1.

3.3 IR spectra of the adducts with sulphur donors

In sulphur donor ligands (TU) absorption at 1069 cm-1 report- ed to posses equal contribution from ν(CN) and ν(CS). These remains unaffected on adduct formation and appear at 1075 cm-1. When coordination occurs through sulphur atom, the ν(CN) suffers a positive shift while ν(CS) suffers an almost equal negative shift. As a consequence to this resulting absorp- tion remains apparently unchanged [5]. The positive shift of ν(NH) from 3360 cm-1 and 3300 cm-1 in free ligand to 3410 and 3370 cm-1 in its adducts indicates absence of coordination through N atom of the ligand and indirectly suggest
Sb S bonding.
However on the basis of some previous observation and pre-
sent studies, the Sb S bond is assigned at 380 cm-1 [5]. The diagnostic frequencies due to NCS group bound to antimony appears around at 2080, 840 and 475 cm-1 which could be at- tributed to asymmetric (NCS), symmetric(C-S) and bending modes NCS respectively. The pattern and intensity does not shoe any significant change reported earlier for (C6F5)3Sb(NCS)2 compounds [8]. Sb-C bond appear in the range 445-465 cm-1 [6], [7].

3.4 Stereochemistry of neutral molecular adducts


It has been assumed that the addition of Lewis base L, to the central atom in a trigonal bipyramidal molecule acquire a trig- onal plane and steric and electrostatic factor play an im-
portant role in determining the position of entry of L. It is well established that the more electronegative group goes to the axial position and less electronegative on equatorial positions. Therefore base should settle in the equatorial position. It is also supported by a tentative assignment of Sb-N bond at 326 cm-1 appearing in all the spectra and attributed to NCS present in axial position. In view of the above idea the nucleophilic attack at the position between two fluoro groups to produce structure (fig. 1) appears to be most favorable, since Rf is less electronegative than any halogen atom directly bonded to metal.
Thus analytical, conductance measurement, molecular weight determination and IR data clearly indicates that the newly synthesized complexes have hexacoordination environment around antimony with octahedral configuration as has been suggested for R2SbCl3.L complexes [5]. It is generally accepted that the tris(pentafluorophenyl)antimony(V) diisothiocyanate have a geometry of a trigonal bipyramidal with two thiocya- nate group occupying axial position. In adduct formation as indicated number to six for hexacoordinated complex. A tenta- tive assignment of octahedral structure may be represented as in figure 1.


Tris(pentafluorophenyl)antimony(V) dichloride, (C6F5)3SbCl2 and tris(pentafluorophenyl)antimony(V) diisothiocyanate, (C6F5)3Sb(NCS)2 was prepared by the reported methods [9], [10]. All the ligands were of reagent grade and used without further purification. The solvents were purified and dried be- fore use. All manipulations were conducted in an atmosphere of nitrogen and stringent precautions were taken to exclude moisture. Conductivity data were obtained in acetonitrile with the help of Philips magic eye type PR 950 Conductivity Bridge using a dip type conductivity cell. Molecular weights were determined cryoscopically in benzene. IR spectra were record- ed on a Perkin Elmer 577 spectrophotometer in the range4000-
200 cm-1. Typical experimental details of the reactions are de-
scribed below. All other complexes/adducts were prepared in
similar fashion. Analytical data are given in Table 1, 2, 3.

4.1 Reaction of (C6F5)3Sb(NCS)2 with DMF ligand

In an oxygen free atmosphere a solution of tris(pentafluorophenyl)antimony(V) diisothiocyanate (0.738 g,
0.5 mmol) in methanol (25 cm3) and DMF (0.0731 g, 0.5 mmol) in the same solvent (25 cm3) were stirred together at 80°C for 3 h. After that it was filtered off. The filtrate on concentration in vacuo yielded a white crystalline solid was recrystallised from petroleum ether (40-60°C) to afford tris(pentafluorophenyl)antimony(V) diisothiocyanate- dime- thyl formamide adduct, (C6F5)3Sb(NCS)2.DMF .

4.2 Reaction of (C6F5)3Sb(NCS)2 with Ph3PO ligand

A solution of tris(pentafluorophenyl)antimony(V) diisothiocy-
anate (0.738 g, 0.5 mmol) in methanol (25 cm3) and a solution
of the same solvent (25 cm3) of triphenylphosphine oxide
(0.278 g, 0.5mmol) were stirred together at 80°C for 3 h under
nitrogen. It was filtered off and the filtrate on concentration in

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vacuo afforded a light brown solid and was recrystallized from solvent ether to give tris(pentafluorophenyl)antimony(V) diisothiocyanate-triphenylphosphine adduct.
Molecular weight, conductance measurements of molecular adducts of tris(pentafluorophenyl)antimony(V) diisothiocya- nate adducts


Figure 1 showing octahedral disposition of groups around antimony.

S. No. Complex(Adducts) Rf=C6F5

Molar conductance

(ohm-1cm2mol-1) In acetonitrile

Molecular weight in nitrobenzene Found(Calculated)

I (Rf)3Sb(NCS)2.HCON(C3H7)2 20.6 865.70(868.29) II (Rf)3Sb(NCS)2.α-C6H7N 22.2 825.75(832.22) III (Rf)3Sb(NCS)2.β- C6H7N 22.3 825.75(832.22) IV (Rf)3Sb(NCS)2.γ C6H7N 22.7 825.75(832.22) V (Rf)3Sb(NCS)2.HCON(CH3)2 28.9 805.10(812.19) VI (Rf)3Sb(NCS)2.(C6H5)3PO 25.2 1014.75(986.40)

VII (Rf)3Sb(NCS)2.(C6H5)3AsO 24.4 1069.60(1061.33) VIII (Rf)3Sb(NCS)2. (CH3)2SO 27.6 812.75(817.23) IX (Rf)3Sb(NCS)2.NH2CSNH2 30.8 813.75(815.21)

X (Rf)3Sb(NCS)2.C5H5N 29.9 815(818.19)


Analytical data for tris(pentafluorophenyl)antimony(V)
TABLE 3 IR spectra for (Rf)3Sb(NCS)2.mL (cm-1)

diisothiocyanate neutral adducts

S. No. ν(Sb-C) ν(Sb-S)/ ν(Sb-

O)/ ν(Sb-N)



S.No. Adducts


mp(°C) Yield


Analysis: Found (Calculated :%)

Ligand complex

I 458ms 385ms 1660(1612) II 461ms 382 1615

IV (Rf)3Sb(NCS)2.γPic 188 71 37.50(37.52) 0.82(0.85) 5.03(5.05) V (Rf)3Sb(NCS)2.DMF 197 66 34.00(34.01) 0.85(0.87) 5.15(5.17) VI (Rf)3Sb(NCS)2.Ph3PO 200 56 46.25(46.27) 1.50(1.53) 2.82(2.84) VII (Rf)3Sb(NCS)2.Ph3AsO 165 57 42.85(43.00) 1.40(1.42) 2.62(2.64) VIII (Rf)3Sb(NCS)2.DMSO 180 69 32.30(32.33) 0.73(0.74) 3.40(3.43) IX (Rf)3Sb(NCS)2.TU 183 71 30.92(30.94) 0.45(0.49) 6.85(6.87) X (Rf)3Sb(NCS)2.Py 207 72 36.68(36.70) 0.60(0.62) 5.12(5.14)



The replacement of aryl group (C6H5, p-CH3C6H5) from (C6H5)3SbCl2 by pentafluorophenyl group enhances the Lewis acidity to such an extent as so to facilitate the formation of complexes but replacement of chloride by thiocyanate group permit only one ligand to attach with antimony atom due to high electron density & steric factor. These complexes are monomeric and stable to atmospheric moisture and oxygen.

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The author is thankful to the Head, Chemistry Department, Lucknow University & I. E. T., Constituent college of G. B. T. U. ,Lucknow for providing necessary laboratory facilities, and special thanks to Ram Nath Prasad Yadav who previously done this work but not previously published in any journal, which was further reinvestigated and extended by author.


[1] Isha Rizvi, Amarika Singh, N. Singh, Dharmendra K. Srivastava, Internation al Journal of Scientific & Engineering Research, V-3, Issue-5, May-2012.

[2] Ashvin Kumar Aggarwal Ph. D. Thesis: Studies on organo-antimony(III) and antimony(V) derivatives, Lucknow University, Lucknow, India (1990).

[3 ] Prem Raj, A. K. Aggarwal, A. K. Saxena, J. Fluorine. Chem. 42 (1989) 163-172. [4] A. Otero, P. Royo, J. Organomet. Chem. 154 (1978) 13-19.

[5] B. A. Nevett, A. Perry, Spectrochim. Acta. A 31 (1975) 101-106. [6] Prem Raj, N. Misra, Ind. J. Chem., 30A (1991) 901-903.

[7] M. Hall, D. B. Sowerby, J. Organomet. Chem. 347 (1988) 59-70; W. S. Sheldrick, C. Martin, Z. Natureforsch Teli B 46 (1991) 639-646.

[8] M. Nunn, M. J. Begley, D. B. Sowerby, Polyhedron 15 (1996) 3167-3174. [9] Prem Raj, A. K. Saxena, K. Singhal, A. Ranjan Polyhedron 4 (1985) 251-


[10] M. Fild, O. Glemser, G. Christoph, Angew. Chem. 76 (1964) 953.

[11] Ashok Ranjan, Ph. D. Thesis: Synthesis and reactivity of some non-transitional

Organometallic compounds, Lucknow University, Lucknow (1984).

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