On the Lewis acidity of tris(pentafluorophenyl)antimony(V) dichloride towards neutral monodentate O, N and S donor ligands

Journal of Fluorine Chemistry 121 (2003) 131–134

On the Lewis acidity of tris(pentafluorophenyl)antimony(V) dichloride towards neutral monodentate O, N and S donor ligands Kiran Singhala,*, Ram Nath P. Yadavb, Prem Raja, A.K. Agarwala a

b

Chemistry Department, Lucknow University, Lucknow 226007, India Chemistry Department, Tribhuwan University, T.R.M. Campus, Birganj, Tribhuwan, Nepal

Received 24 June 2002; received in revised form 9 October 2002; accepted 18 October 2002

Abstract Hexa-coordinate neutral adducts (C6F5)3SbCl2
L(I) [L ¼ DMSO, DBSO, Ph3AsO, Ph3PO, DPF, DMF, Py, 3-Pic, TU] and penta-coordinate cationic complexes [(C6F5)3Sb(Ph3AsO)2][ClO4]2 (II) and [(C6F5)3Sb(bipy)][ClO4]2 (III) have been synthesised. Molecular adducts are monomeric in benzene and non-electrolyte in acetonitrile. IR spectra and conductance measurement suggest the absence of coordination of ClO4 group to Sb atom in cationic complexes. Spectroscopic data conform to the requirement of octahedral configuration for neural complexes (I) and a trigonal bipyramidal structure for complex cations (II) and (III). # 2002 Published by Elsevier Science B.V. Keywords: tris(Pentafluorophenyl)antimony(V) dichloride; Molecular adducts; Monomeric; Non-electrolyte; Octahedral; Cationic complexes; Ionic perchlorate; Trigonal bipyramidal

1. Introduction The Lewis acidity of the RnSbX5–n towards various ligands increases with decreasing value of n [1]. With monodentate oxygen donors viz.; DMSO, Popov and Kondratenko established the sequence as RSbF4 > R2 SbF3 > R3 SbF2 [2]. Attempts to demonstrate the existence of Ph3SbCl2
L have not yet been successful [1,3]. However, complex cations of the type [Ph3SbL2]2þ in combination of bulkier, anions [BPh4, BF4 and ClO4] have been isolated [4]. The paucity of published data coupled with our interest on organoantimony(III) and antimony(V) compounds [5–7] prompted us to explore the Lewis acidity of tris(pentafluorophenyl)antimony(V) chlorides, (C6F5)3SbCl2. In the present communication, we report for the first time the isolation of neutral adducts of (C6F5)3SbCl2 with O, N and S donor neutral monodentate ligands. Representative cationic complexes of the type [(C6F5)3Sb(Ph3AsO)2]2þ and

* Corresponding author. Tel.: þ91-522-327193. E-mail address: [email protected] (K. Singhal).

0022-1139/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 1 1 3 9 ( 0 2 ) 0 0 2 7 5 - 0

[(C6F5)3Sb(bipy)]2þ have also been isolated in combination of balancing [ClO4] anions. 2. Results and discussion Under anhydrous oxygen free conditions neutral adducts of type (I), could readily be obtained by the interaction of tris(pentafluorophenyl)antimony(V) dichloride, (C6F5)3SbCl2 with an equivalent of neutral monodentate ligands in anhydrous methanol (Eq. (1)). Methanol

ðC6 F5 Þ3 SbCl2 þ L ! ðC6 F5 Þ3 SbCl2
L

(1)

ðIÞ

where L: dimethyl sulfoxide (DMSO), dibutyl sulfoxide (DBSO), triphenylarsenic oxide (Ph3AsO), diphenylformamide (DPF), dimethyl formamide (DMF), pyridine (Py), 3-picoline (3-Pic) and thiourea (TU). The cationic complexes of the type (II) and (III) could readily be obtained by the interaction of preformed solution of tris(pentafluorophenyl)antimony(V) dichloride, neutral monodentate (Ph3AsO) or bidentate (bipyridyl) ligands in the appropriate stoichiometry and the two equivalents of

132

K. Singhal et al. / Journal of Fluorine Chemistry 121 (2003) 131–134

silver perchlorate (Eqs. (2) and (3)) under anhydrous and oxygen free conditions. ðC6 F5 Þ3 SbCl2 þ 2Ph3 AsO þ 2AgClO4 Benzene

! ½ðC6 F5 Þ3 SbðPh3 AsOÞ2 ½ClO4 2 þ 2AgCl

(2)

1610 5 cm1 [14]. In addition to this a band at 3310

10 cm1 assignable to n(NH) mode of free ligand is shifted to slightly lower frequency, 3010 30 cm1. 2.4. IR spectra of the adducts with sulphur donor

ðIIÞ

ðC6 F5 Þ3 SbCl2 þ bipy þ 2AgClO4 Benzene

! ½ðC6 F5 Þ3 SbðbipyÞ ½ClO4 2 þ 2AgCl

(3)

ðIIIÞ

All the reactions were found to proceed smoothly under mild conditions. The products can be crystallised from petroleum ether (40–608) or diethyl ether. In general, mp of the adducts or complexes are high and few of them were found to decompose without melting. The neutral adducts and cationic complexes are readily soluble in polar solvents. These are stable towards atmospheric oxygen and moisture. The neutral adducts are monomeric in freezing benzene. Conductance measurement values of 103 M solution of the type (I) in acetonitrile suggest the absence of ionic species while complexes of the types (II) and (III) behave as 1:2 electrolytes in dichloromethane [8].

In sulphur donor ligand (TU), an absorption at 1069 cm1 is reported to possess equal contribution from n(CN) and n(CS). This remains unaffected on adduct formation and appears at 1073 cm1. When coordination occurs through sulphur atom [15,16], the n(CN) suffers a positive shift while the n(CS) suffers an almost equal negative shift. As a consequence to this the resulting absorption remains apparently unchanged. The positive shift of n(NH) from 3360 and 3300 cm1, in free ligand, to 3400 and 3380 cm1, respectively, in its adduct, indicates absence of coordination through N-atom of the ligand and indirectly suggests S (Sb S) bonding. However, on the basis of some previous observations [17] and present studies, the (Sb–S) bond is assigned at 380 cm1. 2.5. Stereochemistry of the neutral molecular adducts

2.1. IR spectroscopy The IR absorptions due to C6F5 groups attached to antimony atom are almost identical and do not differ significantly from those observed for pentafluorophenylantimony(V) compounds earlier [7,9,10]. 2.2. IR spectra of the adducts with oxygen donors The n(C¼O) modes in various amide bases [4,11] appearing at 1650 15 cm1 undergo negative shift and are identified at 1608 10 cm1 in the spectra of the adducts suggesting weakening of the C¼O bond and coordination through the oxygen atom of the base. On the basis of Dn(CO) [D(n)(CO) ¼ n(CO) free ligand—n(CO) complex] the DPF was found better donor as compared to DMF. An absorption of strong intensity for n(S¼O) [12], n(As¼O) and n(P¼O) [4,12] lying at 1045, 880 and 1195 cm1, respectively, in the spectra of the free ligands, undergoes a distinct negative shift on complexation. The corresponding absorption in the spectra of the adducts appears at 940, 835 and 1122 cm1 suggesting coordination from oxygen atom of the base. The relative donor abilities of the ligands as appears from the value of Dn(XO) [Dn(XO) ¼ n(XO) free ligand—n(XO) complexed; X¼S, P or As], follow the sequence DMSO > Ph3 PO > Ph3 AsO. On the basis of present and some previous studies [4,12,13] a medium strong band in the region 380– 410 cm1 is assigned to n(Sb–O) stretching frequency.

It has been assumed that the addition of a Lewis base, L, to the central atom in a trigonal bipyramidal molecule takes place in a trigonal plane and steric and electrostatic factor play an important 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 L should settle in the equatorial position. It is also supported by a tentative assignment of Sb–Cl band at 325 cm1 appearing in all the spectra and attributed to the chlorine atoms present in the axial positions [10] (Fig. 1). In view of the above idea, the nucleophilic attack at the position between the two pentafluorophenyl groups to produce octahedral structure with trans chloro groups appears to be most favourable. A similar structure has been suggested for R2SbCl3
L adducts by Nishii et al. [12]. 2.6. IR spectra of complexes (II) and (III) The appearance of strong band around 1100 and 620 cm1 strongly suggest the presence of ionic perchlorate group. The absence of any degeneracy or splitting of IR bands rules out

2.3. IR spectra of the adducts with nitrogen donors The n(CN) frequency in (C6F5)3SbCl2
Py and (C6F5)3SbCl2
3-Pic is seen to decrease significantly to

Fig. 1. Suggested structure of R3SbCl2
L. R ¼ C6F5; L ¼ DMSO, DBSO, Ph3AsO, Ph3PO, DPF, DMF, Py, 3-Pic, TU.

K. Singhal et al. / Journal of Fluorine Chemistry 121 (2003) 131–134

133

Fig. 2. Suggested structure of [R3Sb(L0 )2]2þ cation. R ¼ C6F5; L0 ¼ Ph3AsO. Fig. 3. Suggested structure of [R3SbL00 ]2þ cation. R ¼ C6F5; L00 ¼ bipyridyl.

the possibility of coordination of oxygen atom of OClO3 group to the antimony atom of complexes II and III [9]. In the IR spectra of (II) the diagnostic frequency n(As–O) appearing at 880 cm1 in free ligands is shifted to 790 cm1 on complexation, suggesting coordination of oxygen to the antimony atom. A medium band appearing at 420 cm1 is assigned to Sb–O stretching frequency [4]. For the cationic complex (X) with bidentate ligand (2,2-bipyridyl), characteristic frequencies associated with n(C¼C), n(C¼N) and ring stretching appear at 1582, 1560 and 1540 cm1 show a positive shift on coordination and appears at 1610, 1580 and 1568 cm1, respectively, and are in good agreement with the observation made by Schilt and Taylor [18]. This may be attributed to electron donation from nitrogen atom to the antimony atom. The assignment for (Sb–N) band could not be made with certainty due to complex nature of the spectra in this region. In the IR spectra of complex (IX), absorption corresponding to nsym (Sb–C) was found missing, and thus it may reasonably be concluded that the penafluorophenylantimony(V) complex cation have a trigonal bipyramidal structure with all the three C6F5 groups occupying an equatorial positions (SbC3) with trans L groups [9] (Fig. 2). In sharp contrast to this, presence of absorption band around 270 in case of complex (X) suggest the absence of (SbC3) equatorial array in a trigonal bipyramidal system (Fig. 3(a) or (b)). A decision between the two structures 3(a) or 3(b) could not be arrived at on the basis of present studies. C–Sb–C skeleton frequency could not be assigned with certainty for structure 3(b). However, structure 3(b) seems to be more plausible on

the basis of previous reports on similar hydrocarbon based ligands [19].

3. Experimental tris(Pentafluorophenyl)antimony(V) dichloride, (C6F5)3SbCl2 was prepared by the method reported by Nevett and Perry [10]. Anhydrous silver perchlorate was prepared by the standard method. Al the ligands were of reagent grade and used without further purification. The solvents were purified and dried before 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 a Philips magic eye type PR 950 conductivity bridge using a dip type conductivity cell. Molecular weight were determined cryoscopically in benzene. IR spectra were recorded on a Perkin-Elmer 577 spectrophotometer in the range 4000–200 cm1. Typical experimental details of the reactions are described below. All other complexes were prepared in similar fashion. Analytical data are given in Table 1. 3.1. Reaction of (C6F5)3SbCl2 with DMSO ligand In an oxygen free atmosphere, a solution of tris(pentafluorophenyl)antimony(V) dichloride (0.3469 g, 0.5 mmol) in anhydrous methanol (25 cm3) and DMSO (0.039 g, 0.5 mmol) in the same solvent (25 cm3) were stirred together at 80 8C for 3 h. After that it was filtered off, the filtrate on concentration

Table 1 Analytical data for tris(pentafluorophenyl)antimony(V) neutral adducts and cationic complexes S. No.

Adducts/complexes

mp (8C)

Yield (%)

Analysis: found (calculated; %) C

I II III IV V VI VII VIII IX X a

(C6F5)3SbCl2
DMF (C6F5)3SbCl2
DPF (C6F5)3SbCl2
Ph3PO (C6F5)3SbCl2
Ph3AsO (C6F5)3SbCl2
DMSO (C6F5)3SbCl2
3-Pic (C6F5)3SbCl2
Py (C6F5)3SbCl2
TU [(C6F5)3Sb(Ph3AsO)2] [ClO4]2 [(C6F5)3Sb(bipy)] [ClO4]2 Decomposed without melting.

a

320 225 235 240a 225 175a 178 210 205 196–199

65 60 75 75 80 60 65 70 70 65

32.66 41.56 44.25 41.90 30.75 36.00 35.55 30.10 44.58 34.65

H (32.86) (41.76) (44.46) (42.54) (31.09) (36.61) (35.71) (29.62) (44.21) (34.37)

0.98 1.12 1.40 1.36 0.71 0.75 0.58 0.48 2.25 0.95

N (0.92) (1.23) (1.54) (1.48) (0.78) (0.89) (0.65) (0.52) (2.05) (0.82)

1.62 1.30 – – – 1.66 1.75 – – –

(1.82) (1.57)

(1.78) (1.81)

134

K. Singhal et al. / Journal of Fluorine Chemistry 121 (2003) 131–134

under reduced pressure yielded a crystalline solid which was recrystallised from petroleum ether (40–608) to afford tris(pentafluorophenyl)antimony(V) dichloride, dimethylsulfoxide adduct.

laboratory facilities; the Director, RSIC, Lucknow, for obtaining IR spectra and microanalysis.

References 3.2. Preparation of [(C6F5)3Sb(Ph3AsO)2][ClO4]2 (IX) tris(Pentafluorophenyl)antimony(V) dichloride (0.6937 g, 1 mmol) and triphenylarsine oxide (0.644 g, 2 mmol) were dissolved in benzene (40 cm3) and then a solution of silver perchlorate (0.415 g, 2 mmol) in the same solvent (20 cm3) was added to this solution. After stirring the mixture for about 2 h under anhydrous and dark conditions, silver chloride and the desired complex were precipitated. The precipitate was filtered and washed with benzene and then dissolved in absolute ethanol. On concentration of the ethanolic solution in vacuo a solid was obtained, which was recrystallised from petroleum ether (40–608) to give desired complex. 3.3. Preparation of [(C6F5)3Sb(bipy)][ClO4]2 tris(Pentachlorophenyl)antimony(V) dichloride (0.6973 g, 1 mmol) and 2,20 -bipyridyl (0.156 g, 1 mmol) were dissolved in benzene (50 ml, and then a solution of silver perchlorate (0.415 g, 2 mmol) in the same solvent (20 cm3) was added to this solution. After stirring the mixture for 3 h in an inert and dark atmosphere, silver chloride and desired complex were precipitated. The precipitate was filtered and washed with benzene and then dissolved in absolute ethanol. On concentration of the ethanolic solution in vacuo a solid was afforded, which was recrystallised from petroleum ether (40–608) to give desired complex. Acknowledgements The authors are thankful to the Head, Department of Chemistry, Lucknow University, for providing necessary

[1] J.L. Wardell, in: G. Wilkinson, F.G.A. Stone, E.W. Abel. (Eds.), Arsenic, Antimony, Bismuth in Comprehensive Organometallic Chemistry, vol. 2, Pergamon, London, 1995, pp. 321–347, 681– 763. [2] V.I. Popov, N.V. Kondratenko, Zh. Obshch. Khim. 46 (1976) 2597–2601. [3] A.K. Aggarwal, Ph.D. Thesis, Lucknow University, Lucknow, India, 1991. [4] R.G. Goel, H.S. Prasad, J. Oragnomet. Chem. 59 (1973) 253– 257; P. Raj, A.K. Aggarwal, Synth. React. Inorg. Met. Org. chem. 22 (15) (1992) 543–547. [5] P. Raj, A.K. Saxena, K. Singhal, A. Ranjan, Polyhedron 4 (1985) 251–258. [6] P. Raj, R. Rastogi, K. Singhal, A.K. Saxena, Polyhedron 5 (1986) 1581–1585. [7] P. Raj, A.K. Aggarwal, A.K. Saxena, J. Fluorine Chem. 42 (1989) 163–172. [8] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81–122. [9] A. Otero, P. Royo, J. Organomet. Chem. 154 (1978) 13–19. [10] B.A. Nevett, A. Perry, Spectrochim. Acta A 31 (1975) 101–106. [11] W.F. Currier, J.H.S. Weber, Inorg. Chem. 6 (1967) 1539–1543. [12] N. Nishii, Y. Matsumura, R. Okawara, J. Organomet. Chem. 30 (1971) 59–65. [13] E. Maslowsky Jr., J. Organomet. Chem. 70 (1974) 153–207. [14] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed., Wiley/Interscience, New York, 1986, pp. 71–121. [15] G. Marcotrigiano, R. Battistuzzi, J. Inorg. Nucl. Chem. 36 (1974) 3719–3723. [16] T.N. Srivastava, P.C. Srivastava, S.K. Srivastava, Indian J. Chem. A 20 (1981) 443–445. [17] G.E. Forster, M.J. Begley, D.B. Sowerby, J. Chem. Soc., Dalton Trans. (1995) 1173–1176. [18] A.A. Schilt, R.C. Taylor, J. Inorg. Nucl. Chem. 9 (1959) 211–221. [19] P. Raj, A.K. Aggarwal, K. Singhal, Synth. React. Inorg. Met. Org. chem. 22 (10) (1992) 1471–1494.