Vibrational spectra of the trihalocyclopropenium ions [C3X3]+ (X = Cl, Br or I)

Specwochimica Acta, Vol. 48A, No. 2, pp. 205211. 1992 Printed in Great Britain

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Vibrational spectra of the trihalocyclopropenium ions [C,X,]’ (X=Cl, Br or I) M. J. TAYLOR* Department of Chemistry, University of Auckland, Auckland, New Zealand and

P. N. GATES and P. M. SMITH Boume Laboratory,Royal Holloway and Bedford New College, Egham, SurreyTW2OOEX, U.K. (Received 14 May 1991; in final form and accepted 27 August 1991)

Abstract-Vibrational spectra are reportedand assigned for the planarD,,, symmetrycyclopropcniumcations [C&l’ (X= Cl, Br or I) from investigations of the compounds C,Cl+UCl,, CQGaCl,, C$&FeCl,, C,Cl,SbC&,,C,Br&Br, and CJ,, using conventional infraredand Raman spectroscopyand Fouriertransform Raman spectroscopy.The symmetricGX stretchingmodes of [C&J’ occur at 458,269 and 18Ocm-’ and the ring-breathingmodes at 179O,l732 and ca. 1650cm-’ in [C&I,]+, [GBr,]+ and [C&l+. respectively. A normal coordinate calculation is performedfor [C,Cl,]’ .

INTR~DUC~ON CYCLOPROPENIUMcations are of experimental and theoretical interest as small rings in which there is delocalised bonding, associated with the presence of (4n + 2) 3celectrons, in the case where II = 0. The vibrational spectra of such three-membered rings have been reviewed [l]. Reasonably complete spectra are available for the [C&X3]+ions where X = Ph [2], Me2N [3], H [4] or F [5]. Other halocyclopropenium ions were studied by WEST et al. [6] who observed five of the seven allowed vibrational bands of [C&l,]+ and two infrared (IR) bands of [C3Br3]+. fie data were used in calculations on the [C&l,]+ ion by YOSHIDA et al. [7]. The preparation of compounds of [GX,]’ (X= Cl or Br) from the volatile liquid tetrahalocyclopropenes, C&b or C3Br4 requires the introduction of a strong Lewis acid such as AK& to remove a halide ion [6]. In contrast, the iodide, C& is a salt-like compound [C&1+1- as reported by WEISS et al. [8]. We had also prepared C,I, during a study of the interactions of boron trihalides with the derivatives of cyclic unsaturated compounds [9], but our initial sample exploded under laser irradiation while attempting to characterise it by Raman spectroscopy. In the present work we have prepared crystalline compounds of the [GX3]+ ions (X = Cl or Br) and [C&]+I-, and obtained the Raman and IR spectra. Assignments of the fundamental vibrations are proposed and supported by a normal coordinate calculation for [C,Cl,] + .

EXPERIMENTAL Tetrachlorocyclopropeene, C3C14, was purchased from Aldrich Chemicals, or prepared from pentachlorocyclopropane [lo]. Tetrabromocyclopropene, Car,, was prepared from C&Y, by treatment with boron tribromide [ 111. Dichloromethane was used as solvent, and after removal of the solvent under vacuum the C3Br4 product was purified by distillation at reduced pressure (boiling point 55 “C at 0.1 mmHg). Tetraiodocyclopropene, CJI, was obtained by combining CQ, with boron tri-iodide [8]: 3C&

+ 4B13+ 3C314+ 4BC13.

* Author to whom correspondenceshould be addressed. 205

M. J. TAYLOR et al.

206

Toluene or CH& was used as solvent from which CJ., precipitated as a crystalline solid which varied in colour from yellow to dark brown, probably due to traces of iodine, which is incorporated as the t&iodide ion to form [C313]‘[13]-. The formation of iodine was minimised by pretreatment of the B13 solution with mercury, maintaining a nitrogen atmosphere over the reaction mixture, washing the precipitate with dry solvent, and drying the product under vacuum. Due to the risk of explosion [8], no more than 1 mmol of C314was made at a time. Chloro- and bromocyclopropenium salts were prepared following WEST et al. [6]. C&l~AlCl,,, which had been obtained previously as an off-white powder from C&b and AK& without solvent [ll] or slurried in CHrClr [12], was prepared using Schlenk apparatus as colourless crystals by reaction in thionyl chloride: C&l, + AlCl+

Q&AlCl,+

The product appeared several minutes after mixing 1 mmol quantities of the reagents in 5 ml of SOCII. The crystals were rinsed with small volumes of solvent, then CH&l,, and dried under vacuum. Modifications of this procedure were used to prepare other cyclopropenium salts employing the Lewis acids GaC&, FeCl,, SbC&, and AlBr,. CQFeCl,+, pale green crystals, separated from SOClz solution when the solvent was removed under vacuum. C3C13SbC&formed an immediate white precipitate on mixing SOClr solutions of C3C14 and SbC&. C3Br3A1Br4 crystallised on combining CsBr4 and AlBr3 in CH#&. All the halocyclopropenium salts are very susceptible to hydrolysis by atmosphere moisture, re-forming C3C14or C&. Crystalline solid samples were sealed under nitrogen into capillary tubes for Raman investigation. Infrared spectra were recorded from mulls prepared in a dry-box using nujol pre-dried with sodium. KBr plates were employed for the range 4000-4OOcm-’ and polythene or paraffin wax supports for the SIO-SOcm-’ range. Decomposition tended to give spurious peaks attributable to hydrolysis products. These grew in successive IR spectra and so could be distinguished from the initial bands due to the cyclopropenium compounds.

Observing the Rarnan spectrum of C314presented some difficulty because several samples detonated on exposure to the laser beam (Kr+ 647 nm radiation), and our experience of handling this compound serves to reinforce the previous warning that it is a hazardous material [8]. Very small samples loosely packed in the capillary tube were used, and laser power and exposure time to the red line were kept as low as possible while scanning the spectrum from 50 to 250cm-’ in Raman shift. The range from 100 to 2OOOcm-’ was recorded by Fourier transform Raman spectroscopy without any evidence of sample decomposition by the near-IR line of the neodynium YAG laser (1064 nm radiation). Infrared spectra were recorded on Perkin-Elmer 983, Bruker 113V and Digilab FlS-60 spectrometers. Raman spectra were measured conventionally using a Jobin Yvon UlOOO Raman spectrometer system, or a Spex Ramalog V spectrometer, operating with an argon-ion laser tuned to the green line at 514 nm with a power of 20-200 mW, or a krypton-ion laser tuned to the red line at 647 nm with a power of 50 mW. Fourier transform Raman spectra were obtained from a Bruker IFS66 + FRA106 Raman module with excitation at 1064 nm with a power of 80 mW. This system permits the observation of Raman shifts to within 100 cm-’ of the exciting line. Conventional and FT Raman spectra of C3X3A1X4(X = Cl or Br) are shown in Fig. 1.

RESULTS AND DISCUSSION

Vibrational spectra and assignment

Incomplete vibrational spectra of the cyclopropenium ions, [C,X,]’ (X = Cl, Br or I) appear in the prior publications dealing with these species [6,8]. Our results (Tables 1 and 2; Fig. 1) confirm these data, extending the IR measurements into the low-frequency region, and include the Raman spectra of all three species. The only previous Raman spectrum is that of C3C13AlC14in liquid SO2 solution using mercury arc excitation [6]. Our purpose in investigating compounds of [C$&]+ with several different counter ions (see Table 2) is to allow for the possible overlap of bands, especially in the low-frequency region. The trihalocyclopropenium ions are expected to be planar with D3,, symmetry, as shown in Scheme 1. We have confirmed this structure for [C,Cl,]+ by X-ray crystallo-

Vibrational spectra of trihalocyclopropenium

ions

207

(b)

Ln.

Ic 4

-

2000

I

1800

1600

1400

1200 RAHRN

600

1000 SHIFT

600

400

200

CM-l

Fig. 1. Raman spectra of C&13A1Cl, and ~Br,AlBq as crystalline solids. (a) FT Raman spectrum of C,CI,AlCl., obtained by NdlYAG laser excitation at 1064nm; 200 scans at 1 per second. (b) Conventional Raman spectrum of C&4lCI, obtained by Ar+ laser excitation at 514 nm; 1 scan occupying 40 min. (c) FT Raman spectrum of C&AlBr, obtained by NdlYAG laser excitation at 1064 MI; 200 scans at 1 per second.

graphic

of C$&AlC14 [13]. The vibrational representation for this structure polarised) +a; (inactive) +3e’ (IR Ra, depolarised) +a$ (IR) +e” (Ra,

analysis

is 2a;(Ra,

depolarised) .

M. J. TAYLOR et al.

208

The [C,Cl,]’ ion. a; modes. The two a; vibrations are assigned to the intense Raman bands at 1790 and 458 cm-’ first observed, and shown to be polarised, by WESTet al. [6]. They represent predominantly C-C ring breathing and C-Cl symmetric stretching modes, respectively. a; mode. The single a; vibration is not expected to be active in IR or Raman spectra. A value of 6OOcm-’ is inferred from the possible involvement of this mode in a Fermi resonance involving the e’ modes. This assignment is supported by the comparison with [C3FJ]+ in Table 2. e’ modes. All three e’ modes are observed in IR and Raman spectra, as expected. The values, which average 1315, 733 and 16Ocm-‘, alter only slightly with change of the counter anion. The highest IR band is accompanied by a weaker component at 1348 cm-‘. This can be attributed to Fermi resonance of the fundamental with a combination involving the 733 cm-’ mode and the inactive a; mode, assuming this to have a value of cu. 600 cm-‘. This a; + e’ combination has the required e’ symmetry, whereas other possibilities such as e’ + e”, 2e’ or 2e” do not, or else do not match the 1315 cm-’ frequency closely enough to permit resonance. Separation of the e’ fundamental into peaks at 1310 and 1324 cm-’ in the Raman spectrum of C&l~AlC& crystals is possibly a case of factor group splitting, since the unit cell contains two [C,Cl,]’ ions. a; mode. The IR band at 190cm-’ is assigned to this out-of-plane bending mode. Previous workers [6] assigned this band to the e’ species and commented that experimental factors prevented their observing it in the Raman spectrum. Our results show Table 1. Vibrational spectra (cm-‘) of crystalline trichiorocyclopropenium CsClsAICl, Raman IR

CQGaCi, Raman IR

(;ClsFeCi, Raman IR

C,Cl,SbC& Raman IR

1789 s

1786 s

1786 s

1792 s

1349 VW 1324 m 1310 m 733 m

1348~ 131s s 733 m

1345 m 1310 m

1318 m

134s w 1305 m

738 w

73s s

731 w

728s

52Svw

524vW 490vw

134s VW 1318 m 1305 m

1320m

1346 w 1312 s 734 m

458 s

458 s 385 s

w3~~31+

v5 (e’) of [C$l,]+

v2 of [SbCi& t

195 m

19Om

(a;? of w41+ v4of [AK&] -

v7

178 vs 180 vs 176 s 161 vs 151 m

126 m 11Sm

*

vj of [SbQ]-

v1 of [SbC&]-

284m

*

133 m 120 sh

of

v, of [MC4](M = Al, Ga or Fe)

334 vs

349 vs

158 m

v4 WI

vs of [MC4](M = Ga or Fe)

388 s

330 vs

160vs

ofPm31 +

vz (ai) of Will’

346s

184s

Vl(4)

v, of [AK41 457 s

350 s

Assignment

vs (e”) of [C$I,]+

520 VW

485 VW

458 s

salts

161 vs

163m

138 m

135 vs

155 m

v4 of [SbC&] vs of [sbCb,-

159 m

vs (e’) of [CsCl,]’ v4 of [GaCh-

150 vs

118s 1lOsh

* Region masked by an adjacent band of the anion.

v4 of [FeCl,] v4 of [MC41 (M = Al, Ga or Fe)

Vibrational spectra of trihalocyclopropenium ions

209

Table 2. Fundamental frequencies (cm-‘) of trihalocyclopropenium ions Species

[C3Clj]+obs.

[C,ClJ’ talc.

2014

1790

1790

1732

1650

v2

752

458

463

269

180

4

v3

e’

V4

811 1590

600 1315

1238

1276

1205

VS

999

733

733

580

446

v6

287

160

160

100

85

V7

239 642

190 524

190 524

140 -

ai

Mode [GFJ” VI

V8

[GBrj]+ [C&1$ Potential energy distribution (%)

64-m

-

-

75 sym. CCC str., 25 sym. CC1 str. 75 sym. CCI str., 25 sym. CCC str. 100 a bend 70asym. CC str., 30 asym. CC1 str. 68 asym. CC1 str., 28 asym. CC str., 4 bend 96 a bend, 3 CC1 str., 1 CC str. 100 /3 bend 100 j3 bend

* Reference [5]. Internal coordinates: r = C-C, R = C-Cl, a = Cl-C-C in-plane, /?= Cl-ring out-of-plane. Force constants: k,=5.948, k,=4.436, k,=O.148, ka=0.092, k,,=-0.207, kRR=0.843, k,R=0.058, kas= -0.014, k,== 1.164, kRa=0.295, in units of mdyn A-’ for str. and str.-str.; mdynA-’ radian2 for bend; mdynlradian for str.-bend. k, is small and can be neglected.

(X=Cl,BrcrI)

Scheme 1.

conclusively that this mode is present only in the IR spectrum, which confirms its identity as the single al fundamental. e” mode. Following the suggestion of WENT [6], we initially sought this fundamental below 2OOcm-‘. However, the comparison with [GF3]+ where this mode occurs at 642 cm-’ [5], suggests a higher value. The weak Raman band at 524 cm-’ is tentatively assigned to the e” mode. The [Car,]+ ion. Intense Raman bands at 1732 and 269 cm-’ can be assigned with confidence to the two a; modes. The three e’ modes are recognised by their presence in IR and Raman spectra at 1276,580 and 100 cm-‘. The highest of these bands is a doublet in the Raman spectrum of C&#&r4 as in the chloride case. A band at 140 cm-’ in the far-IR region is assigned to the al mode. The [C&l+ ion. Due to practical difficulties (the tendency of CJ., to detonate when irradiated and to react with the medium during mulling for IR analysis) only incomplete spectra were obtained. An intense Raman band at 180 cm-’ probably arises from the ai of C-I symmetric stretching mode. A band at 165Ocm-’ detected against a high background in the FT Raman spectrum is likely to be due to the other a; mode. Two e’ modes are found in the IR spectrum at 1205 and 446 cm-‘, and a further band is expected in the far-IR but was not positively identified due to the poor quality of the spectra in this region. Raman studies confirmed the identity of the 446cm-’ fundamental, and also revealed an intense band at 85 cm-‘. This is assigned to the third e’ mode, making it the counterpart of the bands at 160 and lOOcm-’ assigned to this mode in the spectra of [C3C13]+and [GBr,]‘, respectively. Table 2 summarises the assignments for the trihalocyclopropenium ions and includes the approximate descriptions of the modes based on the calculated potential energy

210

M. J. TAYLOR et al.

distribution for [C,Cl,]+ described below. Points to note are the consistently high frequency of the ring breathing mode (1600-2000 cm-‘), and the changing C-X symmetric stretching frequencies. These values in the [CJ3]+ series (X= Cl, Br or I) are almost the same as the symmetric stretching frequencies of the tetrahalomethanes, CCL, (459 cm-‘), CBr, (267 cm-‘) and CL, (178 cm-‘) [14,15]. Normal coordinate calculations

Normal coordinate calculations were performed using programs based on the FG-matrix method [14]. The structural parameters used in generating the G-matrix of the [C,Cl,]’ ion consisted of the C-C and C- Cl bond distances, the Cl-C-C angles (1500) and the Cl-ring out-of-plane angles (the angle between the Cl-C bond and the plane of the C3 ring). The bond lengths C-C = 1.356 A and C-Cl = 1.631 A were obtained from a single crystal X-ray study [13], which also confirmed the planarity and strict D3,, symmetry of the structure. A valence field of four force constants, associated with C-C and C-Cl stretching and the deformation of Cl-C-C in-plane and out-of-plane angles, was used initially to estimate the primary force constants. This was extended into a general valence force field treatment which yielded the values of force constants, and calculated frequencies given in Table 2. Several shallow minima are present in the potential field and although the solution given here provides the best fit, others almost as good could have been chosen. This would have changed the force constants slightly, especially the values of the interaction constants. We intend to explore the force field of halogenated cyclopropenium ions more fully by means of ab initio calculations. Meanwhile the present normal coordinate calculation lends support to the vibrational assignment of [C,ClJ’ and, by analogy, to the assignments of [C3Br3]+ and [C313]+. The potential energy distribution in the normal modes of [C&l,]’ given in Table 2 reveals considerable mixing of C-C and C-Cl stretching, which is to be expected for a ring system. The extent of mixing (75 : 25% in the case of the a, modes) is marginally less than in [C3F3]+where the ratio is 73:27% [5]. The C-C ring force constant of 5.95 mdyn A-’ in [C,Cl,]’ according to the present analysis is considerably smaller than the value in [C3F3]+ (7.71 mdyn A-‘) [5] or in [C,H,]+ (7.87 mdyn A-‘) [4]. Anion bands. The vibrational frequencies of the complex anions, [AlCh]-, [GaClJ, [FeCl,]-, [SbC&]- and [AlBrd- are well known [14] and so could be easily recognised in the spectra of the present compounds. The frequencies of the [MCI,,]- species in [C&l,]’ salts are similar to those observed in compounds with other large cations, or in solution [16,17]. Acknowledgements-We are grateful to Dr R. Grinter, University of East Angha, for the FT Raman spectra, to Dr S. Best, University College and Dr W. P. Griffith, Imperial College, London, for assistance with other spectroscopic measurements. We thank Dr P. D. W. Boyd and Dr P. Schwerdtfeger for performing the normal coordinate analyses.

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Vibrational spectra of trihakxyclopropeniu

ions

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