The Raman and far i.r. spectra of dichloroiodoarenes

Spccmcbimiea

Acta,

Vol.33A.

pp. 183 to 187. Pergamon

The Raman

Press 1977. Primed

in Northern

Ireland

and far i.r. spectra of dichloroiodoarenes

A. LOEWENSCHUSS*,R. J. MUREINIK and J. SHAMIR Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel (Received 3 February 1976) Abstrnti-The Raman and far i.r. spectra of four dichloroiodoarenes are presented. The symmetric and asymmetric stretching modes and the bending mode of the IClz moeity are assigned. The spectra are discussed in the light of the T-shaped structure reported for these molecules, and are compared with those of other iodine compounds. The ICI2 moeity shows greater similarity to the IClr- ion than to compounds containing iodine in the formal +I11 oxidation state. INTRODUCDON

There has been considerable interest in the few interhalogen molecules having the empirical formula XY,. Whereas ClF3 and BrF, have been shown to be essentially T-shaped [l-8], “IC&” differs in that it possesses a dimeric structure similar to that of Au&l6 [9, lo]. Dichloroiodoarenes, ArIC12, which can be regarded as monosubstituted “IC&” derivatives have, however, been reported to be T-shaped [ll, 121. Vibrational group frequencies of organic compounds containing polyvalent iodine have been studied [13-161, but only HIKAL et al. have discussed [16] the ICI, group in dichloroiodoarene compounds. The spectral region investigated by HIKAL et al. [16] does not cover all the ICIZ vibrational modes. In this paper, we report the far i.r. and Raman spectra below 300 cm-’ of four dichloroiodoarenes, and assign the stretching and bending modes of the ICI2 moeity. The assignments are discussed in the light of the reported T-shape of the molecule. EXPERIMENTAL

Dichloroiodobenzene was prepared by bubbling chlorine through a chloroform solution of iodobenzene [ 171. Substituted dichloroiodoarenes were obtained by the same general reaction. These species are more soluble in chloroform than the unsubstituted dichloroiodobenzene, and were isolated by partial evaporation of the solvent followed by addition of petroleum ether. Infrared spectra of the dichloroiodoarenes and their mother aryliodides (where these are solids) were obtained from Vaseline mulls on polyethylene plates using a Perkin-Elmer 180 i.r. spectrophotometer equipped with the far i.r. option. * To whom correspondence should be addressed.

Infrared spectra of the liquid aryliodides were obtained from films between polyethylene plates. Raman spectra were obtained using Ar laser excitation (5145 A). Some of the dichloroiodoarenes decomposed on prolonged exposure to the laser beam and spectra were obtained using the rotating sample technique, which has been previously described together with other instrumental details [18]. Polarisation measurements were performed on dimethylformamide solutions.

RESULTS AND DISCUSSION

BAKER et al. [15] have noted that dichloroiodobenzene has no absorption bands that cannot be correlated with those of the mother compound, iodobenzene, in the region 3300-660 cm-‘, and HIKAL et al. [16] have shown that this is essentially true down to 300 cm-‘. In this paper we deal with the vibrations of the ICI, moeity, and only the spectral region below 300 cm-’ is discussed. The Raman and i.r. spectra obtained for the four compounds are shown in Figs. 1 and 2, respectively, and the numerical frequency data are summarized in Table 1. Vibrational spectra have been reported by HIKAL et al. [16] for dichloroiodobenzene and dichloroiodo(p-chloro-)benzene but these spectra are limited to the region above 250 cm-‘. However, even in the spectral region where our study overlaps with that of HIKAL et al., there is no correspondence in the observed frequencies. Thus, for example, HIKAL et al. report only one band in the i.r. at 260 cm-’ and one band at 259cm-’ in the Raman for dichloroiodobenzene. Our Raman study (Fig. 1) clearly shows two strong lines, the higher of which is split by 5 cm-‘. T’wo strong bands are also observed in the i.r. (Fig. 2). For

183

A. LOEWENSCHVSS, R. J. MUREINIK and J. SHAMIR

184

ir r W

d

(.,

M””

XT””

d

200

300

100 cm-'

200

300

c,

cm-t

100

Fig. 1. Raman spectra of some dichloroiodoarenes.

Fig. 2. Far i.r. spectra of some dichloroiodoarenes.

dichloroiodo(p-chloro-)benzene the single band observed by HIKAL et al. at 278 cm-’ (i.r., 275 cm-‘) has no counterpart in our spectra. The intermolecular iodine-chlorine distance of 3.40 A found in the X-ray studies by Archer and van Schalkwyk [ll] of C6H51C12show that a bond-

ing interaction exists between pairs of C6H51Cl, molecules. This “dimeric” unit qualitatively resembles the I&l6 dimer, but there are significant quantitative differences. The data in Table 2 show that the bridging chlorine is equidistant from both iodine atoms in I&. This is not the case in

Table 1. Far i.r. and Raman bands of dichloroiodoarenes

IR 276 s

260 sh 251 “S

133 m 122 sh

C;Hs’Cp 276 271

R(soln)

100 83

258 256 22s

sh 57 4

211 217

:

127

10

108 100

sh 38

;: 54

:: 11

274~

2S6dp

IR 294~s

p-C’-C6H;1C12 R 294 289

100 sh

269 s 216m

260 217 (222)

86 8

18Svw 142

191 140

1 19

122 132

:zl

127

64

R(soln)

272 p

22op

IR

m-CyC3sI;IJClz

288 “S 277sh 264 vs (260)

275 272

98 100

R(soln) 275~

12C16

p-CH,+$HJCl; IR 295~s

283 279

[p,‘”

‘;I

340

344

1c1*I”ic26,;91 Assignment

100 96

267 “I-sym.

SW.

7a* 222

2Slvs 251 214~ 204 VW 18Ovw 130s 118s 118 111

44

2S4dp

:“6

236 272mm

19svw 132 s 126 s

::

33

Frequencies in parentheses are bands obscwed in the parent arylhalide. * Vibrational modes of mother compound: Numbering corresponds to usage of reference

30.

240 260 (246)

:z

327

141 127 122

sh 16 12

133

88

24

62

42

314

126 117

“~-asym. SW. 7a’

vz-bend

lattice vibrations or 12x12 ring vibrations

The Raman and far ix. spectra of dichloroiodoarenes Table 2. Some bond lengths in I&l6 and [C,H,ICl,], We

Terminal I-Cl distance Bridging I-Cl distances

WI

LWC~212

2.39 2.70 2.70

Dll

2.45 2.45 3.40

C6H51C12 where the bridging chlorine is much closer to one iodine atom than to the other. Thus, it is probably justified to deal with the I-Cl vibrations as resulting principally from an independent IC12 functional group perpendicular to the phenyl ring. The symmetry is therefore essentially C,,, for which three vibrations are expected, all of them active in both the i.r. and the Raman. Three bands are indeed observed and good coincidence is found between i.r. and Raman frequencies. Polarisation measurements on the absorption bands at 275-295 cm-’ and 256-260 cm-’ enable us to assign these bands as symmetric and asymmetric stretching vibrations of the IC12 group, respectively (Fig. 3). The band at about 130cm-’ is assigned as the bending mode. The’I-Cl vibrations in the Raman are extremely intense, and obscure even relatively intense lines present in the parent aryliodide. However, some weaker lines in the spectra are correlated with similar lines observed in

I

I

300

I

I

I

I

I

250

I

1

cm-’

Fig. 3. Polarisation measurements on m-dichloroiodotoluene.

185

the parent compounds, which are listed in parentheses in Table 1. Comparison of the frequency data in Table 1 shows that substitution of the benzene ring by substituents of considerably different electron donating or attracting properties has no systematic effect on the positions of the various bands. This is in keeping with the small differences found in rate or equilibrium constants for dissociation of m- and p-substituted dichloroarenes [ 19,201 and with the observation that “F chemical shifts are almost identical for m- and p-fluoro(dichloroiodo)benzenes [21]. These findings support our contention that the ICI, group may be treated separately. The X-ray data do, however, point to the existence of two types of chlorine in ArICl,, one involved in bridging, albeit weak, and one terminal. Evidence in support of the non-equivalence of the two chlorines and hence of the dimeric structure has been adduced from the n.q.r. measurements of EVANS and Lo [22]. The observed splitting of the vi band can be explained as resulting from inphase and out-ofphase coupling of the symmetric vibrations of the two ICI, groups in the dimeric unit. Splitting of the v, band cannot be discerned in solution, either because of dissociation of the dimer or because of the greater width of the band. In general, the observed I-Cl stretching vibrations in the dichloroiodoarenes are of lower frequencies than those observed for the terminal I-Cl vibrations in 12C16 [lo], or in ICl,+ [23]. One reason for this frequency relationship is the greater mass of the phenyl group as compared to chlorine. In addition, the IC12 group withdraws electron density strongly from the phenyl ring [19,20], and the iodine atom in ArIC12 is thus rendered less positive than that in 12C16.Lowering of the oxidation state of the central atom in similar species results in lower stretching frequencies [24,25]. Indeed, the frequencies ‘observed for ArICl, show a far greater resemblance to those of the ICI,- ion (Table l), where the iodine is formally in the +l state, than to those of I,Cl+ The dichloroiodoarenes also resemble ICI,- in the frequency relationship found between the symmetric and asymmetric vibrations. In centro-symmetric linear XY, species, the symmetric stretching mode almost invariably occurs at a lower frequency than the asymmetric mode. The only exceptions to this generalisation appear to be IC12- and BrCl,- in some of its compounds [26]. In both these species, as well as in the other centrosymmetric trihalide ions where v, < vg, but the two frequencies are very

186

A. L~EWEN~CHIJSS, R. J. MIJREINIK and J. SHAMIR

close to each other, interaction force constants, fiz, are calculated to be as much as 36-50% of the stretching force constant, fl. For ArICl,, ratios of fizlf, of about 0.30 can be calculated from the spectral data. This has been attributed to the delocalisation of the bonding electrons [26]. It should be noted, that in however, the noncentrosymmetric trihalide ions, 13-, IBr,-, Br,- and BrCl*-, v1 is generally observed at higher frequencies than vg, but these species must be regarded as XYY’ molecules, for which v1 and vg do not describe vibrations strictly comparable to those of the XY, moeity. In IzC16, the bond length between a terminal chlorine atom and iodine differs from the length of the bond between the same iodine atom and a bridging chlorine atom. In [ICI,+][SbClJ the fouriodine atom attains square-planar coordination by interaction with one chlorine atom from each of two [SbClJ octahedra. In this case, too, there is considerable difference between the length of the bond between iodine and the terminal chlorine atom, and the distance between the iodine and the bridging chlorine [27]. In this respect, ArIClz does not resemble the above trivalent iodine compounds. The X-ray data of ARCHER and VAN SCHALKWYK [l l] do not distinguish between the lengths of the bonds from the same iodine atom to chlorine atoms in bridging or terminal positions. The spectroscopic data are thus best explained on the basis of a highly polarised structure in the sense Ar6+-ICl,s-. This is in keeping with the high dipole moment measured for the compounds [28]. The strong electron-withdrawing ability of the ICI, group confers on the iodine atom oxidation-state character much less than +I11 and approaching in large measure iodine (+I). Some inconsistent aspects of the X-ray data should be pointed out. On the one hand, the dimerit structure of CsH,ICl, “probably indicates the main binding force in the crystal” [ll]. In this case each iodine possesses four-coordination, resembling all known iodine (III) compounds. The iodine in such structures is actually in a site of distorted octahedral symmetry, where the four halogens are almost planar and two pairs of non-bonded electrons occupy trans positions. It seems that this four-coordination is essential for the existence of trivalent iodine, as neither monomeric ICI, nor stable IF, (in contrast to the stable ClF, and BrF,) are known. On the other hand, in T-shaped compounds such as ClF3 and BrF,, which ArICl, appears to resemble, the structure is trigonal bipyramidal in which two halogens occupy the api-

cal positions, with two pairs of non-bonding electrons in cis positions at wmers of the equatorial triangular plane forming an angle of about 120”. Basing the interpretation of the spectra on the X-ray result that the Cl-I-Cl group is linear, one must conclude that the oxidation state of the iodine is closer to a (+I) state as in the IC12- anion, rather than the (+III) state that may be intuitively deduced from the structural formula. It should be pointed out that for dichloroiodo(pchloro-)benzene, two polarised lines are observed. The lower one at 220 cm-’ is not an I-Cl stretching mode, but a ring vibration present in the parent p-chloroiodobenzene. The polarised band at 272 cm-’ is at a frequency intermediate between the values of 269 and 294 cm-’ observed for the solid. The splitting observed in the bending mode of all the compounds probably reflects different kinds of bending motion. In addition to the ICI, bond-angle deformation, other deformation modes occur, in which the ICI, group as a whole moves with respect to the plane of the phenyl ring. The Raman spectra also show bands of wnsiderable intensity at low frequencies. It is possible that some or all of these bands derive from deformations of the I&l2 ring of the dimeric species, similar to the bands so assigned in the spectrum of I$& [lo]. We do not make definite assignments of these bands, as lattice modes are expected in this region as well.

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The Raman and far i.r. spectra of dichloroiodoarcms

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