Mössbauer effect in complexes of antimony trihalides with aromatic compounds

J. inorg, nucl. Chem., 1974, Vol. 36, pp. 101 105. Pergamon Press. Printed in Great Britain.

MOSSBAUER EFFECT IN COMPLEXES OF ANTIMONY TRIHALIDES WITH AROMATIC COMPOUNDS L. H. BOWEN,* K. A. TAYLOR, H. K. CHIN and G. G. LONG Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27607

(First received 21 February 1973; in revisedform 29 March 1973) Abstraet--A series of complexes of SbCI3 and SbBr3 with aromatic hydrocarbons, ethers, ketones, and aniline have been studied by the M6ssbauer technique at 4°K. Both isomer shifts (IS) and quadrupole coupling constants (e2qQ)indicate little change in electron distribution about Sb upon complexation, and indeed little difference between the two halides, apart from a generally lower coupling constant for the bromide complexes. There appear to be more variations in absorption intensity than in IS or e2qQ, but even these do not correlate simply with structural information. INTRODUCTION THE SOLUBILITYof SbCI3 or SbBr3 is extremely high in many aromatic hydrocarbons, ethers, ketones, and amines (L). The phase diagrams of a number of these systems were investigated in the early 1900's by Menshutkin[1], who established the existence of stoichiometriccompounds, usuallyoftheform2SbX 3 . L or S b S 3 . L. Bonding in these Menshutkin complexes is generally assumed to be due to weak charge transfer, but its exact nature is uncertain and indeed may vary depending on the aromatic compound used. Raman[2] and i.r.[3,4] spectroscopic studies have been made of a number of the complexes. Nuclear quadrupole resonance (NQR) has been used to study both polycrystalline samples[5-9] and single crystals[10,11]. Although a number of X-ray structures have been reported for these complexes[12-15], a general picture of the bonding is still not clear-cut. In different compounds the number and closeness of intermolecular Sb- - -X contacts varies. The presumed n interaction of the ring with Sb in one case distorts the ring in that region[13], while in another[14] it does not. In complexes of SbC13 with CloHs[13] and C14Hlo[14 ] the Sb-C1 bond opposite the ring is slightly longer while in 2SbBra. C16Hlo the corresponding Sb-Br bond is not[15]. Coordination about Sb may be a distorted trigonal bipyramid with the lone pair in the equatorial plane[13] or, counting intermolecular contacts but not the lone pair, a distorted octahedron[15]. There may even be an interaction between the ring and the halogen [15,10]. In the present work we focus on the bonding effects of complexation at the Sb site. The M6ssbauer effect *Author to whom correspondence should be sent at North Carolina State University.

in 121Sb exhibits isomer shifts (IS) sensitive to ligand electronegativity and geometry[16-19]. The Mcnshutkin complexes provide a sensitive test series for seeing the effect of weak charge transfer on the value of IS and 5s electron density. In addition, if quadrupole coupling occurs to any extent, both the magnitude and the sign of the coupling constant, e2qQ, may be determined from the 12 t Sb M6ssbauer spectrum, providing information about the distribution of 5p electrons around Sb. EXPERIMENTAL All solvents were dried over molecular sieves and distilled before use. The heptane and ligroin (b.p. 77.3-98.5 °) were pretreated with conch. H2SO4 to remove unsaturated hydrocarbons. Reagent grade SbC13 was distilled just before use. Either reagent grade SbBr~ or a sample made from the elements was used. For the latter antimony metal and bromine were refluxed in benzene, the solvent was partially evaporated, and the crystals which precipitated were filtered off, dried, and sublimed. All complexest were prepared in a similar fashion : the SbC13 or SbBr 3 was dissolved in warm solvent, a 50 per cent excess of the aromatic compound was added, and the solution cooled. Crystals were filtered off, dried, and analyzed for Sb. In some cases recrystallization was necessary. All samples used gave an analysis agreeing to within 0.5 per cent of the calculated percent Sb. The preparation and manipulation of the complexes were carried out in a dry nitrogen atmosphere. Solvents used were ligroin (2SbC13. CxoHa), heptane (SbCI 3 complexes with C6H6, C12Hlo, Cx4Hlo, (C~Hs)2CH2, 1,3,5-(CH3)3C6H6, and p-(CH3)2C6H4), chloroform (SbCI 3 complexes with C6HsNH2 and C6HsOC2Hs, SbBr 3 complexes with C6H6, p-(CH3)2C6H4, (C6Hs)2CHz, and 1,3,5-(CHa)3C6H3), and carbon tetrachloride (SbC13 complexes with C~HsCOCH 3 and (C6H5)2CO, and SbBra. (C6H5)20). Absorbers were prepared in a dry atmosphere from t CloHs = naphthalene, Ct2Ht0 = biphenyl, C14H~o = phenanthrene.

101

L. H. BOWEN, K. A. TAYLOR, H. K. CHIN and G. G. LONG

102

weighed amounts of crushed compound mixed with polyethylene powder to give 10 mg Sb/cm 2, and were cooled to 77°K just before inserting in the cryostat. We used a glass cryostat (Kontes/Martin) based on a reported design [20], but modified with an inner chamber so that absorber and source were immersed in 1 atm helium gas surrounded by a liquid helium jacket. Hold time for about 2-5 liters liquid helium was 15-20 hr. The temperature measured at the absorber was 4.2 + 0.50K. We collected counts from both the 37 key photopeak and the 8 keV escape peak of 121Sb with a xenon-methane proportional counter. The constant acceleration drive velocity was calibrated using the magnetic splitting of 57Fe in iron metal. The source used for the 121Sb experiments was Ni21121Sn2B 6 [21], which gives an isomer shift with an InSb absorber of -1.68 ___0.02 mm/sec. All shifts have been converted to InSb using this value. Runs were made for 12-16 hr, with about 30,000 counts collected per channel (about 300 counts/sec total). The spectra after folding were fit with several function forms, using the series approximations of Shenoy and Dunlap [22]. In most cases, the 8-line quadrupole splitting (assuming ~/= 0) for a 7/2-5/2 transition gave reasonable fits, using 1-34 as the ratio of quadrupole moments [23]. However, since the asymmetry parameter is known to be non-zero for some of the compounds, in those cases ~/was fixed at the known value. Due to overlapping of lines, no significant improvement was obtained with values of r/ in the range 0.1-0.2. When ~/was allowed to vary independently, it either iterated to ~ 0 or to a much higher value (~0-4) than observed by NQR[5,6]. We conclude that r/ cannot be determined for these compounds by the M6ssbauer technique. As an alternative approach to improving the fit, we analyzed the spectra allowing the n(Am = 1) and tr(Am = 0) transition intensities to vary independently. Invariably the ratio IJl, was less than unity, but in only a few cases, discussed below, was there any significant improvement. The values of IS and e2qQwere insensitive to the method of fitting and thus could be obtained from the simple 8-line fit. A typical spectrum with its computer fit is shown in Fig. 1.

.. ,...?.':"

o

• . • ...

2 4

6 8

I0

12

-16

I

-12

-8

I

-4

I

Velocity,

I

I

O

4

I

8

I

12

mm/sec

Fig. 1. Sample M6ssbauer spectrum at 4°K with computer fit. The spectrum of 2SbBr 3 . C6H6 was fit with intensity 18.8 ~ , width 2.7 mm/sec, IS = - 7.81 (relative to the source) and e2qQ of 12.8 mm/sec. The value of IS relative to InSb would be -7.81 + 1.68 = -6.13 mm/sec. RESULTS Table 1 r e p o r t s the results for the complexes with SbCI 3 a n d Table 2 for the ones with SbBr3. Previously results have been r e p o r t e d for b o t h pure trihalides [17,24,25]. H o w e v e r , these m e a s u r e m e n t s were m a d e with oxide sources, w h i c h place the trihalide a b s o r p t i o n at a very negative velocity ( - 14 to - 16 m m / s e c relative to the source) thus i n t r o d u c i n g e r r o r i n t o the determ i n a t i o n of peak position. In addition, the earlier runs were m a d e at 77°K with c o n s e q u e n t l y lower intensity. T h e fact that SbBr3 a n d SbC13 have essentially the same IS is in a g r e e m e n t with previous results from our l a b o r a t o r y [ 1 7 ] , a l t h o u g h it is difficult to c o m p a r e absolute values due to the reasons given above. The p r e s e n t values should be m o r e accurate due to the

Table 1. M6ssbauer parameters for SbC13 complexes at 4°K. Isomer shifts are reported relative to InSb. The values of IS and e2qQare in most cases averages of several runs. Estimated errors are 0.1 mm/sec for IS and 1 mm/sec for e2qQ.The N Q R values of e2qQhave been converted to the same units for comparison Intensity (%)

Width (mm/sec)

IS (mm/sec)

e2qQ

e2qQ(rl)

(mm/sec)

(NQR results)

SbCla 2SbCI 3 . C6H 6

15 24

2.8 2.8

- 5.9 - 5.9

13-9 13.1

12.79(0.188)* 13.11(0.103)* 12.97(0.161)

2SBC13. p-(CHa)2C6H 4 2SbC13 . C10H s 2SbC13 . Ct2H10

25 30 131"

2.7 3.1 3.1

- 5.9 - 5.7 - 5.9

13.2 13.9 13.9

2SbC13 . Ct4H10 2SbC13 . (C6Hs)2CH 2 2SbC13 . 1,3,5-(CH3)3C6H 3 SbC13 . C6HsNH2 SbCI 3 . C6HsOC2H 5 SbCI 3 . (C6Hs)2CO SbCla. C6HsCOCH 3

19 31 21 24 23 31 31

3.0 2.9 2.8 3.0 2.9 2-8 2.8

- 6"3 - 6"0 - 5'7 -6.3 - 6-1 - 5"8 -6.5

13"4 13-2 13-3 12.0 13.2 14"0 14.6

Compound

.'~

13.22(0)* 12.11(0.137)* 12.88(0.099)

10.87(0.03):~ 12-29(0.118)* 13.05(0-23)*

* Reference [6] (values at 77°K). t This intensity is not directly comparable, being an early run with different geometry. :~T. B. Brill, Department of Chemistry, University of Delaware, Private communication (value at 300°K).

M6ssbauer effect in complexes of antimony

103

Table 2. M6ssbauer parameters for SbBr3 complexes at 4°K. Isomer shifts are reported relative to InSb. The values of IS and e2qQare in most cases averages of several runs. Estimated errors are 0.1 mm/sec for IS and 1 mm/sec for e2qQ

e2qQ

Compound

Intensity (~)

Width (mm/sec)

IS (mm/sec)

(mm/sec)

SbBr 3 2SbBr 3 . C 6 H 6 2SbBr a . p-(CHa)2C6H 4 2SbBr 3 . (C6Hs)2CH2 2SbBr 3 . 1,3,5-(CHa)aC6H 3 SbBr3. (C6H5)20

28 19 33 22 16 30

2.8 2.7 2.8 2-9 3.0 3.0

- 5.9 - 6.1 - 6.2 - 5-9 -6.3 - 6.1

11.6" 12.8t 11.8 11.5 12.9 11.9

* Value reported from NQR at 77°K is 11.48 (q = 0-100) [5]. t Value reported from NQR at 77°K is 10.79 (q = 0.204) [6]. source used and the larger intensities observed. In the earlier report as well as an independent oncE24] evidence was presented for a linear variation in isomer shift with electronegativity in the case of Sb(III) compounds. Although the general variation with electronegativity is valid, certainly the electronegativity difference between C1 and Br seems to have essentially no effect on the isomer shift. The variation in IS for the SbCI 3 complexes, for example, is greater than the variation as a whole between Table 1 and Table 2. Recent results on salts.containing SbC16 -3, SbBr 6 -3, SbC15- 2, Sb2C19- 3 and Sb2Br 9- 3 [ 18,191 indicate that coordination and geometry have a much stronger effect on IS than does a small electronegativity difference., Thus the shifts for both SbC16 -3 and SbBr6 -a are considerably more negative by 3~5 mm/sec than those for the trihalides. This difference could be explained if the lone pair in S b S 6 - 3 occupies the 5s orbital[181, whereas in SbX 3 it has appreciable p~ character. The shifts of SbC16 -3 are generally lower than those of SbBr 6- 3, a fact ascribed to the greater electronegativity of C1[18,191. Since the s character of the lone pair in SbX 3 compounds is not necessarily fixed, its variations are probably the decisive factor in determining IS for these, rather than electronegativity differences. The isomer shifts of SbCI3 and SbBr3, - 5.9 mm/sec, correspond, according to the calibration of Ruby[26], to a 5s electron density at Sb between 1.0 and 1-5, depending on the amount ofp shielding, with the latter value probably more realistic, considering the large positive values of eZqQ.Since the S b ~ l bonds are not actually identical in SbC13 the lone pair and z axes probably do not coincide, although the positive sign of e2qQ indicates the excess p electron density is in the z direction (that is, the lone pair has considerable p~ character). The maximum change in IS for the complexes, - 0 . 6 mm/sec for SbCl 3 . C6H5COCH 3 relative to SbC13, and - 0 . 4 for 2SbBr 3 . 1,3,5-(CH3)3C6H a relative to SbBr3, corresponds to less than 0.1 s electron increase, and thus could be explained by a simple weak donation from the aromatic compound to Sb. However, both SbC13 . C6HsNH2 with a long S b - N bond [121 and 2SBC13. C14H19 with apparently a n interaction between Sb and the aromatic ring[14] have more negative IS than SbC13 itself. Small changes in bond

length, hybridization, or even intermolecular interactions (some of the intermolecular Sb- - -C1 distances are shorter than Van der Waals' radii) could well account for these effects. Although 5d orbitals of Sb may participate in bonding when other groups interact with an SbC13 molecule, the IS results here do not indicate any clear influence they might have on shielding the 5s orbitals. Certainly the 5s electron density at Sb changes very little on complexation. Reproducibility of values for e2qQby the M6ssbauer technique was about 0-5 mm/sec, although the error is estimated at 1 mm/sec due to the variation between these Values and those obtained by the N Q R technique. The assumption of Lorentzian line shapes in the M6ssbauer fitting routine generally seems to overestimate e2qQ(Tables 1 and 2). Fits could be obtained constraining both e2qQand r/to the known values, but Z2 for these fits was somewhat large. The values of e2qQ from M6ssbauer measurements, although not precise, do show the broad trends. Thus SbBr a complexes generally have lower absolute values than SbC13 complexes (in keeping with the lower electronegativity of Br compared to C1 [5]). There is relatively little change in e2qQupon complexing either SbC13 or SbBr3, and the changes which do occur are not correlated with changes in IS. Thus, the same IS corresponds both to an increase in e2qQ (2SbC13 . C6H6) and an average decrease (2SbCl3. C12Hx0) over SbC13 . A reduction in IS corresponds in one case to an increase in e2qQ (SbC13 . C6HsCOCH3) and in another to a decrease (SbCI 3 . C6HsNH2). This lack of correlation seems to corroborate the earlier statement that bond hybridization varies within the complexes. The unusually small values of e2qQand r/for C6HsNH2 complex with SbC13 (Table 1), and its structure[12], in which all bonds are on one side of a plane through Sb, agree with a a donation from the nitrogen to Sb through an orbital with appreciable p~ character. The M6ssbauer values of e2qQ for the SbBr 3 complexes with C6H6 and 1,3,5-(CHa)aC6H 3 (Table 2) are slightly larger than for the others. However, at least for the C6H 6 complex this larger value is apparently an artifact of the M6ssbauer spectrum as the more precise N Q R result (6) is in fact smaller than that for SbBr3. Since neither the structure or N Q R measurements have been reported for the

104

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BOWEN,

K. A.

TAYLOR,

1,3,5-(CH3)3C6H 6 complex, it appears unwarranted to assume the small increase, although reproduceable, is in fact significant. Considering all cases studied, the 5p electron distribution at Sb changes very little upon complcxation. In Table 3 are listed those compounds for which improvement in fit was obtained by allowing the ~ and transition intensities to vary independently. A ratio I,~/I~ different from unity in a randomly oriented, powdered sample could be caused by a GoldanskiiKaryagin effect[27]* where a value less than unity would indicate that the recoilless fraction was less and the Sb mean-square vibrational amplitude greater in the z direction than x or y (that is, greater near the lone pair axis, a not unreasonable postulate in these cases). However, there is a close relation between total intensity per 10 mg Sb/cm 2 and 1,,/I~,. All compounds studied which gave intensities greater than 28 per cent are listed in Table 3 and those with the most improvement in fit (the SbCla complexes with CaoHs, (C6Hs)2CH2, (C6Hs)2CO, and C6HsCOCH3) had intensities of at least 30 per cent. For compounds with intensities less than 23 per cent the improvement was negligible. Since it is likely that at these large intensities saturation occurs, at least part of the effect may be due to unequal saturation of the n and tr lines. For thinner samples this part would vanish, but unfortunately the low counting rate of the source precluded running samples much thinner with comparable counting statistics. Thus, it is uncertain whether or not a G - K effect exists in any of these complexes. Observation of the G - K effect in 121Sb will require compounds with larger e2qQ than these halide complexes. Although G-K effects in Sb compounds have been reported[23], they could have been due in part to thickness saturation also. Some of the overall intensities (Tables 1 and 2) do vary outside the range of experimental reproducibility (about 5 per cent). The low absorption intensity of Table 3. Ratio of intensities I,Jlo compared to intensity calculated from fit assuming I,,/I~ = 1. The )~2 values are given following the intensities. Only cases where Z2 was reduced by at least 20 are included Compound 2SbC13 . C 6 H 6 2SBC13. CtoH s 2SBC13. ( C 6 H s ) 2 C H 2 SbC1 a . C 6 H s N H 2 SbCl 3 . C 6 H s O C 2 H ~ SbCI3. (C6H5)2CO

SbC13. C6HsCOCH 3 2SbBr 3 . p-(CHa)2C6H 4 SbBr 3 . ( C 6 H 5 ) 2 0

% intensity (X2) 24 (298) 30 (315) 31 (392) 24 (312) 23 (256) 31 (373) 31 (326) 33 (292) 30 (315)

I,,/I,(A)~2) 0-71 ( - 25) 0.74 ( - 53) 0.62 ( - 122) 0-73 (-21) 0.69 (-26) 0.74 ( - 70) 0-81 (-33) 0.77 (-22) 0'62 (-- 21)

*The method of sample preparation, crushing and intimately mixing with powdered polyethylene, should insure the random orientation of the crystallites required to observe the G-K effect.

H. K. CHIN and G. G. LONG SbC1 a itself indicates an especially small recoilless fraction. However, SbBr 3 gave an absorption more intense than some of its complexes, so that no simple correlation of intensity with, for example, melting point, can be made. Menshutkin complexes thus exhibit only small variations in their 1215b M6ssbauer spectra, but these effects indicate a variation of both the bond hybridization and recoilless fraction, depending on the aromatic compound used for complexation. The 5d orbitals of Sb likely participate in bonding in the complexes (intermolecular Sb---C1 and Sb---aromatic bonds). However, there is no evidence that the presumed ~ interaction ofthearomatic ring with Sb directly affects the M6ssbauer parameters. Acknowledgements--This work was supported by the National Science Foundation. The assistance of Mrs. Delores E. Knight in the antimony analyses is gratefully acknowledged. REFERENCES

1. B. NI Menshutkin, Izv. petrogr, politekh. 13, 277 (1910); Zh. russk.fiz.-chim. Obshch. 43, 1303, 1329, 1805 (1910); 44, 1079, 1113, 1128 (1912) [Chem Abstr. 5, 1434 (1911); 6, 734, 735, 1281, 3403, 3404 (1912)]. 2. Sh. Sh. Raskin, Optika Spektrosk. 1, 516 (1956). 3. H. H. Perkampus and E. Baumgarten, Z. phys. Chem. 39, 1 (1963). 4. L. W. Daasch, Spectrochim. Acta 9, 726 (1959). 5. S. Ogawa, J. phys. Soc. Japan 13, 618 (1958). 6. V. S. Grechishkin and I. A. Kyuntsel, Optics Spectrosc. 16, 87 (1963). 7. V. S. Grechishkin and I. A. Kyuntsel, J. struct. Chem. 5, 45 (1964). 8. D. Biedenkapp and A. Weiss, Z. Naturf 19a, 1518 (1964). 9. H. Negita, T. Okuda and M. Kashima, J. chem. Phys. 45, 1076 (1966). 10. T. Okuda, A. Nakao, M. Shiroyama and H. Negita, Bull. chem. Soc. Japan, 41, 61 (1968). 11. T. Okuda, H. Terao, O. Ege and H. Negita, Bull. chem. Soc. Japan 43, 2398 (1970). 12. R. Hulme and J. C. Scruton, J. chem. Soc. (A), 2448 (1968). 13. R. Hulme and J. T. Szymanski, Acta Crystallogr. 25B, 753 (1969). 14. A. Demald6, A. Mangia, M. Nardelli, G. Pelizzi and M. E. Vidoni Tani, Acta Crystallogr. 28B, 147 (1972). 15. G. Bombieri, G. Peyronel and I. M. Vezzosi, Inorg. Chim. Acta 6, 349 (1972). 16. G. G. Long, J. G. Stevens, R. J. Tullbane and L. H. Bowen, J. Am. chem. Soc. 92, 4230 (1970). 17. L. H. Bowen, J. G. Stevens and G. G. Long, J. chem. Phys. 51, 2010 (1969). 18. T. Birchall, B. Della Valle, E. Martineau and J. B. Milne, J. chem. Soc. (A), 1855 (1971). 19. J. D. Donaldson, M. J. Tricker and B. W. Dale, J. chem. Soc. Dalton 893 (1972). 20. B. J. Zabransky and S. L. Ruby, Rev. scient. Instrum. 41, 1359 (1970). 21. L.H. Bowen, K. A. Taylor, H. Z. Dokuzoguz and H. H. Stadelmaier, In M6ssbauer Effect Methodology (Edited by I. J. Gruverman) Vol. 7, p. 233. Plenum Press, New York (1971).

M6ssbauer effect in complexes of antimony 22. G. K. Shenoy and B. D. Dunlap, Nucl. Inst. Meth. 71, 285 (1969). 23. J. G. Stevens and S. L. Ruby, Phys. Lett. 32A, 91 (1970). 24. V. Kothekar, B. Z. Iofa, S. I. Semenov and V. S. Shpinel, Soy. Phys. JETP 28, 86 (1969). 25. G. K. Shenoy and S. L. Ruby, In M6ssbauer Effect

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Methodology (Edited by I. J. Gruverman) Vol. 5, p. 77. Plenum Press, New York (1970). 26. S. L. Ruby, In M6ssbauer Effect Methodology (Edited by I. J. Gruverman) Vol. 3, p. 203.Plenum Press, New York (1967). 27. S. V. Karyagin, Soviet Phys. Dokl. 148, 110 (1964).