The vibrational spectra of some manganese(III) chloro complexes

1. inorg, nucl. Chem., 1977, Vol. 39, pp. 773-775. Pergamon Press. Printed in Great Britain

THE VIBRATIONAL SPECTRA OF SOME MANGANESE(III) CHLORO COMPLEXES C. F. BELL and D. N. WATERS School of Chemistry, Brunel University, Kingston Lane, Uxbridge, UB8 3PH, England

(Received 4 July 1976) Abstraet--Raman and IR spectra of some manganese(III) chloro complexes have been measured and are discussed in the light of known or probable structures. Assignments of (Mn-CI) stretching frequencies are made in the case of MnC152- and MnC163- ions and for some chlorine-bridged dimeric complexes. hydrogen chloride above 90°C and are completely converted to Mn L2 C13 by heating for 90 min at 100°C. The stoichiometries of Mn phen H2OCI~, Mn phen C13 Mn bipy H20 C13 and Mn bipy CI~ were confirmed by elemental analysis carried out by Butterworth Microanalytical Consultancy, Ltd, Teddington. Extracts of a chloromanganate(III) complex in tfi-n-butyl phosphate (TBP) were prepared in two ways: (i) a deep brown solution containing Mn(III) was prepared[4] by addition of 2g potassium permanganate to 20 cm3 hydrochloric acid (10moldm-3). The mixture was allowed to stand at room temperature until the reaction subsided (about 15 min), then filtered and 5 cm3 of the filtrate immediately extracted by TBP pre-saturated with concentrated hydrochloric acid. The green TBP extract was used for Raman measurements. (ii) 2g potassium permanganate was added to 6.5 g manganese(II)chloride tetrahydrate in 25cm 3 concentrated hydrochloric acid[2]. The brown solution was filtered and immediately extracted as above with TBP saturated with hydrochloric acid.

INTRODUCTION The colour and instability of most manganese(liD chloro complexes have precluded Raman spectroscopic measurements and consequently published data on their vibrational spectra are incomplete. The hexachloromanganate(IIl) ion is stabilised in a crystal lattice when combined with large counter ions like Co(pn)33÷ and Co(enh 3÷(pn = 1,2 diaminopropane; en = 1,2 diaminoethane)[l, 2]. The pentachloromanganate(III) ion is known, inter alia, in the complexes [N(C~Hs),]2[MnCIs], [phenH2][MnCls] and [bipyH2] [MnCIs] (phen = I,I0 phenanthroline; bipy = 2,2' bipyridyl)[3-5]. Structural studies on the last two complexes[6,7] have established that MnCI52- is square pyramidal with four short equatorial and one long axial Mn-CI bonds. The stereochemistry of the anion appears to be influenced by hydrogen bonding between the axial chlorine and protons attached to the two nitrogens of the heterocyclic base. The aforementioned complexes characteristically show a marked degree of thermal instability. Some chloro complexes of Mn(lll) which are much more stable are exemplified by Mn L2 CI3H20 and Mn L2 C13 (L2 = phen or bipy) [4]. These are believed to be dimeric or polymeric, with bridging chlorines linking adjacent manganese atoms. Mn phen Cl3 and Mn bipy C13 are of special interest because of their anfiferromagnefism [8]. We now report the results of a Raman spectroscopic study of some manganese(m) chloro complexes and some additional IR data, which together provide further insight into the vibrational spectral characteristics and structures of these species.

EXPERIMENTAL

Preparations [Bipy H2][MnCIs] and [phen Hz][MnCI,] were prepared according to Goodwin and Sylva's method[4]. Cs2MnCls was made from MnO(OH) by the method of Levason and McAuliffe[5]. [N(C2H,)4h[MnCId and [N(CH3)a]2[MnCIs] were prepared according to Gill's procedure[3]. Co(en),MnCl¢2H20 and Co(pn)3MnCl6 were prepared by addition of Co(en)3C13 and Co(pn)~Cl3 respectively to solutions containing chloromanganate(Ill) ions[l, 2]. MnphenH2OCl~ and MnbipyH2OC13 were prepared by Goodwin and Sylva's methnd[4]. Mn phen Cl3 and Mn bipy C13 were prepared by controlled thermal decomposition of [phenH2][MnC15] and [bipyH2][MnCls] respectively. We have independently confirmed by thermogravimetric analysis Akabori's observations [8] that Mn phen CI~ and Mn bipy C13 are formed as intermediates in the thermal breakdown of the salts containing diprotonated base. When heated in air both salts lose 1 mole of 773

Spectra IR spectra were run as paraffin wax discs or Nujol mulls, either on a Perkin-Elmer model 220 or a Unicam model SP 2000 instrument. For the region < 200 cm-I a Beckman--R1 IC model FS 720 interferometer was employed. Raman spectra were recorded with a Spex "Ramalab" instrument with excitation by the 5145 or 6471 ,~ lines of a Coherent Radiation Laboratories model 52 MG Argon-krypton laser. Studies on the unstable compounds were made immediately after preparation. A rotating disc technique [9] was used for obtaining the Raman spectra of all the solid compounds; the technique was complemented by the use of a cylindrical lens in the laser beam [10] to reduce still further the incident power density at the surface of the samples. Initial attempts to record the Raman spectrum of the TBP extract showed that the extracted species was light-sensitive; the initially green solution was rapidly bleached by the incident radiation. To overcome this problem a simple flowsystem was used; the solution was allowed to flow under gravity from a reservoir mounted a few cm above the horizontal capillary sample tube. Polarization measurements on the solution were made by the use of a half-wave plate in the incident beam.

Thermogravimetric analyses These were performed using a Stanton Redcroft TG 750 thermobalance. RESULTS AND DISCUSSION Spectroscopic data for the complexes examined are summarised in Table 1.

Pentachloromanganate(Ill) The compounds [bipy H2][MnCIs] and [phen H~j[MnCIs] are characterized by strong IR absorption at ca. 360 cm -1 and a strong Raman band at 296 cm -~. Similar features are

C.F. BELL and D. N. WATERS

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Table 1. Vibrational spectra of manganese(Ill) complexes below 500 cm-~ (frequenciesin cm-~) Compound [bipyHdlMnCls] [phen H2]IMnCI~]

Cs2MnCls

MnCI~2- in TBP [N(C2Hs),]2[MnCI~] [N(CH3),]2[Mn(CIs]

Co(pn)sMnCl~ Co(en)sMnClt.2H20 Mnphen H20 Cb

[Mn phen Cls]2

Mn bipy H20 CIs

[Mn bipy CI3L

Raman 296 ms 218vw limit 200 365 vw 296 s 235 w limit 130 294 vs 235 w 195 m limit 150 292 (pod 289 s 233 vw limit200 360 vw 291 s 232 w limit 200 284 m limit200 280 w limit 200 438m 365s 300w 230 s 190w 180w 160s limit 130 438 mw 363 ms 300w 228s 162 ms limit 130 386 w 358 m 266ms 212w limit 180 359 s 261 s 207m limit 180

IR 362 s, br 305 w 260w 235w limit200 412m 360s 350w 300 w 250 w 227 m limit200 375vs 365vs 257 w 230 w limit200 345s 330s 290 m 257 s limit 200 360t 280t 333 360~ 335~: 436 ms 370 s, br 356 sh 300 vw 256 w 232 sh 212m 190s limit 180 436 m 368 s 348 sh 230 w 210 mw limit 200 423 w 384 s 368 w 357 s 270 w 263 w 214w limit200 421m 378sh 373s 361s 350 sh 268 w 240 mw 221 vw limit200

tData from Ref. [5]. CData from Ref. [2]. observed in the IR and Raman spectra of Cs2MnCI5, [N(CeHs)A2[MnCId and [N(CHs)4MMnCIs] with only minor frequency variations amongst the five compounds. Weaker features of both the Raman and IR spectra appear fairly consistently throughout these compounds. It appears therefore that all contain the square pyramidal MnC152- ion. Some slight alterations in its geometry in the last three compounds compared with the first two are to be expected because of the absence of specific cationanion interactions involving hydrogen bonding and these could account for the minor spectroscopic differences. The vibrational representation, with spectral activities, for an isolated species of this geometry (point group C~) is I'o =3A~ (R, IR) + 2Bt (R) + B2(R) + 3E(R, IR). Of these 2A~, 1B t and 1E represent stretching modes and the remainder are deformations. If there is little mixing of axial and equatorial At symmetry coordinates, one of the At stretching modes will be of predominantly axial stretching character; the other will be mainly equatorial

stretching. Consideration of the likely changes in molecular dipole moment and polarizability accompanying symmetry coordinate distortions, suggests that At axial stretching should be relatively intense in the IR while At equatorial stretching should be weaker; in the Raman spectrum these relative intensities should be reversed. On this basis we assign the band near 360 cm -t (strong in the IR) to the At axial stretching mode and the band near 292 cm -t (strong in the Raman spectrum) to the A~ equatorial stretch. The reduced masses for totally symmetric vibrations in fragments of the types CI

I I

Mn--C1 and C1--Mn---C! (square-planar) CI are, respectively, 21.6 and 35.5; whilst these may be considered as at best crude approximations to the actual vibrations, the calculations lend support to the assignments. The A~ symmetry of the 292cm -t band is demonstrated by the observed polarization of the line from the TBP extract. Both the Raman and the IR spectra show a band near 230 cm-~; although weak in most of the spectra it appears consistently. In accordance with the selection rules we assign it to the E stretching mode. The remaining stretching mode, of Bt symmetry, should occur only in the Raman spectrum; either it is too weak to be observed or it lies within the "tail" of the exciting line. The intensity of primary scattering precluded Raman observations below 200 cm-~ except for two compounds and so bands associated with the deformation modes of MnCls2- were not generally observed. In Cs2MnCIs, the Raman band at 195 cm -t is probably one of these, but the low frequency band at 160 cm -t in [phen H2][MnCIs] may be attributable to the cation rather than the anion. The Raman spectra of TBP extracts show one strong peak at 292 cm -t TBP itself shows a broad Raman band in the region 200-300 cm-t, but this is relatively weak and did not seriously interfere. The depolarisation ratio, p, for the peak at 292 cm ~l was found to be =0.3, thus establishing the assignment to a totally symmetric mode. At the same time the octahedral MnC163- and tetrahedral MnCL- ions are excluded as possible species giving rise to this band. (Both these latter species would be expected to give p values close to zero). Thus it appears that, in the TBP extracts, manganese(Ill) is largely present in the form of the MnCI52- ion. This ion is also likely to be a component of the strongly acidic aqueous solution from which these extracts can be prepared. It is interesting to note that Levason and McAuliffe prepared an acetone solution containing MnCi52- identified by its electronic spectrum.

Hexachloromanganate( llI) For a regular octahedral complex MX6 (point group OD there are two Raman-active stretching modes, ~1(AI,) and v2(Eg) and one IR active stretching mode, u3 (Ft,). In the case of the manganese(III) complex, MnCk3-, Jahn-Teller distortion to a tetragonal form is expected[ll] but so far no crystallographic data have been reported on compounds containing this ion. In the two hexachloromanganate complexes examined, identification of bands with MnCI63- stretching modes is not completely straightforward. Thus Co(pn)sMnCl6

The vibrationalspectra of some manganese(liDchlorocomplexes shows IR absorption between 330 and 375 cm-1 by the cation as well as the anion and Co(en)3MnClc2H20 shows a weak Raman band at ca. 280cm -~ due to Co(en)~3÷, almost coincident with the Raman band which we attribute to MnCI~3-. In addition, the compounds are markedly unstable upon laser irradiation and it has been possible to observe one only of the two predicted Raman stretching modes (at 282 cm-~). The IR band at 333 cm -~ (recorded at 342cm -~ by Adams and Morris[Ill and at 350cm -~ by Levason et al. [2]) in Co(pn)3MnCl6 may be identified with v3 but we cannot decide whether the Raman band at 282cm -z corresponds with p~ or v2. Attention has been drawn earlier [11] to the difficulty of assigning these modes even in the more favourable cases where two bands are observable. Chlorine-bridged dimeric complexes

There are close resemblances between the vibrational spectra of the four complexes containing coordinated bipyridyl or phenanthroline. This supports Goodwin and Sylva's conchision[4] that manganese has a similar environment in each complex. [Mn phenCl3]2 and [Mnbipy C1312 are believed to be dimeric and to contain octahedral manganese[4,8]. Although four different structures (I-IV) must be considered [12], the far IR spectrum of each compound is

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not as complex as would be expected for structures of such low symmetry as either III or IV. Therefore we conclude that both compounds probably have either structure I or II. The Raman spectrum of [Mn phen C1312is characterized by strong bands at 363 and 228 cm-~; that of [Mn bipy C1312 by similarly intense lines at 359 and 261 cm -1. The higher frequency bands in each case are due to Mn-CI terminal stretching; the bands at lower frequencies may be due either to Mn-CI-Mn bridging or Mn-N vibration. The near coincidence, for both compounds, of several IR and Raman bands is to be interpreted in terms of similarity in frequency of gerade and ungerade fundamentals rather than as evidence of non-centrosymmetric structures. [Mn phen Cl~]z and Mn phen HzO CI~ have very similar spectra, as do [Mn bipy C1312and Mn bipy H20 C13. This, together with other similarities reported earlier[4], suggests that both Mn phen H20 C13 and Mn bipy H20 C13 contain dimeric [MnL2Ci3]2 molecules, water being incorporated in the crystal lattice but not directly coordinated to manganese. Thermogravimetric analysis confirms that water is loosely held and hence is unlikely to be part of the coordination shell around the metal. Thus Mn bipy H20 C13 loses water between 65 and 110" when heated in air at 10°C/min and Mnphen H20 CI~ under similar conditions loses water between 85 and 13&C. Acknowledgements--The authors thank Mr. W. G. Hooper and

Mr. D. S. J. T. Gregoryfor their assistance withpreparative work and thermogravimetricanalysis.We wishalso to thank P. C. M. U. Harwell for measurement of some of the far IR spectra. REFERENCES

1. W. E. Hatfield,R. C. Fay, C. E. Pflugerand T. S. Piper, J. Am. Chem. Soc. 85, 265 (1963). 2. W. Levason, C. A. McAuliffeand S. G. Murray, lnorg. Nucl. Chem. Lett. 8, 97 (1972). 3. N. S. Gill, Chem. Ind. 989 (1961). 4. H. A. Goodwin and R. N. Sylva, Aust. J. Chem. 18, 1743 (1965). 5. W. Levason and C. A. McAuliffe, J. Chem. Soc. (Dalton Trans.) 455 (1973). 6. M. Matsui, S. Koda, S. Ooi and H. Kuroya, Chem. Lett. 51 (1972). 7. I. Bernal, N. Elliott and R. Lalancette, Chem. Comm. 803 (1971). 8. K. Akabofi, J. Sci. Hiroshima Univ., Ser. A 39, 73 (1975). 9. W. Kiefer and H. J. Bernstein, Appl. Spectroscopy 25, 609 (1971). 10. D. N. Waters, Spex Speaker 20, No. 3, 7 (1975). 11. D. M. Adamsand D. M. Morris,J. Chem. Soc. (A),694 (1968). 12. J. A. Broomhead and W. Grumley, lnorg. Chem. I0, 2002 (1971).