J . Chem. SOC., Faraday Trans. I, 1984, 80, 3257-3274 Vibrational Study of the Methyl Viologen Dication MV2+ and Radical Cation MVo+ in Several Salts and as an Intercalate in Some Layered MPS, Compounds BY OLIVIER POIZAT* Laboratoire de Spectrochimie Infrarouge et Raman, CNRS, 94320 Thiais, France AND CLAUDE SOURISSEAU Laboratoire de Spectroscopie Infrarouge, LA 124, Universite de Bordeaux I, 33405 Talence, France AND YVES MATHEY Laboratoire de Physicochimie Minerale et ERA 672, Universite Paris-Sud, 91405 Orsay, France Received 8th February, 1984 Infrared and Raman spectra (1 800-200 cm-l) of methyl viologen (1,l’-dimethyl-4,4’- blpyridyl dication or MV2+) ([lH8] and [2H,] derivatives) have been recorded for the chloride, iodide and palladate salts and for the intercalated compounds of formula Ma.,, PS,-(MV),, 16 (with M = Mn and Cd), FePS,-(MV),.,, and Zn,,,, PS,-(MV),,,,.Complete assignments have been proposed for all these species. The methyl viologen appears to be intercalated in its dicationic form and is weakly interacting with the host lattice. In the ZnPS, system the MV2+ pyridyl rings are twisted while in MnPS,, CdPS, and FePS, the cation rings are coplanar and parallel to the sulphur layers. From these results, a model of packing of MV2+ in the MPS, host lattices has been established which accounts for the different intercalation rates. Finally, the reduced product, MV’+, has been studied in both the chloride salt and the CdPS, intercalation compound; in the latter case the radical cation behaves as though diluted and trapped inside an inert matrix, demonstrating unexpected stability.Extensive studies of the methyl viologen cation, (CH,-NC,H,-C,H,N-CH,)i+, have been reported over the last decade in relation to its ability to be photoreduced by transition-metal complexes, which is an important primary process for the splitting of water and the evolution of hydrogen.l-, Other studies have been devoted to the layered metal phosphorus trichalcogenides MPS,, where MI1 is a transition metal, because of the ability of these two-dimensional phases to intercalate reversibly different ions or metal complexes in the van der Waals For instance, we have recently reported an infrared, Raman and resonance Raman study of the Ru(2,2’- dipyridyl)z+ cation intercalated in MnPS,. This work is thus part of a general vibrational investigation of layered MPS, systems intercalated with various molecular entities.In recent studies we have shown that structural, dynamic and electronic information concerning the guest species can be obtained usinginfrared, Raman and neutron-scattering It thus appeared interesting to focus our attention on the potentially reactive 1,l ’-dimethyl-4,4’-dipyridyl cation or methyl viologen (abbreviated hereafter as MV2+) with the aim of under- standing the changes in its photophysical and electronic properties which occur upon 106 3257 FAK 13258 VIBRATIONAL STUDY OF MV2+ AND MV.+ intercalation. We were also concerned with the stabilization of the radical cation MV'+ inside the van der Waals gaps. The vibrational spectra of MV2+ halides have already been analysed.12-14 Recently, Hester and Suzuki15 have performed a normal-mode calculation for the in-plane vibrations of both MV2+ and MV'+. However, all these previous studies are incomplete because of the lack of data on isotopic derivatives. We have thus prepared [lH,] and [,H,] samples of MVCl,, MVI, and MVPdC1, compounds and carried out a complete vibrational investigation of these salts. It is now established that the conformation of MV2+ depends on the nature of the counter ion: the two pyridyl rings of the dication are coplanar in the halide salts16 and twisted (8 = 50") in the palladate c0mp1ex.l~ We have thus been looking at modifications of the charge-transfer strength when changing the anions. Moreover, we have carried out the intercalation of MV2+ uia either the 'metal-vacancies-creation ' route4? for the MnPS, CdPS, and ZnPS, lattices, or the 'direct' route6 for the FePS, system.In the former cases new compounds of general formula M,-,PS, (MV, nH,Q),, where x = 0.16 for M = Mn, Cd and x = 0.29 for M = Zn, were obtained, while in the latter case FePS, (MV, nH,Q),. 15 was prepared. The corresponding vibrational spectra have been carefully analysed in order to determine the structural conformation and packing of MV2+ in the gaps and to estimate the interactions occurring between the guest cation and the host lattices. Finally, the radical cation MV'+ ([lH,] and [,H,] derivatives), in both the chloride salt and the intercalated CdPS, compound, has been generated. The Raman spectra were recorded in order to evaluate the extent to which the electronic and photophysical properties of this radical cation are perturbed upon intercalation.EXPERIMENTAL MATERIALS The [IH,] and [,HE] derivatives of methyl viologen dichloride, MVCl,, were synthesized as follows: 4,4'-bipyridyl ([lH,] or [,HE] derivative) ( lo-, mol) prepared from pyridyl ([lH5] or [,HJ derivative) according to the literature method,', was added to a solution of 6 cm3 CH,Cl in 5 cm3 H,O and heated at 130 "C for 8 h in an evacuated ampoule. The excesses of CH,Cl and H,O were evaporated and the quaternized salt (C,H,N-CH,),Cl, [or (C,D,N-CH,),Cl,] was recrystallized several times from methanol. Anhydrous salts were easily obtained by gently heating (ca. 60 "C) powder or crystal samples for 20 min; water is reabsorbed as soon as the complex is left under an ambient atmosphere.MVI, (['H,] or [,HE] derivative) was obtained from MVCl, by halogen exchange.lg MVPdCl, ([lH,] derivative) was prepared by mixing hot solutions of MVCl, ( lo-, mol) and Na,PdCl, (1 O-, mol) in 6 mol dm-, HCl(1 cm3) and allowing them to cool slowly. Brown crystals of MVPdCl, were isolated and washed. Finally the free radical [lH,] or [,HE] MV'+ was obtained by direct sublimation from the chloride MV2+ salt. The layered MPS, system with MI1 = Mn, Cd, Fe and Zn, were synthesized according to previously described procedures20- and were characterized by X-ray powder diffraction and chemical analysis. Intercalation in MnPS,, CdPS, and ZnPS, was performed by treating the pure materials (powder or platelet) with lo-, mol dm-, aqueous solutions of methyl viologen chloride ([IH,] and [,HE] derivatives) at 60 "C for 15 days.After washing and drying, intercalates of general formula MI-,PS,(MV, nH,O), with x = 0.16 for MI1 = Mn, Cd and x = 0.29 for MI1 = Zn were obtained. The value of n was 4 for the Cd compound. This intercalation route requires the creation of M2+ intralamellar vacancies in the and corresponds to the following reaction : MPS, + XMVE; + M,-,PS, (MV, nH,O), + xM::. Intercalation in FePS, was carried out in presence of sodium dithionite as reducing agent and led to the compound FePS, (MV, nH,O),,,,. This last intercalation route has been described0. POIZAT, C. SOURISSEAU AND Y. MATHEY 3259 as the ‘direct’ or ‘reduction’ route;6 it corresponds to the reaction FePS, + xMVt: + 2xe- -+ FePS, (MV, nH,O), then 2x electrons are accommodated by the FePS, lattice. All these compounds were charac- terized by chemical analysis and powder X-ray diffraction.The sets of (001) lines observed in the X-ray diffraction patterns indicate increases of 3.3 A in the basal spacing upon intercalation in MnPS,, CdPS, and FePS, and an increase of 3.7 A in ZnPS, The following analyses were obtained. Mn,,,,PS,(MV),~,,, calc. : Mn 17.55; P, 1 1.91 ; S, 36.93; C, 8.80; N, 1.63; found: Mn, 17.51; P, 11.87; S , 37.14; C, 8.92; N, 1.68. Cd,.,,PS,(MV, 4H20)0,16, calc.: Cd, 35.87; P, 11.91; S, 36.92; C, 8.80; N, 1.63; H, 1.34; found: Cd, 35.84; P, 11.80; S, 37.10; C, 8.75; N, 1.57; H, 1.30. Zn,.~,,PS,(MV),,,,, calc.: Zn, 17.98; P, 11.89; S, 36.92;C, 16.24;N,2.84;found:Zn, 17.71;P, 11.84;S,38.41;C7 16.12;N72.85.FePS,(MV),.,,, calc.: Fe, 20.43; P, 10.73; C, 7.61; found: Fe, 20.47; P, 10.74; C, 7.39. The reduction of the already intercalated MV2+ was performed by treating a powder sample of Cd,,,,PS,(MV),,,, with a tetrahydrofuran solution of butyl-lithium. The formation of intercalated MV+ is well characterized by its deep blue colour and its Raman spectrum, and charge neutrality is maintained because of the concomitant intercalation of Lif ions. The radical cation appears to be quite stable in air since, from repetitive Raman measurements, a half-life of more than three weeks has been estimated. SPECTRA Infrared spectra were recorded on Perkin-Elmer 225 and 180 spectrometers.Polycrystalline samples were dispersed in Nujol and Fluorolube and spectra of oriented platelets (ca. 2 x 2 x 0.03 mm) were obtained using a beam condenser. Raman spectra were recorded on a Dilor RTI triple monochromator equipped with a krypton laser (6471 A) and an ionized argon laser (5145 A). In order to minimize any local heating effects, Raman spectra of absorbing compounds were measured by means of the rotating sample technique and using low incident laser power (I d 50 mW). Inelastic neutron scattering (INS) spectra were obtained from powdered samples contained in flat thin-walled aluminium alloy cans using a beryllium filter detector spectrometer at the Institut Laue-Langevin (Grenoble, France). RESULTS AND DISCUSSION MV2+ SALTS The band wavenumbers and proposed assignments corresponding to the infrared and Raman spectra (1700-200 cm-l) of the anhydrous and hydrated MVCl, ([‘HJ and [2H8] derivatives) and of the MVPdC1, ([lH,] derivative) complexes in the solid state are presented in table 1.Inelastic neutron-scattering results for anhydrous MVCl, ([lH8] and [2H8] derivatives) are also included. Some related vibrational spectra are shown in fig. 1. SYMMETRY AND METHOD OF BAND ASSIGNMENT The MV2+ cation is planar in its various dihalide salts16 and belongs to the Dzh symmetry group. In contrast, in the PdCli- salt the two pyridyl rings are twisted1’ in such a way that the symmetry is lowered to D,. For the sake of convenience, we have used the notation already employed for the vibrational modes of pyridine22 and N-methylpyridinium 24 The correlation table between the C,, symmetry group of the methylpyridinium cation and the D,, or D, groups of MV2+ is presented in table 2.Moreover, site and intermolecular effects are expected in the solid compounds; however, they must be negligible since the vibrational spectra of MVCl, in the solid state and in ~ o l u t i o n ~ ~ , ~ ~ are quite similar. The only reported changes have been explained by a lowering of the symmetry of MV2+ in solution because of a loss of 106-2w Table 1. Infrared and Raman band wavenumbers (cm-l) and assignments for chloride and palladate salts of MV2+ ([lH8] and [2Hg] derivatives) and inelastic neutron-scattering wavenumbers for anhydrous MVCl, ['H,]MVCl, [2H,]MVC12 anhydrous hydrate anhydrous hydrate ['H,]MVPdCI, Raman i.r.neutron Raman i.r. Raman i.r. neutron Raman i.r. Raman i.r. assignmentb 1655 br - - - - 1650 br 1619sh - - 1603m - 1610 m) 1649s - 1599s - 1628 m - - 1602 s - 1654 s 1654 s 1651 s 1643 s - - 1563 m 1634 s - 1623 vw 1584 vw - 1577 vw - - 1575 vw - - - - - - 1573m - 1530m - - 1534 m - ET) - 1 4 8 6 ~ - - 1458m - - - - - - ( 1516m - 1459m - 1435sh - 1 4 2 0 ~ 1447sh 1 4 5 0 ~ - - - 1436 m 1372 sh - 1366m - 1425m 1430br - 1430 sh 1335s - - 1343 s 1284m - - 1287 m - - - - - - - - - - - - - - - - - - - - - 1290 w - 1221 s - - 1 2 8 0 ~ - 1211 vs - - - - 1558 w 1554 w 1531 m 1514 m 1465 sh 1445 m 1418 vw 1368 sh 1357 m ) - - - 1492 vw 1446 vw 1428 vw 1363 vw - - - 1490 vw - 1420 s 1370 br - 1427 vw 1340 w 1328 w - 1418 vw) - BCH, 1326 s 1299 s 1278 w 1266 sh - - 1291 s 1281 sh 1303 s 1285 w 1264 vw - 1270 m 1242 m - - - 1274 w - 1134s - - 1140 s 1240m - 1 1 5 2 ~ - - 1 1 3 8 ~ - - 1224 m 1220 sh - 1233a w- 1189m 1143 m 1086 vw I - - 1040 vw 972 w - - - - - - - 884 vw - - - 825 s - - - - 790 sh - - 710 w - - 669 w - - 1190 s 1153 s - - - - 1057 w 1037 vw - - - - - - - - - 873 w 840 s - - - - - 813 vw 793 w - - 720 vw - - 672 vw 661 m - rCH3 3 BCD + Vring 5 YCH + rring m 0 C 6b Aring + Vringw h, m h, Table 1.(cont.) ['H,]MVCl, [2H8]MVC12 anhydrous hydrate an hydrous hydrate [ 'H,]MVPdCl, Raman i.r. neutron Raman i.r. Raman i.r. neutron Raman i.r. Raman i.r. assignmentb - - - 561 w - - 485m - - - 468 sh 470 w - 405w 410s 4oovw - 345vw - 348 w - - - - - - - - - - 278vw 278 w 280s - 217 vs - - 531 vw - 5 2 6 ~ - 5 0 2 ~ ~ - 482 w - - 475m - 430 sh 430 sh, w - 464m - 418s - - - - - - - - - - - - - - - 3 9 2 ~ ~ 402s - - - - - 400aw - 3 4 0 ~ ~ - 331 w - 332w 324vw - - - - 3 5 0 ~ ~ - - - - - - - - - - - - - 282 w - 273 w - 274s 278 w - - - 2 1 3 ~ ~ - - - 5 2 4 ~ ~ - 424s - '468 vw I 392vw 429vw - - 353vw - 308 vs - 283vs F1'] - - - 268 s - a Bands observed only in the spectra of the solution. v = stretch; d,A = in-plane deformations; y, = out-of-plane deformations; r = rocking.0.POIZAT, C. SOURISSEAU AND Y. MATHEY 3263 I I I I I I I 1600 12 00 800 4 00 wavenumber/cm -' Fig. 1. Infrared and Raman spectra (A, = 6471 A), in the 1700-200 cm-l region, of anhydrous MVCl, {(a) ['H,] and (b) [2H,]} and of MVPdC1, ( c ) in the solid state. cation ~1anarity.l~ In agreement with the expected selection rules, the vibrational spectra of MVC1, and MVI, are quite simple and show complete exclusion between infrared and Raman wavenumbers, while band splittings and infrared-Raman coincidences are observed for many modes with the palladate complex.Our band assignments were established using isotopic data (table 2) and by comparison with previous vibrational results for pyridine,,, p i c ~ l i n e , ~ ~ biphenyl,26 2,2'-di~yridyl,~~ 4,4'-dip~ridyl,~~ 1,l 'dimethyl-4,4'-dipheny128 and N-methylpyr- i d i n i ~ m . ~ ~ ? 24 In fact, very characteristic group-frequency sequences are commonly encountered in these six-membered aromatic molecules. In particular a very close analogy is noted between MV2+ and methylpyridinium data.This behaviour has beenw h) o\ P Table 2. Correlation diagram and selection rules for the MV2+ cation in both the planar and the twisted conformations, from the methylpyridinium ion symmetry (v = stretch; 6, A = in-plane bends; r = rocking; t = torsion; y, r = out-of-plane bends) planar methyl viologen, Dzh twisted methyl viologen, D, methyl- pyridinium, CH, activity symmetry C,,,: symmetry activity ring CH CH, ring- N- ring CH, symmetry ring CH, ring ring- N- CH 3v+2A w v 3v+lA A A 2 r r r i r r - ir - - 2 r r r 3v+lA A A 3 ~ + 2 A - v 2v+ 26 2v+ 26 2Y 2Y 2Y 2Y 2v + 26 2v+26 lv+16 R(p) lv+16+lr R lv+lG+lr R It R It - I v + 16+ Ir i.r. I v + 16+ Ir i.r. lv+ 16 1.r. R(p) 3 v + 2 A + l r o+T v 2~+26+2y lu+16+lt R,i.r. 3v+2A+lr - v 2v+26+2y lv+IG+lt R, i.r.3v+ l A + 2 r A+T A+T 2v+26+2y lv+ 16+2r R , i . r . 3v+lA+2r A + r A+T 2v+26+2y lv+16+2r + B,,0. POIZAT, C. SOURISSEAU AND Y. MATHEY 3265 Table 3. Comparison of the band wavenumbers (cm-l) corresponding to the YCH,CD (vll), v ~ - ~ ~ ~ and ~ 5 % ~ ~ modes for various MV2+ complexes and intercalation compounds MVCl, i anhydrous 1 hydrated MVI, MVPdCl, MV2+ in CdPS, MV2+ in MnPS, MV2+ in FePS, MV2+ in ZnPS, 825 850 810 848 813 816 814 675 1242 688 1240 668 1229 1220 - (1224 668 1224 670 1224 - 1223 1216 - 1226 - 1134 (:it; 1425 1436 1430 1140 (:ili 1128 1352 1412 - 1326 - 1332 1144 1344 1144 1340 1345 - 1331 - 1335 - of great help as a reliable assignment is known for the methylpyridinium, supported by a valence force-field calculation performed using the four derivatives C,H,N-CH,, C,D4N-CH,, C,H,N-CH, and C,D,N-CD,.28 Finally the assignments of the CH,-group deformations have been confirmed by comparison of the inelastic neutron-scattering spectra of the [lH,] and [2H,] derivatives, where these modes give rise to the most intense signals.VIBRATIONAL RESULTS The experimental results allow us to propose new assignments for several modes in disagreement with previous studies. In particular, we assign the strong absorption band at ca. 850 cm-l to the in-phase out-of-plane CH deformation (BEH, vll), which generally leads to the most intense infrared band in aromatic mo1ecules.22-28 This band cannot be connected with the almost coincident Raman peak at 842 cm-l, which corresponds to a typical in-plane vibration.This precision is supported by the spectra of deuterated MV2+, where the absorption shifts to ca. 675 cm-l (pPHID = 1.26) while the Raman band shifts to 808 cm-l (pH,,, = 1.05). A second major remark concerns the two infrared bands observed at 1459 and 1145 cm-l ([lH,] derivative), which have been omitted in the normal-mode calculation of Hester and Suzuki.', We assign the former band to the ring vibration vlgb by analogy with the results for various aromatic m01ecules.~~-~~ The latter band, whose intensity depends on the degree of hydration of the complex (see table l), corresponds to a strong signal on the inelastic neutron-scattering spectrum and it is thus assigned to a CH, rocking mode. The frequencies of three infrared-active vibrations, namely v,, (yCH), vN-cH, and d$H,, are remarkably sensitive to modifications of the surroundings (hydration rate, nature of the counter ion and twist-angle between the two pyridyl rings) of the MV2+ cation (see table 3).This behaviour is due partly to variations of the charge-transfer strength, which occur when the molecular conformation and the counter-anion are modified. vN-CH, and BEH, modes have frequencies which are dependent on the electronic charge of the nitrogen atom. Such specific perturbations indicate that the charge transfer is mainly localized on the N atoms. This conclusion is strengthened3266 VIBRATIONAL STUDY OF MV2+ AND MV.+ by a comparison of the vibrational spectra of MV2+ and of the isoelectronic molecule 1,l '-dimethyl-4,4'-diphenyl: in agreement with the quasi-similarity of their atomic masses and molecular geometries, their Raman spectra are almost identical.In contrast, their infrared spectra are different and reveal important variations of the dipole moments in these molecules. This confirms the localized character of the charge-transfer mechanism in MV2+ salts. In addition, MVPdCl, vibrational spectra exhibit several characteristics of the non- planarity of the cation: first, the two absorption bands for out-of-plane ring deformations (v, and 1'16) and previously situated at ca. 468 and 485 cm-l (halide salts) are now observed at 534 and 561 cm-l, respectively. A second typical shift occurs for the in-phase ScH3 absorption band, observed in the 1365-1 340 cm-l range for planar MV2+ and lowered to 1326 cm-l in MVPdCl,.Similar behaviour has been observed with MV2+ intercalation products of mica-type ~ i l i c a t e s : ~ ~ while planar in mont- morillonite, the cation is twisted in vermicullites and the vibration is shifted from 1354 to 1332 cm-l. Finally, the non-planarity of the MV2+ cation is characterized by the presence of a Raman component of the vNPcH, mode; situated at 1224 cm-l for MVPdCl,, this mode is also observed at 1233 cm-l in the Raman spectrum of MVCl, in solution. Note that most of the in-plane ring vibrations, which are expected to be sensitive to the electronic delocalization within the rings, do not show significant frequency variations. In contrast the frequency increase of the r ring modes indicates strengthening of the ring-deformation constraints, which may be caused by crystal packing effects.INTERCALATED MV2+ COMPOUNDS Infrared and Raman spectra (1 700-200 cm-l) of ZnPS, and MnPS, intercalated with MV2+ are shown in fig. 2. The corresponding band wavenumbers and proposed assignments are reported in table 4, together with the infrared data for the intercalated FePS, analogue. No Raman spectra were obtained for this last compound because of its opacity. Some infrared results obtained with polycrystalline samples and quasi-monocrystalline platelets of intercalate MnPS, are compared in fig. 3. These spectra seem to arise from the superposition of bands characteristic of the host lattices and those of the MV2+ cation. However, some changes are noted in the MPS, vibrations [splittings of the vdps3 and SS,,, modes and the presence of new low-frequency i.r.bands] with respect to the pure lattice spectra. Similar results have already been reported for the CoCp: and CrBz: intercalates in the corresponding host structures.89 lo, l1 We can thus easily compare the spectra of Mn,~,,PS,(CoCp,),,,, with those of Mno~,,PS,(MV)o~16 on the one hand, and the spectra of Zn,,,,PS,(CoCp,),~,, with those of Zn,~,lPS,(MV),~,g on the other hand, despite the fact that the CoCp: intercalation induces a larger increase in the basal spacing (ca. 516 A) than the MV2+ intercalation (ca 3.3 A). Vibrational spectra thus appear to be more sensitive to the nature of the host system, i.e. the nature of the MI1 transition metal constituting the lattice, than to the amount and geometry of the intercalated species. MV2+ INTERCALATES OF MnPS,, CdPS, AND FePS, In all cases, the MV2+ intercalated species ([lH,] and [2H,] derivatives) are well characterized by vibrational bands whose frequencies and relative intensities are close to those observed for the halide salts.This indicates that in these compounds the two pyridyl rings are again coplanar and that the cations interact only weakly with the host lattices, which behave as regular counter-ions. However, some frequency shifts are noted for the above mentioned three vibrations yCH, vN-CH, and SCH3 for their energy dependence on the cation surroundings (see table 3).0. POIZAT, C. SOURISSEAU AND Y. MATHEY 3267 / r L V I I I 1 I 1 1 1 1600 1200 800 400 Fig. 2. Infrared and Raman spectra (A, = 6471 A), in the 1700-200 cm-l range, of the MV2+ cation intercalated in (a) MnPS, and (b) ZnPS, host lattices. w avenumberlcm-’ For the polarized infrared spectra (EHlayer planes) of platelets of the Mn and Cd compounds (fig.3), the vp-p (ca. 450 cm-l) and C ~ S , , ~ (ca. 317 cm-l) bands are extinguished, as expected from geometric lo Similarly, bands corre- sponding to the two characteristic out-of-plane MV2+ vibrations, yCH(vl1) and rring (v16a)7 at 813 and 475 cm-l and at 668 and 424 cm-’, respectively, for the [lH,] and [2Hg] derivatives of MV2+ in CdPS,, nearly disappear. We conclude that the two coplanar pyridyl rings are parallel to the host layer planes. A schematic picture of such a cation lying in the interlamellar spacing and undergoing van der Waals interactions with the sulphur atoms of the lattice is represented in fig.4. The interlamellar distance estimated from this representation is nearly 3.3 A, in agreement with the value obtained from powder X-ray diffraction data. This confirms the validity of the proposed structural model. According to the calculated values of the area of a flat MV2+ on the one hand and of the layer plane area per primitive cell on the other hand, we can deduce that an intercalation rate of x z 0.16 corresponds satisfactorily to maximum filling of the available interlayer space; nevertheless, there are still empty regions, around the nitrogen atoms, able to accommodate up to four water molecules between the cations. The MV2+ intercalates of MnPS,, CdPS, and FePS, reach the same intercalation rate with x = 0.16 & 0.01 ; hence, whatever the intercalation route (‘direct’ route or ‘metal-vacancies-creation’ route) the reactions are likely to be limited3268 VIBRATIONAL STUDY OF MV2+ AND MV.+ Table 4.Infrared and Raman band wavenumbers (cm-l) and assignments for the MnPS,, ZnPS, and FePS, compounds intercalated with the methyl viologen (MV2+) cation MV2+ in MnPS, MV2+ in ZnPS, MV2+ in FePS, Raman 1.r. Raman i.r. i.r. assignmentsa - 1640 s 1620 m 1570 m 1507 m 1436 m 1340 w - - - - - 1278 m 1224 w - - 1194 m 1052 vw - - - - - - - - 816 s 798 sh 694 w - - 606 vs - - - - 557 vs - I 475 w 467 sh 451 m 380 w - 1649 s - - - 1530 m - - 1352 w (:E m) 1298 s 1290 sh - - 1226 w 1188 s 1154 vw 1064 vw 1038 vw - - - - 873 vw - - 837 m - - 705 w 678 vw 657 m 625 w - - - 573 sh 563 m 554 sh - - 473 m - - - 1635 s 1601 m 1560 m 1499 m 1440 m 1350 sh 1331 m - - - 1273 m 1267 sh 1226 w 1216 m 1188 sh 1185 m 1065 vw 1038 vw 979 vw 950 vw 872 w 863 vw 847 w 837 vw - (K 3 G: 3 793 vw - - 625 s 605 vs 596 vs 583 vs 573 vs 558 sh 518 w 506 w 450 s 380 vw - - - 1640 s 1620 br 1507 m 1436 m 1345 w} - - - - 1276 m) 1192 m 1055 vw 1032 vw - - - - 698 m 614 sh 604 vs - 480 sh} 468 m 446 s 414 m 380 w 8a H2O 19a 8a 19 b &Ha C’ 3 ‘N-CHa 9a 9b 18 a 5 17a ‘CH3 YCH ‘d’ YCH 6a 6b VdPSa ‘e’ 4+16a CPS3 VSPS30.POIZAT, C. SOURISSEAU AND Y. MATHEY 3269 Table 4. (con?.) MV2+ in MnPS, MV2+ in ZnPS, MV2+ in FePS, Raman i.r. Raman i.r. i.r. assignmen tsa - - 330 s __ 298 m 300 s - - - 317 m 306 s 306 s 307 s) 6% - - 272 s - - 286 s 273 vs (276 sh - - - - 257 sh 239 vs - 219 m - TiY+RiY 222 s - 240 s 237 229 s s 230 m - ,,,q (PS,) - - - a v = stretch; 6, y = in-plane and out-of-plane deformations; r = rocking; T', R' = external translation and rotation.mainly by the steric constraints of the cations in the gap. Similarly the intercalation of organometallic species in various MPS, systems*q lo, l1 led to the common formula M,-,,,PS, (cation),, with x = 0.35 k0.05 for CoCpz and x = 0.30+0.03 for CrBzi; note that these intercalation rates are approximately twice those reached with the MV2+ intercalates, in agreement with the fact that the organometallic cations occupy roughly half the area of MV2+. MV2+ INTERCALATE OF ZNPS, In contrast to the preceeding intercalated systems, i.r. and Raman bands of MV2+ intercalated in ZnPS, (fig.2) are comparable to those recorded with the palladate salt: they exhibit infrared and Raman coincidences, complex structure and numerous vibrational components. In addition, several vibrational frequencies are shifted with respect to the frequencies observed with the intercalates of MnPS,, CdPS, and FePS,. These shifts are similar to those mentioned above for the palladate salt: the Tring(v4,v16) and B&, modes are situated at 506/518 cm-l and at 1331 cm-l, respec- tively, and the v ~ - ~ ~ , vibration leads to a Raman peak at 1226 cm-l (see table 3). We thus conclude that the pyridyl rings of the MV2+ cation intercalated in ZnPS, are likely to be twisted as in the PdCli- salt. Note that in this case an important sharpening of most infrared and Raman bands is also observed.In agreement with the high rate of intercalation (x = 0.29) found with this host system, all these results are indicative of the high order and compactness of the cations in the ZnPS, gap. The non-planarity of the MV2+ pyridyl groups leads to a possible partial overlap of the cations and consequently allows more compact arrangement in this interlayer space than in the MnPS,, ZnPS, and FePS, gaps. ' FREE ' AND INTERCALATED MV' -k RADICAL Raman spectra (1800-180 cm-l) of the MV'+ radical ([lH,] and [2H,] derivatives) intercalated in CdPS,, recorded using the 6471 A excitation line, are shown in fig. 5. 'The corresponding band wavenumbers are reported in table 5 and compared with those of the 'free' species generated from the chloride salts.Tentative assignments are also given.3270 VIBRATIONAL STUDY OF MV2+ AND MV.+ 80 0 600 400 750 550 3 50 wavenum berlcm -' Fig. 3. Infrared spectra of the (a) ['H,] (900-400 cm-l) and (b) [2Hg] (760-340 cm-l) derivatives of MV2+ intercalated in CdPS,. Upper traces, polycrystalline samples ; lower traces, platelets @//lattice layers). 5. --- gap 3.3 A C a d 0 5 A Fig. 4. Packing model for a planar methyl viologen cation inside the interlayer spacing of the MPS, host lattice, as viewed along the b direction. MV2+ van der Waals contours and sulphur atom van der Waals radii are represented to point out the interaction between the guest species and the host lattice.0. POIZAT, C. SOURISSEAU AND Y. MATHEY 327 1 * 1600 1200 8 00 wavenumberlcm-' 400 Fig.5. Resonance Raman spectra (A, = 6471 A) of the (a) ['H,] and (b) [2H,] MV" species intercalated in CdPS, (asterisks indicate bands from the host lattice). The MV'+ radical exhibits a strong electronic absorption band30 centred at ca. 605 nm which accounts for the resonant character of the Raman spectra.14 Hence, as pointed out by Forster et aZ.,14 the observed Raman peaks correspond to vibrations mainly involved in the electronic transitions responsible for this absorption band : only those modes which contain high contributions from the chromophore coordinates (ring and N-CH, stretches and bends) are expected to be enhanced and thus to be observed. Our assignments are in agreement with this assumption and have been established according to the experimental isotopic shifts and comparison of the MV2+ and MV'+ data.These assignments roughly correspond to the potential-energy description computed by Hester and Suzuki.15 The main difference comes from the observation of several new bands, which precludes an assignment of all the bands to only totally symmetric modes. In any case, the great complexity of the excitation profiles obtained by Forster et all4 for the MV'+ Raman peaks, in the contour of the visible electronic band, indicates that this absorption results from the overlap of several electronic transitions. As the nature and the symmetry of these transitions are not yet established, it is not possible to determine the resonance Raman scattering mechanism. However, since our results show that both totally symmetric and non-symmetric modes are enhanced, we believe that this mechanism is more complex than previously suggested and is probably caused by interfering phenomena.,l Moreover, it is evident from table 5 that the 'free' and intercalated MV'+ spectra are very similar in wavenumbers, relative intensities and enhancement factors.We conclude that, in both the chloride salt and the CdPS, intercalation system, the3272 VIBRATIONAL STUDY OF MV2+ AND MV.+ Table 5. Raman band wavenumbers (cm-l) and assignments of the MV'+ chloride salt and of the reduction product of MV2+ intercalated in CdPS, [IHJ MV'+ [,H,] MV'+ chloride in CdPS, chloride in CdPS, assignmen tsa 1660 m 1656 sh 1534 s 1430 w - - 1357 m 1250 w 1210 vw 1046 sh 1028 m 818 vw 682 w - - 554 vw 430 vw 385 vw 282 w - - - - 1652 m 1528 s 1427 vw 1373 sh 1365 vw 1349 m 1245 vw 1038 sh 1022 m 682 w 562 vw - - - - - - - 381 m 281 w 273 w 269 sh 243 vw 1623 w 1603 sh 1475 s 1432 w - - - - - 866 m 920 w 794 vw 669 vw 615 vw - - - - - ,2w1 - - 1618 m 1475 s 1429 w - - - - - - 861 m 924 w 668 vw 562 vw - - - - 381 m 272 m 242 w ' c ' Vring-ring 'N-CH3 9a + Vring 18a ~ C H + Vriy 1 ring breathing 'd' (ring) 6b Aring CdPS, lattice ' e ' (ring) AN-CH~ + A r i n g AN-CH3 - CdPS, CdPS, CdPS, a v = stretch; 6, A = in-plane deformation; r = out-of-plane deformation.reduction of MV2+ induces identical electronic perturbations in the intramolecular bonds. Hence, the structural and electronic properties of the radical cation are not markedly changed upon intercalation.Finally, the half-life of the radical species in the atmosphere has been estimated by measuring the intensity decay of the MV'+ Raman spectrum with time. Such kinetic measurements have been carried out under identical experimental conditions with the free chloride salt and with the intercalate of CdPS,. A half-life as long as three weeks is estimated for the intercalated MV'+ species, in contrast with the very short lifetime (less than one second under ambient laboratory conditions) of the free-radical cation. Such stability reveals that no electron transfer occurs between the guest species and the host lattice, and confirms once more the absence of strong electronic interaction. Under these conditions it appears that the radical cation has the appearance of a diluted and trapped species inside an inert matrix.This material thus provides an adequate starting system for performing interlamellar photochemical studies, and work towards this goal is in progress.3273 0. POIZAT, C. SOURISSEAU AND Y. MATHEY CONCLUSIONS In this spectroscopic study, the assignment for the vibrational modes of the methyl viologen cation (MV2+) has been improved owing to new data from deuterated molecules. The frequencies of three i.r.-active vibrations, namely yEH (v,,), v ~ - ~ ~ , and &,, are particularly sensitive to modifications of the external environment of the cation, and several characteristic band structures and frequency shifts provide convincing evidence for a transformation from planar to twisted MV2+ structural conformation.Spectra of the intercalated compounds nearly correspond to the superposition of those of the host lattice and of the guest species; this suggests that the intercalated cation interacts only very weakly with the host systems, which behave as regular counter-ions. The orientation and conformation of the guest molecules in the different lattices have been inferred from polarized infrared results and thus can account for the different observed intercalation rates. In the ZnPS, host system the MV2+ pyridyl groups appear to be twisted, while in the MnPS,, CdPS, and FePS, lattices the cation is planar and parallel to the layers. Finally, chemical reduction of the guest MV2+ cation in CdPS, leads to the intercalated MV'+ radical cation, whose vibrational properties are similar to those of the 'free' radical (chloride salt).However, the radical cation trapped inside the interlamellar space possesses a substantially longer lifetime. This last result may be of great interest in the field of electron-transfer processes involving the MV2+/MV'+ redox couple, e.g. in the photochemical reduction of water, especially if we consider intercalation of the Ru(bipy)i+ cation, which is an oxidizing agent commonly associated with MV2+ in such photoreduction reactions. Therefore additional experi- ments are currently in progress with the aims of co-intercalating both these reactive entities and then performing photochemistry inside these layered compounds. We thank Mrs J. Belloc for technical assistance in the synthesis, Dr H.Jobic for the inelastic neutron-scattering data and Dr R. Clement for helpful discussions. D. 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