J . CHEM. SOC. DALTON TRANS. 1992 1217Preparation and Properties of Tetrathia- and Tetramethyl-tetraselena-fulvalene Salts of [ M,O,,]*- (M = Mo or W) tSmail Triki,a Lahcene Ouahab,"," Jean-Francois Halet,"aa Octavio Pena,a Jean Padiou,"Daniel Grandjean,a Chantal Garrigou-Lagrange" and Pierre Delhaes **ba Laboratoire de Chimie du Solide et lnorganique Moleculaire, U.R.A. 254 CNRS, Universite de Rennes I,35042 Rennes Cedex, FranceCentre de Recherche Paul Pascal, CNRS, Avenue Albert Schweitzer, 33600 Pessac, FranceThe preparation, X-ray crystal structures, conductivity and optical properties of the salts, [ttf],[w,o1,] 1,[ttfJ,[Mo,O,,] 2 (ttf = tetrathiafulvalene) and [trntsf],[W60,,]~~dmf 3 (tmtsf = tetramethyltetra-selenafulvalene) are reported. Crystal data: 1, triclinic, space group P1, a = 9.965(3), b = 10.503(3), c =10.634(3) A, a = 71.93(2), p = 78.63(2), y = 63.38(3), Z = 1, R = 0.020; 2, triclinic, space group P i , a =monoclinic, space group P2,/c, a = 11.589(4), b = 19.385(5), c = 13.681 (3), p = 99.53(2), Z = 2 andR = 0.049.In all the salts the organic molecules form trimerized stacks. A classical ring-over-double bondoverlap is observed in 3, in contrast with 1 and 2 which present an unusual intra-trimer criss-cross overlap.In compound 3 the intra-trimer Se Se (3.73-3.81 A) contacts are shorter than those observed in[tmtsf],X (X = PF, or BF,) series. The inter-trimer Se Se contacts (3.94-4.1 2 A) are in the rangeof the van der Waals separation (4 A). Short 0 Se (3.1 7-3.30 A) contacts are observed. Bothelectrical and optical measurements and also electronic band-structure calculations reveal that thethree salts are semiconductors.9.942(3), b = 10.417(3), c = 10.601 (3), a = 72.33(2), p = 78.77(2), y = 63.52(3)", Z = 1, R = 0.030; 3,We are currently studying the charge-transfer complexes arisingfrom organic donors derived from the tetrathia(se1ena)ful-valenes and inorganic anions which have specific properties.Our goal is to obtain new materials containing organic andinorganic blocks in the mixed-valence states.Iu3 In this work theinorganic counterparts that are chosen are the polyoxometalatedianions [M6OI9l2- (M = W or Mo) which result from thecondensation of six M 0 6 octahedra sharing common verticesand adopt the so-called Lindquist Known for overa century, the polyoxometalates are still of interest because oftheir high electron-acceptor a b i l i t ~ .~ The presence of inorganicacceptor anions in charge-transfer salts can induce the co-existence of mixed-valence states in the organic and inorganicsystems. The stabilization of such a mixed-valence state is one ofthe prerequisite conditions for electron delocalization (electricalconductivity) or unpaired spin localization on the transitionmetal d orbitals (magnetic properties). Since our first at-tempt to synthesise new adducts combining tetrathia-fulvalene derivatives and x-Keggin 4b phosphotungstate orLindquist hexametalate polyanions several other organic-inorganic salts have been characterized in our laboratory andby Bellito e t ~ 1 .~ " Following a preliminary note,2a we re-port here in detail the preparation, X-ray crystal structure,conductivity. magnetic and spectroscopic characterizationsof [ttf]3[W6019] 1, [ttf]3[Mo,0,,] 2 and [tmtsf],-[W,O ,]-2dmf 3 { ttf = tetrathiafulvalene [2-( 1',3'-dithiol-2'-y1idene)- 1,3-dit hiole], tm tsf = tetramethyl tetraselenafulvalene[2-(4',5'-dimethyl- 1 ',3'-diselenol-2'-ylidene)-4,5-dimethyl- 1,3-diselenole] and dmf = dimethylformamide).ExperimentalPreparations.-The compounds ttf and tmtsf (Fluka) wereused as received. Acetonitrile, dichloromethane and dimethyl-formamide (Fluka) were distilled and passed through activatedneutral alumina before use. The compounds [NEt4]2[Mo,019],[ N B U ~ ] ~ [ M O ~ O ~ ~ ] and [NBU~]~[W~O~,] were prepared andrecrystallized according to refs.6 and 7. Their stoichiometrieswere determined by elemental analysis [Calc. (Found) for48.93 (48.75); N, 2.40 (2.30). For [NBu,]~[Mo~O,,]: C, 28.15(28.40); H, 5.30 (5.35); Mo, 42.20 (43.25); N, 2.05 (2.05). For(1.35); W, 58.35 (54.65)%]. The electrocrystallizations werecarried out in a U-shaped cell (25 cm3) with the two electrolyticcompartments separated by glass frits (porosity 3). Theelectrodes comprised platinum wire (diameter 1 mm) immersedin the degassed solution to a depth pf 2.5 cm. The constantcurrent was stabilized by a regulator made in our laboratory.[ttf]3[W6019] 1 and [ttf]3[Mo6019] 2. Anodic oxidation ofthe organic donor ttf (2 x mol dm-3) at a low constantcurrent ( I = 1 PA) in the presence of the tetraalkylammoniumsalt of the [M6019]2- dianion mol dm-3) as supportingelectrolyte in acetonitrile led to polyhedral block-shaped blackcrystals after several weeks.The stoichiometries were deter-mined unambiguously by X-ray crystal structure analysis. Thetwo compounds can be obtained either with [NBu,]~[M~O,,]or [NEt4]2[M6019] as supporting electrolyte.[tmtsf]3[W60,,]~2dmf 3. The anodic oxidation of tmtsf(2 x mol dm-3) at a low constant current (I = 1.25 PA) inthe presence of [ N B U ~ ] ~ [ W ~ O ~ , ] mol dm-3) assupporting electrolyte led to black needle crystals withhexagonal sections (1-2 mm long) after 1 week. A mixture ofdmf and CH2C12 (ratio 3 : l ) was used [Calc. (Found) for(0.90); Se, 32.70 (33.20); W, 38.05 (37.65)%].Attempts toprepare the corresponding [Mo601 9]2- salt under the sameexperimental conditions failed.[NEt4]2[M~6019]*2H20: C, 16.35 (16.40); H, 3.75 (3.90); MO,[NBU4]2[W60,9]: c , 20.30 (20.70); H, 3.80 (4.10); N, 1.50C36H,,N2O21Se12W6: c , 14.90 (15.15); H, 1.70 (1.55); N, 0.95t Supplenicwrrwj rlrrtu available: see Instructions for Authors, J . Chew.Soc., Dulton Trans., 1992, Issue 1, pp. xx-xxv.Noti-SI units cwiplojed: eV z 1.60 x J, Oe = lo3 A m-'.X-Ray Crystal Structure Analysis.-Black crystals of com-pounds 1-3 were mounted on an Enraf-Nonius CAD4 dif-fractometer equipped with graphite-crystal-monochromatize1218 J. CHEM. SOC. DALTON TRANS. 1992Table 1 Crystal and refinement data for compounds 1-3Compound Lttf13[w60191 ~ t f f ~ 3 ~ M 0 6 0 1 9 ~ ~tmtsf]3[W60,9]*2dmf 3(a) Crystal dataA4 2020.15 1492.69 2897.42Crystal system Triclinic Triclinic Monoclinic4 9.965(3) 9.942(3) 11.589(4)10.503(3) 10.4 17(3) 19.385(5)C I A 10.634( 3) 10.601(3) 13.68 l(3)4" 71.93(2) 72.33(2) 90Pi" 78.63(2) 78.77( 2) 99.5 3( 2)rl' 63.3 8( 3) 63.52( 3) 90ujA3 943.8 933.9 3030.9Z 1 1 2DJg ~ m - ~ 3.554 2.654 3.175F ( o w 908 716 2600(h) Data collection and refinement parametersCrystal size/mm 0.3 x 0.1 x 0.1p( M o-Kx)/cm-' 193.23 26.29 187.94Formula C18H12019S12W6 Cl8Hl2Mo6019S12 C36H50N2021Se12W6Space group PT PT p2 1 /cb/A0.15 x 0.15 x 0.1 0.35 x 0.1 x 0.1h,k,l Ranges (r13,-13t013,-14t014 -13t013, -13t013,&14 &13,&23,-16t01626 Limits/" 2-50 2-50 2-50No.of reflections:C F O 3 no(F0)3unique 336! 4244 50452729,n = 6 2294,n = 3 3380,n = 3Rint 0.047 0.013 0.026No. variables 25 1 25 1 310R(F)," R'(F) 0.020,0.030 0.030,0.046 0.049,0.063s 0.739 1.118 1.314A i o 0.10 0.01 11.61Aple A-3 1.179 1.616 2.7 12"R = C[lIFoI - IFcll]/CIFoI. K' = [Cw(IF0I - IFcI)2/Xw~lFo12]f, where M' = 4F02/Lo2(I) + (0.071F,12)2]. Goodness of fit (s) = [Xw(1F0I - lFc1)2/( N o h - N"a,>14.'++++%+++ ++++++ + ++++-I-2 3 4 5 61 O ~ T -klFig. 1 The d.c. conductivity of Ltmtsf],[W3OI9]~2dmfMO-KR radiation (A = 0.710 73 A). The cell dimensions havebeen determined and refined by the least-squares method fromsetting angles of 25 centred reflections.The intensities werecollected by 0-20 scans. Three standard reflections measuredevery hour revealed no fluctuations in intensities. One set ofreflections were collected up to 20 = 50". Lorentz, polarizationand absorption corrections were applied. The absorptioncorrections were performed using the y-scan * O method forcompounds 1 and 3 and the DIFABS procedure '' for 2. Crystaldata are summarized in Table 1.The structures were solved by direct methods (MULTAN84) and successive Fourier difference syntheses. Everystructure was refined by weighted anisotropic full-matrix least-squares methods. After refinement of positional and anisotropic( p i j ) thermal parameters for all non-hydrogen atoms, thepositions of the H atoms were calculated [dC-H) = 1 A; Be, =4 A2] and included as a fixed contribution to F,.In compound3 the atoms of the dmf molecule and 0(4), C(1) and C(6) wererefined isotropically. Scattering factors and corrections foranomalous dispersion were taken from ref. 10. The molecularand crystal structure illustrations were drawn with ORTEP.'All the calculations were performed on a PDPl1/60 computerusing the SDP programs described by Frenz.12 The positionalatomic coordinates are given in Table 2, bond lengths andangles in Table 3 (see also Figs. 5 and 11).Additional material available from the Cambridge Crystal-lographic Data Centre comprises H-atom coordinates, thermalparameters and remaining bond lengths and angles.Electricul Conductivity.-The d.c. electrical conductivitieswere measured by use of a standard four-probe technique. Allsalts 1-3 are semiconducting with absolute room-temperaturevalues ( ~ 7 ~ ~ ~ ~ ) equal to 1.4 x 0.5 x and 1.3 Q-'cm-', respectively.The temperature dependence of the single-crystal d.c. electrical conductivity along the needle axis ispresented in Fig. 1 for 3 and yields an activation energy of 0.2 eV.Optical Propertiex-An IR Nicolet MX 1 interferometer(3504800 cm-') and a UV/VIS Perkin-Elmer 350 spectrometer(385G25 OOO cm-') were used for comparative transmissionmeasurements of finely ground KBr pellet samples. The IRspectra were recorded at different temperatures down to 15 Kwith a 4He cryostat.The electronic spectra of the three salts are illustrated in Fig.2, except the vibrational bands, observed at lower frequenciesJ .CHEM. SOC. DALTON TRANS. 1992 1219Table 2 Atomic coordinates with standard deviationsAtom XILl( ( 1 ) Cttfl,Cw,o,YlThe anionW(2) 0.01 5 56(2)W(1) -0.253 36(2)W(3) -0.062 89(2)O(1) -0.439 2(5)(3.2) - 0.024 8( 5 )O(3) 0.107 4( 6)O(4) -0.214 9(4)O(5) -0.192 l(4)O(6) -0.153 9(5)O(7) - 0.252 9(4)O(8) 0.059 2(4)00) 0.037 3(4)O( 10) 0( h ) CttfI,CMo,O,,lThe anionM o ( l ) 0.252 87(5)Mo(2) 0.020 23(5)O( 1 ) 0.437 2(4)O(2) 0.027 4(4)O(4) 0.215 6(4)O(6) 0.157 I(4)o ( 7 ) 0.253 7(4)O(8) 0.054 2(4)O(9) -0.038 9(4)O(10) 0Mo(3) -0.063 8 5 ( 5 )O(3) -0.105 l(5)O(5) 0.194 4(4)( c ) [tmtsf],[W60,,].2dmfThe anionW( 1 ) 0.126 7(1)W(7) 0.032 I ( I )W(3) 0.I 56 O( I )O(1) 0.2 18(2)O(3) 0.267(2)O(5) 0.127(2)O(6) 0.223(2)O(2) - 0.059(2)O(4) * 0.080( 1 )O(7) - 0.023 2)O(8) O.loo(1)O(9) 0.1 54( 2)O( 10) 0tmtsf, ASe( 1 )Se( 2)C(1)*C(2)(23)C(4)( 3 5 )- 0.034 3( 3)- 0.079 7( 3)- 0.022( 2)- 0.094( 3)-0.1 1 l(3)-0.1 19(3)- 0.165(3)* Atoms refined isotropicallyYlh0.160 98(2)0.052 13(2)0.278 8(6)0.335 9(5)0.087 2(5)0.170 4(4)0.284 4(4)0.199 3(4)0.1 14 3(4)0-0.193 33(2)-0.091 O(5)- 0.028 O(4)-0.162 80(5)0.049 27(5)-0.191 84(5)-0.275 9(5)0.087 l(4)-0.334 8(4)-0.085 3(4)- 0.167 9(4)-0.285 l(4)0.030 l(4)0.201 4(4)-0.1 15 5(4)00.057 61(6)0.083 58(6)0.099( 1 )- 0.063 70( 6)- 0.147( 1)O.llO(1)-0.021 l(9)0.1 13 8(9)0.099 8(9)0.013 8(9)0-0.005 6(9)-0.1 16 80.069 9( 1)0.087 9(2)0.032( 1)0.155( 1)0.161 ( 1 )0.209( 2)0.224(2)Z i C-0.001 32(2)-0.230 03(2)0.014 38(2)0.398 3(4)0.185 l(4)- 0.002 4(4)-0.025 7(6)- 0.184 2(4)-0.01 1 2(4)0.0 12 5(4)-0.196 O(4)0.172 7(4)00.005 lO(5)0.013 67(5)0.003 O(4)0.024 2(5)0.184 6(4)0.008 7(4)-0.229 86(4)-0.395 l(4)-0.186 6(3)- 0.0 1 6 O(4)- 0.196 8(4)-0.173 8(4)0-0.087 16(9)0.123 20(9)0.082 64(8)- 0.1 50( 2)-0.21 l(1)0.143( 1)0.027( 1 )- 0.002( 1 )-0.168( 1)-0.139(1)- 0.032( 1 )0.165( I )00.628 7(2)0.397 5(2)0.505(2)0.579(2 10.480(2)0.649(2)0.419(3)Xla0.736 8(2)0.541 3(2)0.559 2(6)0.813 l(7)0.725 6(8)0.175 3(2)0.430 8(2)0.649 9(2)0.400 8(2)0.367 2(6)0.139 8(8)0.256 O(8)0.461 l(6)0.685 l(7)0.572 9(8)0.737 8(2)0.542 9(2)0.561 O(6)0.814 O(7)0.726 8(8)0. I 74 2(2)0.432 O(2)0.649 O(2)0.395 6(2)0.366 O ( 5 )0.143 O( 7)0.257 2(8)0.458 3(6)0.681 8(7)0.565 7(7)0.289 7(3)0.243 3(3)0.406 4(3)0.354 3(3)0.303( 2)0.234(2)0.213(3)0.21 l(3)0.1 72( 3)0.349(3)0.437(3)0.4 1 5( 3)0.49 l(4)0.448(3)0.539( 3)0.487( 2)0.413(3)0.633(3)0.472( 3)Y/b0.352 2(2)0.553 O(2)0.481 6(6)0.359 2(7)0.449 l(7)0.371 5(2)0.299 2(2)0.080 3(2)0.156 7(2)0.274 4(6)0.436 5 ( 8 )0.402 O ( 8 )0.184 l(6)0.023 0(8)-0.009 3(7)0.352 l(2)0.551 O(2)0.479 l(5)0.354 5(6)0.446 3(7)0.370 8(2)0.297 6(2)0.079 6(2)0.158 7(2)0.274 l(5)0.437 8(7)0.402 5(7)0.182 9(5)0.027 6(6)-0.008 2(6)0.072 9(2)0.095 O(2)- 0.086 O( 2)- 0.060 I(2)0.040( 1 )0.1 58( 1)0.169(1)0.215(2)0.23 5( 2)-0.025( 1)--0.157(2)- 0.145(2)- 0.2 19( 1 )- 0.193(2)0.43 l(2)0.44 1 ( 1 )0.404(2)0.40 1 (2)0.496( 2)Z/C0.497 l(2)0.286 2(2)0.453 l(5)0.335 9(7)0.241 l(7)0.466 5(2)0.272 2(2)0.502 9(2)0.701 9(2)0.437 8(6)0.300 7(8)0.212 6(7)0.535 5(5)0.665 9(8)0.757 6(7)0.499 l(2)0.285 7( 1)0.453 7(5)0.340 7(7)0.239 8(7)0.465 4(2)0.272 2( 1)0.506 7(2)0.702 7(2)0.436 2(5)0.299 7(8)0.21 1 6(7)0.538 8 ( 5 )0.670 4(8)0.759 3(6)0.629 2(2)0.397 7(2)0.593 7(2)0.364 O(2)0.501 (2)0.577(2)0.480(2)0.653(2)0.420(3)0.49 1 (2)0.502(3)0.406(2)0.560( 3)0.329( 3)0.644(3)0.4 16( 2)0.635(2)0.636(2)0.361(3)We note the presence of both the ‘A’ and ‘B’ charge-transfer (c.t.)bands (Torrance’s notation,13 respectively around 6000 and12 500 cm-’ for the ttf salts and around 3800 and 12 000 cm-’for the tmtsf salts.The observed frequencies for the ‘A’ band1220,-.J. CHEM. SOC. DALTON TRANS. 1992Wave nu m berlcm-’Fig. 2 Plots of the mean electronic absorption coefficient (ii in cm-I)determined for KBr pellets at room temperature uersus the energy (inwavenumbers) for [ttf],[W,O,,] (-), [ttf]3[Mo,0,,] (. . . .) and[tmtsf],[W,O,,].2dmf (- - - -)I l l4400 3200 2000 1400 800Wavenumberlcm-’[ttf13[w,o,,l, ( h ) [fff13[M060191 and (c) [tmtsf13[W,0,91.2dmf atFig.3room temperature (the anion bands are labelled with asterisks)Infrared absorption spectra between 4800 and 400 cm-’ of ( a )1390..-~~ - ~~1500 1400 1300 1200W aven u m be rlcm-’+ + + + + + + +* ++ + +-ma + +ooa a + +a m aa a m *++m a m a ++a a m ++ +++ +am + ++ + + amaa m aa ma maam aa m13701 I I I0 100 200 300TIKFig. 4 (a) Infrared spectra between 1500 and 1200 cm-’ of[ttf]3[W6019] (upper) and [ttf],[Mo,O,,] (lower) at 15 (- -~ -) and300 K (---). (h) Temperature dependence of the vibronic bandfrequency ag v(C=C) for [ttf],[W,O,,] ( + ) a n d [ttf]3[Mo6019] (e)are almost identical to those observed for [tmtsf],[M(CN),](M = Pt or Ni)14-16 or [ttf]14[MC14]4 (M = Mn or Co)salts: they are characteristic of a mixed-valence state.So far,however, we have not observed simultaneously the ‘A’ and ‘B’bands in the absorption spectra of ttf and tmtsf salts.From Fig. 3, showing the room-temperature infrared spectraof our three Lindquist salts, we can immediately recognize thev(C=O) band of the solvent molecule (dmf) present in thecrystal structure of [tmtsf],[W,01,]~2dmf and the charac-teristic bands of the [W,Ol,]z- and [ M O , O , ~ ] ~ - dianionsJ. CHEM. SOC. DALTON TRANS. 1992 1221Scheme 1These bands have been attributed by comparison with literaturedata. Fig. 4(u) presents the expanded region between 1500 and1200 cm-’ at room temperature and at 15 K for the two ttf salts1 and 2.The lower band situated around 1355 cm-’ whosefrequency does not vary with temperature is probably due to acombination of the F l u and A, vibrations of the anions ataround 880 and 600 cm-’ respectively.’8 On the other hand, thewavenumber of the other component of the doublet is 15 cm-’higher at 15 K than at room temperature. This band is due tothe vibronic ag v(C=C) vibration. ’’ Its temperature dependenceis also given in Fig. 4(6).The v(C=C) ag band of the tmtsf salt 3 is observed around1335 cm ’ and its frequency does not vary with temperature.Therefore, we can assume that no structural phase transitionoccurs for this salt.Mcigtwric. Susceptihilit~.-Magnetic susceptibility measure-ments were performed in the range 2 < T < 300 K using avariable-temperature SQUID susceptometer (SHE-VTS-906)under a magnetic field strength of 1 or 5 kOe.An almostconstant diamagnetic signal was observed over the wholetemperature range. In order to get a more precise estimation ofthe diamagnetic contribution of the compounds, additionalmeasurements were performed at fixed temperatures (300 and100 K ) at 20 and 50 kOe, yielding diamagnetic contributions of9.32 x lW9, 4.57 x and 4.26 x m3 kg-’ mol-’ atroom temperature for salts 1, 2 and 3 respectively. Such valuesare in reasonable agreement with the total diamagneticcontributions calculated using Pascal’s tables.Elrctronic Bmd-structure Calculations.-All molecular andsolid tight-binding l 9 calculations were carried out within theextended-Huckel formalism 2o using standard atomic param-eters for H, C , S and Se.The exponent (6) and the valence-shellionization potential ( H i i in eV) were respectively: 1.3, - 13.6 forH Is; 1.625, - 21.4 for C 2s; 1.625, - 11.4 for C 2p; 1.817, -20.0for S 3s; 1.81 7, - 13.3 for S 3p; 2.44, - 20.5 for Se 4s and 2.07,- 13.2 for Se 4p. A set of 8k points, chosen in the irreducible partof the oblique Brillouin zone, according to the Ramirez andBohm method ” was used to calculate the atomic charges of theorganic ( t t f ) 3 slab present in 1 and 2. The symbols r, X and Yrefer to points of the Brillouin zone of coordinates (0, 0), (a*/2,0) and (0, h*/2) respectively. A set of 9k points was used tocalculate the atomic charges of the organic (tmtsf), chainencountered in 3.For computational reasons, the methylgroups of the tmtsf molecules were replaced by hydrogen atoms.Results and DiscussionDescription of the M6019’- Anions.-The structure of the[M601912- anions (Scheme 1) consists of a central oxygenatom 0, [e.g. 0(10)] surrounded octahedrally by six metalatoms which are bonded through oxygen bridges Ob [e.g. 0(4)].One terminal oxygen atom 0, [e.g. O(l)] is bonded to eachmetal. The structure can be described as a condensation of sixdistorted MO, octahedra sharing common vertices at thecentral oxygen atom. Each M 0 6 octahedron has common edgeswith four neighbouring ones. The bond distances and angles aregiven in Table 2. In the [w6019]2- units the average bonddistances observed in 1 and 3 [W-W 3.292, 3.286; W-0,2.323(1) A] are in good agreement and compare well with thecorresponding ones found as in [NBU,+]2[W,O19] or[N(PPh3),],[W6019].226 In the Mo60,, unit (compound 2)the average Mo-Mo (3.277 A), Mo-0, [1.678(5) A], Mo-0,[ 1.924(4) A] and Mo-0, C2.3 17( 1) A] bond distances are closeto the corresponding ones found, for instance, in [HN3P3-1.705(5), 1.68(2); w-ob 1.924(5), 1.92(2); and w-0, 2.328( l),(NMe2)6],[M0601 9] 23a or [AsPh4]2[M06019].23b[ttf]3[W6Ol9] 1 and [ttf]3[Mo6019] 2.-Crystal structures.The bond distances and angles are given in Fig.5 for the ttfmolecules of [ttf]3[W6019] 1 and [ttf]3[Mo6019] 2. Both saltsare isostructural. The crystal structures represented in Figs. 6and 7 are built from [w6O19l2- or [Mo,O,,]’- anionslocated at the origin of the lattice, and two independent ttf unitsdenoted A and B: one (A) positioned on the (4, 4, )) inversioncentre and the other (B) in general position.The bond distances of the two independent ttf molecules inboth compounds 1 and 2 are averaged in the rnrn symmetry andcompared in Table 4 with the corresponding distances in ttf”’salts (x = 0 or l).24-28 We note a slight difference in the C-Cand S-C bond distances between the A and B ttf molecules forcompounds 1 and 2.Molecules A and B form a stack of trimerswith a BAB-BAB sequence (Fig. 6). The overlap betweensuccessive trimers, i.e. between the molecule B and itscentrosymmetry-related one can be described as a ring-over-double bond overlap followed by a transversal deplacement.2“The intra-trimer overlap, i.e.between molecules A and B, is ofcriss-cross type with an eclipse angle of 90” (Fig. 7). This modeof overlap has been observed for instance in [tmtsf][FeCI,]and its sulfur analogue^,,^ [tmtsf][Re04]-0.25C2H,C13 3o and[tmtsf],[PW 2040].3 Such a packing leads to strong intra-trimer S S interactions (Fig. 6). The intertrimer S Scontacts are longer than the van der Waals separation of 3.8 A.Additionally, there are short anion+ation S - 0 and inter-chain S - - - S contacts (Figs. 6 and 8).Theoretical band calculations. A pertinent question is theassignment of the oxidation state of the ttf molecules con-stituting the organic (ttf)32+ stack in [ttf]3[W6019] 1 and[ttf],[Mo6Ol9] 2.Is there a mixed-valence t t f t f , ttf’, ttf%+stryctur?, corTesponding to inequivalent sites, or a statisticalttf”, ttf”, ttf’+ one for equivalent sites in compounds 1 and 2?In order to elucidate this point we performed tight-bindingcalculations within the extended-Huckel formalism on the two-dimensional organic sublattice (ttf)32’ present in 1 and 2 (seeFig. 8).Since the (ttf)32 + chains in this sublattice are constructedfrom the stacking of nearly isolated criss-cross (ttf)32 + trimers(see above), the natural starting point is to look at the bondingfeatures of an idealized pseudo-molecular (ttf)32 + model a ofsymmetry D,, made of two ‘outside’ ttf molecules (B and B’) andan ‘inside’ one (A). Identical central C-C, C-S and edge C-Cbond distances (A) of 1.35, 1.72 and 1.313 respectively weretaken for all of them.Consideration only of the highest occupiedmolecular orbital (HOMO) of the neutral ttf molecule isgenerally sufficient to discuss the factors affecting the electronicand structural properties of organic salts., ’The HOMO of ttf is concentrated heavily on the p, atomicorbitals of the S and central C atoms.,’ Being bonding betwee1222 J. CHEM. SOC. DALTON TRANS. 1992Table 3 Bond distances (A) and selected bond angles (") in the M,OI9*- units (see Scheme 1 for labelling)1(M = W) 2(M = Mo) 3(M = W) l ( M = W) 2(M = Mo) 3(M = W)O(l)-M(l)-O(4)O( 1 )-M( I )-O( 5 )O( 1 )-M( I )-0(6)O( 1 )-M( 1 )-O( 7)O( I)-M( 1)-O( 10)O(4)-M( 1)-O(5)O(4)-M( 1)-0(6)O(4)-M ( 1 )-O( 7)O(4)-M( 1)-O( 10)O(5)-M( 1)-0(6)O(5)-M( 1)-0(7)O(5)-M( 1)-O( 10)O( 6)-M ( 1 )-O( 7)O( 6)-M ( I )-O( 10)O(7)-M( 1)-O( 10)0 (2 )- M (2 )-O( 4)O( 2)-M( 2)-O( 5 )0(2)-M(2)-0(8)0(2)-M(2)-0(9)0(2)-M(2)-0( 10)0(4)-M(2)-0(5)0(4)-M(2)-0( 8)0(4)-M(2)-0(9)0(4)-M(2)-0( 10)1.705(4)1.939(4)1.905(4)I .924(6)1.942(5)2.325( 1)1.702(4)1.9 lO(4)1.933(3)104.0( 2)103.7( 2)103.5(2)180.0( 1 )I52.4(2)86.2(2)86.2(2)76.1 (1)87.5(2)87.0(2)76.3( 1)152.4(2)76.6( 1 )75.q 1)104.0(2)103.6( 2)104.0(2)103.7(2)1 52.4( 2)88.2(2)86.2(2)76.6( 1)104.1(2)179.3( 1)1.680(4)1.987(4)1.870(4)1.888(5)1.956(5)2.322( 1)1.669(4)1.864(3)1.980( 3)102.7(2)104.4( 2)104.1(2)102.7(2)177.6(1)152.7( 1 )8 5.7( 2)83.2(2)75.4( 1)90.8(2)87.9(2)7 7 3 1)152.6( 1)77.2( I )75.8( I )10432)102.3(2)I04.0( 2)102.9(2)177.7( 1)153.1( I )90.1 (2)88.5(2)77.7( 1)1.67(2)1.91(2)1.90(2)I .92( 2)I .97(2)2.325( 1 )1.7 1 (2)1.95(2)1.94(2)103.2(9)10339)105.3( 9)102.6(9)I 80.0( 5 )153.4(8)85.6(7)88.3(7)76.8(6)87.0(7)86.4( 7)76.6(6)152.1 (8)74.7(6)77.4(6)106.1(9)10 1.7(9)104.4( 8)104.5(8)177.8(7)152.1(7)85.0( 7)87.8(7)76.1 ( 5 )0(5)-M(2)-0(8)O( 5)-M (2)-0(9)O( 5)-M (2)-O( 10)0(8)-M(2)-0(9)0(8)-M(2)-0( 10)0(9)-M(2)-0( 10)0(3)-M(3)-0(6)O( 3)- M (3)-O( 7)0(3)-M(3)-0(8)O( 3)-M( 3)-0(9)0(3)-M(3)-0( 10)O(6)-M( 3)-0(7)0(6)-M(3)-0(8)0(6)-M(3)-0(9)0(6)-M(3)-0( 10)O(7)-M( 3)-O(8)0(7)-M(3)-0(9)0(7)-M(3)-0( 10)0(8FM(3)-0(9)0(8)-M(3)-0( 10)0(9)-M(3)-0( 10)1.929(5)2.325( 1)1.708(6)1.929(4)1.9 1 O(3)1.920(4)2.334( I )85.8(2)86.7(2)75.8(2)152.3(2)76.2( 1)76.1(1)103.2(2)1O4.3( 2)103.6(2)104. I(3)179.5(2)152.4(2)86.2(2)86.1 (2)76.3(2)87.1(2)87.6( 2)76.2(2)152.3(2)76.1(1)76.2( 2)I .935(5)1.9 I 7(4)1.900( 5)2.320( I )1.68 l(5)1.964(4)1.890(3)I .932(4)I .9 1 5(3)2.3 1 O( 1 )84.9(2)84.1(2)75.5( 1)152.6(2)76.6( 1 )76.3( 1)104.3(2)103.4(2)103.0( 2)178.4(1)153.5(2)85.1(2)85.9( 1)76.1( 1)87.9(2)89.0( I )77.3(2)153.3(2)76.3( 1)7 7 4 I )1.947(4)102.2(2)I .92(2)1.97(2)2.323( 1)1.67( 2)1.87(2)1.94(2)1.90(2)1.88(2)2.322( 1 )86.4( 8)87.1(8)76.0( 5 )I51.1(7)75.4(5)75.7(5)105.5(9)101.1(9)104.2(8)102.6(8)179.0( 8)153.4(7)86.3(7)86.6(8)7 5 3 5 )87.3(7)87.7(8)77.9(5)153.2(7)75.9( 5 )77.3(5)24( 10)S(3)B BFig.5 The constituent molecules, atomic numbering, bond lengths (A) and bond angles (") for (a) [ttf]3[W,0,,] and (b) [ttf],[Mo,O,,J . CHEM. SOC. DALTON TRANS. 1992 1223Fig. 6 The ttf stacks in salts 1 and 2 showing the shortest intermolecular contacts (A): d, [S(l') - S(5')] = 3.315(3), 3.304(3); d,[S( 1') * * S(3")] = 3.528(3), 3.502(3); d, cS(2') - S(6")] = 3.392(3), 3.385(3); d4 CS(2') - * * S(4')] = 3.358(3), 3.326(3); d, [S(S') - S(6"')] =4.016(3), 3.999(3)]; d6 CS(2) O( l)] = 3.094(4),v 3.098(4);'" d, [S( 1) * - 0(2)] = 3.056(5),"' 3.075(3);" d, CS(4') * * * C(8"')l = 3.55(1), 3.52(1); d9CS(4') - C(9"')] = 3.52(1), 3.49(1); d , , [S(5') C(7"')] = 3.57(1), 3.53(1).Symmetry codes: I x, y , z; I1 1 - x, 1 - y,l - 2; 111 1 - x , --y, 1 - z;IV .\-. 1 + is, z; v -s, 1 - y, -2Fig. 7 Stereoscopic view of salts 1 and 2 showing the intratrimer criss-cross overlapsTable 4 Comparison of averaged bond lengths (A) in ttf"1 2A BPhU 1.379(7), 1.353(7)h 1.7 17(5), 1.73 l(6)l , 1.725(7), 1.720(8)d 1.299(9), 1.324(10)Ref. This workA B ttf CttflCtcnsl0 + 0.591.396(7), 1.383(7) 1.349(3) 1.369(4)1.717(5), 1.724(5) 1.757(2) 1.743(4)1.71 1(7), 1.716(7) 1.726(4) 1.736(5)1.346(9), 1.33(1) 1.314(3) 1.323(4)This work 24 25The nrni symmetry has been imposed.Electronic charge.[ttf]C104 A B+ I + 1 01.404(13) 1,393(3), 1.347(3)1.7 1 3(9) 1.715(2), 1.755(2)1.725(12) 1.719(2), 1.737(2)1.306(16) 1.330(3), 1.322(3)26 27A B + $ (mean)1.357(24), 1.360(9)1.736( lo), 1.735(5)1.71 7( 16), 1.726(6)1.326( lo), 1.345(9)28C atoms and antibonding between C and S atoms, electronremoval from ttf usually results in a lengthening of the C-Cbonds and a shortening of the C-S bonds.31The interaction of the HOMOS of the three ttf moleculesstacked in a criss-cross fashion leads to the orbital diagramshown in Fig. 9. The HOMO of the inside ttf molecule (A) o1224 J. CHEM. SOC. DALTON TRANS. 1992Fig. 8 Projection of the structures of salts 1 and 2 onto the ah plane,showing the interstack S - - S contacts: d,, [S(3)-S(3’)] = 3.430(2) and3.415(2) A for 1 and 2 respectively; I -I, 1 - J’, 1 - z-gt1bluBI aFig. 9 Interaction diagram of the HOMOs of three ttf moleculesb,, symmetry interacts with the in-phase combination of theHOMOs of the two outside molecules (B and B’) of the samesymmetry.The out-of-phase combination of ag symmetryremains unperturbed after interaction. With a four-electronsystem in a low-spin state, the mixed-valence ttf*+, ttf+, ttf*+structure is straightforward. With two bonding and two non-bonding electrons, the situation is reminiscent of what isencountered with a linear H,- molecule.33 Note that the samesituation would prevail if the three ttf molecules were eclipsed asin [ttf]3[SnC16]28 because of the local D,, symmetry, i.e.thepresence of an inversion centre on the inside ttf molecule. Achange in the electronic distribution would occur with a high-spin state. We do not envisage this situation here sincecompounds 1 and 2 are diamagnetic materials.A glance at the overlap populations between atoms in thetriad a (BAB), shown in Scheme 2, indicates that the asymmetricelectronic distribution induces some differences between thebonds of the outside molecules and the corresponding ones ofthe inside molecule. For instance, the central C-C overlappopulation is 1.1 1 for molecule A, somewhat weaker than the1.17, computed for the corresponding bond in molecules B andB’. On the other hand, the C-S overlap populations are slightlylarger in the inside molecule than in the outside ones.Consequently, this difference in bond strengths might leadto different bond distances,33 in accord with the observeddistances in 1 and 2.Though less pronounced in 2 than in 1, thecentral C-C bond is longer in ttf A than in B and B’, while theaverage C-S bonds are slightly shorter (see above).Tight-binding calculations on the two-dimensional donorsublattice of compound 1 confirm the assigned mixed-valencestructure. Charges of 1.12+ and 0.44+ are computed for theA and B ttf molecules respectively. The shift of the chargescompared to those obtained for model a is mainly due tointramolecular structural relaxation observed in 1. The HOMOof the A ttf is no longer degenerate with the HOMOs of the Band B’ ttf, but slightly higher in en erg^.^' Consequently, theoccupied bonding combination ( l b l , MO in a) becomes morelocalized on the B and B’ ttf molecules, leading to reduction ofthe positive charge, while the reverse happens for molecule A.The bands derived from the HOMOs of the ttf triads areshown in Fig.10(a). The r ---+ X direction and the - Ydirection in reciprocal space correspond to the inter-stackcoupling a axis and the stacking b axis respectively. Thoughthere are some relatively short non-bonding S S interstackcontacts of 3.43 8, in the a direction ( d , in Fig. 8), the bands arenearly flat along the - X direction. This reflects negligibleinterstack interactions. The weak band dispersion comes fromthe fact that the interstack S S overlap along that direction isessentially K in nature and therefore not very important.This isnot the case along the - Y direction where some banddispersion is noted due to intertrimer interactions betweensulfur and carbon atoms which overlap in a more or less ofashion (see S C contacts in Fig. 6). We conclude that salts 1and 2 present quasi-one-dimensional band structures with anarrow band width (A d 0.5 eV).The band gap of about 0.45 eV calculated for the organicslabs of salt 1 is in agreement with the semiconductingproperties of compounds 1 and 2.IR ubsorption bands. The criss-cross trimer arrangement inthe crystal structure of the ttf salts 1 and 2 is a particular andunusual case of centrosymmetrical trimers which has not yetbeen considered theoretically.When the electron-molecularvibration linear coupling is introduced, it gives rise to theclassical vibronic The only example known so far is[tmtsf],[PW ,,04,] which is an i n s ~ l a t o r . ~ In the spectrum ofthis salt there is only a c.t. ‘B’ band and a relatively weakvibronic band at around 1340 cm-’, the frequency of which isalmost unchanged with temperature.”’ The existence of avibronic band for a criss-cross trimer proves that electron-molecular vibration coupling is allowed in the present situ-ation.Indeed for our ttf salts 1 and 2 the vibronic band is observedat around 1380 cm-’. This frequency and the associated ‘A’band are the same as those observed in the case ofperpendicular trimers such as [ttf]14[MC14J4 (M = Mn orCo) l 7 for which no temperature dependence of the vibronicband is observed.In the present situation a weak temperaturJ. CHEM. SOC. DALTON TRANS. 1992 1225c, Scheme 2-9.52 -.xQ)wP10.-1 0.5 ~ -11ACI151.... ............................. ......... 1Fig. 10 Electronic band structure for ( a ) the (ttf)32+ slabs in[ttf],[W60,,] (the Brillouin zone is shown in the inset) and (h) the( t r n t ~ f ) , ~ + chain in [tmtsf],[W60,,].2dmfdependence is observed [see Fig. 4(b)] which could beattributed either to some molecular charge in the trimer becauseof a strong thermal dilatation, or to a modification of theelectron-molecular vibration coupling due to a differentbehaviour of the c.t.bands. In the absence of any experimentaldata at low temperature we are unable to confirm or reject thislast hypothesis.[tmtsf] ,[ W 6 0 ,]-2dmf 3.-Crystal structure. The bonddistances and angles are given in Fig. 11 for the tmtsf moleculesof [tmtsf],[W60,,]-2dmf 3. The crystal structure shown inFigs. 12 and 13 is built from W6OIg2- anions located at theorigin of the lattice, and two independent tmtsf units denoted Aand B: molecule A is centred on the (O,O,$) inversion centre andB in general position. The molecules of solvent (dmf) are locatedin the middle of the unit cell.The bond distances of the two independent tmtsf moleculesare averaged in the mrn symmetry and compared in Table 5 withthe corresponding distances encountered in other tmtsf"+ salts(x = 0 or 1).16*35b*37 These bond distances compare ratherwell with those observed in other 3 : l salts, especially with[tmtsf],[Pt(CN),].l 6 The tmtsf molecules form trimerized andslipped stacks (Fig. 13) parallel to the [loo] direction. Thespacing between adjacent tmtsf molecules inside a trimer (3.53BFig. 1 I(A) and bond angles (") for [tmtsf],[W60,,]~2dmfThe constituent molecules, atomic numbering, bond lengthsand 3.63 A) and the intra- and inter-trimer ring-over-centralbond overlaps (Fig. 12) are comparable to those found in othertmtsf salts.35 This mode of overlap generates in the present com-pound slipped stacks similar to that observed in the black phaseof CtmtsflCtcnq] (tcnq = tetracyanoq~inodimethane),~~ in con-trast with the zigzag ones observed commonly in the[(tmt~f)~]+X- (X = PF6 or BF4) series3' or in other[ ( t m t ~ f ) ~ ] ~ + X ~ - [X = Pt(CN), or Ni(CN),] compounds.'6The intra-trimer Se Se contacts range between 3.73 and 3.818, (Fig.13). They are shorter than those commonly observed inthe 2: 1 salts.35 The inter-trimer Se Se contacts (3.944.12 A)are in the range of the corresponding van der Waals separations(4 A). Short 0 Se anion-cation interactions are alsoobserved: Se(6) O(3) 3.17(2), Se(4) O(2) 3.21(2) andSe(1) O(7) 3.21(2) A (van der Waals distance: 3.4 A).Theoretical band calculations. As in compounds 1 and 2, amixed-valence structure is computed for the ( t m t ~ f ) , ~ ' stackpresent in 3. Molecular calculations on the ( t m t ~ f ) , ~ ' triad bmade of two 'outside' tmtsf molecules B and B' and one 'inside'molecule A, give the tmtsf*+, tmtsf+, tmtsf*+ charge sequence.Here again the unsymmetrical electron distribution is due tothe presence of an inversion centre located on the A molecule.With identical separations for both inside and outside tmtsfmolecules (central C-C, C-Se and edge C-C bond distances of1.35, 1.90 and 1.33 8, respectively), the overlap populationsbetween different atoms shown in Scheme 3 indicate that aweak intramolecular structural relaxation might occur., Bonddistances determined by X-ray crystallography are not accurateenough to reveal this structural def~rmation.~'In agreement with the molecular calculations, a mixed-valence tmt~f'.'~+, t m t ~ f ' .~ ~ +, t m t ~ f ' . ~ ~ ' structure is obtainedfrom tight-binding calculations on the one-dimensional chainof ( t m t ~ f ) , ~ + triads encountered in salt 3. Combinations of theHOMOS of the three tmtsf molecules constituting each trimerlead to three molecular orbitals (one bonding, one non-bondingand one antibonding) which mainly comprise the three bandsshown in Fig. 10(b). The band dispersion which is observedtranslates some intertrimer interactions predominantly through(T overlap between Se atoms separated by 3.94 8, (d' contactsin Fig. 13). Nearly zero but positive overlap population iscomputed between these Se atoms.With a formal oxidation state of (tmt~f),~', the two lowe1226 J. CHEM. SOC. DALTON TRANS.1992Table 5 Comparison of averaged bond lengths (A) in tmtsf *A B tmtsf [tmtsf] , PF, - [tmtsf] 3[ Pt(CN),]I I 1.36(4) 1.39(4) 1.35(1) 1.369( 14) 1.352(11), 1.373(11)c 1.88(3) 1.89(3) 1.906(7) 1.893( 10) 1.901(8), 1.887(8)h 1.86(3) 1.87(3) 1.892(7) 1.875(10) 1.878(8), 1.859(8)c/ 1.33(4) 1.32(4) 1.32 1.329( 15) I .323( 12), 1.346( 1 1)Ref. This work 37 35(h) 16* The )~ii)i symmetry has been imposed.a bFig. 12 Stereoscopic view of the structure of salt 3 showing the intermolecular overlapsFig. 13 Side view of the tmtsf stack showing shortest intra- and inter-trimer contacts (A): (/, [Se(I') . - - ~e(3')] = 3.753(5), d, [Se(I') - - .Se(6")] = 3.731(5), ti, [Se(2')... Se(4')] = 3.745(5), d, CSe(2') - -Se(5")] = 3.807(5), d, [Se(S') - Se(6"')I = 3.940(4), d, CSe(4') - - .Se(5"')] = 4.047(5), LI, CSe(3') * Se(6"')I = 4.120(5), d, = 3.53, d, =3.63.Symmetry codes: I .I-, y, z; I1 -.I-, - 19, 1 - z; I11 1 - s, -j; I - 1Scheme 3bands are filled and the upper one empty. This is in accordancewith the semiconducting behaviour of compound 3. Thecalculated gap of 0.14 eV is in accordance with the activationenergy of 0.2 eV measured experimentally (see Fig. 1).IR absorption bands. In this compound the moleculararrangement of the trimers is very similar to that encountered in[(tmtsf),][M(CN),] (M = Pt or Ni) salts." The electronic'A' band and the vibronic mode are observed at about the sameenergy. The c.t. 'B' band (Fig. 2) can be related to the hopping ofcharges towards the oxidized molecule in order to get a doubleoccupancyJ.CHEM. SOC. DALTON TRANS. 1992ConclusionUsing the standard electrocrystallization technique, we haveobtained new 3 : 1 salts of ttf and tmtsf with Lindquist-typeW 6 0 , 9 and Mo,OI9 dianions. These salts show one-dimensional trimerized organic stacks with both unusual criss-cross and ring-over-double bond intratrimer overlaps in the ttfand tmtsf salts respectively. Mixed-valence (3 +, 1 +, 3 +)structures for the organic donor trimers are determined on thebasis of their bond lengths and electronic band-structurecalculations. The three compounds are diamagnetic semi-conductors: this semiconducting behaviour associated with alocalized mixed-valence state in these salts is confirmed by thespectra which show rather classical vibronic modes induced bytwo charge-transfer bands.These results are in agreement withthe Robin-Day classification 38 which is based on the degree ofn charge transfer associated with each site. Organic conductorswhich present a metallic or even a supraconducting state belongto class I11 where the mean degree of charge transfer ishomogeneous (or constant), whereas the present salts exhibit alocalized mixed-valence behaviour as in class 11. This largelyconfirms that the same classification can be used for bothorganic and inorganic compounds.AcknowledgementsThanks are expressed to A. Flattet and C. Hervieu for theirtechnical assistance in electrochemical experiments andL. Hubert for the illustrations.ReferencesL.Ouahab, M. Bencharif and D. Grandjean, C. R. Acad. 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