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Synthetic Metals 162 (2012) 1577– 1581

Contents lists available at SciVerse ScienceDirect

Synthetic Metals

journa l h o me page: www.elsev ier .com/ locate /synmet

tructure and spectral properties of [Fe(dipy)3](TCNQ)4·{(H3C)2CO}nion-radical salt

. Lewandowskaa,∗, B. Barszcza, G. Vasyletsb, I. Potocnákc, V.A. Starodubb,d, A. Grajaa

Institute of Molecular Physics, Polish Academy of Science, 60-179 Poznan, PolandV.N. Karazin Kharkiv National University, Faculty of Chemistry, 61022 Kharkiv, UkraineP.J. Safárik University in Kosice, Institute of Chemistry, 041 54 Kosice, SlovakiaInstitute of Chemistry, Jan Kochanowski University, 25-406 Kielce, Poland

r t i c l e i n f o

rticle history:eceived 3 April 2012ccepted 17 June 2012

a b s t r a c t

New TCNQ salt [Fe(dipy)3](TCNQ)4·{(H3C)2CO} containing solvent molecules in the crystal structurewas synthesized and characterized. The crystal structure can be described as formed from alternating[Fe(dipy)3]2+ cations and acetone molecules and two types of stacks of (TCNQ)2

− dimers that are parallel

vailable online 19 August 2012

eywords:CNQ anion-radical salte dipyridile cationrystal structure

one to another. Analysis of the vibrational IR absorption and Raman scattering spectra shows that theoptical conductivity spectrum is highly anisotropic. It is characteristic for the salt under investigationthat the bands attributed to [Fe(dipy)3]2+ cations are relatively strong. This effect is particularly distinctin the Raman scattering spectra.

© 2012 Published by Elsevier B.V.

R spectral properties

. Introduction

Field of donor–acceptor compounds involving metallic frag-ents and organic acceptor molecules challenges syntheticork to design supramolecular systems which, depending on

he relative arrangement, present various physical properties1–3]. Conjugated polynitril ligands such as 7,7,8,8-tetracyano-p-uinodimethane (TCNQ) [1] and its derived molecules are veryseful for designing such systems because of their variable coor-ination behavior, proven ability to bridge up to four metalenters, tendency to form aggregates via �–� interaction, andacile reduction to anion radicals and further to dianionic forms4,5]. TCNQ anion-radical salts and complexes are of interest sinceheir electrical properties vary from insulating to highly conduct-ng, with room temperature conductivities in the range 10−14 to03 S cm−1; metallic behavior has been also observed in some TCNQalts [6].

Within last two decades the ability of anion-radical salts to formlso ordered magnetic structures and ferromagnetic state has beeniscovered [7–9]. In this context, TCNQ salts, that contain complexations based on transition metals, cause special interest. Two mag-

etic subsystems – anion-radical and cationic – exist in the crystal

attice of such kind of salts. It can result in formation of unusualagnetic structures. The anion-radical salts with such cations as

∗ Corresponding author.E-mail address: kornelia.lewandowska@ifmpan.poznan.pl (K. Lewandowska).

379-6779/$ – see front matter © 2012 Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.synthmet.2012.06.017

[Fe(dipy)3]2+ and [Fe(phen)3]2+ (where dipy is 2,2′-dipyridile andphen is 1,10-phenantroline) are unique systems due to their abil-ity to form spin isomers. For example, [Fe(dipy)3]Cl2 complex saltis in the equilibrium between low spin 1A1g (t2g

6) and high spin5T2g (t2g

4eg2) states. The magnetic susceptibility of the paramag-

netic salt [Fe(dipy)3](TCNQ)2 is described by Curie low for isolatedspins S = 2 [10]. This distinguishes it from similar in composition[Fe(phen)3](TCNQ)2·2(CH3OH) salt, in which anion-radicals arebonded in diamagnetic dimers TCNQ–TCNQ with carbon–carbonbond length equal to 1.644 A. For the first time the formation ofthese dimers was observed in (NEP)(TCNQ) crystal, where NEP is N-ethylphenazinium-cation [11]. Thus, minor structural modificationof complex cation significantly affects the structure and propertiesof TCNQ anion-radical salt.

One of the most interesting electron donors is dipyridine andderived molecules. Dipyridines are a family of chemical compoundswhich are formed by the coupling of two pyridine rings. Among sixisomers of dipyridine, two isomers are prominent: 2,2′-dipyridinewhich is a chelating ligand that forms complexes with most tran-sition metal ions and 4,4′-dipyridine which is redox active andcan bridge between metal centers to give coordination species. Atone time, several dipyridinium-TCNQ salts have been synthesizedand investigated by Ashwell et al. [12–14] and Mihály et al. [15].These authors have shown that, by alternating the cation geometry

and consideration to the effect on the stacking characteristics ofthe counter ion, chain stabilization and conductivity enhancementwere achieved. The conductivity of dipyridinium-TCNQ salts is inthe range 10−5 to 103 S cm−1 at 300 K [13].

1578 K. Lewandowska et al. / Synthetic Metals 162 (2012) 1577– 1581

} anio

sii

2

[

3

biio

mweocCc––

nc

td

Fig. 1. Crystal structure of [Fe(dipy)3](TCNQ)4·{(H3C)2CO

The aim of this paper is to synthesize new and interesting TCNQalt [Fe(dipy)3](TCNQ)4·{(H3C)2CO}, containing solvent moleculesn the crystal structure, to determine its crystal structure and tonvestigate its spectral properties.

. Experimental

The monocrystalline samples of the anion-radicalFe(dipy)3](TCNQ)4·{(H3C)2CO} salt were obtained by the reaction:

[Fe(dipy)3]I2 + 8TCNQ + (CH3)2CO

= 2[Fe(dipy)3](TCNQ)4∗(CH3)2CO ↓ + [Fe(dipy)3](I3)2

For the synthesis of the salt the reduction of TCNQ recoveryy iodide-anions was applied. During the reaction iodide-anion

s oxidizing to free iodine that rapidly associates with the rest ofodide-anions. Excess of TCNQ in reaction mixture causes formationf complex anion-radical (TCNQ)2

−.Equimolar quantities of reagents were dissolved apart and

ixed. As a solvent mixture of acetone and methanol in ratio 4:1as used. After mixing, a vessel with the solution was left for slow

vaporation. After four days black needles of the product formedn the bottom of the vessel. Back product has higher solubility inomparison to anion-radical salt. Mother solution was removed.rystals of the salt were washed with diethyl ether and dried. Saltomposition was determined by elemental analysis. Calculated: C

69.52%, N – 22.03%, H – 3.32%; found: C – 68.97%, N – 23.813%, H 3.19%.

The single crystals of [Fe(dipy)3](TCNQ)4·{(H3C)2CO} salt showeedle-like shape with length between 0.5 and 1 mm and black

olor.

The crystal structure was determined using an Oxford Diffrac-ion Xcalibur2 diffractometer equipped with a Sapphire2 CCDetector. Crystalline CCD was used for data collection while

n-radical salt (view along b-axis (a) and along a-axis (b)).

Crystalline RED was used for cell refinement, data reduction andabsorption correction [16]. The structure was solved by the directmethod with SHELXS97 and subsequent Fourier syntheses usingSHELXL97 [17]. Anisotropic displacement parameters were refinedfor all non-H atoms. The H atoms of aromatic cycles were placed incalculated positions and refined riding on their parent C atoms withC–H distances of 0.93 A and Uiso(H) = 1.2Ueq(C). An analysis of bonddistances and angles was performed using SHELXL97. DIAMOND[18] was used for molecular graphics.

The infrared reflection spectra of the crystalline samples wererecorded in the frequency range from 400 to 7500 cm−1, at roomtemperature, for two polarizations of the light beam: parallel andperpendicular to the longest crystal axis. These investigations wereperformed with FT-IR Bruker Equinox 55 spectrometer equippedwith Hyperion 1000 microscope. The spectral resolution of IR spec-tra was 2 cm−1. The Kramers–Krönig analysis of the reflectance datawas carried out to obtain the frequency dependent optical conduc-tivity. A standard PEAKFIT computer program was used for spectralanalysis.

The ab initio calculations of optimal geometry and normal modesfrequencies of the dipyridine derived cations were performed usingthe Gaussian 03 program [19]. Calculations were carried out on anisolated molecules using the density functional theory (DFT) withBecke’s three parameter exchange functional combined with theLee–Yang–Parr correlation functional (B3LYP). The 6-311++G(d,p)basis set was used for optimizations and all calculations. The normalmodes assignment was performed based on the experimental andtheoretical data.

3. Remarks on the crystal structure

[Fe(dipy)3](TCNQ)4·{(H3C)2CO} salt crystallizes in the triclinicP-1(2) space group. The crystal structure can be described as formed

K. Lewandowska et al. / Synthetic Metals 162 (2012) 1577– 1581 1579

ping o

ftasiaocacTAdakl(bas

4

smui

Fig. 2. Tetramerized TCNQ stacks (view along c-axis (a)) and overlap

rom alternating [Fe(dipy)3]2+ cations and acetone molecules andwo types of stacks of (TCNQ)2

− dimers that are parallel one tonother; it is seen in Fig. 1. The quasi-one dimensional (Q-1D)tacks of dimers are oriented along the a-axis. As it is shownn Figs. 1 and 2a there are four crystallographically inequivalentnion-radicals of TCNQ− in the lattice. We can distinguish TCNQ−

f A, B, C and D types due to bond lengths inside TCNQ ion radi-als and their positions in the unit cell. Total electrical charge onll of those four TCNQ molecules equal to −2. First kind of stacks isreated by A and B anion-radicals ordered in tetrads of overlappedCNQ− anions: . . .ABBA. . .ABBA. . . (Fig. 2b). The anion-radicals of

type are overlapped with anion-radicals of B type with shortestistances 3.175 and 3.316 A; between the B-molecules there arelso short contacts (3.337 and 3.362 A). Thus, we can consider suchind of stacks as tetrameric. Another stacks are created by over-apping of C and D anion-radicals in order CC. . .DD. . .CC. . .DD. . .Fig. 2a). Shortest distances inside CC dimers are 3.296 and 3.304 Aut inside DD dimers – 3.323 and 3.335 A. CC and DD dimers havelso short contacts 3.368 and 3.377 A. The conclusion is that theecond kind of stacks has overlapping along whole stack.

. Vibrational spectra

Spectral properties of low-dimensional ion-radical salts are

trictly related to electron–electron interactions and electron-olecular vibrational couplings. These fundamental intermolec-

lar interactions are strongly dependent on the crystal directionn anisotropic crystals. Therefore, both infrared and Raman

f TCNQ anions in CCDD and ABBA tetramers (view along a-axis (b)).

vibrational spectra are sensitive not only to changes in the chargedistribution on sites but also to the crystal orientation. The crystalorganization of [Fe(dipy)3](TCNQ)4·{(H3C)2CO} salt is Q-1D and forthis reason one can expect distinct anisotropy of the spectra. Thisis why we have recorded IR reflection spectra for light polarizationparallel and perpendicular to the longest crystal axis.

The polarized reflection spectra shown in Fig. 3 are restricted tothe most interesting spectral range from 600 to 4500 cm−1. As typi-cally for Q-1D crystals, an optical reflectance of the investigated saltis small and almost frequency independent when the electric vectorof external radiation is perpendicular to the stacks, but for parallelpolarization one observes an overdamped electronic charge trans-fer (CT) band and a number of lines with considerable half-width,at frequencies close to those of totally symmetric (Ag) modes ofisolated TCNQ molecules; there are also some bands of Fe(dipy)3cation and strongest bands of (H3C)2CO solvent. For TCNQ andother symmetrical molecules, � molecular orbitals occupied by theradical electron are nondegenerated; so linear electron-molecularvibration (e–mv) coupling is possible only for the totally symmetricmodes [2,3,20,21]. Electron-molecular vibration (e–mv) couplingdetermines the optical properties of low-dimensional organic con-ductors in the middle infrared region. It leads to the activation ofAg modes that are normally inactive in the IR and these modesdominate the spectrum of the salt.

The optical conductivity spectrum of[Fe(dipy)3](TCNQ)4·{(H3C)2CO} salt, typical for the family ofTCNQ salts, is presented in Fig. 4. This spectrum was calculatedfrom reflectance data measured for two light polarizations, using

1580 K. Lewandowska et al. / Synthetic Metals 162 (2012) 1577– 1581

1000 1500 2000 2500 3000 3500 4000 4500

Re

fle

cta

nce

Wavenumber (cm-1)

parallel polarization

perpendicular polarization

Fig. 3. Polarized IR reflection spectra of [Fe(dipy)3](TCNQ)4·{(H3C)2CO} anion-rc4

tfttAtatacmvcatn

bpp

Frf

Table 1Frequencies and attributions of the main bands of [Fe(dipy)3](TCNQ)4·{(H3C)2CO}anion-radical salt determined from experimental IR reflectance and Raman scatter-ing spectra; the frequencies of selected calculated modes are also given.

Wavenumber (cm−1) Band assignment

Raman IR

3050 (3204)a �1 C H s2209 (2325)a 2047 �2 C N s; C CW s1624 (1647) C C s; C N s; C C H b in Fe(dipy)3

1607 (1665)a 1552 �3 C CR s; C CR s; C C H b1563 (1524) C C s; C N s; C C H b in Fe(dipy)3

1490 (1489)a 1285 �4 C CW s; C CW s1321 (1348) C C s; C C H b in Fe(dipy)3

1170 (1231)a 1057 �5 C C H b; C C N b; C CR s1024 (1046) Breathing modes in Fe(dipy)3

960 (971)a �6 C CR s; C CW s716 (724)a �7 C CR s; C CW s; C C CR b658 (615)a �8 C CW s; C C H b; C C CW b381 (368) Fe N s in Fe(dipy)3

371 (364) Fe N s in Fe(dipy)3

329 (335)a �9 C C CR b; C C H b; C CW s

comparison the calculated spectrum of the cation is also shown.

adical salt recorded for light polarization parallel and perpendicular to the longestrystal axis of the salt (restricted to the most interesting spectral range from 600 to500 cm−1).

he Kramers–Krönig analysis with typical approximations forrequencies below and above measured in the experiment. Dueo the strong e–mv coupling the IR conductivity spectrum ofhe salt contains almost only bands related to totally symmetricg modes of the electron acceptor and the strongest bands ofhe donor. TCNQ vibrations, that are normally infrared active,re less intensive in the polarized reflectance and thereforehey are slightly visible in the conductivity spectrum. Strongnisotropy of the optical conductivity spectra caused by Q-1Dharacter of the salt is also seen in Fig. 4. The optical conductivityeasured in the direction perpendicular to the TCNQ stacks is

ery low, in particular in the middle IR region, and does notontain vibrational bands. In opposite, it shows a continuousbsorption starting from about 400 cm−1. Thus, we can concludehat [Fe(dipy)3](TCNQ)4·{(H3C)2CO} anion-radical salt appears asarrow-gap semiconductor or metal.

Totally symmetric modes of TCNQ appear as broad, intensive

ands shifted towards lower wavenumbers comparing to theirositions in the Raman spectra. The magnitude of these shift forarticular mode depends on the e–mv coupling constant for the

1000 1500 2000 2500 3000 3500 4000 4500

0

80

160

Optical C

onductivity (

Ω-1cm

-1)

Wavenumber (cm-1)

parallel

perpendicular

ig. 4. Optical conductivity spectrum of [Fe(dipy)3](TCNQ)4·{(H3C)2CO} anion-adical salt obtained by the Kramers–Krönig analysis of the reflection data recordedor the direction parallel and perpendicular to the TCNQ stacks.

�n , Ag modes; s, stretching; b, bending; W, wing; R, ring; () calculated. TheoreticalIR intensities for Ag modes are 0.

a [23].

mode and thus is not the same for various Ag vibrational modes ofTCNQ. The wavenumber of �2 mode of the salt is about 160 cm−1

lower than its Raman position recorded for non-bonded TCNQ (seeTable 1) but the �3 mode is shifted by about 60 cm−1. The biggestshifts are observed for the �4 mode related to the symmetricalstretching of the wing C C bonds; there is about 200 cm−1. For �5the shift is about 170 cm−1. On the other hand, nearly undetectableshift is observed for �6 mode. It is difficult to discuss the shifts inthe case of modes from �8 to �10 because they are less intensiveand not so well resolved in the Raman spectra (see Raman spec-tra in Fig. 5). Similar situation has been observed also for adequatemodes in several TCNQ salts with substituted pyrazines [22,23].The conductivity spectrum of the investigated salt contains alsodistinct bands of Fe(dipy)3 cation localized, e.g. at 3171, 1624, and1325 cm−1.

Raman scattering spectrum of [Fe(dipy)3](TCNQ)4·{(H3C)2CO}anion-radical salt for two light polarization is shown in Fig. 5; for

The spectrum of the salt is very rich and the Raman lines arenarrow and perfectly separated. The scattering spectra recordedfor two perpendicular light polarizations show distinct anisotropy

Fig. 5. Raman scattering spectrum of [Fe(dipy)3](TCNQ)4·{(H3C)2CO} anion-radicalsalt for two light polarizations; for comparison the calculated spectrum of the elec-tron donor molecule is also shown.

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dequate to orientation of cations or anions. It is characteristic thathe anisotropy of anion lines is bigger in comparison with cationines. It seems to be clear because TCNQ stacks are ordered and ori-nted in direction of the a-axis. In opposite, the cations form dimersith differently oriented species.

Wealth of the Raman spectra of the salt and dipyridine moi-ty as well as changes in the cation spectra after salt formationeem to be clear tacking into consideration the results of resonance-nhanced Raman spectra reported by Basu et al. [24] for severalubstituted complexes of bipyridine-Ru ligands. It was shown thator the nature of the bipyridine ring substituent effects on the elec-ronic properties of the complexes are important. It was also shownhat the ground-state properties are perturbed by ligands giving theppearance of additional resonance enhanced Raman bands.

. Conclusions

New TCNQ salt [Fe(dipy)3](TCNQ)4·{(H3C)2CO} containing sol-ent molecules in the crystal structure was synthesized andharacterized. The crystal structure can be described as formedrom alternating [Fe(dipy)3]2+ cations and acetone molecules andwo types of stacks of (TCNQ)2

− dimers that are parallel one tonother. This quasi one dimensional structure of the salt deter-ines its physical properties, in particular spectral properties.

omplex analysis of the vibrational IR absorption and Raman scat-ering spectra shows that the optical conductivity spectrum isighly anisotropic – the optical conductivity in the middle IRegion measured in the direction perpendicular to the TCNQ stackss very low and nearly does not contain vibrational bands. Inpposite, for parallel polarization one observes an overdampedlectronic charge transfer (CT) band and a number of lines withonsiderable half-width, at frequencies close to those of totallyymmetric (Ag) modes of isolated TCNQ molecules; there are alsoome bands of Fe(dipy)3 donor and strongest bands of (H3C)2COolvent. It is characteristic for the salt under investigation thathe bands attributed to [Fe(dipy)3]2+ cations are relatively strong.his effect is particularly distinct in the Raman scattering spec-ra. Comparison of the Raman spectra of the Fe(dipy)3 and thealt [Fe(dipy)3](TCNQ)4·{(H3C)2CO} gives an evidence of distincthanges in the nature of the bipyridine ring and in the role ofhe substituent effects on the electronic properties of the com-lex.

cknowledgment

One of us (G.V.) thanks the International Visegrad Fund for finan-ial support.

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tals 162 (2012) 1577– 1581 1581

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