Intermolecular Hydrogen-Bond Networks and Physical Properties of

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Intermolecular Hydrogen-Bond Networks and Physical Properties of BF4− and TCNQ•− Salts of Three-Fold Symmetric Tris(alkylamino) phenalenyliums Tsuyoshi Murata, Eigo Miyazaki, Takuji Yokoyama, Kazuhiro Nakasuji, and Yasushi Morita* Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *

ABSTRACT: Synthesis, redox properties, and crystal structures of tris(alkylamino)phenalenyliums (TAP) having alkyl groups (n-Pr, i-Pr, t-Bu) and their charge-transfer salts with tetracyanoquinodimethane radical anion (TCNQ•−) were investigated. The electrochemical measurements revealed that TAP exhibits two irreversible reduction processes to neutral radical and anion species. The introduction of an alkylamino group caused a large negative shift of the first reduction potential and a significant decrease of the on-site Coulomb repulsion because of the electron-donating nature of amino groups and the extension of the π-electronic system. In the crystal structures of the BF4− salts, the TAP skeleton possesses a nearly 3-fold symmetric molecular plane indicating the delocalization of positive charge. The face-to-face stack of TAP formed π-dimer or columnar structures, which were connected through intermolecular N−H···F hydrogen bonds with BF4− to construct multidimensional network structures. The TCNQ•− salts prepared by the metathesis method were characterized as fully ionic salts with a 1:1 component ratio. In the crystal structures, both TAP and TCNQ•− molecules formed π-dimers, and the intermolecular hydrogen bonds between TAP and TCNQ•− constructed a two-dimensional sheet.

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INTRODUCTION Phenalenyl 1 is a planar and highly symmetric (D3h) hydrocarbon composed of three multiply fused benzene rings.1 The odd-alternate π-electronic structure of 1 causes amphoteric redox ability to afford three thermodynamically stable species, anion, neutral radical, and cation (Chart 1), the former two of

interaction of the SOMO, which delocalized around the molecular plane, induced a π-dimer formation with 12-center−2electron long C−C bond.3 In order to explore intriguing functions based on the electronic spin of the phenalenyl system, we investigated the chemical modifications on the skeleton of 14 such as nitrogen atom incorporation5 and oxo substitution.6 The 1 system has attracted much attention also as a component of organic conductors. The first proposal of a 1-based organic conductor was presented by Haddon,7 and the high conductance in the charge-transfer (CT) complex of 1,9dithiophenalenyl with tetracyanoquinodimethane (TCNQ) was demonstrated.8 The F4TCNQ complex of an acetylene-linked phenalenyl dimer exhibited a metallic conductivity with a metal−insulator transition associated with the intermolecular C−C bond formation.9 The phenalenyl system afforded also single-component organic conductors, zwitterionic bis(phenalenyl)boron complexes10 and singlet biradical bisphenalenyls,11 where the linear π-stack motifs provided the passage of high electrical conductivity. Introduction of hydrogen-bond (H-bond) interaction into CT complexes and salts is recognized as an effective methodology in the exploration of new organic conductors by controlling the relative molecular orientation as well as the

Chart 1. Chemical Structures of Phenalenyl Derivatives

Received: August 26, 2011 Revised: November 8, 2011 Published: November 22, 2011

which were successfully isolated with the aid of steric protection by bulky substituent groups.2 The strong intermolecular © 2011 American Chemical Society

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EI-MS, m/z 335 (M+ − HBF4, 15%), 320 (M+ − HBF4 − CH3, 100%); IR (KBr) 3390, 3089, 2976, 2939, 1602, 1575, 1520, 1084 cm−1; UV (KBr) 284, 490(sh), 516 nm. Anal. Calcd for (C22H30N3)(BF4−): C, 62.42; H, 7.14; N, 9.93. Found: C, 62.18; H, 7.14; N, 9.76. 6c·BF4−. Red powder (94% yield). Single crystals suitable for X-ray measurement were obtained by vapor diffusion using a 1:1 mixture of ethyl acetate and MeOH and hexane or benzene. Mp >300 °C; 1H NMR (270 MHz, DMSO-d6) δ 1.52 (s, 27), 7.20 (d, 3, J = 9.7 Hz), 7.50 (s, 3), 8.50 (d, 3, J = 9.7 Hz); EI-MS, m/z 377 (M+ − HBF4, 17%), 362 (M+ − HBF4 − CH3, 100%); IR (KBr) 3411, 2979, 2937, 1604, 1575, 1514, 1084 cm−1; UV (KBr) 282, 490(sh), 514 nm. Anal. Calcd for (C25H36N3)(BF4−): C, 64.52; H, 7.80; N, 9.03. Found: C, 64.53; H, 7.74; N, 8.93. Typical Synthetic Procedure for TCNQ•− Salt; 1,4,7-Tris(propylamino)phenalenylium−TCNQ •− . 6a·TCNQ •− . Salt 6a·BF4− (46 mg, 0.11 mmol) was placed in a 30-mL Schlenk tube and dissolved with MeOH (3 mL). To this mixture, the solution of Li·TCNQ•− (24 mg, 0.11 mmol) in MeOH was added dropwise. The mixture was stirred at room temperature and was left to stand at 5 °C overnight. The resulting precipitate was collected by filtration and then washed with MeOH to give 6a·TCNQ•− (99% yield) as black crystals; single crystals suitable for X-ray measurement were obtained by the slow diffusion of 6a·BF4− and Li·TCNQ•− in MeOH as deep green crystals. Mp 248−249 °C; IR (KBr) 3346, 2960, 2176, 2152, 1602, 1576, 1520, 1505 cm−1; UV (KBr) 904, 666, 516, 370, 282 nm. Anal. Calcd for (C22H30N3)(C12H4N4): C, 75.53; H, 6.34; N, 18.13. Found: C, 75.51; H, 6.26; N, 18.01. 6b·TCNQ•−. Deep green crystals (92% yield). Single crystals suitable for X-ray measurement were obtained by the slow diffusion of 6b·BF4− and Li·TCNQ•− in MeOH. Mp 248−251 (dec) °C; IR (KBr) 3397, 3347, 2973, 2179, 2155, 1601, 1573, 1505 cm−1; UV (KBr) 282, 372, 488, 670, 888 nm. Anal. Calcd for (C22H30N3)(C12H4N4)(H2O)0.2: C, 75.02; H, 6.37; N, 18.01. Found: C, 74.91; H, 6.21; N, 17.97. Cyclic Voltammogram. Cyclic voltammetric measurements were made with an ALS electrochemical analyzer model 612A. Cyclic voltammograms were recorded with a 1.6 mm diameter gold working electrode and a Pt wire counter electrode in CH3CN containing 0.1 M Bu4NClO4 as the supporting electrolyte at room temperature. The experiments employed a Ag/AgNO3 reference electrode, and the final results were calibrated with a ferrocene/ferrocenium (Fc/Fc+) couple. The measurements using Pt and glassy carbon disk working electrodes instead of the gold disk did not make any remarkable differences in the cyclic voltammograms. X-ray Crystallographic Analysis. X-ray crystallographic measurements were made on a Rigaku Raxis-Rapid imaging plate or Rigaku Mercury CCD area detector with graphite-monochromated Mo Kα radiation (λ = 0.71070 Å). Structures were determined by a direct method using SHELXS-9719 or SIR-2004.20 Refinements were performed by full-matrix least-squares on F2 using SHELXL-97.21 All non-hydrogen atoms were refined anisotropically. Positional parameters of hydrogen atoms were calculated with sp2 or sp3 configuration of the bonding carbon and nitrogen atoms, and hydrogen atoms were refined using the riding model. In the refinement procedures, isotropic temperature factors with magnitudes of 1.2 times of the equivalent temperature factors of the bonding atoms were applied for hydrogen atoms. Selected crystal data and data collection parameters are given in Table 1.

electronic state of component molecules.12,13 We have recently demonstrated the introduction of nitrogen atoms as H-bonding sites into the 1 system. The protonated cations of diazaphenalenyls (2 and 3), in which two N−H groups were incorporated at the α-positions of 1 (Chart 1), formed H-bonded assemblies with TCNQ•− radical anion and afforded highly conductive CT complexes with TCNQ.14 Hexaazaphenalenyl (4), a derivative of 1 with six nitrogen atoms incorporated, constructed multidimensional H-bond networks in the protonated state and metal complexes.15 An alkylamino group substituted derivative, trimethoxy tris(n-propylamino)phenalenylium (5), exhibited N−H···O intramolecular H-bonds between amino and methoxy groups (Chart 1).16 Steric hindrance of the substituent groups and coexistence of protondonor and -acceptor sites in one molecule prevented 5 from forming intermolecular H-bonds. Our preliminary study on the BF4− salt of tris(n-propylamino)phenalenylium (6a), which preserved the 3-fold symmetry of the phenalenyl system, revealed that the amino group formed intermolecular H-bonds to construct an assembled network in the crystal structure.17 In the present study, we have prepared tris(alkylamino)phenalenylium derivatives (TAP, 6) with alkylamino substituents in a 3-fold symmetric manner. Redox properties, electronic structure, and intermolecular H-bond networks in the crystal structures of 6a−6c·BF4− were investigated. We have also elucidated the optical properties and crystal structures of TCNQ•− salts of 6a and 6b.

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EXPERIMENTAL SECTION

General. The synthetic intermediate, 1,4,7-trimethoxyphenalenylium tetrafluoroborate (7·BF4−), was prepared according to the previously reported procedure (Scheme 1).18 Melting points were measured with a hot-stage apparatus and uncorrected. 1H NMR spectra were obtained on a JEOL EX-270 with DMSO-d6 using Me4Si as an internal standard. Infrared spectra (IR) were recorded on JASCO FT/IR-660M or Perkin-Elmer 1600 series using KBr plates (resolution 2 or 4 cm−1). Electronic spectra were measured by a Shimadzu UV3100PC spectrometer using KBr plates. Electron ionization (EI) mass spectra were taken at 70 eV by using a Shimadzu QP-5000 mass spectrometer. Elemental analyses were performed at the Graduate School of Science, Osaka University. Solvents were dried (drying agent in parentheses) and distilled under an argon prior to use: MeOH (Mg and I2); CH2Cl2 (CaH2). n-Propylamine, isopropylamine, and tert-butylamine were used after passing through a short alumina column (ICN, super-I). All reactions requiring anhydrous conditions were conducted under an argon atmosphere. Typical Synthetic Procedure for 1,4,7-Tris(alkylamino)phenalenylium Tetrafluoroborate. 6a·BF 4 − . Intermediate 7·BF4− (898 mg, 2.62 mmol) was placed in a 20-mL Schlenk tube and dissolved with CH2Cl2 (3 mL). To this mixture was added n-propylamine (2 mL, 24.2 mmol), and the mixture was stirred at room temperature for 10 min. The residual propylamine and solvent were removed under reduced pressure to give 6a·BF4− (1.03 g, 93%) as a red solid. Single crystals suitable for X-ray measurement were obtained by vapor diffusion using a 1:1 mixture of ethyl acetate and MeOH and hexane or benzene. Mp 261−262 °C; 1H NMR (270 MHz, DMSO-d6) δ 0.96 (t, 9, J = 6.4 Hz), 1.67 (m, 6), 3.51 (m, 6), 7.03 (d, 3, J = 9.6 Hz), 8.45 (s, 3), 8.51 (d, 3, J = 9.6 Hz); EI-MS, m/z 335 (M+ − HBF4, 12%), 306 (M+ − HBF4 − C2H5, 100%); IR (KBr) 3407, 3268, 2963, 2875, 1605, 1577, 1523, 1084 cm−1; UV (KBr) 284, 488, 520(sh) nm. Anal. Calcd for (C22H30N3)(BF4−): C, 62.42; H, 7.14; N, 9.93. Found: C, 62.29; H, 7.09; N, 9.85. 6b·BF4−. Red powder (80% yield). Single crystals suitable for X-ray measurement were obtained by vapor diffusion using a 1:1 mixture of ethyl acetate and MeOH and hexane or benzene. Mp 276−278 °C; 1H NMR (270 MHz, DMSO-d6) δ 1.32 (d, 18, J = 6.1 Hz), 4.13 (m, 3), 7.02 (d, 3, J = 9.3 Hz), 7.99 (d, 3, J = 7.6 Hz), 8.57 (d, 3, J = 9.3 Hz);

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RESULTS AND DISCUSSION Synthesis. Staab et al. synthesized tris(N,N′-dimethylamino)phenalenylium tetrafluoroborate, 6d·BF4−, by the reaction of 1,4,7-trimethoxyphenalenylium salt 7·BF4− with dimethylamine.18 Following this previous synthetic procedure, TAP derivatives 6a−6c were synthesized using primary amines (Scheme 1). Slow diffusion of MeOH solutions of the BF4− salts and Li·TCNQ•− afforded the TCNQ•− salts of 6a and 6b as deep green block crystals (Scheme 1). Powder samples of 805

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Table 1. Crystallographic Data of BF4− and TCNQ•− Salts crystal formula formula weight crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalc (g cm−3) μ (mm−1) temp (K) unique reflns reflns used (I > 2.0σ(I)) params R1, wR2 GOF

6a·BF4−

6b·BF4− (H2O)

6c·BF4−

6a·TCNQ•−

6b·TCNQ•−

C22H30BF4N3 423.30 monoclinic P21/a 16.24(2) 16.730(7) 8.650(3) 105.23(7) 2268(2) 4 1.240 0.096 288 2269 2267 274 0.092, 0.227 1.213

C22H33BF4N3O 442.32 cubic Pa3̅ 16.693(7)

C25H36BF4N3 465.38 monoclinic P21/n 11.098(2) 16.579(3) 13.670(3) 98.731(9) 2486.2(8) 4 1.243 0.094 200 5636 5117 325 0.092, 0.196 1.227

C34H34N7 540.68 monoclinic P21/n 15.995(2) 9.3589(8) 19.313(1) 104.949(2) 2793.2(4) 4 1.286 0.079 173 6295 3645 370 0.075, 0.185 1.029

C34H34N7 540.68 monoclinic P21/n 16.4664(7) 9.8133(4) 18.6732(7) 104.736(1) 2918.1(2) 4 1.231 0.075 296 6642 4679 370 0.056, 0.152 1.018

4651(3) 8 1.263 0.099 200 1782 1089 94 0.060, 0.179 1.053

Scheme 1. Procedure for the Preparation of TAP Derivatives 6 and TCNQ•− Salts

intermolecular H-bond is the most plausible origin of this behavior. The solid-state IR spectra of the (6a−6c)·BF4− salts showed broad absorption bands at 3200−3400 cm−1 assignable to the N−H stretching modes forming intermolecular H-bonds (Figure S1, Supporting Information). With increase of the steric hindrance of the alkyl groups, the N−H stretching modes showed a decrease in intensity and a slight increase in frequencies. In the case of 6c·BF4−, the broad N−H mode was observed as a weak shoulder band, because the H-bonds were very weak. This behavior corresponds to the relationship between bulkiness of the alkyl groups and H-bond distances in the crystal structures (vide infra). These results indicate that the alkyl groups of TAP have a large effect on the strength of H-bonds in the solution and solid states. Electrochemical Properties. Cyclic voltammograms of (6a−6c)·BF4− exhibited two reduction peaks corresponding to the reduction processes from cation to neutral radical (E1) and from neutral radical to anion species (E2) (Figure 1). The

TCNQ•− salts were also obtained by the metathesis method between the BF4− salts and Li·TCNQ•− in MeOH. Effect of Alkyl Groups on the H-Bond Strength. The 3-fold symmetry of 6a−6c in a solution state was confirmed by the 1H NMR spectra of BF4− salts. The spectra of 6a−6c showed only three kinds of signals assignable to aromatic C−H (7.0−7.2 and 8.5−8.6 ppm) and N−H (7.5−8.5 ppm) protons on the TAP skeleton. This feature indicates that three equivalent structures resonate as shown in Scheme 2 and that Scheme 2. Three Resonance Structures of TAP

Figure 1. Cyclic voltammograms of (6a−6c)·BF4− (V vs Fc/Fc+).

first reduction process was reversible. However, the second reduction process was irreversible, and a faint return peak was observed. This result implies the instability of the anion species.2a Table 2 summarizes the redox potentials of 6a−6c and related compounds. Both E1 and E2 values of 6a−6c were

the positive charge delocalizes around the TAP skeleton. The signal of N−H proton showed a significant upfield shift with increasing the bulkiness of alkyl group on the amino group (6a 8.45, 6b 7.99, and 6c 7.50 ppm). The weakening of the 806

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Table 2. Redox Potentials and Potential Gap (E1 − E2) of 6·BF4− and Related Compoundsa E1 (V) 1a 6a 6b 6c 6db 7 4c TCNQ TTF

E1 − E2 (V)

E2 (V)

−0.23 (−0.18) −1.45d (−1.41)e −1.44d (−1.40)e −1.38d (−1.34)e −1.20d (−1.14)e −0.82d (−0.77)e d

e

−0.23d (−0.19)e −0.03d (−0.07)e

−1.70 (−1.66) −2.08d −2.05d −1.97d −1.91d −1.82d (−1.78)e +1.2c −0.77d (−0.74)e +0.34d (+0.30)e d

result that the reduction potentials (E1 and E2) of 6a−6c in the cyclic voltammetry were lower than those of 1a. The coefficient of frontier MOs locates also on the amino group, indicating that the amino substitution causes effective π-extension. The reduction of E1 − E2 in the cyclic voltammetry corresponds to this behavior. Crystal Structure of TAP Salts: Bond Alternation and Charge Delocalization. To investigate the electronic structure and H-bond nature of TAP in crystal structures, we have carried out X-ray crystal structure analysis of the BF4− and TCNQ•− salts of 6. The TAP skeleton possessed a planar structure having a 3-fold symmetry in 6b·BF4− salt. In the other salts, although the 3-fold symmetry was crystallographically broken due to the configuration of the alkyl groups, an almost planar TAP skeleton with a nearly 3-fold symmetry was observed. Table 3 summarizes the C−C and C−N bond lengths in the TAP skeleton. Each bond in all compounds possessed nearly constant lengths indicating the delocalization of the positive charge around the TAP skeleton. A bond alternation, where the bonds a and c (ca. 1.35 Å) are shorter than bonds b and d (ca. 1.40 Å), indicates that the iminium character had a larger effect as shown in Scheme 2. Crystal Structure of 6a·BF4−. The asymmetric unit of this crystal was composed of one 6a and one BF4− molecule. The TAP skeleton stacked to form a one-dimensional column along the c-axis with nearly staggered stacking (stack-A) and small overlap manners (stack-B) (Figure 3a and Figure S2, Supporting Information). The face-to-face distances were 3.45 Å for stack-A and 3.38 Å for stack-B. The 6a molecules in the stack-A dimer were linked through double N1−H···F1−B− F2···H−N3 intermolecular H-bonds (N···F distances, 3.01 and 3.19 Å, Figure 3a). The stack-A dimers were connected by the N2−H···F3 H-bonds (N···F distance, 2.99 Å) to form a twodimensional layer (Figure 3b). Crystal Structure of 6b·BF4−. This crystal consisted of a crystallographically independent 6b, BF4−, and crystalline water molecules. The face-to-face dimer of 6b was formed in a staggered manner (interplanar distance, 3.43 Å, Figure 4 and Figure S2, Supporting Information). Interdimer π-stacks were not found in this crystal. The dimers were connected by the intermolecular H-bonds through BF4− of the H-bond distance 3.09 Å to form a three-dimensional network of this crystal (Figure 4). Crystal Structure of 6c·BF4−. One crystallographically independent 6c and disordered BF4− moieties existed in the asymmetric unit. The 6c molecule formed a face-to-face dimer of the nearly staggered stack (interplanar distance, 3.56 Å) (Figure 5 and Figure S2, Supporting Information). The dimers were linked through N−H···F−B−F···H−N intermolecular H-bonds to construct a two-dimensional structure (Figure 5). Due to the steric hindrance of the tert-butyl group, the H-bond distances (3.35 and 3.43 Å) were longer than those in the BF4− salts of 6a and 6b. Crystal Structure of 6a·TCNQ•−. This salt was composed of 6a and TCNQ•− with a 1:1 ratio. The bond length analysis of the TCNQ•− moiety indicated that TCNQ•− was a fully ionized radical species (Table S1, Supporting Information).22 The 6a molecule stacked to form a one-dimensional column with two kinds of stacking patterns, in which TAP moieties overlapped with the one benzene ring (stack-A, 3.46 Å) and the edge of the molecule (stack-B, 3.45 Å) (Figure 6a and Figure S2, Supporting Information). TCNQ•− molecules formed a face-to-face dimer with a nearly eclipsed stacking manner and

e

1.47d (1.48)e 0.63d 0.61d 0.59d 0.71d 1.00d (1.01)e 0.54d (0.55)e 0.36d (0.37)e

a Conditions: concentration, 1 mM; solvent, MeCN (0.1 M Bu4N· ClO4−); scan rate, 20−30 mV/s; reference electrode, Ag/AgNO3 (0.01 M in MeCN); counter electrode, Pt wire; working electrode, Au disk; the results were calibrated with Fc/Fc+ couple. Redox potentials of 1a·BF4−, 6d·BF4−, 7·BF4−, TCNQ, and TTF were measured by cyclic voltammetry in our laboratory. The measurement conditions are identical to those of 6a−6c. bSalt 6d·BF4− was prepared according to the Staab’s method.18 cSee ref 15a for the cyclic voltammetry of 2. The oxidation process from neutral radical to cation species of 2 was not observed because of too strong electron affinity. dPeak potential. eHalf-wave potential.

close to each other and showed large low-potential shifts compared with those of 1a and 7 due to the strong electrondonating effect of amino group. The E1 values of −1.20 to −1.45 V vs Fc/Fc+ of 6a−6d indicate that the neutral radicals of TAP derivatives are much stronger electron-donor molecules than tetrathiafulvalene (TTF, E1 = −0.03 V). The E2 values of 6a−6c were much lower than those of nitrogen-atom incorporated derivative 4 (+1.2 V),15a of which frontier molecular orbital energy markedly decreases. Furthermore, the difference between E1 and E2 (E1 − E2), a simple indicator of amphotericity, became much smaller than that of 1. The origin of this behavior would be the decrease of on-site Coulomb repulsion because of the π-extension by the amino-group substitution. Molecular Orbital (MO) Calculation. In order to elucidate the electronic effect of amino substitution on the TAP skeleton, MO energy levels were calculated by density-functional theory (DFT) calculation. Figure 2 illustrates the energy diagram

Figure 2. MO energy levels of HOMOs and LUMOs for 1a (left) and 6a (right) calculated at the B3LYP/6-31G(d,p) level of theory.

and the pictures of frontier MOs of 1a and 6a. Both HOMO and LUMO levels of 6a were significantly higher than those of 1a because of the strong electron-donating effect of the amino group. This result is in good agreement with the experimental 807

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Table 3. C−C and C−N Bond Lengths (Å) of TAP Moieties

a (Å) 6d·BF4−18 6a·BF4− 6b·BF4− 6c·BF4− 6a·TCNQ•− 6b·TCNQ•−

1.351(4), 1.357(4), 1.360(4) 1.345(7), 1.345(6), 1.344(6) 1.344(3) 1.358(3), 1.342(3), 1.349(3) 1.342(4), 1.348(4), 1.345(4) 1.351(2), 1.351(2), 1.350(2)

b (Å) 1.400(5), 1.408(5), 1.407(5) 1.409(8), 1.402(7), 1.403(7) 1.415(3) 1.406(3), 1.416(3), 1.415(3) 1.410(4), 1.421(4), 1.422(4) 1.421(2), 1.414(3), 1.416(2)

c (Å) 1.351(5), 1.356(5), 1.350(5) 1.344(7), 1.355(7), 1.362(7) 1.358(3) 1.369(3), 1.360(3), 1.365(3) 1.366(4), 1.364(4), 1.369(4) 1.363(3), 1.363(2), 1.358(3)

d (Å) 1.406(5), 1.398(5), 1.414(5) 1.404(7), 1.405(7), 1.416(7) 1.413(3) 1.410(3), 1.413(3), 1.406(3) 1.414(4), 1.410(4), 1.421(4) 1.415(2), 1.413(2), 1.412(2)

e (Å) 1.427(4), 1.426(5), 1.427(4) 1.433(7), 1.431(7), 1.426(6) 1.435(3) 1.433(3), 1.445(3), 1.432(3) 1.423(4), 1.430(4), 1.426(4) 1.432(2), 1.430(2), 1.431(2)

Figure 4. H-bond network in the crystal structure of 6b·BF4− viewed along the (111) direction. H-bonds are illustrated in red-dotted lines. H-atoms of C−H bonds are omitted for clarity. Figure 3. Crystal structure of 6a·BF4−: (a) one-dimensional π-stack columns; (b) H-bond network of the stack-A dimers. H-bonds are illustrated in red-dotted lines. H-atoms of C−H bonds are omitted for clarity.

significantly lower than that of neutral TCNQ•− (2227 cm−1) indicating the highly ionized state and strong dimerization of the TCNQ•− species (Figure 7a).23 The bands in the CC and CN stretching region (1505, 1520, 1570, and 1600 cm−1) were assigned as those of 6a·BF4− (1523, 1577, and 1605 cm−1) and K·TCNQ•− (1508 and 1578 cm−1) (Figure 7b).24 Figure 8 shows the electronic spectra of TCNQ•− salts of 6a and 6b. The spectrum of 6a·TCNQ•− at the >13.0 × 103 cm−1 region was explained by the simple superposition of those of 6a·BF4− and K·TCNQ•−.25 The intermolecular CT transition within the TCNQ face-to-face dimer was observed at 11.6 × 103 cm−1. In the case of K·TCNQ•− forming a uniform TCNQ•− stacking column, the intermolecular CT band appeared at 7.6 × 103 cm−1. The difference in the CT energies of 6a·BF4− and K·TCNQ•− originated from that in the stacking structures of TCNQ•−. The spectral features of 6b·TCNQ•− were very similar to those of 6a·TCNQ•−.

an interplanar distance of 3.24 Å (Figure S3, Supporting Information). TCNQ•− dimers were arranged parallel to the TAP column (Figure 6a). The N−H···NC intermolecular H-bonds (3.05−3.23 Å) between 6a and TCNQ•− formed a two-dimensional H-bond network on the ac-plane (Figure 6b). The TCNQ•− salt of 6b was isostructural to 6a·TCNQ•− (Figure S4, Supporting Information). The H-bond distances (3.19−3.52 Å) were longer than those in 6a·TCNQ•− because of the larger steric hindrance of the isopropyl group. Optical Properties. Figure 7 compares the CN (∼2200 cm−1), CC, and CN stretching (1500−1700 cm−1) modes in the IR spectrum of 6a·TCNQ•− with those of related compounds. The CN stretching mode at around 2180 cm−1 was 808

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Figure 7. IR spectra (KBr pellet) of 6a·TCNQ•− and related compounds: (a) CN stretching region and (b) CC and CN stretching region.

Figure 5. Two-dimensional H-bond network of the π-stack dimer in the crystal structure of 6c·BF4−. H-bonds are illustrated in red-dotted lines. H-atoms of C−H bonds and disordered F atoms are omitted for clarity. F2−F4 atoms disordered into two atoms, and the atoms of smaller site occupancy factors are omitted. In the measurement of Hbond distance, the disordered F atoms of larger site occupancy factors were used.

Figure 8. Electronic spectra (KBr) of 6a·TCNQ•−, 6b·TCNQ•−, and related compounds.

enhancement of amphoteric redox ability of the neutral radical species of 6. The self-assembled structures constructed by the π-stacks of the phenalenyl skeleton and H-bonds at the amino groups demonstrated the structural regulation ability of TAP in CT complexes and salts. The difference of the alkyl group on the amino group greatly affected the H-bonded networks in the BF4− salts. These features of 6 derivatives show the high potential of the TAP system as a component of electronic molecular materials such as organic conductors and semiconductors. Further investigations are currently in progress for the TAP-based conductive CT complexes and cooperative protoncoupled electron-transfer system.26 Furthermore, the highly symmetric molecular structures and nitrogen atoms for coordination sites of 6 are expected to provide self-assembled coordination cages.27

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ASSOCIATED CONTENT * Supporting Information IR spectra of 6·BF4− and 6·TCNQ•−, overlap modes in the crystal structures of 6·BF4− and 6a·TCNQ•−, illustrations of the crystal structure of 6b·TCNQ•−, and a list of intramolecular C−C bond lengths of the TCNQ skeletons in 6a·TCNQ•−, 6b·TCNQ•−, and related compounds and X-ray crystallographic data for each structure in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. S

Figure 6. Crystal structure of 6a·TCNQ•−: (a) one-dimensional π-stack columns of 6a and TCNQ•− dimers; (b) H-bond network of 6a and TCNQ•−. H-bonds are illustrated in red-dotted lines. H-atoms of C−H bonds are omitted for clarity.

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CONCLUSION Amino-substituted derivatives of the phenalenyl 1 system, TAP derivatives 6 having alkylamino groups, were investigated as a new countercation of CT salts having intermolecular H-bond ability. The TAP skeleton possessed planar and 3-fold symmetric structures showing the delocalization of the positive charge around the molecular plane. The CV measurements indicated that the amino substitution caused a significant strengthening of the electron-donating ability and the

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AUTHOR INFORMATION Corresponding Author *Mailing address: Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan. Phone: +81-6-6850-5393. Fax: +81-66850-5395. E-mail: [email protected]. 809

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Crystal Growth & Design

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Article

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ACKNOWLEDGMENTS This work was partly supported by Grants-in-Aid for Scientific Research (20550051) and Challenging Exploratory Research (21655015) from the Japan Society for the Promotion of Science and for Scientific Research on Innovative Area (20110006) and Elements Science and Technology Project from the Ministry of Education, Culture, Sports, Sciences and Technology, Japan.

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