A New Class of TCNQ Derivatives Easily Generated from TCNQH2

Mar 9, 2009 - at room temperature, with no precaution to exclude air, afforded an air-stable, diamagnetic, ... and bond length data for TCNQ0 and for ...
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A New Class of TCNQ Derivatives Easily Generated from TCNQH2 Containing Discrete TCNQ2- Anions and Noncoordinating Cations Timothy A. Hudson and Richard Robson* School of Chemistry, UniVersity of Melbourne, Victoria 3010, Australia

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 4 1658–1662

ReceiVed December 24, 2008; ReVised Manuscript ReceiVed February 26, 2009

Intense interest was aroused by reports appearing in the early 1960s concerned with the synthesis and properties of tetracyanoquinodimethane, TCNQ, I (see Chart 1).1 The promise of unusual and possibly useful electronic, magnetic, and electrical properties arising from the ready accessibility of the stable, long-lived radical anion, TCNQ-•, lies behind much of this interest, which has continued more or less unabated to the present day. By the use of TCNQ itself or various salts of the TCNQ-• radical anion as starting materials, very large numbers of derivatives, both metal-containing and purely organic, have been isolated and studied in which the TCNQ component is almost always involved in some close π-π association and generally has an average oxidation state in the range minus one to zero.2 We proposed recently that the reduced form of TCNQ, viz. (CN)2CH · C6H4 · CH(CN)2, structure II, referred to here as TCNQH2, may provide a very convenient entry into new types of coordination polymers based on the rare TCNQ2- anion and reported the first example of the use of TCNQH2 for this purpose.3 TCNQH2, despite its reduced oxidation state, is quite stable in air and can be easily obtained from TCNQ without precaution to exclude air. The TCNQ2- anion in solution, on the other hand, was long ago reported to be sensitive to aerial oxidation.4 Undeterred by this dispiriting information, we surmised that if appropriate metal cations with good affinity for negatively charged nitrogen donor centers were available in the reaction mixture at the instant the TCNQ2- was generated, the anion might be immediately bound to metal and thereby stabilized against such oxidation. We found that the very simple reaction between TCNQH2, Cd(NO3)2, (Ph3PMe)Br, and Li(CH3CO2) in MeOH/DMF at room temperature, with no precaution to exclude air, afforded an air-stable, diamagnetic, crystalline coordination polymer of composition (Ph3PMe)2[Cd2(TCNQ)3] in which the TCNQ was present as the dianion.3 We have subsequently made several other TCNQ2--based coordination polymers from TCNQH2 and determined their structures.5 We now report that TCNQH2 affords new types of crystalline solids in which discrete TCNQ2- anions associate with various cations (mostly organic) by noncoordinative interactions. The TCNQ2- appears in these cases to be stabilized in the lattice by a combination of simple anion-cation electrostatic attraction and van der Waals interactions; in some of them, the TCNQ2- unit participates in hydrogen bonding, in others in π-π association and in yet others neither of these interactions is present. Structural and spectroscopic data discussed below leave no doubt that the TCNQ is present in these crystalline products as essentially the discrete dianion (except in one interesting case). Reaction of TCNQH2 with dabco (1,4-diazabicyclo[2.2.2]octane) in DMF solution at room temperature, with no precaution to exclude air, affords good yields (65-70%) of colorless, crystalline (dabco · H)2(dabco)(TCNQ) that shows no sign of decay after months in air; its structure is shown in Figure 1. All atoms within the TCNQ units are essentially coplanar. There are no π-π interactions, a fact in marked contrast to the large numbers of previously reported “organic” derivatives of TCNQ where extensive π-π interactions are a common if not universal structural feature. * To whom correspondence should be addressed. Fax: 61 3 9347 5180. Phone: 618344 6469. E-mail: [email protected].

Figure 1. Part of the structure of (dabco · H)2(dabco)(TCNQ) showing the internally hydrogen bonded (dabco · H+ · · · dabco · · · +H · dabco) cations and the planar strips of TCNQ2- units.

The TCNQ units are arranged into planar strips as can be seen in Figure 1, with N · · · N contacts of 3.37 Å. The strips are all parallel and are isolated from each other by cationic [(dabco.H)2(dabco)]2+ units, which, as can be seen in Figure 1, consist of a central neutral dabco molecule hydrogen bonded to two flanking (dabco · H)+ cations (N · · · N separation 2.70 Å), all six nitrogen atoms being colinear. The lack of color in this compound and the obvious 2+ charge on the [(dabco · H)2(dabco)]2+ units (the two protons on nitrogen were clearly revealed) strongly suggest that the TCNQ is present as the dianion, which is confirmed by IR and structural data discussed below. It has been noted previously6 that VCN stretching frequencies and CC and CN bond distances (see a-e in I) within the TCNQ unit give an indication of the negative charge formally associated with it: the higher the negative charge the lower is νCN, the longer are bonds a, c, and e, and the shorter are bonds b and d. If one compares bonds a-e in the representation of TCNQ0 in I with the analogous bonds in TCNQ2- seen in resonance structures IIIa and IIIb, this lengthening of bonds a, c, and e and shortening of b and d in the dianion can be appreciated; analogous bonds in TCNQ-• are affected to an intermediate extent. Table 1 presents some νCN and bond length data for TCNQ0 and for some TCNQ-• and TCNQ2- systems, including the new compounds we report below. A very wide range of crystallographically characterized compounds in which the formal charge associated with TCNQ lies between minus 1 and zero have been reported and typical data for this class are included in Table 1. Very few structural studies have been carried out on derivatives in which the dianionic state of the TCNQ component is well authenticated; examples in this category included for comparative purposes in Table 1 are (Ph3PMe)2[Cd2(TCNQ)3],3 solvated Zn(TCNQ)(4,4-bipy),7 solvated MnIII2(salen)2(TCNQ),6 and [CoIII(C5Me5)2]2(TCNQ).8 Also included in Table 1 are νCN data for Na2TCNQ,9 which is colorless as expected for TCNQ2-, but unfortunately, crystallographic data for this compound are not available. The structure of a TCNQ-derived and dabco-containing compound of composition (dabco · H)2(TCNQ)3 related to our (dabco · H)2(dabco)(TCNQ) referred to above has been previously reported,10a in which the cations are hydrogen-bonded together to give a linear [(dabco.H)+]∞ polymer with all nitrogen atoms colinear and in which the TCNQ units appear closely spaced in π-π stacks running parallel with the polymeric cationic chains. The formal

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Figure 4. TCNQ2- anion surrounded by four neighboring VII2+ cations in the structure of (VII)(TCNQ).

Figure 2. Stack of alternating anions and cations in (IVa)(TCNQ). The TCNQ2- and IV2+ components are not exactly parallel and atom · · · atom contacts between neighbors vary from 3.27 Å (the shortest) to approximately 3.5 Å. Figure 5. Two adjacent, internally hydrogen bonded (Et3NH+ · · · (TCNQ2-) · · · +HNEt3)0 units in the structure of (Et3NH)2(TCNQ).

Figure 3. Stack of alternating TCNQ2- and Pt(bipy)22+ units in (VI)(TCNQ). The closest C · · · C contact between neighbors is 3.28 Å and the closest Pt · · · C contact is 3.49 Å.

average charge per TCNQ unit in this case is -2/3, in stark contrast to the -2 charge in (dabco · H)2(dabco)(TCNQ); the data for these two related compounds presented in Table 1, in light of the considerations above, clearly reflect these different charges. It is significant that (dabco · H)2(TCNQ)3, containing stacked and strongly interacting TCNQ units each with an average charge of -2/3, is black, whereas (dabco•H)2(dabco)(TCNQ) containing TCNQ2- is colorless. The new TCNQ2--derivatives listed in Table 1 were all obtained extremely simply from TCNQH2 by reaction at room temperature with sources of various cations (some of which are shown in structural figures IV-X) in a variety of media such as methanol,

ethanol, DMF or dioxane or various mixtures thereof as indicated in the Supplementary Data Section. In some cases the acetate ion was used to deprotonate the TCNQH2 and in others, where the cation was a protonated nitrogenous base, the base itself performed this role. With the cations IV-VI, we have generated from TCNQH2 a number of very dark, almost black crystalline compounds, listed in Table 1, in which the TCNQ2- makes π-π contact with the cation to produce stacks consisting of alternating anion and cation, illustrated for compounds (IVa)(TCNQ) and (VI)(TCNQ) in Figures 2 and 3 respectively. The data in Table 1 for these almost black π-π stacked compounds clearly indicate that the TCNQ is present as the dianion, despite the intense visible absorption, which presumably has a charge transfer donor-acceptor origin in which TCNQ2- acts as the donor, transferring an electron to the π-interacting cation in the excited state. We have likewise generated from TCNQH2 and the cations VII-IX, crystalline TCNQ2- derivatives in which the cation, being too twisted to allow direct π-π interaction with the dianion, instead makes contact via edges. The compounds (VII)(TCNQ) and (VIII)(TCNQ) are both almost black (dark red when crushed) and (IX)(TCNQ) is bright red; it appears therefore that, despite the lack of direct π-π contact, charge transfer donor-acceptor absorption in which TCNQ2- acts as the donor is still possible. To illustrate this class of structures in which TCNQ2- is not involved in faceto-face π-π associations, Figure 4 shows the surroundings of the TCNQ2- unit in (VII)(TCNQ). Reaction of TCNQH2 with triethylamine in ethanol solution at room temperature affords colorless crystalline (Et3NH)2(TCNQ) with the structure shown in Figure 5. The TCNQ2- ions, with a center of symmetry at the center of the C6 ring, are in this case slightly twisted out of coplanarity, two centrosymmetrically related nitrile groups being each hydrogen bonded to an Et3NH+ cation (N · · · N, 2.86 Å) to generate overall electrically neutral [Et3NH+ · · · (TCNQ2-) · · +HNEt3]0 units, as can be seen in Figure 5. The TCNQ2- ions are not involved in any π-π contact.

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Compounds of compositions (enH)2(en)(TCNQ) (en ) ethylenediamine), which is pale yellow, and [(HOCH2CH2)3NH]2(TCNQ)(dioxan)3, which is colorless, also have structures in which the TCNQ2- units are hydrogen-bonded to the cations. An internally hydrogen bonded, overall electrically neutral {[(HOCH2CH2)3NH]+ · · · (TCNQ)2- · · · +[HN(CH2CH2OH)3]]0 unit is shown in Figure 6. The mode of preparation and the lack of color in several of the compounds above together with the obvious dipositive charge associated with the cations, provide persuasive evidence that the TCNQ is present as the -2 anion, but this is emphatically confirmed by the data in Table 1; despite the variety of environments in which

the TCNQ2- is found, the bond length and νCN data are remarkably invariant. It is clear the same anion with a -2 charge is present in the black compounds as in the colorless ones. More detailed descriptions of all these structures are presented in the Supporting Information. In contrast to the above consistency of bond length and νCN data, the compound obtained from the dication X of composition (X)(TCNQ), which is very dark brown, appears to be very much the odd man out, showing νCN bands and bond lengths suggesting a charge on TCNQ approximating to -1 (see Table 1). The dication [C6H2(NH2)4]2+ seen in X would be expected to be readily reduced to the radical-monocation [C6H2(NH2)4]+• and it appears therefore that the (cation+•)(TCNQ-•) oxidation level

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Crystal Growth & Design, Vol. 9, No. 4, 2009 1661 Table 1. Infrared and Structural Data νCN

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TCNQ TCNQ-• (6) Na2TCNQ(9) (Ph3PMe)2[Cd2(TCNQ)3](3) Zn(TCNQ)(4,4′-bipy) · solvate(7) MnIII(salen)2(TCNQ)(6) [CoIII(C5Me5)2]2(TCNQ)(8) (dabco.H)2(dabco)(TCNQ) (dabco.H)2(TCNQ)3a (IVa)(TCNQ) (IVb)(TCNQ) (V)(TCNQ) (VI)(TCNQ) (VII)(TCNQ) (VIII)(TCNQ) (IX)(TCNQ) (Et3NH)2(TCNQ) (en.H)2(en)(TCNQ) [(HOCH2CH2)3NH]2(TCNQ)(diox)3 (X)(TCNQ) a

2220-2170 2175-2150 2164, 2096 2176, 2113 2194, 2121 2100 2113 2167, 2107 2160, 2105 2152, 2104 2197, 2166 2158, 2107 2158, 2116 2154, 2110 2154, 2111 2151, 2108 2156, 2110 2158, 2112 2160, 2108 2168, 2100 2181, 2146

a (Å)

b (Å)

c (Å)

d (Å)

e (Å)

1.330-1.37 1.358-1.375

1.436 -1.453 1.40-1.425

1.34 -1.388 1.405-1.438

1.415-1.433 1.40-1.48

1.126-1.19 1.126-1.156

1.387 1.380 1.380 1.371 1.36, 1.42 1.42, 1.42 1.388 1.350, 1.365 1.386 1.383 1.381 1.393 1.386 1.386 1.388 1.387 1.387 1.380 1.366

1.404 1.402 1.395, 1.38

1.471 1.472 1.491, 1.481

1.403, 1.402 1.405 1.39, 1.37

1.156, 1.157 1.158, 1.163 1.167, 1.14

1.38, 1.46

1.43, 1.45

1.402, 1.429, 1.400, 1.403, 1.407, 1.414, 1.407, 1.408, 1.402, 1.406, 1.406, 1.403, 1.418,

1.468 1.402, 1.429 1.460 1.464 1.463 1.456 1.463 1.465 1.464 1.472 1.470 1.466 1.435

1.41, 1.42 1.44, 1.42 1.403, 1.402 1.425, 1.416 1.389, 1.390 1.402, 1.398 1.409, 1.402 1.422, 1.396 1.403, 1.400 1.404, 1.401 1.406, 1.393 1.408, 1.397 1.402, 1.401 1.404, 1.404 1.416, 1.402

1.19, 1.16 1.10, 1.13 1.160, 1.161 1.149, 1.158 1.165, 1.163 1.154, 1.158 1.160, 1.154 1.157, 1.146 1.159, 1.159 1.160, 1.159 1.154, 0.1.144 1.156, 1.156 1.163, 1.154 1.156, 1.156 1.148, 1.146

1.401 1.422 1.394 1.402 1.406 1.403 1.406 1.408 1.400 1.399 1.402 1.397 1.402

IR spectra and bond distances obtained from refs 10b and 10a, respectively.

Figure 6. Internally hydrogen bonded, overall electrically neutral {[(HOCH2CH2)3NH]+ · · · (TCNQ)2- · · · +[HN(CH2CH2OH)3]]0 unit in the structure of [(HOCH2CH2)3NH]2(TCNQ)(dioxan)3.

presumed roughly to represent the charge transfer donor-acceptor excited state in the colored examples above, in the case of (X)(TCNQ) represents the ground state. Figure 7 shows the structure of (X)(TCNQ), which consists of stacks of alternating cations and anions. Within a stack, all planar TCNQ-• anions are parallel with each other and the interspersed planar [C6H2(NH2)4]+• cations are close to, but not exactly, parallel to the anions, the separation between cation and anion being of the order of 3.2-3.3 Å. All stacks are equivalent and parallel but the inclination of the components within half the stacks differs from that in the other half, as can be seen in Figure 7, which shows two adjacent, parallel, but differently oriented stacks. Each stack participates in extensive hydrogen bonding to adjacent stacks, the amino NH groups in one acting as hydrogen bond donors to the TCNQ-• nitrogen atoms in the neighbor (N · · · N, 2.94, 3.03, 3.05, 3.14 Å). The work reported here opens the way for the generation, via the TCNQH2 approach, of large numbers of new TCNQ derivatives with a wide variety of counter cations, affording structures quite different in many cases from the numerous presently known ones. As illustrated by the dabco and Et3NH+ examples described above, even when the new TCNQ2- system shares the same cation that is present in an already known TCNQ compound, the structure will undoubtedly be very different. Although the spin-paired TCNQ2-based systems per se may not offer directly the exciting electronic, magnetic and electrical possibilities promised by the odd electron

Figure 7. Stacks of alternating anion and cation in the structure of (X)(TCNQ). All stacks are equivalent and parallel but the inclination of the components common to half the stacks differs from that common to the other half.

TCNQ-•-based and mixed oxidation state systems, the range of new structural types afforded by the TCNQH2 approach provides many opportunities for subsequent modification by doping or intercalation so as to generate radical species in circumstances quite different from those previously seen (e.g., in ways analogous to the doping of conjugated polymers to increase their electrical conductivities many orders of magnitude). The strong π donor properties of TCNQ2- and its potential to participate in Mulliken-type donor/ acceptor complex formation have already been pointed out by others7-in this connection there is much scope for the insertion of acceptors of various sorts into TCNQ2- derivatives obtained by the TCNQH2 approach. In addition, the production directly from TCNQH2 of a material [viz. (X)(TCNQ)] in which the TCNQ is present in an oxidation state approximating to the radical monoanion is suggestive of new entry routes, by the use of counter cations

1662 Crystal Growth & Design, Vol. 9, No. 4, 2009 with appropriate oxidizing properties, into TCNQ-•-based and mixed oxidation state materials (possibly including novel TCNQ-•/ TCNQ2- mixed states that are to be contrasted with the common TCNQ-•/TCNQ0 mixed states). Undoubtedly, the TCNQH2 approach will greatly extend the number and types of accessible TCNQ-based materials-and this will be achieved in a very simple manner.

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(2) (3) (4)

Acknowledgment. The authors gratefully acknowledge support from the Australian Research Council.

(5) (6)

Supporting Information Available: Syntheses and crystallographic details (PDF); crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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References (1) (a) Acker, D. S.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Melby, L. R.; Benson, R. E.; Mochel, W. E. J. Am. Chem. Soc. 1960, 82, 6408. (b) Acker, D. S.; Hertler, W. R. J. Am. Chem. Soc. 1962, 84,

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3370. (c) Melby, L. R.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Benson, R. E.; Mochel, W. E. J. Am. Chem. Soc. 1962, 84, 3374. (a) Herbstein, F. H.; Kapon, M. Crystallogr. ReV. 2008, 14, 3. (b) Ballester, L.; Gutierrez, A.; Perpinan, M. F.; Azcondo, M. T. Coord. Chem. ReV. 1999, 190, 447, and references therein. Abrahams, B. F.; Hudson, T. A.; Robson, R. Cryst. Growth Des. 2008, 8, 1123. Suchanski, M. R.; Van Duyne, R. P. J. Am. Chem. Soc. 1976, 98, 250. Hudson, T. A.; Robson, R. Unpublished results. Oshio, H.; Ino, E.; Ito, T.; Maeda, Y. Bull. Chem. Soc. Jpn. 1995, 68, 889. Also see refs 2a and 7. Shimomura, S.; Matsuda, R.; Tsujino, T.; Kawamura, T.; Kitagawa, S. J. Am. Chem. Soc. 2006, 128, 16416. Miller, J. S.; Zhang, J. H.; Reiff, W. M.; Dixon, D. A.; Preston, L. D.; Reis, A. H.; Gebert, E.; Extine, M.; Troup, J.; Epstein, A. J.; Ward, M. D. J. Phys. Chem. 1987, 91, 4344. Khatkale, M. S.; Devlin, J. P. J. Chem. Phys. 1979, 70, 1851. (a) Akutagawa, T.; Takeda, S.; Hasegawa, T.; Nakamura, T. J. Am. Chem. Soc. 2004, 126, 291. (b) Bandrauk, A. D.; Truoung, K. D.; Carlone, C.; Jandl, S. J. Phys. Chem. 1985, 89, 434.

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