A New Approach to TCNQ-Based Coordination Polymers via TCNQH2 Brendan F. Abrahams, Timothy A. Hudson, and Richard Robson* School of Chemistry, UniVersity of Melbourne, Victoria 3010, Australia
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1123–1125
ReceiVed January 14, 2008; ReVised Manuscript ReceiVed February 19, 2008
ABSTRACT: 2,2′-(p-Phenylene)bis(propanedinitrile), (NC)2CH · C6H4 · CH(CN)2 (referred to as TCNQH2), upon reaction with Cd(NO3)2, [(C6H5)3PMe]Br, and Li(CH3CO2) in MeOH/DMF at room temperature in air yields an air-stable product of composition (Ph3PMe)2[Cd2(TCNQ)3] in the form of rhombohedral single crystals amenable to X-ray diffraction studies. In the anionic three-dimensional (3D) {[Cd2(TCNQ)3]2-}∞ coordination network Cd centers are in an approximately octahedral coordination environment consisting of N atoms from six TCNQ2- units each of which is in turn attached to four Cd2+ centers at the corners of a quadrilateral that is almost a square. The disposition of the Cd centers is only very slightly distorted from primitive cubic. The results constitute a “proof of concept” for our proposition that easily accessible, air-stable TCNQH2 may afford a very convenient general entry into new types of coordination polymers based on the TCNQ2- anion and that these in turn may be amenable to redox intercalation to afford materials in which TCNQ is present either as the TCNQ- radical or in mixed oxidation states. In the early 1990s when the foundations for the crystal engineering of an apparently unlimited range of potentially very useful coordination polymers were being laid,1 one general possibility we had in mind was that interesting and conceivably useful electronic/ magnetic properties might arise when the metal centers and ligands forming the network either (a) both contained unpaired electrons or (b) were both capable of relatively stable existence in more than one oxidation state. In this regard, the stable tetracyanoquinodimethane radical anion, TCNQ–., was one of a number of very attractive candidates for investigation. We already had underway exploration of the coordination networks formed by a number of other poly cyano building blocks with varying geometries and connectivities, such as the linear NCMCN- (M ) Ag or Au) binding two metals,2 the tricyanomethanide anion, C(CN)3-, binding a trigonal group of metals,3 Pt(CN)42- binding four metals at the corners of a square,4 metalloporphyrins with 4-cyanophenyl substituents at the four meso positions also binding four metals at the corners of a square5 and (4-NC · C6H4-)4C, binding four metals at the corners of a tetrahedron.1a All of these approaches at that time were proving fruitful and we were expecting the planar TCNQ–. radical anion, like the planar 4-connecting Pt(CN)42- and porphyrin examples just mentioned, to bind four metal centers at the corners of a rectangle thereby affording PtS-related coordination networks, if the connecting metal centers had a preference for tetrahedral geometry. On close examination of the crystal structure of AgTCNQ, determined by Shields in 1985,6 it was apparent to us that this did indeed consist of two independent and interpenetrating PtS-related networks,7 to the best of our knowledge a previously unrecognized fact, entirely consistent with the net-based ideas we were developing at that time. Unfortunately our early work with TCNQ was disappointing in that we were unable to obtain metal derivatives in a form suitable for single crystal X-ray diffraction studies and we abandoned the area in the middle 1990s; we suspect other workers may have encountered similar difficulties. Although TCNQ/metal associations have been very extensively investigated, relatively few structural studies of coordination polymers of this type have been reported,8 possibly because of the difficulty of obtaining these compounds in a pure and suitably crystalline form. The TCNQ/metal field shows great promise for the generation of solids with interesting and possibly useful electronic/magnetic properties, for example, (a) thin films of Cu(TCNQ) and Ag(TCNQ), when subjected to an electric field,
switch at a certain threshold potential from a high resistance state to a low resistance state;8d (b) in 2003 Dunbar and co-workers reported the synthesis, magnetic properties, and crystal structure of a TCNQ/rare-earth metal (Gd) coordination polymer, which behaved as a magnet below 3.5 K;8e (c) in 2005 Miller and coworkers reported an interesting family of M(TCNQ)n (M ) Mn, Fe, Co, Ni) magnets (Tc 8–60 K), but unfortunately these materials were ill-defined, with compositions varying according to the starting material used and the structures could not be determined.9 There is clearly a pressing need, in this TCNQ/metal area, for reliable and simple synthetic approaches that provide pure, crystalline products amenable to single crystal X-ray studies. The central proposition of the present contribution is that the reduced species (NC)2CH · C6H4 · CH(CN)2 (referred to below as TCNQH2), structure I, may afford a very convenient entry into new types of coordination polymers based on the TCNQ2- anion and that these in turn may be manipulated to afford materials in which TCNQ is present either as the TCNQ–. radical or in mixed oxidation states. TCNQH2 is readily accessible10 and, despite its reduced state, is quite stable in air. In earlier work we found that the ligands C(CN)3- and N(CN)2very readily gave crystalline coordination polymers;3,11 what we are proposing in the present paper in relation to the future general use of TCNQH2 as a source of TCNQ2- is essentially a variant on that earlier fruitful work. One virtue that we see in this TCNQH2 approach is that the more highly charged TCNQ2- dianion may be realistically expected to interact with metal cations more strongly than does either the TCNQ–. radical monoanion or the neutral TCNQ0 molecule and may be expected to afford crystalline networks correspondingly more readily.
* To whom correspondence should be addressed. Fax: +61 3 9347 5180. Phone: +61 3 8344 6469. E-mail:
[email protected].
In 1979 some very air-sensitive, almost black materials, described as transition metal derivatives of the TCNQ2- dianion, were
10.1021/cg800048e CCC: $40.75 2008 American Chemical Society Published on Web 03/01/2008
1124 Crystal Growth & Design, Vol. 8, No. 4, 2008 reported that were obtained from the reduction of TCNQ0 by various reactive organometallic compounds;12 no crystallographic structural work was reported, the compounds being characterized by magnetic and spectroscopic techniques. X-ray structural studies of some nonpolymeric vanadium complexes of TCNQ2- were reported in 2006.13 In the same year Kitagawa and co-workers reported the generation and X-ray crystal structures of TCNQ2--based coordination networks in which the TCNQ source was the mono anion radical TCNQ–. (in the form of the lithium salt); they observed, “The production process of resulting TCNQ2- is not clear, but that may be a disproportionation reaction of TCNQ-.”8g Very recently Dunbar reported a similar reaction process with the tetrafluoro derivative of TCNQ (i.e., TCNQF4) in which a manganese coordination network involving dianionic TCNQF42- bridging ligands was generated from a monoanionic starting material, i.e., Li(TCNQF4); uncoordinated TCNQF4–. was present in the crystal lattice of this Mn/TCNQF4 compound, making π–π contact with the dianionic TCNQF42- species that formed part of the coordination network.8f We draw attention to the fact that the dianion binds four Mn2+ cations while the monoanion radical, also present within the same crystal, fails to associate with the cation. We take encouragement from this preference, which supports our proposal that the TCNQ2- dianion generated in situ from TCNQH2 may interact more strongly with metal cations than the TCNQ–. monoanion radical does. A classical route to a wide variety of materials with enhanced electrical conductivities is the intercalation of oxidants or reductants into solid starting materials, e.g., such intercalation into graphite greatly increases its electrical conductivity and likewise intercalation of elemental alkali metals into solid C60 in amounts appropriate to the formation of C60M3 (M ) K, Rb, or Cs) generates superconducting materials with Tc’s in the range 20–30 K.14 We propose that a general route either to mixed TCNQ2-/TCNQ–. networks or to entirely TCNQ–. networks or to mixed TCNQ–./TCNQ0 networks might consist of first using TCNQH2 to generate the TCNQ2network, then intercalating the appropriate amount of an oxidant such as an elemental halogen. Another attractive feature of the TCNQH2 approach is the possibility, arising from the “high” 2 minus charge on the ligand, of generating many types of previously unknown anionic coordination networks [(Mn+)x(TCNQ2-)y]m- (one example of which is reported below). In such a case the countercation used could be a crucial structural determinant and choice of cation may therefore afford a means of controlling the structure of the anionic network. The vast range of possibilities offered by the TCNQH2 approach will be apparent. We report here that the very simple reaction between TCNQH2, Cd(NO3)2, (Ph3PMe)Br, and Li(CH3CO2) in MeOH/DMF at room temperature in air yields an air-stable product15 of composition (Ph3PMe)2[Cd2(TCNQ)3] in the form of rhombohedral single crystals amenable to X-ray diffraction studies.16 From the synthetic viewpoint it is interesting, and it will be no doubt very useful in future work to know, that a base as weak as acetate ion, in the presence of the metal cation, is sufficient to generate TCNQ2- from TCNQH2. The structural solution reveals that the disposition of the cadmium centers is only very slightly distorted from primitive cubic, as can be seen in the representation of the anionic 3D {[Cd2(TCNQ)3]2-}∞ network shown in Figure 1. All the TCNQ2units are equivalent and are attached to four Cd2+ centers at the corners of a quadrilateral that is almost a square (Cd · · · Cd · · · Cd angles 88.55 and 91.46°). Two types of Cd2+ are present, both having an approximately octahedral coordination environment consisting of N atoms from six TCNQ2- units [N-Cd-N, 85.4° and 94.6° for both Cd(1) and Cd(2)]. All Cd centers are located on 3-fold axes, an example of which is indicated in Figure 1. The pseudo cubic arrangement seen in Figure 1 can be considered to consist of eight “octants”: as can be seen in Figure 1 each octant has three square faces occupied by µ4-TCNQ2- ligands arranged around the 3-fold axis and three square faces that are vacant,
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Figure 1. Representation of the anionic 3D {[Cd2(TCNQ)3]2-}∞ coordination network. (Cd(1) centres red and Cd(2) green). The almost primitive cubic arrangement of Cd centers can be seen. A 3-fold axis passing through the center of the figure is shown.
Figure 2. The location of the triphenylmethylphosphonium cation in the central region of an octant. The P-CMe bond is coincident with the 3-fold axis, the Me group being directed towards Cd(2) and away from Cd(1). Broken lines indicate two types of close nonbonded interactions– one type from a methyl C-H to a TCNQ2- aromatic carbon atom (C · · · C, 3.55 Å), the other type from a TCNQ2- aromatic carbon atom to a coordinated N atom just outside the octant (C · · · N, 3.40 Å; only the N and C atoms of three involved TCNQ2- units outside the octant are shown).
likewise arranged around the 3-fold axis. A triphenylmethylphosphonium cation, as shown in Figure 2, occupies the central region of every octant; as can be seen, the phosphorus atom is located close to the center of the octant, the P-CMe bond lying on the 3-fold axis with its methyl group directed toward Cd(2) and away from Cd(1). The triphenylmethylphosphonium cation makes close nonbonding contact via an aromatic C-H, meta to phosphorus, to a coordinated N atom (C · · · N, 3.40 Å) as is indicated in Figure 2. The three methyl C-H bonds of the countercation are directed toward the aromatic π systems of the three surrounding TCNQ2units, the closest CMe · · · Carom separation of 3.55 Å being indicated in Figure 2. The IR spectrum of TCNQH2 is unusual in that the νC-H stretching band at 2891 cm-1 is the strongest in the entire spectrum whereas the νCN bands at 2258 and 2267 cm-1 are very weak. In
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the spectrum of (Ph3PMe)2[Cd2(TCNQ)3] the νC-H band has, of course, disappeared and now the νCN bands are very strong and have moved to lower frequency at 2176 and 2113 cm-1. As a general rule the νCN bands are expected to fall as the negative charge on the TCNQ increases. There can be little doubt that the TCNQ in (Ph3PMe)2[Cd2(TCNQ)3] is present as TCNQ2-, given the mode of preparation from TCNQH2 and the fact that the compound is diamagnetic and very pale in color, and these “lowered” νCN frequencies are in good agreement with those quoted by Kitagawa8g and Dunbar8f to support (convincingly) the proposed reduction of their 1- oxidation state starting materials to the 2- state in the coordination framework; thus, bands at 2194 and 2121 cm-1 in Kitagawa’s Zn(TCNQ)(4,4′-bipyridine) were taken to indicate the TCNQ2- oxidation state8g and a band at 2161 cm-1 in the spectrum of Dunbar’s Mn/TCNQF4 compound was ascribed to the TCNQF42- component of the coordination network, while the noncoordinated TCNQF4–. lattice components showed higher frequency νCN bands at 2211 and 2202 cm-1.8f Some electrochemically synthesized materials described as Ni(TCNQ) and Ni3(TCNQ)2 and presumed to contain TCNQ reduced beyond the monoanion radical level showed multiple νCN bands in the range 2250–2000 cm-1.17 Thermogravimetric analysis of (Ph3PMe)2[Cd2(TCNQ)3] indicates the compound is stable at temperatures up to approximately 300 K, above which rapid decomposition occurs. In conclusion, the results presented in this preliminary report provide “proof of principle” for the proposal that easily accessible, air-stable TCNQH2 may provide a very convenient source of new types of TCNQ2--based coordination polymers. We have successfully used TCNQH2 to generate other TCNQ2--based coordination polymers, which we are presently characterizing and whose generation we are attempting to optimize. It is known that the “free” TCNQ2- is sensitive to oxidation by O2,18 but we were optimistic that in the presence of the metal cation, the in situ-generated TCNQ2- might be coordinated at all times and thereby protected; in the present work, it was unnecessary to exclude air in the reaction generating (Ph3PMe)2[Cd2(TCNQ)3] and this may well be generally the case for the TCNQH2 route with a variety of metal cations other than Cd2+. From the point of view of synthetic procedure it is interesting and it will no doubt be very useful in future work to know that a base as weak as acetate ion, in the presence of the metal cation, is sufficient to generate TCNQ2- from TCNQH2.We note that the general proposition presented here can be equally well applied in principle to the reduced form of N,N′-dicyanoquinodiimine, that is, NC-NH-C6H4-NH-CN (or DCNQIH2), which like TCNQH2 is easily accessible and air-stable.
Acknowledgment. The authors gratefully acknowledge support from the Australian Research Council. Supporting Information Available: X-ray crystallographic information files (CIF) and TGA data are available free of charge via the Internet at http://pubs.acs.org.
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(2) (a) Hoskins, B. F.; Robson, R.; Scarlett, N. V. Y. Chem. Commun. 1994, 2025. (b) Hoskins, B. F.; Robson, R.; Scarlett, N. V. Y. Angew. Chem. Int Ed. 1995, 34, 1203. (3) (a) Batten, S. R.; Hoskins, B. F.; Robson, R. Chem. Commun. 1991, 445. (b) Batten, S. R.; Hoskins, B. F.; Moubaraki, B.; Murray, K. S.; Robson, R. J. Chem. Soc., Dalton Trans. 1999, 2977. (4) Gable, R. W.; Hoskins, B. F.; Robson, R. Chem. Commun. 1990, 763. (5) Abrahams, B. F.; Hoskins, B. F.; Michail, D. M.; Robson, R. Nature 1994, 369, 727. (6) Shields, L. J. Chem. Soc., Faraday Trans. 1985, 81, 1. (7) Robson, R. ComprehensiVe Supramolecular Chemistry; Lehn, J.-M. Pergamon Press: U.K., 1996; Vol. 6,Chapter 22, p 733. (8) (a) Heintz, R. A.; Zhao, H.; Ouyang, X.; Grandinetti, G.; Cowen, J.; Dunbar, K. R. Inorg. Chem. 1999, 38, 144. (b) Zhao, H.; Heintz, R. A.; Ouyang, X.; Grandinetti, G.; Dunbar, K. R.; Campana, C. F.; Rogers, R. D. Chem. Mater. 1999, 11, 736. (c) Miyasaka, H.; CamposFernandez, C. S.; Clerac, R.; Dunbar, K. R. Angew. Chem., Int. Ed. 2000, 39, 38311. (d) O’Kane, S. A.; Clerac, R.; Zhao, H.; Ouyang, X.; Galan-Mascaros, J. R.; Heintz, R.; Dunbar, K. R. J. Solid State Chem. 2000, 152, 159. (e) Zhao, H.; Bazile, M. J.; Galan-Mascaros, J. R.; Dunbar, K. R. Angew. Chem., Int. Ed. 2003, 42, 1015. (f) Lopez, N.; Zhao, H.; Prosvirin, A. V.; Chouai, A.; Shatruk, M.; Dunbar, K. R. Chem. Commun. 2007, 4611. (g) Shimomura, S.; Matsuda, R.; Tsujino, T.; Kawamura, T.; Kitagawa, S. J. Am. Chem. Soc. 2006, 128, 16416. (h) Oshio, H.; Ino, E.; Maeda, Y. Bull. Chem. Soc. Jpn. 1995, 68, 889. (9) Vickers, E. B.; Giles, I. D.; Miller, J. S. Chem. Mater. 2005, 17, 1667. (10) Acker, D. S.; Hertler, W. R. J. Am. Chem. Soc. 1962, 84, 3370. (11) Batten, S. R.; Jensen, P.; Moubaraki, B.; Murray, K. S.; Robson, R. Chem. Commun. 1998, 439. (12) Siedle, A. R.; Candela, G. A. Inorg. Chim. Acta 1979, 35, 125. (13) Choukroun, R.; Lorber, C.; de Caro, D.; Vendier, L. Organometallics 2006, 25, 4243. (14) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600. (15) Synthesis of (Ph3PMe)2[Cd2(TCNQ)3]: To a solution of H2TCNQ (200 mg, 0.97 mmol) and (Ph3PMe)Br (548 mg, 1.53 mmol) in DMF (5 ml) was added a solution of Cd(NO3)2 · 6H2O (133 mg, 0.430 mmol) and Li(OAc) · 4H2O (548 mg, 7.53 mmol) in MeOH (20 mL). The yellow crystals were collected after 10 min and washed with cold MeOH (yield 312 mg, 70%). Anal. Calcd for C74H48Cd2N12P1: C, 63.9; N, 12.1; H, 3.5. Found: C, 62.9; N, 12.2; H, 3.8. FT-IR (KBr): 2980, 2905, 2176 (vs), 2113 (vs), 1653 (m), 1576 (m), 1503 (m), 1442 (m), 1279 (m), 1118 (m), 828 (m), 742 (m), 497 (m) cm-1. Larger crystals suitable for single crystal X-ray analysis were obtained by the slow diffusion of a solution of Cd(NO3)2 · 6H2O, (Ph3PMe)Br and Li(OAc) · 4H2O in MeOH (4 mL) into a solution of H2TCNQ in DMF (1.5 mL). (16) Crystal data for [P(C6H5)3CH3]2[Cd2(C12H4N4)3]; Mr 1391.98, rhombohedral (hexagonal setting, R3j, a ) b ) 12.6751(6), c ) 32.243(3) Å, V ) 4486.1(5) Å3, Z ) 3, Fcalcd ) 1.546 Mg/m3, µ ) 0.822 mm-1, λ(MoKR) ) 0.71073 Å, F(000) ) 2106, yellow hexagonal plate, 0.22 × 0.22 × 0.075 mm3, 2θmax ) 55.0 °, T ) 130(1) K, 9516 reflections measured, 2294 independent reflections, and 2015 observed reflections (I > 2σ(I)), wR2 (all data) ) 0.0767, R1(I > 2σ(I)) ) 0.0314. The structure was solved using direct methods and refined using all data in full-matrix least squares procedure.19 CCDC-668920 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. (17) Long, G.; Willett, R. D. Inorg. Chim. Acta 2001, 313, 1. (18) Suchanski, M. R.; Van Duyne, R. P. J. Am. Chem. Soc. 1976, 98, 250. (19) Sheldrick, G. M. SHELX-97 - Programs for Crystal Structure Analysis (Release 97-2); Institüt für Anorganische Chemie de Universitat: Göttingen, Germany, 1998.
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