DOI: 10.1021/cg9015004
A New Approach to DCNQI-Based Coordination Polymers via DCNQIH2
2010, Vol. 10 1468–1470
Brendan F. Abrahams,* Timothy A. Hudson, and Richard Robson* School of Chemistry, University of Melbourne, Victoria 3010, Australia Received December 1, 2009; Revised Manuscript Received February 3, 2010
ABSTRACT: Readily available, air-stable DCNQIH2 (i.e., 1,4-NC 3 NH 3 C6H4 3 NH 3 CN) affords, in a very simple manner, crystals of [Mn(DCNQI)(bridge)]2DMF (where bridge = 4,40 -bipyridine or 1,2-bis(4-pyridyl)ethylene) in which there are [(Mn2þ)(DCNQI2-)]n sheets held together by 4,40 -bipyridine or 1,2-bis(4-pyridyl)ethylene pillars. This approach via DCNQIH2 promises convenient general entry into new types of DCNQI-based coordination polymers and could lead to new materials with interesting and possibly useful magnetic/electronic properties. Derivatives of tetracyanoquinodimethane, I (represented as TCNQ), and dicyanoquinodiimine, II (represented as DCNQI), have been intensively studied for many years, largely because they promise interesting and potentially useful magnetic and electrical properties arising from the accessibility of relatively stable, longlived radical anions, TCNQ•- and DCNQI•-. Generally, the TCNQ or DCNQI component has an average oxidation state in the range of -1 to 0 and almost always is involved in close π-π association either with others of its own kind or with internally πbonded molecules of some other type. The discovery of metal-like electrical conductivity in some of these derivatives stimulated intense research activity.1 We proposed recently that the reduced form of TCNQ, viz. (CN)2CH 3 C6H4 3 CH(CN)2, structure III, referred to here as TCNQH2, may provide a very convenient entry into new types of coordination polymers based on the rare TCNQ2- anion, and we reported the first example of the use of TCNQH2 for this purpose.2 TCNQH2 is easily obtained from TCNQ and is airstable; in solution however the TCNQ2- anion is sensitive to aerial oxidation.3 We surmised that if appropriate metal cations with affinity for negatively charged nitrogen donor centers were available in the reaction mixture at the instant the TCNQ2- was generated from TCNQH2, 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 LiOAc in MeOH/DMF at room temperature, with no precaution to exclude air, afforded an air-stable, diamagnetic, crystalline compound of composition (Ph3PMe)2[Cd2(TCNQ)3] in which the TCNQ was present as the dianionic component of a [Cd2(TCNQ)3]2- three-dimensional (3D) coordination network.2 The synthetic approach via TCNQH2 is at present promising to be widely applicable, and this tactic has already allowed us to isolate and structurally characterize a considerable number of TCNQ2--based coordination polymers. Our longer term objective is to subject well characterized coordination polymers of TCNQ2- to oxidative intercalation (chemical or electrochemical) to generate materials in which, we hope, the initial coordination network remains intact and in which the TCNQ2- component has been oxidized to the radical anion state (TCNQ•-) or to a mixed oxidation state such as or (TCNQ•-)x(TCNQ2-)y or (TCNQ•-)x(TCNQ0)y; these would be systems containing TCNQ•- radicals in entirely new sorts of environments. In the present report, we describe preliminary investigations of the possibility of gaining entry, in a manner *To whom correspondence should be addressed. Fax: þ61 3 9347 5180. Phone: þ61 8344 6469. E-mail:
[email protected] (R.R.); bfa@ unimelb.edu.au (B.F.A.). pubs.acs.org/crystal
Published on Web 02/18/2010
analogous to that described above for TCNQ derivatives, into new types of coordination polymers of DCNQI2- using the readily accessible DCNQIH2, IV, as the starting material.
When a solution of Mn(OAc)2 in methanol is diffused into a DMF solution containing DCNQIH2 and 4,40 -bipyridine (hereafter bipy), yellow crystals of composition [Mn(DCNQI)(bipy)]2DMF form.4 Although the crystals appear indefinitely stable in the mother liquor, crystallinity is lost upon their removal from the solution. This loss of crystallinity appears to be associated with the loss of DMF from the crystal, a supposition supported by thermogravimetric analysis, which reveals the absence of solvent in the sample (see Supporting Information). Crystals, suitable for X-ray diffraction studies, can be protected from solvent loss by transferring the crystals directly from the mother liquor into a protective oil and immediately cooled. The crystal structure5 reveals coordination polymer networks in the form of two-dimensional (2D) sheets (an example of which is shown in Figure 1a), linked together by bipy pillars as shown in Figure 1b. As can be seen in Figure 1a the DCNQI2- units, all of which are equivalent, are attached to four equivalent Mn centers each of which lies on a center of symmetry. The attachment of the two terminal nitrogen donors to metal centers is a common feature in the DCNQI/metal derivatives reported in the literature, but the additional attachment of metal centers, seen here, to the two “elbow nitrogen atoms” (i.e., the ones directly attached to the C6H4 ring) is unusual. To the best of our knowledge, coordination of the elbow nitrogen atom has not previously been observed in the r 2010 American Chemical Society
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Figure 2. The 3D structure of [Mn(DCNQI)(bpe)]2DMF. Color code: Mn purple, O red, N blue, C black. Hydrogen atoms have been omitted.
Figure 1. (a) A [Mn(DCNQI)]n sheet in the structure of [Mn(DCNQI)(bipy)]2DMF. (b) Sheets linked together by 4,40 -bipyridine pillars in the 3D structure of [Mn(DCNQI)(bipy)]2DMF. Disordered DMF molecules occupy the channels. Color code: Mn purple, O red, N blue, C black. Hydrogen atoms have been omitted.
structurally characterized metal derivatives of either unsubstituted DCNQI or a wide range of substituted DCNQIs, except for one example in which a thioether substituent in the C6 ring forms a bond to a Agþ cation, which also then interacts with the adjacent elbow nitrogen to form a chelate ring.6 In the relatively few examples of metal/DCNQI2- associations so far reported, all of which are nonpolymeric, the elbow nitrogen atom does not interact with the metal.7 As can be seen in Figure 1, eight-membered rings (viz. Mn-N-C-N-Mn-N-C-N-) are formed and at the center of each is a center of symmetry. Within these eight-membered rings, the terminal nitrogen atom of one DCNQI component makes very close contact with the cyanamide carbon of its partner (C 3 3 3 N, 2.92 A˚). The penta-atomic Mn-N-CN-Mn system is essentially planar, and those at opposite ends of a DCNQI unit are essentially coplanar, the C6 rings being inclined at ca. 20° to this common plane. The Mn centers are in a distorted octahedral coordination environment of six nitrogen donors. The cis angles at the metal are in the range 85.0-95.0° and the Mn-N bond distances range from 2.16 A˚ (to the terminal DCNQI nitrogen atom) to ca. 2.36 A˚ (the other four Mn-N bonds) - a coordination geometry typical of MnII in a pseudo-octahedral N6 environment. DMF molecules, disordered over two orientations sharing a common position for the oxygen atom, occupy the spacious channels, as can be seen in Figure 1b.
When a solution of Mn(OAc)2 in methanol is diffused into a DMF solution containing DCNQIH2 and 1,2-bis(4-pyridyl)ethylene (bpe), crystals of composition [Mn(DCNQI)(bpe)]2DMF form, slightly more orange in color than the yellow bipy-derived crystals referred to above.4 The crystal structure, determined by single crystal X-ray diffraction,8 is very similar to that of the bipy derivative above, with almost identical, slightly undulating [Mn(DCNQI)] sheets connected together by bpe pillars, that are more inclined to the average plane of the sheets than are the bipy pillars, as can be seen in Figure 2. Again, disordered DMF molecules occupy the channels. With regard to future syntheses of further DCNQI2- coordination polymers, it is useful to know, as the present work indicates, that a base as weak as the acetate ion is sufficient to doubly deprotonate the DCNQIH2, provided the metal cation is on hand to bind the DCNQI2- formed. The length of the CN bond between the C6 ring and the elbow nitrogen, which we refer to below as CN bond a, appears to be the structural parameter most sensitive to the formal negative charge associated with the DCNQI unit in a wide range of derivatives.7 In the DCNQI2- dianion, the C6 ring is essentially benzenoid and the CN bond a is essentially single, whereas this bond acquires quinonoid character in the neutral and radical monoanionic forms when it would be expected to be shorter. In a wide range of DCNQIn- derivatives (n = 0.5-1), the lengths of CN bond a are in the range 1.31-1.35 A˚, whereas for binuclear DCNQI2-bridged metal derivatives the range is 1.33-1.47 A˚.7 The essentially single CN bonds a in the DCNQI2- compounds reported in this paper have lengths close to 1.43 A˚. We note that the range of metal/DCNQI coordination polymers reported in the literature is surprisingly narrow in view of the intense research devoted to this area. The vast majority of crystallographically characterized DCNQI/metal coordination polymers contain either d0 or d10 metal centers (such as alkali metal cations, CuI, AgI, TlI), whereas potentially more interesting metal centers with incompletely filled d shells and unpaired electrons are conspicuously rare, the only examples being a MnII porphyrin9 and a CuII macrocyclic derivative,10 both with DCNQI μ2 bridges axial to the macrocyclic system. In conclusion, as with our work with the TCNQ2- systems, the preliminary results presented in this report indicate that the DCNQIH2 approach will open up for synthesis and study many new types of metal/DCNQI coordination polymers with new structural features and with a much wider range of metal
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components. An attractive feature of the DCNQIH2 approach is the possibility, arising from the “high” two minus charge on the ligand, of generating many types of previously unknown anionic coordination networks [(Mnþ)x(DCNQI2-)y]m-; in such a case the countercation could be a crucial structural determinant and choice of that cation may therefore afford a means of controlling the structure of the anionic network. Generally, the DCNQI component in known coordination polymers (a) has an average oxidation state in the range -1 to 0, (b) is almost always involved in close π-π associations, and (c) almost never associates with metal centers via its elbow nitrogen. The compounds described in the present report differ from previous examples in regard to all three of these criteria - the DCNQI component (a) is in oxidation state -2, (b) is not in any π-π association, and (c) does associate with metal centers via its elbow nitrogen. One of our longer term objectives is to subject structurally characterized coordination polymers of DCNQI2- to oxidative intercalation (chemical or electrochemical) to generate materials in which, we hope, the initial coordination network remains intact and in which the DCNQI2- component has been oxidized to the radical anion state (DCNQI•-) or to a mixed (DCNQI•-)x(DCNQI2-)y or (DCNQI•-)x(DCNQI0)y state. With regard to electronic communication between DCNQI components in such mixed oxidation state systems, the very close proximity, reported above, of the NCN side arm of one DCNQI unit to that of a neighbor may have interesting consequences. These would be systems containing DCNQI•- radicals in entirely new sorts of geometrical arrangements and chemical environments; their properties could be fascinating. Acknowledgment. The authors gratefully acknowledge the financial support of the Australian Research Council. Supporting Information Available: Crystallographic data (in CIF format) for compounds [Mn(DCNQI)(bipy)]2DMF and [Mn(DCNQI)(bpe)]2DMF and thermogravimetric information are available free of charge via the Internet at http://pubs.acs.org.
References (1) An entire issue of Chemical Reviews was devoted to molecular conductors: Chem. Rev. 2004, 104, 4887-5782. (2) Abrahams, B. F.; Hudson, T. A.; Robson, R. Cryst. Growth Des. 2008, 8, 1123. (3) Suchanski, M. R.; Van Duyne, R. P. J. Am. Chem. Soc. 1976, 98, 250.
Abrahams et al. (4) Synthesis of [Mn(DCNQI)(bridge)] 3 2DMF: A solution of Mn(OAc) 3 4H2O (155 mg, 0.632 mmol) in MeOH (15 mL) was diffused into a solution of DNCQI H2 (100 mg, 0.632 mmol) and 4,4-bipy (99.0 mg, 0.632 mmol) or bpe (115 mg, 0.632 mmol) in DMF (3 mL). Yellow crystals separated from the solution overnight, which were collected, washed with MeOH, and dried in air. Anal. Calcd for C18H15MnN6O1.5, that is, [Mn(DCNQI)(bipy)] 3 1.5H2O: C, 54.8; H, 3.8; N, 21.3. Found: C, 54.9; H, 4.1; N, 21.5. (5) Crystal data for [Mn(DCNQI)(bipy)] 3 2DMF: Mr 513.47, triclinic, P1, a = 5.5655(3) A˚, b = 9.3702(5) A˚, c = 11.7543(5) A˚, R = 94.321(4)°, β = 96.734(4)°, γ = 100.837(4)°, V = 594.91(5) A˚3, Z = 1, Fcalcd = 1.433 Mg/m3, μ = 4.835 mm-1, λ(CuKR) = 1.54184 A˚, F(000) = 267, yellow rod, 0.29 0.20 0.13 mm3, θmax = 67.49°, T = 130(2) K, no. of measured (and independent) reflections 5533 (2129), wR2 (all data) = 0.1274, R1 [I > 2σ(I)] = 0.0418, max/min residual electron density: 0.624/-419 e A˚3. (6) Mori, T.; Inokuchi, H. Chem. Lett. 1990, 2077. (7) (a) Kato, R.; Kobayashi, H.; Kobayashi, A. J. Am. Chem. Soc. 1989, 111, 5224. (b) Aumuller, A.; Erk, P.; Klebe, G.; Hunig, S.; von Schutz, J. U.; Werner, H. P. Angew. Chem., Int. Ed. 1986, 25, 740. (c) Miyasaka, H.; Campos-Fernandez, C. S.; Galan-Mascaros, J. R.; Dunbar, K. R. Inorg. Chem. 2000, 39, 5870. (d) Ouyang, X.; Campana, C.; Dunbar, K. R. Inorg. Chem. 1996, 35, 7188. (e) Mori, T.; Inokuchi, H. Chem. Lett. 1990, 2077. (f) Hunig, S.; Meixner, H.; Metzenthin, T.; Langohr, U.; von Scgutz, J. U.; Wolf, H. C.; Tillmanns, E. Adv. Mater. 1990, 2, 361. (g) Fabre, M.; Jaud, J.; Hliwa, M.; Launay, J. P.; Bonvoisin, J. Inorg. Chem. 2006, 45, 9332. (h) Aquino, M. A. S.; Crutchley; Lee, F. L.; Gabe, E. J.; Bensimon, C. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1993, 49, 1543. (i) Evans, C. E. B.; Yap, G. P. A.; Crutchley, R. J. Inorg. Chem. 1998, 37, 6161. (j) Aquino, M. A. S.; Lee, F.; Gabe, E. J.; Greedan, J. E. J. Am. Chem. Soc. 1992, 114, 5130. (k) Rezvani, A. R.; Bensimon, C.; Cromp, B.; Reber, C.; Greedan, J. E.; Kondratiev, V. V.; Crutchley, R. J. Inorg. Chem. 1997, 36, 3322. (8) Crystal data for [Mn(DCNQI)(bpe)] 3 2DMF: Mr 539.50, triclinic, P1, a = 5.6153(3) A˚, b = 9.3336(5) A˚, c = 13.3398(7) A˚, R = 106.030(4)°, β = 93.611(4)°, γ = 102.050(4)°, V = 651.73(6) A˚3, Z = 1, Fcalcd = 1.375 Mg/m3, μ = 4.442 mm-1, λ(CuKR) = 1.54184 A˚, F(000) = 281, yellow rod, 0.22 0.15 0.10 mm3, θmax = 69.98°, T = 130(2) K, no. of measured (and independent) reflections 5270 (2450), wR2 (all data) = 0.1546, R1 [I > 2σ(I)] = 0.0602, max./min residual electron density = 1.10/-0.556 e A˚3. Both structures were solved using direct methods and refined using all data in full-matrix least-squares procedure.11 Crystallographic information files (CIF) can be found in the Supporting Information. (9) Sugiura, K.; Mikami, S; Johnson, M. T.; Raebiger, J. W.; Miller, J. S.; Iwasaki, K.; Okada, Y.; Hino, S.; Sakata, Y. J. Mater. Chem. 2001, 11, 2152. (10) Oshio, H. Inorg. Chem. 1993, 32, 4123. (11) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Analysis (Release 97-2); Insitut fur Anorganische Chemie der Universitat: Gottingen, Germany, 1997.
