Acid/Base versus Redox Reaction-The Case of Chloranilic Acid and Hydrazine: A Supramolecular Decision? Olaf Ku¨hl*,† and Sigrid Goutal‡
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1875-1879
Institut fu¨ r Angewandte Photophysik, TU Dresden, D-01069 Dresden, Germany, and Institut fu¨ r Organische Chemie, TU Dresden, D-01062 Dresden, Germany Received April 18, 2005;
Revised Manuscript Received July 5, 2005
ABSTRACT: The reaction between the oxidative chloranilic acid and the reductive base hydrazine can in principle follow either of two pathways, a redox or a neutralization reaction. We anticipated the redox reaction and were quite surprised to obtain the salt instead. However, after the salt’s crystal structure was solved, it was quite obvious that the reaction is supramolecularly controlled by the formation of a three-dimensional charge-assisted hydrogen bond network between the “polymeric” hydrazinium cations and the parallel π-π stacks of chloranilate anions. Introduction Noncovalent bonding interactions play a major role in the formation of solid state structures. Although they are significantly weaker than covalent bonds, strong superstructures can be found when the existence of multiple donor and acceptor centers within the constituting molecules permit the formation of extended one-, two-, or three-dimensional networks.1,2 Judicious choice of hydrogen bonding or π-π stacking synthons allows a degree of structure control and prediction and is believed to facilitate engineering of materials with desirable properties for commercial applications soon.1,3-5 In particular, the formation of extended threedimensional networks based on multiple hydrogen bonding should render quasi-polymeric materials that feature drastically reduced solubility in all known solvents as one of the key properties. Such a material should form quickly and precipitate before the onset of any appreciable side reaction. In addition, examples are known where the formation of hydrogen bond networks is aided by electrostatic forces. These networks are known as charge-assisted hydrogen bond networks.6 The engineering of these charge-assisted hydrogen bonds to extended networks can be facilitated by the utilization of a Bronsted acid/ base pair for charge separation featuring several hydrogen bond donors and acceptors for the formation of the extended hydrogen bond network. To this purpose, we designed an experiment with two possible and forseeable outcomes, one based on the genesis of an extended three-dimensional hydrogen bonding network with charge separation and the other on a competing redox process that results in the complete decomposition of one of the starting materials (Scheme 1).The reaction of chloranilic acid (pKa1 ) 0.58, pKa2 ) 3.18)7 with a base is well documented and numerous examples whereby the resulting salt was structurally analyzed by X-ray diffraction techniques can be found in the literature.8-14 However, chloranilic * To whom correspondence should be addressed. Mailing address: Olaf Ku¨hl, TU Chemnitz, Anorganische Chemie, Strasse der Nationen 62, D-09111 Chemnitz, Germany. E-mail:
[email protected]. † Institut fu ¨ r Angewandte Photophysik. ‡ Institut fu ¨ r Organische Chemie.
Scheme 1. Possible Pathways for Redox (Top) and Neutralization (Bottom) Reaction
acid is equally well-known as an oxidizing agent,15,16 whereas hydrazine is a powerful reducing agent that can act as both a weak base and an acid against very strong bases such as NaH or NaNH2 (pKa1 ) 6.07, pKa2 ) 15.08 not feasible in water).17 It was expected that the reaction between the two molecules would proceed as reduction of chloranilic acid (0 ) 4 mV18) by hydrazine according to eq 1. As can be seen from the
redox potential, the oxidation of hydrazine should proceed better in a more basic solution but is more than sufficient at neutral pH. The alternative reaction requires the formation of an extended hydrogen bonding network without nitrogen evolution. Ideally, no electron transfer should occur. Chloranilic acid has, in its dianionic form, six hydrogen bonding acceptors, A (four oxygen and two chlorine atoms), whereas the hydrazinium cation has five hydrogen bonding donors, D, and one acceptor (the nitro-
10.1021/cg0501581 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/20/2005
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Ku¨hl and Goutal
Figure 1. Resonance structures for the chloranilate dianion.
gen atom of the amino group). In addition, chloranilate frequently forms π-π stacks that allow for the cations and solvent molecules to occupy the resulting channels forming hydrogen bonds with interstitional water and other D centers with all of its six possible acceptor atoms.7-9,19-21 For hydrazine, and the hydrazinium cation, structures are reported where the N2H5+ cation forms onedimensional tapes reminiscent of polymeric cations that would fit into the channels created by the chloranilate dianions.22-25 Such a polymeric cation would provide a stabilizing one-dimensional scaffold capable of forming three-dimensional networks with the anions through multiple hydrogen bonding. Whereas the bond energies of true covalent bonds are on the order of 400 kJ/mol, the energetic advantage due to intermolecular forces is much smaller. Cl-Cl interactions typically contribute only a few kilojoules per mole and even the comparatively strong O(N)-H‚‚‚O(N) hydrogen bonds contribute only 40-90 kJ/mol for a dimeric pattern.26,27 In extended 2D or 3D supramolecular networks, the additive nature of these energetic gains can accumulate to a total effect approaching or exceeding the strength of covalent bonds. Results and Discussion The reaction proceeded via the neutralization pathway. In a first step, hydrazine was protonated to the hydrazinium ion, NH2NH3+, and subsequently the resulting salt precipitated. No reduction was observed. The salt is practically insoluble in all organic solvents, water, and aqueous sodium hydroxide.28 The title compound, C6O4Cl2‚2NH2NH3, 1, crystallizes in the monoclinic crystal system in the space group P21/c with Z ) 2. The asymmetric unit consists of half the chloranilate anion and one NH2NH3+ cation.29-32 Within the crystal, the chloranilate ions form parallel stacks in the direction of the b axis with an interplane distance of 327 pm (Figure 2). The chloranilate dianions stack in a slipped fashion with the chlorine atoms directly above and underneath the C1 to C3 entity of adjacent molecules. In similar structures, the distance between two chloranilates ranges from 325 pm in chloranilic acid and chloranilic acid dihydrate to 337 pm in dipyridinium acetylide salts of chloranilic acid.7-9,19-21 Along the a axis, these stacks are parallel to each other. Along the c axis, however, these stacks are staggered by b/2 and are tilted by an angle of 80.7° toward each other. The bond lengths and angles within the chloranilate anions are quite unspectacular and within the range of similar structures.7-14,19 A chloranilate anion should essentially be a fully conjugated system (see Figure 1) having two main resonance structures. In reality, the carbon-carbon bond lengths are as follows: C1-C2 140.33(19) pm, C2-C3 154.23(18) pm, and C1-
Figure 2. The hydrogen bond network in 1.
C3# 139.41(18) pm. The respective carbon-oxygen bond lengths are C2-O1 123.89(16) pm, and C3-O2 125.34(16) pm. The difference in C-O bond lengths is significant but small. The structure of the chloranilate anion can therefore be described as two acetylacetonate units linked by a pair of carbon-carbon single bonds. This structural motif is common for all structurally determined chloranilate compounds.33 The packing of the chloranilate anions creates channels in the middle of parallelograms formed by four such stacks. These channels are filled by the hydrazinium ions in two antiparallel bands such that the NH2NH3+ cations form a zigzag ribbon along the b axis. The hydrazinium ions are held together by alternating N-H‚‚‚N hydrogen bonds between the nitrogen center of a NH2-terminus and a hydrogen atom of a NH3+terminus in the antiparallel hydrazinium column below it (hydrogen bonds 4 and 5 in Figure 2, for hydrogen bond lengths and angles see Table 1). Each NH2NH3+ cation acts simultaneously as a Lewis acid (with its NH3+-terminus) and as Lewis base (with its NH2terminus) against either of its two neighboring hydrazinium cations. The two hydrogen bonds formed by every hydrazinium ion with the neighboring NH2NH3+ cations of the antiparallel column lends this hydrazinium tape great stability. It has the appearance of a positively charged polymeric chain. The structural motif of a polymeric zigzag chain has been observed before for the hydrazinium cation.22-25 [N2H5]+2[N4C-NdNCN4]2-‚2H2O is a hydrazinium 5,5′-azotetrazolate whose hydrazinium cations form hydrogen bonds with themselves and the dianions in a manner similar to 1. The water molecules serve as hydrogen acceptors to a hydrogen from the NH3+-terminus. When the water molecules are substituted by additional hydrazine molecules, the polymeric tape structure of the cations breaks down in favor of hydrazinium-hydrazine interactions (Figure 3).22 The hydrazinium ions not only form hydrogen bonds among themselves but also with all the oxygen and chlorine atoms of the chloranilate anions. These hydrogen bonds serve to stabilize the stacks along their stacking axis (the b axis of the crystal), as well as linking neighboring stacks and stacks that are diagonally related. For a better understanding of these, the
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Table 1. Hydrogen Bond Lengths, Angles, and Numbering H-bond no. name length [pm] angle [deg]
1 H3′′′-O2 214.2 143.18
2 H3′′′-O1 215.4 125.05
3 H4′′′-O2′′′ 191.8 159.97
Table 2. Hydrogen Bond Connectivities between the Anionic Stacksa stack A B C D
A
B 1,2,7/6,8
C 1,2,7/10 6,8/10
D 1,2,7/3,9 6,8/3,9 10/3,9
a Hydrogen bonds 4 and 5 are invoked to stabilize the hydrazinium ribbons.
Figure 3. Molecular structure of 1sORTEP plot showing the numbering scheme. Thermal ellipsoids are drawn at 50% probability.
stacks are labeled A-D in Figure 2. Two molecules of stack A are bridged by a single hydrazinium ion using the hydrogen bonds 1, 2, and 7. Stack B is internally stabilized by the hydrogen bonds 6 and 8 to different NH2NH3+ cations. Since both these hydrogen bonds are formed by the same molecule in B, stabilization occurs via the hydrazinium backbone and ultimately through hydrogen bonds 4 and 5. The same is true for stack D where the hydrogen bonds to hydrazinium are 3 and 9 (9 is generated from 6 and 8 from 3 by a C2 axis along the b axis in the middle of the channel). Stack C is stabilized by a single hydrogen bond to the hydrazinium backbone, indexed as 10 (generated from 7). Diagonal and neighboring linking of stacks occurs across the hydrazinium backbone (for a listing see Table 2). The bond lengths of all hydrogen bonds are in the range expected for strong hydrogen bonds (O‚‚‚H hydrogen bond length typically 216-265 pm with NsH slightly and Cl‚‚‚H significantly longer).34-36 However, there are obvious geometric constraints explaining the wide range of bond lengths observed in 1 (23.6 pm for H-E‚‚‚H, E ) O, N). The main constraint is the size of the channel where the “polymeric” cation is located. Its dimensions are fixed by the closest approach of the chloranilate anions in the anionic sublattice (A-B and C-D Cl-Cl distance 347.8 pm; A-D and B-C O-O distance 333.7 pm). For comparison, the Cl-Cl distance in solid chlorine is 334 pm.37 Cl-Cl interactions are discussed for distances of 330-350 pm,36 the upper limit being the sum of van der Waals radii.38 Recently, bonding interactions are increasingly discussed for interatomic distances that exceed the sum of van der Waals radii by about 10%.39 In comparison, the O-O distance is relatively long and reflective of the anionic repulsion meaning that the negative charge is mainly
4 H5′′′-N1′′′′ 210.6 163.30
5 N1′′′-H5′′′′ 210.6 163.30
6 H2′′′-Cl1′′ 256.2 162.48
7 H1′′′-O1 198.5 167.37
located on oxygen. Since the “polymeric” cation forms hydrogen bonds to the neighboring anions in all directions, shortening one hydrogen bond means elongating several others. Thus, the existing set represents the best compromise. In other words, the two hydrazine molecules are a perfect match for the size of the channel created by the chloranilate moeities. Similar considerations hold true for the bond angles of these hydrogen bonds. They deviate significantly from linear with 3-8 ranging from 159.97° to 167.37°. For the hydrogen bonds 1 and 2, the situation is different. They are bridging hydrogen bonds where the hydrogen donor atom (N) cannot align linearly with both hydrogen acceptors (O) at the same time. However, in the present case the bridge is asymmetric and low in angles.34 Each stack forms hydrogen bonds to the hydrazinium cations of four adjacent cation channels. The individual contributions are different as can be seen from Figure 2 and Table 1, but the sum of all hydrogen bonding interactions for each chloranilate anion can safely be assumed to approach the strength of a covalent bond.40 In addition, the total effect is spread out over many individual interactions that have to break down collectively, if the total structure is to disintegrate completely. This statistical element, which is a property of the supramoloecular network, adds additional stability to the complex. The structure of 1 is reminiscent of that of chloranilic acid dihydrate. The hydrazinium cations take the place of the neutral water molecules and there is no proton transfer to water as can be deferred from the C-O bond lengths in chloranilic acid indicating a C-OH group.7,19 From an electrostatic point of view the structure of 1 consists of a B centered anionic sublattice with the “polymeric” cation occupying the two channels between the corners and the middle of the ac plane (see Figure 2). The electrostatic attraction is strengthened by the multiple hydrogen bonding between the anions and the cationic center. Similar host-guest hydrogen-bonded interactions forming three-dimensional networks are seldom reported in the literature.22,39,41 Treatment of the hydrogen bonding pattern by graph theory normally leads to a better understanding of the supramolecular ordering within the structure. In graph theory, ordering is achieved by determining a repetitive sequence formed by hydrogen donors and acceptors forming rings, chains, and discrete structures, abbreviated as R, C, and D, respectively. The number of hydrogen acceptors is indicated as superscript, the number of donors as subscript, and the number of atoms added in brackets.4,42 Thus, the hydrazinium backbone can be described as C22 (6). Applied to the entire structure, a large number of these patterns can be formulated. The main reason for this is that any atom in the structure of 1 is linked to any other atom by a series of covalent and hydrogen bonds. Rings include R21 (5) [1,2] and R33 (11) [3,5,9/6,4,8] and rings that go around a stack, for example, R1212 (40) [1,4,10/1,4,10/1,4,10/1,4,10] or R1212 (44) [2,4,10/2,4,10/
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2,4,10/2,4,10]. Chains include C22 (8) [6,7/2,3], C22 (9) [3,6/8,9], C33 (10) [1,4,10/7,5,9], C33 (11) [2,4,10], and C33 (12) [7,4,10/6,5,9]. Most of the chains and rings identified by graph theory make use of the hydrogen bonds 4 and 5 of the cationic polymer ribbon emphasizing the importance of the hydrazine system to the stability of the structure. Indeed, one can go from smaller chains and rings to larger ones almost at will, but only by incorporating the same seven basic hydrogen bonds. All others are generated from them by symmetry operations inherent to the crystal. Thus, identifying more intricate and complicated ring and chain patterns may be a marvelous application of graph theory, but it does not disclose new information for the understanding of the supramolecular structure displayed by 1. In fact, it confuses the facts already established. These are that the stability of the structure stems from the fact that all five hydrogen atoms of the NH2NH3+ cation form hydrogen bonds to O, N, and Cl atoms of neighboring molecules (anions and cations alike) incorporating every potential hydrogen acceptor on either ion and that the pair of hydrazinium cations per chloranilate anion is a perfect steric match for the channels created by the anions. In most other structures involving the chloranil molecule, the crystals contain water molecules that form hydrogen bonds to the oxygen atoms of the chloranilate. The exception is anhydrous chloranilic acid where there is no solvent molecule. Compound 1, however, does not incorporate water, although it is crystalized from aqueous solution. Hydrazine fits better into the structure and thus stabilizes the crystal more than water does. For this very reason, hydrazinium is not easily displaced, and 1 is scarcely soluble in most solvents (hydrocarbons, chlorinated hydrocarbons, acetonitrile, DMF, DMSO, alcohols, water) including concentrated NaOH. Likewise, the expected redox reaction involving the strong reducing agent hydrazine and the strong oxidizing agent chloranilic acid is prevented by very low concentrations of the free ions NH2NH3+ and chloranilate and thus by the exceptionally strong supramolecular forces within the crystal. Formation of the supramolecular network needs to be rapid to prevent the redox process to occur kinetically, even though it is less favored thermodynamically. And indeed, formation of the orange precipitate is immediate upon mixing of the two solutions.
Ku¨hl and Goutal
Acknowledgment. We thank Novaled GmbH Dresden, Germany, for financial support. Supporting Information Available: Crystallographic details in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28)
Conclusion We have shown that the expected redox reaction between the oxidizing agent chloranilic acid and the reducing agent hydrazine is efficiently prevented by the facile formation of a very stable supramolecular structure generated from the neutralization reaction between chloranilic acid and the base hydrazine. The key element of this supramolecular structure is the formation of a polymeric hydrazinium cation through hydrogen bonding. The three-dimensional charge-assisted hydrogen bonding network formed by the polymeric cation and the six hydrogen bond acceptors of the anion is responsible for the exceptional stability of the supramolecular structure and the remarkable resistance toward the redox reaction.
(29)
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