J. Phys. Chem. 1986, 90, 4199-4201
4199
A Two-Carrier Fast Ion Conductor: Coumarin-Alkali Iodide-Iodlne Complexes C. Wu and M. M. Lab-* Department of Chemistry, Temple University, Philadelphia, Pennsylvania 191 22 (Received: November 12, 1985; In Final Form: February 18, 1986)
By placing samples of solid complexes of coumarin with alkali iodides and iodine between different concentrations of electrolytes, it was ascertained that both the alkali and iodide ions are mobile. Thus a typical complex such as C4Rb14(where C = coumarin) has an ac (1 kHz) conductivity of approximately 0.04 i2-l cm-l and conducts by the migration of both Rb+ and I-. The organic lattice framework is responsible for this unusual behavior. In contrast, complexes of benzophenone with alkali iodide and iodine are cation conductors.
Introduction Solid complexes of carbonyl compounds such as benzophenone or coumarin with alkali iodides and iodine, although known since the early 19OOs,' did not attract attention until their interesting crystallography was mentioned in a 1974 thesis by Kapom2 Subsequently, we reported that these complexes showed both electronic and fast ion cond~ctivity,~,~ and crystallographic studies of the benzophenone and coumarin poly(iodide) complexes were reported by Coppens et al., giving a complete description of the three-dimensional s t r ~ c t u r e . ~These ,~ complexes feature both cationic and anionic columnar arrays. A schematic describing cross sections of the structures is given in Figure 1. Attempts at directly measuring ionic transport through these solids have been only qualitatively successful. Faradaic experiments using platinum electrodes or solid proton injecting electrodes have given evidence of product at both cathode and anode. However, the degree of transport changed from 5 to 70% from sample to sample and with current density in an irreproducible manner? A benzophenone complex was sectioned into multiple layers, and an iodine concentration gradient was found, but the results were again very irreproducible.3 In this work, transport through these complexes was studied by utilizing two solution electrolyte contacts a t different concentrations. It was our hope to be able to identify quantitatively the degree of charge transport by following concentration changes, but this required long-term stability of the complexes in contact with the aqueous electrolyte. Unfortunately, the solids show signs of significant degradation after 1 or 2 h contact with the electrolyte. The instantaneous potentials, on the other hand, were quite reproducible and scaled with the concentration gradients. The unique feature of these complexes is the possibility that, by virtue of the organic framework, both cations and anions are mobile. It is the purpose of this paper to demonstrate that such a situation is indeed realized in the coumarin complexes of the alkali iodides and iodine, but not in the benzophenone complexes of the same species. Experimental Section The organic carbonyl compound, alkali iodide, and iodine are simply mixed in stoichiometric amounts in methanol, heated until
all ingredients dissolve, and cooled slowly to room temperature. Slow solvent evaporation gives well-defined crystals. Details of the stoichiometry of these complexes have been presented in previous papers. A concentration cell is made by using a 13-mm-diameter X -2.0-mm-thick pelletized sample of the complex, made with a die typically used in preparing KBr disks for infrared spectroscopic studies. The pellet is placed between two plastic chambers with 10-mm holes on the side of each chamber. Each chamber is 1 X 2 X 2 in. and contains the appropriate aqueous electrolyte and electrode. The potential difference between the two chambers was monitored with a Keithley 616 electrometer.
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Results In a concentration cell in which there is no liquid junction potential, the emf depends solely on the difference in concentration (activities) of two solutions of the same electrolyte. When the cell operates, the concentrations of the electrolyte move toward equalization; that is, there must be a transport mechanism for both cation and anion. The electrodes in such solutions are chosen to be reversible with respect to one ion, and the contact between the solutions must allow the migration of the other ion (or both ions). If a fast ion conductor is chosen as the barrier between the two concentrations of electrolyte, then one can determine if a given charge carrier is transported through the barrier. The emf of the cell should follow the Nernst relationship: E = nF In
Thus if one plots In C2/Cl (assuming activity concentration) vs. E for a given electrolyte and obtains a good comparison with the Nernst slope, it is reasonable to conclude that the ion in question is a charge The following cells were chosen and will be designated by the indicated Roman numeral in the discussion that follows. The coumarin series of complexes is designated by the symbol CIMI, where M is the (variable) alkali cation, and the benzophenone series is designated by the symbol BIMI. Cell I .
anode (1) Clover, A. M. J. Am. Chem. Soc. 1904, 31, 256. (2) Kapon, M. Ph.D. Thesis, Technion Institute of Technology, Haifa, Israel, 1974. Herbstein, F. H.; Kapon, M. Nature (London)1972, 239, 153.
(3) Labes, M. M.; Jones, M.; Kao, H. I.; Nichols, L.; Hsu, C.; Poehler, T. 0. Mol. Cryst. Liq. Crysr. 1979,52, 115. Kao, H. I. Ph.D. Thesis, Temple University, Philadelphia, PA, 1978. (4) Wu, C.; Kim, B.; Kao, H. I.; Griffin, C. W,; Jones, M.; Labes, M. M. Mol. Cryst. Liq. Cryst. 1982, 88, 317. (5) Coppens, P.; Leung, P. C.W.; Ortega, R.;Young, W. S.; Laporta, C. J. Phys. Chem. 1983,87, 3355. ( 6 ) Coppens, P.; Li, L.;Petricek, V.; White, J. A. Synth. Mer. 1986, 14, 215.
0022-3654/86/2090-4199$01.50/0
(:)
- -' I-
K+
CI
I- cathode
c 2
In cell I, anion imbalance can lead to I- injection or removal from the electrodes, provided that cations can flow through CIMI. The passage of 1 F through the cell causes a net change in the (7) Yde-Anderson, S.; Lundsgaard, J. S.;Malling, J.; Jensen, J. SolidSrare Ionics 1984, 13, 8 1. ( 8 ) Croce, F.; Cigna, G. Solid Stare Ionics 1982, 6, 201.
0 1986 American Chemical Society
4200 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
a
Wu and Labes
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-E
80
W
0.4
b
I
I
1
I
I
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Log (Cl/CZ) Figure 2. Potential vs. log C,/C2 for a series of coumarin complexes using cell I: (A)Li; (X) Na; (0) K (0)Rb; (V) Cs. Solid line represents Nernst potential calculated from E = 0.059 log C , / C , .
dolo
130 r 110
90
5
-E
70
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Figure 1. Views of the cross section of BIMI and CIMI complexes according to crystal determinations of Coppens et al.5*6(a) Projection (001) of the structure of the potasium and ammonium benzophenone iodine complexes. The cations are in a column at the center of the benzophenone oxygen atoms. Benzophenone molecules in a wand layer are in staggered positions and complete an antiprismatic coordination of the cations. (b) Projection of the structure of CIRbI down the c axis. Large circles denote minor iodine sites. The frame indicates projections of the a and b translation vectors on the plane perpendicular to the c axis. The oxygen atoms of the coumarin molecules are indicated as filled circles.
30 10
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I
I
I
I
I
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1.0
1.4
1.8
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Log (CdCz) Figure 3. Potential vs. log C,/C2 for a series of benzophenone complexes using cell I: (A)Li; (+) Na; ( 0 )K; (m) Rb; (V)Cs; ( 0 )NH4. Solid line represents Nernst potential calculated as in Figure 2. 140 r
concentration as follows (n+ is the transport in aqueous solution): anolyte by electrode process -Iby ionic migration -n+M+ + (1 - nt)Inet change -ntMI
number of the cations catholyte -1-
+n+M+ + (1 - nt)I+n+MI
V
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- -
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-
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In cell 11, since Zn2+ can be injected from the anode and removed a t the cathode, a Nernst potential will only be achieved provided that I- transports through CIMI. Cell III. Zn(ZnBrz(C1)ICIMI(ZnBrz( C2)lZn Since we do not expect Br- or Zn2+to move through the CIMI lattice, no Nernst potential is anticipated in such a cell. Such a cell will then provide a check of any trivial potential such as ion transport around the edges of the pellet of CIMI. Similar cells were also tested for the benzophenone complexes “BIMI”. The anticipated results, then, are as follows: A Nernst potential should be observed in cell I if M+ can migrate through the solid electrolyte and in cell I1 if I- can (also) migrate. No Nernst potential should be observed in cell 111. Figures 2 and 3 show results for cell I for a series of coumarin and benzophenone complexes. For all samples, a Nernst potential
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;
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Log (Cl/CZ) Figure 4. Potential vs. log C,/C2 for BIMI and CIMI complexes using cell 11: (A)CiLiI; ( X ) CINaI; (0)CIKI; (0)CIRbI; (V) CICsI; (0) CINH41;( 0 )BIKI; (A) BILiI. Solid line represents Nernst potential using E = 3/2(0.059)log C,/C,. is observed up to a concentration ratio of approximately 50:1, beyond which there is a deviation from Nernst behavior. It appears that cations can migrate in both series of complexes. However, there is a clear distinction between these two series of complexes with respect to I- migration as is shown in Figure 4. CIMI complexes allow the migration of I-, whereas BIMI
J. Phys. Chem. 1986,90, 4201-4205 complexes are not I- carriers. Measurements of such complexes in cell I11 show only a small concentration-independent polarization potential as is anticipated.
Discussion In our earlier work, it was concluded that BIMI and CIMI complexes are mixed ionic and electronic conductors and that the most likely conduction process involved hopping in the iodine sublattice. Complete crystal structure information on these materials was not available at that time, but Coppens et al.5*6have now determined a number of these structures. Both classes of compounds are best understood as consisting of two sublattices. The cations are contained in columnar arrays coordinated by the parts of the organic molecules. The poly(iodide) polar (44) moieties of the lattices are interacting with the hydrophobic (aromatic) portions of the organic molecules and again are columnar networks whose detailed structure varies from complex to complex. Both cation and anion columns show vacancies and/or deficiencies. In the benzophenone series, some solvent molecules may be incorporated into the cationic columns, but there is no evidence for that incorporation in the coumarin complexes. Thus the
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qualitative picture of these complexes is that a lattice exists, stabilized by organic molecules, in which both cation and anion are columnar and disordered. The first evidence that there are members of these series in which both carriers can indeed migrate is presented in this paper. Several details are unclear. It is puzzling that the BILiI complexes show cationic conductivity in light of their tetrahedral crystal structures. It is also unclear why the entire BIMI series shows no I- conductivity, whereas the entire CIMI series does show such conductivity, since in both series there are considerable variations in the precise iodide stacking arrangements. The existence of a class of inorganic ionic conductors in which an organic lattice is the important structural factor stabilizing the lattice is an interesting new approach to fast ion conductors. Extension of these types of structures to polymeric structures and attempts to create more stable complexes are under investigation. Acknowledgment. This work was supported by the National Science Foundation, Solid State Chemistry, under Grant No. DMR83-02329. Registry No. Benzophenone, 119-61-9.
Steric Effects In Electron-Transfer Reactions. 3. Use of Electrochemlcally Measured Dlffuslon Coefflclents To Estimate the Average Radii of Transition-Metal Complexes Carl A. Koval,* Michael E. Ketterer, and Cindy M. Reidsema Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 (Received: November 21, 1985)
The estimation of van der Waals radii for transition-metalcomplexes is critical for comparison of the electron-transfer reactivity of these molecules with theoretical predictions. Presently, these radii are usually estimated with X-ray crystallography and/or molecular models. When the complexes are not spherical or when the ligand structure has large voids, these estimates may not accurately represent the true average radius. We report the diffusion coefficientsfor a variety of transition-metalcomplexes in aqueous solutions containing low concentrations (0.05-0.12 M) of 1:l electrolytes. The diffusion coefficients, D,were measured electrochemically by utilizing a rotating glassy-carbon-disk electrode. Most of the complexes examined constitute structurally related redox series in which the complexes differ only in the size of the organic substituents bound to the ligands. The uncorrected diffusion coefficients range from 2.2 X 10" to 8.3 X lod cm2 s-l, while the average radii, ( r ) ,estimated from models range from 3.4 to 9.1 A. The use of the diffusion coefficients to calculate limiting ionic conductivities and average radii is discussed.
Introduction The detailed interpretation of rate constants for electron-transfer reactions in solution that involve diffusing species depends on the ability to estimate the size of the molecules involved. (See Discussion.) When these molecules have spherical shapes, the van der Waals or contact radius of this sphere can be estimated from space-filling (SF)models or crystallographic distances, if available. For nonspherical molecules, an average radius, ( r ) , is often calculated. This average radius represents the average distance from the redox center to the periphery of the molecule. The average radius is often determined by equating the volume of the molecule, calculated from the appropriate geometric formula, to the volume of a sphere. The average radius of the molecule is then the radius of the equivalent sphere. For example, the value of ( r ) for ellipsoidal molecules can be obtained with eq 1,
where d is the distance along the axes of the ellipsoid.' Even for spherical molecules, these methods for calculating ( r ) have a (1) Brown, G.
M.;Sutin, N. J. Am. Chem. Soc. 1979, 101, 883. 0022-3654/86/2090-4201$01.50/0
significant flaw because they do not account for voids in the molecular structure. For example, ( r ) for metal tris( 1,lOphenanthroline) complexes is usually taken to be the distance from the metal atom to the edge of the phenanthroline ligand, even though there are gaps between these ligands that could be penetrated by solvent molecules or small reactants. At present, there are no accepted experimental procedures for estimating values of ( r ) other than X-ray crystallography. In order to study distance effects, or more generally steric effects, in electron-transfer reactions, we have synthesized several series of transition-metal c o m p l e x e ~ . ~Within -~ each series, the complexes vary only in the size of the organic substituents that are bound to the ligands in positions remote from the donor atoms. Except for molecular size, all members of each structurally related redox series (SRRS) have similar physical properties, resulting in similar electron-transfer reactivity based on energetic factors. Therefore, any steric effects, which may be relatively small, can be identified and studied by examining the homogeneous and heterogeneous electron-transfer reactivity of these complexes. (2) Koval, C. A.; Pravata, R. L. A.; Reidsema, C. M. Inorg. Chem. 1984, 23, 545. (3) Koval, C. A.; Ketterer, M.E . J. Electroanal. Chem. 1984, 175, 236.
0 1986 American Chemical Society
