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Mixed-valence states of polynuclear iron complexes
Nuclear Instruments North-Holland
and Methods
in Physics Research
RIOMI B
B76 (1993) 408-414
Beam Interactions with Materials&Atoms
Mixed-valence
states of polynuclear
iron complexes *
Satoru Nakashima Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 724, Japan
The valence-delocalization in mixed-valence binuclear ferrocene derivatives is investigated by using several physicochemical results. It is demonstrated that the mixed-valence state is controlled by the environment in the solid state. The mechanism of the valence-delocalization in l’,l”‘-diethylbiferrocenium triiodide is discussed. The difference between the valence-delocalization accompanied by the symmetry change of the counter anion and the valence-delocalization without connection with the symmetry of the counter anion is discussed. The results are compared with those of oxo-centered trinuclear iron carboxylates.
1. Introduction Binuclear ferrocene derivatives and oxo-centered trinuclear basic iron carboxylates have been studied mainly by using 57Fe Miissbauer spectroscopy for elucidation of their electronic states in the solid state [l]. These complexes have been intensively investigated because some of them convert from being valencetrapped at low temperatures to valence-detrapped at high temperatures. It is also interesting because these complexes have the potential for application to materials science. An interesting example by using ferrocene derivatives is the case of decamethylferrocenium-tetracyanoethenide, in whcih ferromagnetism is observed at low temperatures [2]. On the basis of 57Fe MGssbauer spectroscopy, the mixed-valence states can be classified into two types of valence states of the iron atoms: one is a trapped-valence type and the other is an averaged-valence type. In the former, two different valence-states of iron atoms corresponding to ferrocene-like bivalent and ferrocenium-like tervalent iron atoms are involved, while only an equivalent valence-state of iron atoms is found in the latter. Some derivatives show valence-delocalization with increasing temperature. There are two types of valence delocalization in addition to the relaxation process. One is the fusing type and the other is the coexisting type. In the fusing type, two quadrupole-split doublets, observed at low temperatures, approach each other with increasing temperature to converge into one doublet without significant broadening of the half-width in “Fe MGssbauer spectra in the averaging process. In the coexisting type, two types of valence states coexist
* Invited presentation at the Third International Symposium on the Industrial Applications of the Massbauer Effect, Otsu, Japan, August 24-27, 1992. 0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
and the rate of intramolecular electron-transfer increases with increasing temperature. In the present paper, the packing effect to the mixed-valence states of a series of binuclear ferrocene derivatives as shown in fig. 1 is demonstrated and the mechanism of the valence-delocalization in l’,l”‘-diethylbiferrocenium triiodide is discussed by using several physicochemical results. The comparison of the results with those of oxo-centered basic iron carboxylates is also discussed.
2. Examples of the packing effect to mixed-valence states of a series of l’,l”‘-dialkylhiferrocenes The mixed-valence states of a series of l’,l”‘-dialkylbiferrocenes have been investigated using 57Fe MGssbauer spectroscopy [3]. The mixed-valence states cannot be explained by the electronic effect of the substituent. The mixed-valence states of l’,l”‘bis(methylbenzyl)biferrocenium triiodides are affected by the position (o-, m-, or p-) of the methyl group to the benzyl substituent [4]. The mixed-valence states are also affected by the counter anion, e.g., triiodide salt of l’,l”‘-dipentylbiferrocene shows temperature-independent trapped-valence state, while its 7,7,8,&tetracyanop-quinodimethane salt shows temperature-independent averaged-valence state [5]. These results cannot be explained by the nature of intramonocation alone. Monocation-monocation and/or monocation-anion interaction must be considered. The packing effect was confirmed by a dispersion experiment, e.g., l’,l”‘-diisobutylbiferrocenium triiodide in crystalline state shows valence-delocalization, while the salt dispersed in poly(methy1 methacrylate) shows the temperature-independent trapped-valence state [6]. The packing effect was also confirmed by mixed-crystals experiment [7] B.V. All rights
reserved
409
S. Nakashima / Mixed-valencestatesof Fe complexes
and by ESR samples [6].
experiment
of crystalline
and dispersed
2.1. Mixed-valence states of biferrocene derivatirles with long alkyl chains It is not easy to study the packing effect systematically because of the difficulty in design of the crystal structures. One of the approaches, however, is to use long alkyl substituents. The biferrocene derivatives with long alkyl chains are considered to have a layer structure. The structure was confirmed using X-ray powder patterns. The inter-layer lengths are summarized in fig. 2. The results of the samples obtained from the hexane solution are indicated by the triangles, while those of the samples which are recrystallized from the dichloromethane solution are indicated by the circles. Shortening of the inter-layer length by recrystallization from dichloromethane is observed in l’,l”‘-ditetradecyland l’,l”‘-dihexadecylbiferrocenium triiodide. Two samples of l’,l”‘-dioctadecylbiferrocenium triiodides from hexane and from dichloromethane belong to the crystal form with longer inter-layer length. 57Fe Mijssbauer spectra showed that the samples with a longer inter-layer length exhibit a temperature-independent trapped-valence state and the samples with a shorter inter-layer length show a fusing type valencedelocalization [8,9]. 2.2. Polymorphism and mixed-oalence states of l’,l’“-dibutylbiferrocenium triiodide l’,l”‘-Dibutylbiferrocenium triiodide recrystallized from dichloromethane gave two kinds of crystal forms. Needle-like crystals (crystals A) are found to show a temperature-independent trapped-valence state, while plate-like crystals (crystals B) show a fusing type valence-delocalization [6], as shown in fig. 3. The crystal structures for both crystal forms in l’,l”‘-dibutylbiferrocenium triiodide were analyzed [lO,ll]. They are shown in fig. 4. The structure of monocation for crystals A is asymmetric. On the other hand, in crystals B
IL
16
IS
“c
Fig. 2. Inter-layer distances (ref. [9]). The “nc” is the number of carbons in the alkyl chain. Triangles indicate the results of the samples obtained from hexane solution, while circles
indicate the results of the samples recrystallized dichloromethane solution.
there are two crystallographically independent monocations in a unit cell. Although the symmetry of the two monocations is better than that of crystals A, butyl groups of one of the two monocations in a crystal unit of crystals B are relatively asymmetric. The packing arrangements are also different from each other. The asymmetric interaction between monocation and asymmetric triiodide anion is found in crystals A, which agrees with the result showing a temperature-independent trapped-valence state. Although the interaction between monocation and anion was not symmetric in crystals B, an appreciable interaction seems to exist between the cyclopentadienyl ring and the butyl group of the adjacent monocation. We suppose that the asymmetric interaction between monocation and asymmetric triiodide anion causes the mixed-valence state to be a trapped-valence type and the intermonocation interaction in stack plays an important role for the valence-delocalization. It is considered that the role of the monocation-monocation interaction for the valence-delocalization overcomes the asymmetric monocation-anion interaction in crystals B.
3. Mechanism of valence-delocalization ylbiferrocenium triiodide [121
Fig. 1. Biferrocenium and disubstituted biferrocenium salts. R = H or alkyl substituents. Y = I, or (TCNQ), (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane).
from a
in l’,l”‘-dieth-
l’,l”‘-Diethylbiferrocenium triiodide shows a fusing type valence-delocalization [13]. l’,l”‘-Diethylbiferrocenium triiodide is a good compound to establish the mechanism of the valence delocalization, because so far polymorphism is not observed in l’,l”‘-diethylbifer-
S. Nakashima
410
states of Fe complexes
/ Mixed-valence
Temperature dependence of the quadrupole splittings is shown in fig. 5a. This shows that the rate of approach of the two doublets becomes drastic in the
rocenium triiodide and both the crystal structures at room temperature and at low temperatures are already reported [ 141.
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Velocily B (b) of l’,l”‘-dibutylbiferrocenium
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(ref. [6]).
411
S. Nakashima / Mixed-valence states of Fe complexes
- 150 to - 280 K range. It can be seen from fig. Sb that the slope of the logarithmic value of the relative area1 intensity is altered at - 200 and - 270 K. This fact indicates that there are changes in the state of lattice vibrations around those temperatures. This is interesting because temperatures 200 and 270 K are the starting and finishing temperatures for valence-delocalization, respectively. Variable temperature far infrared spectra of l’,l”‘diethylbiferrocenium triiodide are shown in fig. 6. A linear, symmetric triatomic ion such as I; has three fundamental modes of vibration; the symmetric stretching mode (vi), the doubly degenerate deformation mode (~a), and the asymmetric stretching mode (v,). Examination of the activity of the vi, vz, and vs bands for I; ion has a diagnostic value for the determination of the I; structure. Three peaks centered at 109, 58, and 147 cm-’ obtained at 93 K can be assigned to the vi, vz, and vs mode, respectively. The intensity of the vi mode in the far infrared spectra decreases with increasing temperature and finally, at 298 K, the peak due to the vi mode disappears. The result shows that the asymmetric I; at low temperatures becomes symmetric with increasing temperature. Variable temperature Raman spectra agree with the results of far infrared spectra. The temperature region where the asymmetric I; becomes symmetric corresponds to that of valence-delocalization of monocation. It can be concluded that the intramolecular electron-transfer in l/,1”‘-diethylbiferrocenium cation is coupled with a symmetry change of the I; anion. However, the results of s7Fe Mossbauer, far infrared, and Raman spectra are in conflict with the results of X-ray structural
(a) 2.0 Ferrocene-like
iron
”
Ferrocenium-1 0.5
ike
iron
--•
r/K
Fig. 5. Temperature dependence of the quadrupole splittings (a) and of area1 intensity relative to that at 78 K (b) for l’,l”‘-diethylbiferrocenium triiodide (ref. [12]).
b)
a 1
Fig. 4. The structure for crystals A (a) and crystals B (b) of l’,l”‘-dibutylbiferrocenium
triiodide (ref. [ll]).
analysis at 140 K. X-ray structural analysis shows that both the monocation and the triiodide anion sit on a crystallographic center of symmetry, both at 298 and at 140 K [14]. One of several explanations is that the X-ray structural analysis at 140 K might reflect a statistical average of the asymmetric structure. In the case of biferrocenium triiodide, far infrared and Raman spectra showed that the triiodide ion remains symmetric from low temperatures to room temperature 1151, whereas the 57Fe Miissbauer spectra showed that the monocation converts from being in a valence-trapped state at low temperatures, to a dynamically interconverting valence-detrapped state at high temperatures. One may ask what is the difference between the valence-delocalization accompanied by the symmetry change of the counter anion and the valence-delocalization without a connection with the symmetry of the counter anion. One of several possible explanations is that the structural difference between
412
S. Nakashima / Mixed-valence states of Fe complexes
biferrocenium triiodide and l’,l”‘-diethylbiferrocenium triiodide and thus the difference in crystal packing might be responsible for the way of valence-delocalization. In the biferrocenium triiodide crystal, the monocations and triiodide anions are stacked in independent columns [lo]. On the other hand in l’,l”‘-diethylbiferrocenium triiodide crystal, each monocation is surrounded by triiodide anions and vice versa. Therefore the difference between biferrocenium triiodide and l’,l”‘-diethylbiferrocenium triiodide is that the monocations in the former can interact directly while the monocations in the latter need to interact through the triiodide ions. The importance of intermonocation interaction was discussed in section 2.2. The molar heat capacity at constant pressure of l’,l”‘-diethylbiferrocenium triiodide was measured. Excess heat capacity, AC,, of l’,l”‘-diethylbiferrocenium triiodide beyond the normal heat capacity curve was calculated by an effective frequency distribution method [16]. There exists a sharp heat-capacity peak centered at 67.2 K and a broad anomaly over a wide temperature region above - 200 K. The broad anomaly corresponds to the temperature region where the “Fe Miissbauer spectra are altered from a trapped- to an averaged-valence state and the triiodide ion is altered from an asymmetric to a symmetric structure. The
J
Fig. 6. Variable temperature far infrared absorption spectra for l’,l”‘-diethylbiferrocenium triiodide in the 300-30 cm-’ range. The vl, Y*, and vj indicate the symmetric stretching, deformation, and asymmetric stretching modes of the I; ion, respectively (ref. [12]).
(A)
(B)
E(Q) I
E (Q)
Fig. 7. Adiabatic
potential energy surface, E(Q), for the l’,l”‘-diethylbiferrocenium monocation plotted as a function of the out-of-phase combination (Q = QA- Q,) of hvo symmetric metal-ligand breathing vibrational modes (QA and Q,) of the two halves of a binuclear mixed-valence species (ref. [17]). Diagram A is for a symmetric mixed-valence complex in the absence of environmental effects. Diagram B results if the environment about the binuclear mixed-valence complex is asymmetric.
entropy gain above - 200 K is quite small. AC, at 300 K is about 3-4 JK-’ mol-‘. The value is very close to the maximum value of a two-level Schottky anomaly, in which the degeneracies of the ground and excited states are identical. The infrared spectrum at room temperature shows two independent CH bending modes which are ascribed to the ferrocene and ferrocenium parts, although 57Fe Mijssbauer spectroscopy cannot distinguish the two irons at this temperature. Therefore, the adiabatic potential energy surface of the monocation is characterized not by one minimum but by two minima. The two vibronic states are shown as [FegFeg’] and [Fei’Feg]. Kambara and Sasaki [17] derived the energy difference by intramolecular coupling, where both the ground and excited states correspond to the delocalized electron-tunneling splitting as shown in fig. 7a. At low temperatures, however, since the I; ion has an asymmetric form and thus the environment about the mixed-valence monocation is asymmetric, the electronic energy of either [Fe:FeE’] or [Fez’Feg] would be low in comparison to that of the remaining one. Thereby, the adiabatic potential energy surface would have the shape shown in fig. 7b. With increasing sample temperature, the energy difference, A E, of scheme (B) in fig. 7 is cooperatively reduced as the valence-delocalization proceeds through coupling with a change in the symmetry of the I; ion. And finally when AE becomes nearly equal to the energy difference, ho, of scheme (A) in fig. 7, the adiabatic potential surface would be altered from scheme (B) to (A). In such a scheme, characterized by a tunneling splitting, one can expect a heat capacity anomaly of the Schottky type.
S. Nakashima / Mixed-uaience states of Fe complexes
-1
*
100
300
200
400
I
500
T/K
Fig. 8. Semi-logarithmic plot of the excess heat capacity, AC,, of l’,l”‘-diethylbiferro~enium triiodide (ref. [121>.The broken line indicates the heat capacity anomaly calculated on the basis of a Schottky anomaly due to tunneling-splitting, in which the energy difference is 750 cm-‘. The energy difference has been determined so that the two shaded areas and thus the entropy gains are almost equal.
Fig. 8 illustrates the excess heat capacity against the loga~thmic temperature scale. In this figure, area corresponds to entropy. As shown by a broken curve in fig. 8, a Schottky heat capacity curve was determined so that the areas of two shaded portions become equal. In this case, the energy difference, hw, was estimated to be 750 cm-‘. The problem, however, is that 750 cm-’ is too large to be due to tunneling splitting.
413
triangular form. Therefore, the deformation of a molecule in the crystal lattice is much more difficult in the oxo-centered trinuclear basic iron carboxylates than the bin&ear ferrocene complexes. In mixed-valence binuclear ferrocene derivatives, a slight change in the symmetry of the anions can affect the electronic state of the cations. On the other hand, since the trinudear oxo-centered basic iron carboxylates complexes consist of neutral molecules, there does not exists such an electrostatic interaction. However, those oxo-centered trinuclear basic iron carboxylates complexes, which exhibit temperature-dependent valence-deIocalization, contain solvate molecules. Orientational disordering of these solvate molecules cooperatively couples with the intramolecular electron-transfer event. One of the exceptional cases hitherto reported is oxo-centered trinuclear basic iron complexes having long alkyd chains 1201. In this case, cooperative conformational change in the long alkyl chains might play the role of solvate molecules.
Acknowledgements The author would like to thank Professor II. Sano of Tokyo Metropolitan University, Professor M. Konno of Ochanomizu University, and Professor M. Sorai of Osaka University for kind guidance and valuable discussion.
References 4. Comparison
of mixed-valence binuclear ferrocene derivatives with oxo-centered trinuclear basic iron carboxylates
The situation for the valence delocalization of the mixed-valence binuclear ferrocene derivatives is quite different from that for oxo-centered basic iron carboxylates. In the case of trinuclear oxo-centered basic iron carboxylates complexes, the valence-delo~l~ation occurs through phase transition(s) [18], while for l’,l”‘-diethylbiferrocenium triiodide it proceeds without a phase transition. Although in biferrocenium triiodide [151 and l’, l‘~-diiodobife~ocenium hexa~uoroantimonate [19] the phase transition is observed, the entropy gains are much smaller than those in oxo-centered basic iron carboxylates. When the intramolecular electron-transfer occurs, the molecular structure is inevitably distorted from its original form. In the binuclear ferrocene monocations, this distortion is established by the aid of the antisymmetric breathing vibrational mode in binuclear ferrocene derivatives, whereas in oxo-centered trinuclear basic iron carboxylates the distortion is accompanied by a drastic change in its
[l] H. Sano, Hyperfine Interactions 53 (1990) 97. [2] J.S. Miller, J.C. Calabrese, H. Rommelmann, S.R. Chittipeddi, J.H. Zhang, W.M. Reiff and A.J. Epstein, 3. Am. Chem. Sot. 109 (1987) 769. [3] S. Nakashima, M. Katada, I. Motoyama and H. Sano, Bull. Chem. Sot. Jpn. 60 (1987) 2253. [4] S. Nakashima, M. Katada, I. Motoyama and H. Sano, Bull. Chem. Sot. Jpn. 59 (1986) 2923. [S] S. Nakashima, S. Iijima, I. Motoyama, M. Katada and H. Sano, Hyperfine Interactions 40 (1988) 315. [6] S. Nakashima, Y. Masuda, I. Motoyama and H. Sano, Bull. Chem. Sot. Jpn. 60 (1987) 1673. [7] S. Nakashima and H. Sano, Buti. Chem. Sot. Jpn. 62 (1989) 3012. [81 S. Nakashima and H. Sano, Chem. Lett. (1989) 1075. [9] S. Nakashima and H. Sano, Hyperfine Interactions 53 (1990) 367. [lOI T.-Y. Dong, D.N. Hendrickson, K. Iwai, M.J. Cohn, S.J. Geib, A.L. Rheingold, H. Sano, 1. Motoyama and S. Nakashima, J. Am. Chem. Sot. 107 (1985) 7996. [ill S. Nakashima, M. Konno and H. Sane, Hyperfine Interactions 68 (1991) 205. 1121 S. Nakashima, A. Nishimori, Y. Masuda, H. Sano and M. Sorai, J. Phys. Chem. Solids 52 (1991) 1169.
414
S. Nakashima / Mixed-valence states of Fe complexes
[13] S. Iijima, R. Saida, I. Motoyama and H. Sano, Bull. Chem. Sot. Jpn. 54 (1981) 1375. [14] M. Konno and H. Sano, Bull. Chem. Sot. Jpn. 61 (1988) 1455. [15] M. Sorai, A. Nishimori, D.N. Hendrickson, T.-Y. Dong and M.J. Cohn, J. Am. Chem. Sot. 109 (1987) 4266. [16] M. Sorai and S. Seki, J. Phys. Sot. Jpn. 32 (1972) 382. [17] T. Kambara and N. Sasaki, J. Coord. Chem. 18 (1988) 129.
[18] M. Sorai and D.N. Hendrickson, Pure & Appl. Chem. 63 (1991) 1503. [19] R.J. Webb, P.M. Hagen, R.J. Wittebort, M. Sorai and D.N. Hendrickson, Inorg. Chem. 31 (1992) 1791. [20] T. Nakamoto, M. Katada and H. Sano, Chem. Lett. (1990) 225.
and Methods
in Physics Research
RIOMI B
B76 (1993) 408-414
Beam Interactions with Materials&Atoms
Mixed-valence
states of polynuclear
iron complexes *
Satoru Nakashima Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 724, Japan
The valence-delocalization in mixed-valence binuclear ferrocene derivatives is investigated by using several physicochemical results. It is demonstrated that the mixed-valence state is controlled by the environment in the solid state. The mechanism of the valence-delocalization in l’,l”‘-diethylbiferrocenium triiodide is discussed. The difference between the valence-delocalization accompanied by the symmetry change of the counter anion and the valence-delocalization without connection with the symmetry of the counter anion is discussed. The results are compared with those of oxo-centered trinuclear iron carboxylates.
1. Introduction Binuclear ferrocene derivatives and oxo-centered trinuclear basic iron carboxylates have been studied mainly by using 57Fe Miissbauer spectroscopy for elucidation of their electronic states in the solid state [l]. These complexes have been intensively investigated because some of them convert from being valencetrapped at low temperatures to valence-detrapped at high temperatures. It is also interesting because these complexes have the potential for application to materials science. An interesting example by using ferrocene derivatives is the case of decamethylferrocenium-tetracyanoethenide, in whcih ferromagnetism is observed at low temperatures [2]. On the basis of 57Fe MGssbauer spectroscopy, the mixed-valence states can be classified into two types of valence states of the iron atoms: one is a trapped-valence type and the other is an averaged-valence type. In the former, two different valence-states of iron atoms corresponding to ferrocene-like bivalent and ferrocenium-like tervalent iron atoms are involved, while only an equivalent valence-state of iron atoms is found in the latter. Some derivatives show valence-delocalization with increasing temperature. There are two types of valence delocalization in addition to the relaxation process. One is the fusing type and the other is the coexisting type. In the fusing type, two quadrupole-split doublets, observed at low temperatures, approach each other with increasing temperature to converge into one doublet without significant broadening of the half-width in “Fe MGssbauer spectra in the averaging process. In the coexisting type, two types of valence states coexist
* Invited presentation at the Third International Symposium on the Industrial Applications of the Massbauer Effect, Otsu, Japan, August 24-27, 1992. 0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
and the rate of intramolecular electron-transfer increases with increasing temperature. In the present paper, the packing effect to the mixed-valence states of a series of binuclear ferrocene derivatives as shown in fig. 1 is demonstrated and the mechanism of the valence-delocalization in l’,l”‘-diethylbiferrocenium triiodide is discussed by using several physicochemical results. The comparison of the results with those of oxo-centered basic iron carboxylates is also discussed.
2. Examples of the packing effect to mixed-valence states of a series of l’,l”‘-dialkylhiferrocenes The mixed-valence states of a series of l’,l”‘-dialkylbiferrocenes have been investigated using 57Fe MGssbauer spectroscopy [3]. The mixed-valence states cannot be explained by the electronic effect of the substituent. The mixed-valence states of l’,l”‘bis(methylbenzyl)biferrocenium triiodides are affected by the position (o-, m-, or p-) of the methyl group to the benzyl substituent [4]. The mixed-valence states are also affected by the counter anion, e.g., triiodide salt of l’,l”‘-dipentylbiferrocene shows temperature-independent trapped-valence state, while its 7,7,8,&tetracyanop-quinodimethane salt shows temperature-independent averaged-valence state [5]. These results cannot be explained by the nature of intramonocation alone. Monocation-monocation and/or monocation-anion interaction must be considered. The packing effect was confirmed by a dispersion experiment, e.g., l’,l”‘-diisobutylbiferrocenium triiodide in crystalline state shows valence-delocalization, while the salt dispersed in poly(methy1 methacrylate) shows the temperature-independent trapped-valence state [6]. The packing effect was also confirmed by mixed-crystals experiment [7] B.V. All rights
reserved
409
S. Nakashima / Mixed-valencestatesof Fe complexes
and by ESR samples [6].
experiment
of crystalline
and dispersed
2.1. Mixed-valence states of biferrocene derivatirles with long alkyl chains It is not easy to study the packing effect systematically because of the difficulty in design of the crystal structures. One of the approaches, however, is to use long alkyl substituents. The biferrocene derivatives with long alkyl chains are considered to have a layer structure. The structure was confirmed using X-ray powder patterns. The inter-layer lengths are summarized in fig. 2. The results of the samples obtained from the hexane solution are indicated by the triangles, while those of the samples which are recrystallized from the dichloromethane solution are indicated by the circles. Shortening of the inter-layer length by recrystallization from dichloromethane is observed in l’,l”‘-ditetradecyland l’,l”‘-dihexadecylbiferrocenium triiodide. Two samples of l’,l”‘-dioctadecylbiferrocenium triiodides from hexane and from dichloromethane belong to the crystal form with longer inter-layer length. 57Fe Mijssbauer spectra showed that the samples with a longer inter-layer length exhibit a temperature-independent trapped-valence state and the samples with a shorter inter-layer length show a fusing type valencedelocalization [8,9]. 2.2. Polymorphism and mixed-oalence states of l’,l’“-dibutylbiferrocenium triiodide l’,l”‘-Dibutylbiferrocenium triiodide recrystallized from dichloromethane gave two kinds of crystal forms. Needle-like crystals (crystals A) are found to show a temperature-independent trapped-valence state, while plate-like crystals (crystals B) show a fusing type valence-delocalization [6], as shown in fig. 3. The crystal structures for both crystal forms in l’,l”‘-dibutylbiferrocenium triiodide were analyzed [lO,ll]. They are shown in fig. 4. The structure of monocation for crystals A is asymmetric. On the other hand, in crystals B
IL
16
IS
“c
Fig. 2. Inter-layer distances (ref. [9]). The “nc” is the number of carbons in the alkyl chain. Triangles indicate the results of the samples obtained from hexane solution, while circles
indicate the results of the samples recrystallized dichloromethane solution.
there are two crystallographically independent monocations in a unit cell. Although the symmetry of the two monocations is better than that of crystals A, butyl groups of one of the two monocations in a crystal unit of crystals B are relatively asymmetric. The packing arrangements are also different from each other. The asymmetric interaction between monocation and asymmetric triiodide anion is found in crystals A, which agrees with the result showing a temperature-independent trapped-valence state. Although the interaction between monocation and anion was not symmetric in crystals B, an appreciable interaction seems to exist between the cyclopentadienyl ring and the butyl group of the adjacent monocation. We suppose that the asymmetric interaction between monocation and asymmetric triiodide anion causes the mixed-valence state to be a trapped-valence type and the intermonocation interaction in stack plays an important role for the valence-delocalization. It is considered that the role of the monocation-monocation interaction for the valence-delocalization overcomes the asymmetric monocation-anion interaction in crystals B.
3. Mechanism of valence-delocalization ylbiferrocenium triiodide [121
Fig. 1. Biferrocenium and disubstituted biferrocenium salts. R = H or alkyl substituents. Y = I, or (TCNQ), (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane).
from a
in l’,l”‘-dieth-
l’,l”‘-Diethylbiferrocenium triiodide shows a fusing type valence-delocalization [13]. l’,l”‘-Diethylbiferrocenium triiodide is a good compound to establish the mechanism of the valence delocalization, because so far polymorphism is not observed in l’,l”‘-diethylbifer-
S. Nakashima
410
states of Fe complexes
/ Mixed-valence
Temperature dependence of the quadrupole splittings is shown in fig. 5a. This shows that the rate of approach of the two doublets becomes drastic in the
rocenium triiodide and both the crystal structures at room temperature and at low temperatures are already reported [ 141.
0
I
2
Velocity
3
4
I mm.s-’
I
n
’
n
.
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:
+A.+. >. . r: .’ .,.
k v) z z v
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10.7
’
:
.. .*
’
’
1
5
* ~_~~*-TY, :
;‘,
147K
: * .. . . . . : *. ‘: . .
J
’
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’
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spectra
’
-1
*
’
I 0 Velocity
for crystals
0
’
3 2 I mm.51
I
A (a) and crystals
I
I
-3
1
1
*
-2
-I
0
’
’
’
1
2
3
Velocily B (b) of l’,l”‘-dibutylbiferrocenium
I
I mm-s-’ triiodide
(ref. [6]).
411
S. Nakashima / Mixed-valence states of Fe complexes
- 150 to - 280 K range. It can be seen from fig. Sb that the slope of the logarithmic value of the relative area1 intensity is altered at - 200 and - 270 K. This fact indicates that there are changes in the state of lattice vibrations around those temperatures. This is interesting because temperatures 200 and 270 K are the starting and finishing temperatures for valence-delocalization, respectively. Variable temperature far infrared spectra of l’,l”‘diethylbiferrocenium triiodide are shown in fig. 6. A linear, symmetric triatomic ion such as I; has three fundamental modes of vibration; the symmetric stretching mode (vi), the doubly degenerate deformation mode (~a), and the asymmetric stretching mode (v,). Examination of the activity of the vi, vz, and vs bands for I; ion has a diagnostic value for the determination of the I; structure. Three peaks centered at 109, 58, and 147 cm-’ obtained at 93 K can be assigned to the vi, vz, and vs mode, respectively. The intensity of the vi mode in the far infrared spectra decreases with increasing temperature and finally, at 298 K, the peak due to the vi mode disappears. The result shows that the asymmetric I; at low temperatures becomes symmetric with increasing temperature. Variable temperature Raman spectra agree with the results of far infrared spectra. The temperature region where the asymmetric I; becomes symmetric corresponds to that of valence-delocalization of monocation. It can be concluded that the intramolecular electron-transfer in l/,1”‘-diethylbiferrocenium cation is coupled with a symmetry change of the I; anion. However, the results of s7Fe Mossbauer, far infrared, and Raman spectra are in conflict with the results of X-ray structural
(a) 2.0 Ferrocene-like
iron
”
Ferrocenium-1 0.5
ike
iron
--•
r/K
Fig. 5. Temperature dependence of the quadrupole splittings (a) and of area1 intensity relative to that at 78 K (b) for l’,l”‘-diethylbiferrocenium triiodide (ref. [12]).
b)
a 1
Fig. 4. The structure for crystals A (a) and crystals B (b) of l’,l”‘-dibutylbiferrocenium
triiodide (ref. [ll]).
analysis at 140 K. X-ray structural analysis shows that both the monocation and the triiodide anion sit on a crystallographic center of symmetry, both at 298 and at 140 K [14]. One of several explanations is that the X-ray structural analysis at 140 K might reflect a statistical average of the asymmetric structure. In the case of biferrocenium triiodide, far infrared and Raman spectra showed that the triiodide ion remains symmetric from low temperatures to room temperature 1151, whereas the 57Fe Miissbauer spectra showed that the monocation converts from being in a valence-trapped state at low temperatures, to a dynamically interconverting valence-detrapped state at high temperatures. One may ask what is the difference between the valence-delocalization accompanied by the symmetry change of the counter anion and the valence-delocalization without a connection with the symmetry of the counter anion. One of several possible explanations is that the structural difference between
412
S. Nakashima / Mixed-valence states of Fe complexes
biferrocenium triiodide and l’,l”‘-diethylbiferrocenium triiodide and thus the difference in crystal packing might be responsible for the way of valence-delocalization. In the biferrocenium triiodide crystal, the monocations and triiodide anions are stacked in independent columns [lo]. On the other hand in l’,l”‘-diethylbiferrocenium triiodide crystal, each monocation is surrounded by triiodide anions and vice versa. Therefore the difference between biferrocenium triiodide and l’,l”‘-diethylbiferrocenium triiodide is that the monocations in the former can interact directly while the monocations in the latter need to interact through the triiodide ions. The importance of intermonocation interaction was discussed in section 2.2. The molar heat capacity at constant pressure of l’,l”‘-diethylbiferrocenium triiodide was measured. Excess heat capacity, AC,, of l’,l”‘-diethylbiferrocenium triiodide beyond the normal heat capacity curve was calculated by an effective frequency distribution method [16]. There exists a sharp heat-capacity peak centered at 67.2 K and a broad anomaly over a wide temperature region above - 200 K. The broad anomaly corresponds to the temperature region where the “Fe Miissbauer spectra are altered from a trapped- to an averaged-valence state and the triiodide ion is altered from an asymmetric to a symmetric structure. The
J
Fig. 6. Variable temperature far infrared absorption spectra for l’,l”‘-diethylbiferrocenium triiodide in the 300-30 cm-’ range. The vl, Y*, and vj indicate the symmetric stretching, deformation, and asymmetric stretching modes of the I; ion, respectively (ref. [12]).
(A)
(B)
E(Q) I
E (Q)
Fig. 7. Adiabatic
potential energy surface, E(Q), for the l’,l”‘-diethylbiferrocenium monocation plotted as a function of the out-of-phase combination (Q = QA- Q,) of hvo symmetric metal-ligand breathing vibrational modes (QA and Q,) of the two halves of a binuclear mixed-valence species (ref. [17]). Diagram A is for a symmetric mixed-valence complex in the absence of environmental effects. Diagram B results if the environment about the binuclear mixed-valence complex is asymmetric.
entropy gain above - 200 K is quite small. AC, at 300 K is about 3-4 JK-’ mol-‘. The value is very close to the maximum value of a two-level Schottky anomaly, in which the degeneracies of the ground and excited states are identical. The infrared spectrum at room temperature shows two independent CH bending modes which are ascribed to the ferrocene and ferrocenium parts, although 57Fe Mijssbauer spectroscopy cannot distinguish the two irons at this temperature. Therefore, the adiabatic potential energy surface of the monocation is characterized not by one minimum but by two minima. The two vibronic states are shown as [FegFeg’] and [Fei’Feg]. Kambara and Sasaki [17] derived the energy difference by intramolecular coupling, where both the ground and excited states correspond to the delocalized electron-tunneling splitting as shown in fig. 7a. At low temperatures, however, since the I; ion has an asymmetric form and thus the environment about the mixed-valence monocation is asymmetric, the electronic energy of either [Fe:FeE’] or [Fez’Feg] would be low in comparison to that of the remaining one. Thereby, the adiabatic potential energy surface would have the shape shown in fig. 7b. With increasing sample temperature, the energy difference, A E, of scheme (B) in fig. 7 is cooperatively reduced as the valence-delocalization proceeds through coupling with a change in the symmetry of the I; ion. And finally when AE becomes nearly equal to the energy difference, ho, of scheme (A) in fig. 7, the adiabatic potential surface would be altered from scheme (B) to (A). In such a scheme, characterized by a tunneling splitting, one can expect a heat capacity anomaly of the Schottky type.
S. Nakashima / Mixed-uaience states of Fe complexes
-1
*
100
300
200
400
I
500
T/K
Fig. 8. Semi-logarithmic plot of the excess heat capacity, AC,, of l’,l”‘-diethylbiferro~enium triiodide (ref. [121>.The broken line indicates the heat capacity anomaly calculated on the basis of a Schottky anomaly due to tunneling-splitting, in which the energy difference is 750 cm-‘. The energy difference has been determined so that the two shaded areas and thus the entropy gains are almost equal.
Fig. 8 illustrates the excess heat capacity against the loga~thmic temperature scale. In this figure, area corresponds to entropy. As shown by a broken curve in fig. 8, a Schottky heat capacity curve was determined so that the areas of two shaded portions become equal. In this case, the energy difference, hw, was estimated to be 750 cm-‘. The problem, however, is that 750 cm-’ is too large to be due to tunneling splitting.
413
triangular form. Therefore, the deformation of a molecule in the crystal lattice is much more difficult in the oxo-centered trinuclear basic iron carboxylates than the bin&ear ferrocene complexes. In mixed-valence binuclear ferrocene derivatives, a slight change in the symmetry of the anions can affect the electronic state of the cations. On the other hand, since the trinudear oxo-centered basic iron carboxylates complexes consist of neutral molecules, there does not exists such an electrostatic interaction. However, those oxo-centered trinuclear basic iron carboxylates complexes, which exhibit temperature-dependent valence-deIocalization, contain solvate molecules. Orientational disordering of these solvate molecules cooperatively couples with the intramolecular electron-transfer event. One of the exceptional cases hitherto reported is oxo-centered trinuclear basic iron complexes having long alkyd chains 1201. In this case, cooperative conformational change in the long alkyl chains might play the role of solvate molecules.
Acknowledgements The author would like to thank Professor II. Sano of Tokyo Metropolitan University, Professor M. Konno of Ochanomizu University, and Professor M. Sorai of Osaka University for kind guidance and valuable discussion.
References 4. Comparison
of mixed-valence binuclear ferrocene derivatives with oxo-centered trinuclear basic iron carboxylates
The situation for the valence delocalization of the mixed-valence binuclear ferrocene derivatives is quite different from that for oxo-centered basic iron carboxylates. In the case of trinuclear oxo-centered basic iron carboxylates complexes, the valence-delo~l~ation occurs through phase transition(s) [18], while for l’,l”‘-diethylbiferrocenium triiodide it proceeds without a phase transition. Although in biferrocenium triiodide [151 and l’, l‘~-diiodobife~ocenium hexa~uoroantimonate [19] the phase transition is observed, the entropy gains are much smaller than those in oxo-centered basic iron carboxylates. When the intramolecular electron-transfer occurs, the molecular structure is inevitably distorted from its original form. In the binuclear ferrocene monocations, this distortion is established by the aid of the antisymmetric breathing vibrational mode in binuclear ferrocene derivatives, whereas in oxo-centered trinuclear basic iron carboxylates the distortion is accompanied by a drastic change in its
[l] H. Sano, Hyperfine Interactions 53 (1990) 97. [2] J.S. Miller, J.C. Calabrese, H. Rommelmann, S.R. Chittipeddi, J.H. Zhang, W.M. Reiff and A.J. Epstein, 3. Am. Chem. Sot. 109 (1987) 769. [3] S. Nakashima, M. Katada, I. Motoyama and H. Sano, Bull. Chem. Sot. Jpn. 60 (1987) 2253. [4] S. Nakashima, M. Katada, I. Motoyama and H. Sano, Bull. Chem. Sot. Jpn. 59 (1986) 2923. [S] S. Nakashima, S. Iijima, I. Motoyama, M. Katada and H. Sano, Hyperfine Interactions 40 (1988) 315. [6] S. Nakashima, Y. Masuda, I. Motoyama and H. Sano, Bull. Chem. Sot. Jpn. 60 (1987) 1673. [7] S. Nakashima and H. Sano, Buti. Chem. Sot. Jpn. 62 (1989) 3012. [81 S. Nakashima and H. Sano, Chem. Lett. (1989) 1075. [9] S. Nakashima and H. Sano, Hyperfine Interactions 53 (1990) 367. [lOI T.-Y. Dong, D.N. Hendrickson, K. Iwai, M.J. Cohn, S.J. Geib, A.L. Rheingold, H. Sano, 1. Motoyama and S. Nakashima, J. Am. Chem. Sot. 107 (1985) 7996. [ill S. Nakashima, M. Konno and H. Sane, Hyperfine Interactions 68 (1991) 205. 1121 S. Nakashima, A. Nishimori, Y. Masuda, H. Sano and M. Sorai, J. Phys. Chem. Solids 52 (1991) 1169.
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S. Nakashima / Mixed-valence states of Fe complexes
[13] S. Iijima, R. Saida, I. Motoyama and H. Sano, Bull. Chem. Sot. Jpn. 54 (1981) 1375. [14] M. Konno and H. Sano, Bull. Chem. Sot. Jpn. 61 (1988) 1455. [15] M. Sorai, A. Nishimori, D.N. Hendrickson, T.-Y. Dong and M.J. Cohn, J. Am. Chem. Sot. 109 (1987) 4266. [16] M. Sorai and S. Seki, J. Phys. Sot. Jpn. 32 (1972) 382. [17] T. Kambara and N. Sasaki, J. Coord. Chem. 18 (1988) 129.
[18] M. Sorai and D.N. Hendrickson, Pure & Appl. Chem. 63 (1991) 1503. [19] R.J. Webb, P.M. Hagen, R.J. Wittebort, M. Sorai and D.N. Hendrickson, Inorg. Chem. 31 (1992) 1791. [20] T. Nakamoto, M. Katada and H. Sano, Chem. Lett. (1990) 225.