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The localized-to-delocalized transition in mixed-valence chemistry.

Medium Effects on Charge Transfer in Metal Complexes.

Metal−Metal Interactions in Linked Metallocenes

Making crystals from crystals: a green route to crystal engineering and polymorphism

Resonance Raman (RR) spectra are reported for myoglobin reconstituted with seven heme isotopomers which are labeled with 15N and meso-D4 in the porphyrin skeleton or at the vinyl and propionate substituents. The RR bands are assigned to the porphyrin in-plane and out-of-plane modes as well as to the internal vibrations of substituents on the basis of the observed isotope shifts. The issue of vinyl substituent effects is revisited, and bands are assigned to the 2- or 4-vinyl group from selective deuteration shifts. Contributions of the aliphatic propionate groups are also revealed in the RR spectrum. The protein influence on the heme structure is reflected in the activation of several out-of-plane modes in the low-frequency region.

Assignment of Protoheme Resonance Raman Spectrum by Heme Labeling in Myoglobin

Data are presented to indicate that the rate of inrramolecular electron transfer in mixed-valence biferrocenium triiodide, 1, and four mixed-valence dialkyl-substituted biferrocenium triiodide salts is controlled by dynamics in the solid state. The X-ray structure of 1 at 296 K has been determined: Pi, a = 7.5779 (20) A, b = 8.4742 (14) A, c = 9.5577 (21) A, LY = 112.619 (14)', p = 104.646 (20)O, and y = 94.610 (19)'; Z = 1, Dcalcd = 2.32 g ~m-~, R,, = 0.042 for 2185 observed (F, 2 2.5u(F0)) reflections. The mixed-valence biferroceqium cation assumes a trans conformation where the Fe(q5-C5Hs) moieties are on opposite sides of a planar fulvenide ligand. The two rings bonded to each Fe atom are eclipsed. Both the cation and the anion sit on centers of symmetry in 1. The 300 K Mossbauer spectrum of 1, however, shows equal-area Fe" and Fe"' doublets. It is probable that a well-formed single crystal has Mossbauer-delocalized molecules, whereas a microcrystalline sample tends to have localized molecules. Several different samples of 1 were examined with the Mossbauer technique to find that at 300 K the ratio of delocalized to localized molecules can vary from -0 to - 1 depending on the history of the sample. At temperatures above 300 K the Mossbauer spectrum of compound 1 does change to eventually become a single average-valence doublet at -357 K. Differential scanning calorimetry curves in the range of -303 to -363 K were run for several samples of 1. For samples that show 300 K Mossbauer spectra comprised essentially of a localized signal, a relatively sharp endothermic peak is seen at -335 K. Samples of 1 which show appreciable amounts of a signal for the delocalized species at 300 K give a broad DSC peak shifted to lower temperature than -335 K. In one case the peak was apparently even so broad as to be not detectable. The X-ray structure of 1',6'-di-n-butylbiferrocenium triiodide, 7, has been determined at 150, 298, and 363 K. At all three temperatures 7 crystallizes in the P2,/c space group. At 150 K the structure of 7 is characterized as a = 10.0201 (8) A, b = 13.5203 (9) A, c = 21.6080 (23) A, and p = 106.015 (8)'; Z = 4, RF = 0.042, and RwF = 0.052 for 5145 observed (IqZ > 2.0~14~) reflections. The mixed-valence di-n-butylbiferrocenium cation assumes a trans conformation with a planar fulvenide ligand. The two halves of the cation are inequivalent at 150 K by virtue of the positioning of the 13- anion and differences in the configuration of the two butyl groups. Analysis of the X-ray diffraction results for 7 at 298 and 363 K shows that at these higher temperatures positional disorder sets in for both the cation and the anion. One of the butyl groups develops pronounced positional disorder at the higher temperatures. It is suggested that this butyl group is dynamically disordered at the higher temperatures. This is supported by the observation that the c axis of a crystal of 7 expands appreciably from 150 to 363 K. Both 7 and 1',6'-dibenzylbiferrocenium triiodide, 8, exhibit an interesting phenomenon. Microcrystalline samples of 7 and 8 give 300 K Mossbauer spectra which show appreciable intensity signals from valence-localized species, whereas recrystallized samples give 300 K Mossbauer spectra exhibiting only a signal for a delocalized species. At temperatures below -200 K there are two doublets for crystalline 7 and 8. The two doublets became a single doublet at -275 and -260 K, respectively, for 7 and 8 as the temperature is increased. The above results indicate that these mixed-valence compounds are undergoing phase transitions. It is suggested that the observed temperature dependence of the Mossbauer spectra for the dialkyl-substituted complexes 7 and 8 and the analogous ethyl- and propyl-substituted complexes results from I< anions undergoing a transformation from a static situation which tends to localize the charge in the mixed-valence cation to a dynamic situation where on the average the environment about each half of a mixed-valence cation is identical. ~~~~~~~ ~

Mixed-Valence Dialkylbiferrocenium Salts: An Explanation for the Observed Temperature Dependence of Electron-Transfer Rates

Determination des structures cristallines du triiodure de diiodobiferrocenium et de l'octaiodure de dichlorobiferrocenium. Spectres Mossbauer, RPE et IR

Mixed-valence 1',6'-dihalobiferrocenium salts: the effects of the solid-state environment on electron-transfer rates

The effects on the rate of electron transfer of replacing 1,by Br21in the mixed-valence compounds It,6’-diiodobiferrocenium triiodide (3), 1’,6’-dibromobiferrocenium triiodide (4), and 1’,6’-dichlorobiferrocenium triiodide (2) are examined. Replacing I,by Br21in the first two complexes leads to an appreciable decrease in the rate of electron transfer, whereas in the case of the third complex an increase in the rate is seen. The triiodide complex 3 shows a single “average-valence” 57Fe Mossbauer doublet at 4.2 K; that is, the electron-transfer rate is in excess of lo7 s-l at 4.2 K. Two doublets are seen at 4.2 K for 1’,6’-diiodobiferrocenium dibromoiodate (9). It is necessary to heat 9 to -80 K so that the rate exceeds I O 7 s-I. Two Mossbauer doublets are also seen at 4.2 K for 1’,6’-dibromobiferrocenium dibromoiodate (lo), whereas 4 shows one doublet. The mixed-valence complex 10 needs to be heated to -180 K to have the rate be in excess of lo7 s-’. Complex 2 is Mossbauer-localized up to -340 K; however replacing the I,by Br21to give 8 changes this such that 8 begins to show some Mossbauer-delocalized species at 100 K, and at 300 K there are approximately equal amounts of Mossbauer-localized and -delocalized species present. This increase in rate by replacing I,by Br21is what was seen before for biferrocenium triiodide (1). A qualitative model is presented to explain these anion effects on the intramolecular electron-transfer rate of mixed-valence biferrocenes. It is shown that the difference in response of I< replacement by Br21reflects the different packing arrangements in the solid state. The Occurrence of a phase transition is strongly indicated. In 1 there are stacks of mixed-valence cations, and cation-cation interactions are important. The greater electron-transfer rate in biferrocenium dibromoiodate than in biferrocenium triiodide could be a reflection of three possibilities: 1, charge oscillation between two asymmetric forms of the trihalide anion is easier in Br21than in I<; 2, the Br21ion begins to move between two lattice positions at lower temperatures than the 1,anion; and 3, the replacement of 1,by BrJ reduces the intrastack cation-cation interactions which lead to valence trapping. The packing arrangement in 3, 4, 9, and 10 is comprised of alternating stacks of cations and anions. Unlike the situation for 1, IR and EPR results for these complexes show that the mixed-valence cation is appreciably influenced by changing the anion. The origin of this is the appreciable in-stack interaction between the terminal atom of the trihalide anion and the iodine atom on the cyclopentadienyl ring. Thus, changing Ijin 3 to give the Br21salt 9 leads to an increase in the in-stack cation-anion interaction, and a higher temperature is needed for 9 to achieve an electron-transfer rate in excess of I O 7 s-l. In the last year or two there has been considerable progress made in understanding what factors control the rate of intramolecular electron transfer in the solid state for mixed-valence biferrocenes”’ and oxo-centered trinuclear iron acetates.’-I2 The ( I ) O n sabbatical leave from the Department of Engineering Physics, The University of Electro-Communications, Chofu, Tokyo 182, Japan. (2) Dong, T-Y.; Cohn, M. J.; Hendrickson, D. N.; Pierpont, C. G. J. Am. Chem. SOC. 1985, 107, 4171. (3) Cohn, M. J.; Dong, T.-Y.; Hendrickson, D. N.; Geib, S. J.; Rheingold, A. L. J . Chem. SOC., Chem. Commun. 1985, 1095. (4) Dong, T.-Y.; Hendrickson, D. N.; Iwai, K.; Cohn, M. J.; Rheingold, A. L.; Sano, H.; Motoyama, I.; Nakashima, S . J . Am. Chem. Soc. 1985, 107, 1996. (5) Dong, T.-Y.; Hendrickson, D. N.; Pierpont, C. G.; Moore, M. F. J. Am. Chem. SOC. 1986, 108, 963. (6) Moore, M. F.; Wilson, S. R.; Cohn, M. J.; Dong, T.-Y.; MuellerWesterhoff, U. T.; Hendrickson, D. N. Inorg. Chem. 1985, 24, 4559. (7) Hendrickson, D. N.; Oh, S. M.; Dong, T.-Y.; Kambara, T.; Cohn, M. J.; Moore, M. F. Comm. Inorg. Chem. 1985, 4 , 329. (8) (a) Oh, S. M.; Hendrickson, D. N.; Hassett, K. L.; Davis, R. E. J . Am. Chem. SOC. 1984, 106,1984. (b) Oh, S. M.; Hendrickson, D. N.; Hassett, K. L.; Davis, R. E. J . Am. Chem. SOC. 1985, 107, 8009. (9) Oh, S. M.; Kambara, T.; Hendrickson, D. N.; Sorai, M.; Kaji, K.; Woehler, S. E; Wittebort, R. J . J . Am. Chem. SOC. 1985, 107, 5540. (10). Kambara, T.; Hendrickson, D. N.; Sorai, M.; Oh, S . M. J. Chem. Phys., in press. exact positioning of a counterion, a ligand, or even a solvate molecule relative to the mixed-valence complex in the solid state can have a dramatic impact on the rate of intramolecular electron transfer. When there is an onset of dynamics associated with the c o ~ n t e r i o n , ~ , ~ ligand,7s8 or solvate molecule~~”J2 this probably will influence the rate of intramolecular electron transfer. In fact, because there are interactions between molecules in the solid state, frequently changes in the rate of intramolecular electron transfer are coupled to phase transition^.^*^^^^-' Mixed-valence compound 2 crystallizes with half a mole of 12. The X-ray structure’ of this compound shows that not only is the I,anion asymmetric but the 1,counterion sits closer to one half of the cation, the iron(II1) metallocene moiety. As a result, mixed-valence compound 2 is localized on the Mossbauer time scale even a t 340 K.5si3 The ground-state double-well potential-energy surface for the mixed-valence cation in 2 is asymmetric (11) Sorai, M.; Kaji, M.; Hendrickson, D. N.; Oh, S. M . J. A m . Chem. (12) Woehler, S. E.; Wittebort, R. J . ; Oh, S. M.; Hendrickson, D. N.; (13) Motoyama, I . ; Suto, K.; Katada, M.; Sano, H. Chem. Lett. 1983, SOC. 1986, 108. 702. Inniss, D.; Strouse, C. E. J. Am. Chem. SOC. 1986, 108, 2938.

Mixed-valence 1',6'-dihalobiferrocenium cations: counterions micromodulating intramolecular electron transfer.

The effect of changing the counterion from I,to 12Br-, Br21-, or PF6upon the rate of intramolecular electron transfer in the mixed-valence biferrocenium cation is examined. Room temperature powder X-ray diffraction data are presented to show that the 1,Brand BrJsalts are isostructural to biferrocenium triicdide. The 4.2 K X-band EPR spectrum of biferrocenium BrzIis axial (g,, = 3.60 and g, = 1.75) and very similar to that for the Issalt. However, the g-tensor anisotropy (g,, = 3.23 and g, = 1.90) observed for the 4.2 K EPR spectrum of biferrocenium 12Bris appreciably reduced compared to that of the I,and BrJ salts. This probably reflects the low-symmetry environment provided by the asymmetric I-I-Branion. The 57Fe Mossbauer characteristics of biferrocenium IzBrresemble those of the 1,salt; however, everything is shifted to lower temperatures. The 150 K spectrum essentially consists only of two doublets (one Fe" and one Fell'). A third average-valence doublet grows in with increasing temperature Until at -340 K only this average doublet is seen. A temperature of only -220 K is needed to give only an average-valence doublet for the Br21salt. Thus, replacing the 1,by the Br21anion leads to a dramatic reduction by 150 deg in the temperature where the mixed-valence biferrocenium cation transfers an electron faster than the Mossbauer technique can sense. A qualitative model is developed to explain the effect of the anion replacement. The importance of the cation-cation and cation-anion interactions, as well as the intrinsic charge-oscillation barrier heights in the mixed-valence cations and anions, is discussed relative to the phase transition that is believed to be present in these compounds. In very recent studieszw7 it has been shown that the solid-state environment plays a crucial role in determining the rate of intramolecular electron transfer in various mixed-valence biferrocenium salts. Triiodide salts (Y= 13-) were studied. The

Counterion effects on the intramolecular electron-transfer rate of mixed-valence biferrocenium salts: micromodulation and phase transitions

The results of the measurement of heat capacity at constant pressure, C,,, of the mixed-valence compound biferrocenium triiodide with an adiabatic calorimeter between 14 and 360 K are reported. This compound is known to show a trapped-valence state at low temperature on the 57Fe Mossbauer time scale and a detrapped average valence state at high temperatures. Heat capacity data are presented to show that biferrocenium triiodide exhibits a phase transition in the same temperature region where the valence detrapping occurs on the S7Fe Mossbauer time scale. The present calorimetric measurements indicate a main C, peak centered at 328 K and two subsidiary small peaks at 312 and 346 K. These three peaks may be attributed to the phase transition arising from different sample histories (mainly different grain sizes of the specimen). The phase transitions are characterized by weak cooperativity; the excess heat capacity due to the phase transition is observed over a wide temperature region from -220 to -360 K. The transition entropy was found to be ASm = (1.77 & 0.06) J K-' mol-'. This value is unexpectedly small even if one takes into account only the contribution from the onset of intramolecular electron transfer in the binuclear cations, which amounts to R In 2 (=5.76 J K-' mol-'), where R is the gas constant. Based on the temperature acquisition of AS,,,, the average of the potential energy difference between two electron-localized states, ( FeA"FeB"') and ( FeArrrFeB"), was estimated. The result indicates incompleteness of the phase transitions. A qualitative microscopic model for the phase transition is presented. Intermolecular interactions in the stacks of mixed-valence cations are suggested to be important. In order to have a phase transition it is also important to have interactions between a given cation and its neighboring 1,- anions. In order to examine the role of the 13- counteranion, far-infrared and Raman spectra have been recorded in the range below 400 cm-' as a function of temperature. The symmetric stretching mode (ul) of the I,- anion was observed at 110 cm-I in the Raman spectrum while the deformation (u2) and asymmetric stretching (u,) modes were observed at 55 and 136 cm-' in the IR spectrum. The observation of such a mutual exclusion of vibrational modes between the IR and Raman experiments suggests that the I,- anion in biferrocenium triiodide has nearly a linear symmetrical form, and thus a charge oscillation between two distorted forms I--I-I and I-I--I- does not occur in the range 101-340 K. At the very least the distortion of the 1,- anion in biferrocenium triiodide is not as appreciable as it is in other statically distorted cases. IR and Raman data are also presented for the analogous mixed-valence compound 1',6'-dichlorobiferrocenium triicdide hemiicdine. In keeping with the X-ray structural results, which show an asymmetric It- anion in this compound, all three I,- vibrational modes are seen both in the IR and Raman spectra.

Teng Yuan Dong

Papers

Mixed-Valence Dialkylbiferrocenium Salts: An Explanation for the Observed Temperature Dependence of Electron-Transfer Rates

Mixed-valence 1',6'-dihalobiferrocenium salts: the effects of the solid-state environment on electron-transfer rates

Mixed-valence 1',6'-dihalobiferrocenium cations: counterions micromodulating intramolecular electron transfer.

Counterion effects on the intramolecular electron-transfer rate of mixed-valence biferrocenium salts: micromodulation and phase transitions

Heat Capacity and Phase Transition of the Mixed-Valence Compound Biferrocenium Triiodide'