3181
Acknowledgments. This work was supported by the National Science Foundation (Grant GP-41661 X) and the U.S. Energy Research and Development Administration. We wish to thank Bernice Mills and David A. Shirley for providing binding energy data in advance of publication. References and Notes (1) Data for the boron compounds are from ref 2, for the fluorocarbons from ref 3, for EtO(CO)CF3 from ref 4, for EtOH and CH3C02H from ref 5. and for the remaining compounds from this work. (2) D. A. Allison, G. Johansson, C. J. Allan, U. Gelius, H. Siegbahn. J. Allison, and K. Siegbahn, J. Electron Spectrosc. Relat. Phenom., 1, 269 (1973). (3) D. W. Davis, Ph.D. Thesis, University of California, Berkeley, 1973. (4) K. Siegbahn, J. Electron Spectrosc. Relat. Phenom.,5 , 3 (1974).
(5) K. Siegbahn et al.. "ESCA Applied to Free Molecules", North-Holland Publishing Co.. Amsterdam, 1969. (6) W. L. Jolly and D. N. Hendrickson, J. Am. Chem. SOC.,92, 1863 (1970); W. L. Jolly in "Electron Spectroscopy", D. A. Shirley, Ed., North-Holland Publishing Co., Amstdam, and American Elsevier, New York, N.Y.. 1972, p 629. (7) J. A. Pople and M. Gordon, J. Am. Chem. Soc.. 89,4253 (1967); R. T. C. Brownlee and R. W. Taft. J. Am. Chem. Soc..92,7007 (1970); W. J. Hehre and J. A. Pople, /bid., 92, 2191 (1970). (8) D. W. Davis, D. A. Shirley, and T. D. Thomas, J. Am. Chem. Soc.. 94,6565 (1972); D. W. Davis, M. S. Banna, and D. A. Shirley, J. Chem. Phys., 60, 237 (1974); S.A. Holmes and T. D. Thomas, J. Am. Chem. SOC.,97,2337 (1975). (9) W. L. Jolly and W. B. Perry, Inorg. Chem., 13, 2686 (1974). (10) W. L. Jolly, T. F. Schaaf, and W. B. Perry, unpublished calculations. (11) C. G.Swain and E. C. Lupton. Jr., J. Am. Chem. SOC..90, 4328 (1968). (12) W.B.Perryand W.L. Jolly, Inorg. Chem., 13, 1211 (1974). (13) W. B. Perry, T. F. Schaaf, and W. L. Jolly, J. Am. Chem. SOC.,97, 4899 (1975).
Bis( fulvalene)diiron, Its Mono- and Dications. Intramolecular Exchange Interactions in a Rigid System Carole LeVanda,la Klaus Bechgaard,la Dwaine 0. Cowan,*18 Ulrich T. Mueller-Westerhoff,*lb Peter Eilbracht,lb George A. Candela,lc and R. L. Collinsld Contribution f r o m the Department of Chemistry, The Johns Hopkins Unioersity, Baltimore, Maryland 21 218, IBM Research Laboratory, San Jose, California 951 93, National Bureau of Standards, Washington, D.C. 20234, and Department of Physics, The Unicersity of Texas, Austin, Texas 71712. Receioed August 18, 1975
Abstract: Biferrocenylene [bis(fulvalene)diiron, BFD]was synthesized by two independent routes: an Ullman coupling of dibromoferrocene and the reaction of the fulvalene dianion with ferrous chloride. It was chemically oxidized to the mixed valence monocation and to the dication. These derivatives were characterized by optical, Mossbauer. ESR, and x-ray photoelectron spectra and magnetic susceptibility. The Mossbauer spectra of the mixed valence salts at 298 and 77 K indicate that both iron atoms are equivalent. X-Ray photoelectron spectra similarly attest to this equivalence. An asymmetry in the intensity of the Mossbauer lines is due to a Karyagin effect. The Mossbauer spectrum of the dication shows a quadrupole splitting of 3.0 m m / s which is unusually large for a ferrocenium-type derivative. The magnetic susceptibility of BFD (2,3) picrate, measured in the 2-300 K range, follows a Curie law with a room temperature moment very close to the spin-only value. The dicationic fluoroborate salt is diamagnetic. The ESR spectra of the monocationic picrate and fluoroborate salts are characterized by narrow lines and a small rhombic anisotropy. An absorption in the near-infrared centered at 1550 nm is observed in the spectra of the monocations, but not the neutral or dicationic derivatives. The assignment of this band is discussed with respect to the results of the other physical measurements.
By definition, mixed valence compounds contain two or more atoms of the same element in different formal states of oxidation.2 Theory3 predicts that new (e.g., magnetic and spectroscopic) properties will arise from interactions between valence electrons in unique oxidation states. Many inorganic mixed valence systems have been found to possess properties beyond those derived by simple addition from the component parts of the m o l e c ~ l e . ~ The term "mixed valence" was chosen by Robin and Day3b with the intent that it be all encompassing. Systems ranging from no interaction and firmly trapped valences to complete delocalization and nonintegral valences are included. The former group is termed class I; the latter is class 111. An intermediate classification, class I I, houses those compounds in which there is some delocalization, but the properties of the components are still discernible. Our objective has been to investigate the variation of properties with change in structure in binuclear mixed valence metallocenes. According to the theory of the rate of electron transfer in mixed valence systems depends on the amount of reorganizational energy necessary to make the mixed valence sites identical; i.e., if the coordination geometry
of the two sites is very different, the rate of transfer is very slow. I n this regard, the iron group metallocenes are notable candidates for rapid electron transfer since crystallographic data on ferrocene5 and ferrocenium salts6 have shown that the oxidation state of iron has a small effect on interatomic distances. The organometallic mixed valence compound biferrocene [Fe(II)Fe(III)] picrate (Ib) has been characterized as a class I I species2 Its Mossbauer,' ESCA,* m a g n e t i ~and , ~ opticallo properties have features attributable to the constituent ferrocene and ferrocenium portions in addition to features ascribable to its mixed-valence nature. Moreover, the fully oxidized biferrocene [2Fe(III)] salt (IC), although unstable in solution and more difficult to characterize, displays properties that are essentially the sum of two ferrocenium This is in sharp contrast to the observed behavior of the mono- (IIb) and dioxidized (IIc) salts of biferrocenylene (bis(fulvalene)diiron, BFD), whose syntheses and properties are discussed herein.
Synthesis Bis(fulva1ene)diiron (Ha) has been reported in the literature as a product of the Ullmann coupling of 1,l'-diiodoferrocene'
'
Cowan, Mueller- Westerhoff, et a1 f Bis(fulua1ene)diiron
3182 Table 1.
Mossbauer Parameters
BFD BFD (2,3) picrate BFD (2,3) (TCNQ)2 BFD (2,3) BF4
Ia,n=O
BFD (2,3) 13' BFD (3,3) 2BFdC
11%n -0
b,n-l c,n=2 and as a by-product in the pyrolysis of polymercuriferrocenylene.I2 In the course of our studies, two other methods of preparation were developed.'3,14 The yield of BFD from the Ullmann coupling ranged from 2 to 30% depending on the purity of the 1,I'-diiodoferrocene. I 5 The major impurity in diiodoferrocene when prepared by the method of Kovar et a1.I6 is iodoferrocene which lowers the yield and complicates the isolation of BFD by increasing polymer formation. As diiodoferrocene is difficult to purify by conventional measures, 1,1'-dibromoferrocene was used instead. This compound is a solid and can be purified by fractional sublimation and repeated recrystallizations. The coupling of the dibromide (eq 1) requires higher temperature than that of the diiodide, and was carried out with biphenyl as solvent. Reaction for 19 h a t 180-190' in the presence of activated copper gave a 20-25% yield of biferrocenylene (IIa). b,n=l c,n=2
I
Fe I
cu
IIa
~ 1 9 0 ~
The second approach, through the intermediacy of the fulvalene dianion and without the isolation of any intermediates, proved to be a most efficient route (eq 2).14
-
I11
FeC1,,2THF
IIa (2)
J Y Sodium cyclopentadienide (CpNa) was converted to dihydrofulvalene (111) according to the procedure given by Doering and Matzner.I7 This reaction was found to proceed with approximately 70-80% conversion (based on the ratio of the final ferrocene products). The first step of this reaction presumably involves the initial formation of 5-iodocyclopentadiene which was described as stable by Breslow.I8 W e considered that the inverse addition of C p N a to iodine might lead to a more complete conversion, but no major change in product distribution was achieved in this case. Addition of n-butyllithium afforded the fulvalene dianion, contaminated chiefly by cyclopentadienide. Addition of the stable FeC12.2THF complex'9 in T H F produced BFD together with ferrocene and polyferrocenes. The yield of BFD was 20-40% based on starting CpNa. Journal of the American Chemical Society
/
98:l I
/
Ferrocene" Ferrocenium DDQ" Biferrocene (2,3) picratee Fe( I I ) Fe( I I I ) Biferrocene (3,3) 2BF4
T,K
QS, m m / s
IS," m m / s
298 77 298 77 298 77 298 17 4.2 298 77 77
2.44 2.46 1.75 1.78 1.759 1.781 1.772 1.803 1.756 2.945 3.025 2.40 0
0.455 0.535 0.436 0.525 0.440 0.5 12 0.439 0.5 16 0.542 0.468 0.534 0.475 0.466
71 17 298
2.14 0.288 0.568
0.510 0.518 0.392
77
0.581
0.489
I7
Isomer shift relative to iron foil. Reference 24. Contaminated with (2,3) salt. Reference 23. e Reference 7.
This synthetic scheme has proved to be general and was also used successfully by Davison and Smart20 in the synthesis of bis(fulvalene)dicobaIt(III,III) bishexafluorophosphate, the isoelectronic cobalt analogue of BFD. Monocationic salts of BFD were obtained by oxidation with benzoquinone in the presence of picric acid or boron trifluoride etherate to give the (2,3) picrate and fluoroborate salts, respectively. (For simplicity, the notation (2,2) is used for neutral BFD, IIa, with formal iron(I1)-iron(I1) oxidation states. Similarly, the notations (2,3) and (3,3) denote the monocation, I Ib, and dication, IIc, respectively.) Recrystallization from chloroform, acetonitrile, or methanol gave dark green needles which decomposed without melting above 200 OC. BFD was readily oxidized by T C N Q ( T C N Q = tetracyanoquinodimethane) in methylene chloride or acetonitrile to give the 1:2 complex salt, BFD (2,3) (TCNQ)2. BFD was fully oxidized in acetonitrile to the (3,3) fluoroborate salt by excess benzoquinone in the presence of boron trifluoride etherate.2' The salt was isolated as thin leaflets by slow cooling to -30' or as microcrystalline powder by the addition of dry ether. Pure (3,3) salt appears to be stable in air a t room temperature but darkens above 250 'C without melting. In the absence of an oxidizing agent, solutions of BFD (3,3) salts decompose rapidly, forming the (2,3) salt. Similar behavior was observed for biferrocene (3,3) 2BF4.' The Oxidation State of Iron in the [2,3]Salts: Trapped or Fractional Valency? Mossbauer Spectra. It is well known that Mossbauer spectroscopy is often able to identify the oxidation states of iron in mixed valence compounds.22The quadrupole splittings and isomer shifts are generally very different for Fe(I1) and Fe( 111). For ferrocene derivatives, the quadrupole splitting parameter has been particularly useful in defining the 3d configuration of the iron atom. Ferrocene itself has a large quadrupole splitting (see Table I), while removal of an electron to form the ferrocenium ion leads to a vanishingly small quadrupole splitting.23 Biferrocene picrate (Ib) was shown to have "trapped" valences by Mossbauer s p e c t r o ~ c o p y . Its ~ . ~four-line ~ spectrum is a composite of ferrocene and ferrocenium transitions. The rate of electron transfer in the solid must therefore be slower
May 26, 1976
3183
1
Biferrocenylene (2,3)(BF
