Tetraferrocenylethylene, a Chiral, Organometallic Propeller: Synthesis, Structure, and Electrochemistry

4334 Organometallics 1995, 14, 4334-4342

Tetraferrocenylethylene, a Chiral, Organometallic Propeller: Synthesis, Structure, and Electrochemistry

Benno Bildstein,*lt Peter Denifl,? Klaus Wurst,? Max Andre,$ Martin Baumgarten,s Jan Friedrich,$ and Ernst Ellmerer-Miillerl'

Institut f i r Allgemeine, Anorganische, und Theoretische Chemie, Universitat Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria, Biochemie GmbH, A-6250 Kundl, Austria,

Max-Planck-Institut f i r Polymerforschung, 0-55021 Mainz, Germany, and Institut f i r Organische Chemie, Universitat Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria

Received April 24, 1995@

Tetraferrocenylethylene is synthesized from diferrocenyl ketone by three different reductive carbon-carbon bond-forming methodologies: (a) an ultrasound-promoted McMurry reaction with low-valent titanium, (b) a modified Clemmensen reduction with zinc and trimethyl- chlorosilane, and (c) an aluminum-assisted oxygen-tellurium exchange in diferrocenyl ketone and subsequent thermolysis. Mechanistically, the first two methods involve carbenoid intermediates, whereas the third method consists of a twofold extrusion process from a preformed cyclic dimer of diferrocenyl telluroketone. Tetraferrocenylethylene shows spectral properties which are in accord with a sterically highly congested conformation. Noteworthy features include the very low C-C stretching vibration of 1474 cm-' in the Raman spectrum, indicative of an elongated and weak C-C double bond, and the magnetic inequivalence of the lH and 13C NMR signals of the hydrogens and carbons of the substituted cyclopentadienyl rings, indicative of a frozen molecular propeller conformation. An X-ray single-crystal structure analysis shows tetraferrocenylethylene to be a chiral, strongly twisted, and sterically congested olefin. The bond length of 138.1 pm of the central double bond and the angles of twisting and torsion are close in value to those of the most distorted olefins known. The helical chirality stems from the uniform twisting of the four alternatingly arranged ferrocenyl substituents. Electrochemically, tetraferrocenylethylene can be oxidized to the tetracation in accord with the number of ferrocenyl units. The donor ability of tetraferrocenylethylene compared to ferrocene itself is strongly enhanced with A,?P1/2 = -220 mV.

Introduction

The normal properties of the olefinic double bond in ethylene can be substantially altered by fourfold at- tachment of sterically demanding, electron-donating or electron-accepting groups. Perturbation of the preferred molecular geometry by bulky substituents can lead to twisting of the double bond or to pyramidalization of the olefinic carbon atoms and to elongation of the C-C double bond. Electron-donating substituents reduce the oxidation potential, whereas electron-accepting substituents increase the electron affinity of the olefinic double bond. In comparison to ethylene these effects result in molecular distortion1 of these tetrasubstituted olefins and in unusual redox behavior and reactivitya2

In this context, tetraferrocenylethylene is an interest- ing target compound because of the electron-donating properties and steric requirements of the ferrocenyl moiety. Structurally, the fourfold substitution of the carbon-carbon double bond should result in a highly strained molecule with an exceptionally elongated olefinic bond. Stereochemically, the twisting of the cyclopentadienyl rings in relation to the C=C plane should cause atropisomerism. Electronically, the powerful electron-donating capacity of the ferrocenyl moieties3 will substantially ease oxidation.

Here we report the synthesis, characterization (NMR, IR, W-vis, Raman, MS), structure (X-ray), attempted separation of enantiomers by HPLC, and electrochemistry (CV, PES) of tetraferr~cenylethylene.~

' Institut fur Allgemeine, Anorganische, und Theoretische Chemie, Universitat Innsbruck. * Biochemie GmbH.

8 Max-Planck Institut fur Polymerforschung. I' Institut fur Organische Chemie, Universitat Inssbruck. @ Abstract published in Advance ACS Abstracts, July 15, 1995. (1) (a) Luef, W., Keese, R. Strained Olefins: Structure and Reactivity

of Nonplanar Carbon-Carbon Double Bonds. Top. Stereochem. l S S l p 0 , 231. (b) Bock, H.; Ruppert, K.; Nather, C.; Havlas, Z.; Herrmann, H.- F.; h a d , C.; Gobel, I.; John, A.; Meuret, J.; Nick, S.; Rauschenbach, A.; Seitz, W.; Vaupel, T.; Solouki, B. Angew. Chem. 1992, 104, 564; Angew. Chem., Int. Ed. Engl. lSS2,31, 550. (c) Borden, W. T. Chem. Rev. 1989,89, 1095.

( 2 ) (a) Wiberg, N. Angew. Chem. 1968,80,809; Angew. Chem., Int. Ed. Engl. 1968, 7 , 766. tb) Hoffmann, R. W. Angew. Chem. 1968,80, 823;Angew. Chem., Int. Ed. Engl. 1968, 7 ,754 . (c) Hocker, J.; Merten, R. Angew. Chem. 1972,84, 1022; Angew. Chem., Int. Ed. Engl. 1972, 1 1 , 964. (d) Lappert, M. F. J. Orgunomet. Chem. 1988, 358, 185. (e) Fatiadi, A. J. Synthesis 1986, 249. (0 Fatiadi, A. J. Synthesis 1987, 85. (g) Fatiadi, A. J . Synthesis 1987, 959.

0 276-7333195123 14-4334$09.00/0

Results and Discussion

Preparation of Tetraferrocenylethylene (1). Com- pound 1 can be prepared by three different synthetic routes, as outlined in Scheme 1.

Method a. The ultrasound-promoted McMuny reac-

( 3 ) (a) Watts, W. E. J. Organomet. Chem. Libr. 1979, 7 , 399. (b) Watts, W. E. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, U.K., 1982; Vol. 8, Chapter 59, p 1051.

( 4 ) Part of this work has been presented as a poster at the XVth International Conference on Organometallic Chemistry, Warsaw, Poland, Aug 9-14, 1992 and at the Xth FECHEM Conference on Organometallic Chemistry, Agia Pelagia, Crete, Greece, Sept 5-10, 1993.

0 1995 American Chemical Society

Tetraferrocenylethylene Organometallics, Vol. 14, No. 9, 1995 4335

Scheme 2. Twofold Te-Extrusion Mechanism for Method ca

r 1

Scheme 1"

Fc + Fc, ,Fc

Fc Fc Fc F=C, Fc

2 3 I ' a Legend: (a) TiC1~~3THF/Li/DME/ultrasoun& (b) Zn/Me&CY

THF; (c) Med-Te-AlMeddioxane; Fc = ferrocenyl.

tion5 of diferrocenyl ketone gives a high yield of 1 (80%) in a mixture with tetraferrocenylethane6 (2) and traces of hexaferrocenylcyclopropane (3). Unsuccessful attempts to synthesize compound 1 from diferrocenyl ketone or from diferrocenyl thioketone by a McMurry reaction have been r e p ~ r t e d . ~ The failure to obtain olefinic products has been attributed to steric hindrance, but the combination of the proper choice of reducing agent (TiCly3THFLi versus the more common hydride- containing TiCldLiAlH4 or versus TiClJZn) and im- proved reaction conditions (ultrasonic enhancements of the heterogeneous McMurry reaction) results in complete consumption of the starting material diferrocenyl ketone with a good yield of a product mixture of 1, 2, and 3. Although the exact nature of the low-valent Ti reagent and the mechanism of the McMurry reaction is still under discus~ion,~ the small traces of hexaferrocenylcyclopropane (31, a formal trimer of diferro- cenylmethylidene, indicates the involvement of carbenoid intermediates, which either couple to olefin 1 or oligomerize to cyclopropane 3. Ethane 2 is formed most likely by hydrogen abstraction from the solvent.

Method b. Reduction of diferrocenyl ketone by Z d Me3SiCl similarly affords a mixture of mainly 1 with 2 as byproduct. Mechanistically, this reaction can be viewed as a modificationlo of the more familiar Clem- mensen reduction, in which the proton has been re- placed by a silicon electrophile, whose high oxophilicity removes the carbonyl oxygen as hexamethyldisiloxane and generates an organozinc carbenoid, which forms products 1 and 2 in a manner analogous to that in method a. In this case, no trimerization product 3 can be detected.

Method c. Thermolysis of diferrocenyl telluroketone, prepared from diferrocenyl ketone by reaction with bis- (dimethylaluminum) telluride,ll yields almost quanti-

( 5 ) (a) McMurry, J . E. Chem. Rev. 1989, 89, 1513. (b) Lenoir, D. Synthesis 1989, 883. (c) Betschart, D.; Seebach, D. Chimia 1989, 43, 39. ( d ) McMurry, J. E. Acc. Chem. Res. 1983, 16, 405. (6) Paulus, H.; Schlogl, K.; Weissensteiner, W. Monatsh. Chem.

1982, (7)

Sato, (8)

113, 767. (a) Lenoir, D., Burghard, H. J . Chem. Res. Synop. 1980,396. (b) M.; Asai, M. J . Organomet. Chem. 1992, 430, 105. Navak. S. K.: Banerii, A. J . 0r.g. Chem. 1991, 56, 1940.

(9) Fuistner, A. Angew."Chem. 1953, 105, 171; Angew. Chem., Int. Ed. Engl. 1993, 32, 164.

(10) (a) Motherwell, W. B.; Nutley, C. J. Contemp. Org. Synth. 1994, 1,219. (b) Motherwell, W. B. Aldrzchim. Acta 1992.25, 71. (c) Banerjee, A. K.; de Carrasco, M. C. S.; Frydych-Houge, C. S. V.; Motherwell, W. B. J . Chem. SOC., Chem. Commun. 1986, 1803. (11) Denifl, P.; Bildstein, B. J. Organomet. Chem. 1993, 453, 53.

Fc [2 + 21

Fc > W C 2 :C=Te -

Fc, ,Fc - Te

Fc'""Fc -

Fc = ferrocenyl.

Fc, ,Te Fc

Fc Fc ,C-'<

tatively elemental tellurium and olefin 1 and traces of 2. Probably this redox disproportionation is analogous to other reductive carbon-carbon bond-forming reactions of selenoketones by a twofold extrusion process.12 First the telluroketone reacts in a [2 + 21 cycloaddition to give the dimer 2,2,4,4-tetraferrocenyl-1,3-ditellur- etane. This proposed reaction is based on the recent mass spectroscopic evidence and X-ray structure13 of the first 1,3-ditelluretanes and on the analogous reactions of sterically protected germanethione, germaneselone, stannanethione, and stannaneselone compounds.14 Tet- raferrocenylethylene (1) is formed from this cyclic dimer of diferrocenyl telluroketone by stepwise extrusion of elemental tellurium; the first extrusion yields tetra- ferrocenyltellurirane, which fragments further to afford 1 in a second extrusion step (Scheme 2).

In terms of availability of starting materials, yield of ethylene 1, and convenience of reaction conditions, method b is the preferred preparation of compound 1. Both methods (a) and (c) involve a multiple-step procedure: either the preparation of the McMurry reagent from TiC13*3THF and Li powder or the cumbersome and stench-affected synthesis of bis(dimethylaluminum1 telluride from bis(triorganostanny1) telluride and tri- methylal~minum,'lJ~~ respectively.

For all three synthetic routes the purification of 1 necessitates its separation from the byproduct 2. Un- fortunately, these two compounds have very similar physical properties and are therefore difficult to separate by conventional means. After prepurification of l by fractional crystallization and column chromatography on silica the remaining traces of byproduct 2 are removed either chemically or physically. Chemically, extended treatment with palladium on activated carbon in refluxing decalin selectively reduces 2 t o diferrocenylmethane, which can be separated from 1 by conventional column chromatography. Physically, HPLC purification of 1 is achieved on a 150 mg scale on Nucleosil c18 (10 pm) as stationary phase with THF/ H2O as mobile phase, taking into consideration the low solubility of 1, 2, and 3 in conventional HPLC solvent mixtures and the oxidative decomposition of 1 in polar

(12) (a) Guziez, F. S., Jr.; SanFilippo, L. J.; Murphy, C. J.; Moustakis, C. A,; Cullen, E. R. Tetrahedron 1985,41,4843. (b) Back, T. G.; Barton, D. H. R.; Britten-Kelly, M. R.; Guziec, F. S., Jr . J . Chem. SOC., Perkin Trans. 1 1976, 2079. (13) (a) Segi, M.; Koyama, T.; Takata, Y.; Nakajima, T.; Suga, S. J .

Am. Chem. SOC. 1989,111, 8749. (b) Boese, R.; Haas, A,; Limberg, C. J . Chem. SOC., Dalton Trans. 1993, 2547. (14) Okazaki, R.; Tokitoh, N.; Ishii, A,; Ishii, N.; Matsuashi, Y.;

Matsumoto. T.: Suzuki, H. Phosphorus; Sulfur Silicon Relat. Elem. 1992, 67, 49.

4336 Organometallics, Vol. 14, No. 9, 1995 Bildstein et al.

I --__._ __....__. " ....-_-_--.-_.......-. - ---- -- F l O

IO@-

6G-

6 0.

48

2P

E

1 1 4

3

Figure 1. FAELMS of hexaferrocenylcyclopropane (3).

solvents. In the case of method a cyclopropane 3 can be obtained in a total yield of 7 mg from collecting the corresponding fractions of three chromatographic separations. Due to this limited amount of 3 only characterization by FAELMS (Figure 1) was possible.

Properties of Tetraferrocenylethylene. Com- pound 1 remains unchanged in the solid state (mp > 350 "C) under an inert atmosphere (Ar) at ambient temperature but decomposes in oxygen-containing solution over time to insoluble unidentified oxidized products. A preliminary screening of the chemical properties of 1 include the following reactions. The typical reactivity of ferrocene is partially retained in 1; fourfold Friedel-Crafts acylation with acetyl chloride affords tetrakid 1-acety1ferrocenyl)ethylene together with some less acetylated products in 75% overall isolated yield. Unexpectedly, a Mannich reaction with biddimethyl- amino)methane/H3POJCH&O2H or with a preformed iminium salt15 (MeZN=CHz)+ gives no aminomethylated product. The central double bond in olefin 1 is unre- active, probably for steric reasons. Attempted complex formation with transition-metal carbonyls2d under thermal or photochemical activation and attempted diox- etane formation with singlet oxygen16 give no reaction.

In the 'H NMR spectrum of 1 (Figure 2) the 16 hydrogens of the substituted cyclopentadienyl rings are magnetically equivalent for all four ferrocenyl substituents and magnetically inequivalent for the four hydrogens of each cyclopentadienyl unit, resulting in one set of four slightly broadened singlets. No resolved coupling for this ABCD spin system could be obtained, although the usually observed lH-lH coupling con- stants for ferrocene derivatives are in the range of 1-3

(15) Kinast, G.; Tietze, L.-F. Angew. Chem. 1976, 88, 261; Angew. Chem., Int. Ed. Engl. 1976, 15, 239.

(16) (a) Adam, W. In Small Ring Heterocycles. Part III. Ozirenes, Arene Oxides, Oxaziridines, Dioxetanes, Thietanes, Thietes, Thiazetes, and Others; Hassner, A,, Ed.; Wiley-Interscience: New York, 1985; Chapter IV, p 351. (b) Adam, W.; Cilento, G. Angew. Chem. 1983,95, 525; Angew. Chem., Int. Ed. Engl. 1983, 22, 529. ( c ) Adam, W.; Encarnacion, L. A. A. Chem. Ber. 1982, 115, 2592.

' . E t 6

1 -3

61 1 69 sBl 8

THF B

5 . 4 5 . 1 5 . 0 4 . 8 4 . 6 4 . 4 4 . 2 4 . 0 1 .8 1 6

F 1 I M "

Figure 2. Pulsed field gradient enhanced HSQC of tetraferrocenylethylene (1).

Hz.17 Cooling does not change the appearance of these signals, indicating no exchange equilibrium. The half- width of the residual proton of the solvent CDCl3 indicates paramagnetic impurities (partially oxidized 1)

(17) Levenberg, M. I.; Richards, J. H. J . Am. Chem. SOC. 1964,86, 2634.

Tetraferrocenylethylene

as being responsible for the unresolved coupling in the lH NMR spectrum. We tried to remove these detrimen- tal traces by treating a THF solution of 1 with excess Li powder as a reducing agent and filtering the resulting suspension under an atmosphere of argon, but no improvement in the signal width was observed. The 13C NMR spectrum (Figure 2) shows five signals for the substituted cyclopentadienyl ring in addition to one signal for the unsubstituted cyclopentadienyl ring. We interpret this as evidence for a “frozen propeller’’ conformation in solution, where each ferrocenyl group is locked in a position with little or no rotational freedom due to steric interactions. Similar arguments have been reported for other tetrasubstituted olefinic molecular propellers.ls The 13C NMR shift of the olefinic carbon is observed at 133.7 ppm, which is rather unexceptional for a C=C bond, in accord with the location of the corresponding 13C NMR signals in other highly strained olefins.la In contrast, the Raman-active C=C stretching vibration of 1474 cm-l is very low in comparison to those for other distorted olefins,la indicating reduced n-bond order and considerable twist caused by steric repulsions of the four ferrocenyl groups.

A detailed interpretation of the W-vis and PE spectral data is hampered by the ferrocenyl substituents. Comparison of the visible absorptions of 1 with W-vis data for sterically congested alkeneslaJg and for metallocenes20 hints at an assignment of the absorption at 503 nm (log E = 3.50) as indicative of a strongly twisted and partially conjugated C=C bond. The PE spectrum of 1 shows the expected ionizations for the ferrocenyl moiety (6.74 and 9.00 eV):21 one shoulder at 6.14 eV and one band at 10.06 eV. Vertical ionization potentials of sterically strained olefins show decreased values (8.8-7.9 eV)la in comparison to that of ethylene (10.51 eV). A further shifting of the first ionization to lower energy (6.1 eV for tetrakis(dimethy1amino)ethylene, for example) has been observed in electron-rich olefins,22 which makes these compounds strong reducing agents comparable to alkaline-earth metals (Ca, 6.1 eV). In our case, an assignment of the 6.14 eV band in 1 to the ionization of the C=C bond is not corroborated by cyclic voltammetric measurements (see Electrochemis- try) and by the unobserved chemical reactivity of 1 as a reducing agent (with the exception of the partial oxidative decomposition of 1 in solution). The 10.06 eV band in 1, on the other hand, seems to be too high in energy for a sterically congested and electron-rich olefinic compound such as tetraferrocenylethylene.

Electrochemistry. Oligoferrocenes, such as ferrocene itself, usually undergo reversible one-electron oxidations of each ferrocene unit, where the separation of the consecutive half-wave potentials may vary over a wide range, depending on the interaction and substi-

Organometallics, Vol. 14, No. 9, 1995 4337

Table 1. Half-Wave Potentials 0” of 1 in Comparison to Model Compounds 2,4,5,6, and 7b

(18) (a) Gur, E.; Kaida, Y.; Okamoto, Y.; Biali, S. E.; Rappoport, Z. J . Org. Chem. 1992, 57, 3689. (b) Maeda, K.; Okamoto, Y.; Toledano, 0.; Becker, D.; Biali, S. E.; Rappoport, Z. J. Org. Chem. 1994,59, 5473. (c) Columbus, I.; Biali, S. E. J . Org. Chem. 1994, 59, 3402.

(19) Beck, A.; Gompper, R.; Polborn, K.; Wagner, H.-U. Angew. Chem. 1993,105, 1424; Angew. Chem., Int. Ed. Engl. 1993,32, 1352.

(20) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. SOC. 1971,93, 3603.

(21) Zerner, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-Westerhoff, U. T. J . Am. Chem. SOC. 1980, 102, 589.

(22) (a) Cetinkaya, B.; King, G. H.; Krishnamurthy, S. S.; Lappert, M. F.; Pedley, J. B. J . Chem. SOC., Chem. Commun. 1971, 1370. (b) Bock, H.; Borrmann, H.; Havlas, Z.; Oberhammer, H.; Ruppert, K.; Simon, A.Angew. Chem. 1991,103,1733;Angew. Chem., Int. Ed. Engl. 1991, 30, 1678.

1 2 4 5 6 7 ~~~

E’iiz 0.09 0.20 0.35 0.26 0.25 0.28 E21/z 0.26 0.37 0.48 0.37 0.41 0.28 E3 O.4lc 0.52c E4 0.62c 0.57c

a Versus SCE at 100 mV/s in 0.1 M [TBAIPF~CHZC~Z at -30 “C. Legend: 2, tetraferrocenylethane; 4,1,1-diferrocenylethylene; 5, diferrocenylmethane; 6, trans-1,2-diferrocenylethylene; 7, 1,2- diferrocenylethane. Determined by rectangular voltammetry because of partial adsorption on electrode surface.

: -5.000

-4.000

- 3 -3.000 1

- 1 -2.000

-1.000 c 1

i - I 1 I I , -0.300 -0.100 0.100 0.300 0.500 0.700 0,900

E (VI

Figure 3. Rectangular voltammogram of tetraferrocenylethylene (1).

tution pattern of the ferrocenyl subunits.23 Electro- chemical measurements of 1 yield four distinct oxidation steps with formation of the tetracation (see Table 11, demonstrating the highly efficient donor capacity of tetraferrocenylethylene. The good donor ability is also established by a drastically diminished first half-wave oxidation potential of 0.09 V compared to ferrocene itself (0.31-0.32 VI. Under standard conditions used for cyclic voltammetric detection of the oxidation potentials in solution, the last step occurs somewhat irreversibly, probably due to decreased solubility leading to adsorption phenomena of the compound on the electrode surface. The rectangular voltammogram (Figure 31, on the other hand, enables the clear determination of the fourth half-wave potential a t 0.62 V. The separation of the half-wave potentials do not allow us to identify a clear assignment of the interactions between the cis/ trans forms of the 1,2-substituted ethylene or with the 1,l’-substituted units. Therefore, the following model compounds were included in the studies to serve as references for specific ferrocene-ferrocene interactions: tetraferrocenylethane (21, 1,l-diferrocenylethylene (41, diferrocenylmethane (51, trans-1,2-diferrocenylethylene (61, and 1,2-diferrocenylethane (7) (Table 1). The saturatively bridged tetraferrocenylethane (2) is characterized by four oxidation steps, as found for 1, but the complete oxidation process occurs in a much smaller potential range (A[E1-E41 = 370 mV, versus 530 mV for 1). Thus, the ethane moiety diminishes the interaction between the ferrocene substituents as expected and shows the smaller oxidation power indicated by a positively shifted first oxidation potential. The relatively large difference of the first two oxidation potentials of 2 stem from a considerable Coulomb

(23) (a) Levanda, C . ; Beechgard, K.; Cowan, D. 0. J . Org. Chem. 1976, 41 , 2700. (b) Jaitner, P.; Schottenberger, H.; Gamper, S.; Obendorf, D. J. Orgunomet. Chem. 1994, 475, 113.


Figure 4. Molecular structure of tetraferrocenylethylene (11, showing the two independent molecules A and B and the two THF molecules in the asymmetric unit.

interaction (in contrast to 71, which seems somewhat unusual for ethane-bridged ferrocenes but may be explained by the large steric demands of two ferrocenyl moieties on each carbon center, such that no anti conformation can be chosen as in 1,2-diferrocenylethane (7), where no such interaction can be envisaged.

X-ray Diffraction Structure of Tetraferrocenyl- ethylene. The above results are in accord with a conformationally distorted olefin. For an exact descrip- tion of the molecular structure of 1, a single-crystal X-ray diffraction study proved necessary. The growth of suitable crystals of 1 is difficult, because this compound tends to form very thin platelets and decomposes partially in solution. After many attempts, suitable crystals were finally obtained from a concentrated solution of 1 in a mixture of THF and H20, kept overnight under argon at 4 "C. Figure 4 shows the contents of the asymmetric unit; the crystallographic data are listed in Table 2, and the final positional parameters are given in Table 3. Compound 1 is chiral, displays 0 2 molecular symmetry in the crystal, and crystallizes as the racemate with two crystallographi- cally independent molecules A and B and with two molecules of THF. The THF molecules and the two structurally slightly different molecules A and B seem to be necessary to obtain a sufficient crystal packing. Figures 5 and 6 show the two independent molecules of the asymmetric unit in the same orientation, il- lustrating their similarity. Molecules A and B differ only slightly in their torsion angles and in the bond length of the C=C double bond (Table 4). The central double bond is elongated to 138.4(7) pm in molecule A and 137.8(8) pm in molecule B (mean value 138.1 pm). Comparably high values are 140.2 pm for bis[l,3-bis-

Table 2. Crystal Data and Structure Refinement for 1

mol formula fw cryst system space group unit cell dimens

vol z temp radiation density (calcd) abs coeff F(OO0) color, habit cryst size 0 range for data collection index ranges

no. of rilns collected no. of indep rflns no. of rilns with I > 2 d I ) abs cor refinement method datdrestraintdparameters goodness of fit on F final R indices (I > 2dI ) ) R indices (all data) largest diff peak and hole

[C42H36Fe41'[C4H~Ol [764.141.[72.111 triclinic P1 (No. 2) a = 1162.4(8) pm b = 1581.3(9) pm c = 2067.'0(10) pm a = 107.02(5)" /5' = 94.8(7)" y = 99.21(4)" 3.551(4) nm3 4 183(2) K Mo Ka (1 = 71.073 pm) 1.564 Mg/m3 1.639 mm-' 1728 red platelet 0.8 x 0.4 x 0.1 mm 5.02-23.03" -12 5 h 5 12, -16 5 k 5 16,

-22 5 1 5 22 10 546 8924 (R,,t = 0.0289) 6788 DIFABS full-matrix least-squares on F 8260/0/919 1.039 R1 = 0.0413, wR2 = 0.0890 R 1 = 0.0687, wR2 = 0.1215 393 and -422 e nm-3

(di~yanomethylene)indan-2-ylideneI~~ and 136.gZ4 and 137.0 pm25 for tetrasilylethylenes. The molecular struc-

(24) Sakurai, H.; Ebata, K.; Kabuto, C.; Nakadaira, Y. Chem. Lett.

(25) Sakurai, H.; Tobita, H.; Nakadaira, Y.; Kabuto, C. J . Am. Chem. 1987,301.

Soc. 1982, 104, 4288.

Tetraferrocenylethylene Organometallics, Vol. 14, No. 9, 1995 4339

Table 3. Atomic Coordinates ( x and Equivalent Isotropic Displacement Parameters (pm2 x 1O-I) for la

4668(1) 7008(1) 9111(1)

10114(1) 7224(4) 8276(4) 4633(5) 3569(5) 3013(5) 3726(5) 4722(5) 6082(5) 5053(5) 4538(5) 5261(4) 6238(4) 6075(6) 6377(5) 7610(6) 8079(5) 7137(6) 8034(5) 7580(6)

13682(1) 13790( 1) 12323(1) 9181(1)

12651(4) 11839(4) 12 146( 5) 13090(5) 13958(5) 13570(5) 12446(5) 13479(5) 14510(6) 15264(5) 14714(4) 13599(5) 15593(5) 15246(5) 14603(5) 14535(6) 15161(5) 13056(5) 12779(5)

13592(7) 13685(10) 12742(13)

12311(8) 11879(8) 12926(7)

5916(1) 3328(1) 7060(1) 6590(1) 5438(3) 6043(3) 6600(4) 6588(4) 5684(4) 5130(4) 5694(4) 6597(3) 6582(4) 5677(4) 5134(3) 5684(3) 2973(4) 2210(4) 2342(4) 3195(4) 3584(4) 4086(3) 3231(4)

11325(1) 9486(1)

12877(1) 11359( 1) 11035(3) 11476(3) 10997(4) 11415(4) 10883(4) 10138(4) 10217(4) 12313(3) 12586(4) 11969(4) 11320(3) 11528(3) 9826(4) 8965(4) 9097(4)

10023(4) 10462(4) 9404(3) 8572(3)

10244(5) 10663(7) 10155(6)

4523(5) 5007(6) 5667(6)

1778(1) 1521(1) 877(1)

3230(1) 1797(2) 1903(2) 1081(3) 1380(3) 1240(3) 861(3) 761(3)

2472(3) 2797(3) 2695(3) 2316(3) 2181(2) 2233(3) 1783(3) 1824(3) 2299(3) 2554(3) 1077(3) 605(3)

4953(1) 2481(1) 2382(1) 3501(1) 3429(2) 3214(2) 5330(3) 5868(3) 5774(3) 5178(3) 4910(3) 4536(3) 5017(3) 4807(3) 4187(3) 4000(3) 2713(3) 2195(3) 1636(3) 1794(3) 2461(3) 3332(3) 2813(3)

370(3) -130(5) -682(5)

5548(5) 5129(5) 5085(4)

Molecule A 6351(6) 6022(5) 7092(5) 9231(5) 9623(6) 8677(7) 7719(6) 8062(5)

10182(5) 10439(5) 9415(5) 8510(5) 8970(4)

11653(6) 11808(6) 10996(6) 10343(6) 10743(5) 9551(4) 9712(5) 8977(5) 8365(5) 8700(4)

12166(5) 12066(5) 12600(4) 11031(5) 11894(7) 12962(6) 12761(6) 11559(6) 11564(5) 12354(5) 13495(5) 13419(5) 12227(4) 9189(5) 9728(5) 8907(5) 7849(5) 8014(5) 9694(5) 8635(5) 8838(5)

10036(5) 10595(4)

11763(10) 12516(11)

13795(8) 13519(10)

3110(3) 3900(3) 4523(3) 8403(3) 8295(4) 7780(4) 7558(4) 7943(4) 6452(3) 6354(4) 5895(4) 5718(3) 6055(3) 6238(49 6644(5) 6135(5) 5408(5) 5474(4) 7508(3) 7844(3) 7244(4) 6529(3) 6687(3)

8689(3) 9609(3)

10061(3) 12618(5) 13346(4) 13032(4) 12113(4) 11862(4) 13118(3) 13898(3) 13699(3) 12788(3) 12421(3) 11685(4) 12442(4) 12642(4) 12021(4) 11428(4) 11216(3) 10610(4) 10051(4) 10305(3) 11028(3)

9791(9) 9590(8)

5717(7) 4849(8)

527(3) 950(3)

1291(3) 1399(3) 756(3) 255(3) 583(3)

1289(3) 1373(3) 702(3) 242(3) 625(3)

1340(2) 2919(3) 3630(3) 3906(3) 3370(4) 2766(3) 2804(3) 3524(3) 3773(3) 3207(3) 2596(2)

2235(3) 2396(3) 3088(3) 1584(3) 1592(3) 1527(3) 1479(3) 1514(3) 3262(3) 3256(3) 3203(3) 3173(3) 3228(2) 4530(3) 4358(3) 3914(3) 3813(3) 4188(3) 2561(3) 2508(3) 2910(3) 3210(3) 2988(3)

-334(8) 212(6)

5640(5) 5754(6)

a Ueq) is defined as one third of the trace of the orthogonalized U, tensor.

ture of 1 can be described as a twisted olefin (molecule A, 33.5'; molecule B, 36.7'; mean value 35.1'; Figure 71, which is substituted by four ferrocenyl groups; each of these ferrocenyl substituents shows no out-of-plane bending with the twisted central double bond. In other words, no pyramidalization of the olefinic sp2 carbons occurs, but each ferrocenyl group is uniformly twisted in reference to the C=C plane (molecule A, 21.8'; molecule B, 24.4'; mean value 23.1'; Figure 8) due to steric hindrance of the inner ortho hydrogens of the substituted cyclopentadienyl rings. The four ferrocenyl substituents are alternatingly transoid attached to the central double bond, resulting in a helical vinyl propeller system, which exists in two enantiomeric forms,

differing only in the sign of the twist angle of the four ferrocenyl substituents. Comparable vinyl propellers are fourfold aryl-substituted sterically congested olefins, which have been recently reported.ls

Attempts at Chromatographic Separation of the Enantiomers of Tetraferrocenylethylene. Since attempts to separate tetraferrocenylethane (2) by column liquid chromatography on triacetylcellulose as chiral stationary phase (CSP) failed,6 several chiral stationary phases were tested for the separation of the enantiomers of tetraferrocenylethylene (1) by HPLC. No chiral separation was obtained on (+I-poly(tripheny1- methyl methacrylate) (CHIWPAK OT(+)), which


C i l l l

CItOl

c1311

Figure 5. Molecular structure of molecule A of tetraferrocenylethylene (11, showing the atom-numbering scheme.

CIS11

C1831

21

C1701

C161 I711

CI c1721

Figure 6. Molecular structure of molecule B of tetraferrocenylethylene (l), showing the atom-numbering scheme.

proved an appropriate CSP for the analogous dissymmetric tetramesitylethyleneIs as well as for planar chiral ferrocene compounds.26 Also, cellulose carbamate (CHIRALCEL OD-R) in the reversed-phase mode ex- hibited no enantiomeric selectivity. In addition, cyclo- dextrins as CSP have been examined, assuming a possible separation via inclusion complexes in the reversed-phase mode. No chiral separation was obtained wi th a y-cyclodextrin bonded phase, recom- mended for the chiral separation of dissymmetric molecules,2' o r with permethylated 8-cyclodextrin. Usually permethylated B-cyclodextrin should exhibit enhanced chiral selectivity a n d shorter retention times for very apolar molecules compared to unmodified B-cyclodextrin used for separation of assymmetric metallocene com-

(26) Yamazaki, Y.; Morohashi, N.; Hosono, K. J. Chromatogr. 1991,

(27) Stulcup, A. M.; Jin, H. L.; Armstrong, D. W. J. Liq. Chromatogr. 542, 129.

1990, 13(1), 473.

pounds.28 In all cases unambiguous assignment of enantiomeric peaks by online polarometric detection w a s not possible due to enhanced pseudorotation ef- f e c t ~ . ~ ~ The so fa r unsuccessful resolution of 1 is understandable by inspection of a van der Waals plot of 1. The highly symmetrical ball-like shape of tetraferrocenylethylene leaves little opportunity to form a diastereomeric complex with a chiral stationary phase. In further experiments capillary electrophoresis will be examined as an alternative separation system.

Experimental Section General Comments. All the reactions were carried out

in the absence of air using standard Schlenk techniques and vacuum-line manipulations. Solvents were deoxygenated, purified, and dried prior to use. Instrumentation: Bruker AC 200, Bruker AM 300, Varian UNITYplus 500 ('H and 13C NMR); Nicolet 510 FT-IR (IR); Nicolet 910 FT-Raman (Ra- man); Bruins Instruments Omega 20 (UV-vis); Varian CH-7, Finnigan MAT 95 (MS); Siemens P4 (X-ray); Waters 510/486/ 746 (HPLC); EG&G Princeton Applied Research Potentiostat (CV). Microanalyses were obtained from the Department for Microanalysis, University of Vienna, Vienna, Austria. Diferro- cenyl ketone,30 diferrocenyltelluroketone," Tic1~.3THF,~l 1,l- diferrocenylethylene,32 diferro~enylmethane,3~ trans- 1,2-difer- r~cenylethylene,~~ and 1,2-diferro~enylethane~~ were prepared according to literature methods or were obtained as side products and had properties in accord with literature values.

Tetraferrocenylethylene (1). Method a. A suspension of 1.13 g (5 mmol) of TiC1303THF in 100 mL of DME (dimeth- oxyethane) was reduced with 0.11 g (15 mmol) of lithium powder by immersing the reaction vessel in an ultrasonic cleaning bath (Bandelin SONOREX SUPER RK 255H) at room temperature for 30 min. To the resulting black suspension was added 0.5 g (1.25 mmol) of diferrocenyl ketone, and sonication was continued for an additional ' / z h. The mixture was poured into water, the organic materials were extracted with three portions of ether, and the combined organic layers were washed with water and dried with CaC12. After removal of all volatile materials in uucuo, the product mixture was prepurified by flash chromatography on silica with ethedn- hexane (3/5) as eluent, affording 0.45 g of red, microcrystalline material, consisting of mainly ('80%) tetraferrocenylethene (11, tetraferrocenylethane (2), and traces ( ~ 1 % ) of hexaferrocenylcyclopropane (3). For a relative amount of 80% in this product mixture this corresponds to a 360 mg, 0.47 mmol, 75% yield of 1, based on starting diferrocenyl ketone.

Method b. A Schlenk vessel was charged with 100 mg of Zn powder, 0.10 mL of freshly distilled HC1-free trimethyl- chlorosilane, and 30 mL of THF. To this stirred suspension was added dropwise a solution of 100 mg (0.25 mmol) of diferrocenyl ketone, dissolved in 15 mL of THF. After the mixture was stirred at 0 "C for 2 h, workup similar to that in method a yielded 83 mg of a red amorphous product mixture,

(28) Armstrong, D. W.; DeMond, W.; Czech, B. P. Anal. Chem. 1985, 57. 481.

(29) Dappen, R.; Voigt, P.; Maystre, F.; Bruno, A. Anal. Chim. Acta 1993.282. 47.

(30) &onnor Salazar, D. c.; Cowan, D. 0. J. Organomet. Chem. 1991, 408, 219 and references cited therein.

(31) Manzer, L. E. Inorg. Synth. 1982, 21, 137. (32) 1,l-Diferrocenylethylene was prepared by Wittig olefination of

diferrocenyl ketone and had properties in accord with values reported by: Rinehart, K. L.; Kittle, P. A.; Ellis, A. F. J. Am. Chem. SOC. 1960, 82, 2082.

(33) Diferrocenylmethane was obtained by reduction of 2 with palladium on activated carbon (see Experimental Section) and had properties in accord with values reported by: Pauson, P. L.; Watts, W. E. J . Chem. SOC. 1962, 3880.

(34) trans-1,2-Diferrocenylethylene, with 1,2-diferrocenylethane as byproduct, was obtained by reductive coupling of ferrocenecarboxal- dehyde with Zdtrimethylchlorosilane (analogous to method b for the preparation of 1) and had properties in accord with values reported by: Pauson, P. L.; Watts, W. E. J . Chem. SOC. 1963, 2990.

Tetra ferrocenylethylene Organometallics, Vol. 14, No. 9, 1995 4341

Table 4. Selected Bond Lengths (pm) and Angles (deg) for 1 molecule A molecule B

C(l)-C(2)/C(3)-C(4) C(l)-C(19)/C(3)-C59) C( l)-C(29)/C(3)-C(69) C(2)-C(39)/C(4)-C(89) C(2)-C(49)/C(4)-C(79) Fe( l)/Fe(5)-cent(subst CpP Fe( l)/Fe(5)-cent(unsubst CpP Fe(2)/Fe(G)-cent(subst CpP Fe(2)/Fe(6)-cent(unsubst CpP Fe(3)/Fe(8)-cent(subst CpP Fe(3)/Fe(8)-cent(unsubst CpP Fe(4)/Fe(7)-cent(subst CpP Fe(4)/Fe(7)-cent(unsubst CpP mean Cing-Cing in subst Cp of Fe(l)/Fe(5) mean Cing-Cfing in unsubst Cp of Fe(l)/Fe(5) mean Cing-Cfing in subst Cp of Fe(2)/Fe(6) mean Cfing-Cing in unsubst Cp of Fe(2)/Fe(6) mean Cfing-Cfing in subst Cp of Fe(3)/Fe(8) mean Cing-Cing in unsubst Cp of Fe(3)/Fe(8) mean Cfing-Cing in subst Cp of Fe(4)/Fe(7) mean Cing-Cfing in unsubst Cp of Fe(4)/Fe(7)

Bond Lengths 138.4( 7) 148.6(10) 149.3(7) 147.3(10) 148.3(9) 164.9( 19) 165.3(19) 165.0(19) 165.3(19) 165.1(19) 165.6(20) 164.8(19) 165.0( 19) 14 1.8(8 14 1.6( 9) 14 1.8 9 140.7( 9) 142.0(9) 140.4(11) 141.8(8) 140.3(10)

137.8(8) 147.8( 12) 148.4(7) 147.5(9) 148.1( 7) 165.6(20) 165.9(20) 165.0( 19) 165.5(20) 164.8( 19) 165.3(20) 165.4(19) 165.5(20) 142.2(9) 142.2(9) 142.0(8) 141.9(9) 142.2(9) 14 1.0(9) 142.3(7) 140.4(9)

Angles twist angle C(1)-C(2)/C(3)-C(4)b 33.5U0.69) pyramidalization of C( 1)/C(3F 0.43(1.12) pyramidalization of C(2)/C(4F O.lg(0.97) torsion angle C(l)/C(3)-Cp of Fe(lYFe(5) 22.95(0.80) torsion angle C(l)/C(3)-Cp of Fe(2)/Fe(6) 21.97(0.81) torsion angle C(2)/C(4)-Cp of Fe(3)/Fe(8) 20.3U0.77) torsion angle C(2)/C(4)-Cp of Fe(4)/Fe(7) 2 1.78(0.85)

36.75(0.74) 0.85( 1.11) 1.50(0.89)

24.83(0.94) 23.79(0.87) 23.72(0.85) 25.38(0.85)

a cent = centroid of Cp. Twist angle = mean value of dihedral angles C(29)-C(l)-C(2)-C(39) and C(19)-C(l)-C(2)-C(49) for molecule A and mean value of dihedral angles C(69)-C(3)-C(4)-C(89) and C(59)-C(3)-C(4)-C(79) for molecule B. Pyramidalization is defined as in ref la.

I I

Figure 7. Molecular structure of molecule A of tetraferrocenylethylene (11, viewed down the olefinic C(l)-C(2) bond, showing the twist angle.

Figure 7. Molecular structure of molecule A of tetraferrocenylethylene (11, viewed down the olefinic C(l)-C(2) bond, showing the twist angle.

consisting of mainly ( =. 80%) tetraferrocenylethene (1) and tetraferrocenylethane (2). For a relative amount of 80% in this product mixture this corresponds to a 66 mg, 0.087 mmol, 70% yield of 1, based on starting diferrocenyl ketone. Method c. A Schlenk vessel with an attached reflux

condenser was charged with 30 mL of toluene, 1.57 g (3.27 mmol) of bis(tri-n-butylstannyl) telluride, and 1.63 mL (3.27 mmol) of a 2.0 M trimethylaluminum-toluene solution. After the mixture was stirred at 110 "C for 16 h, the solvent and all volatile materials were removed on a vacuum line, leaving a white solid residue, which was dissolved in 30 mL of dioxane. To the resulting colorless solution was added 0.65 g (1.63 mmol) of diferrocenyl ketone, and within a few minutes the

A

I

(iFe'4'

\ I

\

Figure 8. Molecular structure of molecule A of tetraferrocenylethylene (11, showing the torsion angle (twisting in reference to the C=C plane) of the ferrocenyl substituents. blue color of diferrocenyl telluroketone indicated initation of the reaction. Stirring was continued for 15 min at room temperature and for 3 h at 95 "C, during which time the color of the reaction mixture changed from dark blue to dark red with precipitation of elemental tellurium in the form of a metallic mirror on the wall of the reaction vessel. After the mixture was cooled to cool to room temperature, the following workup was performed without exclusion of air. The reaction mixture was poured into ice/water and extracted with four portions of ether; the combined organic layers were washed with three portions of water, dried with Na2S04, and evapo- rated to dryness, leaving an oily crude product, which contains

4342 Organometallics, Vol. 14, No. 9, 1995

1 and 2 in addition to organotin compounds and diferrocenyl ketone, formed by hydrolysis of diferrocenyl telluroketone. The organotin compounds were separated by flash chromatography on silica.35 First the organic stannanes were eluted with n-hexane, and second the mixture of 1, 2, and diferrocenyl ketone was eluted with dichloromethane. This mixture was separated into a diferrocenyl ketone fraction and a product fraction by a second flash chromatography on silica with dichloromethaneln-hexane (2/1) as eluent, yielding 0.41 g of a red amorphous product mixture, consisting of mainly (> 80%) tetraferrocenylethene 1 and tetraferrocenylethane (2). For a relative amount of 80% in this product mixture this corresponds to a 328 mg, 0.43 mmol, 53% yield of 1, based on starting diferrocenyl ketone.

Separation of Tetraferrocenylethylene (1) from Tetra- ferrocenylethane (2) by Selective Reduction of 2. A Schlenk vessel with an attached reflux condenser was charged with a solution of 380 mg of a mixture of 1 and 2 dissolved in 20 mL of dry, deoxygenated decalin and with 300 mg of palladium (10%) on activated carbon. After 70 h at reflux temperature (bp 191 "C) the solvent decalin was removed on a vacuum line at 90 "C bath temperature; the residue was dissolved in a small volume of dichloromethane and purified by chromatography on silica with dichloromethaneln-hexane (1/2) as eluent. The first fraction consists of diferrocenylmethane,33 directly followed by the second fraction of tetraferrocenylethylene (1). Recrystallization from THF/water (lo/ 1) yields 174 mg (46% yield based on 380 mg starting material) in the form of thin, red platelets.

Separation of Tetraferrocenylethylene (1) from Tetra- ferrocenylethane (2) by Preparative HPLC. The low solubility of 1,2, and 3 in conventional HPLC solvent mixtures, the oxidative decomposition of 1 in polar solvents, and the similarity in molecular shape of 1 and 2 makes the separation by HPLC difficult. The optimization of HPLC parameters results in a compromise in high loadability, efficiency of separation, and tolerance of 1 toward oxidative decomposition: column dimensions, 250 x 16 mm; stationary phase, Nucleosil CIS (lopm); mobile phase, 60% THF/40% HzO; flow rate, 16 mumin; detection, UV (300 nm); column temperature, ambient; injection volume, 2 mL, 150 mg in 2 mL of THF. Product mixtures from McMurry reactions (method a for the synthesis of tetraferrocenylethylene) contain small traces of hexaferrocenylcyclopropane (31, which are easily separated from 1 and 2 by HPLC. A total yield of 7 mg of 3 could be obtained from collecting the corresponding fractions of three chromatographic separations.

Characterization of 1. Mp: >350 "C. Anal. Calcd for C42H36Fe4: C, 66.02; H, 4.75. Found: C, 65.43; H, 4.61. UV- vis (hexane, 1") [ndlog E]: 203/4.56, 314/3.70,351/3.61,399/ 3.65, 503/3.50. MS (EI, 70 eV) [miz]: 764 (M+), 643 (M+ - CpFe), 578 (M' - (Cp)zFe), 382 (M+/2), 317 ((M+/2) - CpH). MS (FAB) [m/z] : 764 (M+). IR (KBr) [cm-'1: 3097 m, 1560 vw, 1510 vw, 1460 w, 1412 w, 1396 w, 1295 w, 1108 s, 1052 m, 1000 m, 920 w, 814 s. Raman (KBr) [cm-'I: 1474 s, 1386 m, 1269 m, 1210 m, 1168 s, 1058 m, 326 m. lH NMR (CDC13; 500 MHz) [ppml (Figure 2): 4.13 (s, 20 H, unsubst Cp); 4.03, 4.1, 4.43, 5.27 (each signal: unresolved m, 4 H, C(2)-C(5) of subst Cp). 13C NMR (CDC13; 50 MHz, 75 MHz) [ppm] (Figure 2): 69.6 (unsubst Cp); 66.9,68.3,69.1,72.4, (C(2)-C(5) of subst Cp); 90.3 (C(1) of subst Cp); 133.7 (olefinic C). CV (CH2Clz;

Bildstein et al.

(35) Farina, V. J . Org. Chem. 1991, 56, 4985.

250 K) [VI (Table 1, Figure 3): +0.09, +0.26, +0.41, +0.62. PE (HeI; 320-340 "C) [eVl: 6.14, 6.74, 9.00, 10.06.

Characterization of 2. Spectral parameters and a single- crystal structure analysis concur with published data.6

Characterization of 3. MS (FAB) [m/zl (Figure 1): 1146 (M+).

X-ray Structure Analysis of 1. A Siemens P4 diffracto- meter with graphite-monochromatized Mo Ka radiation (,I = 71.073 pm) was used for data collection (Table 2). The unit cell parameters were determined and refined from 25 ran- domly selected reflections, obtained by P4 automatic routines. Data were measured via o-scan and corrected for Lorentz and polarization effects. Scattering factors for neutral atoms and anomalous dispersion corrections were taken from ref 36, and an empirical absorption correction37 was made. The structure was solved by direct methods, SHELXS-86,38 and refined by a full-matrix least-squares procedure using SHELXL-93.39 All non-hydrogen atoms were refined with anisotropic displacement parameters (Tables 2-4). Hydrogen atoms were placed in calculated positions.

Cyclic Voltammetry. The cyclic (CV) and rectangular voltammetric experiments were carried out with an EG&G Potentiostat (Princeton Applied Research). All the measurements of the ferrocenes were taken under strictly inert conditions (Nz, Ar) in dichloromethane (CH2C12) at -30 "C with 0.1 M tetrabutylammonium hexafluorophosphate ([TBAIPFs) as the conducting salt. As a reference for the absolute calibration of the redox potentials we used cobaltoceniud cobaltocene versus a standard calomel electrode (SCE; Ell,z = -1.040 V). The measurements were performed with a scan rate of typically 100 mV/s. A gold disk (1 mm) was used as the working electrode and a platinum wire as the counter electrode. A silver wire was employed as a quasi reference electrode for the calibration of cobaltoceniudcobaltocene.

Acknowledgment. We thank the FWF, Vienna, Austria ( Grant No. P10182) for financial support. We are grateful to Prof. Laszlo Szepes, University of Buda- pest, for PE measurements, Dr. Bill Willis, University of Connecticut, IMS, Storrs, CT, for preliminary valence band XPS measurements, Prof. F. R. Kreissl, Anorgan- isch-Chemisches Institut der Technischen Universitat Miinchen, Miinchen, Germany, for FAB mass spectra, and Dr. Fleischer, Nicolet Instrument GmbH, Offen- bach, Germany, for FT-Raman spectra.

Supporting Information Available: Tables of crystal data and structure refinement details, anisotropic thermal parameters, fractional atomic coordinates and isotropic thermal parameters for the non-hydrogen atoms, all bond lengths and angles, and fractional atomic coordinates for the hydrogen atoms (15 pages). Ordering information is given on any current masthead page.

OM950296V

(36) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. IV, pp 72-98.

(37) North, A. C. T.; Phillips, D.; Mathews, F. S. Acta Crystallogr. 1968, A24, 351.

(38) Sheldrick, G. M. SHELXS-86: Program for Crystal Structure Solutions; University of Gottingen, Gottingen, Germany, 1986.

(39) Sheldrick, G. M. S H E U - 9 3 : Program for the Refinement of Crystal Structures; University of Gottingen, Gottingen, Germany, 1993.

Documents

Tetraferrocenylethylene, a Chiral, Organometallic Propeller: Synthesis, Structure, and Electrochemistry