19F NMR Spectroscopy as a Characterization Tool for Substituted

Chapter 20 19

F NMR Spectroscopy as a Characterization Tool for Substituted Ferrocene E. J. Hawrelak

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Department of Chemistry, Bloomsburg University of Pennsylvania, Bloomsburg, PA 17815

The reaction of sodium cyclopentadienide (NaCp) and iron(II) bromide (FeBr ) in refluxing THF for 1 h afforded ferrocene (Cp Fe). The Cp ligands of ferrocene were lithiated via the reaction between ferrocene and n-butyllithium. The dilithioferrocene product ([C H Li] Fe) was reacted with the fluorinated phenyl compounds, octafluorotoluene (CF C F ) and hexafluorobenzene (C F ) to afford two fluoro-susbstituted ferrocene products, 1,1'-octaflourophenylferrocene ([{CF C F }C H ] Fe) and 1,1'-pentaflurorphenylferrocene ([{C F }C H ] Fe. Both H and F NMR were used to characterize the compounds. 2

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Introduction This experiment introduces a class of compounds known as metallocenes. Metallocenes are compounds formedfromtwo cyclopentadienyl ligands and a metal ion. The cyclopentadienyl (Cp) ligand is a monoanionic ligand with the formula C H \ A common starting material for the preparation of metallocenes is sodium cyclopentadienide, NaC H , produced via the reaction of cyclopentadiene, C H with sodium hydride, NaH (eq 1). This simple 5

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5

6

NaH + C H 5

6

• NaC H 5

5

(1)

"sandwich" structural motif (A, Figure 1) is known for many transition metals. 288

© 2007 American Chemical Society

In Modern NMR Spectroscopy in Education; Rovnyak, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

289 Among these, the best characterized are M = V, Cr, Fe, Co, Ni, Ru, and Os. Others have been observed or proposed as reactive intermediates.


Figure 1. Various Structures of "Metallocenes " Variations on the metallocene structure are shown in Β - G. The metallocenium ions (B) are simply charged metallocene species; examples include M = Fe and M = Co. "Bent" metallocenes (C) have been extensively studied with metals of groups 3-6, lanthanide elements, thorium, and uranium. Bent metallocenes have applications as catalysts. "Half metallocenes (D) are a huge category that includes any complex with just one cyclopentadienyl ligand. Two especially well studied examples are CpMn(CO) and CpFe(CO) X, where X = CI, Br, or CH . Metallocenes may be ring-substituted (E), which changes their physical properties and chemical reactivity. Because of the isoelectronic relationship between Cp and benzene, many of the same structural motifs are found in the compounds formed when benzene bonds to transition metals (F, G). 3

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Bonding in Metallocenes 1

The Cp ligand may bond to metal ions in a variety of ways, η (monohapto), η (trihapto), or η (pentahapto). The most common boding mode for Cp is η , for which several different resonance structures can be drawn for the bonding of an T]-Cp ligand to a transition metal ion (Figure 2). The actual bonding picture of a metallocenes is complex, requiring an analysis of various metal-ligand interactions. The molecular orbital (MO) diagram of ferrocene, which best describes the bonding in a Cp complex, will be used as an example. Consider the three types of MO's of a Cp ligand (20 MO's are possible), zero nodes, one node, and two nodes (Figure 3). Generating all 20 MO's for the Cp ligand is an excellent exercise for students. The mixing of these Cp MO's with the s, p, and d orbitals of iron generate the appropriate molecular orbital diagram for the representative metallocenes, ferrocene. One such interaction is between the orbital of iron and the appropriate group Cp orbital, one of the 1 nodal surfaces (Figure 4). The complete molecular orbital diagram, representing the bonding in 3

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Figure 2. Resonance Structures of Cp Bonding

Figure 3. Molecular Orbital Types for Cp Ligands

Figure 4. Bonding and Antibonding Orbital Overlap within Ferrocene


291 ferrocene is shown in Figure 5 (appendix). Generating the MO diagram for Ferrocene is another good exercise for students. More details of the bonding picture can be found in any good inorganic chemistry textbook.


Ferrocene The synthesis and structural characterization of ferrocene (1,2) was one of a relatively small number of truly surprising discoveries that led to the emergence of organometallic chemistry as a separate discipline with its own guiding principles, empirical knowledge base, practitioners, and applications. The idea of ligand face-bonding was not easily accepted by leading scientists of the day, partly because the initial "proof of the structure rested heavily on a relatively new technique for the organic chemists of that time - infrared spectroscopy. Within a few years the molecular structure had been unambiguously determined using X-ray diffraction, and numerous congeners had been prepared, so that even the most skeptical were well satisfied. The synthesis of ferrocene has been accomplished so many different ways that it would be difficult to know which is best. However, one of the most general and reliable methods of preparing many different metallocene complexes is a simple substitution process, in which halide ligands are replaced by Cp anions. This is the method we will follow in the synthesis of ferrocene (eq 2). In both FeBr and Cp Fe, iron is considered to have an oxidation state of (+2) and the two ligands are each (-1). 2

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2 NaC H + FeBr 5

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• (r| -C H ) Fe + 2 NaBr 5

(2)

5 2

Ferrocene is a volatile, air-stable, orange solid, easily purified by sublimation, recrystallizationfromhexanes, or chromatography on alumina. The best-known reactions of ferrocene are Friedel-Crafts acetylation (eq 3) using aluminum chloride as the catalyst. In fact, the Friedel-Crafts "aromatic" substitution reaction originally led chemists to propose the name "ferrozene" in analogy to benzene, even though most other typical aromatic substitution conditions (bromination, nitration, sulfonation) lead instead to oxidation. The bestcontrolled of these oxidations uses sulfuric acid (eq 4). Cp Fe + CH3COCI Cp Fe + H7SO4 (cone) 2

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• (X] -CH COC H )(T] -C H )FQ 5

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• [Cp Fe] S0 + H 2

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(3) (4)

The oxidation of ferrocene (eq 5) is electrochemically reversible, and its potential can be adjusted by attaching substituents to the Cp ligands. Also, synthetic methods for attaching ferrocene to other organic molecules are well developed. These two facts have enabled organic chemists to prepare substances that have interesting, adjustable, and possibly useful electrochemical properties. This idea probably accounts for most of the ongoing research in ferrocene chemistry today.


292 5

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(n -C H ) Fe + e 5

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• 0i -C H ) Fe

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+

E° = 0 mV

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(5)

Experimental Objective In this sequence, ferrocene (A where M = Fe) and two substituted ferrocenes in which each Cp ligand bears one pentafluorophenyl or a perfluorotoluene substituent (Ε, M = Fe, R = C F or CF C F ), will be prepared. These three complexes will be compared using NMR spectroscopy and can also be compared by electrochemistry if desired. This experiment will also introduce F NMR. The magnetic properties of F are similar to *H, resulting in well resolved and easily integrated NMR spectra Table 1. 6


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Table 1. Comparison of Magnetic Properties of *H and F Isotope Ή F 19

Natural Abundance 99.985 100.00

Relative Sensitivity 1.0 0.83

Frequency 250 MHz 400 MHz 250.00 400.00 235.19 376.31

Discussion 5

While the physical and chemical properties of η -0 Η metal complexes with varying electron-donating Cp ring substituents are well known (3), complexes with Cp ring substituents of electron-withdrawing character (4) are underrepresented in the literature. The isolation of the Cp ligand as a stable cyclopentadienyl anion, and the stability of the Cp ring substituents toward electrophilic/oxophilic transition metalfragmentsare two challenges that plague this class of ligands. The pentafluorophenyl (5) and perfluoro-4-tolyl (6) substituted Cp ligands are not hindered by these issues and are straightforward to prepare. Ferrocene, l,r-bis(pentafluorophenyl)ferrocene, and l,r-bis(perfluoro-4tolyl)ferrocene are each prepared via a ligand-substitution procedure (7) (eq 2) with the appropriate ligand sodium salt. Each compound can be isolated as an air-stable reddish-orange solid. While ferrocene remains stable in solution, the pentafluorophenyl- and perlfuoro-4-tolyl-susbstituted ferrocenes will slowly decomposed in solution. The *H NMR spectrum of ferrocene (Figure 6) shows a singlet at 4.16 ppm corresponding the ten equivalent hydrogen atoms on the two Cp rings. The Cp rings can easily "spin" above and below the iron center (barrier to rotation -1 kcal/mol). The symmetry of ferrocene is lowered by the addition of either 5

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293 Ο CO

Η 0 Downloaded by UNIV LAVAL on October 20, 2015 | http://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0969.ch020

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Figure 6. *HNMR Spectrum of(C H ) Fe in CDCl 5

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the pentafluorophenyl or perfluoro-4-tolyl substituent on the Cp ring. The respective *H NMR spectra demonstrate this fact, showing two peaks for the two sets of equivalent hydrogen atom on the Cp rings (Figure 7 and 8). While the H NMR spectra provides evidence of a mono-substituted Cp ring, they provide no evidence as to the identity of the substituents. The question that needs to be answered is "did the fluorinated groups remain intact during the synthesis of the ferrocenes?" !

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Figure 7. HNMR Spectrum of [(C F H ] Fe in CDCl 6

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hexane acetone ι • ι ' I ι » ι ι I

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Figure 8. HNMR Spectrum of[(CF C F )C H ] Fe 3

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Analysis of l,r-bis(pentafluorophenyl)ferrocene, and l,l'-bis(perfluoro-4tolyl)ferrocene using F NMR will address the unanswered question. Similar tô *H NMR and contrary to C NMR, F NMR peaks can be integrated to determine the number of fluorine atoms corresponding to a particular peak. Also, F NMR peaks demonstrate similar splitting patterns as *H NMR peaks, following the n+1 rule. This is true when the spectra are proton-decoupled and the only splitting is the result of neighboring fluorine atoms. Each of the spectra reported here are proton-decoupled. The F NMR spectrum of l,r-bis(pentafluorophenyl)ferrocene shows three peaks corresponding to the three fluorine environments of the pentafluorophenyl moiety (Figure 9). The ortho-fluorine atoms can be assigned to the doublet at - 140.8 ppm due to the integration of 4 (2 fluorine atoms per ring) and splitting by the meta-fluorine. The multiplet at -159.3 ppm with an integration of 4 fluorine atoms corresponds to four meta fluorine atoms of the two rings. The final peak, a triplet with an integration value of 2 represents the para-fluorine atoms for each ring. The NMR spectrum of l,r-bis(perfluoro-4-tolyl)ferrocene also contains three peaks (Figure 10). The ortho-fluorine atoms are assigned to the multiplet at -139.5 ppm and the meta-fluorine atoms to the mutliplet at -143.1 ppm due to the integration values of 4 and the trend that ortho-fluorine atoms are further downfield than meta-fluorine atoms. The trifluoromethyl moiety is represented by the peak at -57.4 ppm based on the integration value of 6. 19

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Figure 9. FNMR Spectrum of [(C F )C H ] Fe in CDCl 6

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Figure 10. F NMR Spectrum of [(CF C F )C H ] Fe in CDCl 3

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References: 1. Kealy, T.J.; Pauson, P.L. Nature, 1951, 168, 1039. 2. Wilkinson, G.; Rosenblum, M.; Whiting, M.C.; Woodward, R.B. J. Am. Chem. Soc., 1952, 74, 2125. 3. (a) Möhring, P.C.; Covill, N.J. Coord. Chem. Rev. 2006, 250, 18. (b) Zachmanoglou, C.E.; Docrat, Α., Bridgewater, B.M. J. Am. Chem. Soc. 2002, 124, 9525. (c) Meier, E. J. M.; Koźmiński, W.; Linden, Α.; Lusternberger, P.; von Philipsborn, W. Organometallics, 1996, 15, 2469. (d) Möhring, P.C.; Coville, N. J. J. Organometal. Chem. 1994, 479, 1. (e) Maitlis, P. M. Acc. Chem. Res. 1978,11,301. (f) King, R.B.Coord.Chem. Rev. 1976, 20, 155. 4. (a) Baschky, M. C.; Sowa, J. R., Jr.; Gassman, P. G.; Kass, S. R. J. Chem. Soc. Perkin Trans 2 1996, 213. (b) Herberich, G. E.; Fischer, A. Organometallics 1996, 15, 58. (c) Schut, D. M.; Weakley, T. J. R.; Tyler, D. R. New J. Chem. 1996, 20, 113. (d) Hughes, R. P.; Trujillo, H. A. Organometallics, 1996, 15, 286. (e) Oberoff, M.; Duda, J. K.; Mohr, R.; Erker, G.; Fröhlich, R.; Grehl, M. Organometallics 1996, 15, 4005. (f) Barthel-Rosa, L. P.; Catalano, V. J.; maitra, K.; Nelson, J. H. Organometallics, 1996, 15, 3924. (g) Hauptman, E.; Waymouth, R. M.; Ziller, J. W. J. Am. Chem. Soc. 1995, 117, 11586. (h) Miquel-Garcia, J. Α.; Adams, H.; Bailey, Ν. Α.; Mailtis, P. M. J. Chem. Soc. Dalton Trans. 1994, 116, 385. (i) Lee, I.-M.; Gauthier, W. J.; Ball, J. M.; Iyengar, B.; Collins, S. Organometallics, 1992, 11, 2115. (j) Wei, C.; Aigbirhio, F.; Adams, H.; Bailey, Ν. Α.; Hempstead, P. D.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1991, 883. (k) Lichtenberger, D. L.; Renshaw, S. K.; Basolo, F.; Cheong, M. Organometallics, 1991, 10, 148. (l) Piccolrovazzi, N.; Pino, P.; Consiglio, G.; Sironi, Α.; Moret, M. Organometallics, 1990, 9, 3908. (m) Burk, M. J.; Arduengo, A. J., III; Calabrese, J. C.; Harlow, R. L. J. Am. Chem.Soc.1989, 111, 8938. (n) Finch, W. C.; Anslyn, Ε. V.; Grubbs, R. H. J. Am. Chem. Soc. 1988, 110, 2406. (o) Bernheim. M.; Boche, G. Angew. Chem. Int. Ed. Engl. 1980, 19, 1010. 5. (a) Deck, P.A. Coord. Chem. Rev. 2006, in press, (b) Deck, P.A.; Jackson, W.F.; Fronczek, F.R. Organometallics, 1996, 15, 5287. 6. (a) Deck, P.A.; McCauley, B.D.; Slebodnick, C. J. Organomet. Chem.

2006, 691, 1973. 7.

(a) Gubin, S. P. Dokl. Akad. Nauk SSSR Ser. Khim. 1972, 205, 346. (b) Organomet. Synth. 1965, 1, 64. King, R. B., ed. New York: Academic Press, (c) Newmeyanov, A. N. Dokl. Akad. Nauk SSSR Ser. Khim. 1964, 154, 646. (d) Fischer, E. O.; Fellmann, W. J. Organomet. Chem. 1963, 1, 191. 8. (a) Deck, P.A.; Lane, M.J.; Montgomery, J.L.; Slebondnick, C.; Fronczek, F.R. Organometallics, 2000, 19, 1013. (b) Guillaneux, D.; Kagen, H.B.; J.


297 Org. Chem. 1995, 60, 2502. (c) Riant, O.; Argouarch, G.; Guillaneux, D.; Samuel, O.; Kagan, H.B. J. Org. Chem. 1998, 63, 3511. (d) Dietz, S.D.; Bell, W.L.; Cook, R.L. J. Organomet. Chem. 1997, 546, 67. (e) Rausch, M.D.; Moser, G.A.; Meade, CF. J. Organomet. Chem. 1973, 51, 1. (f) Slocum. D.W.; Englemann, T.R.; Ernst, C.; Jennings, C.A.; Jones, W.; Koonsvitsky, B.; Lewis, J.; Shenkin, P.J. Chem. Educ. 1969, 46, 145.


Appendix: Experimental Section 5

Preparation of Ferrocene, (n -C H ) Fe s

5 2

A dry 100-mL Schlenk flask is fitted with a stir bar, sealed with a rubber septum, and flushed with nitrogen. The flask is evacuated and transferred to the glove box. Inside the glove box, the flask is charged with 5.0 mmol of FeBr . Outside the box, the flask is connected to a nitrogen line, and about 50 mL of THF is admitted using a canulla and "two-needle" technique. Then, 11.0 mmol of NaCp is added as a 2.0 M solution in THF (5.5 mL) using a syringe and "twoneedle" technique. A condenser is fitted, and the solution is stirred under reflux, under N for about 1 h and then cooled. The solvent is then removed simply by connecting the Schlenk flask to the rotary evaporator (only the reactants are air sensitive — the product can survive exposure for short periods). Apply the vacuum cautiously to avoid bumping. After evaporating all of the THF, add about 10 mL of hexane to the flask and evaporate again. This is a useful trick for removing the last traces of THF. About 50 mL of dichloromethane is then added to the flask to dissolve the product. The dark residue is then filtered through a short (3-5 cm) column of alumina. You should use another 50 mL portion of dichloromethane to rinse out the flask and make sure all the product is eluted. The eluent is evaporated in a (pre-weighed) round-bottom flask to obtain the ferrocene product. Sublime a small sample (perhaps 20-50 mg - enough for NMR) and characterize the compound by H NMR in CDC1 or C D . 2

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Preparation of Ι,Γ-dilithioferrocene, l,r-^ -C H Li) Fe 5

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A dry 50 mL Schlenk flask is fitted with a stir bar, sealed with a rubber septum, and flushed with nitrogen. The flask is charged with 3.0 mmol of (C H ) Fe, 900 of freshly distilled tetramethylethylenediamine, TMEDA, and approximately 15 mL of pentane. The solution is cooled in an ice bath and 5

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298 6.6 mmol of 1.6 M n-BuLi in hexanes is syringed in dropwise. The ice bath is removed and the reaction mixture is stirred for 18 h at ambient temperature. When the stirring is stopped, the orange solid settled to the bottom of the flask, and the solvent is carefiilly decanted off under a nitrogen purge. The solid is washed with pentane (2 χ 20 mL) to remove excess TMEDA and n-BuLi. The dilithioferrocene product can be isolated and stored under an inert atmosphere or it can be used immediately for the preparation of l,r-[(C F )C5H ] Fe or Ι,Γ[(CF C F4)C5H ] Fe. 6

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Preparation of l,r-Bis(perfluoro-4-tolyl)ferrocene, 1,Γ-[η 6

(CF C F )C5H ] Fe Downloaded by UNIV LAVAL on October 20, 2015 | http://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0969.ch020

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A dry 100-mL Schlenk flask is fitted with a stir bar, sealed with a rubber septum, and flushed with nitrogen. The flask is charged with 21.0 mmol of perfluorotoluene, CF C F and approximately 20 mL of THF. The solution is cooled in an ice bath and 3.0 mmol of l,r-[r) -C H Li) Fe as a slurry in approximately 40 mL of pentane is added using a canulla and "two-needle" technique. The ice bath is removed and the reaction is stirred for 18 h at ambient temperature. The solvent is then removed simply by connecting the Schlenk flask to the rotary evaporator (only the reactants are air sensitive ~ the product can survive exposure for short periods). The dark residue is dissolved in a mixture of hot hexane/toluene and filtered through a short (3-5 cm) column of alumina. The solution is transferred to a pre-weighed round-bottom flask and placed in the freezer until the next lab period. Isolation of the deep reddishorange solid (68% literature yield, 21 % undergraduate student yield) can be achieved via vacuum filtration. Finally, the substituted ferrocene should be characterized by H and F NMR in CDC1 or C D . The NMR spectrum should be obtained quickly as the product will slowly decompose in solution. H NMR (CDC1 ) δ 4.93 (m, 4H), 4.52 (t, 4H). F NMR (CDC1 ) δ -55.6 (t, 6F), -138.6 (m, 4F), -142.3 (m, 4F). 3

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Synthetic Procedure for l,r-Bis(pentafluorophenyl)ferrocene, 1,Γ-[η 5

(C F )C5H ] Fe 6

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For the substituted ferrocene, (C F C H4) Fe, the procedure is essentially the same as for l,r-fa -(CF C F4)C H4] Fe, few details are different. First, 21.0 mmol of hexafluorobenzene, C F , is used instead of octafluorotoluene. The l,r-[(C F )C H4] Fe is best purified by recrystallization from hexane. Dissolve your sample in a minimal amount of warm hexane, stopper the flask, label it, and place it in the refrigerator until the next lab period. Isolation of the deep reddish-orange solid (88% literature yield, 25 % undergraduate student yield) can be achieved via vacuumfiltration.Finally, the substituted ferrocene 6

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299 should also be characterized by Ή NMR, also in CDC1 or QD . The NMR spectrum should be obtained quickly as the product will slowly decompose in solution. Ή NMR (CDC1 ) δ 4.78 (tt, 4H), 4.42 (t, 4H). F NMR (CDC1 ) δ140.67 (d, 4F), -159.17 (t, 2F), -164.07 (m, 4F). 3

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Figure 5. Molecular Orbital Diagram of Ferrocene


19F NMR Spectroscopy as a Characterization Tool for Substituted

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