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Experiment 9. Synthesis and Electrochemical Characterization of Ferrocene. The two most common ligands to appear in organometallic complexes of the transition metals are the carbonyl ligand, :CO, and the cyclopentadienyl ligand, C5H5. Elucidation of the structure of ferrocene, Fe(5-C5H5)2, in the early 1950's provided a leap forward in understanding the structure of unsaturated organic ligands coordinated with metals through -orbitals. For this, Geoffroy Wilkinson received the Nobel Prize in 1973. Prior to Wilkinson's characterization it was thought that the structure might resemble that of Hg(1-C5H5)2, with the H Hg H C5H5 rings coordinated through a single carbon atom as a localized carbanion. Early 1H-NMR characterization on Hg(1-C5H5)2 indicated that all five ring protons were equivalent at room temperature, as might be expected for a pentahapto (5) structure. However, at low temperature the A2B2X pattern expected for the monohapto (1) could be resolved. Compounds of this type were called "Whizzers" by Cotton. In contrast, ferrocene gave only a sharp singlet in the NMR indicating that all five carbon atoms were equivalent at all temperatures. The "sandwich" structure of ferrocene has been extended to a wide class of metallocenes shown below. Molecular orbital calculations on ferrocene revealed

that the metal more closely resembled d6 Fe(II), rather than d8 Fe(0), since the ligand-based orbitals at lowest energy in the diagram below are filled with 12 e-, and the metal-localized d-orbitals contain 6 e- with dz2 (a1g) as the filled HOMO (highest occupied molecular orbital). Consequently, the electronic structure of ferrocene is best described as FeII((5-C5H5-)2. Intuitively, this is quite reasonable since the cyclopentadienide ligands have an aromatic 6 electron configuration and the complex as a whole, satisfies the EAN rule. Looking at other members of the metallocene series above one can find other 6 rings including C6H6 (benzene), C7H7+ (tropylium cation), and C4H42- (cyclobutadienide dianion), and even 10 C8H82- (cyclooctatetraene dianion) for larger actinide metals.

Metallocene complexes of general form M(C5H5)2 are now known for first row transition metals ranging from Ti(II) to Ni(II). "Nickelocene" is an interesting example since, if both C5H5 rings were pentahapto, the complex would exceed the EAN rule with 20 e-. Instead, one ring "slips", coordinating with the metal asymmetrically through only 3 of the 5 carbons, contributing 4 e- as an allyl anion. In this way the EAN is satisfied. More generally, metallocene "ring slipped" structures provide a mechanism for creating a vacant coordination site at the metal for substrate ligand addition in a catalytic cycle. As may be seen from the diagram above, the HOMO of ferrocene is the axially-symmetric a1g (dz2) orbital. Ferrocene is easily oxidized to the ferrocenium cation, [Fe(5-C5H5)2]+, containing a single dz2 electron. This electrochemical couple is quite reversible, and it is used as a reference in the characterization of other compounds. In our synthetic procedure we will prepare ferrocene and use our sample in a cyclic voltammetric experiment to study its electrochemical properties.

Part I. The synthesis of ferrocene. Cyclopentadiene is most stable in the form of the Diels-Alder dimer, C10H12, shown below. Usually, the first step in the synthetic procedure is the formation of

heat KOH

K+

Fe

2 K

+

+

Fe

2+

cyclopentadiene by the pyrolysis of dicyclopentadiene in a high-boiling point solvent (nujol). Next, the cyclopentadiene obtained will be treated with KOH to form potassium cyclopentadienide (Cp-). Ferrocene will be formed by treating the K+Cp- solution with Fe2+. In our procedure we will begin with a preprepared solution of sodium cyclopentadienide in THF, and add this to a DMSO solution containing Fe2+. Equipment: Two-neck round bottom flask, magnetic stirring bar, mineral oil bubbler, N2, syringe. Materials: 2.0 M sodium cyclopentadienide solution in THF, FeCl2 · 6 H2O, DMSO, 6 M HCl Procedure: Cap both necks of the round-bottom flask with septum caps. Use one neck as the N2 inlet through a syringe tip; the other neck will be the outlet with the flow of N2 monitored using a mineral oil bubbler. Bubble N2 through the flask for approximately 1 min to create an inert atmosphere. Inject into the flask 12.5 mL of 2.0 M Na(C5H5) (25 mmol) in THF. Caution ­ the Na(C5H5) solution is extremely air and moisture sensitive. Transfer to the reaction flask should be carried out under an atmosphere of N2. Prepare a solution of 2.50 g (12.5 mmol) FeCl2 · 6 H2O in 7 mL of DMSO in an erlenmeyer flask and degas the solution with a flow of N2. Using a syringe, add the DMSO solution to the solution containing sodium cyclopentadienide. Addition should be dropwise over the period of 15 min. Continue stirring the solution for 20 min and transfer the mixture to a 100 mL beaker containing 30 g of ice. Test the solution with pH paper. If the solution is basic add 6 M HCl to neutrality. Orange crystals of ferrocene will have formed. Collect the crystals by filtration, and wash them with distilled water, small portions of cold isopropanol, and diethyl ether. Characterization Record the melting point temperature of the ferrocene (174° C) and the infrared spectrum (KBr).

Part II. Study of the Electrochemical Properties of Ferrocene using Cyclic Voltammetry. Cyclic voltammetry (CV) is the tool most commonly used in the electrochemical characterization of redox-active compounds. We will begin by considering the redox couple formed by the oxidation of ferrocene to the ferrocenium cation. Both the neutral ferrocene molecule and the cation are stable in solution, and the redox couple is said to be reversible. Irreversible couples arise when one of the species formed by oxidation (or reduction) undergoes a chemical change. The species that is then re-reduced is chemically different from the species oxidized and reduction takes place at a voltage different from the oxidation voltage. In principle, the oxidation and reduction of ferrocene take place at the same potential. However, there is typically a finite solution resistance between the working and reference electrodes resulting in a slight difference in potential between the oxidation and reduction steps (E). A typical CV for ferrocene is shown below. The horizontal coordinate of the plot is electrochemical potential presented in units of volts or millivolts. The more negative the reduction potential, the more difficult the species is to reduce, and the higher the energy of the electronic orbital receiving the electron. Species that are reduced at high negative potentials are typically strong reducing agents in their reduced form. Sodium ion is reduced to sodium metal at -2.71 V relative to the reduction potential of hydrogen ion. Species that are easily reduced undergo reduction at a positive potential, relative to hydrogen ion. Fluorine is reduced to fluoride at +2.87 V. Fluoride is a poor reducing agent, but fluorine is a strong oxidizing agent. This behavior is summarized in a standard table of reduction potentials that may be found typically as an appendix in most general chemistry texts.

The vertical coordinate is current in units of microamps. Current is a measure of the number of electrons transferred in the process, and it is typically quite small since the number of electroactive species absorbed on the electrode

surface is small. CV plots are conventionally presented with voltage increasing negatively to the right. The current increase above the horizontal is associated with a reduction process as the scan in potential is made in a negative direction and it is said to be cathodic. Once all the species on the electrode has been reduced the scan may be reversed in direction so as to oxidize the species that was just reduced. The electrons produced by the oxidation step appear as an anodic current below the horizontal. If the couple, consisting of oxidation and reduction steps, is reversible the cathodic and anodic currents will be the same. Further, if the couple is truly reversible the oxidation and reduction steps will occur at nearly the same potential or with a Nernstian E no greater than 57 mV. In practice, solution resistance often results in a slightly greater E, and the couple is said to be quasireversible. A third property of the CV is scan rate. A typical value for scan rate is 100 mV/sec. Often, if the reduction (or oxidation) is irreversible due to a chemical transformation, it may become more reversible at a rapid scan rate as oxidation takes place before transformation. The potential at which a reversible redox couple takes place is recorded at the midpoint of E. However, the zero point potential is somewhat arbitrary. Conventionally, potentials are referenced to the Normal Hydrogen Electrode (NHE), but experimentally it is usually difficult to assemble a hydrogen electrode for comparison. Other reference electrodes commonly used are the Standard Calomel Electrode (SCE) which utilizes the reduction potential of Hg+ at 0.241 V vs NHE and the Ag/AgCl reference at 0.197 V vs NHE. All of these are cumbersome to assemble, and the Fc/Fc+ couple of ferrocene has become a useful standard since both ferrocene and ferrocenium are chemically inert. In practice, the CV of a compound of interest is recorded. Ferrocene is then added to the solution and the redox potentials of the compound are referenced to the Fc/Fc+ couple as zero. Or, if desired, the experimental couple can be referenced to any of the other standard reference electrodes by noting that the Fc/Fc+ couple appears at 0.400 V vs NHE. Shown below are the oxidation and reduction steps of Ru(acac)3 (a), the same CV for the solution containing an equal concentration of ferrocene (b), the Ru(acac)3/ferrocene solution referenced to SCE (c) and the Ru(acac)3/ferrocene solution referenced to a Cu wire.

In our experimental procedure we will be using the electrochemical cell described below. The cell consists of a working electrode (WE), a counter electrode (CE), and a reference electrode (RE).

The potential at the WE is monitored and controlled precisely with respect to the RE via the potentiostat (P). The potentiostat is typically interfaced to a computer which controls the scan in potential at the WE and records current transfer. The counter electrode is used to compensate for the resistance of the solution. Typically, electrochemical measurements are recorded under N2 to avoid redox interference from O2. Solvents used must be redox inactive over a broad range in potential, and capable of dissolving the compound of interest and a relatively high concentration of electrolyte to reduce solution resistance. Solvents commonly used include acetonitrile, dichloromethane, and sometimes DMSO. The salts commonly used as electrolytes are tetraalkylammonium salts due to their redox inactivity. In our experimental procedure we will use acetonitrile as the solvent, with a solution 0.1 M tetrabutylammonium perchlorate (TBAP), as the

electrolyte, and 0.001 M in ferrocene. Once prepared, the solution may be added to the cell for electrochemical characterization. Questions 1. The molecule W(C5H5)2(CO)2 shows a single 1H-NMR resonance for the Cp rings at room temperature, but the resonance splits at lower temperature to give one sharp resonance and one that is significantly broadened. Suggest a structure for W(C5H5)2(CO)2 that satisfies the EAN rule. 2. Suggest a structure for nickelocene, Ni(C5H5)2, that satisfies the EAN rule. 3. Ru(acac)3 is observed to undergo reversible oxidation at 0.602 V and reduction at -1.157 V vs Fc/Fc+ (see the figure above). Where would these couples appear if referenced to the NHE? 4. Can ferrocene reduce H+?

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