Exploring Ferrocene Chemistry: Properties, Synthesis, and Reactions

1. Lecture 15a Metallocenes

2. Ferrocene I • Ferrocene • It was discovered by two research groups by serendipity in 1951 • P. Pauson: Fe(III) salts and cyclopentadiene • S. A. Miller: Iron metal and cyclopentadiene at 300 oC • It is an orange solid • Thermodynamically very stable due to its 18 VE configuration • Cobaltocene (19 VE) and Nickelocene (20 VE) (and their derivatives) on the other side are very sensitive towards oxidation because they have electrons in anti-bonding orbitals • Ferrocene can be oxidized electrochemically or by silver nitrate to form the blue ferrocenium ion (FeCp2+)

3. Ferrocene II • Pauson proposed a structure containing two cyclopentadiene rings that are connected to the iron atom via s-bonds • The diene should undergo Diels-Alder reaction, but ferrocene does not! Instead it undergoes aromatic substitution i.e., FC-acylation • During the following year, G. Wilkinson (NP 1973) determined that it actually possesses sandwich structure, which was not known at this point • The molecule exhibits D5d-symmetry, but is highly distorted in the solid state because of the low rotational barrier around the Fe-Cp bond (~4 kJ/mol) • All carbon atoms have the same distance to the Fe-atom (204 pm) • The two Cp-rings have a distance of 332 pm (ruthenocene: 368 pm)

4. Ferrocene III • In solution, a fast rotation is observed due to the low rotational barrier around the Fe-Cp axis: • One signal is observed in the 1H-NMR spectrum (d=4.15 ppm) • One signal in the 13C-NMR spectrum (d=67.8 ppm) • Compared to benzene the signals in ferrocene are shifted upfield • This is due to the increased p-electron density (1.2 p-electrons per carbon atom in ferrocene vs. 1 p-electron per carbon atom in benzene) • The higher electron-density causes an increased shielding of the hydrogen atoms and carbon atoms in ferrocene • The shielding is larger compared to the free cyclopentadienide ligand(NaCp: dH=5.60 ppm (THF), dC=103.3 ppm)

5. Ferrocene IV • Cyclopentadiene • It tends to dimerize (and even polymerize) at room temperature via a Diels-Alder reaction • It is obtained from the commercially available dimer by cracking, which is a Retro-Diels-Alder reaction (DH>0, DS>0) • The monomer is isolated by fractionated distillation (b.p.=40 oC vs. 170 oC (dimer)) and kept at T= -78 oC prior to its use • Note that cyclopentadiene is very flammable, forms explosive peroxides and also a suspected carcinogen

6. Ferrocene V • Acidity of cyclopentadiene • Cyclopentadiene is much more acidic (pKa=15) than other hydrocarbon compounds i.e., cyclopentene (pKa=40) or cyclopentane (pKa=45) • The higher acidity is due to the resonance stabilized anion formed in the reaction • The cyclopentadienide ion is aromatic (planar, cyclic, conjugated, possesses 6 p-electrons)

7. Ferrocene VI • The high acidity implies that cyclopentadiene can be deprotonated with comparably weak bases already i.e., OH-, OR- • Potassium cyclopentadienide is ionic and only dissolves well in polar aprotic solvents i.e., DMSO, DME, THF, etc. • The reaction has to be carried out under the exclusion of air because KCp is oxidized easily, which is accompanied by a color change from white over pink to dark brown

8. Ferrocene VII • The actual synthesis of ferrocene is carried out in DMSO because FeCl2 is ionic as well • The non-polar ferrocene precipitates from the polar solution while potassium chloride remains dissolved in this solvent • If a less polar solvent was used (i.e., THF, DME), the potassium chloride would precipitate while the ferrocene would remain in solution

9. Ferrocene VIII • Infrared spectrum • n(CH, sp2)= 3085 cm-1 • n(C=C)= 1411 cm-1 • asym. ring breathing: n= 1108 cm-1 • C-H in plane bending: n= 1002 cm-1 • C-H out of plane bending: n= 811 cm-1 • asym. ring tilt: n= 492 cm-1 (E1u) • sym. ring metal stretch: n= 478 cm-1 (A2u) • Despite the large number of atoms (21 total=57 modes total), there are only very few peaks observed in the infrared spectrum….why? • Point group: D5d: 4 A1g, 2 A1u, 1 A2g, 4 A2u, 5 E1g, 6E1u, 6 E2g, 6 E2u • Only the modes highlighted in bold red are infrared active! n(CH, sp2) n(C=C) asym. ring breathing

10. Synthesis I • Alkali metal cyclopentadienides • Alkali metals dissolve in liquid ammonia with a dark blue color due to solvated electrons that are trapped in a solvent cage • The addition of the cyclopentadiene to this solution causes the color of the solution to disappear as soon as the alkali metal is consumed (‘titration’) • Magnesium • It is less reactive than sodium or potassium because it possesses often a thick oxide layer and does not dissolve well in liquid ammonia  • Its lower reactivity compared to alkali metals demands elevated temperatures (like iron) to react with cyclopentadiene

11. Synthesis II • Transition metals are generally not reactive enough for the direct reaction except when very high temperatures are used i.e., iron (see original ferrocene synthesis) • A metathesis reaction is often employed here • The reaction of an anhydrous metal chloride with an alkali metal cyclopentadienide • The reaction can lead to a complete or a partial exchange • The choice of solvent determines, which product precipitates

12. Synthesis III • Problem: Most chlorides are hydrates, which react with the Cp-anion in an acid-base reaction  • The acid strength of the aqua ion depends on the metal and its charge • The smaller the metal ion and the higher its charge, the more acidic the aqua complex is • All of these aquo complexes have higher Ka-values than CpHitself (Ka=1.0*10-16), which means that they are stronger acids

13. Synthesis IV • Anhydrous metal chlorides can be obtained from various commercial sources but their quality is often questionable  • They can be obtained by direct chlorination of metals at elevated temperatures (200-1000 oC) • The dehydrating of the metal chloride hydrates with thionyl chloride or dimethyl acetal to consume the water in a chemical reaction • Problems: • Accessibility of thionyl chloride (restricted substance because it used in the illicit drug synthesis)  • Production of noxious gases (SO2 and HCl) which requires a hood  • Very difficult to free the product entirely from SO2  • Anhydrous metal chlorides are often not very soluble 

14. Synthesis V • The hexammine route circumvents the problem of the conversion of the hydrated to the anhydrous forms ofthemetal halide • The reaction of ammonia with the metal hexaaqua complexes affords the hexammine compounds • Color change: dark-redtopink (Co), greentopurple(Ni) • Advantages: • A higher solubility in some organic solvents  • The ammine complexes are less acidic than aqua complexes because ammonia itself is less acidic than water!  • They introduce an additional driving force for the reaction  • Disadvantage: • [Co(NH3)6]Cl2 is very air-sensitive because it is a 19 VE system. It changes to [Co(NH3)6]Cl3 (orange) upon exposure to air

15. Synthesis VI • The synthesis of the metalloceneuses the ammine complex • The solvent determines which compound precipitates • THF: the metallocene usually remains in solution, while sodium chloride precipitates • DMSO: the metallocene often times precipitates, while sodium chloride remains dissolved • The reactions are often accompanied by distinct color changes i.e., CoCp2: dark-brown, NiCp2: dark-green • Ammonia gas is released from the reaction mixture, which makes the reaction irreversible and highly entropy driven 

16. Properties I • Alkali metal cyclopentadienides are ionic i.e., KCp, NaCp, etc. • They are soluble in many polar solvents like THF, DMSO, etc. • They are insoluble in non-polar solvents like hexane, pentane, etc. • They react readily with protic solvents like water and alcohols • They react with chlorinated solvents • Many divalent transition metals form sandwich complexes i.e., ferrocene, cobaltocene, etc. • They are non-polar • They are soluble in non-polar solvents like hexane, pentane, etc. • They are poorly soluble in polar solvents for most parts • Their reactivity towards chlorinated solvents varies greatly because of their redox properties • Many of the sandwich complexes can also be sublimed because they are non-polar i.e., ferrocene can be sublimed at ~80 oC in vacuo (MCp2: DHsubl.= ~72 kJ/mol (M=Fe, Co, Ni)

17. Properties II • Cobaltocene is a fairly strong reducing reagent (E0= -1.33 V vs. FeCp2) • 19 valence electron system with the highest electron in an anti-bonding orbital • The oxidation with iodine leads to the light-green cobaltocenium ion, which is often used as counter ion to crystallize large anions • The reducing power can be increased by substitution on the Cp-ring i.e., Co(CpMe5)2: (E0= -1.94 V vs. FeCp2) • Cobaltocene is paramagnetic while the cobaltocenium ion is diamagnetic

18. Properties III • HgCp2 can be obtained as a yellow solid from aqueous solution, but is sensitive to heat and light! • HgCp2undergoes Diels-Alder reactions but only exhibits one signal in the 1H-NMR and the 13C-NMR spectrum (d=5.8 ppm,116 ppm), which in indicative of 1, 5-bonding • A similar mode is found in Zn(CpMe5)2

19. Applications I • Cp2TiCl2 • Used to prepare the Tebbe reagent (used to convert keto to alkene functions) • Used to prepare Cp2TiS5, which is a precursor to cyclic sulfur allotropes (i.e., S6, S7, S9-15, S18, S20) • Used to prepare CpTiCl3, which is used for the syndiotacticpolymerization of styrene

20. Applications II • Cyclopentadiene compounds of early transition metals i.e., titanium, zirconium, etc. are Lewis acids because of the incomplete valence shell i.e., Cp2ZrCl2 (16 VE) • Due to their Lewis acidity they were used as catalyst in the Ziegler-Natta reaction (Polymerization of ethylene or propylene) • Of particular interest for polymerization reactions areansa-metallocenes because the bridge locks the Cp-rings and also changes the reactivity of the metal center based on X • Variations of the cyclopentadiene moiety leads to the formation of catalyst that yield different forms of polypropylene (atatic, isotactic, syndiotactic)

21. Applications III • Metallocenes are used to prepare thin films of metals or metal oxides via OMCVD (Organometallic Chemical Vapor Deposition) • Pyrolysis (ansa-Zr and Hf metallocenes) • Reduction with hydrogen (i.e., NiCp2, CoCp2) • Catalysis of formation of carbon nanotubes (i.e., FeCp2)