Lecture 15a Metallocenes

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 o C 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 (FeCp 2 + )

Ferrocene II Pauson proposed a structure containing two cyclopentadiene rings that are connected to the iron atom via  -bonds The diene should undergo Diels-Alder reaction, but ferrocene does not! Instead it undergoes aromatic substitution i.e., Friedel-Crafts 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 D 5d -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)

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 1 H-NMR spectrum (  =4.15 ppm) One signal in the 13 C-NMR spectrum (  =67.8 ppm) Compared to benzene the signals in ferrocene are shifted upfield This is due to the increased  -electron density (1.2  -electrons per carbon atom in ferrocene vs. 1  -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:   =5.60 ppm (THF),  C =103.3 ppm)

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 (  H o = 77 kJ/mol,  S o = J/mol*K,  G o = 34.6 kJ/mol, K eq (25 o C)=8.6*10 -7, K eq (180 o C)=3.6*10 -2 ) The monomer is isolated by fractionated distillation (b.p.=40 o C vs. 170 o C (dimer)) and kept at T= -78 o C prior to its use Note that cyclopentadiene is very flammable, forms explosive peroxides and also a suspected carcinogen

Ferrocene V Acidity of Cyclopentadiene Cyclopentadiene is much more acidic (pK a =15) than other hydrocarbon compounds i.e., cyclopentene (pK a =40) or cyclopentane (pK a =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  -electrons)

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

Ferrocene VII The actual synthesis of ferrocene is carried out in DMSO because FeCl 2 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

Ferrocene VIII Infrared spectrum (CH, sp 2 )= 3085 cm -1 (C=C)= 1411 cm -1 asym. ring breathing:  1108 cm -1 C-H in plane bending:  1002 cm -1 C-H out of plane bending:  811 cm -1 asym. ring tilt:  492 cm -1 (E 1u ) sym. ring metal stretch:  478 cm -1 (A 2u ) 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: D 5d : 4 A 1g, 2 A 1u, 1 A 2g, 4 A 2u, 5 E 1g, 6 E 1u, 6 E 2g, 6 E 2u Only the modes highlighted in bold red are infrared active! (CH, sp 2 ) (C=C) asym. ring breathing

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 The THF adduct of MgCp 2 displays one cyclopentadienide ligand bonded via a  -bond (  1 ) and the second one via pentahapto (  5 )

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

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 K a -values than CpH itself (K a =1.0* ), which means that they are stronger acids Aqua complexKaKa [Fe(H 2 O) 6 ] * (~hydrocyanic acid) [Fe(H 2 O) 6 ] *10 -3 (~phosphoric acid) [Co(H 2 O) 6 ] *10 -9 (~hypobromous acid) [Ni(H 2 O) 6 ] * (~hypoiodous acid) [Al(H 2 O) 6 ] *10 -5 (~ acetic acid)

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 ( o C) 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 (SO 2 and HCl) which requires a hood  Very difficult to free the product entirely from SO 2  Anhydrous metal chlorides are often not very soluble 

Synthesis V The hexammine route circumvents the problem of the conversion of the hydrated to the anhydrous forms of the metal halide The reaction of ammonia with the metal hexaqua complexes affords the hexammine compounds Color change: dark-red to pink (Co), green to purple (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(NH 3 ) 6 ]Cl 2 is very air-sensitive because it is a 19 VE system. It changes to [Co(NH 3 ) 6 ]Cl 3 (orange) upon exposure to air

Synthesis VI The synthesis of the metallocene uses 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., CoCp 2 : dark-brown, NiCp 2 : dark-green Ammonia gas is released from the reaction mixture, which makes the reaction irreversible and highly entropy driven 

Properties I Alkali metal cyclopentadienides are ionic i.e., LiCp, NaCp, KCp, etc. They are soluble in many polar solvents like THF, DMSO, etc. but they are insoluble in non-polar solvents like hexane, pentane, etc. They react readily with protic solvents like water and alcohols (in some cases very violently) Many of them react with chlorinated solvents as well because of their redox properties KCp LiCp, NaCp

Properties II Many divalent transition metals form sandwich complexes i.e., ferrocene, cobaltocene, etc. They are non-polar They are often soluble in non-polar or low polarity solvents like hexane, pentane, diethyl ether, dichloromethane, etc. but are usually poorly soluble in polar solvents Their reactivity towards chlorinated solvents varies greatly because of their redox properties The M-C bond distances differ with the number of total valence electrons (i.e., FeCp 2 : ~204 pm, FeCp 2 + : ~207 pm; CoCp 2 : ~210 pm, CoCp 2 + : ~203 pm, NiCp 2 : ~214 pm, NiCp 2 + : ~206 pm ) Many of the sandwich complexes can also be sublimed because they are non-polar i.e., ferrocene can be sublimed at ~80 o C in vacuo (MCp 2 :  H subl.= ~72 kJ/mol (M=Fe, Co, Ni)

Properties III Cobaltocene is a fairly strong reducing reagent (E 0 = V vs. FeCp 2 ) 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(CpMe 5 ) 2 : (E 0 = V vs. FeCp 2 ) Cobaltocene is paramagnetic while the cobaltocenium ion is diamagnetic Placing electron-accepting groups on the Cp-ring make the reduction potential more positive i.e., acetylferrocene (E 0 = 0.24 V vs. FeCp 2 ), cyanoferrocene (E 0 = 0.36 V vs. FeCp 2 )

Properties IV HgCp 2 can be obtained as a yellow solid from aqueous solution, but is sensitive to heat and light The X-ray structure displays two  -bonds between the mercury atom and one carbon atom of each ring HgCp 2 does undergo Diels-Alder reactions as well as aromatic substitution (i.e., coupling with Pd-catalyst) In solution, HgCp 2 only exhibits one signal in the 1 H-NMR and the 13 C-NMR spectrum (  =5.8 ppm,116 ppm), which in indicative of  1,  5 -bonding A similar mode is found in BeCp 2 and Zn(CpMe 5 ) 2

Applications I Cp 2 TiCl 2 Used to prepare the Tebbe reagent (used to convert keto to alkene functions) Used to prepare Cp 2 TiS 5, which is a precursor to cyclic sulfur allotropes (i.e., S 6, S 7, S 9-15, S 18, S 20 ) Used to prepare CpTiCl 3, which is used for the syndiotactic polymerization of styrene

Applications II Cyclopentadiene compounds of early transition metals i.e., titanium, zirconium, etc. are Lewis acids because of the incomplete valence shell i.e., Cp 2 ZrCl 2 (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 are ansa-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)

Applications III Mechanism of Ziegler-Natta polymerization of ethylene

Applications IV 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., NiCp 2, CoCp 2 ) Catalysis of formation of carbon nanotubes (i.e., FeCp 2 ) Petasis Reagent (Cp 2 Ti(CH 3 ) 2 ) and Tebbe Reagent (Cp 2 TiCH 2 AlCl(CH 3 ) 2 ) are used to convert carbonyl groups to alkenes via a titanium carbene complex (Cp 2 Ti(=CH 2 ))