lecture 15a metallocenes. ferrocene i ferrocene it was discovered by two research groups by...
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Lecture 15a
Metallocenes
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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
+)
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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., 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 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)
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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)
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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 (DHo= 77 kJ/mol, DSo= 142.3 J/mol*K, DGo = 34.6 kJ/mol, Keq(25 oC)=8.6*10-7, Keq(180 oC)=3.6*10-2)
• 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
O
O
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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)
HHH
+ OH-
-H2O
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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
H H
+ KOH K + H2O
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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
FeFeCl2 + 2 K +Cp- + 2 KCl
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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, 6 E1u, 6 E2g, 6 E2u
• Only the modes highlighted in bold red are infrared active!
n(CH, sp2) n(C=C)
asym. ring breathing
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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 elevatedtemperatures (like iron) to react with cyclopentadiene
• The THF adduct of MgCp2 displays one cyclopentadienide ligandbonded via a s-bond (h1) and the second one via pentahapto (h5)
M + C 5 H 6 NH 3 (l) M C 5 H 5 + 1/2 H 2 M=Li, Na, K
M + 2 C 5 H 6 500 o C
M ( C 5 H 5 ) 2 + H 2 M=Mg, Fe
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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
I
MCl 2 + 2 NaC 5 H 5 Solvent
M ( C 5 H 5 ) 2 + 2 NaCl M=V, Cr, Mn, Fe, Co, Ni Solvent= THF, DME, NH 3 (l)
FeCl 2 + C 5 H 6 + 2 Et 2 NH F e ( C 5 H 5 ) 2 + 2 [ E t 2 N H 2 ] C l
M Cl 4 + 2 NaC 5 H 5 T o l u e n e
M= Ti, Zr (C5H5)2MCl2 + 2 NaCl
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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 CpH itself (Ka=1.0*10-16), which means that they are stronger acids
Aqua complex Ka
[Fe(H2O)6]2+ 3.2*10-10 (~hydrocyanic acid)[Fe(H2O)6]3+ 6.3*10-3 (~phosphoric acid)[Co(H2O)6]2+ 1.3*10-9 (~hypobromous acid)[Ni(H2O)6]2+ 2.5*10-11 (~hypoiodous acid)[Al(H2O)6]3+ 1.4*10-5 (~ acetic acid)
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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
CoCl2*6 H2O + 6 SOCl2 CoCl2 + 6 SO2 + 12 HCl
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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(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
[M(H2O)6]Cl2 + 6 NH3 [M(NH3)6]Cl2 + 6 H2O (M=Co, Ni)
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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., CoCp2: dark-brown, NiCp2: dark-green
• Ammonia gas is released from the reaction mixture, which makes the reaction irreversible and highly entropy driven
[M(NH3)6]Cl2 + 2 NaCp MCp2 + 2 NaCl + 6 NH3(g)
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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
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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., FeCp2: ~204 pm, FeCp2
+: ~207 pm; CoCp2: ~210 pm, CoCp2+: ~203
pm, NiCp2: ~214 pm, NiCp2+: ~206 pm )
• 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)
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Properties III
• 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
• Placing electron-accepting groups on the Cp-ring make the reduction potential more positive i.e., acetylferrocene (E0= 0.24 V vs. FeCp2), cyanoferrocene (E0= 0.36 V vs. FeCp2)
2 CoCp2 + I2 2 CoCp2+ + 2 I-
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Properties IV
• HgCp2 can be obtained as a yellow solid fromaqueous solution, but is sensitive to heat and light
• The X-ray structure displays two s-bonds between the mercury atom and one carbon atom of each ring
• HgCp2 does undergo Diels-Alder reactions as well as aromatic substitution (i.e., coupling with Pd-catalyst)
• In solution, HgCp2 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 BeCp2 and Zn(CpMe5)2
HgCl2 + 2 TlCp HgCp2 + 2 TlClH2O
Hg
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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 syndiotactic polymerization of styrene
Na
TiCl4+2 Ti
Cl
+ 2 NaCl
Cl
TiCl4
Ti
S S S
SS
Li(HBEt3)/S8
S
S S S
SS
SCl2
TiCl
Cl
ClS2Cl2
SS
S
SSSS
+ 2 Al(CH3)3- CH4- Al(CH3)2Cl
Ti
Cl
CH2
AlCH3
CH3
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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 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)
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Applications III
• Mechanism of Ziegler-Natta polymerization of ethylene
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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., NiCp2, CoCp2)
• Catalysis of formation of carbon nanotubes (i.e., FeCp2)
• Petasis Reagent (Cp2Ti(CH3)2) and Tebbe Reagent (Cp2TiCH2AlCl(CH3)2) are used to convert carbonyl groups to alkenes via a titanium carbene complex (Cp2Ti(=CH2))