Transcript Chem 174-Lecture 15a..

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 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+)
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)
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)
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
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)
Ferrocene VI
• The high acidity implies that cyclopentadiene can be
deprotonated with comparably weak bases already
i.e., OH-, ORH
H
+ KOH
K
+ H2O
• 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 FeCl2 is ionic as well
FeCl2 + 2 K +Cp-
Fe
+ 2 KCl
• 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
• n(CH, sp2)= 3085 cm-1
n(CH, sp2)
n(C=C)
• n(C=C)= 1411 cm-1
• asym. ring breathing: n= 1108 cm-1
asym. ring
• C-H in plane bending: n= 1002 cm-1
breathing
• 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!
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’)
M + C 5H 6
M + 2 C 5H 6
•
NH3(l)
500 oC
MagnesiumMCl2 + 2 NaC 5H5 Solvent
•
•
•
MC 5H5
+ 1/2 H 2
M(C 5H5)2 + H2
M(C 5H5)2 + 2 NaCl
M=Li, Na, K
M=Mg, Fe
M=V, Cr, Mn, Fe, Co, Ni
Solvent= THF, DME, NH 3(l)
possesses
It is less reactive than sodium or potassium because it
often a thick oxide layer and does not dissolve well in liquid
ammoniaFeCl
 2 + C 5H6 + 2 Et2NH
Fe (C 5H5)2 + 2 [Et2N H2] Cl
Its lower reactivity compared to alkali metals demands elevated
temperatures (like iron) to react with
cyclopentadiene
To l u
ene
(C5H5)2MCl2 + 2 NaCl
MCl4 + 2 NaC 5H 5
The THF adduct of MgCp2 displays one cyclopentadienide
ligand
1
bonded via a s-bond (h ) and the second one via pentahapto (h5)
M= Ti, Zr
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
NH3(l)to a complete or a partial exchange
• The reaction
can lead
M + C 5H 6
MC 5H5 + 1/2 H 2
M=Li, Na, K
• The choice of solvent
determines, which product precipitates
500 oC
M + 2 C 5H 6
MCl2 + 2 NaC 5H5
Solvent
M(C 5H5)2 + H2
M=Mg, Fe
M(C 5H5)2 + 2 NaCl
M=V, Cr, Mn, Fe, Co, Ni
Solvent= THF, DME, NH 3(l)
FeCl2 + C 5H 6 + 2 Et2NH
I
MCl4 + 2 NaC 5H 5
Fe (C 5H5)2 + 2 [Et2N H2] Cl
To lue n e
(C5H5)2MCl2 + 2 NaCl
M= Ti, Zr
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
Aqua complex
[Fe(H2O)6]2+
[Fe(H2O)6]3+
[Co(H2O)6]2+
[Ni(H2O)6]2+
[Al(H2O)6]3+
Ka
3.2*10-10 (~hydrocyanic acid)
6.3*10-3 (~phosphoric acid)
1.3*10-9 (~hypobromous acid)
2.5*10-11 (~hypoiodous acid)
1.4*10-5 (~ acetic acid)
• 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
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 
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
Synthesis VI
• The synthesis of the metallocene uses the ammine complex
[M(NH3)6]Cl2 + 2 NaCp
MCp2 + 2 NaCl + 6 NH3(g)
• 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 
Properties I
• Alkali metal cyclopentadienides are ionic i.e., LiCp, NaCp, KCp, etc.
KCp
LiCp, NaCp
• 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
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)
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 antibonding 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)
Properties IV
•
HgCp2 can be obtained as a yellow solid from
aqueous 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 h1, h5-bonding
•
•
•
A similar mode is found in BeCp2 and Zn(CpMe5)2
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
CH3
Cl
Al
Ti
CH3
C
H2
+ 2 Al(CH3)3
- CH4
- Al(CH3)2Cl
Cl
+
2
TiCl 4
+
Ti
Na
2 NaCl
Cl
TiCl4
Li(HBEt 3)/S8
Ti
S
S
S
S
S
Ti
Cl
S2Cl 2
SCl2
S
S
S
S
S
S
S
S
S
S
S
S
S
Cl
Cl
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)
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., 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))