Download Chem 174-Lecture 15a..

Metallocenes
 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+)
 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)
 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)
 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
 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)
 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
 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
 Infrared spectrum
 n(CH, sp2)= 3085 cm-1
 n(C=C)= 1411 cm-1
 asym. ring breathing: n= 1108 cm-1
n(CH, sp2)
n(C=C)
 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)
asym. ring
breathing
 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!
 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
 MagnesiumMCl2 + 2 NaC 5H5
NH3(l)
500 oC
Solvent
MC 5H5
+ 1/2 H 2
M=Li, Na, K
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)
 It is less reactive than sodium or potassium because it possesses often
a thick oxide
not dissolve
in2 liquid

FeCl2 +layer
C 5H 6 +and
2 Et2does
NH
Fe (Cwell
[Et2N H2] Cammonia
l
5H5)2 +
 Its lower reactivity compared to alkali metals demands elevated
To l ue n e
(C H ) MCl + 2 NaCl
MCl4 +(like
2 NaCiron)
M= Ti, Zr
temperatures
to react
with cyclopentadiene
5H 5
5
5 2
2
 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
NH (l)
M + C H
MC H + 1/2 H
M=Li, Na, K
 The reaction can lead to a complete or a partial exchange
500 C
M + 2C H
(C H ) + H
M=Mg, Fe
 The choice of solvent Mdetermines,
which
product precipitates
3
5 6
5 5
2
o
5 6
MCl2 + 2 NaC 5H5
5 5 2
Solvent
M(C 5H5)2 + 2 NaCl
FeCl2 + C 5H 6 + 2 Et2NH
I
MCl4 + 2 NaC 5H 5
2
M=V, Cr, Mn, Fe, Co, Ni
Solvent= THF, DME, NH 3(l)
Fe (C 5H5)2 + 2 [Et2N H2] Cl
To lue n e
(C5H5)2MCl2 + 2 NaCl
M= Ti, Zr
 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
[Ni(H2O)6]2+
[Co(H2O)6]2+
[Al(H2O)6]3+
[Fe(H2O)6]3+
Ka
2.5*10-11
1.3*10-9
1.4*10-5 (~ acetic acid!)
6.3*10-3 (~phosphoric 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
 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 
 The hexammine route circumvents the problem of the conversion of
the hydrated to the anhydrous forms ofthe metal halide
 The reaction of ammonia with the metal hexaaqua 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
 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 
 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)
 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
 HgCp2 can be obtained as a yellow solid from aqueous
solution, but is sensitive to heat and light!
 HgCp2 undergoes 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, 5bonding
 A similar mode is found in Zn(CpMe5)2
 Cp2TiCl2
Al
CH3
C
H2
 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
Ti
+ 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
 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)
 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)