Download Reactivity of Transition Metal Organometallics L. J. Farrugia MSc

L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Reactivity of Transition Metal Organometallics
L. J. Farrugia MSc Core Course C5
Text books : Inorganic Chemistry - Housecroft & Sharpe Ch 23 - the very basics
Inorganic Chemistry - Shriver & Atkins Ch 16 - the very basics
This course assumes familiarity with the Level-2 and Level-3
courses on Organometallic Chemistry, and this is covered in the
above texts
Organometallics - Elschenbroich & Salzer (library) - much more
useful
Topic 1 - Introduction to Cyclopentadienyl Compounds
First part of course covers the role of the ligand cyclopentadienyl (Cp) and its
derivatives. Cp is one of the most important ligands in organometallics after CO.
A considerable percentage of organometallic compounds contain this ligand - it is
also a good ligand for main group metals and the f-block metals (lanthanides &
actinides).
H
H
H
Fe
CO
H
OC
Cyclopentadiene
CO
Cyclopentadiene complex (4 e donor ligand)
very rare as a ligand
H
H
Na / -H2
-
Na+
acidic H atom
planar aromatic
(6-pi electron)
dienyl anion
The anion C5H5- is a very useful synthetic reagent. It is usually treated as
equivalent to occupying THREE coordination sites, so that C5H5 ≡ 3(CO). In
electron counting terms, it can be treated as either as a 6-e donor ANION or a 5e donor NEUTRAL molecule. The latter is the recommended approach because
it is simpler (do not need to worry about oxidation levels).
1.1 Bonding in cyclopentadienyl compounds
Cp has 5 π electrons in the 5 out-of-plane p-orbitals on the C atoms. These 5
orbitals combine as a1 + e1 + e2 under five-fold symmetry. With a single Cp ring,
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
inspection shows the possible combinations with the metal d-orbitals are shown
below
Cp orbitals
a1
e1
e2
Metal orbitals
pz dz2, s
dxz dyz px py
dx2-y2 dxy
Symmetry of bond
σ
π
δ
For two Cp rings in a metallocene M(C5H5)2 the rings may be either staggered or
eclipsed - Fe(C5H5)2 is eclipsed but Co(C5H5)2 and Ni(C5H5)2 are staggered.
The bonds can also be divided into sigma, pi and delta symmetry as shown in
OHP #1
OHP # 2 shows the formal MO interaction diagram for ferrocene - Fe(C5H5)2
The main points to remember
(i) the 9 filled orbitals have 18 electrons - hence the rule !
(ii) the LUMO is a doubly degenerate π* orbital, so Ni(C5H5)2 is paramagnetic
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
This shows how the metal and ring π-orbitals match by symmetry. The resultant
MO scheme for ferrocene shows the way that the basis orbitals having the same
symmetry can combine to give new orbitals (NOT necessary to remember the
MO scheme)
The filling of the nine bonding orbitals in ferrocene explains the high stability of
this compound. The mixing of metal and Cp orbitals indicates a strong covalent
character to the transition metal - cyclopentadienyl bond.
In Co(C5H5)2 ONE electron fills the e*1g while in Ni(C5H5)2 there are TWO
unpaired electrons - hence both compounds are paramagnetic
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
+
Co(C5H5)2 readily loses an electron to give the cobalticinium cation [Co(C5H5)2]
an 18e cation. Similarly Ni(C5H5)2 loses one electron to give a 19e cation, but
further oxidation results in decomposition.
1.2 Structural types of Cp compounds
(i) Metallocenes MCp2
These are known for the metals Ti, V, Cr , Mn, Co and Ni. Staggered or eclipsed
rings leads to D5h or D5d symmetry with virtually NO barrier to rotation of the Cp
ring about the metal-Cp axis. So all Cp protons appear as equivalent in the 1H
NMR spectrum
V, Cr, Fe, Co and Ni give the "classic" sandwich compounds illustrated above.
The exceptions are
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
(a) Titanocene "TiCp2"
This is really a fulvalene complex made by reducing Cp2TiCl2. It contains bridging
hydrides and a Ti-Ti bond - gives 16 e Ti atoms. Real "titanocene" TiCp*2 has
recently been made but is extremely reactive.
(b) Manganocene MnCp2
This is ionic at low temperature, with a polymeric chain like structure
Above 159oC it becomes isomorphous with ferrocene, so must adopt a sandwich
structure.
All the sandwich metallocenes apart from ferrocene are paramagnetic
No. unpaired electrons
3e
2e
5e
1e
2e
Cp2V
Cp2Cr
Cp2Mn
Cp2Co
Cp2Ni
high spin Mn2+
(ii) 'bent' metallocenes
These have non-parallel rings due to the presence of other ligands. Some
examples are
H
W
Cl
Ti
H
18 e
V
CO
Re
Cl
16 e
17 e
5
18 e
H
L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
(iii) half-sandwich compounds - piano stool compounds
These have one Cp ring and a variety of other ligands. Some examples are
V
OC
CO
OC
CO
4-legged stool
Mn
Ru
Co
PPh3
CO Cl
PPh3
OC
CO
3-legged stool
Chem-3 lab
OC
CO
2-legged stool
The 17 electron species CpMo(CO)3 and CpFe(CO)2 are not stable as such, but
dimerise to give the familiar compounds with a Mo-Mo or Fe-Fe metal-metal
bond. This is typical behaviour of odd-electron organometallic species.
(iv) other types of bonding mode
So far only the so-called eta-5 η5-C5H5 bonding mode has been illustrated, where
(in principle) all five C atoms are equally bonded to the transition metal. However
there are other possibilities, most common are η3-C5H5 and η1-C5H5
H
M
M
η1-C5H5
1 e donor - like alkyl group
η3-C5H5
3e donor - like allyl group
H
Mo
ON
One good example is Mo(NO)Cp3 - with "normal" η5-C5H5 bonding modes,
electron counting give a 24 electron compound (!!) - simply not possible. In fact it
is an example of a compound containing all three types.
η1-C5H5 ligands are usually fluxional. Consider the compound (C5H5)4Ti - again
cannot be 4 η5-C5H5 bonding modes because it would be a 24 e compound. In
this case it has two η5-C5H5 and two η1-C5H5 ligands.
Ti
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
At the very lowest temperature measured, the 1H NMR spectrum shows a singlet
for the two (equivalent) η5-C5H5 groups and an AA'BB'C multiplet for the two
(equivalent) η1-C5H5 groups (three different H environments).
This compound shows two fluxional processes
(i) migration of metal atom around the η1-C5H5 group via 1,2 shifts (ring whizzing)
(ii) exchange of the η1-C5H5 and η5-C5H5 groups
Process (i) is a very low energy process, which is frozen out only at the lowest
temperatures. Process (ii) occurs at higher temperatures ~ room temperature.
The net result of (i) and (ii) is that all 20 protons appear equivalent at the highest
temperatures measured.
Process (i) could in principle occur also by 1,3 shifts or random shifts. How can
we tell which ? We can if it is possible to assign the protons of the AA'BB' signal
M
c
M
c
M
a
a
a
a
c
b
a
b
b
b
b
b
a
1, 2 shift
The resulting exchanges are :
a→c
a→b
i.e. both a type protons exchange
b→a
b→b
i.e. only half of b type protons exchange
c→a
So... the rate of broadening for the a type protons is twice as fast as for the b
type protons
The actual spectrum of (η5-C5H5)(η1-C5H5)Fe(CO)2
exchange is shown below.
7
which exhibits a similar
L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
1.3 Oxidation states in Cp compounds
Since Cp is formally charged with a -ve charge, each Cp ligand present in a
compound contributes a charge of +1 towards the formal oxidation state of the
metal. Hence a metal must be in a +ve oxidation level. The only exception would
be with +ve charged ligands - NO+ is the only common example. CpNi(NO) has a
zerovalent Ni atom.
In terms of its ability to stabilize oxidation states, Cp is comfortable with both low
and high oxidation states of the metal (unlike many pi-acid ligands like CO which
are only found for low oxidation states). In general Cp is a good sigma- and pidonor, but a poor pi-acceptor. Substituting H for Me on the Cp ring makes it an
even better sigma-donor - Cp* is the common symbol for C5Me5 .
H2O2
CO
Re
CO
Re
O
OC
O
CO
Re(+7) oxo complex
Re (+1)
8
O
L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Topic 2 - Reactivity of Cyclopentadienyl-type Ligands
Very often Cp is a spectator ligand (i.e. it does not take part in the reaction and it
is unchanged at the end). Under some circumstances however the ring will react.
Ferrocene Fe(C5H5)2 has been the most studied (because it is a stable 18 e
metallocene) but ruthenocene Ru(C5H5)2 and osmocene Os(C5H5)2 will give
similar reactions - they are in the same periodic group ! However, most Cp
compounds will not survive the reaction conditions described below.
2.1 Electrophilic substitution at ring
This is a very facile process, occuring some 3×106 times faster than with
benzene.
E
E+
Fe
E
H
Fe +
Fe
- H+
E+ is a general electrophile, see E/S p 328-330. A concrete example is a FriedelCrafts acylation
O
O
C
C
CH3COCl/AlCl3
Fe
Fe
+
Fe
O
C
The mechanism probably involves exo attach at the ring C-atom, followed by a
movement of the proton down to the metal.
+
Fe
E
E
H
+
Fe
H
E
Fe
The hydride intermediate can be made by treating FeCp2 with very strong acids.
The hydride signal in the 1H NMR spectrum comes aroound -2.1 ppm, which is
typical of metal hydrides.
This reaction does not work if the electrophile is an oxidising electrophile as
many are such as NO2+. These reagents oxidise the complex to give the
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
ferricinium cation [FeCp2]+ and the +ve charge now on the rings makes
electrophilic attack very difficult.
Another example and indication of the high reactivity of FeCp2 is that the
Vilsmeier reaction works
O
O
C
(i) POCl3
H
C
+
Fe
MePhN
Fe
(ii) H2O
H
substituted formamides
This reaction only works well with highly activated aromatics such as amines
(anilines) and phenols.
Because of the high reactivity of the Cp rings, there is little of the selectivity
observed in aromatic compounds.
4 .2
1.4
Et
for Friedel-Crafts acylation
Fe
CH3COCl/AlCl3
1.0
2.2 Reactivity of attached organic groups
Organic functional groups attached to Cp undergo many of the most common
reaction types, e.g. below (non-oxidising conditions)
H
O
C
H
H
Fe
Fe
PhCOMe/base
O
Ph
aldol
condensation
H
H
Fe
R
Ph3P=CHR
Wittig ylid
O
O
C
CH3
Fe
H
Fe
RCHO/base
H
R
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
2.3 Metallation of ferrocene
Further substituents may be introduced by the very useful reaction with butyl
lithium
Li
CO2H
CO2
Fe
Fe
Fe
BuLi
N 2O4
NH2OMe
2 BuLi
NH2
NO2
Fe/HCl
Fe
Fe
Li
PPh2
Fe
PPh2
Fe
Fe
Fe(CO) 4
Fe(CO) 5
PPh2Cl
Li
PPh2
PPh2
Note that it is not possible to introduce the NO2 group directly by nitration as the
electrophile NO2+ is oxidising
2.4 Stabilization of α-carbonium ions
OAc
Fe
H
+
R
R
H
H2O/H+
OH
OH-
Fe
H
Fe
R
carbonium ion intermediate
The hydrolysis of the aceto substituted ferrocene proceeeds seven times faster
than the solvolysis of Ph3C-OAc (this is a classic example of SN1 hydrolysis via
the stable carbonium ion Ph3C+ the trityl ion). So we conclude that the carbonium
ion intermediate is more stable that the trityl cation.
Reactions involving vinylic substituents also proceed through α-carbonium ions,
e.g. the reaction with CH3CO2H
+
Fe
CH3
H
H2O/H+
Fe
OAc
OAcFe
carbonium ion intermediate
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H
CH3
L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
The carbonium ion can actually be isolated and crystallised as the BF4- or PF6salts. The crystal structures show clear evidence of an "interaction" between the
exo- C-C bond and the Fe atom - "bends" towards the Fe atom.
Ph
overall 6 e donor
Ph
Fe +
160 deg
Fe
crystal structure
There is also NMR evidence for the presence of a π-bond between the exo-C
atoms and the Fe atom. Uses 57Fe-13C coupling constants, which was observed
to be 1.5 Hz in the above cation. Clearly indicates that the bond between the
carbonium carbon and the Fe is nearer to a π-bond.
J(57Fe-13C) /Hz
~9
1.5 - 4.5
Type of bond
σ Fe-C
π Fe-C
2.5 The "indenyl" effect and ring slippage
Indene is a bicyclic hydrocarbon closely related to cyclopentadiene
base
H
H
indenyl anion Ind
Indenyl forms complexes which are similar to Cp, but substitution reactions are
much faster (up to 108 times !!).
Example
(Ind)Rh(CO)2 + PPh3 → (Ind)Rh(CO)(PPh3) + CO
This is an SN2 reaction, i.e. 2nd order and associative. Therefore the rate
determining step involved addition of the phosphine
Example
(Cp)Mn(CO)3 + PBu3 → no reaction
(Ind)Mn(CO)3 + PBu3 → (Ind)Mn(CO)2(PBu3) + CO
(Flr)Mn(CO)3 + PBu3 → (Flr)Mn(CO)2(PBu3) + CO
Fluorenyl (Flr) is 250 times faster than the indenyl reaction
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Fluorenyl anion Flr
What is the reason for this ? - ring slippage shown in the mechanism below
eta-5
eta-3
eta-3
PPh3
Rh
OC
18 e
Rh
CO
OC
Rh
OC
CO
18 e
16 e
CO
PPh3
eta-5
- CO
Rh
OC
PPh3
18 e
The C5 ring is moving from eta-5 to eta-3 back to eta-5 coordination to the
rhodium atom. The driving force is that the benzene ring can become fully
aromatic when the ring is slipped. This is even more so in fluorenyl.
Evidence for ring slippage
The ring slipped complex is usually a reactive intermediate and is not observed
as a stable compound. However in the following reaction the ring slipped
compound can be isolated
(Ind)Ir(PMe2Ph)2 + PMe2Ph → (Ind)Ir(PMe2Ph)3
18 e
20 e ??
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
The X-ray structure shows that only 3 C atoms of the C5 ring are coordinated to
the iridium
PMe2Ph
[Ir(PMe2Ph)4]+
Ind-
Ir
16 e
PMe2Ph
PhMe2P
PMe2Ph
18 e
The eta-3 coordination means that the bond between the Ir and the indenyl group
is weakened and it may be displaced by a further mole of phosphine. The
displacement of Cp or Cp related ligands is highly unusual !
2.6 Heterocycles as Cp analogues
The CH group in Cp may be replaced by hetero-atoms to give heterocycles
N (P, As etc) ≡ CH (isolectronic)
N
C4H4NC4H4PC4H4As-
N
P
As
pyrrolyl
phospholyl
arsolyl
These anions form π-complexes which are directly analogous to Cp
N
CpFe(CO)2I
heat
+
Fe
- 2CO
N
N
OC
OC
sigma-complex
Aza - ferrocene
Ph
[CpFe(CO)2]2
P
150 C
+
Fe
P
Phospha - ferrocene
Li
P
P
FeCl2
Fe
+2
Fe
P
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
The P (and indeed N) atoms have lone pairs and so are capable of acting as
ligands in their own right. May be thought of as derivitised phosphine ligands.
P
P
Fe(CO)4
P
Fe(CO)4
+ 2 Fe(CO)4(THF)
Fe
Fe
Source of 16 e "Fe(CO)4"
P
Thiophene C4H4S has one extra electron and so behaves as a six-electron donor
analogous to benzene
S
S
Cr(CO)3
Cr(CO)3
Boroles C4H4BR have one less electron and so behave as four-electron donors.
So compare Cp2Fe(CO)4 with (C4H4BMe)2Co(CO)4 - isostructural and isoelectronic compounds.
BMe O
C
O
C
Fe
OC
Fe
C
O
Co
OC
CO
BMe
Co
C
O
CO
There are a large number of heterocycles which can act as ligands (see E/S p
376-385 for other examples). The replacement of CH by P for example can go
the whole way.
150 C
[Cp*Fe(CO)2]2
+
P4
Fe
P
P
P
P
P
Penta phospha - ferrocene
All the P atoms are equivalent and give a singlet at 153 ppm in the
spectrum.
15
31
P NMR
L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Topic 3 - Stabilisation of Unstable Molecules by Complexation
Transition metals have the remarkable ability to stabilise unstable, unknown or
highly reactive organic (and inorganic) molecules. This is because coordination
to a metal changes the electron density in the ligand. This facet of organometallic
chemistry will be illustrated by a few pertinent examples.
3.1 Unstable molecule - Cyclobutadiene
This is a highly strained and unstable molecule. It is non-aromatic, i.e. doesn't
obey the (4n+2) π-electron Hückel rule. In fact it is anti-aromatic and is
destabilised by a square-planar arrangement of C atoms. This is easily seen by
thinking about the π-orbitals in this symmetry (D4h)
bonding
slightly antibonding
v antibonding
ψ−4
ψ−3, ψ−2
ψ−1
ψ−4
ψ−3, ψ−2
ψ−1
The four π-electrons thus give rise to an expected triplet state, which is subject to
Jahn-Teller distortion. In fact C4H4 is not square but rectangular with localised
double bonds and D2h symmetry,
H
H
t-Bu
H
t-Bu
H
highly reactive
t-Bu
H
stable because of steric restraints
In 1955, Longuett-Higgins and Orgel predicted theoretically that the aromatic
square form of C4H4 would be stabilised by complexation to a transition-metal.
The synthesis of such a complex was soon accomplished.
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
H3C
CH3
Cl
Cl
Cl
Ni
+ Ni(CO)4
Cl
H3C
Ni
Cl
Cl
CH3
In 1959 the nickel complex of tetramethylcyclobutadiene was synthesised and
the X-ray crystal structure determined. The Ni(CO)4 also abstracts the two
chlorine atoms and NiCl2 is a by-product. The C-C distances are ~ 1.45Å which is
intermediate between a single and double bond.
The first complex of the parent unsubstituted cyclobutadiene was made in a
similar way
Cl
+ FeCl2
+ Fe2(CO)9
+ 6 CO
Fe
Cl
OC
CO
CO
This compound is a low melting yellow crystalline complex with a signal at 3.91
ppm in the 1H NMR spectrum and two ν(CO) stretches in the IR spectrum,
consistent with the pseudo three-fold symmetry. This reaction also proceeds by
halogen abstraction and it is a general reaction for the synthesis of complexes of
cyclobutadiene (and substituted cyclobutadienes)
Cl
+
2 Na [Mo(CO)5]
2-
OC
OC
Cl
Mo
CO
CO
The anion [Mo(CO)5]2- is a powerful nucleophile. Another synthetic route for
cyclobutadiene complexes is through the dimerisation of alkynes. This is a
thermally forbidden reaction (Woodward-Hoffman rules) but with the mediation of
a transition metal, this may be overcome
R
CO
Co
R
CO
+ 2 RC
CR
R
Co R
3.1.1 Physical properties
In complexes the ring is essentially square-planar. If all distances are
approximately equal, this indicates bond delocalisation and a fully conjugated πsystem. Some structures however show partial double bond localisation. The ring
has aromatic properties, for example the 1H NMR signals of the CH ring protons
are in the range 4 - 6 ppm, similar to Cp protons.
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
3.1.2 Chemical reactivity
3.1.2.1 Spectator role
The cyclobutadiene ligand is often a "spectator" ligand in much the same way as
Cp
+ PPh3
Fe(CO)3
Fe(CO)2(PPh3)
hv
O
C
Fe
Fe
C
C
O
O
18 e with triple FeFe bond
In the latter complex the butadiene ligand act as steric "plugs" preventing access
to the metal atom
3.1.2.2 Reactivity at ring
Electrophilic substitution occurs very readily and in a very similar fashion to Cp.
For example the Vilsmeier reaction gives aldehyde substitued ring product
O
H
Ph
+
N
Me
Fe(CO)3
H
POCl3
O
H2O
C
Fe(CO)3
Organic functional groups also show the same reactivity as with Cp rings.
O
CH2OH
H
BH4-
Fe(CO)3
CH2Cl
HCl
Fe(CO)3
Fe(CO)3
LiAlH4
CH3
Fe(CO)3
Likewise the stabilisation of the α-carbonium ion occurs in a similar fashion to Cp
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
CH2+
CH2Cl
CH2
SbCl5
Fe(CO)3
HCl
+
Fe(CO)3
Fe(CO)3
OHCH2OH
Fe(CO)3
3.1.2.3 Release of free cyclobutadiene
Low temperature oxidation with mild oxidants such as Cerium(+4) oxidises the Fe
atom and releases free cyclobutadiene. This is highly reactive and it is prepared
in situ with the reactive substrate. This reaction may be used in organic
synthesis. A nice example is the preparation of cubane in E/S p315
2. Unstable molecule - Trimethylene methane
This molecule C4H6 is an isomer of butadiene. It is extremely unstable in the free
state since it doesn't obey the conventional rules of bonding.
H2C
C
CH2
CH2
butadiene
trimethylene methane
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Stable complexes of this ligand (and derivatives) can be easily made, in an
extension of the routes used to make the cyclobutadiene complexes.
CH2Cl
H2C
CH2
H2C
+ Fe2(CO)9
C
CH2
CH2Cl
Fe
(CO)3
FeCl2 is formed as a byproduct from halide abstraction. The four C atoms are not
quite coplanar so the shape is a little like an umbrella, with the exterior CH2
groups bending towards the Fe atom. The whole ligand acts as a four electron
donor
The crystal structure of this complex is shown above. The six H atoms are
equivalent and the complex adopts a staggered geometry, as shown by a view
down the 3-fold axis
H H
OC
CO
Fe
H
H
CO
H
H
How is this odd molecule bonded to the transition metal ? Need to consider the
lowest lying π-orbitals
e symmetry non-bonding
a1 symmetry bonding
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
The a1 is the σ-donor orbital and the e set are the π-acceptor orbitals. The
Fe(CO)3 fragment has a similar set (as far as symmetry is concerned) of orbitals
which provide a perfect match
high lying
sigma acceptor
high lying pi-acceptor and low lying pi-donor
derived from eg
sp hybrid
The bonding is thus the "classical" σ-donotion from ligand and π-back-donation
from metal. This orbital approach provides a delocalised view of the bonding and
avoids the problems with the conventional view of localised bonds.
The coordination geometry of the Fe atoms is roughly octahedral and there is a
high barrier to rotation about the 3-fold axis. This barrier cannot be observed in
the Fe(CO)3 complex, because all 6 protons are equivalent anyway. In order to
observe any barrier, we need to make them chemically inequivalent. So choose
an ML3 fragment which does not have 3-fold symmetry
H H
CH2SiMe3
IrCl(CO)(PPh3)
OC
- SiMe3Cl
PPh3
Ir
+ H2C
CH2Cl
H
H
H
Cl
H
All six H's are now chemically inequivalent (the compound is chiral) and even at
elevated temperatures, there are 6 signals between -0.1 and 3.5 ppm in the 1H
NMR spectrum, showing that if rotation occurs that it must have a high barrier.
3. Unstable molecule - Benzyne
This molecule is highly strained and hence very reactive, but is stable in an inert
matrix at 8o K. The alkyne group should be linear - hence the strain. Benzyne can
be made in situ by abstraction of HCl from chlorobenzene, using a very strong
base such as sodamide NaNH2. The generated benzyne may be trapped using a
diene such as cyclopentadiene by the Diels-Alder reaction
Benzyne
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Simple and stable benzyne complexes of transition metals are known, for
example
heat
H3C
Ta
CH3
H3C
- CH4
Ta
H3C
H3C
Benzyne has obvious similarities to alkynes in its complexes. Quite a few are
known for cluster compounds
Os3(CO)12 + C6H6 → H2Os3(CO)9(C6H4)
The alkyne "dances" around the three Os-Os edges
4. Unstable molecule - E2 (E=As,Sb, Bi) analogues of N2
While N2 is the stable form of elemental nitrogen, the corresponding molecules
P2, As2, Sb2 and Bi2 are not stable at room temperature. While P2, As2, Sb2 have
been observed in mass spectra at high temperatures, Bi2 is wholly unknown.
However, when coordinated to transition metals, these molecules may be
isolated at room temperature.
Co(CO)3
(OC)3Co
Co(CO)3
(OC)3Co
CH
Bi
C
H
Bi
The Bi2 ligand with a formal Bi≡Bi triple bond is analogous to alkynes RC≡CR
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
The "Mercedes Benzenes" have a similarly E2 group (E=As, Sb, Bi) coordinated
to three M(CO)5 groups (M = Mo, W). They are made by the simple reaction
ECl3 + [M2(CO)10]2- → E2[M(CO)5]3
The structures of typical examples are shown below, both schematically and in
3D.
Mercedes Benzene
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Topic 4 - Reactivity of Coordinated Olefin Complexes
4.1 Introduction
The coordination of ligands to metals can, in general, quite substantially change
not only the stability (as shown in previous topic) but also the reactivity of the
ligand. The reason for this is that coordination to a metal changes the electron
distribution within the ligand. As an example to show why this happens, consider
the case of butadiene
free ligand
(OC)3Fe
The ligand π-orbitals (frontier orbitals) are the most important in bonding to the
metal. Assuming the conformation found in complexes, we get
ENERGY
(+)
(-)
Ψ-4
(+)
(δ)
(-)
(+)
(-)
Ψ-3
LUMO
(π)
Ψ-2
HOMO
(π)
(+)
(-)
(-)
(-)
(+)
(+)
(+)
(+)
Ψ-1
(+)
(+)
π-orbitals of butadiene
coefficients are either 0.37 or 0.60
24
(σ)
L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Ψ-4
0.44
Ψ-3
1.48Å
1.34Å
0.89
Ψ-2
π-bond order
Ψ-1
bond lengths
ground state
Ψ-4
0.72
Ψ-3
1.39Å
1.45Å
0.44
Ψ-2
Ψ-1
π-bond order
bond lengths
excited state
The coefficients of each basis p-orbital on the C atoms tells us qualitatively about
the bonding/antibonding nature of each of the orbitals
•
•
•
•
Orbital Ψ-1 provides π-bonding between all atoms, but more so between the
two inner C-atoms.
Orbital Ψ-2 provides π-bonding between the inner and outer C-atoms, but is
antibonding between the two inner atoms.
Orbital Ψ-3 provides π-bonding between the two inner C-atoms, but is
antibonding between the inner and outer C-atoms.
Orbital Ψ-4 is antibonding between all C-atoms but is not occupied or used.
For the ground state, the bonding is the sum of Ψ-1 and Ψ-2, which leads to a
higher π-bond order between the inner and outer atoms (as expected from
conventional ideas).
For the excited state, the partial population of Ψ-3 and partial depopulation of Ψ2 leads to an inversion of π-bond orders compared with the ground state.
This is the simple Hückel picture of bonding. Coordination of butadiene to a metal
fragment such as Fe(CO)3 uses the same orbitals as previously shown
high lying
sigma acceptor
high lying pi-acceptor and low lying pi-donor
derived from eg
sp hybrid
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
These orbitals interact with the ligand orbitals to give delocalised bonds of σ- and
π-symmetry (the δ-orbital is too high lying and has no symmetry match with any
metal orbital). The synergic bonding between metal and ligand redistributes the
electron population of the butadiene in a similar fashion as that explained above
for the excited state.
Thus qualitatively we can see that :
(a) changes in the relative populations of the frontier orbitals of the ligand can
change the bonding density in the ligand. Some examples show the effects on
the bond lengths.
1.45
1.41
1.40
1.45
Cp2Zr
(OC)3Fe
The arrows show the direction of flow of electrons and the numbers are the bond
lengths in Ă units. The Fe fragment is electron rich and the Zr fragment is
electron deficient. In the case of the Fe compound there is thus a small
population of the LUMO of butadiene, while in the Zr compound there is a small
withdrawal from the HOMO of butadiene.
In terms of the valence bond approach, it is possible to view the bonding as
arising from two extreme canonical forms
MLn
MLn
B
A
Early transition metals such as Zr which are electron deficient are more like B,
while later transition metals such as Fe are electron rich and more like A
(b) The orbital approach also allows us to rationalise the site of attack of the
incoming nucleophile. The LUMO of butadiene has most of the wavefunction on
the outer C-atoms and the lone pair of the nucleophile will seek this, hence
leading to preferential attack at the outer carbon atom. The reaction of
nucleophile with electrophile is a Lewis acid-base interaction.
Nu
better overlap here
than at the inner C
atom
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
A number of MO studies of individual complexes has led to a set of simple rules
for determining the site of nucleophilic attack at unsaturated hydrocarbon ligands
in cationic metal complexes.
4.2 Green/Mingos/Davies Rules (GMD) for Nucleophilic Addition
Normally, unsaturated hydrocarbons are not at all susceptible to nucleophilic
addition. On coordination into a CATIONIC metal complex this changes, and
attack by nucleophiles to give addition products is well known. The increase in
susceptibility to nucleophilic attack arises because there is a net flow of electrons
from the ligand to the metal (i.e akin to introducing electron withdrawing
substituents). An example from organic chemistry of this enhancement is the
attack on the bromonium ion by the relatively weak nucleophile Br- during the
bromination of alkenes.
BrBr
The nucleophilic additions to transition metal complexes are generally very regiospecific, i.e only one out of a possible number of products are formed. For attack
on 18-electron organometallic cations, where reactions are kinetically controlled,
the most favourable position for nucleophilic attack is given by the GMD rules.
Some definitions are needed first. Hydrocarbon ligands described as even or odd
and as open or closed
Even n = 2,3,4 ......
Odd n = 3,5,7 .... refers to the η number, i.e. how many C-atoms of ligand are
bonded to the metal
Open
not cyclically conjugated
Closed
cyclically conjugated
Rule1
Nucleophilic attack prefers EVEN polyenes with no unpaired
electrons in their HOMO.Cyclo-C4R4 only common example)
EVEN before C4R4 before ODD
Rule2
OPEN before CLOSED
Rule3
For even open polyenes, attack occurs at terminal C-atom.
For odd open polyenes, attack occurs at terminal C-atom only
if MLn is strongly electron withdrawing
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
eta-5
eta-5
M
eta-5
M
odd closed
odd open
odd open
M
M
M
even closed
eta-6
M
even open
even open
eta-4
eta-4
Rules are best illustrated by examples
eta-5 odd
Et
H
EtMgCl
Fe
Fe
Et
Fe
cyclohexadienyl
H
eta-6 even
Et
Even before odd. NOTE that the nucleophile attacks from the EXO position
eta-5 closed
H
H
Rh
NaBH4
Rh
Rh
H
eta-5 open
Open before closed Attack occurs at terminal C-atom of conjugated system
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Rules need to be applied sequentially. Consider the case of the Mo+ cation
shown below when treated with hydride source (BH4-)
eta-6 even closed
BH4
Mo
Mo
eta-3 odd open
eta-4 even open
Me
Rule 1
Rule 2
Rule3
even before odd - therefore allyl NOT attacked
open before closed - therefore butadiene attacked
terminal C-atom - therefore product the methylallyl complex
Another way of expressing Rules 1 & 2 is that the order of reactivity of
unsaturated hydrocarbons coordinated to cations is as shown below
It can be seen that Cp is the least reactive - this explains it's stability and role as
a "spectator" ligand. It may be used to design complexes for nucleophilic addition
to even ligands and allyl and pentadienyl ligands.
CAVEAT
Cations which contain only ONE unsaturated hydrocarbon ligand and at least
ONE carbonyl ligand may undergo nucleophilic attack at the CO. This most often
occurs with nucleophiles having heteroatoms as the nucleophilic site (e.g.
methoxide -OCH3) and when choice is between a CO and an eta-5 ligand.
MeO
OMe
slow isomerisation
Os
Os
Os
O
CO
OC
CO
CO
OC
CO OMe
OC
CO
When there is more than one unsaturated ligand, the normal MGD rules apply
again, e.g. for [(C5H5)Mo(CO)3(C2H4)]+ attack occurs at the ethene ligand.
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Topic 5 - Carbene and Carbyne Complexes
Textbooks : E/S pages 210 - 220
5.1 Introduction
Carbene :CH2 is a highly reactive and unstable molecule. Carbene itself can be
made by the thermal decomposition of diazomethane
CH2N2 → N2 + :CH2 → (CH2)n polymerises to "polythene"
Chlorocarbene is also easily made
CHCl3 + strong base → :CCl2
(reactive intermediate)
Carbenes may be stabilised by coordination to transition metals, as in this
OC
O
C
Rh
Rh
+ CH2N2
Rh
C
O
Rh
C
H2 CO
example of the formation of a carbene complex directly from carbene. This
reaction is the organometallic analogue of the reaction of carbenes with alkenes
to give cyclopropanes.
Carbenes are thus further example of unstable molecules which are stabilised by
coordination to transition metals. Relatively recently, stable carbenes at room
temperature have been synthesised by Arduengo - contain N-heterocycles.
R
N
N
R
The terms carbene complex and alkylidene complex and also carbyne complex
and alkylidyne complex are used interchangeably.
5.2 Fischer Synthesis of Carbene Complexes
Carbene complexes were first made in 1964 by E.O. Fischer who subsequently
won a Nobel Prize for his work. The synthesis is based on the nucleophilic
addition of heteroatom nucleophiles to coordinated CO (see last part of Topic 4).
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
CO
OC
CO
W
OC
CO
CO
(i)
OC
OC
(ii)
CO
CO
OC
OC
CO
CO
CO
W
W
CO
C
C
R
R
OLi
OCH3
(i) nucleophilic addition to CO with LiR
(ii) methylation using Me3O+ BF4-
The lithium intermediate is unstable and methylation with Me+ reagents result in
thermally stable and isolable carbene complexes. Many examples of this type are
now known, most with OR or NR2 substituents. These have several resonance
forms.
OR
OR
LnM
LnM
C
OR
C
LnM
R
R
C
R
The latter canonical form bears a positive charge on the carbene carbon, and so
may be expected to be electrophilic at this atom.
5.3 Schrock Synthesis of Carbene Complexes
This is the other very important route to carbene complexes, of quite a different
character to those made by Fischer. The reaction was the treatment of the
Cl
TaNp5
CMe3
HC
LiNp
(Np)2ClTa
(Me3CH2)3Ta
H
C
H2CMe3
Cl
Np = neopentyl = Me3CCH2H
TaNp3
C
CMe3
CMe3
C
LiNp
- CMe4
H
Np2Ta
Cl
neopentyl (Np) alkyl complex of tantalum Np3TaCl2 with two moles of the lithium
reagent LiNp. The original purpose of the experiment was the synthesise TaNp5
but the actual product was much more interesting (as is often the case in
organometallic chemistry). The first stage involves an α-deprotonation step,
whereby a proton from one neopentyl group is transferred to another neopentyl
group. This releases the volatile hydrocarbon neopentane (tetramethyl methane)
which is one of the driving forces for this reaction. This α-deprotonation of alkyl
groups ONLY occurs with very sterically bulky groups and is unusual.
A related reaction forming carbenes is that of hydride abstraction from methyl
groups
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
+ Ph3CH
Ph3C BF 4
Re
ON
L
Re
CH3
ON
L
CH2
The two research groups of Fischer and Schrock have developed the chemistry
of these carbenes, which are intrinsically different. The most important
differences lie in their reactivity.
•
•
Fischer carbenes have π-donor hetero-substituents and are electrophilic at
the carbene
Schrock carbenes have alkyl (or H) substituents and are nucleophilic at the
carbene
5.4 Evidence for Multiple Bonding
1. X-ray Structures. These show (i) planar trigonal sp2 C
(ii) M=C bonds is shorter than a single bond,
but not as short as M-CO bond
2. 13C NMR chemical shifts are in the region 200-400 ppm - cited as evidence for
a δ(+) charge on C
3. MO picture predicts a barrier to free rotation in Fischer carbenes, which is
observed in NMR spectra. Also the observed inequivalence of CR2 groups in
Schrock carbenes in the NMR spectra also indicates a very high barrier to
rotation about M=C bond
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
5.5 Why the distinction between Fischer and Schrock carbenes ?
The table below lists their general properties and differences
Property
Nature of carbene carbon
Typical R group
Typical metal
Typical ancilliary ligands
Electron count
Oxidation state change
on addition of CR2 to
metal
Fischer
Electrophilic
π-donor (e.g. -OR)
Mo(0), Fe(0)
Good π-acceptors - CO
2e
0
Schrock
Nucleophilic
Alkyl, H
Tav(V), W(VI)
Cl, Cp, alkyl
2e
+2
Actually not easy to provide a convincing explanation for the difference. The MO
scheme suggests that while both ligands are effective σ-donors
•
•
the Fischer carbene is a π-acid
the Schrock carbene is a π-base
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
5.6 Reactivity of Fischer carbenes
Fischer carbenes are readily susceptible to nucleophilic substitution
OCH3
NHEt
NH2Et
(OC)5Cr
(OC)5Cr
CH3
+ MeOH
CH3
PhLi
Ph
(OC)5Cr
+ MeOH
CH3
Ph
Mechanism involves the intermediate
(OC)5Cr
OCH3
CH3
Similar to amminolysis of esters
OCH3
O
OCH3
H
O
NH2R
N
HR
CH3
CH3
NHR
O
CH3
The analogy between metal carbenes and organic ketones is quite strong. For
another example, compare their reactions with P-ylides:
OCH3
OCH3
Ph3P=CH2
H2C
(OC)5W
+ (CO)5WPPh3
Ph
Ph
compare with Wittig reaction
Ph
Ph3P=CH2
O
Ph
+ OPPh3
H2C
Ph
Ph
Another important reaction is with alkenes to give metallocycles, which in turn
decompose to give re-arranged alkenes. This is the so called "alkene
metathesis" reaction, which is of major commercial importance.
The resultant olefin has the =CPh(OMe) group from the carbene complex and the
=CH2 group from the starting CH2=CH(OR) alkene. This re-arrangement involves
breaking a very strong C=C double bond, and is impossible without the presence
of the organometalic carbene catalyst
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
OR
OCH3
(OC)5Cr
(OC)5Cr
C
Ph
OR
Ph
OCH3
OR
OR
OCH3
(OC)5Cr
+
(OC)5Cr
H
Ph
C
Ph
OCH3
Carbene complexes are excellent catalysts for the general alkene metathesis
reaction
R2C=CR'2 + R''2C=CR'''2 → R2C=CR''2 + R'2C=CR'''2 etc
5.7 Reactivity of Schrock carbenes
Schrock carbenes behave much like ylides
They are easily attacked by electrophiles (i.e. behave as nucleophiles) - two
example reactions are
One of the most useful carbene compounds is Tebbe's reagent.
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
The reagent is released by the treatment with ammine bases NR3. It behaves as
a carbene transfer reagent, and has a number of specialised uses.
"Cp2Ti=CH2" + RC(O)OR'
→
R(OR')C=CH2
This has the useful advantage over Wittig's reagents in the same reaction in that
it works !!. When used with enolisable ketones it does not result in racemisation.
While the distinction between Fischer and Schrock carbenes is a useful one, it
should be realised that this distinction is not rigid, but merely represents two
extremes. An example of an "in-between" carbene is Roper's carbene based on
osmium, which shows both types of reactivity, depending on the substrate.
SO2 here is acting as an electrophile, while CO is acting as a nucleophile. The
"in-between" character of Roper's carbene is sensible in terms of the descriptions
given in the Table, as there are both π-donors (chloride ligand) and π-acceptors
(nitrosyl ligand) and the carbene carbon has no π-donor substituents.
5.8 Carbyne complexes
Carbyne complexes have a CR group attached to a metal atom, with a triple M≡C
bond. Their chemistry is closely related to those of carbenes :
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
The evidence for a triple bond comes from crystal structures, which show a very
short M-C bond. Moreover the M-C-R angle is close to 180o indicative of linear sp
hybridisation. The bonding of the C atom to a metal is quite similar to that of CO,
with a sigma-donation and pi-back donation to the (unhybridised) p-orbitals of the
carbyne atom
The reactions of carbyne complexes show some similarities with carbene
complexes. Thus alkyne metathesis with carbyne complexes as catalysts is
possible.
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
This is usually only possible with alkyl or aryl R groups.
Carbyne complexes may be thought of as "half-inorganic" analogues of alkynes
For instance the analogy is obvious by comparing these reactions
The PtW 2 compound can also be viewed as a cluster of 3 metals with two
bridging CR groups
In fact, carbynes (or alklidynes) are very common as bridging ligands. One
particular well know series of very stable compounds can be easily made
Co2(CO)8 + CRX3 (X=Cl, Br, I) →
CoX2 + Co3(µ3-CR)(CO)9 - "Fred"
A huge variety of derivatives with different R groups have been made and
extensive chemistry is known. The structure of the simplest CH derivative is
shown. The three cobalt atoms and the bridging carbon make up a tetrahedron
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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
Using the reactions described above it was possible to synthesise a related
compound with three different metal atoms. Because the vertices of the
tetrahedron were all different, the compound is chiral and it is possible to resolve
these chiral clusters by making derivatives with homo-chiral phosphines PRR'R''.
They have proved to be some of the most chiral molecules made (i.e. have the
highest molar rotation coefficients).
39