Transcript Organometallic Chemistry between organic and inorganic

Ligand Substitution
Reactivity of Coordinated Ligands
2-
Cl
Cl
Pd
Cl
Cl
C2H4
Cl
Cl
Pd
H2O
Cl
Cl
H2O Pd
Cl
OHPd(0)
+
+H +
H2O + 2 Cl-
Cl
H2O Pd
- 2 e (CuCl2  CuCl)
Cl
H2O Pd
OH
Cl
- Cl-
H
-
Cl
OH
Cl
H2O Pd
CH3CHO
-H elim
OH
ClCl
H2O Pd
O "-H elim"
H
Cl
H2O Pd
OH
ins
Cl
H2O Pd
H
Peter H.M. Budzelaar
Why care about substitution ?
Basic premise about metal-catalyzed reactions:
• Reactions happen in the coordination sphere of the metal
• Reactants (substrates) come in, react, and leave again
• Binding or dissociation of a ligand is often
the slow, rate-determining step
This premise is not always correct, but it applies
in the vast majority of cases.
Notable exceptions:
• Electron-transfer reactions
• Activation of a single substrate for external attack
– peroxy-acids for olefin epoxidation
– CO and olefins for nucleophilic attack
2
Ligand Substitution
Dissociative ligand substitution
Example:
LnM CO
18 e
LnM
16 e
+ CO
L'
LnM L'
18 e
Factors influencing ease of dissociation:
• 1st row < 2nd row > 3rd row
• d8-ML5 > d10-ML4 > d6-ML6
• stable ligands (CO, olefins, Cl-) dissociate easily
(as opposed to e.g. CH3, Cp).
3
Ligand Substitution
Dissociative substitution in ML6
16-e ML5 complexes are usually fluxional;
the reaction proceeds with partial inversion, partial retention of
stereochemistry.
or
16-e
18-e
oct
SP
distorted
TBP
The 5-coordinate intermediates are normally too reactive to be
observed unless one uses matrix isolation techniques.
4
Ligand Substitution
Associative ligand substitution
Example:
LnM
L'
LnM L'
16 e
-L
18 e
Sometimes the solvent is involved.
Reactivity of cis-platin:
Ln-1M L'
16 e
Br(NH3)2PtCl2
(NH3)2Pt(Cl)(Br)
- Clslow
Br- - H O
H2O - Cl
2
fast
(NH3)2Pt(Cl)(H2O)+
- Cl-
NucleoBase - H O
2
fast
slow
(NH3)2Pt(Cl)(NB)+
5
Ligand Substitution
Ligand rearrangement
Several ligands can switch between n-e and (n-2)-e situations,
thus enabling associative reactions
of an apparently saturated complex:
M
N O
M N
3-e
1-e
O
O
CO
M
R
M
M
M
5-e
3-e
R
(1+2)-e
6
1-e
Ligand Substitution
Redox-induced ligand substitution
Unlike 18-e complexes, 17-e and 19-e complexes are labile.
Oxidation and reduction can induce rapid ligand substitution.
LnM
- e-
+
17-e
L'
LnM L' +
19-e
LnM
18-e
+ e-
LnM19-e
Ln-1M- + L
17-e
• Reduction promotes dissociative substitution.
• Oxidation promotes associative substitution.
• In favourable cases, the product oxidizes/reduces
the starting material  redox catalysis.
7
Ligand Substitution
Redox-induced ligand substitution
Fe(CO)4L
CO
Fe(CO)5
Fe(CO)4
Fe(CO)5
Fe(CO)4L
L
Initiation by added reductant.
Sometimes, radical abstraction
produces a 17-e species
(see C103).
8
Ligand Substitution
Photochemical ligand substitution
Visible light can excite an electron from an M-L bonding orbital to
an M-L antibonding orbital (Ligand Field transition, LF).
This often results in fast ligand dissociation.
M(CO)6
d
h
d
Requirement: the complex must absorb, so it must have a colour!
or use UV if the complex absorbs there
9
Ligand Substitution
Photochemical ligand substitution
Some ligands have a low-lying * orbital and undergo
Metal-to-Ligand Charge Transfer (MLCT) excitation.
This leads to easy associative substitution.
– The excited state is formally (n-1)-e !
– Similar to oxidation-induced substitution
M(CO)4(bipy)
d
HOMO
* h
d
LUMO
M-M bonds dissociate easily (homolysis) on irradiation
 (n-1)-e associative substitution
10
Ligand Substitution
Electrophilic and nucleophilic attack
on activated ligands
Electron-rich metal fragment:
ligands activated for electrophilic attack.
+
++
N
N
Rh
N
N
N
H+
S
N
S
Rh
N
N
H2O is acidic enough to protonate this coordinated ethene.
Without the metal, protonating ethene requires H2SO4 or similar.
11
Ligand Substitution
Electrophilic and nucleophilic attack
on activated ligands
Electron-poor metal fragment:
ligands activated for nucleophilic attack.
Bu
H
BuLi
Cr
Cr
OC
OC
Li+
CO
OC
OC
CO
BuLi does not add to free benzene, it would at best metallate it
(and even that is hard to do).
12
Ligand Substitution
Electrophilic attack on ligand
Hapticity may increase or decrease.
Formal oxidation state of metal may increase.
+
MI
H+
MI
+
M(0)
13
H+
MII
Ligand Substitution
Electrophilic addition
+
O
OEt
Et3O+
Fe(CO)3
Fe(CO)3
• Is formally oxidation of Fe(0) to FeII (the ligand becomes anionic).
• Ligand hapticity increases to compensate for loss of electron.
14
Ligand Substitution
Electrophilic abstraction
+
Electrophilic abstraction
also by
Ph3C+,
+
H+
Me
Cp2Zr
B(C6F5)3
MeB(C6F5)3-
Cp2Zr
Me
Me
16 e
14 e
Alkyl exchange also starts with electrophilic attack:
Me
Zn
Me
15
Zn
Me
Me
Me
Me
Zn
Zn
Me
Me
Ligand Substitution
Electrophilic attack at the metal
If the metal has lone pairs, it may compete with the ligand
for electrophilic attack
Transfer of the electrophile to the ligand may then still occur
in a separate subsequent step
+
Fe
H+
Fe
H
+
?via?
+
Ni
16
H
Ni
Ligand Substitution
Electrophilic attack at the metal
Can be the start of oxidative addition
I2(CO)2Rh
Me
I
I2(CO)2RhMe + I-
(although this could also happen
via concerted addition)
I3(CO)2RhMe
HI
CH3COOH
H2O
CH3COI
Key reaction in the
Monsanto acetic acid process:
HI
MeOH + CO
MeCOOH
"Rh"
CH3I
Rh(CO)2I2-
MeCORh(CO)2I3-
CO
17
CH3OH
MeRh(CO)2I3-
MeCORh(CO)I3-
Ligand Substitution
Nucleophilic attack on ligand
Nucleophile "substitutes" metal
 hapticity usually decreases
Oxidation state mostly unchanged
Competition: nucleophilic attack on metal
usually leads to ligand substitution
18
Ligand Substitution
Nucleophilic abstraction
-
Mostly ligand deprotonation
Na+
NaH
Cr
Cr
OC
OC
Cp2WH2
19
CO
BuLi
OC
OC
Cp2WH
CO
Li
Ligand Substitution
Nucleophilic addition
OH
(H2O)Cl2Pd
+ OH-
(H2O)Cl2Pd
Key reaction of the Wacker process:
C2H4 + ½ O2
20
PdCl2, H2O
CH3CHO
CuCl2
Ligand Substitution
The Wacker process
2-
Cl
Cl
Pd
Cl
Cl
C2H4
Cl
Cl
Pd
H2O
Cl
Cl
H2O Pd
Cl
OHPd(0)
+
+H +
H2O + 2 Cl-
Cl
H2O Pd
- 2 e (CuCl2  CuCl)
Cl
H2O Pd
OH
Cl
- Cl-
H
-
Cl
OH
Cl
H2O Pd
CH3CHO
-H elim
OH
ClCl
H2O Pd
21
O "-H elim"
H
Cl
H2O Pd
OH
ins
Cl
H2O Pd
H
Ligand Substitution
The Wacker process
Characteristics of the Wacker process:
• The oxygen in the product derives from water,
not directly from the oxygen used as oxidant
• higher olefins yield ketones, not aldehydes
• large amounts of halides required: corrosive
• side products resulting from nucleophilic attack
of halide on olefin
No longer important for acetic acid synthesis
Several variations (with more complicated nucleophiles)
used in organic synthesis
22
Ligand Substitution
Nucleophilic attack on the ligand
How can you distinguish between internal and external
attack of OH- ?
Cl
OHH2O
Pd
OH-
-
Cl
H2O Pd
Cl
Cl-
OH
Cl
??
-
Cl
H2O Pd
-
ins
OH
Use trans-CHD=CHD and trap the intermediate
OH
Cl
Pd-C-C-OH with CO:
H2O
Pd
Cl
-
Cl
H2O Pd
H
O
CO
- Cl-
OH
Pd
CO
ins
HO
O
O
23
nucl
attack
Pd
O
Ligand Substitution
Using isotopic labelling
to study mechanisms
HO
D
OH
Pd
D
D
Pd
D
-
O
CO
O
D
D
OH
Pd
D
D
HO
Pd
Pd
D
24
HO
D
CO
D
D
O
D
O
D
Ligand Substitution
Using isotopic labelling
to study mechanisms
Could acetaldehyde be formed directly as vinyl alcohol ?
OH
OH
diss
Pd
H
H2O
O
CH3
H
ins
OH
Pd
"-H elim"
O
Pd
diss
H
CH3
H
Perform reaction in D2O:
OD
OD
D2O
diss
Pd
O
H
O
CH2D
H
ins
Pd
OD
"-H elim"
Pd
D
25
O
diss
H
O
CH3
Ligand Substitution