ligand substitution reactions: dissociative
DESCRIPTION
Ligand substitution reactions: dissociative. Example: Factors influencing ease of dissociation: 1 e row < 2 e row > 3 e row d 8 -ML 5 > d 10 -ML 4 > d 6 -ML 6 stable ligands (CO, olefins, Cl - ) dissociate easily (as opposed to e.g. CH 3 , Cp). Dissociative substitution in ML 6. - PowerPoint PPT PresentationTRANSCRIPT
04/22/23 Ligand Substitution and Activation 1
Ligand substitution reactions:dissociative
Example:
Factors influencing ease of dissociation:• 1e row < 2e row > 3e row
• d8-ML5 > d10-ML4 > d6-ML6
• stable ligands (CO, olefins, Cl-) dissociate easily (as opposed to e.g. CH3, Cp).
LnM CO LnM LnM L'+ COL'
18 e 16 e 18 e
04/22/23 Ligand Substitution and Activation 2
Dissociative substitution in ML6
16-e ML5 complexes are usually fluxional;the reaction proceeds with partial inversion, partial retention of stereochemistry.
18-e
oct
16-e
SP distortedTBP
or
04/22/23 Ligand Substitution and Activation 3
Example:
Sometimes the solvent is involved:
Ligand substitution reactions:associative
16 e 18 e
L'Ln-1M L'LnM LnM L'
- L
16 e
L2MCl2 L2M(Cl)(Br)
L2M(Cl)(H2O)+
Br-
- Cl-
H2O - Cl- Br-- H2O (cis-platin !)
04/22/23 Ligand Substitution and Activation 4
Ligand rearrangement
Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactions:
M N
O
M N O
3-e 1-e
M M
3-e5-eMCO
RM
O
R
(1+2)-e 1-e
04/22/23 Ligand Substitution and Activation 5
Ligand substitution reactions:redox-induced
For 17-e ⇋ 19-e usually fast equilibrium:
• Reduction promotes dissociative substitution.
• Oxidation promotes associative substitution.
• In favourable cases, the product oxidizes/reduces the starting material redox catalysis.
LnM L'LnM + L'
04/22/23 Ligand Substitution and Activation 6
Initiation by added reductant.
Sometimes, radical abstraction produces a 17-e species(see C103).
CO
L
Fe(CO)4L
Fe(CO)4L
Fe(CO)4
Fe(CO)5
Fe(CO)5
04/22/23 Ligand Substitution and Activation 7
Ligand substitution reactions:photochemical
Visible light can excite an electron from an M-L bonding orbital to an M-L antibonding orbital (LF transition). This results in fast ligand dissociation.
Requirement: the complex must absorb,so it must have a colour.
d
d
h
M(CO)6
04/22/23 Ligand Substitution and Activation 8
Some ligands have a low-lying * orbital and undergo MLCT excitation. This leads to easy associative substitution.
The excited state is formally (n-1)-e !
M-M bonds dissociate easily on irradiation (n-1)-e associative substitution
d
d
h
M(CO)4(phen)
*
04/22/23 Ligand Substitution and Activation 9
Ligand activation:electrophilic and nucleophilic attack
• Electron-rich metal fragment:ligands activated for electrophilic attack.
N4: strong -donor
NN
N
Rh SN
++
SH+N
N
N
RhN
+
04/22/23 Ligand Substitution and Activation 10
Ligand activation:electrophilic and nucleophilic attack
• Electron-poor metal fragment:ligands activated for nucleophilic attack.
-
CrOCOC
CO
Bu
H
Li+
CrOCOC
CO
BuLi
CO: strong -acceptor
04/22/23 Ligand Substitution and Activation 11
Electrophilic attack on ligand
• Hapticity may increase or decrease.
• Formal oxidation state of metal may increase.
H+MI
+
MI
H+M(0) MII
+
04/22/23 Ligand Substitution and Activation 12
Electrophilic addition
Electrophilic abstraction
Alkyl exchange also starts with electrophilic attack.
O
Fe(CO)3
OEt+
Fe(CO)3
Et3O+
Cp2ZrMe
Me
B(C6F5)3Cp2Zr
Me
+
MeB(C6F5)3-
16 e 14 e
+
04/22/23 Ligand Substitution and Activation 13
Competition: attack of electrophile on metal(may be followed by shift to ligand)
Fe
+
Fe H
Ni
H+
H+
Ni
+
04/22/23 Ligand Substitution and Activation 14
Electrophilic attack on metal
Can be the start of oxidative addition
Key reaction in the Monsanto acetic acid process:
I2(CO)2Rh Me I I2(CO)2RhMe + I- I3(CO)2RhMe
MeOH + CO MeCOOHHI
"Rh"
04/22/23 Ligand Substitution and Activation 15
MeOH
MeI
HI
H2O
MeCOOH
MeCOI
Rh(CO)2I2
Rh(CO)2I3Me Rh(CO)2I3(COMe)
CO
oxidativeaddition
reductiveelimination
insertion
04/22/23 Ligand Substitution and Activation 16
Nucleophilic attack on ligand
Nucleophile "substitutes" metal hapticity usually decreases
Oxidation state mostly unchanged
Competition: nucleophilic attack on metalusually leads to substitution
04/22/23 Ligand Substitution and Activation 17
Nucleophilic abstraction
NaH
CrOCOC
CO
Na+
-
CrOCOC
CO
Cp2WH2BuLi
Cp2WH Li
04/22/23 Ligand Substitution and Activation 18
Nucleophilic addition
Key reaction of the Wacker process:
(H2O)Cl2Pd + OH- (H2O)Cl2Pd
OH-
C2H4 + ½ O2PdCl2, H2O
CH3CHOCuCl2
04/22/23 Ligand Substitution and Activation 19
H2O PdCl
Cl
H2O PdH
ClOH
H2O PdH
Cl O
Cl Pd ClCl
Cl 2--
Cl PdCl
Cl
H2O PdCl
OH
H2O PdCl
OH-H elim
- 2 e (CuCl2 CuCl)
C2H4 H2O
H2O PdCl
ClOH
-
ins
Cl-CH3CHO
H2O Pd ClH
Cl-
Pd(0) + H+ +H2O + 2 Cl-
OH-
- Cl-
-H elim
04/22/23 Ligand Substitution and Activation 20
How can you distinguish between internal and external attack of OH- ?
Use trans-CHD=CHD and trap the intermediatePd-C-C-OH with CO:
H2O PdCl
Cl
H2O PdOH
Cl-
OH-H2O Pd
Cl
ClOH
-
-
H2O PdO
Cl
H
ins
OH- - Cl-??
H2O PdCl
ClOH
-
PdCO
OH
O
O
CO
- Cl-
Pd
O
HO
ins
nuclattack
04/22/23 Ligand Substitution and Activation 21
OH-
O
OD
D
Pd
HO
D
D
Pd
HO
D
D
Pd
D
D
O
OD
D
PdD
HO
D
Pd
OHD
D
CO
CO
04/22/23 Ligand Substitution and Activation 22
Could acetaldehyde be formed directly as vinyl alcohol ?Perform reaction in D2O:
diss-H elim OPd
D
ins
CH3
OH
Pd
OD
D2O
CH2D
OHdiss
ODOD
Pd
H