Transcript lecture 5 ligand substitution

LECTURE 5: ORGANOMETALLIC REACTIONS I
LIGAND SUBSTITUTION
LIGAND SUBSTITUTION
Cl
Cl
2Pd
Cl
Cl
C 2H 4
Cl
Cl
H 2O
Pd
Cl
Cl
H 2O
Pd
Cl
OH-
P d (0 ) + H + +
H 2 O + 2 C l-
Cl
H 2O
OH
Cl
H 2O
- 2 e (C u C l 2  C u C l)
Cl
- C l-
Pd
Pd
OH
Cl
Cl
H 2O
H
Pd
C H 3C H O
 -H e lim
C lCl
H 2O
O "  -H e lim "
Pd
H
OH
OH
Cl
H 2O
Pd
in s
Cl
H 2O
Pd
H
LIGAND 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
Ligand Substitution
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LIGAND SUBSTITUTION
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 MAIN MECHANISTIC PATHWAYS
ASSOCIATIVE (A):
LnML’ + L’’
LnML’L’’
slow
DISSOCIATIVE (D):
L’’
LnML’
slow
LnM
+ L’
fast
LnML’’
fast
LnML’’ + L’
DISSOCIATIVE LIGAND SUBSTITUTION
Example:
L nM
CO
18 e
L nM
+
CO
16 e
L'
L nM
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).
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DISSOCIATIVE SUBSTITUTION at ML6
16-e ML5 complexes are usually fluxional;
the reaction proceeds with partial inversion, partial
retention of stereochemistry.
or
1 6 -e
1 8 -e
oct
SP
Ligand Substitution
d is to rte d
TBP
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ASSOCIATIVE LIGAND SUBSTITUTION
Example:
L nM
16 e
L'
- L
L nM
L'
L n -1 M
18 e
16 e
(N H 3 ) 2 P tC l 2
L'
B r- C ls lo w
H 2O - C l
Sometimes the solvent
is involved.
Reactivity of cis-platin:
(N H 3 ) 2 P t(C l)(B r)
B rfa s t - H 2 O
(N H 3 ) 2 P t(C l)(H 2 O ) +
- C l-
N u c le o B a s e
fa s t
s lo w
- H 2O
(N H 3 ) 2 P t(C l)(N B ) +
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trans influence and trans effect
• In square planar complexes, some ligands direct
substitution to a position trans to themselves.
• When reaction is controlled by factors influencing the
ground state energy of the complex – trans influence
• Reaction is controlled by factors affecting the transition
state energy.
trans influence
Ligands that form strong  bonds or  tend to weaken the
metal ligand bond trans to the metal.
In the ground state this property is called the trans
influence.
H- > PR3 > SCN- > I-, CH3-, CO, CN- > Br- > Cl- > NH3 >
OH-
trans kinetic effect
• Tendency of certain ligands to direct incoming groups
to trans position with reactions under kinetic control.
C2H4, CO > CN- > NO2- > SCN- > I- > Br- > Cl- > NH3>
OH-
overall trans effect
CO, CN-, NO C2H4 > PR3, H- > CH3-, S=C(NH2)2 > PhNO2- SCN-, I-, > Br- > Cl- > Py, NH3, OH- H2O
Exercise 7.1
ASSOCIATIVE LIGAND SUBSTITUTION
Example:
L nM
16 e
L'
- L
L nM
L'
L n -1 M
18 e
L'
16 e
Sometimes the solvent is involved.
Reactivity of cis-platin:
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rate
Rate = ks[ML4] = k1 [ML4][Y]
Mechanism:
Square pyramidal – trigonal bipyramid – with retention of
configuration.
Associative substitution with 18 e systems
• Can occur if the metal can delocalize a pair of electrons
onto one of its ligands
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
O
3 -e
1 -e
O
CO
M
R
M
M
M
5 -e
3 -e
R
(1 + 2 )-e
1 -e
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DISSOCIATIVE LIGAND SUBSTITUTION
Example:
L nM
CO
18 e
L nM
+
CO
16 e
L'
L nM
L'
18 e
Rate = k [ML6]
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Rate of substitution of Ligands
Rate of substitutions of a particular ligand is a function of
ligand type.
Ligands that are nuetral in their free state dissociate
rather easily.
Redox-induced ligand substitution
Unlike 18-e complexes, 17-e and 19-e complexes are
labile.
Oxidation and reduction can induce rapid ligand
substitution.
L'
L nM
- e-
+
1 7 -e
L nM
L'
+
1 9 -e
L nM
1 8 -e
+ e-
L nM 1 9 -e
L n -1 M - + L
1 7 -e
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Redox-induced ligand substitution
F e(C O )4L
F e(C O )5
CO
F e(C O )4
F e(C O )5
F e(C O )4L
L
Initiation by added reductant.
Sometimes, radical abstraction
produces a 17-e species
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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 (C O )6
d
h
d
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