lecture 5: organometallic reactions i ligand substitution
DESCRIPTION
LECTURE 5: ORGANOMETALLIC REACTIONS I LIGAND SUBSTITUTION. LIGAND SUBSTITUTION. 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 - PowerPoint PPT PresentationTRANSCRIPT
LECTURE 5: ORGANOMETALLIC REACTIONS ILIGAND SUBSTITUTION
LIGAND SUBSTITUTION
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
Ligand Substitution 4
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 oftenthe slow, rate-determining step
LIGAND SUBSTITUTION
This premise is not always correct, but it appliesin 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’’ slow LnML’L’’ fast LnML’’ + L’
DISSOCIATIVE (D):
L’’
LnML’ slow LnM fast LnML’’
+ L’
Ligand Substitution 7
DISSOCIATIVE LIGAND SUBSTITUTION
Example:
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).
LnM CO LnM LnM L'+ COL'
18 e 16 e 18 e
Ligand Substitution 8
DISSOCIATIVE SUBSTITUTION at ML6
16-e ML5 complexes are usually fluxional;the reaction proceeds with partial inversion, partial retention of stereochemistry.
The 5-coordinate intermediates are normally too reactive to be observed unless one uses matrix isolation techniques.
18-e
oct
16-e
SP distortedTBP
or
Ligand Substitution 9
ASSOCIATIVE LIGAND SUBSTITUTION
Example:
Sometimes the solvent
is involved.Reactivity of cis-platin:
16 e 18 e
L'Ln-1M L'LnM LnM L'
- L
16 e(NH3)2PtCl2 (NH3)2Pt(Cl)(Br)
(NH3)2Pt(Cl)(H2O)+
Br-
- Cl-
H2O - Cl- Br-- H2O
NucleoBase - H2O
(NH3)2Pt(Cl)(NB)+
slow
fast
fast- Cl-
slow
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 > Ph-
NO2- SCN-, I-, > Br- > Cl- > Py, NH3, OH- H2O
Exercise 7.1
Ligand Substitution 15
ASSOCIATIVE LIGAND SUBSTITUTION
Example:
Sometimes the solvent is involved.Reactivity of cis-platin:
16 e 18 e
L'Ln-1M L'LnM LnM L'
- L
16 e
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 Substitution 18
LIGAND REARRANGEMENT
Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactionsof an apparently saturated complex:
M N
O
M N O
3-e 1-e
M M
3-e5-eMCO
RM
O
R
(1+2)-e 1-e
Ligand Substitution 19
DISSOCIATIVE LIGAND SUBSTITUTION
Example:
Rate = k [ML6]
LnM CO LnM LnM L'+ COL'
18 e 16 e 18 e
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.
Ligand Substitution 22
Redox-induced ligand substitutionUnlike 18-e complexes, 17-e and 19-e complexes are labile.
Oxidation and reduction can induce rapid ligand substitution.
• Reduction promotes dissociative substitution.
• Oxidation promotes associative substitution.
• In favourable cases, the product oxidizes/reducesthe starting material redox catalysis.
L'
LnM
LnM+ LnM L' +
LnM- Ln-1M- + L
- e-
+ e-18-e
17-e 19-e
19-e 17-e
Ligand Substitution 23
Redox-induced ligand substitution
CO
L
Fe(CO)4L
Fe(CO)4L
Fe(CO)4
Fe(CO)5
Fe(CO)5
Initiation by added reductant.
Sometimes, radical abstractionproduces a 17-e species
Ligand Substitution 24
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.
Requirement: the complex must absorb, so it must have a colour!or use UV if the complex absorbs there
d
dh
M(CO)6