Ch12 Lecture 2 The Chelate Effect
I.
How to Make a Strong Complex
A.
Factors Effecting M—L Binding Strength = Molecular Organization
1) Complementarity = sum of size, geometry, and electronic matching between the
metal ion and the ligand(s)
a) The individual components are simple and can be predicted or found
experimentally
b) Example: HSAB Theory predicts Fe3+/O2- is more complementary than
Fe3+/S2c) Example: d8 Ni2+ should have good complementarity with cyclam
d) Complementarity is only the first step towards complex stability
2)
Constraint = the number of and flexibility between ligand donor atoms
a) Topology = interconnectedness of donor atoms
b) Rigidity = how fixed in space donor atoms of the ligand are with respect to
each other
c) These constraint factors are more difficult to grasp than complementarity
d) Maximizing these factors can lead to huge increases in complex stability
Constraint and Binding Affinity
Rigidity
Binding
Affinity
Topology
Electronics
Size
Geometry
Complementarity
Constraint
B.
Topological Effects
1) The Chelate Effect
a) Two donor atoms linked together = a chelate (claw)
b) Chelate ligands form much more stable metal complexes than monodentate
related ligands (up to 105 times as stable)
better
2+
Ni + L Formation Constants:
complementarity
L =
NH3
en
trien
2,3,2
log b
8.12
13.54
13.8
16.4
H2N
H2N
NH2
en
c)
NH
trien
NH2
H2N
NH
NH
2,3,2
Thermodynamic Reasons for the Chelate Effect = Entropy
H2N
NH3
+
M
NH
NH3
2 particles
H2N
+
M
H2N
H2N
3 particles
2 NH3
NH2
DG
DH
DS
Ni(NH3)22+
-6.93
-7.8
-3
Ni(NH3)42+
-11.08
-15.6
-15
Ni(NH3)62+
-12.39
-24
-39
Ni(en)2+
-10.3
-9.8
Ni(en)22+
-18.47
Ni(en)32+
-24.16
d)
OH2
M
OH2
A
D(DG)
D(DH)
D(DS)
+4
-3.1
-1.2
+7
-18.3
+3
-7.4
-2.7
+18
-28.0
-10
-11.8
-4
+29
~1.3/ring
(small)
Largest
Effect
Kinetics and the Chelate Effect
i. Chelate complex formation
+
k1
L
L
M
L
k -1
B
k2
OH2
AB
L
k -2
L
M
L
C
ii.
The Steady-State Approximation yields:
dC k1k 2 [A][B] k -1k -2 [C]


dt
k -1  k 2
k -1  k 2
Or rewriting with formation (kf) and dissociation (kd) constants:
dC
 k f [A][B] - k d [C]
dt
k 1k 2
k -1k -2
kf 
and k d 
k -1  k 2
k -1  k 2
iii. Assuming a chelate effect, k2 >>k-1
k 1k 2
kf 
 k1 (the same as for monodentat e ligands)
k -1  k 2
k -1k -2
k -1k -2
kd 

k -1  k 2
k2
iv. kf is not the source of the chelate effect. It is the same as other ligands
NH3+
v. kd must be the source of the chelate effect (dissociation is slow!)
k-1 is the same as for monodentate ligands
N
M
N
k-2 (ring opening) is the same as for monodentate ligands:
+
H
2+
Ni(trien)(H2O)2
-1
Ni(Htrien)(H2O)23+
15 s
H+
+
-1
4s
N
+
H
H
2 s-1
2 s-1
2+
Ni(H2O)6
+
Data for k2 (ring closing)
N
N
Pt
N
NH2
+
ka
Cl
N
N
Pt
2+
N
+
Pt(NH3)3Cl
NH3
kb
-1
ka = 0.73 s
N
+2
Pt(NH3)4
-1 -1
kb = 5.4 x 10 M s
ka
0.73 s 1
3


1.4
x
10
M  Effective Concentrat ion
4
1 1
k b 5.4 x 10 M s
Huge Concentration!
-4
4+
H4trien
k2, the formation of the second M—L bond, has been shown to be extremely large
compared to a second monodentate ligand binding. This is due to the large
“effective concentration” of the second donor atom of a chelate
NH3
NH2
M
M
NH2
chelate
NH3
monodentate
If k2 is large, kd must be small;
Very fast bond reformation after the first donor dissociates is the kinetic
source of the chelate effect
vi. Data
M+
L
kf
kd
Fe2+
HCO2-
7 x 103
10
Fe2+
C2O42-
2 x 104
3 x 10-3
2)
The Macrocycle Effect
a) Macrocyclic chelate complexes are up 107 times more stable than non-cyclic
chelates with the same number of donors
Ni(trien)2+ + H+
Ni2+ + H4trien4+
t½ = 2 seconds
Ni(cyclam)2+ + H+
Ni2+ + H4cyclam4+
t½ = 2 years
b)
Connecting all of the donors (having no end group) makes k-2 important
i. Breaking the first M—L bond requires major ligand deformation
ii. The increase in Ea required greatly slows down k-2
N
NH2
N
N
N
M
N
N
M
N
N
N
N
M
M
N
N
N
N
chelate
N
macrocycle
c)
A macrocycle is still a chelate, so it still has the k2 chelate effect going
d)
The result is a very stable complex as kd becomes miniscule
3)
The Cryptate Effect
a) Additional connections between donor atoms in a macrocycle further enhance
complex stability by making dissociation even more difficult
R
a)
N
b)
NH
HN HN
N
M
M
NH
HN HN
N
N
R
b)
My own research and others’ seek to take advantage of this stability
c)
Data
Cu(H2O)62+ water substitution t½ = 1.4 x 10-9 s
Cu(Me4Cyclam) in 1 M H+ t½ = 2 s
Cu(Bcyclam)Cl+ in 1 M H+ t½ > 6 years = 1.9 x 108 s
d)
Usefulness of such stable complexes
i. Oxidation catalysis in harsh aqueous conditions (H+ or OH-)
ii. MRI Contrast agents that must not dissociate toxic Gd3+
C.
Rigidity Effects
1) More rigid ligands (assuming complementarity) make more stable complexes
2) Data
M
L
t½
Cu2+
en
0.006 s
Cu2+
bipy
0.025 s
x3
Cu2+
spartiene
295 min
x 106
Ni2+
dien
0.07 s
Ni2+
tach
7 min
x (6 x 103)
Ni2+
TRI
90 days
x 108
H2N
NH2
en
N
difference
N
N
N
bipy
spartiene
N
NH2
H2N
NH
dien
N
NH2
NH2
NH2
tach
N
TRI
D.
Encompassing Principles
1) Multiple Juxtapositional Fixedness (Busch, 1970) = lack of end groups and
rigidity effects leads to more stable complexes for topologically complex ligands
if complementarity is satisfied
2)
II.
Pre-Organization (Cram, 1984): Ligands pre-formed into size and geometry
match for the metal ion do not require entropically costly reorganization to bind.
This savings in entropy leads to more stable complexes
Problem: Too much pre-organization can make it hard to get the metal in!
Stereochemistry of Reactions
A.
Retention or Inversion of Stereochemistry is possible during substitution reactions
1) Retention is usually favored for D and ID mechanisms
2) Inversion is often the result of the SN1CB mechanism
3) The orientation of the ligand entering the trigonal bipyramidal intermediate
determines the outcome
B.
Substitution in trans complexes
1) 3 possible substitution reactions for trans-[M(LL)2BX] + Y
a) Retention of configuration with
a square pyramidal intermediate
b) Trigonal bipyramidal intermediate
with B in the plane gives a mixture
of products
c) Trigonal bipyramidal intermediate
with B axial leads to cis product
2)
Experimental Data
a) Many factors determine the mixture of isomers in the product
b) Example: Identity of X
c)
C.
Prediction is very difficult without experimental data on related complexes
Substitution in cis complexes
1) The same 3 possibilities exist as for trans
1) The products are just as hard to predict
D.
Isomerization of Chelate Complexes
1) One mechanism is simple dissociation and reattachment of one donor of the
ligand. This would be identical to any other substitution reaction
2)
Pseudorotation
a) “Bailar Twist” = Trigonal twist = all three rings move together through a
parallel intermediate
b) Tetragonal Twists = one ring stays the same and the others move
Bailar Twist
Tetragonal Twist
Tetragonal Twist
Bailar Twist