Cation Binding Hosts

Question 1

Why bind cations?

Answer 1

Biological processes
Medically important
Extract valuable metal ions
Extract toxic/pollutant metal ions

Question 2

What affects stability of metal complexes?

Answer 2

1) Basicity of donor atoms
2) Covalent tendency of metal ion
3) Charge neutralisation on complex formation
4) Sterics
5) Increase in translational entropy
6) Preorganisation of the ligand

Question 3

How does preorganisation affect entropy and enthalpy?

Answer 3

Decreases solvation energy, steric strain and dipole-dipole repulsions
Lower entropy of ligand favours complex formation

Question 4

Why are s-block cations important? Why are they hard to bind?

Answer 4

For biological roles (Na+/K+)

Have little tendency towards covalency, and low charge

Question 5

What is the chelate effect?

Answer 5

The enhanced stability of a complex containing chelate rings, compared to similar systems containing fewer or no rings

Question 6

What are the entropic and enthalpic contributions to the chelate effect?

Answer 6

Entropy - a multidentate ligand will release more species into solution than several monodentate ligands
Enthalpy:
For a multidentate ligand, lone pairs are closer together compared to seperate monodentate ligands so mutual repulsion has been overcome.
Increase in basicity of more alkylated chelate ligand

Question 7

How does the chelate effect change?

Answer 7

For a 5m ring upwards, it decreases in magnitude.

Longer chains means higher configurational entropy so ring formation becomes increasingly improbable

Question 8

Why are 4m rings less stable?

Answer 8

They are very strained

Question 9

What is the macrocyclic effect?

Answer 9

Increased thermodynamic stability of macrocyclic systems compared to their acyclic systems

Question 10

What are the entropic and enthalpic contributions to the macrocyclic effect?

Answer 10

Entropy:
A macrocyclic ligand is less flexible and has less disorder to lose on complexation
Enthalpy:
Less heavily solvated - less energy needed to desolvate
Lone pairs closer together so part of mutual repulsion has been overcome
Increase in basicity of more alkylated ligand

Question 11

What are the types of cation binding hosts?

Answer 11

Crown ether
Cryptand
Spherands

Question 12

What is the optimal spatial fit?

Answer 12

The better the fit of the cation guest, the stronger the complex is

Question 13

What happens when the cation isn’t the right size for crown ether?

Answer 13

Too small - crown will wrap itself around, maximises electrostatic interaction, increases strain of ligand
Too big - has to perch, decreases electrostatic interaction

Question 14

Why is cryptand stable?

Answer 14

Entropy - lone pairs almost exactly where they need to be to bind cation so very little reorganisation
Enthalpy - bound cation is poorly solvated with a cryptand so it is favourable to bind

Question 15

How is polymerisation in cryptand synthesis prevented?

Answer 15

1) Run reactions at high dilution - add dropwise to large volumes of solvent
2) Template synthesis

Question 16

How is template removed in cryptand synthesis?

Answer 16

1) Weakly coordinated - wash organic solution of complex with water
2) If macrocycle coordinating to cation through amines, add an acid
3) Addition of a competitive ligand

Question 17

What is Cram’s principle of preorganisation?

Answer 17

The more highly hosts and guests are organised for binding and low solvation prior to complexation, the more stable the complex

Question 18

Why is spherand most stable?

Answer 18

Enforced spherical cavity - lone pairs are forced to be exactly in right place for binding