Topic 2- host guest recognition

Question 1

A host complex will be more selective for binding a certain ion if the Ka of the ion is what?

Answer 1

higher

eg
log ka Na+ = 4 ish
log Ka K+ = 6 ish
host is selective for k+ over Na+

Question 2

What group are crown ethers good at binding?

Answer 2

Group one metal cations

Question 3

Why are crown ethers good at binding group one metal ions?
What type of interactions occur?

Answer 3

The lewis basic O atoms donate electron density to positively charged M cation.
Macrocycles, entropically favourable, rigid, less flexibility, high preorganisation.
Ion dipole interactions.

Question 4

What key things should we always consider when deciding how optimal a host is going to be for binding?

Answer 4

PREORGANISATION- the more preorganised the beter, less enthalpy cost to organise and can often help linearity of halogen or hydrogen bonds which strengthen interactions.

COMPLEMENTARITY, size shape polarity and electrostatics.
size could effect if it is convergent which ramps up binding.

SOLVENT- always consider polarity, hydrophobic or hydrophillic properties of solvent. If a solvent better solvates an ion, its harder to get it to bind in the host.
Consider hydrophobic effects too

Question 5

4 types of interactions that we could employ to design a good host for cation binding:

Answer 5

1)Ion dipole interactions
2) Cation-pi interactions
3) electrostatic interactions
4) lewis base coordination

Question 6

4 types of interactions that we could employ to design a good host for cation binding:

Ion dipole interactions… how do they work?

Answer 6

EDG connected to atom to create a negative dipole. This can interact with the positive cation.

Question 7

4 types of interactions that we could employ to design a good host for cation binding:

Cation- pi interactions … how do they work?

Answer 7

e rich aromatic ring interacts with the cation due to electronic complementarity between them.
pi system donates electron density to the positive cation.

Question 8

4 types of interactions that we could employ to design a good host for cation binding:

Lewis base coordination… how does it work?

Answer 8

Lewis base eg N or O atom has a lone pair.
The lone pair interacts by donating its e density to the positive cation guest.

Question 9

Crown ethers
how are they named?

Answer 9

number in the middle refers to how many atoms are in the ring, number at the end refers to how many donor atoms there are in the ring.
eg dibenzo[18]crown-6
18 atoms in ring
6 oxygen donor atoms.

Question 10

If a metal cation is too big for a crown ether, what may happen to the structure of the complex?

Answer 10

In the event of Cs+ cation with 18crown6, the Cs+ ion is too big for the crown ether so it complexes but slightly above the macrocycle.
Another macrocycle then complexes above that, resulting in 2 macrocycles to one Cs+ ion.

Stoichiometry changes to a 2:1 host guest complex.

Question 11

What is the binding strength of a cryptand like in comparison to that of a macrocycle/ crown ether?

why is this?

Answer 11

Cryptands have higher binding strength. Larger Ka values.

3rd “strap”/ bridge provides additional rigidity and preorganisation as well as increased amount of donor atoms. Thermodynamically the extra preorganisation is favourable.

Question 12

In a 2,2,2 cryptan which is the cryptan analogous to 18 crown ether, what particular M ion does it coordinate well? How does this compare to the crown ether?

Answer 12

Cryptands exhibit stronger binding and better selectivity for K+ than the crown ether anologue does.

The 3D shape provides a very complementary size for the K+ ion.

Question 13

How does the strength of binding for a spherand compare to that of a crown ether or cryptand?

why is this?

Answer 13

Spherands are even more preorganised than cryptands/ crown ethers / normal macrocycles. There is no degree of freedom to undergo change.
Everything is locked in position which results in an extremely well defined cavity in the middle to bind M cations. They are extremely selective.

Question 14

Are spherands selective? why?

Answer 14

yes, they are extremely preorganised and have an extremely well defined cavity for M+ cation.

Question 15

Spherands are more selective for Li+ over Na+ true or false?

Answer 15

true, the high preorganisation results in a very small unchangeable cavity for M+ cation. Li+ fits this better than Na+ does.

Question 16

order these hosts in order of increasing selectivity / increasing binding constants for group 1 metal ions:

Spherands, crown ethers, cryptands

Answer 16

lowest binding constant
crown ethers
cryptands
spherands
highest binding constant

Question 17

what are the main differences between TM or g1 M ions and organic cations?

Answer 17

organic cations are more hydrophobic and do not have a spherical charge distribution.

Question 18

Explain the structure of a bipyridinium cation. (cationic guest)

Answer 18

cationic charges on the Ns at both ends of the mollecule.
Hydrophobic section in the middle. worse interactions with water.

+ ———-+

(middle is negative or neutral)

Question 19

How is a host for cationic binding of an organic cation organised?

Answer 19

A macrocycle is formed of repeating units.
The repeating units themselves have polar headgroups which are hydrophillic at the top and the bottom (such as carbonyls which have an electron rich oxygen atom).

between these headgroups there is a hydrophobic middle of the repeating unit.

when the units are put together a macrocycle is formed much like the shape of a barrel, the top and bottom are made of the hydrophillic oxygen atoms and a non polar hydrophobic cavity is created in the middle.

Question 20

How does a host for cationic binding of an organic cation bind with said cation?

Answer 20

HOST: A macrocycle is formed much like the shape of a barrel, the top and bottom are made of the hydrophillic oxygen atoms and a non polar hydrophobic cavity is created in the middle. This is created from repeating units of the same makeup.

The organic cation guest takes a similar form, in that it has two positively charged ends and a hydrophobic component in the middle.

The host and the guest have complementary electronics and sizes so the hydrophobic cavity of the host encapsulates the hydrophobic part of the guest molecule.
The hydrophillic parts of the host and guest also line up out the top and bottom of the barrel shape.

Question 21

How does a host for cationic binding of an organic cation bind with said cation?

What type of effects and interactions create this strength of bonding?

Answer 21

the hydrophobic effect (of different parts of host and guest) and the complementarity in charge creates ion dipole interactions.
Both factors together are the driving force for very strong binding.

Question 22

How does the ionic radius of an anion compare to that of a cation?

how does this effect binding strengths?

Answer 22

Ionic radii are larger for anions as they have a lower charge density.

this may cause weaker binding as there are weaker electrostatic interactions due to a lower charge density.

Question 23

How does the solvation of anions compare to the solvation of cations?

Answer 23

anions are better solvated.

They can form better H bonds.

Question 24

why is the better solvation of anions compared to cations potentially an issue?

Answer 24

Better solvated means they make better H bonds which need a larger amount of energy to overcome.
larger delta H in these processes.

Question 25

How does the geometry of anions differ to those of cations?

Answer 25

cations are largely spherical
anions have far more varied geometires eg NO3- is trigonal planar. PF6- is octahedral.

Question 26

why is the more varied geometry of anions compared to cations potentially an issue?

Answer 26

There may be an enthalpic cost to reorganise the geometry of the host for optimum binding of the anion.
eg halogen bonding and h bonding have strict linearity, if the host cant accomodate this it may have to change.

Question 27

How does ph dependence compare for an anion to a cation? why may this be an issue?

Answer 27

anions can be protonated so may not always be in the anionic form which may present challenges when binding.

Question 28

how does coordination preference differ for anions compared to cations?

Answer 28

Anions are coordinatively saturated so we have to rely on weaker non covalent interactions to bind an anion than we would for cations.

Question 29

4 types of interactions that we could employ to design a good host for anion binding:

Answer 29

1) Anion- pi interactions
2) Hydrogen Bonding
3) Halogen bonding
4) electrostatic interactions

Question 30

4 types of interactions that we could employ to design a good host for anion binding:

Anion- pi interactions … how do they work?

Answer 30

electron poor aromatic ring due to the EWG substituent on the ring. e density from the anion given to the pi system of the aromatic.

Question 31

4 types of interactions that we could employ to design a good host for anion binding:

Electrostatic interactions … how do they work?

Answer 31

Electronic complementarity.
positive and negative charges interact in an ion ion interaction (most likely, could be ion dipole)

Question 32

4 types of interactions that we could employ to design a good host for anion binding:

Halogen bonding … how does it work?

Answer 32

Highly directional.
EWG attatched to halogen (most likely Br or I) withdraws electron denisty resulting in a positive dipole on the halogen.
This positive dipole then interacts electrostatically with the negative anion.

Question 33

4 types of interactions that we could employ to design a good host for anion binding:

Hydrogen bonding … how does it work?

Answer 33

EWG withdraws electron density from H making a positive dipole on H.
This positive dipole undergoes a ion dipole interaction with the anion guest.

Question 34

To see how a solvent is going to interact with both cations and anions we can use what?

Answer 34

the Hofmeister series

Question 35

Types of hosts for anions:

the electrostatic host:

How is the example we have studied structured?

Answer 35

a cryptand structure with 4 positively charged ammonium groups around the structure.
There is an anion binding cavity in the middle like a normal cryptand.

Question 36

Types of hosts for anions:

a hydrogen bonding host: (most common)

How is the example we have studied structured?

Answer 36

3 convergent H bonds from amide groups converge on amide guest.
Both host and guest have C3V symmetry, complementary shape means H bonds can occur linearly as they have a strict linearity.

Question 37

Types of hosts for anions:

NH hydrogen bonding host:
what motifs are often used in these examples?

Answer 37

urea motifs eg urea and thiourea

Question 38

Types of hosts for anions:

CH hydrogen bonding host:

How does it work?
What effect does the R group have on strength of H bonding?

Answer 38

EWG—–C—–H
d- d+

(—– = bond)
(//// = interaction)

EWG—–C—–H/////// A-
d- d+
EWG inductively withdraws e density through aromatic system. Creates larger d+ on H. Stronger bond with guest anion.

Question 39

Types of hosts for anions:

Halogen bonding host:
How does it work?

Answer 39

EWG attatched to halogen (most likely Br or I) withdraws electron denisty resulting in a positive dipole on the halogen.
This positive dipole then interacts electrostatically with the negative anion.

Question 40

Types of hosts for anions:

Hydrophobic host:
How is a host like this structured?

Answer 40

Similarly to when we made a host for organic cations. A large barrel like macrocycle will be formed which is hydrophobic and has a hydrophobic cavity for the anion.

Above this there may be a hydrophillic section to make it soluble in H2O.

Question 41

Types of hosts for anions:

the electrostatic host:

What are some drawbacks/ issues with this type of host?

Answer 41

In reality, each positive ammonium group in the cryptand is going to be stabilised by a negative counteranion.
These will compete with our central anion X- for the position in the cavity. This is counterproductive and can reduce the binding strength of X-.

In addition, they are hard hosts to make as there is repulsion between the positively charged ammonium groups.

Question 42

Types of hosts for anions:

the electrostatic host:

what can we do to fix the drawbacks that we see from this type of host?

Answer 42

use zwitterionic hosts
(will need updating once the topic is finished)

Question 43

Types of hosts for anions:

NH hydrogen bonding host:
How is a urea motif good for H bonding?

Answer 43

Complementary shape to many oxo anions

eg Os on acetate can line up linearly with H on urea for optimum linear H bonding.

Question 44

Types of hosts for anions:

NH hydrogen bonding host:

why does thiourea (X=S) have stronger binding than urea? (X=O)

Answer 44

Delocalisation of the lone pair on N is better when X=S than when X=O.
this better stabilises the reosnance form in adjacent rings
This will polarise the bond more
which results in a stronger H bond.

Question 45

Types of hosts for anions:

CH hydrogen bonding host:
Charge assisted HB?
How does it work?

Answer 45

Works in exactly the same way as when there is no charge on the group attached to carbon:
EWG—–C—–H
d- d+

(—– = bond)
(//// = interaction)

EWG—–C—–H/////// A-
d- d+
EWG inductively withdraws e density through aromatic system. Creates larger d+ on H. Stronger bond with guest anion.

BUT ALSO:
There is now an electrostatic component in which the positive charge on the species containing the H can interact electrostatically with the negative anion.

The COMBINATION of both these factors makes charge assisted HB stronger than normal CH hydrogen bonding.

Question 46

Types of hosts for anions:

Halogen bonding host:
How does a host like this affect selectivity of anions?

Answer 46

Allows for different selectivity than usual. Typically favours coordination of softer halides such as Br- or I-. Usually it is quite difficult to form complexes with these anions.

Question 47

Types of hosts for anions:

Halogen bonding host:

How does this type of host link to preorganisation?
What other bonds can effect this preorganisation?

Answer 47

halogen binding is a strictly linear process.
Therefore the host must be preorganised for bonding to occur. This means its selective.

hydrogen bonds in the further field may hold certain structures in place and impact the direction in which the halide is pointing. This will ramp up the binding as the H bonds will be convergent and preorganised.

Question 48

Types of hosts for anions:

Halogen bonding host:

what happens if a TM complex is added to the molecule containing the halide?

Answer 48

TM complex (d+) will likely withdraw negative charge from the Halide. This will cause a larger d+ and lead to stronger halogen bonding.

Question 49

Types of hosts for anions:

Hydrophobic host:
How can we tell what ions will bind most strongly with these hosts?

Answer 49

the host has a hydrophobic cavity for the anion.
we can use the Hofmeister series to see the order of hydrophobicity of our anions.
More hydrophobic anions will bind more strongly (higher Ka) to the hydrophobic cavity.

Question 50

Types of hosts for anions:

Hydrophobic host:

In what situations is a hydrophobic host used to bind anions?
why is this useful?

Answer 50

Used to bind anions in water (H2O).
This is useful as anions are often well solvated in H2O so this process is usually difficult.

Question 51

end of pre lecture notes:)))))