Metal Helicates Flashcards
What other orientation is available for this MOF?
- If we link two monodentate ligands via a flexible linker, then the ligands has many options
- It can birdge between the two metals to form an intramolecular chelate
Why would we use chelating ligands to avoid making complex mixtures?
- Monodentate chelating ligand do not impose many restrictions of metal geometry, meaning ligands often span out in a range of difference coordination environments
- This leads to a complex mixture of products
- Chelating ligands are rigid with restricted amounts of flexability
Metal helicates are typically made from
bidentate or tridentate chelating ligands with short flexible linkers (gives spiral shape)
[Ru(bipy)₃²⁺ is an ocetahedral metal centre with 3 bidentate ligands which is chiral
meaning…
it exist as two enantiomeric forms
If we draw each octehedral centre from the C₃ axis of rotation (120° rotation = identical)
3 ligands can point towards us and 3 point away
The rotation comes from the direction between the front and back pair of the bidentate ligand
What is the names of these two enantiomers?
- Enantiomer with clockwise rotation is deta (or P for plus)
- Enantiomer with anticlockwise roation lambda (or M for minus)
- (this is the helicity of this metal centre)
If there is 2 centres with this helical chiality, we can connect these bidentate groups resulting in 3 possibilities
They are…
- Both centres ΔΔ
- Both centres ΛΛ
- One Δ and one Λ
What are the names of the structures formed from connecting 2 bidentate groups together?
- When both metal centres have the same helicity, they are called helicates - there are chiral structures due to ΔΔ helicate being the mirror image of the ΛΛ helicate
- When the two centres have a different helicity, the structures are called mesocates - achiral due to having a mirror plane
Out of the 3 possibilities, which compound will form the most?
- The two helicate enantiomers have the same energy (and would therefore be formed in the same amount, i.e., a racemic mixture)
- Mesoates are more strained and higher in energy than helicates
Why is the helicase lower in energy?
In a helicate, a ligand has a gradual curvature throughout the structure, whereas in a mesocate, the curvature or the ligand must change direction midway through, hence increasing strain
What would be the instance where only helicates form?
If the flexible linkers between the rigid chelating groups are short, then only helicates can form
Called mechanical coupling
(coupling can exist between many metal centres in a row and also not limited to octahedral metal centres)
Ni(II) often adopts ocehedral coordination
We can form only a helicate with Ni(II) and ligand shown below
There are 3 metal centres, so the helicate we get are a racemic mixture of…
- ΔΔΔ
- ΛΛΛ
We can classify these helicates further as…
- Trinuclear - due to three metal ions
- Triple helicate - as 3 ligands around each metal
Are these structures charged?
- In shorthand we could describe this cationic helicate as [M₃L₃]⁶⁺
- The full formula of this compound must include anions (i.e. 3SO₄²⁻)
- In solution, the anions move around independently of the helicate
- Therefore the structures are not charged
Can a tetrahedral metal centre also have helicity?
- Tetrahedral metal centre can also have helicity
- This occurs when the two bidentate ligands coordinating at the metal centre are different at each end
- In this case one has an R group attached, and this would be where the rest of the ligand attached for making a helicate
- (common for Cu(I) and Ag(I) which form tetrhedral complexes with pyridine-based ligands)
How would we name this structure?
- Trinuclear double helicate
- It has 3 bidentate groups on a ligand which makes an [M₃L₂]³⁺ complex as a mixture of ΔΔΔ and ΛΛΛ isomers
If we link a greaer number of bidentate groups together, then we can construct even longer helicates
What is a requirement for this to hapen?
For all the centres to be of the same helicity
How would we name this structure?
- Tetranuclear double helicate
- With 4 bidentate groups on a ligands
- Make an [M₄L₂]⁴⁺ complex as a mixture of ΔΔΔΔ and ΛΛΛΛ isomers
How would we name this structure?
- Pentanuclear double helicate
- with 5 bidentate groups on a ligand we make a [M₅L₂]⁵⁺ complex
- as a mixture of ΔΔΔΔΔ and ΛΛΛΛΛ isomers
What is self-recognition?
- If ligands of different lengths are mixed (2, 3, 4, and 5 chelating groups)
- if the system is given enough time to reach equilibrium then all ligands pair up selectively with a ligand of the same length
- Can also occur between different ligands and different metal ions
If helicates formed from ligands of different lengths, this would leave some metals with not enough donor groups and some of the otther ligand donor atoms not coordinated to metals
What does this result in?
There would be an enthalpic pentalty for this as fewer M-L bonds would have been formed
Hence the overall free energy (ΔG) is lowest if self-recognition occurs first
What is the similarities and differences between a metal helicate and a DNA double helix?
- Recognition in DNA is controlled by H-bonding between base pairs
- However, in the metal helicates all the donor groups are the same and there is onl one type of M-L interaction
- The assembly of both structures is also favoured by aromatic interactions (π-π stacking0 and solvophobic/hydrophobic effect
What is the difference between ligand A and ligand B
- Ligand A leads to more steric bulk immediately around the metal due to being linked via the 6-position
- Ligand B leads to lower steric bulk immediately around the metal due to being linked via the 5-position
Why do triple helicates not form around octahedral metal centres?
There is greater steric hindrance in the formation of triple helicates around octahedral metal centres than in the formation of double helicates around tetrahedral metal centres, as you must fit three ligands around one metal
If we want to design an optimal ligands to encourage double helicate formation…
… then we often link at the 6-position and select an appropriate metal
If we want to design an optimal ligand for triple helicate formation…
… we often link at the 5-position and select and appropriate metal
If ligands A and C are mixed with copper(II) then…
exclusively mixed ligands double helicates are formed
Due to a 5 coordinate environment is suitable for Co(II) and this is achieved in this mixed ligand helicate
This is better than having copper (II) 50% tetrahedral and 50% octrahedral
What happens when we mix this ligand with copper?
- We can construct helicates where not all the metal centres in one helicate are the same
- Copper can readily undergo oxidation in air to give Cu(II)
- When some Cu(I) is mixed with this pentapyridine ligand in the presence of air, a helicate forms that has 1x Cu(I) ion and 1x Cu(II) ion
What is the geometry of the copper centres in the complex formed?
- Copper (I) ion has a distored tetrahedral geometry
- Copper (II) ion has a distored octahedral geometry
It is also possible to have circuar helicates
What is the requirements for this?
- All metal centres in circular helicates are of the same helicity
- Fe(II) has a preference of octahedral coordination and forms strong M-L bonds
- One Cl- sits in the middle and templates the formation of the circular helicate due to attractive electrostatic interactions with the positive metal centre
How would we describe this helicate?
- Pentanuclear (5-metals)
- Circular (not linear)
- Triple helicate (3x ligands at each metal)
- known as [M₅L₅ ͻ Cl]⁹⁺
If the same ligand instead was mixed with Fe(BF₄)₂ then a different structure is formed
Hexanuclear circular triple helicate
(but in this case the BF₄⁻ anion is larger than the Cl⁻ anion, it templaces this larger helicate)
