Metal Grids

Question 1

What are the two key features of a metal grid?

Answer 1

• Two dimensional array of metal ions
• Ligands orientated in two different directions

Question 2

What is the difference between ligands which form a metal helicate vs a metal box?

Answer 2

• The blue bonds in both ligands have free rotation, but once a chelate-metal bond is formed this restriction motion around these blue bonds
• The helicate ligands has four bonds in red that link two chelating groups - there is free rotation around each of these bonds which allows the ligand to spiral between adjacent metal centres in the helicate
• There is no flexibility in the ligand for the grid. Once coordinated to metals it must be esstentially planar

Question 3

How is a metal grid structured?

Answer 3

• Tetrahedral geometry at the metal with two metal bidentate ligands sitting at an angle of 90° to each other
• For a 2x2 grid, a rhombus shape is seen rather than a square
• This is due to distortion from bringing two co-planar ligands closer together so they are at an optimal distance to maximise the aromatic interactions (π-π stacking)

Question 4

How does a metal grid form?

Answer 4

This grid can be self-assembled by mixing 4eq of Cu(I) with 4eq ligand to give an [M₄L₄]⁴⁺ structure

Question 5

If we form a metal grid with silver(I) ions?

Answer 5

• Ligands can extend to include a 3 bidentate group
• This self assembles to form a 3x3 grid which can be described as a [M₉L₆]⁹⁺
• Again it distorts towards rhombus-shape

Question 6

What about if we wanted to prepare a grid that was based on rectangular array of metal ions rather than a sqaure array?

Answer 6

This 2x3 grid can be prepared by combining 2 ligands of different lengths in the correct proportions
[M₆L₂L’₃]⁶⁺

Question 7

In the case of combining a tridentate and tetradentate ligand combining together with a metal, what structures can be formed

Answer 7

• In this case it is possible to form a 2x2 grid, 3x3 grid and 2x3 grid
• However, we see a great deal of selectivity for the mixed 2x3 grid over the 2x2 and 3x3 grids
• This is because the 3x3 grids is higher in energy (less stable)

Question 8

Why is the 3x3 grid not as favourable?

Answer 8

The central Ag(I) in the 3x3 grid is the only one entirely coordinated by pyridazine (pz) nitrogens
These are less electron rich than pyridine (py) nitrogens and so this is coordinated less strongly making the complex less stable

Question 9

What are the requirements for constructing grids around octahedral metal ions?

Answer 9

• Each metal ion is coordinated by two tridentate ligands in a meridional manner
• The ligand need to be rigid and planar with limited conformational flexibility
• The metal ion here needs to easily adopt octahedral geometry i.e. Zn(II)

Question 10

This 2x2 grid can be described as

Answer 10

[M₄L₄]⁸⁺

Question 11

Why does the ligand require an addition phenyl group when forming a grid with an octehedral centre?

Answer 11

• Each ligand has a phenyl substituent in the iddle which oritentates 90° to the ligand it is attached to and coplanar with the liganding running in the other direction
• This allows for good π-π stacking in between the aromatic rings

Question 12

Why does manganese need to be paired with a ligand with harder anionic oxygen donors?

Answer 12

Because it is an earlier transition metal

Question 13

How does the following ligand bond to metals

Answer 13

Oxygen is coordinating to two different metal centre - one LP involved in the coordination to each metal ion

Question 14

What are some unusal features formed by the 3x3 grid with this ligand and Mn
And what is the overall structure?

Answer 14

• Oxygen coordinating to 2 different metal centres - where there is one LP involved in coordinating to each metal ion
• The ligand now had a charge of -2
• This means that theres is an overall charge contribution of -12 from the 6 ligand. This overall structure is [M₉L₆]⁶⁺
• (note grids with too high a positve charge can become unstable)

Question 15

We can use ¹H NMR spectroscopy for metal-coordinate complexes
What is the differences between this and organic molecules?

Answer 15

• Most organic molecules are diamagnetic (all electrons paired in orbitals)
• Metal-organic coordination complexes may contain paramagnetic metal ions with unpaired electrons spins
• These unpaired spins interact with nuclear spins that give rise to NMR signals and alter the spectrum
• ¹H NMR of diamagnetic species gives signals in the range of 0-10ppm
• ¹H NMR of paramagnetic species gives signals spread over many hundreds of ppm which are much broader (faster relaxation)

Question 16

Silver is diamagnetic
What would the ¹HNMR integral ratio be of a 3x3 grid?

Answer 16

• In a 3x3 grid there a two distinct positions that the ligands can have: an interior position and an exterior position
• There are twice as many exterior positions than interior positions
• This means for each environment on the ligand there will be 2 signals from this grid, in an integral ratio of 2:1

Question 17

M-L coordination bonds are sufficiently strong that if we run a mass spectrum under mild conditions then all these coordination bonds can survive, and we can see molecular ion peaks
This method of ionisation we typically use is electrospray ionisation (ESI)
What is the main differences between mass spectra of organic molecules and metal-organic coordination complexes?

Answer 17

• Aleady charged (do not need to pick up a H⁺/Na⁺ to form ion
• Often have a charge greater than one (m/z x-axis)

Question 18

Why might we used X-ray crystallography to study metal complexes?

Answer 18

• Single crystal X-ray crystallography is a great method for structural determination and all the 3D structures throughout this module are from this method
• This is a powerful technique as it gives information on the precise 3D arrangement of atoms in space
• It can tell us about coordination geometries and bond lengths in a way that no other method can

Question 19

What are the drawbacks of X-ray crystallography?

Answer 19

• The picture we build is based on the average structure in the crystal - this may be different to the structure in solution
• Is also does not tell us about flexibility
• i.e. Although the silver grids have a rhombus shape in the solid state, they are very flexible and rapidly interconvert from a rhombus to a square.

Question 20

What is scanning tunnelling microscopy (STM)?

Answer 20

• involves depositing the grid on a surface such as gold or graphite
• A fine conducting tip is brought close to the surface in a vacuum
• Electrons can then tunnel across the gap allowing the surface to be imaged
• In the CITS the image shows sharp contrast at the positions of the Mn centres
• The combined image has metal ions stand out due to electrons in the 3d orbitals