Low-coordinate Lanthanides

Question 1

How do the major bulk of lanthanides coordinate

Answer 1

1. In aqueous
2. Low-coordinate= rarer

Question 2

What are the general principles of coordination

Answer 2

1. Lanthanide ions behave as hard Lewis acids. They thus have high affinity for hard bases such as F- and H2O.
2. The bonding is largely non-directional and electrostatic in origin. Coordination geometries are largely determined by the size and shape (steric demands) of the coordinating ligands.
3. The f-elements form large cations and thus support high coordination numbers. The size of the Ln3+ cations decreases across the series leading to higher charge densities and stronger ionic bonds for the heavier members.

Question 3

What is important for low-coordinate species

Answer 3

1. Need to keep water and air out the way

Question 4

What are the different reactions of Ln3+ complexes

Answer 4

1. Oxidative insertion
2. protonolysis
3. Reductive transmetallation
4. schlenk-type equilibrium
5. salt metathesis
6. adduction
7. dimerisation

Question 5

What is used to install ligands

Answer 5

1. Salt metathesis

Question 6

Describe synthesis of Sm(II) Aryloxide

Answer 6

1. Form LnI2 - oxidative addition
2. Salt metathesis with THF to install ligands
3. Protonolysis to remove other bits
4. Look at diagrams

Question 7

Describe ligands needed for non-aqueous coordination chemistry of Ln

Answer 7

1. Steric production (bulky) which are often solubilising hydrocarbon substituents.
2. Tend to rely on hard donors (N, O) at the metal for good Ln ligand bonding.
3. Allow access to unusual geometries and coordination numbers- the bigger the ligand the lower the coordination number.
4. Geometries driven by sterics- no preference to the direction of bonding.
5. Low coordinate lanthanides are often air and moisture sensitive requiring specialised techniques.

Question 8

Describe oxidative addition

Answer 8

1. Reaction of a reducible substrate with a metal leading to oxidation of the metal and insertion into the substrate.
2. Can also be halogens such as I2.
3. Sm + ICH2CH2I –> SmI2(THF) + H2C=CH2

Question 9

Describe Schlenk-type eq

Answer 9

1. “Scrambling” of ligands around a lanthanide centre which reflects ionic bonding and can be inhibited by steric demand of substituents.
2. LnR3 + LnR3’ <–> LnR2R’ + LnR2’R <–> etc

Question 10

Describe protonolysis

Answer 10

1. Deprotonation of a substrate by a more basic Ln-X bond.
2. Deprotonation ability roughly follows electronegativity i.e. the more stable anion will form.
3. Ln-C will deprotonate H-N, Ln-N will deprotonate H-O etc
4. Driven by pKa- more e-neg atom the happier the anion
5. Ln(N(SiMe3)2)3 + MeOH –> Ln(N(SiMe3)2)2(OMe)

Question 11

Describe salt metathesis

Answer 11

1. Reaction of a more electrophilic metal-element bond with a lanthanide halide.
2. Relies on the high lattice energy of simple halides such as LiCl to drive the reaction forward
3. LnCl3 + 3 LiN(SiMe3)2 –> Ln(N(SiMe3)2)3 + 3LiCl

Question 12

What is lattice energy

Answer 12

1. energy gap between the energy of the separate gaseous ions and the energy of the ionic solid.

Question 13

Describe Adduction

Answer 13

1. Generally, good adduct formers are hard 2 electron donors like phosphine oxides, cyclic ethers.
2. Can be inhibited by extremely bulky ligands or high coordination numbers.
3. Replacement of bridging atoms in oligomers by donor molecules leads to monomers, albeit without reducing coordination number.
4. e.g. adds O=PPh3

Question 14

Describe oligomerisation

Answer 14

1. Essentially a type of adduct formation where the 2-electron donor is a ligand on another metal centre acting as a bridge.
2. Size of oligomer determined by steric demands.
3. Can lead to dimers, trimers etc

Question 15

Describe reductive transmetallation

Answer 15

1. Reduce one metal to 0 OS as ligand swaps to other

Question 16

Describe Ligands for non-aqueous chemistry

Answer 16

1. Big, hard, greasy
2. Slight pyramidal structure

Question 17

What methods can be used when doing low coordinate Ln chem

Answer 17

1. Schlenk line- replaces air
2. Schlenk glass- what reaction is done in
3. Glove box - full of inert gas