Low-coordinate Lanthanides Flashcards
1
Q
How do the major bulk of lanthanides coordinate
A
- In aqueous
- Low-coordinate= rarer
2
Q
What are the general principles of coordination
A
- Lanthanide ions behave as hard Lewis acids. They thus have high affinity for hard bases such as F- and H2O.
- 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.
- 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.
3
Q
What is important for low-coordinate species
A
- Need to keep water and air out the way
4
Q
What are the different reactions of Ln3+ complexes
A
- Oxidative insertion
- protonolysis
- Reductive transmetallation
- schlenk-type equilibrium
- salt metathesis
- adduction
- dimerisation
5
Q
What is used to install ligands
A
- Salt metathesis
6
Q
Describe synthesis of Sm(II) Aryloxide
A
- Form LnI2 - oxidative addition
- Salt metathesis with THF to install ligands
- Protonolysis to remove other bits
- Look at diagrams
7
Q
Describe ligands needed for non-aqueous coordination chemistry of Ln
A
- Steric production (bulky) which are often solubilising hydrocarbon substituents.
- Tend to rely on hard donors (N, O) at the metal for good Ln ligand bonding.
- Allow access to unusual geometries and coordination numbers- the bigger the ligand the lower the coordination number.
- Geometries driven by sterics- no preference to the direction of bonding.
- Low coordinate lanthanides are often air and moisture sensitive requiring specialised techniques.
8
Q
Describe oxidative addition
A
- Reaction of a reducible substrate with a metal leading to oxidation of the metal and insertion into the substrate.
- Can also be halogens such as I2.
- Sm + ICH2CH2I –> SmI2(THF) + H2C=CH2
9
Q
Describe Schlenk-type eq
A
- “Scrambling” of ligands around a lanthanide centre which reflects ionic bonding and can be inhibited by steric demand of substituents.
- LnR3 + LnR3’ <–> LnR2R’ + LnR2’R <–> etc
10
Q
Describe protonolysis
A
- Deprotonation of a substrate by a more basic Ln-X bond.
- Deprotonation ability roughly follows electronegativity i.e. the more stable anion will form.
- Ln-C will deprotonate H-N, Ln-N will deprotonate H-O etc
- Driven by pKa- more e-neg atom the happier the anion
- Ln(N(SiMe3)2)3 + MeOH –> Ln(N(SiMe3)2)2(OMe)
11
Q
Describe salt metathesis
A
- Reaction of a more electrophilic metal-element bond with a lanthanide halide.
- Relies on the high lattice energy of simple halides such as LiCl to drive the reaction forward
- LnCl3 + 3 LiN(SiMe3)2 –> Ln(N(SiMe3)2)3 + 3LiCl
12
Q
What is lattice energy
A
- energy gap between the energy of the separate gaseous ions and the energy of the ionic solid.
13
Q
Describe Adduction
A
- Generally, good adduct formers are hard 2 electron donors like phosphine oxides, cyclic ethers.
- Can be inhibited by extremely bulky ligands or high coordination numbers.
- Replacement of bridging atoms in oligomers by donor molecules leads to monomers, albeit without reducing coordination number.
- e.g. adds O=PPh3
14
Q
Describe oligomerisation
A
- Essentially a type of adduct formation where the 2-electron donor is a ligand on another metal centre acting as a bridge.
- Size of oligomer determined by steric demands.
- Can lead to dimers, trimers etc
15
Q
Describe reductive transmetallation
A
- Reduce one metal to 0 OS as ligand swaps to other
