Lanthanide Complexes 2 Flashcards
Describe bonding in lanthanide complexes
- 4f orbitals are deep-seated and hence little involved in ligands binding
- Bonding in lanthanide complexes and solid state compounds is ionic in nature
- Largely non-directional as this suits the sterics the best e.g. not square planar
- Coordination chemistry is more in common with that of groups 1,2 and 13 than that of d-block
What are the 3 principles that determine bonding
- 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.
Are Lns contracted
- No
- The 4f orbitals are
- The Ln are big ions
What ligands are the Ln happiest to bind to
- Cl, CCPh, Br, Ou
What ligands are the Ln happiest to bind to
- Cl, CCPh, Br, OBu
What happens when LnCl3 is dissoluted in water
- Dissolution of LnCl3 in H2O results in the formation of [Ln(H2O)x]3+
- where x = 9 for the first half of the series
- and x = 8 from Gd onwards.
What is structure of hydrated lanthanide ions
- Tricapped trigonal prismatic - tysonite
Describe pH of aqueous Ln
- Acidic as a result of hydrolysis
- Due to highly polarising nature of Ln3+ cations
- [Ln(H2O)9]3+ <–> [Ln(H2O)8(OH)]2+ + H+ <–> [Ln(H2O)7(OH)2]+ + 2H+ etc
- As water coordinates to lanthanide oh bonds weaken as electron density from Oxygen stabilising OH-
What happens to pH of aqueous solutions across the group
- Increasing acidity
- Ln3+ ions reduce in size- increasing charge density
- example of lanthanide contraction
Describe of aqueous compounds are labile or stable
- Aqueous compounds are labile
- Have high rate of exchange
. exchange of water - diffusion controlled
Describe the stability constant of lanthanide complexes
- A stability (formation) constant, K1, can be written for any complexing reaction between a Ln3+ ion and a ligand Ln–.
- M3+(aq) + Ln–(aq) <–>ML(3–n)+ (aq)
- K1 = [ML(3-n)+(aq)] / ([M3+(aq)][Ln–(aq)] )
What is needed for a stable Ln complex
- The noted high rates of exchange of ligands in Ln3+ complexes makes the isolation of their coordination complexes difficult – a kinetic effect.
- Polydentate ligands form especially stable complexes because of the chelate effect i.e. the favourable entropy change involved in the process – ie thermodymanic stabilisation.
- May have strong affinity for F- but not stable so would just be hydrated
What halogens do lanthanides form most stable complexes with
- The lanthanides form more stable complexes with hard fluoride than the softer halogens.
Does Lu or La have higher stability constants
- The values for Lu are consistently higher than for La as expected for the smaller ion with a higher charge/radius ratio.
Where are irregularities seen in stability constants
- Discontinuity often seen between Eu/Gd/Tb called the “gadolinium break”
- originates from variation in x for [Ln(H2O)x]3+ from 9 to 8 at Gadolinium.
- Decrease in H2O coordination number - decrease in positive value of deltaS
What are ion exchange methods
- Spin off from manhattan project
- Makes use of lanthanide contraction - entropic drive- chelate effect
- THe mixed lanthanides are loaded on to a cation-exchange resin and then eluted with a suitable complexing agent like EDTA or citric acid
- Heavier, smaller lanthanides are better Lewis acids
- Removed from resin first in order of their stability constants i.e. decreasing atomic number
What can access 4+ oxidation states
1 Only Ce4+ stable in aqueous solution for weeks, for example cerium ammonium nitrate (CAN)- important oxidant:
2. Ce2O3 + HNO3 + NH3 –> Ce(NO3)6(NH4)2 + H2O
3. Ce4+ has higher charge density than Ln3+
4. Much better at ionising water due to higher charge density
5. Stable for weeks at a time (12 coordinate)
6. Also can cerium ammonium sulphate
Why is +4 OS accessible for Ce
- Chemically accessible due to higher energy of 4f orbitals at start of series - similar in energy to 5d orbital
- Not sufficiently stable to prevent the loss of the 4th e-
- Ce3+ has 1 f electron - only one away from completely empty shell - closed shell is stable
Which elements can access Ln(II) chemistry
- Sm, Eu, Yb
Show how Ln2+ can be accessed
- LnCl3 + 1/2 Zn –> Ln2+
Describe stability of Ln(II) Aqueous complexes
- not stable long term,
- with Eu being stable for hours
- Sm, Yb for minutes - faster oxidation
- Eu 3+ –> Eu2+ E0 is less negative than for other two
What is Sm(II) Iodide used for
- Important reagent in organic synthesis
- Very bright colour- good colour changes + most reactive
Why is Eu2+ aqueous complex more stable than Sm and Yb
- Eu>Yb>Sm
- Eu more stable with respect to own +3 forms
- Eu> Sm due to higher exchange energy
- Eu> Yb : 7 rounds of pairing energy gives stability away
- Pairing energy not as important as exchange energy why Yb>Sm
- Sm is stronger reducing agent
Describe paramagnetism of Ln3+ compounds and consequence for spectra
- Ln3+ compounds are strongly paramagnetic which precludes useful NMR data (peaks are broadened and shifted).
How can Ln be used in NMR
- Lanthanide shift reagents can be added to diamagnetic samples to induce chemical shift changes via through-space interactions.
- Occurs via transient coordination of substrate to Ln centre.
- Interaction between proton spin and spin of magnetic moment- sharpens peaks and moves peaks apart- spreads out
What are the most common chemical shift reagents
- Eu3+ and Pr3+ ions with biketonate (acac) ligands
- Have short relaxation times reducing broadening in 1H NMR spectra
- Eu3+ shifts downfield
- Pr3+ upfield
Give example of when chemical shift reagents are used
- Chiral compounds
- Allows determination of ee by integration
- Normally wouldn’t see both enantiomers
- One enantiomer reacts more readily with Sm than the other
- Relies on dynamic coordination
What is needed for ligands in aqueous Ln chemistry
- Must overcome hydrophilicity
- Tend to rely on hard donors (N,O)
- Tend to be polydentate
- Allows applications in medicine - stops Ln interacting with body as stable
Describe how MRI works
- Contrast in a magnetic resonance image (MRI) is due to the different relaxation times of water protons in different environments within the body
- Protons with short relaxation times give rise to brighter images
- Enhance the contrast in a magnetic resonance image with ‘contrast agent’
- Complexes of Gd3+ are widely used due to paramagnetic (4f7 configuration)
- Cause greatly shortened 1H NMR relaxation times for coordinated H2O molecules - rapid exchange with bulk H2O
What is needed from a ligand in in vivo applications
- Necessary to have a complex which is soluble in physiological media and in which Gd3+ is strongly bound - stop release in body
- Must have at least one free site for coordination of H2O
- Ligands must be water stable, and give solubility in i.e. blood such as DTPA (above).
Which of the Ln 2+ ions is the most useful reducing agent
- three lanthanides form stable but reducing Ln2+ ions.
- Sm2+ is the least stable (Sm3+/Sm2+ – 1.48 V) and has found use as a selective organic reductant, usually in the form of SmI2.
How can SmI2 be made
- SmI2 may be conveniently prepared as a deep blue ether solution by reaction of Sm metal and ICH2CH2I.
- Sm + ICH2CH2I –> CH2=CH2 + SmI2(THF)x
- Need to be free of air otherwise Sm2+ oxidises rapidly to become Sm3+
Complete equations for reduction by SmI2
Ph2S=O + SmI2
PhCH2I + SmI2
- Ph2S=O + 2SmI2 –> Ph2S + SmOI
- 2PhCH2I + 2SmI2 –> PhCH2CHPh + 2 SmI3
What oxidants are widely used
- Ceric ammonium nitrate [Ce(NH4)2(NO3)6] (CAN) and the analogous sulphate (CAS) are widely used as selective one electron oxidants.
- Recall Ce4+ (4f0) is the only member of the lanthanide series with a well developed 4+ chemistry.
- Can turn OH –> =O
What are 3 organic oxidations that CAN or CAS can do
- Benzoin condensations (CHO -> 2x C=O
- Ortho oxidations (OH –>=O
- alpha-C oxidations ( C=O added to carbon chain)
Describe how self cleaning ovens work
- Oxidises fat to water
- Rely on oxidising ability of Ce4+ oxide - Walls are coated in CeO2
- 2CeO2 + R2CH2 (Fat) –> Ce2O3 + H2O + R2C
- 4CeO2 + R2C –> 2Ce2O3 + CO2 + R2
- 3Ce2O3 + 1 1/2 O2 –> 6CeO2
- Overall R2CH2 + 1 1/2 O2 –> R2 + H2O + CO2
- Completely catalytic process
Describe how catalytic converters work
1.
