6: Small molecule activation

Question 1

Why is small molecule activation important?

Answer 1

some of the most challenging reactions are some of the simplest
by studying the interaction and reactions of small molecules with metal complexes
-we gain an oppurtunity to learn about new mechanism to transform them into more useful chemicals
H2 N2 CO CO2 CH4

Question 2

Difference between actinides and lanthinides

Answer 2

Early TM’s are more like TM’s- easier for small model backbonding etc

Question 3

What are the underpinning reactivities

Answer 3

1) Adduct fformation
2) Insertion into M-X
3) Electron transfer
4) Sigma bond metathesis

Question 4

Adduct formation

Answer 4

-Coordination
Ln-M + CO –>Ln-M-CO
If donation and back-donation can occur, an adduct can be formed

Question 5

Insertion

Answer 5

Insertion into M-X bonds
cf. migratory insertion
Ln-M-X + CO –> Ln-M-(CO)-X
X=H or R, NHR,etc etc

Question 6

Electron transfer

Answer 6

Single electron transfer

Ln-M(n) + CO2 –> Ln-M(n+1)-O=C:O

Question 7

Sigma Bond Metathesis

Answer 7

Ln-M-X +CH4 –> Ln-M-Ch3 + X-H

X=H,R

Question 8

What are good synthetic starting points

Answer 8

Lanthanides
LnCl3 +2 KCp* --> dimeric structure 
Single halogen left for reactive site (3 Cp* around lanthanide not possible)
Thorium 
ThCl4 +2KCp* -->ThCl2Cp*2
Th(IV) complexes diamagnetic 
Uranium 
UCl4 + 3 KCp --> UCp3Cl
single halogen left for reactive site

Question 9

Actinide Carbonyls

Answer 9

• Mainly U and Th
• Mainly +4 but +3 increasing
• Display some dependancy on the symmetry and availability of the 5f orbitals
• U(CO)6 unstable but matrix studies show some degree of backbonding

-UCp3(CO) is stable at room temperature, carbonyl stretching frequency suggests backbonding
UCp3Cl+Na –> UCp3 +CO -> UCp3Co
Na reduced the U U(IV) –> U(III)

Backbonding = U(5f) –> CO(pi*

Question 10

Lanthanide Carbonyls

Answer 10

-4f orbitals to core-like and contracted to interact with the pi* orbitals of CO

Unstable Ln(CO)n (n=1-6) have been isolated by co-condensation of lanthanide vapours with CO in a argon matrix at temperatures below -40C

CO stretching frequencies increase with n suggesting competition for 4f or 5d electron density and their IR spectra is similar to those of TM- suggesting some level of back donation

None of these complexes have been definitely characterised as they decompose upon raising the temperature

suggest not 100% ionic if backbonding

Cp3Nd (iso to UCp) carbonyl complexes have been attempted but only lead to a formation of a product from CO insertion and degradation of the Cp* ligand.

Question 11

What is backbonding for CO- what does it involved

Answer 11

picture

Question 12

The big difference between uranium and TM

Answer 12

Majority of TM complexes are bound through the CO carbon

However uranium(III) compound incorporates a 6 coordinate triazacyclononane ligand = LnU
2 U(III)Ln +CO –> LnU(III)-C≡O-U(IV)Ln
- the product containds both carbonyl and isocarbonyl coordination modes
- a single electron transfer to CO results in the mixed valence

Question 13

Reductive coupling of CO

Answer 13

w/ modification of the ligands of U - it has been shown that electron transfer may be coupled to C-C bond formation
These reaction form a series of of dianions of reduced CO
Called cyclic aromatic oxocarbons of the form [CnOn]2-

Usually involves U(III)(Cp*)(COT)(solv-THF) +CO –> oxocarbons

This rxn is TF unfave in absense of metal and deliberate OrgSynth of oxocarbons is otherwise very challenging

Question 14

How to vary which oxocarbon is made 

Answer 14

2- Ynediolate O-C≡C-O (2-)
3-deltate
4-squarate
5- croconate

others 2x single C-O facing outwards joined double bond
all other C=O

all aromatic 4n+2 and 2- charge

vary by the sterics of the ligands and reaction conditions/reaction stoichiometry- eg - one less Me on Cp* or different equivalents of CO

some simple coordination complexes eg U(III) (N(SiMe2)2)3 have also been shown to couple 2 equivalents of CO to produce the corresponding ynediolate
sandwiched between two uranium - U(III) gives a single electron to each CO- they couble to form a carbon carbon bond - then more CO you add bigger etc etc

Question 15

Carbon dioxide activation

Answer 15

CO2 coordination to TM is rare
Using a U(III) triazanonane ligand previously mentioned
CO2 has been coordinated to U in an eta-1 fashion U-OCO
The reaction is propsed to occur with a single electron transfer (U(III) to U(IV))
PICTURE
end on coordination v rare - usually side on through pi bond

Question 16

Dinitrogen activation

Answer 16

End on
Dinitrogen complexes of Uranium form reversibly
At higher pressures the equilibrium is forced to the complex
-The N2 bond is slightly longer and the N2 frequency is slightly smaller

Side on
Dinitrogen complexes of U form reversibly, high p =eqm forced to the complex

These reactions proceed by oxidation of U centres to U(IV)

Question 17

Pentalene and N2

Answer 17

Picture of pentalene
With these structures, dinitrogen binding has also been observed with reversible reduction of N2
PICTURE

Question 18

Lanthanide Metallocenes

Answer 18

need 2+ –> Sm, Eu and Yb

2CpK +LnI2 –> Cp-Ln-Cp* (w/ two solvent -THF)

-Bent structure-may be explained by the polarisation ion model
-Bonding - non directional - maximise bonding if they both sit on one side of the unit,
They can polarise the cation, meaning one side more electropositive, maximise ionic interaction

Question 19

Lanthanide metallocenes and N2

Answer 19

LnCp*2 exposed to N2 reversibly forms a dinuclear bridged complex
N2 is bound very weakly and can be removed in the solid state under vacuum

TO DO
and pictures

Question 20

H2 activation

Answer 20

Th(IV) are diamagnetic meaning they can be studied via NMR spectroscopy
ThCl2Cp2 -> ThCp2R2 where R= Me or CH2CMe3

Hydrogenolysis of dialkyls yields the dimeric dihydride
via a sigma bond metathesis mechanism

NMR studies suggest that in solution there is rapid exchange between Hterminal and Hbridging hydrides and also with H2 gas dissolved in solution