Small molecule activation

Question 1

N2

Answer 1

Isoelectronic with CO (but non-polar, weaker sigma-donor and weaker pi-acceptor)
Usually coordinates end-on as a terminal ligand, side on (eta2) coordination is rare (draw)
Metal tends to be in a low oxidation state (i.e. electron rich) to allow backbonding (draw)

Question 2

N-N dissociation energy

Answer 2

E = 945.4 kJmol-1

Question 3

Interesting reactivity of N2 complexes

Answer 3

Due to partial -ve charge on N

Draw

Question 4

N2 activation scheme

Answer 4

Draw

Question 5

Examples of N2 complexes

Answer 5

RuCl3 + N2H2 —> [Ru(NH3)5(N2)]2+
[Ru(NH3)5(OH2)]2+ + N2 —> [Ru(NH3)5(N2)]2+

N2 is non-activated in both systems
Displacing the coordinated H2O is facile because the electron-rich Ru would prefer to swap a sigma-donor for a pi-acceptor

Question 6

Displacement of N2 by various ligands

Answer 6

e.g. PMe3, CO, imines, isocyanides

Draw

Question 7

Isocyanides

Answer 7

Can displace CO as well as N2
Isocyanides are stronger sigma-donors and weaker pi-acceptors than CO – but their pi-accpetor character is influenced by the nature of R

Question 8

Isocyanide resonance forms

Answer 8

Draw

Question 9

Making NH3 from N2 using homogeneous complexes

Answer 9

N2 + 6H+ + 6e- —> 2NH3

We need:
Source of H+, where the counter ion X- can’t coordinate to the metal centre
Reducing agent (i.e. source of electrons), whose oxidised species also can’t form a complex (or pick up the H+)

Question 10

Problems associated with homogeneous complexes for N2 activation

Answer 10

Can disproportionate and become deactivated (draw)

Question 11

Ideal position of ligands with respect to coordinated N2

Answer 11

Want the ligand to be trans to N2
Sigma donors can push electron density onto the metal, which would allow increased back-bonding to N2 and therefore weaken the N-N bonds
So generally use tridentate/tetradentate ligands

Question 12

Stoichiometric N2 activation using [MoCl3(THF)3] complex as starting material

Answer 12

Draw

Mo is biomimetic
Metal centre is a low oxidation state (Mo0)
N ligand is sigma-donating
BUT get pi-backbonding into sigma* of phosphine ligand, rather than onto N :(

Question 13

Catalytic N2 activation

Answer 13

Reducing agent = CoCp2 (Cp = bulky which prevents the reducing agent from forming other complexes)
Proton source = 2,6-lutidinium tetraarylborate
Ligand = tetradentate amide. Amides = good sigma and pi donors, phosphine ligands would limit the amount of back bonding to N2 because phosphines can accept electron density themselves. Bulky tetradentate ligand = stabilises the complex, forms a ‘cage’ around the metal centre that means N2 can only bind in a linear fashion

Question 14

Side reaction in catalytic N2 activation

Answer 14

CoCp*2 can react with the H+ source and produce H2 rather than NH3 (loss of efficiency)
Only 4 full turnovers of catalytic cycle possible to produce 8 eq of NH3

Question 15

Intermediates in catalytic N2 activation

Answer 15

All neutral intermediates can be crystallised

Mo-N bonds length decreases and N-N bond length increases from M

Question 16

Mossbauer spectroscopy

Answer 16

Versatile technique used to study nuclear structure via the absorption and re-emission of gamma rays
Especially useful for analysis of 57Fe

Question 17

Characterisation of CO2-containing complexes

Answer 17

Elemental analysis
Mass spectrometry
IR spectroscopy
NMR spectroscopy
X-ray diffraction
Evolution of CO2 by acidolysis, oxidation with I2 or ligand replacement (i.e. kicking CO2 out of complex)

Question 18

13C NMR free CO2

Answer 18

124 ppm

Question 19

13C NMR eta2-CO2

Answer 19

150-250 ppm

Downfield from free CO2 due to interactions with the metal centre

Question 20

How to distinguish between CO2 and CO3(2-)

Answer 20

Using C-P coupling constants

i.e. they will have different couplings to phosphorus ligands on the metal

Question 21

Binding modes of CO2

Answer 21

Side-on (pi-complexes), usually with an electron rich metal M–>C back bonding
End-on (sigma-complexes), usually with electron poor metals (M

Question 22

Bridging modes of CO2

Answer 22

Draw

Question 23

Stoichiometric CO2 activation

Answer 23

Via nucleophilic attack from an amide ligand to form a carbamate
OR
Insertion into M-O bond to form a carbonate

Question 24

Catalytic CO2 activation

Answer 24

CO2 can be used to open epoxides, using a Zn catalyst, to make polycarbonates
Was investigated using stoichiometric reactions, which also helped to elucidate the mechanism of catalysis

Question 25

Stoichiometric reactions to investigate catalytic CO2 activation

Answer 25

1. Cyclohexane oxide inserts into the bridging carboxylate - showed that is was possible to ring-open an epoxide using CO2
2. CO2 inserts into a bridging alkoxide - showed that CO2 could be activated
3. Cyclohexane oxide can form a bridging alkoxide

Altogether these experiments suggested that catalysis was possible, and that it may involve a mixture of mono- and dinuclear Zn complexes

Question 26

Catalytic CO2 activation cycle

Answer 26

Draw

Question 27

Why are metal carbonyls important?

Answer 27

Starting material for low valent (0 oxidation state) complexes which are reactive
Cheap
Can be substituted to change the reactivity of the complex e.g. by Lewis bases, olefins, arenas
Intermediates in catalysis
Complexes can be dynamic in behaviour - change geometry/coordination number

Question 28

Resonance forms of metal carbonyl complexes

Answer 28

Draw

Question 29

Various bridging forms of metal carbonyls and their IR stretching frequencies

Answer 29

See flashcard

Question 30

Why is bridging of CO more common for 1st row TMs rather than larger metals?

Answer 30

Larger metals prefer not to bridge because of the M-M bond length and the M-C-M angle

Question 31

Reactions of metal carbonyls

Answer 31

Substitution
Electrophilic attack at O
Nucleophilic attack at C
Migratory insertion

Question 32

Substitution reactions of metal carbonyls

Answer 32

CO can dissociate under thermal or photochemical conditions
Dissociation of CO from an 18 VE complex occurs to generate a coordinately unsaturated intermediate
Weak, neutral donors (i.e. ethers/nitriles e.g. THF, Et2O, MeCN) can facilitate CO loss by stabilising the coordinatively unsaturated intermediate before they are substituted themselves
An associative process can occur with unsaturated complexes e.g. 4/5 coordinate, <18 VE

Draw

Question 33

Electrophilic attack at O

Answer 33

e.g. with AlMe3
Bulky Lewis acid binds at O
(H+ will also bind at O)

Draw

Question 34

Nucleophilic attack at C

Answer 34

Hydride can attack CO to form an aldehyde ligand
Other reactions:
1. Carbene formation
2. Rare example of a chemical method to remove CO and generate a vacant site

Question 35

Migratory insertion reactions of metal carbonyls

Answer 35

Reagent = e.g. nucleophilic phosphine
Draw
Can study whether the Me or CO ligand migrates using 13C-labelled compounds