Small molecule activation Flashcards
N2
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)
N-N dissociation energy
E = 945.4 kJmol-1
Interesting reactivity of N2 complexes
Due to partial -ve charge on N
Draw
N2 activation scheme
Draw
Examples of N2 complexes
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
Displacement of N2 by various ligands
e.g. PMe3, CO, imines, isocyanides
Draw
Isocyanides
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
Isocyanide resonance forms
Draw
Making NH3 from N2 using homogeneous complexes
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+)
Problems associated with homogeneous complexes for N2 activation
Can disproportionate and become deactivated (draw)
Ideal position of ligands with respect to coordinated N2
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
Stoichiometric N2 activation using [MoCl3(THF)3] complex as starting material
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 :(
Catalytic N2 activation
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
Side reaction in catalytic N2 activation
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
Intermediates in catalytic N2 activation
All neutral intermediates can be crystallised
Mo-N bonds length decreases and N-N bond length increases from M
Mossbauer spectroscopy
Versatile technique used to study nuclear structure via the absorption and re-emission of gamma rays
Especially useful for analysis of 57Fe
Characterisation of CO2-containing complexes
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)
13C NMR free CO2
124 ppm
13C NMR eta2-CO2
150-250 ppm
Downfield from free CO2 due to interactions with the metal centre
How to distinguish between CO2 and CO3(2-)
Using C-P coupling constants
i.e. they will have different couplings to phosphorus ligands on the metal
Binding modes of CO2
Side-on (pi-complexes), usually with an electron rich metal M–>C back bonding
End-on (sigma-complexes), usually with electron poor metals (M
Bridging modes of CO2
Draw
Stoichiometric CO2 activation
Via nucleophilic attack from an amide ligand to form a carbamate
OR
Insertion into M-O bond to form a carbonate
Catalytic CO2 activation
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
Stoichiometric reactions to investigate catalytic CO2 activation
- Cyclohexane oxide inserts into the bridging carboxylate - showed that is was possible to ring-open an epoxide using CO2
- CO2 inserts into a bridging alkoxide - showed that CO2 could be activated
- 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
Catalytic CO2 activation cycle
Draw
Why are metal carbonyls important?
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
Resonance forms of metal carbonyl complexes
Draw
Various bridging forms of metal carbonyls and their IR stretching frequencies
See flashcard
Why is bridging of CO more common for 1st row TMs rather than larger metals?
Larger metals prefer not to bridge because of the M-M bond length and the M-C-M angle
Reactions of metal carbonyls
Substitution
Electrophilic attack at O
Nucleophilic attack at C
Migratory insertion
Substitution reactions of metal carbonyls
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
Electrophilic attack at O
e.g. with AlMe3
Bulky Lewis acid binds at O
(H+ will also bind at O)
Draw
Nucleophilic attack at C
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
Migratory insertion reactions of metal carbonyls
Reagent = e.g. nucleophilic phosphine
Draw
Can study whether the Me or CO ligand migrates using 13C-labelled compounds
