Introduction to Homogeneous Catalysis Flashcards
What is a Pre-Catalyst?
A pre-catalyst is a chemical compound that is not initially active as a catalyst, but transforms into the active catalyst under reaction conditions
This activation often involves changes such as ligand dissociation, oxidation/reduction, or reaction with a substrate
A faster catalyst means what for turnover and Gibbs energy?
- Mean more product per unit hr (go round the cycle more times)
- Smaller barrier to catalysis = higher turnover frequency (how many catalytic cycles per hr)
What is selectivity?
Why is Homogenous catalysis used over heterogenous catalysis?
- With homogeneous catalysis it is often easier to optimise for very selective reactions
- BUT the product/solvent separation costs energy and £s compared to heterogenous catalysis
- Thus, homogeneous catalysis often used for lower volume ‘fine’ chemicals (10³-10⁶ tons yr⁻¹ at >£500 per ton)
Molecular catalysts which have single sites are modulated by?
- Size of ligand
- Donor Ability of ligand
- Lewis acidity or metal (PPh₃ is a pi acceptor)
What are 3 spectator ligands you need to know about?
(sometimes monophosphines fall off the metal to easily, so diphosphines are used - chealating effect)
What are 4 reactive ligands you need to know about?
How many electrons do these two complexes have?
Rh = 9
PPh₃ = 2
Cl = 1
LHS = 16e-
RHS = 14-
What oxidation state is Rh in, in the LHS complex?
LHS = Rh¹ (d⁸ complex)
16e- is stable for a d⁸ comlex
(don’t want something stable for catalysis, hence RHS would be good for catalysis with a free coordination site)
Why use ‘Spectator’ ligands?
‘Spectator’ ligands allow us to fine-tune the reactivity of the metal complex , by altering its electronic and steric properties
we can change the electronics at the metal centre by using electron donating or electron withdrawing ligands
How?
- We can use n-accepting ligands to reduce electron density at the metal centre through n-backbonding
- We can use n-donating ligands to increase electron density at the metal centre
What affect does decreasing vs increasing the electron density at the metal have?
- Decreasing electron density at the metal favours steps like oxidative addition, that adds electrons to the metal
- Increasing electron density at the metal favours step like reductive elimination, that removes electron density from the metal
What is a ‘Ligand Accelerated Catalysis’ (LAC)
- Some ligands (L) make MLn react faster than ‘naked’ M
- Useful feature (if attained) in asymmetric catalysis with a chiral ligand
Bulky ligands favour…
complexes with lower coordination numbers
This causes ligand dissociation to form active catalytic species
If we have a very high oxidation state on the metal…
ligands can help stabilise catalytic intermediates
(prevent reactions such as dimerisation or unwanted solvent coordination)
Tolman defined a ‘cone angle’ for phosphine ligands by considering the Van der Waals surface of the R groups
What is the difference between R = OMe and R = OC₆H₄Me
R = OMe then θ = 107 °
R = OC₆H₄Me (ortho) then θ = 141 °
(Basically, the bigger the group, the larger the angle
The more a ligand donates into the antibonding orbital of the metal, does the triple C-O bond become stronger or weaker?
Weaker
Therefore the Ni-C≡O vibrational mode of Ni(CO)₃PR₃ is proportional to e- density at the metal
Tolmans electronic parameter (TEP) is a measure of the e- donating or withdrawing ability of a ligand
What two things does this plot show?
- The change in the substituent on the phosphine can make the ligands a better sigma donor, which means it will stick to the metal more
- Or it can make it a better pi acceptor, allowing it to stabilise low oxidation states and weaken the C≡O bond
What is the trans effect?
- The trans effect refers to the ability of a ligand in a square planar or octahedral metal complex to influence the rate of ligand substitution at the position trans to itself
- A strong trans-effect ligand accelerates the substitution of the ligand opposite to it
- Certain ligands can also lengthen or weaken the bond trans to them due to electronic effects
How can you alter the bite angle of a metal complex
- By using the sterics of different chelating ligands
- Different chelating ligands will have different Bite angles depending on the backbone
- (has major influence on catalysis)
Carbenes are very reactive and will flood electron density onto the metal using its lone pair
How is this stabilised?
- By using a N-heterocyclic carbene (NHC) using the lone-pair on nitrogen to interact with the p-orbital of carbon
- This makes these ligands very sticky
How can you increase the donor strength on a N-heterocyclic carbene?
- Adding R groups (i.e. big bulky R groups)
- And removing conjugation
