Transition metal catalysis Flashcards
If we want to break a π-bond, what do you have to do?
- Attack the anti-bonding orbital
How strong is a M-R bond
R = H, alkyl
= 30-50 kcal mol⁻¹
If we have large ligands, which region on the metal complex will it occupy
Large ligands will occupy the equatorial region
(anionic ligands in the axial positions)
Why are transition metals used in catalysis?
- Metals can bind to organic substrates because they are lewis acids
- Gives close proximity of different reagents, due to shape of the d-lobes
- Variable oxidation state and coordination number (allowing the structure to alter during catalytic processes so reagents can bind and product can leave)
What is ligand association and its opposite process?
- Ligand association - ligand binds to metal
- Ligand dissociation - ligand dissociates from metal
What is oxidative addition and its opposite process?
- Oxidative addition - Addition of R-X bond across a metal with oxidation of metal. Involves metal-mediated breaking of a σ-bond anf formation of 1/2 metal-ligand σ-bonds, resulting in an increase in OS by 2
- Reductive elimination - Elimination of R-X bond across a metal with reduction of metal. Where two metal-ligand bonds are broken to form one substrate σ-bond, resulting in a decrease in OS by 2
What is Migratory Insertion and its opposite process?
- Migratory Insertion - Migration of a ligand from a metal to insert into another functional group bound to the metal
- Migratory elimination - The reverse - group eliminated from ligands and adds to metal
What is oxidative coupling and its opposite process?
- Oxidative Coupling - Coupling of two ligands bound to the metal into one ligand with oxidation of metal
- Reductive Cleavage - Cleavage of a ligand into two ligands, both bound to the metal, with reduction of the metal
Is this a stable or an unstable complex
- 1 x charged ligand of Cl-
- Makes this Rh¹ - d⁸
- A 16e-, d⁸ metal is very stable as the dz orbital is near impossible to occupy in d⁸
- Therefore it is this 14e compound (RHS) which undertakes catalysis
What is a terms for an SN2 reaction in inorganic chemistry?
IA - Intermediate Associative process
Most catlytic reactions occur via…
concerted oxidative addition (generally in a non-polar media)
How does electron count, oxidation state and charge change within this reaction
Do large ligands favour oxidative addition or reductive elimination?
Reductive elimination
Large ligands will result in a large cone angle, which will push the ligand out
What is the trend of these d⁸ square planar compounds for the rate of reductive elimination
Increase in rate from right to left
(M-H bonds are super weak)
For reductive elimination the groups being eliminated must be…
in a mutually cis orientation
What is a requirement for reductive elimination to occur?
Both ligands are cis to each other (hence cis-ligand migration)
What orbital interactions occur during a β-elimination
- Weaker M-H/M-R bonds broken as fragments attacks a π-antibonding orbital (forming a stronger bond)
- The C=C double bond from the alkene uses to 2e- to bond with the metal
- Through this interaction it results in the π-antibonding orbital being just above the weak M-H bond
- The vacant coordination site is then filled by a solvent molecule
Despite no seeming entirely favourable, why does this reaction go
- Electrophilic metal and empty d-orbital abstract 2e- from σ-bond
- The creation of π-bond compensates for weak M-H bond generated
What is the products of this type of reaction?
What is the product of this type of reaction
Sqaure planar geometry is favoured when…
For Rh(I), Ir(I), Ni(I), Pd(II) and Pt(II) with square planar geometry are favoured when 16e-
What are two core requirements of catalysts?
- Catalysts should bind a substrate selevtively… BUT not bound too tightly
- Centres more ready to exist in a coordinatively unsaturated state, e.g. square planar
- (generally second row transition metals are more active than third row - stonger bonds in latter)
How can we activate this pre-catalyst?
- React it with H₂ (50 atm) at 50 °C
- Imaging the H-H sticking up in the TS, where it could easily react with Cl, to leave as HCl
The following is an exemplary catalytic cycle: using RuHCl(PPh₃)₃
What are the names of the 4 reactions involved in this cycle?
- Ligand association - C=C acts as a 2e- donor
- Cis-Ligand migration - π antibondin orbitals buldge out towards to hydrogen to attack
- Ligand association: put H-H close to the other reactive ligand for sigma metathesis to occur
