Homogeneous Catalysis
In homogeneous catalysis, the catalyst and the reactants are in the same phase (usually liquid). The catalyst enters the mechanistic cycle by forming intermediates and is regenerated at the end. The reaction rate increases, but the thermodynamics (ΔG°) remains unchanged.
General Principles
A catalyst:
1. Provides an alternative reaction pathway with a lower activation energy. 2. Is not consumed overall (net zero in the stoichiometry). 3. Has no effect on thermodynamic equilibrium (K values are unchanged).
Selectivity is often superior in homogeneous catalysis compared to heterogeneous: ligand design gives precise control over regio- and stereoselectivity.
Transition-Metal Complexes
The most important homogeneous catalysts are organometallic complexes of transition metals (Pd, Rh, Ir, Ru, Ni). Key elementary steps in a catalytic cycle:
- Oxidative addition: metal oxidizes from +n to +(n+2) by inserting into a σ bond (e.g. Ar–X + Rh^I → Ar–Rh^III–X).
- Migratory (1,2-)insertion: an alkene inserts into an M–H or M–C bond.
- Reductive elimination: metal reduces by forming the C–C or C–X bond of the product.
- Transmetalation: organic ligand exchange between two metals.
| Step | Metal oxidation state change | Example |
|---|---|---|
| Oxidative addition | M^n → M^(n+2) | Rh^I + H₂ → H₂Rh^III |
| Reductive elimination | M^(n+2) → M^n | H₂Rh^III → Rh^I + H₂ |
| Migratory insertion | unchanged | Rh–H + alkene |
Wilkinson Hydrogenation
Wilkinson's catalyst, RhCl(PPh₃)₃, hydrogenates terminal alkenes with excellent selectivity under mild conditions (25 °C, 1 atm H₂). The catalytic cycle:
1. Phosphine dissociation: RhCl(PPh₃)₃ → RhCl(PPh₃)₂ + PPh₃ (activation) 2. Oxidative addition of H₂: Rh^I → Rh^III dihydride 3. Alkene coordination then migratory insertion 4. Reductive elimination → alkane + regenerated Rh^I
Rhodium (Rh) remains coordinated by the phosphines throughout the cycle, conferring stability and selectivity.
Other Industrial Examples
- Monsanto process: carbonylation of methanol to acetic acid (Rh or Ir catalyst).
- Olefin metathesis (Grubbs): Ru catalyst — Nobel Prize 2005.
- Wacker oxidation: Pd/Cu in aqueous solution; ethylene → acetaldehyde.
Homogeneous catalysis enables fine engineering of the active site through ligand design, at the cost of more complex catalyst separation compared to heterogeneous systems.