Metal-allyl complexes Flashcards
Why are they important?
Important precursors/intermediates during homogeneous catalysis catalytic cycles
General propertjes of allyl ligands
Can be anionic, neutral (radicals) or cationic
Contain an even number of valence electrons (pi electrons)
Contain an odd number of carbon atoms (3C and 5C most important)
Unsaturated
Binary allyl complexes are labile - the metal is released by dimerisation of the organic ligand (i.e. so are important precursors for homogeneous catalysis)
Methods of synthesis of metal-allyl complexes (with anionic allyl ligand)
- Substitution
- Sigma-allyl to pi-allyl rearrangement
- Pi-olefin to pi-allyl rearrangement
- Substitution reactions for synthesis of metal-allyl complexes
- X- ligand replaced by allyl- ligand
- Metal salt + organometallic reagent = a type of salt metathesis (d.f.), good way of preparing binary allyl complexes (therefore synthesis and isolation must be carried out at low T because these complexes are thermolabile - will decompose to give oligomers of the allyl ligand and M(0))
- Sigma-allyl to pi-allyl rearrangement for synthesis of metal-allyl complexes
- Carbonyl metalate + allyl halide. Driving force = salt formation
- Metal hydride + diolefin
Metalate
= any complex anion that contains a metal ligated to several atoms/small ligands
- Pi-olefin to pi-allyl rearrangement for synthesis of metal-allyl complexes
- Electrophilic/nucleophilic addition to diolefins (requires a Lewis base to donate electron density and provide a more stable coordination number)
- Hydrogenation of an electron-excess complex
Psi1
0 nodes
Sigma-bond from ligand to metal s, pz and dz2 orbitals
Psi2
1 node
(HOMO?)
Pi-bond from ligand to metal py and dyz orbitals
Psi3
2 nodes
(LUMO?)
Pi-backbonding from metal to ligand px and dxz
Metal-ligand interactions in metal-allyl complexes
The MO interactions result in an electronic rotational barrier
Syn and anti protons are inequivalent, although dynamic behaviour is often observed in solution so they appear equivalent on the NMR timescale
Dynamic behaviour in metal-allyl complexes
Pi-sigma-pi rearrangement
C-C and C-M bond rotation in the sigma-complex allows syn and anti protons to change positions
Reactions of metal-allyl complexes
MOST metal-allyl complexes are attacked by nucleophiles (basically anything with a lone pair)
Fe allyl anions attack electrophiles (i.e. Fe attacks E+, the complex itself acts as a nucleophile)
Dienyl and trienyl allyl complexes
Eta5 and eta7 complexes are prepared using similar reactions to those of the eta3 complexes
e.g. hydride addition to an eta6 ligand —> goes to eta5 allyl
Pentadienyl complexes
e.g. Cp
Can exist as: closed (i.e. Cp itself), open bridged and open
Open pentadienyl complexes
Opening leads to more steric bulk - open metallocenes are more sterically shielded
Form coordinatively unsaturated species i.e. they are less likely to form (eta5-C5H7)2MLn
More stable than closed analogues
Why are complexes formed with open pentadienyls more stable than closed pentadienyls?
Ring opening perturbs the pi MOs on Cp
Cleaving the bonding pi MOs results in their energy being raised
Cleaving the associated anti-bonding MOs lowers their energy
The HOMO/LUMO gap decreases - therefore open pentadienyls are better pi donors (higher energy HOMOs) and better sigma acceptors (lower energy LUMO)
An open cyclopentadienyl therefore has stronger M-L bonds, but the ligand is also more reactive (c.f. Cp which is less reactive because it is aromatic and wants to preserve this)
DGM rules
Can predict the direction of kinetically controlled nucleophilic attack at 18 VE cationic complexes
Applying DGM rules
- Even coordinated polyenes react first (i.e. eta2/4/6)
- Open polyenes react preferentially over closed polyenes
i. e. so the order is Even Open, Even Closed, Odd Open, Odd Closed - For even and open polyenes, attack takes place at the terminal carbon
- For odd and open polyenes, attack only takes place at the terminal carbon if MLn+ is a strongly electron-withdrawing fragment
C3H5 radical nucleophilic attack
C3H5 radical = ‘odd & open’
Ligand HOMO is singly occupied and low lying so can easily accept electron density from other ligands on the metal (to become C3H5-) or can have electron density removed from the HOMO by other ligands on the metal (to become C3H5+)
C4H6 nucleophilic attack
C4H6 = ‘even & open’
LUMO is high energy so cannot accept electron density
Therefore, the only choice is to remove electron density from the HOMO, meaning the terminal carbon becomes delta+
DGM points to note
Regio- and chemoselectivity of attack is determined by the unequal positive charge on ligand carbon atoms as a result of metal coordination
(In free alkyls/polyenes/cyclic systems, the charge distribution is approx. equal)
Open polyenes are attacked preferentially over closed polyenes because the charge distribution is less even
