I4 Flashcards
What is a sigma complex?
Type of coordination complex where a metal directly interacts with the sigma bond of a molecule namely (H-H) (fancy) n^2 the bond remains intact.
What are the main issues for the activation of hydrocarbons?
1) Relatively inert due to strong bonds
2) No rxn at high temp- only unselective transformation?
3) Formed products always more reactive
What is the activation of a C-H bond in a coordinated ligand called?
Cyclometallation
What is agnostic bonding?
C-H bond interacting weakly with a metal centre, form due to metal having deficient d-electrons, sigma bind donation stability and relieves ring strain
How does agnostic bonding activating C-H?
Agostic interaction aligns C-H bond due to the metal electron density donated into empty metal orbital.
Metal can then cleave C-H bond
Describe stereochemical Non-rigidity (fluxonial behaviour)
Rapid reversible movement of ligand/atoms within a molecule making them appear equivalent in NMR- lower temp shows 2 peaks as the complex isn’t equivalent
What is the Indenyl effect?
Fluxonial behaviour changes its hapacityfrom fancy n 5 to fancy n 3, doesn’t lose aromatic its so is facile.
What are the main differences between heterogenous + homogeneous catalysts?
Het- Catalyst can be separated readily, amendable
Disadvantages- low specifity, high rxn temp, mechanisms hard to determine.
Homo- High specifity, low rxn temp, mechanisms easier to study.
Disadvantages- Removal of catalyst problematic, less amendable to continuous processes.
What are the steps in the catalytic cycle?
Catalyst + substrate(association) —> cat. Substrate —> cat. Intermediate (principle rxn) —> cat. Product—> Dissociation product leaves catalyst
What are the 3 mechanisms for oxidative addition?
Concerted- 3 centre- TS. - stereospecific
Ionic mechanisms/nucleophillic attach(SN2)- favoured when polar
Radical chain rxn- complete loss stereospecific
What is the structure of a Fischer Carbene?
X
/
Heteroatom stabilised L-type - it is a poorer TT acceptor and better sigma donor than CO [M]=C
\
OR
Metal double bonded to a carbon with OR and CH3
How do you synthesise a Fischer carbene?
Rxn occurs from CO—> MeLi then OLi and C-Me and the carbon is double bonded to the metal
What is a N- Heterocyclic carbene?
Pentagon with 2N’s at the top 1,3 respectively these have tuneable appending groups and the point of the 2 position has a stabilised + localised lone pair which is an excellent sigma donor ligand
How are free NHC’s synthesised?
Deporotonation of an imidazoloum salt
Metal complexation is achieved via direct rxn with pre-isolated free NHC
Or
In-situ deprotonation of imidazoloum salt, followed by rxn with a metal complex
NHC binding modes? (3)
Singlet lone pair binds to metal
Singlet lone pair binds to metal centre which back bonds
And both from NHC (pi acceptor)
Schrock Alkylidenes- what are they?
Triplet carbene, high metal O.S, electron rich, alkyl+H
What is the synthesis of Schrock alkylidenes?
Ta+ Bu3, Cl2 (image in camera roll)
What is Olefin metathesis?
C=C bonds are broken and reformed resulting in a rearrangement of alkene fragments
What drives ring closing metathesis?
Removal of volatile C2H4
What drives ROMP?
Release of ring strain- all steps in equilibrium but shifts to favour the release of strain
What metals participate in sigma-bond metathesis?
d0 oxidative addition not possible, Sc, lanthanides, Grp 4: Ti, Zr
Concerted rxn via 4 centre (box) TS, square exchange
What are the reactants for lanthanide catalysts for alkene polymerisation?
2LnCl3 + 4Na(fancy n5 - C5Me5) in THF. forms cp square with Cl’s bridging Ln’s, react with 2 LiMe to swap out Cl’s for Me’s multiple insertions and ends with hydrolysis
What is Ziegler Natta alkene polymerisation?
Zirconocene dichloridenpre-catalyst reacts with excess MeAlO which comes from AlMe3 + H2O —> [MeAlO] (sets of 3 linked by O’s)
What is a key reagent in the synthesis of alkyl cations?
B(C6F5)3 - takes away methyl’s of Zr metal which has 2 cp’s on it
