Metal-Carbon Multiple bonds Flashcards
What are the features of M=C carbene compounds?
- two general types of carbene ligand:
(a) the atom adjacent to the sp2 carbene carbon has p-donor properties (e.g. O or N)
(b) those without p-donor stabilization - generally: - early transition metals form complexes with the unstabilized carbene ligands
- for the later transition metals complexes of p-stabilized ligands dominate
How are carbene ligands π-stabilised?
- originally made by nucleophilic attack on a carbonyl ligand
- this was the method used by E. O. Fischer in the original synthesis of metal carbene complexes
- isonitrile (isocyanide) ligands are similar to CO in electronic structure
How to convert isonitrile ligands to carbenes (π-stabilised)?
- isonitrile (isocyanide) ligands are similar to CO in electronic structure
- can often be converted to carbenes by reaction with alcohols
How are non-stabilised carbene ligands made?
- may be made using a carbene source (e.g. diazomethane CH2N2 - readily eliminates N2)
- also hydride abstraction from a neutral or anionic metal alkyl complex
- need a H– abstracting reagent such as stable carbocation [Ph3C]+
What is a closely related appraoch to non-stabilised carbene ligand synthesis?
α-proton abstraction
Deprotonation of a neutral or cationic alkyl complex
* particularly common in high oxidation state complexes of early TMs with bulky ligands
Describe features of the metal carbon multiple bond
- bond between the metal and the carbene ligand has both s- and p- components
- compared with the CO ligand, carbenes are better s-donors and/or weaker p-acceptors
- structural evidence shows that the carbene–metal (double) bond
is significantly shorter than a metal–C s-bond - there is restricted rotation about the metal–carbene bond
- all evidence for the double character of the bond
Describe features of the reactivity of π-stabilized carbene ligands
- often relatively unreactive,
- when they do react, they often function as electrophiles
e.g. in the reaction below the amine attacks the carbene carbon as the first step in the replacement of the alkoxy group:
Draw mechanism for carbene stabilisation with N or O
Nucleophilic attack on carbene carbon
Describe features of unstabilised carbenes
- often very reactive and have nucleophilic character
i.e. the carbene carbon carries a δ– charge and is, for example, easily protonated - also undergo other reactions typical of nucleophiles (e.g. they will attack electrophilic carbon centres)
What are features of N-heterocyclic carbene (NHC) compounds?
- Arduengo isolated stable heterocyclic carbene molecules (NHCs) – often based on imidazole and related heterocycles
- stable carbenes protected by bulky substituents on the nitrogen atoms
- this development reignited interest in N-stabilised carbene complexes
- led to the discovery that ligands of this type were excellent and versatile ancillary ligands
in many situations, comparable with phosphines
Describe the ruthenium complex
1 x N-stabilized NHC drawn with a single bond to the Ru 1 x unstabilized carbene drawn with a double bond.
* the distinction is only one of degree, although the two ligands behave very differently
* this complex is very active in the alkene methathesis reaction
* known as the second generation Grubbs catalyst
What is aklene (olefin) metathesis and show an example
What are the uses of aklene (olefin) metathesis?
- possibility exists of forming complex mixtures from simple alkenes…
- applied industrially in the SHOP (Shell Higher Olefins Process)
- used to convert ethene into C14-C18 alkenes and C15-C19 terminal alcohols
- the later are used to manufacture biodegradable detergents
- SHOP is a combination of up to four different catalytic processes,
- can be tuned to deliver different ranges of alkenes and alcohols as the market demands
What are the steps of aklene (olefin) metathesis?
- Ethene oligomerization : affords a broad range of terminal alkenes (only mid range useful)
- The high and low MW terminal alkenes are isomerized catalytically to give internal alkenes that are suitable for metathesis
- Combination of short and long alkene fragments by metathesis gives useful mid-range alkenes.
- Internal alkenes are isomerized, hydroformylated and hydrogenated in one process to yield straight-chain terminal alcohols
* the metathesis reaction involves a carbene and a metallacyclic intermediate:
Draw the metal analogues of allene and higher cumulenes
What are the features of M≡C carbyne complexes and draw examples?
- first derived accidentally by Fischer from
p-stabilized carbene complexes - high oxidation state early TM complexes accessed by removal of the a-proton in an alkylidene complex by a strong base:
