Catalysis in Acyclic Diene Metathesis (ADMET) Polymerization - Late Transition Metal Polymerization Catalysis - page 210

Fig. 6.3 N-Heterocyclic carbenes: numbering and crude
orbital picture neglecting C4 and C5.

erature as to the role of conjugation, but it is commonly accepted that interaction
of the two filled

-N( ) orbitals with the unfilled C( ) orbital renders the energy

of the C( ) orbital presumably

leads to the observed singlet ground state and the tendency of the NHC to form

) orbital quite high. The high energy of the C(

bonds with metals (Fig. 6.3). Saturation of the carbon-carbon double bond (C4-C5
in Fig. 6.3) leads to a lower singlet-triplet gap, however, which seems to contradict
the former statement. There is even apparent controversy over the nomenclature;
the form used in the majority of the metathesis literature will be used herein [92].
Many groups have used NHC ligands in complexes with almost every transition
metal and many other elements. The carbene unit as a ligand in metal complexes
is very -accept-

ing than PMe 3 and even nitriles as judged by trans -CO stretching frequencies in
Ni complexes. They are also very nucleophilic and basic. The p K a of the conjugate
acid has been measured to be 24 in DMSO- d 6 , and is therefore one of the strong-
est known neutral bases (phosphazenes are more basic than NHCs, however) [92].
Since the metathesis activity of the bis-phosphine ruthenium metathesis catalysts
increases with the

-donating and very poorly -accepting. These complexes are less

-donating properties of the phosphines, the use of NHC li-
gands was a quite logical extension. Nolan et al. determined that for Cp*RuClL
(Cp*=pentamethylcyclopentadienyl) complexes the substitution L=PCy 3 to L=1,3-
dimesityl-imidazol-2-ylidene (IMes) is exothermic by 5 kcal mol -1 , demonstrating
the strong bonding of this carbene to the ruthenium atom [93]. Herrmann and
co-workers have contributed greatly to the NHC field in general and reported the
first metathesis catalysts containing NHC ligands, including complex 8 [94].
These complexes showed little improvement over complex 6 in metathesis reac-
tions, but the mixed ligand complexes of the type RuCl 2 (NHC)(PR 3 )CHR 1 have
shown enormous benefits in activity and stability compared to 6 . The Nolan
group reported the mixed ligand complex 9 [93, 95], which contains the 1,3-dime-
sitylimidazol-2-ylidene (IMes) ligand. The Grubbs group independently reported
the saturated analogue, complex 10 [96], which contains the 1,3-dimesityl-4,5-dihy-
droimidazol-2-ylidene (SIMes) ligand. This saturated analog of the IMes ligand
was expected to be more basic than IMes due to the reduced conjugation through
the NHC ring and hence a better ligand than IMes for olefin metathesis with
these complexes. A large number of synthetic variations have since been reported
for these ligands [97], and although activity profiles throughout this series of com-
plexes are not straightforward, complex 10 is generally the most active and popu-
lar complex of this type.
Although the NHC ligands are carbenes, the metal-carbon bond of the NHC is
completely different than that of the ruthenium benzylidene bond, displayed in