Chemistry Reference
In-Depth Information
a) NHC
M
σ
-donation
d
NN
R
R
b) NHC
M
π
-dona tion
d
N
N
R
R
c) M
NHC
π∗
-backdonation
NN
R
R
d
Figure 4.3
Simplified depiction of the bonding molecular orbitals of the M-NHC
bond.
metals such as Pd(0), the contribution of the p-interaction has been esti-
mated at 15 and 20%, respectively.
9d,20
The NHC
M p-donation results
from the overlap between the filled p-orbital of the NHC and a d-orbital of
the metal. The M
-
NHC p*-backdonation results from the overlap between
a p* orbital of the NHC and the HOMO frontier orbital of the metal. The
latter represents 475% of the global M-NHC p-interaction for late transition
metals.
9d,20
Several methods exist to quantify the electronic properties of the NHC
ligands,
21
and notably include methodologies based on pK
a
or nucleophili-
city measurements, IR and NMR spectroscopy and electrochemistry. The
most commonly used metric is the Tolman electronic parameter (TEP),
22
which consists of the measurement of the IR stretching frequency of the
carbonyl ligand(s) for complexes of formula [M(CO)
n
L]. The more electron
donating the ligand is, the lower the stretching frequency of the carbonyl
ligand(s) will be. The TEP is the frequency of the A
1
vibrational frequency
and is typically found between 2050 and 2100 cm
1
. To measure the TEP,
metal complexes of Ni, Rh or Ir can be prepared.
21
The steric parameters of NHCs are also of crucial importance. Especially in
palladium catalysis, bulky yet flexible
23
NHCs are believed to have a positive
influence on the reactivity of the complexes in catalysis. This is why recent
efforts have been devoted towards the development of a second generation
of bulky NHC ligands (Figure 4.2).
9e,24
Compared with phosphines, the steric
configuration of the NHCs is completely different. Indeed, for the phos-
phines, the bulk expands away from the metal centre. For the NHCs, the
bulk is directed towards the coordination sphere of the metal (Figure 4.4). In
order to evaluate and compare the bulk of the ligands, Tolman developed the
cone angle for phosphine ligands.
22
Because phosphines and NHCs are
geometrically different, the Tolman cone angle cannot be used to quantify
-
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