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NN
NN
R
R
R
R
H
H
X
CCl
3
NN
R
R
NN
NN
R
R
R
R
S
CO
2
Scheme 4.1 Routes for formation of free NHCs.
(4.1)
Pd
L
+
C
Pd
NHC
+L
L = Labile ligand such as dba, P R
3
,
μ
-C l
Ba se
-Base.HX
(4.2)
Pd
+
NHC.HX
Pd
NHC
(4.3)
Pd
R
+
.
X
X
Pd
NHC
R = Basic group such as AcO
+LPd
(4.4)
X
Ag
NHC
Pd
NHC
NHC.HX
+
1/2
Ag
2
O
-1/2 H
2
O
-LAgX
Scheme 4.2 Routes to Pd-NHC.
then transmetallated to Pd in a second step. The disadvantages of the
method are the cost and the light sensitivity of the silver complexes. Method
(4.4) should be only used when routes (4.1), (4.2) and (4.3) fail or are
impossible.
4.2.3 Electronic and Steric Parameters of the NHCs
Electronically, the Pd-NHC bond is composed of three main components: (a)
the NHC
-
NHC p*-backdonation.
9d,20
A simplified diagram depicting the bonding
molecular orbitals of the M-NHC bond is presented in Figure 4.3. Most of
the M-NHC bond is comprised of the NHC
-
M s-donation; (b) the NHC
-
M p-donation; and (c) the M
M s-dative bond. NHCs are
stronger s-donors than the most basic phosphines. As a consequence, an M-
NHC bond is typically stronger than an M-phosphine bond and the dis-
sociation of the NHC from the metal centre is more dicult. This behaviour
is presumed to have a significant positive impact on catalysis. The overall M-
NHC p-interaction (NHC
-
NHC p*-backdonation)
is relatively small compared with the s-interaction. Nevertheless, it is non-
negligible.
9d,20
For the formally d
8
metals such as Pd(II) and the formally d
10
-
M p-donation and M
-
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