Dissertation

|3.3 NN-NHC-Ligand bbip: Toward Second Generation Catalysts|
NHC complexes are known from literature. [178] These strategies include isolation of the free
carbene with subsequent complexation to an unsaturated metal fragment, cleavage of the
tetraaminoetene-derivatives (from the Wanzlick equilibrium) with suitable metal precursors,
preparation of a carbene transfer agent for transmetalation or the synthesis of the NHC-ligand at
the metal center. [179, 180, 181, 182, 183] Several different synthetic routes where studied, attempting the
transformation of the imidazolium substructure into the NHC unit.
The deprotonation of the imidazolium salt represented the first attempt to isolate a free carbene
species (see figure 87).
N
N
N
N
Ru 2+
N
N
N
N
H
NaH
N
N
N
N
Ru 2+
N
N
N
N
Figure 87: Attempted deprotonation of the imidazolium salt for the preparation of the NHC-species
of Ru(bbip).
A metal hydride reacts irreversibly with the loss of hydrogen to give the desired carbene, with
the inorganic salt by-products and excess hydride being removed by filtration. [179] Unfortunately,
deprotonation in dry THF under inert conditions at -80°C with NaH as base did not lead to a
persistent carbene, as experienced by other work groups. [184] This case, the reaction was possibly
too slow in tetrahydrofuran due to the relative insolubility of the metal hydride and the salt.
Otherwise, a fast consecutive reaction of the highly reactive carbene with traces of impurities
could disguise its intermediate formation. Performance of this reaction in the presence of the
previously prepared carbene complex precursor [Mo(thf)CO 5 ] (to trap the carbene intermediate)
did not yield the desired carbene complex (see figure 88).
The second attempt aimed at the recently reported preparation of the NHC⋅B(Et) 3 adduct. This
method was developed by Yamaguchi and Ito et al. and yielded in their case a stable adduct
in moderate yields (46 - 80%) as could be confirmed through X-ray analysis. Furthermore, the
|125|