Dissertation

|3.3 NN-NHC-Ligand bbip: Toward Second Generation Catalysts|
In addition to the preparation of the palladium complex, the silver precursor was used to
synthesize a rhodium-NHC-complex [(tbbpy) 2 Ru(µ-bbip)Rh(cod)Cl] 2+
(Ru(bbip)Rh) using
[Rh(cod)Cl] 2 (COD = 1,5-cyclooctadiene) for the transmetalation reaction. Similar work-up gave
a red solid compound which was characterized by NMR- and MS-methods. In this case all CH 2 -
signals of the benzyl group are shifted with respect to the asymmetric NHC-bound {Rh(cod)Cl}-
metal fragment so that all four protons give independent signals with the characteristic 2 J-coupling
to the respective geminal proton (δ Ru(bbip)Rh
CH 2
(d, 1H, 2 J = 20 Hz) and δ Ru(bbip)Rh
CH 2 ”
= 7.20 ppm (d, 1H, 2 J = 20 Hz), δ Ru(bbip)Rh
CH 2 ’
= 6.75 ppm
= 7.05 ppm (d, 1H, 2 J = 20 Hz), δ Ru(bbip)Rh
CH 2 ”’
= 6.95 ppm (d, 1H,
2
J = 20 Hz), see figure 95) in contrast to the silver complex (δ Ru(bbip)Ag
CH 2
= 6.50 ppm, (s, 4H)).
Additionally, it was possible to identify the COD-related signals which further supported the
formation of the desired compounds.
Mass spectrometric analysis revealed the characteristic pattern (for details see figure 97)
which correspond to the calculated isotopic patterns for the expected binuclear complex
Ru(bbip)Rh. Furthermore, unexpected peaks which refer to AgCl-adducts and Rh(cod)Cladducts
of Ru(bbip)Rh were found. It was concluded that undesired soluble [AgCl 2 ] − or higher
AgCl-cluster counter ions which did not precipitate from the solution were still present in the
product, which is a common problem in transmetalation reactions with formation of silver
halogenides. [188, 191] Unfortunately, it was not possible to remove the impurities from the mixture
by further work up steps such as dissolving the complex in dichloromethane and reprecipitation
with diethyl ether. Counter ion exchange with a [PF 6 ]-salt did not succeed.
3.3.8 Spectroscopic Characterization
For the series of this new ruthenium chromophores photophysical properties were determined.
Special interest was placed on the influence of the electronic transformations at the imidazole
moiety in the N,N’-coordinated ruthenium complexes with the new ip-type ligands. Preliminary
insights were derived from steady-state absorption and emission studies in dichloromethane and
acetonitrile (see figures 98 and 99, compare table 14 on page 143).
At first glance, all described complexes show the typical broad 1 MLCT absorption between 400
and 500 nm which give rise to similar 3 MLCT emission in the 550 to 800 nm range (for an example
see figure 98). [36] Upon closer analysis, however, subtle changes become visible as the ligand
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