electronic supplementary material

Catalytic Hydrocarbon Oxidation by Palladium-bis-NHC-Complexes
Dominik Munz, Thomas Strassner*
Physical Organic Chemistry, Technical University Dresden, Bergstrasse 66, 01069 Dresden, Germany
* Corresponding Author: thomas.strassner@chemie.tu-dresden.de
1. Formation of L2PdIVX4 species after addition of K2S2O8 to L2PdIIBr2
Below, five 1H NMR spectra are given, measured at room temperature in HOTFA with a 300 MHz Bruker
AC 300P NMR spectrometer. The reactions have been performed in screw-cap NMR tubes in a mixture of
trifluoroacetic acid (0.36 mL) and trifluoroacetic acid anhydride (0.04 mL) with a catalyst loading of
31 mol. The NMR tubes contained a capillary filled with DMSO-d6 in order to provide a lock signal. The
golden and red spectra were measured after heating at 60 °C overnight, whereby the NMR tubes were
kept under constant vibration. All manipulations have been performed on air.
0.0010 NMR_Spektra_PalladiumIV_corrected.010.001.1r.esp
Normalized Intensity
Chemical Shift (ppm)
Assignments of the signals are given for the green Pd(II) complex. The methylene bridge splits up in two
doublets (6.1 ppm and 6.5 ppm). The considerable shift can be attributed to the coordination of bromide,
which is much less basic than trifluoroacetate. For a thorough discussion see Kraft et al. (JACS 2011, 133,
1832.). Without halides present in solution, a singlet is obtained for the methylene bridge as shown in the
blue spectra after the addition of two equivalents of AgBF4. The 1H NMR spectra for the corresponding
trifluoroacetate or acetate complexes are basically equivalent. This means that the two protons of the noncoordinated CH2 group become equivalent due to a ring flip of the six-membered palladacycle. Likewise,
a singlet is obtained in strongly coordinating solvents like DMSO-d6.
After addition of bromine (grey) a reddish precipitate forms instantaneously. However, (at least one)
second, soluble species forms. Most remarkable is the shift of the CH3 wingtip signal from 3.9 ppm to
4.2 ppm, and a shift of the two imidazolylidene protons to 8.0 ppm and 7.6 ppm (see integration). This
species is assigned to a soluble PdIV complex. The shift of the signals is in line with the results by Kraft et
al. (J. Am. Chem. Soc. 2011, 133, 1832), who reported a similar shift to lower field for a soluble L2PdIVCl4
complex (with long alkyl wingtips, in order to improve solubility) in regard to L2PdIICl2. The
palladium(IV) complex L2PdIVBr4 cannot be isolated, as it eliminates molecular bromine in vacuo at room
temperature (cf. J. Am. Chem. Soc. 2011, 133, 1832). But we have been successful in isolating the
corresponding L2PdIVCl4 chloro complex (light green), which is also insoluble in HOTFA. After several
hours at 60°C and addition of AgOTf we received the spectrum given above for comparison. Furthermore,
the formation of the palladium(II) complex L2PdCl2 is observed, most probably due to reductive
elimination of Cl2.
After addition of K2S2O8 to the complex L2PdIIBr2 and heating over night at 60 °C (red), (at least one)
new soluble species with very similar 1H-NMR shifts forms: Signals are obtained at 4.2 ppm, 7.6 ppm,
8.0 ppm (broad). Likewise to the experiment with the addition of bromine, an insoluble reddish precipitate
is observed.
We therefore conclude that the addition of K2S2O8 to L2PdIIX2 in HOTFA unambiguously leads to the
formation of L2PdIVX4 species. There is very strong indication that the reddish precipitate corresponds to
L2PdIVBr4, which can be transformed to better soluble species in HOTFA by exchange of at least one
bromido ligand, e.g. for one trifluoroacetato ligand. The latter species correspond to the palladium(IV)
complexes evidenced by the 1H NMR spectra shown above.
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