Dinuclear Palladium(II)
and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature
Direct 2‑Phenylation of 1‑Methylindole
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Abstract
New dinuclear palladium(III) compounds of general formula
Pd<sub>2</sub>[(C<sub>6</sub>H<sub>4</sub>)PPh<sub>2</sub>]<sub>2</sub>[O–O]<sub>2</sub>Cl<sub>2</sub>, O–O being chelating
phenolates C<sub>6</sub>H<sub>4</sub>OC(O)R (R = CH<sub>3</sub>, <b>3a</b>; R = C<sub>2</sub>H<sub>5</sub>, <b>3b</b>; R = OPh, <b>3c</b>) or acetylacetonates RC(O)CHC(O)R (R = CH<sub>3</sub>, <b>4a</b>; R = CF<sub>3</sub>, <b>4b</b>; R = C(CH<sub>3</sub>)<sub>3</sub>, <b>4c</b>), have been obtained by oxidation
with PhICl<sub>2</sub> of the corresponding palladium(II) compounds.
The stability of the new compounds has been studied by <sup>31</sup>P NMR spectroscopy from 200 to 298 K. DFT calculations of the stability
of the complexes have also been performed. In agreement with these
calculations, only compound Pd<sub>2</sub>[(C<sub>6</sub>H<sub>4</sub>)PPh<sub>2</sub>]<sub>2</sub>[(CF<sub>3</sub>C(O)CHC(O)CF<sub>3</sub>]<sub>2</sub>Cl<sub>2</sub>, <b>6b</b>, showed the highest
thermal stability. <b>6b</b> was characterized by X-ray diffraction
methods, presenting the longest Pd–Pd distance, 2,6403(6) Å,
observed among the already described discrete Pd<sub>2</sub><sup>6+</sup> compounds. The isolated palladium(II) and -(III) compounds have
been tested at room temperature in the catalytic 2-phenylation of
1-methylindole with [Ph<sub>2</sub>I]PF<sub>6</sub>. With <b>3a</b> as precatalyst the reaction was completed in 2 h with a 93% isolated
yield. The results were compared with those obtained with other orthometalated
dinuclear and mononuclear palladium compounds
