Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature Direct 2‑Phenylation of 1‑Methylindole

New dinuclear palladium­(III) compounds of general formula
Pd2[(C6H4)­PPh2]2[O–O]2Cl2, O–O being chelating
phenolates C6H4OC­(O)­R (R = CH3, 3a; R = C2H5, 3b; R = OPh, 3c) or acetylacetonates RC­(O)­CHC­(O)­R (R = CH3, 4a; R = CF3, 4b; R = C­(CH3)3, 4c), have been obtained by oxidation
with PhICl2 of the corresponding palladium­(II) compounds.
The stability of the new compounds has been studied by 31P 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 Pd2[(C6H4)­PPh2]2[(CF3C­(O)­CHC­(O)­CF3]2Cl2, 6b, showed the highest
thermal stability. 6b was characterized by X-ray diffraction
methods, presenting the longest Pd–Pd distance, 2,6403(6) Å,
observed among the already described discrete Pd26+ compounds. The isolated palladium­(II) and -(III) compounds have
been tested at room temperature in the catalytic 2-phenylation of
1-methylindole with [Ph2I]­PF6. With 3a 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.